OPTICAL FIBER COMMUNICATION SYSTEM

OPTICAL FIBER COMMUNICATION SYSTEM
SEM-VII, 2012-13
B.TECH EXAMINATION
UTTARAKHAND TECHNICAL UNIVERSITY

Time: 3 Hours
Total Marks:100
Attempt any four parts:
  1. Discuss Dispersion mechanism with respect to single mode fiber indicating the dominating effects and then show how inter-modal dispersion may be minimize within the single mode region.
  2. Briefly discuss with the aid of a suitable diagram what is meant by the acceptance angle for an optical fiber. Show how this is related to the fiber numerical aperture and the refractive indexes for the fiber core and cladding?
  3. A step index with a large core diameter compared with the wavelength of the transmitted light has an acceptance angle in air of 22 degree and a relative refractive index difference of 3%. Estimate the numerical aperture and the critical angle at the core-claddinginterface of the fiber.
  4. What are the modes in optical fiber? Calculate number of modes for a graded index optical fiber if core diameter (d = 62.5 μ m), NA =0.275 and Operating wavelength = 1300 nm.
  5. Two polarization maintaining fibers operating at the wavelength of 1.3 micrometer have beat length of 0.7 mm and 0.8 mm. Determine the briefringence in each case and comment the result.
  6. Derive an expression for the acceptance angle for skew rays which changes direction by an angle of 2γ at each reflection in step index fiber in terms of NA and γ.
Attempt any four parts:
  1. Explain how the optical and electrical confinement is achieved in a LASER.
  2. List the three major cause of attenuation in an optical fiber and explain their mechanism.
  3. What is the difference between a surface emitting LED and edge emitting LED? An injection loss has a total efficiency in a GaAs active region as 20%. The volume applied to the device is 2.2 V and the band gap energy of GcAs is 1.43 ev. Find the external power efficiency of the device.
  4. Explain the basic construction of Laser diode and also discuss Einstein's relation with suitable expression.
  5. What are the advantages of S C laser over LED for communication and explain the mechanism of light from LED.
  6. Explain modulation bandwidth of an LED with the help of suitable frequency response curve, clearly marking optical and electrical bandwidth.
Attempt any two parts:
  1. Explain the working of a P-I-N photodiode. Also explain the factors that limit the speed of response of photodiode.
  2. Explain the impact ionization in avalanche photodiodes. Define photo multiplication factor and cutoff over length of photodiode.
  3. Discuss the expression for the SNR in an APD receiver. How the signal to noise ratio may be modified to give the optimum avalanche multiplication factor.
Attempt any two parts:
  1. Explain the necessity of preamplifier in an optical receiver. Mention the types of preamplifier used and explain the working of any one of them.
  2. Describe surface LED and PIN photodiode always operative in reverse bias region.
  3. Explain the various noise and disturbances associated with signal detection in optical receiver. Briefly discuss Quantum noise and consider this phenomenon with respect to digital signaling.
Attempt any two parts:
  1. Describe the optical power budgeting for digital optical fiber systems.
  2. How internal and external quantum efficiency is laser diode. Determine and describe Quantum well laser.
  3. Draw the block diagram of detection principle of coherent optical fiber systems.
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