Flowchart: Alternate Process: DECEMBER 2007
Code: 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.       Critical angle is given by

 

                   (A)                       (B)  

(C)                         (D) 

       

b.      Mode hopping is concerned with

                          

                   (A)  Injection Laser.                            (B)  Surface Emitter LED.

(C)  Edge Emitter LED.                       (D)  Photodiodes.

                 

             c.   The refraction index of glass is 

                                      

(A)    1.00                                             (B)  1.33

(C)  1.50                                             (D)  2.42

 

             d.   Unit of attenuation coefficient is 

 

(A)    dB/km                                        (B) km

(C)  nepers/km                                   (D) Watt/km

 

             e.   The direct energy bandgap for GaAs material is 

                                      

(A)     1.43 eV                                       (B) 0.73 eV

(C)  0.35 eV                                       (D) 0.18 eV

 

             f.    Fractional gain for laser oscillation is

 

(A)                                         (B) 

(C)                                (D)

 

             g.   The threshold for direct absorption of Germanium occurs at

 

(A)  *                                        (B) 

(C)                                         (D) 


 

 

             h.   The average number of electron hole pairs created by a carrier per unit distance travelled is called the 

 

(A)     avalanche effect                            (B) impact ionization

(C)  ionization rate                               (D) none of these

 

             i.    Which device is used at the interconnection point to translate the control information from one protocol to another

 

(A)   active coupler                               (B) passive coupler

(C) FDDI                                            (D) router

                  

             j.    The transmission bit rate of the basic SONET signal is

 

(A)  26.24 Mbps                                 (B)  51.84 Mbps

(C)  155.52 Mbps                               (D)  622.08 Mbps

 

 

 

 

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

 

 

 

 

 

  Q.2     a.   Explain bending losses in details.                                                                         (8)

 

             b.   A continuous 12-km-long optical fiber link has a loss of 1.5dB/km.

 

(i)      What is the minimum optical power level that must be launched into the fiber to maintain an optical power level of  at the receiving end?

 

(ii)    What is the required input power if the fiber has a loss of 2.5 dB/km?           (8)

                                                                                                                      

  Q.3     a.   Draw and explain structure of glass fiber lasers.                                                  (8)

             b.   A GaAs laser operating at 850 nm has a  length and a refractive index n=3.7. What are the frequency & wavelength spacings. If, at the half-power point, what is the special width ‘σ’ of the gain?                                              (8)

                                                                                                                            

  Q.4     a.   Draw and explain schematic diagram of a typical optical receiver.                        (8)

 

             b.   A photodiode has a quantum efficiency of 65% when photons of energy 1.5 x 10–19 J are incident upon it

 

                    (i)    at what wavelength is the photodiode operating?

 

                    (ii)    calculate the incident optical power required to obtain a photocurrent of  when the photodiode is operating as described above.      (8)

 

  Q.5     a.   Explain Link Power Budget in detail along with suitable diagram.                         (8)          

 

             b.   A90-Mb/s NRZ data transmission system that sends two DS3 channels uses a GaAlAs laser diode that has a 1-nm spectral width.  The rise time of the laser transmitter output is 2ns.  The transmission distance is 7km over a graded-index fiber has an 800-  bandwidth-distance product.

                   (i)   If the receiver bandwidth is 90MHz and the mode-mixing factor q=0.7, what is the system rise time?  Does this rise time meet the NRZ data requirement of being less than 70 percent of a pulse width?

                   (ii)  What is the system rise time if there is no mode mixing in the 7-km link; that is, q=1.0?              (8)

                                                                             

  Q.6     a.   Describe fusion splicing technique.                                                                      (8)

 

             b.   Describe structure of a concentric sleeve connector in detail.                               (8)

 

  Q.7     a.   With the aid of suitable diagram, discuss the principles of operation of the injection laser.                 (8)

 

             b.   Compare the ratio of the threshold current densities at 20ºC & 80ºC for a AlGaAs injection laser with T0=160K and the similar ratio for an InGaAsP device with T0=55K.                                  (8)

            

  Q.8     a.   Discuss the operation of silicon RAPD. Outline its advantages & drawbacks as a detector for optical fiber communication.                                    (8)

 

             b.   An InGaAs pin photodiode has the following parameters at a wavelength of 1300 nm; ID=4nA, η=0.90, RL=1000Ω & the surface leakage current is negligible. The incident optical power is 300nW (–35 dBm), and the receiver bandwidth is 20MHz. Find out (i) IP,  (ii) IQ, (iii) IDB & (iv) IT .         (8)

 

  Q.9     a.   Discuss Wavelength Division Multiplexing (WDM) in detail.                                (8)

 

             b.   Consider a tunable DBR laser operating at 1550 nm that has a linewidth (frequency spread) of 1.25 GHz.  If the maximum index change is 0.55 percent, how many wavelength channels can this laser provide if the channel spacing is 10 times the source spectral width?                                         (8)