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1997 Course Fiber Optic Communication (Elective II)

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Total No. of Questions : 10] P1271 [Total No. of Pages : 4 [3864]-37 B.E. (E & TC) FIBER OPTIC COMMUNICATION (1997 Course) (404191) (Elective - II) Time : 3 Hours] [Max. Marks : 100 Instructions to the candidates: 1) 2) 3) 4) Solve section-I and section-II separately. Solve any three questions from section-I and any three questions from section-II. Assume suitable data if necessary. Figures to the right indicate full marks. SECTION - I Q1) State whether the following statements are true or false and give reasons for the same (any three) : [18] a) Si cannot be used for optic transmission system components in the third window of transmission. b) Single mode fibers are most widely used fiber types. c) Optical Communication is feasible only in the IR range. d) Fiber Optic Communication is restricted to only a few transmission windows. Q2) a) Draw the spectral attenuation curve of optical fiber and mention the different types of loss mechanisms. Explain how these losses can be minimized. [8] b) Determine the cutoff wavelength for the single mode operation of an optical waveguide having radius 8 m, core refractive index 1.47 and fractional index difference of 0.002. [8] Q3) a) Describe in detail the various modes of propagation in optical fibers.[8] b) Explain the difference between : LASER and LED. [8] Q4) a) A silicon p-i-n photodiode has a quantum efficiency of 85% at the operating wavelength of 0.85 m. Calculate : [8] i) ii) Its responsivity at 0.85 m, The received optical power if the mean photocurrent is 5 A and the received photons at this wavelength. P.T.O. b) Explain the factors that determine the response time of a photodiode. Draw the equivalent circuit of a simple photo diode receiver and find out its bandwidth in terms of circuit parameters. [8] Q5) Write short notes on : [16] a) E/O modulation Techniques. b) DH structure of LED. SECTION - II Q6) Answer the following questions (any three) : [18] a) State and explain the major elements of optical fiber transmission link. Comment on the selection of each of these elements. b) State and explain the various methods for measurement of attenuation in fiber. Comment on the merits and demerits of each of these methods. c) Explain the homodyne and heterodyne detection schemes used in optical communication. d) A 32 32 port multimode fiber transmission star coupler has 1mW of optical power launched into a single input port. The average measured optical power at each output port is 14 W. Calculate total loss incurred by the star coupler and average insertion loss through the device. Q7) a) An analog optical fiber link employing D-IM has a p-i-n photodiode receiver in which thermal noise is dominant. [10] The system components have the following characteristics and operating conditions : p-i-n photodiode with quantum efficiency = 60% Effective load impedance for photodiode = 50 K Preamplifier noise figure = 6dB Operating wavelength = 1 m Operating temperature = 300oK Receiver post detection Bandwidth = 10 MHz Modulation index = 0.5 Estimate the required average incident power at the receiver in order to maintain an SNR, defined in terms of the mean square signal current to mean square noise current of 45 dB. [3864]-37 2 b) What are the necessary requirements to be fulfilled by the material being used for manufacturing optical fibers? How can the refractive index of glass be varied to achieve different properties of fiber? Describe any one method of fiber fabrication technique. [6] Q8) a) Explain the following terms used in fiber optic domain : [8] i) C/N ratio. ii) RIN. b) State the principles of good connector design. State the different types of optical fiber connectors. [8] Q9) a) Find the optical gain at threshold of a LD having following parametric values : [8] Reflectivities at both ends = 32% Absorption coefficient of material = 10cm 1 Length = 500 m. What is the effect observed if the length is reduced down to 100 m? b) Choose the correct alternative from the multiple options for each of the following : [8] i) Which of the following cables will have the highest launch power capability? 1) 50/125/0.2 2) 85/125/0.275 3) 62.5/125/0.275 4) 100/140/0.3 ii) Equilibrium Mode Distribution is best described by which statement? 1) 70% of the core diameter and 70% of the fiber NA should be filled with light. 2) 70% of the fiber diameter and 70% of the cone of acceptance should be filled with light. 3) 70% of the input light should be measured at the output. 4) 70% of the unwanted wavelengths should be attenuated by the fiber. [3864]-37 3 iii) iv) The higher the index number : 1) The higher the speed of light. 2) The lower the speed of light. 3) Has no effect on the speed of light. 4) The shorter the wavelength of propagation. The term dispersion describes the process of : 1) Separating light into its component frequencies. 2) Reflecting light from a smooth surface. 3) Absorption of light by an uneven rough surface. 4) Light scattering. Q10) Write notes on (any two) : [16] a) Optical Amplifiers. b) Wavelength Division Multiplexing. c) Non Linear optical effects. xxxx [3864]-37 4

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