Subject: Optical Communication and Networking Subject code: EC 2402

Class: VII SEM ECE

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

PART-A
1) A multimode step index fiber with a core diameter of 80 µm and a relative index difference
of 1.5% is operating at a wavelength of 0.85 µm. If the core refractive index is 1.48
determine (AU April/May 2010)
(a) Normalized frequency of fiber.
(b) The number of guide modes.
V= (2πa/λ) n1 (2Δ)1/2 =75.7954
M= V2/2 = 2872
2) Define the numerical aperture of a step index fiber. (AU April/May 2005 & 2010)
It is the relationship between the acceptance angle and the RI of the three media involved
namely core, cladding and air. The NA is a dimensionless quantity, which is less than unity
with values ranging from 0.14 t0 0.50.
NA = (n1 2 – n2 2) ½
3) Mention the major advantages of optical fiber communication system over microwave
communication system. (AU Nov/Dec 2010)
 Small size and weight
 Electrical isolation
 Immunity to interference and cross talk
 Signal security and low transmission loss
 Ruggedness and flexibility
 System reliability and ease of maintenance
4) Mention the advantages and disadvantages of monomode fiber over multimode fiber.
(AU Nov/Dec 2010 AU Apr/May’2008 R-2004)
Single mode fiber sustains only one mode of propagation. Multimode fiber contains
hundreds of modes.
Advantages of monomode fiber:
1. Monomode fiber is free from inter modal dispersion
2. Higher bandwidth is possible in monomode fiber.
Disadvantages of Monomode fiber:
1. Only LASER optical source has to be used which is costly than LED source.
2. Smaller core radii prevents easier launch of optical power in to the fiber.
5) Define acceptance angle and critical angle for fiber. (AU Apr/May2009)
Acceptance Angle: Light acceptance is how much light can get into the fiber for
transmission.
Critical angle: At which angle the refracted ray lies in the core cladding interface, that
angle is called as critical angle. Sinθc = n2 / n1
6) What is tunnel effect? (AU Nov/Dec 2009)
The leaky modes are continuously radiating their power out of the core as they propagate along
the fiber. This power radiation out of the waveguide in quantum mechanical phenomenon is
referred to as ‘tunnel effect’.
7) What is skew ray? (AU Nov/Dec 2009)
Skew rays are not transmitted through the fiber axis. The skew rays follow a helical path in
the optical fiber. It is very difficult to track the skew rays as they do not lie in a single plane.
8) A Silica optical fiber with a core diameter large enough to be considered by ray theory
analysis has a core refractive index of 1.5 and a cladding refractive index of 1.47. Determine
the numerical aperture and acceptance angle in air of the fiber.
(AU Apr/May’2008,Nov/Dec 2007 R-2004)
NA=√ (n12-n22)
n1=1.5, n2=1.47
NA=√ (1.52-1.472) =0.3
Acceptance angle θ=sin-1(NA) = sin-1(0.3) =17.46o.

9) A typical relative refractive index difference for an optical fiber designed long distance
transmission is 1%.Estimate the numerical aperture for the fiber when the core index is
1.47. (AU Nov/Dec 2008)
1/2
N.A=n1 (2Δ) = 0.20788
10) What are the advantages and disadvantages of the ray optic theory?(AU Nov/Dec 2008)
Advantage:
It is easy to track as it travels along the fiber because it lies in a single plane.
The bounded rays are trapped in the core and propagate along the fiber axis according to the laws
of geometrical optics. (Meridional ray).
Disadvantage:
Skew rays are not confined to a single plane. These rays follows helical path along the fiber.
These rays are very difficult to track as they travel along the fiber, since they do not lie on the
single plane.
11) A step index fiber has a normalized frequency V=26.6 at 1300 nm wavelength .If the core
radius is 25µm, Find the numerical aperture. (AU Apr/May 2007)
Use the formula V= (2πa/λ)*NA
V=26.6, a=25µm, λ=1300 nm.
NA= (λ/2πa)*V= (1300nm/2*π*25 µm) * 26.6=0.169
12) Define Mode-Field Diameter? (or) What is the fundamental parameter of single mode
fiber? (AU April/May 2005, April/May 2007)
The fundamental parameter of single mode fiber is the mode field diameter (MFD). This
parameter can be determined from the mode field distribution of the fundamental LP 01 modes.
The MFD is the cross sectional dimension 2wo, Where the beams intensity drops to 1/e 2 =
0.135 of its peak value.
13) Give the expression for the effective number of modes guided by a curved multimode fiber
“a”. (AU Nov/Dec 2004, April/May 2005, Nov/Dec 2005)
M = V2 /2
Where V = (2πa/) (n1 2 – n2 2) ½
14) Consider a parabolic index wave-guide with n1 = 1.75, n2 = 1.677 and core radius 25
micrometer. Calculate the numerical aperture at the axis and at a point 20 micrometer
from the axis. (AU Nov/Dec 2005)
Given n1 = 1.75, n2 = 1.677, a= 25µm,
Numerical aperture at the axis NA (0) = n1 (2*Δ) 1/2
 Δ= (n1-n2)/ n1= (1.75-1.677)/1.75=0.04171
NA (0) =1.75 * (2*0.04171)1/2=0.5054
NA (20 µm) = NA (0)* √(1-(r/a)2=0.5054*√(1-(20/25)2=0.3324.
15) It is desired to make a single –mode fiber at an operating wavelength = 1300 nm with n core
=1.505 and nclad = 1.502. Find the numerical aperture and core radius.
(AU Nov/Dec 2006)
NA=√ n21−n22 =√1 . 5052−1. 5022=0. 095
λ V 1300 nm 2. 405
a= = =5 .2 μm
2 π NA 2 π 0. 095
16) Give the refractive index expression for graded index fiber. (AU Nov/Dec 2006)

{ [ ( )] }
α 1/2
r
n(r )=¿ n1 1 −2 Δ for 0≤r≤a ¿ ¿ {}
a
17) What is meant by mode coupling? What causes it? (AU Nov/Dec 2006)
Mode coupling refers to interaction of higher order modes in the core medium with
the radiation modes in the cladding. Harmonic variation of higher order mode results when the
mode travels through the core and it undergoes exponential decay in the boundary which interacts
with the radiation mode and results in mode coupling.
Boundary condition n2 k< β <n1 k.
18) A point source of light is 12 cm below the surface of a large body water (n=1.33). What is
the radius of the largest circle on the water surface through which the light can emerge?
(AU Nov/Dec 2004 ,Nov/Dec 2005)
n1 sinθ1= n2 sinθ2
n1=1.33,
sinθ1=12*10-2/x
1.33*(12*10-2/x) =1
x=16 cm.
19) Write the expression for refractive index in graded index fiber?
(AU Nov/Dec 2004)
In GI fiber the RI decreases gradually with increasing radial distance r from the core axis.
But it is constant in the cladding. The most commonly used construction for the RI
variation in the core is the power law relationship.

n(r) = n1 [ 1- 2  (r/a)  ] ½ for 0r a

n1(1 – 2  ) ½ = n2 for ra
Where “r” radial distance from the fiber axis
a - core radius
n1 – R1 of the core, n2 – R2 of the cladding
20) Give the relationship between rays and modes. (AU Nov/Dec’2007 R-2004)
Modes: Propagation of light waves along the wave guide described in terms of set of
guided electromagnetic waves is called mode. Each guided mode is a pattern of electric and
magnetic field lines that is repeated along the fiber at intervals equal to wavelength
Ray congruence: A light ray can be associated with any plane wave that is perpendicular
to the phase front of the family of plane waves corresponding to a particular mode forms a set
of rays.
For a monochromatic light field of radian frequency ω a mode travelling in positive z
direction is given by ej (ωt-βz).β- z component of wave propagation constant.
 21) Why do we prefer step index single mode fiber for long distance communication?
Step index single mode fiber has a) low attenuation due to smaller core diameter b) higher
bandwidth and c) very low dispersion.
22) Define relative refractive index difference.
Thus relative refractive index difference is the ratio between the refractive index difference (of core
and cladding) and refractive index of core.
23) What is Snell’s law?
The relationship at the interface is known as Snell’s law and is given by n1sinΦ1=n2 sinΦ2
24) What is the necessity of cladding for an optical fiber?
a) To provide proper light guidance inside the core
b) To avoid leakage of light from the fiber
c) To avoid mechanical strength for the fiber
d) To protect the core from scratches and other mechanical damages
25) Among microwave and light waves which have high bit rate distance product? Why?
Light waves have high bit rate distance product because light waves have high frequency(1014
Hz) than microwave frequency(1011 Hz) and information carrying capacity of an electromagnetic
wave is directly proportional to its frequency.
26) Mention the advantages of graded index fiber.
(a) Signal distortion is low because of self focusing effect.
(b) B.W is from 200 MHz to 600 MHz
(c) Attenuation is less
(d) Numerical aperture is less
27) What is the condition for TIR?
(a) Light should travel from denser medium to rarer medium.
(b) The angle of incidence should be greater the critical angle of the denser medium
28) Define V-number.
Its defined as the normalized frequency of the fiber giving the cutoff wavelength condition and
the relation for number of modes propagating through the fiber.
29) Write the adv and disadv of MM Fiber?
ADV:
(a) It allows a large number of paths or modes for the light rays travelling through it.
(b) Launching of light into fiber and jointing of two fibers are easy in these fibers.
(c) Fabrication is less difficult
(d) Cost is low
DISADV:
(a) The core diameter and the relative refractive index difference are larger than the SM fiber
(b) There is signal degradation due to multimode dispersion and material dispersion
(c) These are less suitable for communication and can be generally used in the local area
networks
30) Define cutoff conditions.
Cutoff wavelength of a single mode fiber is the minimum wavelength that can be
transmitted through the fiber. Wavelengths greater than the cutoff wavelength can be transmitted.
For MM fibers, it’s the wavelength at which it behaves as a single mode fiber(transmits only LP 01
mode)
 31) Why do we use LP01 mode for long distance comm.?
LP01 mode has uniform intensity distribution of transverse electric field throughout the core
of the fiber and has low loss and low dispersion during transmission
32) What are the 3 operating windows?
The 3 operating windows are: 850nm,1310nm and 1550nm.

UNIT II TRANSMISSION CHARACTERISTICS OF OPTICAL FIBERS

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

PART-A
1) The optical power launched into the fiber is 10 µm. The transportation distance is 10 km.
The optical power at the output of fiber is 2 µm. (AU Apr/May2010)
(a) Calculate the signal attenuation/ unit km length.
(b) Calculate the overall signal attenuation.
α= (10/Z) log(p(0)/p(Z)) => α=0.6989dB/Km
Overall attenuation (For 10 Km) = 6.989dB/Km

2) What is intramodal and intermodal dispersion? How the effect can

be minimized? (AU Apr/May 2010)
Intramodal dispersion is the dispersion that arises due to the action of the components
present within the fibers.
Intermodal dispersion is the dispersion that occurs due to the presence of many modes in a
fiber.
3) What is group delay? (AU Nov /Dec 2008 & 2010)
When the light propagates along the fiber, each spectral component can be assumed to
travel independently, and to undergo a time delay or group delay per unit length.
4) Mention two causes of intra modal dispersion:(AU Nov/Dec 2010 &AU Apr/May 2007)
The two main causes of intramodal dispersion are
1. Material dispersion:
Material dispersion arises from the variation of refractive index of core material as a
function of wavelength. This causes wavelength dependence of group velocity of any given mode.
Pulse spreading occurs even when different wavelength follow the same path.
2. Waveguide dispersion:
Waveguide dispersion occurs because a single mode fiber only confine about 80% of
optical power to the core .Dispersion thus arises since 20% of the light propagating in the cladding
travels faster than the light confined to the core.

5) A 100 Km fiber is used in a communication system. The fiber has 3.0dB/km loss.
What will be the output power, when the input power fed at the input of fiber is
500 µW? (AU Apr/May2009)
α=3dB/Km; α = 4.343 αp; αp = 0.6907
p(Z)=p(0) e-αpz = 5.037*10-34Watts
6) What is the need for mode coupling in optical fiber? (AU Apr/May2009)
 An optical fiber mode coupling device, capable of being readily connected to a
conventional optical fiber with a high degree of ruggedness, is provided. The mode coupling
device only allows transmission of at least one supported fiber mode there through, and is
preferably configured to maximize the coupling, of at least one desired fiber mode, to the at least
one supported fiber mode.
7) What are micro bends? How they are formed? (AU Nov/Dec 2009)
Micro bends are repetitive small scale fluctuations in the radius of curvature of the fiber axis. They
are formed either by non-uniformities in the manufacturing of the fiber or by non-uniform lateral
pressures created during cabling of fibers.
8) A multimode GI fiber exhibits a total pulse broadening of 0.1µsec over a distance of 15 km.
Estimate maximum possible bandwidth on the line. (AU Nov/Dec 2009)
-6
Bandwidth Bopt = BT = (1/2ζ) = (1/2x0.1x10 ) = 5MHz.
9) Distinguish intrinsic and extrinsic absorption: (AU Apr/May’2008,R-2004)
Intrinsic absorption is due to the absorption of basic constituent atoms of the fiber material
extrinsic absorption is by the absorption of impurity atoms in the glass material.
10) What is inter modal dispersion? What causes it? (AU Apr/May’2008 R-2004)
Modes in a given optical pulse arrive at the fiber end at slightly different times thus
causing the pulse to spread out in time as it travels along the fiber. This is known as intermodal
dispersion.
11) A multimode graded index fiber exhibits total pulse broadening of 0.1µs over a distance of
15 km. Estimate the maximum possible bandwidth on the link assuming RZ coding without
inter symbol interference. (AU Nov/Dec 2008)

Bandwidth Bopt = BT = (1/2ζ) = (1/2x0.1x10-6 ) = 5MHz.

12) Write the expression for attenuation (or ) Define fiber loss: (AU Apr/May2007)
Signal attenuation is defined as the ratio of the optical output power from a fiber of length L to
the optical input power .This is the function of wavelength.
Attenuation is given by,
α = 10/L log (Pin/Pout)
The basic attenuation mechanisms are
 absorption
 scattering
 radiation losses
13) What do you mean by Polarization mode dispersion? (AU Nov/Dec’2007,R-2004)
Dispersion is defined as pulse spread as a function of wavelength and is measured in
ps/km/nm.
1 dτg
D=
L dλ
Polarization mode dispersion defines dispersion due to orientation of electric field in one
particular direction.
14) Distinguish dispersion shifted and dispersion flattened fiber.
(AU Nov/Dec’2007,R-2004)
Dispersion shifted fiber : Material dispersion depends only on the composition of the
material ,waveguide dispersions a function of core radius ,refractive index difference and shape of
refractive index profile .Waveguide dispersion can vary dramatically with fiber design
parameters .By shifting waveguide dispersion to longer wavelength and assuming a constant value
 for material dispersion ,addition of waveguide and material dispersion can produce zero total
dispersion at 1550 nm .Resulting optical waveguides are known as dispersion shifted fibers.
Dispersion flattened fiber: An alternative to reduce fiber dispersion is by spreading the
dispersion minimum out over a broad range .This approach is known as dispersion flattening.

15) Find the coupling loss for two fibers having core refractive index profiles αE=2.0 and αR=1.5
(AU May/June 2006)

{ ( )
L F ( α )=¿ −10 log
α R ( α E +2 )
α E ( α R +2 ) }
for α R ≤α E ¿ ¿{}

{
L F ( α )= −10 log
((
1 . 5 ( 2+2 )
2 1 .5+2 ) )}
=0 .6694 dB

16) List the basic attenuation mechanisms in an optical fiber. (AU Nov/Dec 2006)
The basic attenuation mechanisms are
1. absorption
2. scattering
3. radiation losses
17) What are the causes of absorption? (AU April/May 2005)
Absorption occurs when the incoming photon has such a frequency that its energy is equal
to the energy gap of the material.
Classification of absorption:
 Atomic defects.
 extrinsic absorption
 intrinsic absorption
18) Define normalized propagation constant. (AU Nov/Dec 2005)
The number of modes that can exist in a waveguide as a function of V may be
conveniently represented in terms of a normalized propagation constant defined by

b = a2 w2 / v 2 = (β /k) – n2 2/ n1 2 – n2 2
19) An optical signal has lost 55% of its power after traversing 3.5 Km of fiber. What is the loss
in Db/Km of this fiber? (AU Nov/Dec 2004)
1 P 1 P
log i log i
Attenuation α= L P 0 = 3. 5 Po = 0.55

Loss=0.55*3.5=1.9 Db/km
20) What is fiber birefringence and fiber beat length? (AU Apr/May2009)
In the multi mode fiber modes propagate with different velocity. So the difference between their
effective refractive indices as called as fiber birefringence.
Bf = ny - nx
The length over which the beating occurs is called as fiber beat length. Lp = 2π / β

21) Commonly available single mode fibers have beat lengths in the range 10 cm<L P<2 cm.
What range of refractive index differences does this correspond to for λ=1300nm?
(AU Apr/May 2006)
Fiber birefingence Bf= ny-nx = (λ/Lp)
When Lp= 10 cm, Bf = (1300 *10-9/ 10*10-2) = 130*10-7
 When Lp= 2 cm, Bf = (1300 *10-9/ 2*10-2) = 650 * 10-7
22) List the different types of mechanical misalignments that can occur between two joined
fibers (AU Nov/Dec’2007 R-2004)
 Lateral misalignment
 Longitudinal misalignment
 Angular misalignment

23) Calculate the ratio of stimulated emission rate to the spontaneous emission rate for lamp
operating at a temperature of 1000 K. Assume average operating wavelength 0.5
micrometer. (AU May/June 2006)
(Refer page number 288 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)

24) Name few splicing methods in fiber optics. (AU Nov/Dec 2006)
The different fiber Splicing techniques are
 Fusion splicing
 V groove and tube mechanical splicing
 Elastic tube splicing
 Rotary splicing
25) What are splices? What are the requirements of splices?
The splices are generally permanent fiber joints, whereas connectors are temporary fiber joints.
Splicing is a sort of soldering. The requirements of splices are:
Should cause low attenuation
Should be strong & light in weight
 should have minimum power loss
Should be easy to install
26) Define Raleigh scattering loss.
It’s the dominant loss mechanism in the ultraviolet region. Its tail extends upto infrared
region. Its inversely proportional to the fourth power of wavelength. It arises due to the
microscopic inhomogeneties caused by density fluctuations, refractive index fluctuations and
compositional variations.
27) Define Mie scattering loss
It’s a linear scattering which arises from the inhomogenities, which are comparable in size
to the guided wavelength, in the forward direction. Its also due to the imperfect cylindrical
structure of the waveguide, irregularities in the core-cladding interface, core-cladding refractive
index difference along the fiber and diameter fluctuations. It can be reduced by defect free fiber
fabrication and increasing the relative refractive index difference.
28) How are micro-bending losses reduced?
These are the losses due to the bends in the fiber axis, during cabling and stress acting on
the fiber. These produce mode coupling and radiation losses. These can be reduced by extruding a
compressible jacket over the fiber. When external forces are applied, the jacket will be deformed
but the fiber will tend to stay relatively straight.

UNIT III SOURCES AND DETECTORS

Optical sources: Light Emitting Diodes - LED structures - surface and edge emitters, mono
and hetero structures - internal - quantum efficiency, injection laser diode structures -
comparison of LED and ILD Optical Detectors: PIN Photo detectors, Avalanche photo
diodes, construction,
 Characteristics and properties, Comparison of performance, Photo detector noise –Noise
sources, Signal to Noise ratio, Detector response time.

PART-A
1) A lens coupled surface emitting LED launches 190µm of optical power into a multimode
step index fiber when a forward current of 25mA is flowing through the device. Determine
the overall power conversion efficiency when the corresponding forward voltage across the
diode is 1.5v. (AU April/May 2010)
(Refer page number 397 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)

2) Draw the three key transition processes involved in laser action. (AU April/May 2010) (AU
Nov/Dec 2005) (AU April/May 2009)
The three key transition processes involved in laser action
 Absorption
 Spontaneous emission
 Stimulated emission
3) Write short notes on Direct and Indirect band gap materials.
(AU April/May2005, 2006, Nov/Dec 2005, 2006 &2008 & 2010)
The simplest and most probable recombination process ill be where the electron and hole have the
same momentum value. This is called direct band gap material.
Indirect band gap materials are materials whose conduction band minimum and the valence band
maximum energy levels occur at the different values of momentum. Here the band-to-band
recombination must involve a third particle to conserve momentum, since the photon momentum
is very small.
4) A double hetero junction InGaAsP LED emitting at a peak wavelength of 1310 nm has
radioactive and nonradioactive recombination times of 30 and 100 ns respectively.
Calculate the bulk recombination time. (AU Nov/Dec 2010)
1/τ=(1/τr)+(1/τnr) = 0.23ns
5) When a LED has 2V applied to its terminals, it draws 100mA and produces 2 mW of
optical power.Determine the LED conversion efficiency from electrical to optical power?
(AU April/May 2009)
Pin = V*I = 2*100m = 200 mW
Pout = 2 mW (given)
η = pout/pin = 0.01
6) Define three modes of cavity. (AU Nov/Dec 2009)
Longitudinal modes are related to the length L of the cavity.
Lateral modes lie in the plane of the PN junction. These modes depend upon the side wall
preparation and width of the cavity.
Traverse modes are associated with the electromagnetic field and beam profile in the direction
perpendicular to the plane of the PN junction. These modes determine the radiation pattern of the
laser.
7) What does population Inversion mean?
(AU Nov/Dec 2004, April/May 2007, Apr/May’2008 R-2004)
To achieve optical amplification, it is necessary to create a non equilibrium distribution of
atoms such that the population of the upper energy level is greater than that of the lower energy
level. This is known as population Inversion.
8) Compare LED and LASER diodes: (AU Apr/May 2007 Apr/May’2008 R-2004)
LED can be used in optical communication systems where bit rate is less than 100
Mb/s to 200Mb/s together with multimode fiber coupled optical power. Laser diodes can be used
 for systems with bit rate greater than 200 Mb/s with single mode fiber. LED s require less
complex drive circuit than laser diodes since no thermal or optical stabilization are
required .LED s can be fabricated less expensively than LASER diodes with higher yields.

9) An LED has radiative and nonradiative recombination times of 30 and 100 ns

respectively.Determine the internal quantum efficiency. (AU Nov/Dec 2008)

ηint = 1/(1+(τr/τnr)) = 1/(1+(30ns/100ns)) = 0.7692

10) What do you mean by heterojunction? Mention its advantages.

(AU Nov/Dec’2007 R-2004)
A heterojunction consists of two adjoining semiconductor materials with different band
gap energies .These devices are suitable for fiber transmission system because they have adequate
output power for a wide range of applications, their optical power output can be directly
modulated by varying the input current to the device they have a high efficiency and their
dimensional characteristics are compatible with those of the optical fiber .

11) Define internal quantum efficiency of an LED. (AU Nov/Dec 2006)

The Internal Quantum efficiency in the active region is the fraction of the electron-hole
pairs that recombine radiatively. If the radiative recombination rate is R r and the non radiative
recombination rate is Rnr, then the internal Quantum efficiency ηint is the ratio of radiative
recombination rate to the total recombination rate
ηint = Rr/ Rr + Rnr
12) What is meant by double heterostructure? (AU April/May 2005)
A heterojuntion consists of two adjoining semiconductor materials with different band gap
energy.
13) Mention the benefits and drawbacks of avalanche photodiode. (AU April/May 2010)
Benefits:
Receiver sensitivity is high.
Carrier multiplication.
Avalanche effect
Drawbacks:
Total current gain is reduced at the shorter wavelength.
14) Define responsivity. (AU Nov/Dec 2005 , 2004,2008 & 2010, April/May 2005 &2010)
The responsivity is defined as the ratio of output photocurrent to the incident optical power.
R=Ip /Po
Where Ip- average photocurrent generated by a steady average optical power . Po
incident on a photo detector
15) Define the quantum efficiency of a photo detector.
(AU Nov/Dec 2004&2005, April/May 2010)
Quantum efficiency η is the number of electron hole pair generated per incident
photon of energy hγ is given by

No. of electron hole pairs generated Ip/q

η= ------------------------------------------- = -------
No. of incident power Po/ hγ
16) In a 100ns pulse, 6x106 photons at a wavelength of 1300 nm fall on an InGaAs
photodetector.On the avarege, 5.4x106 electron hole pairs are generated. Find the
quantum efficiency. (AU Nov/Dec 2010)
 Refer example 6.2 (PG NO:247 in 3rd edition Gerd Keiser)
η = (No of electron-hole pairs generated) / (no of incident photons)
 5.4*106/(6*106)
 0.9
17) A silicon avalanche photodiode has a quantum efficiency of 65% at a wavelength of 900
nm. If 0.5µW optical power produces a multiplied photocurrent of 10 µA. Determine its
primary photon current and multiplication factor. (AU April/May 2009)

Ip = (ηq/(hν))p0 = (0.65*1.6*10-19*9*10-7*5*10-7)/(6.625*10-34*3*108)

 0.235µA
M=IM/IP = 10µA/0.235µA = 43
18) Define quantum efficiency and responsivity of photo detector. (AU April/May 2009& 2008)
Quantum efficiency is defined as the ratio of number of electron hole pair generated to
number of incident photons
η = (IP/q)/(PO/ hν).
Responsivity specifies the photo current generated per unit optical power
R= IP/ PO

19) List out the values of Operating wavelength and Responsivities of Si,Ge and InGaAs
Photodiodes. (AU Nov/Dec 2009)
Si: Operating wavelength = 400-1100nm & Responsivity = 0.4-0.6
Ge: 800-1650nm & 0.4-0.5
InGaAs: 1100-1700nm & 0.75-0.95
20) What is meant by (1/f) noise corner frequency? (AU Nov/Dec 2009)
The (1/f) noise corner frequency fc is defined as the frequency at which (1/f) noise,which
dominates the FET noise at low frequencies and has (1/f) power spectrum becomes equal to the
high frequency channel noise given by Г.
21) Ga As has a band gap energy of 1.43 eV at 300K. Determine the wavelength above which
an intrinsic photo detector fabricated from this material will cease to operate.
(AU Apr/May’2008 R-2004,Apr/May 2007)
Given Eg=1.43 eV=1.43*1.602*10-19 J =2.29086*10-19 J.
Eg=hν=hc/λ.
h=6.625*10-34 J-s.
c=3*108 m/s.
λ= hc/ Eg=6.625*10-34*3*108/2.29086*10-19=8.6757*10-7.
λ=8.6757*10-7 m.

22) Compare the performance of APD and PIN diode. (AU Nov/Dec 2008)
Refer table 6-1. Page No: 267
23) Define quantum limit. (AU Nov/Dec’2007 R-2004)
The minimum received optical power required for specific bit error rate performance in a
digital system is known as quantum limit.
24) What are the desired features of a photodetector? (AU Nov/Dec’2007 R-2004)
 High response
 Minimum addition of noise
  Fast response speed
 Sufficient bandwidth
 Insensitive to variation in temperature.

25) What is meant by quantum limit? (AU May/June 2006)

The minimum received optical power required for a specific bit error rate performance in a
digital system is known as the Quantum Limit.
26) An APD generates a current of 100 nA when the incident power is 5 nw. The operating
wavelength is 1.5 micro meter. Find its reponsivity. If the quantum efficiency is 0.7, find
the multiplication factor. (AU Nov/Dec 2006)
ηq 0 .7∗1 . 6∗10 ∗1 .5∗10−6∗5∗10−9
−19
I P = P0 = =4 nA
hν 6 . 625∗10−34∗3∗108
I M 100 nA
Multiplicationfactor= = =25
I P 4 nA

27) A given APD has a quantum efficiency of 65% at a wavelength of 900 nm. If 0.5
microwatts of optical power produces a multiplied photocurrent of 10 microamperes, find
the multiplication M. (AU April/May 2005)
ηq ηqλ (0 .65 )∗(1. 6∗10−19 c )∗(9∗10−7 m)
P 0= P0 = −34 8
∗5∗10−7 W=0. 235 μA
Ip=RP0= hν hc (6 . 625∗10 J −s )∗(3∗10 m/s )

I M 10 μA
= =43
Multiplication M= I P 0. 235 μA
28) Define long wavelength cut off related to photo diode. (AU Nov/Dec 2004)
The upper wavelength cutoff λC is determined by the band gap energy Eg of the material. If
Eg is expressed in units of electron volts (eV) then c is given in units of micrometers (μ m) by
hc 1.24
λC(μm) = ---- = ----------
Eg Eg (e V)
29) What does bulk dark current mean? (AU Nov/Dec 2004)
The bulk dark current iDB arises from electrons and/or holes which are thermally generated in
the pn junction of the photodiode.
30) Define radiance. (AU Nov/Dec 2004)
Radiance is the optical power radiated into a unit solid angle per unit emitting surface area
and is generally specified in terms of watts per square centimeter per steradian. Since the optical
power that can be coupled into a fiber depends on the radiance (i-e, on the spatial distribution of
the optical power), the radiance of an optical source rather than the total output power is the
important parameter when considering source to fiber coupling efficiencies.
31) Define modal noise and mode partition noise. (AU April/May 2009 & 2010)
Modal Noise: Interference of a multimode optical communications fiber with a laser light when a
speckle pattern in the light intensity in the fiber alters because of motion of the fiber or changes in
the laser spectrum. Also known as modal distortion.
Mode partition noise: In an optical communications link, phase jitter of the signal caused by the
combined effects of mode hopping in the optical source and intramodal distortion in the fiber.
32) What is meant by modal noise? (AU April/May 2005, April/May 2007)
 Modal noise arises when light from a coherent laser is coupled into a multimode fiber. This
is generally not a problem for links operating below 100 Mbps but becomes disastrous at speeds
around 400 Mbps and higher. The following factors can produce modal noise in an optical fiber
link
 Mechanical disturbances along the line
 Fluctuations in the frequency of an optical source

33) Differentiate LEDs and Laser diodes.

LED Laser diode
The output obtained is incoherent. The output obtained is coherent.
Less expensive and less complex More expensive and more complex.
Long life time. Less life time.
34) .Define photocurrent.
The high electric field present in the depletion region causes the carriers to separate and be
collected across the reverse-biased junction. This gives to a current flow in the external circuit, with
one electron flowing for every carrier pair generated. This current flow is known as photocurrent
35) Define impact ionization.
In order for carrier multiplication to take place, the photo generated carriers must traverse a
region where a very high electric field is present. In this high field region, a photo generated electron
or hole can gain energy so that it ionizes bound electrons in the valence band upon colliding with
them. This carrier multiplication mechanism is known as impact ionization.
36) Define avalanche effect.
The newly created carriers are accelerated by the high electric field, thus gaining enough energy
to cause further impact ionization. This phenomenon is called avalanche effect.
37) What is p+ Π p n+ reach- through structure?
The reach –through avalanche photodiode (RAPD) is composed of a high resistivity p-type
material deposited as an epitaxial layer on a p+ substrate. A p-type diffusion is then made in the
high resistivity material, followed by the construction of an n+ layer. The configuration is called p+
Π p n+ reach- through structure.
38) Define ionization rate.
The avg. no. of electron hole pairs created by a carrier per unit distance traveled is called
ionization rate.
39) What are the conditions to be met for a high signal- to- noise ratio in a photo detector?
The photo detector must have a high quantum efficiency to generate a large sign al power. The
amplifier noises should be kept as low as possible.
40) What is meant by error rate?
An approach is to divide the number Ne of errors occurring over a certain time interval t by the
number Nt of pulses transmitted during this interval. This is called either the error rate or the bit
error rate.
Bit error rate BER =Ne / Bt Where B= 1/Tb
41) What is meant by excess noise factor?
The ratio of the actual noise generated in an avalanche photodiode to the noise that would exist if
all carrier pairs were multiplied by exactly m is called the excess noise factor (F).
42) Define multiplication M.
The multiplication M for all carriers generated in the photodiode is defined by M = I M /IP
IM average value of the total multiplied output current
IP primary un multiplied photocurrent
43) What are the characteristics of light sources?
 (a) The spectral line width of the source should be as small as possible because dispersion is
directly proportional to it.
(b) Its size and configuration should be compatible with the launching light and highly
directional
(c) It must accurately track the electrical input signal to minimize distortion and noise
(d) It should be capable of simple signal modulation over a wide B.W
(e) It must couple sufficient optical power to overcome attenuation
44) What is meant by diffusion length?
As the charge carriers flow through the material of a PIN photodetector due to photo, some
electron-hole pairs will recombine and hence disappear. The charge carriers move a distance L n or
Lp for electrons and holes respectively. This distance is called diffusion length.
45) Define transit time.
Its defined as the time taken to transfer the charge carriers within the depletion region. The
response speed of a photodiode is limited by the time it takes photogenerated carriers to travel
across the depletion region. This transit time td depends on the carrier drift velocity vd and the
depletion layer width w and is given by,
td=w/vd
46) A photodiode is constructed of GaAs which has a bandgap energy of 1.43eV at 300K.
Determine the long wavelength cutoff.
λc=hc/Eg
= [(6.625*10-34)(3*108)]/(1.43*1.6*10-19)=869nm
UNIT IV FIBER OPTIC RECEIVER AND MEASUREMENTS
Fundamental receiver operation, Pre amplifiers, Error sources – Receiver Configuration –
Probability of Error – Quantum limit. Fiber Attenuation measurements- Dispersion
measurements – Fiber Refractive index profile measurements – Fiber cut- off Wave length
Measurements – Fiber Numerical Aperture Measurements – Fiber diameter measurements.

PART-A
1) List the important requirements of an optical receiver. (AU Nov/Dec 2006)
It has the task of first converting the optical energy emerging from the end of a fiber into
an electrical signal, and then amplifying this signal to a large enough level. The BER of a optical
receiver should be less.
2) What are the benefits of transimpedance amplifier? (AU April/May 2007)
 It has a wide dynamic range compared to the high –impedance amplifier
 Usually little or no equalization is required because the combination of Rin and the feedback
resistor Rf is very small, which means the time constant of the detector is also small.
 The output resistance is small, so that the amplifier is less susceptible to pickup noise ,cross
talk ,electromagnetic interference etc
 The transfer characteristic of the amplifier is actually its transimpedance, which is the
feedback resistor. Therefore, the transimpedance amplifier is very easily controlled and stable.
 Although the transimpedance amplifier is less sensitive than the high impedance amplifier, this
difference is usually only about 2 to 3 dB for most practical wide band designs.
3) What is meant by pre-amplifier? What are the advantages of pre-amplifier?
Pre-amplifiers are the circuits that are designed to maximize the receiver sensitivity while maintaining
a suitable bandwidth since the receiver’s sensitivity and B.W are dominated by noise sources. Its
advantages are:
(a) Has low noise level and high gain
(b) Has high B.W
 (c) Has high dynamic range
(d) Has high sensitivity to avoid non-linearities
4) What are the drawbacks of high impedance amplifier?
It produces a large input RC time constant, the front-end B.W is less than the signal B.W. Thus,
the input signal is integrated and equalization techniques must be employed to compensate for
this.
5) Define extinction ratio
Its defined as the ratio of the optical power in a 0 pulse to the power in a 1 pulse. Biasing the light
source slightly on during a 0 time slot results in a non-zero extinction ratio ,є. Its effect is a power
penalty in receiver sensitivity.
6) A 2km length of multimode fiber is attached to apparatus for spectral loss measurement the
measured output voltage from the photo receiver using the full 2km fiber length is 2.1v at a
wavelength of 0.85 micrometer. When the fiber is then cut back to leave a 2km length the
output voltage increases to 10.7V.Determine the attenuation per km for the fiber at a
wavelength of 0.85 micrometer and estimate the accuracy of the result.
Solution:
αdb=10/(L1-L2)log10(V2/V1) =10/(1.998)log10(10.7/2.1) =3.5 db km-1
7) A He-Ne laser operating at a wavelength of 0.63 µm was used with a solar cell cube to
measure the scattering loss in a multimode fiber sample. With a constant optical output
power the reading from the solar cell cube was 6.14 nV.The optical power measurement at
the cube without scattering was 153.38 µV .The length of the fiber in the cube was
2.92cm.Determine the loss due to scattering in dB km -1 for the fiber at a wavelength of 0.63
µm.
Solution:
αsc= 4.343 X 105 /L(cm) (Vsc/Vopt)
=4.343 X 105/2.92 (6.14 x 10-9/153.38 x 10-6)
= 6.0 db km-1
8) A trigonometrical measurement is performed in order to determine the numerical aperture
of a step index fiber. The screen is positioned 10.0 cm from the fiber end face. When
illuminated from a wide angled visible source the measured output pattern size is 6.2 cm.
Calculate the approximate numerical aperture of the fiber.
Solution:
NA= A/(A2+4D2) (1/2)
=6.2/(38.44+400) (1/2)
=0.30
9) The shadow method is used for the on-line measurement of the outer diameter of an optical
fiber. The apparatus employs a rotating mirror with an angular velocity of 4 rads -1 which is
located 10cm from the photo detector. At a particular instant in time a shadow pulse of
width 300µs is registered by the photo detector. Determine the outer diameter of the optical
fiber in µm at this instant in time.
Solution:
ds/dt = L . Dф/dt = 0.1 x 4 = 0.4 ms-1 = 0.4 µm µs-1
hence the fiber outer diameter do in µm is ,
do = We.ds/dt = 300µs x 0.4 µm µs-1 = 120 µm.
UNIT V OPTICAL NETWORKS
Basic Networks – SONET / SDH – Broadcast – and –select WDM Networks – Wavelength
Routed Networks – Non linear effects on Network performance – Performance of WDM +
EDFA system – Solitons – Optical CDMA – Ultra High Capacity Networks.
 PART-A
1) What are the main parameters used for characterizing the performance of optical amplifiers
in a communication system? (AU April/May 2010)
 External pumping and amplifier gain are the main parameters of optical amplifier.
2) What is meant by ‘soliton’? (AU April/May 2005 & Nov/Dec 2010)
A soliton is a non dispersive pulse that makes use of nonlinear dispersion properties in a fiber
to cancel out chromatic dispersion effects. It refers to a special kind of wave that can propagate
undisturbed over long distances and remain unaffected after collisions with each other.
3) What are the advantages of using soliton signals through fiber? (AU April/May 2009)
(i) Solitons are very narrow, high intensity optical pulses that retain their shape through the
interaction of balancing pulse dispersion with the nonlinear properties of an optical fiber.
ii) It can propagate undistorted over long distances and remain unaffected after collisions
with each other.
4) What is chirping? (AU Nov/Dec 2009)
A laser which oscillates in a single longitudinal mode under CW operation may experience
dynamic line broadening when the injection current is directly modulated. This line broadening is a
frequency chirp associated with modulation induced changes in the carrier density.
5) What is EDFA? (AU Apr/ May’2008,R-2004)

An important class of fiber amplifiers makes use of rare earth elements as again medium by
doping the fiber core during the manufacturing process. Amplifier properties such as the operating
wavelength and the gain bandwidth are determined by the dopants rather than by silica fiber which
plays the role of host medium. Erbium is one of the rare earth elements which is doped along with the
fiber core. and they operate near the wavelength region near 1.55µm
6) List down the system requirements needed in analyzing a point-to-point link
(AU Nov/Dec 2005, Apr/May 2007)
 Desired (or) possible transmission distance
 Data rate or channel BW
 Bit error rate (BER).
7) What are the advantages of WDM? (AU Nov/Dec’2007 R-2004)
WDM corresponds to the scheme in which multiple optical carriers at different
wavelengths are modulated using independent electrical bit streams and are then transmitted over
the same fiber. WDM has the potential of exploiting the large bandwidth offered by optical fibers
WDM was used to transmit two channels in different transmission windows of optical fiber.
8) Distinguish fundamental and higher order solitons. (AU Nov/Dec’2007 R-2004)
When an input pulse having an initial amplitude U (0, τ) =N sech (τ) is launched in to the
fiber its shape remains unchanged during propagation when N=1 is called fundamental soliton ,but
follows a periodic pattern for integer values of N>1 are called higher order solitons.
9) A soliton communication is operating at 1.55 micro meter by using fibers
β2 = -1 ps2 and r=2/ (w-km). What is the peak power required to maintain a fundamental
soliton of width (TFWHM) 10 ps? (AU Nov/Dec 2006)
The FWHM pulse width and T0 are related by TFWHM=1.763T0.
T0= TFWHM/1.763= 10ps/1.763= 5.67 ps
Ppeak=1/(2*5.672)=15.5 mW.

10) Define Kerr effect.

The non-linearity in the refractive index of any optical material is known as the Kerr
nonlinearity. This nonlinearity produces a carrier-induced phase modulation of the propagating
signal which is called Kerr effect. In single-wavelength links, this gives rise to self-phase
modulation (SPM), which converts optical power fluctuations in a propagating light wave to
spurious phase fluctuations in the same wave.
11) Define frequency chirping
As the phase fluctuations are intensity-dependent, different parts of the pulse undergo
different phase shifts. This is caused by varying index change resulting in varying phase change.
This leads to Frequency chirping. In this, the rising edge of the pulse experiences a red shift in
frequency and the trailing edge of the pulse experiences a blue shift in frequency. Freq chirping’s
intensity depends on transmitted power.
12) Define dynamic range
The performance quantity of interest is defined as the system dynamic range(DR). Its due
to the serial nature of the linear bus, the optical power available at a particular node decreases with
increasing distance from the source. This is the maximum optical power range to which any
detector must be able to respond. The worst case DR is,
DR=10log(P1,2/P1,N)=(N-2)(Αl+2LC+Lthru+Li)
13) What are the challenges in designing networks?
(a) Transmission of the different wavelength channels at the highest possible bit rate
(b) Transmission over the longest possible distance with the smallest number of optical
amplifiers
(c) Network architectures that allow simple and efficient network operation, control and
management.
14) What are the non-linear effects in optical network?
(a) Group velocity dispersion(GVD) limits the bit rate by temporarily spreading the
transmitted optical pulse
(b) Non-uniform gain across the desired wavelength range of EDFAs in WDM links
(c) Polarization-mode dispersion(PMD) arises from orthogonal polarization modes travelling
at slightly different speeds owing to fiber birefringence
(d) Reflections from splices and connectors cause instabilities in Laser sources
15) Define threshold power
The threshold power Pth is defined as the signal power at which the back-scattered light equals
the fiber input power. It’s the point at which the SBS becomes problem. Its approximation is
given by,
Pth=21(Aeffb/gbLeff)(1+∆vsource/∆vB)
16) What are the schemes available for reducing the power penalty effects of SBS
(a) Keep the optical power per WDM channel below the SBS thresholds. For the long haul
systems, it reduces amplifier spacing
(b) Increase the line width of the source as the gain BW of SBS is very small. Its achieved
through direct modulation of source as it causes line width to broaden because of chirping
effect
(c) Slightly dithering the laser output in frequency. This is effective in SBS as it’s a
narrowband process
17) Define FWHM
The full-width half maximum (FWHM) of a pulse is defined as the full width of the pulse at its
half-maximum power level.

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

PART-B ** indicates that give first preference while studying.

1) (i)With the help of suitable diagrams, explain the following concepts in optical fiber
transmission.(12) ** (AU Nov/Dec 2009)
(1)Evanescent field (Refer page number 30 - 35 of the book “optical fiber communication” by
John. M. Senior, 2nd Edition)
(2) Goos – Haenchen shift and (Refer page number 35 - 36 of the book “optical fiber
communication” by John. M. Senior, 2nd Edition)
(3) Mode coupling (Refer page number 43 - 44 of the book “optical fiber communication” by
John. M. Senior, 2nd Edition)
(ii)A multi mode graded index fiber has an acceptance angle in air of 8°. Estimate the
relative refractive index difference between the core axis and the cladding when the
refractive index at the core axis is 1.52 (04)
Formula used Sinθc = n2 / n1 and ∆ = (n1-n2)/ n1
2) (i)Explain with simple ray diagrams (1) the multimode SI fiber and (2) the single mode SI
fiber. Compare the advantages and disadvantages of these two fibers.(10)
Refer page number 37 of the book “optical fiber communication” by Gerd Keiser, 3 rd edition
TMH 2000
(FOR Adv and Disadv please refer notes)
(ii)A single mode step index fiber has core and cladding refractive indices of 1.498 and 1.495
respectively. Determine the core diameter required for the fiber to permit its operation over
the wavelength range 1.48 and 1.60µm. Calculate the new fiber core diameter to enable
single mode transmission at the wave length of 1.3 µm.(06)
(AU Nov/Dec 2009)
FORMULA USED V = (2πa/) (n1 2 – n2 2) ½ ∆ = (n1-n2)/ n1
( V= 2.406)
3) Discuss the mode theory of planar wave-guides also discuss phase velocity and group
velocity? (AU April/May 2004, April/May 2007) **
(Refer page number 26-30 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
4) Discuss the ray theory of transmission. (AU April/May 2004) **
(Refer page number 14-23 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
5) List the advantages of optical fiber communication. ** (AU May/June 2006)
(Refer page number 7-10 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
6) A Step –index fiber has a normalized frequency 26.6 at a 1300 nm wave length. If core
radius is 25 µm find the numerical aperture and mode volume.
(AU Nov/Dec 2006)
 Given V=26.6,λ=1300nm , a=25 μm
2 λ
2 1/2 1300∗26 . 6 −3
NA =(n1 −n 2 ) = V= ∗10 =0 . 22
2 πa 2∗π∗25
−6 2
2∗π∗a2 2∗π∗(25∗10 )
Modevolume= 2 = =2323 . 668
λ (1300∗10−9 )2
7) Explain with a neat block diagram the fundamentals of optical fiber communication.(8) **
(Nov 2008)
(Refer page number 5-7 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
8) Explain with a neat diagram the elements of an optical fiber transmission link.
(AU Nov/Dec’2007,R-2004)
Refer page number 9-10 of the book “Optical fiber communication” by Gerd Keiser, 3 rd edition
TMH 2000.
9) Compare the configuration of different types of fibers. ** (AU Nov/Dec 2005)
(Refer XEROX copy)
10) Briefly explain the evolution of fiber optic system. **
(AU Nov/Dec 2005, Nov/Dec’2007 R-2004)
(Refer page number 1-5 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
11) The relative refractive index difference between the core axis and the cladding of a graded
index fiber is 0.7% when the refractive index at the core axis is 1.45. Estimate values for the
numerical aperture of the fiber along the axis when the index profile is assumed to be
triangular. (AU Apr/May’2008 R-2004)
(Solve Yourself)
12) Calculate the numerical aperture, cutoff parameter and number of modes supported by a
fiber having µ1(core)= 1.54, µ2(cladding)=1.5, core radius 25µm and operating
wavelength1300nm. (May/June 2009)
(Solve Yourself)

13) A multimode step index fiber with a core diameter of 80 µm and a relative refractive index
difference of 1.5% is operating at a wavelength of 0.85 µm. If the core refractive index is
1.48, estimate the normalized frequency for the fiber and number of guided modes.
(AU Nov/Dec’2007R-2004)
(Refer page number 46 of the book “optical fiber communication” by John. M. Senior, 2nd Edition)

14) Discuss the electromagnetic mode theory of optical propagation (AU April/May 2004)
**
(Refer page number 23-25 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)

15) Briefly explain SM Fibers in optical communication ** (May/June 2009)

(Refer page number 56-77 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
Notes: Should cover all headings, apart from that Gaussian approximation (67-73) ** and ESI
(73-77) ** may ask separate 8 mark questions)
16) Discuss the modes in cylindrical fibers. **
(AU Nov/Dec 2005, Nov/Dec’2007 R-2004)
 (Refer page number 36-54 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
Notes: Should cover all headings, apart from that WKB theory (50-55) ** and electric field
configuration and its profile (38-43) may ask separate 8 mark questions)

UNIT II TRANSMISSION CHARACTERISTICS OF OPTICAL FIBERS

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

PART-B ** indicates that give first preference while studying.

1) Explain modal birefringence. ** (AU Nov/Dec 2006)
(Refer page number 136-141 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
2) (i)Describe the linear and non-linear scattering losses in optical fiber.(08) **
(Refer page number 91-96 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
(ii)Silica has an estimated fictive temperature of 1400 K with an isothermal compressibility
of 7x10-11 m2N-1. The refractive index and the photo – elastic coefficient for silica are 1.46 and
0.286 respectively. Determine the theoretical attenuation in dB/km due to the fundamental
Rayleigh scattering in silica at optical wavelengths 850nm, 1310nm and 1550nm. (08)
(AU Nov/Dec 2009)
(Refer page number 92 of the book “optical fiber communication” by John. M. Senior, 2nd Edition)
3) (i)Describe the intermodal pulse broadening in multimode SI and GI fibers and derive the
expressions for their delay difference between the extreme meridional ray and the axis ray
and the rms pulse broadening at the fiber output.(08)
(Refer page number 110-119 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
(ii)The beat length in a single mode fiber is 9 cm when light from an injection laser with a
spectral line width of 1nm and a peak wavelength of 1550 nm is launched into it. Determine
the modal birefringence and estimate the coherence length in this situation. Also, calculate
the difference between the propagation constants for the two orthogonal modes and check
the result. (08) (AU Nov/Dec 2009)
(Refer page number 138 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
4) Write in detail about information carrying capacity and group delay of optical fiber.
Refer page number 105 - 108 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
5) What is meant by wave-guide dispersion? Derive the expression for it. **
(AU Nov/Dec 2004 April/May 2005)
Refer page number 109 - 111 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
6) When the mean optical power launched in to an 8 km length of fiber is 120 µW , the mean
optical power at the fiber output is 3 µW. Determine
 (1) Overall signal attenuation in Db/km and
(2) The overall signal attenuation for a 10 km optical link using the same fiber with splices at
1km intervals each giving an attenuation of 1db. **
(AU Nov/Dec’2007 R-2004)
(Refer page number 87 of the book “optical fiber communication” by John. M. Senior, 2nd Edition)
7) Discuss in detail intra-modal dispersion with relevant expressions and diagrams **
(AU Nov/Dec’2007 R-2004)
(Refer page number 102-110 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition) for Material Dispersion
Refer page number 109 - 111 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000 ) for waveguide Dispersion
8) Discuss various kinds of losses that an optical signal might suffer while propagating through
the fiber. Which is most important one? What is the effect of these losses on light power and
pulse shape? (OR) With the aid of diagrams discuss the various losses occurring in optical
fibers. (OR) Explain with suitable diagrams the different mechanisms that contribute to
attenuation in optical fibers. ** (16)
(AU April/May 2007, Apr/May 2008 R-2004 Nov/Dec’2007 R-2004)
(Refer page number 86-99 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
9) A 6 km optical link consists of multimode step index fiber with a core refractive index of 1.5
and a relative refractive index difference of 1%. Estimate the delay difference between the
slowest and fastest modes at the fiber output and the rms pulse broadening due to
intermodal dispersion on the link. Also derive the expression involved in it. **
(AU Apr/May’2008 R-2004)
(Refer page number 114 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
10) What are the losses on signal attenuation mechanisms in a fiber? Explain in detail. **
(May/June 2009)
(Refer page number 86-99 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
11) Explain the effects over all signal dispersion in optical wave guide. (May/June 2009)**
(Refer page number 121-129 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
12) Explain about material absorption losses in silica glass fibers with relevant diagrams(8)**
(Refer page number 88-91 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
13) Explain non linear phenomena in optical fibers (8) **
(Refer page number 145-149 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
14) Explain the effect of mid-infrared and far-infrared transmission in fiber materials
(Refer page number 99-102 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
15) Write technical note on optical fiber connectors. ** (AU Nov/Dec 2004)
(Refer page number 237-244 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
16) Explain the various types of fiber splicing techniques.***
 (AU Nov/Dec 2004, Apr/May 2007, Apr/May 2005)
(Refer page number 227-237 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
17) Write technical note on optical fiber couplers. *** (AU Nov/Dec 2004)
(Refer page number 249-270 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
18) Explain the various types of fiber alignment and joint losses.***
(AU Nov/Dec 2004, Apr/May 2007, Apr/May 2005)
(Refer page number 212-226 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
19) Write a short note on expanded beam connectors (8)
(Refer page number 244-249 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
UNIT III SOURCES AND DETECTORS
Optical sources: Light Emitting Diodes - LED structures - surface and edge emitters, mono
and hetero structures - internal - quantum efficiency, injection laser diode structures -
comparison of LED and ILD Optical Detectors: PIN Photo detectors, Avalanche photo
diodes, construction,
Characteristics and properties, Comparison of performance, Photo detector noise –Noise
sources, Signal to Noise ratio, Detector response time.
PART-B ** indicates that give first preference while studying.
1) (i)Describe the characteristic of responsivity against wavelength for an ideal silicon
photodiode with the help of its expression. Explain the Direct and Indirect absorption in Si
and Ge.(10) (AU Nov/Dec 2009)
Refer page number 244-248 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
Also refer
(Refer page number 423-425 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
(ii)A photodiode has a quantum efficiency of 73% when photons of energy 1.5x10 -19 J are
incident upon it. Estimate the wavelength at which the photo diode operates and calculate
the incident optical power required to obtain a photocurrent of 2.2 µA when the photo diode
is operating as above.(06) **
(Refer page number 429 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
2) Discuss the requirements of optical detector. (AU Nov/Dec 2004)
(Refer page number 420 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
3) Draw the schematics of PIN photodiode and APD and explain. ***
(AU Nov/Dec 2004, Nov/Dec 2006, Apr/May’2008 R-2004, Nov/Dec’2007 R-2004)
Refer page number 244 - 252 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
4) Given silicon APD has a quantum efficiency of 65% at a wavelength of 900 nm. If 0.5 µw of
optical power reduces a multiplied photo current of 10, find the multiplication factor M.
(AU Nov/Dec 2006)
Refer page number 250 - 251 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
5) A Silicon p-i-n photo diode incorporated in to an optical receiver has a quantum efficiency
of 60% at a wavelength of 0.9 µm. The dark current is 3 n A and load resistance is 4 KΩ.
The incident optical power is 200 n W and the receiver bandwidth is 5 MHz. Determine
(1) Mean square quantum noise current
(2) Mean square dark current and
(3) Mean square thermal noise current at a temperature of 20oC.
(AU Nov/Dec’2007 R-2004) **
Refer page number 256-257 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
6) Discuss the different noise sources and disturbances in the optical pulse detection
mechanism. ** (AU Nov/Dec’2007 R-2004)
Refer page number 253-256 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
7) Explain briefly the three key processes involved in the laser action. Describe for a fabry
perot resonator laser diode, modes and threshold conditions. Obtain its rate equations for
steady state output **
(AU Nov/Dec’2007 R-2004, Apr/May 2007 Apr/May’2008 R-2004)
Refer page number 161-169 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
8) Derive an expression for the internal optical power level generated in LED s. **
(AU Apr/May’2008,R-2004)
(Refer page number 377-384 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
9) Draw and explain the LED structures based Double hetero structure configuration. (OR)
What type of materials is used for optical sources .What are the advantages of double
hetero structure. **
(AU Nov/Dec’2007, R-2004 Apr/May’2008, R-2004)
(Refer page number 384-385 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
10) Discuss the structure of edge emitting and surface emitting LEDs and explain. **
(AU April/May2004, Nov/Dec 2005)
(Refer page number 386-393 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition) &
Refer page number 150-153 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
11) Explain detector response time of photo detector?** (AU Nov/Dec 2009)
Refer page number 257-262 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
12) The quantum efficiency of a particular silicon RAPD IS 80% for the detection of radiation
at a wavelength of 0.9 µm when the incident optical power is 0.5 µW. The output current
from the device (after avalanche gain) is 11µA.Determine the multiplication factor of the
photo diode under these conditions. ** (AU
Apr/May’2008 R-2004)
(Refer page number 454 of the book “optical fiber communication” by John. M. Senior, 2nd Edition)
 13) When 3x1011 photons each with a wavelength of 0.85µm are incident on a photodiode, on
average 1.2x1011 electrons are collected at the terminals of the device. Determine the
quantum efficiency and responsivity of the photo diode at 0.85µm.(6) ** (Nov 2008)
(Refer page number 428 of the book “optical fiber communication” by John. M. Senior, 2nd Edition)
UNIT IV FIBER OPTIC RECEIVER AND MEASUREMENTS
Fundamental receiver operation, Pre amplifiers, Error sources – Receiver Configuration –
Probability of Error – Quantum limit. Fiber Attenuation measurements- Dispersion
measurements – Fiber Refractive index profile measurements – Fiber cut- off Wave length
Measurements – Fiber Numerical Aperture Measurements – Fiber diameter measurements.
PART-B ** indicates that give first preference while studying.
1) Discuss the performance of digital receiver by defining the probability of error?(16) **
(AU April/May 2005,Apr/May 2007 )
Refer page number 282 - 287 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
2) Discuss the about optical receiver operation with neat diagram. (16) **
(AU Nov/Dec 2004, Nov/Dec 2005)
Refer page number 275 - 281 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
3) Explain the error sources in the optical receiver.(8) (AU Nov/Dec 2006 Apr/May’2008 R-
2004) **
Refer page number 277- 279 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
4) What is known as quantum limit? Digital fiber optic link operating at 850 nm requires a
maximum BER of 10-9. Find the minimum incidental optical power P 0 to achieve this BER at
a data rate of 10 Mb/s for a simple binary level signaling scheme.(η=1),(1/τ=B/2). **
(AU Apr/May 2007)
Refer page number 288 of the book “optical fiber communication” by Gerd Keiser, 3 rd edition
TMH 2000
5) Draw and explain the high impedance pre amplifier designs based on BJT and FET.(8)**
(Nov 2008) (AU Apr/May’2008 R-2004)
Refer page number 307-310 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
6) Write a brief notes on transimpedence amplifier.(8) ** (Nov 2008)
Refer page number 310-311 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
7) Discuss with the aid of a suitable diagram the cut-back technique used for the measurement
of the total attenuation in an optical fiber. Indicate the differences in the apparatus utilized
for spectral loss and spot attenuation measurement. (16)**
(Refer page number 779-783 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
8) Briefly outline the principle behind the calorimetric methods used for measurement of
absorption loss and scattering in optical fibers. (16) **
(Refer page number 783-789 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
9) Discuss with the aid of suitable diagrams the measurement of dispersion in optical fibers.
Consider both and frequency domain measurement techniques. (16)**
(Refer page number 789-796 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
10) Compare and contrast the major techniques employed to obtain a measurement of a
refractive index profile for an optical fiber. In particular suggest reason why the refracted
near field method has been adopted as the reference test method by the EIA. (16)**
(Refer page number 796-802 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
11) Compare and contrast two simple techniques used for the measurement of the numerical
aperture of optical fibers. (16)**
(Refer page number 807-810 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
12) Discuss with the aid of suitable diagrams the measurement of fiber diameter (16)**
(Refer page number 810-812 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
13) Discuss with the aid of suitable diagrams the measurement of fiber cut off wavelength
(16)**
(Refer page number 802-807 of the book “optical fiber communication” by John. M. Senior, 2 nd
Edition)
UNIT V OPTICAL NETWORKS
Basic Networks – SONET / SDH – Broadcast – and –select WDM Networks – Wavelength
Routed Networks – Non linear effects on Network performance – Performance of WDM +
EDFA system – Solitons – Optical CDMA – Ultra High Capacity Networks.
PART-B ** indicates that give first preference while studying.
1) Explain the salient features of solitons using relevant expressions and diagrams. ** (16)
(Nov 2008)
Refer page number 505-513 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
2) Give a brief account of SONET. **
(AU Nov/Dec’2007 R-2004 Apr/May’2008 R-2004)
Refer page number 467-477 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
3) Explain various types of topologies in network also explain how these topologies are
performed by passive linear buses and star architectures
Refer page number 459-466 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
4) Explain the working principle of optical CDMA **
Refer page number 514-516 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
5) Write short notes on ultra high capacity network. **
Refer page number 516-519 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
6) Explain how broadcast and select WDM network overcome limitation of SONET network?
Refer page number 477-482 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
7) Explain how wavelength routed networks overcome limitation of broadcast and select WDM
network?
Refer page number 482-488 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
8) Explain various nonlinear effect on optical network **
Refer page number 488-502 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000
(Note: Each heading may ask in 8 marks questions)
9) Write notes on Performance of WDM on EDFA system **
Refer page number 502-505 of the book “optical fiber communication” by Gerd Keiser, 3 rd
edition TMH 2000

Note: For problems please refer notes which I gave for all units.
For pass mark, First concentrate unit 3, 4 & 5