Wednesday, December 9, 2009


Seventh Semester

(Regulation 2004)

Electronics and Communication Engineering


(Common to B.E.(Part-time)Sixth semester Regulation 2005)

Time: Three hours Maximum: 100

Answer All Questions

PART-A-(10*2=20 marks)

1. What are the advantages and disadvantages of the ray optics theory?

2. A typical relative refractive index difference for an optical fiber designed for long

distance transmission is 1%.Estimate the numerical aperture for the fiber when the core

index is 1.47.

3. Define group delay.

4. A multimode graded index fiber exhibits total pulse broadening of 0.1 us over a

distance of 15 km. Estimate the maximum possible bandwidth on the link assuming RZ

coding without intersymbol interference.

5. Distinguish direct and indirect band gap materials.

6. An LED has radiative and nonradiative recombination times of 30 and 100 ns

respectively. Determine the internal quantum efficiency.

7. Define responsivity of a photo detector.

8. Compare the performance of APD and PIN diode.

9. What is the significance of rise time budget?

10. Compare doped fiber amplifiers and conventional repeaters.

PART-B---(5*16=80 marks)

11. a) (i) Explain with neat block diagram the fundamentals of optical fiber


(ii) Discuss the mode theory of circular wave guides (8)


b) (i) Discuss briefly about linearly polarized modes. (6)

(ii) Draw the structures of single and multimode step index fibers and graded index fiber

with typical dimensions.


(iii) Mention the advantages of optical fiber communication systems (4)

12. a) (i) Explain the scattering and the bending losses that occur in an optical fiber with

relevant diagrams and expressions (8)

(ii) Discuss polarization mode dispersion and its limitations (8)


b) (i) Discuss material and waveguide dispersion mechanisms with necessary

mathematical expressions (8)

(ii) Write brief note on pulse broadening in graded index fibers. (8)

13. a) (i) Draw and explain the different structures used to achieve carrier and optical

confinement in laser diodes


(ii) Discuss the effects of temperature on the performance of a laser diode (8)

(iii) Give a brief account of the modulation of an LED (4)


b) (i) Derive expressions for the power coupled from a surface emitting LED into step

index and graded index fibers (10)

(ii) Explain the mechanical misalignments that can occur between two joined fibers with

necessary diagrams. (6)

14. a) (i) Discuss with necessary expressions the different types of noise that affect the

performance of a photo detector. (10)

(ii) When 3´1011photons each with a wavelength of 0.85 mm are incident on a

photodiode, on an average 1.2´1011 electrons are collected at the terminals of the device.

Determine the quantum efficiency and responsivity of the photodiode at 0.85 mm (6)


b) (i) Draw and explain the high impedance preamplifier designs based on BJT and FET


(ii) Write brief note on Tran impedance amplifier (8)

15. a) (i) Explain the salient features of solitons using relevant expressions and diagrams



b) (i) Discuss the concept of WDM with neat diagram (6)

(ii) Give a brief account of the different types of losses to be considered in the design of

an optical link. (6)

(iii) Draw and explain the basic format of an STS-N SONET frame (4)

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