Unit-1 and 2 Optical

Unit-1

UNIT – I INTRODUCTION • Introduction, • Ray theory transmission- • Total internal reflection-• Acceptance angle –• Numerical aperture – • Skew rays – • Electromagnetic mode theory of optical propagation –• EM waves – • modes in Planar guide – • phase and group velocity – • cylindrical fibers –• SM fibers.

Fig. 1-2: Digital transmission hierarchy

Fig. 1-3: Operating ranges of components

Fig. 1-5: Major elements of an optical fiber link

Fig. 1-6: Optical fiber cable installations

Fig. 1-7: History of attenuation

Fig. 1-8: Optical multiplexing

Evanescent wave

UNIT 2

TRANSMISSION CHARACTERISTICS OF OPTICAL FIBERS

UNIT 2• Attenuation – • Material absorption losses in silica glass fibers – • Linear and Non linear Scattering losses – • Fiber Bend losses – • Midband and farband infra red transmission –• Intra and inter Modal Dispersion – • all Fiber Dispersion – • Polarization- • non linear Phenomena. • Optical fiber connectors, • Fiber alignment and Joint Losses – • Fiber Splices• – Fiber connectors – Expanded Beam Connectors – • Fiber Couplers.

Absorption in

Infrared region

Absorption

Atomic Defects

Extrinsic (Impurity atoms)

Intrinsic Absorption

Absorption in

Ultraviolet region

Attenuation

Scattering Losses

Compositional fluctuations in material

Inhomogeneities or defects

in fiber

Radiative losses/ Bending

losses

Macroscopic bends

Microscopic bends

Pulse broadening and attenuation

1. Raleigh scattering 1. Mie Non linear scattering : 1. Stimulated Brillouin scattering2. Stimulated Raman scattering

89

2.4.3 Stimulated Roman Scattering (SRS) SRS will deplete short wave power and amplifier long wave.

90

2.4.1 Stimulated Brillouin Scattering (SBS)

The scattering interaction occurs with acoustic phonons over Δf =15 MHz, at 1.55μm, stokes and pump waves propagate in opposite directions.

If spacing > 20 MHz => no effects on different channels

Ps(0) Pp(L)

pumping

SBS

A compressible jacket extruded over a fiber reduces microbending resulting from external forces.

Minimizing microbending losses:

Macrobending due to poor reeling

Minimum safe bend radius —shown full size

Bends are shown full size — and may have caused damage to the fiber

Making use of bending losses

There are many uses of bending losses which are based on either the increase in the attenuation or on making use of the light which escapes from the optic fiber.

Radiative losses / Bending Losses

A fiber optic pressure sensor

Active fiber detector

This makes use of the increased attenuation experienced by the fiber as it bends.

This uses the escaping light.

Pressure causes loss at the bends

Is the fiber in use?

Signal Distortion/

Dispersion

Polarization-modeDispersion

Intramodal Dispersion/

Chromatic DispersionIntermodal Delay/

Modal Delay

Material Dispersion

Waveguide Dispersion

Dispersion

Attenuation only reduces the amplitude of the output waveform which does not alter the shape of the signal.

Dispersion distorts both pulse and analog modulation signals.

The basic need is to match the output waveform to the input waveform as closely as possible.

It is noted that actually no power is lost to dispersion, the spreading effect reduces the peak power.

In a pulse modulated system, this causes the received pulse to be spread out over a longer period.

Dispersion

Dispersion results when some components of the input signal spend more time traversing the fiber than other components.

Dispersion

The difference in width of an input pulse with the width of the same pulse at the output, measured in time, is the dispersion characteristic for that piece of fiber.

Pulse dispersion is usually specified in terms of “Nanoseconds-per-kilometer”.

Dispersion

Dispersion of optical energy within an optical fiber falls into following categories:

Intramodal Dispertion or Chromatic DispersionMaterial DispertionWaveguide Dispertion

Intermodal Delay or Modal Delay)

Dispersion

Polarization –Mode Dispersion

Intermodal delay/ modal delay

Dispersion

Intermodal distortion or modal delay appears only in multimode fibers.

This signal distortion mechanism is a result of each mode having a different value of the group velocity at a single frequency.

Group Velocity: It is the speed at which energy in a particular mode travels along the fiber.

The amount of spreading that occurs in a fiber is a function of the number of modes propagated by the fiber and length of the fiber

Intermodal delay/ modal delay

The maximum pulse broadening arising from the modal delay is

the difference between the travel time Tmax of the longest ray and

the travel time Tmin of the shortest ray .

This broadening is simply obtained from ray tracing for a fiber of length L:

∆T= Tmax – Tmin = n1/c ( L/sinøc –L) = (Ln12/cn2)∆

∆T= Tmax – Tmin = (Ln12/cn2)∆

Intermodal delay/ modal delay

Fiber Capacity:

Fiber capacity is specified in terms of the bit rate-distance product BL.

(Bit rate times the possible transmission distance L)

For neighboring signal pulses to remain distinguishable at the receiver, the pulse spread should be less than 1/B.

Or

Pulse spread should be less than the width of a bit period.

∆T < 1 /B General requirement∆T ≤ 0.1 /B For high performance link

Bit rate distance product BL < n2 c/ n12 ∆

Light rays with steep incident angles have longer path lengths than lower-angle rays.

How to minimize the effect of modal dispersion?

1. Graded index fiber

2. Single mode fiber

Answer is

How to get one mode and solve the problem

V = 2πa / λ x (n12 – n2

2)1/2 = 2πa / λ x (NA)

we could decrease the number of modes by increasing the wavelength of the light.

Changing from the 850 nm window to the 1550 nm window will only reduce the number of modes by a factor of 3 or 4.

Change in the numerical aperture can help but it only makes a marginal improvement.

We are left with the core diameter. The smaller the core, the fewer the modes.

When the core is reduced sufficiently the number of modes can be reduced to just one.

How to get one mode and solve the problem

Step Index Multi-mode

Graded Index Multi-mode

Q: Consider a 1 Km long multimode fiber in which n1= 1.480 and ∆ = 0.10 , so that n2= 1.465.

Then find ∆T= ?

∆T = (Ln12/cn2)∆

Where:

L = 1 Km

n1 = 1.480

n2= 1.465

∆ = 0.10

How to characterize dispersion?• Group delay per unit length can be defined as:

• If the spectral width of the optical source is not too wide, then the delay difference per unit wavelength along the propagation path is approximately For spectral components which are apart, symmetrical around center wavelength, the total delay difference over a distance L is:

d

d

cdk

d

cd

d

Lg

2

1

ω

2

[3-15]

d

d g

2

2

2

22

22

d

dL

V

L

d

d

d

d

d

d

d

d

c

L

d

d

g

g

[3-16]

• is called GVD parameter, and shows how much a light pulse broadens as it travels along an optical fiber. The more common parameter is called Dispersion, and can be defined as the delay difference per unit length per unit wavelength as follows:

• In the case of optical pulse, if the spectral width of the optical source is characterized by its rms value of the Gaussian pulse , the pulse spreading over the length of L, can be well approximated by:

• D has a typical unit of [ps/(nm.km)].

2

2

2

d

d

22

211

c

Vd

d

d

d

LD

g

g

[3-17]

g

DLd

d gg [3-18]

Dispersion

Intramodal Dispersion or Chromatic Dispersion

This takes place within a single mode.

Intramodal dispersion depends on the wavelength, its effect on signal distortion increases with the spectral width of the light source.

Spectral width is approximately 4 to 9 percent of a central wavelength.

Two main causes of intramodal dispersion are as:

1. Material Dispersion

2. Waveguide Dispersion

t

Spread, ²

t0

Spectrum, ²

12o

Intensity Intensity Intensity

Cladding

CoreEmitter

Very shortlight pulse

vg(2)

vg(1)Input

Output

All excitation sources are inherently non-monochromatic and emit within aspectrum, ² , of wavelengths. Waves in the guide with different free spacewavelengths travel at different group velocities due to the wavelength dependenceof n1. The waves arrive at the end of the fiber at different times and hence result ina broadened output pulse.

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Material Dispersion

Intramodal Dispersion or Chromatic Dispersion

Material Dispersion:

This refractive index property causes a wavelength dependence of the group velocity of a given mode; that is,

Pulse spreading occurs even when different wavelength follow the same path.

Material dispersion can be reduced:

•Either by choosing sources with narrower spectral output widths OR

•By operating at longer wavelengths.

LASER source will produce far less spectral dispersion or intramodal dispersion than an LED source since it is more nearly monochromatic

Material Dispersion

• The refractive index of the material varies as a function of wavelength,

• Material-induced dispersion for a plane wave propagation in homogeneous medium of refractive index n:

• The pulse spread due to material dispersion is therefore:

)(n

d

dnn

c

L

nd

dL

cd

dL

cd

dLmat

)(2

22ω

22

[3-19]

)(2

2

mat

matg DL

d

nd

c

L

d

d [3-20]

)(matD is material dispersion

Material dispersion as a function of optical wavelength for pure silica and 13.5 percent GeO2/ 86.5 percent SiO2.

Intramodal Dispersion or Chromatic Dispersion

Waveguide Dispersion:

Dispersion arises because the fraction of light power propagating in the cladding travels faster than the light confined to core.

It causes pulse spreading because only part of the optical power propagation along a fiber is confined to core.

Single mode fiber confines only 80 percent of the power in the core for V values around 2.

The amount of waveguide dispersion depends on the fiber design.

Waveguide Dispersion

• Waveguide dispersion is due to the dependency of the group velocity of the fundamental mode as well as other modes on the V number, (see Fig 2-18 of the textbook). In order to calculate waveguide dispersion, we consider that n is not dependent on wavelength. Defining the normalized propagation constant b as:

• solving for propagation constant:

• Using V number:

21

22

22

1

22

22 //

nn

nk

nn

nkb

[3-29]

)1(2 bkn [3-31]

2)( 22/12

22

1 kannnkaV [3-33]

Waveguide Dispersion• Delay time due to waveguide dispersion can then be expressed as:

dV

Vbdnn

c

Lwg

)(22 [3-34]

Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

Signal Distortion in single mode fibers

• For single mode fibers, waveguide dispersion is in the same order of material dispersion. The pulse spread can be well approximated as:

2

22 )(

)(dV

VbdV

c

LnDL

d

dwg

wgwg

[3-25]

Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

)(wgD

Over all Fiber distortion

Over all Fiber distortion

Chromatic & Total Dispersion• Chromatic dispersion includes the material & waveguide dispersions.

• Total dispersion is the sum of chromatic , polarization dispersion and other dispersion types and the total rms pulse spreading can be approximately written as:

LD

DDD

chch

wgmatch

)(

)(

[3-28]

LD

DDD

totaltotal

polchtotal

...[3-29]

Polarization Mode dispersion

Core

z

n1 x

// x

n1 y

// y

Ey

Ex

Ex

Ey

E

= Pulse spread

Input light pulse

Output light pulset

t

Intensity

Suppose that the core refractive index has different values along two orthogonaldirections corresponding to electric field oscillation direction (polarizations). We cantake x and y axes along these directions. An input light will travel along the fiber with Ex

and Ey polarizations having different group velocities and hence arrive at the output atdifferent times

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Polarization Mode dispersion

• The effects of fiber-birefringence on the polarization states of an optical are another source of pulse broadening. Polarization mode dispersion (PMD) is due to slightly different velocity for each polarization mode because of the lack of perfectly symmetric & anisotropicity of the fiber. If the group velocities of two orthogonal polarization modes are then the differential time delay between these two polarization over a distance L is

• The rms value of the differential group delay can be approximated as:

gygx vv and pol

gygxpol v

L

v

L [3-26]

LDPMDpol [3-27]

Optical Fiber Connectors

• Some of the principal requirements of a good connector design are as follows:

1- low coupling losses

2- Interchangeability

3- Ease of assembly

4- Low environmental sensitivity

5- Low-cost and reliable construction

6- Ease of connection

Fiber Joints

• Fibers must be joined when– You need more length than you can get on a

single roll– Connecting distribution cable to backbone– Connecting to electronic source and transmitter– Repairing a broken cable

Splices v. Connectors

• A permanent join is a splice• Connectors are used at patch panels, and can

be disconnected

Optical Loss

• Intrinsic Loss– Problems the splicer cannot fix

• Core diameter mismatch• Concentricity of fiber core or

connector ferrules• Core ellipticity• Numerical Aperture mismatch

– Images from LANshack and tpub.com (links Ch 6a & 6c)

Optical Loss

• Extrinsic Loss– Problems the person doing the

splicing can avoid• Misalignment• Bad cleaves• Air gaps• Contamination: Dirt, dust, oil, etc.• Reflectance

Measuring Reflectance

• The reflected light is a fraction of the incoming light– If 10% of the light is reflected, that is a reflectance

of 10 dB– If 1% of the light is reflected, 20 dB– Reflectance is not usually a problem for data

networks, but causes ghosting in analog cable TV transmission

– Angled connectors reduce reflectance

Acceptable Losses

Fiber & Fiber & JointJoint

Loss (max)Loss (max) Reflectance Reflectance (min)(min)

SM spliceSM splice 0.15 dB0.15 dB 50 dB50 dB

SM connectorSM connector 1 dB1 dB 30 dB30 dB

MM spliceMM splice 0.25 dB0.25 dB 50 dB50 dB

MM MM connectorconnector

0.75 dB0.75 dB 25 dB25 dB

Connectors

• There are four types– Rigid Ferrule (most common)– Resilient ferrule– Grooved plate hybrids– Expanded beam

• Top image shows ferrules from swiss-jewel.com (link Ch 6e)

• Lower image shows LC, SC, Biconic, and the obsolete Deutsch 1000

– From thefoa.org (link Ch 6d)

Rigid Ferrule Connectors

• 2.5 mm ferrule»ST

»SC

»FC• Images from thefoa.org (link Ch 6d)

Rigid Ferrule Connectors

• 1.25 mm ferrule• Small Form Factor

»LC

»MU

»LX-5

• Images from thefoa.org (link Ch 6d)

Obsolete Connectors

• Simplex (1-fiber)»SMA

»D4

»Biconic

• Images from thefoa.org (link Ch 6d)

Duplex Connectors

• Old, bulky»FDDI

»ESCON

• Images from thefoa.org (link Ch 6d)

Duplex Connectors

• Newer, smaller• Small Form Factor

»MT-RJ

»Opti-Jack

»Volition

• Images from thefoa.org (link Ch 6d)

Duplex Connectors

• New, popular• Small Form Factor

»Duplex LC

• Images from globalsources.com (link Ch 6f)

Ferrule Polish

• To avoid an air gap• Ferrule is polished flat, or • Rounded (PC—Physical Contact),

or• Angled (APC)

– Reduces reflectance– Cannot be mated with the other

polish types• Image from LANshack (link Ch 6a)

FOCIS

• Fiber Optic Connector Intermateability Standard– A document produced by a connector

manufacturer so others can mate to their connector

– Connectors with the same ferrule size can be mated with adaptors

– But 2.5 mm ferrules can not be mated with 1.25 mm ferrules

Telecommunications• In telecommunications, SC

– and FC

– are being replaced by

– LC• in the USA

– MU• in other countries

Data

• In data communications, SC and ST– are being replaced by

– LC

Connectorizing a Cable

• Epoxy-polish process (Proj. 4)– Strip cable, strip and clean fiber– Inject adhesive, put primer on fiber, insert fiber– Crimp connector, cleave protruding fiber– Air polish, final polish– Clean and inspect by microscope– Test connector loss with power meter

Cable Type and Connectors

• Epoxy-polish process requires a cable jacket and strength member to make the connector durable– It works for simplex, zip, or breakout cables– But loose-tube cables and ribbon cables contain bare fiber,

and cannot be connectorized this way– Distribution cables contain 900 micron buffered fiber – can

be connectorized, but the connectors are not very strong and must be protected by hardware such as a junction box

Breakout Kit

• Provides tubing that protects the bare fiber so it can be terminated– Picture from

fonetworks.com (link Ch 4d)

Mounting Methods for Connectors• Adhesives

– Epoxy (room temperature-cure or oven-cure)– Quick-curing anaerobic adhesives (we used this

method in Proj 4)– Hot-Melt adhesive

• Crimping to hold the fiber– Like the Unicam – see link Ch 6h

• Splicing to preconnectorized pigtails– Image of pigtail from fiberdyne.com (link Ch 6g)

Mounting Methods Comparison• Epoxy-Polish

– Takes longer, but costs less and has lowest loss and reflectance

• Anaerobic adhesive– Faster than epoxy-polish but higher loss because polishing

is difficult

• Crimping– Easier, but more expensive and more loss

• Splicing to preconnectorized pigtail– Very easy, but expensive and higher loss

Strip, Clean and Cleave

• Strip – remove 900 micron buffer (if present) and 250 micron coating

• Clean with alcohol and lint-free wipe• Cleave – scribe and snap; goal is a 90 degree

flat break

End-Face Polish

• Polish on a flat glass plate for a flat finish• Polish on a rubber mat for a domed PC finish

(Physical Contact)• Angled PC finish is tilted at 8 degrees to avoid

reflectance (difficult to field-terminate)

Cleaning Connectors

• Keep dust caps on• Use lint-free wipes and reagent-grade

isopropyl alcohol to avoid attacking epoxy• “Canned air” has propellant, so does

compressed air from a hose

Splices

• Splices are a permanent join of two fibers– Lower attenuation and reflectance than

connectors– Stronger and cheaper than connectors– Easier to perform than connectorization– Mass splicing does 12 fibers at a time, for ribbon

cables

Mass Fusion Splicing

• Video from fitel.fiberoptic.com (link Ch 6i)

Fusion Splicing

• Melts the fibers together to form a continuous fiber

• Expensive machine• Strongest and best join for singlemode fiber

– May lower bandwidth of multimode fiber

• Corning videos 1-7 & 12

Mechanical Splicing

• Mechanically aligns fibers• Contains index-matching gel to transmit light• Equipment cost is low• Per-splice cost is high• Quality of splice varies, but better than

connectors• Fiber alignment can be tuned using a Visual

Fault Locator

Connector Return Loss

Coupling Efficiency

s

F

P

P

sourse thefrom emittedpower

fiber theinto coupledpower [5-1]

Source Optical Fiber

sPFP

Examples of possible lensing schemes used to improve optical source-to-fiber coupling efficiency

Fiber-to-Fiber Joint

• Fiber-to-Fiber coupling loss:

• Low loss fiber-fiber joints are either:

1- Splice (permanent bond)

2- Connector (demountable connection)

FFL log10]dB[ [5-8]

Different modal distribution of the optical beam emerging from a fiber lead to different degrees of coupling loss. a) when all modes are equally excited, the output beam fills the entire output NA. b)

for a steady state modal distribution, only the equilibrium NA is filled by the output beam.

Mechanical misalignment losses

Lateral (axial) misalignment loss is a dominant Mechanical loss.

2/12

2step, 21

2arccos

2

a

d

a

d

a

d

a

AcommF

[5-9]

Longitudinal offset effect

Losses due to differences in the geometry and waveguide characteristics of the fibers

ERE

RF

ERE

RF

aL

aaa

aaL

NANAfor )NA

NAlog(20)(

for )log(10)(

[5-10]

E & R subscripts refer to emitting and receiving fibers.

Fiber end face

Fiber end defects

Fiber splicing

Fusion Splicing

V-groove optical fiber splicing

Optical Fiber Connectors

• Some of the principal requirements of a good connector design are as follows:

1- low coupling losses

2- Interchangeability

3- Ease of assembly

4- Low environmental sensitivity

5- Low-cost and reliable construction

6- Ease of connection

Connector Return Loss

198

Connectors

A mechanical or optical device that provides ademountable connection between two fibers or

a fiber and a source or detector.

199

Connectors - contd.

Type: SC, FC, ST, MU, SMA• Favored with single-mode fibre• Multimode fibre (50/125um) and (62.5/125um)• Loss 0.15 - 0.3 dB• Return loss 55 dB (SMF), 25 dB (MMF)

Single fibre connector

200

Connectors - contd.

• Single-mode fiber• Multi-mode fiber (50/125)• Multi-mode fiber (62.5/125)

• Low insertion loss & reflection

MT-RJ Patch Cord MT-RJ Fan-out Cord

201

Coupler• Uses

– Splitter: (50:50)– Taps: (90:10) or (95:05)– Combiners

• An important issue: – two output differ /2 in phase

• Applications: – Optical Switches, – Mach Zehnder Interferometers, – Optical amplifiers, – passive star couplers, ...

202

Coupler Configuration

P1P2

P3

P1P2

P3

1

2

3

1

2

n

1 ……n

203

Coupler - Integrated Waveguide

Directional Coupler

P2 = P0 sin2 kz P1 = P0 - P2 = P0 cos2 kz

k = coupling coefficient = (m + 1)/2

P0

P1

P2

P3

zP4

204

Coupler - Integrated Waveguide Directional Coupler

• A directional coupler

• Different performance couplers can be made by varying the length, size for specific wavelength.

G Keiser

205

Couplers - Fabrication

• Wavelength independent, depends on how light is launched • In the coupling region

– Higher order modes are trapped at the outer surface of the cladding: thus becoming cladding modes

– Lower order modes remain in the original fibre (as the incident angles are still > the critical angle)

• Cladding modes are converted back into core modes at the output ports. • The splitting ratio is determined by the

– length of the taper – thickness of the cladding.

• Multimode Fibres

Cladding modes

Source: Australian Photonics CRC

206

Couplers - Fabrication

• It is wavelength dependent. Resonance occur when the two fibres are close to each other.

• Single Fibres

100% coupling

The coupling length for 1.55 µm > the coupling length for 1.3 µm: – 100 % of light coupling for 1.3 µm to the core of fibre B, and to the core of

fibre A. – 100% of light coupling for 1.55 µm to the core of fibre B

Source: Australian Photonics CRC

207

Couplers - Fabrication• The amount of power transmitted into fibres depend on

the coupling length• The coupling length changes with the wavelength. • The splitting ratio can be tuned choosing the coupling

length.• By choosing carefully the coupler length, it is possible to

combine or separate Two different wavelengths

208

Coupler - Performance Parameters

Coupling ratio or splitting ratio

outT

t

P

PCR

portsalltooutpowerTotal

outputsingleanyfromPower

• Excess LossoutT

ie P

PL

routputpoweTotal

powerInput

21

01010

PP

PLe log

21

21010

PP

PCR logIn dB For 2 x 2 coupler

209

Coupler - Performance Parameters

• Isolation Loss or Crosstalk

• Insertion Loss

i

ti P

PL

inputPower

outputsingleanyfromPower

3

01010

P

PLiso log

portinputotherintobackpowerReflected

portoneatpowerInputisoL

In dB

210

Generic 2X2 Guided-Wave Coupler

Sab

,b

2

1

b

band

a

a,a

2

1

2221

1211

ss

ssSwhere

There are altogether eight possible ways(two ways) for the light to travel.

a1

a2

b1

b2

Inputfield

strengths

Outputfield

strengths

s22

s21 s12

s11

211

Generic 2X2 Guided-Wave CouplerAssume: Fraction (1- ) of power in the input port 1 appears at output port 1,and the remaining power at the output port 2

1

1

j

jS

If = 0.5, an d input signal defined in terms of filed intensity Ei, then

2

1

2

1

1

1

2

1

,

,

,

,

i

i

o

o

E

E

j

j

E

E

Let Eo,2 = 0, thus in term of optical power

021

12

21

111 PEEEP iooo ,,*

,,

021

12

21

222 PEEEP iooo ,,*

,,

Half the input power appears at

each output

212

Tree and Branch Coupler

Coupling ratio; 1:1 or 1: n, where n is some fraction

Fibre

213

Star Couplers

• Optical couplers with more than four ports.

• Types of star couplers: – transmission star coupler

the light at any of the input port is split equally through all output ports.

– reflection star coupler

214

Fibre Star CouplerCombines power from N inputs and divided them between M outputs

NN

CR 1010 101

10 loglog

Coupling ratio

N

i iout

ine

P

PL

,

log1010Excess loss

1

N

1

N

P1

PN

Power at any one output ).......(, Nio PPPn

P 211

215

Star Coupler - 8 X 8

1

2

3

4

5

6

7

8

1, 2, ... 8

1, 2, ... 8

No of 3 dB coupler NN

N dBc 23 2log

N/2

N2log

Star couplers are optical couplers with more than four ports

216

Star Coupler - 8 X 8 - contd.

• If a fraction of power traversing each 3 dB coupler = Fp, where 0< Fp < 1.

Then, power lost within the coupler = 1- Fp. Excess loss )(log log N

pe FL 21010

NN

CR 1010 101

10 loglog

Coupling ratio

(splitting loss)

Total loss = splitting loss + excess loss

NFLT 10103223110 log)log.( NFLT 10103223110 log)log.(

217

Reflection Star Couplers

The light arriving at port A and is reflected back to all ports.

A directional coupler separates the transmitted and received

signals.

Source: Australian Photonics CRC

218

Y- Couplers

1 X 8 coupler

Y-junctions are 1 x 2 couplers and are a key element in networking.

Ii

I1

I2

219

Coupler - CharacteristicsDesign class No. of CR Le Isolation

port (dB) directivity (-dB)

2 x 2 2 0.1-0.5 0.07-1.0 40 to 55Single mode

2 x 2 2 0.5 1-2 35 to 40Multimode

N x N 3-32 0.33-0.03 0.5-8.0Star

220

Splitters• The simplest couplers are fiber optic splitters. • They possess at least three ports but may have more than 32 for more

complex devices. • Popular splitting ratios include 50%-50%, 90%-10%, 95%-5% and 99%-1%;

however, almost any custom value can be achieved. • Excess loss: assures that the total output is never as high as the input. It

hinders the performance. All couplers and splitters share this parameter. • They are symmetrical. For instance, if the same coupler injected 50 µW

into the 10% output leg, only 5 µW would reach the common port.

OutputOutput

Input

Irfan khan

Launching optical power from source into fiber needs following considerations:

Fiber parameters:

• Numerical aperture

• Core size

• Refractive index profile

• Core cladding index difference

Source parameters:

• Size

• Radiance

• Angular power distribution

Irfan khan

Coupling efficiency:

It is the measure of the amount of optical power emitted from a source that can be coupled into a fiber .

η = PF / PS

PF =Power coupled into the fiber

PS = Power emitted from the light source

Coupling efficiency depends on:

1. Type of fiber that is attached to the source

2. Coupling Process (e.g. lenses or other coupling improvement schemes)

Irfan khan

Flylead / Pigtail:

Short length of optical fiber attached with the source for best power coupling configuration.

Thus

Power launching problem for these pigtailed sources reduces to a simpler coupling optical power from one fiber to another.

Effects to be considered in this case include:

1.Fiber misalignments:

a. Different core sizes

b. Numerical apertures

c. Core refractive index profiles

2.Clean and smooth fiber end faces:

a) perfectly perpendicular to the axis

b) Polished at a slight angle to prevent back reflections

Irfan khan

Optical fiber receptacles:

An alternate arrangement consist of light sources and optical fiber receptacles that are integrated within a transceiver package.

Fiber connector from a cable is simply mated to the built in connector in the transceiver package.

Commercially available configurations are the popular small form factor (SFF) and the SFF pluggable (SFP) devices.

Photodiode, PIN, 1310/1550 nm, LC, SC or FC Receptacle

SFP ,Transceiver, 155 Mb/s STM-1

Irfan khan

Laser diodes with pigtails and Receptacle

Irfan khan

Source to fiber power launchingOptical output of a luminescent source is usually measured by its radiance B at a given diode current.

Radiance: It is the optical power radiated into a unit solid angle per unit emitting surface area and is generally specified in terms of watts per square centimeter per steradian.

Radiance = Power / per unit solid angle x per unit emitting surface area

Solid angle is defined by the projected area of a surface patch onto a unit sphere of a point.

The angle that, seen from the center of a sphere, includes a given area on the surface of that sphere. The value of the solid angle is numerically equal to the size of that area divided by the square of the radius of the sphere

Irfan khan

Radiance (Brightness) of the source

• B= Optical power radiated from a unit area of the source into a unit solid angle [watts/(square centimeter per stradian)]

Irfan khan

Surface emitting LEDs have a Lambertian pattern:

cos),( 0BB [5-2]

Irfan khan

Edge emitting LEDs and laser diodes radiation pattern

LT BBB cos

cos

cos

sin

),(

1

0

2

0

2

For edge emitting LEDs, L=1

[5-3]

Irfan khan

Power Coupled from source to the fiber

rdrdddB

dAdABP

s

r

s

A

sssF

m

f f

max0

0

2

0

2

00

sin),(

),([5-4]

source theof angleemission solid and area : and ssA

fiber of angle acceptance solid

and area : and ffA

Irfan khan

Power coupled from LED to the Fiber

rdrdB

rdrdB

rdrddBP

s

r

s

r

s

r

s

s

s

22

00

0

2

0

max02

0

0

2

0 0

0

0

NA

sin

sincos2max0

210

2220

22stepLED, 2)NA( nBrBrP ss [5-5]

Irfan khan

Power coupling from LED to step-index fiber

• Total optical power from LED:

sincos2

sin),(

2/

0

022

02

2

0

2/

0

BrdBrP

ddBAP

sss

ss

[5-6]

arP

r

a

arP

Pss

s

ss

if )NA(

if )NA(

2

2

2

stepLED, [5-7]

Irfan khan

Power coupling from LED to graded-index fiber

• Power coupled from the LED to the graded indexed fiber is given as

• If the medium between source and fiber is different from the core material with refractive index n, the power coupled into the fiber will be reduced by the factor

a

rnP

rdrnrnBP

ss

r

oginLED

s

2

212

2

21

0

22

22,

2

1

1

nn

nnR

Irfan khan

Power Launching Vs Wavelength

• Optical power only depends on the radiance and not on the wavelength of the mode. For a graded index fiber number of modes is related to the wavelength as

• So twice as many modes propagate for 900 nm as compared to 1300 nm but the radiated power per mode from a source is

• So twice as much power is launched per mode for 1300nm as compared to the 900nm

2

12

2

an

M

2os B

M

P

Irfan khan

Equilibrium Numerical aperture

• For fibers with flylead attachments the connecting fiber should have the same NA. A certain amount of loss occurs at this junction which is almost 0.1 – 1dB. Exact loss depends on the connecting mechanism.

• Excess power loss occurs for few tens of meters of a multimode fiber as the launched modes come to the equilibrium.

• The excess power loss is due to the non propagating modes• The loss is more important for SLED.• Fiber coupled lasers are less prone to this effect as they have very few

non propagating modes.• The optical power in the fiber scales as

Irfan khan

Equilibrium Numerical Aperture

Irfan khan

Lensing Scheme for Coupling Improvement

Several Possible lensing schemes are:

1. Rounded end fiber

2. Nonimaging Microsphere (small glass sphere in contact with both the

fiber and source)

3. Imaging sphere ( a larger spherical lens used to image the source on

the core area of the fiber end)

4. Cylindrical lens (generally formed from a short section of fiber)

5. Spherical surfaced LED and spherical ended fiber

6. Taper ended fiber.

Irfan khan

Examples of possible lensing scheme used to improve optical source to fiber coupling efficiency

Irfan khan

Problem in using lens:

Lensing Scheme for Coupling Improvement

One problem is that the lens size is similar to the source and fiber core dimensions, which introduces fabrication and handling difficulties.

In the case of taper end fiber, the mechanical alignment must be carried out with great precision

Irfan khan

Non Imaging Microsphere• Use for surface emitter is shown• Assumptions: refractive indices shown in

the fig. and emitting area is circular• To collimate the output from the LED,

the emitting surface should be located at the focal point of the lens which can be found as

• Where s and q are object and image distances as measured from the lens surface, n is the refractive index of the lens, n/ is the refractive index of the outside medium and r is the radius of curvature of the lens surface

Irfan khan

continued

The following sign conventions are used1. Light travels from left to right2. Object distances are measured as positive to the left of a vertex and negative to

the right3. Image distances are measured as positive to the right of a vertex and negative

to the left4. All convex surfaces encountered by the light have a positive radius of curvature,

and concave surfaces have a negative radius.

For these conventions, we can find the focal point for the right hand surface of the lens shown in the last fig. We set q = infinity, solve for s yields

s = f = 2RL

So the focal point is at point A. Magnification M of the emitting area is given as

Irfan khan

continued

Using eq. 5.4 one can show that, with the lens, the optical power PL that can be coupled into a full aperture angle 2θ is given by

For the fiber of radius a and numerical aperture NA, the maximum coupling efficiency max is given by

So when the radius of the emitting area is larger than the fiber radius, there’ll be no improvement in the coupling efficiency with the use of lens

Irfan khan

Laser diode to Fiber Coupling

• Edge emitting laser diodes have an emission pattern that nominally has FWHM of

• 30 – 50o in the plane perpendicular to the active area junction• 5 – 10o in the plane parallel to the junction

• As the angular output distribution of the laser is greater than the fiber acceptance angle and since the laser emitting area is much smaller than the fiber core, so that one can use

• spherical lenses• cylindrical lenses• Fiber taper

to improve the coupling efficiency between edge emitting laser diodes and optical fibers

• Same technique is used for vertical cavity surface emitting lasers (VCSELs).

Irfan khan

continued

• Mass produced connections of laser arrays to parallel multimode fiber has efficiencies of 35%

• Direct (lensless) coupling from a single VCSEL source to a multimode fiber results into efficiencies of upto 90%.

• The use of homogeneous glass microsphere lenses has been tested in series of several hundred laser diode assemblies.

• Spherical glass lens of refractive index 1.9 and diameters ranging between 50 and 60μm were epoxied to the ends of 50 μm core diameter graded index fibers having NA of 0.2. The measured FWHM values of the laser output beams were as follows

• b/w 3 and 9μm for the near field parallel to the junction• b/w 30 and 60o for the field perpendicular to the junction• b/w 15 and 55o for the field parallel to the junction Coupling efficiencies in these experiments ranged between 50 and 80%.

Irfan khan

Fiber-to-Fiber Joints

Interconnecting fibers in a fiber optic system is another very important factor. These interconnects should be low-loss. These interconnects occur at

• Optical source• Photodetector• Within the cable where two fibers are connected• Intermediate point in a link where two cables are connected

The connection can be

• Permanent bond: known as SPLICE

• Easily demountable connection: Known as CONNECTOR

Irfan khan

continued• All joining techniques are subject to different levels of power loss at the

joint. These losses depend on different parameters like

• Input power distribution to the joint• Length of the fiber between the source and the joint• Geometrical and waveguide characteristics of the two ends at the joint• Fiber end face qualities

• The optical power that can be coupled from one fiber to the other is limited by the number of modes that can propagate in each fiber

• A fiber with 500 modes capacity connected with the fiber of 400 modes capacity can only couple 80% of the power

• For a GIN fiber with core radius a, cladding index n2, k=2π/, and n(r) as the variation in the core index profile, the total number of modes can be found from the expression

5.18

Irfan khan

continued

• Eq. 5.18 can be associated with the general local numerical aperture to yield

• As the different fibers can have different values of a, NA(0) and α, so M can be different for different fibers

• The fraction of energy that can be coupled is proportional to the common mode volume Mcomm. The fiber-to-fiber coupling efficiency F is given by

Where ME is the number of modes in the emitting fiber. The fiber-to-fiber coupling loss LF is given in terms of F as

LF = -10 log F

Irfan khan

Case a: All modes equally excited, joint with fiber of the same size having even slight mechanical misalignment can cause power loss

Case b: Propagating modes in the steady state have an equilibrium NA. Joining with an optical fiber of the same core size and same characteristics will face a NA of larger size in the receiving fiber and even a mechanical misalignment cannot cause the power loss.

case b is for longer fibers. Power loss will occur when in the receiving fiber, steady state will be achieved

Irfan khan

Mechanical Misalignment• Mechanical alignment is the major problem when joining two fibers

considering their microscopic size. • A standard multimode GIN fiber core is 50 - 100μm in diameter (thickness

of the human hair)• Single mode fiber has core dia of 9 μm • Radiation losses occur because the acceptance cone of the emitting fiber

is not equal to the acceptance cone of the receiving fiber.• Magnitude of radiation loss depends on the degree of misalignment• Three different types of misalignment can occur

• Longitudinal Separation• Angular misalignment• Axial displacement or lateral displacement

Irfan khan

Axial displacement

• Most common misalignment is the axial displacement.

• It causes the greatest power loss

Illustration:

• Axial offset reduces the overlap area of the two fiber-core end faces

• This in turn reduces the power coupled between two fibers.

Irfan khan

continued

• To illistrate the effect of misalignment, consider two identical step-index fibers of radii a.

• Suppose the axes are offset be a separation d• Assume there is a uniform mdal power distribution in the emitting fiber.• NA is constant for the two fibers so coupled fiber will be proportional to

the common area Acomm of the two fiber cores

• Assignment: show that Acomm has expression

• For step index fiber, the coupling efficiency is simply the ratio of the common core area of the core end face area

Irfan khan

continued

• For Graded Index Fiber the calculations for the power loss between two identical fibers is more complex since n varies across the end face of the core.

• The total power coupled in the common area is restricted by the NA of the transmitting or receiving fiber at the point, depending which one is smaller.

• If the end face of the GIN fiber is uniformly illuminated, the optical power accepted by the core will be that power which falls within the NA of the fiber.

• The optical power density p(r) at a point r on the fiber end is proportional to the square of the local NA(r) at that point

Where NA(r) and NA(0) are defined by eqs. 2.80. p(0) is the power density at the core axis which is related to the total power P in the fiber by

Irfan khan

We can use the parabolic index profile (α=2.0) for which p(r) will be givn as

p(r) = p(0)[1 – {r/a}2 ]P will be calculated as

P = (πa2 / 2) p(0)

The calculations of received power for GIN fiber can be carried out and the result will be

Where P is the total power in the transmitting fiber, d is the distance between two axes and a is the radius of fiber

Irfan khan

continued

The coupling loss for the offsets is given as

For Longitudinal misalignment:For longitudinal misalignment of distance s, the coupling loss is given as

Where s is the misalignment and θc is the critical acceptance angle of the fiber

Irfan khan

Angular misalignment at the joint

When the axes of two fibers are angularly misaligned at the joint, the optical power that leaves the emitting fiber outside the acceptance angle of the receiving fiber will be lost. For two step index fibers with misalignment angle θ, the optical power loss at the joint will be

where

Irfan khan

Fiber Related LossesFiber losses are related to the

• Core diameter• Core area ellipticity, numerical aperture• Refractive index profiles• Core-cladding concentricity

Fiber losses are significant for differences in core radii and NADifferent core radii: Loss is given as

Different NA: Power loss is given as

Irfan khan

Different core index profiles: Coupling loss will be given as

Irfan khan

Fiber End Face Preparation• End face preparation is the first step before splicing or connecting the

fibers through connectors.

• Fiber end must be • Flat• Perpendicular to the fiber axis• Smooth

• Techniques used are• Sawing• Grinding• Polishing

• Grinding and Polishing require a controlled environment like laboratory or factory

Irfan khan

continued

• Controlled fracture techniques are used to cleave the fiber

• Highly smooth and perpendicular end faces can be produced through this method

• Requires a careful control of the curvature and the tension• Improperly controlled tension can cause multiple fracture and can leave a

lip or hackled portion

Irfan khan

Fiber Splicing

Three different types of splicing can be done

• Fusion splicing• V-groove mechanical splicing• Elastic tube splice

Irfan khan

Self-Centering Effect

Influences on Fusion Process

The self-centering effect is the tendency of the fiber to form a homogeneous joint which is consequently free of misalignment as result of the surface tension of the molten glass during the fusion bonding process

Core Eccentricity

The process of aligning the fiber cores is of great importance in splicing. Fibers with high core eccentricity can cause , depending on the position of the relating cores, increased splice losses due to the core offset within the splice

Fiber End Face Quality

The end face quality of fibers to be fused directly influences the splice loss. Thus when cleaving fibers for splicing, the end face of the fiber has to be clean, unchipped, flat and perpendicular to the fiber axis

Irfan khan

Influences on Fusion Process

Fiber Preparation Quality

When preparing the fibers for splicing, it is necessary to ensure that no damage occurs to the fiber cladding

Any damage to the unprotected glass of the fiber can produce micro cracks causing the fiber to break during handling, splicing or storage

Dirt Particles or Coating Residues

Any contamination on the fiber cladding or in the v-grooves can lead to bad fiber positioning.

This can cause fiber offset (fiber axis misalignment) and can influence the fusion process extremely like bad cleave angles

Irfan khan

Influences on Fusion Process

Fiber Melting Characteristics

When fibers are brought together for splice some air gaps are present, called gas bubbles

Electric arc should not be too intense or weak.

When electric arc melts the fibers, the glass tends to collapse inwards, filling the gap

Electrode Condition

High quality splices require a reproducible and stable fusion arc.

Fusion arc is influenced by electrode condition.

Electrode cleaning or replacement is necessary from time to time.

Irfan khan

Fusion Splicing

• It is the thermal bonding of two prepared fiber ends

• The chemical changes during melting sometimes produce a weak splice• Produce very low splice losses

Irfan khan

V-groove splicing

• The prepared fiber ends are first butt together in a V-shaped groove• They are bonded with an adhesive• The V-shaped channel is either grooved silicon, plastic ceramic or metal

substrate• Splice loss depends on the fiber size and eccentricity

Irfan khan

Elastic Tube splicing

• It automatically performs lateral, longitudinal and angular alignment• It splices multimode fiber with losses in the range as commercial fusion

splice• Less equipment and skills are needed• It consists of tube of an elastic material• Internal hole is of smaller diameter as compared to the fiber and is

tapered at two ends for easy insertion of the fiber• A wide range of fiber diameters can be spliced• The fibers to be spiced might not be of the same diameter, still its axial

alignment will be maximum

Irfan khan

Optical Fiber Connectors

Principle requirements of a good connector design are as follows:

Coupling loss: The connector assembly must maintain stringent alignment tolerances to ensure low mating losses. The losses should be around 2 to 5 percent (0.1 to 0.2 dB) and must not change significantly during operation and after numerous connects and disconnects.

Interchangeability: Connectors of the same type must be compatible from one manufacturer to another.

Ease of assembly: A service technician should be able to install the connector in a field environment, that is, in a location other than the connector attachment factory.

Irfan khan

Low environmental sensitivity:Conditions such as temperature, dust, and moisture should have a small effect on connector loss variations.

Low cost and reliable construction:The connector must have a precision suitable to the application, but it must be reliable and its cost must not be a major factor in the system.

Ease of connection:Except for certain unique applications, one should be able to mate and disconnect the connector simply and by hand.

Irfan khan

Connector components

Connectors are available in designs that screw on, twist on, or snap in place. The twist-on and snap-on designs are the ones used most commonly.

The basic coupling mechanisms used belong to either butt-joint or the expanded-beam classes. The majority of connectors use a butt-joint coupling mechanism.

The key components are a long, thin stainless steel, glass, ceramic, or plastic cylinder, known as a ferrule, and a precision sleeve into which the ferrule fits.

This sleeve is known variably as an alignment sleeve, an adapter, or a coupling receptacle.

The center of the ferrule has a hole that precisely matches the sizeof the fiber cladding diameter.

Butt-joint connector:

Irfan khan

Connector components

Expanded beam connector :

Employs lenses on the end of the fiber.

These lenses either collimate the light emerging from the transmitting fiber, or focus the expanded beam onto the core of the receiving fiber.

Optical processing elements, such as beam splitters and switches, can easily be inserted into the expanded beam between the fiber ends.

Irfan khan

Connector types

• Connector are available in designs that screw on, twist on, or snap into place

• Most commonly used are twist on, or snap on designs• These include single channel and multi channel assemblies• The basic coupling mechanism is either a Butt joint or an expanded beam

class• Butt joint connectors employ a metal, ceramic or a molded plastic Ferrule

for each fiber

Irfan khan

Expanded Beam Fiber Optic connector

• Expanded beam connector employs lenses on the end of the fibers.• The lenses collimate the light emerging from the transmitting fiber and

focuses the beam on the receiving fiber• The fiber to lens distance is equal to the focal length• As the beam is collimated so even a separation between the fibers will not

make a difference• Connector is less dependent on the lateral alignment• Beam splitters or switches can be inserted between the fibers

Irfan khan

Optical Connector Types

There are numerous connector styles and configurations.

The main ones are ST, SC, FC, LC, MU, MT-RJ, MPO, and variations on MPO.

ST is derived from the words straight tip, which refers to the ferrule configuration.

SC mean subscriber connector or square connector, although now the connectors are not known by those names.

A connector designed specifically for Fibre Channel applications was designated by the letters FC.

Since Lucent developed a specific connector type, they obviously nicknamed it the LC connector.

ST

SC

FC

LC

The letters MU were selected to indicate a miniature unit.MU

Irfan khan

The designation MT-RJ is an acronym for media termination—recommended jack.

Optical Connector Types

MT-RJ

The letters MPO were selected to indicate a multiple-fiber, push-on/pull-off connecting function.

MPO

Irfan khan

SC connectorST connector

Irfan khan

FC connector LC connector

Irfan khan

MU

MT-RJ

MPO

Irfan khan

Summary Coupling efficiency

Flylead / Pigtail

Optical fiber receptacles

Source to fiber power launching

Power coupling calculations

Lensing Scheme for Coupling Improvement

Fiber SplicingSplicing techniquesGood Splice RequirementsSplice PreparationInfluences on Fusion ProcessFusion Splicing Methods

Optical Fiber Connectors

Connector componentsOptical Connector Types

Coupling Losses

Intrinsic lossesExtrinsic losses