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2003 Course Optical & Microwave Communication

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Total No. of Questions : 12] P991 [Total No. of Pages : 4 [3664]-205 B.E. (E & T/C) OPTICAL AND MICROWAVE COMMUNICATION (2003 Course) Time : 3 Hours] [Max. Marks : 100 Instructions to the candidates: 1) Attempt Q. 1 or Q. 2, Q. 3 or Q. 4, Q. 5 or Q. 6 from section-I and Q. 7 or Q. 8, Q. 9 or Q. 10, Q. 11 or Q. 12 from section-II. 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) Explain in detail the structure of edge emitting LED. [6] b) A GaAs planar LED emitting at a wavelength of 0.85 m has an internal quantum efficiency of 60% when passing a forward current of 20 mAs-1. Estimate the optical power emitted by the device into air, and hence determine the external power efficiency if the potential difference across the device is 1V. It may be assumed that the transmission factor at the GaAs - Air interface is 0.68 and that the refractive index of GaAs is 3.6. [6] c) Explain the working of avalanche photodiode. Which are three factors that limits the response time of the device? [6] OR Q2) a) What are the advantages of semiconductor injection laser over other semiconductor sources that may be used for optical communications?[6] b) The rediative and nonradiative recombination lifetimes of the minority carriers in the active region of a double heterojunction LED are 50 ns and 110 ns respectively. Determine the total carrier recombination lifetime and the power internally generated within the device when the peak emission wavelength is 0.87 m at a drive current of 40 MA. [6] c) With the help of neat diagram explain the construction and working of p-i-n photodiode. [6] P.T.O. Q3) a) Explain material absorption losses in silica glass fibers. [6] b) Compare MMSIF, SMSIF, MMGIF with reference to the pulse broadening due to intermodal dispersion. Also comment on bandwidthlength product for these three fibers. [6] c) A 5km optical link consists of multimode step index fiber with a core refractive index of 1.5 and a relative refractive index difference of 1%. Estimate : [4] i) The delay difference between the slowest and fastest modes at the fiber output. ii) The rms pulse broadening due to intermodal dispersion on the link. iii) The maximum bit rate that may be obtained without substantial errors on the link assuming only intermodal dispersion. iv) The bandwidth-length product corresponding to (iii). OR Q4) a) What is a fiber splice? Explain any two splicing techniques. b) Explain in detail optical timedomain reflectometer. [6] [6] c) A step index fiber has a core refractive index of 1.5 and a core diameter of 50 m. The fiber is jointed with a lateral mis alignment between the core axes of 5 m. Estimate the insertion loss at the joint due to the lateral mis alignment assuming a uniform distribution of power between all guided modes when : [4] i) ii) There is a small air gap at the joint. The joint is considered index matched. Q5) a) What are the key system requirements that are needed in analyzing a point-to-point link? Explain the point-to-point link design with reference to the choice of components and their associated characteristics. [8] b) Explain the operational principles of WDM. [8] OR Q6) a) What are the three classes of optical amplifiers and explain their applications. [8] b) Explain 2 2 fiber coupler in detail. [8] SECTION - II Q7) a) Express Maxwell s Curl equations for E and H fields, in free space, in scalar form in a rectangular co-ordinate system. [4] b) What is critical about the number KC for an e.m.wave to travel in a rectangular waveguide? [4] [3664]-205 2 c) A section of X-band waveguide with dimensions a = 2 .286cm, b = 1.016cm. has perfectly conducting walls and is filled with a lossy dielectric ( d = 367.5 s / m, r = 2.1, r = 1). Find the attenuation factor in dB/m, for the dominant mode of propagation at a frequency of 9GHz. [10] OR Q8) a) Write the S-matrix of an N-port Network and state how to determine a [6] specific element sij of the [S] matrix? b) A two-port network has the following scattering matrix : 0.15 0o 0.85 45o [S] = o 0.2 0o 0.85 45 Determine if the network is reciprocal and lossless. If the port two is terminated with a matched load, what is the returnloss seen at port 1?[6] c) Draw a neat diagram of a co-axial to waveguide transition and explain its principle of working. [6] Q9) a) Write down an S-matrix of a magic tee. [4] b) Define the following parameters which are used to characterize a directional coupler : [6] i) Coupling. ii) Directivity. iii) Isolation. c) For a directional coupler the incident power is 350 mwatt. Calculate the power in the main arm and auxiliary arm. The coupling factor is 20dB.[6] OR Q10) a) Explain the principle of working of a Reflex-Klystron. Where it is used. [8] b) An X-band cylindrical magnetron has the following operating parameters : [8] Anode voltage Beam current Max. Flux-density Cathode radius Anode radius Vo = 32 kV. Io = 84 A. Bo = 0.01 wb/m2. a = 6 cms. b = 12 cms. [Given data : e/m = 1.759 1011]. [3664]-205 3 Compute : i) The cyclotron angular frequency. ii) The cutoff voltage for a fixed Bo. iii) The cutoff magnetic flux density for a fixed Vo. Q11) a) Describe with constructional details the performance characteristics of a microwave transistor. Also give its power-frequency limitations. [8] b) Explain with the help of block-diagram the working of a typical wireless microwave link for long-distance communication. [8] OR Q12) Describe the salient features and applications of the following microwave devices : [16] a) TWT - Amplifier. b) PIN - diode. c) Microstrip line. d) Gunn - diode. xxxx [3664]-205 4

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