Advances in original fibres fabrication using innovative techniques

Opt Quant Electron (2007) 39:1033–1045DOI 10.1007/s11082-007-9157-6

Advances in original fibres fabrication using innovativetechniques

Christine Restoin · Jean-Louis Auguste · Gurvan Brasse · Stéphanie Hautreux ·Sébastien Février · Jean-Marc Blondy · Alexandre Boulle · André Lecomte

Received: 17 September 2007 / Accepted: 16 November 2007 / Published online: 5 December 2007© Springer Science+Business Media, LLC. 2007

Abstract The development of original technologies to fabricate new kinds of fibres ispresented here. First of all, the sol–gel process is developed to achieve fibres with originalproperties of waveguiding (new wavelength of emission of rare earths or transition metals,ultraviolet waveguiding): either a fibre composed of doped nanocrystals in a silica matrix orfibres composed of a one dimensional photonic bandgap structure. In this way, high refractiveindex dielectric oxides like ZrO2, TiO2 are studied. Secondly, the core suction technique asso-ciated with the stack and draw process is developed to fabricate fibres with various glassesand then, original profiles of refractive index to achieve multiwavelength lasers.

Keywords Sol–gel technology · Nanocrystals doped silica fibre · Hollow core fibre ·Core suction technique

1 Introduction

In the last years, air silica microstructured fibres called photonic crystal fibres (PCF) haveshown a lot of original properties thanks to the interaction between the air holes and theelectric field of the guided modes. Their realization by the stack and draw technique hasopened a wide field of investigation especially in the improvement of the fabricated fibre byusing new materials to obtain composite fibres. By this way, the sol–gel technology and thecore suction technique are going to be developed. The study of three kinds of original fibresis going to be done:

– active fibres composed of doped nanocrystals in a silica matrix are going to be studied inorder to get new wavelength and higher level of emission.

C. Restoin (B) · J.-L. Auguste · G. Brasse · S. Hautreux · S. Février · J.-M. BlondyXlim UMR 6172, 123 avenue A. Thomas, 87060 Limoges, Francee-mail: [email protected]

A. Boulle · A. LecomteSciences des Procédés Céramiques et Traitement de Surface (SPCTS), UMR 6638, ENSCI,47-73 Avenue A. Thomas, 87065 Limoges cedex, France

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– hollow core fibres where the propagation of light is based on a 1D photonic bandgap struc-ture with a high refractive index difference (few 10−1 for ZrO2/SiO2 and TiO2/SiO2) canbe achieved.

– higher nonlinear fibres composed of non pure silica glasses exhibiting important refractiveindex will be developed.

2 Fibres developed from the Sol–gel technology

2.1 Nanocrystals doped silica core fibres

There has been an intense interest in the development of new materials with a highefficiency. Nanocrystalline materials, and especially ceramics built with nanocrystallites(Bhargava 1998; Sun et al. 2000) have attracted special attention because a significantenhancement of the optical properties occurs when the crystallite size is reduced to a fewnanometers. Due to size effects, these materials behave differently in various spectroscopicprocesses than their large grain counterparts (Goldburt et al. 1997; Yang et al. 1999). A spe-cial form of nanocomposites attract more and more attention, silica glass doped with lightemitting nanocrystallites (Fu et al. 2003; Hreniak et al. 2003).

Many nanocrystalline materials have been studied: ZrO2, TiO2 and ZnO, CdS, SnO2 arerespectively the principal ones for the dielectric and the semiconductors media. Rare earthssuch as Eu3+ or metal transition like Ni2+ have been inserted in these kinds of nanocrystalsvia a sol–gel route or a coprecipitation method.

For example, Gueu et al. (2007) and Nogami et al. (2002) have studied the fluorescenceproperties of Eu3+ in bulk nanocrystallised SnO2–SiO2 glass ceramics. They show that whenthe Eu3+ ions and nanosized SnO2 codoped glasses are optically excited, the energies of theelectron-hole pairs formed in the crystals is effectively transferred into the Eu3+, resultingin the enhanced fluorescence intensity.

All the studies have been done on bulk materials, layers or powders and until now, no fibreshave been drawn with an active core composed of nanocrystals. Rare earth doped fibres arecommonly achieved using the MCVD (Modified Chemical Vapour deposition) process. How-ever, the sol–gel technology permits to achieve preforms with a sufficient quality for activefibre fabrication using low cost equipments compared to the MCVD technique.

The first developed fibres have a core composed of doped nanocrystals of ZrO2 in a silicamatrix (Fig. 1). Zirconia is a very interesting host material as it has a good chemical stabilityand a very high boiling temperature (higher than the fibre drawing temperature of silica).Moreover, its high refractive index (n = 2.15 at λ = 0.633µm) allows to get fibres with astrong refractive index difference. The insertion of rare earths and metal transition are alsofavoured in zirconia.

2.1.1 Experimental sol–gel process

The sol–gel process involves the transition of liquid sol into solid gel phase. The precur-sors are subjected to a series of hydrolysis and polymerisation reactions to form the desiredcompound.

The preparation of the SiO2 sols is obtained by mixing tetraethylorthosilicate (TEOS),hydrochloric acid, water and ethanol in various molar ratio. The solution is first aged at 70◦Cfor 2 h and then at room temperature for 24 h. After that, a remaining of ethanol is added inorder to lower its viscosity. The ZrO2 sols are prepared by adding zirconium n-propoxyde,

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Advances in original fibres fabrication using innovative techniques 1035

Fig. 1 TEM observation of a bulk material composed of 30 mol% ZrO2 and 70 mol% SiO2 (Gaudon 2005)

acetylacetone and propan-1-ol in glove box to get a 0% humidity as the alcoxyde of zirconiais highly reactive. TiO2 sols are prepared in the same way from titanium n-propoxide. Manysols with a concentration varying from 0.25 to 1 mol.l−1 were synthesized.

The gels of silica–zirconia (70–30 mol% ) are prepared from mixing the sol of SiO2 inthe sol of ZrO2.

Xerogels are achieved after drying the gels at a 100◦C temperature without any caution.The evaporation of the organic compounds and the strong variation of the dimensions createcracks in the structure of the gel and then the break of the gel structure: the powder is thenannealed to eliminate the organic radicals.

Two kinds of fibre fabrication have been explored: one way is based on the drawing ofpreforms where the core is coated by layers of ZrO2–SiO2 nanocomposites. Each layer whichis formed by dip coating is dried and then annealed at 1000◦C in order to evaporate all theorganic compounds and to form the desired phase. The preform is then collapsed during thedrawing of the fibres.

The second process consists on filling the core of the preform by the xerogel powder ofZrO2–SiO2 nanocomposites and drawing the fibre.

2.1.2 Simulations

The fibres composed of ZrO2 nanocrystals doped silica core have been simulated usingCOMSOL Multiphysics software. Many profils showing a random distribution of the nano-crystals of zirconia have been studied. The properties of waveguiding (effective refractiveindex, number of guided modes, …) have been explored as a function of:

1. The diameter of the nanocrystals,2. The nanocrystals doping percentage,3. The diameter of the core.

The calculations show that a refractive index variation as high as 0.16 in the visible rangefor a 30% zirconia ratio can be reach (Fig. 2).

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Fig. 2 Variation of the refractive index as a function of the wavelength for ZrO2/SiO2 (30 mol%/70 mol%)

Fig. 3 Simulated electric field in a fibre for a low concentration of nanocrystals (few percents)

Single mode fibres are achieved for low ratio of nanocrystals until the near IR region. Inall the simulations, the crystals has a 10–20 nm diameter, what is in good agreement withthe experimental observations. In this way, the losses induced in this kind of medium do notincrease compared to a classical doped silica fibre. For example, Fig. 3 shows the simulatedelectric field profile in a quarter of the core where only the fundamental mode is guided.

2.1.3 Results and discussions

The fibres where the core is achieved by dip coating layers of the nanocomposite have beendrawn with a 125µm and a 350µm external diameter and the influence of the number offilms have been studied. Observations of the profile have been done using a scanning elec-tron microscope (SEM). It appears that the diameter of the core is about 200 nm and 1.5 µmwhen 1 and 5 layers are respectively deposited on the internal face of the preform. These

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Fig. 4 Refractive index profile of a fibre obtained from dip coating 5 layers of ZrO2/SiO2 (30 mol%/70 mol%), (350µm external diameter)

thicknesses were predicted taking into account the thickness of each layer in the preformand the step of diameter reduction during the fibre drawing. The refractive index profile havebeen measured and, as it is shown on Fig. 4, it is equal to 0.018.

The second process based on xerogel that has been tested is particularly suitable to achievefibres with large cores. In this way, the distribution of ZrO2 in the core will be easily observedusing SEM. The measured variation of refractive index between the core and the cladding isequal to 0.01 as it has been predicted by computation for such composition (Fig. 5). The littledifference between computed and experimental results may be linked to the weak variationof the ZrO2 content. The SEM photograph (Fig. 6) and the chemical analysis realized byElectron Dispersive X-ray (EDX) show that ZrO2 is in the aggregates form and few holesappear because of gaseous inclusions. The problem of intragranular porosity have also beenshown and is probably due to the polymeric way that permits to achieve the xerogel.

The first results show the possibility to fabricate fibres composed of ZrO2 nanocrystalsin a silica matrix by the sol–gel process using two methods that can be complementary as afunction of the dimensions of the core desired. The sol–gel route via the colloidal process isgoing to be studied and compared to the polymeric way in order to decrease the intragranularporosity of the xerogel.

2.2 Hollow core fibres

Hollow core fibres where the propagation of light is based on a photonic bandgap (PBG)structure are more suitable for the propagation of high power UV light than classical fibres(Cregan et al. 1999). Indeed, nonlinear effects and the deterioration of silica do not appear.

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Fig. 5 Refractive index profile of a fibre achieved from a xerogel ZrO2/SiO2 (30 mol%/70 mol%)

Fig. 6 SEM micrograph of a fibre achieved from a xerogel ZrO2/SiO2 (30 mol%/70 mol%)

PBG fibres with one-dimensionally periodic cladding, the so-called Bragg fibres, arecomposed of a succession of high and low index layers which acts as a cylindrical Braggmirror (Yeh et al. 1978; Fink et al. 1998; Deopura et al. 2001). These omniguide fibres havefirst been implemented experimentally by the group of Joannopoulos at the MIT institutein the IR range (Temelkuran et al. 2002). They show that the mechanism of guidance onlydepends on the cladding: the refractive index contrast and the number of layers are the mostimportant parameters. The higher the index contrast, the wider the stop-band of high reflec-tance is; moreover, the reflectance within the stop-band increases with the number of layers.The two PBG structures chosen are then composed of TiO2/SiO2 and ZrO2/SiO2.

Their design has been studied using a method developed by Marcou et al. (2001) wherewe only consider the propagation of the single HE11 mode. The thickness of each layer isthen calculated from the refractive index distribution and the electric field profile.

The sol–gel technology is particularly suitable to achieve this kind of multilayers. Thiswet chemical method allows to fabricate very thin films with a very high homogeneity andpurity at low processing temperature (Rabaste et al. 2002; Almeida et al. 2003, 2004; Zhanget al. 2000; Doeuff et al. 1987).

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First of all, we report the study of planar multilayers: the reflectance has been measuredaccording to the number of layers and their thickness. The properties of these 1D PBGstructures have been determined by computing the reflectance spectra and by an electronicmicroscopy study. Then, the development of a 1D PBG inside a fibre will be presented:the conditions of layer formation are studied, especially the process of sol extrusion andannealing conditions.

2.2.1 Experimental

The sols of ZrO2 and SiO2 are prepared as presented in the last part. TiO2 sols are preparedin the same way from titanium n-propoxide. Many sols with a concentration varying from0.25 to 1 mol.l−1 were synthesized.

The planar multilayers are fabricated by dip coating method onto slide glass or siliconwafer cleaned in ultrasonic bath.

The fabrication of multilayers inside a capillary and a fibre is studied in different ways.Two techniques are studied to fill the fibre: the capillarity and the suction of the sol controllingthe velocity. A fast annealing is achieved after each layer deposited.

The reflectance spectra were measured using a UV-Vis spectrometer (PERKIN ELMER,Lambda 40) at 8 degrees of incidence in the wavelength range 200–1100 nm. The surfacesof the sample were characterized using an optical microscope, a SEM and a transmissionelectron microscope (TEM).

2.2.2 Reflectance analysis on planar multilayers

The reflectance of ZrO2/SiO2 and TiO2/SiO2 multilayers has been measured according to thenumber of layers, the concentration of various sols, the temperature and the time of annealing.

The maximum of reflectivity is about 92% for ZrO2/SiO2 (17 layers) (Fig. 7) and 95% forTiO2/SiO2 (13 layers). A higher reflectance is achieved for TiO2/SiO2 with a lower numberof layers because the refractive index difference between these two materials is higher thanfor ZrO2/SiO2. The maximum of reflectance is not achieved because on one hand the numberof layers is not enough and on the other hand, a significant roughness appears at the interfaceof the layers near the substrate (Fig. 8).

Moreover, the reflectance increases with the annealing length until a limit which cor-responds to the maximum of layers densification. For longer annealing time, cracks areinduced in the structure because of the difference of thermal expansion coefficients and then,the reflectivity decreases.

The evolution of the reflectivity as a function of the concentration of the sols shows thatthe maximum is shifted to shorter wavelengths when the concentration decreases, that is tosay when the thickness of the layers decreases. In this way, a maximum of reflectivity in theUV range is achieved around 280 nm for ZrO2/SiO2 multilayers.

The computations of the reflectivity spectra are developed to determine the quality of thestructures in a non destructive way. These results are compared to the microscopic study.The simulations are based on a matrix description of the phenomenon of transmission andreflection at the interfaces. The variation of the refractive index versus the wavelength istaken into account.

The reflectance of the structure can then be written R = |M21/M11|2, where Mi j are theelements of the transfer matrix of the multilayer which is deduced from the transfer matrixof each medium as M = Mair M1 M2 · · · MN Msubstrate. The transfer matrix of the medium nis then:

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Fig. 7 Reflectance spectra of ZrO2/SiO2 multilayers annealed at 600◦C

Fig. 8 TEM micrograph of a ZrO2/SiO2 multilayer composed of 9 layers on a silica substrate

Mn =(

t ′−1n exp (ikntn) r ′

j t′−1j exp (ikntn)

r ′nt ′−1

n exp (−ikntn) t ′−1n exp (−ikntn)

)

where kn is the wave vector of the medium n and tn is the thickness of the n layer. r′n andt′n are the Fresnel coefficient of reflection and transmission including a random variation ofthickness (roughness) which follow a Gaussian distribution:

t ′n = tn exp

[1

2σ 2

n (kn − kn+1)2]

r ′n = rn exp

[−2σ 2n knkn+1

]

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Fig. 9 Experimental (grey continuous line) and calculated reflectance spectra: (a) taking into account theroughness (black continuous line) (b) without the roughness (dashed grey line)

Moreover, the variations of the roughness and the density of each layer according to theirposition in the multilayer are taken into account because of the process of fabrication (anneal-ing after each layer deposited). The expression of the refractive index n(λ) includes then thedensity parameter; coherent results are achieved in this study with the following expression:

n(λ) = 1 − ρ(1 − nth(λ)).

In this way, the reflectance spectra are perfectly simulated; a few nanometers roughness per-mits to explain the decrease of reflectance (Fig. 9) compared to the ideal multilayer. Thesimulations show that this roughness is twice higher at the interface of the layers near thesubstrate than near the surface. The photographs presented in Fig. 8 show the same results.This difference of roughness is explained by a number of thermal annealing n times higherfor the nth layer than for the first one, which induces a more important crystallisation andcrystalline growth.

2.2.3 XRD analysis

The crystalline data were obtained by X-ray diffractometer (XRD; Inel CPS 120 with CuKα; 37.5 kV, 28 mA).

The XRD study shows that ZrO2 only crystallises in the cubic phase for a 600◦C annealingtemperature; the monoclinic phase appears from 900◦C or for the more important annealinglength (20 min per layer). The crystalline size of cubic ZrO2 calculated by the Scherrer for-mula, increases with the annealing temperature from 12 to 20 nm in the studied range. Thesame behaviour is observed concerning TiO2 where the anatase phase is present at 600◦Cand is replaced by the rutile phase for higher and longer annealing. However, the reflectancedoes not increase with the annealing temperature even if the refractive index of monoclinicZrO2 and rutile TiO2 is higher because of the increase of the roughness of the layers as it ismentioned before.

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Fig. 10 Scanning Electron Microscope observation of a ZrO2/SiO2 multilayer composed of 11 films insidea 25µm radius fibre

2.2.4 Multilayers inside a fibre

The reflectance analysis has shown that ZrO2/SiO2 multilayers presents the better reflectancein the UV range; in this way, the study has been developed in fibres for this pair of materials.

The first results about the fabrication of multilayers in a circular geometry are presentedin Fig. 10. First, 7 layers of ZrO2/SiO2 have been deposited in a 125µm radius capillary.The thickness of the layers is not regular because the velocity of sol suction is not constant.However, the thicknesses of the 6 first layers near the air core lie between 30 and 40 nmfor ZrO2 and are about 80 nm for SiO2, which are the dimensions required to get a PBGwaveguiding at 330 nm.

The deposit of the layers on the internal face of a fibre or a capillary depends on the aircore diameter. In this way, the formation of the ZrO2/SiO2 multilayer has been studied insidea 50µm diameter fibre, especially according to the extrusion process of the sols. It appearsthat a limited number of layers is achieved when the sols is thrown out the fibre withoutexerting a pressure because the fibre is blocked. However, if a pressure is applied at a bottom,this problem does not occur. As it has been difficult to keep a constant pressure until now:the velocity of the layers formation varied and that is why the thickness of the films is notconstant.

The drying and annealing conditions also play an important part. The range of dryingtemperature studied is 20–170◦C; it appears that a drying at room temperature is the bestway to achieve films without cracks. Moreover, a thermal annealing under air ventilation inthe conditions defined for planar structures promotes the organic elements venting for manymeters length fibres.

In this way, multilayers composed of 11 layers has been achieved (Fig. 10). The opti-cal measurements could not yet underline the PBG effect because of the variations of thethickness layers.

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Fig. 11 Experimental setup of the core suction technique

3 Fibres using the core suction technique

Another way to achieve original light sources is the use of the core suction technique whichwas first described by Goel and Stolen (Goel et al. 2006). One of the main advantages of thistechnique is to be able to insert several kinds of glasses in the preform, with a low cost dueto the small quantity involved, in order to achieve multiwavelength lasers.

This technique consisted in pumping melted glass in silica tube to obtain a core glass fibresurrounded by a silica cladding (Fig. 11).

The first works have been developed to realize a glass core surrounding by a silica clad-ding. The preform filled on 80 cm by a borosilicate glass has been drawn into fibres. Fewmeters of fibres has then been achieved. The SEM photograph of the profile shows that thecore is perfectly homogeneous (Fig. 12).

Computations are now developed in order to determine the best composition of the glass.On one hand, the difference between the expansion coefficient of silica and the doped glassmust be minimized to avoid cracks in the preform and then in the fibre. On the other hand,the refractive index of the glass must be as high as possible.

These first results have not only shown the potential of this method but also, the possibilityto adapt the experimental setup to the kind of glasses according to the working temperature(tubular or inductive furnace).

The main objective is to realize glass–silica–air fibres with a glass core or to create highrefractive index zones in the claddings.

4 Conclusion

The work presented here shows the potential of the sol–gel process and the core suctiontechnique to elaborate original fibres. In this way, fibres with a core composed of ZrO2 nano-crystals in a silica matrix have been achieved. Incorporation of luminescent ions like rare

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Fig. 12 Photograph of the fibre with a borosilicate glass core achieved with the core suction technique

earth ions or metal transition in the nanocrystals is now the most important next step of thestudy.

Hollow core fibres can be achieved, using the dip coating method of sols, with a highefficiency because high refractive index oxides can compose the PBG structure. This methodcan be broadened to the fabrication of preform which will then be drawn into few meters offibers.

The core suction experiment is now finalized from the experimental point of view: varioussilica glasses compositions are going to be prepared to get the best properties in term of highrefractive index and low difference of expansion coefficients between silica and doped silicaglasses.

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