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. Free space optical signal propagation is generally not preferred because of

(A)  higher attenuation                          (B)  lower bandwidth

(C)   higher dispersion                          (D)  radiation hazard

b. The human eye responds to wavelengths of electromagnetic energy that are in the range of

                   (A)  300 to 3000°A                             (B)  3000 to 4000°A

(C)  4000 to 7700°A                           (D)  7700 to 10000°A

             c.   Discrete bundles of energy produced by a source of light are called

(A) Ions                                             (B)  Electrons

(C)  Neutrons                                      (D)  Photons

             d.   Light is best described as a form of

(A)  Work                                          (B) Energy

(C)  Waves                                         (D) Force

             e.   Cladding helps in providing

(A) mechanical strength                     

(B) protects core from absorbing surface contaminants

(C) reduces scattering                        

(D) All of the above

             f.    One of the following is not considered for launching optical power into a fiber

(A) numerical aperture.                       (B)  core size.

(C)  optical receiver.                            (D)  refractive index profile.

             g.   Pulse broadening in Graded Index fibers is due to

(A) Intermodal dispersion                    (B)  Intramodal dispersion

(C)  Both (A) and (B) (D)  Chromatic dispersion

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

             i.    LASER is a

(A) a coherent light                            

(B) originated from stimulated emissions of radiation

(C) a non coherent light                       

(D) Both (A) and (B)

             j.    The material which added to silica to increase the refractive index of the material is

(A)  GeO₂ & P₂O₅ (B)  GeO₂   & B₂O₃

(C)  P₂O₅ & B₂O₃ (D)  B₂O₃ & F

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q.2     a.   With the help of a block diagram discuss various components of an optical communication system. How are optical fibers advantageous in communication application?                                                  (8)

     b. Calculate the number of modes at 870nm and 1.5μm in a Graded Index Fiber with  a parabolic-index profile, α=2 with core radius of 25μm; n₁=1.5; n₂=1.47                                                      (8)

Q.3     a.   Explain what is meant by modal birefringence and the beat length in single mode fibres. Two polarizations maintaining fibres operating at a wavelength of 1.3m have beat length of 0.7 mm and 80 m. Determine the Modal birefringence in each case.                                                                   (4+4)

     b. Find the total intermodal, intramodal and total dispersion for a fiber with 1.5 km length;   line width 30 nm;   intermodal & intramodal dispersion  of

                   6 ns/km and 85 ps/km.                                                                                       (8)

Q.4     a.   (i) Explain the following:                                                                                     (5)

Pumping, stimulated emission, spontaneous emission related to LASERs with the help of energy level diagram.                                             

                   (ii) List the advantages of injection laser diode over light emitting diode.               (3)

b.   Power generated internally with a LED is 28.4 mW at a drive current at 40mA. Find the peak emission wavelength from the device when radiative and non-radiative combination of lifetimes, if minority carriers in the active region are 30ns and 50ns respectively.                                                (8)

Q.5     a.   (i) 800 photons per second are incident on a PIN photodiode operating at a wavelength of 1.3μm. They give 550 electrons per second. Calculate the responsivity of the device.                                     (4)

                   (ii) Calculate the efficiency of a PIN silicon photodiode if the responsivity is 0.274A/W at 0.85μm wavelength.                                                                                                        (4)

             b.   Discuss the operation of the silicon RAPD (Reach Through Avalanche Photodiode) compare it with p-i-n diode.                                                  (8)

  Q.6     a. An optical transmission system is considered to have 500 GHz channel spacings. How many wavelength channels can be utilized in the 1536-to-1556-nm spectral band?                                         (8)

             b.   Explain the salient features and operational principle of WDM Techniques. Also explain how is it different from FDM.                                          (8)

Q.7     a.   What are the key system requirements needed in analyzing a point-to-point link.                 (8)

     b.   A 5 km length optical fiber link has a fiber cable which has an attenuation of 4dB/km. The splice losses are 0.7dB/km and the connector losses at the source and detectors are 4dB and 3.5dB. Considering no dispersion on the link, calculate the total channel loss.                                                         (8)

Q.8     a.   Explain the spectral characteristics of different codes used in optical communication. Discuss the RZ, NRZ, AMI line codes with example.              (8)

b.   Silica has an estimated fictive temperature of 1400K with an isothermal compressibility of 710^–11 m²N^–1. The  refractive index is 1.46 and photoelastic cofficient is 0.286. Determine the theoretical attenuation in dB/km due to the fundamental Rayleigh scattering in silica at optical wavelength of 0.85μm and 1.30μm. Boltzmann’s constant is 1.38110^–23 JK^–1.                                                                        (8)

Q.9     a.   Describe a semiconductor injection laser. Find its internal quantum efficiency and show how it is related to the differential external quantum efficiency. What is the total efficiency? Explain.   (8)

b.   Define the noise equivalent power (NEP) for a photodetector. Starting with photocurrent, derive an expression for NEP of a photodiode in which the dark current noise dominates.                  (8)