VIII Semester B.E. (E & C) Degree Examination, June/July 2013 (2K6 Scheme) EC 802 – OPTICAL COMMUNICATION Time : 3 Hours
Max. Marks : 100 Note : i) Answer any 5 questions, selecting atleast two questions from each Part. ii) Missing data if any may suitably be assumed. PART – A
1. a) Compare optical communication with satellite communication.
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b) Define ‘Numerical aperture’. Deduce an expression for the same.
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c) A step index fiber has a normalized frequency V = 28 at a 1550 nm wave length. If the core radius is 25 μ m, find the ‘Numerical Aperture’. Also calculate the number of modes ‘M’ entering the fiber.
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2. a) With a neat schematic diagram explain the fabrication process of a fiber using ‘Modified CVD method.
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b) List the mechanical properties of a fiber.
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c) With the help of a cross-sectional view, explain the structure of a single core optical fiber.
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3. a) Why silicon is not used to fabricate a photo source ? Explain.
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b) With the help of Energy band-diagrams, differentiate spontaneous emission from stimulated emission.
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c) With the help of a figure, explain ‘distributed feedback (DFB) semiconductor LASER.
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4. a) With the help of energy band diagrams explain the principle of operation of an “Avalanche photodiode’ (APD) as a photo dector. 10 b) Define quantum efficiency of a PD. Deduce an expression for the same.
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c) Calculate the responsivity of a p-i-n photodiode at 1.3 μm and 1.55 μm if the quantum efficiency is 70%. For which wave length photodiode has move responsivity and why ?
P.T.O.
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*JE805*
JE – 805 PART – B
5. a) Explain the working principle of a photo transmitter giving the details of a Laser diode driver circuit.
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b) Explain any two lensing schemes used for launching the optical signal on to the fiber.
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c) Briefly discuss the different ‘optical signal degradation’ mechanisms.
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6. a) Distinguish clearly the differences between intra and intermodal dispersions.
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b) A multimode fiber with a 50- μm core diameter is designed to limit the intermodal dispersion to 10 ns/km. What is the numerical aperture of this fiber ? What is the limiting bit rate for transmission over 10 kms at 0.88 μm ? Refractive index of the cladding is 1.45.
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c) Explain a digital optical receiver with the help of a block diagram.
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7. a) What is the requirement of the design of the front end of an optical receiver ? Explain the different front ends that are commonly used . (2+9) b) An optical link has a distance of 100 kms. A LASER launches an average optical power of 15 dBM into the fiber. ‘ α ’ of the cable is 0.1 dB/km. There is a connector each at the transmitter and receiver with 0.1dB loss. There is a splice for every 20 kms with a loss of 0.2 dB. The APD used at the receiver has a sensitivity of 40 dBm. Find the system power margin. Explain the power budget with the help of a graph. 8. a) What is WDM ? Explain its 4 salient features. b) What is subcarrier multiplexing ? Explain both the transmitter and receiver used for this type of optical link. c) What is the concept of All Optical Network (AON) ?
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