Optical Communication & Networks - May 2013

Electronics & Telecomm. (Semester 7)

TOTAL MARKS: 80
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any three from the remaining questions.
(3) Assume data if required.
(4) Figures to the right indicate full marks. 1(a) Discuss the basic block diagram of optical communication system.(5 marks) 1(b) Draw the refractive index profile for the step index and graded index fiber. For each type give typical core and cladding diameters.(5 marks) 1(c) Differentiate between spontaneous and stimulated emissions.(5 marks) 1(d) Derive expression for the responsivity of an intrinsic photodetector in terms of quantum efficiency and wavelength.(5 marks) 2(a) Draw refractive index profile of a graded index fiber and show with neat diagram transmission of light through this fiber. Explain how GRIN fiber has transmission bit rate much higher than multimode step index fiber.(10 marks) 2(b) Find the core radius necessary for single mode operation at 820nm of step index fiber with n1 = 1.482 and n2 = 1.474. What is the numerical aperture and maximum acceptance angle of this fiber ? Calculate the corresponding solid angle.(10 marks) 3(a) List the important factors responsible for power loss in optical fiber. Explain each factor briefly.(10 marks) 3(b) Explain intermodal and intermodal dispersion. How does dispersion affect the transmission bandwidth of optical fibers.(10 marks) 4(a) What is the basic principle on which optical sources work ? With the help of a LED structure explain its working.(10 marks) 4(b) Draw the structure of Avalanche Photo Diode (APD) along with the electric field profile that exist in the various regions of APD structure. Explain the working.(10 marks) 5(a) Explain modified chemical vapour deposition (MCVD) method of fiber fabrication in detail.(10 marks) 5(b) Discuss a popular non-destructive technique for attenuation measurement.(10 marks) 6(a) Describe two methods of splicing individual fibers together. What are the advantages and disadvantages of each method ?(10 marks) 6(b) What are the desirable requirements of a good fiber optic connector ? What are the lensing schemes for coupling environments ?(10 marks)

Write short notes on any two:

7(a) OTDR(10 marks) 7(b) Link Power Budget(10 marks) 7(c) Coherent and Noncoherent optical transmission.(10 marks)