Code: A-24 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. SDH is a standard for ______networks
(A) Twisted pair cable. (B) Coaxial cable.
(C) Ethernet (D) Fiber optic cable.
b. Optical fibers, unlike wire media, are highly resistant to_________.
(A) High frequency transmission. (B) Refraction.
(C) Electro magnetic interference. (D) Low frequency transmission.
c. In _________propagation, the beam of propagated light is almost horizontal and the low-density core has a small diameter compared to the cores of the other propagation modes.
(A) Multimode step index. (B) Multimode graded index.
(C) Multimode single index. (D) Single mode.
d. The performance of transmission media can be measured by
(A) Throughput. (B) Propagation speed.
(C) Propagation time. (D) all of the above.
e. The speed of a photo detector is not dependant on
(A) the carrier drift in the depletion region.
(B) carrier diffusion in the non-depleted region.
(C) diode geometry.
(D) the RC time constant of circuit impedance and diode capacitance.
f. The wavelength of green light in air is ______the wavelength of green light in fiber optic cable.
(A) Less than (B) greater than
(C) equal to (D) none of the above.
g. In FTTC _________is the medium from the cable company office to the subscriber’s curb.
(A) Co-axial. (B) twisted pair.
(C) untwisted pair. (D) optical fiber.
h. The numerical aperture can be expressed as
(A)
(B)
(C) 
(D) 
i. The power emitted by an optical source is measured to be 0 dBm. In linear scale this corresponds to
(A) Zero milli watt. (B) one watt.
(C) 0.1 watt. (D) one milli watt
j. Which of the following is a tunable filter in the context of WDM system?
(A) dielectric thin film. (B) acousto-optic device.
(C) diffraction grating. (D) none of the above.
Answer any FIVE Questions out of EIGHT Questions.
Each question carries 16 marks.
Q.2 a. Define the terms numerical aperture, critical angle and propagation modes in the context of optical fibers. Draw schematic diagrams of cross section and refractive index profile for step-index single mode and multimode fibers and graded index fiber. Indicate typical values for the same. (10)
b. Find
the core radius for highly multimode that can support 1000 modes at an
operating wavelength of 1.3 
m if the refractive index of the core
is 1.5 and that of the cladding is 1.48. (6)
Q.3 a. Draw any two structures each for LED and LD. Compare the properties of LED and LD and comment on their suitability for optical communication. (7)
b. The band gap of a material
of which a laser diode is made is 1.3eV. The cavity length is 0.4 mm and the
reflectivities on the ends is 0.5 on each side. If the loss coefficient is 3/mm,
find the gain coefficient at the threshold. If the threshold current density is
30 
105
amp/m2, find the threshold current for the laser active area
of 0.2 0.5
mm2. Also find the emission wavelength of the above laser. (9)
Q.4 a. Write short notes on the following:
(i) Sub carrier modulation.
(ii) Fiber splicing and joint losses. (8)
b. Discuss in detail, types of single mode fibers. What is fiber birefringence and fiber beat length. What is mode field diameter and why is it an important parameter in single mode fibers? (8)
Q.5 a. Discuss the drive circuit used in modulation of lasers and LED’s. (9)
b. Consider a LED source of emitted power 1mW, with rise time 20 ns and spectral width of 20 nm. Let the detector be a PIN diode with a rise time of 1 ns and sensitivity of –30 dBm. The fiber being used is a step-index multimode fiber of core index 1.46, NAA = 0.2 and is available in lengths of 2km. Let the source coupling loss be 3dB, system margin is 5dB, splice loss is 0.2 dB per splice and attenuation is 2 dB/km at an operating wavelength of 850 nm. Calculate the maximum permissible link length and data rate. (7)
Q.6 a. Write down and explain the link design equations in a point-to point communication link, based on power budget and rise time budget considerations. (9)
b. Discuss the basic network topologies used in LAN’s. Give a brief account of FDDI, DQDB and star coupler and explain how these systems help in connecting different nodes. (7)
Q.7 a. Describe the different mechanisms that limit the frequency response of a photodiode. Derive the necessary equations and discuss the effect of transit times on the frequency response of photodiodes. Indicate how the speed can be improved by proper choice of parameters. (10)
b. A APD generates a current
of 100 nA when the incident power is 5nW. The operating wavelength is 1.5 . Find its
responsivity. If the quantum efficiency is 0.7, find the multiplication factor. (6)
Q.8 a. Discuss the advantages of PIN photodiode over p-n structure in a photo diode. (5)
b. Define and explain the meaning of responsivity and quantum efficiency of a photodiode, and derive the expressions for the same. (7)
c. A photo diode has a quantum
efficiency of 0.6 at 0.85 m. Find the
responsivity. (4)
Q.9 a. What are the underlying principles of the WDM technique? What are its various advantages? How is it different from FDM techniques? (8)
b. Consider the low loss of a silica fiber communication system to be in the region of 1520-1580 nm. Find the number of channels which can be multiplexed by FDM, if the channel spacing is required be to 75 GHz. (4)
c. What is optical TDM? Compare it with WDM. (4)