OPTICAL FIBRE COMMUNICATION (OFC) MAY 2012 ELECTRONICS AND TELECOMMUNICATION SEMESTER 8

OPTICAL FIBRE COMMUNICATION (OFC) MAY 2012 ELECTRONICS AND TELECOMMUNICATION SEMESTER 8

Con.3705-12.                              (REVISED COURSE)                                    GN-7643

                                                             (3 Hours)                                  [Total Marks:-100]

N.B:- (1) Question No.1 is compulsory.
          (2) Attempt any four from remaining six.
          (3) All questions carry equal marks.

1. (a) Explain with a neat sketch the two categories of front end amplifiers used in optical 
         fiber communication systems. [5 Marks]
    (b) State the spectral band designations used in optical fiber communications. [5 Marks]
    (c) Discuss in brief single mode step index fiber and multimode step index fiber. [5 Marks]
    (d) Name the key parameters for describing the signal transmission in single mode fiber and 
          multimode fibre. [5 Marks]

2. (a) Define the terms numerical apertures, critical angle, propagating modes and microbands 
          in the context of an optical fiber. [10 Marks]
    (b) Calculate the required ∆ if a fiber with a 8 µm core and a 125 µm cladding is to be single 
          mode at 1300 mm. Assume that the core index is 1.46. [5 Marks]
     (c) A 45˚ - 45˚ - 90˚ prism is immersed in alcohol (n=1.45). What is the minimum 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]
    (b) With a neat sketch explain fiber optical cable. [5 Marks]
    (c) Name five connectors used in optical fiber communications. [5 Marks]
    (d) What does the coupling efficiency equation? = (pf/ps) = (NA)² signify where pf is the 
           power coupled to the source ps source power. [5 Marks]

4. (a) What do you understand by double hetrostructure? State its limitations. [5 Marks]
    (b) State the difference between LED and LASER. [5 Marks]
    (c) A light source generating an optical power output equal to 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˚ equal to 15 ˚ [5 Marks]
      (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]
    (b) Derive an expression for the responsibility "R" of the photo detector. [5 Marks]
    (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 [10 Marks]
             (I) Digital signalling
             (ii) Analog transmission. 

6. (a) In a point-to-point communication link it is given that launched power is - 10 dBm, 
         receiver sensitivity is - 40 dBm and the length of the link is 10 km. If the total losses in 
         the link add upto 27dB. Find the safety margin. If the fiber bandwidth is 1000 MHz.km 
         what is the maximum permissible data rate. If the risk time due to the source and the 
         detector is neglegible. [10 Marks]
     (b) Draw and explain the test-set up for measuring the chromatic dispersion. [10 Marks]

7. Write short notes on any two:- [20 Marks]
     (a) Linearly Polarized modes.
     (b) Numerical aperture in GIF.
     (c) Scattering losses in optical fiber.
     (d) Distributed feed back laser.