(a) Prove the 3-dB advantage of homodyne detection by showing that the average electrical power generated by a coherent receiver is twice as large for homodyne detection as for heterodyne detection under identical operating conditions. (4) (b)Indicate the major problems that can be encountered in realizing the coherent homodyne detection scheme. How these problems can be overcome? (3) (c) The incoming signal power to an ASK optical heterodyne receiver operating at its noise limit is 1.28 nW for a received SNR of 9 dB. Determine the transmission wavelength of the ASK system if the quantum efficiency of the photodetector is 75% at this wavelength and the transmission bandwidth is 400 MHz. (5) 7 (a) Briefly outline the possible techniques to overcome the temperature dependence of lasers while designing the laser driving circuit. (3) (b) Illustrate the major considerations in the design of digital drive circuits for an LED source and laser source? (4) (c) A 1.3-?m digital receiver is operating at 200 Mb/s and has an effective noise bandwidth of 60 MHz. The p–i–n photodiode has negligible dark current and 100 % quantum efficiency. The load resistance is 100 ? and the amplifier noise figure is 3 dB. Calculate the receiver sensitivity corresponding to a BER of 10?9 5 (a) Briefly indicate any two methods to provide the single-frequency operation in the semiconductor lasers? (4) (b) The threshold current of a semiconductor laser doubles when the operating temperature is increased by 50 oC. What is the characteristic temperature of the laser? (2)(c) Why semiconductors-based photo detectors preferred to other types of photo detectors. (3) (d) What is the role of intrinsic layer in PIN photodiodes? Comment on what should be the thickness of this layer.
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