AMIETE – ET (OLD SCHEME)

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 the best alternative in the following:                                 (2 10)

a. A glass fiber has refractive indices  of 1.5 and  of 1.  Assuming .  The multipath time dispersion will be

(A)  2.5 ns/m.                                      (B) .

(C) .                                      (D)  .

b. As a result of reflections from a plane conducting wall, electromagnetic waves acquire an apparent velocity greater than the velocity of light in space.  This is called

                   (A)  velocity of propagation.                (B)  normal velocity.

(C)  group velocity.                              (D)  phase velocity.

             c.   A certain fiber has refractive index of clad  and that of

                   core . Its numerical aperture will be 

(A) 0.8575.                                        (B)  0.9260.

(C)  0.3500.                                        (D)  0.1585.

             d.   A single fiber can handle as many voice channel as

(A)  a pair of copper conductors.        (B) a 500-pair cable.

(C)  a 1500-pair cable.                       (D) a 1000-pair cable.

             e.   The term ‘power budgeting’ refers to  

(A) The cost of cable, connectors, equipment and installation.      

(B) The loss of power due to defective component.

(C) The total power available minus the attenuation losses.           

(D) The comparative costs of fiber and copper installations.

             f.    The direct modulation rate of laser diodes depends on

(A) spontaneous carrier lifetime.         

(B) stimulated emission carrier lifetime.

(C)  photon lifetime.                            

(D)  all of above.

             g.   In semiconductor lasers the line width is increased by a factor ___________, where  is the line width enhancement factor. 

(A) (B)

(C) (D)

             h.   A photodiode is constructed of GaAs, which has a band-gap energy of        1.43 eV at 300 K.  The long-wavelength cut off will be 

(A) 869 nm.                                       (B) 820 nm.

(C)  769 nm .                                      (D) 720 nm.

             i.    Read the following statements for pre amplifier used to placed ahead of photodiode, and choose the correct combinations

                   1.  Boost the optical signal level before photodectection take place.

                   2.  Suppressed the signal-to-noise ratio degradation caused by thermal noise.

                   3.  Provides a larger gain factor and a broader bandwidth.

(A) Statement 1 and 2 are correct.       (B) Statement 2 and 3 are correct.

(C) Statement 1 and 3 are correct.       (D) Statement 1, 2 and 3 are correct.

             j.    Which of the following can be used to determine the dispersion limitation of an optical fiber link

(A)  sub carrier multiplexing.                (B)  rise-time budget analysis.

(C)  preamplifiers.                                (D)  none of these.

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q.2     a.   Explain the role of following elements of optical fiber communication link.

(i)    Optoelectronic sources and detectors.

                   (ii)   Channel couplers.

                   (iii)  Fiber-optic information channel.

                   (iv)  Repeaters.                                                                                    (4 2  = 8)

     b. Calculate the total pulse broadening due to material dispersion for a

                   graded–index fiber of total length 80 Km when a LED emitting at    (i) 

                   and   (ii) , is coupled to the fiber.  In both cases, assume .  The material dispersion parameters of the fiber for the two wavelengths are –105.5 ps  and –2.8 ps  respectively. [ : full width at half minimum (FWHM)]                                     (8)          

Q.3     a.   What is the difference between the propagation phase constant and the normalized propagation parameter b?  How are they related?                    (8)

     b. A multimode step-index fiber has a relative-refractive index difference of 2% and a core refractive index of 1.5.  The number of modes propagating at a wavelength of 1.3  is 1000.  Calculate the diameter of the fiber core.                                                                                                          (8)

Q.4     a.   Explain following:

(i)   Distinguish between a splice and a connector.

                   (ii)  How can one avoid or reduce loss due to fresnel reflection at a joint.

                   (iii) Distinguish between fusion and mechanical splicing of optical fiber. 

                          Discuss the advantages & drawbacks of these techniques.         (2 + 2 + 4 = 8)

b.   A typical gas laser is emitting a spectral line centered at 632.8 nm, whose gain curve has a half-width of  if the cavity length of the laser is 20 cm, calculate the number of longitudinal modes excited.  Take the refractive index inside the gas medium to be 1.                                      (8)

Q.5     a.   (i)   Discuss unique properties of the p-n double heterostructure LED and sketch the energy-level diagram of such a configuration.

                   (ii)   Derive the threshold condition for laser action.                                 (4 + 4 = 8)

             b.   The maximum 3-dB bandwidth permitted by an InGaAs photo-conducting detector is 450 MHz when the electron transit time in the device is 6 ps.  Calculate

(i)   The gain G.

                   (ii) The output photocurrent when an optical power of 5 at a wavelength of 1.30 is incident on it, assuming quantum efficiency is 75%.                                                                   (8)

  Q.6     a. Distinguish between a p-n diode, a p-i-n diode, and an APD.  Is it possible to make these three types of photodiodes using the same semiconductor?                                                       (8)

             b.   A fiber has a 50-core diameter and a 0.2 NA value.  The beam is expanded to a 2-mm diameter.  Design the lens arrangement and compute the allowable lateral offset for a 0.5 dB loss.       (8)

Q.7     a.   Draw & discuss the design approaches of front-end amplifier to be used within an optical receiver.             (8)

     b.   A system consist of LED emitting 10 mW at 0.85 , a fiber cable with 20 dB loss and a PIN photodetector of responsivity  0.5 A/W.  The detectors dark current is 2 nA.  The load resistance is 50 .  The receivers bandwidth is 10 MHz & its temperature is 300 K .  The system losses in addition to the fiber attenuation include a 14 dB loss due to source coupling and a 10 dB loss caused by various splices and connectors, compute

                   (i)   The received optical power.

                   (ii)  The detected signal current and power.

                   (iii) The shot-noise power, thermal-noise power and SNR.               (2 + 2 + 4 = 8)

Q.8     a.   Explain link power budget analysis and rise-time budget analysis?  Discuss in details.                       (8)

b.   Discuss different type of fiber-optic communication system architectures, suggest the application of each of these topologies.                                  (8)

Q.9     a.   Discuss in brief:

                   (i)    Line coding.

                   (ii)   Optical TDM.                                                                                (4 2 = 8)

b.   A 80/125  graded index (GI) fiber with a NA of 0.25 and profile parameter  of 2.0 is jointed with a 60/125  GI fiber with an NA of 0.21 and  of 1.9.  The fiber axis are perfectly aligned and there is no air gap.  Calculate the insertion loss at a joint for the signal transmission in the forward and backward directions.                                                                                                          (8)

SOME USEFUL DATA

(i) Planck’s constant,

(ii) Velocity of light

(iii) Electron charge,