optical fiber communication part 1 optical fiber fundamentals

OPTICAL FIBER

COMMUNICATION

• Wavelength of operation

• Propagation of light in fibre

• Types of fibre

• Ray theory

• Mode theory

• Attenuation and dispersions

• Fibre Manufacturing

• Fibre to fibre coupling

• Splices and connectors

PART I:-

OPTICAL FIBRE

ADVANTAGES OF OPTICAL FIBER

COMMUNICATIONS

Low transmission loss and wide bandwidth.

Small size and weight.

Immunity to interference.

Electrical isolation.

Signal security.

Abundant raw material.

BLOCK DIAGRAM

Drive

r

Circui

t

Light

Source

LED/LD

Optical

Receive

r

Detecto

r

Processin

g Circuit

Light

Sourc

e

Amplifier Detector

Input

signal

o/p

Optica

l Fiber

Repeater

E/O E/O O/E

O/E

FREQUENCY

OF

OPERATION

FREQUENCY OF OPERATION

ATTENUATION OF

SIGNAL

ATTENUATION OF SIGNAL OFC Transmits all wavelengths from 800nm to 2.5µm.

Attenuation offered by different wavelengths are different.

Windows of wavelengths are used.

Earlier minimum attenuation sensed at 800nm to 900nm.

Concentration of hydroxyl ions and metallic ions impurities

reduced later .

Glass is further purified.

1100nm to 1600nm region gave lesser loss.

Two popular windows centered around 1300nm and

1550nm.

BASIC OPTICAL LAWS

n1sinφ1 = n2sinφ2

BASIC OPTICAL LAWS

Refractive index n = c/v

c = 3 X 108m/s speed of light in vacuum/free space.

v = Speed of light in material.

n(air) = 1

n(pure glass) = 1.5

n1 > n2

n1sinφ1 = n2sin φ2

PROPAGATION OF LIGHT THROUGH OFC

OPTICAL LAWS

φ1

φ2

OPTICAL LAWS

no sinθo = n1sin θ

no sinθo = n1sin(90-φ)

no sinθo = n1cos φ1

Also n1sinφ1 = n2sinφ2

θo is gradually increased.

θ increases and φ1 reduces till critical angle.

Limiting stage is when light refracts.

θo = θomax

φ1 = φc

φ

1

φ

2

NUMERICAL APERTURE – STEP INDEX

FIBER

no sinθomax = n1cos φc

n1sin φc = n2sin90

sin φc = n2/ n1

cos φc = √(n21 - n

22)

/ n1

Hence no sinθomax = √(n21 - n

22)

For air no = 1

sinθomax = √(n21 - n

22) = NA

sinθomax = √ n1(2∆ ) = NA

∆ = (n1 - n2) / n1 – Core cladding index difference

∆2 being small, neglected

n1

(n21 - n

22)

1/2

n2

φc

θ

WAVE PROPAGATION

FIBER STRUCTURE

In principle, clad is not necessary for light to propagate.

Light can propagate through core-air interface.

Clad is required for –

It reduces scattering losses due to dielectric

discontinuities at core surface.

Adds mechanical strength to fiber.

Protects core from absorbing external light.

FIBER STRUCTURE

Low loss fiber –

Made from glass core – glass cladding.

Medium loss fiber –

Glass core – plastic cladding (Plastoclad)or

plastic core – plastic cladding.

Has high loss.

Cheaper as clad covering is elastic abrasion resistant

plastic material.

Gives strength and protects fiber from geometric

irregularities, distortion and roughness of adjacent

surface.

TYPES OF FIBER

TYPES OF FIBER STRUCTURE

Step index Fiber–

RI is constant throughout and changes abruptly at

interface.

Constant in cladding.

Graded Index Fiber—

RI reduces gradually from center to interface and

constant in clad.

GRADED INDEX FIBER

NUMERICAL APERTURE – GRADED INDEX FIBER

More complex.

Function of position across core face.

Light will propagate as guided mode at r only if it is

within local NA(r) defined as -


Single mode Fiber–

Thin core.

Uses high power through precision LASER.

Low distortion.

Low intermodal dispersion.

High Bandwidth.


Multimode Fiber—

Large core diameter.

Easy power launching.

Easy connectorization.

Cheap.

Uses cheaper light source as LED and less complex

circuitry.

But power output is low.

Intermodal dispersion high.

Graded index fiber reduces dispersion hence has high

BW.

RAYS AND MODES

Light travels in form of ray with total internal reflection.

During launching, infinite number of rays launch inside.

Few discrete rays travel down the fiber.

Propagation of light uses set of electromagnetic waves .

Called Modes of waveguide.

Or trapped modes of waveguide.

RAYS AND MODES

Each guided mode has a pattern of E and H field lines

repeated along the fiber in interval of wavelength.

Certain discrete number of modes can propagate along the

fiber.

They are those EM waves satisfying

homogeneous wave equations in the fiber.

Boundary conditions at waveguide surface.

Propagation characteristics of light in OFC can be explained

by

Ray optics.

Electromagnetic field theory.

RAY OPTICS

Light rays are perpendicular to phase front of the wave.

Family of waves for one mode gives a set of light called Ray congruence.

Each ray of a set travels at same angle relative to fiber axis.

Discrete number of ray sets exist inside fiber due to Phase condition.

RAY OPTICS

•Two types of phase changes.

•One while reflection.

•Other while travelling

RAY OPTICS

RAY OPTICS

Phase shift 1: Totally internally reflected twice.

Depends upon whether polarization is normal or

parallel to plane of incidence.

With n = n1/n2 and θ1 < θc , Phase change at each

reflection:

RAY OPTICS

Phase shift 2: due to wave travel from A to B and B to

C.

δ2 = k1s

K1 = Propagation constant in medium of RI n1.

s= total distance travelled.

Total phase change must be integral multiple of 2Π.

RAY OPTICS

Total phase change must be integral multiple of 2Π.

Other angles cancel out each other.

Total angle of Π, 3Π etc will cancel out completely.

Hence..

M = number of discrete ray sets allowed to propagate inside fiber.

MODE THEORY FOR CIRCULAR WAVE GUIDE

Ray optics has limitations.

It does not deals with coherence or interference

phenomenon.

It doesn’t give the field distribution of individual mode.

Doesn’t show coupling of power between modes of wave

guides.

Hence the mode theory.

TYPES OF RAYS - MERIDIONAL RAYS

Confined to the meridional planes of the fiber, i.e. planes

containing axis of symmetry of fiber, core axis.

A given Meridional ray propagates in a single plane

along fiber axis, hence easy to track.

Bound Rays – Trapped in fiber core according to Snell’s

law of reflection and refraction.

Unbound Rays –Rays refracted out of fiber core

according to Snell’s law of refraction and can not be

trapped in core.

TYPES OF RAYS - SKEW RAYS

Propagates without passing through core of fiber.

Not confined to single plane, but follow helical path along fiber.

Difficult to track these rays as they do not lie in single plane.

SKEW RAYS.

•Direction of ray changes by angle 2γ at each reflection

where γ is angle between projection of ray in two

dimensions and radius of fiber core.

•Skew rays show smoothening effect on distribution of light

transmitted even if light launched in fiber is not uniform.

• Numerical aperture of skew rays is greater than

meridional rays.

ACCEPTANCE ANGLE OF SKEW RAYS.

cos ɸ = RB/AB = RB/BT * BT/AB

Under limiting condition ɸ becomes ɸc.

ACCEPTANCE ANGLE OF SKEW RAYS.

sin ɸc = n2/n1

Also no sinθo = n1sin θ

Under limiting condition -

sinɵas = NA/cosγ


• Field pattern of three modes shown.


Three categories of mode:

Bound modes are those modes which are confined in

core of waveguide.

Refracted modes are those which are scattered out of

clad due to roughness of surface or absorbed by coating

of clad.

Leaky modes are those which are partially confined to

core region

attenuate continuously, radiating their power out of

core as they propagate.

Due to tunnel effect.


• For a particular mode to be confined , the condition is:

• β is propagation constant.

• If β < n2k, power leaks out of core into cladding

region.

•Significant power loss due to leaky modes.

•Modes that sustain have very small loss throughout

fiber propagation.


Assuming linear isotropic dielectric material having no

current and free charge, Maxwell's equations are:

ELECTROMAGNETIC WAVE PROPAGATING ALONG

CYLINDRICAL WAVEGUIDE

1

2

3

4

5

6


CYLINDRICAL WAVEGUIDE

c2 = 1/μϵ

WAVE EQUATIONS FOR CYLINDRICAL OPTICAL FIBER

WAVEGUIDE.

TEM MODE

Occurs through free space/ parallel wire/ co-axial cable.

Ez = 0, Hz = 0.

Et , Ht are perpendicular to each other and to direction

of propagation.

TE MODE

Occurs through metallic waveguide.

Ez = 0, Hz = finite.

Electric field lies entirely in transverse plane.

Magnetic field vector has component in direction of Z as well as transverse.

Propagation in z direction takes place with group velocity vg.

E-H plane moves at angle normal to itself with speed of light.

TM MODE

Occurs through metallic waveguide.

Hz = 0, Ez = finite.

Magnetic field lies entirely in transverse plane.

Electric field vector has component in direction of Z as well as transverse.



HYBRID MODE

Hz = finite, Ez = finite.

Both Magnetic field and Electric field vectors have

components in direction of Z as well as transverse.

Et > Ht --- EH mode

Ht > Et --- HE mode



NOTE:

Meridional rays take place only in TE and TM mode

which is completely guided.

Skew rays propagate entirely in the hybrid HE and

EH mode. May contribute to losses through leakage

and radiation.

SOLUTION FOR WAVE EQUATIONS FOR CYLINDRICAL

OPTICAL FIBER WAVEGUIDE.

Putting value of Ez in wave equation, we get

•Similar equation for Hz.

•Bessel’s equation, whose Solutions are called Bessel's

functions.

REQUIREMENTS FROM SOLUTION

To sustain, field inside core must be sinusoidal.

Field should be exponentially decaying outside the core

i.e. in cladding.

Depending on q, we have to chose that only that

possibility which satisfies above two equations and

find q to achieve this solution.

Various possibilities for the solution are:--

1. q is real

Bessel function of order ν and argument qr. OR

Newmahn function of order ν and argument qr.

1. Q IS REAL

Bessel Function:-

Oscillatory behavior inside core.

Amplitude reduces as order ν increases.

For ν = 0, i.e., lowest order mode Jo is finite = 1.

Favorable inside core.

Neumann Function:-

Oscillatory behavior inside core.

Amplitude reduces as order ν increases.

For ν = 0, i.e., Neumann function tends to -∞

Not desirable condition as field strength along axis in

infinite.

HENCE:- Bessel function as solution to Bessel

equation inside core with q = real.

2. q is imaginary

Modified Bessel function of first kind OR

Modified Bessel function of second kind

qr/j is real

2. Q IS IMAGINARY

Modified Bessel function of first kind:

q is imaginary hence argument qr/j is real.

Monotonically increasing function of argument.

Not desirable in cladding.

Modified Bessel function of second kind

Monotonically decreasing function of argument.

Desirable in cladding.

HENCE:- Modified Bessel function of second

kind

as solution to Bessel equation inside cladding

with q = imaginary.


CYLINDRICAL WAVEGUIDE – INSIDE CORE

Field must be finite, hence sinusoidal inside core as r →0.

Same as Bessel function.

For r < a, solutions are Bessel function of first kind of order ν.

F1(r) = J ν(ur)


CYLINDRICAL WAVEGUIDE – INSIDE CLADDING

Field must decay exponentially outside core as r →∞.

Same as Modified Bessel function of second kind

Hence For r > a, solutions are Modified Bessel function

of second kind

F1(r) = Kν(wr)

CONDITION ON Β

From definition of Modified Bessel Function of Second kind, Kν(wr) = e-wr .

e-wr → 0 when r → ∞

Hence w2 = β2 – k22 must be >0.

Hence β ≥ k2 and defines cutoff condition.

Cutoff condition is the condition when mode is no longer bound to the core region.

Condition on J ν(ur) can be deduced from the fact that u must be real inside core for F1 to be real.

Hence k1 ≥ β

Therefore -

MODAL EQUATION

Solution for β will depend upon boundary conditions.

Tangential component Eɸ and Ez of E inside and outside of interface at r = a must be same.

Similarly Tangential component Hɸ and Hz of E inside and outside of interface at r = a must be same.

Let Ez = Ez1 inside core and Ez = Ez2 outside core-clad boundary.

• Inside core q2 is given

by-- • Outside core q2 is given

by--

----1

• Hence In cladding, q2 = - w2.

MODAL EQUATION

Hence condition on Eɸ1 and Eɸ2 at r = a is

---2

Similarly

---3

---4

MODAL EQUATION

Four unknown coefficients A, B, C, D.

Solution will exist if the determinants of these

coefficients is zero.

MODAL EQUATION

Evaluating this determinant gives eigenvalue equation for β--

MODAL EQUATION

Solving eigenvalue equation for β indicates---

Only discrete values β is allowed within the range

k2 ≤ β ≤ k1

Equation for some lower order modes can be given as -

BESSEL FUNCTION OF ORDER Ν

ROOTS OF BESSEL FUNCTION OF ORDER Ν

MODES IN STEP INDEX FIBER

J-Type Bessel function similar to harmonic function.

Oscillatory behavior for real k.

Hence m roots for each ν.

Roots given as βνm

Corresponding modes are either TEνm, TMνm, HEνm, or EHνm.

For dielectric fiber waveguide, all modes are hybrid except ν=0.

For ν = 0 -- 0

MODES IN STEP INDEX FIBER

CUT-OFF CONDITION-NORMALIZED FREQUENCY OR V

NUMBER Cutoff condition for a mode:

at which mode is no longer confined to core / guided region.

Field no longer decays outside core region.

Related to a parameter called V number or Normalized

frequency.

and w2a2

Dimensionless number V determines how many modes the fiber can support .

V can also be expressed as Normalized Propagation

Constant b as

b = 1 – a2u2/V2 V2 = a2(u2 +

w2)

NORMALIZED PROPAGATION CONSTANT B

Also

CUT-OFF

CONDITION

•Each mode can exist only for the value of V that exceeds the

limiting value.

•Mode is cut-off when β/k = n2.

•HE11 has no cut-off. It ceases to exists when core dia of fiber

is zero.

DESIGN OF SINGLE MODE FIBER

From β/k Vs V graph, there is only one mode HE11 till

V=2.405.

PROB – A step index fiber has normalized

frequency of 26.6 at wavelength 1300nm.If the

core radius is 25m, find numerical aperture.

NUMBER OF MODES M IN MM FIBER

A ray will be accepted by the fiber if it lies

within angle θ defined by NA .

• The solid acceptance angle of fiber -

NUMBER OF MODES M IN MM FIBER

For electromagnetic radiation of wavelength λ from a laser

or fiber, number of modes per unit solid angle is 2A/ λ2.

A = πa2

2 is because plane wave can have 2 polarization

orientations.

POWER FLOW IN STEP INDEX FIBER Power flowing in core and cladding can be obtained by

integrating poynting vector in axial direction.

M = V2/2

SIGNAL DEGRADATION IN OPTICAL FIBER

Signal attenuation.

Determines maximum repeater less distance

between Transmitter and Receiver.

Signal Distortion due to Dispersion (Pulse

broadening).

Determines information carrying capacity.

Bandwidth.

ATTENUATION

Expressed as α dB/Km.

L = fiber length.

Caused by

Absorption

Scattering

Bending

ATTENUATION- ABSORPTION

Absorption by atomic defects or imperfections

as Missing molecules.

High density cluster of atom groups.

Oxygen defects in glass.

Negligible w.r.t. other causes.

ATTENUATION- ABSORPTION

Extrinsic absorption of photons by impurity

atoms in glass as Iron, chromium, cobalt, copper

OH ions from hydro-oxygen flames.

Absorption results in energy level transition of electrons.

Charge exchange between OH ions.

Less than 0.5dB/Km in range of operation

with better methods.

VAD SILICA FIBER WITH VERY LOW OH ION

CONTENT

ATTENUATION - ABSORPTION

Intrinsic absorption by glass materials itself.

Due to absorption bands in ultraviolet region (Energy level transition).

Tail of the curves enter the operation region.

Small as compared to IR absorption.

E and loss inversely proportional to wavelength.

Typically 0.1dB/Km at 1200nm.

Follows empirical relation as: Urbach’s rule (E-Photon Energy)

SIGNAL DEGRADATION - ABSORPTION

Intrinsic absorption by glass materials itself.

Crystal lattice vibration in Infra red region

If frequency lies within resonant frequency of vibration.

Tail of the curves enter the operation region.

Typically 0.1dB/Km at 1500nm.

SIGNAL DEGRADATION - SCATTERING

Microscopic variations in material density.

Glass is randomly connected network of molecules having

higher or lower than average density.

Compositional fluctuations of SiO2, GeO2, and P2O5.

Give refractive index fluctuations.

If fluctuation distance very small w.r.t wavelength,

cause Rayleigh-type scattering of light.

i.e. photons moving in all directions.

Effective signal strength gradually reduces.

Proportional to λ-4.

Reduces with increase in wavelength.

SIGNAL DEGRADATION - SCATTERING

MIE scattering

When RI fluctuation distance comparable to wavelength.

Can be reduced by-

Reducing imperfections during manufacturing.

Carefully controlled extrusion and coating.

Increasing fiber guidance by increasing ∆.

OPTICAL

FIBER

ATTENUATION

CHARACTERIS

TICS OF LOW

LOSS, LOW OH

SILICA FIBER

SIGNAL DEGRADATION - BENDING

1.MACROSCOPIC BENDING

Bending radius larger than fiber diameter.

Coiling, corner turns.

No longer supports Total internal reflection for few rays.

Light refracts and power is lost.

Can be explained by mode theory.

SIGNAL DEGRADATION - BENDING



The radiation loss is present in every bent fiber no matter

how gentle the bend is.

Radiation loss depends upon how much is the energy

beyond xc.

For a given modal field distribution if xc reduces, the

radiation loss increases.

The xc reduces as the radius of curvature of the bent fiber

reduces, that is the fiber is sharply bent.


Lower order modes - fields decay rapidly in the cladding,

more confined in core.

Higher order modes - more slowly decaying energy in the

cladding .

The higher order modes hence are more susceptible to the

radiation loss compared to the lower order modes.

The number of modes therefore reduces in a multimode

fiber in presence of bends.

Energy on outer part of cladding has to travel faster than

light to keep pace with energy in core.

Not possible, hence gets lost.

2.MICROSCOPIC BENDING

Small scale fluctuations in radius of curvature of fiber axis.

Or non uniform lateral pressure during cabling.

Can not maintain Total Internal Reflection if ray hits bends.

Due to bends, Power couples from guided modes to leaky modes

0.1 to 0.2dB/Km

CORE CLADDING LOSSES

Core and clad have different composition and RI.

Hence different attenuation coefficients α1 and α2.

Loss for mode (vm) - P is total power.

SIGNAL DISTORTION IN OPTICAL FIBER

GROUP DELAY

Group delay is time required for a mode to travel along fiber length L.

Assume modulated optical signal excites all modes equally at input.

Each mode carries equal amount of energy through fiber.

Each mode contains all spectral components in the wavelength band of source.

Phase velocity – at which phase of a particular frequency travels in space.

vp = ω/ β = c/n1

Group velocity – at which overall wave (group of frequencies) travels in space.

Vg = ɗω/ ɗβ = c ɗk/ ɗβ

Each spectral component travels independently.

GROUP DELAY

Each spectral component undergoes time delay or group

delay per unit length in direction of propagation given as-

• Group velocity depends on wavelength.

• Each spectral component of a mode take different

amount of time to travel.

• Pulse spreads.

GROUP DELAY

Assuming spectral width is not too wide –

Delay difference per unit wavelength = dτg/ dλ

For spectral components which are δλ apart and lie δλ/2 above and below central wavelength λ0, total delay difference δτ over distance L is -

• If spectral width δλ of an optical source is characterized

by rms value σλ, then pulse spreading can be

approximated by rms pulse width.

GROUP DELAY

Dispersion = pulse spread as function of wavelength.

Measured in picoseconds/km/nm.

= pulse broadening per unit distance per unit spectral width

D

1

INTRAMODAL DISPERSION – MATERIAL DISPERSION

Also called chrominance dispersion or spectral dispersion.

RI varies for wavelength and phase velocity.

vp = ω/ β = λ/T, n= c/ vp

Source has finite spectral width.

Different wavelength travels with different phase velocities.

Delay. Shorter wavelength more delay.

LASER better than LED

VARIATION OF RI

WITH WAVELENGTH

FOR SILICA.

NOTE THE FLATTER

REGION OF LEAST

VARIATION AROUND

WAVELENGTH OF

OPERATION.

MATERIAL DISPERSION

Assuming dispersion is due to only material dispersion.

τg = τmat

Time delay per unit length = τmat /L = 1/ Vg = dβ/dω

• ω = 2πc/ λ

• dω = -2 π c/ λ2 dλ

• τmat /L = -λ2/2 π c dβ /dλ

MATERIAL DISPERSION

Total pulse spread σmat is fractional material dispersion per

unit spectral width taken over entire spectral width σλ .

• D is material dispersion per unit length per unit

spectral width.

• Dmat = ?

• Material dispersion can be reduced either by

• Choosing source with narrower spectral width σλ,

• Or by operating at longer wavelength.

• Proportional to curvature of RI profile.

DISPERSION VS

WAVELENGTH

If D is less than zero, the medium is said to have

positive dispersion.

Light pulse is propagated through a normally

dispersive medium, the result is the lower wavelength

components travel slower than the higher wavelength

components.

RI increases with reduction in wavelength.

If D is greater than zero, the medium has negative

dispersion.

Pulse travels through an anomalously dispersive

medium, lower wavelength components travel faster

than the higher ones.

RI increases with increase in wavelength.

Pulse spreads in both case.

n= c/ vp

MATERIAL DISPERSION REDUCES WITH WAVELENGTH

INTRAMODAL DISPERSION – WAVEGUIDE

DISPERSION Assuming RI of material independent of wavelength.

80% power in Core.

20% power in clad.

RI of clad is smaller.

Clad power travels faster than core power.

Cause pulse spreading.

INTRAMODAL DISPERSION – WAVEGUIDE

DISPERSION Need to make group delay independent of fiber configuration.

Group delay expressed in terms of normalized propagation

constant b.

Solving for β---

Δ≈ (n1-n2)/n2

• Group delay due to Wave guide dispersion =

WAVEGUIDE DISPERSION

• β is obtained by eigenvalue equations and expressed in

terms of Normalized Frequency V.

• In multimode fiber, waveguide dispersion is very small

w.r.t. material dispersion, hence ignored.

DISPERSION IN SINGLE MODE FIBER

Waveguide dispersion and material dispersions are

of same order.

Pulse spread σwg occurring over wavelength σλ derived

from derivative of group delay w.r.t. wavelength -


It is 0.2 to 0.1 for V from 2 to 2.4.

Find waveguide dispersion at V=2.4, Δ=0.001, n2=1.5

Material dispersion at 900nm =

DISPERSION VS WAVELENGTH


• Material dispersion dominates at 900nm.

• At longer wavelength as 1.310μm, total dispersion is

almost zero.

• It is operating wavelength for single mode.

INTERMODAL DELAY

In MM Step index fiber, each mode has different group velocity.

Higher order mode, slower axial group velocity due to steeper angle of propagation.

Higher order modes travels slower than lower order modes.

Pulse spreads.

Can be eliminated in MM Graded index fiber or single mode fiber.

FIBER MANUFACTURE – REQUIREMENTS FROM

MATERIAL

Must be possible to make long thin flexible fiber.

Transparent at particular optical wavelength.

Able to make physically compatible material having

slightly different refractive indices for core and cladding.

FIBER MANUFACTURE

I- Glass-Glass Fiber

Glass core and glass cladding.

Fragile, needs heavy strengthening covering.

Least attenuation.

Longer distance transmission.

FIBER MANUFACTURE


Glass as silica (SiO2) with RI of 1.458 at 850nm

Addition of GeO2 and P2O5 increases RI.

Addition of B2O3 and fluorine decreases RI.

FIBER MANUFACTURE


Combinations can be—

GeO2 - SiO2 core, SiO2 Cladding.

P2O5 - SiO2 core, SiO2 Cladding.

SiO2 core, B2O3 -SiO2 Cladding.

GeO2 - B2O3 - SiO2 core, B2O3 -SiO2 Cladding….

FIBER MANUFACTURE

II- Plastic clad Glass Fiber

Glass core and plastic cladding.

Higher losses.

Short distance (several hundred meters).

Reduced cost.

Core Silicon resin RI = 1.405 at 850nm

Clad is Teflon FEP (Perfluoronated ethylene propylene)

with RI = 1.338.

Large NA with large RI difference.

Core dia of 150 to 600µm.

LED as source.

FIBER MANUFACTURE

III- Plastic Fiber

Very short distance (100m max).

High attenuation.

Low cost, tough, durable, inexpensive.

Core dia of 110 to 1400µm.

LED as source.

Polystyrene core (1.6), methyl methacrylate clad (1.49).

NA = 0.6.

Polymethyle methacrylate core(1.49), its co-

polymer(1.40), NA = 0.5

FIBER MANUFACTURE

Preforms are made with core and cladding.

By reacting pure vapours of metal halides(SiCl4 , POCl3

and GeCl4) with oxygen.

Vapours are collected to make a loose structure.

Sintered at 1400⁰C to make clear glass rod.

10 to 25mm in diameter and 60 to 120cm long.

FIBER FABRICATION – OVPO

OUTSIDE VAPOUR PHASE OXIDATION


Graphite rod or ceramic mandrel used to deposit soot.

Impurity levels controlled to make core and cladding.

Mandrel is removed and Porous tube is sintered in dry

atmosphere .

Equations-

SiCl4↑ + O2↑ → SiO2 + 2Cl2↑

GeCl4↑ + O2↑ → GeO2 + 2Cl2↑

4POSiCl3↑ + 3O2↑ → 2P2O5 + 6Cl2↑

4BBr3↑ + 3O2↑ → 2B2O3 + 6Br2↑

POSiCl3 – Phosphorous Oxychloride

2BBr3- Boron Tribromide

FIBER FABRICATION – OVPO


Sintered in dry atmosphere

above 1400ºC Fiber drawing

FIBER

FABRICATION –

VPAD

VAPOUR PHASE

AXIAL DEPOSITION

VAPOUR PHASE AXIAL DEPOSITION

Soot deposited axially.

Two separate torches for clad and core.

Preform continuously rotated for uniform deposition.

Torches are correspondingly fed with metal halides.

Advantage:

No central hole.

Continuous process so low production cost.

Better yield.

No gap between torch chamber and sintering chamber.

Clean environment.

VAPOUR PHASE AXIAL DEPOSITION

Equations –

SiCl4↑ + 2H2O↑ → SiO2 + 2H2↑ + 2Cl2↑

GeCl4↑ + 2H2O↑ → GeO2 + 2H2↑ + 2Cl2↑

2POSiCl3↑ + 3H2O↑ → P2O5 + 3H2↑ + 3Cl2↑

2BBr3↑ + 3H2O↑ → B2O3 + 3H2↑ + 3Br2↑

POSiCl3 – Phosphorous Oxychloride

2BBr3- Boron Tribromide

FIBER FABRICATION – MCVD

MODIFIED CHEMICAL VAPOUR DEPOSITION

MODIFIED CHEMICAL VAPOUR DEPOSITION

Most widely method.

Clear glass tube as clad taken.

Metal halide with oxygen is flown into it.

Soot deposited uniformly as tube is rotated.

Burner sinters the soot to clear glass continuously.

Later hole tube is heated strongly to collapse it to solid

rod.

Equations same as OVPO

FIBER FABRICATION – PCVD

PLASMA ACTIVATED CHEMICAL VAPOUR

DEPOSITION

PCVD - PLASMA ACTIVATED CHEMICAL VAPOUR

DEPOSITION

Gas molecules or atoms turn into a plasma containing

charged particles, positive ions and negative electrons,

when heated or under strong electromagnetic field.

The presence of a non-negligible number of charge

carriers makes the plasma electrically conductive.

Very small grains of silica within a gaseous plasma will

also pick up a net negative charge.

They act like a very heavy negative ion components of

the plasma.

PCVD - PLASMA ACTIVATED CHEMICAL VAPOUR

DEPOSITION

Moving microwave resonator at 2.45GHz generates

plasma inside tube to activate chemical reaction.

Gaseous ions escape through exhaust while silica heavy

plasma move along with M/W resonator and get

deposited inside tube.

Silica tube at 1000 to 1200⁰C to reduce mechanical

stress in growing glass film.

Deposits clear glass directly on tube wall till desired

thickness achieved.

No soot, no sintering.

At end tube is collapsed into a Preform.

DIRECT FIBER FABRICATION –

DOUBLE CRUCIBLE METHOD

DOUBLE CRUCIBLE METHOD

Glass rods of core and clad material are separately

made by melting mixtures of purified powders of

required composition.

Rods used as feedstock for two concentric crucibles.

Fibers are drawn from molten state through orifices of

crucibles.

Has advantage of being continuous process.

Requires careful attention to avoid contamination.

Contamination can be from furnace environment or

crucible.

Glass crucible used to make rods.

Platinum crucibles used in furnace to melt and draw

fiber.

FIBER DRAWING

FIBER DRAWING

Preform is softened in drawing furnace till it is

possible to draw thin filament.

Turning speed of drum decides thickness of fiber.

Speed regulation is done by thickness monitor in

feedback loop.

A thin elastic coating is applied to protect from dust

and water vapour.

These fibers are later bound into cable.

FIBER TO FIBER COUPLING

If all modes are equally excited, optical beam fills

entire NA of emitting fiber.

Perfect mechanical alignment required.

Geometrical and waveguide characteristics must

exactly match.

In case of equilibrium state, energy in central region.

Fills only equilibrium NA of next fiber.

No joint loss for Slight misalignment or slight

variation in characteristics.

Further power loss in new fiber after new steady state.

TYPES OF MISALIGNMENT

AXIAL OR LATERAL MISALINGMENT

MECHANICAL MISALIGNMENT – AXIAL

OR LATERAL MISALIGNMENT

STEP INDEX FIBER-( Constant NA)

Most common in practice.

Greatest power loss.

Assuming uniform modal power distribution—

Coupled power proportional to common area.

Coupling efficiency is ratio of common core area

to receiving core end face area.

ηF step = Acomm/πa2

MECHANICAL MISALIGNMENT – AXIAL

OR LATERAL MISALIGNMENT

GRADED INDEX FIBER-( Variable NA)

Power coupled restricted by NA of transmitting and

receiving fiber whichever is smaller at that point.

For uniform illumination optical power accepted by core

is that power that falls within the NA of that fiber.

Optical power density p(r ) at a point r on the fiber end

face is proportional to the square of local NA.

AXIAL OR LATERAL MISALIGNMENT

GRADED INDEX FIBER-( Variable NA)

In area A1

NA of transmitting fiber is more than receiving fiber.

Receiving fiber will accept only part of transmitted

power that falls within its own NA.

In area A2

NA of receiving fiber is more than transmitting fiber.

Receiving fiber will accept all of transmitted power in

this region.

LONGITUDINAL SEPERATION

LONGITUDINAL SEPERATION All higher order modes optical power emitted in the

ring of width x will not be intercepted by receiving fiber.

Loss is given by--

FIBER RELATED LOSS

Due to difference in geometrical and wave guide

related characteristics as--

Core diameter variation*

Core area ellipticity.

NA variation*

RI profile variation

Core cladding concentricity

FIBER RELATED LOSS – COUPLING

LOSS If aE ≠ aR

And

But


LOSS

If NAE(0) ≠ NAR(0)

And

But

aR = aE


LOSS

And

But aE =

aR

If αE ≠αR

For αR <αE , number of modes that can be supported by receiving fiber is less

than number of modes in emitting fiber.

FIBER END FACE PREPARATIONS

For splicing or connectorisation, end face must

be : Flat

Perpendicular to fiber axis

Smooth

Techniques: Sawing

Grinding

Polishing

Controlled fracture

FIBER PREPERATION

CONTROLLED FRACTURE TECHNIQUE

• Fiber scratched to create pressure concentration.

• Uniform Tension is applied to two ends of fiber kept on curved base.

•Maximum stress occurs at scratched point.

• Crack propagates through the fiber.

• Highly smooth and perpendicular end face can be achieved.

IMPROPERLY CLEAVED FIBER END

Non uniform stress applied.

Curvature of fiber not proper.

LIP:

Sharp protrusion from edge of cleaved fiber.

Prevents proper contact with adjoining fiber.

Can cause fiber damage.

HACKLE:

Severe irregularity across fiber face

Smooth Surface

Hackled Surface

IMPROPERLY CLEAVED FIBER END

ROLL-OFF:

Rounding off of edge of fiber, condition opposite to lip.

Also called Break-over.

Can cause high insertion or splice loss.

CHIP:

Localized fracture or break at end of cleaved fiber.

MIST:

Less severe hackle.

SPIRAL or STEP:

Abrupt changes on fiber end faces topology.

SHATTERING:

Due to uncontrolled fracture, fiber face has no definable cleave

or surface characteristics.

FIBER SPLICING

Fusion splicing

V-Groove splicing

Tube mechanical splicing

Elastic tube splicing

Rotary splicing

FUSION SPLICING

Fiber end-face prepared and aligned microscopically.

Joint then heated by electric arc or laser pulse.

Joint momentarily melts and joins.

Very low splice loss 0.06dB.

Weak splice may result if end face not clean and

prepared, and uncontrolled heating etc

V-GROOVE SPLICING

Temporary splice needed during testing.

V-shaped channel made of silicon, plastic, ceramic, metal substrate.

Bonded together with adhesive or held in place with cover plate.

Splice loss depends on outer dimension of fiber, eccentricity.

ELASTIC TUBE SPLICING

Automatically performs, laterally, longitudinal and angular alignment.

Splices multimode fibers with good accuracy.

Less equipment and skills needed.

Uses elastic tube with hole slightly smaller than fiber with taper on each end for easy insertion.

Fibers to be joined need not have same outer dimensions.

OPTICAL FIBER CONNECTOR

Requirements of a good connector are:

Low coupling loss

Interchangeability – compatibility from manufacturer

to manufacturer.

Ease of assembly, even on field, independent of

operator skill.

Low environmental sensitivity- temperature, dust,

moisture have no effect on connector losses.

Low cost and reliable construction

Ease of connection

CONNECTOR TYPES

Single channel and multichannel assemblies in

Screw-on

Bayonet-mount

Push-pull

Basic mechanisms are

Butt joint – more common

Expanded beam

BUTT JOINT-STRAIGHT SLEEVE CONNECTOR

Metal, ceramic or molded-plastic ferrule for each fiber.

Precision sleeve into which ferrule fits.

Fiber epoxied into hole drilled in ferrule.

SM and MM fibers.

Length of sleeve and guide ring on ferrule determine

the end separation of fibers.

BUTT JOINT-TAPERED SLEEVE CONNECTOR

Metal, ceramic or molded-plastic ferrule for each fiber.

Taper sleeve to accept and guide tapered ferrule.

SM and MM fibers.

Length of sleeve and guide ring on ferrule determine

the end separation of fibers.

EXPANDED-BEAM CONNECTOR

Lens on the end of fiber.

Lenses collimate or focus expanded beam into

receiving core.

Fiber to lens distance is equal to focal length of lens.

Connector less dependent on lateral alignment.

Beam splitters and switches can easily be inserted into

the connector.

optical fiber communication part 1 optical fiber fundamentals

Engineering