Flowchart: Alternate Process: DECEMBER 2008Code: AE24                                                                       Subject: OPTO ELECTRONICS AND

                                                                                                    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.       Single-mode step-index optical Fibre Cable has a core diameter in the range of

 

                   (A)  100 to 1000 m                          (B)  50 to 100 m

(C)    2 to 15 m                                 (D)  0.1 to 2 m

       

b.      The key system features of WDM are given. Select the wrong one

                          

                   (A)  capacity upgrade                          (B)  Transparency

(C)  wavelength routing                        (D)  bandwidth increase

                 

             c.   The factors to be considered for coupling optical power from one to another fibre are given. Select the wrong one. 

                                      

(A)    Different bandwidths                     (B)  Fibre misalignment

(C)  Different core sizes                       (D)  Numerical Apartures

 

             d.   The advantage of a laser diode over an LED is

 

(A)    Narrow spectral width of output 

(B)  Output power level is strongly dependent on temperature

(C)  Both (A) and (B)                        

(D)  None of these

 

             e.   For multimode step index Fibre with V=20 has _______ total number of guided modes. 

                                      

(A)     10                                                (B) 20

(C) 200                                               (D) 400

 

             f.    In a photodiode, main source of noise is

 

(A)     Dark current noise                        (B) Shot noise

(C)  Thermal noise                               (D) All of above.

 

             g.   Intermodal dispersion may occur in 

 

(A)   Glass Fibre                                   (B)  Plastic Fibre

(C)  Step-index Fibre                          (D)  All types of Optical Fibres

 

             h.   For graded-Index fibres NA is a function of  

 

(A)     position across core and face        (B) (core radius)2

(C)  diameter of the core                      (D) cross section of the core

 

             i.    OTDR is an instrument used for

                           

(A)   Fibre drawing                               (B) Splicing

(C) Attenuation measurement               (D) None of these

                  

             j.    In optical fibre, the propagating mode is

 

(A)  TE                                               (B)  TM

(C)  TEM                                            (D)  HE

 

 

 

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

 

  Q.2     a.   Describe the advantages and disadvantages of optical system.  Also discuss the application of optical fibre system.                                                (8)

 

             b.   A step-index fibre has core refractive index of 1.44 and clad refractive index of 1.40. Calculate

                   (i)  The N.A.                                      

                   (ii) (core cladding Index difference)

                   (iii) The acceptance angle of the fibre    (8)                                                                                                

 

  Q.3     a.   What is V number?  Explain its significance.  A multimode step index fibre with a core diameter of 80 and a relative index difference of 1.5% is operating at a wavelength of 0.85.  If the core refractive index is 1.48.  Determine the number of guided modes and normalized frequency for the fibre.                                                                   (4+2+2 = 8)

       

             b.   With respect to optical fibre communication system, define and explain the following:-

                   (i)  Ray leigh scattering and Mie scattering.

                   (ii) Intermodal dispersion and intramodal dispersion.                                 (4+4 = 8)

                                                                                                                                                           

  Q.4     a.   Explain the structure of DFB LASER with neat sketch.                                       (8)

 

             b.   Compare LED and LASER as a light source for the optical communication system.            (4)

 

             c.   Describe the various modes in step-index fibre.                                                   (4)

       

  Q.5     a.   Derive an expression for the power coupled from LED into a step index fibre.                    (8)

 

             b.   An LED with a circular emitting area of radius 20 m has a lambertian emission pattern with a 100 W/() axial radiance at a 100-mA drive current. How much optical power can be coupled into a step-index fibre having a 100 m core diameter and NA=0.22? How much optical power can be coupled from this source into a 50-m core-diameter graded index fibre having   and ?                                                            (8)                                                             

                                                                                                                                                

  Q.6     a.   Derive the equation for the power coupled between the Joints of two G.I. Fibres having an axial offset of d mm at the joint.                                    (8)

 

             b.   Describe the constructional requirements for a good photo diode and features of PIN photodiode with neat sketch.  Also define the quantum efficiency and responsivity.                                               (8)       

 

  Q.7     a.   Analyse an APD based receiver for optical communication systems in terms of various noise currents and derive the formula for the SNR.            (8)

 

             b.   The quantum efficiency of a particular RAPD is 80% for the detection of radiation at a wavelength of 0.9 m when the incident optical power is 0.5 mw, the output current from the device is 11 A. Determine the multiplication factor of the photodiode under these conditions.                             (6)

            

             c.   Describe optical TDM.  Compare it with WDM.                                                (2)

 

  Q.8     a.   A graded Index Fibre has a core with a parabolic refractive index profile which has a diameter of 50 m.  The fibre has a numerical aperture of 0.2. Estimate the total no. of guided modes propagating in the fibre when it is operating at a wavelength of 1 m.                                                                     (6)

 

             b.   Distinguish between optical bandwidth and electrical bandwidth.                         (3)

 

             c.   Describe the optical power budget in detail.                                                        (7)

 

  Q.9     a.   Describe various Fibre splicing techniques.                                                         (8)

 

             b.   Explain

                   (i)   Sub carrier multiplexing.

                   (ii)  SONET / SDH.                                                                                           (8)