Optical Fibre Communication - May 2012

Electronics Engineering (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) Explain with a neat sketch the two categories of the front-end amplifiers used in optical fiber communication systems.(5 marks) 1(b) State the spectral band designations used in optical fiber communication(5 marks) 1(c) Discuss in brief single mode step index fiber and multimode step index fiber.(5 marks) 1(d) Name the key parameters in describing the signal transmission in single mode fiber and multimode fiber.(5 marks) 2(a) Define the terms numerical aperture, critical angle, propagating modes and microbands in the context of an optical fiber.(10 marks) 2(b) Calculate the required Δ if a fiber with a 8 μm core diameter and a 125 μm cladding is to be single mode at 1300 mm. Assume that the core index is 1.46.(5 marks) 2(c) A 45⁰-45⁰-90⁰ prism is immersed in alcohol (n = 1.45). What is the min. refractive index the prism must have if a ray incident normally on one of the short faces is to be totally reflected at the long face of an prism ?(5 marks) 3(a) What do you understand by degenerating modes in step index fiber ?(5 marks) 3(b) With a neat sketch explain fiber optic cable.(5 marks) 3(c) Name five connectors used in optical fiber communication(5 marks) 3(d) What does the coupling efficiency equation η = (pf / ps) = (NA²) signify where pf is the power coupled to the source ps source power.(5 marks) 4(a) What do you understand by double heterostructure? State its limitations(5 marks) 4(b) State the difference between LED and LASER.(5 marks) 4(c) A light source generating an optical power output equal to 1 μW is coupled into an optical fiber with a cross sectional area larger than the active area of the light source.
Determine the power coupled into the fiber. θ_a equal to 15⁰.(5 marks) 4(d) Draw the current optical power output curve for Fabry Perot Laser and explain to relationships.(5 marks) 5(a) List all the parameters that contribute to photo current gain of APD(5 marks) 5(b) Derive an expression for the responsivity R of the photo detector.(5 marks) 5(c) Briefly discuss the possible sources of noise in optical fiber receiver. Describe in detail what is meant by quantum noise. Consider this phenomenon with regard to:
(i)Digital Signalling
(ii)Analog transmission(10 marks) 6(a) In a point-to-point communication link it is given that launched power is -10dBm, receiver sensitivity is -40dBm and the length of the link is 10km. If the total losses in the link add upto 27dB. Find the safety margin. If the fiber bandwidth is 1000MHz.km what is the maximum permissible data rate. If the risk time due to the source and the detector is negligible(10 marks) 6(b) Draw and explain the test-set up for measuring the chromatic dispersion(10 marks)

Write short notes on any two:

7(a) Linearly polarized modes(10 marks) 7(b) Numerical aperture in GIF.(10 marks) 7(c) Scattering losses in optical fiber.(10 marks) 7(d) Distributed feed back laser.(10 marks)