Code: A-24 Subject: OPTO ELECTRONICS AND
OPTICAL COMMUNICATION
Time: 3 Hours Max. Marks: 100
NOTE: There are 11 Questions in all.
Question 1 is compulsory and carries 16 marks.
Answer to Q. 1. must be written in the space provided for it in the answer book
supplied and nowhere else.
Answer any THREE Questions each from Part I and Part
II. Each of these questions carries 14 marks.
Any required data not explicitly given, may be
suitably assumed and stated.
Q.1 Choose the correct or best alternative in the following: (2x8)
a. One of the following is used as the principle photo detector in optical fiber link.
(A) Avalanche diode. (B) Varactor diode.
(C) Semiconductor PIN diode. (D) Zener diode.
b. Cladding helps in providing
(A) mechanical strength
(B) protects core from absorbing surface contaminants
(C) reduces scattering
(D) all of above
c. Single mode fibers do not suffer from problems of __________
(A) intermodal
dispersion
(B) connecting
similar fibers
(C) efficient launching of optical power
(D) fiber birefringence
d. One of the following is not a technique for fabrication of glass optical wave guide
(A) Vapor phase oxidation method (B) Direct melt method
(C) Epi-taxial layering method (D) Chemical vapor deposition
technique.
e. The transmission losses per km of optical fiber cables are about
(A) 10
dB. (B)
1 dB.
(C) 6 dB. (D) 0.1 dB.
f. What is the
fiber-to-fiber coupling loss (assuming size and NA mismatch) for two fibers of
NA 0.2 and 0.1 while core diameters of 12 
and 6 
.
(A) 12.04
dB. (B) 6.02 dB.
(C) 3.01 dB. (D) 9.03 dB.
g. Identify which of the following is/are involved in lasing action.
(A)
photon absorption. (B)
spontaneous emission.
(C)
stimulated emission. (D)
all of above.
h. One of the following is not considered for launching optical power into a fiber
(A) numerical
aperture. (B) core size.
(C) optical receiver. (D) refractive index profile.
PART I
Answer any THREE Questions. Each question carries 14 marks.
Q.2 a. Give an account of optical fiber modes. What are hybrid modes? (5)
b. Explain
the basic optical principles used to transmit optical fiber power along the
fiber. (5)
c. A single mode fiber has a beat length of 8 cm at 1300 nm/, what is
the modal birefringence of the fiber. (4)
Q.3 a. List the basic methods of fabricating glass
optical wave-guides. Explain plasma
activated CVD method. (10)
b. Consider a 30 Km long optical fiber that has an attenuation of 0.8
dB/Km at 1300 nm. Find the output
optical power for an input launched power 
. (4)
Q.4 a. Explain what is intramodal dispersion and
discuss the two main causes for the same. (6)
b. Why
are LEDs and ILDs suitable for fiber transmission system? Draw and explain the edge emitter LED
configuration. (8)
Q.5 a. Describe the basic detection process in a silicon reach through APD (RAPD). How does it differ from the p-i-n photodiode? Given that the following measurements were taken for an APD, calculate the multiplication factor for the device.
Received optical power at 
.
Corresponding
output photocurrent = 
.
(after avalanche gain)
Quantum
efficiency at 
. (7+7)
Q.6 a. What are the impediments in the
implementation of coherent optical communication as compared to RF
systems? What are the underlying
principles of WDM technique? What are
its advantages? How is it different
from FDM techniques? (8)
b. Consider the low loss region of a silica fiber communication
system to be in the region of 1520-1580 nm.
Find the number of channels which can be multiplexed by FDM, if the
channel spacing is required to be 75 GHz.
If each channel carries a 2.5
Gbps signal, calculate the effective bitrate-distance product taking a power
margin of 30 dB and fiber loss of 2.5 dB/Km. (6)
PART II
Answer any THREE Questions. Each question carries 14 marks.
Q.7 a. Define and explain the meaning of
responsivity and quantum efficiency of a photo diode, and derive expressions
for the same. (5)
b. Consider a silicon PIN and an APD to detect light at a wavelength
of 0.85 
. When the incident
light intensity is 0.1 mW / 
, the photocurrent generated by the PIN and APD are 10 
and 500 
respectively. The
detector area is 0.2 mm2. Find the
quantum efficiency and the avalanche multiplication factor. (9)
Q.8 a. Define the terms-numerical aperture, critical
angle, total internal reflection, propagation modes and micro bends in the
context of an optical fiber. (5)
b. Define
dispersion. What is material
dispersion? State the expression for
material dispersion. (5)
c. A graded index optical fiber is being modelled using the Gaussian
function for the mode field. Determine
its effective radius if the V-number is 2 and actual radius is 4 
. (4)
Q.9 a. What is AGC and Equalisation? With the help of a circuit diagram explain
their importance in communications. (6)
b. Discuss in detail, the power budget for optical fiber links. (8)
Q.10 a. Describe
with neat sketches the WDM Network Architecture used in optical communication. (7)
b. Discuss the various noise effects on system performance. (7)
Q.11 Write
short notes on the following:
(i)
Optical fiber splicing techniques. (5)
(ii)
Modulation of Lasers and LEDs. (4)
(iii)
Receiver performance calculations. (5)