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1997 Course Fiber Optic Communication

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Total No. of Questions : 10] P1339 [Total No. of Pages : 3 [3664]-36 B.E. (E & TC) FIBER OPTIC COMMUNICATION (1997 Course) (404191) Time : 3 Hours] [Max. Marks : 100 Instructions to the candidates: 1) Answer any 3 questions from each Section. 2) Answers to the two sections should be written in separate books. 3) Neat diagrams must be drawn wherever necessary. 4) Figures to the right indicate full marks. 5) Use of logarithmic tables, slide rule, Mollier charts, electronic pocket calculator and steam tables is allowed. 6) Assume suitable data, if necessary. SECTION - I Q1) Give reasons for the following: [16] a) The manufacturer specifies RL when characterizing the bandwidth of a photodiode. b) Optical fiber cable manufacturer specify bending losses in dB but not in dB/km. c) Laser diodes present more problems to the circuit designer than LED s. d) Light from usual light sources is in co-herent. Q2) What are the requirements of: [16] a) Sources for optical communication. b) Good connector design. c) Detectors for optical communication. d) Optical fibers for communication. P.T.O. Q3) a) Prove that f3dB(electrical) = 0.707 f3dB (optical). [8] b) A GaAs laser diode has a 450 m cavity length, which has an effective absorption coefficient of 10/cm. For uncoated facets the reflectivities are 0.32 at each end. What is the optical gain at lasing threshold? If internal quantum efficiency is 0.60% what is the external quantum efficiency. If by addition of a coating the reflectivity is increased, what is the effect on the optical gain at lasing threshold and the external quantum efficiency. [8] Q4) a) Explain the various fundamental structures of confining optical waves in the lateral direction for laser light. [8] b) A single mode step index fiber has n1 = 1.447 and n2 = 1.442; operating wavelength is 1300 nm. If the core diameter is 7200 nm, confirm that, the fiber will permit single mode operation. Estimate the range of wavelengths over which this will occur. If core diameter is doubled will it support single mode operationExplain. [8] Q5) a) A standard single mode fiber offers per km attenuation of i) 10 dB at 600 nm ii) 3 dB at 800 nm iii) 1.8 dB at 900 nm iv) 0.7 dB at 1100 nm v) 0.55 dB at 1250 nm vi) 0.5 dB at 1300 nm vii) 0.55 dB at 1350 nm viii) 1.8 dB at 1400 nm ix) 0.35 dB at 1500 nm x) 0.18 dB at 1550 nm xi) 0.35 dB at 1650 nm xii) 2.0 dB at 1800 nm [10] at wavelength given above. Plot the graph and mark the three windows of optical transmission. Comment on the attenuation-wavelength relation. State the reasons for higher attenuation at wavelengths of 600 nm, 1400 nm and 1800 nm. b) Compute the allowed misalignment for a multimode step index fiber if each type of error is allowed to contribute 0.25 dB of loss. The radius [8] of the cable is 50 m and NA is 0.24. [3664]-36 -2- SECTION - II Q6) Explain in brief: a) Operating principle of p-i-n photodiode. b) Optical Directional Coupler. c) EDFA d) Photonic switching. [16] Q7) For a long haul single mode optical fiber system operating at 1300 nm: [16] Mean power launched from laser is 500 W. Cable fiber loss is 0.4 dB/km. Splice loss is 0.1 dB/km. Connector loss at transmitter & receiver is 1 dB each. Mean power required at APD is 55 dBm when operating at 35 Mbits/sec (BER = 10 9). a) Assuming no dispersion equalization penalty. b) Assuming 1.5 dB dispersion equalization penalty. Estimate the maximum possible link length without repeaters at 35 Mbits/ sec (BER=10 9) Q8) a) Explain the various methods of measurement of attenuation of optical fiber. Comment on the accuracy of the result in each case. [8] b) Explain Eye pattern in detail. [8] Q9) a) What is co-herent optical detection? How many kinds of co-herent detection schemes may be designed? Explain the advantages of coherent detection system over direct detection system. [8] b) Discuss briefly the possible sources of noise in optical fiber receivers. [8] Q10) Write short notes on (Any 3): a) Splicing Techniques. b) Transimpedance Amplifier. c) WDM. d) Role of cladding in optical fibers. rrr [3664]-36 -3- [18]

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