Development of rare-earth doped microstructured optical fibres

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UNIVERSITY OF SOUTHAMPTON

FACULTY OF ENGINEERING AND APPLIED SCIENCE

OPTOELECTRONICS RESEARCH CENTRE

DEPARTMENT OF ELECTRONICS AND COMPUTER SCIENCE

DEVELOPMENT OF RARE-EARTH DOPED MICROSTRUCTURED OPTICAL FIBRES

by

Kentaro Furusawa

Thesis for the degree of Doctor of Philosophy

August 2003

UNIVERSITY OF SOUTHAMPTON ABSTRACT

FACULTY OF ENGINEERING, SCIENCE AND MATHEMATICS OPTOELECTRONICS RESEARCH CENTRE

Doctor of Philosophy DEVELOPMENT OF RARE-EARTH DOPED MICROSTRUCTURED OPTICAL FIBRES

By Kentaro Furusawa This thesis describes the development of novel optical fibres, microstructured optical fibres (MOFs), and

demonstrates device applications based on these structures. A particular emphasis is made on incorporating

rare-earth ions within these fibres in order to realise novel active devices. Together with the development of

the fabrication technique, characterisation and applications of these radically different fibre types are

presented.

First, the fabrication techniques of MOFs, which heavily rely upon fibre drawing, are studied. A

mathematical model developed for the capillary drawing process is experimentally examined. Good

agreement is obtained whilst it is also found that the model provides useful physical insights for determining

the fibre draw parameters even for MOFs with complex geometries. Details of the fabrication techniques

developed to optimise fibre structures are also presented.

Transmission properties of highly nonlinear MOFs are then studied experimentally. It is found that the

transmission losses are strongly influenced by the core dimensions due to the high Rayleigh scattering

coefficient that originates from the holey cladding. A simple model is used to explain the observations. In

addition, a continuous effort towards reducing OH-induced losses of this fibres type is outlined.

Rare-earth doped highly nonlinear MOFs are fabricated and characterised. Then, three device demonstrations

are carried out for the first time. These include a mode-locked ytterbium doped MOF laser, a nonlinear

amplifier based on an ytterbium doped MOF, and a continuous wave erbium doped MOF laser with a very

low threshold and high efficiency. Using the ytterbium doped MOF, wide tunability of ultrashort pulses from

1µm to 1.58µm is demonstrated using the soliton self frequency shift effect. For the erbium doped MOF, a

pump power threshold of 0.5mW and a slope efficiency of 57% are demonstrated.

Novel cladding pumped fibres, air clad MOFs, which use a conventional inn er cladding and a holey outer

cladding, are developed aiming at improved performance of cladding pumped fibre lasers. Wide tunability

over 110nm and pure three level operation at 980nm of ytterbium doped cladding pumped fibre lasers are

demonstrated.

Finally, the fabrication and characterisation of large mode area microstructured fibres (LMA-MOFs) are

described, and a comparison with conventional counterparts is made in terms of bend losses and

corresponding effective mode areas. The results show that a slight refractive index difference introduced in

the core region of this fibre type strongly modifies its waveguide characteristics. By applying this knowledge,

a novel ytterbium doped cladding pumped fibre, which uses different sizes of air holes to define the inner and

outer cladding, is developed. A continuous wave output power in excess of 1W is obtained. Results

concerning various forms of pulsed laser operation using this fibre are presented and future possibilities are

discussed.

List of Contents List of contents Acknowledgements Chapter.1 Introduction

1.1 Introduction................................................................................................................................. 1 1.2 Properties of MOFs ..................................................................................................................... 2 1.2.1 Single mode operation ......................................................................................................... 2 1.2.2 Group velocity dispersion (GVD)......................................................................................... 3 1.2.3 Nonlinearity ........................................................................................................................ 4 1.3 Applications ................................................................................................................................ 5 1.3.1 Nonlinear devices ................................................................................................................ 5 1.3.2 Optical devices .................................................................................................................... 6 1.4 Outline of the thesis ..................................................................................................................... 7

Chapter.2 Fabrication of microstructured optical fibres

2.1 Introduction............................................................................................................................... 10 2.2 Fibre draw tower ....................................................................................................................... 12 2.3 Capillary drawing ...................................................................................................................... 14 2.3.1 Mathematical model .......................................................................................................... 15 2.3.2 Analytical solution............................................................................................................. 16 2.3.3 Numerical solution using the FEM package........................................................................ 18 2.3.4 Comparison between the experiment data and the analytical model..................................... 19 2.3.5 Practical issues for capillary drawing ................................................................................. 22 2.3.6 Summary........................................................................................................................... 23 2.4 Preform fabrication.................................................................................................................... 24 2.4.1 Cleaning............................................................................................................................ 24 2.4.2 Preform assembly.............................................................................................................. 25 2.5 Caning and Fibre drawing.......................................................................................................... 29 2.5.1 Single-step fibre drawing ................................................................................................... 30 2.5.2 Two-step drawing.............................................................................................................. 31 2.5.3 Summary........................................................................................................................... 35 2.6 Conclusions............................................................................................................................... 35

Chapter � 3 Transmission properties of highly nonlinear microstructured optical fibres

3.1 Introduction............................................................................................................................... 36 3.1.1 Overview of the fibre with high nonlinearity ...................................................................... 36 3.1.2 The loss mechanisms within HNL-MOFs........................................................................... 37 3.1.3 Outline .............................................................................................................................. 39 3.2 Scattering losses ........................................................................................................................ 40 3.2.1 Introduction....................................................................................................................... 40 3.2.2 Scattering mechanisms....................................................................................................... 41 3.2.3 Cut-back loss measurement of HNL-MOFs........................................................................ 47 3.2.4 Back-scattering measurement for HNL-MOFs.................................................................... 53 3.2.5 Conclusions....................................................................................................................... 59 3.3 Reducing the OH induced losses in MOFs.................................................................................. 59 3.3.1 Mechanism........................................................................................................................ 60 3.3.2 Process considerations ....................................................................................................... 61 3.3.3 Experimental observations ................................................................................................. 66 3.3.4 Summary........................................................................................................................... 67 3.4 Conclusions............................................................................................................................... 68

Chapter.4 Small core rare-earth doped microstructured optical fibres

4.1 Introduction............................................................................................................................... 70 4.2 Fabrication of doped highly nonlinear microstructured fibres...................................................... 73 4.2.1 The ytterbium doped MOF................................................................................................. 73 4.2.2 The erbium doped MOF..................................................................................................... 75

4.3 Optical properties...................................................................................................................... 77 4.3.1 Dispersion ......................................................................................................................... 77 4.3.2 Effective mode area........................................................................................................... 78 4.3.3 Modal birefringence........................................................................................................... 79 4.4 A mode locked ytterbium doped MOF laser...............................................................................81 4.4.1 Experimental setup............................................................................................................81 4.4.2 Laser characteristics...........................................................................................................83 4.4.3 Discussion......................................................................................................................... 85 4.4.4 Summary........................................................................................................................... 87 4.5 A nonlinear amplifier based on a ytterbium doped MOF............................................................. 87 4.5.1 Experimental setup............................................................................................................ 87 4.5.2 Operating principles........................................................................................................... 88 4.5.3 Single Raman soliton generation – forward pumping configuration ....................................89 4.5.4 Backward pumping configuration ......................................................................................92 4.5.5 Multiple Raman soliton generation.....................................................................................92 4.5.6 Summary........................................................................................................................... 95 4.6 A low threshold, high efficiency erbium doped MOF laser......................................................... 95 4.6.1 Absorption characteristics.................................................................................................. 95 4.6.2 A high efficiency, low threshold laser based on erbium doped MOF...................................96 4.6.3 Summary.........................................................................................................................101 4.7 Conclusions.............................................................................................................................101

Chapter.5 Air-clad microstructured optical fibres

5.1 Introduction.............................................................................................................................104 5.1.1 The tunability of the Ytterbium doped cladding pumped fibre lasers (YDCPFLs)..............105 5.1.2 In-fibre gratings in MOFs................................................................................................108 5.1.3 Outline of this chapter......................................................................................................109 5.2 Design and fabrication............................................................................................................. 110 5.3 A cladding pumped ytterbium doped laser using air-clad MOFs............................................... 114 5.3.1 Properties of ytterbium doped air-clad MOFs................................................................... 114 5.3.2 Experimental setup..........................................................................................................116 5.3.3 A cladding pumped ytterbiund doped fibre laser with a wide tuning range........................117 5.3.4 A cladding pumped 980nm ytterbium doped fibre laser....................................................118 5.3.5 Summary......................................................................................................................... 120 5.4 Fibre Bragg grating in air-clad MOFs....................................................................................... 120 5.4.1 A GeO2-B2O3 co-doped air-clad MOF.............................................................................. 120 5.4.2 The effect of the air-silica interface.................................................................................. 121 5.4.3 Summary......................................................................................................................... 122 5.5 Conclusions............................................................................................................................. 123

Chapter.6 Large mode area microstructured optical fibres

6.1. Introduction............................................................................................................................. 124 6.2. Fabrication of large mode area microstructured optical fibres...................................................126 6.2.1. Refractive indices of silica based materials.......................................................................126 6.2.2. Evolution of the LMA-MOFs fabrication process.............................................................128 6.3. Effective mode area................................................................................................................. 134 6.3.1. Measurement via nonlinearity .......................................................................................... 135 6.3.2. Measurement via mode field diameter (MFD) ..................................................................138 6.4. Bend losses.............................................................................................................................. 141 6.4.1. Bend losses at 1550nm..................................................................................................... 142 6.4.2. Wavelength dependence of the bend losses...................................................................... 144 6.4.3. Discussion.......................................................................................................................146 6.5. Transmission losses................................................................................................................. 148 6.6. Conclusions............................................................................................................................. 150

Chapter.7 An ytterbium-doped all-glass double-clad large mode area microstructured optical fibre

7.1. Introduction............................................................................................................................. 152 7.2. Fabrication.............................................................................................................................. 155 7.2.1. Fabrication of the core..................................................................................................... 155

7.2.2. Fabrication of the fibre..................................................................................................... 156 7.3. Optical properties .................................................................................................................... 158 7.3.1. Modal characteristics ....................................................................................................... 158 7.3.2. Absorption and background losses ................................................................................... 163 7.4. CW laser characteristics........................................................................................................... 164 7.4.1. Core pumping at 976nm................................................................................................... 164 7.4.2. Cladding pumping at 915nm............................................................................................ 165 7.5. Pulsed laser characteristics....................................................................................................... 166 7.5.1. Q-switching..................................................................................................................... 166 7.5.2. Mode-locking .................................................................................................................. 170 7.6. Discussion ............................................................................................................................... 172 7.7. Conclusions............................................................................................................................. 174

Chapter.8 Conclusions and future directions ................................................................... 176 Appendix.A Effective index model ................................................................................. 180 Bibliography................................................................................................................... 183 List of Publications......................................................................................................... 202

Acknowledgement

The last three years has passed so quickly just like travelling with light. I believe that this is

primarily because things have been moving very quickly in the field related to holey fibres.

Working in such an environment has been a little bit challenging, but stimulating and has also been

the greatest opportunity to learn a lot of things, from establishing methodology and time

management to practical and technical things.

To Prof.Dave Richardson, who gave this great opportunity to me, I would express my best

gratitude. This expression might sound like usual acknowledgement, but I am confident that I

would have been unable to make a better choice than doing my PhD at Southampton with him. I

would also like to thank Dr.Tanya Monro, from whom I learnt how important it is to be optimistic.

I have always tried to exercise my ingenuity in fabrication with positive thinking after interacting

with her. Dave and Tanya’s optimistic views have always provided something I could challenge

with fresh mind, possibly owing to their non-fabricator’s perspective.

I would also gratefully acknowledge many help from the people in the fabrication area, which was

indispensable for my activity i n the cleanroom. Dr.Jayanta Sahu and Paul Turner showed me their

gifted talents as fabricators. Without observing their activities, it would have been difficult for me

to make usable fibres. The successive head of the silica groups, Dr.Duncan Harwood and

Dr.Richard Williams who have always retained a better environment for silica fibre fabrication, are

also greatly appreciated.

Thanks are also to the (former and current) members of the advanced fibre technologies and

applications group, the high power fibre lasers group, and the novel fibres and waveguides group. I

would like to thank Dr.Neil Broderick, for his introduction to fibre experiments, Dr.Girberto

Brambilla for the grating experiments, and Jonathan Price for the nonlinear amplifier work,

Dr.Cyril Renaud and Romeo Selvas for the JAC fibres applications, Joanne Baggett for the work of

the large mode area fibres, and Vittoria Finazzi for the discussions of the optical properties of holey

fibres in general. The enthusiasm of many people at the ORC, who kindly dared to use/investigate

my more speculative creations, is also greatly appreciated.

I am deeply indebted to Prof.Alistair Fitt and Prof.Colin Please of the Department of Mathematics

for modelling of capillary drawing, which is described in the Chapter 2. I also grateful to Ping Hua,

Dr.Barbara Cressay, and Dr.Richard Pierce for passing their SEM skills to me, which I have used

every occasion throughout my study at the ORC. I would also like to acknowledge Dr.Eleanor

Tarbox, who kindly reading my thesis through. Her suggestions were also quite helpful.

I would like to thank those who encouraged me to go to Southampton: Dr.Marrku Oksanen, Dr.Ari

Tervonen, and Prof.Isao Endo. With their suggestions and encouragement, I decided to take this

opportunity in Southampton. I would also like to thank Prof.Minoru Obara. By working with him,

my interest in the field of optoelectronics has been triggered on.

Finally, I would like to thank to my family for their endless mental support. This thesis is dedicated

to them in token of my gratitude and heartfelt respect.

“Try to do your best, because that’s all part of the fun.” Hermann.A.Haus

DECLARATION OF AUTHORSHIP

I, Kentaro Furusawa, declare that the thesis entitled Development of rare-earth doped microstructured optical fibres and the work presented in it are my own. I confirm that:

� this work was done wholly or mainly while in candidature for a research degree at this University;

� where any part of this thesis has previously been submitted for a degree or any other

qualification at this University or any other institution, this has been clearly stated;

� where I have consulted the published work of others, this is always clearly attributed;

� where I have quoted from the work of others, the source is always given. With the exception of such quotations, this thesis is entirely my own work;

� I have acknowledged all main sources of help;

� where the thesis is based on work done by myself jointly with others, I have made clear exactly what was done by others and what I have contributed myself;

� parts of this work have been published as indicated in the list of publication. Signed: ……………………………………………………………………….. Date:…………………………………………………………………………….

Chapter.1

Introduction

1.1 Introduction

Fibre technology has already found widespread use in a variety of advanced applications in various

industries ranging from telecommunication to medical services. Since the early 70’s, the

continuous development of low loss optical fibres, birefringent fibres, photosensitive fibres, and

rare-earth doped fibres has offered many fibre based device applications providing novel, practical

and ever more sophisticated functions. Recently, a novel type of optical fibre has been introduced:

microstructured optical fibres (MOFs), which possess a transverse microstructure defined by air

holes running along their entire lengths. MOFs were initially named as photonic crystal fibres

(PCFs) and are also referred to as holey fibres (HFs).

By introducing the microstructure within the optical fibres, the guidance mechanisms can be

provided by the air holes, differing from the conventional optical fibres in which different materials

are used to provide an index boundary between the core and the cladding. When the

microstructures possess a solid core, the air holes serve as index decreasing elements leading to a

modified form of total internal reflection. On the other hand, by appropriately arranging the air

holes, it is also possible to guide light within a certain air hole by taking advantage of Bragg

scattering. Such MOFs are specifically referred to as photonic band gap fibres (PBGFs). This thesis

focuses upon the former type of MOFs.

It is interesting to note that both ideas of introducing the air holes within the fibres and of guiding

light using Bragg scattering can be found in the 1970’s[1,2]. However, due to the technical

difficulties in realising such structures and the lack of interesting optical properties discovered

within such structures, these directions had not been explored until recently.

A breakthrough was made by Knight et al., who demonstrated the first microstructured optical

fibres possessing unique optical properties, which cannot be obtained by conventional fibre designs

Chapter.1 Introduction 2

in 1996[3]. With strong demands on high performance optical fibres, the importance of MOFs has

become widely recognised during the last couple of years.

Fig.1.1.1 shows the rapidly increasing number of scientific publications related to microstructured

optical fibres. The exponential growth of the number indicates that MOFs have captured research

interests very rapidly and that a number of important technological breakthroughs are anticipated to

enable useful applications. The work presented in this thesis has been carried out during this

exciting time period.

��

�� !#"$� % & ' ( ") * + % !#� ( ,#� ,#" %' * ! - " % " ! ' ".,#� ,#" %/ * �0" 1 & ' ) * + % ! � ( ,#� ,#" %

Fig. 1.1.1 The number of scientific publications related to MOFs during recent years.

Below, a brief review of MOFs in terms of properties and applications is followed by a summary of

the following chapters.

1.2 Properties of MOFs

1.2.1 Single mode operation

The first interesting property found in MOFs was ‘endlessly single mode’ guidance[3] in fibres

whose cladding consists of a hexagonally packed array of air holes. This cladding structure can be

characterised by the diameter d and the pitch Λ of the air holes, as shown in Fig.1.2.1. The MOF

cladding thus contains discrete and periodic changes of refractive index profile across the fibre

cross section, prohibiting simple waveguide analyses. Birks et al. introduced the effective index

model[4,5], where the complex air-silica cladding structure is regarded as a medium with an

effective refractive index neff and then the entire structure is approximated as a step index fibre, to

which an exact analytical solution exists. With this significant simplification, they demonstrated

that the effective V value (normalised frequency) 243556 7789 : ; ;<#= > = ?.@: ; ; AAB CDE can always be


below a certain value; an appropriately designed MOF can thus always be single-moded

(Veff<2.405) regardless of the operating wavelength λ.

Fig. 1.2.1 SEM of the core region of the index guiding MOF with an air hole diameter d and a hole to hole spacing Λ (These parameters range approximately from 1µm to 10µm.)

This interesting wavelength dependence of Veff originates from the fact that neff approaches nsilica as

the wavelength reduces. Although there used to be long lasting discussions about the definition of

the Veff value for MOFs, the fact that the cladding index displays a strong wavelength dependence

was confirmed by experimentally characterising the far-field diameter and the beam divergence[6].

This robust single mode operation uniquely possible using MOFs led to the first large mode area

(LMA) MOFs[7]. Chapter 6 and 7 of this thesis are devoted to exploring this direction.

Since waveguide properties are determined by the structural scale with respect to operating

wavelengths, terahertz pulse propagation within a polymer MOF has recently been reported[8].

MOFs based on new materials are currently being developed[8-11], good transparency in infrared

spectral range should be obtainable using these materials. An important fact here is that it is

possible to realise waveguides using a single material: rather than solving all the issues related to

material combinations to realise the waveguides. This flexibility may be very important for some

cases. Thus, the introduction of these new materials to MOFs offers an alternative route for

realising practical fibres in various wavelength ranges.

1.2.2 Group velocity dispersion (GVD)

The fact that the cladding refractive index is strongly wavelength dependent also leads to unusual

dispersion properties. While the effective index model provides some important insights to MOFs

(and more detailed methods for deriving the indices of the space filling modes have been

developed[12,13]), a number of numerical methods have been proposed for accurately calculating

the optical properties[14-31].


Thanks to the earlier development of the numerical models, some interesting features of the MOFs

were discovered. Mogilevstev et al. showed that MOFs can exhibit interesting dispersion properties

such as the zero dispersion wavelength (ZDW) below 1.28µm[32]. This is not obtainable using

conventional fibres with single mode operation, and the first demonstration of supercontinuum

generation took advantage of this feature combined with the high peak powers of the ultrashort

laser pulses from a Ti:sapphire laser[33]. In addition, interferometric dispersion characterisation

confirmed anomalous dispersion around 800nm in such fibres[34-36].

It has been shown that the MOFs with a very small core and large air holes can be modelled by

approximating the structure by a silica rod. Such fibres can exhibit a large normal dispersion at

1550nm[37], providing an alternative approach for the dispersion compensation in existing

telecommunication systems. Fibres with controlled dispersion characteristics around 1550nm have

been developed for the last couple of years[38]. Chapter 3 discusses the loss properties of such

MOFs.

These dispersion properties of MOFs imply that it is possible to flatten the dispersion profile over a

wider spectral range than possible using conventional fibres. Indeed, it has been shown that a very

small dispersion over a 400nm bandwidth covering both the 1.3 and 1.55µm telecommunication

bands is possible by optimising the hole size and spacing of the holey cladding[39]. Such a fibre

has recently been realised demonstrating ±1.2ps/nm/km over a 600nm spectral range from 1µm to

1.6µm[40].

1.2.3 Nonlinearity

Fibre nonlinearity can most conveniently be characterised by the effective mode area Aeff since the

effective nonlinearity per unit length � �� is inversely proportional to the effective

mode area[41], where n2 and λ are the nonlinear index coefficient and the wavelength. Although

the intrinsic n2 of the silica based fibre materials is not high, the tight modal confinement

achievable within MOFs leads to high effective nonlinearity compared with conventional fibres.

This tight modal confinement is very sensitive to the fibre structure and therefore the waveguide

dispersion becomes strongly dependent on the operating wavelengths. In other words, the

interesting dispersion properties are often naturally accompanied by high nonlinearity, and which

has led to numerous applications described in Section 1.3.


In order to increase the effective nonlinearity per unit length, new glass materials have been

exploited[9]. Owing to high intrinsic nonlinearity of high index glasses[42], the effective

nonlinearity γ achieved in a MOF now exceeds 500W-1km-1 at 1550nm[43].

On the other hand, at the other extreme, very low nonlinearity can also be achieved by an

appropriate choice of the structural parameters owing to the inherently low nonlinearity of high

silica glass. As described, combined with the robust single mode operation, a novel opportunity for

the development of the large mode area fibres has been found and we have demonstrated γ as low

as 0.1W-1km-1 at 1550nm[44]. Thus, MOFs can play a role in extending both the high and low

nonlinearity extremities possible within optical fibres.

1.3 Applications

1.3.1 Nonlinear devices

The combined action of the high nonlinearity and the unusual dispersion properties of MOFs offers

new opportunities for developing useful nonlinear optical devices. The importance of the

nonlinearity was pointed out and characterised in ref.[45], where the effective mode area estimated

from the nonlinear phase shift data agreed well with the numerical simulations, assuming the

intrinsic nonlinearity n2~2.3x10-20 m2/W.

The high effective nonlinearity allows us to readily access various nonlinear optical effects and the

following nonlinear devices have been demonstrated by many groups around the world.

�

All optical switching[46-48] �

Raman amplification[49] �

Parametric devices[50-52] �

Soliton generation and squeezing[53-61] �

Supercontinuum (SC) generation[62-84]

Due to the possibilities of reduced power requirements and/or device lengths for these MOF-based

devices, the first three examples could be very attractive for signal processing for future optical

networks. Soliton effects that can be realised over a broad spectral range are also useful for

ultrashort pulse applications. In particular, utilisation of soliton self frequency shift (SSFS), with

moderate power requirements, allows us to access wavelength ranges where ultrashort pulses were

difficult to obtain previously. As a result, the demonstrated tuning range for fibre sources is now

from 800nm to 1.7µm[53-55,57]. Recently, soliton squeezing has been demonstrated using self

phase modulation within MOF[59-61]. Owing to the reduced soliton energy, this offers ease of


implementation in generating bright entangled states as required for many quantum optics

applications experiments.

The combination of an ultrashort pulse oscillator and a highly nonlinear MOF has become a

popular tool for generating broadband SC because of the modest power requirements. Under the

appropriate conditions, the spectrum broadens over more than an octave with well defined mode

spacing without loosing the coherence of the pump. By taking a beat of the octave frequency, the

caesium clock can then be extended directly to the optical domain by controlling the repetition rate

of the mode-locked oscillator[87-90] using a phase lock loop. This has enabled extremely accurate

spectroscopic measurements[91,92]. This strategy has also been used for stabilising the carrier

envelope of the ultrashort pulses[93-96] over a long period of time. This approach may pave the

way for accurate control over ultrafast phenomena in an attosecond regime[97].

Broadband SC sources have been used for optical coherence tomography (OCT) with a depth

resolution as short as 2µm around a 1.3µm wavelength[98] and ~0.75µm at a 0.72µm

wavelength[99]. The extremely broadband radiation available from this type of SC sources may

allow us to add spectroscopic diagnostics into OCT.

Driven by such a wide range of applications, a large body of work related to SC generation in

MOFs has been reported, including both theoretical[64-66,73,82-84] and experimental work[33,62-

84]. The reasonable agreement between the theory and the experiment indicates that now the SC

process can be qualitatively explained as a combined action of various nonlinear optical effects,

depending on the pump wavelength and the dispersion properties of MOFs.

It has become common knowledge that use of short pump pulses and a short section (~1cm) of

MOF is essential to obtain flatter spectra[85], in order to take advantage of the dominant effect of

self-phase-modulation, and is also important to retain a high degree of coherence[82,86]. Then,

these conditions automatically require the coincidence between the pump wavelength and ZDW of

the fibre. Thus, the design of the MOF with appropriate dispersion characteristics for a given pump

source is becoming very important.

1.3.2 Optical devices

By taking advantage of the interesting optical properties of MOFs, a variety of other optical devices

have also been reported, demonstrating unique opportunities as follows.

Chapter.1 Introduction 7 �

Fibre gratings[101-108] �

Long period gratings (LPGs) �

Bragg gratings �

Tunable polarisation/attenuation controller[109-112] �

Sensors[113-117] �

Lasers[118,119]

Fibre gratings were inscribed by using MOFs with a germanium doped core[101]. The

demonstrated applications include environmentally stable long period gratings that do not require

hermetic coatings[102], and tunable Bragg gratings made by filling a polymer inside the air

holes[104]. Furthermore, combined with the tapered MOFs, tunable devices such as a birefringent

controller and a variable attenuator have been demonstrated[109-112]. On the other hand, high

performance rocking filters (~23dB) have been demonstrated using MOFs[108], in which a

periodic perturbation was uniquely provided by the structural deformation of the holey

cladding[107].

Twin core fibres can readily be made using MOF technology[116]. An all-fibre curvature sensor

has been demonstrated[117] using this MOF type. Furthermore, evanescent-wave gas

sensing[113,114] has been proposed and it has been demonstrated that 65 times better sensitivity is

achievable compared to conventional D-shaped fibre[115].

Finally, lasers and an amplifier have been demonstrated by introducing doped sections within the

MOF cores[118,119]. Both large mode area doped MOFs and highly nonlinear doped MOFs are of

great interest for developing new types of light sources. This has been a major objective of this

thesis, and this work is described in Chapters 4, 5, and 7.

1.4 Outline of the thesis

The activities presented in this thesis all relate to the development of MOFs and MOF-based

devices. A particular emphasis is made on the development of rare-earth doped MOF devices. The

introductions to the individual chapters are given separately below.

Chapter 2 describes the fabrication method of MOFs, in which glass capillaries are stacked together

and then pulled. Although new approaches such as direct synthesis of the preform using the sol-gel

method and the glass extrusion technique have been proposed, this simple method allows for the

greatest flexibility in realising a wide range of fibre geometries. However, practical issues such as

good control over the fibre geometries and also preparation of the raw glass materials were not well

understood prior to this study .


In order to solve this issue, the process of simple capillary drawing is experimentally studied first.

A mathematical model is developed to predict the final geometry of the capillaries for the given

drawing parameters. The author shows that this model also provides some useful insights into

controlling the geometry of the complex fibre structure. In addition, preform preparation methods

are presented.

Chapter 3 focuses upon the transmission loss properties of highly nonlinear MOFs (HNL-MOFs),

which also possess unique dispersion properties. Although these fibres are very attractive for

nonlinear signal processing applications, little was known about their loss mechanisms. It is found

that the losses of these fibres are significantly dependent on their structural dimensions. A simple

model is developed to qualitatively explain the observations.

As in conventional optical fibres, MOFs also suffer from the losses induced by water content. The

final part of this chapter is devoted to the fabrication efforts to reduce the OH-loss levels within

MOFs.

In Chapter 4, rare-earth doped highly nonlinear MOFs are presented, where highly nonlinear

ytterbium and erbium doped MOFs are developed. The efficient operation of both fibre lasers is

demonstrated. The first mode-locked operation is presented for an ytterbium doped MOF laser

while the first continuous wave laser action is reported for the erbium doped MOF with extremely

low threshold and high efficiency.

It is well known that a combination of anomalous dispersion and active fibre allows us to

implement useful pulsed devices. Taking advantage of an anomalously dispersive ytterbium doped

MOF, tunable soliton generation within the ytterbium doped MOF based amplifier is presented in

conjunction with a fibre based ultrashort pulse source. The tuning range spans from 1.06 to 1.58µm

encompassing a spectral range previously inaccessible using conventional fibre technology.

Chapter 5 describes the development of air-clad MOFs, in which the conventional doped preform

forms the core and the inner cladding whilst the air holes define the outer cladding. Taking

advantage of such a structure as a cladding pumped fibre, laser oscillation at 980nm and a wide

tunability of ytterbium fibre laser sources are demonstrated. In order to allow for further integration,

a preliminary study on the fabrication of fibre Bragg grating in such a fibre structure is also

described.

Chapter 6 focuses on the fabrication and characterisation of large mode area MOFs. Basic optical

properties are experimentally studied and appropriate comparisons with conventional fibre are


carried out. The contents provide understanding of the practical issues related to LMA-MOFs, and

form a foundation for the fibre type developed in Chapter 7.

In Chapter 7, a novel cladding pumped ytterbium doped MOF is developed for the first time, in

which inner and outer claddings are formed by arranging the different sizes of air holes. Under

continuous wave operation, an average power of more than 1W is demonstrated. Furthermore, Q-

switched and mode-locked operation of the laser are studied.

Finally, conclusions and future directions are given in Chapter 8.

Chapter.2

Fabrication of microstructured optical fibres

2.1 Introduction

This chapter describes the fabrication of microstructured optical fibres (MOFs) that the author has

used and refined during his PhD period. MOFs are drawn from a preform, with millimetre-scale

features, on a fibre draw tower, in much the same way that conventional optical fibres are drawn

from a solid glass preform[3]. A structured preform could potentially be synthesised by a variety of

methods such as the sol-gel[120] and VAD methods, or be fabricated from a bulk material by using

extrusion[121] or direct drilling[122]. However, the most commonly used method to date is by

stacking glass capillaries thanks to its flexibility and low capital cost requirement.

Although it is an open question what is the best method for fabricating high quality low loss MOFs,

the capillary stacking method is very useful at the early stage of development, where the funture

potential of MOFs is investigated. Furthermore, continuous refinement of this approach has led to a

loss level of MOFs as low as 0.3dB/km for some ranges of structural dimensions[123]. Thus,

comparable loss levels to conventional fibres can still be anticipated using the stacking method by

further technological refinement.

The entire fabrication procedure can be divided into two major steps: preform preparation and fibre

drawing. The preform preparation usually includes drawing capillaries from a glass tube, cleaning

and stacking them. Capillaries of the order of 1mm diameter are also drawn from a glass tube

(~10mm O.D.) on the fibre draw tower. Therefore, the fabrication technology heavily relies upon the

drawing process. The assembled preform typically has a ~10mm diameter and is drawn either

directly into a fibre (~100µm) or via a microstructured cane (~1mm), which is again pulled after

jacketing to obtain small scale structures (~1µm) within the final fibre.

It is worth pointing out that the idea of this fabrication method originates from nano-channel glass

technology[124]. In addition, there have been several reports on structures that contain air holes

Chapter.2 Fabrication of microstructured optical fibres 11

within optical fibres such as side-pit fibres[125], an early variant of birefringent fibre. Furthermore,

the rod stacking process was often used in the fabrication of PANDA fibres[126], the standard

method of which involves drilling a pair of air holes and inserting stress rods (then the air holes are in

this case collapsed in the draw). However, the amount of air and the number of rods within the cross

sectional area of these fibres were much smaller than MOFs. On the other hand, commercially

available imaging fibres consist of a bundle of ~10000 fibres that are closely packed and then fused

together. However, no air holes are present in the fibres within the bundle except interstitials, whilst

the control of air holes is crucial for MOFs. In these regards, recent MOF fabrication poses slightly

different technological challenges from any of the fabrication processes described above.

In conventional optical fibre fabrication, the key to achieving the high quality fibres lies mostly in the

fabrication of the preforms. This has led to a dramatic improvement of transmission losses[127], as

well as allowing for the incorporation of various active and passive dopants within their cores[128].

On the other hand, the drawing process has largely been considered as a trivial problem since when

using a solid preform the dimension of the fibre is simply controlled according to the mass

conservation law

��

��

��

��

��

, (2.1)

where φf and φp are the outer diameters of the fibre and the preform, uf and ud denote feed and draw

speeds, respectively. Here, the volume reduction ratio η is defined for convenience. Since

conventional fibre performs are prepared so that the ratio between the fibre outer diameter (O.D.) and

the core diameter is exactly the same as that desired in the drawn fibre, both the outer diameter and

the core size of the fibre are tuned by optimising either the feed or draw speeds.

However, in the case of MOFs, eq.(2.1) does not generally hold due to the existence of the air holes

that may be either intentionally or unintentionally collapsed during the draw process due to the

combined effects of the viscosity and the surface tension of the glass. Although the total glass

volume of the individual glass elements within the preform may satisfy the above equation, the final

structural dimensions of the drawn fibre cannot be defined without taking the air holes into account.

For this reason, when the author started his PhD, even reliable and reproducible glass tube (capillary)

drawing was a non-trivial problem since the final size of the central air hole could not be easily

predicted, even though this process is a basic building block of the preparation of the MOF preforms.

Accurately predicting the shape of a complex air glass structure during fibre drawing is thus even

more difficult.


The aim of this chapter is to refine the MOF fabrication technology based upon the capillary

stacking technique. The main focus is upon the drawing process since the stacking process heavily

relies on a manual procedure, which could, in principle, be automated by using the packing

techniques employed in the manufacture of imaging fibres. First, the capability of the fibre draw

equipment is described in Section 2.2. Then, capillary drawing is described in Section 2.3 as it forms

a foundation for the draw process. The mathematical model for the capillary drawing, which was

developed in collaboration with the Mathematics Department at the University of Southampton, its

experimental verification and further practical issues related to this process are presented. In Section

2.4, the method for preparing MOF preforms is described. Fibre drawing and caning are discussed in

Section 2.5, where it is shown that the capillary drawing model described in Section 2.3 can provide

some important insights. General guidelines for obtaining better control over the structures of MOFs

are also discussed. Finally, conclusions from this chapter are given in Section 2.6.

2.2 Fibre draw tower

The configuration of the fibre draw tower is first outlined. The limits and practical ranges of fibre

draw parameters available on the tower are also discussed.

Fig. 2.2.1 Schematic of fibre draw tower.

The 5m fibre draw tower that was mainly used for MOF fabrication is shown in Fig.2.2.1; it is also

used for conventional fibre fabrication. The fibre preform is clamped by a chuck onto a motorised

stage so that the feed speed uf can be accurately controlled. The reliable speed ranges span

approximately from 0.4mm/min to 15mm/min. The lower limit is given by the minimum step size of

Tensioner Bobbin

Capstan wheel

Thermo-furnace

UV oven Coating dye Coating cup Diameter gauge

Furnace

Top Iris

Hot zone

~150mm

Bottom Iris

Chuck Feed uf

ud


the stepping motor, while the maximum is limited by the motor driver itself. Note that using high

feed rates, fibres are prone to break since the surface quality of the glass cannot be improved

through fire-polishing, and this fact sets the practical limit for the high feed rates for a given fibre

diameter.

The preform is initially positioned centrally inside a graphite induction furnace with a drop rod that

is attached on the bottom of the preform, and which falls by its own weight due to gravity when the

furnace is heated to near ~2000°C. The drop is collected from the bottom of the furnace and the fibre

drawing process starts after the elongated preform end is taken up by a capstan wheel at the bottom of

the tower, which controls the draw speed ud. The drawn fibre is taken up by a bobbin synchronised to

the capstan wheel such that an appropriate amount of tension is applied to the fibres while they are

wound on the bobbin.

The hot zone of the furnace is located approximately 150mm below the top iris and the length of the

furnace element is ~50mm. The distance between the top and bottom iris is ~300mm. The

temperature profile measured at low temperature (~1400°C) showed a triangular shape along the

furnace. Note that the measurement temperature was limited by the available high temperature

compatible thermocouple. The furnace is always purged by argon, in order to avoid any oxidation of

the furnace elements at high temperatures (~2000°C) and to prevent the incorporation of impurities

into the glass[129]. The furnace temperature control provides high relative accuracy, although the

accuracy in terms of absolute temperature depends on the value chosen for the emissivity of silica

and the alignment of a pyrometer. Therefore, the temperature values used in the following sections

is with reference to the drop temperature, at which the fibre drawing was started, although this does

not provide any absolute measure.

The relative temperature stabilit y of the furnace needs to be very good in order to allow for stable

fibre drawing. Because the viscosity depends exponentially on the temperature as described in

eq.(2.7), any temporal temperature variation leads to fibre diameter fluctuations due to resultant

changes in fibre tension. The typical diameter fluctuation observed for conventional fibres drawn on

this tower is ±1µm although, of course, it depends on the draw parameters such as draw speeds and

temperature (thus tension). A diameter gauge is placed between the bottom of the furnace and the

coating cup, and is also used to monitor the concentricity of the fibre.

A polymer coating can be applied by using either UV curing (acrylate) or thermally curable polymer

(silicone) with a coating dye that is placed at the bottom of the coating cup, and through which the

polymer is extruded. The coating material is then immediately cured through either a UV curing unit

or low temperature furnace (~500°C). The coating quality contributes to the mechanical reliability of

the fibre[131] in addition to the surface quality of the glass after drawing. Hence, optimising the


coating is important. Since the coating may contain some defects (or bubbles)[132], at least ~50µm

thickness is required for good mechanical strength for a 125µm fibre after curing[131]. Although the

optimum thickness may be dependent on the fibre diameter and the coating material, a thickness of

~40% of the fibre diameter is typically enough for laboratory use for most fibres with a diameter

less than 500µm. However, this coating requirement ultimately limits the available ud as described

below.

There are three factors that need to be taken into account to apply a good coating. The first factor is

the volume shrinkage of the coating. This depends on the curing time, which is thus proportional to

ud. The shrinkage factor is 2~3 after sufficient curing for the high index coating polymer that is

commonly used at the ORC (Desotech: DSM 3-14). The second factor is the viscosity of the polymer

material, and the third factor is the surface temperature of the fibre. The viscosity of the polymer

determines the maximum ud for a given die dimensions. If the die is too large with respect to the fibre

diameter, the polymer slips at the die, resulting in an insufficiently thick coating. The other issue is

the formation of bubbles within the coating cup. These degrade the quality of the coating[133]. These

issues can be solved by using a pressurised coating die.

However, in the case where the pressurised coating is used, the maximum ud is limited by the surface

temperature of the fibre which must be sufficiently cool as it enters the coating cup. Too high a

temperature may result in a fire or slippage of the polymer due to the locally increased viscosity of

the polymer within the coating cup. Therefore, a thermally conductive gas flow such as helium or

carbon dioxide is often used to cool down the fibre before it arrives at the coating material. Because

the heat removal primarily depends on the distance between the furnace and the coating cup, the

height of the fibre draw tower ultimately determines the fastest ud. Using the equipment described in

Fig.2.2.1, the highest possible ud was ~60m/min. using a pressurised coating system (without

actively cooling the fibre).

2.3 Capillary drawing

Capillary drawing is a fundamental process that can greatly impact the MOF fabrication since the

precise control of capillary dimensions and the uniformity of capillaries are essential for structuring

regularly arranged air holes within the stacked MOF preforms. However, the capillary drawing

process itself was not so well controlled and understood prior to this study. It was particularly

difficult (and tedious) to obtain the desired dimensions of capillaries (both ID and OD) because the

draw parameters had to be obtained empirically in order to obtain desired capillary dimensions.

A mathematical model for the capillary drawing process is introduced in Section 2.3.1, and its

solution is given in Section 2.3.2. A numerical solution using the finite element method (FEM) is


presented in Section 2.3.3. The experimental data is compared with the analytical solution to

examine the validation of the model. Finally, practical fabrication issues are described and discussed

in Section 2.3.4.

2.3.1 Mathematical model

In general, the fluid dynamics for both gases and liquids are described by the Navier-Stokes

equations[134] as follows.

�� and (2.2)

�� , (2.3)

where ρ, p, g and µ(Τ) are the fluid density, pressure, gravity and viscosity, respectively. The

viscosity is temperature dependent, as given in eq.(2.7). u=(u,w) denotes the velocity vector. The

first equation (2.2) expresses the local preservation of kinematic momentum, whilst the second

equation (2.3) results from the incompressibility of the fluid. To include the temperature distribution

across the medium of interest, which can be a three dimensional coordinate function, it is also

necessary to take the heat conduction equation into account, based on Fourier’s law,

��

� �� , (2.4)

where c(T) and k(T) are temperature dependent heat capacity at constant volume and thermal

conductivity of the liquid, respectively. R is the heat source, which is described as a product of

absorptivity and blackbody radiation energy emitted by the furnace element. Generally this equation

is called the “energy equation” , in contrast to the “momentum equation” (2.2). By considering all

these equations, one can develop a detailed non-isothermal model for the capillary drawing process.

However, the system becomes rather too complex to understand from an analytical perspective, even

though the coupling between the energy and momentum equations is only through the viscosity.

Therefore, the isothermal model is considered here by excluding eq.(2.4).

Both numerical and analytical approaches were examined to solve the equations described above.

However, it has been proven that only the analytical approach is useful in terms of accuracy and

computing time. The mathematical derivation of the analytical solution for the capillary drawing is

based upon perturbation analysis, which is beyond the scope of this chapter, and the details are

provided in ref.[135,136]. The numerical approach uses the Finite-element method (FEM), where the

object is first meshed into small pieces, to which the partial differential equations (2.2) and (2.3) are


applied using a variational theorem. Then the boundary conditions relate the individual meshes,

leading to the global system energy through the matrix operation. By minimising this energy, the

solution is obtained. Since this was performed by a commercially available software package

(Polyflow), the detailed formulation is not discussed here; this is provided in ref.[137]. In the

following section, the derivation of the analytical solution is qualitatively explained based upon the

physical insights.

2.3.2 Analytical solution

The physical model and the parameters used within the model are illustrated in Fig.2.3.1. The

boundary conditions are given by the feed speed uf and the draw speed ud, while the initial conditions

are the inner and the outer diameter (h1(0) and h2(0), where the origin is taken to be the top of the hot

zone). Other physical parameters in the model include the viscosity µ(Τ), surface tension γ, and hot

zone length L. In the model, boundary conditions are also applied to the side walls of the tubes. These

include the kinematic conditions and stress conditions. The former amounts to the fact that the total

derivative of each boundary is zero. This is equivalent to stating that the direction of the velocity

vector at the boundary determines the evolution of the glass shape along the z axis in steady state.

The stress conditions may include two vector components (normal and tangential stress) at the

boundary, and are expressed in a quadratic form, namely the stress tensor. By applying the normal

and/or tangential unit vector to the stress tensor, one can obtain both normal and tangential stress

components. For the sidewalls, the normal stress component balances with the sum of the surface

tension and the internal pressure of the gas within the furnace for the outer wall or for the inner wall.

At the inlet and the outlet, it is assumed that there is no tangential stress. The importance of

including these boundary conditions is that it still provides the possibility of defining the pressure

inside the tube. However, for brevity, atmospheric pressure at the inner wall is assumed as this is the

usual condition used for ordinary capillary drawing.

By applying these boundary conditions and the initial conditions uf, ud, h1(0), and h2(0), we obtain

the inner and outer radii (h1(L) and h2(L)) at the drawn end (z=L), beyond which it is assumed that

no further deformation of the object occurs.

� � � � � ��

��

� �

��

�� , (2.5)

where

� ��

�� !!"""

""#$ %&&%& '

'(*) + . (2.6)


Fig. 2.3.1 Definitions used for the model.

The temperature dependence of the viscosity for silica at a temperature T°C is approximated by the

following equation,

� � � �� [poise], (2.7)

in the range from 1600 to 2500 °C for silica with a small amount of OH content (3x10-4 wt%) [138].

The surface tension coefficient γ is commonly estimated to be ~0.3N/m [138]. Hence, by using the

ratio between the surface tension and the viscosity γ/µ as a parameter, the experimental data can be

fitted as a quasi-temperature dependent model.

By analysing the solution, it is possible to explain some aspects of the capillary drawing process.

Assuming x=O(L) and �� !#"" , which is a typical condition for capillary drawing, the

collapse ratio C can be defined as

$ % &'()*+ ,-./ 0213 13 10 0213 4456874 99::: ;99 99< =>=? @ . (2.8)

C is a measure of the change in capillary geometry during drawing and reflects the sensitivity of the

collapse to the relevant draw parameters. Note that C=0 means that the relative dimensions of the

inner and outer diameter are preserved after drawing whereas C=1 corresponds to complete

collapse of the air hole. Note that eq.(2.8) is independent of the position x, which suggests that any

collapse occurs mainly in the upper part of the hot zone[135]. Although the effect of the surface

tension becomes more significant as the radius of the capillary decreases, the viscous force that

originates from the longitudinal tension applied by the continuous pulling action increases more

uf

h1(x) h2(x)

ud

z Drawing wheels

µ(T), γ L:hot zone


rapidly with the reduced capillary dimensions. As a result, the influence of the surface tension

becomes less significant as the capillary diameter decreases. In other words, the geometry of the

capillary is nearly constant over the neck-down region. It is clearly seen that C is more sensitive to uf

than to ud, and that the short hot zone length L can help to avoid collapse. The term within the

square bracket explains how C depends on the initial tube dimensions. Therefore, it can be

understood that it becomes increasingly difficult to avoid the collapse when the initial dimensions

are smaller. However, this may be compensated for by decreasing the hot zone length L and

increasing the viscosity µ, or the feed speed uf.

2.3.3 Numerical solution using the FEM package

In the FEM software package, a similar set of boundary conditions to the analytical model was

imposed as follows.

Inlet: uf and σt=0

Outlet: ud and σt =0

Inner wall: vn=0, fn=γ/R1(x)

Outer wall: vn=0, fn =γ/R2(x) (2.9)

where σt is the tangential stress, v and f are the velocity and the force vector, respectively, and the

subscript n denotes the normal outward component. R(x) is the radius of the curvature of the side

walls. Here, the presence of the pressure is omitted because the external force of the gas pressure is

expected to be negligibly small compared to the viscous force of the glass, as long as the top of the

preform tube is left open, which is usually the case.

The calculations were carried out by starting from a uniform glass tube and by increasing ud for a

given value of viscosity. Because the FEM package calculates the dynamics (temporal evolution of

the drawing process), the convergence and stability of the solution was examined by stopping the

changes in the draw parameter (ud) and by allowing for spontaneous evolution of the solution with

time (for instance, 10 minutes in draw time). Note that the spontaneous evolution depends on the rate

at which the draw parameter is incremented. In order to include the other effects such as surface

tension, the converged solution for a given draw parameter was used as an initial condition, and the

value of the surface tension was again gradually increased until it reached the physically relevant

value. The numerical stability of the solutions was also examined by observing the temporal

evolution.

The velocity boundary condition vn=0 in eq.(2.9) corresponds to the kinematic condition described

above. In terms of the stress boundary condition, fn=γ/R is also the same, since the effect of the gas


pressure is also omitted in the analytical model. Despite these similarities in the boundary

conditions, the calculated final dimensions using the FEM package are always ~10% smaller than the

analytic solutions regardless of the initial dimension of the tubes. Even when the mesh density was

increased, the discrepancy was not reduced particularly in the vicinity of the neck-down region.

Although it is not clear and it is difficult to specify this reason due to the black box nature of the

automated software package, the discrepancy between the FEM and the analytical models could be

due to the numerical inaccuracy in FEM, given the intensive computation requirement.

It is also possible to examine the non-isothermal model using FEM. However, the required

computation task is almost doubled. The entire simulation takes at least 1 hour for a set of parameters

(feed speed, draw speed, and temperature) for an isothermal model. In the case of a non-isothermal

model, it normally takes one day including the stability check. Therefore, the non-isothermal

modelling was unrealistic. In addition, the temperature profile was measured at low temperatures,

the reason for which was described in Section 2.2.1. Therefore, it must be assumed that there are no

changes in profile even at high temperatures. Given the accuracy and time consumption required

for the FEM modelling, we decided to use only the analytical model for the comparison with the

experimental results, below.

2.3.4 Comparison between the experiment data and the analytical model

In order to examine the validity of the mathematical models, a simple experiment was performed by

drawing a high-quality silica tube (Suprasil® F300: O.D. 28mm, I.D. 24mm) under several different

drawing conditions on the drawing tower, and measuring the resulting capillary dimensions. The top

end of the tube was left open. The diameter gauge on the tower was used to monitor the final O.D. of

the capillary whilst the final I.D. was measured under the optical microscope, and both of which

were calibrated using callipers. ud was fixed by passing the drawn capillaries through a tractor (the

surface of these wheels is designed to prevent any slippage).

In this experiment, uf was varied in the range from 2 to 8 mm/min, ud was varied from 0.6m/min to

1.2m/min and the furnace temperatures of 1900, 1950 and 2000°C were used. For each combination

of drawing conditions, once these conditions were set, the measurement of the dimensions was

performed after the process was stabilised. Due to the relatively long distance between the drawing

wheels and the neck-down region, the capillary was slightly perturbed transversely by the air

turbulence during the draw, affecting the uniformity along the length. This was particularly

significant when either the temperature was high, or ud was low. Note that the diameter fluctuation

due to this effect is much faster than the dynamics induced by the draw parameter changes, and is

therefore cancelled out by averaging. On the other hand, when the temperature is too low, the

circularity of the capillaries is poorly degraded, possibly due to the very high tension. Nevertheless,


the validity of the ID measurements was confirmed using the law of mass conservation described in

eq.(2.1), and the deviations of the data from this requirement were only ±2% at most.

An example of the fit data for the different uf is shown in Fig.2.3.2. Good agreement was obtained by

taking γ/µ as a fitting parameter, as shown in Table.2.3.1, from which the temperature dependence

of the viscosity can be fitted by using eq.(2.7) and assuming γ to be a constant value

(~0.3N/m)[140].

�� !�� "$#%� �&#%� #�#%� '(#%� )

*+ ,-./ .01 --2

��

#%� �

#%� �

'��

'��

)�� 3�465 7�8986: 8<; =%> ?@ 465 7�8986: 8<; =%> ?3�465 A�8986: 8<; =%> ?@ 465 A�8986: 8<; =%> ?3�465BC8986: 8<; =%> ?@ 465BC8986: 8<; =%> ?3�465 7�8986: 8<; =%> ?@ 465 7�8986: 8<; =%> ?3�465 A�8986: 8<; =%> ?@ 465 A�8986: 8<; =%> ?3�465BC8986: 8<; =%> ?@ 465BC8986: 8<; =%> ?

Fig. 2.3.2 ID and OD of the capillaries at different ud for various uf (2, 4 and 8 mm/min.). The dots correspond to the experimental data, while the curves are calculated by use of the analytic model. The experiment was performed at 1950 °C.

Table. 2.3.1 Fitting parameters used for the data, and comparison of the extrapolated characteristic temperatures with the published data in ref.[138].

Temperature [°C] 1900 1950 2000

γ/µ [m/s] 1.61x10-6 3.85x10-6 1.12x10-5

strain point annealing point softening point

µ [Pa sec] 1015.5 1014 108.6 Extrapolated [°C] 1195.8 1278.1 1669.7

Ref [°C] 1108 1190 1670

It was found that the best fit can be obtained by introducing a slight offset (-26.5°C) in eq.(2.7).

The extrapolated strain, annealing, and softening points are also shown in Table.2.3.1. The

comparison with the published data shows good agreement. This indicates that the pyrometer

reading is reasonably accurate in terms of the absolute value, given that eq.(2.7) is applicable for

the range 1600~2500°C. Note that this offset sensitively depends upon the alignment of the

pyrometer and the emissivity of the furnace element. Once the temperature dependence of the

viscosity is known, it is possible to predict the optimum draw parameters at different temperatures .

Next, we study the impact of the different uf and temperatures on the collapse. Fig.2.3.3 (a) shows

the relation between the ratio I.D./O.D. and uf for different ud at 1950°C. It can be understood that


below 2mm/min, the collapse becomes increasingly significant. This implies that drawing in this

range is unstable since a slight change in uf leads to significantly different I.D./O.D. Furthermore,

the dependence on ud is very small. This confirms that uf has a significant impact on the collapse, as

predicted by the model.

Fig.2.3.3(b) displays the equivalent dependence on the draw temperature, by assuming the

temperature dependence of µ given by eq.(2.7). A high draw temperature leads to low viscosity and

thus to more collapse for a given value of uf. Thus, the final structure becomes sensitive to any

variation of the draw temperature. Since the relative temperature control is very accurate, it is

anticipated that the temperature tuning can provide fine control over the collapse of the capillary

geometry. In these two cases, it is clear that the ratio I.D./O.D. asymptotically approaches that of

the original value of the tube by either reducing the temperature or increasing uf.

Fig. 2.3.3 Collapse dependence on uf at 1950°C (a) and on the draw temperature with ud=1m/min.(b).

Although experiments using tubes with thick wall thickness were not carried out, we note that the

analytical solution converges to eq.(2.1) as the wall thickness increases. Therefore, it is clear that the

model is also applicable to the case of thick wall thickness.

��

�� !" � �

��# ��# ��$��# %��# %�$��# ��# ��$��# �� &

'('() '+* ,�-. '('() '+* ,�-/ '('() '+* ,�-&'('() '+* ,�-. '('() '+* ,�-/ '('() '+* ,�-

0+1�1�2435�161�287 949�:;94<>=@? AB C D E F

G�HI�HJK HI�H

B�L E�BB�L E�MB�L N�BB�L N�MB�L F�BB�L F�M

O - P'() '+* ,�-�Q R S�T�UVO - / '() '+* ,�-�Q R S�T�UVW - O '() '+* ,�-�Q R S�T�UVW -&'() '+* ,�-�Q R S�T�UVO - P'() '+* ,�-�Q T�XY�VO - / '() '+* ,�-�Q T�XY�VW - O '() '+* ,�-�Q T�XY�VW -&'() '+* ,�-�Q T�XY�V

(a)

(b)


2.3.5 Practical issues for capillary drawing

The above discussions were focused upon the control of the draw parameters. However, in practice,

other issues such as surface treatment and dimensional homogeneity along the length are also very

important for assembling a good quality preform. Here, the author discusses the choice of the

material first, and then processing that improves the quality.

Low qualit y grade silica tubes (HLQ-210) were often used for initial trials. However, the default

uniformity of the diameter per unit length is typically measured to be ± ~5%/m. Although it is

possible to adjust ud so that the outer diameter is always within 10µm, this may lead to a variation in

wall thickness. The yield of the capillaries with sufficient uniformity is ~60% after screening,

during which the inner and the outer diameters are characterised by the microscope and the

diameter gauge, respectively, and the capillaries that satisfy the acceptable range (<1%) are chosen.

Although callipers were initially used to measure O.D., it is recommended to use diameter gauges

since the head of the callipers may scratch the surface. Note that diameter gauges can now measure

I.D. and O.D. simultaneously and non-invasively. Note that the uniformity of the low quality silica

tubes can be improved by circularising the preform tube on a glass lathe by applying the optimum

pressure for a given temperature and by collapsing the tube slightly. In this case, the uniformity can

be imporoved to be ~1%/m for both HLQ-210 and Vycor® tubes, improving the total yield of the

capillaries.

Suprasil® F300 tubes possess better initial uniformity, where the diameter deviation is typ.

±~0.2%/m. However, due to the limited accuracy of the draw parameters, some transverse

deformation induced through the drawing process degrades the uniformity of capillaries to the level

of ~0.5%/m. Nevertheless, more than 80% of the drawn capillaries can still be used after screening.

Therefore, the yield can be improved by using high qualit y silica tubes.

Next, the surface quality is discussed as it directly affects the optical losses as shown in Chapter 3.

When the glass tubes are shipped, they contain some surface scratches (particularly in the case of

low quality tubes), and these are known to be the cause of imperfection or scattering losses for

conventional MCVD fibres[139]. These surface defects can be eliminated by fire-polishing tubes

followed by pre-baking. Although the capillary drawing process itself involves heating a glass tube,

the capillaries have to be pulled at reasonably low temperatures and using a high feed speed, in order

to preserve the geometry and to ensure the stability. This makes it difficult to improve the surface

quality during the drawing process. Thus, use of glass tubes with good surface quality is essential.

The pre-baking process greatly impacts the quality of the surface after fire-polishing. A glass tube

was heated up to ~1700°C on the MCVD lathe. The oxy-hydrogen burner carriage was traversed

approximately 5~8 passes with a speed of 150mm/min., depending on the surface condition of the


tube and the oxygen flow inside the tube (~500cc/min.). If the temperature is too high (~1750°C), air

can be trapped within the scratch, forming bubbles that are diffcult to eliminate. If the temperature is

too low (1600°C), no changes occur to the surface since the visocity of the glass is too high. The

scratch dimensions vary depending on the tubes and their handling prior to the pre-baking process.

Therefore, a choice of an appropriate temperature is important. After prebaking the tube, the inner

surface of the tube can be etched off (~100µm at 1950°C) using SF6 and can then be fire-polished

(2050°C), through which process the presence of the defects/bubbles can readily be assessed.

Using appropriately prebaked tubes, no scattered light is observed at the bottom of the furnace when

the capillaries are drawn from the tower. However, the prebaking process incorporates a significant

amount of water into the glass due to the oxy-hydrogen burner heat source. Therefore, this process

may better be performed on the fibre drawing tower or using a dry heat source such as a plasma

torch[140] or a furnace. Recently, a technique that allows for mechanical polishing of silica tubes

(both outer and inner walls) to a high quality has been developed[141]. Such a low temperature

process is particularly attractive for obtaining good quality preform tubes since diffusion of the

water impurity is negligible in addition to the improved homogeneity.

After drawing capillaries, they are cut into suitable lengths for the preform assembly stage.

However, the cut end may possess sharp edges that can cause additional scratches when the

capillaries are stacked to form a preform. Therefore, the edges are trimmed by using a flame burner.

Although the impact of trimming is unknown, a lot of scatches as well as tiny silica particles were

observed within the assembled preforms without trimming. In contrast, they are perfectly

eliminated by the use of trimmed capillaries. Mechanical trimming is not recommended as it

generates micron-size silica powder that is difficult to remove due to static charges on a silica

surface.

2.3.6 Summary

The capillary drawing process has been discussed. The mathematical model for the process and the

derivation of its solution have been qualitatively described. It has been proven that it is difficult to

use numerical solvers based on the FEM method for analysing the practically important information

such as capillary dimensions with the desired accuracy. In contrast, the analytical solution provides

useful physical insights to qualitatively understand the process mechanism, which will be

demonstrated in the following sections. Finally, practical aspects of the capillary drawing process

have been presented. These form foundations for understanding the fabrication process and realising

a wide range of different MOFs.


2.4 Preform fabrication

Preform fabrication involves preparing capillaries and stacking them into a jacket tube. In this

section, practical issues related to the preform preparation are described, in order to neatly prepare

the structure as it has a direct influence on the fibre performance. The cleaning of capillaries and the

jacket tube is first discussed, and preform assembling is then presented.

2.4.1 Cleaning

Cleaning the capillaries is important in order to avoid contaminants that can be incorporated into

MOFs. Due to the elevated temperature in the furnace, any residue within a preform can be burnt and

some elements can diffuse into the glass, which leads to extra losses. In reality, however, it is

difficult to clean the inner wall of the capillaries because of their small inner diameter (<<1mm).

Consequently, only the outer surface of the capillaries is cleaned using acetone and iso-propanol

(IPA). Acetone removes most contaminants, while IPA removes acetone and then naturally

evaporates. Note that the evaporation of alcohol leads to condensation of water from the atmosphere.

When the furnace itself is contaminated, a colouring of the capillary surface is observed, primarily

due to the carbon inside the furnace. In this case, diluted hydrofluoric acid (~5%) can be used to

etch off the capillary surface by ~1µm prior to the normal cleaning procedure. Note that

over-etching leads to degradation of the surface, which is clearly observable since it loses its

specularity[142]. Inorganic contaminants such as iron that are trapped on the silica surface can also

be removed by using this process[143]. This contamination occurs only when the furnace element

is about to be exhausted and can be prevented by replacing the furnace element and cleaning the

inside of the furnace.

Both acetone and IPA have to be perfectly removed by drying capillaries since they decompose into

a powder form of carbon at the high temperature in the furnace, which not only contaminates the

glass, but may also cause instabilities by blocking the air holes during the drawing. In order to dry the

capillaries after cleaning, dried gas flow from the MCVD system can be used to purge the

capillaries. Once the capillaries are dried, one end may be sealed using a burner. By heating

capillaries, scratches and contaminants within the capillaries are illuminated by the black-body

radiation, and which allows us to select clean capillaries. Care must be taken so that the diameter of

the sealed end is not greater than the capillary diameter since overheating leads to globule formation,

which prohibits us from neatly staking capillaries into a jacket tube.

The jacket tube can be cleaned in a similar manner using acetone and IPA. Pre-baking a tube not only

improves the surface quality but also helps to clean it. Although a tube has to be dismantled from the


lathe after pre-baking, the jacket tube should be separated by using a burner rather than by cutting the

fused joint using a saw with a diamond coated blade. Cutting with a diamond coated dice generates

silica powders, which not only contaminate the surface, but also scratch the tube despite the use of

lubricant. Note that pre-baking the jacket tube is very important for obtaining a sufficiently strong

fibre, particularly when it is pulled at low temperatures.

If the preform is pulled into a cane for the two step drawing approach, the jacket tube can be etched

in diluted hydrofluoric acid prior to cleaning. This allows the elimination of adsorbed contaminants

from the surface, which are generally difficult to remove by using both acetone and IPA, as

described in relation to the capillary cleaning.

2.4.2 Preform assembly

General guidelines for the preform assembling are given here. The structural dimensions that are

related to the final fibre structure, which determines its optical properties, are excluded as they vary

substantially, depending on the target structures and the drawing strategies.

a) Choice of materials

In order to retain the air-silica structure during the fibre drawing process, there are two options in

terms of material combinations as follows.

�

Sealed silica capillaries + a silica jacket �

Unsealed silica capillaries + a Vycor® jacket

The key difference is whether to use sealed capillaries or not. For many MOFs, it is important to be

able to collapse interstitial holes and gaps between the jacket and capillaries, and this can be

accomplished by using sealed capillaries. Furthermore, this allows us to fabricate purely

single-material fibres (Fig.2.4.1 left). On the other hand, the interstitials can intentionally be left

open using a combination of unsealed silica capillaries and a Vycor® jacket (Fig.2.4.1 right). The

reason for this is explained below.

Eq.(2.8), discussed in Section 2.3, tells us that a tube with larger dimensions experiences less

collapse. Since this argument directly applies to the individual elements within a preform assembly,

it is clear that the capillaries are more collapsed than the jacket if both are made of the same material.

Thus, the structure cannot be maintained during the fibre drawing/caning because the capillaries

can freely move within the jacket tube. This issue can be overcome by pressurising the individual

capillaries so that the inner structure is less collapsed than the jacket tube, and which can be


accomplished by sealing the capillary ends. Then, the inner structure can be fitted to the jacket,

maintaining the structure. Otherwise, it is impossible to pull a single material structure without

sealing the capillaries, which was experimentally confirmed.

Fig. 2.4.1 Two types of possible preforms in terms of material combinations. An all silica fibre made of sealed capillaries (left) and a fibre made of unsealed capillaries using a Vycor® jacket (right).

Vycor® tubes contain ~4% of boron oxide, and this results in significantly lower viscosity than that of

silica for a given temperature. This leads to significant collapse of a Vycor® jacket, despite the large

initial dimensions, allowing the jacket tube to be fitted to the unsealed silica capillary bundle. Note

that the mechanical force from the jacket is negligibly small due to its significantly lower viscosity.

With the combination of the silica capillaries with a Vycor® jacket, it is not necessary to seal

capillaries. Below, preforms made of this combination are referred to as a composite preform.

Depending on the possible material combinations, the draw conditions can vary significantly. In the

case of unsealed capillaries, the viscosity and the surface tension of the glass material primarily

determine the evolution of the structure during the draw.

On the other hand, the balance between the pressure within the air holes and the mechanical forces

within the glass are the primary factors for the resultant geometry in the case of the preform with

the sealed capillaries. The pressure within the capillaries is inversely proportional to the length of

dc

Vycor jacket

Λc Silica rod (core)

Silica jacket Sealed silica capillaries

Silica capillaries (Unsealed)

Silica rods


the remaining preform, by ignoring the pressure decay inside the capillaries. Since most of the

pressures are cancelled out by the adjacent air holes, the net pressure imposed on the jacket tube is

not determined by the total volume of the air holes but by the dimensions of a single air hole.

Therefore, the pressure built up within the preform can well be compensated by either use of thick

enough jacket or by incorporating a large number of capillaries (>~50) to reduce the dimensions of

the air holes. On the other hand, when the air hole diameters of the capillaries within the preform

are small (<1mm), the pressure decay along the length of the capillaries can become significant due

to the temperature gradient along the preform length. This effect can be relaxed by sufficiently

warming up the preform before pulling it. Care must be taken when sealed silica capillaries are

used together with a Vycor® jacket tube. In this instance, the tube must be sufficiently thick.

Otherwise, the increasing pressure within capillaries can readily blow up the entire preform in this

case due to the small viscosity of the jacket tube.

b) Assembly

There are two important considerations for preform assembly. One consideration is the absolute

dimensions of the preform. The other issue is related to the practical care that must be taken during

the preform assembly.

The preform dimensions are determined by the capillary dimensions and the number of air holes

required within the structure. Although in general, the use of larger capillaries (~2mm) makes

assembly easier, there is a limit for the maximum possible preform dimensions because of either of

the following two reasons. One is the bore of the furnace, and the other is the thermal gradient

within the preform (the transverse temperature distribution) when it is within the furnace. Consider

that 2mm capillaries are stacked to form 8 rings of air holes. The I.D. of the jacket has to be 34mm.

Since the furnace bore used for the work in this thesis is 35mm, the jacket thickness must be as thin

as 0.5mm, which will cause a problem because of the possible expansion (or explosion) of the

preform due to the residual pressure when the sealed capillaries are used. The number of capillaries

can be increased by using smaller capillaries. However, it becomes increasingly difficult to

assemble them as their dimensions are reduced.

The preform dimensions are more severely restricted by the temperature gradient within the preform

in the case of single material preforms. Both large diameter preforms and ones that contain more air

holes are significantly affected by the temperature difference between the vicinity of the core and

that of the jacket tube, the latter of which is hotter and tends to be unstable. This is more apparent

when a vacuum is applied within the preform (see Section.2.5) and the resultant pressure is too low,

as the slight difference in viscosity leads to a significant transverse deformation through the

drawing. As a result, the air holes near the jacket tube collapse more than those near the centre.


If a thermal gradient exists along the length of the preform, the deformation induced by this effect

can be recovered by continuously pulling over a long length until the temperature distribution

within the preform reaches a steady state. However, any deformations caused by the transverse

thermal gradient cannot be recovered. A large bore furnace would help solve this issue since the

magnitude of the thermal gradient is primarily determined by the dimensions of the furnace

element.

As the preform dimensions increase, it typically takes more time to stabilise the draw process due to

the increased glass volume. Therefore, it is always better to use relatively smaller preform

dimensions (<20mm) with a sufficiently long length (>20cm) in all cases. The smallest capillary

dimensions that have been used for stacking to date are ~500µm. This allows us to reduce the

preform dimensions significantly so that no problems associated with the thermal gradient are

present. However, as the number of the air holes increases, this issue may have to be studied more

fully along with the temperature dynamics.

Additional care that should be taken during the preform assembly includes minimising accidental

scratches and breaks, and aligning the capillary bundle without any twists. The former can mostly

be avoided by trimming the ends of the capillaries as described in Section 2.3. The capillary bundle

needs to be held at the top end, to which the sealed ends are aligned in the same position so that there

is no significant pressure difference between the capillaries during the draw. This can be done by

slightly collapsing (<1% in diameter) one end of the jacket tube and inserting the last two or three

capillaries from the top end. When the cladding region is shaped, additional rods are inserted near

the jacket tube (see Fig.2.4.1 left), and which can conveniently be used to fix the stack to the jacket.

These additional rods are also useful for preventing any unexpected collapse of air holes near the

jacket tube that may occur due to the thermal gradient.

c) drop

The drop is a silica rod attached on the bottom of the preform, as discussed. For MOFs, the

temperature required for the drop is typically higher than the draw temperatures. In the case of

preforms with sealed capillaries, it is possible that pressure will build up within the air holes due to

the heat, and so there is a possibility of blowing up the preform before starting to draw it. There are

many options for preventing this accident from happening and some examples are given below.

First, the initial neckdown should be on the preform (not at the drop itself) since the heat capacity of

the MOF preform is much smaller. This allows us to start drawing fibres at lower temperatures

(~-100°C). Second, a large enough drop should be used for the given preform dimensions to apply

sufficient tension to counteract the increased internal pressure. Third, the preform should be warmed


up prior to pulling if the capillaries are sealed. Alternatively, a vacuum may be applied to the inside

of the preform to reduce the pressure, although this leads to substantially different draw conditions.

The internal pressure also depends sensitively on the water trapped inside the preform. Therefore, it

is imperative to warm up the preform prior to making a joint so that no water condensation occurs.

d) handle

In order to clamp the preform at the top of the tower, a handle needs to be attached to the preform.

The diameter of the handle tube should ideally be the same as that of the jacket tube, since it provides

a smooth transition in diameter. The argon flow within the furnace can greatly influence the stability

of the draw[144], so a smooth transition at the joint is important since it does not require any changes

of the iris aperture, thereby minimising the instability . The position of the joint should well be

separated from the capillaries (>3cm). Otherwise, if the jacket is overheated, the capillaries can be

badly deformed due to strong surface tension.

2.5 Caning and Fibre drawing

An appropriately prepared preform can be drawn into either a cane or a fibre on the fibre draw tower.

Depending upon the final dimensions of the structure in the fibre, either a one-step or a two-step

drawing approach is used. In the former case, the preform is directly drawn into a fibre. In the latter

case, the microstructured cane is drawn first from a preform, and is then pulled again by jacketing

with another tube. This approach is particularly effective for obtaining the small structures with Λ of

less than 2µm, the reason for which can be understood by considering the maximum draw ratio

required to achieve the small scale structures.

The smallest capillary dimensions, which can practically be stacked to form a preform, are limited

to ~500µm, as discussed in Section 2.4. Because of the initial capillary dimensions, a volume

reduction ratio η has to be ~10-7 to achieve Λ~1.5µm, for instance. However, as can be understood

from eq.(2.8), uf must be fast enough to preserve the structure. This leads to the requirement for

extremely fast draw speeds (ud >>100m/min.), which are not possible due to the restrictions on the

height of the fibre draw tower used for the work described in this thesis, as discussed in Section 2.2.

Hence, a two step approach has to be taken.

In this section, general guidelines for determining the draw parameters are presented. First, the

one–step fibre drawing process is discussed. Then, the two-step approach is discussed together with

the caning process.


2.5.1 Single-step fibre drawing

Fibres with relatively large scale structures can directly be pulled from preforms on the fibre draw

tower. Comparing the capillary drawing and the one-step fibre drawing in terms of eq. (2.8); the

one-step fibre drawing involves fibres with Λ>~3µm, the volume reduction ratio η ranging from

10-4 to 10-5, compared with ~10-3 in the capillary drawing. Furthermore, the minimum structural

dimensions within the fibre preform (~dc) are also an order of magnitude smaller (100~1000µm)

than those of the glass tubes (~10mm) used in the capillary drawing. Given that uf is not

significantly different in these two processes, eq.(2.8) suggests that more than 10 times higher

viscosity is required in one-step drawing to obtain the same collapse ratio, and this corresponds to

~200°C lower temperature. Therefore, it is understood that the temperature for the one-step fibre

drawing has to be significantly lower than the temperature required for the capillary drawing.

The above argument is reasonably applicable for composite preforms as their dynamics are

primarily determined by the unsealed silica capillaries. This in turn gives a straightforward way of

determining the fibre draw parameter by substituting (Λ/Λc) for (φ/φc) in eq.(2.1). Indeed, without

sealing, it has experimentally been observed that the temperature has to be 150°C lower than the drop

temperature in order to retain the structure with a high fraction of air (Fig.2.4.1 right) using uf

~4mm/min., which is comparable to the speed used in the capillary drawing. Thus, the draw

parameters can be well predicted by considering the individual capillaries within the preform.

On the other hand, for a single material preform, the final fibre diameter sensitively depends upon

the balance between the pressure built up within the capillaries and the other mechanical forces due

to the viscosity and the surface tension of the glass, as discussed. Since the pressure depends on the

size of the air holes within the preform, the optimum draw parameters must be tuned depending on

the preform design by monitoring the fibre cross section.

In general, owing to the presence of the pressure, the fibre can be pulled at higher temperature than

that used for the composite preform. However, it is also true that the structure in the fibre is more

sensitive to both uf and the temperature. The draw parameters can also be estimated by considering

the ratio (Λ/Λc)2, as the same as the composite preform. Drawing at higher temperatures or with uf

that is too low (<2mm/min.) may result in instabilities, where collapse and expansion of the air holes

occurs irregularly along the length of the fibre because the viscosity of the glass is too low. In

practice, this can be observed as diameter fluctuations. On the other hand, when temperatures are

too low, a structure is somewhat blown up. Since the collapse ratio of the jacket is inherently

smaller than that of the capillaries, a reduced temperature allows its collapse ratio to be almost

unity. This allows the capillaries within the preform to expand to fill the interstitials. Further

reductions in the temperature can result in the onset of interstitials.


As a result, the practically useful temperature range for stable fibre drawing can be very narrow for

single material preforms. For example, the optimum temperature range was as narrow as 10°C for

the preforms which contained 8 rings of air holes of dc/Λc~0.2 with a jacket thickness of ~3Λc. This

is because the small i nner diameter of the capillaries (~100µm) led to the onset of the instabilities

at lower temperature, whilst the interstitials open up at higher temperature. Thus, in general, when

either Λc or dc/Λc is small, the temperature range tends to be narrow.

To widen the stable drawing temperature range, either a thicker jacket whose ratio between I.D.

and O.D is more than dc/Λc can be used or a well controlled pressure, lower than atmospheric

pressure, can be applied inside the preform. However, in the former case, the fibre diameter change

becomes insensitive to the inner structure, making it difficult to observe the stability of the

structure along the length. In addition, when d/Λ is required to be <0.5, the fibre can become

unacceptably thick.

When a preform is pulled over a length of a couple of centimetres, the temperature may need to be

slightly adjusted (~10°C), so that the balance between the forces and the internal pressure within the

individual capillary is retained. When a lower pressure than the atmospheric pressure is applied

inside the preform, the pressure can alternatively be adjusted. Note that the required pressure

changes are slow compared with the other diameter fluctuation mechanisms. Therefore, in principle,

feed back control could be implemented to control the furnace temperature or the pressure inside the

preform, in order to obtain a uniform structure over a longer length.

2.5.2 Two-step drawing

The total volume reduction factor, a product of two steps as ηt =η1 η2, varies from 10-6 to 10-7 in the

two-step approach, where the subscript number corresponds to each step. The problem then

becomes how to allocate the volume reduction factor for each step that provides optimal structural

control in achieving the small scale structure in the final fibre.

As discussed, a smaller η permits a smaller collapse ratio, thereby better preserving the structure.

On the other hand, the collapse ratio is also inversely proportional to the initial structural

dimensions. Therefore, it is intuitive that η2 should be as small as possible since the default

structural scale involved in the second step is small. However, the maximum value of η is limited

to be ~10-4, assuming that the lower limit of uf is ~2mm/min. whilst the highest ud is ~60m/min. as

described. Therefore, η1 must be at least 10-2, which corresponds to an order of magnitude

reduction in scale at the first step. Because η2 is comparable to that of the single step approach, this


suggests that an order of magnitude higher viscosity is required to retain the same collapse ratio at

the second stage. In practice, this means that the fibre has to be pulled more than 250°C below the

drop temperature, which is impractical because of the high tension that would be required. (The

fibres pulled with the two step approach used tension >120cN that is beyond the measurement

range of the available tension gauge. Therefore, it is impossible to specify the actual values here.)

This implies that it is necessary to seal the cane to preserve the structure, and it is difficult to

reliably pull the fibre with a small η2 value.

Fig. 2.5.1 SEM photographs of early two-step drawn fibres.

In order to cope with this issue, some techniques were developed. One is to use additional

capillaries for the fibre preform so that the cane is thermally isolated as shown in Fig.2.5.1 (a). This

method also allows us to reduce the fibre tension because of the small heat capacity due to the

reduced silica volume in the region surrounding the sealed cane. However, this is accompanied by

the reduced mechanical strength of the fibre, which makes use of this fibre type impractical. To

reinforce the fibre, a solid Vycor® jacket was also used (Fig.2.5.1 (b)). Although this approach also

greatly reduces the fibre tension while improving the mechanical strength, preparation of the jacket

is very difficult because of the poor qualit y of the starting tubes, whose circularity needs to be

improved on the lathe prior to stacking. Furthermore, a Vycor® tube is not a sufficiently transparent

glass to allow for the ultimate low loss fibres, given the diffusion length of OH ions during the

fibre drawing process, as described in Chapter 3. Single material fibres, ideally all silica fibres, are

desired.

In order to reduce the tension on the fibre during the second stage of the drawing process, there are

two options. One is to use a small preform (<<10mm), since the preform is more uniformly heated

for the given furnace bore. Another option is to reduce ud. These options can be applied by

decreasing η1 to ~10-3, or by correspondingly enlarging η2 to ~10-3. By drawing small canes, it is

possible to use ud as slow as ~10m/min. where the fibre tension is low enough to allow us to pull at

(a) (b)


low temperatures. Although the initial dimensions at the second step are smaller than 100µm, and

correspondingly the temperature has to be reduced (~250°C below the drop temperature), it has

become possible to reliably draw truly single material MOFs with extremely small dimensions,

combined with the use of high qualit y (pre-baked) jacket tubes (see Fig.2.5.2).

Fig. 2.5.2 SEM photographs of all silica MOFs pulled using two-step approach.

Since the cane must be sealed to prevent the structure within the cane from collapsing, care must be

taken at the second step to achieve good structural control. The hole-to-hole spacing Λc within the

cane can be used as a guide to determine the draw parameters by comparing it to the target

parameters Λ.

The collapse ratio of the jacket is nearly zero at a low temperature. Thus, the cane must be tightly

fitted within the jacket. Otherwise, the sealed cane can expand during drawing while preserving the

glass volume according to eq.(2.1). This results in a larger fraction of air in the fibre than in the

original cane (Fig.2.5.2 (a)). However, because of the small cane dimensions (<1mm), it is difficult

to prepare a jacket tube so that it perfectly fits the cane. To overcome this problem, the fibre is

pulled by using a relatively loose jacket (more than ~300µm greater than the cane diameter), and

applying a lower pressure (~300mbar) inside the preform to intentionally collapse the jacket so as to

allow the jacket to fit onto the cane, as shown in Fig.2.5.3. Note that the accuracy of the pressure

gauge reading can be poor due to possible leaks and to the long length (typically >1m) of the

preform. The resultant structure is shown in Fig.2.5.2 (b).

Under this condition, the expansion of the cane structure is sensitively determined by the pressure

and the temperature for a given thickness of the jacket for a given value of the draw speed (i.e.

tension). In other words, the balance between these three factors determines the cladding structure.

The pressure, at which the jacket tube fits the cane, varies from 400 to 700mbar depending on the

thickness of the jacket tube. Naturally, use of thick jacket tubes requires a smaller pressure inside

the preform at a given temperature.

(a) (b)


Fig. 2.5.3 Schematic of the pressure control inside the preform for the two-step approach.

Although the jacket fits the cane well by applying the low pressure, the cane structure can still be

expanded depending on the jacket thickness, particularly when the pressure is too low. The reason

may be attributed to the fact that the cane starts expanding before the jacket shrinks because of the

low pressure. Thus, there is an optimum pressure, where the jacket is sufficiently collapsed while

suppressing the expansion of the cane, and which can be found by accurately controlling the

regulator valve. From the above observations, the range of this optimum pressure is expected to be

very narrow, depending on the preform (the initial air hole diameter within the cane and the

thickness of the jacket tube). Fixing the draw speed is also important since a slight difference in

draw speed modifies the effect of the surface tension of the jacket tube. Therefore, the expansion of

the cladding structure is also sensitively affected by the draw speed or the applied tension (i.e.

when the tension is too high, the cladding structure tends to expand.)

A systematic study of the pressure control (and the effect of tension) is ongoing. The structures

fabricated so far have been controlled primarily by increasing the jacket thickness to suppress the

expansion (compare the fibre diameters in Fig.2.5.2 (a) and (b)) by fixing the draw speed. This has

allowed us to obtain similar d/Λ to dc/Λc (the difference is <10%) for structural ranges of

~0.2<d/Λ<~0.9 and Λ<2.5µm. By improving the pressure control, more accurate control over the

structural dimensions can be obtained without significant restrictions in fibre diameters (and thus

jacket thickness).

Pressure gauge

Argon supply

Vacuum pump

Microstructured cane (sealed & fused)

Jacket tube

Valve

(Bubblers) Regulator

valve


2.5.3 Summary

General guidelines for determining the parameters for fibre drawing and caning have been presented.

It has been shown that the analytical model for capillary drawing can be applied for the individual

components within the preform, and can conveniently be used to consider the draw parameters.

Sealing capillaries or canes has allowed us to realise single material fibres, which provides more

ideal realisations of the unique characteristics of MOFs as demonstrated in Chapter 5. For the

two-step approach, it has been found that the key issue depends on relaxing the fibre tension at low

temperatures, and that precise pressure control is required for more accurately controlling the

cladding structure.

2.6 Conclusions

The fabrication technology of MOFs has been discussed and the steps taken to refine the process and

to realise a wide range of different structures of MOFs have been described.

First, a mathematical model for a capillary drawing process has been developed. Experiments have

been carried out to examine the validity of the model. It has been demonstrated that the model can

predict the draw parameters required to obtain the desired dimensions of capillaries for given

dimensions of a tube. Although a numerical approach was also taken to obtain solutions to the model,

it has been proven to be impractical primarily due to the intensive computational tasks involved.

Practical aspects of capillary drawing such as characteristics of raw materials and surface quality

improvement have also been discussed, where the importance of the use of high quality silica tubes

and of a pre-baking process has been emphasised.

Secondly, general guidelines for the preform preparation, caning, and fibre drawing have been

presented. It has been shown that physical insights obtained from the capillary drawing model can be

conveniently used for determining the draw parameters. The dynamics involved in the fibre

drawing were qualitatively discussed based upon the observations for both single and two-step

approaches. It has been shown that the single step approach possesses certain advantages, and that

the use of a high speed fibre drawing system will allow the fabrication of fibres with any structural

dimensions in a single step. Despite the difficulties involved in the two-step approach, it has been

demonstrated that, by optimising the draw parameters to relax the fibre tension and by improving the

quality of the jacket tube, it is possible to reliably fabricate MOFs with a wide range of structures.

Chapter.3

Transmission properties of

highly nonlinear microstructured optical fibres

3.1 Introduction

3.1.1 Overview of the fibre with high nonlinearity

Recent advances in wavelength division multiplexing (WDM) and time division multiplexing

(TDM) technologies have made it possible to transmit more than 1Tbit/sec through a single mode

optical fibre over several tens of kilometres or more[145,146]. The control of optical nonlinear

effects is one of the key enabling technologies for these ultrahigh bit-rate systems, allowing for

ultrafast signal processing, which is difficult to perform electronically, to be performed directly in

the optical domain.

In these systems, conventional fibre types have often been used as an optical nonlinear medium.

Although the nonlinearity of silica-based optical fibres is not particularly high relative to many

other materials, the long interaction lengths that can be achieved by virtue of the low transmission

losses of the fibres, combined with their potential low cost, makes fibres very attractive options as

nonlinear media.

However, due to the inherently low nonlinearity of the high silica fibres, kilometre scale lengths are

typically required to take advantage of the nonlinearity of these conventional fibres. Such a long

length requirement leads to several limitations such as accumulated chromatic dispersion, which

typically limits the useful bandwidth of such devices. For this reason, highly nonlinear dispersion

shifted fibres (HNL-DSFs) have been developed, which possess a high nonlinearity (γ~20W-1km-1)

combined with a small amount of dispersion (D<2ps/nm/km) by achieving high NA (~0.4) while

retaining compatibility with conventional fibres[147].

Chapter.3 Transmission properties of small core microstructured optical fibres 37

MOFs provide a simple route for realising highly nonlinear fibres relative to conventional HNL-

DSF, since it is possible to fabricate fibres with very small effective mode areas due to the high

index contrast between the silica and air[33], and we specifically refer to such highly nonlinear

MOFs as HNL-MOFs. In addition, high nonlinearity in MOFs may be accompanied by interesting

dispersion properties[32,37,38], which can potentially be tailored over wider spectral ranges than

conventional fibres. To date, it is commonly accepted that higher γ can be achieved within HNL-

MOFs than in HNL-DSFs and that the fibre based nonlinear device lengths can significantly be

shortened using HNL-MOFs. The highest nonlinearity theoretically achievable within pure silica

based HNL-MOFs is predicted to be γ~52W-1km-1 at 1550nm (i.e. Aeff~1.7µm2 assuming n2~2.2x10-

20m2/W)[148]. By using a highly germanium doped core, a further factor of two increase in

nonlinearity could be anticipated[41].

However, there are currently several serious drawbacks in silica based HNL-MOFs, compared with

HNL-DSFs: interface issues and losses. The losses are in particular a severe factor since the useful

fibre nonlinearity is determined by the product of the nonlinearity and the effective device length.

Apart from dispersion properties, which make the most impact for some applications, one possible

approach to evaluate the fibre for nonlinear device applications, in which the phase-matching

condition is automatically satisfied, is to define the figure of merit (FOM: γ/α) since the effective

length (Leff=(1-exp(-αL))/α) is ultimately limited by the attenuation length (i.e. α-1). HNL-DSFs

exhibit FOM=25~30dB-1W-1[147]. Given the current loss levels of HNL-MOFs (10~100dB/km),

their FOMs are <3dB-1W-1. Thus, if the loss level can be reduced by a factor of ten or more, and the

interface issues can be solved, then HNL-MOFs will be in the position to compete with or even

improve upon the performance of HNL-DSFs. Thus, it is of paramount importance to reduce the

current loss levels of HNL-MOFs. These parameters are summarised in Table.3.1.1.

Table. 3.1.1 A comparison of the fibre parameters between HNL-DSF and HNL-MOF (typical).

HNL-DSF HNL-MOF γ [W-1km-1] 10~25 ~50 D [ps/nm/km] <2 -300~100 α [dB/km] 0.5~1 10~100 FOM [dB-1W-1] 25~30 <3

3.1.2 The loss mechanisms within HNL-MOFs

The total losses of optical fibres can be written as a sum of different factors[149].

� � �� , (3.1)


where αUV, αIR, and αS are the losses due to the electron transitions in the ultraviolet, multi-phonon

absorption in the infrared, and scattering losses, respectively. αIM and αC denote the losses induced

by the impurities such as hydroxyl ions and confinement losses, respectively. Throughout this

study, it has become apparent that except for αUV, αIR, which are intrinsic to the core material of the

fibre, the other three components in MOFs substantially differ from those of the conventional fibres,

as described below.

The scattering mechanisms that occur within MOFs are complicated when compared to

conventional fibres. In addition to the Rayleigh scattering, that originates from nanometer-scale

refractive index inhomogeneities inside the glass due to its random molecular structure,

perturbations to the guided modes can result from any fluctuations in the fibre geometry along the

fibre length. In particular, a longitudinal variation of the structure promotes modal coupling to high

order or radiation modes, resulting in excess losses. Therefore, the scattering is no longer intrinsic

to the material in MOFs as it is in the conventional fibres. In Section 3.2, the author investigates the

scattering loss properties of HNL-MOFs.

The losses induced by impurities have been a major issue to be solved to widen the transparent

spectral window of conventional silica based optical fibres and continuous efforts have been made

to improve conventional silica fibres for more than 30 years[130,140]. The unique fabrication

method for MOFs, which the author has used (the capillary stacking technique), begins with

synthesised silica tubes or rod, differing from the conventional technologies where vapour phase

halide materials are synthesised in order to ensure the material purity and, for some cases, to reduce

incorporation of hydroxyl ions (i.e. MCVD). In addition, the capillary stacking method consists of

multiple heating steps such as capillary drawing, caning and fibre drawing. Therefore, a

substantially different approach must be taken in order to eliminate these impurities. Primarily

owing to the high purity of the raw glass materials that we used, no significant traces of transition

metals have been observed, but hydroxyl ions have been and present a significant problem for

telecommunication applications. Section 3.3 is devoted to the development of the fabrication

method for low-OH HNL-MOFs.

Finally, the most unique loss mechanism in MOFs is the confinement losses[150], which arises

from the fact that the core of single material MOFs has the same refractive index as the region

beyond the finite holey cladding. For this reason, the modes within MOFs are inherently leaky.

Although this type of loss can potentially be minimised by appropriate cladding designs (solely by

increasing the number of air holes), or introducing a doped section within the core, the practical

number of air holes that can be arranged within a fibre cross section is limited by the fabrication

difficulties.


Because the modal response of the holey cladding is a strong function of wavelengths, the

confinement losses exhibit a strong wavelength dependence, as shown in Fig.3.1.1. The cut-off

appears at long wavelengths since the modal field significantly penetrates into the holey cladding

and thus starts leaking to the jacket. The fibres studied in this section all possess negligible

amounts of confinement losses primarily because the fraction of air was high and precautions were

taken to incorporate sufficient numbers of air holes. However, when the dispersion properties are of

major interest as well as the nonlinearity, it may be necessary to reduce the fraction of air within

the fibre structure[40]. In such cases, it may be necessary to take the effect of the confinement

losses into account and to adopt the other strategies to reduce them. An apparent approach includes

gradually increasing the size of air holes in a radial direction towards the jacket. As shown in

Chapter 6 and 7, incorporating different sizes of air holes and arranging them is feasible.

Fig. 3.1.1 Example of confinement losses in white light transmission over a 3m length of the MOFs (left).

3.1.3 Outline

This chapter describes continuous efforts toward reducing the loss level of HNL-MOFs. Although

very low loss MOFs have recently been reported[123,151], they are not in parameter ranges

suitable for achieving short device lengths in nonlinear fibre device applications. More importantly,

it is not well understood how the losses depend on the structure within the highly nonlinear regime

and how the fabrication process can be improved. To solve these issues, it is very important to

understand the possible loss mechanisms within HNL-MOFs.

Therefore, the aim of this chapter is to understand the loss mechanisms involved within MOFs, in

particular in the highly nonlinear regime, and to consider possible improvements in fabrication to

reduce these losses. In Section 3.2, the relation between the losses of HNL-MOFs and their

structural parameters is presented. In Section 3.3 the OH induced losses in MOFs are discussed,

and some improvement of the fabrication is demonstrated. Finally, conclusions are given in Section

3.4.

��

� �� !" #$%

& ��

& ')(

& ' �

& � (

& ��Confinement loss edge


3.2 Scattering losses

3.2.1 Introduction

Scattering losses is a term that is used to refer to Rayleigh scattering in optical fibres, but the other

mechanisms such as imperfections can also lead to scattering losses. Examples of such

imperfections in conventional fibres are bubbles and diameter transitions, which can cause

coupling to the radiation modes. HNL-MOFs may be multimoded and higher order modes may

suffer from excess losses due to their relatively poor confinement. Hence, any modal coupling to

higher order modes in HNL-MOFs effectively contributes to the total losses, which may take the

form of coupling to radiation modes in single mode waveguides.

Love et al introduced a generalised low loss criterion for single mode waveguides, where they

showed that the perturbation length scale Ln should satisfy the following inequalities[152]: (a)

Ln<λ/n or (b) Ln>zc=2π/(β1- β2), where λ is the wavelength, n the refractive index of the core

material, zc the beat length between the two modes of interest, whose propagation constants are β1

(fundamental mode) and β2 (cladding mode), respectively. (a) represents the fact that if the length

scale of a waveguide nonuniformity is longer than the wavelength of the incident light, there will

be significant scattering or loss because the length scale of the field is small enough to detect the

non-uniformity. On the other hand, (b) denotes the length scale with which modal coupling occurs.

These loss mechanisms are all related to the nonuniformity/perturbation length scale with respect to

the modal properties of the waveguides. This in turn implies that the scattering loss characteristics

within HNL-MOFs are very much wavelength dependent since the modal properties, such as

propagation constants and mode field diameters, vary drastically depending on the operating

wavelengths and the structural scales. This suggests that without any knowledge of the modal

properties, it is hard to interpret the detailed loss mechanisms from observations.

In order to quantitatively understand the loss mechanisms, the length scales of any perturbation

within the fibre must be known. Techniques for experimentally characterising the perturbation

length scales exist for conventional waveguides/fibres. The longitudinal variations of the core-clad

interface profile are measured by some means, which is described later, and so the profile function

of the roughness f(z) can be obtained. The general approach to analyse such random variations is to

take the correlation function � � � � � �� of the profile function[153,154]. In general,

Cf is a monotonically decreasing function since there are similar features within the surface profile

(if it had perfect randomness, Cf would be a delta function), and in which the characteristic decay

length, correlation length, Lc can be defined. By taking its Fourier transform, the power spectrum of

the longitudinal variation can be obtained as ��

�� . It has been shown that


the spectral components that fall within the range �� promote coupling

between the two modes with propagation constants β1 and β2 as a result of scattering[155]. Note

that the lower and upper limits correspond to the forward coupling (sub-millimetre scale) and

backward coupling (sub-micron scale), respectively. The former provides the length scale zc. For

q>>β (nanometre scale), the scattering can be categorised as Rayleigh scattering, which results in

scattered radiation in all directions.

Several methods have been proposed to characterise the profile function f(z). For instance, to assess

cabling issues, conventional diameter gauges can be used[153]. Since the ratio between the core

and the cladding is well defined in conventional fibres, the core diameter fluctuation can be

quantified by measuring the sub-micron scale fluctuations of the fibre diameter. For more precise

measurement, use of whispering-gallery modes has recently been proposed providing information

about nanometre scale fluctuations, although the length scale is limited to centimetres[156]. On the

other hand, for planar waveguides, atomic force microscopes can be used[154].

In the case of MOFs, it is currently difficult to directly assess the core diameter profiles along the

length, since the core dimensions and the cladding structures are not necessarily reflected by the

fibre outer diameter. Recently, use of surface third harmonic generation[157], by scanning the

focused femtosecond laser pulse beam across the fibre cross section, has been proposed[158].

However, the longitudinal resolution of this method is approximately limited by the beam spot size.

Furthermore, a large number of air holes involved in HNL-MOFs (to prevent the confinement

losses) may weaken the pump beam before reaching the vicinity of the core. Thus, it would be

difficult to characterise the structural fluctuation with sub-micron length scales.

Given these experimental difficulties for HNL-MOFs, one must deduce the loss characteristics of

HNL-MOFs from experimental observations and theoretical considerations. Below, several

scattering mechanisms are considered for HNL-MOFs. Then, the experimental observations are

presented including cut-back and back-scatter measurements. By using simple theoretical estimates,

it is shown that the Rayleigh scattering is the predominant loss mechanism in HNL-MOFs. Finally,

conclusions are given.

3.2.2 Scattering mechanisms

We have three scattering loss components as follows.

�� (3.2)


where αf and αb denote the coupling losses to the forward and the backward radiation or high order

modes, respectively. �� is the effective Rayleigh scattering coefficient. Below, each component is

separately discussed.

A) Forward scattering

The generalised low loss criterion tells us that the forward scattering process can be prevented by

ensuring that Ln>zc=2π/(β1- β2). Mortensen et al. suggested that this rule can be used for the low

loss criterion for single mode MOFs[159], where β2 can be taken as the propagation constant of the

fundamental space filling mode in the single mode regime. However, multi-mode regimes have not

been considered. To investigate the multi-mode regime, which is the often the case for HNL-MOFs,

the neighbouring high order mode (quasi-TE mode[160]) should be considered. After calculating

the effective cladding index using the effective index model[13], the dispersion formula for TE

modes within a perfectly circular step index fibre can then conveniently be used to estimate the

value of β2.

��

� ��

�

��

��

�� ! "$#�%�� &�� &' "$#�%�� (�� (! "$#�%

Fig. 3.2.1 Normalised relation between the coupling lengths and the wavelengths.

Fig.3.2.1 shows the normalised relation between the coupling lengths and the wavelengths in the

multimode regimes using the estimated values of β1 and β2 as described above. The core radius was

assumed to be )* [23]. Note that we have ignored the wavelength dependence of the silica

refractive index and taken nsilica=1.45 since this ensures scale invariance of the wave equation[161].

As we increase d/Λ, the coupling length is significantly reduced because the difference of the

propagation constants between the fundamental modes and high order modes/radiation modes

becomes greater. This indicates that fibres with large d/Λ have wider low loss ranges in terms of


the perturbation length scale, as anticipated. It is clearly seen that there are minima with respect to

λ/Λ for a given value of d/Λ. This results from the fact that their dispersion relations are different:

the two modal indices of the guided and cladding(/high order) modes take values between the

refractive index of silica at the high frequency limit and the volume average index of the cladding

at the low frequency limit with different dependencies on the frequency, according to the

dispersion formula. Thus, there is an intermediate frequency at which the difference between the

two modal indices becomes a maximum that corresponds to the shortest beat length between these

modes.

Also, as we decrease λ/Λ, each line splits into two, which represents the onset of the quasi-TE

modes. For this reason, the perturbation scale zc sharply increases with reduced λ/Λ. The pitches Λ

that define the modal cut-offs of the quasi-TE modes at 1550nm are estimated to be ~1.1µm for

d/Λ=0.9 and ~1.55µm for d/Λ=0.7, which can be used as a measure of the accuracy of the model.

Since HNL-MOFs are operated in the range where λ/Λ>1.0 with d/Λ>0.9, the coupling length scale

is found to be less than 10 times the wavelength, which is far shorter than those of the conventional

fibres (~103 times greater than the wavelength), despite the presence of the lossy high order modes.

This suggests that only a narrow range of spatial frequency components q can promote the coupling

from the fundamental mode to radiation(/high order) modes and lead to losses. Therefore, the

losses due to the forward scattering αf can be smaller in HNL-MOFs than in conventional fibres for

a given perturbation spectrum and coupling strength.

B) Backward scattering

Backward scattering has been discussed in the context of surface roughness within planar

waveguides[154,162]. Although the mechanism involves coupling to all of the backward

propagating modes in reality, we restrict ourselves to the coupling to the fundamental mode only

for brevity.

Roughness is generally described in terms of axial changes in the core dimensions[154]. The

commonly accepted model for the correlation function within planar waveguides is an exponential

decay function � � � �� , where δf is the variance of the roughness[162]. It has

been shown that the attenuation coefficient per unit length due to the local reflections caused by the

roughness is expressed by the Fourier transform of the correlation function of the coupling

coefficient[162].

Under the assumption of weakly guiding waveguides, the coupling coefficient for the circular step

index fibre can be derived as follows,


��

��

� � � �� !" , (3.2)

where k is the wave number, aeff the effective core radius (due to the hexagonal shape of the core in

a MOF), nco is the core refractive index (=nsilica for single material MOF.) All the other parameters

are defined in Appendix A. In this way, the surface profile function and the coupling coefficient are

linked. The attenuation coefficient per unit length can then be obtained by taking the Fourier

transform of the correlation function of the above relation as

#�$#�$ %%%%%% %&'(% % )* *+ ,,-. - -,�/. - -0 112334 3452687 9:

;9:< =>>?@

AABC >>?@

AABC =D . (3.3)

Thus, for given parameters d, Λ, λ, and the parameters for roughness (i.e. δf and Lc), it is possible to

roughly estimate the resulting losses due to the backward coupling. Note that this model only

accounts for coupling to the backward propagating fundamental mode. Also note that δf and Lc

must be characterised by some means, as discussed. By taking a derivative with respect to Lc, it is

readily understood that αb possesses a peak value at Lc=1/2β, which is of the order of 100nm.

Fig.3.2.2 shows the dependence on Λ for the structure with d/Λ=0.9. aeff is approximated to be EFG[23]. In Appendix A, a more reliable approach is discussed in order to obtain an adequate

value of aeff. The roughness parameters δf=5nm and Lc=100nm are tentatively used. Given the

observed roughness in planar waveguides[162], δf is most likely overestimated while Lc is most

likely underestimated.

HJILKNMPO MPQSRUT VXW Y[Z]\^P_ `Lâ^P_ bcêdf_ ^L^gdf_ hL^gdf_ iLêdf_ `L^gdf_ bc^jhP_ ^L^k lmn opq rs

^P_ Ûd^P_ ddd�^d�^c^

t�uwv x y z {|�u�} x ~ z {|�u�} x � z {|�u�} x � z {|�u�} x v z {|�u�} x y z { ��w�[�]�

�f� � �f� � �f� � �f� � �f� � �P� ��

��L��L��L�

Fig. 3.2.2 Calculated attenuation spectrum assuming δf=5nm and Lc=100nm for d/Λ=0.9 with different Λ (left), and its Λ dependence at 1550nm (right).


Typical fibre diameter fluctuations observed during the HNL-MOF drawing were of order ~1µm

over several millimetres (determined by the indicator clock frequency, and ultimately limited by

the operation clock frequency of the diameter gauge ~1kHz). Assuming that this fluctuation is

reflected in the central structure, the fluctuation of Λ can be ~60nm for an 8 ring structure and

~75nm for 6 rings, corresponding to ~Λ/10 for HNL-MOFs. Since the core volume of the MOF is

dependent on the volume change of the surrounding capillaries, the variance of the roughness (or

corrugation amplitude) can be greater than 1nm, although the longitudinal length scale of such

fluctuation within MOFs is unknown.

As expected, αb sharply increases with the effective core radius aeff due to the inverse quartic

dependence (and thus is roughly proportional to ~Λ-4). This can be interpreted as being caused by

the increased modal overlap with the perturbed region (the air-silica interface for high d/Λ). For the

same reason, the attenuation coefficient is greater at longer wavelengths, since modal penetration

into the cladding is more significant. This indicates that the surface condition of the air-silica

interface can significantly influence the total losses within HNL-MOFs, particularly at long

wavelengths and for small core dimensions. Since αb is also proportional to δf2, it is understood that

δf must be below 1nm to achieve αb <1dB/km for Λ~1.2µm at 1550nm.

C) Uniform scattering

Rayleigh scattering is described by classical theory in terms of electric dipoles driven by

electromagnetic waves propagating through an amorphous material. Due to microscopic

inhomogeneities in the refractive index, adjacent dipoles are not perfectly cancelled out, resulting

in radiating waves. Since the local index inhomogeneity is significantly smaller than the

wavelength of light, the inhomogeneity acts to scatter the incident wave in all directions.

Although the detailed Rayleigh scattering mechanism due to the holey cladding is not well

understood, it is possible to take into account such an effect by assuming the effective scattering

medium for the holey cladding that follows the Rayleigh law[163]. The effective Rayleigh

scattering coefficient �� can then be written as the intensity weighted mean value[164]:

��

�

� ��

� �� !�� !�� ""

#$$"

#"

, (3.4)


where �� is the confinement factor, and �� and �� are the

respective Rayleigh scattering coefficients within the core and the cladding. Notice that here we

assume that the holey cladding exhibits uniform scattering (although there are unlikely to be any

scattering events within the air holes.) Note also that when d/Λ is smaller, the scattering elements

(air-silica interfaces) are spatially more localised. Although these effects are difficult to quantify, it

is likely that the accuracy of the model is worse for the fibres with small d/Λ.

The Rayleigh scattering coefficient of carefully prepared conventional fibres is ~0.8dBµm4/km,

which results in �� ~3.2x10-5m-1 at 1550nm[165], and which may be used as an estimate for ��

in MOFs. Slightly higher values (e.g. �� ~4.5x10-5m-1 at 1550nm) are observed for commercially

available telecom grade fibres[166] possibly due to their germanium content and high tension used

during the fibre drawing[165]. Although no experimental values have yet been reported for ��

within MOFs, a significantly higher value can be expected.

Fig.3.2.3 shows calculated �� for fibres with d/Λ=0.9 and different Λ, where

�� =3000dBµm4/km and �� =0.8 µm4/km are assumed, as experimentally observed in Section

3.2.3. It is clear that for a fixed value of �� , �� significantly increases as the pitch Λ (or the

core dimensions) decreases since the modal overlap with the cladding increases (i.e. Γ decreases).

Furthermore, it is clearly seen that the curves display a square root dependence at long wavelengths

with respect to λ-4 whilst showing a linear dependence at short wavelengths.

�� "!�#%$&� �'(*) ( (*) + ,-) ( ,-) + ./) (0 12234 567 89

:;<:=>:?:=�;<:@�:?:@*;<: A/B<C�D E>F-GA/B�H�D C�F-GA B�H�D I F GA/B�H�D J�F-GA/B�H�D K�F-GA/B�H�D E>F-G

Fig. 3.2.3 Predicted loss spectra from eq.(3.4) for various dimensions of the fibres with d/Λ=0.9.

Because L�M�N OPQ << R�S T�UVQ is assumed, the term proportional to R�S T�UVQ effectively dominates the

Rayleigh scattering losses, and thus its wavelength dependence can strongly be modified by (1-Γ).


Γ is in general nearly unity at short wavelengths since the light is well confined within the core,

and it is reduced toward long wavelengths. It was found that within the wavelength range where

Γ>>0, (1-Γ) may be approximated by a quadratic function of λ. Therefore, the wavelength

dependence of the Rayleigh scattering losses can be proportional to λ-2 within a certain wavelength

range as � ��

� � �� . Thus, �� can display a

square root dependence with respect to λ-4. Note that the square root dependence is more

significant for the fibres with smaller Λ since (1-Γ) more significantly changes with wavelength

within such fibres.

By analysing eq.(3.4), it is understood that � � �� >> �� is required for the λ-4

dependence to be significantly modified due to the strong wavelength dependence of Γ. Although

the fibres with d/Λ=0.9 are only discussed in Fig.3.2.3. It is apparent that for small values of d/Λ

(e.g. ~0.4), Γ/(1-Γ) is smaller since the modal confinement is weaker compared to the fibres with

d/Λ=0.9. For instance, Γ/(1-Γ) is ~25 for Λ=2µm and d/Λ=0.9 whilst ~4 for Λ=2µm and d/Λ=0.4 at

1550nm. This implies that the wavelength dependence of �� is stronger for the fibres with small

d/Λ for a given value of � � �� .

The general conclusion here is that for both small Λ and d/Λ, Γ/(1-Γ) becomes smaller and that the

resultant Rayleigh scattering loss spectra are thus more strongly influenced by a give value of

� � �� . However, the author notes that � � �� itself should be a function of d/Λ and Λ since the area

of the air-silica interface depends on these parameters. For this reason, it is anticipated that an

optimum combinations of Λ and d/Λ exists, with which �� is minimised for a given wavelength.

Note that these parameter sets can differ from the ones obtained by considering αf (thus minima of

zc) as discussed for Fig.3.2.1.

3.2.3 Cut-back loss measurement of HNL-MOFs

The losses for a range of HNL-MOFs were measured with a white light source over >60m lengths

on fibre bobbins (~50cm diameter) using the cut-back technique. The SEM photographs of the

fibre samples are given in Fig.2.5.2 and Fig.3.2.5. More than 25m of fibre was left on the drum

remained to allow any cladding modes excited at the launch to be attenuated. Because of the poor

launch efficiency of the white light source to the very small cores of these fibres, the dynamic

range for the signal was limited to ~10dB.

Fig.3.2.4 shows loss spectra taken for several different fibres, whose core dimensions are

1.22µm(A), 1.37µm(B) and 1.65µm(C), respectively (d/Λ>0.9). As expected, the loss spectra can


be divided into two regimes depending on the wavelength range. At long wavelengths, the loss

spectra show a square root dependence to λ-4, indicating the λ-2 dependence of the losses. At short

wavelengths, a conventional linear fit to λ-4 is possible. Qualitative agreement with Fig.3.2.3

indicates that the losses are predominantly due to the large value of � � �� combined with the

wavelength dependent change in the confinement factor Γ. Note that the determination of the

fitting ranges for these two regimes is performed empirically by visually inspecting the loss spectra.

Note also that the large water absorption peaks in these loss spectra were eliminated prior to fitting.

Fig. 3.2.4 Loss spectra of the MOFs with different core diameters (A: 1.22µm, B: 1.37µm, and C: 1.65µm.)

The applicable range for λ-2 fitting is dependent on the core dimensions of the fibres. As expected,

this range is located at longer wavelengths for the fibres with larger core dimensions. Notice that

fitting for the fibre A is relatively poor at the long wavelength range (λ-4<0.2) compared with the

others, possibly because the assumption of the quadratic dependence of (1-Γ) on the wavelengths

becomes inadequate (i.e Γ<<1).

The fitting parameters for a selection of fibres are summarised in Table.3.2.1, where the following

notation has been used for convenience.

� � � � �� (3.5)

Note that the intercept values α0’ are interpreted to be the background losses, and represents the

loss components that are insensitive to wavelength[139]. However, for the fibres discussed here,

this interpretation is inadequate since α0’ can be greater than the actual losses at long wavelengths

due to the square root dependence. Therefore, α0’’ sh ould be considered as the background losses.

For the fibre with smaller core dimensions (i.e. <1.22µm), the square root fitting was only

applicable because of the large losses at the short wavelengths, where the λ-4 dependence ought to

�� "! ��#$&% $ $&% ' $&% ( $&% ) $&% * +�% $ +�% '

, -../0 123 45

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A

B C

A B

C


be observed. In addition, poor qualit y of the fitting at long wavelengths was more apparent for

these fibres similarly to that of the fibre A, resulting in greater loss values than the data.

Table. 3.2.1 Fitting parameters for the loss spectra.

Core diamater [µm]

α0'' [dB]

αR'' [dBµm2/km]

α0' [dB]

αR' [dBµm4/km]

1.06 6.50 336.45 NA NA 1.12 9.79 292.93 NA NA

1.22 (A) 24.89 130.70 147.49 19.22 1.37 (B) 11.96 100.81 103.86 16.25 1.65 (C) 11.79 69.87 71.09 9.63

2.00 11.56 14.56 30.73 6.26

It is clearly seen that αR’’ sharply increases with reduced core dimensions whereas the increase of

αR’ is not as strong as that of αR’’ . Note that α0’’ varies rather randomly for the fibres with small

core dimensions (i.e. below 1.22µm). This indicates the poor reliability of the fitting, particularly at

long wavelengths for these fibres. However, the weak structural dependence of α0’’ above a core

diameter of 1.37µm implies that the background losses are similar in these fibres. This is expected

since these fibres are drawn from the same batch of microstructured cane.

Given the sharp wavelength dependence of the losses at long wavelengths, fitting must be accurate

to provide quantitative information about the background losses. Fitting should ideally be

performed by using a theoretically derived Γ without separating the two regimes. Note that this

would also allow us to estimate ��

�� , used in eq.(3.4). Although the author tried to fit the loss

spectra with a single curve using theoretically estimated Γ using the localised function method[16]

it was found to be difficult for reasons described below.

Fig. 3.2.5 An example of slightly deformed core region of HNL-MOFs.


First, the core dimensions are often smaller than that of the idealised structure, as shown in

Fig.3.2.5, due to the structural expansion of the cladding and resultant deformation of air holes

around the core (observe their elliptical shapes compared with the others). This leads to a slightly

higher effective cladding index than the estimated value for the idealised structures, and results in

deeper modal penetration into the cladding at longer wavelength, where modal penetration into the

cladding becomes significant. Therefore, the actual wavelength dependence of Γ may differ from

the theoretically estimated values. In fact, the significant λ-2 dependence actually appears at shorter

wavelengths than expected for the given core dimensions. Second, as discussed in Section 3.2.2(C),

eq.(3.4) can be insufficient due to the assumption of the homogeneous distribution of the scattering

elements becomes more inadequate due to the longer strands that support the core.

An alternative approach to deduce the Rayleigh scattering coefficient of the holey cladding �� ,

is to fit the loss values at a fixed wavelength by assuming a constant value of �� for similar

fibres. Fig.3.2.6 shows the data at 1550nm for various fibres drawn from two different preforms.

The groups A and B correspond to the fibres pulled from different preforms (DHLP01 and

DHLP00, respectively, see Section 3.3), and d/Λ of these fibres are all similar (>0.9).

Fig. 3.2.6 Correlation between the losses and the core diameter.

Fitting was carried out by using the calculated confinement factor Γ for d/Λ=0.9. �� was

estimated to be ~3600dBµm4/km for group A and ~3050dBµm4/km for group B, respectively. For

the group B, the deviation between the fitting curve and the data points can primarily be explained

by the slight deformation of the structure around the core, as shown in Fig.3.2.5. For instance, the

fibres a and b had greater (~1.5times) Λ than their core diameters whereas the fibre c had Λ almost

equal to its core diameter. Nevertheless, the fitting results are still in reasonably good agreement

taking into account the measurement errors in losses, SEM characterisation, and the accuracy of the

model.

�� ! " �! # �! $ �! % �! & # " # # # $

')(**+*,- ./012

"#�"$�"%�"&!"��"!"��#!"��$!"��%!"

3�4�565�7683�464�5�7683�96565�768: ; < 3: ; < =

A

B

a b c


For group A, the best fit was obtained by introducing the background losses as an ‘offset,’ which

amounted to 28dB/km. Note that this offset was negligibly small for group B. This difference can

be attributed to the use of different preforms for the respective fibre groups. In fact, a wet etching

process was used in the preform fabrication of the preform for DHLP01, from which the fibres of

group A were drawn. The etching process using hydrofluoric acid leads to surface degradation[142],

and which resulted in the higher value of ��

�� . This indicates that air-silica interfaces are not

sufficiently fire-polished during the fibre drawing due to the low draw temperature requirement for

these fibres. Note that the amount of etching was very slow (1µm/10min.) and small (<10µm) in

order to minimise the surface degradation. In fact, the capillaries were still transparent after etching,

and which means that the roughness within the capillaries is dimensionally smaller than the visible

wavelength range. Although etching was introduced in order to eliminate the OH rich layer of the

surface of the capillaries, as discussed in Section 3.3, this result suggests that there is a trade-off

between the OH induced losses and the scattering losses as long as the wet etching process is used.

This observation also indicates that a good surface qualit y of the capillaries is critical for reducing

the scattering losses.

In order to study the impact of the choice of d/Λ on the losses, the loss spectrum of a fibre with a

low fraction of air (DHNL09: cane DHLP-02, Λ=2.0µm, and d/Λ~0.44, see Fig.3.2.7) is shown in

Fig.3.2.8. The losses are found to be the lowest around 1550nm at ~14dB/km.

Fig. 3.2.7 SEM photographs of DHNL09.

It was found that λ-2 fitting for the λ>1µm range fits this loss spectrum well (see Fig.3.2.8). αR’’

and α0’’ were estimated to be 20.3dBµm2/km and 6.01dB from λ-2 fitting, which can be compared

with the 2µm core fibre (~20dB/km at 1550nm) in Table.3.2.1. αR’’ is ~30% higher whereas α0’’ is

about half the value of the fibre with d/Λ~0.9. The former result indicates that despite the deeper

modal penetration into the cladding, the net increase in the scattering coefficient is small possibly

due to the reduced area of the air-silica interface in DHNL09. Furthermore, it was found that


extension of the square root fitting resulted in an underestimate of the losses at short wavelengths,

differing from the tendency observed for the fibres with large d/Λ (see Fig.3.2.4). This implies that

there is an additional loss mechanism other than the Rayleigh scattering at these wavelengths.

Fig. 3.2.8 Transmission spectrum of DHNL09.

Since the NA of this fibre is not as high as the fibres shown in Fig.3.2.5, radiation losses can be

more significant for this fibre type for the given longitudinal uniformities particularly at short

wavelengths (the coupling spectrum for this fibre type is much broader than that for the fibres with

high d/Λ (see Fig.3.2.1)). Thus, by improving the longitudinal uniformity, the losses at short

wavelengths may further be reduced.

The losses increase above 1.6µm, differing from the others shown in Fig.3.2.4. The loss values of

more than 15dB/km above 1600nm are much greater than the intrinsic IR absorption losses

(~0.5dB/km) in this wavelength range. The author believes that this is due to the onset of

confinement losses, as discussed in Section 3.1.2. Because of the reduced index contrast compared

with the other fibres studied here, the modal penetration of the guided modes into the cladding is

more significant, allowing for more power leakage to the jacket. In fact, the fibres with smaller d/Λ

(~0.35), or smaller Λ (~1.8µm) showed significant confinement losses around 1400nm, as shown in

Fig.3.1.1.

It was again impossible to fit the loss spectrum using the theoretically estimated Γ. From the

previous discussions (see Section 3.2.2(C)), fitting for the fibres with small d/Λ is anticipated to be

even more difficult, although the structure is more ideal in this case. A more detailed model for

Rayleigh scattering losses is required in order to estimate �� from a single loss spectrum of a

fibre.

�� !��#"$��%"�&��%"�'��%"$��

( )**+, -./01

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243 576�8�9:3 5�;<�= <><�= ?><�= @A<�= B><�= CED!= <FD!= ?ED!= @

G HIIJK LMNOP

Q$RQ�ST�RT�SU�RU�SV�R


3.2.4 Back-scattering measurement for HNL-MOFs

Although the cut-back method using a white light source provides informative spectral information,

it is hard to fully understand the origin of the loss mechanisms by considering it in isolation.

Optical time domain reflectometry (OTDR) provides complementary information to the cut-back

measurement, including the loss distribution along the length[167] and the amount of the back-

scattered signal[168].

Initially, the author deduced that there was substantial backward scattering in the HNL-MOFs

studied herein due to the roughness of the air-silica interface, as discussed in Section 3.2.2 (B).

However, the presence of such a loss mechanism cannot be readily revealed using a simple cut-

back technique, particularly when the other effects such as uniform scattering are superimposed.

Therefore, the aim of this measurement is to examine the losses in more detail by measuring the

back scattered power.

In OTDR, the power returning to the detector is given by[169]

�� , (3.6)

where P0 is the launched power and τ is the pulse width. α(z) is the local attenuation coefficient of

the fibre as described in eq.(3.1), and is obtained as one of the fitting parameters to the OTDR trace.

η(z) is the backscatter factor, which is given by

!#" !�"$ %&'( )*+) ,,- ./ 0 1 . (3.7)

Here vg (=c/n) is the group velocity and B(z) the backscatter capture fraction that is calculated as

the overlap of the far-field distributions of the guided mode and of the dipole. In this case, where

the cross sectional distribution of the scattering element is non-uniform, B(z) is dependent on the

field distribution as follows[164].

2�3 24352432435243 24366 6 77 789:;<=> ?5@? @? A@@BDC EGF�EEEGF�EEE EGF�EEEHIKJLMN O O

PPQPQ

, (3.8)


where V is the normalised frequency. αR and φ(r) are the Rayleigh scattering coefficient and the

field, respectively. By combining eqs.(3.4,6-8), the fraction of the backscattered power at the

launched end, which is also obtained as another fitting parameter to the OTDR trace, is given by

��

��

��

��

� ��

��

�!"#��# $%$% &$$$$% ')( *+,-.+/-

. (3.9)

Note that the first term in the bracket originates from Rayleigh scattering and is proportional to aeff-2,

whilst the second term that is due to the roughness has the aeff-4 dependence, by recalling eq.(3.3).

Thus, by observing the structural dependence of the backscattered power, it may be possible to

distinguish these two components.

Finally, the author notes that these expressions are only valid within the weakly guiding

approximation, which does not strictly apply to the fibres considered herein. However, the

vectorial expression is rather complex and requires intensive numerical integration to obtain the

coupling coefficient between the mode and the dipole. Therefore, we use eq.(3.9) as a first order

approximation.

A) Experimental set-up

The schematic of the measurement set-up is shown in Fig.3.2.9. The output from a DFB laser

operating at 1536nm (with a dν~30MHz) was modulated by an electro-optic modulator (EOM) to

obtain 10ns rectangular pulse trains with a variable repetition rate. Two stage EDFAs were used to

amplify the pulse train. Acousto-optic modulators (AOMs) were inserted after every stage of

amplification, in order to eliminate the ASE pedestal components. The first AOM cuts off the

trailing edge while the second AOM cleans the leading edge. Furthermore, a bulk tuneable filter

was inserted, in order to clean the pulse in the spectral domain before launching into a 3dB coupler.

The output port of the coupler was angle-cleaved to avoid any reflection from the end facet, and

then coupled into MOFs via free space launch optics. The end facet of the MOF was also angle-

cleaved. The coupling efficiency was <20%, because of the combined effects of the angled end

facet and small core dimensions. Although the MOFs under the test were all successfully angle-

cleaved, scattering from the launch end facet was still very high compared with conventional

counterparts, resulting in an increased dead zone in the OTDR trace. Nevertheless, when the effect

of the end facet reflection was examined at the output end, a suppression of more than 10dB was

observed. The reason for this can be explained by the combined action of the poor launch

efficiency and the multiple scattering of the resultant radiation within the holey cladding.


Fig. 3.2.9 Experimental setup for the OTDR measurement.

Due to the strong attenuation of the fibre, it was not necessary to angle-cleave the output end of the

fibre. In addition, a normal flat cleaved end was often useful for identifying the position of the end

in the backscattering trace. The backscattered signal was measured using a calibrated 125MHz

bandwidth detector (New focus, 1811-FC) via the other port of the 3dB coupler.

OTDR traces were taken by averaging over 512 cycles using an oscilloscope connected to the

detector. Then, the powers at both the output and the launch were measured using a power meter.

In particular, the measured launched power was used to calculate the backscattered power. The

transmitted pulse spectra were checked using a spectrum analyser. No significant spectral

broadening due to nonlinear effects was observed for any of the fibres.

A system check was performed by launching pulses with ~500mW peak powers into a 1.3km

single-mode-fibre, from which the attenuation of 0.64dB/km was measured and the back-scatter

capture fraction B of 1.22x10-3 is estimated using �� =4.5x10-5m-1[164]. Note that this fibre

length was too short to accurately estimate the losses of the fibre and the estimated value is slightly

higher than expected. On the other hand, the back-scattered factor is in good agreement with the

value predicted from the above formula (e.g. eq.(3.8)), where B=1.12x10-3 is obtained using

�� =4.5x10-5m-1, NA=0.1, and n=1.45.

B) Application to HNL-MOFs

The author examined a range of HNL-MOFs (γ~50/W-1km-1), most of which exhibited normal

dispersion around 1550nm, an example of which is shown in Fig.2.5.2 (b) (DHNL08) and Fig.3.2.5

(DHNL07).

DFB-LD

EOM

EDFA

AOM

EDFA

Isolator

AOM

Filter

50/50

Detector

Fibre under test


Fig.3.2.10 shows examples of the measured OTDR traces obtained from HNL-MOFs. Even for the

angle cleaved front end facet, a strong back reflection can clearly be seen in the trace of DHNL09

(left). For the normal end facet at the output of this fibre, an ASE pedestal and a resultant dead

zone can be seen. For the fibres with high NA (i.e. with large d/Λ), the strong back reflection at the

front end facet typically results in a dead zone more than 20m, which was eliminated before fitting

exponential curves to the data (DHNL8-2). For this reason, combined with the high losses caused

by the small core dimensions, the effective length that could be used for fitting to the exponential

curve was very short (60m~120m). It can also be seen that strong coherence noise was present

since the coherence length of the source is longer than the resolution of the detector. Despite the

noise and the short length of the fibres, it was still possible to fit an exponential curve to the data

although the detailed loss distribution along the fibre length could not be evaluated.

Fig. 3.2.10 Examples of the measured OTDR traces for two different fibres (DHNL09:left and DHNL08-2:right).

Fig.3.2.11 shows the attenuation coefficients obtained from the OTDR traces by fitting and the

losses obtained by the cut-back method as a function of core diameter. Because of the presence of

the noise, the fitting parameters varied depending on the start points of the fitting. Changing the

starting data points for the fitting from 20~40m, the average, the maximum, and the minimum of

the fitting parameters were obtained. These values reflect the vertical error bars of the circles,

whereas the power fluctuation during the measurement corresponds to the vertical error bars of the

diamonds. The horizontal error bars are due to the variation of the core diameters observed in their

SEM photographs, and which were taken at both ends of the fibres.

Because of the short lengths used for the fibres with smaller cores, due to their relatively higher

losses, the measurement for these fibres was more sensitive to the noise. Consequently, the vertical

error bars are greater in fibres with a smaller core. However, the general trend, where the losses

increase as the core dimensions decrease, is consistent with the data studied in the previous section.

Fitting resulted in �� ~5700dBµm4/km with an offset of 17.7dB/km, respectively, using the

�� !�#"$��%"��&�%"��&�%"$��

' () *(+, ) -. /01324(, ) +5

�&6 �

�&6 �

�&6 �

�&6 �

�&6 �

798�:<;&=�>@? A�BC�D E�D F�D G!D H$D�D

I JK LJMN K OP QRSTUJN K MV

W D&X D�YD&X D�DD&X D�YD&X H$DD&X H�YD&X C�DD&X C�YD&X Z�D

Normal end facet

Dead zone

Angled end facet


calculated Γ. The value of � � �� is slightly higher than that obtained from Fig.3.2.5, which implies

extra losses caused by the rewinding process, as described later.

�� ! � � " #�� #�� # #�� $ #�� % #�� &

' ())*+ ,- ./

0 1 00 1 20 1 30 1 40 1 5 6 7 8:9�; 7=<:>:?:@>=A�; B C <D>=EF�G=H I J�K�L�L:LF�G=H I J�M�L�L:LF�G=H I J�N�L�L:L

Fig. 3.2.11 Comparison of the attenuation coefficients obtained from OTDR trace with the ones obtained from the cut-back measurement.

The backscattering due to the modal coupling αb is proportional to aeff

-4 and thus shows a sharper

dependence on the core dimensions than that of the Rayleigh scattering. When αb (i.e. eq.(3.3)) is

included in (3.9), the quality of the fitting was rather poor. Therefore, αb is unlikely to play a

substantial role.

Fig.3.2.12 shows the ratio of the back-scattered power to the launched power at the input end

(=Pb(0)/P0). The error bars are again due to the variation of the fitting parameters and the structural

dimensions. It can be seen that the ratio also increases with decreasing core diameter. The lines

correspond to the calculated values for Pb(0)/P0 for different ODP Q�RST from eq.(3.9) by assuming αb=0.

UV�W�XZY\[=] ^�X�_�X W�`:a�^�bc�d e c�d f g�d c g�d=g g�d h g�d i g�d j

kmln opq ksr

tDuwvt=x�ut=x vtDy utDywv z�{D| } ~ � ��

��=� � � � �� =� � � � �� w�D��

Fig. 3.2.12 Measured back-scattered signal and predicted trends for different αRcl.


It was found that most of the data points fall between 1000~2000dBµm4/km. A comparison to the

value obtained from Fig.3.2.11 ( ��

�� ~5700dBµm4/km) indicates that the majority of the losses are

not reflective components: the backscattered power is weaker than anticipated from the attenuation

data. Therefore, the reason for the large Rayleigh scattering coefficient estimated from Fig.3.2.11 is

possibly due to the substantial forward scattering losses despite the high NA designs of the fibres.

Also, the reasonable quality of fitting in Fig.3.2.11 suggests that the forward scattering losses also

exhibit a strong dependence on the structural dimensions similar to the Rayleigh scattering losses.

It is difficult to predict the loss spectra of the forward scattering losses αf without any knowledge of

perturbation spectra. However, since the modes confined within small cores can result in deeper

modal penetration into the cladding in this regime, they possess greater modal overlap with

radiation modes, leading to stronger modal coupling to radiation modes. This suggests that the

strength of the modal coupling (and thus the forward scattering losses αf) is also strongly dependent

on the core dimensions and increases as the core dimensions are reduced. Therefore, the structural

dependence of the forward scattering losses can be similar to that of the Rayleigh scattering losses.

As discussed, the fibre with high d/Λ>0.9 has a cut-off about Λ~1.1µm and the fibres examined

here are therefore mostly single mode fibres. From Fig.3.2.1, the perturbation length scale is below

10µm since λ/Λ~1.5. Given this length scale, it is hard to consider any reasons for the perturbation

since fibres should be smooth over this length scale. One possible cause of the forward scattering

losses can be micro-fractures of the holey cladding. Even when their length scales are greater than

the beat length zc, a perturbation strength given by these micro-fractures can be so significant that

the forward scattering can still be induced by them. In fact, the author observed a loss increase after

re-winding. The fibres used for this experiment were all rewound whilst the fibres used in the

previous section (i.e. Fig.3.2.5) were immediately characterised soon after drawing and without

rewinding.

It is necessary to further improve the measurement accuracy in order to confirm the above

discussions. For instance, by characterising the loss distribution along the length, it should possible

to identify the approximate positions of any imperfections/micro-fractures that lead to radiation

losses. The necessary system improvements are discussed below. First, the coherence noise must be

suppressed. Although we tried to use time-gated ASE sources, it was difficult to obtain the short

pulse duration relevant for the available short lengths of the fibres in order to achieve high

resolution. Possible solutions to this problem would be to use a multimode laser, or to frequency

modulate the DFB lasers with RF frequency to generate sidebands.


Second, the high effective nonlinearity of the fibre also limits the fibre length since the peak power

can be reduced due to self phase modulation. To cope with this issue, use of a 95/5 coupler rather

than a 3dB coupler could help since they allow us to launch less pump power without reducing the

OTDR signal. Finally, the fibre alignment was so critical due to the extremely small core

dimensions that the averaging time was limited. To improve the quality of the OTDR traces, longer

time averaging (~104 frames in typical OTDR systems[169]) are necessary. Thus, a robust

motorised stage or fusion splicing will be required to improve these measurements further.

3.2.5 Conclusions

The scattering losses within HNL-MOFs were studied. The various possible scattering loss

mechanisms within HNL-MOFs were discussed. Cut-back measurements using a white light source

showed a significant increase in loss with reduced structural dimensions. By using a simple model,

it has been shown that the wavelength dependence of the Rayleigh scattering is modified due to the

significant change in modal confinement at long wavelengths and that λ-2 fitting can be used for a

limited range of spectrum whilst the conventional λ-4 fitting can be used at short wavelength.

Furthermore, it has been found that the Rayleigh scattering is sensitively affected by the wet

etching process used for some fibres.

Back scattering measurements were also performed for HNL-MOFs. It was found that the amount

of the backscattering is smaller than expected from the total losses of the fibres. This implies that

there are substantial radiation losses. At the same time, the losses due to backward scattering were

found to be relatively small, compared to the Rayleigh scattering.

Due to the issues related to the stability of the measurement combined with the large coherence

noise present in the OTDR traces, the measurement was limited in terms of its accuracy. Further

study is required to fully understand the loss mechanisms within HNL-MOFs. Other approaches

such as distributed Brillouin scattering measurements are ongoing using a commercial OTDR

system, in which the same measurement can also be performed. Along with more understanding of

the loss mechanisms, it is expected that important information for improving the fabrication

technology could be obtained.

3.3 Reducing the OH induced losses in MOFs

This section discusses the incorporation of water content that occurs throughout the fabrication

process. It is of paramount importance to reduce the OH induced losses in order to widen the usable

bandwidth around 1550nm, where the intrinsic losses of silica are the lowest[130]. The mechanism

of OH incorporation is first reviewed in Section 3.3.1. Then, possible improvements in the


fabrication process are discussed in Section 3.3.2. The experimental results from the process

modification are presented in Section 3.3.3. Finally, a summary is given.

3.3.1 Mechanism

Surface chemistry allows us to understand the mechanisms of OH incorporation at an air-silica

surface[170]. The evolution of the surface states at elevated temperatures is schematically shown in

Fig.3.3.1. At room temperature, the water content in the atmosphere can be physically adsorbed or

chemically bound at the surface. When the glass is heated to 150°C, physically adsorbed water

molecules are removed through evaporation. The chemically bonded Si-OH groups start to

dehydrate at about 400°C, as a result of pairing of neighbouring hydroxyl molecules. However,

there can still remain some isolated OH ions, and which start diffusing into the glass as the

temperature is increased.

Fig. 3.3.1 Surface characteristics of silica at different temperatures.

Therefore, simple heating alone does not allow for complete removal of hydroxyl ions from the

surface, and may indeed increase the OH content in the glass. Indeed, it has been reported that

consolidation of VAD preforms in dry atmosphere (without dehydration) results in 5~30ppm of

OH content since the soot contains some hydroxyl ions as a result of flame hydrolysis[132].

Similarly, the observed OH content for the author’s MOFs without dehydration was ~30ppm

despite the use of high quality synthetic silica tubes (i.e. F300/F320 see Table.6.2.1) that contain

only ~0.2ppm of OH content by default.

Next, the length scale over which OH ions can diffuse into the glass is considered. The diffusion

length of the OH species is defined as �� , where D is the diffusion coefficient and T is

the time period over which heat is applied to the glass[140]. Thus, in the drawing process, the

diffusion length depends on the feed rate of the tube and the drawing temperature. The feed speed

range typically used for MOF fabrication is 2~10mm/min. and the hot zone length of the furnace is

~30mm. Therefore, T lies within the range of 180~900sec. Using D~10-7cm2/sec at 2000°C[140],

Si

OH

Si Si

O

H H

O H H

O

Si

OH

Si Si

O

Isolated OH ion

Si

OH

Si Si

O

H H

O H H

O

At room temperature 150� C >400� C


Ld is 42~94µm, the lower limit of which is comparable to the values reported for conventional

fibres[130].

In MOFs, the nearest air-silica interface from the core is located at a distance of ~Λ/2. This length

scale is much shorter than the diffusion length. This implies that the distribution of the OH ions

across the fibre core becomes almost uniform after drawing a preform. Therefore, all surfaces

within MOF preforms must be free from hydroxyl ions. Furthermore, holey claddings typically

span only ~20µm in HNL-MOFs where the two-step drawing approach is used (see Chapter 2).

Since the distance between the outer surface of the microstructured cane and core is of the order of

10µm, care must also be taken to eliminate the OH ions at the interface between the jacket tube and

the microstructured cane particularly when lower feed speeds and/or small preform dimensions are

used. Note that this value could be an underestimate, since the diffusion process can actually

commence far from the hot zone of the furnace.

Understanding the above mechanism suggests two approaches to eliminate hydroxyl ions: either to

dehydrate the glass prior to heating, or to eliminate an appropriate thickness of the glass surface

after heating. Note that the glasses are heated up to high temperatures (>1500°C) several times

throughout the entire MOF fabrication process. The examples of the heat treatment include pre-

baking, capillary drawing, sealing, caning, and fibre drawing. Thus, removal of the surface layer, in

which high proportion of OH ions are present, apparently complicates the fabrication process. It is

also apparent that the two-step approach (see Chapter 2) is more prone to suffer from hydroxyl

contaminations because of the small scale structures and the additional drawing process involved.

Below, possible improvements and precautions related to the fabrication process are described

following the categories used in Chapter 2 (capillary drawing, caning, and fibre drawing). The

author notes that it is difficult to examine in isolation the effects of the capillary preparation and

caning as their contamination cannot be directly assessed. After considering the process,

experimental observations for a range of fibres prepared by different procedures are described.

3.3.2 Process considerations

A) Capillary preparation

There are three principal possibilities for incorporating hydroxyl ions.

1) From the burner to the outer wall of a tube during the pre-baking process

2) From the top and bottom ends of the tube during the drawing

3) From the outer wall of the surface that is exposed to the furnace element.


The importance of the pre-baking process was discussed in Chapter 2. An oxygen-hydrogen burner

generates a high water content as a consequence of combustion. Due to the high temperature

(~1700°C, where D~10-8cm2/sec) used in the pre-baking process, diffusion also occurs, and the

resulting diffusion length is ~50µm, assuming the heating time to be 30 minutes. In addition, the

OH ions can be diffused further into the glass at the drawing process. As a consequence, more than

~10µm thickness of the outer wall will contain high water content for ~1mm capillaries after the

tube is drawn.

The removal of the surface layer of silica tubes can be performed either before or after the capillary

drawing by using hydrofluoric acid. There is a trade-off between the etching depth and the surface

quality in the etching process and it was described in Section 3.2 that over-etching of capillaries

results in increased scattering losses. Thus, the amount of etching that can be used without loss

penalties is actually limited for silica capillaries. On the other hand, the surface quality can be

somewhat recovered through the capillary drawing process by applying high temperatures and low

feed speeds although the collapse involved such parameters needs to be taken into account. Thus,

etching a glass tube before capillary drawing (but after the pre-baking process) seems a better

option than the capillary etching. However, due to the large amount of glass required for capillary

drawing, a large scale etching environment must be prepared, and which had not been available for

most of author’s PhD period.

There are a couple of possibilities for minimising the incorporation of water before drawing into

capillaries. Two examples are described below. First, the burner used in the pre-baking process can

be substituted by either a plasma torch[140] or a furnace. By incorporating these devices, the OH

incorporation can be reduced. The second possibilit y is to use mechanical polishing, instead of pre-

baking[142]. This allows us to eliminate a heat source for improving the surface qualit y of the glass

tube. Thus, the original low OH content of the glass tube is essentially preserved until it i s drawn

into capillaries.

In reality, it is difficult to dehydrate the outside of the tube since it cannot be isolated from the

atmosphere. Therefore, post-etching may be necessary for the outer surface[189]. For the inner

surface of the tube, dehydration can be performed although the pulling end of the capillary has to

be open unless sealing is performed while drawing capillaries. Primarily for this reason, the

dehydration of the tube has not yet been applied. To date, preform tubes have been purged with

argon during drawing followed by applying a vacuum as an alternative approach to minimise the

OH incorporation.


B) Caning

The drawn capillaries are exposed to the atmosphere and are typically sealed using a torch.

Therefore, the assembled preform must be dehydrated before caning. The dehydration process of

the assembled preform is described below.

There have been several different reagents that have been used for dehydrating silica

materials[130]. These include brominating, fluorinating, and chlorinating reagents. The chemical

reaction strength is proportional to the electronegativity (i.e. Br < Cl < F). However, the chemical

reaction of fluorine is so strong that it is accompanied by reactions with silicon atoms on the

surface, resulting in etching. Furthermore, fluorine is made available by heating up SF6 gas (SF6-

>SF4+F2), and this increases the local pressure due to the molecular decomposition, making it

difficult to apply to microstructured materials. Chlorine (Cl2) and thionyl chloride (SOCl2) have

been frequently used for conventional fibre fabrication. Electronics grade gas is available for Cl2,

while an ordinary bubbling purification can be done for SOCl2. The author used Cl2 to dehydrate

the preform due to the limited availability of SOCl2. The chemical reaction of chlorine is described

as follows.

2Si-OH + Cl2 � 2Si=O + 2HCl

Hence isolated hydroxyl ions can be halogenated, thereby generating hydrochloric acid. The

chemical equilibrium can be shifted toward the right hand side by continuously flowing fresh

chlorine gas while applying heat. By this means, the hydroxyl concentration can exponentially be

reduced over time. The time scale required for the dehydration is dependent on the temperature and

gas flow rates, which are discussed later. Furthermore, it is important to remove all the

hydrochloric acid components after the dehydration. This step is referred to as dechlorination

process.

In order to perform the dehydration process, the MCVD method was adapted by mounting a MOF

preform (either cane or fibre preform) on a glass lathe and by connecting it to the gas delivery of

the MCVD system (see Fig.3.3.2). The gas flow consists of oxygen, chlorine, and helium with the

ratio of 10:2:1. Oxygen is a carrier gas and helps remove any organic impurities through

combustion. Helium helps to improve thermal conductivity of the gas, since the central part of a

MOF preform is thermally well-isolated. The total gas flow rate was always fixed to be 260cc/min.,

and this was practically limited by the head-stock pressure built up within the preform. This is due

to the small amount of the interstitials within the preform, through which the gas can flow to the

extract. With this flow rate, the head-stock pressure was always below 10mbar even when the heat

was applied to the preform. Note that the slow flow rate is also important to reduce the thermal

gradient within the preform.


The ratio of the constituent gas mixture was set to be the same as the values used for MCVD

preforms[171]. However, the temperature range used in the MCVD method is completely different

(~2200°C), since the dehydration process is usually applied at the final collapse stage in the MCVD

method. Although the relative amount of the chlorine may be increased to accelerate the

dehydration process, this approach was prevented due to possible risks of leakage.

Heat was applied by a travelling burner. The temperature was ~1000°C, which was the lower limit

of the pyrometer detection, and the burner was slowly scanned (25mm/min.) along the preform for

~1hour, depending on the structure. At the initial stage of dehydration, the evaporation of the water

content from the surface was clearly observable as the preform became more transparent. The time

duration of the process was set by considering the time scale used for both the VAD soot preforms

(~several hours) and the MCVD (sintered) preforms (~10minutes), and can be optimised in the

future. Note that the surface area inside the MOF preform is much greater than the glass tube (used

in MCVD) whereas it is much less than the VAD soot preforms.

Dechlorination was performed by flowing dry oxygen (~500cc/min.) for more than 24hours.

Completion of the process was checked using the chemical sensor (detection limit ~0.2ppm) near

the extract. Although this process is necessary, it involves some OH incorporation since the extract

side of the end of the preform is left open. This could be prevented by facilitating a vacuum

channel near the headstock (see Fig.3.3.2 (B)) so that the dehydration and the dechlorination

process could be held in an enclosed environment. Use of a furnace type heat source is also

advantageous since the uniformity of the process along the length can be improved without any

diffusion of OH ions from outside the preform.

Fig. 3.3.2 Schematic of dehydration process for MOF preforms. (A) a method used for the present work, and (B) a proposed method for future improvement.

Vacuum

Cl2 +O2+ He

Furnace

Cl2 +O2+ He Extract

~1000�

C

Rotary seal

Burner

(A)

(B)


The preform was then pulled on the tower by applying an appropriate internal pressure after

evacuating the preform as shown in Fig.2.5.3. Caning again involves similar limitations to those of

the capillary drawing: hydroxyl ions can be incorporated from the outside of the preform and the

draw end. The former can be compensated by post-etching. For the latter, a technique that allows

for in situ sealing (sealing the draw end of the cane while it is being pulled) may need to be

developed, as discussed in the previous section.

C) Fibre drawing

MOF preforms contain two components: canes and jacket tubes. Therefore, the canes and the inner

walls of the jacket tubes need to be dehydrated.

Because of the caning process, the outside of the cane contains a thin OH-rich layer of the order of

>10µm, similarly to that of the capillaries as described at the beginning of this section. Since the

holey cladding within the small scale MOFs typically spans ~20µm or less within a final fibre, it is

smaller than the possible diffusion length. Therefore, hydroxyl ions within the thin OH-rich layer

can diffuse into the core during the fibre drawing. Therefore, removal of this layer using

hydrofluoric acid is important.

Since the pull end of the cane is open to the atmosphere, the inside of the cane can also become wet

again after the caning process. Therefore, dehydration should ideally be performed before pulling

fibres for the inside of the cane. However, it turned out to be difficult to dehydrate inside the cane

since its inlet dimensions are typically too small (~100µm) to apply sufficient pressure. Although

the cane ends can be left open for a long time under the chlorine atmosphere, it is difficult to

monitor the progression of the dehydration and/or dechlorination processes because of the small

dimensions. When this approach was examined, the author observed the onset of hydrogen

absorption peaks and associated huge increases in background losses in the drawn fibre. This

probably resulted from insufficient dechlorination.

Therefore, it has so far only been possible to perform the dehydration for the outside of the cane

and the inside of the jacket tube. The dehydration process is the same as that explained in the cane

preform. The cane is inserted from the downstream of the pre-baked jacket tube and is then dried or

dehydrated.


3.3.3 Experimental observations

Based upon above discussions, a series of experiments was carried out using dehydrated canes

(DHLP00/01 O.D.~1.8mm) and by applying different fibre drawing procedures. These canes

consisted of a F300 rod and F320 capillaries (dc/Λc~0.8) with a F300 jacket tube. The difference

between these glasses (i.e. F300 and F320) is described in Section 6.2. For DHLP01, the outer

surfaces of the capillaries were etched (~10µm) using diluted hydrofluoric acid (~5%) prior to the

stacking. The numbers of rings of air holes were 6 and 8 for DHLP00 and DHLP01, respectively.

DHLP00: 6 rings, F300 core, F320 capillaries, F300 jacket (dc/Λc=0.8, Λc~80µm)

DHLP01: 8 rings, F300 core, F320 capillaries (etched <10µm), F300 jacket (dc/Λc=0.8, Λc~70µm)

The fibres were pulled at ~200°C below the drop temperature after jacketing these canes with

6x3mm F300 tubes, resulting in structures with Λ~1.5µm and d/Λ>0.9. The ends of the canes were

sealed. In order to minimise additional OH incorporation through the sealing process, precautions

were taken such that no water vapour penetrates into the inside of the cane, except for one fibre

preform (DHNL01, see Table.3.3.1). Note that the ends of the canes used for these fibres were left

open after caning (before sealing). Al l fibres were pulled within one month after caning.

Although the resultant structural dimensions were not precisely identical, these fibres are similar

enough for comparison purposes. Owing to the high fraction of air within these fibres and the large

number of air holes, no traces of confinement losses were observed.

Table. 3.3.1 Attenuation characteristics of the fibres pulled by different procedures.

Core

diameter Losses @ 1550nm

Losses @ 1250nm

Estimated OH content

Cane conditions

[µm] [dB/km] [dB/km] [ppm]

DHNL01 1.67 43.2 164.5 28 DHLP00 as drawn

DHNL03 1.56 38.9 103.4 14.6 DHLP00 vacuum

DHNL04 1.65 40.3 74.9 6.3 DHLP00 dehydration+vacuum

DHNL05 1.53 62.6 112.6 6.9 DHLP01 vacuum

The attenuation was characterised by the cut-back method using a white light source over more

than 100m of the fibres. The losses at 1550nm and 1250nm for these fibres are summarised in

Table.3.3.1. The losses at 1550nm reflect the background losses since the effect of OH induced

losses are relatively small whereas the losses at 1250nm are direct indications of the OH content.

Note that the attenuation at 1550nm due to hydroxyl ions is approximately a three order of

magnitude smaller than that at 1380nm and ~40 times smaller than that at 1250nm[172]. Therefore,


the contribution of the OH induced losses to the background losses are estimated to be <5dB/km

for the fibres examined here. To estimate the OH content from these values, the conventional

conversion factor (~2.5dB/km/ppm[172]) was used after subtracting the background losses,

although this value should in practice depend to some extent upon the modal overlap with the fibre

material. The peak at 1380nm cannot be accurately characterised due to the existence of the

cladding modes within the short length of the fibre, over which the losses at 1380nm can be

measured due to the strong attenuation at this wavelength. Note that none of these fibres exhibited

the loss minima at 1550nm but at far longer wavelengths due to scattering losses, as shown in

Fig.3.2.4.

It is easily understood that the application of vacuum is very effective for reducing the OH content

by comparing DHNL01 with 03. Combined with the preform dehydration, the OH content is

further reduced by a factor of four (DHNL04) compared with the ‘as drawn’ fibre. The background

losses are almost the same for all the fibres except DHNL05. A comparison between DHNL03 and

05 provides an estimate of the impact of the capillary etching since their fibre drawing procedures

were the same. Although the OH content is less in DHNL05, the background losses are

substantially higher due to the increased surface roughness resulting from capillary etching. This

implies a trade-off between the OH content and the surface quality that increases the background

losses for this range of structural dimensions. The reduced OH content in DHNL05 also indicates

that a fair amount of the OH content comes from the capillary drawing stage, although further

improvements should be expected at the other stages such as caning and fibre drawing.

Further reduction of the OH content was observed for the other structures with different structural

dimensions as shown in Fig.3.2.8. (DHNL09: seal + vacuum without dehydration, d/Λ~0.4, OH

content~2.4ppm). In this case, the dehydrated preform (DHLP02: all F300) was pulled into a cane,

and was immediately sealed after drawing. Although a direct comparison for the hydroxyl content

cannot be made due to the different structural dimensions, the value is certainly lower than the

other fibres described above. This may be because DHLP02 contains smaller (~50µm) air holes,

into which the penetration of the atmosphere is slower and the absolute surface area that hydroxyl

ions can attach to is also less. Recalling that preform dehydration leads to reduction of hydroxyl

ions by more than a factor of two (compare DHNL03 and 04), it is anticipated that an OH level as

low as 1ppm can be anticipated by dehydrating this preform. Note that one of the lowest OH

contents ever reported within MOFs so far is ~1ppm[123].

3.3.4 Summary

The OH induced losses within MOF with a small structural scale have been described. By

considering the process, dehydration techniques for the MOF preforms have been developed. It has


been shown that, combined with the dehydration technique, by minimising the physical exposure of

the glasses to the atmosphere, it is possible to reduce the OH content to the level of a few ppm.

3.4 Conclusions

The continuous effort to reduce the loss levels of HNL-MOFs has been described. The two major

causes of the loss mechanisms within HNL-MOFs have been studied including scattering losses

and OH-induced losses.

Scattering losses: possible causes of scattering losses including the coupling to forward and

backward propagating modes were considered. It was found that owing to high index contrast

between the air and the silica, the beat length between the forward propagating modes can be as

short as several times the wavelength. The backward propagating modes can also be excited due to

the high frequency components of longitudinal variation at the air-silica interfaces that surround the

core. However, the backscattering measurement indicates that this loss mechanism is also

negligible.

Rayleigh scattering losses were studied by considering a simple model in which the scattering

element is homogeneously distributed around the core. By analysing this model, it was found that

the Rayleigh scattering losses exhibit λ-2 dependence at long wavelength (>1µm) and the model

qualitatively agreed with the experimental results. The effective cladding Rayleigh scattering

coefficient was estimated to be ~3000dBµm4/km by fitting theoretical curves to measured loss

values for various fibres with different dimensions pulled from the same preforms. This result

implies that the majority of the losses are due to the Rayleigh scattering. However, it was also

found that the model is not accurate enough to predict the loss spectra with single curves over an

entire spectral window. Further improvement is required.

The backscattering measurement showed that the actual attenuation coefficient is more than

expected from the backscattered power at the launched end. This implies that there are substantial

radiation losses possibly due to the longitudinal fluctuation of holey cladding structures. A further

study is necessary to understand this radiation loss mechanism.

From a practical point of view, it was found that the surface quality of the capillaries is the most

crucial to achieve low losses.

OH induced losses: By considering the OH incorporation mechanism in the silica based glasses,

the improvement of each fabrication step has been discussed. It was found that dehydration of the

preforms drastically reduces OH incorporation. Although it was also found to be effective to etch


off the surface layer that contains OH ions, there is a compromise with the increased losses because

of the increased surface roughness induced by the etching process. By minimising the exposure to

the atmosphere combined with the dehydration process developed, an OH concentration as low as

1ppm can be anticipated in HNL-MOFs even using the two-step drawing approach. For further

reduction of the OH incorporation, exposure to the atmosphere during the capillary drawing and

caning must be prevented.

Chapter.4

Small core rare-earth doped

microstructured optical fibres

4.1 Introduction

This chapter concerns the fabrication, characterisation, and applications of rare-earth doped

microstructured fibres (MOFs) with small cores. There are several distinctly different opportunities

offered by small core MOFs, compared with the conventional step index fibres including: unique

dispersion properties[35,37,38] and high nonlinearity[33].

The unique dispersion properties of small core MOFs originate from the large values of waveguide

dispersion that arise from the small scale dimension and high index contrast between glass and

air[33]. This large waveguide dispersion can dominate the material dispersion of silica even at

short wavelengths into the visible regions of the spectrum where the material dispersion itself

becomes very large. This leads to an extended anomalous dispersion range below the typical

~1300nm ZDW of conventional silica based fibres. Such dispersion properties have been

experimentally confirmed using low coherence interferometry[34-36]. Recent intense studies on

white light supercontinuum generation mostly use this feature of MOF to place the ZDW around

800nm so as to facilitate the use of Ti:sapphire based ultrashort pulse lasers as pump

sources[33,65,67,70,78].

The strong waveguide dispersion can also result in a large amount of normal dispersion at longer

wavelengths when appropriate structural parameters are chosen for MOFs. This is accompanied by

Chapter.4 Small core rare-earth doped microstructured optical fibres 71

the onset of the so-called second ZDW, in addition to the first ZDW described above. The second

ZDW has also recently been utilised in supercontinuum generation in a tapered fibre[81].

The waveguide dispersion of the conventional step index fibres displays the maximum normal

dispersion at 1550nm V~1.13[173]. Due to the small index contrasts achievable within

conventional fibre types, a low V value must be used in order to obtain normal dispersion at

1550nm (i.e. to compensate for the anomalous material dispersion at this wavelength). However,

such fibres cannot be operated practically due to the onset of appreciable bend loss sensitivity.

Therefore, inverse and reversed dispersion fibres, for instance, have required more complex

designs of their refractive index profiles to overcome this issue.

By contrast, owing to the high index contrast between the core and the cladding within MOFs,

large normal dispersion can still be obtained with reasonably high V values[38]. As a result, not

only the second ZDW, but also large normal dispersion are readily accessible at important

wavelengths in the near IR without any penalties in terms of bend sensitivity. Therefore, by

minimising the losses of MOFs, a required length of dispersion compensation fibres may

significantly be shortened for a given amount of dispersion to be compensated.

These unique dispersion properties of MOFs offer new possibilities for developing fibre devices

over an extended spectral range.

On the other hand, due to the tight modal confinement achieved by the presence of air, small core

MOFs naturally exhibit very high nonlinearity[33], particularly when a large fraction of air is

incorporated. It has been theoretically predicted that it is possible to obtain an effective nonlinear

coefficient γ=52 W-1km-1 at 1550nm in a pure silica MOF; which is ~50 times higher than that of a

conventional SMF[148].

By combining high nonlinearity and the unique dispersion properties of MOFs with rare-earth

doped fibres a number of exciting short pulse device opportunities can be anticipated. In particular

it should be possible to utilise soliton techniques developed for pulse generation, amplification,

compression and switching [174-179] of short pulses at 1550nm through the development of

erbium doped fibres, and to extend these to wavelengths in the near IR and visible regions of the

spectrum. The possibility of developing soliton based devices operating at 1060nm based on either

ytterbium or neodymium doped fibres appears particularly attractive since these fibres are

extremely efficient and can be used for truly high power operation.

To date, fibre based laser work at 1 µm has concentrated on the stretched pulse mode-locking

technique[180-183] and has required the use of intracavity bulk dispersion compensating elements


for dispersion management purposes. The need for these bulk components clearly compromises

cavity compactness and robustness. Note that although a neodymium doped fibre based soliton

laser has been reported, by incorporating a chirped fibre grating that exhibit anomalous

dispersion[184], the achievable bandwidth may be limited. Anomalously dispersive ytterbium

doped MOFs should provide a solution to these problems and represent a route to both all fibre

stretched pulse and soliton ytterbium fibre lasers. Although continuous laser operation using a

small core ytterbium doped MOF has recently been reported[118], no demonstration of

mode-locked operation of ytterbium doped MOF had been reported until the author conducted the

work described in Section 4.4 of this thesis.

Combining the use of soliton effects with amplification for frequency conversion purposes is

another very exciting possibility. For example, following the discovery of the soliton self frequency

shift (SSFS)[185], an all fibre tunable soliton source has been developed using conventional fibre

combined with a passively mode-locked erbium doped fibre laser as a seed[175]. To date, compact,

continuously tunable femtosecond soliton pulses are now commercially available covering the

wavelength range spanning from 1.55µm to 2.2µm, where conventional fibres exhibit anomalous

dispersion.

Using periodically poled lithium niobate (PPLN), this system is also used to seed high power fibre

based femtosecond amplifiers at 1.06µm with an average power of a watt level[186]. These fibre

based sources now serve as valuable laboratory tools for optical coherence tomography, and

multi-photon microscopy, pulsed laser deposition (laser CVD), and THz generation as an

alternatative to conventional solid-state femtosecond lasers[187]. By taking advantage of SSFS

within the broader anomalous dispersion regimes available within MOFs, the tuning range of

femtosecond soliton pulse sources can be further extended.

Using passive MOFs, several tunable femtosecond sources based on SSFS have been reported to

date, in the ranges from 0.8 to 1.1µm and from 1.3 to 1.6µm[55-57]. However, none of these

results employed fibre based seed sources. The author demonstrates in Section 4.5 that it is

potentially possible to implement an all fibre wavelength tunable femtosecond source operating at

wavelengths between 1.06µm and 1.33µm using an ytterbium doped femtosecond fibre

source[182] and a nonlinear fibre amplifier based upon an anomalously dispersive ytterbium doped

MOF. This wavelength range was previously inaccessible using fibre based systems.

Although it is interesting to apply MOF technology to short wavelength rare-earth doped fibres

such as the ytterbium system, high nonlinearity is still of great relevance for erbium doped fibres.

For instance, the thresholds and the lengths of the optical switching devices can further be reduced,

which is attractive for future telecommunications. Note that the highly nonlinear dispersion shifted


fibres (HNL-DSF)[147] use the VAD method for fabrication and given that these fibres already

contain a large amount of germanium, it would be very difficult to realise an efficient rare-earth

doped variant. On the other hand, efficient erbium doped fibres must contain very low OH content

since erbium has a spectral overlap with hydroxyl ions in the 2.7µm region. Possibly for this reason,

the first attempt to fabricate an erbium doped MOF did not result in lasing action[188]. By

applying the technology developed in Chapter.3, the author demonstrated the first laser based upon

an erbium doped MOF with a very high efficiency and a very low threshold due to the tight mode

confinement achievable within these fibres. These results are described in Section 4.6.

The organisation of this chapter is as follows. The fabrication of small core doped MOFs is first

described in 4.2 covering both ytterbium and erbium doped MOFs, and the optical properties of the

fabricated ytterbium doped MOF are then presented in 4.3. Then, by use of this fibre, two

applications are demonstrated, which include the first demonstration of a mode-locked MOF laser

(Section 4.4) and tunable soliton generation in a nonlinear amplifier configuration (Section 4.5).

Preliminary results on continuous wave operation of the erbium doped MOF are presented in 4.6.

Finally, the conclusions of this work are given in 4.7.

4.2 Fabrication of doped highly nonlinear microstructured fibres

The fibres were fabricated using the two step approach in order to realise small scale structures, as

explained in Chapter.2. However, since the ytterbium doped MOF was fabricated far earlier within

the author’s PhD period and the erbium doped MOF was developed more recently, their fabrication

methods are substantially different. The author therefore describes the detailed fabrication

processes for each fibre individually below.

4.2.1 The ytterbium doped MOF

In order to incorporate an ytterbium doped core into the microstructure, a central region (~2mm

diameter) of a solution doped MCVD preform (HD549: aluminosilicate host, NA~0.08, ytterbium

concentration: ~2000ppm by weight, core diameter~1.2mm) was extracted using an ultrasonic

drilling machine and was fire-polished by scanning a torch to reduce the surface roughness. The

resulting core rod was then inserted into the centre of an unsealed capillary bundle jacketed by a

Vycor® tube. The preform and draw parameters used for the cane fabrication are summarised in

Table.4.2.1. Caning was performed at as a low temperature as possible, in order to retain as high a

fraction of air within the structure as possible. The lowest temperature was limited by the fragilit y

of the cane, which arose from both its poor surface qualit y of the jacket tube(see Chapter 2) and the


reduced glass volume for the given structural dimensions. The resultant cane diameter was 2.4mm

and the diameter of the internal air holes was ~200µm.

Table. 4.2.1 Preform and draw parameters used for ytterbium doped microstructured cane.

Preform parameters: capillaries (F300) jacket (Vycor®)

I.D. [mm] O.D. [mm] I.D. [mm] O.D. [mm]

2 2.4 22 23.5

Draw parameters:

vf [mm/min.] vd [mm/min.] Temperature [°C]

9 0.8 ∆100

The fibre preform consisted of an unsealed capillary assembly within a jacket tube, and the sealed

cane was inserted into the central capillary, as shown in Fig.2.5.1. The reason that the fibre preform

was partly composed of capillaries was to attempt to thermally isolate the cane from the jacket tube,

as discussed in Section 2.5. The preform and draw parameters are given in Table.4.2.2. Over 300m

of fibre was drawn and a conventional high index polymer coating was applied.

Table. 4.2.2 Preform and draw parameters used for Yb-HNL-00.

Preform parameters:

capillaries (Vycor®) jacket (Vycor®) I.D. [mm] O.D. [mm] I.D. [mm] O.D. [mm]

1.8 2.4 12 15

Draw parameters and the final fibre dimensions:

vf [mm/min.] vd [mm/min.] Temperature [°C] d [µm] Λ [µm] Fibre O.D. [µm] 9 0.8 ∆100 ~2.3 ~2.7 ~125

An SEM photograph of the central structure of the fibre is shown in Fig.4.2.1. The resultant

structure had a 2.6x1.5µm core size with Λ=2.7µm and d=2.3µm, respectively, and thus d/Λ~0.85.

The resultant highly elliptical core (ellipticit y ~0.58) can be attributed to the following reason.

Since the core was inserted into a central capillary within the capillary bundle, two extra

interstitials with different initial dimensions to the others were formed within the cane. This

resulted in two diametrically opposed forces acting on the core and led to deformation due to the

lower viscosity of the core material relative to that of the capillaries. This ellipticit y of the core led

to high birefringence as discussed in Section 4.3.3. Assuming that the hexagonal core is made of

the initial rod which was inserted within the capillary bundle, then the doped section within the

core corresponds to ~70% of the hexagonal region within the final fibre and is calculated to be

~2.14µm2.


Fig. 4.2.1 SEM photograph of Yb-HNL-00.

4.2.2 The erbium doped MOF

The erbium doped MOF also used a solution doped aluminosilicate based MCVD preform (LF15:

NA~0.14, erbium concentration ~1000ppm, core diameter ~1mm). Note that the aluminium

concentration is expected to be just below the phase separation limit[189]. The core was

mechanically polished to form a hexagonal rod after drilling out the central part of the preform

(~2.5mm diameter). By this means, the fire-polishing step can be avoided, preventing the doped

section from being contaminated by the hydroxyl ions due to the use of an oxy-hydrogen

burner/torch. Furthermore, an excellent surface quality is obtained. The extracted core is shown in

Fig.4.2.2. By evenly lapping off from all directions, the core can be polished and shaped to be a

nearly perfect hexagon with critical dimensions of 1mm (flat to flat). Thus, the doped section

accounts for ~90% of the cross sectional area of the hexagon.

Fig. 4.2.2 Extracted doped core by polishing.

��

��

x

y


It is known that a slightly higher index layer of the order of 50nm is formed at the polished surface

due to the alumina or cerium oxide slurry used for polishing process[190]. Although the final

polish is performed using colloidal silica suspension (Syton), there is a possibilit y that this high

index layer may remain, which can cause extra losses due to chemical reactions and/or diffusion

that can occur at the elevated temperature during the fibre drawing. Therefore, the extracted core

was etched in a diluted hydrofluoric acid (~5%) solution to etch off a layer of ~50nm thickness

from the surface.

The extracted core was inserted into a sealed capillary bundle that comprised 8 rings of air holes.

The preform was dehydrated using chlorine for 1 hour at ~800°C, dechlorinated for more than 24

hours, and was then caned to obtain a ~1.9mm diameter cane by taking into account shrinkage of

the structure due to the collapse of the interstitials. The preform was purged with argon during the

drawing after applying a vacuum to evacuate the water content. The parameters are summarised in

Table.4.2.3. Note Suprasil F300 glass was used everywhere within the cane other than the core.

Table. 4.2.3 Parameters used for erbium doped microstructured cane.

Preform parameters:

Capillaries (F300) Jacket (F300) I.D. [mm] O.D. [mm] I.D. [mm] O.D. [mm]

0.4 1 19 25

Draw parameters:

vf [m/min.] vd [m/min.] Temperature [°C]

9.6 1.5 ∆100

After sealing both ends of the drawn cane, ~50µm thickness of its surface was again etched off by

using 10% hydrofluoric acid solution. The cane was then inserted into a pre-baked jacket tube to

form a fibre preform and was dried using dried oxygen (~400cc/min.) over ~12hours. The fibre was

drawn while applying a vacuum inside the preform (i.e. between the cane and jacket). The draw

parameters are given in Table.4.2.4.

Table. 4.2.4 Preform, draw, and resultant structural parameters used for Er-HNL-00.

Preform parameters: Cane Jacket

dc [µm] Λc [µm] O.D. [mm] I.D. [mm] O.D. [mm]

30 80 1.8 2 6

Draw parameters and final fibre dimensions:

vf [mm/min.] vd [m/min.] Temperature [°C] D [µm] Λ [µm] O.D. [µm] 4.3~7.8 10 ∆120∼140 1 2 135~170


SEM photographs of the fabricated fibre are shown in Fig.4.2.3. The doped area is calculated to be

~2.16µm2 for the 135µm fibre, corresponding to ~25% of the core area defined by the glass region

within the first ring of the air holes. Note that the fraction of the doped section is naturally reduced

when the cladding geometry contains a lesser fraction of air.

Fig. 4.2.3 SEM photographs of the fabricated erbium doped MOF.

4.3 Optical properties

This section studies the optical properties of the fabricated ytterbium doped fibre (Yb-HNL-00) in

terms of dispersion, effective mode area, and birefringence since they are relevant for the

subsequent sections.

4.3.1 Dispersion

Although it is difficult to measure the dispersion at the wavelengths of interest due to the

absorption of any probe signal, our vector model[18] predicts that the zero dispersion wavelength

of Yb-HNL-00 lies between 750nm and 850nm, depending on the polarisation axis, as shown in

Fig.4.3.1(left). This leads to a large anomalous dispersion 40~80ps2/nm/km at the ytterbium laser

wavelengths. The dispersion coefficient of the x-axis is in fact twice as large as that of y-axis at

1.03µm due to the large ellipticity of the core. From the trend, the anomalous dispersion range is

expected to extend up to at least λ>1.7µm, and this corresponds to a total span of over 1000nm. In

addition, the dispersion slope is clearly greater on the ‘x-axis’ (see Fig.4.2.1), which is reflected by

the smaller interstitials and can be explained in relation to the effective mode area as below.


��

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7A2 @

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Fig. 4.3.1 Calculated wavelength dependence of dispersion and effective mode area for the two orthogonal polarisation axes. (dashed: x-polarised mode, solid: y-polarised mode.) (Courtesy T.M.Monro)

4.3.2 Effective mode area

The effective mode area Aeff of Yb-HNL-00 is also calculated from the model and is nearly linearly

increasing with the wavelength, as shown in Fig.4.3.1 (right). The values at the laser wavelengths

are 2.3~2.4µm2 for both polarisation axes, with the x-polarised mode being slightly smaller.

Interestingly, in the range λ<1.25µm, the x-polarised mode has a smaller Aeff than the y-polarised

mode while the opposite occurs when λ>1.25µm, although the difference is smaller than 5%. This

can be attributed to the small interstitials that are diametrically located around the core.

In a perfect hexagonal structure, the modal degeneracy (the equivalence of the propagation

constants for the two orthogonal polarisation modes) has been numerically confirmed while the

field distribution of the two orthogonal modes has been found to be substantially different[191,192].

This suggests that the direction of the modal fields is strongly affected by the air-silica boundary,

particularly when the core dimensions are small. Indeed, it has been numerically found that the

wavelength dependence of the field distribution is more pronounced when small scale structures

are incorporated and when their air silica interfaces are orthogonal to the field direction[193].

In the present case, the x-polarised mode results in a smaller effective mode area than the

y-polarised mode at short wavelengths. This is because the x-polarised modal field can decay more

strongly into the small interstitials at short wavelengths. On the other hand, when the air hole

dimensions of these interstitials become large enough with respect to the wavelength, the

x-polarised modal field extends more deeply into the interstitials and eventually tunnel through

them. The continuity condition tells us that the electric (or magnetic) field components parallel to

the boundary are only continuous across the boundary. Since the interstitials are elliptical, the


x-polarised mode can more readily flip its field direction across the air silica boundary than the

y-polarised mode as long as the operating wavelength is short enough to resolve the structure. This

results in a smaller mode area for the x-polarised mode at the short wavelengths.

Notice that the wavelength, at which the effective mode area of the x-polarised mode becomes

greater than the y-polarised mode, roughly coincides with the turning point of the dispersion curve

(see the dashed curve in Fig.4.3.1.) and that the x-polarised mode exhibits stronger wavelength

dependent dispersion. This can be understood by considering the relatively stronger wavelength

dependence of the effective mode area (thus the modal confinement) of the x-polarised mode,

which reflects the strong wavelength dependence of the transverse wave vector and thus the

propagation constant. Therefore, there is a correlation between the dispersion curves and the

effective mode areas.

The predicted Aeff values are more than 10 times smaller than those of conventional single mode

fibres in this wavelength range. Therefore, the effective nonlinearity γ is expected to be ~10 times

greater. From the physical dimensions, the geometric core area is ~3.06µm2. Hence, it is

understood that the mode is tightly confined within the core and that a good modal overlap with the

doped section (~2.14µm2) is obtained.

4.3.3 Modal birefringence

Birefringence of fibres in general originates from the following two factors. One is form

birefringence that is induced by any asymmetry of the core structure. The other is stress-induced

birefringence, and this is widely used for conventional polarisation preserving fibre types. It is

generally known that the former dominates the latter when the index difference between the core

and cladding is more than 2% in conventional step index fibre[173]. The large index contrast

between air and silica therefore implies that the addition of any geometric asymmetry within small

core MOFs results in large values of birefringence in such fibres[173,63].

In ref.[194], high birefringence has been induced in the MOF by placing different sizes of air holes

around the core. The reported value for the beat length was 0.4mm at 1530nm. However, this

approach has a potential disadvantage since the guidance becomes weaker in the direction where

the smaller air holes are arranged. Such structures may thus suffer from greater confinement

losses[150] and may possess an orientation dependent bend loss. An alternative approach is to use

an elliptically shaped core (see Fig.4.2.1) which should be less prone to such issues but be able to

provide similarly high values of birefringence.


Fig. 4.3.2 Experimental setup for the modal birefringence measurement (above), and the measured beat

spectrum around 1545nm for 1.1m length of the fibre (bottom). (Courtesy P.Petropoulos)

To confirm this we measured the birefringence of Yb-HNL-00 (again see Fig.4.2.1) around 1.55µm

using an ASE source with a pair of polarizers, as shown in Fig.4.3.2. When both axes are equally

excited by incident polarised broadband light, the polarisation state evolves along the length of the

fibre as a result of the relative difference in propagation constants of the modes. By inserting a

polarizer at the output spectral beating is obtained across the transmitted spectrum. The period of

this spectral beat gives information on the fibre beat length as follows.

The beat length LB is defined by[41]

�� , (4.1)

where β and n are propagation constants and refractive indices for the different axes, respectively.

B is generally referred to as the modal birefringence. The measured beat spectrum is a result of the

wavelength dependent phase difference, which is accumulated by propagation along the fibre. The

phase difference φ at a fixed wavelength λ is given by

� �� . (4.2)

Differentiating with respective to λ and substituting ∆φ into 2π leads to

0.4nm

Pol.

Pol.

ASE source OSA


��

�� , (4.3)

If we assume that LB is proportional to λk. Then, the LB is given by

�� . (4.4)

Further, k=1 can be used if the wavelength range of the measurement is sufficiently narrow. By

putting the values, ∆λ=0.4nm, L=1.1m, and λ=1545nm; LB~0.3mm (B~5x10-3) is obtained, which

is shorter than the value reported in ref.[194,195] and is approximately 5 times shorter than that of

conventional high-birefringence fibres.

4.4 A mode locked ytterbium doped MOF laser

This section describes a mode locked laser based on the fabricated ytterbium doped MOF. First, the

experimental setup is presented, together with the principle of laser operation. The observed laser

characteristics are then presented and discussed. Finally, a brief summary is given.

4.4.1 Experimental setup

Fig.4.4.1 shows the experimental setup of the mode-locked laser which was based on a simple

Fabry-Perot cavity. The mode-locking mechanism employed relied upon frequency shifted

feedback into the cavity[196]. The cavity contained ~1m of Yb-HNL-00 and which corresponded

to approximately one absorption length at the pump wavelength of 966nm. The pump laser itself

was a single mode laser diode based MOPA capable of delivering up to 300mW. The pump

wavelength was detuned from the ytterbium absorption peak at 976nm so that the high nonlinearity,

anomalous dispersion and reasonably high CW efficiency could be achieved.

Coupling of the pump beam, and the signal beam into and out of the fibre was accomplished using

an appropriate choice of aspheric, achromatic lenses (f=4.5mm, NA=0.45) at both ends of the fibre

since the effective NA of the fibre was predicted to be ~0.4. Laser output was extracted from the

pump end of the cavity using a dichroic mirror (HT at 980nm and HR at 1030nm), that was angled

at 20% relative to the pump beam. Note that the required feedback from the pump end of the cavity

was provided by the ~4% Fresnel reflection from the cleaved fibre end.


Fig. 4.4.1 Experimental setup for the ytterbium mode-locked holey fibre laser (left) and the

operation principle (right).

An acoustic optic tunable filter (AOTF) with a 3nm bandwidth (FWHM) was inserted in the cavity

and acted as both a frequency shifter and a polarizer. The AOTF could be tuned over the entire gain

bandwidth of the ytterbium transition by tuning the frequency of the RF drive by ±1MHz, about the

operating frequency of 110MHz. Thus, the AOTF provided a ~220 MHz frequency downshift per

cavity round-trip. The transmission at the central wavelength was ~90%; by maximising the

transmission using a λ/2 plate placed between the AOTF and the fibre. Combined with the high

birefringence of the fibre, this allowed us to achieve single polarisation operation of the laser at low

output powers.

The frequency shift feed back mode-locking can qualitatively be explained as follows. Under CW

operation, spectral components are continuously shifted out of the filter pass band of the AOTF as

the light makes repeated passes of the cavity, resulting in a high loss. By contrast, under pulsed

operation, new frequency components are generated via self-phase modulation, which reduces the

effective round trip loss of the cavity due to the filter. Pulsed operation is thus favoured and a

stable pulse forms within the cavity for which the effects of the amplification, SPM,

frequency-shifting and filtering per round trip balance[197]. This form of mode-locking is similar

to the idea of sliding guiding filters that was developed for soliton transmission systems[198].

Combining the above approach with nonlinear polarisation evolution (NPE) to provide saturable

absorber action, pulse durations as short as 68fs have now been obtained �� pulse cavities based on conventional ytterbium doped fibre (still with bulk dispersion compensating

elements)[183]. In the anomalous regime, ~1ps pulses with a pulse energy >1nJ have been obtained

at 1.56µm using NPE and soliton effects in an erbium doped LMA fibre without the need for any

dispersion compensating components[199].

f

Gain spectrum Pulse spectrum

∆f ~220MHz

MOPA @966nm

DM

HR

Yb3+HF

AOTF

Output

L1

L2

RF

λ/2

4% Fresnel reflection


4.4.2 Laser characteristics

Fig.4.4.2 shows the laser output characteristics obtained at 1038nm. By assuming a coupling

efficiency of 50%, the CW laser threshold and the slope efficiency with respect to the absorbed

pump power are estimated to be ~7.7mW and ~63%, respectively. These values are relatively poor

compared with conventional fibres, implying a substantial amount of background loss. Although it

was difficult to accurately characterise the transmission losses of this fibre, the background loss at

1.5µm was roughly estimated to be ~1dB/m from the cut-back measurement using a white light

source. Nevertheless, the output could be scaled up to ~100mW. Given the fact that the pump beam

diameter was greater than the clear aperture of the focusing lens, and that the fibre outer cladding

comprised relatively thick support strands that can also guide light, it is likely that an element of

pumping to the system was provided by light propagating in the cladding. However, the relative

contribution of the different pumping mechanisms to the overall laser performance was difficult to

quantify in practice.

Interestingly, the fibre often spontaneously broke when the pump radiation exceeded a certain level

(>250mW). This indicates that this fibre type possesses limited power handling capability because

of the increased thermal isolation of the core, due to the large amount of air in the cladding and the

small glass volume associated with the core. However, ten times less average power is required to

obtain the same level of nonlinearity in this fibre relative to the same length of conventional fibre

and hence the fibre is thermally capable of handling the power over the operation range of interest.

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Fig. 4.4.2 The output characteristics of ytterbium doped MOF laser.

On examining the temporal characteristics of the laser output using a fast photodetector and a fast

oscilloscope, it was found that stable self-start mode-locking could be reliably obtained at average

output powers in excess of 17mW (corresponding launched pump power ~70mW) by appropriate

control of the half-waveplate. The mode-locking operation was immediately stopped when either

the pump power was reduced from this level, or the orientation of the half-waveplate was changed.


Single pulse operation was observed at the fundamental repetition rate (Fig.4.4.3 left) for launched

pump powers of up to ~100mW.

Fig. 4.4.3 Photograph of the mode-locked pulse trains (left) near the mode-locking

threshold and (right) at high output powers (5nsec/div.).

Above this pump power level multiple pulses were observed (Fig.4.4.3 right). By further increasing

the pump powers, the mode-locked operation became unstable indicating the onset of parasitic

continuous laser oscillation. It was also found that with a pump power of more than 120mW single

polarisation mode operation was no longer retained, and this may be explained by the scattering

from the main lasing axis to the orthogonal axis.

Fig. 4.4.4 Tuning curve obtained using a 1m length of the fibre and typical ML spectrum (inset).

The spectral bandwidth of the pulses was ~0.1nm as shown in the inset in Fig.4.4.4. This

corresponds to a pulse duration of ~15ps (assuming transform-limited Gaussian pulses). The laser

wavelength could be readily tuned simply by changing the RF frequency applied to the AOTF. The

mode-locked operation was obtained over a 20nm wavelength range from 1030nm to 1050nm.

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Laser oscillation at longer wavelengths (up to 1.1µm) could be obtained using longer lengths (~5m)

of the fibre due to the re-absorption of short wavelength ASE components. However, it proved

difficult to achieve mode-locking using such long fibre lengths, since the power that was fed back

through the AOTF became too weak for the given amount of the cavity losses.

4.4.3 Discussion

Unfortunately, further pulse shortening within our current cavity was not possible since the high

birefringence prohibited us from taking advantage of NPE as an effective saturable absorber.

Within our current cavity, the dispersion due to the reflection from a dielectric stack typically

amounts to ~1x10-27sec2, and 10mm transmission of AOTF (made of fused silica) which presents

an even lower amount of dispersion than the fibre. On the other hand, a 1m length of the fibre

provides -2~4x10-26sec2 depending on the polarisation axes. Thus, the anomalous fibre dispersion

dominates the overall intracavity dispersion in the current cavity. The nonlinear coefficient of the

fibre is estimated to be γ~50W-1km-1. Using the standard soliton equations[41] an average power of

~0.06mW should be sufficient to support fundamental solitons of 1ps at the fundamental repetition

rate.

However, the observed mode-locking threshold of ~17mW was far too high even though the

threshold was minimised by optimising the fibre length. Thus, pulses suffer from excessive

nonlinearity than that required for mode-locked operation. This prohibited the pulses from taking

advantage of the soliton pulse shaping effects, which depend on the balance between the intracavity

dispersion and the nonlinearity.

The cause of the high mode-locking threshold is most likely the stray intracavity reflections from

the fibre cleave at the AOTF end of the cavity, which effectively weakened the influence of the

frequency shifted feed back components, frustrating the initiation of the mode-locking at low

power levels. In order to prevent stray reflection, the fibre ends ideally need to be angle-cleaved.

However, it was difficult to angle cleave or polish Yb-HNL-00 due to its poor mechanical strength

for this particular structure. It is also difficult to suppress the stray reflection in high NA fibres, in

general. High NA means that the reflection from the angled end facet can be captured back to the

guided mode if the angle is too small.

Fig.4.4.5 shows the required angle as a function of the effective cladding index. The required angle

θ can be defined such that the normal incidence to the fibre end facet is reflected out of the capture

angle defined by the NA: � � ��

. For Yb-HNL-00, the required angle to


suppress the recapture is ~15° while the conventional fibre with NA~0.1 requires 3.5° using this

definition.

� � � ��

� ��

� !�"��#��#��" ��!��

Fig. 4.4.5 The required angle as a function of effective cladding index.

Therefore, future fibre designs for this application may need to minimise the NA whilst still

ensuring that anomalous dispersion is achieved at 1µm wavelength. Furthermore, because of the

birefringence of the fibre, the cleave angle has to be well defined with respect to the polarisation

axes. Such cleaving techniques also need to be developed for free space coupling.

It has recently been reported that unexpected excitation of the orthogonal polarisation mode can

impede the mode-locking operation[200]. Although this effect can be overcome in the stretched

pulse cavity solely by increasing the average power, it is a severe limitation for soliton lasers, in

which the optimum power level tends to be much lower. The observation that single polarisation

operation stopped at relatively low powers indicates that this effect might well have been a

contributing factor to the increased mode-locking threshold in our cavity.

However, further pulse shortening will certainly require the use of an effective saturable absorber

to initiate and stabilise the mode-locking process[201]. Popular examples that are compatible with

the polarisation preserving format include the NOLM[176], NALM[ 177] and semiconductor

saturable absorber mirrors (SESAM)[202]. Incorporating the former two elements is challenging

since a reliable interface between conventional fibres and MOFs will need to be established.

Nevertheless, successful mode-locked lasers that use un-doped MOFs have recently been reported

that use fusion splicing to perform the necessary fibre interconnections[200,203]. Hence, it should

also be possible to realise short pulse fibre lasers using doped MOFs. The recent doped MOFs are

made with a full silica jacket as shown in Section.4.6 and the interface to doped MOFs will

hopefully improve in due course.


4.4.4 Summary

The first mode-locked operation of small core ytterbium doped MOF laser was demonstrated. The

laser was tunable over 20nm around 1040nm and had a repetition rate of ~60MHz. The lack of

pulse shortening (nonlinear pulse shaping effects) was attributed to the high birefringence of the

fibre and the relatively high mode-locking threshold resulting primarily from stray reflections at the

fibre end facets. These issues may be overcome by optimising the fibre design and by developing

better end preparation techniques.

4.5 A nonlinear amplifier based on a ytterbium doped MOF

Soliton formation and wavelength tuning within an anomalously dispersive ytterbium doped MOF

are demonstrated. A broadly tunable ultrashort pulse source that has the potential for an all fibre

system implementation is presented.


Fig.4.5.1 shows the experimental setup. The amplifier consisted of 5~10m lengths of the

Yb-HNL-00 that was forward pumped using a laser diode MOPA (the same pump source used in

the laser experiments). Both ends of the fibre were hand cleaved normal to the fibre axis.

Fig. 4.5.1 Experimental setup of the nonlinear amplifier based on the ytterbium doped MOF.

The seed signal was generated by a stretched pulse mode-locked ytterbium doped fibre laser[182].

This laser operated at 54MHz repetition rate and emitted an average power of ~2mW,

corresponding to the pulse energy of ~45pJ. The seed pulse had a 13nm bandwidth and a central

MOPA (@966nm)

DM

Ytterbium doped-MOF

Output

SESAM

λ/4 λ/2

HR

FI Yb3+ fibre

Grating pair

pump

SMF

λ/2

λ/2


wavelength of 1055nm. The initial pulse duration and chirp were ~2.4psec and +0.18ps/nm and the

pulse could be compressed down to 110fs through the use of a grating pair. In this experiment, the

duration/chirp of the pulses launched into the MOF was controlled by inserting an appropriate

length of SMF prior to the MOF launch. Using a dichroic mirror (HT@980nm, HR@1060nm), the

pump and signal beam were combined and then coupled into the MOF. We examined the output

characteristics by modifying the lengths of the fibre, the initial chirp, and the pump power.

4.5.2 Operating principles

Under this experimental configuration, the launched pulses simultaneously experience linear

compression, amplification and nonlinear evolution, leading to Raman soliton formation and

subsequently SSFS. Although this complicated interplay between gain, dispersion, and nonlinearity

can be described by the extended form of the nonlinear Schrodinger equation[41], the author rather

concentrates on explaining the operating principles qualitatively below.

A schematic of the pulse evolution is described in Fig.4.5.2. The launched pulse first undergoes

simultaneous linear pulse compression and amplification. The peak power of the pulse is not high

enough to initiate significant nonlinear evolution and thus retains an original smooth spectral shape.

The ytterbium doped MOF compensates for the initial chirp of the pulse due to its anomalous

dispersion while simultaneously providing distributed gain. A rough estimate of this length scale

can be obtained as follows. From the spectral width of the pulse (~15nm), we have potentially

pulses of ~125fs when optimally compressed within the fibre. Thus, the dispersion length

�� is ~0.35m for D~80ps/nm/km. Furthermore, a fibre length of ~2.3m is required to

linearly compress the pulses for the initial pulse chirp of 0.18ps/nm.

As a result of the gain and the pulse compression, the peak power of the pulse rapidly increases

leading to the formation of a fundamental or even a high order soliton thereby further accelerating

the nonlinear pulse evolution. The fibre Aeff~2.5µm2 corresponds to a nonlinear coefficient

γ~50W-1m-1. The nonlinear length is then given by �� , where P0 is the peak power of the

pulse. In order to excite a fundamental soliton of 125fs duration (N=1, �� ) a peak

power of ~60W is required. This corresponds to a pulse energy of ~8pJ or an average power of

~0.5mW assuming the given repetition rate of 54MHz. Assuming a coupling efficiency of 20%, we

were able to launch ~0.5mW of the average laser power. Hence, the coupled pulse energy to the

MOF on its own without further amplification is nearly sufficient to form a fundamental soliton

through lossless pulse compression.


Fig. 4.5.2 Schematic of the pulse evolution with propagation along the ytterbium doped MOF amplifier.

(courtesy J.H.V.Price)

Once the pulse is sufficiently amplified/compressed the pulse starts to experience intrapulse Raman

scattering – a process that starts transferring the energy from the high frequency part of the pulse

spectrum to the low frequency part. This eventually leads to a break-up of the pulse into discrete

Raman soliton and dispersive waves (pedestals). The Raman soliton is a stable entity, which

continuously shifts its carrier wavelength due to SSFS as it propagates along the anomalously

dispersive fibre. The wavelength shift rate is proportional to τ-4, where τ is the pulse duration

(FWHM)[204].

The final wavelength at the amplifier output sensitively depends on the gain distribution (thus the

pump power) for a given length. This is because the initial resonant gain determines the position at

which the Raman soliton is formed, its pulse duration, and thus its wavelength shift rate. The

Raman soliton may experience further resonant gain until either the gain is saturated or the pulse

leaves the gain window (up to ~1.12µm). From then on the Raman soliton travels as though it is

propagating along a passive fibre. By contrast the ASE components and the dispersive waves are

further absorbed by unpumped ytterbium ions. Thus, spectral filtering can be simultaneously

performed using a longer length of the doped fibre.

4.5.3 Single Raman soliton generation – forward pumping configuration

Single pulse operation was accomplished using a 4.7m length of the fibre. The spectral evolution

with the pump power is shown in Fig.4.5.3. At the maximum pump the soliton peak shifts up to

1.33µm. The remaining peaks around 1060nm correspond to ASE or dispersive wave components.

The average power from the output was ~20mW, and which is dominated by these extra


components. This implies that the fundamental soliton peak power required within the MOF is

indeed very small, and that the amplifier gain is also very small. Given the estimated average

power for the fundamental soliton (~0.5mW), the portion of the signal in the total output should be

less than 1mW. This is consistent with the observed spectra, where the soliton peaks are ~10dB

lower than the ASE peaks.

Fig. 4.5.3 Raman soliton spectra for different pump powers (left) and tuning curve (right).

The roll-off in the peak spectral intensities from 1060nm to ~1150nm (Fig.4.5.3 left) implies that

the increased gain leads to the formation of the Raman soliton occurring at an earlier stage of the

amplification, thus resulting in an earlier departure of the Raman soliton from the amplifier gain

window. At high pump powers the pulses quickly move across and throughout the gain bandwidth.

However, the overall gain is maintained resulting in a constant spectral power above ~1150nm.

This is in contrast to the case without amplification, where the different seed powers lead to

different excitation efficiencies meaning that the output pulse energy depends on the

wavelength[55,56]. Note that the pulses below 1150nm are rather more amplified, due to the

relatively long interaction with the gain, despite the corresponding low pump powers. This can be

attributed to the fact that the pulses are possibly too broadened in the time domain due to the

reabsorption and the long propagation distance when the pulse has acquired sufficient pulse energy

to form a Raman soliton. Note that this also leads to the reduced wavelength shift rate.

Also shown in Fig.4.5.3 are the central wavelength λ0 and the spectral bandwidth of the soliton as a

function of the incident pump power. The central wavelength changes almost linearly with the

pump power, whereas the spectral bandwidth is almost constant (~14nm) above ~100mW of

incident pump, at which level the carrier wavelength shifts to ~1150nm. Note that this coincides

with the spectral intensity ‘rol l off’ wavelength (~1150nm) discussed above. The narrower spectral

bandwidth indicates the reabsorption as discussed.

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In order to evaluate the pulse quality, we performed autocorrelation measurements, a summary of

which is shown in Fig.4.5.4. The pulse duration and the time-bandwidth product of the pulse are

plotted in accordance with the output central wavelength. The maximum pulse compression

(τ~140fs) occurred at 1140nm, where the time-bandwidth product is also a minimum (~0.4). The

autocorrelation trace showed small satellite peaks in the time domain, indicating the walk off of the

Raman soliton from the original pulse in the time domain. Note that this wavelength again

coincides with the ‘roll off’ wavelength, although the spectral width is not the broadest.

Fig. 4.5.4 Pulse width (FWHM) and time bandwidth product for

different wavelengths. (courtesy J.H.V.Price)

The pulse quality is somewhat degraded at longer wavelengths. The most likely reason for this is

that the soliton suffers from the losses once it is out of the amplifier gain bandwidth and will thus

begin to broaden in the time domain, as previously discussed. Note that the water absorption peak

is located at 1.24µm, which is clearly evident in the pulse width data as the pulse duration sharply

increases in this wavelength region. The fact that pulse durations can be maintained above this

lossy wavelength range results from the fast passage of the Raman soliton through the loss peaks.

Despite these issues the pulse duration was less than 200fs over the entire tuning range, and the

pulse quality is reasonable throughout (∆ν∆τ<0.58).

In order to extend the tuning range, the initial chirp of the seed pulse can be reduced so that the

linear pulse compression takes place at an earlier stage within the amplifier. This was accomplished

by extracting the negatively chirped pulses from a different output port of the oscillator and by

transforming these pulses to have a slight positive chirp using SMF prior to seeding the amplifier.

However, this resulted in additional losses for the seed and any potential benefit was lost with the

current seed source since the interaction length with the gain is also an important factor. Thus, in

our current system at least, an appropriate amount of seed power must be launched for the given

dispersion properties of the fibre.

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4.5.4 Backward pumping configuration

We also examined the amplifier experiment using a backward-pumping configuration. The length

of the fibre was 1.8m and linearly chirped 2.4ps pulses (direct output from the oscillator) were

used. Use of the short fibre length was essential in this case since the signal pulses suffered from

losses at the initial unbleached part of the fibre. Thus, there were no unpumped ‘passive’ sections

of amplifier in these experiments. As a consequence, the tuning range was restricted up to 1.2µm

due to the frustrated soliton formation and showed a steeper dependence on the pump power,

compared with Fig.4.5.3. However, due to the very high gain available from the backward pumping

scheme, SPM of the chirped pulse plays an important role, significantly increasing the spectral

bandwidth to values in excess of 35nm with the maximum pump power as shown in Fig.4.5.5.

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Fig. 4.5.5 Output spectrum obtained using the backward pumping configuration using the

maximum pump power.

As a result of this more complex nonlinear evolution of the initially chirped pulse, the initial

quadratic phase profile of the pulse can readily be distorted making good compression difficult.

However, by minimising the pre-chirp of the seed pulse, the large effect of SPM provided by

backward pumping can efficiently be utilised for pulse compression. Note that the optimum pulse

compression would result in sub 50fs pulses in this case. Also note that the spectral peak intensity

of the Raman pulse is greater than the dispersive or ASE components in this instance (as compared

to the forward propagating pump results).

4.5.5 Multiple Raman soliton generation

It was found that multiple soliton generation was also possible by carefully aligning the launch of

both the signal and the pump to achieve maximum coupling efficiency (~30%) into a ~9m length of


the fibre in the forward pumping configuration with the same 2.4ps pulses. Individual wavelength

shifted pulses at wavelengths as long as 1.58µm were obtained as shown in Fig.4.5.6, and this

corresponds to a~30% reduction in the original carrier frequency. Note that although the spectra

were taken by butt-coupling the output into SMF (and then coupling this fibre to the OSA), the

relative intensity will not be quantitatively accurate because of the strongly wavelength dependent

NA and mode area of the MOF relative to SMF. By increasing the pump power, the number of the

soliton peaks gradually increases whilst existing peaks also shift toward longer wavelengths.

Fig. 4.5.6 Multiple Raman solitons with the pump power of 200mW. The longest soliton

peak is reached at 1.58µm.

The increased coupling efficiency of the signal to the MOF, compared with the experiment

described in Section 4.5.3, allows for more efficient energy extraction by the pulse within the gain

bandwidth and nonlinear pulse evolution starts quickly at an early stage of amplification.

According to the recent study using numerical simulation[205], the pulse evolution can be

categorised into two regimes depending on the gain[206]. When the gain is lower, the soliton

number (thus peak power) does not change over one soliton period and the amplified pulses may

repetitively generate Raman solitons as they are sufficiently amplified. The output wavelengths of

the solitons in this instance are thus reflected by the time when they have been split off from the

original pulse (thus the propagation distance with which to further shift their carrier wavelengths).

However, when the gain is high enough that the soliton number increases rapidly within a distance

shorter than one soliton period, a higher order soliton is formed, which breaks up into more than

one discrete Raman soliton during the fission process and that evolve separately as they propagate

further through the amplifier. This is possibly the cause of this multi-colour soliton generation

regime.

By continuously changing the pump power a range of interesting dynamic soliton effects was

observed. For example consider the following observation relating to Fig.4.5.6. Peak (b) was

originally a soliton peak rapidly shifting toward longer wavelengths whilst peak (a), initially the

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most rapidly moving soliton, appeared to have stopped moving once it had reached a wavelength of

1450nm. It remained at this wavelength for further increases in pump power until peak (b)

approached it – however once peak (b) got close- it moved off again to far longer wavelengths as

shown in Fig.4.5.6. Although the dynamics needs to be studied in more detail the obvious

explanation is that peak (a) was pumped by peak (b) due to Raman gain created by peak (b). Note

though that peak (b) itself stopped around 1380nm. Notice that the water absorption peak at

1380nm is also significant. This may be the reason why the peak (a) stopped around 1450nm and

the part of the reasons for the slow down of peak (b).

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Fig. 4.5.7 Examples of the supercontinuum spectra obtained from the nonlinear amplifier.

Finally, we observed continuum generation extending up to 1.6µm by further optimising the

incident signal polarisation. Examples of the spectra are shown in Fig.4.5.7. Recent studies on

supercontinuum generation in MOFs suggest that soliton fission, and the resultant four wave

mixing, play an important role in the transition from the multi-colour soliton regime to the

continuum regime[64-66]. Our observations are fully consistent with these suggestions. Given that

these other studies are performed using nanojoule pulses as pump sources this implies that the

pump requirement can be reduced by two orders of magnitude using an active MOF.

The last section highlights the fact that our tunable pulse source is an extremely complex nonlinear

system and a detailed numerical model is required to obtain a full understanding of precisely how

the dispersive, nonlinear and gain effects interact for a given system configuration. Although the

author has only provided a heuristic picture of the dominant physical mechanisms that occur within

a number of our experiments, some of the author’s colleagues have recently been working on a

detailed numerical study of the system which support the author’s conclusions.

Important issues for system improvement also remain for the fibre design and fabrication. The

design issues include flattening of the soliton peak power (proportional to λ3DAeff) over a wide

spectral range (up to the infrared absorption edge of the aluminosilicate glass[207]) by optimising

the wavelength dependence of effective mode area and dispersion. Note that this may need to take


the quantum efficiency of the successive Raman amplification into account. Low loss fibres over a

wide range must be fabricated so that the deceleration of the frequency shift rate are minimised

once the ideal fibre design is realised.

4.5.6 Summary

A nonlinear amplifier based upon the ytterbium doped MOF was studied. The simple spectroscopic

structure of ytterbium doped fibres combined with MOF technology allowed us to generate Raman

solitons tunable over a wide range of spectrum using only picojoule seed pulses in the forward

pumping configuration. It was found that a variety of operating regimes can be obtained by

changing the pumping schemes and by controlling the initial chirp of the seed pulse. Mono-colour

soliton tunable from 1.06 to 1.33µm was obtained with the output pulse duration shorter than 200fs.

Multi-colour soliton generation was observed by optimising the system, the longest wavelength of

which reached 1.58µm. Furthermore, supercontinuum spanning from 1~1.6µm was also observed

by further increasing the pump power.

The major limitation of both the pulse quality and the overall system performance was attributed to

the losses and the interface of the MOF. In order to study this system in more detail, more practical

and robust fibres are required. In addition, more optimal wavelength dependence of the dispersion

and of the effective mode area can also be designed to suit this purpose. By improving all these

factors, practical all fibre tunable ultrashort pulse sources can be implemented using ytterbium

doped MOF.

4.6 A low threshold, high efficiency erbium doped MOF laser

This section describes the first continuous wave laser operation of erbium doped MOF

(Er-HNL-00), the fabrication of which is described in Section 4.2. First, the absorption

characteristics of Er-HNL-00 are compared with those of their conventional counterparts in terms

of relative absorption strengths. Then, the results of laser experiments are presented. The

advantages of doped small core MOFs in terms of reduced laser threshold and slope efficiency are

demonstrated. Finally, a summary is given.

4.6.1 Absorption characteristics

Absorption measurements were performed using a simple cut-back method on both Er-HNL-00 and

a conventional form (NA~0.14, core diameter~7µm) of the fibre which was drawn from the same


MCVD preform. Although the conventional fibres had a cut-off wavelength at 1225nm, SMF

(cut-off @ 915nm, NA~0.17) was spliced at the pump launch end to minimise the excitation of

high order modes at 980nm. Furthermore, care was also taken to ensure that no mode coupling due

to bends or twists occurred.

Fig.4.6.1 shows a comparison between two absorption spectra. A small amount of excitation of

higher order modes is visible around 1200nm in the conventional fibre despite the precautions

described above. A similar anomaly is seen around 900nm due to the cut-off of the spliced SMF

fibre. The MOF has weaker absorption than the conventional fibre due to the lower ratio of doped

area to mode area within the core and which arises from the stacking procedure used to fabricate

MOFs. Note that OH absorption is completely negligible in both cases.

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Fig. 4.6.1 Comparison of absorption spectra between the conventional fibre (EDF) and Er-HNL-00 (EDHF).

The ratios between the two peaks at the same wavelengths provide an estimate of the relative

modal overlap factors (assuming that there is no excitation of high order modes at 980nm in the

conventional fibre variant). The ratios are ~0.47 at 1550nm and ~0.69 at 980nm. Given the fact that

the high order modes are slightly excited in the conventional fibres and that high order modes

suffer less absorption, the ratio at 980nm could in fact be slightly larger than indicated by these

values. The relatively good modal overlap at 980nm compared with that at 1550nm results from the

strong wavelength dependent modal confinement in MOFs. This is favoured from a device

perspective since it compensates for the smaller doped area in this MOF allowing a shorter fibre

length to be used than might otherwise be needed.

4.6.2 A high efficiency, low threshold laser based on erbium doped MOF

Continuous wave laser operation was accomplished using a Fabry-Perot cavity. The set up is

shown in Fig.4.6.2. A fibre pigtailed single mode laser diode was used as a pump source and was


coupled into the MOF via an aspheric lens. A 980nm optical isolator was used to isolate the laser

from the pump and a 45° angled dichroic mirror was placed between the lens and isolator to extract

the laser signal.

Fig. 4.6.2 Experimental setup for the erbium doped MOF laser.

The fibre end facets were cleaved normal to the fibre axis using a commercial mechanical cleaver

(Sumitomo:FCH-9). The cavity was closed by a high reflector at 1550nm and the ~4% Fresnel

reflection at the pumped end. The maximum pump coupling efficiency was ~50% using a lens pair

with f=12.5mm and f=3mm, and the coupling efficiency was found to be extremely sensitive to the

quality of the cleaved end of the MOF. The slope efficiency was optimised by changing the fibre

length

Fig. 4.6.3 Output characteristics of the erbium doped MOF laser at 1535nm using the 3.4m length. (left: near the threshold, and right: over the entire pump range).

Fig.4.6.3 shows the laser output obtained from a 3.4m length of Er-HNL-00, through which ~90%

of the pump power was absorbed. The laser wavelength was 1535nm and the quantum efficiency at

this wavelength is ~63.8%. A slope efficiency of 57.3% with respect to the absorbed pump power

and a threshold of 0.55mW were estimated from this data. (Note that the accuracy of the laser

output power measurement close to threshold was compromised by the stability of the laser diode

when operated at lower powers).

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Experiments were also performed by varying the length of fibre used. It was found that slope

efficiencies of more than 40% can be obtained using lengths between 3m and 4.5m. In fact, laser

oscillation was still observable using just a 1.9m length of fibre with a pump power threshold of

3mW and a slope efficiency of 26% measured in this instance.

Low threshold laser action was also confirmed by taking the output spectra and by monitoring the

laser output using a photo detector. Fig.4.6.4 shows the growth of the spectral peak near the laser

threshold for different absorbed pump powers taken using a 4.5m length of the fibre. It is clear that

the lasing peak quickly grows for absorbed pump powers of 0.5mW and above. Laser action was

also confirmed by inserting a rotating mechanical chopper into the cavity between the high

reflector and the fibre end and monitoring the laser output using a photodiode and an oscilloscope.

Relaxation oscillations, characteristic of laser operation, were clearly visible at times when the

chopper was out of the feedback beam and disappeared when the chopper blocked the feedback,

allowing us to accurately determine the onset of laser action as the pump power was increased.

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Fig. 4.6.4 Laser output spectra for different pump powers near the threshold.

Laser operation using the conventional fibre (fibre O.D.~125mm, core diameter~12microns) was

also investigated using the high power 980nm fibre laser source described in Chapter 5. The

maximum slope efficiency achieved in this fibre was ~51% with a threshold of ~82mW using a

fibre length of ~7.5m. The reason for the longer fibre length was that low index coating was

applied to this fibre and which effectively allowed this fibre to be both core and cladding pumped.

~90% of the pump was absorbed with 70% coupling efficiency. The high threshold is also due to

the long fibre length. This indicates that the original preform possesses low background losses

despite the high erbium and aluminium concentration.


It can be shown that for a three level fibre laser systems the laser threshold with respect to the

absorbed pump power and the slope efficiency are roughly proportional to Aeff(λp)/Γp and

Aeff(λs)Γp/Aeff(λp)Γs respectively, where Γp, Γs are the overlap factors for the pump and signal

respectively. Despite the smaller Γp in the MOF compared with the conventional fibre, the reduced

Aeff(λp) achievable in MOFS such as Er-HNL-00, (which can be ~10 times smaller than those of

conventional doped fibres) greatly reduces the lasing threshold. Given that typical erbium fibre

laser threshold are of order several milliwatts for similar cavity geometries[208] the measured

threshold value of ~0.55mW appears reasonable. Furthermore, in terms of the slope efficiency, the

strong wavelength dependence of Aeff in small scale MOF also leads to both increased Aeff(λs)

/Aeff(λp) and Γp/Γs.

In order to examine the tunability of the laser, a bulk angle-tuned dichroic filter was inserted within

the cavity. In order to suppress parasitic laser oscillations the fibre end was angle cleaved although

the angle had to be sufficiently large (>15°) to be effective. Unfortunately, this large cleave angle

resulted in increased intracavity losses which slightly degraded the laser performance (the slope

efficiency was now ~40% and the threshold ~2mW for a fibre length of 3.5m). Fig.4.6.5 shows

output laser spectra using 4.5m of the fibre. The maximum tuning range was limited by the tuning

range of the filter from 1530 to 1562nm. The author would anticipate a broader tuning range should

be achievable with a more widely tunable filter due to the high aluminium concentration of the

doped section[209].

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Fig. 4.6.5 Tuned spectra of the erbium doped MOF fibre laser (two laser spectra at the tuning edges).

In order to investigate the polarisation properties, the laser output was characterised by using a

cross polarizer whilst changing the polarisation state of the pump laser beam using a half-waveplate.

Note that the single polarisation of the pump beam is ensured by the beam splitter within the

isolator. It was found that the polarisation state of the laser output was only weakly dependent on


the pump polarisation and that the extinction improved as the output power increases and saturates

at higher powers, as shown in Fig.4.6.6.

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Fig. 4.6.6 Extinction ratio as a function of the laser output power.

Although the birefringence of the present fibre has not been measured, a similar undoped variant

(DHNL09 see Chapter 3) displayed a beat length of ~1mm. Thus, the present fibre is expected to

be highly birefringent. It has been shown that using conventional Hi-Bi erbium doped fibre the

polarisation state of the laser follows that of the pump when the pump polarisation is aligned with a

principal axis, and that the linear polarisation output is favoured even when a large angle is given

between the pump polarisation and the principal axes of the fibre[210]. The results obtained here

are thus qualitatively different.

The reason may be attributed to the excitation of two polarisation axes (for both signal and pump)

at the fibre end facet. Although the fibre itself is birefringent, the orthogonal polarisation

components can be excited at the free space launch due to the multiple scattering from the holey

cladding. Given the relatively poor coupling efficiency into MOF (~50%) compared to what can be

achieved for the conventional fibres (~70%) and the number of air-silica boundaries involved

within the cladding that can scatter the radiation modes, it is not surprising that a substantial

amount of radiated power undergoes multiple reflection at the air-silica interface, leading to some

power to being coupled back into the guided mode.

Studies on polarisation properties of the MOFs with small cores are ongoing. It is anticipated that

good single polarisation operation (>25dB) can be accomplished by improving both understanding

and interfaces to this fibre type.


4.6.3 Summary

Continuous wave operation of an erbium doped MOF laser was demonstrated for the first time.

Owing to improvements in fabrication it has been shown that it now is possible to fabricate doped

MOFs with a low OH content and with background losses comparable to those of the original

MCVD perform.

Comparison of the absorption spectra between the conventional step index fibre and the fabricated

MOF showed that a significantly better modal overlap with the doped section is achieved at the

pump wavelength than at the signal wavelength. This led to slope efficiencies as high as 57.3% and

the absorbed pump power thresholds as low as 0.55mW for CW laser operation. The tunability of

the laser was examined and was proven to be limited by the available tuning devices (over 32nm).

The output polarisation was also characterised. It was shown that although single polarisation mode

operation could be achieved, the extinction ratios achieved (~13dB) are relatively poor. Further

research is necessary to understand this result.

4.7 Conclusions

Rare-earth doped MOFs with a small scale structure were studied. Ytterbium doped and erbium

doped MOFs with core dimensions of ~2µm were fabricated. Using these fibres, three device

demonstrations were carried out including the first mode-locked operation of ytterbium doped

MOF laser, tunable Raman soliton generation based upon the ytterbium doped MOF amplifier

covering 1.06µm~1.33µm range, and the first continuous wave operation of erbium doped MOF

laser.

In Section 4.2, the detailed fabrication method of doped MOFs was described. In the erbium doped

MOF, the core extraction was performed by polishing. This step ensured a negligible amount of

hydroxyl contamination in the final fibre.

In Section 4.3, the optical properties of the fabricated ytterbium doped MOF were studied in terms

of dispersion, effective mode area, and birefringence. Anomalous dispersion at 1.06µm was

predicted for both optical axes, and which was subsequently verified in Section 4.5. It was found

that the orthogonal modes possess different effective mode areas due to the small interstitial air

holes around the core, and that the dimensions of these air holes relative to the operating

wavelength determine which mode possesses the greater effective area. It was also shown that the

properties of the effective mode area are deeply related to the dispersion characteristics. The


measured birefringence of 0.3mm at 1550nm in this ytterbium doped MOF is one of the shortest

values ever achieved, and is due to the strong form birefringence within this high air fill fraction

fibre.

In Section 4.4, mode-locked operation of an ytterbium doped MOF laser was demonstrated using

the frequency shift feed back technique. Tunable mode-locked pulsed output ranging from 1.03µm

to 1.05µm, with more than 63% efficiency slope efficiency with respect to the absorbed pump

power was obtained. However, no nonlinear pulse shaping owing to the anomalous dispersion was

observed, and which the author primarily attributed to the high mode-locking threshold (~17mW)

caused by stray reflections at the fibre end facets. Possibilities for improving the laser performance

were also discussed.

In Section 4.5, tunable femtosecond Raman soliton generation was demonstrated using less than

10m of the ytterbium doped MOF as an amplifier medium. The combined effects of anomalous

dispersion and high nonlinearity within the MOF allowed us to generate Raman solitons tunable

over a wide range of spectrum using only picojoule level seed pulses. It was found that continuous

tuning from 1.06µm to 1.33µm was obtainable in the single soliton regime by using a forward

pumping configuration. In contrast, the tuning range was severely limited in the backward pumping

configuration although SPM significantly enlarges the spectral bandwidth of the pulse. In the

multiple soliton regime, frequency shifted pulses up to a wavelength of 1.58µm were observed,

which corresponds to a shift of one third of the initial carrier frequency. Direct evidence of high

OH absorption peaks within the fibre was observed in the spectrum, and which probably reduced

the tuning range of the system. It is anticipated that by introducing the improved fabrication

technique, as described for the erbium doped MOF, overall system performance could be improved

further.

In Section 4.6, continuous wave operation of the erbium doped MOF laser was investigated for the

first time. It was found that the strong modal confinement characteristics of small scale MOFs

result in substantially modified absorption characteristics compared to their conventional

counterparts. This favours high slope efficiency operation (~57.3%) and low laser thresholds.

Absorbed pump power thresholds as low as 0.55mW were observed due to the small effective

mode area of the fibre. Tunability over 32nm was obtained and was limited by the available tuning

optics. It was found that polarisation purity of the laser output was not as good as anticipated.

Further investigations into the polarisation properties of the small core MOFs are still required.

The investigation into rare-earth doped MOFs described herein has revealed a lot of issues that

need to be overcome in order to benefit from useful optical properties of MOFs. These issues cover

all aspects of the MOFs; design, fabrication and devices. However, by overcoming these issues, the


author believes that compact active devices can be realised operating over an extended spectral

range and with less power requirements than that demonstrated to date.

Chapter.5

Air-clad microstructured optical fibres

5.1 Introduction

Cladding pumped fibre lasers (CPFLs) are set to play a key role in scaling up the output powers

achievable from diode-pumped solid-state lasers. CPFLs act as brightness compressors to convert

the output from low brightness, but high average power, laser diodes into a high-brightness, high

power diffraction-limited beams[211]. Combined with their inherently good thermal dissipation,

owing to their large surface area to volume ratio, single mode average output powers in excess of

100W have so far been reported[212-214], and the extension to the kW power level with a high

beam quality is optimistically anticipated[215,216].

The fact that shoe-box sized multi-watt light sources can readily be made using cladding pumping

fibres (CPFs) has already resulted in a range of fruitful (and lucrative) applications for the

technology, including optical coherence tomography[98-100], laser ranging, laser radar[217], laser

surgery and dentistry[218]. Moreover, the diffraction limited beam quality from the fibre lasers

allows for efficient nonlinear parametric wavelength conversion, further extending the available

spectral range from the visible out well into the mid infrared[219-221]. This has resulted, for

example, in the development of suitcase sized sub-ppb level gas sensing systems[222,223].

The current research into CPFLs is now focused primarily on their power scaling, however

improvements in their operating performance characteristics, including extension of CPFL

technology to new, or previously difficult to access spectral regimes is feasible. For instance,

CPFLs possess serious drawbacks when operated at wavelengths for which small signal absorption

is large (relative to the pump absorption). Examples include the pure three level operation of

ytterbium around 980nm and the quasi-three level operation of erbium from 1480 to 1530nm. At

these wavelength ranges, high power lasers and amplifiers are very important for immediate

applications within telecommunications. The former provides an opportunity for scaling up the

output from the existing (core-pumped) C- and L-band EDFAs, and the latter for extending the

Chapter.5 Air-clad microstructured optical fibres 105

signal bandwidth (with high power capability) to the S-band telecommunication window. Both of

these possibilities are very attractive for use within future dense wavelength division multiplexing

(DWDM) systems.

Another key advantage of fibre laser systems is that they can utilise well established fibre

techniques that have been developed for fibre telecommunication and sensor applications during

the last two decades. A good example is the in-fibre UV written grating, which has been developed

and exploited for its valuable wavelength selective and dispersive properties[224]. Such

components can be used within CPFLs in order to realise sophisticated high power laser and

amplifier devices.

Based upon the above technical and commercial background this chapter is devoted to the

development of novel CPFLs using air-clad microstructured optical fibres (MOFs). The author

demonstrates that it is possible to overcome several key issues within CPFLs by using air clad

MOFs. Furthermore, a possibility for inscribing in-fibre gratings using photosensitive air-clad

MOFs is also demonstrated. A combination of these technologies may allow for truly integrated

microstructured CPFLs with improved performance. Below, the specific issues associated with

conventional CPF technologies are discussed and a review of related works is presented. Finally, an

outline of this chapter is given.

5.1.1 The tunability of the Ytterbium doped cladding pumped fibre lasers (YDCPFLs)

The extremely broadband gain available from ytterbium doped fibres is one of their most unique

and attractive features[225]. This broadband gain results from a range of the three and quasi four

level transitions between two manifolds that both exhibit relatively large Stark level splittings. For

example there is an effectively pure three level 980nm transition that can be accessed using 915nm

pump sources whereas transitions at longer wavelengths (beyond 1030nm) are largely quasi four

level in nature.

The major applications of broadband emission of ytterbium doped fibres include optical coherence

tomography (since the ytterbium emission wavelength lies in a low attenuation region between the

overtones of water absorption), and high power ultrashort pulse lasers. On the other hand, the pump

absorption bands of the rare-earth ions such as erbium (980nm), praseodymium (1010nm) and

thulium (1060~1140nm) also lie within the emission band of ytterbium doped silica.

Praseodymium and thulium doped fluoride fibres are also key components to obtain amplification

within the 1.3µm and 1.45µm telecommunication bands[226,227]. Furthermore, these ions are also

useful for upconversion lasers in a ZBLAN glass host and which operate at a wavelength range

spanning from the violet to near infrared[228,229]. Thus, high power YDCPFLs with wide


tunablity and a narrow line width could provide beneficial options for a range of laser and amplifier

applications.

Arguably one of the most interesting commercial opportunities for ytterbium CPFLs deployment

relates to the development of short wavelength 980nm pump devices for high power EDFA

applications. Driven by the rapid growth of the telecommunication market much effort has been

devoted to techniques to scale up the output powers from single mode laser diodes where the need

for efficient high gain broadband amplifiers is ultimately anticipated. State-of-the-art

semiconductor technology currently allows for ~500mW fibre coupled output powers[230] with

sufficient reliability, while ~1W outputs have been reported in the laboratory developments[231].

At the same time other routes for realising Watt class high power single mode lasers at 980nm have

been explored including optically pumped InGaAs based vertical-cavity surface emitting

lasers[232] (OP-VCSELs) and YDCPFLs.

OP-VCSELs are pumped by 800nm laser diodes and are attractive for their inherent compactness

and for their capability to generate the watt level outputs[232]. However, the control over the

longitudinal modes is challenging under high power operation. In addition, there is a substantial

amount of loss when the output is coupled to SMFs as required for most applications.

YDCPFLs are an attractive option both with regard to high power operation and for stable low loss

interfacing to SMFs. The former is due to the high internal efficiency of ytterbium doped silica that

can be pumped at 915nm and the excellent thermal dissipation properties of fibre. The latter is due

to the robust single mode laser output which allows for efficient coupling to EDFs by fusion

splicing.

Although 980nm laser operation of conventional ytterbium doped fibres has already been

studied[233], it has been difficult to realise in the cladding pumped variant for the following

reasons[234]. Firstly, the laser threshold is unavoidably high in three level systems since more than

half of the population has to be inverted to provide any gain. The small core clad area ratio of

conventional CPFs leads to low optical pump power densities and this results in the laser threshold

being simply too high for efficient operation using conventional low-brightness broad-stripe pump

diodes. Secondly, the signal absorption at 980nm is also very high (2~3 orders of magnitude

greater than at 1µm wavelengths), and thus strong reabsorption of 980nm emissions prevents the

fibre from lasing unless a sufficiently high excitation level is retained along the entire length, or a

short fibre length is used. Finally, due to the high pump power requirements, the glass is prone to

suffer from non-saturable quenching[235], which can result in increased thresholds and reduced

device efficiencies.


The same argument can also be applied to the tunability of YDCPFLs, where signal reabsorption

limits their short wavelength operation. A typical end-pumped YDCPFL allows for only 40nm

tuning ranges even when pumping from both ends[236]. Techniques such as a V-groove pumping

technique[237] and fibre disk configuration[215] should allow more distributed pumping to be

achieved along the full device length. A tunability of over 60nm has been reported using an

all-fibr e ring cavity configuration with the V-groove technique[238]. Recently, tunability as wide

as 92nm has been reported from a ring cavity with end pumped configuration[239]. However, the

pump absorption was compromised to be (~80%) in this instance. In order to efficiently increase

their tuning range, the pump absorption needs to be enhanced.

To overcome these issues strategic doping techniques such as ring doping have been proposed[240],

where the active ions are doped in a ring region around the core. This reduces the modal overlap

with the signal thereby preventing the gain from concentrating at longer wavelengths. This

technique has also led to increased pulse energies from fibre amplifiers[240] and possibly improves

the concentration quenching due to the reduced modal overlap with the doped region[235].

However, this technique allows us neither to reduce the lasing threshold nor to shorten the fibre

length.

Traditional end pumping techniques remain the most attractive approach due to their inherent

simplicity. With the end pumping scheme, the minimum inner cladding (IC) dimension that can be

used is restricted compared to the other pump coupling schemes. In order to realise small I C

dimensions without compromising pump launch efficiency the IC NA has to be significantly

increased. However, the cladding area cannot be reduced to less than ~3000µm2 for conventional

CPFs for the reasons described below.

There are two types of CPFs described here: those that use a low refractive index polymer coating

as an outer cladding and those that use a fluorine doped silica layer instead, and each possesses

different limitations with respect to the minimum IC that can be used. A polymer coating allows us

to achieve an NA as high as 0.4[241]. However, the smallest IC dimensions that can be used in

practice is limited to around 70µm by handling considerations – smaller fibres are just too difficult

to work with. Furthermore, the pump power density within such a small IC is relatively high and

thus results in direct exposure of the polymer outer cladding to high optical power densities.

Consequently the long-term reliabilit y and durability of the polymer will be compromised[242]. By

contrast, no such limitations exist for fluorine doped silica based CPFs. However, their IC NA

cannot be more than 0.26[243], which represents the practical upper limit in terms of incorporation

of fluorine in silica[244] for the given process (i.e. PCVD). This NA is too low to permit a great

reduction in IC dimension.


To overcome these limitations, tapered multimode fibre lasers have recently been developed[245]

in which a multimode high NA fibre receives the pump beam whilst the fundamental mode is

selectively excited owing to the taper along the fibre length. Using this approach output powers of

more than 1W have now been reported[246]. However, the problem of ensuring a good interface to

SMFs remains to be solved and is likely to be difficult for the following two reasons. Firstly, the

glass is based upon multi-component silicate glass that softens at much lower temperatures than

silica. Secondly, its refractive index is too high compared to silica (~1.5). For these two reasons,

combined with the small dimensions of the tapered end, it is challenging to splice such fibres to

SMFs with negligible back reflections.

As a better approach, the author developed a novel type of CPF using silica based MOF technology.

By introducing air-silica microstructure within the CPF, a small IC and a high NA can

simultaneously be obtained whereas handling of the fibre remains practical (see Fig.5.1.1).

Fig. 5.1.1 Comparison between conventional CPFs (top:polymer cladding, middle: fluorine doped silica cladding) and an air-clad CPF (bottom).

The increased core cladding area ratio resulting from the small IC makes it possible to shorten

typical fibre device lengths and prevents signal reabsorption. As a result, extended tuning ranges

for quasi-four level and efficient pure three level operation (around 980nm) of ytterbium doped

CPFLs can be obtained using the cladding pumping scheme. Furthermore, it was shown that such

fibres can be fusion spliced to SMF with a loss as small as 0.1dB[105], thereby providing a ready

and robust interface to other conventional fibre types.

5.1.2 In-fibre gratings in MOFs

Fibre Bragg gratings (FBGs) and long period gratings (LPGs) are becoming increasingly important

as wavelength selective optical components in optical telecommunication and sensor systems[224].

n Low index polymer

F/Si

Air holes

High index polymer


The former couples the forward propagating mode to the backward propagating mode, whilst the

latter provides the coupling to the forward propagating high order modes (or cladding modes) of

the fibre. These couplings are induced by the periodic modulation of the effective core index along

the fibre length, which can be inscribed into a fibre with a photosensitive core by irradiating it

(from the side) with suitably patterned UV light.

FBGs have so far been used within a number of active fibre devices including distributed feedback

(DFB) lasers[247], cascaded Raman lasers and amplifiers[248], and as in-fibre reflectors to control

bandwidth and intracavity dispersion for mode-locking[184]. Furthermore, the output wavelengths

of semiconductor lasers are frequently stabilised using external FBGs, particularly at 980nm for

EDFA pumping applications[230]. In addition, LPGs are often used as gain-equalisers for

EDFAs[249]. All of these features can also be used to good effect in CPFLs providing techniques

that allow for compact integration in an efficient manner.

In-fibre gratings within MOFs have in fact already been demonstrated and studied in terms of their

modal properties[101-106]. As described in Chapter 1, it has been demonstrated that LPGs

inscribed in an air-clad MOF do not require particular hermetic coatings to ensure reliable

operation[102]. For a similar reason, there are some advantages to inscribing grating structures in

air-clad MOFs, compared to conventional CPFs. In particular, the protective coating can be

stripped off without affecting the cladding modes. Thus, a UV transparent low index polymer

coating is not required for grating inscription. This also provides flexibility in both grating

fabrication and device design.

In addition, a range of devices have been developed in recent years by filling some liquids or

polymers into the air holes and also by applying heat. Such devices have included variable

attenuators[112], polarisation controllers[111], and tunable gratings[110] that have been used for

the tunable fibre Raman lasers[248]. Such devices are very attractive for incorporation into CPFLs,

and it is anticipated that the combination of air-clad MOFs and in-fibre gratings could lead to a new

generation of more sophisticated CPFLs, including for instance, electrically tunable narrow

linewidth high power fibre lasers.

5.1.3 Outline of this chapter

The following sections of this chapter describe the development of ytterbium doped air-clad MOFs

that allow for a high IC NA and thus small IC dimensions. These fibres are capable of operating in

a three-level system and of extending the tuning range of YDCPFLs. First, the design criteria and

fabrication of the air-clad MOFs are described in Section 5.2. Two examples of laser operation are

then presented in Section 5.3. These include a description of a YDCPFL laser providing

continuous-wave operation with a tuning range of over 110nm, and an efficient, high power,


980nm YDCPFL. In Section 5.4, the feasibility of inscribing gratings within MOFs is studied and

the results of an initial trial are presented. Finally, a summary is given in Section 5.5.

5.2 Design and fabrication

The author begins by giving a few quantitative examples of some of the issues and important

parameter values that are relevant to the design of YDCPFLs. There are three main factors to be

taken into account when designing a CPF: pump absorption, brightness of the laser diode it is

designed to be used with, and the cladding design.

The pump absorption of a CPF can be well approximated by[250]

��

�� , (5.1)

where a is the core radius, which is chosen by considering the core NA and criteria for stable single

mode operation, which is given by ~1.5< �� "!<2.405. Acore and Aclad are the core and

cladding areas, respectively. N is the dopant concentration, while σp(λ) is the absorption cross

section at the pump wavelength λ. In order to increase αeff, it is clear that Aclad is effectively the

only parameter that can be modified since the dopant concentration of rare-earth ions is limited in

general by concentration quenching (thus αcore is correspondingly limited)[235]. The ytterbium

absorption at 915nm is ~23dB/km/ppm, as described. An ytterbium concentration of ~5000ppm

(typical of a high concentration preform for CPFs) within the core corresponds to αcore~100dB/m.

This means that the area ratio between the core and the cladding (Acore/Aclad) needs to be below 0.1

to obtain 10dB/m of pump absorption. A typical device length is determined by the efficient pump

absorption (~10dB): for a 1m device length an IC diameter of less than ~6.6a is required. For a

typical core NA of 0.1, the core diameter can be as large as 9µm for single mode operation at 1µm.

This implies that a fibre with a cladding diameter of at most ~30µm is required in order to be able

to realise such a short device length.

It is worth noting that the effective core NA is increased when the distance between the core and

the air-silica interface is kept short. This therefore represents a practical upper limit to the value of

the core clad area ratio while retaining single mode operation. The shortest possible distance

between core and outer cladding depends predominantly on the core NA. A rough estimate based

upon the equivalent step index approach indicates that the cladding diameter should be >6a to

suppress the effect of the air-silica interface on the core for an NA~0.1[251]. Although this

provides a reasonable ‘rule of thumb,’ a more accurate numerical model will need to be developed

to obtain more accurate limits, if required, in the future.


From the perspective of pump brightness consider the following. A state-of-the-art commercial

pigtailed broad stripe laser diode typically provides a 100µm spot diameter with a NA of 0.22, and

this corresponds to a typical M2 of ~1000 (in both transverse dimensions). The maximum available

input pump power for such a fibre from a single stripe diode is currently ~5W. The pump

brightness can equivalently be expressed as a product of the beam NALD and the spot radius wLD and

this needs to be matched to the ‘acceptance brightness’ defined by the CPF i.e.

�� , (5.2)

where NAcl and D are the IC NA and the IC diameter respectively.

To realise a structure that can accept such a low brightness beam using a holey outer cladding, it is

intuitively understood that the outer cladding should possess small air hole spacings and a high

fraction of air, so that the effective index of the outer cladding is low enough compared to pure

silica. This means that the IC NA is increased as the fibre is drawn into smaller dimensions for

given preform dimensions.

Although a detailed numerical method is necessary to quantitatively predict the waveguide

properties of the IC, a first order approximation can be made again using the effective index

model[13]. Fig.5.2.1 shows the relationship between the hole spacing Λ and the achievable IC NA

at 915nm, which is calculated from the index of the fundamental space-filling-mode

�� of the

outer cladding as � � �� . Note that the model assumes a triangular air hole lattice,

although, in reality, the air-hole cladding is instead constituted by cylindrically symmetric arrays of

air holes, in which the absolute fraction of air can be smaller. Although interstitials may be present,

their effect is typically small. Thus, this model may provide an overestimate for NAcl.

!#" $&%(')+* , -�,.* , -�).* , /0,.* , /0).* , 10,.* ,

23546

,+* ,,+* /,+* 7,+* 8,+* 9-* , : ; <.=0>? @�A: ; < =0>? @�B A: ; <.=0>? @: ; <.=0>? C�A

Fig. 5.2.1 Relationship between the air-hole spacing Λ and the NAcl calculated by the effective index model.


The curves in Fig.5.2.1 are strong functions with respect to both Λ and d/Λ. By substituting these

relations into eq.(5.2), it is possible to establish the minimum IC diameter D for sufficient pump

coupling for the given values of Λ and d/Λ. This is shown in Fig.5.2.2 assuming NALD=0.22 and

wLD=50µm. The minimum IC diameter scales almost linearly with Λ, with the gradient sharply

increasing with reducing d/Λ. By using Λ~7µm with d/Λ=0.95, an IC diameter as small as ~30µm

can accept the pump beam.

Notice that the linearity of the curves is lost for Λ<10µm. This means that the NA of the focused

pump beam becomes too high below 30µm to allow further reduction in D by an appropriate

reduction in Λ. In other words, although the IC NA does increase by drawing the fibre to smaller

dimensions �� the change in NA is not sufficient to maintain good pump coupling into the

IC and a resultant loss in coupling efficiency would be obtained. This implies that for a given

preform structure there will be a minimum usable IC dimension.

��

� !"#

$�%�& %

'�%�& %

(�%�& %

)�%�& %

*�%�%�& %

+ , -�.�/�0 12 3 4�.�/�0 52 3 4�.�/�0 576�82 3 4�.�/�0 578

Fig. 5.2.2 Minimum cladding diameter with respect to Λ, assuming wLD=50µm, NALD=0.22.

Additional design considerations may include the issue of confinement losses for the pump

radiation, since the highly multimoded pump beam is expected to be more leaky as high order

modes possess greater values of transverse wave vector components than the fundamental modes.

By increasing the thickness of the holey cladding (i.e. by incorporating more air holes), this can be

prevented. This also helps increase the NA that can be achieved.

The number of the air-hole rings m that can physically fit within the fibre cross section can roughly

be evaluated from the expression 9�: ;�< =>?@ ABCDC�EE, where J and φfibre are the thickness of

the jacket and the fibre diameter, respectively. The author found, by fabricating a number of fibres,

that J>15µm is required in order to obtain a sufficiently strong and mechanically robust fibre with

φfibre~125µm, which is a useful fibre diameter for splicing purposes. Thus one can write mΛ~30µm


by assuming D=30µm. This implies that a practically possible value of m is of order 3 to 4 (i.e.

Λ~10µm). Although most of the fabricated air-clad MOFs to date contain only m=3, no significant

confinement losses have been observed over the relevant length scales for the experiments. This is

possibly because of the short device lengths of air-clad MOFs used in our experiments to date, over

which leaky modes can also be absorbed by the doped section.

Air-clad MOFs can be readily fabricated using the single step approach presented in Chapter 2. In

order to avoid any effects of the transverse temperature distribution within the furnace during fibre

drawing (see Section 2.4.2), a relatively small preform (<15mm) is used. The jacket tube is

typically obtained by collapsing a tube on the lathe. The conventional doped fibre preform is

stretched to obtain the desired core dimensions with respect to the jacket tube and then etched

down to appropriate dimensions (~5mm). The preform can be polished if desired prior to etching in

order to shape the cladding geometry so as to further enhance the pump absorption and to obtain

excellent surface quality. Since a large amount of etching degrades the surface quality of the

preform, the surface quality needs to be recovered (by any means) if etching is used in order to

minimise the propagation losses of the pump.

Cleaned and sealed silica capillaries are stacked together, with the processed doped preform inside

the jacket tube, in such a way that the doped preform locates itself at the centre of the jacket tube.

The temperature should be set as low as possible during the draw process such that the capillaries

inflate only slightly to reduce the thickness of the strands. If the temperature is correctly set then

the initial internal preform structure can be reasonably well preserved in the drawn fibre, and the

core and IC dimensions roughly scale in proportion with the external diameter. Note that although

the interstitials will also appear when the temperature is very low, they actually help to reduce the

outer cladding index slightly (to further increase the NA). Examples of air clad MOFs are shown in

Fig.5.2.3. It can be seen that IC of LF_14_JAC is not bright under backlight illumination,

indicating a low NA and strong leakage because of both the large Λ and single ring of air holes

(m=1).

Fig. 5.2.3 Optical microscope photographs of HD_669_SQJAC (a) and LF_14_JAC (b), taken by the transmission mode of optical microscope.

(a) (b)


5.3 A cladding pumped ytterbium doped laser using air-clad MOFs

Two examples of novel CPYDFLs (a broadly tunable CPYFDFL and a 980nm CPYDFL) are

presented based on ytterbium doped air-clad MOFs. These device examples were chosen to

demonstrate the advantages of these fibres relative to conventional CPFs. First, the characteristics

of the fabricated fibres are described, and the laser characteristics are then presented.

5.3.1 Properties of ytterbium doped air-clad MOFs

a) Absorption

The small signal absorption per unit length of ytterbium doped air clad MOFs is typically more

than five times greater than in conventional CPFs because of the reduced inner cladding

dimensions. Since the IC dimensions are small, the number of modes is also reduced compared

with the conventional CPFs.

One concern is that differential mode attenuation (DMA) could prove an issue for achieving

efficient pump absorption along the fibre length. In order to establish whether this was the case the

author measured the pump absorption as a function of fibre length for fixed pump launch

conditions. An example of the measured attenuation curve for HD669_SQJAC (see Fig. 5.2.3) is

shown in Fig.5.3.1, which fits well with an exponential curve. This shows that the absorption

length is short enough that the effect of DMA is negligible for laser implementation.

��

� �� !"� #$ %&'

��

Fig. 5.3.1 Absorption characteristics of the ytterbium doped air-clad MOF at 915nm without coiling. The solid line corresponds to the exponential fit.

Although this particular fibre possesses a square shaped cladding, which ensures good modal

overlap with the doped core, significant DMA has not so far been observed in any other air clad

MOFs even with nominally circular ICs. The reason for this is attributed to the fact that the IC


edges are non-uniform due to the small scale capillaries, which naturally break the circular

symmetry of the IC thereby promoting good mode-mixing along the fibre length.

For the above measurement, the dopant concentration of the core was estimated to be ~3000ppm

from the original preform, corresponding to an absorption of 69dB/m at 915m for core pumping.

The actual core diameter in HD_669_SQJAC was 30µm, whilst the cladding was shaped to be

60x65µm. From eq.(6.1), the author estimates a net absorption of ~12.5dB/m which implies an

optimum length of 80cm. An exponential fit to the data shown in Fig.5.3.1 leads to an estimate of

11.1dB/m. The slight discrepancy can be attributed to experimental uncertainties in the dopant

concentration measurement of the conventional fibre type. Because of the high absorption per unit

length (typically the fibre length <50mm) combined with the low NA core in the core pumped

configuration, the coupling efficiency from a spliced SMF was difficult to evaluate.

b) Numerical aperture

The NA is measured by using a beam analyser to characterise the far-field intensity distribution

from the fibre end while overfilling the input. The ratio of the beam radius r, at which the intensity

is 5% of its peak, to the distance z (>>a2/λ) from the fibre end facet gives NAcl to be

�� [252]. Two examples are studied below.

Fig. 5.3.2 SEM photographs of (a) HD652_2_JAC (NAcl=0.4) and (b) HD709_3_JAC (NAcl=0.5).

The measured NAcl was 0.4 for HD_652_2_JAC and 0.5 for HD709_3_JAC at 808nm, where the

absorption is negligibly small. The IC diameters are 30µm and 28µm, respectively, whilst the air

hole diameters in the first ring are ~15µm and ~8.3µm respectively. The smaller air hole spacing

results in the higher NA, since IC is more isolated from the jacket due to thinner silica bridges

(a) (b)


within the structure, assuming the same values of d/Λ. Thus, the measured values are consistent

with the previous discussions.

Although it is difficult to define the air hole diameter from these SEMs due to the irregularity of the

geometry; by assuming d/Λ=0.925, an NA of 0.33 for Λ=15µm and an NA of 0.52 for Λ=8µm are

calculated using the effective index model at 915nm. Given the fact that the NA becomes relatively

smaller at short wavelengths (i.e. 808nm), the measured values of NA are higher compared to the

estimates from the model, contrary to what was discussed in Section 5.2. In the IC NA

measurement for standard fibres, a length of 2~3m is used to eliminate any leaky modes from the

measurement process[252]. However, because of the short length requirement for the air-clad

MOFs, it is not appropriate to take such measures to avoid the impact of leaky modes and this may

provide one reason for the high NA values observed, and the NA may indeed gradually decrease

along the length[253], particularly for HD652_2_JAC. In practice, however, leaky modes may well

be efficiently absorbed within the core and still effectively contribute to the total pump. Therefore,

the author believes that the measurement should provide reasonable accuracy for calculating the

coupling efficiency of the pump.


The laser setup is as shown in Fig.5.3.3. The output beam from a pigtailed pump diode (wLD=50µm,

NALD=0.22) was collimated by an 11mm focal length lens with 0.25NA and then focused and

launched into the end of the fabricated fibres with a 0.55NA 4.5mm focal length lens. The

perpendicularly cleaved launch end of the fibre also served as the output coupler. A dielectric

mirror separated the output beam from the path of the pump beam. The other fibre end was angle

cleaved to suppress back reflections. An external bulk grating (600lines/mm, blazed at 1µm)

provided wavelength selective feedback under the Littrow configuration. A dichroic mirror was

used to reflect back unabsorbed pump light into the fibre via a diffraction grating, allowing for a

double pass pump configuration to be achieved and to further increase pump absorption.

Fig. 5.3.3 Schematic of the experimental setup for cladding pumped ytterbium doped air-clad MOF lasers.

Air-clad MOF DM

DM

Output

Pump

Signal

Pump


5.3.3 A cladding pumped ytterbium doped fibre laser with a wide tuning range

The fibre used in this experiment was based on an aluminosilicate core with NA~0.08 and had an

ytterbium concentration of ~8000ppm (NL-22-JAC in Fig.5.3.4). The inner cladding diameter was

~36µm, and its NA was measured to be 0.5 at 808nm. The doped core was very elliptical in shape

(~9µm x ~5µm). The absorption was ~5dB/m at 915nm, which agreed well with the estimate from

the independent measurement of both area ratio and ytterbium concentration.

Fig. 5.3.4 Optical microscope photograph of NL-22-JAC.

A total pump absorption of 8.5dB was achieved by taking the double pass pump through a 1.7m

length of the fibre. Fig.5.3.5 shows the output characteristics of the laser. A slope efficiency of

67% and a threshold of 0.2W were obtained. A reference double-clad fibre was pulled from the

same MCVD preform, by applying a low index polymer coating, for comparison purposes (10µm

core, 125µm O.D. and αeff~1.2dB/m at 915nm). Using a 10m length of this fibre a slope efficiency

of 61% and a threshold of 1.2W were measured from a cavity formed by two perpendicularly

cleaved ends. Therefore, the relatively low efficiency of the air-clad MOF can be attributed to that

of the original preform.

�� !��"�!��#�!��"�!��

$ %& '%& '()*+, -./

�

��

��

��

��

�!��

02143�5�6

Fig. 5.3.5 Output characteristics of laser based on NL29_JAC at 1060nm. (Courtesy C.C.Renaud)


The laser was broadly tunable from 1010nm to 1120nm with a double-pass pump (FWHM~100nm),

as shown Fig.5.3.6. The tuning range dropped to ~75nm (FWHM) when a single-pass pump was

used. This mainly reduced the tuning range at short wavelengths. This clearly indicates the impact

of the pump feedback which greatly reduces signal reabsorption at the far end of the fibre and

allows the laser to operate at wavelengths below 1040nm. The tuning range of over 75nm in the

single-pass pump configuration is still very much broader than the 30nm obtained using the

reference fibre, with its length optimised with respect to efficiency in the same cavity. Thus, the

impact of the short fibre length is proportionately greater than that obtained due to the double-pass

pumping scheme and results in a 45nm extension of the tuning range.

�� !�� "��

# $% &$% &'()*+ ,-.

�

��

��

!��

"��

��

Fig. 5.3.6 Tuning range using a 1.7m of NL_29_JAC. (Filled dots: with a double-pass pump. Open dots: with a single-pass pump) (Courtesy C.C.Renaud)

5.3.4 A cladding pumped 980nm ytterbium doped fibre laser

Laser operation at 980nm was achieved by adjusting the bulk grating to an optimum angle so that

980nm radiation is fed back into the fibre. The fibre used for this experiment was HD709_JAC (see

Fig.5.3.2). The preform contains a slightly complicated core structure as explained below.

The core of the preform is composed of two layers of different materials as shown in Fig.5.3.7: a

boro-germanosilicate layer in the centre and an ytterbium doped alumino-germanosilicate layer that

surrounds the former (refractive index difference ~10-3). The innermost cladding layer is made of

an ytterbium doped alumino-borosilicate layer. The core layers that contain germanium serve as a

photosensitive core for inscribing in-fibre grating. The ytterbium ions are distributed such that their

overlap with the modal peak is prevented whilst achieving better modal overlap and greater doped

area than the ring doped fibres (referred to at the ORC as ‘extended doping.’). By approximating

the core structure as a single layer since the difference in their refractive indices is only 10-3, an

effective core NA is estimated to be ~0.11.


Fig. 5.3.7 Refractive index profile of the preform HD-709. (Courtesy J.K.Sahu)

The core, cladding and outer diameters of the fibre are 6.5µm, 28µm and 125µm, respectively. The

total absorption at 915nm is >4dB/m. Using the same set up as shown in Fig.5.3.3, the maximum

output power of 2W at 976.8nm was obtained for a 45cm length of the fibre. The maximum

launched pump power was 7W with a pump coupling efficiency of ~50%.

Further improvement was made by substituting the previously used pigtailed diode with two

brighter, low NA laser diode bars (wLD=25µm and NALD=0.25). The diode bars were polarisation

multiplexed via a polarisation beam splitter to double the available pump power into the same NA

and spot size. The total available incident pump power was 14W. A maximum output power of

3.5W was obtained for an absorbed pump power of ~6W using just a 45cm length of the fibre, as

shown in Fig.5.3.8. The slope efficiency of 58.7% and a threshold of 218mW were calculated from

the output characteristics. By making use of the double pass pump configuration a pump absorption

of more than 7dB was achieved with respect to launched pump. The output was nearly diffraction

limited, M2 <1.2 was measured. The output spectrum was almost a single peak and its bandwidth

was less than 0.2nm. Furthermore, the output was tunable from 976nm to 980nm.

�� ! �� "�� #��!$�� %��!&��

' () *() *+,-./ 012

�$��$�� $��"��"�$��#��

Fig. 5.3.8 The output characteristics of the 980nm laser using HD709_JAC. (Courtesy R.Selvas-Aguilar)

35476�8 9�:5; <�<�=> ? >�@ A @ ?

B C

D E�F EGE7HE�F EGEGEE�F EGE7HE�F EGEGIE�F EGEGJE�F EGEGKE�F EGE�LE�F EGEGM

B/Ge/Si

Yb3+:Ge/Al/Si

Yb3+:B/Al/Si


5.3.5 Summary

Several ytterbium doped air-clad MOFs have been fabricated and characterised and both extended

tuning range and 980nm laser operations of YDCPFLs have been demonstrated, which are difficult

to achieve using conventional CPFs. It has been shown that the measured NA is greater than

expected based on simple theoretical estimates, which the author attributes to the presence of leaky

modes that may also be well absorbed within a short device length. A broad tuning range over

110nm for an YDCPFL has been demonstrated using a double-pass pump configuration through

1.7m of fibre. Under 980nm operation, a maximum output power of 3.5W has been obtained using

only 45cm of the fibre with a slope efficiency of 58.7% and a threshold of 218mW with respect to

the absorbed pump power.

5.4 Fibre Bragg grating in air-clad MOFs

As discussed in Section 5.1.2, a possible disadvantage of inscribing gratings within photosensitive

air-clad MOFs would be the scattering of the UV light at the air-silica interfaces. In fact, although

the author fabricated several photosensitive MOFs, few gratings were successfully written even

when using a KrF excimer laser. Thus, it was important to investigate the extent to which cladding

structures would degrade the quality of the UV writing processes. Here, the author evaluates the

impact of the air-silica interface and demonstrates that it is possible to inscribe the gratings within

these MOFs even without hydrogen loading by developing an ultra-photosensitive air-clad MOF.

5.4.1 A GeO2-B2O3 co-doped air-clad MOF

For long period fibre gratings, the temperature sensitivity (~0.05nm/°C) is often a problem for

telecommunication applications although it is used to beneficial effect for sensor applications[254].

This effect is induced primarily by the temperature dependence of the refractive index of the glass

(dn/dT). Although improved environmental stability of LPGs inscribed in an air-clad MOF has

been demonstrated[102], improved temperature stabilit y cannot be obtained solely by isolating the

inner cladding that supports the cladding modes. Moreover, for laser applications it is anticipated

that due to the reduced thermal capacity, because of the small glass volume, and the resultant poor

cooling rate the temperature stabilit y may indeed set a limit for high power operation.

Conventional fibres that consist of a SiO2 or GeO2/SiO2 core exhibit positive dn/dT, however it has

been shown that by doping the core with GeO2-B2O3[255] or GeO2-P2O5[256], dn/dT can be

flattened over a range of 0~100°C due to the negative dn/dT induced by boron or phosphorous

doping. Therefore, by combining this idea with MOFs, it should be possible to realise gratings with

improved stability with respect to both temperature and environment. Furthermore, GeO2-B2O3

co-doped silica can contain more germanium, which is the main contributor to the fibre


photosensitivity, than conventional fibres without increasing the NA. This is because boron doping

decreases the refractive index. Thus, GeO2-B2O3 co-doped MOFs are anticipated to possess three

advantages: high photosensitivity, better temperature stability , and increased environmental

stability.

A GeO2-B2O3 co-doped preform (HD514: NA~0.15) was stretched on the lathe and mechanically

polished to form a hexagonal rod. The diagonal distance of this rod was ~2.5mm, whilst the core

diameter was ~0.7mm. The other parameters used in the preform and the fibre drawing are

summarised in Table.5.4.1. The cut-off wavelength for the core in the final drawn fibre was set to

be 1370nm. The relatively high temperature used in the draw of this fibre enabled high speed

drawing, which resulted in relatively expanded cladding air holes. The cladding material was made

of F300 glass to ensure good transparency at UV wavelengths down to well below 240nm. The

fabricated fibres are shown in Fig.5.4.1.

Table. 5.4.1 Parameters of the preform elements, the fibre drawing, and the fibre dimensions. *: the diagonal distance of the hexagon, **: compared with the drop temperature.

Preform parameters:

Capillary Jacket Preform

I.D O.D. I.D. O.D. core diameter O.D.*

2.2mm 2.5mm 7.5mm 14mm 0.7mm 2.5mm

Draw parameters and the resultant structural dimensions:

vf vd Temperature** core diameter IC diameter Fibre O.D.

2mm/min. 49.5m/min. � 40°C 7µm 25µm 100µm

Fig. 5.4.1 A SEM photograph of GeO2-B2O3 co-doped air-clad MOF (HD514_JAC).

5.4.2 The effect of the air-sili ca interface A 6mm long grating was written by using a fixed phase mask and irradiation with pulses from a

KrF excimer laser providing a fluence of 0.5J/cm2 at 20Hz for five minutes. No care was taken


concerning the orientation of the fibre during the grating fabrication. The grating was characterised

using a white light source. The output was collected by butt-coupling the fibre to SMF, in order to

selectively characterise the core mode. The measured reflection spectrum is shown in Fig.5.4.2 (a).

A peak reflectivity of ~5dB was obtained. Note that the background ‘beat’ in the transmission

spectrum is due to the high order modes within the IC. The half width was measured to be ~0.5nm,

which is comparable to that obtained for gratings written in conventional fibres using the same

equipment under the same conditions of UV exposure.

In order to investigate the impact of the air-silica interfaces for UV writing, the air holes within

HD514_JAC were intentionally collapsed by slightly tapering the fibre at a low temperature, which

resulted in the core and fibre diameters of 4µm and 60µm, respectively. Then, the grating was

inscribed using the same dose, and was characterised using the same method described above. The

observed reflection of 3dB is somewhat smaller than in the original fibre due to the relatively poor

modal overlap with the photosensitive region due to the reduced core dimension, as shown in

Fig.5.4.2(b). However, this implies that there is no significant intensity attenuation of the UV li ght

through the air-silica interface for this type of MOF, (Note that the total number of air-silica

interfaces in the original MOF is three versus one for the collapsed fibre). Note that the different

Bragg wavelength is due to the use of a different phase mask in this instance although its alignment

was ensured by examining grating inscription within a SMF prior to the experiment.

Fig. 5.4.2 Transmission spectra from the grating inscribed in the air-clad MOF (a) and that from the grating inscribed in the ‘collapsed’ air-clad MOF (b).

5.4.3 Summary

The author has described the initial trials for fabricating fibre Bragg gratings within air-clad MOFs.

The GeO2-B2O3 co-doped air-clad MOF has been fabricated and a fibre Bragg grating with a

reasonable strength (~5dB) at 1550nm was successfully inscribed, without hydrogen loading, using

a fixed phase mask and a KrF excimer laser. Comparison with gratings produced in a conventional

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fibre counterpart, which was prepared by slightly tapering the fabricated fibre and collapsing the air

holes, indicates that the air-cladding within this MOF, which comprises a single ring of air holes

with a structural scale of ~20µm, has had little effect on the quality of the grating writing process.

Unfortunately, due to failure of the excimer laser half-way through this study it was impossible to

study the detailed characteristics of the MOFs gratings with respect to the dose. In addition,

experiments could not be carried out on the temperature or environmental stability. The laser has

just recently been replaced and the investigations will be resumed in due course.

5.5 Conclusions

Two types of novel air-clad microstructured optical fibres have been developed. It has been shown

that, based upon a crude effective index model, high IC NAs in excess of 0.5 can be achieved. The

design criteria for CPFs have been discussed, and it has been shown that, although the NA can be

increased by decreasing the structural dimensions within the air hole cladding, the smallest inner

cladding diameter that can be used in practice will ultimately be determined by the pump

brightness.

Several ytterbium doped air-clad MOFs have been fabricated. We have experimentally shown that

the inner cladding NA can be as high as 0.5, while the inner cladding diameter is reduced to be as

small as 28µm. Using these fibres, we have shown extended tunability and 980nm operation of

ytterbium doped cladding pumped fibre lasers and that were only possible due to the short

(sub-metre) device lengths enabled by the combination of the small inner cladding dimensions and

high NA.

A photosensitive GeO2-B2O3 air-clad MOF has been fabricated, and the effect of the multiple

air-silica interfaces on UV written grating fabrication has been studied. It was found that it is

possible to inscribe a reasonably strong grating (~5dB) without hydrogen loading in this fibre.

Hence the air-silica interfaces do not degrade the grating writing process at least when the number

of the air silica interfaces within the fibre is small. Further study is required to investigate the effect

of increasing the number of air-silica interfaces, on the scale of the air holes within cladding, and

how the maximum strength of the grating changes with respect to total UV dose. This research is

on-going at the ORC, and an examination of the temperature and environmental stability of the

gratings written in this novel photosensitive air-clad fibre will soon be conducted.

Chapter.6

Large mode area microstructured optical fibres

6.1. Introduction

Large mode area (LMA) optical fibres are becoming increasingly important as a high power

transmission medium. One major driving force results from the constant increase in the output

powers from laser diodes that allows for higher output powers from diode-pumped solid-state

lasers. As a result, higher demands are being placed on the power handling capability of optical

fibres. For instance, the recently developed high average power ultrashort pulse sources has

effectively utilised a LMA fibre as a nonlinear fibre to compress the pulse duration below

50fs[257], a pulse duration that is difficult to obtain directly from high average power oscillators.

Moreover, for medical and industrial use of high power lasers such as excimer, Nd:YAG, Er:YAG,

and CO2 lasers, optical fibres are expected to provide a convenient and flexible means of handling

the light without the difficulties of alignment. Traditionally, hollow fibres with appropriate inner

coatings or hollow tapers have been used[258-260]. However, given the recent advances in diode-

pumped solid state lasers with significantly improved beam quality, in conjunction with the

nonlinear wavelength conversion technique[261], single mode optical fibres with low nonlinearity

such as LMA fibres need to be developed for a range of wavelengths.

LMA fibres are also becoming increasingly important as an active medium due to the recent

interest in the possibility of very high power fibre lasers. Single mode output powers of more than

100W are now routinely reported[212-214] and 1kW output has been reported from a multimode

fibre laser[215]. For many applications, a diffraction limited quality of the output beam is one of

the key features that the active optical fibres can provide, since it allows for the maximum

brightness achievable for a given fluence or average power. It is thus important to increase the

mode size of the fundamental mode, in order to avoid the limiting factors of both optical damage

and nonlinear thresholds.


125

So far, conventional LMA optical fibres have been fabricated using the MCVD technique. These

fibres are characterised by a very low numerical aperture (NA<0.1) and an additional ring profile

(see Fig.7.2.1). In order to control the desired refractive index profile, a precise flow control of the

chemical vapour is required during the deposition process. Unless fabrication systems are fully

optimised, it is challenging to reliably fabricate such low contrast refractive index profiles,

particularly at short wavelengths. Despite this, an optimised MCVD process, combined with the

solution doping technique, pioneered here in Southampton[128], has allowed for an increase in the

effective area by a factor of four using LMA fibres at 1550nm[262].

High power single mode operation is now routinely achieved using single mode excitation of step

index multimode fibres[263,264]. As a result, millijoule pulse energies can now be obtained from a

Q-switched fibre laser[265], an ultrashort pulse amplifier[266], and a MOPA configuration[267]. In

particular, nearly diffraction limited beam quality has been obtained with the aid of preferential

gain[268] in the case of active fibres, where the modal overlap with the doped region is designed

such that only the fundamental mode is efficiently amplified even when several modes are

supported within the fibre.

However, it has been shown that the single mode propagation distance in MCVD based multimode

fibres is proportional to Ls~2x1015xD6λ4/(ρ10n6), where D and ρ are the fibre and core radii, λ and n

are the wavelength and the refractive index of the core[263]. It is readily recognised that D must be

very large (>>125µm) in order to achieve Ls>10m at 1.06µm, and that it is increasingly difficult to

achieve single mode operation at short wavelength due to the λ4 dependence. In ref. [262], a tight

bend was imposed on the fibre to prevent high order modes. For passive applications, these

restrictions can severely limit the usefulness of MM fibres. Furthermore, for active fibres, large

cladding dimensions suggest reduced absorption and thus long device lengths are required. This

requirement can impose other limitations such as Brillouin scattering[41]. (Further issues

associated with use of the long active device lengths are also discussed in Chapter 5)

MOF technology provides an alternate route for low-NA LMA fibres, and possesses an advantage

that perfectly homogeneous pure silica core fibres can be readily made. This implies that MOFs are

inherently suitable for pulsed and/or short wavelength operations owing to the wide electron band

gap of pure silica and to the fact that the refractive index contrast is not limited by doping levels.

Although large mode area microstructured fibres (LMA-MOFs) have been already

demonstrated[7], practical issues such as macroscopic bend loss and power handling of such fibres

were not well understood prior to this study. Furthermore, no active devices based on LMA-MOF

had been reported.


126

This chapter describes the fabrication and characterisation of LMA-MOFs, aiming at forming a

foundation for the work on active LMA-MOFs that is presented in Chapter 7. The author begins by

discussing the fabrication issues related to LMA-MOFs in Section 6.2. The basic optical properties

of LMA-MOFs are experimentally characterised including effective mode area in Section 6.3 and

bend losses in Section 6.4, in both of which a comparison with conventional LMA fibres is carried

out. Furthermore, the transmission losses of LMA-MOFs are presented in Section 6.5. Finally, the

conclusion is given in Section 6.6.

6.2. Fabrication of large mode area microstructured optical fibres

In this section, the refractive indices of the materials that can be used for silica based LMA-MOFs

are reviewed first, as they can greatly influence the performance of LMA-MOFs. In addition, a

doped LMA-MOF incorporates a core material with different refractive indices, as shown in

Chapter 7. As we learnt more about the sensitive characteristics of LMA-MOFs, a number of

modifications of the fabrication process became necessary. In the near future, further modifications

are likely to be required. Therefore, in Section 6.2.2, an overview of the evolution of the fabrication

process so far is presented.

6.2.1. Refractive indices of silica based materials

As is in any low NA fibre, any index difference within the transverse index profile of the LMA-

MOF can strongly influence the modal properties of the fibres. This is because the index contrast

created by the air holes is typically very small and may be comparable to any slight index

difference introduced by the use of different silica based materials within the fibre cross section. It

is thus naturally important to know the refractive indices of the start materials that can be used

within such fibres. The measurement of the refractive indices of various silica glasses is presented

below.

The author made a trial preform, which consisted of several different silica glasses by collapsing

and jacketing different tubes on the lathe. Then, the refractive index profile of this preform at

633nm was measured using a preform index analyser (P-104, York technology). We also made an

index measurement on bulk samples using a refractometer, in which the sodium D-line (589nm)

was used as a light source. Although the relative index differences between some of the glasses

were near to the resolution limit (∆n~10-4) of these two measuring techniques, the index values and

differences with respect to the high quality silica are summarised in Table.6.2.1. The absolute

offset of the respective measurements was eliminated for the purpose of comparison.


127

Suprasil® F300 and F320 are synthesised silica glasses with an extremely low OH content

(<0.2ppm), and the difference between them are due to the agents used during their dehydration

process. Consequently, F300 contains a substantial amount of chlorine (~1500ppm) whilst F320

contains an amount of fluorine (~3500ppm). Suprasil® F100 is also a synthesised silica made

without dehydration, containing <1000ppm of OH ions. However, the transmission window of

F100 extends below 200nm, thereby it is often used for UV optics and fibres. HLQ-210 and

Vycor® are fused silica, and they contain significant amounts of impurities as well as high water

content. The latter contains ~3wt.% of boron and ~4wt.% of sodium, leading to a significantly

reduced softening temperature, compared with the others glasses discussed here.

Table. 6.2.1 Measured refractive indices for different raw materials. ∆ is the index difference with respect to the values measured for Suprasil® F300. (*unknown)

Vycor® F100 HLQ-210 F300 F320 Sellmeier's

formula Cl content [ppm] - - - 1500 -

F content [ppm] - - - - 3500

OH content [ppm] * <1000 ~30 <0.2 <0.2

Refractometer 1.456 - 1.4572 1.4575 - 1.4586@589nm

Preform analyser 1.4571 1.4578 1.458 1.4581 1.4572 1.4573@633nm

∆n (Refractometer) -1.03x10-3 - -2.06x10-4 0.0 -

∆n (Preform analyser) -7.03x10-4 -2.18x10-4 -3.43x10-5 0.0 -8.58x10-4

Both chlorine and sodium increase the refractive index of silica whilst fluorine and boron reduce it,

as long as the overall silica content is high enough. Note that the degree of index modification can

be dependent on both the dopant concentrations and the species. The measured values can be well

explained by taking into account these dopants. The relative refractive index differences measured

from the preform analyser are smaller than those measured from the refractometer. This can be

attributed to the diffusion of impurities that took place when the glass was heated to prepare the

sample used for the preform analyser, and which led to a reduction in the refractive index contrasts.

Note that when different materials are used in the same MOF preform, heat is also applied to the

glasses although the effect is not as significant as that during the collapsing process on the lathe.

Throughout both of the measurements, it was found that the refractive index of HLQ-210 is not as

spatially uniform as those of the synthesised glasses. This resulted in a noisy index profile,

particularly in the low quality silica layer, in the preform analyser. The reading was also blurred in

the refractometer for HLQ-210. This may be attributed to some inhomogeneously localised

impurities.

In fabricating MOFs, there are three possible combinations of materials as follows:


128

A) Single material

B) High index material core and low index material cladding

C) Low index material core with high index material cladding

Most of MOFs reported to date use type A). Type C) has been reported using F320 in the core with

F300 in the cladding, where an antiguide was formed at short wavelengths since the index

difference created by the air holes is compensated by the index difference between the core and

cladding materials[269]. Type B) is also a typical case when doped fibres are made, as described

Chapter 4 and 7. However, the index difference has to be sufficiently small to still receive the

benefits of the MOFs since their NAs are increased by the doped sections. Amongst the materials

investigated here, the highest index difference is between those of F300 and F320 (~8.6x10-4). The

corresponding NA is ~0.05. To achieve rigorously single mode operation at 1µm using

conventional step index fibre designs, the core diameter has to be less than 15µm for this material

combination. When air holes are introduced in such a step index fibre, it is likely to become

multimoded due to the extra contribution of the air holes to the total NA. Thus, particularly in

LMA fibres, it is not so trivial to incorporate air holes together with a high index section created by

the use of different silica based materials. In the following sections, we demonstrate types A) and

B), and examine the difference in their optical properties.

6.2.2. Evolution of the LMA-MOFs fabrication process

Using the materials described in the previous section, a range of LMA-MOF s have been fabricated.

The fabricated fibres are summarised below.

A) The first generation

At the very early stage of LMA-MO F development, the preform consisted of a F300 core, HLQ-

210 capillaries, and a Vycor® jacket since it was anticipated to be the cheapest combination (from a

cost perspective) whilst achieving an acceptable loss level. The capillaries were prepared by

drilling a 13mm diameter air hole within a 25mm O.D. HLQ-210 rod using an ultrasonic drilling

machine and then by drawing it into ~2mm O.D. capillaries after a fire-polishing step since drilling

results in surface roughness of ~1µm. Due to the thick walls of the tube, it was difficult to fire-

polish the inner wall on the lathe. This may be improved by flowing helium inside the tube during

the fire-polishing stage due to the increased thermal conductivit y of such gases. The reason why

this approach was taken is because it is the most straightforward way of obtaining a uniform thick

walled tube (I.D./O.D. < 0.5) with a large O.D.


129

It was difficult to stack these 2mm thick walled capillaries to form the preform, since they do not

deform significantly unlike thin walled capillaries. Combined with the poor qualit y of the

unprocessed Vycor® jacket tube, the resultant preform assembly was hopelessly disordered. Note

that it is now possible to improve the circularity of Vycor® tubes on the lathe. The preform and the

draw parameters are summarised in Table.6.2.2.

Table. 6.2.2 The fibre draw parameters, the preform dimensions, and the fibre dimensions of LMA- 0001.

Draw parameters:

vf [mm/min.] vd [m/min.] Temp [°C]

1.216 10~30 ∆75~85

Preform dimensions:

Core (F300) Capillaries (HLQ-210) Jacket (Vycor®)

O.D. [mm] I.D. [mm] O.D. [mm] I.D. [mm] O.D. [mm]

~2.0 ~0.5 ~2.0 ~18 ~20

Final fibre dimensions:

O.D. [µm] Λ [µm] d/Λ

125~200 8.5~15 0.1~0.2

A range of different dimensions of fibre were fabricated by varying the draw temperature and the

draw speed, and an example of this is shown in Fig.6.2.1 (LMAHF-0001). Although the structure

was approximately retained along the entire length of the fibre (~500m), unexpected gaps were

formed between the jacket tube and the inner capillary bundle in addition to the interstitial air holes

between the capillaries.

Fig. 6.2.1 SEM photographs of LMAHF-0001. (a) the entire cross section and (b) the core region of the fibre. The fibre O.D. was varied from 125µm to 200µm by varying the drawing conditions.

(a) (b)


130

As discussed in Chapter.2, the idea of the collapse ratio can be applied to the individual preform

components. By dividing the collapse ratio of the capillaries by that of the jacket tube, a ratio of

~0.07 is obtained assuming the same viscosity and the surface tension. This means that the

capillaries collapse far more than the jacket, which is the cause of the gaps between the stack and

the jacket tube and interstitials. Since the jacket almost fitted on to the capillary stack (i.e. nearly

the same collapse ratio), the viscosity difference between F300 and Vycor® is estimated to be

nearly a factor of 10 at these temperatures. The gaps between the capillary stack and the jacket tube

are mainly attributable to the low filling factor of the initial preform due to the poor quality of the

stack. This could be eliminated by using a thicker jacket so that the collapse ratio of the jacket can

be slightly increased.

B) The second generation

A second generation of LMA-MOFs was developed with the aim of realising a single material fibre,

the development of which is described in Chapter 2. The difference from the previous generation

was that it became possible to collapse a sufficiently long tube on the lathe, so that higher quality

capillaries can be obtained. The difficulty involved in this process is to obtain good control of the

inner diameter without cutting the tube, since the O.D. of the tube does not change as much as the

I.D. during the collapsing process. However, recently developed laser diameter gauges allow us to

measure both I.D. and O.D. of the tubes simultaneously using the beam deflection technique. Thus,

this issue should soon be overcome by utilising such a device.

Another technique introduced in this generation was a screening procedure. By using short lengths

(~10cm) of capillaries selected in terms of their dimensions, it becomes possible to stack the

capillaries neatly. The author initially concentrated on fabricating fibres with a very low fraction of

air (d/Λ<0.1). However, it turned out to be difficult to guide light within these fibres because their

extremely low NA leads to significant bend losses (and possibly also because of the confinement

losses) (see Fig.6.2.2 (a)).

These losses can be significantly reduced by increasing the number of rings of air holes in the

cladding[150]. Therefore, the number of the rings was increased up to seven (>150 elements).

However, this approach was not successful in helping the light guidance.

The other option to cope with this issue could be to use a double clad structure as shown in

Fig.6.2.2 (c). By this means, it was anticipated that the modal leakage could be reduced owing to

the stronger modal confinement in the outer cladding without significantly influencing the mode.

However, the mode was strongly confined within the inner cladding rather than in the core. As a


131

result, it was impossible to determine the guidance properties of the core mode due to the bend

losses that couples its power into the cladding modes within the inner cladding.

Fig. 6.2.2 Impractical fibres because the air holes are too small. The single clad fibre: (a) and (b). The double clad fibre: (c) and (d).

By comparing with LMA-0001, which possesses similar structural parameters in terms of d and Λ,

it is understood how significantly the wave guidance in LMA-0001 was assisted by the slight index

difference between F300 and HLQ-210.

It was found that it is necessary to increase d/Λ to >0.2 in order to realise practically usable single

material fibres. Note that the practical choice of d/Λ can be dependent on Λ. Seven rings of air

holes were used in the examples shown in Fig.6.2.3 (LMAHF-0100 and LMAHF-0101). The

typical draw parameters used for these fibres are summarised in Table.6.2.3. Due to the large

number of capillaries, it became difficult to align the core centrally with respect to the jacket tube.

This was found to be the cause of the orientation dependent bend losses, described in Section

6.4[270].

Finally, it should be noted that when using this fibre, some of the author’s colleagues found that it

was difficult to obtain a reasonable agreement between the theoretical predictions (localised

function method[18] and multi-pole method[25,26]) and the experimental measurements using a

(a) (b)

(c) (d)


132

mode field diameter (MFD) measurement[44] and a scanning near field microscope (SNOM)[271].

This problem was solved using the fibre that was made of all F300, in the third generation. The

refractive index inhomogeneity within the HLQ-210 rod may be the reason for this discrepancy, as

discussed in the refractive index measurement section.

Table. 6.2.3 The draw parameters used for LMAHF-00 and 01.

Draw prameters:


1.5~2.5 20~30 ∆10~20

Preform dimensions:

Capillaries (sealed) Jacket I.D. [mm] O.D. [mm] I.D. [mm] O.D. [mm]

~0.3 ~1.0 ~17 ~20


O.D. [µm] Λ [µm] d/Λ 125~200 7.5~12 ~0.25

��

��

��

��

Fig. 6.2.3 LMAHF-0100: (a) the entire cross section and (b) the core region, and LMAHF-0101 : (c) the entire cross section and (d) the air-clad region.


133

C) The third generation

The third generation was developed using only Suprasil® F300 glass as a raw material. Capillaries

are drawn from the tube that was prepared by slightly etching, using hydrofluoric acid (<5µm), and

then fire-polishing in the furnace by purging with argon. The jacket tube was prepared in the same

fashion. The reasons why the preparation was modified are explained below. When gas phase

etching is applied using SF6, a thin fluorine doped silica layer is formed, as is found in many

refractive index profiles of MCVD preforms (see Fig.7.2.1). This low index fluorine doped silica

layer remains during the fabrication process, and may help trap the cladding modes when such a

tube is used as a jacket.

By using a wet etching process, the formation of this fluorine layer can be prevented. In addition,

the OH ions in the final fibre can also be reduced as described in Chapter 3. The only drawback of

this method is that it is difficult to judge the completion of the fire-polishing of the etched tube as

the furnace is enclosed. By observing the scattered light when capillaries are drawn, the

temperature was adjusted so as to reduce the amount of scattered light.

In addition, in order to reduce the orientation dependence of the bend losses, as described in the

previous section, the boundary between the cladding and the jacket was hexagonally shaped by

inserting rods in the vicinity of the jacket tube. By using sealed capillaries with dc/Λc~0.4, high

temperatures were used to collapse the air holes in order to obtain d/Λ=~0.3. However, the collapse

of the air holes begins at the outer-most ring as the temperature is higher here due to the presence

of the temperature gradient within the preform, as discussed in Chapter 2. This degrades the

transverse uniformity of the structure, leading to the partial collapse of the air holes near the jacket

tube due to the lower viscosity. Furthermore, the drawing becomes somewhat unstable due to the

reduced tension at high temperatures. These two factors limited the highest temperature that could

be used. The examples of the draw parameters and the fabricated fibres (LMA_0200) are shown in

Table.6.2.4 and Fig.6.2.4, respectively.

At low temperatures, interstitial air holes appeared within the jacket region, where the glass rods

are stacked in order to shape the cladding.

Thus, the useful temperature range, where no interstitials appear and the partial collapse is achieved,

is at most within ±10°C in this example (d/Λ=0.25~0.35). The temperature range is generally

narrower when the difference in dimensions between the capillary and the jacket is larger (i.e.

when a substantial number of silica rods were inserted to shape the cladding boundary, which

enlarges the absolute difference in dimensions between the jacket and the capillaries). Such a

preform contains a greater difference in the collapse ratio in comparison with LMAHF-0100,


134

making interstitials appear more easily at low temperatures. In addition, the effect of the thermal

gradient across the preform is more pronounced at high temperatures.

Table. 6.2.4 The preform and the fibre draw parameters used for LMAHF-0200.

Draw prameters:


2.605 15 ∆0~60

Preform dimensions: Capillaries (sealed) Jacket

I.D. [mm] O.D. [mm] I.D. [mm] O.D. [mm]

~0.4 ~0.94 ~16 ~20


O.D. [µm] Λ [µm] d/Λ

230~250 11~13 0.25~0.5

Fig. 6.2.4 SEM photographs of LMAHF-0200 (Λ=11.3µm, d/Λ=0.35).

The optical properties of these fibres are currently under investigation.

6.3. Effective mode area

Here, experimental measurements of the effective mode areas of LMA-MOFs are presented.

Initially, fibre nonlinearity measurements were used to characterise the effective mode area.

However, the measurement technique has been subsequently shifted to mode field diameter (MFD)

measurement using a knife-edge technique, as is commonly used for standard telecommunication

fibres. These two different measurement techniques are compared and the measured effective mode

areas are discussed with respect to their structural parameters.

(a) (b)


135

6.3.1. Measurement via nonlinearity

The effective mode areas of the fibres drawn from LMA-0001 were characterised by measuring the

four wave mixing (FWM) of intense two colour pulses propagating through the fibres[272]. The

experimental set-up is shown in Fig.6.3.1.

Fig. 6.3.1 Experimental setup for the nonlinearity (effective mode area) measurement. DFB: distributed feed-back laser, EESL: grating stabilised edge-emitting-semiconductor laser, EDFAs: erbium doped amplifier modules, AOM: acousto-optic modulator, FI: Faraday isolator, LM-EDFA: Large mode area EDFA, P.M.: power meter, and OSA: optical spectrum analyser.

A master oscillator power amplifier (MOPA) configuration was used. The CW semiconductor laser

outputs were combined and then gated by using an electric-optical modulator (EOM) to generate

5ns pulses, which were then seeded into an 3-stage erbium doped fibre amplifier (EDFA)

chain[262]. At the output we obtained micro-joule level pulses with a variable repetition rate, and

thus a variable pulse energy. An acousto-optic modulator (AOM) was inserted prior to the LMA-

EDFA to suppress the amplified spontaneous emission (ASE).

By launching the two colour pulses into a sample fibre, one could observe the generation of side

bands via FWM. The ratio between the main peak Imain and side-band peak Iside is related to the

nonlinear phase shift φ of the pulses by the relation

� � � ��

��

��

��

��

� � ��

�� , (6.1)

where Jn is the nth Bessel function of the first kind. Thus, using this equation, the launched power

versus the nonlinear phase shift can be obtained. The nonlinear phase shift is written as follows.

LMA-EDFA

EESL @λ2

EOM (∆t :5nsec)

Ti:sapphire @976nm

AOM (Rep.R.:1~100kHz) FI

DFB @λ1

OSA

P.M. LMA-MOF

[email protected]µm

λ/2 λ/2

λ/4 HT@980nm

ω ω

Imain

Iside

EDFAs


136 � �� , (6.2)

where P is the launched pump power and

��

is the effective mode area of the fibre, respectively.

λ, α and L are the pump wavelength, absorption coefficient, and the length of the fibre,

respectively. Therefore, by assuming the nonlinear coefficient (n2~2.16x10-20 [m2/W]) for pure

silica[45], the effective mode area of the sample fibre can be estimated[272].

The accuracy of this measurement technique is limited by the inherent low nonlinearity of the

LMA-MOFs, which was comparable to the background nonlinear phase shift accumulated by the

propagation of the incident pulses through the amplifier chain. Though the nonlinear phase shift

within the LMA-MOFs can be increased by reducing the repetition rate (and thus increasing the

pulse energy), this is limited by the distortion in the pulse shape due to the saturation of the

amplifier. In addition, the background phase shift is also increased, leading to high order side band

generation.

The measurement was performed so that the dynamic range was as large as possible for a given

length of the fibre (~3m), whilst the pulse shape was retained. However, the bend loss present in

some of the fibres with the largest

��

prevented us from accurately characterising them.

Nevertheless,

��

as large as 436µm2 could be measured using this approach for fibres drawn

from LMA-0001. A typical plot of the nonlinear phase versus internal peak power is shown in

Fig.6.3.2. The measurement error is mainly due to the power fluctuation of the Ti:sapphire laser,

which pumped the final amplifier. The estimated

��

for these fibres with a range of different

dimensions are summarised in Table.6.3.1.

Fig. 6.3.2 Measured nonlinear phase shift with respect to the launched peak power of LMA-

MOF. The data was taken at the repetition rate of 15kHz and the calculated

��

is 436µm2.

� � !! � "! � #! � $! � "

� ! � � " � � % � � # � � & � �

' ( ) *+ , - . */ 0 (1324 5 6 7 4 83936 5 6 6 4 : ; < =?> 2@6 5 A < 8 B ;132C6 5 A ; A 7 A3936 5 6 6 4 : B B 4 =D> 2C6 5 A B 4 7 ;132C6 5 A B 6 8 4E936 5 6 6 4 : : < : =D> 2C6 5 A A 7 ; F

GH IJKL MINPOQ

RTSVU@W XY Z\[


137

Note that because of the irregularities present in the transverse structure of the fibre, it is difficult to

explicitly quantify d and Λ for these fibres. The limit of the measurement can clearly be observed

from the fibres E and J, where the core area A is more than ~450µm2. ��

values are far smaller

than those of D and I, which have smaller core areas than E and J, respectively. Although the

author set the fibre such that the bend curvature was very loose (>50cm) and almost homogeneous

along the length, these results indicate that the bend losses were present during the measurement

for these fibres, leading to underestimates of ��

(see Eq.(6.2)).

The results obtained from the other fibres qualitatively follow the anticipated tendency that the

value of ��

increases with increased Λ or decreased d/Λ. For instance, the fibres C (Λ=11.36µm,

d/Λ=0.189) and D (Λ=12.57µm, d/Λ=0.187) have a similar d/Λ, and the measured values of ��

are 268µm and 311µm, respectively. By comparing the fibres C and I (Λ=11.01µm, d/Λ=0.105),

the measured ��

values are 268 and 294µm2, respectively. This confirms that the use of small

d/Λ drastically increases the effective mode area. These examples imply that ��

is either a strong

function of d/Λ or Λ, or both, depending on these parameters.

Table. 6.3.1 Summary of the fibre characterisation. O.D.: outer diameter, Λ: hole spacing (average), d: hole diameter (average), A: calculated geometric core area from the SEM photographs, Aeff

N: effective mode area via nonlinearity measurement, and Aeff

M: effective mode area via MFD measurement.

O.D. Λ D d/Λ A AeffN Aeff

M

[µm] [µm] [µm] [µm2] [µm2] [µm2]

A H2 108 8.67 1.786 0.206 203 123 126 B H6 145 10.38 2.19 0.211 282 190 195 C 163 11.36 2.147 0.189 336 268 - D H7 173 12.57 2.357 0.187 401 311 300 E 198 14.71 2.088 0.142 577 220 - F 193 14.59 1.967 0.135 587 - - G H4 170 14.09 2.237 0.159 533 282 378 H 158 11.46 1.546 0.135 353 243 - I H5 168 11.01 1.159 0.105 350 294 305 J H3 168 12.58 1.334 0.106 447 232 - K 188 13.76 1.391 0.101 508 436 - L H1 195 12.97 1.246 0.096 536 - 680

Indeed, our theoretical model has revealed such characteristics[45]: when d/Λ is >~0.12, Aeff is

relatively insensitive to d/Λ, whilst it becomes a strong function of both Λ and d/Λ when d/Λ

becomes smaller, as shown in Fig.6.3.3. This, in turn, suggests that it is difficult to precisely

control Aeff in the region of d/Λ <~0.1 in practice, because of the increased sensitivity to both d and

Λ. Note that as long as collapse is utilised, Λ changes with d. However, d is more directly affected


138

by the fluctuation of the collapse than Λ. Hence, it is expected that the control over Aeff can be

improved by increasing the value of d/Λ.

Fig. 6.3.3 Predicted effective mode area as a function of structural parameters d and Λ at 1.55µm wavelength. Contour level unit is µm2. Labels H1-H6 corresponds to the measured values for some LMAHF-0001 with different dimensions. (Courtesy J.C.Baggett)

In fibres drawn from LMAHF-0001, the preform parameter dc/Λc~0.5 was used and the control

over the fibre structure heavily relied on the collapse of the air holes during the fibre drawing

process. Thus, d and Λ could not be controlled independently and are also sensitive to the draw

temperature. Note that the temperature range used for these fibres was only within ±5°C. If the

degree of the collapse was smaller, the sensitivity of Λ to the draw temperatures would be reduced

and thus the change in Λ would be negligible compared with that of d. Therefore, the preform

parameter dc/Λc should ideally be similar to d/Λ in the final fibre.

6.3.2. Measurement via mode field diameter (MFD)

The mode field diameter of the fibre can be characterised by measuring the divergence of the

output beam versus the distance from the fibre end assuming Gaussian beam propagation in free

space. A Gaussian beam, which has a beam radius of w0 and radius of curvature �� at z=0,

expands as follows. �� , (6.3)

where �� . By differentiating by z, we obtain

� � ��

�!! "

#$%& (z>>zR). (6.4)


139

Thus, by measuring dw/dz, w0 is obtained, from which the effective mode area and the mode field

diameter are immediately calculated using ��

� � � � and MFD=2w0, respectively.

Fig. 6.3.4 Experimental setup for the MFD measurement.

The experimental setup is shown in Fig.6.3.4. By detecting the beam through a beam chopper

blade, which rotates with a well defined frequency, the signal rise follows the integrated Gaussian

function, in which the half width positions correspond to a height of 16 to 84%. Here, we assume

that the chopper diameter is sufficiently large, compared to the beam diameter. Therefore, by

measuring the rise time of the leading edge of the signal at different distances from the fibre end, it

is possible to measure FWHM of the beam diameter with the known blade speed. The advantages

that this method includes are as follows.

Its adaptability to any light sources as long as their linewidth is narrow enough.

The fibre length is not a severe parameter as long as the cladding modes are stripped off.

The measurement is independent of bend losses.

Disadvantages can be the fact that a relatively small beam divergence is required to ensure

adequate beam capture by the lens behind the chopper blade, and that accurate measurement of

both the distance in the z direction and the small beam diameter with respect to the chopper blade is

required. An additional approximation results from the non-circular modal shapes in MOFs.

Furthermore, Gaussian beam propagation of the modes is assumed. However, the error caused by

these facts is theoretically estimated to be 10% at most for LMA-MOFs[44]. Therefore, it can be

concluded that this technique is particularly suitable for the characterisation of LMA fibres.

For some of the LMAHF-0001 fibres and conventional LMA fibres, we examined the wavelength

dependence of the ��

measurement as shown in Fig.6.3.5 using various lasers of different

2w

84%

16% Power transmission through the chopper

Fibre Time

Chopper blade

Light source

Detector Collecting lens


140

wavelengths. For comparison, MOFs, which had similar characteristics at 1550nm to the

conventional LMA, were chosen. Note that �

� � ��

as large as 680µm2 was measured at 1550nm.

Fig. 6.3.5 Wavelength dependence of the effective mode area measured by MFD measurement. H1-4: LMA-MOF (see Table.6.3.1), S1: conventional LMA (NA=0.06,

a=9µm, �

� � ��

=405µm2 at 1550nm), and S2: SMF (NA=0.11, a=4µm, �

� � ��

=126µm2 at

1550nm). (courtesy J.C.Baggett)

The conventional fibres are limited below 1.3µm due to the presence of the modal cut-off. Several

tens of modes are present in the visible wavelengths in these fibres, making it difficult even to

selectively excite the fundamental mode. Although the set of fibres LMA-0001 contain an index

difference between the core and the cladding materials and thus display cut-off, it was confirmed to

be only few moded at 488nm. Thus, it was still possible to measure the effective mode areas at

short wavelengths in the LMA-MOFs.

By referring to Table.6.3.1, the following trends can be understood. If d/Λ is very small, as it is for

H1, �

� � ��

drastically increases at longer wavelengths, while the fibres with d/Λ ~0.2 give a

relatively flattened wavelength dependence of �

� � ��

over a wide spectral range[44]. This is because

the modal penetration into the cladding at longer wavelengths can be suppressed with these

structural parameters. Generally, smaller d/Λ also allows for deeper modal penetration into the

cladding. On the other hand, when d/Λ is relatively larger, modal confinement at shorter

wavelengths is significant. Thus, there is a certain value for which both of these characteristics are

less pronounced. Note also that the strong wavelength dependent confinement of light is

particularly significant for smaller Λ. Therefore, Aeff can be flattened over a wide range of the

spectrum by controlling the value of d/Λ for a given core diameter (or Λ), especially for relatively

large core fibres. This may be useful for maximising nonlinear optical parametric processes within


141

the fibre[273], where two or more wavelength components interact with each other, and

upconversion type lasers, where the pump and signal wavelengths are significantly different.

6.4. Bend losses

Bend losses are an important factor in LMA-MOFs as they practically limit the largest mode areas

that can be used. The bend losses are customarily categorised into two components: micro-bend

loss and macro-bend losses, depending on the physical scale of the bend, with respect to the core

diameter. The former is primarily related to the issues arising from cabling and installation, where

the propagating modes within the fibre continuously suffer from modal distortion, whereas the

latter results from the bends that are significantly greater in scale than the wavelength of the light

guided by the fibre and represents the factors that limit the practical handling.

Macro-bend losses can be categorised into two components: transition loss and pure bend loss[155].

The former is induced when the mode enters or exits a bend due to the abrupt change in curvature,

which leads to a change in modal shape. The latter happens continuously as the mode propagates

along the bend, and this is known to be the main contributor to macro bend losses for practical

situations[270]. This mechanism can alternatively be explained as follows. On a straight fibre, the

modal field at every point in the cross-section propagates parallel to the fibre axis with a constant

phase velocity, thus the phase front is orthogonal to the propagation axis. Under the presence of the

bend, the phase front has to rotate about the centre of curvature of the bend. This means that the

phase velocity parallel to the fibre axis increases with distance from the centre of curvature. Once

the phase velocity matches that of the cladding modes, a large loss occurs due to the coupling to the

cladding and radiation modes.

In Ref.[14], it has been predicted that the bend losses in MOFs are qualitatively different from the

conventional step index fibres, as they exhibit a short wavelength edge in the bend loss spectrum.

In conventional fibres, the bend loss is dominant at long wavelengths due to the fact that the

refractive index difference or NA of the core becomes small. This leads to low V values, in which

the guided mode is prone to be affected by environmental perturbations. At the same time, the

transverse wave vector components of the guided mode become small, allowing for deep modal

penetration into the cladding. This increases the modal overlap with the radiation modes. As a

result, the bend loss is also increased due to the enhanced coupling.

However, in single material MOFs, the NA decreases with the wavelength. This fact leads to an

additional bend loss edge at short wavelengths, whilst the bend loss edge due to the deep modal

penetration into the cladding at the long wavelengths also exists. The bend losses at short

wavelengths in MOFs can also be understood by considering the hole spacing Λ with respect to the


142

mode field diameter MFD. If MFD<<Λ, the mode can leak out from the core through the silica

bridges in the holey cladding. Note that the MFD is small at short wavelengths despite the weak

guidance since it is determined by the transverse wave vector components of the mode, which is

inversely proportional to the NA in endlessly single mode MOF since the V value is constant at the

short wavelength limit. Indeed, one physical interpretation to the large transverse wave vector

components is that the light on the short wavelength edge can ‘resolve’ the holey cladding and thus

escape from the core.

Although quantitative predictions for the bend loss calculation have been controversial even for

conventional fibres, at least qualitative agreements have been obtained by several authors[274,275].

By applying one of the theoretical models to the MOFs, the presence of both the short and the long

wavelength bend loss edges has been predicted, indicating that the bend loss minimum is located

around ~Λ/2[276]. Given the Λ scale in LMA-MOFs (~10µm), the transmission window of the

silica fibres lies on the short wavelength edge. Thus, it appears that the bend loss characteristics of

MOFs are likely to be worse than conventional fibres.

However, the refractive index profiles of the depressed-cladding fibres (or W-fibres)[277] have

been engineered such that desired dispersion characteristics at a certain wavelength range are

obtained while retaining low bend loss. Therefore, it is anticipated that LMA-MOFs can also be

engineered to obtain a large Aeff while the bend loss is reduced. To start such engineering, it is

important to quantify how the bend losses vary with the structure. Below, the bend loss of LMA-

MOFs are first characterised at a fixed wavelength (1550nm) and then compared with their

conventional counterparts. Then, the measurement of the bend loss spectra for different types of

LMA-MOFs is presented.

6.4.1. Bend losses at 1550nm

The bend losses were measured for one loop of the fibre on a flat bench, while maintaining as small

a fibre tension as possible. The fibre loop was positioned using pins on a well defined circle that

determined the bend radius. The bend loss characteristics of a selection of LMA-0001 fibres and

the conventional fibres are shown in Fig.6.4.1. The measurement was performed at 1550nm. The

bend losses increase with the mode area, as expected, and it is understood that LMA-MOFs (H2: �

� � ��

~126µm2, H4: �

� � ��

~378µm2) possess almost comparable bend loss characteristics to their

conventional counterparts (S2: �

� � ��

~126µm2, S1: �

� � ��

~405µm2) with the similar values of Aeff.


143

Fig. 6.4.1 Normalised transmission intensity versus the bend radius. (Courtesy J.C.Baggett)

The curve may be not smooth since there is a portion of power that can be coupled back from the

cladding or the radiation modes. The coupling can be rather strong for some angular orientations

due to partial reflections at the air-silica interface. For this reason, the bend losses of LMAHF-0001

(see Fig.6.2.1) displayed clear orientation dependence. However, this effect can, in turn,

immediately be used for improving the bend losses of MOFs. Hoping to confirm the improvement

of the bend loss characteristics, LMAHF-0102 was fabricated as shown in Fig.6.4.2. However, it

was found to be difficult to strip off the cladding modes in this fibre, despite the deliberately thick

bridges in the outer cladding. Note that there is another index difference between the holey

cladding and the jacket of LMAHF-0001 although this is typically small (NA~0.04) since the

fabrication of this fibre was based upon the second generation, as described in 6.2.2.

The orientation dependent variations of the bend losses were also observed for LMAHF-0100 (see

Fig.6.2.3). Furthermore, it has been found that there is a correlation between the extent of the

cladding (the distance between the core and the outer most air hole) and the bend losses[270]. This

suggests that it is important to increase the large cladding dimensions further.

Fig. 6.4.2 The SEM photograph of LMAHF-0102.


144

Fig.6.4.3 shows the relation between �� and the critical bend radii Rcr, which the author here

defines as the radius at which the power reduces by half the initial power observed from a straight

fibre, for different types of LMA-MOFs. Although the critical bend radii are almost the same for

all the fibres with relatively small �� , LMAHF-0100 exhibits a sharp increase with ��

whilst

those of LMAHF-0001 and the conventional fibres increase almost linearly. These different trends

imply the difference in the bend loss mechanisms involved in different fibres. Recalling that

LMAHF-0001 is a hybrid fibre, in which there is a small index difference between the core and

cladding (∆n~2x10-4), the bend losses at this wavelength may be dominated by the long wavelength

edge, resulting in the similar trend to the conventional fibres. On the other hand, LMAHF-0100 is a

single material fibre that is likely to suffer from the short wavelength bend loss edge. Thus, it is

understood that the small index difference between the core and the cladding helps to prevent the

onset of the short wavelength edge at this wavelength.

��

�� !" ##$

%&�%'�%(�%)�%*�%�%

+�,-/.10�2 354�3�3+�,-/.10�2 363�3 471869�:�;69�<�= 869�> ?

Fig. 6.4.3 Relation between the Aeff and Rcr for different fibres (at 1550nm).

6.4.2. Wavelength dependence of the bend losses

The measurement was performed using a white light source, where the dynamic range of the

measurement varies depending on the wavelength. Because of the lack of source power at short

wavelengths, the obtainable intensity contrast dropped (almost exponentially) from ~20dB at

800nm to just 2dB at 400nm. The bend was applied by using several bobbins with different

diameters. The author only discusses qualitative characteristics, which are always observable for

any orientation of the fibre, by neglecting the orientation dependence.

Examples of the bend loss spectra for LMAHF-0001 are shown in Fig.6.4.4. The peaks at 700nm

(a) and at 550nm (b) correspond to the cut-off of the higher order modes. This demonstrates that


145

these fibres are no longer endlessly single moded due to the small index difference between the

core and the cladding although the dimensions of the small air holes within the structures are small.

Notice that the tolerable bend diameters are very different for (a) ~3.3cm and (b) ~15cm due to

their different mode areas. For a relatively small core fibre (H2, �� ~126µm2), significant losses

(>10dB) occurred when the bend radius was 1.65cm and the losses suddenly increased toward

shorter wavelengths. The relative transmission loss around 1550nm was less than 2dB, clearly

indicating the onset of the short wavelength bend loss edge. The many peaks between 800 and

1200nm were possibly induced by the irregular cladding structure since the bend losses are

sensitively dependent on the positions and the dimensions of the air holes. In fact, these peaks

showed orientation dependence.

Fig. 6.4.4 Bend loss spectrum of LMA-0001 for the different bend diameters (units in cm). (a) H2, and (b) Η5. The peak losses between 500~800nm occur due to the leakage of the higher order modes.

On the other hand, for relatively large core fibres (H5, �� ~305µm2) in Fig.6.4.4 (b), the long

wavelength spectral components clearly show larger losses even without a tight bend. The spectral

interference in the bend loss spectrum is also similar to that observed in conventional fibres[275].

The short wavelength bend loss edge is completely negligible in this case. This can be understood

by considering the relative contribution to the NA of this hybrid fibre, as described below.

The NA between F300 and HLQ-210 is ~0.02, which may be regarded as approximately constant

over the wavelength range of interest. By assuming Λ to be the core diameter (the F300 region),

NA~0.08 is calculated from the cut-off wavelength of ~900nm for H2, and is a factor of four higher

than that of the NA due to the material index difference. This implies that the dominant guiding

mechanism is provided by the air holes. On the other hand, for H5, the NA is calculated to be

~0.047 at 700nm. Thus, the NA created by the air holes should be comparable to the NA due to the

material index difference. Note that the relative contribution of the air holes to the total NA can be

increased either by reducing Λ or increasing d/Λ.

��

! "##$% &'

��

�� (*) ��+ ,(*)�-�. - + ,(*) � . / + ,(*)�/ + ,(*) � . � - + ,

�� 0� �� 1��2��2� ��2��

! "##$% &'

� . �� . -� . �� . -� . �� . -/�. � (*) ��+ ,(*) ��+ ,

(a) (b)


146

The same argument can be made by considering the V value. The fundamental mode cut-off of the

step index waveguide is V~0.84[278]. Above this value, the modes are bound to the core defined

by the index difference between F300 and HLQ-210. In this case, the role of the air holes is

supplementary, slightly increasing the effective V value[126,279]. Since the V value is proportional

to the core diameter, there is a limit as to how much the core can be enlarged before the difference

in material index will dominate the guidance provided by the air holes.

��

� ��

�

�

�

!

�

"

�

#%$&!��#%$��

#%$'"��

Fig. 6.4.5 Bend loss spectra of LMAHF-0100 (Λ=7.6µm, d/Λ=0.24).

When LMAHF-0100 was characterised, neither the traces of the long wavelength bend loss edge

nor the cut-off wavelengths were observed for any available fibre dimensions. Fig.6.4.5 shows the

bend loss spectra for LMAHF-0100 (Λ=7.6µm, d/Λ=0.24) for different bend radii. Note that the

water absorption hindered the bend spectra due to the limited dynamic range. Comparing the fibres

with similar structural dimensions (LMAHF-0001:H2 and LMAHF-0100), it can be seen that the

bend characteristics are much better for H2. This is because the total NA in H2 is greater at all

wavelengths than in LMAHF-0100 because of the additional contribution from the index contrast

between F300 and HLQ-210. Although the long wavelength edge due to the index contrast might

be expected to be present in H2, it is compensated by the strong contribution of air holes to the NA

at long wavelengths. Note that the introduction of the slight material index difference effectively

shifts the short wavelength bend loss edge towards shorter wavelengths. However, this

improvement is achieved only by narrowing the single mode wavelength range.

6.4.3. Discussion

There are many combinations of the structural parameters d and Λ for MOFs that can result in the

same Aeff at a given wavelength, as shown in Fig.6.3.3. However, it is not obvious what structural

parameters give the best bend loss characteristics in general, because Aeff is also a strong function of

wavelength.


147

By considering the index of the cladding modes, the following understanding can be obtained.

Because the separation of the propagation constants between the core and the cladding modes is

relatively greater in MOFs with larger d/Λ, a tighter bend radius is required to reduce the modal

effective index so that it coincides with that of the cladding modes. Thus, such fibres should exhibit

more robust transmission for a given bend radius. This indicates that it can be advantageous to use

the fibres with larger d/Λ if Aeff is the same at the fixed wavelength. However, at the same time, the

waveguide tends to be multi-moded for large values of d/Λ. This suggests that a compromise exists

between single mode guidance and the bend loss, as in the conventional fibres. In the case of MOFs,

it is desirable to use d/Λ as large as possible but still retain single mode guidance.

There have been a lot of arguments about to how to rigidly define the endlessly single mode regime

for MOFs and, for Λ=2.3µm, it is now commonly accepted that the endlessly single mode feature

can be obtained with d/Λ<~0.4. However, few studies have been published in the large mode

regime. Therefore, both experimental and theoretical verifications are needed as to what extent d/Λ

can be increased whilst retaining broadband single mode guidance.

As predicted, the short wavelength bend loss edge can play a substantial role in single material

LMA-MOFs, and the net bend losses are likely to be worse than the conventional counterparts

particularly when the effective mode area is large and at short wavelengths for these particular

structures (d/Λ<0.25). One possible improvement can be to increase d/Λ further to the upper limit

of the single mode operation. A further study with the different structures is required: it is likely

that as in conventional fibres, the bend loss could significantly be reduced by modifying the

cladding structure. Moreover, MOFs offer increased flexibility for doing this, as demonstrated in

LMAHF-0102

In the hybrid LMA-MOFs studied here, the bend losses are comparable to those of conventional

fibres at 1550nm. This can be understood by observing the onset of the long wavelength edge (due

to the material index) in hybrid LMA-MOFs with relatively large structural dimensions, where the

dimensions of the slightly high index material can form a well defined core that primarily

determines the modal characteristics. Therefore, there are two factors in the hybrid LMA-MOFs as

summarised below.

�

The short wavelength bend loss edge is shifted towards shorter wavelengths than the single

material fibres, and the amount of this shift is increased by using higher index material within

the core. As a result, the bend losses at the wavelengths within the transparency window of

silica are reduced. However, this limits the usable single mode wavelength range due to the

onset of the modal cut-off.


148

� The introduction of the high index material can form the core by itself when Λ is large, leading

to the onset of the long wavelength edge, which is absent in single material MOFs.

An example of this fibre type is further studied in Chapter.7. However, a more systematic study is

necessary to fully extract the potential of this hybrid LMA-MOFs.

6.5. Transmission losses

The transmission losses of LMA-MOF s have been paid very little attention so far, primarily

because it is envisaged that the majority of applications will use relatively short length (i.e. <50m)

of these fibres. However, in order to use them at extremely high power levels, the transparency of

LMA-MOFs becomes also an important issue, since any absorption due to localised impurities may

lead to an increase in temperature or provide bridges for multi photon absorption process, resulting

in optical breakdown[280]. Furthermore, non-absorptive losses mean that the radiation leaks out

from the fibre core, and that the coating can be damaged. Here, we present the cut-back

measurement performed using LMAHF-0200 made of Suprasil® F300 only. Note that using HLQ-

210 (low quality silica), it is hard to achieve losses below 10dB/km.

The measurement was taken again using a white-light source and the fibres under test were wound

on a 50cm diameter bobbin. Due to the limited length of the fibres (<100m), the minimum losses

that can be accurately measured is ~10dB/km, which corresponds to an absolute intensity

difference of <1dB. To increase the accuracy, the averages were taken by repeating the

measurement 5~10 times. The averaged losses for the different structural dimensions are shown in

Fig.6.5.1.

The sudden increase of losses towards short wavelengths is due to the unique bend losses of single

material MOFs, and which is clearly dependent on d/Λ as Λ is almost the same within all the fibre

samples (~11.5µm). For the fibre with d/Λ>0.4, the loss minima are located around 1600nm and

the values are between 1~10dB/km. Although the detailed loss curves are substantially different for

the different samples, the water absorption peaks at 1.38µm and 1.25µm coincide with each other

as long as the background loss is small enough. This indicates negligible offsets between the

different measurements (fibres) since the same preform was used to fabricate each fibre.

Recall from Chapter 3 that significant scattering losses result from the modal overlap with the air

holes in the small core fibres. Hence, it might be expected here that the greatest d/Λ should result in

the lowest losses because the mode should be more tightly confined within the core (MFD<<2Λ).

However, it was found that the smallest loss was obtained from the fibre with d/Λ~0.4. One


149

possible explanation for this observation is that coupling to lossy high order modes occurs due to

the perturbations along the fibre lengths since high order modes can be supported within these

fibres with d/Λ>0.45. This implies that there may be significant perturbation due to structural

inhomogeneities along the fibre length.

��

� �� ! "#$%&

�(' �

�

��

��

��

) * +�,.-./ 021) * +�,.-./ 32-) * + ,.-./ 321) * +�,.-./ 124

Fig. 6.5.1 Transmission losses of LMAHF-0200 fibres with different dimensions.

The structural stability is anticipated to be better when d/Λ is higher. Since the fibres with larger

d/Λ need to be pulled at lower temperatures for given preform dimensions (in this case dc/Λc~0.4),

the tension imposed on these fibres must be even higher. This prevented significant structural

fluctuation of the fibres. However, the use of low temperatures suggests insufficient firepolishing

through the fibre drawing process. Thus, scattering losses may become significant when fibres are

drawn at too low temperatures. Therefore, an optimum temperature is likely to exist to obtain the

low losses for given preform dimensions.

On the other hand, the fibre with d/Λ~0.35 has its loss minimum around 1µm, qualitatively

differing from the others. The significant loss increase at longer wavelengths indicates that

scattering losses still dominate despite the large structural scale due to the deep modal penetration

into the cladding at these wavelengths. In addition, given that the NA is the lowest of the group, it

is more prone to suffer from perturbations due to the inhomogeneities along the fibre length[159].

Note that this fibre was pulled using substantially lower tension than the others. This clearly

indicates that as we decrease d/Λ, the structure must be more homogenous in order to avoid extra

losses.


150

6.6. Conclusions

The fabrication of LMA-MOFs has been described. First, the refractive indices of the potential

materials were measured, and it was shown that there are three combinations in terms of refractive

index of the materials used in these MOFs. The evolution of the fabrication process was then

presented.

The effective mode area was measured using two different methods, and the results were

compared. It was found that the MFD measurement provides a wide enough dynamic range for

LMA-MOFs. The theoretical simulation predicts that the fabrication tolerances can be relaxed by

choosing an appropriate range of structural parameters (d/Λ>0.15). By using the MFD

measurement, it was also found that the wavelength dependence of Aeff can be relatively flattened

with certain values of d/Λ relative to the core diameter.

The bend loss measurement at 1550nm showed that the bend loss characteristics of the hybrid

LMA-MO F are at least comparable with their conventional counterparts with similar Aeff, while the

single material LMA-MOF s display slightly worse values, particularly when Aeff>200µm2 and at

short wavelengths. However, this may be improved by choosing an appropriate combination of Λ

and d/Λ to obtain the same Aeff values.

By observing the bend loss spectra, it was found that in the case of hybrid fibres, depending on the

relative contribution of the air holes to the total NA, either the long wavelength or short wavelength

bend loss edge can become significant. In the former case, the modes are bound by the core defined

by the index difference between the core and the cladding materials, leading to the similar bend

loss mechanism to the conventional fibres. In the latter case, the dominant effects of air holes result

in expected spectral characteristics due to the fact that the mode can ‘resolve’ the structures.

It was found that using the hybrid MOFs the bend loss characteristics can be improved due to the

increased index contrast at any wavelength. However, this is accompanied by the narrowed single

mode wavelength range.

The transmission losses of LMA-MOF s with different geometries were characterised. The

minimum losses <10dB/km were achieved for the fibres with d/Λ=0.4~0.5. The irregular shape of

the transmission spectra implies that there are substantial amounts of radiation losses caused by the

structural fluctuations along the lengths of these fibres. This suggests that it is important to choose

the appropriate preform parameters, in order to obtain good structural stability.


151

This early study of LMA-MOFs described in this chapter has shown interesting opportunities

offered by LMA-MOFs, but at the same time revealed that significantly challenging issues remain

to be overcome if LMA-MOFs are to be considered to be competitive to the conventional

counterparts in the near infrared wavelength range. However, the author believes that the inherently

broad single mode wavelength range obtainable from LMA-MOFs and future improvements of the

designs (i.e. by arranging appropriate sizes of air holes) will lead to practical LMA-MOFs with a

significantly broader single mode range and less bend sensitivity, compared with the conventional

LMA fibres.

Chapter.7

An ytterbium-doped all-glass double-clad

large mode area microstructured optical fibre

7.1. Introduction

This chapter describes the development of a novel type of cladding pumped fibre which represents

one of the first microstructured active large mode area fibres. The basic optical properties of large

mode area microstructured optical fibres (LMA-MOFs) have been presented in the previous

chapter. Therefore, the motivation of this chapter is to demonstrate a specific device application

and to explore the possibility of high performance lasers based on LMA-MOFs.

During the last few years a vast amount of research has been carried out towards the development

of cladding pumped fibre lasers (CPFLs). This work has focussed essentially on all aspects of

CPFL operation and has included research into novel fibre geometries, pumping schemes, and laser

configurations. In particular, the development of new forms of fibre has provided novel

opportunities to overcome/improve critical issues related to device implementation.

For example, it is has been shown that by shaping the cladding geometry it is possible to improve

the pump power absorption[281], and that robust single polarisation operation of CPFLs can be

achieved in Highly-Birefringent (Hi-Bi) double clad fibre[282]. More recently, beam combining of

a fibre laser array via spectral multiplexing[283] and coherent beam combining, via spatial

multiplexing[284,285] or by means of waveguide structures[216,286-288], have been proposed,

and some of these have been demonstrated to provide efficient power scaling of CPFLs. The

coherent approach requires phase synchronisation between the individual cores and such schemes

are likely to be more readily implemented in novel forms of CPFLs than systems based upon

Chapter.7 A ytterbium-doped all-glass double-clad large mode area microstructured fibre 153

VCSELs[289] or microchip lasers[290] owing to the ease with which evanescent coupling can be

achieved in optical fibres,

The development of conventional LM A fibres has opened up new opportunities for pulsed fibre

laser and amplifier devices due to their reduced nonlinearity, improved power handling and better

energy storage capabilities[262,265]. MOF technology has been proposed as an attractive

alternative to the conventional LMA fibres for such applications ever since the first LMA-MOF

was demonstrated[7]. Whilst LMA-MOFs can indeed exhibit large mode areas and unique

properties relative to conventional LMA fibres, their merits for use in CPFLs have yet to be proven

and further detailed study both of the basic properties and fabrication of MOFs are required.

Critical questions that need to be addressed from a basic waveguide perspective, include firstly

whether the bend loss properties (versus mode area) of LMA-MOF s are better than their

conventional counterparts, and secondly whether the endlessly single mode guidance property can

be used to good effect. From the bend loss perspective, it was shown in Chapter 6 that the

incorporation of a region of raised index within the core, which may be the region doped with

active ions, can greatly modify the bend-loss of an LMA-MOF. When the area of the high index

region is too large, the bend loss spectrum is defined by the high index region, and is thus at least

comparable.

The latter question about the endlessly single mode guidance property obviously indicates that

LMA-MOFs are suited to core pumped laser configurations in which the pump or signal beams are

widely spaced in wavelength (as required for example within upconversion type

lasers[228,229,291], or for long wavelength operation within thulium doped silica fibres[292]).

Although the development of core pumped LMA-MOFs is an interesting topic, and achievable

using appropriate glass materials, it is important to exploit the high pump powers available from

low brightness laser diodes using cladding pumped structures in order to fully take advantage of the

potential of the large mode area as an active gain medium.

From a fabrication perspective, the flexibility offered by the capillary stacking techniques provides

many advantages in terms of defining the fibre structure. Interesting opportunities here include the

possibilit y to fabricate dual-clad LMA-MOFs using air holes of different sizes to define the inner

and outer claddings, and the option to strategically dope the transverse structure of the fibre to

provide preferential gain to the fundamental mode of a multimode structure as discussed in Chapter

6.

However, it is clearly more challenging to achieve good control over refractive index profiles of the

doped section in MOFs relative to the conventional LMA fibres since the index of the doped region

must be sufficiently low and uniform enough. The typical NA between Suprasil® F300 and


ytterbium doped silica used within conventional fibres is of order 0.1 (∆n~0.004) and the

incorporation of a glass with such a relatively high index in the core of a LMA-MOF will prohibit

access to the modal properties unique to MOFs if the core is just replaced by a solid rod. For this

reason, the first reported ytterbium LMA-MOF used hundreds of small sections of doped glass

dispersed across the fibre core to overcome this issue[293]. By distributing the higher index region

of doped glass in this fashion, the presence of a highly localised region of raised index within the

fibre that may affect the MOF guidance is avoided. However, although this approach does help to

reduce the impact of the higher refractive index associated with the doped glass several issues

remain:

�

This fabrication procedure requires a number of additional fibre drawing steps leading to great

scope for degradation in the quality of the original doped MCVD preform in the final fibre

(see Chapter 3). �

The doped area is ultimately limited since the volume average index of the doped core needs

to be kept sufficiently small, and this leads to limited pump absorption and limited energy

storage for pulsed laser operation.

The author emphasises that, in terms of the refractive index of the doped section, the critical issue

is not the detailed form/refractive index distribution of the doped section, but rather the net NA that

results from the combined effects of the refractive index of any doped sections and the holey

cladding. In order to access the full range of parameters possible using the holey cladding the

doped sections must possess the lowest possible NA with respect to the glass used within the rest of

the structure. To ensure this it is thus critical to fabricate an efficient doped preform in which the

refractive index of the doped section is well matched to the glass used for the holey cladding

(practically the difference should as small as possible). Prior to this work, no attempts had been

made to realise an efficient core glass composition for active fibres in this regime.

In order to address a number of the fabrication and device related issues raised above the author

therefore fabricated a dual-clad LMA-MOF in which both the inner and outer cladding were

defined by suitable air hole arrangements, and which incorporated an ultra low-NA ytterbium

doped core that the author developed using the MCVD technique. It is shown that by using the

fabricated ytterbium doped LMA-MOF comparable performance to that of the conventional

ytterbium doped fibres can be obtained. The results obtained from this work hopefully pave the

way to further developments in this interesting research area.

The organisation of this chapter is as follows. First the fabrication of the fibre is described in

Section 7.2, and the fibre characterisation is then presented in Section 7.3. Continuous wave and

pulsed laser characteristics are addressed in Sections 7.4 and 7.5, respectively, the latter of which


includes both Q-switching and mode-locking operations. The issues related to the future

improvement of this technology are discussed in Section 7.6. Finally, the conclusions are given in

Section 7.7.

7.2. Fabrication

7.2.1. Fabrication of the core

A conventional MCVD preform is used here for a number of reasons. First, the optimised solution

doping technique allows one to realise extremely low NA rare-earth doped MCVD preforms with

low back ground loss, from which highly efficient conventional rare-earth doped fibres can be

fabricated. Second, Suprasil ® F300 that is generally used as the substrate tube in MCVD deposition

is the material of choice as a cladding material within MOFs because of its purity and good

structural homogeneity along the length. As a result, the refractive index of the doped section can

be well controlled with respect to that of the holey cladding within the MOFs, and this is essential

to access the full range of the single mode holey fibre structures.

The author fabricated a ytterbium doped aluminosilicate preform, the NA of which can be

controlled by the amount of AlCl 3· 6H2O in the methanol solution if the soot deposition and

consolidation conditions are fixed. Then, the refractive index is linearly proportional to the amount

of AlCl 3· 6H2O[189]. Note that the refractive index contribution in the final doped preform is

dominated by the Al2O3, rather than the rare-earth oxides. Note also that the major role of

aluminium in the rare-earth doped silica glass is to increase the solubilit y of the rare-earth ions by

cancelling out their extra charges within the silica glass matrix[294].

Aluminium can be doped to concentrations of up to ~4mol% in silica without phase-separation,

corresponding to the NA~0.14[189]. The refractive index sensitivity to the amount of AlCl 3· 6H2O

within the solution was reported to be ~2x10-4 100cm3· H2O/g in Ref.[189]. However, recent results

obtained from MCVD lathes at the ORC show a more sensitive dependence (~4x10-4

100cm3· CH3OH/g). This is attributed to the difference in the main deposition burner that allows a

more homogenous soot deposition temperature and that results in a more uniform and porous form

of silica frit, into which both aluminium and the rare-earth ions can be more efficiently

incorporated. Therefore, one can obtain control over the refractive index as precisely as ~3x10-4

from the liquid phase by optimising the process (corresponding to an NA of 0.025~0.03).

Since the aluminium is an index increasing element, its incorporation within low NA preforms

needs to be restricted, and this ultimately limits the concentration of rare-earth ions that can be


incorporated. Nevertheless, the absorption and laser measurements performed on a number of

preforms, fabricated for commercial delivery purposes, showed that it is possible to obtain

~3000ppm (by weight) ytterbium concentration for NA~0.06 without any trace of concentration

quenching[235,295]. In order to obtain higher rare-earth concentrations while retaining such a low

NA an index decreasing element such as boron or fluorine may need to be used as co-dopants. A

further discussion of boron doped silica is given in Section 7.6.

The fibre preform used for the ytterbium doped LMA-MOF had an NA as low as 0.05 at 633nm (as

calculated from the peak refractive index difference shown in Fig.7.2.1). The central dip in the

refractive index profile is deliberately introduced by applying a high consolidation temperature, in

order to suppress the effective NA. Although the author pulled this preform into conventional fibre

forms with 10 and 20µm core diameters, these fibres did not guide light at the wavelength of

interest due to the huge bend losses that resulted from the ultra low NA. Although this is partially

because of the presence of the additional ring structure that enlarges the effective mode area for the

given V values of the cores (1.57 and 3.14 at 1µm wavelength), this observation confirms that the

effective NA is lower than the value estimated as above due to the presence of the central dip.

Therefore, by introducing the core region of this low-NA preform into a MOF preform, it was

anticipated that a ytterbium doped LMA-MOFs can be fabricated, in which the guidance

mechanism predominantly relies upon a holey cladding. The relative contribution of the doped

section (or the holey cladding) to the wave guidance is discussed in Section 7.3.

Fig. 7.2.1 Refractive index profile of HD663. The preform OD was ~12mm. The central region of diameter ~1mm was extracted for use in the MOF.

7.2.2. Fabrication of the fibre

The core was drilled out from the fabricated doped preform using an ultrasonic drilling machine.

The extracted core had a diameter of 3mm. The surface was fire-polished using a flame torch, then

��

� �

� ��

Extracted part (Yb3+~3000ppm)

∆n~8.5x10-4

Substrate (F300)


etched slowly using diluted hydrofluoric acid without heating, to reduce the diameter down to

1mm. Due to the relatively small amount of etching and the slow etching speed (<1µm/min.), the

deviation of the final diameter of the doped core was less than 50µm over a 5cm length, and so it

was possible to insert it directly into the capillary stack despite the tight tolerances in dimensions.

Note that fire-polishing is crucial to obtain an optically smooth surface and thereby to obtain

uniform etching. A starting diameter of 3mm for the extracted core was chosen to make

fire-polishing possible and to ensure that the effect of OH incorporation, due to oxy-hydrogen

flame exposure to the section of doped glass ultimately used within the MOF, was small. A more

recent method for extracting the doped sections (with low OH incorporation) is described in

Section.4.2.

As shown in Fig.6.2.2, it is straightforward to introduce an outer-cladding by inserting layers of

thinner capillaries adjacent to the jacket tube while assembling a preform, in order to form a region

of lower index than the inner holey cladding of the MOF. The advantages of this air hole

arrangement technique include ease of shaping the cladding boundary and the introduction of an

off-set core with respect to the cladding (see Fig.7.2.2). Both techniques are known to enhance the

pump absorption[281].

The fibre fabrication approach applied here corresponds to that of the second generation technique

described in Chapter 5. The preform parameters and the draw parameters are summarised in

Table.7.2.1.

Table. 7.2.1 Preform and the drawing parameters used for Yb-LMAHF-00.

Preform parameters: inner cladding capillaries outer cladding capillaries Jacket

F300 (sealed) Vycor® (sealed) F300

I.D. [mm] O.D. [mm] I.D. [mm] O.D. [mm] I.D. [mm] O.D. [mm]

0.3 1 0.75 1 16 20

Draw parameters and final fibre dimensions:

vf [mm/min.] vd [m/min.] Temperature[°C] d [µm] Λ [µm] d/Λ Fibre O.D. [µm] 1.78 18 ∆20 2.7 9.7 0.278 175

The preform contains 7 rings of air holes (5 rings of inner cladding, and 2 rings of outer cladding).

Note that the fibre diameter is smaller than anticipated, primarily because of the partial collapse of

the outer cladding. Since Vycor® has very low viscosity compared to that of Suprasil® F300, as

discussed in Chapter 5, the outer cladding capillaries were unable to fully counteract the pressure

imposed by the collapse of the jacket. Nevertheless, reasonable degree of isolation between the

inner cladding and the jacket, usable as a cladding pumped fibre, was achieved. A conventional

high-index polymer coating was applied.


The SEM photograph of the fabricated fibre is shown in Fig.7.2.2 (Yb-LMAHF-00). Taking into

account the presence of the air holes in the inner cladding, the area ratio between the doped region

and the cladding was ~0.008, similar to that of conventional cladding pumping fibres. The doped

area is approximately πΛ2/4~73.9µm2.

Fig. 7.2.2 Cross sectional view of Yb-LMA-00.

7.3. Optical properties

7.3.1. Modal characteristics

A) Single mode criterion

Although rigorous numerical analysis[14-31] is in general required to predict the modal properties

of MOFs, the effective index model[4] can conveniently be used to roughly take into account the

effect of the doped core on the total guidance properties of the fibre. The V value of the doped

MOF may be written as

� � �� , (7.1)

where λ is the wavelength, ρ is the core radius (here taken for convenience to be Λ-2d), and ∆n is

the index perturbation introduced by the doped section. The cladding index ncl(λ) is the effective

refractive index of the inner cladding, which can be calculated by several methods[12,13]. Note

that the cross sectional area of the doped section should be included. However, we ignore this

factor for the following reason. The core radius ρ is known to be dependent on both the structure

and the wavelength[20,23] and an appropriate value of ρ at the ytterbium laser wavelength can be

Doped core Inner-cladding

Outer- cladding


obtained by adapting the numerical solutions using the method described in Appendix.A. However,

using e.q.(7.1), the calculation should eventually result in an overestimate for the V value, and thus

can be used with confidence to provide a conservative estimate for the single mode regime.

Fig.7.3.1 shows the wavelength dependence of the V parameter for the relevant fibre types. VHF

corresponds to that of the single material LMA-MOF with Λ=10µm and d/Λ=0.3, and which, as

expected, displays an almost constant value from the near infrared down to visible wavelengths.

Vpreform shows the V value of a step index fibre with ρ=5µm and NA=0.05 (∆n~8.6x10-4, parameters

similar to those of the doped section within the fabricated fibre). The effect of the inverse

proportionality to the wavelength leads to a very narrow wavelength range that can provide useful

single mode operation (~1.5<V<2.405). Finally, VdopedHF is the value calculated from the

approximation in eq.(7.1), from which the cut-off wavelength of Yb-LMAHF-00 is estimated to be

shorter than 900nm. In addition, it can be seen that the VdopedHF lies within a narrow range above the

cut-off wavelength, indicating that the fibre can robustly guide the fundamental mode over a wide

wavelength range. Given the VdopedHF value at the wavelength of interest (2.0~2.3 at 1060nm), the

bend losses are unlikely to be a severe factor.

��

!"#$%& '()* "(+ ,(-. /0

��1 2��1 ��1 2�31 ��31 24�1 �4�1 2�31 ��31 2 5

6�78�9�: ; <= : >8�?= 9�; ? 6�7

Fig. 7.3.1 V values of different fibre types.

By considering the relative contribution to the NA term in eq.(7.1), the following condition can be

used to define the regime where the holey cladding dominates the guidance resulting from the high

index doped core: @ A

BC D C EGFEGDBC D C EGF�HHHH

IJJ KLMN . (7.2)

Therefore, in addition to the single mode condition (V<2.405), the above equation should be

satisfied. In the region beneath the dashed line in Fig.7.3.2, the single mode operation is achieved,

whilst structures that satisfy eq.(7.2) lie under the solid line. Here, Λ=10µm and λ=1.06µm are

assumed. As d/Λ is increased, the possible range of values for ∆n of the doped section increases


due to the strong effect of the holey cladding however the fibre tends towards multi-mode

operation. The fabricated fibre corresponds to the diamond symbol in Fig.7.3.2. Given the fact that

the actual doped section is commonly ~30% of the hexagonal region defined by the inner most air

holes, both curves are underestimated. Therefore, both of these two conditions are well satisfied in

Yb-LMAHF-00.

Fig. 7.3.2 Optimum parameter regime for the doped LMA-MOF (Λ=10µm, λ=1.06µm). The impact of the doped core is relatively small compared to that of the holey cladding for points under the solid line eq.(7.2) whilst robust single mode operation is achieved under the dashed line eq.(7.1).

B) Effective mode area

The effective mode area of Yb-LMAHF-00 was characterised by using the mode field diameter

(MFD) measurement described in Chapter 6. A Nd:YLF laser operating at 1047nm was used.

Fig.7.3.3 shows the experimental plot, from which MFD=12.3µm ( �� =119µm2) was estimated.

The measurement was repeated by rotating the fibre at the output facet and repeating the

measurement >10 times. The error bars correspond to the data range (e.g. maximum and the

minimum values). The large error suggests that the beam is slightly elliptical, as observed in the

following section, although it is to be appreciated that any light associated with excited cladding

modes would also result in an underestimate of the mode-area due to the increased divergence

associated with these modes.

It has been shown that the extent of the cladding impacts the macroscopic bend losses[270].

Therefore, it is fair to consider whether the effective mode area might also be influenced by the

outer cladding structure. Here, we show the measured �� values for LMA-HF-0100 and 0101

(see Fig.6.2.3) with different structural dimensions as given in Table.7.3.1, in order to compare the

effective mode area of undoped LMA-MOFs and undoped double clad LMA-MOFs.

��

� �

�� !�"#!�$ "�� % & '��( )�*�+�� " !�,#)

-�. ,#��!�� $ � ��!


��

� ��

��

Fig. 7.3.3 MFD measurement for Yb-LMAHF-00.

Table. 7.3.1 Measured effective mode areas for a selection of LMA-HF-0100 and 0101.

Λ [µm] d/Λ AeffM [µm2]

LMAHF-0100 7.6 0.23 130 LMAHF-0100 9.7 0.23 215 LMAHF-0100 11.3 0.24 230

LMAHF-0101 (DC) 7.6 0.24 120 LMAHF-0101 (DC) 8 0.23 125 LMAHF-0101 (DC) 9.3 0.25 140

It can be seen that at Λ~7.6µm, � ! !" of the double clad fibre is comparable to that of the single

clad fibre. However, at Λ~10µm, #$ % %" of the single clad fibre drastically increases to a value of

more than 200µm2, whilst that of the double clad fibre is still ~140µm2. Thus, an increase of #$ % %"

is effectively suppressed by the presence of the outer cladding. The measured #$ % %" value for

Yb-LMAHF-00 thus seems reasonable by taking into account the combined action between the

high index doped section and the holey outer cladding.

C) Inner cladding NA

The inner cladding NA can also be roughly estimated by the effective index model to be & ' & '(((*),+),+),+ -.-./ " 0102 33 44 , where nocl is the effective index of the outer cladding as a

function of its hole diameter do and spacing Λo. Note that in reality, the extent of both inner and

outer cladding can affect the NA, but these factors are omitted here as a first order approximation.

Due to the fact that the effective index of the inner cladding is lower than that of silica, the inner

cladding NA of this fibre type is slightly more restricted relative to the case of the air-clad MOFs


that are described in Chapter.5 (i.e. the lower the fraction of air within the inner cladding the higher

the inner cladding NA that can be achieved).

Fig.7.3.4 shows the effective inner cladding NA for the case Λ=10µm and d/Λ=0.3 for the inner

cladding at 915nm. The outer cladding hole pitch is also assumed to be Λo=10µm as an ideal case.

For do/Λo=0.8, NAcl is only ~0.19. Given the deformation involved in Yb-LMAHF-00, it is hard to

predict the actual value since the NA is sensitive to both Λo and do/Λo. However, it is understood

that high d/Λ (>0.9) is essential to be comparable to the conventional cladding pumped fibre with a

polymer coating. One possible approach to improve the value of NAcl is to use smaller capillaries in

the outer cladding than in the inner cladding since the effective index of the outer cladding can be

reduced.

The effect of the index difference between Suprasil® F300 and Vycor® was found to be so small

within the parameter range that there is no actual difference compared with the case in which

Suprasil® F300 is used for both inner and outer cladding. Thus, the use of Suprasil® F300

capillaries should result in higher NA since the deformation of the outer cladding can be prevented.

��

��

��

Fig. 7.3.4 Possible inner cladding NA with respect to the outer cladding structural parameter do/Λo. (Λ=Λo=10µm, d/Λ=0.3, and the wavelength of 915nm is assumed.)

In Chapter 5, it is shown that do/Λo~0.9 is readily achievable in practice within the air clad MOFs.

Therefore, even when a holey cladding is used for both the outer and inner cladding, it is expected

that an inner cladding NA of more than 0.3 can be obtained, although there will be a trade-off with

respect to a corresponding decrease in the effective mode area. This trade-off can be virtually

eliminated by increasing the inner cladding dimensions. However, since the doped section within

this fibre type is relatively small compared with the conventional fibres, the device length is

elongated, which is not a preferred option for this kind of cladding structure since the effective

inner cladding cladding NA is length dependent[253]. Therefore, improving the core structure is a

key issue for the future so that a large doped section can be incorporated.


7.3.2. Absorption and background losses

The absorption of Yb-LMAHF-00 was measured by using the cut-back technique with a white light

source launched into the fibre inner cladding. The ytterbium absorption peak was measured to be

~2.0dB/m at 976nm and ~1.0dB/m at 910nm, respectively, using ~5m of the fibre on an 8cm radius

bobbin. Compared with the absorption cross section data[296] in germanosilicate, the 976nm peak

height is lower, indicating that a fraction of power is not interacting with the core. By slightly

decreasing the bend radius to 7.5cm, the absorption was slightly enhanced to ~3dB/m at 976m and

~1.25dB/m at 915nm.

The area ratio of the inner cladding to the core is readily estimated to be ~3n(n+1)(1-(d/Λ)2)=82,

where n is the number of the rings. Therefore, the dopant concentration can be estimated to be

1900~2900ppm (by weight) for 976nm and 3500~4500ppm for 915nm, respectively, by assuming

the cross section values. Given that the conventional fibres, made using the same ytterbium

concentration in the solution, exhibited ~3000ppm these values seem reasonable. The slightly

higher value at 915nm is presumably due to the increased modal number (~V2/2)[252] and to the

fact that the majority of the extra modes at 915nm overlap well with the core. This in turn implies

that some cladding modes rarely interact with the core.

In fact, the pump absorption measurement along the length using a low brightness laser diode

source (at 915nm) can be best fitted by an exponential curve with a positive offset. This indicates

that there are still some non-absorbed modal components present in the fibre even at 915nm. Thus,

despite an irregular cladding boundary and an offset core, the pump interaction with the core in this

MOF is not as good as anticipated. However, given that the total absorption observed during the

white light measurement at 976nm was ~10dB on the 8cm bobbin with a length of ~8m, the

absorption characteristics resulting from the holey inner cladding seem acceptable from a device

perspective.

Although the author tried to characterise the background losses, it was practically difficult to

measure using the available length of 30m. The accuracy in detection of the white light

measurement is approximately 0.5dB, which restricted the measurement accuracy to ~15dB/km.

The measured loss around 1100nm was ~30dB/km. This is sufficiently low for the given device

lengths of ~10m. On the other hand, the author measured ~30ppm of OH absorption at 1380nm,

which ultimately limits the high power operation due to the energy transfer from the ytterbium ions

because of the spectral overlap at 940nm. The main contribution to the background loss was

primarily due to this OH contribution. It can be seen that there is an overtone peak at 1130nm

which ‘plugs’ the transmission minimum around this wavelength. The long wavelength (>1550nm)

losses are presumably due to the leakage at the outer cladding interface. Thus, by introducing the


dehydration process (see Chapter 3), and by increasing the number of rings in the outer cladding, a

further reduction in the pump propagation losses can be expected.

��

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7.4. CW laser characteristics

7.4.1. Core pumping at 976nm

The author performed laser experiments using a single-mode Ti:sapphire laser at 976nm as the

pump laser. However, it was found to be difficult to efficiently launch the pump light into just the

fundamental mode of the core. As a result, the absorption measurement of the fibre showed an

absorption coefficient of 3.1dB/m with negligible bends and this is only a slightly higher value than

the value given by the white light measurement. This suggests that a large portion of the pump

power is used to excite the cladding modes. The value was relatively insensitive to the focusing

element (f=10mm~20mm). In fact, the diameter of the incident beam was ~5mm. Therefore, the

focusing beam NA was still ~0.1 when using an f=20mm lens. On the other hand, the effective NA

is estimated to be ~0.06 from Fig.7.3.1. Thus, the poor coupling efficiency to the fundamental

mode can be understood. The total coupling efficiency of the pump was measured using a 5cm

piece of the fibre and was 70%.

The laser cavity was formed by a high reflector and 4% Fresnel reflection from the cleaved end of

the fibre, to which the pump laser was coupled. The output power was extracted as a reflection

from a dichroic mirror (HR:1030nm, HT:980nm). Using a 4m length of the fibre, we recorded 82%

slope efficiency with respect to the absorbed pump power, as shown in Fig.7.4.1. Despite the

relatively long length of the fibre, the spectral peaks of the free running laser were around 1040nm,

indicating relatively weak re-absorption. The laser threshold pump power was ~360mW and a

maximum power of ~580mW was obtained.


7.4.2. Cladding pumping at 915nm

A pure cladding pumped laser was demonstrated using a low brightness pump laser diode operating

at 915nm and that had a fibre-pigtailed output with ~100µm spot and 0.22 NA. The pump beam

was imaged on to the fibre end using a 1x telescope. The coupling efficiency was measured to be

65% again using a 5cm piece of the fibre. The slight degradation of the launch efficiency can be

attributed to the higher NA of the pump beam and possibly to the uniform illumination of the pump

beam over the air holes, and which can cause random scattering at the launch interface. The cavity

arrangement was otherwise exactly the same as the one employed for the core pumping

experiments.

The bend was carefully imposed so that no significant leakage of the fundamental mode occurred

while enhancing the pump absorption. It was possible to reduce the bend radius to a value as small

as 7.5cm without significant modal leakage, while enhancing the pump absorption.

The highest slope efficiency was obtained using an 8.5m length of the fibre, where an output power

well in excess of 1W was obtained. Since the absorption cross section is approximately 2.5 times

smaller at 915nm than 976nm, the optimum absorption length should be 2.5 times longer. The

shorter than expected length can be explained by the enhancement of the pump absorption due to

the bend. The slope efficiency and the pump power threshold were calculated to be 70% and

200mW with respect to the absorbed pump power as shown in Fig.7.4.1. The reduction of the slope

efficiency compared with 976nm pumping is primarily because of the different quantum limit

efficiencies (~85% for 915nm pump and 1080nm lasing, ~94% for 976nm pump and 1040nm

lasing). This, in turn, resulted in a smaller pump threshold using the 915nm pump since the laser

operation here is more similar to a four level system than when using the 976nm pump.

The modal profile of the output was taken using an IR camera, and single mode guidance of the

fibre was confirmed, as shown in the inset of Fig.7.4.1. The mode is slightly elliptical although the

hexagonal shape can be deduced from the tail of the modal profile. This is consistent with the MFD

measurement, in which a relatively large variation in angular orientation was observed. The reason

behind this is possibly due to the deformation of the doped section through the fabrication, which

can be induced by the thin silica layer that remains on the surface of the etched preform. The

difference in silica layer thickness can lead to a variation in a compressive force imposed on the

doped section during the drawing. To verify this argument, a SNOM measurement of the near field

modal profile[271] would be necessary.


Fig. 7.4.1 CW laser output power characteristics of Yb-LMAHF-00. The circular dots: core pumping at 976nm using 4m of the fibre. The square dots: cladding pumping at 915nm using 8.5m of the fibre. Inset: the measured output modal profile in the cladding pumped configuration.

7.5. Pulsed laser characteristics

7.5.1. Q-switching

The development of conventional LMA fibres[262] has resulted in major advances in the

performance of high energy pulsed fibre lasers due to the high energy storage capacities and

relatively large saturation energy within these fibres. Using multi-mode ytterbium doped

conventional LMA fibres, nearly single transverse mode, millijoule pulses have been

reported[265,297,298]. The author constructed a Q-switched laser using Yb-LMAHF-00 since its

performance under Q-switched operation provides a direct measure of the energy storage capability

of the fibre. Here, it is shown that despite the relatively small doped area within the current fibre it

is possible to obtain pulses with ~50µJ pulse energies with a peak power of ~1kW.

The extractable pulse energy in a Q-switched fibre laser is roughly determined by the difference

between the stored energy and the energy at which the fibre gain becomes positive to overcome the

signal absorption due to the propagation along the fibre (e.g. a bleaching level)[297]. Note that both

of these quantities are proportional to the doped area. The bleaching level is also proportional to the

saturation energy and the fibre length.

Decreasing the bleaching level in principle provides one way of increasing the extractable energy

as long as either the excitation density is low or the pump is injected within a period shorter than

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the lifetime of the active ions. However, in fibre laser systems due to the small active volume and

continuous pumping, combined with long interaction lengths, the energy is lost as amplified

spontaneous emission (ASE) above a certain pump level, effectively reducing the potential energy

storage. This limits the extractable pulse energies from Q-switched fibre laser systems. One

approach to solve this issue is to increase the effective mode area, in order to reduce the signal

power density within a doped section. Note the reduced signal overlap by means of strategic doping

does not necessarily amount to the reduced amount of ASE in CPFLs, although ASE capture to the

guided modes can be reduced.

From the above discussions, it is clear that the simplest solution to increase the energy storage (for

a given rare earth ion) is simply to increase both the mode area and the doped area[265,298]. Given

the doped area of fabricated Yb-LMAHF-00 (~73.9µm2), high pulse energies cannot be expected

from this fibre compared to those achieved with conventional multi-mode LMA fibre types.

Nevertheless, it was important to understand the Q-switching characteristics of this doped

LMA-MOF for future reference in order to develop new appropriate designs to allow one to obtain

comparable or higher pulse energies in the future. In addition, it was also important to examine the

durability of this fibre type under pulsed operation.

Q-switching operation was achieved by inserting an acousto-optical modulator (AOM) inside a

Fabry-Perot cavity, such that the first-order deflection (with an efficiency of ~75%) could be used

to close the resonator, as shown in Fig.7.5.1. While the pump end of the fibre was cleaved normal

to the fibre axis to provide a 4% Fresnel reflection for the cavity feedback, the facet at the AOM

end of the fibre had to be either angle-cleaved or polished to avoid any parasitic oscillations to

ensure that the feedback is only from the high reflector of the cavity.

Fig. 7.5.1 The experimental setup for the pulsed operations.

Surprisingly, it was possible to angle polish the end face of Yb-LMAHF-00 by use of conventional

polishing pads despite the presence of air holes. The fibre end was soaked after polishing using

de-ionised water to remove any abrasives attached to the surface of the fibre and then dried. The

loss of the polished end face was estimated from a measurement of the slope efficiency of the CW

AOM

Yb-LMAHF-00

Pump at 915nm Output

AOTF

λ/2


laser output using the same length as the previous experiment, which dropped approximately 15%

after polishing the AOM face end. Using a simple Rigrod analysis[299], the author estimates the

transmission loss per pass at the facet to be ~0.45dB. Thus, the loss was not prohibitive and should

be acceptable for a variety of fibre laser devices which in most instances operate with high gain in

any event.

The cavity Q was controlled by triggering the AOM using an electrical pulse generator. The

rise/fall time of the AOM was estimated to be ~100nsec. By changing the repetition rate and the

duration of the trigger pulses (time window), it was possible to obtain stable well defined

Q-switched pulses. The repetition rate was variable up to 100kHz.

A 7.5m length of the fibre was used in the experiments. The pulse quality was checked by

monitoring the laser output with a fast photo-detector (bandwidth~1GHz) and an optical spectrum

analyser. Although the output pulse was in a single transverse mode, a substantial amount of the

ASE components were in fact captured within the inner-cladding. For this reason, it was essential

to measure the output power by inserting a mechanical aperture to eliminate the cladding modes.

The operating wavelength was at 1040nm and which coincided with the ASE peak. The absorbed

pump power was fixed at 1.33W and the average output powers and the pulse duration were

measured as a function of repetition rate, as shown in Fig.7.5.2. The pulse duration shortened as the

repetition rate was decreased from 1.6µs at 80kHz to 100ns at 1kHz The average power dropped at

lower repetition rates and the energy per pulse increased to a maximum pulse energy of ~45µJ at

1kHz. The corresponding peak power at this repetition rate was determined to be almost 1kW. Note

that no fibre damage was observed at the lowest repetition rate despite the high powers and small

glass volume of the fibre.

Fig. 7.5.2 The output characteristics of the Q-switched laser based upon Yb-LMAHF-00.

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At high pump powers and low repetition rates, spurious reflections and resulting parasitic laser

oscillations reduced the stability of the pulse trains and this was visible from the output spectra. In

addition, significant spectral broadening was observed as shown in Fig.7.5.3(a). The operating

wavelength could be shifted by adjusting the beam reflected back into the AOM. Since the ASE

always tends to concentrate around 1040nm, the ASE peak (parasitic laser oscillation) can readily

be distinguished. For example when operating around 1070nm, a parasitic lasing peak was clearly

observed at 1040nm as shown in Fig.7.5.3(b).

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Fig. 7.5.3 The Q-switched pulse spectra for different repetition rates and operating wavelengths (a) 1040nm and (b) 1070nm.

From the plots in Fig.7.5.3 it can be seen that when operating at 1040nm it is possible to reach

repetition rates as low as 1kHz repetition without parasitic lasing effects. By contrast, when

operating at 1070nm parasitic oscillation starts at a repetition rate of 8kHz and at lower repetition

rates the additional available gain contributes to the growth of the ASE peak without increasing the

useful Q-switched output. This destabilises the operation of the laser.

In general, the long wavelength operation provides low background absorption, resulting in a

reduced laser threshold. However, for a given length, there is always a limit to energy storage due

to ASE build up at shorter wavelengths. The lasing threshold of the ASE components can be

slightly increased by using a longer fibre length to shift the effective small signal gain peak. In

other words, the short length of the fibre tends to suffer from ASE because of its low lasing

threshold. Therefore, the author concludes that use of too short a fibre length does not suit the

Q-switching operation unless the nonlinear effects become severe or the shortest possible pulse

duration is required.

If the Q-switched laser is forced to operate at longer wavelengths with respect to the CW lasing

wavelength, the threshold is again increased due to the lack of gain. However, by providing pump

feed back (see Chapter.5), the gain at these wavelengths can be enhanced. Thus, the pump feed


back is effective for Q-switching within a range in which the gain at 1040nm is not too enhanced.

If the pump feed back is used while the gain is sufficient, the onset of ASE significantly limits the

obtainable pulse energy.

Therefore, it may be possible to increase the pulse energy achieved from the current fibre by using

the pump feed back geometry and by using a longer fibre length. However, significant increases

should not be expected. Increasing both the doped and mode area within the LMA-MOFs are the

essential factors and their possibilities are discussed in Section 7.6.

7.5.2. Mode-locking

Ytterbium doped fibre based broadband high peak power sources are attractive for many metrology

and measurement systems. A compact fibre based ultrashort pulse oscillator with low average

power but with high peak power is attractive for optical coherence tomography[98-100] and

nonlinear microscopy[300], particularly for biological imaging applications. Combined with high

power amplifiers, fibre based ultrashort pulse sources are likely to serve as robust tools for material

processing[301,302]. Motivated by these applications, cladding pumped mode-locked lasers have

been actively investigated for the last couple of years[303,304].

However, the long length requirements for the cladding pumped fibres generally mean a large

amount of normal intracavity dispersion within the silica based fibres at the ytterbium laser

wavelength. Moreover, the longer the cavity length the higher the nonlinearity, and this can lead to

instability under mode-locked operation[305,306]. For these reasons, many ultrashort pulse sources

reported to date at 1µm wavelengths either use bulk solid-state lasers or complex erbium doped

fibre laser based systems, (based on mode-locked erbium doped fibre lasers followed by Raman

shifting in fibre and wavelength conversion in PPLN), as seed sources to the ytterbium

amplifiers[307-310]. If a high power fibre based ultrashort pulse oscillator could be realised then it

would allow for drastic simplification of such systems and could open up a very much wider range

of applications.

Here, the author describes a mode-locked laser based on the use of ytterbium doped LMA-MOF.

The outer-cladding design of Yb-LMAHF-00 provides better environmental stability[102], despite

the low NA design, and an increased mode area reduces nonlinear effects. Thus, the fibre is

potentially useful for mode-locking. Stable mode-locked operation at the output power of ~500mW

is obtained owing to the low nonlinearity, and a broad tuning range is achieved.

Mode-locking was achieved by employing the frequency shift feed-back technique[196] as

previously applied to the small core ytterbium doped MOF, which is described in Section 4.4. An


acousto-optic tunable filter (AOTF) and a half wave-plate were inserted, instead of the AOM used

in the previous Q-switching cavity (see Fig.7.5.1), and the fibre length was set to be 6.5m.

Fig. 7.5.4 The output characteristics of the mode-locked laser and the output spectrum (FWHM~0.1nm).

By optimising the half wave-plate, stable, self-starting, fundamental mode-locking at ~13.2MHz

repetition rate was obtained at an output power of ~75mW corresponding to the pump power of

~400mW. The pulse duration was roughly estimated to be ~100ps (precise measurements for pulse

durations in this range are tricky since the dynamic range of an autocorrelator is not generally wide

enough whilst the photodetector response is not fast enough). Note that in general the pulses will be

highly chirped in the time domain due to the lack of a dispersion compensation element within the

cavity[205]. The output characteristics and the spectral shape are shown in Fig.7.5.4. The spectral

bandwidth (FWHM) was 0.1nm, which was comparable to the resolution limit of the optical

spectrum analyser used (0.05nm). Multiple pulsing was observed at an absorbed pump power

>1.4W. The slope efficiency of 48% is slightly lower than that achieved with the Q-switch laser

which is attributed to the short fibre length and to degradation of the fibre end facet. The author

notes a tendency for the end-faces on this fibre to degrade over a time period of a day or so which

seems to result from some aspect of our current end surface preparation procedure. Whilst the

current procedure is acceptable for our own internal research efforts the issue of developing reliable

end face preparation techniques will be a key to the future deployment of this technology.

The wavelength tuning curve was taken by changing the transmission window of AOTF at an

absorbed pump power of 1.33W. Continuous, electronically controlled, wavelength tuning of the

mode-locking operation was achieved over more than 70nm (FWHM: 69.5nm), as shown in

Fig.7.5.5. The broadband tunability was largely achieved owing to the relatively short length of the

fibre and through an appropriate choice of operating pump power - at higher pump powers, the

1040nm peak dominates and thus reduces the tuning range.

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Also the regime for which fundamental mode-locking is obtained is also dependent on the pump

power. Therefore, given the fibre nonlinearity, there exists an optimum pump power, with which

the fundamental mode-locking is achieved and for which a broad tuning range is possible. In

conventional cladding pumped fibres, the optimum power level for the fundamental mode-locking

should be less due to the reduced mode area.

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Fig. 7.5.5 The tuning curve obtained from the mode-locked laser using Yb-LMAHF-00.

An average power in excess of 500mW was obtained from 1030nm to 1090nm with pulse at the

fundamental repetition rate. (Note that the orientation of the wave-plate had to be changed to

recover the mode-locking operation when the wavelength was changed substantially because of the

wavelength dependence of the retardation through the waveplate). Note that the corresponding

pulse energy is more than 40nJ. By employing a more appropriate mode-locking for short pulse

generation and incorporation of suitable dispersion compensation elements within the cavity it

should be possible to obtain reasonably short pulse trains with pulse energies comparable to those

achievable using bulk mode-locked lasers [311].

7.6. Discussion

It has been shown that the novel cladding pumped fibre, that the author has presented herein,

displays comparable CW laser performance to its conventional counterpart, and demonstrates the

high quality of the fabricated MCVD preform and the quality of our MOF fabrication process. This

ensures that the doped core of the preform retains its performance. However, it will be necessary to

improve the structural design (in particular the inner cladding NA) if this technological approach is

to find successful applications.

In terms of modal properties, the limitation of obtaining larger effective mode area arises from both

the high index doped section and the impact of the outer-cladding. The latter may be solved by


increasing the inner cladding dimensions, although this will reduce the net pump absorption

because of the reduced core clad area ratio and the corresponding increase in bend sensitivity.

Furthermore, in Q-switching experiments, the major obstacle for the scaling of the output pulse

energy was primarily identified as the limited doped area in the current fibre.

In order to enlarge the doped section within LMA-MOFs, the index boundary should be placed

outside the MOF core, for instance, between the first and second rings of air holes (see Fig.7.6.1).

By doing this, the effect of the finite refractive index difference between the core and cladding

glasses is different to the current case (dotted line). The impact of the outer cladding on the

effective area will also be different and may slightly modify some of the details in terms of trade

off of mode area versus bend loss, and parameters for which more robust single mode guidance is

achieved. In order to fabricate such structures a fabrication technique that allows us to make large

scale rods of doped glasses is necessary. Thus, the key issues relate to the fabrication of large scale

pieces of doped glasses with an accurately controlled refractive index.

Fig. 7.6.1 The possible area of the doped section with respect to the hexagonal arrangement of air holes (the dotted line corresponds to the one in Yb-LMA-00.)

In terms of controlling the refractive index, it is clear that it will be difficult in practice to reduce

the aluminium content since the dopant concentration of the cladding pumped fibre has to be

reasonably high to reduce the device length. Therefore, incorporation of another index decreasing

element such as boron or fluorine is likely to be required. Fortunately, boron is trivalent and thus

has good characteristics for the incorporation of ytterbium ions[312,313] although fluorine

incorporation through MCVD is difficult since freon can no longer be used. Thus, optimising the

balance between the aluminium and boron, the refractive index of the doped core can be matched

to that of Suprasil® F300 while a sufficiently high ytterbium concentration for cladding pumped

fibres is preserved. Although the boron doped preforms are stressed in general, the boron required

to compensate for the positive contribution from both aluminium and ytterbium should be so small

that the core extraction by polishing is unlikely to be an issue (see Chapter 4 and 5). The author

therefore concludes that there is a possibility for obtaining matched doped glasses without

compromising the dopant concentration.


In terms of the dimensions of the doped glasses, MCVD preforms suffer from the disadvantages of

the solution doping technique that relies upon the deposition of a thin soot layer. Historically, it is

well known that a heat source must be separated from the substrate to deposit thick soot as within

OVD, VAD, and PCVD. However, the author restricts this discussion to the possibilities of using

MCVD systems below.

There are two possible approaches to overcome this issue. One is to increase the thickness of the

silica frit that is to be solution doped. However, it is difficult to obtain a uniform and thick silica

frit because of its poor thermal conductivity. When the deposition temperature is too high, the frit

near the substrate tube becomes insufficiently porous to incorporate the rare-earth ions from the

solution. On the other hand, when the temperature is too low, the inner frit becomes too porous to

attach it to the surface, preventing further deposition. Therefore, thermally conductive gas (such as

helium) may be used during the soot deposition process. Alternatively, water/air cooling of the

deposition tube[314] can help since a large temperature gradient can be created in front of the

deposition burner increasing the thermophoretic force[315]. Another approach is to perform

solution doping many times by repeating the process. However, given that a single step of the

doping process typically takes ~3hours, it is therefore impractical to perform multiple solution

doping stages since the perform core dimensions realised by a single step are typically just

2~3mm2.

Given these difficulties in the MCVD method, it seems that the synthesis of the bulk doped silica

glasses is ultimately needed for the future improvement of the doped LMA-MOFs although the

refractive index control will become more challenging than in the MCVD preform fabrication as a

result. One possible approach is to incorporate organometallic materials as raw materials[316],

which allows for continuous deposition of aluminosilicate and rare-earth incorporation via aerosol

formation[317]. Therefore, it is anticipated that routes forward will be found with MCVD

technique.

7.7. Conclusions

The author has fabricated an efficient all-glass double-clad ytterbium doped LMA-MOF fibre using

the core extracted from a low NA MCVD preform. It was demonstrated that the use of a low NA

doped section is essential, and that LMA-MOFs offer unique opportunities within double clad

structures.

It has been shown that, using the effective index model, it is possible to roughly predict the

optimum structural parameters to preserve single mode guidance while taking advantage of the

holey cladding, and that the V value of the doped LMA-MOF is flattened, allowing for a robust


single mode operation over a wide spectral range. This implies interesting opportunities for core

pumped variants using suitable glass materials.

In laser experiments, a slope efficiency of 80% with respect to the absorbed pump power at 976nm

was obtained for continuous wave laser operation. A corresponding value of 70% was obtained

using a low brightness laser diode at 915nm. The modal profile of the laser output was also

confirmed to be single mode.

Furthermore, both Q-switching and mode-locking operations were demonstrated using the doped

LMA-MOF. In Q-switching experiments, it has been shown that the peak output power obtained of

~1kW was limited by the lasing of ASE components due to the relatively small doped area. In

mode-locking experiments a broad tunability of over 70nm was demonstrated within the

fundamental mode-locking regime.

Finally, the future directions and possibilities for doped LMA-MOF fabrication were discussed.

When the existing fabrication issues are solved, it will open up a host of new opportunities for the

use of doped LMA-MOFs.

Chapter.8

Conclusions and future directions

This thesis has described the fabrication, characterisation, and applications of a broad range of

microstructured optical fibres (MOFs) with a particular emphasis on incorporating rare-earth ions.

Chapter 2: The fabrication related issues were studied. First, a mathematical model of the capillary

drawing process was described, which forms a basic building block for MOF fabrication. Its

experimental verification was carried out and good agreement was obtained.

Second, general guidelines for preform preparation were provided. It was shown that there are two

types of preforms: a single material preform with sealed capillaries or a combination of silica

capillaries and a Vycor® jacket tube since different collapse ratios imposed on the capillaries and

the jacket tube must be compensated either by pressure or viscosity. Practical issues such as

cleaning of the elements and the preform dimensions are also discussed.

Third, caning and fibre drawing processes were discussed. It was found that the capillary drawing

model also provides useful insights for all these drawing processes. General criteria as to how the

drawing parameters can be allotted for the case of the two step drawing approach were discussed. It

was found that reducing the tension of the fibre is a key for the successful drawing of small scale

structures. Precise internal pressure control of the preforms was found to be important for

controlling the final structures in the fibres.

Chapter 3: Investigations into the loss properties of highly nonlinear MOFs (HNL-MOFs) were

presented. The first part focused upon the possible loss mechanisms involved in these fibres.

Forward, backward, and homogeneous scattering mechanisms were individually discussed and the

relevant perturbation scales were identified.

The second part described experimental measurements of losses. Using the cut-back method, the

homogeneous scattering losses were identified as a main contributor to the total losses of these

Chapter.8 Conclusions and future directions 177

fibres since the wavelength dependence of the losses shows λ-2 dependence at long wavelength

range (λ~1.5µm), which agrees with a simple model where the scattering elements are assumed to

be homogeneously distributed within the cladding. Furthermore, significant dependence of the

losses on the structural dimensions of the fibres indicates that the effective Rayleigh scattering

coefficient of holey cladding is more than 1000 times greater than that of conventional fibres. This

indicates that surface treatment during the fabrication process must be improved for these fibre

designs. A comparison with attenuation and backscattering factors within the OTDR trace implies

that there are also substantial radiation losses. To confirm this conclusion, , further details, in

particular for fibres with different d/Λ, need to be studied by improving the backscattering

measurement.

Finally, continuous efforts to reduce OH incorporation in HNL-MOFs were described. Technique

for dehydrating the capillary stack preforms was examined and the concentration of OH ions was

reduced to a few ppm level. Although it was found to be effective to etch off the surface layer that

contains OH ions, there is a trade-off with the increased background losses due to the degradation

of the silica surface. By minimising the exposure to the atmosphere, combined with the developed

dehydration process, an OH concentration as low as 1ppm can be anticipated in HNL-MOFs, even

when the two-step drawing approach is used. For a further reduction of the OH incorporation the

exposure to the atmosphere must be prevented during the capillary drawing and caning.

Chapter 4: rare-earth doped MOFs with small scale cores, thus with high nonlinearity, were

developed and three device demonstrations were carried out using them.

First, the mode-locked operation of an ytterbium doped MOF was presented. By employing the

frequency shift feedback technique, a mode-locking threshold of 17mW and tunability over 20nm

was obtained. The mode-locking threshold was higher than anticipated. This was attributed to end

facet reflection due to the extremely high NA design of the fibre. In order to fullly take advantage

of anomalous dispersion of the fibre in an oscillator, a lower NA design is required but maintains

the anomalous dispersion.

Second, the ytterbium doped MOF was incorporated into an amplifier configuration using a fibre

based ultrashort pulse source as a seed. Taking advantage of the broad anomalous dispersion

regime of the fibre, mono-colour tunable Raman soliton generation was demonstrated covering the

1.06µm~1.33 µm range. Further complex regimes including multi-colour soliton generation up to

1.58µm and even supercontinuum generation were observed by increasing the seed/pump power.

The fibre lengths used for these experiments were less than 10m and seed pulse energy was less

than 10 picojoules. These facts demonstrate the possibility of realising truly compact, widely

tunable ultrashort pulse fibre sources using a highly nonlinear ytterbium doped MOF. For further


improvements, a more detailed understanding of the system is required. In addition, optimisation of

fibre design in terms of effective mode area and dispersion characteristics as well as the fibre

fabrication (i.e. low OH content fibres) are necessary. By overcoming these issues, an all-fibre

femtosecond source, which is continuously tunable from 1µm to 2µm, could be realised.

Finally, the continuous wave operation of an erbium doped MOF laser was presented, resulting in a

pump power threshold of 0.55mW and a slope efficiency of 57.3%. This demonstrates that it now

is possible to fabricate rare-earth doped MOFs with improved performance relative to the fibres

drawn from the original MCVD preform. It was found to be difficult to excite the single

polarisation axis within this fibre despite the high birefringence of these fibre types. These

polarisation issues need to be addressed further for use within a variety of devices.

Chapter 5: Development of the novel cladding pumped fibres; air clad MOFs, which incorporate a

high fraction of air within the outer cladding whilst possessing the conventional core and inner

cladding, were described. This allows one to realise a high NA and small inner cladding at the

same time, which had been difficult to realise using the existing cladding pumped fibres.

Improved laser performance was demonstrated by presenting two examples. A broad tunability

over 110nm was achieved using only 1.7m of the fabricated ytterbium doped air clad MOF, in

cladding pumped configuration with pump feedback. By selecting the wavelength via a bulk

grating, efficient pure three level operation of cladding pumped ytterbium doped fibre at 980nm

was also achieved. The output exceeded 3.5W by employing high quality laser diodes. Furthermore,

a photosensitive air-clad MOF was developed and was used to investigate the impact of air-silica

interfaces during the grating fabrication process. It was found that it is possible to inscribe gratings

within the simplest form of the air clad structure, which contains a single layer of air holes. A

further study is necessary for understanding the effects of the air-silica interface for UV writing. By

overcoming these issues, truly integrated CPFLs with extended performance can be realised.

Chapter 6: Passive large-mode-area MOFs (LMA-MOFs) were fabricated and their optical

properties were studied in terms of effective mode area and bend losses.

Two different approaches to characterise the effective mode areas were examined, and effective

mode areas as large as 680µm2 was measured using the knife-edge method.

It was found that when different silica materials are used in the core and the cladding, the bend loss

characteristics are qualitatively modified and this was qualitatively understood by considering the

relative contribution to the total NA, that can be estimated from the observed cut-off wavelengths.

It was shown that single material fibres suffer from a short wavelength bend loss edge and this


results in worse bend loss characteristics for fibres with d/Λ~0.25 at 1550nm as Aeff is increased.

Comparable bend loss performance can be obtained by using slightly higher index material within

the core. However, this is accompanied by the onset of the multimode regime in visible

wavelengths. By exploring different regimes (near the multimode regime, i.e. d/Λ~0.4), the bend

loss performance is anticipated to be improved. The flexibility of the cladding design of

LMA-MOFs offers further research opportunities for better bend loss performance with greater

effective mode area (see ref.[318]).

Finally, the propagation losses of LMA-MOFs with different structures have been characterised.

The loss at 1550nm varied from 1~10dB/km for fibres with d/Λ>0.4. From the loss spectra, it was

found that the longitudinal uniformities of these fibres are very important because low NA fibres

are prone to suffer from radiation losses when any perturbations are imposed on the fibres.

Chapter 7: A novel cladding pumped ytterbium doped LMA-MOF, which uses different sizes of air

holes to define the inner and the outer cladding, was developed and a range of laser operations were

demonstrated.

By using the low NA MCVD preform, it was possible to retain the single mode output, which was

confirmed during the laser experiments. In continuous wave operation, slope efficiencies of

70~80% were obtained, depending on the pump wavelength, which were comparable to the

conventional counterpart. In cladding pumped operation, an output power in excess of 1W was

obtained. In Q-switching operation, a peak power of ~1kW and a pulse energy of ~50µJ was

obtained. In mode-locking operation, a wide tunability of 70nm was obtained with a fundamental

repetition rate with an average power of ~500mW corresponding to a pulse energy of >40nJ.

Finally, possibilities of improving energy storage capability of these fibres were discussed.

An output power up to 280W has been reported using this fibre type by improving the fibre

design[319]. Thus, it is anticipated that further improvement and refinement of the design will lead

to 1kW output from this fibre type near future.

Appendix.A The effective index model The effective index model is often used to analyse the fibres in this thesis. However, as explained,

this model is not accurate as a modal model when the modal index of the fundamental space filling

mode is calculated[12,13,19,23]. For instance, group velocity dispersion cannot be calculated using

this approach. The reasons for this are:

1. one must first calculate the effective cladding index neff, which may be incorrect, and

2. one must assume a value of the effective core radius aeff, which may be wavelength dependent

for a given structure.

Although the effective index model is not an accurate modal model, it can conveniently be used for

adapting the models for conventional fibres, as shown in Chapter 3. In other words, the effective

index model has a convenient interface to the other models, that can be used for analysing

transmission losses, for instance. Therefore, it is important to improve the accuracy of the effective

index model so that the physical quantities are consistent with the other accurate numerical

methods[14-31].

Below, it is shown that using the propagation constant β and the effective mode area Aeff, both of

which are calculated from the other numerical method, it is possible to determine modal parameters

within the effective index model without any ambiguities. Therefore, it is possible to retain the

consistency between the numerical model and the effective index model.

The dispersion relation of a perfectly circular step index fibre is given by[155]

� �

� � ��

��

��

�

�

� � ��

��

� � ��

��

� ��

��

��

��

��

�

� , (A.1)

where k is the wave vector, aeff the effective core radius, β the propagation constant. Here, unknown

variables are aeff and neff. Therefore, these values are dependent and their relationship can readily be

calculated.

Appendix.A Effective index model 181

The effective mode area is defined as[41]

��

� ��

�, (A.2)

where E is the electric field. Since the field distribution of the perfectly circular fibre is known, Aeff

can be calculated using a tentative combination of aeff and neff and then be compared with the value

obtained from the other modal method. Let the author define a minimisation function as

� �� ! "# , (A.3)

where Aeffo is the effective area obtained from the other model. By minimising f, it is possible to

determine a combination of aeff and neff, with which both β and Aeff are retained to be consistent with

the other modal model.

An example is shown below. The wavelength of 1.06µm and the structure with Λ=2.0µm and

d/Λ=0.4 are used, for which β~8.514852460143605x106 m-1 and $% & &� ~7.61325µm2 is calculated

using the localised function method[19]. In order to evaluate Aeff, the exact solution to the fields is

used and the numerical integration was performed using the adaptive recursive trapezoid

algorithm[316]. The integration range was varied by monitoring the convergence.

Fig.A.1 shows the relation between the core radius aeff and the effective cladding index neff, which

is determined by eq.(A.1).

')( *,+-( ',+-( +,+-( .,+-( /0+-( 1,+-( 20+-( 34+-( 5+-( /-5+-( /76+-( /-*+-( 17'+-( 1-++-( 17.+-( 17/+-( 171

8:9;;

<-= > >@? A-BDC

Fig.A. 1 Relation between aeff and neff that satisfies the dispersion relation (A.1) that gives the same propagation constant as that calculated from the orthogonal function method.

Appendix.A Effective index model 182

Using this relation, the function f is plotted with respect to aeff in Fig.A.2. A local minimum is

clearly seen, which corresponds to the consistent aeff (and thus neff).

��

�

�� !�"�

�� #�

�� "�

��

�

Fig.A. 2 Minimisation function f for a range of aeff.

By applying this technique, the calculated effective core radius aeff as a function of wavelength for

the same structure (Λ=2.0mm, d/Λ=0.4) is shown in Fig.A.3. It is seen that the effective core radius

varies substantially between 0.65~0.7Λ, peaking around 1.1µm. The trend below this wavelength

can be understood as the usual case, where the mode tends to deeply penetrate into the cladding at

longer wavelengths. The decrease of aeff at longer wavelengths is reflected by the fact that the NA

becomes relatively higher than at the short wavelengths whereas Aeff continues to increase with the

wavelength. By recalling that Aeff is almost inversely proportional to the V value, aeff has to be

reduced to match the given Aeff value.

$&%�'�(")*(,+.-./021354*6�7�89+;:�<=�> ? =�> @ =�> A B > = B > C B > ? B > @

D9EFFG HI JKLMNO

B > C�AB > P =B > P�CB > P�?B > P @B > P!AB > ?!=

Fig.A. 3 The calculated effective core radius aeff by hybridising the exact dispersion relation and (A.1).

When the scalar approximation is used for the field, aeff almost linearly scales with wavelength and

results in slightly smaller values being obtained, for instance, aeff~1.26µm at 1µm, that corresponds

to 0.63Λ. This is primarily because the orthogonal polarisation components, which contribute to the

pointing vector, are neglected.

Bibliography

[1] P.Kaiser and H.W.Astle, “Low-loss single-material fibers made from pure fused silica,” Bell System

Tech.J. Vol.53, pp.1021-1039 (1974)

[2] P.Yeh, A.Yariv, and E.Marom, “Theory of Bragg fiber,” J.Opt.Soc.Am. Vol.68, p.1196-1201 (1978)

[3] J.C.Knight, T.A.Birks, P.St.J.Russel, and D.M.Atkin, “All-silica single-mode optical fiber with

photonic crystal cladding,” Opt.Lett. Vol.21, pp.1547-1549 (1996)

[4] T.A.Birks, J.C.Knight, and P.St.J.Russell, “Endlessly single-mode photonic crystal fiber,” Opt.Lett.,

Vol.22, pp.961-963 (1997)

[5] J.C.Knight, T.A.Birks, P.St.J.Russell, J.P. de Sandro, “Properties of photonic crystal fiber and the

effective index model,” J.Opt.Soc.Am. A, Vol.15, pp.748-752 (1998)

[6] M.J.Gander, R.McBride, J.D.C.Jones, T.A.Birks, J.C.Knight, P.St.J.Russell, P.M.Blanchard,

J.G.Burnett, and A.H.Greenaway, “Measurement of the wavelength dependence of beam divergence

for photonic crystal fiber,” Opt.Lett., Vol.24, p.1017-1019 (1999)

[7] J.C.Knight, T.A.Birks, R.F.Cregan, P.St.J.Russel and J.-P. de Sandro, “Large mode area photonic

crystal fiber,” Electron.Lett. Vol.34, pp.1347-1348 (1998)

[8] H.Han, H.Park, M.Cho, J.Kim, “Terahertz pulse propagation in a plastic photonic crystal fiber,”

Appl.Phys.Lett. Vol.80, pp.2634-2636 (2002)

[9] T.M.Monro, Y.D.West, D.W.Hewak, N.G.R.Broderick, and D.J.Richardson, “Chalcogenide holey

fibre,” Electron.Lett. pp.1998-2000 (2000)

[10] M.A. van Eijkelenborg, M.C.J.Large, A.Argyros, J.Zagari, S.Manos, N.A.Issa, I.Bassett, S.Fleming,

R.C.McPhedran, C.M.de Sterke, and N.A.P.Nicorovici, “Microstructured polymer optical fibre,”

Opt.Exp. Vol.9 pp.319-327 (2001)

[11] B.Temelkuran, S.D.Hart, G.Benoit, J.D.Joannopoulos, Y.Fink, “Wavelength-scalable hollow optical

fibres with large photonic bandgaps for CO2 laser transmission,” Nature London, Vol.420 (6916)

pp.650-653 (2002)

[12] M.Midrio, M.P.Singh, and C.G.Someda, “The space fillin g mode of holey fiber: an analytical vectorial

solution,” J.Lightwave.Tech. Vol.18, pp.1031-1037 (2000)

[13] Z.Zhu, and T.Brown, “Analysis of the space filling modes of photonic crystal fibers,” Opt.Exp. Vol.8,

pp.547-554 (2001)

[14] J.Broeng, D.Mogilevstev, S.E.Barkou, and A.Bjarklev, “Photonic crystal fibers: a new class of optical

waveguide,” Opt.Fiber.Tech. Vol.5, pp.305-330 (1999)

[15] A.Ferrando, E.Silvestre, J.J.Miret, P.Andres and M.V.Andres, “Full-vector analysis of a realistic

photonic crystal fiber,” Opt.Lett. Vol.24, pp.276-278 (1999)

[16] T.M.Monro, D.J.Richardson, N.G.R.Broderick, and P.J.Bennett, “Holey optical fibers: efficient modal

model,” J.Lightwave.Tech. Vol.17, pp.1093-1102 (1999)

[17] D.Mogilevstev, T.A.Birks, and P.St.J.Russell, “Localised function method for modelling defect modes

in 2D photonic crystals,” J.Lightwave.Tech. Vol.17, pp.2078-2081 (1999)

Bibliography 184

[18] T.M.Monro, D.J.Richardson, N.G.R.Broderick, and P.J.Bennett, “Modeling large air fraction holey

optical fibers,” J.Lightwave.Tech. Vol.18, pp.50-56 (2000)

[19] F.Brechet, J.Marcou, D.Pagnoux, and P.Roy, “Complete analysis of the characteristics of propagation

into photonic crystal fibers, by the finite element method,” Opt.Fib.Tech. Vol.6, pp.181-191 (2000)

[20] A.Peyrilloux, S.Fevrier, J.Marcou, L.Berhelot, D.Pagnoux, and P.Sansonetti, “Comparison between

the finite element method, the localised function method and a novel equivalent averaged index

method for modelli ng photonic crystal fibres,” J.Opt. A: Pure Appl.Opt. Vol.4, pp.257-262 (2002)

[21] A.Cucinotta, S.Selleri, L.Vincetti, and M.Zoboli, “Holey fiber analysis through the finite-element

method,” IEEE Photon.Tech.Lett. Vol.14, pp.1530-1532 (2002)

[22] D.Ferrarini, L.Vincetti, M.Zoboli, A.Cucinotta, and S.Selleri, “Leakage properties of photonic crystal

fibers,” Opt.Exp. Vol.10, pp.1314-1319 (2002)

[23] M.Koshiba, “Full-vector analysis of photonic crystal fibers using the finite element method,” IEICE

Trans.Electron. Vol.E85C pp.881-888 (2002)

[24] K.Satoh, and M.Koshiba, “Full-vectorial imaginary-distance beam propagation method based on a

finite element scheme: application to photonic crystal fibers,” IEEE J.Quatum.Electron. Vol.38,

pp.927-933 (2002)

[25] T.P.White, B.T.Kuhlmey, R.C.McPhedran, D.Maystre, G.Renversez, C.Martijn de Sterke, and

LC.Botten, “Multipole method for microstructured optical fibers. �

Formulation,” J.Opt.Soc.Am. B

Vol.19, pp.2322-2330 (2002)

[26] B.T.Kuhlmey, T.P.White, G.Renversez, D.Maystre, L.C.Botten, C.Matrijn de Sterke, R.C.McPhedran,

“Multipole method for microstructured optical fibers. � Implementation and results,” J.Opt.Soc.Am.

B Vol.19, pp.2331-2340 (2002)

[27] Z.Zhu, and T.G.Brown, “Multipole analysis of hole-assisted optical fibers,” Opt.Commun. Vol.206,

pp.333-339 (2002)

[28] Y.Zhu, Y.Chen, P.Huray, and X.Dong, “Application of a 2D-CFDTD algorithm to the analysis of

photonic crystal fibers (PCFs),” Micro.Opt.Tech.Lett. Vol.35, pp.10-14 (2002)

[29] F.Fogli, L.Saccomandi, P.Bassi, G.Bellanca, and S.Trillo, “Full vectorial BPM modelling of

index-guiding photonic crystal fibers and couplers,” Opt.Exp. Vol.10, pp.54-59 (2002)

[30] Z.Zhu, and T.G.Brown, “Full-vectorial finite-difference analysis of microstructured optical fibers,”

Opt.Exp. Vol.10, pp.853-864 (2002)

[31] L.Poladian, N.A.Issa, and T.M.Monro, “Fourier decomposition algorithm for leaky modes of fibres

with arbitrary geometry,” Opt.Exp. Vol.10, pp.449-454 (2002)

[32] D.Mogilevtsev, T.A.Birks, and P.St.J.Russell, “Group-velocity dispersion in photonic crystal fibers,”

Opt.Lett. Vol.23, pp.1662-1664 (1998)

[33] J.K.Ranka, R.S.Windeler, A.J.Stentz, “Visible continuum generation in air silica microstructure optical

fibers with anomalous dispersion at 800nm,” Opt.Lett. Vol.25, pp.25-27 (2000)

[34] M.J.Gander, R.McBride, J.D.C.Jones, D.Mogilevtsev, T.A.Birks, J.C.Knight, and P.St.J.Russell,

“Experimental measurement of group velocity dispersion in photonic crystal fiber,” Electron.Lett.

Vol.35, pp.63-64 (1999)

Bibliography 185

[35] J.C.Knight, J.Arriaga, T.A.Birks, A.Ortigosa-Blanch, W.J.Wadsworth, and P.St.J.Russell, “Anomalous

dispersion in photonic crystal fiber,” IEEE Photon.Tech.Lett. Vol.12, pp.807-809 (2000)

[36] D.Ouzounov, D.Homoelle, W.Zipfel, W.W.Webb, A.L.Gaeta, J.A.West, J.C.Fajardo, K.W.Koch,

“Dispersion measurements of microstructured fibers using femtosecond laser pulses”, Opt.Commun.

Vol.192 pp.219-223 (2001)

[37] T.A.Birks, D.Mogilevtsev, J.C.Knight, and P.St.J.Russell, “Dispersion compensation using

single-material fibers,” IEEE Photon.Tech.Lett., Vol.11, pp.674-676 (1999)

[38] A.Ferrando, E.Silvestre, J.J.Miret, J.A.Monsoriu, M.V.Andres, and P.St.J.Russell, “Designing a

photonic crystal fiber with flattened chromatic dispersion,” Electron.Lett. Vol.35, pp.325-327 (1999)

[39] A.Ferrando, E.Silvestre, J.J.Miret, and P.Andres, “Nearly zero ultraflattened dispersion in photonic

crystal fibers,” Opt.Lett. Vol.25, pp.790-793 (2000)

[40] W.H.Reeves, J.C.Knight, P.St.J.Russell, and P.J.Roberts, “Demonstration of ultra-flattened dispersion

in photonic crystal fibers,” Opt.Exp. Vol.10 pp.609-613 (2002)

[41] G.P.Agrawal, “Nonlinear fiber optics,” 2nd edition, Academic Press (1995)

[42] C.C.Wang, “Empirical relation between the linear and the third-order nonlinear optical

susceptibilities,” Phy.Rev. B. Vol.2 pp.2045-2048 (1970)

[43] P. Petropoulos, T. M. Monro, H. Ebendorff-Heidepriem, K. Framoton, R. C. Moore, H. N. Rutt, and D.

J. Richardson, “Soliton-self-frequency-shift effects and pulse compression in an anomalously

dispersive high nonlinearity lead silicate holey fiber,” in Proc: Optical Fiber Communication

Conference, OFC 2003, PD3 (2003)

[44] J.C.Baggett, T.M.Monro, K.Furusawa, D.J.Richardson, “Comparative study of large mode holey and

conventional fibers,” Opt.Lett. Vol.26 pp.1045-1047 (2001)

[45] N.G.R.Broderick, T.M.Monro, P.J.Bennett, and D.J.Richardson, “Nonlinearity in holey optical fibers:

measurement and future opportunities,” Opt.Lett. vol.24, pp.1395-1397 (1999)

[46] P.Petropoulos, T.M.Monro, W.Belardi, K.Furusawa, J.H.Lee, D.J.Richardson, “2R-regenerative

all-optical switch based on a highly nonlinear holey fiber,” Opt.Lett. Vol.26 pp.1233-5 (2001)

[47] J.E.Sharping, M.Fiorentino, P.Kumar, and R.S.Windeler, “All-optical switching based on cross-phase

modulation in microstructure fiber,” IEEE Photon.Tech.Lett. Vol.14, pp.77-79 (2002)

[48] J.H.Lee, P.C.Teh, Z.Yusoff, M.Ibsen, W.Belardi, T.M.Monro, and D.J.Richardson, “A holey

fiber-based nonlinear thresholding device for optical CDMA receiver performance enhancement,”

IEEE Photon.Tech.Lett. Vol.14, pp.876-878 (2002)

[49] Z.Yusoff, J.H.Lee, W.Belardi, T.M.Monro, P.C.Teh, D.J.Richardson, ”Raman effects in a highly

nonlinear holey fiber:amplification and modulation,” Opt.Lett. Vol. 27 pp.424-426 (2002)

[50] J.E.Sharping, M.Fiorentino, A.Coker, P.Kumar, and R.S.Windeler, “Four-wave mixing in

microstructure fiber,” Opt.Lett. Vol.26, pp.1048-1050 (2001)

[51] J.E.Sharping, M.Fiorentino, P.Kumar, and R.S.Windeler, “Optical parametric oscillator based on

four-wave mixing in microstructure fiber,” Opt.Lett. Vol.27, pp.1675-1677 (2002)

[52] K.S.Abedin, J.T.Gopinath, E.P.Ippen, C.E.Kerbage, R.S.Windeler, B.J.Eggleton, “Highly

nondegenerate femtosecond four-wave mixing in tapered microstructure fiber,” Appl.Phy.Lett. Vol.81,

pp.1384-1386 (2002)

Bibliography 186

[53] W.J.Wadsworth, J.C.Knight, A.Ortigosa-Blanch, J.Arriaga, E.Silvestre, and P.St.J.Russell, “Soliton

effects in photonic crystal fibers at 850nm,” Electron.Lett. Vol.36, pp.53-55 (2000)

[54] X.Liu, C.Xu, W.H.Knox, J.K.Chandalia, B.J.Eggleton, S.G.Kosinski, and R.S.Windeler, “Soliton

self-frequency shift in a short air-silica microstructured fiber,” Opt.Lett. Vol.26, pp.358-360 (2001)

[55] B.R.Washburn, S.E.Ralph, P.A.Lacourt, J.M.Dudley, W.T.Rhodes, R.S.Windeler, and S.Coen,

“Tunable near-infrared femtosecond soliton generation in photonic crystal fibres,” Electron.Lett.

Vol.37 pp.1510-1511 (2001)

[56] N.Nishizawa, Y.Ito, and T.Goto, “0.78-0.9µm wavelength-tunable femtosecond soliton pulse

generation using photonic crystal fiber, ” IEEE Photon.Tech.Lett. Vol.14, pp.986-988 (2002)

[57] J.H.V.Price, W.Belardi, T.M.Monro, A.Malinowski, A.Piper, and D.J.Richardson, “Soliton

transmission and supercontinuum generation in holey fiber, using a diode pumped ytterbium fiber

source,” Opt.Exp. Vol.10, pp.382-387 (2002)

[58] J.H.V.Price, K.Furusawa, T.M.Monro, L.Lefort, and D.J.Richardson, “Tunable, femtosecond pulse

source operating in the range 1.06-1.33µm based on an Yb3+-doped holey fiber amplifier,”

J.Opt.Soc.Am. B Vol.19, pp.1286-1294 (2002)

[59] M.Fiorentino, J.E.Sharping, P.Kumar, A.Porzio, and R.S.Windeler, “Soliton squeezing in

microstructure fiber,” Opt.Lett. Vol.27, pp.649-651 (2002)

[60] M.Fiorentino, J.E.Sharping, P.Kumar, and A.Porzio, “Amplitude squeezing in a Mach-Zehnder fiber

interferometer: Numerical analysis of experiments with microstructure fiber,” Opt.Exp. Vol.10,

pp.128-138 (2002)

[61] S.Lorenz, CH.Silberhorn, N.Korolkova, R.S.Windeler, and G.Leuchs, “Squeezed light from

microstructured fibres: towards free-space quantum cryptography,” Appl.Phys. B. (2002)

[62] A.B.Fedotov, A.M.Zheltikov, L.A.Melnikov, A.P.Tarasevitch, D.von der Linde, “Spectral broadening

of femtosecond laser pulses in fibers with a photonic-crystal cladding,” JETP Lett., Vol.71,

pp.281-284 (2000)

[63] J.K.Ranka, R.S.Windeler, A.J.Stentz, “Optical properties of high-delta air-silica microstructure optical

fibers,” Opt.Lett., Vol.25, pp.796-798 (2000)

[64] A.V.Husakou, and J.Hermann, “Supercontinuum generation of higher-order solitons by fission in

photonic crystal fibers,” Phys.Rev.Lett. Vol.87, 203901 (2001)

[65] J.Hermann, U.Griebner, N.Zhavorronkov, A.Husakou, D.Nickel, J.C.Knight, W.J.Wadsworth,

P.St.J.Russell, and G.Korn, “Experimental evidence for supercontinuum generation by fission of

higher-order solitons in photonic fibers,” Phys.Rev.Lett. Vol.88, 173901 (2002)

[66] A.V.Husakou, J.Hermann, “Supercontinuum generation, four-wave mixing, and fission of high-order

solitons in photonic-crystal fibers,” J.Opt.Soc.Am. B Vol.19, pp.2171-2182 (2002)

[67] W.J.Wadsworth, A.Ortigosa-Blanch, J.C.Knight, T.A.Birks, T.-P.Martin Man, and P.St.J.Russell,

“Supercontinuum generation in photonic crystal fibers and optical fiber tapers: a novel light source,”


[68] D.A.Akimov, A.A.Ivanov, M.V.Alfimov, S.N.Bagayev, T.A.Birks, W.J.Wadsworth, P.St.J.Russell,

A.B.Fedotov, A.A.Podshivalov, and A.M.Zheltikov, “Two-octave spectral broadening of subnanojoule

Cr:forsterite femtosecond laser pulses in tapered fibers,” Appl.Phys. B Vol.74, pp.307-311 (2002)

Bibliography 187

[69] A.B.Fedotov, A.N.Naumov, A.M.Zheltikov, I.Bugar, D.Chorvat Jr., D.Chorvat, A.P.Tarasevitch, and

D.von der Linde, “Frequency-tunable supercontinuum generation in photonic-crystal fibers by

femtosecond pulses of an optical parametric amplifier,” J.Opt.Soc.Am. B Vol.19, pp.2156-2164 (2002)

[70] A.Apolonski, B.Povazay, A.Unterhuber, W.Drexler, W.J.Wadsworth, J.C.Knight, and P.St.J.Russell,

“Spectral shaping of supercontinuum in a cobweb photonic-crystal fiber with sub-20fs pulses,”


[71] B.R.Washburn, S.E.Ralph, and R.S.Windeler, “Ultrashort pulse propagation in air-silica

microstructure fiber,” Opt.Exp. Vol.10, pp.575-580 (2002)

[72] S.Coen, A.H.L.Chau, R.Leonhardt, J.D.Harvey, J.C.Knight, W.J.Wadsworth, and P.St.J.Russell,

“White-light supercontinuum generation with 60ps pump pulses in a photonic crystal fiber,” Opt.Lett.

Vol.26, pp.1356-1358 (2001)

[73] S.Coen, A.H.L.Chau, R.Leonhardt, J.D.Harvey, J.C.Knight, W.J.Wadsworth, P.St.J.Russell,

“Supercontinuum generation by stimulated Raman scattering and parametric four-wave mixing in

photonic crystal fibers,” J.Opt.Soc.Am. B Vol.19, pp.753-764 (2002)

[74] J.M.Dudley, L.Provino, N.Grossard, H.Maillotte, R.S.Windeler, B.J.Eggleton, and S.Coen,

“Supercontinuum generation in air-silica microstructured fibers with nanosecond and femtosecond

pulse pumping,” J.Opt.Soc.Am. B Vol.19, pp.765-771 (2002)

[75] L.Provino, J.M.Dudley, H.Maillotte, N.Grossard, R.S.Windeler, B.J.Eggleton, “Compact broadband

continuum source based on microchip laser pumped microstructured fibre,” Electron.Lett. Vol.37

pp.558-560 (2001)

[76] P.A.Champert, S.V.Popov, and J.R.Taylor, “Generation of multiwatt, broadband continua in holey

fibers,” Opt.Lett. Vol.27, pp.122-124 (2002)

[77] P.A.Champert, S.V.Popov, M.A.Solodyankin, J.R.Taylor, “Multiwatt average power continua

generation in holey fibers pumped by kilowatt peak power seeded ytterbium fiber amplifier,”

Appl.Phys.Lett. Vol.81, pp.2157-2159 (2002)

[78] G.Genty, M.Lehtonen, H.Ludvigsen, J.Broeng, and M.Kaivola, “Spectral broadening of femtosecond

pulses into continuum radiation in microstructured fibers,” Opt.Exp. Vol.10, pp.1083-1098 (2002)

[79] X.Gu, L.Xu, M.Kimmel, E.Zeek, P.O’Shea, A.P.Shreenath, R.Trebino, and R.S.Windeler,

“Frequency-resolved optical gating and single-shot spectral measurements reveal fine structure in

microstructure-fiber continuum,” Opt.Lett. Vol.27, pp.1174-1176 (2002)

[80] T.A.Birks, W.J.Wadsworth, and P.St.J.Russell, “Supercontinuum generation in tapered fibers,”

Opt.Lett. Vol.25, pp.1415-1417 (2000)

[81] J.M.Harbold, F.O.Ilday, F.W.Wise, T.A.Birks, W.J.Wadsworth, and Z.Chen, “Long-wavelength

continuum generation about the second dispersion zero of a tapered fiber,” Opt.Lett. Vol.27,

pp.1558-1560 (2002)

[82] J.M.Dudley, and S.Coen, “Numerical simulations and coherence properties of supercontinuum

generation in photonic crystal and tapered optical fibers,” IEEE J.Sel.Top.Quantum.Electron. Vol.8,

pp.651-659 (2002)

[83] J.M.Dudley, and S.Coen, “Coherence properties of supercontinuum spectra generated in photonic

crystal and tapered optical fibers,” Opt.Lett. Vol.27, pp.1180-1182 (2002)

Bibliography 188

[84] A.L.Gaeta, “Nonlinear propagation and continuum generation in microstructured optical fibers,”

Opt.Lett. Vol.27, pp.924-926 (2002)

[85] K.Mori, H.Takara, S.Kawanishi, M.Saruwatari, and T.Morioka, “Flatly broadened supercontinuum

spectrum generated in a dispersion decreasing fibre with convex dispersion profile,” Electron.Lett.

Vol.33, pp.1806-1807 (1997)

[86] K.R.Tamura, H.Kubota, and M.Nakazawa, “Fundamentals of stable continuum generation at high

repetition rates,” IEEE J.Quantum.Electron. Vol.36 pp.773-779 (2000)

[87] D.J.Jones, S.A.Diddams, J.K.Ranka, A.Stentz, R.S.Windeler, J.L.Hall, and S.T.Cundiff,

“Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency

synthesis,” Science, Vol.288 pp.635-639 (2000)

[88] S.A.Diddams, D.J.Jones, J.Ye, T.Cundiff, J.L.Hall JL, J.K.Ranka, R.S.Windeler, R.Holzwarth,

T.Udem, and T.W.Hansch, “Direct link between microwave and optical frequencies with a 300 THz

femtosecond laser comb,” Phys.Rev.Lett. Vol.84 pp.5102-5105 (2000)

[89] R.Holzwarth, Th.Udem, T.W.Hansch, J.C.Knight, W.J.Wadsworth, and P.St.J.Russell, “Optical

frequency synthesizer for precision spectroscopy,” Phy.Rev.Lett. Vol.85, pp.2264-2268 (2000)

[90] S.A.Diddams, D.J.Jones, J.Ye, S.T.Cundiff, J.L.Hall, J.K.Ranka, and R.S.Windeler, “Direct RF to

optical frequency measurements with a femtosecond laser comb,” IEEE Trans.Instr.Meas. Vol.50

pp.552-555 (2001)

[91] J.Stenger, T.Binnewies, G.Wilpers, F.Riehle, H.R.Telle, J.K.Ranka, R.S.Windeler, A.J.Stentz,

“Phase-coherent frequency measurement of the Ca intercombination line at 657 nm with a Kerr-lens

mode-locked femtosecond laser,” Phys.Rev.A Vol.63, art.no.021802 (2001)

[92] P.J.Blythe, S.A.Webster, H.S.Margolis, S.N.Lea, G.Huang, S.K.Choi, W.R.C.Rowley, P.Gill,

R.S.Windeler, “Subkilohertz absolute-frequency measurement of the 467-nm electric octupole

transition in Yb-171(+),” Phys.Rev. A Vol.67 art. no. 020501 (2003)

[93] T.M.Fortier, D.J.Jones, J.Ye, S.T.Cundiff, R.S.Windeler, “Long-term carrier-envelope phase

coherence,” Opt.Lett. Vol.27 pp.1436-1438 (2002)

[94] M.E.Grein, J.A.Jiang, H.A.Haus, E.P.Ippen EP, C.McNeilage, J.H.Searls, R.S.Windeler, “Observation

of quantum-limited timing jitter in an active, harmonically mode-locked fiber laser,” Opt.Lett. Vol.27

pp. 957-959 (2002)

[95] T.M.Fortier, J.Ye, S.T.Cundiff, R.S.Windeler, “Nonlinear phase noise generated in air-silica

microstructure fiber and its effect on carrier-envelope phase,” Opt.Lett. Vol.27 pp.445-447 (2002)

[96] F.W.Helbing, G.Steinmeyer, U.Keller, R.S.Windeler, J.Stenger, H.R.Telle, “Carrier-envelope offset

dynamics of mode-locked lasers,” Opt.Lett. Vol.27 pp.194-196 (2002)

[97] F.Krausz, “ From femtochemistry to attophysics,” Phys.World, Vol.14 pp.41-46 (2001); M.Hentschel,

R.Kienberger, C.Spielmann, G.A.Reider, N.Milosevic, T.Brabec, P.Corkum, U.Heinzmann,

M.Drescher, F.Krausz, “Attosecond metrology,” Nature, Vol.414 pp.509-513 (2001)

[98] I.Hartl, X.D.Li, C.Chudoba, R.K.Ghanta, T.H.Ko, J.G.Fujimoto, J.K.Ranka, and R.S.Windeler,

“Ultrahigh-resolution optical coherence tomography using continuum generation in an air silica

microstructured optical fiber,” Opt.Lett. Vol.26, pp.608-610 (2001)

Bibliography 189

[99] B.Povazay, K.Bizheva, A.Unterhuber, B.Hermann, H.Sattmann, A.F.Fercher, W.Drexler, A.Apolonski,

W.J.Wadsworth, J.C.Knight, P.St.J.Russell, M.Vetterlein, E.Scherzer, “Submicrometer axial resolution

optical coherence tomography,” Opt.Lett. Vol.27 pp.1800-1802 (2002)

[100] Y.M.Wang, Y.H.Zhao, J.S.Nelson, Z.P.Chen, R.S.Windeler, “Ultrahigh-resolution optical

coherence tomography by broadband continuum generation from a photonic crystal fiber,” Opt.Lett.

Vol.28 pp.182-184 (2003)

[101] B.J.Eggleton, P.S.Westvrook, R.S.Wiedeler, S.Spalter, and T.A.Strasser, “Grating resonances in

air-silica microstructured optical fibers,” Opt.Lett. Vol.24, pp.1460-1463 (1999)

[102] R.P.Espindola, R.S.Windeler, A.A.Abramov, B.J.Eggleton, T.A.Strasser, and D.J.DiGiovanni,

“External refractive index insensitive air-clad long period fiber grating,“ Electron.Lett. Vol.35,

pp.327-328 (1999)

[103] B.J.Eggleton, P.S.Westbrook, C.A.White, C.Kerbage, R.S.Windeler, and G.L.Burdge,

“Cladding-mode-resonance in air-silica microstructure optical fibers,” J.Lightwave.Tech., Vol.18,

pp.1084-1100 (2000)

[104] P.S.Westbrook, B.J.Eggleton, R.S.Windeler, A.Hale, T.A.Strasser, and G.Burdge, “Cladding mode

resonances in hybrid polymer-silica microstructured optical fiber gratings,” IEEE Photon.Tech.Lett.,

Vol.12, pp.495-497 (2000)

[105] B.J.Eggleton, A.K.Ahuja, K.S.Feder, C.Headley, C.Kerbage, M.D.Mermelstein, J.A.Rogers,

P.Steinvurzel, P.S.Westbrook, and R.S.Windeler, “Novel waveguide structures for enhanced fiber

grating devices,” IEEE J.Sel.Top.Quantum.Electron. Vol.7, pp.409-423 (2001)

[106] C.E.Kerbage, B.J.Eggleton, P.S.Westbrook and R.S.Windeler, “Experimental and scalar beam

propagation analysis of an air-silica microstructure fiber,” Opt.Exp. Vol.7, pp.113-122 (2000)

[107] G.Kakarantzas, T.A.Birks, and P.St.J.Russell, “Structural long-period gratings in photonic crystal

fibers,” Opt.Lett. Vol.27, pp.1013-1015 (2002)

[108] G.Kakarantzas, A.Ortigosa-Blanch, T.A.Birks, P.St.J.Russell, L.Farr, F.Couny, B.J.Mangan,

“Structural rocking filters in highly birefringent photonic crystal fiber,” Opt.Lett. Vol.28, pp.158-161

(2003)

[109] C.Kerbage, and B.J.Eggleton, “Numerical analysis and experimental design of tunable

birefringence in microstructured optical fiber,” Opt.Exp. Vol.10, pp.246-255 (2002)

[110] P.Mach, M.Dolinski, K.W.Baldwin, J.A.Rogers, C.Kerbage, R.S.Windeler, and B.J.Eggleton,

“Tunable microfluidic optical fiber,” Appl.Phys.Lett. Vol.80, pp.4294-4296 (2002)

[111] C.Kerbage, P.Steinvurzel, P.Reyes, P.S.Westbrook, R.S.Windeler, A.Hale, and B.J.Eggleton,

“Hi ghly tunable birefringent microstructured optical fiber,” Opt.Lett. Vol.27, pp.842-846 (2002)

[112] C.Kerbage, A.Hale, A.Yablon, R.S.WIndeler, and B.J.Eggleton, “Integrated all-fiber variable

attenuator based on hybrid microstructure fiber,” Appl.Phys.Lett. Vol.79, pp.3191-3193 (2001)

[113] T.M.Monro, D.J.Richardson, and P.J.Bennett, “Developing holey fibres for evanescent field

devices,” Electron.Lett. Vol.35, pp.1188-1189 (1999)

[114] T.M.Monro, W.Belardi, K.Furusawa, J.C.Baggett, N.G.R.Broderick, and D.J.Richardson, “Sensing

with microstructured optical fibres,” Meas. Sci. Techno. Vol.12, pp.854-858, 2001.

Bibliography 190

[115] Y.L.Hoo, W.Jin, H.L.Ho, D.N.Wang, R.S.Windeler, “Evanescent-wave gas sensing using

microstructure fiber,” Opt.Eng. Vol.41, pp.8-9 (2002)

[116] B.J.Mangan, J.C.Knight, T.A.Birks, P.St.J.Russell, and A.H.Greenaway, “Experimental study of

dual-core photonic crystal fiber,” Electron.Lett. Vol.36, pp.1358-1359 (2000)

[117] W.N.McPherson, M.J.Gander, R.McBride, J.D.C.Jones, P.M.Blanchard, J.G.Burnett,

A.H.Greenaway, B.Mangan, T.A.Birks, J.C.Knight, and P.St.J.Russell, “Remotely addressed optical

fibre curvature sensor using multicore photonic crystal fibre,” Opt.Commun. Vol.193, pp.97-104

(2001)

[118] W.J.Wadsworth, J.C.Knight, W.H.Reeves, P.St.J.Russell, and J.Arriaga, “Yb3+-doped photonic

crystal fibre laser,” Electron.Lett. Vol.36, pp.1452-1454 (2000)

[119] P.Glas and D.Fischer, “Cladding pumped large-mode-area Nd-doped holey fiber laser,” Opt.Exp.

Vol.10, pp.286-290 (2002)

[120] R.T.Bise, R.S.Windeler, K.S.Kranz, C.Kerbagae, B.J.Eggleton, D.J.Trevor, “Tunable photonic

band gap fiber,” in Proc:Conference of Optical Fiber Communications, OFC’2002 OSA Technical

Digest, Anaheim, CA, pp.466-467 (2002)

[121] K.M.Kiang, K.Frampton, T.M.Monro, R.Moore, J.Tucknott, D.W.Hewak, D.J.Richardson,

“Extruded single-mode non-silica glass holey optical fibres,” Electron.Lett. Vol.38, pp.546-547 (2002)

[122] M.Key, I.G.Hughes, W.Rooijakkers, B.E.Sauer, E.A.Hinds, D.J.Richardson, P.G.Kazansky

“Propagation of cold atoms along a miniature magnetic guide,” Phy.Rev.Lett. Vol.84 pp.1371-3 (2000)

[123] K.Tajima, J.Zhou, K.Nakajima, and K.Sato, “Ultr a low loss and long length photonic crystal

fiber,” in Proc: Optical Fibre Communication Conference, OFC 2003, Atlanta, PD01 (2003)

[124] R.J.Tonucci, B.L.Justus, A.J.Campillo, and C.E.Ford, “Nanochannel array glass,” Science,

Vol.258, pp.783-785 (1992)

[125] T.Hosaka, K.Okamoto, Y.Sasaki, and T.Edahiro, “Single-mode fibers with asymmetric refractive

index pits on both sides of the core,” Electron.Lett. Vol.17, 191-193 (1981)

[126] Y.Sasaki, T.Hosaka, K.Takada, and J.Noda, “8km-long polarization-maintaining fiber with highly

stable polarization state,” Electron.Lett.Vol.19,792-794 (1983)

[127] H.Kanamori, H.Yokota, G.Tanaka, M.Watanabe, Y.Ishiguro, I.Yoshida, T.Kakii, S.Ito, Y.Asano,

S.Tanaka, “Transmission characteristics and reliability of pure silica core single mode fibers,”

J.Lightwave.Tech. Vol.4 pp.1144-1150 (1986)

[128] J.E.Townsend, S.B.Poole, and D.N.Payne, “Solution-doping technique for fabrication of

rare-earth-doped optical fibres,” Electron. Lett. Vol.23, pp.329-331 (1987)

[129] P.Kaiser, A.R.Tynes, H.W.Astle, A.D.Pearson, W.G.French, R.E.Yaeger, and A.H.Cherin,

“Spectreal losses of unclad vitreous silica and soda-lime-silicate,” J.Opt.Soc.Am. Vol.63,

pp.1141-(1973)

[130] T.Izawa, S.Sudo, “Optical fibers: materials and fabrication,” Kulwer Academic, Amsterdam

(1987)

[131] S.Sakaguchi, “Studies on strength characteristics for high-silica optical fibers,” Ph.D. Thesis,

Tohoku university (1984)

[132] M.R.Vastag, “Coating anomalies,” Corning® Optical fiber white paper, WP3450, 6/00 (2000)

Bibliography 191

[133] K.Chida, T.Kimura, M.Wagatsuma, “High-speed optical fiber drawing,” Rev.ECL. Vol.32,

pp.425-431 (1984)

[134] Z.L.Yin and Y.Jaluria, “Thermal transport and flow in high-speed optical fiber drawing,” J.Heat

Transfer Trans. ASME Vol.120, pp.916-930 (1998)

[135] A.D.Fitt, K.Furusawa, C.P.Please, and T.M.Monro, “Modelling the fabrication of hollow optical

fibers: capillary drawing,” J.Lightwave.Tech.Vol.19 pp.1924-1931 (2001)

[136] A.D.Fitt, K.Furusawa, T.M.Monro, C.P.Please, and D.J.Richardson, “The mathematical modelling

of capillary drawing for holey fibre manufacture,” J.Eng.Math. Vol.43, pp.201-227 (2002)

[137] Polyflow Theoriteical Background, ver.3.6.0 (1998)

[138] N.P.Bansal and R.H.Doremus, “Handbook of Glass Properties,” Academic Press, New York

(1986)

[139] T.Miya, “A study on loss reduction and dispersion minimisation of single mode optical fibres for

long-wavelength band,” Ph.D. Thesis, Tohoku University (1983)

[140] G.A.Thomas, B.I.Shraiman, P.F.Glodis, and M.J.Stephan, “Toward the clarity limit in optical

fibre,” Nature Vol.404, pp.262-264 (2000)

[141] A.Sugama, “Polishing equipment for an inner wall of silica glass tubes,” Patent application

JP2000-239033; T.Mizuno, “Polishing method for an inner wall of silica glass tubes,” Patent

application JP2000-239032 (2000)

[142] S.Hopland, “Characteristics of the etching of undoped silica in MCVD-fabricated optical fibers

with buffered hydrofluoric acid,” Mat.Res.Bull. Vol.20, pp.1367-1372 (1985)

[143] J.K.Kang, C.B.Musgrave, “The mechanism of HF/H2O chemical etching of SiO2,” J.Chem.Phys.

Vol.116, pp.275-280 (2002)

[144] K.Imoto, M.Sumi, G.Toda, T.Suganuma, “Optical fiber drawing method with gas flow controlli ng

system,” J.Lightwave.Tech. Vol.7, p.115-121 (1989)

[145] M.Nakazawa, H.Kubota, K.Suzuki, E.Yamada, A.Sahara, T.Yamamoto, K.R.Tamura, “ 1.28

Tbit/s-70 km OTDM transmission using third- and fourth-order simultaneous dispersion compensation

with a phase modulator,” Electron.Lett. Vol.36 pp.2027-2029 (2000)

[146] A.R.Chraplyvy, R.W.Tkach, “Terabits Terabit/second transmission experiments,” IEEE

J.Quantum.Electron. Vol.34, pp.2103-2108 (1998)

[147] T.Okuno, M.Onishi, T.Kashiwada, S.Ishikawa, M.Nishimura, “ Silica-based functional fibers with

enhanced nonlinearity and their applications,” IEEE J.Sel.Top.Quantum.Electron.Vol.5, pp.1385 -1391

[148] V.Finazzi, T.M.Monro, and D.J.Richardson, “Small core silica holey fibers: nonlinearity and

confinement loss trade-offs,” submitted to J.Opt.Soc.Am B.

[149] S.S.Walker, “Rapid modelling and estimation of total spectral loss in optical fibers,”

J.Lightwave.Tech. Vol.4, pp.1125-1131 (1986)

[150] T.P.White, R.C.Mcphedran, C.M.de Sterke, L.C.Botten, M.J.Steel, “Confinement losses in

microstructured optical fibers,” Opt.Lett. Vol.26, pp.1660-1662 (2001)

[151] K.Suzuki, H.Kubota, S.Kawanishi, M.Tanaka, and M.Fujita, “Optical properties of a low-loss

polarisation-maintaining photonic crystal fiber,” Opt.Exp. Vol.9, pp.676-680 (2001)

Bibliography 192

[152] J.D.Love, “Application of a low-loss criterion to optical waveguides and devices,” IEE

Proceedings, Vol.136, Pt.J, pp.225-228 (1989)

[153] P.H.Krawarik, L.S.Watkins, “Fiber geometry specification and its relation to measured fiber

statistics,” Appl.Opt. Vol.17, pp.3984-3989 (1978)

[154] F.Ladouceur, J.D.Love, and T.J.Senden, “Effect of side wall roughness in buried channel

waveguides,” IEE Proc.-Optoelectron. Vol.141, pp.242-248 (1994)

[155] A.W.Snyder and J.D.Love, “Optical waveguide theory,” Kluwer academic publisher, Dordrecht

(1983)

[156] A.W.Poon, R.K.Chang, D.Q.Chowdhury, “Measurement of fiber-cladding diameter uniformity by

use of whispering-galley modes: nanometer resolution in diameter variations along millimetre to

centimetre lengths,” Opt.Lett. Vol.26, pp.1867-1869 (2001)

[157] T.Y.F.Tsang, “Optical third-harmonic generation at interfaces,” Phys.Rev. A Vol.52,

pp.4116-4125 (1995)

[158] R.Barille, L.Canioni, S.Rivert, L.Sarger, F.Brechet, P.Roy, and D.Pagnoux, “Nondestructive

analysis of the transverse structure of novel optical fibers by third-harmonic-generation microscopy,”

Opt.Lett. Vol.27, pp.1391-1393 (2002)

[159] N.A.Mortensen and J.R.Folkenberg, “Low-loss criterion and effective area considerations for

photonic crystal fibers,” J. Opt. A: Pure Appl. Opt. Vol.5, pp.163-167 (2003)

[160] A.Ferrando, E.Silvestre, J.J.Miret, P.Andres, M.V.Andres, “Vector description of higher-order

modes in photonic crystal fibers,” J.Opt.Soc.Am. A Vol.17, pp.1333-1340 (2000)

[161] J.D.Joannopoulos, J.N.Winn, R.D.Maede, “Phtonic crystals: molding the flow of light,” Princeton

Univ. Press (1995)

[162] F.Landouceur and L.Poladian, “Surface roughness and backscattering,” Opt.Lett. Vol.21,

pp.1833-1835 (1996)

[163] B.Chu, “Laser light scattering,” second edition, Academic press, San Diego (1974)

[164] A.H.Hartog and M.P.Gold, “On the theory of backscattering in single-mode optical fibers,”

J.Lightwave.Tech. Vol.2 pp.76-82 (1984)

[165] K.Tsujikawa, K.Tajima, M.Ohashi, “Rayleigh scattering reduction method for silica-based optical

fiber,” J.Lightwave.Tech. Vol.18 pp.1528-1532 (2000)

[166] Personal communication Y.T.Cho

[167] M.P.Gold and A.H.Hartog, “Ultra-long range OTDR in single mode fibers at 1.3µm,”

Electron.Lett. Vol.19, pp.463-464 (1983)

[168] M.P.Gold and A.H.Hartog, “Measurement of backscatter factor in single-mode fibers,”

Electron.Lett. Vol.17, pp.965-966 (1981)

[169] M.E.Fermann, S.B.Poole, D.N.Payne, and F.Martinez, “Comparative measurement of Rayleigh

scattering in single-mode optical fibers based on an OTDR Technique,” J.Lightwave.Tech. Vol.6,

pp.545-551 (1988)

[170] H.L.Hair, “Hydroxyl group on silica surface,” J.Non-cryst.Solids, Vol.19, pp.299-309 (1975)

[171] Personal communication P.Maton

Bibliography 193

[172] O.Humback, H.Fabian, U.Grzesik, U.Hanken, W.Heitmann, “Analysis of OH absorption bands in

synthetic silica,” J.Non-cryst.Solids, Vol.203 pp.19-26 (1996)

[173] K.Okamoto, “Fundamentals of optical waveguides,” Corona Publishing Co. Ltd., Tokyo (1992)

[174] D.J.Richardson, R.I.Laming, D.N.Payne, V.Matsas, and M.W.Phillips, “Self-starting passively

mode-locked erbium fibre ring laser based on the amplifyin g Sagnac switch,” Electron.Lett.

pp.542-543 (1991)

[175] D.J.Richardson, V.V.Afanasjev, A.B.Grudinin, and D.N.Payne, “Amplification of femtosecond

pulses in a passive, all-fiber soliton source,” Opt.Lett. Vol.27, pp.1596-1598 (1992)

[176] N.J.Doran and D.Wood, “Nonlinear optical loop mirror,” Opt.Lett. Vol.13, pp.56-58 (1988)

[177] M.E.Fermann, F.Haberl, M.Hofer, and H.Hochreiter, “Nonlinear amplifyin g loop mirror,”

Opt.Lett. Vol.15, pp.752-754 (1990)

[178] K.R.Tamura and K.Sato, “50GHz repetition rate, 280fs pulse generation at 100mW average power

from a mode-locked laser diode externally compressed in a pedestal-free pulse compressor,” Opt.Lett.

Vol.27, pp.1268-1270 (2002)

[179] Y.Matsui, M.D.Pelusi, and A.Suzuki, “Generation of 20fs optical pulses from a gain-switched

laser diode by a four-stage soliton compression technique,” IEEE Photon.Tech.Lett. Vol.11,

pp.1217-1219 (1999)

[180] M.H.Ober, M.Hofer, and M.E.Fermann, “42-fs pulse generation from a mode-locked fiber laser

started with a moving mirror,” Opt.Lett. Vol.18, pp.367-369 (1993)

[181] V.Cautaerts, D.J.Richardson, R.Paschotta, D.C.Hanna, “Stretched pulse Yb3+:silica fibre laser,”

Opt. Lett. Vol.22 pp.316-318 (1997)

[182] L.Lefort, J.H.V.Price, D.J.Richardson, G.J.Spuhler, R.Paschotta, U.Keller, A.R.Fry, and J.Weston,

“Practical low-noise stretched-pulse Yb3+-doped fiber laser,” Opt.Lett. Vol.27, pp.291-293 (2002)

[183] L.Lefort, A.Albert, V.Couderc, and A.Barthelemy, “Highly stable 68fs pulse generation from a

stretched pulsed Yb3+ doped fiber laser with frequency shifted feedback,” IEEE Photon.Tech.Lett.

Vol.14, pp.1674-1676 (2002)

[184] M.Hofer, M.H.Ober, R.Hofer, M.E.Fermann, G.Sucha, D.Harter, K.Sugden, I.Bennion,

C.A.C.Mendonca, and T.H.Chiu, “High-power neodymium soliton fiber laser that uses a chirped fiber

grating,” Opt.Lett. Vol.20, pp.1701-1703 (1995)

[185] F.M.Mitschke and L.F.Mollenauer, “Discovery of the soliton self-frequency shift,” Opt.Lett.

Vol.11, pp.659-661 (1986)

[186] M.E.Fermann, A.Galvanauskas, M.L.Stock, K.K.Wong, D.Harter, and L.Goldberg, “Ultrawide

tunable Er soliton fiber laser amplified in Yb-doped fiber,” Opt.Lett. Vol.24, pp.1428-1430 (1999)

[187] M.E.Fermann, “Ultrashort pulse fiber lasers,” in Proc: Conference on Lasers and Electro-Optics,

CLEO ‘2001, CWM-1, Baltimore, Maryland (2001)

[188] R.F.Cregan, J.C.Knight, P.St.Russel, and P.J.Roberts, “Distribution of spontaneous emission from

an Er3+-doped photonic crystal fiber,” J.Lightwave Tech. Vol.17, pp.2138-2141 (1999)

[189] J.E.Townsend, “The development of optical fibres doped with rare-earth ions,” Ph.D. Thesis,

University of Southampton (1990)

Bibliography 194

[190] S.Matsui, F.Ohira, K.Koyabu, K.Matsunaga, “Characterisation of machining-damage layer for

optical fiber ends-relation between damaged layer and return loss,” J.Jpn.Soc.Prec.Eng. Vol.64,

pp.1467-1471 (1998)

[191] M.J.Steel, T.P.White, C.M.de Sterke, R.C.Mcphedran, and L.C.Botten, “Symmetry and

degeneracy in microstructured optical fibers,” Opt.Lett. Vol.26, pp.488-490 (2001)

[192] M.Koshiba, K.Satoh, “Numerical verification of degeneracy in hexagonal photonic crystal fibers,”

IEEE Photon.Tech.Lett. Vol.13, pp.1313-1315 (2001)

[193] Personal communication, V.Finazzi

[194] A.Ortigosa-Blanch, J.C.Knight, W.J.Wadsworth, J.Arriaga, B.J.Mangan, T.A.Birks, and

P.St.J.Russell, “Hi ghly birefringent photonic crystal fibers,” Opt.Lett. Vol.25, pp.1325-1327 (2000)

[195] T.P.Hansen, J.Broeng, S.E.B.Libori, A.Bjaklev, J.R.Jensen, and H.Simonsen, “Highly birefringent

index-guiding photonic crystal fibers,” IEEE Photon.Tech.Lett. Vol.13, pp.588-590 (2001)

[196] F.V.Kowalski, S.J.Shattil, and P.D.Hale, “Optical pulse generation with a frequency shifted feed

back laser,” Appl.Phys.Lett. Vol.53, pp.734-736 (1988)

[197] S.H.Yun, D.J.Richardson, D.O.Culverhouse, and B.Y.Kim, “Wavelength-swept fiber laser with

frequency shifted feedback and resonantly swept intra-cavity acoustic tunable filter,” IEEE

J.Sel.Top.Quantum.Electron., Vol.3, pp.1087-1096 (1997)

[198] L.F.Mollenauer, J.P.Gordon, and S.J.Evangelides, “The sliding-frequency guiding filter: an

improved form of soliton jitter control,” Opt.Lett. Vol.17, pp.1575-1577 (1992)

[199] N.G.R. Broderick, H.L.Offerhaus, D.J.Richardson, R.A.Sammut, “Power Scaling in passively

mode locked large mode area fibre lasers,” IEEE Photon.Tech.Lett. Vol.10, pp.1718-1720 (1998)

[200] H.Lim, F.O.Ilday, and F.W.Wise, “Femtosecond ytterbium fiber laser with photonic crystal fiber

for dispersion control,” Opt.Exp. Vol.10, pp.1497-1502 (2002)

[201] F.X.Kartner, J.A.D.Au, U.Keller, “Mode-locking with slow and fast saturable absorbers-what’s the

deifference?” IEEE J.Sel.Top.Quantum.Electron. Vol.4, pp.159-168 (1998)

[202] U.Keller, K.J.Weingarten, F.X.Kartner, D.Kopf, B.Braun, I.D.Jung, R.Fluck, C.Honninger,

N.Matuschek, and J.Aus der Au, “Semiconductor saturable absorber mirrors for femtosecond to

nanosecond pulse generation in solid-state lasers,” IEEE J.Sel.Top.Quantum electron. Vol.2,

pp.435-452 (1996)

[203] A.V.Avdokhin, S.V.Popov, and J.R.Taylor, “Totally fiber integrated, figure-of-eight, femtosecond

source at 1065nm,” Opt.Exp. Vol.11, pp.265-269 (2003)

[204] J.P.Gordon, “Theory of the soliton self-frequency shift,” Opt.Lett. Vol.11, pp.662-664 (1986)

[205] R.Paschotta, J.H.V.Price, and D.J.Richardson, to be published.

[206] personal communication: D.J.Richardson

[207] Y.Ohmori, F.Hanawa, M.Nakahara, “Fabrication of low-loss Al2O3 doped silica fibers,”

Electron.Lett. Vol.18, pp.761 (1982)

[208] W.L.Barnes, P.R.Morkel, L.Reekie, and D.N.Payne, “High-quantum-efficiency Er3+ fiber lasers

pumped at 980nm,” Opt.Lett. Vol.14 pp.1002-1004 (1989)

Bibliography 195

[209] M.Kakui, T.Kashiwada, K.Nakazato, M.Onishi, C.Fukuda, M.Shigematsu and M.Nishimura,

“Design Optimization of Hybrid Erbium-Doped Fiber Amplifier s for WDM Transmission Systems,”

Opt.Fib.Tech. Vol.3 pp.123-133 (1997)

[210] J.K.Sahu, C.C.Renaud, J.Nilsson, W.A.Clarkson, S.A.Alam, and A.B.Grudinin,

“Single-polarisation laser operation in polarization-maintaining single-polarisation pumped

erbium-doped fiber,” in Proc: Conference on Lasers and Electro-Optics, CLEO ‘2001, Baltimore

CWC5 (2001)

[211] H.Po, J.D.Cao, B.M.Laliberte, R.A.Minns, R.F.Robinson, B.H.Rockney, R.R.Tricca, and

Y.H.Zhang, “High-power neodymium-doped single transverse-mode fiber laser,” Electron.Lett. Vol.29,

pp.1500-1501 (1993)

[212] V.Dominic, S.MacCormack, R.Waarts, S.Sanders, S.Bicknese, R.Dohle, E.Wolak, P.S.Yeh, and

E.Zucker, “110W fiber laser,” Electron.Lett. Vol.35, pp.1158-1160 (1999)

[213] J.Limpert, A.Liem, S.Hofer, H.Zellmer, A.Tunnermann, S.Unger, S.Jetschke, and H.R.Muller,

“150W Nd/Yb codoped fiber laser at 1.1µm,” in Proc: Conference on Lasers and Electro-Optics,

CLEO’2002, pp.590-591 Long Beach, CA (2002)

[214] N.S.Platonov, D.V.Gapontsev, V.P.Gapontsev, and V.Shumilin, “135W CW fiber laser with

perfect single mode output,” in Proc: Conference on Lasers and Electro-Optics, CLEO’2002

CPD3-1-4, Long Beach CA (2002)

[215] K.Ueda, H.Sekiguchi, H.Kan, “1kW CW output from fiber-embedded disk lasers,” in Proc:

Conference on Laser and Electro-Optics, CLEO ‘2002 CPD4-1-2, Long Beach CA(2002)

[216] R.J.Beach, M.D.Feit, R.H.Page, L.D.Brasure, R.Wilcox, and S.A.Payne, “Scalable antiguided

ribbon laser,” J.Opt.Soc.Am. B Vol.19, pp.1521-1534 (2002)

[217] C.J.Karlsson, F.A.A.Olsson, D.Letalick, and M.Harris, “All-fibe r multifunction continuous-wave

coherent laser radar at 1.55µm for range, speed, vibration, and wind measurements,” Appl.Opt. Vol.39,

pp.3716-3726 (2000)

[218] T.Sumiyoshi, H.Sekita, T.Arai, S.Sato, M.Ishihara, M.Kikuchi, “High-power continous-wave 3

and 2µm cascade Ho3+:ZBLAN fiber laser and its medical applications,” IEEE

J.Sel.Top.Quantum.Electron. Vol.5, pp.936-943 (1999)

[219] K.P.Petrov, R.F.Curl, F.K.Tittel, and L.Goldberg, “Continuous-wave tunable 8.7µm spectroscopic

source pumped by fiber-coupled communications lasers,” Opt.Lett. Vol.21, pp.1451-1453 (1996)

[220] L.Goldberg, J.Koplow, D.G.Lancaster, R.F.Curl, and F.K.Tittel, “Mid-infrared

difference-frequency generation source by a 1.1-1.5µm dual-wavelength fiber amplifier for trace-gas

detection,” Opt.Lett. Vol.23, pp.1517-1519 (1998)

[221] G.T.Moore, C.A.Denman, and T.Sean Ross, “Using multiple mutually incoherent fiber lasers to

pump a coherent signal beam in an optical parametric oscillator,” IEEE Quantum.Electron. Vol.36,

pp.828-834 (2000)

[222] D.G.Lancaster, D.Richter, R.F.Curl, F.K.Tittel, L.Goldberg, and J.Koplow, “High-power

continuous-wave mid-infrared radiation generated by difference frequency mixing of

diode-laser-seeded fiber amplifier s and its application to dual-beam spectroscopy,” Opt.Lett. Vol.24,

pp.1744-1746 (1999)

Bibliography 196

[223] D.G.Lancaster, D.Richter, and F.K.Tittel, “Portable fiber-coupled diode-laser-based sensor for

multiple trace gas detection,” Appl.Phys. B Vol.69, pp.459-465 (1999)

[224] K.O.Hill , G.Meltz, “Fiber Bragg grating technology fundamentals and overview,”

J.Lightwave.Tech. Vol.15, pp.1263-1276 (1997)

[225] H.M.Pask, R.J.Carman, D.C.Hanna, A.C.Tropper, C.J.Mackechnie, P.R.Barber, and M.Dawes,

“Y tterbium-doped silica fiber lasers: versatile sources for the 1-1.2µm region,” IEEE

J.Sel.Top.Quantum Electron. Vol.1, pp.2-13 (1995)

[226] M.Shimizu, T.Kanamori, J.Temmyo, M.Wada, M.Yamada, Y.Terunuma, Y.Ohishi, S.Sudo,

“28.3dB gain 1.3.µm-band Pr-doped fluoride fiber amplifier module pumped by 1.017µm InGaAs

LDs,” IEEE Photon.Tech.Lett. Vol.5, pp.654-657 (1993)

[227] R.M.Percival and J.R.Williams, “Highly efficient 1.064µm up-conversion pumped 1.47µm

thulium-doped fluoride fiber amplifier,” Electron.Lett. Vol.30, pp.1684-1685 (1994)

[228] D.Piehler, D.Craven, N.Kwong, H.Zarem, “Laser-diode-pumped red and green up-conversion

fiber lasers,” Electron.Lett. Vol.29, pp.1857-1858 (1993)

[229] J.Y.Allain, M.Monerie, H.Poignant, “Blue upconversion fluorozirconate fiber laser,” Electron.Lett.

Vol.26, pp.166-168 (1990)

[230] M.Usami, Y.Matsushima, H.Horie, and H.Kaneda, “Highly reliable and high power 980nm pump

laser diode module for undersea cable systems,” IEICE Trans.Electron. Vol.84-C, pp.639-647 (2001)

[231] B.Sverdlow, B.Schmidt, S.Pawlik , B.Mayer, and C.Harder, “1W 980nm pump modules with very

high efficiency,” in Proc: European Conference on Optical Communication, PD3.6, ECOC’2002

Amsterdam (2002)

[232] W.J.Alford, T.D.Raymond, and A.A.Allerman, “High power and good beam quality at 980nm

from a vertical external-cavity surface-emitting laser,” J.Opt.Soc.Am. Vol.19, pp.663-666 (2002)

[233] D.C.Hanna, R.M.Percival, I.R.Perry, R.G.Smart, P.J.Suni, and A.C.Tropper, “An ytterbium-doped

monomode fibre laser: broadly tunable operation from 1.010µm to 1.162µm and three level operation

at 974nm,” J.Mod.Opt. Vol.37, pp.517-525 (1990)

[234] J.Nilsson, J.D.Minelly, R.Paschotta, A.C.Tropper, and D.C.Hanna, “Ring-doped cladding-pumped

single-mode three-level fiber laser,” Opt.Lett. Vol.23, pp.355-357 (1998)

[235] R.Paschotta, J.Nilsson, P.R.Barber, J.E.Caplen, A.C.Tropper, and D.C.Hanna, “Lifetime

quenching in Yb-doped fibres,” Opt.Commun. Vol.136, pp.375-378 (1997)

[236] C.C.Renaud, R.J.Selvas-Aguilar, J.Nilsson, P.W.Turner, and A.B.Grudinin, “Compact high-energy

Q-switched cladding-pumped fiber laser with a tuning range over 40nm,” IEEE Photon.Tech.Lett.

Vol.11, pp.976-978 (1999)

[237] D.J.Ripin and L.Goldberg, “High efficiency side-coupling of light into optical fibres using

imbedded v-grooves,” Electron.Lett. Vol.31, pp.2204-2205 (1995)

[238] A.Hideur, T.Chartier, C.Ozkul, F.Sanchez, “All-fiber tunable ytterbium-doped double-clad fiber

ring laser,” Opt.Lett. Vol.26, pp.1054-1056 (2001)

[239] M.Auerbach, P.Adel, D.Wandt, C.Fallnich, S.Unger, S.Jetschke, and H.R.Muller, “10W widely

tunable narrow linewidth double-clad fiber ring laser,” Opt.Exp. Vol.10, pp.139-144 (2002)

Bibliography 197

[240] J.Nilsson, R.Paschotta, J.E.Caplen, and D.C.Hanna, “Yb3+-ring-doped fiber for high-energy pulse

amplification,” Opt.Lett. Vol.14, pp.1092-1094 (1997)

[241] Efiron® PC series from Luvantix Co.Ltd.

[242] H.Nagata, “Thermal analysis of jacketing materials for commercial optical fibers,” Opt.Fib.Tech.

Vol.3, pp.87-89 (1997)

[243] Fluorosil® from Heraeus Tenevo AG

[244] R.Setaka, H.Takahashi, T.Sato, S.Yoshida, “Fluorine-doping levels in the low-pressure PCVD

process,” in Proc: Conference on Optical Fiber Communication, OFC’84 p.68 (1984)

[245] M.J.Dejneka, B.Z.Hanson, S.G.Crigler, L.A.Zenteno, J.D.Minelly, D.C.Allan, W.J.Miller, and

D.Kuksenkov, “La2O3-Al 2O3-SiO2 glasses for high-power, Yb-doped, 980nm fiber lasers,”

J.Am.Ceram.Soc. Vol.85, pp.1100-1106 (2002)

[246] L.A.Zenteno, J.D.Minelly, A.Liu, A.J.G.Ellison, S.G.Crigler, D.T.Walton, D.V.Kuksenkov, and

M.J.Dejneka, “1W single-transverse-mode Yb-doped double-clad fibre laser at 978nm,” Electron.Lett.

Vol.37, pp.819-820 (2001)

[247] J.T.Kringlebotn, J.LArchambault, and D.N.Payne, “Er3+:Yb3+-codoped fiber distributed-feedback

laser,” Opt.Lett. Vol.19, pp.2101-2103 (1994)

[248] M.D.Mermelstein, C.Headley, J.C.Bouteiller, P.Steinvurzel, C.Horn, K.Feder, and B.J.Eggleton,

“Configurable three-wavelength Raman fiber laser for Raman amplification and dynamic gain

flattening,” IEEE Photon.Tech.Lett. Vol.13, pp.1286-1288 (2001)

[249] R.Kashyap, R.Wyatt, and R.J.Campbell, “Wideband gain flattened erbium fibre amplifier using a

photosensitive fibre blazed grating,” Electron.Lett. Vol.21, pp.154-156 (1993)

[250] S.Bedo, W.Luthy, and H.P.Weber, “The effective absorption coefficient in double-clad fibres,”

Opt.Commun. Vol.99, pp.331-335 (1993)

[251] Personal communication C.C.Renaud

[252] A.Ghatak and K.Thyagarajan, “Introduction to fiber optics,” Cambridge university press,

Cambridge (1998)

[253] V.A.Kozlov, J.Hernandez-Cordero, R.L.Shubochkin, A.L.G.Carter, T.F.Morse, “Silica-air

double-clad optical fiber,” IEEE Photon.Tech.Lett. Vol.12, pp.1007-1009 (2000)

[254] R.Kashyap, “Fiber Bragg gratings,” Academic Press, San Diego (1999)

[255] K.Shima, K.Himeno, T.Sakai, S.Okude, A.Wada, R.Yamauchi, “Temperature-insensitive

long-period fiber grating,” Fujikura-Giho, Vol.92, pp.11-13 (1997)

[256] T.Enomoto, M.Harumoto, M.Shigehara, S.Ishikawa, and H.Kanamori, “Temperature-insensitive

long-period fiber gratings written in P2O5 co-doped GeO2/SiO2 core optical fibers,” in Proc: Bragg

gratings, photosensitivity, and poling in glass waveguides, BGPP’2000, OSA Trends in Optics and

Photonics series Vol.33, pp.150-152 (2000)

[257] F.Brunner, T.Sudmeyer, E.Innerhofer, R.Paschotta, U.Keller, K.Furusawa, J.C.Bagget,

T.M.Monro, and D.J.Richardson, “Sub-50fs pulses with 24W average power from a passively

mode-locked thin disk Yb:YAG laser with nonlinear fiber compression,” submitted to CLEO’2003

[258] T.Abel, J.Hirsch, and J.A.Harrington, “Hollow glass waveguides for broadband infrared

transmission,” Opt.Lett. Vol.19 pp.1034-1036 (1994)

Bibliography 198

[259] Y.Matsuura and M.Miyagi, “Flexible hollow waveguides for delivery of excimer-laser light,”

Opt.Lett. Vol.23, pp.1226-1228 (1998)

[260] I.K.Ilev, R.W.Waynant, M.N.Ediger, and M.A.Bonaguidi, “Ultraviolet laser delivery using an

uncoated hollow taper,” IEEE J.Quantum.Electron. Vol.36, pp.944-948 (2000)

[261] M.M.Fejer, G.A.Magel, D.H.Jundt, and R.L.Byer, “Quasi-phase-matched 2nd

harmonic-generation –tuning tolerances,” IEEE J.Quantum.Electron. Vol.28 , pp.2631-2654 (1992)

[262] D.Taverner, D.J.Richardson, L.Dong, J.E.Caplen, K.Williams, and R.V.Penty, “158µJ pulses from

a single-transverse-mode large-mode-area erbium-doped fiber amplifier,” Opt.Lett. Vol.22, pp.378-380

(1997)

[263] M.E.Fermann, “Single-mode excitation of multimode fibers with ultrashort pulses,” Opt.Lett.

Vol.23, pp.52-54 (1998)

[264] J.P.Koplow, D.A.V.Kliner, L.Goldberg, “Single-mode operation of a coiled multimode fiber

amplifier,” Opt.Lett. Vol.25, pp.442-445 (2000)

[265] H.L.Offerhaus, J.A.Alvarez-Chavez, J.Nilsson, P.W.Turner, W.A.Clarkson, and D.J.Richardson,

“Multi-mJ, multi-watt Q-switched fiber laser,” Opt.Lett. Vol.25, pp.37-39 (2000)

[266] M.E.Fermann, A.Galvanauskas, M.Hofer, “Ultrashort pulse sources based on multi-mode optical

fibers,” Appl.Phys.B Vol.70, pp.s13-23 (2000)

[267] F.D.Teodoro, J.P.Koplow, and S.W.Moore, D.A.V.Kliner, “Di ffraction-limited, 300kW

peak-power pulses from a multimode fiber amplifier,” Opt.Lett. Vol.27, pp.518-520 (2002)

[268] N.G.R.Broderick, H.L.Offerhaus, D.J.Richardson, R.A.Sammut, J.Caplen, L.Dong,

“Large-mode-area fibers for high power applications,” Opt.Fiber.Tech. Vol.5, pp.185-196 (1999)

[269] B.J.Mangan, J.Arriaga, T.A.Birks, J.C.Knight, and P.St.J.Russell, “Fundamental-mode cutoff in a

photonic crystal fiber with a depressed-index core,” Opt.Lett. Vol.26, pp.1469-1471 (2001)

[270] J.C.Baggett, T.M.Monro, K.Furusawa, D.J.Richardson, “Understanding bending losses in holey

optical fibres,” submitted to Opt.Commun.

[271] C.W.J.Hillman, W.S.Brocklesby, T.M.Monro, W.Belardi, D.J.Richardson, “Structural and optical

characterisation of holey fibres using scanning probe microscopy,” Electron. Lett. Vol.37 pp.1283-4

(2001)

[272] A.Boskovic, S.V.Chrnikov, J.R.Taylor, L.Gruner-Nielsen, and O.A.Levring, “ Direct

continuous-wave measurement of n2 in various types of telecommunication fiber at 1.55µm,” Opt.Lett.

Vol.21, pp.1966-1968 (1996)

[273] T.M.Monro, V.Pruneri, N.G.R.Broderick, D.Faccio, P.G.Kazansky, D.J.Richardson, “Broadband

second harmonic generation in holey optical fibers,” IEEE Photon.Tech.Lett. Vol.13 pp.981-3 (2001)

[274] J.Sakai and T.Kimura, “Bending loss of propagation modes in arbitrary index profile optical

fibers,” Appl.Opt. Vol.17 pp.1499-1506 (1978)

[275] L.Faustini and G.Martini, “Bend loss in single-mode fibers,” J.Lightwave.Tech. Vol.15,

pp.671-679 (1997)

[276] T.Sorensen, J.Broeng, A.Bjarklev, E.Knudsen, and S.E.Barkou Libori, “Macro-bending loss

properties of photonic crystal fibre,” Electron.Lett. Vol.37, pp.287-289 (2001)

Bibliography 199

[277] K.Nakajima and M.Ohashi, “Design consideration for inverse dispersion fiber,” IEICE

Trans.Electron. Vol.E85-C pp.896-902 (2002)

[278] J.D.Love, W.M.Henry, W.J.Stewart, R.J.Black, S.Lacroix, and F.Gonthier, “Tapered single-mode

fibres and devices part1: adiabaticity criteria,” IEE Proceedings-J, Vol.138, pp.343-354 (1991)

[279] T.Hasegawa, E.Sasaoka, M.Onishi, M.Nishimura, Y.Tsuji, M.Koshiba, “Hole-assisted lightguide

fiber for large anomalous dispersion and low optical loss,” Opt.Exp. Vol.9 pp.681-686 (2001)

[280] N.Bloenbergen, “Laser-induced electric breakdown in solids,” IEEE J.Quantum.Electron. Vol.10,

pp.375-386 (1974)

[281] A.Li u and K.Ueda, “The absorption characteristics of circular, offset, and rectangular double-clad

fibers,” Opt.Commun. Vol.132, pp.511-518 (1996)

[282] D.A.V.Kliner, J.P.Koplow, L.Goldberg, A.L.G.Carter, and J.A.Digweed,

“Polarisation-maintaining amplifier employing double-clad bow-tie fiber,” Opt.Lett. Vol.26,

pp.184-186 (2001)

[283] S.J.Augst, A.K.Goyal, R.L.Aggarwal, T.Y.Fan, and A.Sanchez, “Wavelength beam combining of

ytterbium fiber lasers,” Opt.Lett. Vol.28, pp.331-333 (2003)

[284] M.Warage, P.Glas, D.Fischer, M.Leitner, D.V.Vysotsky, and A.P.Napartovich, “Phase locking in a

multicore fiber laser by means of a Talbot resonator,” Opt.Lett. Vol.25, pp.1436-1438 (2000)

[285] P.K.Cheo, A.Liu, and G.G.King, “A high-brightness laser beam from a phase-locked multicore

Yb-doped fiber laser array,” IEEE Photon.Tech.Lett. Vol.13, pp.439-441 (2001)

[286] V.A.Kozlow, J.Hernandez-Cordero, and T.F.Morse, “All-fibe r coherent beam combining of fiber

laser,” Opt.Lett. Vol.24, pp.1814-1816 (1999)

[287] A.Shirakawa, T.Saitou, T.Sekiguchi, and K.Ueda, “Coherent addition of fiber lasers by use of a

fiber coupler,” Opt.Exp. Vol.10, pp.1167-1172 (2002)

[288] D.Sabourdy, V.Kermene, A.Desfarges-Berthelemot, L.Lefort, A.Barthelemy, P.Even, and

D.Pureur, “Efficient coherent combining of widely tunable fiber lasers,” Opt.Exp. Vol.11, pp.87-97

(2003)

[289] S.Sanders, R.Waarts, D.Nam, D.Welch, D.Scrifres, J.C.Ehlert, W.J.Cassarly, J.M.Finlan, and

K.M.Flood, “High-power coherent 2-dimensional semiconductor-laser array,” Appl.Phys.Lett. Vol.64,

pp.1478-1480 (1994)

[290] Y.Kono, M.Takeoka, K.Uto, A.Uchida, and F.Kannari, “A coherent all-solid-state laser array

using the Talbot effect in a three-mirror cavity,” I EEE J.Quantum.Electron. Vol.36, pp.607-614 (2000)

[291] R.M.Macfarlane, F.Tong, A.J..Silversmith, and W.Lenth, “Violet CW neodymium upconversion

laser,” Appl.Phys.Lett. Vol.52, pp.1300-1302 (1988)

[292] D.C.Hanna, I.M.Jauncey, R.M.Percival, I.R.Perry, R.G.Smart, P.J.Suni, J.E.Townsend, and

A.C.Tropper, “Continous-wave oscillation of a monomode thulium-doped fiber laser,” Electron.Lett.

Vol.24, pp.1222-1223 (1988)

[293] W.J.Wadsworth, R.M.Percival, G.Bouwmans, J.C.Knight, and P.St.J.Russell, “Hi gh power

air-clad photonic crystal fiber laser,” Opt.Exp. Vol.11, pp.48-53 (2003)

[294] X.Zou, H.Toratani, “Evaluation of spectroscopic properties of Yb3+ doped glasses,” Phys.Rev.B

Vol.52, pp.15889-15897 (1995)

Bibliography 200

[295] Z.Burshtein, Y.Kalisky, S.Z.Levy, P.Le Boulanger, and S.Rotman, “Impurity local phonon

nonradiative quenching of Yb3+ fluorescence in ytterbium-doped silicate glasses,” IEEE J.Quantum

Electron. Vol.36, pp.1000-1006 (2000)

[296] R.Paschotta, J.Nilsson, A.C.Tropper, and D.C.Hanna, “An ytterbium-doped fiber amplifer,” IEEE

J.Quantum Electron. Vol.33, pp.1049-1056 (1997)

[297] J.A.Alvarez-Chavez, H.L.Offerhaus, J.Nilsson, P.W.Turner, W.A.Clarkson, and D.J.Richardson,

“High-energy, high-power ytterbium-doped Q-switched fiber laser,” Opt.Lett. Vol.25, pp.37-39 (2000)

[298] C.C.Renaud, H.L.Offerhaus, J.A.Alvarez-Chavez, J.Nilsson, W.A.Clarkson, P.W.Turner,

D.J.Richardson, and A.B.Grudinin, “Characteristics of Q-switched cladding-pumped ytterbium-doped

fiber lasers with different high-energy fiber designs,” IEEE J.Quantum.Electron. Vol.37, pp.199-205

(2001)

[299] A.E.Siegman, LASERS: University Science books, California, (1986)

[300] J.Squier, M.Muller, “High resolution nonlinear microscopy: a review of sources and methods for

achieving optimal imaging,” Rev.Sci.Inst. Vol.72, pp.2855-2867 (2001)

[301] S.Nolte, C.Momma, H.Jacobs, A.Tunnermann, B.N.Chichkov, B.Wellegehausen, and H.Welling,

“Ablation of metals by ultrashort laser pulses,” J.Opt.Soc.Am. B Vol.14, pp.2716-2722 (1997)

[302] C.B.Schaffer, A.Brodeur, J.F.Garcia, and E.Mazur, “Micromachining bulk glass by use of

femtosecond laser pulses with nanojoule energy,” Opt.Lett. Vol.26, pp.93-95 (2001)

[303] P.Adel and C.Fallnich, “High-power ultra-broadband mode-locked Yb3+ fiber laser with 118nm

bandwidth,” Opt.Exp. Vol.10, pp.622-627 (2002)

[304] A.Hiduer, T.Chartier, M.Brunel, S.Louis, C.Ozkul, and F.Sanchez, “Generation of high energy

femtosecond pulses from a side-pumped Yb-doped double-clad fiber laser,” Appl.Phys.Lett. Vol.79,

pp.3389-3391 (2001)

[305] A.Hideur, T.Chartier, M.Brunel, M.Salhi, C.Ozkul, F.Sanchez, “Mode-lock, Q-switch and CW

operation of an Yb-doped double-clad fiber ring laser,” Opt.Commun. Vol.198, pp.141-146 (2001)

[306] A.Hideur, T.Chartier, C.Ozkul, F.Sanchez, “Dynamics and stabilisation of a high power

side-pumped Yb-doped double-clad fiber laser,” Opt.Commum. Vol.186, pp.311-317 (2000)

[307] A.Galvanauskas, G.C.Cho, A.Hariharan, M.E.Fermann, and D.Harter, “Generation of high-energy

femtosecond pulses in multimode-core Yb-fiber chirped-pulse amplification systems,” Opt.Lett.

Vol.26, pp.935-937 (2001)

[308] D.Nickel, A.Liem, J.Limpert, H.Zellmer, U.Griebner, S.Unger, G.Korn, and A.Tunnermann,

“Fiber based high repetition rate, high energy laser source applying chirped pulse amplification,”

Opt.Commun. Vol.190, pp.309-315 (2001)

[309] M.E.Fermann, V.I.Kruglow, B.C.Thomsen, J.M.Dudley, and J.D.Harvey, “Self-similar

propagation and amplification of parabolic pulses in optical fibers,” Phys.Rev.Lett. Vol.84,

pp.6010-6013 (2000)

[310] J.Limpert, T.Schreiber, T.Clausnitzer, K.Zollner, H.J.Fuchs, E.B.Kley, H.Zellmer, and

A.Tunnermann, “High-power femtosecond Yb-doped fiber amplifier,” Opt.Exp. Vol.10, pp.628-638

(2002)

Bibliography 201

[311] S.H.Cho, B.E.Bouma, E.P.Ippen, and J.G.Fujimoto, “Low-repetition-rate high-peak-power

Kerr-lens mode-locked Ti : Al 2O3 laser with a multiple-pass cavity,” Opt.Lett. Vol.24 pp.417-419

(1999)

[312] M.J.Weber, J.E.Lynch, D.H.Blackburn, and D.J.Cronin, “Dependence of the stimulated emission

cross section of Yb3+ on host glass composition,” IEEE J.Quantum Electron. Vol.19, pp.1600-1608

(1983)

[313] H.Takebe, T.Murata, and K.Morinaga, “Compositional dependence of absorption and fluorescence

of Yb3+ in oxide glasses,” J.Am.Cer.Soc. Vol.79, pp.681-687 (1996)

[314] Personal communications: E.J.Tarbox

[315] P.G.Simplins, S.Greenberg-Koinski, and J.B.MacChesney, “Thermophoresis: the mass transfer

mechanism in modified chemical vapour deposition,” J.Appl.Phys. Vol.50, pp.5676-5681 (1979)

[316] D.A.Thompson, P.L.Bocko, and J.R.Gannon, “New source compounds of the fabrication of doped

optical waveguide fibres,” in Proc: The intenational society of optical engineering (SPIE) Vol.506,

pp.170-175 (1984)

[317] Personal communications: D.W.J.Harwood, J.K.Sahu, and R.B.Williams.

[318] N.A. Mortensen, M.D. Nielsen, J.R. Folkenberg, A. Petersson, and H.R. Simonsen, “Improved

large-modearea endlessly single-mode photonic crystal fibers,” Opt. Lett. Vol.28, pp.393-395 (2003)

[319] J.Limpert, T. Schreiber, S. Nolte, H. Zellmer, T. Tunnermann, R. Iliew, F. Lederer, J. Broeng, G.

Vienne, A. Petersson, and C. Jakobsen, “High-power air-clad large-mode-area photonic crystal fiber

laser,” Opt.Exp. Vol.11, pp.818-823 (2003)

List of Publications

Journal papers: Z.Yusoff, P.Petropoulos, K.Furusawa, T.M.Monro, and D.J.Richardson, “A 36 channelx10GHz spectrally sliced pulse source based on supercontinuum generation in normally dispersive highly nonlinear holey fibre,” IEEE Photon.Tech.Lett. (accepted for publication) T.Sudmeyer, F.Brunner, E.Innerhofer, R.Paschotta, K.Furusawa, J.C.Baggett, T.M.Monro, D.J.Richardson, U.Keller �

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Nonlinear femtosecond pulse compression at high average power levels using a large mode area holey fiber,” Opt.Lett. Vol.28, pp.1951-1953 (2003) J.C.Baggett, T.M.Monro, K.Furusawa, D.J.Richardson, “Understanding bending losses in holey optical fibers,” Opt.Commun. Vol.227, pp.317-335 (2003) J.H.V.Price, T.M.Monro, K.Furusawa, W.Belardi, J.C.Baggett, S.Coyle, C.Netti, J.J.Baumberg, R.Paschotta, D.J.Richardson, “UV generation in a pure silica holey fiber,” Appl.Phys. B Vol.77 pp.291-298(2003) J.H.Lee, W.Belardi, K.Furusawa, P.Petropoulos, Z.Yusoff, T.M.Monro, D.J.Richardson, “Four-wave mixing based 10Gbit/s tuneable wavelength conversion using a holey fiber with a high SBS threshold,” IEEE Photon.Tech.Lett. Vol.15 pp.440-442 (2003) A.D.Fitt, K.Furusawa, T.M.Monro, C.P.Please, D.J.Richardson, “The mathematical modelli ng of capillary drawing for holey fibre manufacture,” J. Engineering Mathematics Vol.43, pp.201-227 (2002) J.H.V.Price, K.Furusawa, T.M.Monro, L.Lefort, D.J.Richardson, “Tunable femtosecond pulse source operating in the range 1.06-1.33 micron based on an Yb3+-doped holey fiber amplifier,” J.Opt.Soc.Am. B Vol.19 pp.1286-94 (2002) A.D.Fitt, K.Furusawa, T.M.Monro, C.P.Please, “Modeling the fabrication of hollow fibers: Capillary drawing,” J. Lightwave Tech. Vol.19 pp.1924-31 (2001) K.Furusawa, A.N.Malinowski, J.H.V.Price, T.M.Monro, J.K.Sahu, J.Nilsson, D.J.Richardson ”A cladding pumped Ytterbium-doped fiber laser with holey inner and outer cladding,” Opt.Exp. Vol.9 pp.714-20 (2001) J.K. Sahu, C.C. Renaud, K. Furusawa, R. Selvas, J.A. Alvarez-Chavez, D.J. Richardson, J. Nilsson ”Jacketed air-clad cladding pumped ytterbium-doped fibre laser with wide tuning range,” Electron.Lett. Vol.37, pp.1116-7 (2001) K.Furusawa, T.M.Monro, P.Petropoulos, D.J.Richardson, “Mode-locked laser based on ytterbium doped holey fibre,” Electron.Lett. Vol.37 pp.560-561 (2001) J.C.Baggett, T.M.Monro, K.Furusawa, D.J.Richardson, “Comparative study of large mode holey and conventional fibers,” Opt. Lett. Vol.26 pp.1045-7 (2001) P. Petropoulos, T.M. Monro, W. Belardi, K. Furusawa, J.H. Lee, D.J. Richardson, “2R-regenerative all-optical switch based on a highly nonlinear holey fiber,” Opt.Lett. Vol.26 pp.1233-5 (2001) Conference papers: K.Furusawa, J.K.Sahu, T.M.Monro, D.J.Richardson, “A high efficiency low threshold erbium-doped holey optical fiber laser.” CLEO/QELS 2003, Baltimore CTuP (2003)

List of publications 203

Z.Yusoff, P.C.Teh, P.Petropoulos, K.Furusawa, W.Belardi, T.M.Monro, D.J.Richardson, “24 channels x 10 GHz multiwavelength pulse source based on supercontinuum generation in highly nonlinear holey fiber,” OFC 2003 Atlanta 23-28 Mar (2003) E.A.Hinds, B.V.Hall, M.P.A.Jones, D.C.Lau, J.Retter, B.E.Sauer, C.J.Vale, K.Furusawa, P.G.Kazansky,D.J.Richardson, ”Mirror s waveguides and integrated circuits for cold atoms,” Proc. 6th Symposium on Frequency Standards and Metrology 2001 pp.281-7 ed. P Gill, World Scientific, Singapore (2001) W.Belardi, J.H.Lee, K.Furusawa, Z.Yusoff, P.Petropoulos, M.Ibsen, T.M.Monro, D.J.Richardson, “A 10 Gbit/s tuneable wavelength converter based on four-wave mixing in highly nonlinear holey fibre,” ECOC 2002, Copenhagen, PD1.2 (2002) K.Furusawa, A.Malinowski, J.H.V.Price, T.M.Monro, J.K.Sahu, J.Nilsson, D.J.Richardson, ”A highly efficient all-glass double-clad ytterbium doped holey fiber laser,” CLEO 2002 Long Beach, California, CMJ (2002) J.H.V.Price, K.Furusawa, T.M.Monro, C.Netti, A.Malinowski, J.J.Baumberg, D.J.Richardson, “Phased matched UV generation in a silica holey fiber,” CLEO 2002 Long Beach, California, CTuB (2002) J.C.Baggett, T.M.Monro, K.Furusawa, D.J.Richardson, “Distinguishing transition and pure bend losses in holey fibers,” CLEO 2002 Long Beach, California (2002) A.D.Fitt, C.P.Please, K.Furusawa, T.M.Monro, “Modelling fiber drawing: capillary manufacture,” CLEO 2002 Long Beach, California (2002) K.Furusawa, J.H.V.Price, T.M.Monro, P.Petropoulos, D.J.Richardson, “Development and applications of ytterbium-doped highly nonlinear holey optical fibres,” International Workshop on Nonlinear Photonic Crystals - Danish Technical University, Lyngby, (2001) K.Furusawa, J.H.V.Price, T.M.Monro, P.Petropoulos, D.J.Richardson, “A small core Yb3+-doped holey fibre laser and amplifier, IoP Meeting 'In-fibre Bragg gratings and special fibres' - Photonex 2001 Coventry (2001) J.H.V.Price, K.Furusawa, T.M.Monro, L.Lefort, D.J.Richardson, “A tuneable femtosecond pulse source operating in the range 1.06-1.33 microns based on a Yb doped holey fiber amplifier,” CLEO 2001 Baltimore, CPD1 (2001) K.Furusawa, T.M.Monro, J.C.Baggett, P.Petropoulos, P.W.Turner, D.J.Richardson, “A mode-locked ytterbium doped holey fiber laser,” CLEO 2001 Baltimore, CWC2 (2001) T.M.Monro, J.C.Baggett, K.Furusawa, D.J.Richardson, “Comparative study of bend loss in large mode holey and conventional fibres,” CLEO 2001 Baltimore, CTuAA1 (2001) P.Petropoulos, T.M.Monro, W.Belardi, K.Furusawa, J.H.Lee, D.J.Richardson, “A highly nonlinear holey fiber and its application in a regenerative optical switch,” OFC 2001 Anaheim, TuC (2001) Patents: D.J.Richardson, T.M.Monro, J.H.V.Price, and K.Furusawa, “Sources of, and methods for generating, optical pulses,” UK Patent application GB0109082.8 D.J.Richardson, T.M.Monro, J.H.V.Price, and K.Furusawa, “A tunable femtosecond pulse source operating in the range 1.06µm-1.33µm based on an Yb3+ doped holey fibre amplifier,” UK Patent application GB020955.3 D.J.Richardson, T.M.Monro, P.W.Turner, W.Belardi, and K.Furusawa, “Holey optical fibres,” UK Patent application WO0216980

Development of rare-earth doped microstructured optical fibres

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