optical_final.pdf Optical Fiber PPT BECE

Optical Fiber Communication System
At the transmitter section, the electrical signal is converted into an optical
signal using an optical source (intensity modulation).
The modulated optical signal is transmitted through optical fibers to the
receiver.
At the receiver end, the optical signal is reconverted to the electrical signal
for further processing (demodulation) before passing onto destination.

During past three decades, many remarkable and dramatic
changes took place in the electronic communication industry
over the Globe..
Importance Fiber Optic Technology?

•A phenomenal increase in voice, data and video
communication - demands for larger capacity and more
economical communication systems.
•Lightwave Technology: Technological route for
achieving this goal .Most cost-effective way to move huge amounts of
information (voice, video, data) quickly and reliably.

A system with light as carriers has an excessive bandwidth (more than
100,000 times than achieved with microwave frequencies)
The information carrying capacity of a communications system is
directly proportional to its bandwidth;
 Wider the bandwidth, the greater its information carrying capacity.
For good communication a system needs to have following things.
(1)Bandwidh (BW)
(2)Good signal to noise ratio (SNR) i.e. low loss
Since the bandwidth of a system is more or less proportional to the frequency of operation,
use of higher frequency facilitates larger BW.
The BW at optical frequencies is expected to be 3 to 4 orders of magnitude higher than
that at the microwave f requencies (1GHz to 100GHz).
How Optical Transmission fulfill the need?

3,00,000 voice channels on a pair of fiber
Voice/Data/Video Integrated Service
2.5 Gb/s systems from NTT ,Japan; 5 Gb/s System Siemens

Low loss : Information can be sent over a large distance.
Losses ~ 0.2 dB/km
Repeater spacing >100 km with bit rates in Gb/s

Immune to Electromagnetic interference: No cross talk between fibers
Can be used in harsh or noisy environments

Higher security : No radiations, Difficult to tap
Attractive for Defense, Intelligence and Banks Networks
Wide Bandwidth: Extremely high information carrying
capacity (~GHz)
Advantages of Optical Fiber
Interference Free

Advantages of Optical Fiber: Contd..
Compact & light weight
Smaller size : Fiber thinner than human hair
 Can easily replace 1000 pair copper cable of 10 cm dia.
Fiber weighs 28gm/km; considerably lighter than copper
Light weight cable
Environmental Immunity/Greater safety
Dielectric- No current, No short circuits – Extremely safe for
hazardous environments; attractive for oil & petrochemicals
Not prone to lightning
Wide temperature range
Long life > 25 years
Abundant Raw Material : optical fibers made from Sand
Not a scarce resource in comparison to copper.

Some Practical Disadvantages
Optical fibers are relatively expensive.

Connectors very expensive: Due to high degree of precision
involved

Connector installation is time consuming and highly skilled
operation

Jointing (Splicing) of fibers requires expensive equipment
and skilled operators

Connector and joints are relatively lossy.

Difficult to tap in and out ( for bus architectures)- need
expensive couplers

Relatively careful handling required

WDM/DWDM Concept
Typical WDM network containing various types of optical amplifiers.

•A single solid dielectric of two concentric layers. The inner

Fiber Structure
OPTICAL FIBER
layer known as Core is of radius ‘a’ and refractive index „n1‟.
The outer layer called Cladding has refractive index „n2‟.
An optical fiber is a long cylindrical dielectric waveguide,
usually of circular cross-section, transparent to the operating
wavelength.
n2 < n1  condition necessary is TIR

For light propagation through the fiber, the conditions
for total internal reflection (TIR) should be met at the
core-cladding interface
Light Propagation through Optical Fiber

•Core and Cladding materials
•Refractive index profile
•Modes of propagation
CLASSIFICATION OF OPTICAL FIBERS
Classified on basis of

Three Varieties:

•More rugged than glass; attractive to military applications
•Medium attenuation and propagation characteristics

•More flexible and more rugged
•Easy to install, better withstand stress, less expensive, weigh 60%
less than glass
•High attenuation- limited to short runs.
a. Glass core and cladding (SCS: silca-clad silica)
b.Glass core with plastic cladding ( PCS: plastic clad silica) c. Plastic core and cladding
• minimum attenuation & good propagation characteristics
•Least rugged – delicate to handle

Refractive Index Profile: Two types

the radial distance from the centre of the fiber

Mode of propagations : Two types

•Step Index : Refractive index makes abrupt change
•Graded Index : Refractive index is made to vary as a function of

•Single mode : Single path of light
•Multimode : Multiple paths

Light entering from glass-air interface (n
1>n
2) - Refraction

 
2 > 
1
1
2
2
1
n
n
sin
sin



•At 
2 = 90
o
, refracted ray moves parallel to interface between dielectrics and

1<90
o
- Limiting case of refraction

Angle of incidence, 
1  
C ; critical angle
Snell’s Law:

Total Internal Reflection

Value of critical angle (
C ); sin 
C = n
2/n
1

At angle of incidence greater than critical angle, the light is
reflected back into the originating dielectric medium (TIR)
with high efficiency ( 99.9%)
meridional ray Transmission of light ray in a perfect optical fiber
Total Internal Reflection

Not all rays entering the fiber core will continue to be propagated
down its length

ACCEPTANCE ANGLE
Only rays with sufficiently shallow grazing angle ( i.e. angle to the
normal > C ) at the core-cladding interface are transmitted by TIR.
Any ray incident into fiber core at angle > a will be transmitted
to core-cladding interface at an angle < C and will not follow
TIR  Lost

For rays to be transmitted by TIR within the fiber core, they
must be incident on the fiber core within an acceptance cone
defined by the conical half angle “
a” .
•From symmetry considerations, the output angle to the axis will
be equal to the input angle for the ray, assuming that the ray
emerges into a medium of the same refractive index from which it
was input.
Acceptance Angle….
the fiber in order to be propagated  Acceptance angle for
the fiber
a is the maximum angle to the axis at which light may enter

NA = sin 
a = n
2
core - n
2
cladding
Acceptance / Emission Cone
A Very useful parameter : measure of light collecting ability
of fiber.
Larger the magnitude of NA, greater the amount of light accepted by the
fiber from the external source
Numerical Aperture (NA)
•NA varies from 0.12- 0.20 for SMFs and 0.15- 0.50 for MMFs

Where n
0 is refractive index of medium outside the fiber. For
air n
0 =1.

Therefore,

The significance of NA is that light entering in the cone of semi vertical angle im only
propagate through the fibre. The higher the value of im or NA more is the light collected for
propagation in the fibre. Numerical aperture is thus considered as a light gathering capacity
of an optical fibre.
NA= Sin θ
a
where θ
a, is called acceptance cone angle

In terms of relative R.I. difference „‟ between core and cladding, )1for(
n
nn
n2
nn
1
21
2
1
2
2
2
1





NA = n
1(2 )
½

• NA ; independent of core and cladding diameters
• Holds for diameters as small as 8 m

NA and  (Relative R.I Difference)

To obtain an detailed understanding of
propagation of light in an optical fiber

Light as a variety of EM vibrations E and H
fields at right angle to each other and
perpendicular to direction of propagation.

Necessary to solve Maxwell’s Equations

•Very complex analyses - Qualitative aspects only
ELECTROMAGNETIC THEORY

•Light as a variety of EM vibrations E and H at right angle to
each other and perpendicular to direction of propagation.
Field distributions in plane E&H waves

Assuming a linear isotropic dielectric material having no
currents and free charges
Maxwell’s Equations

Substituting for D and B and taking curl of first equation
Using vector identity
We get
Similarly
Wave equations for each component of the field vectors E & H.
Maxwell’s Equations

(a) A plane wave propagating in the guide (b) Interference of
plane wave in the guide ( forming lowest order mode m=0)
A plane monochromatic wave propagating in direction of ray path
within the guide of refractive index n
1 sandwiched between two regions
of lower refractive index n
2
•Wavelength = /n
1

z = n
1k cos

x = n
1k sin
Concept of Modes
•Propagation constant
 = n1k
•Components of  in z
and x directions
•Constructive interference
occurs and standing wave
obtained in x-direction

Concept of Modes
Components of plane wave in x-direction reflected at core-
cladding interface and interfere
Constructive: when total phase change after two reflection is equal to
2m radians; m an integer - Standing wave in x-direction
The optical wave is confined within the guide and the electric field
distribution in the x-direction does not change as the wave propagate in
the z-direction – Sinusoidally varying in z-direction
•Specific mode is obtained only when the angle between the propagation
vectors or rays and interface have a particular value – Discrete modes
typified by a distinct value of 
The stable field distribution in the x-direction with only a periodic z-
dependence is known as a MODE.
•Have periodic z-dependence of exp(-jz z) or commonly exp(-j z)
•Have time dependence with angular frequency , i.e. exp (j t)

For monochromatic light fields of angular frequency , a mode traveling in
positive z-direction has a time and z-dependence given by

exp j(t- z)
•Dominant modes propagating
in z-direction with electric field
distribution in x-direction
formed by rays with m=1,2,3
•m denotes number of zeros in
this transverse pattern.
•It also signifies the order of the
mode and is known as mode
number.
Ray propagation and corresponding TE field
patterns of three lower order modes in planar guide.
Higher Order Modes

Wave picture of waveguides

corresponding component of the magnetic field H is in the
direction of propagation.
Transverse Magnetic (TM) mode: When a component of E field
is in the direction of propagation, but H
z=0.

Transverse ElectroMagnetic (TEM) : When total field lies in the
transverse plane in that case both E
z and H
z are zero.
Transverse Electric mode (TE): When electric field is
perpendicular to the direction of propagation, i.e. E
z=0, but a
TE and TM modes

Low-order TE or TM mode fields

Phase Velocity: For plane wave, there are points of constant phase, these
constant phase points forms a surface, referred to as a wavefront.
Formation of wave packet from combination
of two waves of nearly equal frequencies
Wave packet observed to move at a
group velocity, v
g = / 

V
g is of great importance in study of
TCs of optical fibers as it relates to the
propagation characteristics of
observable wave groups
Phase and Group Velocity
•Non-monochromaticity leads to
group of waves with closely similar
frequencies – Wave Packet
As light wave propagate along a waveguide in the z-direction, wavefront
travel at a phase velocity ;
vp = / 

Considering propagation in an infinite medium of R.I. n
1, 1
p
n
c
v g1
1
g
N
c
d
dn
n
c
v 









Parameter N
g is known as the group index of the guide c
n
2
nkn
111




Group Velocity
Phase velocity :
Propagation constant :
 
Group velocity :

Another phenomenon of interest under conditions of TIR is the
form of the electric field in the cladding of the guide.
The transmitted wave field in the cladding is of the form
B = B
0 exp(-
2x)exp j(t-z)
The amplitude of the field in the cladding is observed to decay exponentially
in the x-direction.- Evanescent Field
Exponentially decaying evanescent field
in the cladding
•A field of this type stores energy and
transports it in the direction of propagation
(z) but does not transport energy in the
transverse direction (x).

•Indicates that optical energy is transmitted
into the cladding.
Evanescent Field

The cladding should be transparent to light at the wavelengths over which
the guide is to operate.
Ideally, the cladding should consist of a solid material in order to avoid
both damage to the guide and the accumulation of foreign matter on the
guide walls.
The cladding thickness must be sufficient to allow the evanescent field to
decay to a low value or losses from the penetrating energy may be
encountered.
The evanescent field gives rise to the following requirements
for the choice of cladding material
Cladding Material
Therefore, the most widely used optical fibers consist of a core
and cladding, both made of glass. Although, it give a lower NA
for fiber, but provides a far more practical solution.

In common with planar waveguide, TE and TM modes are
obtained within dielectric cylinder.
•A cylindrical waveguide is bounded in two dimensions,
therefore, two integers, l and m to specify the modes.
TE
lm and TM
lm modes
These modes from meridional rays propagation within guide
Hybrid modes where E
z and H
z are nonzero – results from skew
ray propagation within the fiber. Designated as
HE
lm and EH
lm depending upon whether the components of
H or E make the larger contribution to transverse field
Cylindrical Fiber
Exact solution of Maxwell’s Eqns. for a cylindrical dielectric
waveguide- very complicated & complex results

Analysis simplified by considering fibers for communication
purposes.
Satisfy, weakly guided approximation , <<1, small grazing angles 
•This linear combination of degenerate modes produces a useful
simplification in the analysis of weakly guiding fibers.
Modes in Cylindrical Fibers
Approximate solutions for full set of HE, EH, TE and TM modes
may be given by two linearly polarized (LP) components
•Not exact modes of fiber except for fundamental mode, however, as  is
very small, HE-EH modes pairs occur with almost identical propagation
constants  Degenerate modes
•The superposition of these degenerating modes characterized by a common
propagation constant corresponds to particular LP modes regardless of their
HE, EH, TE or TM configurations.

Correspondence between the lower order in linearly polarized
modes and the traditional exact modes from which they are
formed.

Linearly polarized Exact

LP
01 HE
11
LP
11 HE
21, TE
01, TM
01
LP
21 HE
31, EH
11
LP
02 HE
12
LP
31 HE
41, EH
21
LP
12 HE
22, TE
02, TM
02
LP
lm HE
2m, TE
0m, TM
0m

Electric field configuration for the
three lowest LP modes in terms of their
constituent exact modes:

•(a) LP mode designations;
•(b) exact mode designations;
•(c) electric field distribution of the
exact modes;
•(d) intensity distribution of E
x for
exact modes indicating the electric
field intensity profile for the
corresponding LP modes.

Intensity Profiles
Field strength in the transverse
direction is identical for the modes
which belong to the same LP mode.

The scalar wave equation for homogeneous core waveguide
under weak guidance conditions is
•The propagation constant for the guided modes  lie in the range
n
2k<  <n
1k
Solution of wave equation for cylindrical fiber have the form

Here,  Represents the dominant transverse electric field component. The
periodic dependence on  gives a mode of radial order l. 








 )ztexp(
lsin
lcos
)r(E  0kn
d
d
r
1
dr
d
r
1
dr
d
222
12
2
22
2







 is the field (E or H).
Solutions of Wave Equation

  0E
r
l
kn
dr
dE
r
1
dr
Ed
2
2
222
12
2







 Introducing the solution to wave equation results in a differential
equation
Important to note is that the field is finite at r =0 and is represented
by the Zero order Bessel function J
0. However, the field vanishes
as r goes to infinity and the solutions in the cladding are therefore
modified Bessel functions denoted by K
l – These modified
functions decay exponentially w.r.t. r.

damped oscillatory functions w.r.t. r)
For a SI fiber with constant RI core, it is a Bessel differential
equation and the solutions are cylinder functions. In the core
region the solutions are Bessel functions denoted by Jl (Gradually

Figures Showing

(a) Variation of the Bessel
function J
l(r) for l = 0, 1, 2, 3
( first four orders), plotted
against r.

(b) Graph of the modified
Bessel function K
l(r) against
r for l = 0, 1.

The electric field is given by
E(r) = GJ
l(UR) for R<1 (core)
= GJ
l(U) K
l(WR)/K
l(W) for R>1(cladding)
where G; amplitude coefficient, R=r/a; normalized radial coordinate, U & W
are eigen values in the core and cladding respectively
U; radial phase parameter or radial propagation constant
W; cladding decay parameter
U = a(n
1
2
k
2
-
2
)
½
and W= a(
2
-n
2
2
k
2
)
½
•The sum of squares of U & W defines a very useful quantity
usually referred to as normalized frequency V
V = (U
2
+W
2
)
½
= ka(n
1
2
-n
2
2
)
½
Bessel Function Solutions

Normalized Frequency, V may be expressed in terms of NA and ,
as
2
1
)2(na
2
)NA(a
2
V
1







•Normalized frequency is a dimensionless parameter and simply
called V-number or value of the fiber.
•It combines in a very useful manner the information about three
parameters, a,  and .
V-Number

The allowed regions for the LP modes of order l = 0,1
against normalized frequency (V) for a circular optical
waveguide with a constant refractive index core (step
index fiber).
Lower order modes obtained in a cylindrical homogeneous
core waveguide
•Value of V, where J
0
and J
1 cross the zero
gives the cutoff point
for various modes.
V = V
c ;

•V
c is different for
different modes
= 0 for LP
01 mode
= 2.405 for LP
11
= 3.83 for LP
02

Allowed LP modes

Fiber with a core of constant refractive index n
1 and a cladding
of slightly lower refractive index n
2 .
Refractive index profile makes a step change at the core-cladding interface
The refractive index profile and ray transmission
in step index fibers: (a) multimode step index fiber.
(b) single-mode step index fiber.
Refractive index profile

n
1 ; r<a (core)
n(r) =
n
2 ; r a (cladding)

• Single mode Step Index
Step Index Fibers
• Multimode Step Index

MM SI fibers allow the propagation of a finite number of
guided modes along the channel.
Number of guided modes is dependent upon the physical parameters ; a, 
of fibers and wavelength of the transmitted light – included in V-number
•The total number of guided modes or mode volume M
s for SI
fiber is related to V-number for the fiber by approximate
expression
M
s  V
2
/2
Allows an estimate of number of guided modes propagating in a particular
MM SI fiber.
For example: A MM SI fiber of core diameter 80m, core refractive index
1.48, relative index difference of 1.5% and operating at 850nm supports 2873
guided modes.
Modes in SI Fibers

GI fibers do not have a constant refractive index in the core, but a
decreasing core index n(r) with radial distance from a maximum
value of n
1 at the axis to a constant value n
2 beyond the core
radius „a‟ in the cladding. – Inhomogeneous core fibers
where,  is relative refractive index difference and  is the profile
parameter which gives the characteristic RI profile of the fiber core.
Index variation is represented as
Graded Index Fiber Structure

Possible fiber refractive index profiles for different
values of 
= ; Step index profile
= 2; Parabolic profile
 =1 Triangular profile
The refractive index profile and ray transmission in a multimode
graded index fiber.

This compensate for the shorter path lengths and reduces
dispersion in the fiber.
An expanded ray diagram showing refraction at the various high to low index
interfacial within a graded index fiber, giving an overall curved ray path.
•In GI fibers, the rays traveling close to the fiber axis have shorter
paths when compared with the rays which travel into the outer
regions of the core. However, the near axial rays are transmitted
through a region of higher refractive index and therefore travel
with a lower velocity than the more extreme rays.

Local numerical aperture
Axial numerical aperture
The parameters defined for SI fibers ( NA, , V) may be applied to GI fibers
and give comparison between two. However, in GI fibers situation is more
complicated because of radial variation of RI of core from the axis, NA is also
function of radial distance.
Number of bound modes in graded index fiber is 
























2
V
2
)kan(
2
M
2
2
1g
•For parabolic profile core ( =2),
M
g=V
2
/4 , which is half the
number supported by a SI fiber
with sane V value
Graded Index Fiber Parameters

•SMFs: Most important for long-haul use (carrier and Internet core).
•Small core (8 to 10 microns) that forces the light to follow a linear
single path down its length.
•Lasers are the usual light source.
•Most expensive and delicate to handle,
•Highest bandwidths (GHz) and distance ratings (more than 100 km).
Single mode (mono-mode) Fibers

•Relatively large diameter core (50 to 100 microns)
•Step-index multimode cable has an abrupt change between core and
cladding. It is limited to about 50 Mbits/sec
•Graded-index multimode cables has a gradual change between core
and cladding. It is limited to 1 Gbit/sec.
SI
GI
Multimode Fibers

For, SM operation only above a theoretical cutoff wavelength, 
c: 
2
1
2
V
na2
c
1
c




c is the wavelength above which a particular fiber becomes single-moded
•At V=2.405: 80% of mode‟s power in core
•At V=1: only 30% power in core;
•Do not want V too small, design compromise: 2<V
SM
SI<2.405
For SI fiber, Vc=2.405, the cutoff wavelength
Cutoff Wavelength
Power distribution:

Ideally suited for high bandwidth, very long-haul applications
using single-mode ILD sources; Telecommunication, MANs
•Step Index Fibers: Best suited for short-haul, limited
bandwidth and relatively low cost applications.
•Graded Index Fibers: Best suited for medium-haul,
medium to high bandwidth applications using incoherent
and coherent sources (LEDs and ILDs); LANs
Application Areas
 Single mode fibers: Mostly Step index type
 Multimode fibers : Step index, Graded index

DESIGNER’S PARAMETERS
0.12-0.15 for SMF, 0.15-0.25 for MMF
Numerical Aperture (NA) : NA = sina = [(n1)2-(n2)2]1/2
Relative Refractive Index Difference ():
 = (n1 –n2)/n ; n- the average refractive index
< 0.4% for SMF, >1% for MMF
Normalized Frequency or V-Number:
V = [(2a)/] NA
V  2.405 for SMF;  10 for MMF

Requirements to be satisfied in selecting materials:
Suitable Materials are either
FIBER MATERIALS
•Glasses or glass like materials ( Silica or Silicates)
•Monocrystalline Structures (Plastics)
•Heavy Metal Fluorides ( Nonsilicates)
•Material must be transparent at a particular wavelength in order for
the fiber to guide light efficiently
•Physically compatible materials having slightly different refractive
indices for the core and cladding must be available.
•It must be possible to make long, thin, flexible fibers from the
materials.

•Refractive Index Profile
•Material Composition and Density fluctuations
•Core-Cladding Interfaces
•Ecentricity
•Diameter
MANUFACTURER’S CONSIDERATIONS

TWO STAGE PROCESS
•Drawing and Pulling Techniques; To acquire end products
METHODS: Two major categories
a) Conventional Glass Refining Techniques (Melting Processes)
- Liquid Phase Techniques (MC Glasses only)
b)Vapour Phase Deposition Methods (VPD)- Silica rich Glasses
- Flame Hydrolysis (VAD, OVPO)
- Chemical Vapour Deposition (MCVD, PCVD)
PREPARATION OF GLASS FIBERS
•Purification of powdered glass
materials and Conversion into rod or Preform

Purification of Fiber Materials
•Preparation of Ultra pure material powders; Usually Oxides or
Carbonates
–SiO
2, GeO
2, B
2O
3 ,Al
2O
3 and F - Silica glass fibers

–Na
2CO
3, K
2CO
3, CaCO
3 and BaCO
3 – Decomposes to Oxides
- MC Glass fibers

•Very High initial purity essential ( ~ PPB) ; lesser for transition
metal ( Cr, Cu, Fe, Ni, Mn, V) impurities.

•Involves combined techniques of fine filtration and
coprecipitation, followed by solvent extraction before
recrystallization and final drying in a vacuum to remove any
residual OH ions.

•Purification accounts for large proportion of material cost;
Commercially available.

Melting Processes
Glassmaking furnace for the production of high
purity glasses.
Silica or Platinum
Crucibles
•Contamination
•Inhomoginities
•Melting high purity
powder to homogeneous,
bubble free glass melt
•Change in R.I. in molten
state by change in
composition of various
constitutents
•Temp: 900 – 1400
O
C

RF Induction Furnace
High-purity melting using a RF induction furnace.
•Radio Frequency of ~
5MHz
•Heated to 1000 OC •Form a thin layer of
solidified pure glass
between melt and
crucible – avoid
contamination

Optical Fiber from a Preform.
Fiber Drawing
•Rod in tube process
•Useful only for Step
Index fibers with
large core and
cladding diameters.
•Bubbles &
particulates at
interfaces
•5-10 dB/km loss

Double Crucible Method for Fiber Drawing
•Core and cladding in form of
separate rods fed into two concentric
platinum crucibles
•Temp. between 800 to 1200
O
C
•Technique for SI & GI Fibers
•Diffusion of mobile ion across core-
cladding interface witihin molten
glass
•Reasonable graded index profile
•Typical losses between 3 –5 dB/km
•Possibility of continuous production
•Lack of precise control-
Double Crucible Approach

The variation in the refractive index of silica using
various dopants.

Doping Materials

Fiber Structure
•An optical fiber is a long cylindrical dielectric waveguide,
usually of circular cross-section, transparent to light over the
operating wavelength.

•A single solid dielectric of two concentric layers. The inner
layer known as Core is of radius ‘a’ and refractive index ‘n
1’.
The outer layer called Cladding has refractive index ‘n
2’.

n2 < n1  condition necessary for TIR

Must meet the conditions for Total Internal
Reflection (TIR)
Light Propagation through Optical Fiber

Step Index / Graded Index

Schematic of Vapour-Phase Deposition Techniques
•For Silica rich
glasses
High transperency
Optimuml optical
properities.

Optical Fiber from a Preform.
•Rod in tube process
•Useful only for Step
Index fibers with
large core and
cladding diameters.
•Bubbles &
particulates at
interfaces
•5-10 dB/km loss
Step-II-Fiber Drawing

Outside Vapour-Phase Oxidation (OVPO) Process
- Uses Flame hydrolysis for ‘Soot’ Formation
• High OH impurity content

•Cracks during mandral
removal
OVPO Process: (a) Soot deposition, (b) Preform Sintering
(c) Fiber Drawing

The VAD Process
•End-on deposition
•Typical OH content between
50-200 PPM
•Reduced by applying chlorine
as drying agent
•Typical Temp. 1500
O
C
•Losses as low as 2 dB/km
VAD Process
Difficulties:
•Cracks while removing mandral
•Depression in RL profile near
centre due to collapsed hole.

MCVD Techniques
•Modified Chemical Vapor Deposition (MCVD)
An inside vapour phase oxidation (IVPO) method
vaporized raw materials are deposited into a pre-made silica tube
•Deposition within an
enclosed reactor-very
clean environment
•Fiber formation 1400-
1600
O
C, drawing at
2000 - 2200
O
C
•Reduced OH impurity
•Minimum losses of only
0.2 dB/km at 1550 nm

PCVD Technique
•Plasma Activated Chemical Vapour Deposition
A variation on the MCVD technique to use plasma to supply
energy for the vapour-phase oxidation of halides.
Film deposition at around 1000
O
C
 Provide controlled and high uniformity of layers

Different types of Commonly used OFs:
•Multimode step index fibers
•Multimode Graded index fibers
•Single mode fibers
•Plastic clad fibers
•All Plastic fibers

Typical structure for a glass multimode step index fiber.
Multimode Step Index Fibers
Structure
Core Diameter : 50 to 400 m
Cladding Diameter : 125 to 500 m
Buffer Jacket : 250 to 1000 m
Numerical Aperture : 0.16 to 0.5
•Fabricated from either from multicomponent glasses or doped silica.
•Have reasonably large core diameters and large NAs to facilitate efficient
coupling to incoherent sources such as LEDs.

Attenuation Spectra for MMSIF:
(a) Multicomponent glass fibers
(b) doped silica fibers.
•Performance characteristics depends on the materials used and the methods of
preparations
•Doped silica fibers exhibit the best performance
Bandwidth : 6 to 50 MHz km.

Applications: Best suited for
• short-haul,
• Limited bandwidth and
• Relatively low cost applications.
Attenuation : 2.6 to 50 dB/km at 850 nm,
• Limited by absorption or scattering.
•Wide variation in attenuation is due
to differences between two
preparation methods

Multimode Graded Index Fibers
Typical structure for a glass multimode graded index fiber.
Structure
Core diameter : 30 to 100 m
Cladding diameter : 100 to 150 m
Buffer jacket diameter : 250 to 1000 m
Numerical aperture : 0.2 to 0.3.
•Fabricated using MC glasses or Doped Silica
•Manufactured from materials with higher purity
•Better performance due to index grading and lower attenuation

Performance Characteristics
Attenuation:
•2 to 10 dB km
-1
at a wavelength of 850nm with generally a scattering
limit.
•Average losses of around 0.4 and 0.25 dB km
-1
can be obtained at
wavelengths of 1.3 and 1.55 m respectively.
Bandwidth: 300 MHz km to 3 GHz km.
Applications:
•Best suited for medium-haul, medium to high bandwidth applications
using incoherent and coherent multimode sources (i.e. LEDs and injection
lasers respectively).

Single-Mode Fibers
Typical structure for a silica single-mode step index fiber.
Structure
Core diameter : 5 to 10 m, typical around 8.5 m
Cladding diameter : generally 125 m
Buffer jacket diameter : 250 to 1000 m
Numerical aperture : 0.08 to 0.15, usually around 0.10.

Single-Mode Fibers
•Have either Step index or Graded index Profile

•GI Profiles
 Provides dispersion modified SMF
 Produce polarization maintaining fibers (PMF)
 Expensive; Not utilized within OFC systems

•Commercially available SMFs are usually SI profile

•High quality fibers; Generally fabricated from doped silica
(SCS)

Attenuation:
- 2 to 5 dB km
-1
with a scattering limit at 850 nm.

- 0.35 and 0.21 dB km
-1
at 1310 and 1550 nm
Bandwidth:
- Greater than 500 MHz km. of 0.85 m.
- More than 10 GHz km at a wavelength of 1.3 m.
Applications:
Ideally suited for high bandwidth very long haul
applications using single-mode injection laser sources.

Performance Characteristics

Plastic Clad (PCS) Fibers
- MMF; either SI or GI profile
Typical structure for a plastic-clad silica multimode step index fiber.
Structure Step Index Graded Index
Core diameter : 100 to 500 m 50 to 100 m

Cladding diameter : 300 to 800 m 125 to 150 m

Buffer jacket diameter: 500 to 1000 m 250 to 1000 m

Numerical aperture : 0.2 to 0.5 0.2 to 0.3.

Performance Characteristics

Attenuation: Step index 5 to 50 dB km
-1

Graded index 4 to 15 dB km
-1

PCS fibers exhibit lower radiation –induced losses than SCS.

• Have improved performance in certain enviornments
Generally cheaper than the corresponding glass fibers

• Have more limited performance characteristics

All-Plastic Fibers (PCP)
Typical structure for an all plastic fiber.

Structure
Core diameter : 200 to 600 m
Cladding diameter : 450 to 1000 m
Numerical aperture : 0.5 to 0.6.

All-Plastic Fibers
•Exclusively of MMF SI type with large core and cladding
diameters.

•Reduced requirement for buffer jacket, protection and
strengthening

•Cheap and Easy to handle

•Limited use in communication applications

•Large NA:- Easy coupling to light sources

•Fabricated with Polymethyl methacrylate (PMMA) and
Fluorinated polymer cladding

Attenuation : 50 to 1000 dB km
-1
at 650 nm
Bandwidth : Not usually specified as transmission is
generally limited to tens of meters.

Applications : only be used for very short haul (i.e. ‘in-
house’) low cost links.

 Fiber coupling and termination are relatively easy and do
not require sophisticated techniques.

Performance characteristics

Transmission Characteristics of
Optical Fiber l

OPTICAL FIBER
A single solid dielectric of two concentric layers. The inner
layer known as Core is of radius ‘a’ and refractive index ‘n
1’.
The outer layer called Cladding has refractive index ‘n
2’.

n
2 < n
1  condition necessary for TIR
An optical fiber is a long cylindrical dielectric waveguide,
usually of circular cross-section, transparent to light over the
operating wavelength.
Fiber Structure

Step Index / Graded Index

Numerical Aperture (NA) :
NA = sin
a = [(n
1)
2
-(n
2)
2
]
1/2
Relative Refractive Index Difference ():
 = (n
1 –n
2)/n ; n- the average refractive index
<0.4% for SMF, >1% for MMF
Normalized Frequency or V-Number:
V = [(2a)/] NA
V  2.405 for SMF;  10 for MMF
DESIGNER’S PARAMETERS
0.10-0.25 for SMF, 0.20-0.50 for MMF

Transmission Characteristics

Dispersion: limit the information – carrying capacity of a
fiber i.e. Bandwidth
Attenuation (or Transmission loss): determines the
maximum repeater less separation between a transmitter and
receiver.
Characteristics of Primary Importance

Fibre Performance
z=0 z=L
Dispersion
z=0 z=L
Attenuation

Optical Attenuation
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
0 5 10 15 20
z (km)
Po
(m
W
)
alpha prime = 0.1
alpha prime = 0.3
alpha prime = 0.5 where,
P(z) : Optical power at distance ‘z’ from input
P
o : Input optical power
 : Fiber attenuation coefficient, [dB/km]
z
P(z)Pine
> Measure of the decay of signal
strength or light power
> Logarithmic relationship between the optical output power
and the optical input power Optical Fiber Attenuation










in
out
P
P
log10
z
1 Attenuation is because of different mechanisms

> Usually, attenuation is expressed in terms
of decibels or mostly dB/km
Optical Fiber Attenuation
Total = absorption + scattering + bending

Basic Attenuation Mechanisms
2. Scattering ( Linear and Non-linear)
3. Bending loss ( Macrobends and Microbends)
1. Material Absorption (Intrinsic and Extrinsic)

Material Absorption

a. Intrinsic : caused by the interaction with one of the major
components of the glass

•Absorption in the IR-wavelength region (Molecular absorption)
•Absorption in UV wavelength region (Electronic absorption)
Results in the loss of some of the transmitted optical power
in the waveguide
A loss mechanism related to the bulk materials and the
fabrication process for the fiber
Absorption of light (optical energy)

b. Extrinsic : caused by impurities within the glass
•Mainly absorption by transition metal impurities (Cr, Cu, Fe,
Mn, Ni, V etc.)

•Another major extrinsic loss mechanism is caused by
absorption due to water ( Hydroxyl- OH ion) dissolved in the
glass

> Reduced to acceptable levels ( i.e. one part in 1015) by
traditional glass refining techniques.
> Hydroxyl groups are bonded to glass structure and have fundamental
stretching vibrations depending on group position.

Material Absorption & Scattering Losses

Attenuation in Silica Fibers
900 1100 1300 1500 1700
0.5
1.0
1.5
2.0
2.5
At
te
nuation
(dB/km)

Wavelength (nm)
“Optical
Windows”
2 3
1
850 nm
1310 nm 1550 nm

Transmission Characteristics of
Optical Fiber lI

Scattering Loss
> Even very small changes in the value of the core’s refractive
index will be seen by a traveling beam as an optical obstacle
and this obstacle will change the direction of original beam.
> Due to Obstacles or inhomogeneities
Scattering effect prevents attainment of total internal
reflection at the core cladding boundary – resulting in power
loss
SCATTERING

Scattering Loss
Dominant intrinsic loss mechanism

Rayleigh Scattering Loss
Rayleigh scattering coefficient (
R) is proportional to (1/
4
) and
is related to transmission loss factor of the fiber as )exp(L
R
Rayleigh scattering component can be reduced by operating
at the longest possible wavelength.
•Theoretical attenuation due to Rayleigh scattering in silica at
different wavelengths:
630nm 5.2 dB km
-1
1000 nm 0.8 dB km
-1
1300 nm 0.3 dB km
-1

Scattering Loss (cont…)
•Controlled extrusion &cabling of the fiber
•Increasing fiber guidance by increasing „‟
occurs at inhomogeneities comparable in
size to guided wavelength
Mainly in the forward
direction

: Usually at high optical power levels
SBS
(of acoustic frequency)
Scattering Loss : Nonlinear

SRS
 Optical phonon
Normally, SBS threshold occurs at 100 mW, and SRS threshold at 1W

Material Absorption & Scattering Losses

Attenuation in Silica Fibers
900 1100 1300 1500 1700
0.5
1.0
1.5
2.0
2.5
At
te
nuation
(dB/km)

Wavelength (nm)
“Optical
Windows”
2 3
1
850 nm
1310 nm 1550 nm

BENDING LOSSES

•Macrobends

•Microbends
Bending an optical fiber introduces a loss in
light power

Micro- and Macro- bending
Microbending - Result of microscopic imperfections in the
geometry of the fiber

Macrobending - Fiber bending with diameters on the order of
centimeters (usually if the radius of the bend is larger than
10 cm)

Power loss in a curved fiber  
2
1
2
2
2
1
2
1
c
nn4
n3
R



Critical radius of curvature :
> the guidance mechanism is
inhibited
Velocity of evanescent field at the bend
exceeds the velocity of light in the cladding
> Designing fibers with large relative refractive index differences
Operating at the shortest wavelength possible.

Microbending losses
Results from non-uniform lateral pressures of fiber surface
(core-cladding interface)
Minimized by extruding a compressible jacket over the fiber.

Losses in fiber are due to
LOSS SUMMARY
* Material Absorption
* Scattering (Linear and Nonlinear)
* Bending (Macrobends & Microbends)
* Interface inhomogenities
> Minimum loss is at 1550 nm
Theoretical minimum loss ( 0.15 dB/km) almost achieved
in practice with Silica based optical fiber.

Transmission Characteristics of
Optical Fiber - II

Pulse Broadening
In the ray model there are a continuum of ray directions between the
axial ray and the critical angle a
c

The axial ray takes the shortest route and arrives at the far end first,
whereas the ray at the critical angle takes the longest route and arrives
last.

A short input pulse will be broadened by the range of paths travelled
t
1 t
2

Dispersion
•Dispersion effects broaden the pulse as it
propagates along the fibre
•The broadening is measured in nsec/km
•After large distance the pulses overlap (intersymbol
interference-ISI) and become indistinguishable
–electrical dispersion
•The broadening, t, limits the maximum data rate: t2
1

T
B

 Dispersion - Spreading of light pulses in a fiber
limits Bandwidth
Most important types
2. Intermodal (Modal) dispersion
1.Intramodal (Chromatic) dispersion
i) Material dispersion
ii) Waveguide dispersion
DISPERSION

Intermodal Dispersion

L
n
1
n
2

c
t c
n
n
n
L
T


2
1 c
n
LT
1
min
 c
c
n
LT
cos
1
max

 1
2
c
n
n
cos c
n
c
n
n
n
L
T





2
1 nc
NA
c
n
c
n
n
n
L
T
2
2
2
1





 T
L
LB

 2
2
NA
nc
T
L
LB 

  
2
1
2
/
2
nn n NA
DT
ns km
L
n c c nc


     kmsMb
NA
nc
T
L
LB )/(
2
2



Intermodal Dispersion

can be minimized by:
mode at wavelengths greater than the cutoff wavelength
Intermodal Dispersion
 Intermodal Dispersion (also Modal Dispersion)
•using a smaller core diameter
•using graded-index fiber (less by a factor of 100)
•use single-mode fiber - single-mode fiber is only single-
When multimode dispersion is present, it usually
dominates to the point that other types of dispersion
can be ignored.

Graded Index Fibers: Solution to modal dispersion
A multimode graded index fiber: (a) Parabolic refractive index
profile; (b) Meridional ray paths within the fiber core.
Core is designed with different refractive index layers so that the
beam traveling the farthest distance does so at the highest
velocity and the beam traveling the shortest distance propagates
at the slowest velocity.
Modal Dispersion in Graded Index Fibers

Intramodal dispersion occurs due to the differing propagation delays of
different wavelengths of light within a single mode (intra-modal)

–Caused by material dispersion and waveguide dispersion

Light sources have a finite spectral width ()
•a fraction of a per cent of the centre frequency for a laser
•several per cent for a LED

•Each spectral component of a pulse travels at a different rate leading to
pulse broadening
Broadband
input pulse
Spectrally dispersed
output pulse
Intramodal Dispersion

•Light sources are NOT monochromatic
(linewidth of source, chirp effects, modulation sidebands)

•Different wavelengths travel at slightly different speeds
(this effect is called “Chromatic Dispersion”)

•Chromatic dispersion causes pulse broadening
(problem at high bit rates over long distances)

•Standard single-mode fiber:
–1300 nm window has lowest CD
–1550 nm lowest loss
Intramodal (Chromatic) Dispersion

Fiber Dispersion: A. Material Dispersion

Material Dispersion (cont.)

The material dispersion parameter for silica as a function
of wavelength 
t
d
d1
m
L
M
is expressed in ps.nm
-1
.km
-1
•Material dispersion
may be minimized
by control of system
parameters.
Material dispersion Parameter (M)

•Light travels at different speeds in core and cladding.

•Variation of propagation constant () with wavelength (),
0
d
d
2
2



Waveguide Dispersion
•Results from the variation in group velocity with wavelength
which leads to a variation in transmission time for the modes.

Fiber Dispersion:
B. Waveguide Dispersion

•Minimum dispersion occurs at
=1.3 mm
–dispersion negligible
–attenuation ~0.3 dB km
-1

•Minimum attenuation occurs at
=1.5 mm
–dispersion 15 ps nm
-1
km
-1

–attenuation 0.2 dB km
-1

•Dispersion flattening
enables 2 ps nm
-1
km
-1

over 1.3-1.6 mm range
–enables low-loss AND low
dispersion at 1.5 mm
ps

nm
-
1

km
-
1

•Dispersion is sum of
material and waveguide
components

D
T = D
M+D
W+D
P ( ps nm
-1
km
-1
)
•In MMFs, the overall dispersion comprises both
Intermodal
Intramodal (Material & Waveguide)

•In SMFs, dispersion is entirely from Intramodal or Chromatic
dispersion
BW is limited by finite spectral width of the source ()
Dominated by material dispersion of fused silica
Zero Material Dispersion by control of dopants
Overall Fiber Dispersion
Total Dispersion
Note: In MMFs, waveguide dispersion is negligible compared to material
dispersion

Schematic diagram showing a multimode step index fiber, multimode graded index
fiber and single-mode step index fiber, and illustrating the pulse broadening due to
intermodal dispersion in each fiber type.

2
2
21
2
1
2
WMT
dV
)Vb(Vd
c
nn
d
nd
c
DDD











 •At wavelengths longer than the ZMD point in most common fiber
designs, the D
M and D
W are of opposite sign and can therefore be
made to cancel at some longer wavelengths.
•Hence, 
ZMD can be shifted to the lowest loss wavelength for
silicate glass fibers at 1550 nm to provide both low dispersion and
low loss fiber.
 Dispersion Modified SM Fibers
Dispersion Modified SMFs
Total Dispersion :
Dispersion Shifted
Dispersion flattened

Total dispersion characteristics for various types of SMFs
•Achieved by mechanisms such as; Reduction in fiber
core diameters, Increase in relative or fractional index difference
and Variation in fiber material composition
Dispersion Shifted& Dispersion Flattened SMFs

TRANSMISSION RATE

SMF : (Standard, 1310 nm Optimized, unshifted)
–Most widely deployed by far distances

SMF DS (Dispersion shifted) :
–For single channel operation at 1550 nm

SMF DF (Dispersion flattened):
–For WDM/DWDM operation in the 1550 nm
region
SMFs For Telecom

Transmission Characteristics
of Optical Fiber II

Intermodal dispersion

Dispersion caused by multipath propagation of light energy is referred to as
intermodal dispersion.

Signal degradation occurs due to different values of group delay for each
individual mode at a single frequency.

In digital transmission, we use light pulse to transmit bit 1 and no pulse for bit
0. When the light pulse enters fiber it is breakdown into small pulses carried
by individual modes. At the output individual pulses are recombined and since
they are overlapped receiver sees a long pulse causing pulse broadening.

Dispersion affect the transmission
bandwidth:

For no overlapping of light pulse down on an optical fiber link,
the Digital bit rate (BT)
BT = 1/2г where 2г is the pulse duration

Maximum bit rate BTmax=0.2/σ bits

where δ represents the rms impulse response for the channel

Multimode step index fiber

Using the ray theory model, the fastest and slowest modes propagating in the step index fiber may be represented by
the axial ray and the extreme meridional ray (which is incident at the core–cladding interface at the critical angle φc)
respectively.. The delay difference between these two rays when traveling in the fiber core allows estimation of the
pulse broadening resulting from intermodal dispersion within the fiber. As both rays are traveling at the same velocity
within the constant refractive index fiber core, then the delay difference is directly related to their respective path
lengths within the fiber. Hence the time taken for the axial ray to travel along a fiber of length L gives the minimum
delay time TMin :

where n1 is the refractive index of the core and c is the velocity of light in a vacuum.

The extreme meridional ray exhibits the maximum delay time TMax

Using Snell’s law of refraction at the core–cladding interface following Eq. (2.2):

where n2 is the refractive index of the cladding. Furthermore, substituting into Eq. (2.21) for cosq gives:

The delay difference dTs between the extreme meridional ray and the axial ray may be obtained by:

where D is the relative refractive index difference. However, when D<< 1, then from the definition given by Eq.
(2.9), the relative refractive index difference may also be given approximately by:

where NA is the numerical aperture for the fiber. The approximate expressions for the delay difference given in Eqs
(2.27) and (2.28) are usually employed to estimate the maximum pulse broadening in time due to intermodal dispersion
in multimode step index fibers. Again considering the perfect step index fiber, another useful quantity with regard to
intermodal dispersion on an optical fiber link is the rms pulse broadening resulting from this dispersion mechanism along
the fiber. When the optical input to the fiber is a pulse pi(t) of unit area, as illustrated in Figure 2.10, then

It may be noted that pi(t) has a constant amplitude of 1/δTs over the range:

The rms pulse broadening at the fiber output due to intermodal dispersion for the multimode step index fiber ss (i.e. the
standard deviation) may be given in terms of the variance ss2:

where M1 is the first temporal moment which is equivalent to the mean value of the pulse and M2, the second temporal
moment, is equivalent to the mean square value of the pulse.
Hence:

The mean value M1 for the unit input pulse of Figure 2.10 is zero, and assuming this is maintained for the output
pulse, then from Eqs (2.30) and (2.32):
Integrating over the limits of the input pulse (Figure 3.12) and substituting for pi(t) in Eq. (2.33) over this range gives:
The pulse broadening is directly proportional to the relative refractive index difference D and
the length of the fiber L. The latter emphasizes the bandwidth–length trade-off that exists,
especially with multimode step index fiber, and which inhibits their use for wideband long-haul
(between repeaters) systems. Furthermore, the pulse broadening is reduced by reduction of the
relative refractive index difference D for the fiber.

Optical Sources

Relation between Einstein’s A and B Coefficient

(Boltzman,s Distribution) divide by B21.N2 1

•B
12
= B
21,
The probability of stimulated emission is same as that of induced
absorption. This means that if these two processes will occur at equal rates, so that
no population inversion can be attained in a two-level system.
•The ratio of spontaneous emission and stimulated emission is proportional to v
3
.

This
implies that the probability of spontaneous emission dominates over induced emission
more and more as the energy difference between the two states increases. 1

Optical Fiber
Tx optical Source
Rx optical detector

•LED (Light Emitting Diodes)

•LASER
(Light Amplification by Stimulated Emission of Radiation)
Types of Optical Fiber
compatible Sources

LEDs
Emits incoherent light through
spontaneous emission.
Used for Multimode systems with
100-200 Mb/s rates.
Broad spectral width and wide
output pattern.
850nm region: GaAs and AlGaAs
1300–1550nm region: InGaAsP
and InP
Two commonly used types:
ELEDs and SLEDs

Optical Source Requirements:

Optical Sources suited to Fiber Optic Communication:
 Light Emitting Diode (SLED, ELED, SLD)
Semiconductor Laser (DFB, DBR, Vertical-Cavity Surface-
Emitting Laser(VCSEL) , Multi Quantum Well , In-Fiber Lasers,
Fiber Ring Lasers)
•Narrow radiation pattern (beam width)
•Ability to be directly modulated by varying driving current
•Fast response time
•Adequate output power to couple into the optical fiber
•Narrow spectral width
•Driving circuit issues
•Stability, Efficiency, Reliability and cost
• Dimensions to suit the optical fiber geometry

•A PN junction acts as the active or recombination region.
•When PN junction is forward biased, electrons and holes
recombine either radiatively (emitting photons) or non-
radiatively (emitting heat). This is simple LED operation.

•Emitted wavelength depends on bandgap energy
•Transitions take place from any energy state in either band to
any state in other band. Results in a range of wavelengths
produced (spontaneous emission). Typically the wavelength
range is more than 80 nm.
LED Basic operation

Light Emitting Semiconductors
Light is emitted at the site of carrier recombination which is
primarily close to the junction.
The amount of radiative recombination and the emission
area within the structure is dependent upon the
semiconductor materials used and the fabrication of the
device.

AlGaInP
GaAs
AlGaAs
InGaAs
0.73 - 1.35
Direct Band gap-
III V compounds like GaAlAs , GaAsP, InGaAsP
TYPES of LEDs
• Edge Emitting LED’s
• Surface Emitting LED’s

Coupling lens used to increase efficiency.
Short optical Links with Large NA fibers.
Data rates less than 20 Mbps.
LEDs for FO Communication

SURFACE EMITTING LED’S
Coupling lens used to increase efficiency.
Short optical Links with Large NA fibers.
Data rates less than 20 Mbps.

Edge Emitting LED’s
Edge-emitting Diode: An LED that emits light from
its edge, producing more directional output
than surface-emitting LED'sthat emit from their
top surface.
Effective Area: Light is not distributed in the fiber
core uniformly. Rather, it follows a distribution that typically
peaks in the center of the core and then tails off near
the core-cladding interface. It usually extends some
short distance into the cladding as well. * active region has more refractive index then other two gyiding layer
* whereas the guiding layer has more refractive index then other than active region

GaAs
P
GaAs
N Carriers are not confined

Light is not confined

LED should have a high radiance (light intensity), fast
response time and a high quantum efficiency for Fiber
Optic Communication system

Structure and index of refraction
in gallium arsenide with a junction
width d
Homojunctions: P- type and N-type from same material

Heterojunction
Heterojunction is the advanced junction design to reduce
diffraction loss in the optical cavity.
This is accomplished by modification in the material to
control the index of refraction of the cavity and the width of
the junction.
The index of refraction of the material depends upon the
impurity used and the doping level.
The Heterojunction region is actually lightly doped with p-type
material and has the highest index of refraction.
The n-type material and the more heavily doped p-type
material both have lower indices of refraction.
This produces a light pipe effect that helps to confine the light
to the active junction region. In the homojunction, this index
difference is low and much light is lost.
Double or single hetero-structure junction with better light
output

Heterojunctions: Different p- and n- materials

•Carriers are confined

•Light is also confined

•Single Heterojunction, Double Heterojunction.

A heterojunction is a junction between two different
semiconductors with different bandgap energies.

The difference in bandgap energies creates a one-way barrier.
Charge carriers (electrons or holes) are attracted over the
barrier from the material of higher bandgap energy to the one
of lower bandgap energy.

Gallium Arsenide-Aluminum Gallium
Arsenide single heterojunction

When a layer of material with a lower bandgap energy is
sandwiched between layers of material with a higher
energy bandgap a double heterojunction is formed. This
is called a double heterojunction because there are two
heterojunctions present - one on each side of the active
material.

The double heterojunction forms a barrier which restricts
the region of electron-hole recombination to the lower
bandgap material. This region is then called the “active”
region
Double Heterojunction

Double Heterojunction
The valence band
of n-InGaAsP is at
a higher energy
than the valence
band of the
adjacent n-InP. The
conduction band is
at a lower energy
level.

p-InP has higher
energy levels than
n-InP but the
bandgap is the
same

Recombination takes place in the n-InGaAsP and
spontaneous emission (or lasing) occurs.

The heterojunction allows to have a small active region
where the light is produced.

The material in the active region has a higher refractive
index than that of the material surrounding it. This means
that a mirror surface effect is created at the junction
which helps to confine and direct the light emitted.
Electrons are attracted across
the left-hand junction from the
n-InP to the n-InGaAsP.

Holes are attracted across the
right-hand junction from the
p-InP into the n-InGaAsP.

Double Heterojunction
Confining and Guiding the
Light within the Device

Within the device the light
must be confined and
directed to the exit aperture
so that it can be directed
into the fibre which is done
using insulating materials
SiO
2 to confine the active
region and the current path.

The active region in a
heterostructure has a higher
refractive index.

This junction forms a mirror
layer and helps to confine
the light to the active layer.
For this reason, the outer
layers are often called
“confinement layers”

LED : Specifications of
importance

•Optical Output Power
•Output Spectrum
•Light coupling into Fiber
•Modulation Bandwidth

Packaging – Microlensed LED

Laser Diodes

Laser Block Components
Major Components:
Active medium
Pumping Source
Mirrors

LDs – Laser Diodes
•Emit coherent light through
stimulated emission
•Mainly used in Single Mode
Systems
•Light Emission range: 5 to
10 degrees
•Require Higher complex
driver circuitry than LEDs
•Laser action occurs from
three main processes:
photon absorption,
spontaneous emission, and
stimulated emission.

Lasing Characteristics
•Lasing threshold is
minimum current that must
occur for stimulated
emission
•Any current produced below
threshold will result in
spontaneous emission only
•At currents below threshold
LDs operate as ELEDs
•LDs need more current to
operate and more current
means more complex drive
circuitry with higher heat
dissipation
•Laser diodes are much
more temperature sensitive
than LEDs

Semiconductor Laser Diode

Edge emitting lasers
•Active layers
very thin
•Light emitting
area ~ 0.5
µm x 5 µm
•Diffraction
causes rapid
beam spread
Current flow
Reflective
facet
Laser action in
Narrow stripe
+ - - +

Source Comparison
LDs. LED SLED LD
Principle of Light
Generation
Spontaneous Emission
Amplified
Spontaneous Emission
Stimulated Emission
Optical Spectrum Broadband Broadband
Narrowband or
multiple Fabry-Perot
modes
Total optical output
power
Medium Medium High
Optical power density Low Medium High
Optical waveguide No Yes Yes
Light Emittance All directions Divergence-limited Divergence-limited
Spatial coherence Low High High
Coupling into single-
mode fibers
Poor Efficient Efficient
Temporal coherence Low Low High
Generation of speckle
noise
Low Low High

Optical Detectors

Optical Detectors

•Optical detector is an essential component of an
optical receiver which converts received optical
signal into an electrical signal.

•Photodetectors can influance the performance
of a fiber optic communication link.

Requirements of Optical detector

High sensitivity at the operating wavelength
High Fidelity
Large electrical response to the optical signal
Short response time to obtain a suitable bandwidth
A minimum noise introduced by the detector
Stability of performance characteristics
Small Size
Low bias voltage
High reliability
Low cost

Type of Optical Detectors

•p-n photodiodes
•p-i-n photodiodes(PIN Structure)
•Avalanche photodiodes(APD)

A reverse bias p-n junction consists of a
region, known as depletion region
Electron-hole pairs are created through
absorption when such p-n junction is
illuminated with light on one side Because
of the large built-in electric field, electrons
and holes generated inside the depletion
region accelerate opposite directions and
drift to n and p sides respectively. The
resulting flow of current is proportional to
the incident optical power A reverse bias
pn
junction acts as a photodetector and is
referred as pn
photodiode
PN Photodiode

PIN photodiode
•PIN photodiode has an intrinsic semiconductor region sandwiched between a
p-doped and an n-doped region .
•PIN photodiode is reverse-biased so that intrinsic (i) region has no free
charges, its resistance is high.
•Since the electric field is high in the i region, any electron-hole pairs
generated by optical radiation in this region are immediately swept away by
the field.
•The resulting flow of current is proportional to the incident optical power

Avalanche Photodiode
P
+
i p N+

Avalanche photodiodes(APD)
The diode which uses the avalanche method to provide extra performance as
compared to other diodes is known as avalanche photodiode.
Avalanche breakdown occurs mainly once the
photodiode is subjected to maximum reverse voltage.
This voltage enhances the electric field beyond the
depletion layer. When incident light penetrates the p+
region then it gets absorbed within the extremely
resistive p region then electron-hole pairs are
generated.
Charge carriers drift including their saturation velocity
to the p n+ region wherever a high electric field exists.
When the velocity is highest, then charge carriers will
collide through other atoms & produce new electron-
hole pairs. A huge charge carrier’s pair will result in
high photocurrent.

The reach-through avalanche photodiode (RAPD) is composed of a high-resistivity p-type
material deposited as an epitaxial layer on a pt (heavily doped p-type) substrate. A p-type
diffusion or ion implant is then made in the high-resistivity material, followed by the
construction of an nt (heavily doped n-type) layer.

A P D
•Drawback of P-I-N photodiode is that it need of an amplifier to
magnify the photocurrent produced by the photodiode.

•The quantum efficiency of the APD is M times larger than that of a P-
I-N photo diode.
R(APD)=M x R (PIN)
M depends upon
1 Accelerating voltage
2 Thickness of the gain region
3 Ratio of electrons to holes participating in the ionization
process.
•M ranges from 10 to 500.

Avalanche Photodiodes (APDs)

•High resistivity p-doped layer increases
electric field across absorbing region

•High-energy electron-hole pairs ionize other
sites to multiply the current -Leads to
greater sensitivity

Characteristics of Photodetector
Quantum Efficiency(η)

Photodetector’s sensitivity can be measured in two concepts: quantum efficiency
and responsivity.
It is defined as the ratio of the number of electron hole pairs generated to the total number of incident photons.
It can by calculated by
R is the reflection coefficient at the air-semiconductor surface ξ is the fraction of the e-h pairs contributes to the photo current
α is the absorption coefficient
ω is the distance where optical power is absorbed n=(Ip/q)/(Popt/hf)

(spectral responsivity) R=Ip/Popt R=n*lambda*q)/h*c

Responsivity

Responsivity is the ratio of electrical output from the detector to the input optical
power.
If the output current varies proportionally to the input, this is measured as amps per
watt (A/W). Since in fiber optic communication systems, input powers are usually in
microwatt level, responsivity is often expressed as uA/uW.
The responsivity ρ is the photo current generated per unit optical power. The following
formula shows how to calculate responsivity.
where
λ
0 is measured in micrometer
η is the quantum efficiency

The figure shows the spectral dependence of responsivity and quantum efficiency for
different semiconductor materials.

Speed of Response

The speed of response and bandwidth of a photodetector depend on three factors.

•The transit time of the photo-generated carriers through the depletion region
•The electrical frequency response as determined by the RC time constant, which
depends on the diode’s capacitance
•The slow diffusion of carriers generated outside the depletion region
Rise Time
Rise time is the time the output signal takes to rise from 10% to 90% of the peak value
after the input is turned on instantaneously.
Fall Time
Fall time is the the time the output signal takes to drop from 90% to 10% of the peak
value after the input is turned off abruptly.

Dark Current

Dark current is the current through the photodiode in the absence of light, when it is
operated in photoconductive mode. The dark current includes photocurrent generated
by background radiation and the saturation current of the semiconductor junction.

EDFA (Erbium Doped Fiber Amplifier)
An optical amplifier amplifies light as it is without converting the optical signal to an
electrical signal, and is an extremely important device that supports the long-distance
optical communication networks .
The major types of optical amplifiers include an EDFA and SOA.
Optical Amplifier
SOA (Semiconductor Optical Amplifier)

OPTICAL FIBER JOINTS &
CONNECTIONS

Basic Fiber Optic Link
Transmitter Connector Cable
ReceiverCableSplice

OPTICAL FIBER JOINTS
Technicalrequirementforbothjointing&termination
oftransmissionmedia
Repeaters Spacing(A continuously increasing parameter)
Ranges from40-60 km at 400 Mbits/s
100 km at 2.4 Gb/s
300 km at 1.7-10 Gb/s using SMDSFs
Number of Joints or Connections
Link length between repeaters
Continuous length of fiber
Length of fiber cable practically or conveniently
installed as continuous length

ManufacturerssupplyElectro-opticaldevices
(SourcesandDetectors)withfiberopticpigtailto
facilitatedirectfiber-fiberconnection
IMPORTANTASPECTISFIBER-TO-FIBERCONNECTION
WITHLOWLOSSANDMINIMUMDISTORTION
•Source-Fiber
•Fiber-Fiber
•Fiber-Detector
FIBER JOINTS

Two major categories of fiber joints
FIBER COUPLERS: Branching devices
Splitters or Combiners
Importance in Networks
FIBER SPLICES: Permanent orSemi-permanent joints
Soldering
FIBER CONNECTORS :Demountable or Removable joints
Plugs or Sockets

Crucial aspect of fiber joints concerning Optical
Lossesassociated with the connection
1.Fresnel Reflection
OpticalLossencounteredattheinterfaces(Evenwhentwofiberends
aresmooth,perpendiculartofiberaxesandperfectlyaligned)
Asmallproportionoflightmaybereflectedbackintotransmitting
fibercausingattenuationatthejoint.
FresnelReflection
•Fiber Alignment
LOSSMECHANISMS ATJOINTS

Occursduetostepchangesinrefractiveindexat
jointedinterface
Glass–Air-Glass
Reflection Loss

Fraction of light reflected at a single interface
n
1
:R.I.ofcore,n:R.I.ofinterfacingmedium(=1forair)2
1
1
nn
nn
r











Loss in decibel due to FR at single interface
Loss
Fres
=-10log
10
(1-r)
Can be reduced to a very low level using index matching fluid
in the gap between jointed fibers.

2. Deviation in Geometrical & Optical Parameters
All light from one fiber is not transmitted to another fiber ;
Because of mismatch of mechanical dimension
a) Core mismatch
b) NA mismatch
c) Index Profile
Three major cases :

Losses due to:
•Fresnel Reflection
•Deviation in Geometrical & Optical parameters
Intrinsic Losses
Minimized using fibers manufactured with
lowest tolerance i.e.(same fiber)

Extrinsic Losses
Losses due to some imperfection in splicing
Caused by Misalignment
Threepossibletypesofmisalignmentatjoint
(a)Longitudinal misalignment
(b)Lateral misalignment;
(c) Angular misalignment

(a)Lossduetolateralandlongitudinalmisalignmentfora50mcore
diameterGIfiber;(b)insertionlossduetoangularmisalignmentforjoints
intwoMMSIfiberswithNAof0.22and0.3.

FIBER SPLICES
A permanent joint formed between two fibers
TWOBROADCATEGORIES
Fibersareheldinalignmentbysomemechanicalmeans
Achievedbyvariousmethods;
oTubeSplices
oGrooveSplices
Accomplished by applying localized heating (a flame or
an electric arc) at the interface between two butted,
prealigned fiber ends causing them to soften and fuse.
•Mechanical Splicing
•Fusion Splicing or Welding

“ Scribe and Break” or“Score and Break”
Scoring of fiber surface under tension with cutting tool
(Sapphire, Diamond or Tungsten Carbide blade)
Optical fiber end preparation: the principle of
scribe and break cutting.
>MUST HAVE SMOOTH AND SQUARE END
FACES
> End preparation achieved using suitable tools -“ Cleavers”

Fiber Cleavers
Handheld Cleaver
One Action Cleaver
Two Action Cleaver:
Fiber cleaving &
Fusion splicing tool

Cable Preparation Equipment
Multipack;
•Enhanced quality to prevent cracks and
fiber strength degradation.
•Allow skill-free operation of factory fiber
prep and field splicing applications.
•Equipped with a high precision tensile
strip and automatic ultrasonic cleaning
action.

Fusion Splicing of Optical Fibers
Electric Arc Fusion splicing
•Require Fiber end surfaces
to be prepared for joint
•Heating of prepared fiber
ends to fusion point with
application of axial pressure
between two fibers.
•Positioning & alignment
using microscopes

Prefusion Method
Prefusion method for accurate splicing
Smaller Fresnel Reflection loss
Typical Losses : 0.1 to 0.2 dB for MMF
No need for end preparation

Fusion Splicers

Drawback: Fiber get weakened near splice (30%)
Fiber fracture occurs near the heat-affected zone adjacent to
the fused joint.
Splice be packaged to reduce tensile loading

Protection Sleeves for spliced fibers
Protection of Joints
Fiber joint enclosures
Underground fiber splice tray

Mechanical Splicing
Uses accurately produced rigid alignment tubes into which the
prepared fiber ends are permanently bonded.
Techniques for tube splicing of optical fibers:
(a) Snug Tube Splice
(b) Loose Tube Splice; Square Cross section Capillary

Comparison of Two Approaches
Snug Tube Splices
•Exhibits problems with
capillary tolerance
requirements
•Losses up to0.5 dB with
Snug tube splice (ceramic
capillaries) using MMGI
and SM fibers.
Loose Tube Splices
•Avoids the critical
tolerance requirements.
•Losses 0.1 dB with loose
tube splice using MMGI
fibers.

Ultra Splice: Reusable mechanical splice.
Average Loss 0.2 dB
Ultra Splice

Groove Splices
V-groovesplices
Insertionlosses0.1dBusingjigsforproducingV-groovesplice.
Use of grooves to secure the fibers to be jointed
better alignment to the prepared fiber ends.

Elastomeric Splice: (a) Cross section (b) Assembly
Fibers of different diameters tend to be centred and hence
successfully spliced.
General loss ~ 0.25 dB for commercial product
Comprises of two elastic parts (inner with V-groove) in
compression to ensure alignment of fibers.
Elastic Tube or Elastomeric Splice

Spring Groove Splice
Springroove Splice: (a) Expanded overview
(b) Cross-section Schematic
Mean Losses 0.05 dB with
MMGI Fibers.
Practically used in Italy.
Utilizes a bracketcontaining two cylindrical pins, which serve
as an alignment guidefor two prepared fibers.
An elastic element(a spring) used to press the fibersinto groove
and maintain alignment of fiber ends.

Alignment of secondary elements around the bare fibers
•Increased ruggedness
•Easy ground and polish of fiber end
•Better termination
Drawbacks:
Time consuming for termination
Increased losses due to tolerances on secondary elements
Fiber misalignment.
Secondary Alignment Techniques

Glass capillary tubes (Ferrules)
MMF mechanical splice using glass capillary tubes.
Fixing of glass ferrules
Alignment sleeve of metal or
plastic in which glass tube
fibers are aligned
Average loss0.2 dB

Rotary Splice
•Built-inoffsetandrotation,for
excellentalignment
•Alignmentaccuracyof0.05m
usingthreeglassrodalignment
sleeve.(necessaryforSMFs;8-10
mMFD)
•MeanLosses0.03dBusing
Indexmatchinggels(Notaffected
byskilllevelsofthesplicer).
Usedinlargeinstallationsin
USA
Rotary Splice for SMF:
(a) Alignment using glass ferrules
(b) Glass rod alignment sleeve
Use glass capillary tubes for fiber termination with small
eccentricity.

MULTIPLE SPLICES
Splice Losses:
• Ranging 0.04 to 0.12 dB-MM GI fibers
• 0.13 to 0.4 dB –SM fibers.
Commercially available for splicing number of fibers
simultaneously
• Simultaneous Splicing of Five fibers in 5 minutes;
• 15 minutes for five single fusion splicing.

A.Silicon Chip Array
MultiplefiberssplicingusingaSiliconchiparray
AverageSpliceloss0.12dB.
Utilize trapezoidal grooves of a silicon chip using a comb structure
for fiber laying and top silicon chip
End faces ground & polished after curing.

Mouldedfromglassfilledpolymerresin
Directmasssplicingof12fiberribbonswithsimultaneousend
preparationusingribbongrindingandpolishingprocedures.
Fiberspositionedingroovesinglassfilledplasticsubstrate.
Vacuumtechniquetoholdfibersatpositionwhilstcoverplateisapplied.
V-groove polymer resin ribbon fiber splice.
Spring clips to hold assembly
and hole in cover plate for index
matching gel.
Average Splice Losses
0.18 dB with MM fiber.
B. V-groove flat Chip

Fiber Splicing and Connectorization kits

More difficult to achieve than fiber splices
Must maintain similar tolerance requirements, but in a removable fashion.
Must allow for repeated connection and disconnection without problems for
fiber alignment -without degradation in performance.
Must protect the fiber ends from damage –due to handling
Must be insensitive to environmental factors ( e.g. moisture & dust)
Must cope with tensile load on the cable and can be fitted with relative ease.
Should ideally be a low cost component,
FIBER CONNECTORS
Demountable fiber connectors

Fiber Termination : protects and locates the fiber ends
Fiber end Alignment :provide optimum optical coupling
Outer shell : maintains the connection and fiber alignment,
protects the fiber ends from the environment and provides
adequate strength at the joint.
Three Major Parts
Losses in the range 0.2 to 0.3 dB

Alignment of two prepared fiber ends in close proximity
(butted) to each other so that the fiber axes coincide.
Butt Jointed Connectors

Utilize interposed optics at the joint in order to expand the
beam from the transmitting fiber end before reducing it
again to a size compatible with the receiving fiber end.
Collimating and refocusing the light from one fiber into
the other.
Expanded-Beam Connectors

FerruleConnectors:(a)structureofabasicferruleconnector;
(b)structureofawatchjewelconnectorferrule.
•Preparation of fiber
ends before fixing the
ferrules
•Insertion Losses1 to 2
dB with MMSIF
•Watch jewel for close
end approach and
tolerance requirement
Glass Ferrules with central drilled hole
Concentric alignment sleeve
Cylindrical Ferrule Connector

Outstanding
•Thermal,
•Mechanical
•Chemical Resistance
ST series multimode fiber connector using ceramic
capillary ferrules.
Ferrules made from ceramic material
End preparation after fixing ceramic ferrules
Average Losses
0.2dBwithMMGI
0.3dBwithSMF
Ceramic Capillary Ferrules

Commonly Used Connectors
FC Connectors
ST Connectors
SC Connector

DIN Connectors
(Spring loaded free-floating
Zirconia ceramic ferrule)
MTRJ Connector
SMA Connector
Biconic Connectors
D4 Connectors

ArosefromdevelopmentofGIfiberwaveguides
Acylindricalglassrod0.5to2mmindiameterwithparabolicrefractive
indexprofile.
Lightpropagationisdeterminedbythelensdimensionandwavelengthof
thelight.

GRIN-rod Lenses
Produce a collimated output beam with divergent angleof1
o
to5
o
from
lightsource.ontotheoppositefaceoflens
An alternative lens geometry to facilitate efficient beam
expansion and collimation

•Raypropagationdeterminedbyparaxialrayequation
Solutionis
r=k
1
cosA
1/2
r+k
2
sinA
1/2
r:Asinusoidalpath
Traversingofonesinusoidalperiod:onefullpitchdr
dn
ndz
rd 1
2
2

GRIN-rod Lenses

0.25,0.23,0.29etc.
SELFOCfromNippon
SheetGlassCo.Ltd.
0.2dBwithMMGI
0.3dBwithSMF
Various fractional pitch GRIN-rod lenses
Losses1dB
AverageLosses

Fiber Reels, Connectors & Patch cords
Connectors
Adapters
Patch cords

Optical Time Domain Reflectometer (OTDR)
Ajeet Kumar Srivastava
Assistant professor
Deartment of Electronics & Communication Engg.

OTDR

•It is a fiber optic instrument used to characterize, troubleshoot and
maintain optical telecommunication networks.
•OTDR testing is performed by transmitting and analyzing pulsed laser light
traveling through an optical fiber.
•The measurement is said to be unidirectional as the light is insert at
extremity of a fiber optic cable link. OTDR-optical time domain reflectometer Operation-->>>

>>> Received signal drwa on the display with x and y axis with attenuation(db) and distance(km) respectivly.

OR d=ct/2*n

-->>Amount of light reflected back to the optical source and expressed as the ratio of power of outgoing signl to power of
reflected signal.

Common Connector Type

Contamination and Signal performance

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