Code: AE-24                                                                     Subject: OPTO ELECTRONICS AND Flowchart: Alternate Process: JUNE 2007

                                                                                                    OPTICAL COMMUNICATION

Time: 3 Hours                                                                                                      Max. Marks: 100

 

NOTE: There are 9 Questions in all.

·      Question 1 is compulsory and carries 20 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.

·      Out of the remaining EIGHT Questions answer any FIVE Questions. Each question carries 16 marks.

·      Any required data not explicitly given, may be suitably assumed and stated.

 

Q.1       Choose the correct or best alternative in the following:                                         (2x10)

                

a.       Optical common systems use wave lengths in the range of

 

                   (A)  ultra violet.                                   (B)  visible.

(C)    x-rays.                                         (D)  infrared.

       

b.      The optical path length in a medium equals

                 

                   (A)  geometric path length x velocity of light in the medium.                                               

                   (B)  geometric path length x refractive index of the medium.

(C)     geometric path length / refractive index of the medium.                                              

(D)    geometric path length x velocity of light in vacuum.

                 

             c.   The unit for specifying material dispersion is

                    

(A)    ns per nm per km.                        (B)  ns per km.

(C)  ns per .                               (D)  none of the above.

 

             d.   The unit for PMD is  

 

(A)    ns per nm per km.                       (B)  ns per km.

(C)  ns per .                               (D)  none of the above.

 

             e.   The range for the diameter of a multimode step index or multimode GI fiber is

                  

(A)     50-200.                                (B)  5-25.

(C)  500-1000.                             (D)  50-200 nm.

 

             f.    The direct bandgap for a GaAeAs LED is 1.6ev.  The emitted  (in nm) is

 

(A)     775                                              (B)  960

(C)  1300                                            (D)  1550

 

             g.   The diffusion length of minority carriers is given by

 

(A)  DZ                                               (B) 

(C)                                               (D) 

                   Where D : Diffusion constant.

                               Z : Life time (average)             

             h.   A laser diode has   

 

(A)     spatial and temporal coherence     (B) only spatial coherence

(C)  only temporal coherence               (D) no coherence

 

             i.    The group velocity  is given by

 

(A)                                                (B)

(C)                                                (D)

                  

             j.    A PN photodiode, on an average, generates one electron-hole pair per five incident photons.  Assuming also the photo-generated holes electrons are collected, the internal quantium efficiency is  

 

(A)  10%                                             (B)  20%

(C)  50%                                             (D)  100%

 

 

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

 

  Q.2     a.   Draw cross section of optical waveguide showing the core and cladding.  Using ray optic representation explain total internal reflection in optical fiber.  Explain the term Numerical Aperture.           (3+3+2)

 

             b.   A silica optical fiber with a core diameter large enough to be considered by ray theory has a core refractive index of 1.50 and a cladding refractive index of 1.47.  Determine

(i)                  the critical angle at the core-cladding interface.

(ii)                N.A. of fiber.

(iii)               Acceptance angle of fiber.                                                          (8)

                                                                                                                                        

  Q.3     a.   As related to working of lasers, explain the following:

                   (i)     absorption.                                  (ii)  spontaneous emission

                   (iii)   stimulated emission                       (iv) pumping.                                           (8)

 

             b.   With a neat sketch, describe working of Fabry-Perot resonator laser.  Derive an expression for threshold gain.                                                    (8)  

                                                                                                                                                           

  Q.4     a.   Why are LEDs commonly used with Multimode fibers?  Explain the heterojunction structure used for LEDs.  What are the parameters, which influence the performance of LEDs?                  (8)

 

             b.   A double-hetero junction InGaAsP LED emitting a peak wave length of 1310 nm has radiative and non-radiative recombination times of 25 and  90 nm, respectively.  The drive current is 35 mA. Find the internal quantum

                   efficiency and the internal power level.  (8)

 

  Q.5     a.   What are the constructional requirements for a good photo diode?  With a neat sketch describe the features of a PIN photo diode and explain its working.  Define quantum efficiency and responsivity.             (12)

 

             b.   A PIN photo diode, on an average, generates one electron-hole pair per two incident photons at a wavelength of 0.85.  Assuming that all the photo-generated electrons are collected, calculate

(i)                  the quantum efficiency of the diode

(ii)                the maximum possible band gap energy (in ev) of the semiconductor, assuming the incident wavelength to be a long-wavelength cut-off.                                (4)

 
 
  Q.6     a.  

                  

                  

 

 

 

 

 

 

 

                   Consider a PD circuit followed by an amplifier as indicated in the Fig.1 above.  Draw an equivalent circuit for the above and derive a general expression for the  ratio at the input to the amplifier.      (8)

 

             b.   Show that the rise time for a fibre and the 3-dB bandwidth are related by the expression:

                  

                   Where  is the full width at half-maximum rise time.  Assume only intermodal dispersion.                                                                 (8)

 

  Q.7     a.   What is splicing?  What are the parameters on which splicing depends.  Explain briefly commonly used splicing techniques.                              (6)                                                                  

 

             b.   Explain synchronous optical network and give the details of SONET frame.   Why are SONET/SDH rings called self healing rings?                       (8+2)   

 

  Q.8           As related to Point to Point fiber optic communication links how are system requirements specified?  Derive an expression for link loss budget, assuming a link margin of 6-8 dB.                                  (16)

 

  Q.9           Write short notes on any FOUR:

(i)                  NRZ codes.                           

(ii)                Wavelength Division Multiplexing.

(iii)               Pre-amplifiers.

(iv)              Mode Hopping.                      

(v)                p-n photodiode.                                                                       (4 x 4 = 16)