1(a) Write a note on
fiber birefringence. (4 Marks)
(b) Differentiate
between stimulated Brillouin scattering and stimulated Raman scattering. (8
Marks)
(c) What is the
necessity of cabling of fibers? (4 Marks)
(d) Assume that a
digital communication system can be operated at a bit rate of up to 1% of the
carrier frequency. How many audio channels at 64 kb/s can be transmitted over a
microwave carrier at 5 GHz and an optical carrier at 1.55 mm.
(4 Marks)
2 (a) Explain the
double heterostructure geometry of Burrus type LED. (8 Marks)
(b) Write a note on
vertical-cavity surface-emitting lasers. (4 Marks)
(c) Explain the terms
extinction ratio and modulation bandwidth. (4 Marks)
(d) The threshold
current of a semiconductor laser doubles when the operating temperature is
increased by 500C. What is the characteristic temperature of the laser? (4
Marks)
3 (a) Explain the
working of avalanche photodiodes. Mention some applications. (8 Marks)
(b)Explain in brief
different noise mechanisms in an optical receiver. (4 Marks)
(c) Explain the term
timing jitter. (4 Marks)
(d) Determine the
responsivity of a p-i-n photodiode at 1.3 and 1.55mm
if the quantum efficiency is 80%. Why is the photodiode more responsive at 1.55mm?
(4 Marks)
4 (a) What are
loss-limited lightwave systems and dispersion-limited lightwave systems. (6
Marks)
(b) Prove that for a
Gaussian optical pulse, the rise time Tr and the 3-dB optical
bandwidth Δf are related by Tr Δf = 0.316. (6 Marks)
(c)Write a note on
prechirp technique. (4 Marks)
(d) What is the
dispersion-limited transmission distance for a 1.55 mm
lightwave system making use of direct modulation at 10 Gb/s? Assume that
frequency chirping broadens the Gaussian-shape pulse spectrum by a factor of 6
from its transform-limited width. Use D=17 ps/(km-nm) for fiber dispersion. (4
Marks)
5 (a) Write a note on
pulse amplification in semiconductor optical amplifiers. (4 Marks)
(b) Explain how
stimulated Raman scattering is used in Raman amplifiers. (6 Marks)
(c) Write a note on
distributed EDFAs. (6 Marks)
(d) The Lorentzian gain
profile of an optical amplifier has a FWHM of 1 THz. Calculate the amplifier
bandwidths when it is operated to provide 20- and 30-dB gain. Gain saturation
is neglected. (4 Marks)
6 (a) What is
wavelength division multiplexing? (4 Marks)
(b) Differentiate
between direct-sequence encoding and spectral encoding. (6 Marks)
(c)Design an add-drop
filter by using a single fiber coupler and two fiber gratings. (6 Marks)
(d) Explain how
stimulated Raman scattering can cause crosstalk in multichannel lightwave
systems. (4 Marks)
7 (a) What are dark
solitons? (4 Marks)
(b) Explain lumped and
distributed amplification schemes for compensation of fiber losses in soliton
communication systems. (6 Marks)
(c) Write a note on
system design issues in high speed soliton systems. (6 Marks)
(d) What is amplified
spontaneous emission? (4 Marks)
8 (a) Explain homodyne
and heterodyne detection techniques. (6 Marks)
(b) What are modulation
formats? Draw ASK, PSK and FSK modulation formats for a specific bit pattern.
(6 Marks)
(c)Draw Bit-error-rate
curves for various modulation formats. (4 Marks)
(d) A 1.55 mm
DFB laser is used for the FSK modulation at 100 MB/s with a tone spacing of 300
MHz. The modulation efficiency is 500 MHz/mA and the differential quantum
efficiency equals 50% at the bias level of 3 mW. Estimate the power change
associated with FSK by assuming that the two facets emit equal powers. (4
Marks)
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