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1997 Course Optical Fiber Communication (Elective II)

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Total No. of Questions : 10] [Total No. of Pages : 3 [3764]-1051 P1840 B.E. (EC) OPTICAL FIBRE COMMUNICATION (404210) (1997 Course) (Elective - II) 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) a) What is the basic principle behind optical transmission? Sketch the structure, index profile and path taken by light ray in SI and GI fibers.[6] b) The refractive index of a core is n1 = 1.48, nclad = 1.46. Under what conditions will light be trapped inside the core? [2] c) Define Numerical aperture and give the expression for the same in both SI and GI fibers. For the plastic fibre, n1 = 1.495, n2 = 1.402. Calculate the acceptance angle and Numerical Aperture. [8] Q2) a) What is meant by group velocity dispersion? What are the causes for intramodal dispersion? Explain how intermodal dispersion is reduced in GI fiber as compared to SI fiber. [8] b) What is meant by leaky modes? How do scattering losses arise? Distinguish Linear and Non-Linear scattering. [8] P.T.O. Q3) a) What are the basic LED configurations being used for fiber optics? An engineer has two Ga 1-x Alx. As LEDS one has a bandgap energy of 1.54ev and other has x = 0.015. i) Find the aluminum mole fraction x and the emission wavelength for the first LED. ii) Find the bandgap energy and the emission wavelength of the other LED. [8] b) Derive Einstein relationship and Threshold condition for Lasing action. Explain in detail the Laser Diode structures and its Radiation Pattern.[8] Q4) a) Enlist and discuss various types of losses that an optical signal might suffer while propagating through fiber. Which is most important one? What is the effect of these losses on optical power and pulse shape? [10] b) Define Excess Loss and Insertion Loss . A coupler has an excess loss of 1dB and 1 : 1 splitting ratio. How much of the input power reaches the two output terminals. [6] Q5) Write short note on the following (Any Three) : [18] a) O.T.D.R. b) Optical pumping. c) Bending losses. d) Eye-Diagram measurement. SECTION - II Q6) a) What is meant by impact ionization in APD? With a neat sketch explain the principle and operation of Avalanche photodiode. [8] b) Discuss in detail the effect of the various noise mechanism in a Photodetector and its signal to noise ratio. [8] Q7) a) Is WDM efficient for multi-wavelength scheme? Discuss the operational principles of WDM highlighting its key features. [8] b) List some of the principle requirements of a good connector design.[4] [3764]-1051 -2- c) List the steps involved in splicing procedure. [4] Q8) a) Explain in detail the rise time budget of a fibre optic point to point link. [8] b) An optical fiber link is designed to operate 8 km length of the link without a repeater. The rise time of different individual components are The rise time of LED source = 5 ns The rise time of p-in photodector = 8 ns Rise time due to modal dispersion = 5 ns/km Rise time due to material dispersion = 1 ns/km Calculate the sytem rise time and maximum bit rate that can be achieved in the link using (i) RZ format (ii) NRZ format. [10] Q9) Explain the use of optical fibers for the following : [16] a) Measurement fiber losses. b) Optical sensor for current measurement. c) Guided weapons system. d) Optical communication for power sector. Q10) a) Discuss in detail Lansing schemes for power coupling improvement. [8] b) A fiber link includes 5 splices at 0.02 dB/splice, four connectors at 0.2 dB/connector, transmitter power at 10dBm, receiver sensitivity of 25dBm. What length of this link will be allowed if a SM fiber with attenuation of 0.3 dB/km is used and the required power margin is 3dB. [8] rrrr [3764]-1051 -3-

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