SPPU Electronics and Telecom Engineering (Semester 8)
Optical Fiber Communication
May 2014
Total marks: --
Total time: --
(1) Assume appropriate data and state your reasons
(2) Marks are given to the right of every question
(3) Draw neat diagrams wherever necessary

Answer any one question from Q1 and Q2
1 (a) A laboratory setup is tuned at 1310 nm, and a 62.5/125 multimode step index fibre with core refractive index 1.48 and clad index of 1.46 is used. Calculate:
i) Critical angle.
ii) Numerical aperture.
iii) Acceptance angle in air.
iv) Normalized frequency.
v) Total number of modes supported by this fibre.
vi) Fraction of power residing in the cladding if the total optical power in the fibre is 1mW
12 M
1 (b) Describe with the aid of a neat diagram the basic principle of total internal reflection that enables the fibre to work as a 'light conduit'.
6 M

2 (a) Explain: MMSI and SMSI type of optical fibres. Determine the cutoff wavelength for a step index fibre to exhibit single - mode operation when the core index and radius are 1.46 and 4.5 m, respectively, with the relative index difference being 0.25%.
12 M
2 (b) Explain what is meant by a graded index optical fibre by giving an expression for refractive index profile n(r). State the major advantage of this type of fibre with regard to multimode propagation.
6 M

Answer any one question from Q3 and Q4
3 (a) Explain the various attenuation mechanisms in optical fibre. State the various techniques deployed for the measurement of attenuation in optical fibres.
8 M
3 (b) Consider a step index optical fibre link 5 km long with core index 1.49 and relative index difference Δ = 1%
i) Calculate the delay difference at the fibre end between slowest and fastest modes.
ii) Calculate the rms pulse broadening caused by intermodal dispersion.
iii) Calculate the maximum bit rate B T that can be transmitted over the fibre without significant errors.
iv) Assuming the maximum bit rate equals the bandwidth, what is the bandwidth distance product of this fibre.
8 M

4 (a) A certain optical fibre has an attenuation of 0.6 dB/km at 1300 nm and 0.3 dB/km at 1550 nm. Suppose the following two optical signals are launched simultaneously into the fibre: an optical power of 150 W at 1300 nm and an optical power of 100 W at 1550 nm. What are the power levels in W of these two signals at
i) 8km and
20 km.
8 M
4 (b) Explain:
i) Dispersion shifted fibres and
ii) Dispersion flattened fibres.
8 M

Answer any one question from Q5 and Q6
5 (a) State and explain the various misalignments and mismatch losses that occur while coupling optical fibres.
8 M
5 (b) Explain the concept of intensity modulation of LEDs and Laser diodes using their I-P characteristics.
8 M

6 (a) Radiative and non radiative recombination lifetimes of the minority carriers in the active region of a double-heterojunction LED are 60ns and 100ns respectively. Determine the total carrier recombination lifetime and the power internally generated within the device when the peak emission wavelength is 1.31μ m at a drive current of 40 mA.
8 M
6 (b) Explain the wavelength and material considerations with reference to optical sources used for communication. Also compare the performance of laser diode versus light emitting diode.
8 M

Answer any one question from Q7 and Q8
7 (a) Explain the working of PIN photo-detector with relevant diagrams.
6 M
7 (b) State the various merits and demerits of p-n, p-i-n and avalanche photo detectors.
6 M
7 (c) An InGaAs Avalanche photo detector has quantum efficiency 90% at a wavelength of 1310nm. If an incident optical powered of 0.5 W produces a multiplied photo current of 15 A. Calculate responsivity and the multiplication factor.
6 M

8 (a) Write short note on Receiver noise.
6 M
8 (b) When photons of energy 1.5 ? 10-19J are incident on a photo diode, its quantum efficiency is found to be 70%. Find out the wavelength of operation, incident optical power and responsivity when the photo current in the diode is 4 μ A.
6 M
8 (c) Draw and explain the nature of responsivity curves of optical detectors.
6 M

Answer any one question from Q9 and Q10
9 (a) Describe the system design considerations involved in establishing point-to-point optical fibre link.
8 M
9 (b) Following components are available for 5 Km link operating at 20Mb/sec:

i) GaAs LED operating at 850 nm and capable of coupling 1mW into fibre.
ii) 10 sections of cable each of which is 500 m long, has 4 dB/km attenuation and has connectors on both ends.
iii) Connector loss of 2 dB/connector.
iv) PIN photo diode receiver (-45 dBm sensitivity).
v) Avalanche photo diode receiver (-56 dBm sensitivity).
Perform Optical power budget and state which receiver should be used if 6-dB system operating margin is required.
8 M

10 (a) Explain the concept of link power budget and rise time budget in optical fibre communication system.
8 M
10 (b) Write short note on multichannel transmission techniques.
8 M

Answer any one question from Q11 and Q12
11 (a) Explain: Principle of operation of erbium doped fibre amplifier.
8 M
11 (b) Write a short note on WDM couplers/splitters.
8 M

Write short notes on (Any two):
12 (a) WDM Technique.
8 M
12 (b) Optical Amplifier.
8 M
12 (c) WDM Components.
8 M

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