Code: A-24                                                                        Subject: OPTO ELECTRONICS AND

                                                                                                    OPTICAL COMMUNICATION

Time: 3 Hours                                                     June 2006                                                 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 fibers uses the ______portion of EM spectrum

 

                   (A)  IR                                                (B)  VHF

(C)    HF                                              (D)  UHF

       

b.      The critical angle for glass (1 = 1.50) and air (2 = 1.0) interface is

            

                   (A)                                               (B)  

(C)                                               (D) 

                 

             c.   For a step-index fiber the NA across the core

                  

(A)    varies                                           (B)  will be opposite

(C)  is constant                                    (D)  is independent

 

             d.   One of the causes for material dispersion in fibers is 

 

(A)    group delay                                 (B)  velocity of wave

(C)  none                                            (D)  material choice           

 

             e.   The surface dark current of a photo diode is related to bandwidth

                  

(A)     directly                                         (B)  inversely

(C)  as its square                                 (D)  inverse square

 

             f.    Pin diodes are used in optical fiber links because of their

 

(A)     resistance                                     (B)  size compatibility

(C)  cost                                              (D)  slow response

 

             g.   Average insertion loss obtained with fibers using ceramic capillaries will be 

 

(A)     as low as 0.1 dB                          (B) more than 1 dB

(C)  equal to 1 dB                               (D) around 2 dB

 

             h.   A preamplifier used with a photo diode should provide

 

(A)    low input impedance                     (B) high input impedance

(C) low output impedance                    (D) high output impedance

 

             i.    Polarization inversion is a property found in

 

(A)   LED                                             (B) LASER

(C) Photodiode                                   (D) FET

 

             j.    The insertion loss of optical fibers varies with misalignment angle

 

(A)  linearly                                         (B)  exponentially

(C)  inversely                                       (D)  constantly

 

 

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

 

  Q.2     a.   Explain mode coupling and evanascent field in optical fiber transmission.              (4)

       

             b.   With a neat block schematic explain the basic elements of an optical fiber transmission link.             (4)

 

             c.   Explain the ray optics representation of skew rays and meridional ray optics representation of waves travelling in a step-index fiber core.      (8)

                                                                                                                 

  Q.3     a.   With necessary expressions, explain the scattering losses in optical fibers.            (8)

 

             b.   Describe the various modes in step-index fibers.                                                 (4)

                  

             c.   Define and explain the terms V number and Numerical Aperture.                        (4)

            

  Q.4     a.   Describe how material dispersion takes place in optical fibers.                             (6)

 

             b.   A silica optical fiber with a core diameter large enough to be considered by ray theory analysis has a core refractive index of 1.50 and cladding refractive index of 1.47.  Determine critical angle at the core clad surface, NA for the fiber, acceptance angle in air for the fiber.                             (4)          

 

             c.   The group delay Tg in an optical fiber is given by

                 

                   Where c is the velocity of light in a vacuum, is the core refractive index and  is the wavelength of the transmitted light. Derive an expression for the rms pulse broadening due to material dispersion in an optical fiber and define the material dispersion parameter.                                              (6)          

 

 

 

  Q.5     a.   Describe with the aid of neat sketches the techniques employed to produce both high and low birefringence PM fibers.                                           (8)

       

             b.   Two polarization maintaining fibers operating at wavelength of 1.3 m have beat lengths of 0.7 mm and 80 m.  Determine the modal birefringence in each case and comment on the results.  (5)

 

             c.   Explain briefly how splicing of fibers is done.                                                       (3)

 

  Q.6     a.   With neat sketches, explain a suitable structure of LED that is suitable for optical fiber communication work.                                                                                                                         (6)

 

             b.   Two LED’s of composition  have x = 0.02 and x = 0.09.  Find  and peak emission wavelength.                                         (4)

 

             c.   Write an explanatory note on modulation of laser diodes.                                    (6)          

 

  Q.7     a.   Discuss, with the aid of a suitable block diagram, a coherent optical fiber communication system. Comment on the differing system requirements to facilitate heterodyne detection in comparison with homodyne detection.                                            (8)   

 

             b.   Explain :

(i)                  Sub carrier Multiplexing

(ii)                SDH signal Hierarchy.                                                                (8)

 

  Q.8     a.   Describe in detail with sketches and necessary expressions the structure of LASER diodes.  Comment on the threshold conditions.                            (8)

 

             b.   Compare the ratio of threshold current densities at  and  for a AlGaAs injection laser with  and the similar ratio for a InGaAsP device.                                                    (8)

 

  Q.9     a.   With neat circuit diagram explain the structure, performance and characterization of an avalanche photo diode.                                                  (6)

 

             b.   Derive an expression for the SNR of photo detector diode.                                 (6)

            

             c.   A photo detector has a Quantum efficiency of 65% when photons of energy  are incident upon it.  At what wavelength is the photo detector operating?  Calculate the incident optical power required to obtain a photo current of 2.5  when the photo detector is thus operating.       (4)