Code: A-24                                                                        Subject: OPTO ELECTRONICS AND

                                                                                                    OPTICAL COMMUNICATION

Time: 3 Hours                                                                                                     Max. Marks: 100

 

NOTE: There are 11 Questions in all.

 

·      Question 1 is compulsory and carries 16 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.

·      Answer any THREE Questions each from Part I and Part II. Each of these questions carries 14 marks.

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

 

 

Q.1       Choose the correct or best alternative in the following:                                           (2x8)

       

a.       The optical spectrum ranges from _________________.

                  

                   (A)  50 nm (uv) to 100  (IR)         (B)  5 nm to 10 nm  in IR

(C)    0.5 nm to 1  in  (uv)              (D)  none of these.

 

             b.   Nowadays optical fibers have losses of

                  

                   (A)  1000 dB/km                                 (B)  100 dB/km

                   (C)  10 dB/km                                     (D)  less than 10 dB/km.

 

             c.  Wave propagation in optical fibers is due to

 

                   (A)  reflection.                                     (B)  diffraction.

(C) total internal reflection.                   (D)  refraction.

 

             d.   Multimode fibers must be used with

 

                   (A)  LED source.                                 (B) LCD source.

(C) LASER source.                             (D) any light source.

                                                                  

             e.   The attenuation in fibers is given by

                  

(A)                         (B)

(C)                                 (D)

 

             f.    Pure silica is highly transparent in ranges of

 

(A)     800-1600 nm                               (B)  800-1600

(C)  80-160 nm                                   (D)  80-1600

 

             g.   Peak emission wavelength of a LED is

 

(A) proportional to EG                         (B) inversely proportional to EG

(C) not related to EG.                           (D) related to (EG)2

       

             h.   For graded-index fibers 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.

 

PART I

Answer any THREE Questions. Each question carries 14 marks.

 

  Q.2     a.   List the advantages of optical fiber communication systems over other systems.                   (6)

 

             b.   Starting from Maxwell’s equations, derive the wave equation for the propagation of waves in fibers.                                                                       (8)

 

  Q.3     a.   With neat sketches make a detailed comparison of single mode and multimode step index and graded-index optical fibers.                                       (8)

 

             b.   Calculate the Numerical Aperture of a step-index fiber having =1.48 and =1.46. find the maximum entrance angle for this fiber if the outer medium is air with .                                        (6)

                                                                             

  Q.4     a.   Write an explanatory note in detail about the scattering losses in fibers.              (10)

       

             b.   An optical signal has lost 85% of its power after travelling 500 m of fiber. What is the loss in dB/km of this fiber?                                                   (4)

            

  Q.5     a.   With neat sketches describe the structure and working of a high-radiant surface-emitted LED.                    (7)

       

             b.  With neat block schematic describe the operation of a coherent optical DPSK detector circuit used in receivers.                                             (7)

 

Q.6       a.   Write an explanatory note in detail about the concept of microbending and hydrogen absorption in optical fibers.                                                  (8)

       

             b.   A single mode fiber has normalized frequency of 2.40, core surface index = 1.46, core diameter of 8 , NA =0.1. Find the total insertion loss of the fiber with lateral misalignment of 1  and angular misalignment     of 10.                                                                                                                 (6)

 

PART II

Answer any THREE Questions. Each question carries 14 marks.

 

  Q.7     a.   Derive an expression for the internal quantum efficiency of a LED.                     (10)

 

             b.   Compare two LEDs made of with x = 0.04 and x = 0.08 for their energy gap and emission wavelength.                                         (4)

       

  Q.8     a.   With a neat structure, explain the principle of operation of laser diode for optical work. Derive the quantum efficiency term.                                    (9)

       

             b.   Write a short note on temperature effects of lasers.                                             (5)

 

  Q.9     a.   Starting from basics, derive the SNR expression at the input of the amplifier used in photo detector receivers.                                                           (8)

            

             b.   Write an explanatory note on detector response time and the bandwidth.             (6)

 

Q.10           a.                                                        Comment on the various noises that affect the performance of a photo detector receiver.                                                                                     (7)

 

             b.   A photo detector has 65% quantum efficiency with photons of energy 1.510-19 Joules incident. Find the operating wavelength and responsivity and optical power required to get 2.5 current.    (7)       

 

Q.11     a.   With relevant sketches and necessary equations describe how time-domain measurements are performed in optical fiber communication systems.    (6)

 

             b.   A 1550-nm single-mode digital fiber optic link needs to operate at         622 Mb/s over 80 km without amplifiers. A single-mode InGaAsP laser launches an average optical power of 13 dBm into the fiber. The fiber has a loss of 0.35 dB/km, and there is a splice with a loss of 0.1 dB every kilometer. The coupling loss at the receiver is 0.5 dB, and the receiver uses an InGaAs APD with a sensitivity of –39 dBm. Excess noise penalties are predicted to be 1.5 dB. Set up an optical power budget for this link and find the system margin. What is the system margin at 2.5 Gb/s with an APD sensitivity of –31 dBm?                       (8)