Optical Fiber Communication Guide

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
AmplifierDetector
Input
signal
o/p
Optica
l Fiber
Repeater
E/O E/OO/E
O/E

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
 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 = √(n2
1 - n2
2) / n1
 Hence no sinθomax = √(n2
1 - n2
2)
 For air no = 1
 sinθomax = √(n2
1 - n2
2) = NA
 sinθomax = √ n1(2∆ ) = NA
 ∆ = (n1 - n2) / n1 – Core cladding index difference
 ∆2 being small, neglected
n1
(n2
1 - n2
2)1/2
n2
φc
θ

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 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.

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
 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

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.
 Propagation in z direction takes place with group velocity
vg.
 E-H plane moves at angle normal to itself with speed of
light.

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
 Propagation in z direction takes place with group velocity vg.
 E-H plane moves at angle normal to itself with speed of light.

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 – k2
2 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 -

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

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 =

• 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
 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.

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
 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 Guide

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Optical Fiber Communication Guide