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Wednesday, 4 December 2013

Fiber-optic communication systems-Model question paper for B.E/B.Tech

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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