2022 May ETC Optical Communication

2022141
IV BE Examination May- 2022

Elex. & Telecommunication Engineering
ETR8C2:Optical Communication
Time: 3 Hours Max. marks:60
Note: All questions are Compulsory. Attempt any two parts from each question. If required,
assumtions may be taken. Symbols/acronyms have usual meaning in the context of the subject
area.
1(a) A rectangular planar waveguide has a relative refractive index of 1.5 % and a core (6)
refractive index of 1.35. When the normalized frequency attains its maximum value, 9
modes propagate in the waveguide corresponding to the light of wavelength 1.55 µm.
Estimate the thickness of the waveguide. Also determine the cut off wavelength for such
waveguide.
(b) What is understood by TE and TM modes in an optical waveguide. Using Maxwell’s (6)
equations, prove for the TE modes - iβHx - (∂Hz)/∂x = iωϵo K(x)Ey.
A step index fiber has a solid acceptance angle in air of 0.115 radians and a relative
refractive index difference of 0.9%. Estimate the speed of light in the fiber core.
(c) Give reasons: (6)
(i) Reduced EMI and RFI in optical fibers (ii) High bandwidth of optical fiber.
(iii) High durabiliy of optical fibers
2(a) Explain the advantage of single mode graded index fiber with respect to single mode (6)
step index fiber. A graded index fiber with a parabolic refractive index profile core has
a refractive index at the core axis of 1.5 and a relative refractive index of 1.1%. Extimate
the maximum possible core diameter which allows single mode operation at a
wavelength of 1.55 µm..
(b) Distinguish between microbending and macrobending loss mechanisms. (6)
-1
A 15 km optical fiber link uses fiber with a loss of 1.5 dBkm . The fiber is jointed every
two kilometers with connectors having atteneuation of 0.8 dB each. Determine the
minimum mean optical power which must be launched into the fiber in order to maintain
a mean optical power level of 0.35 microwatts at the detector.
(c) Explain the origin of waveguide dispersion. In a rectangular step index waveguide, plot (6)
the transverse electrical field variation as a function of x/d for the mode number, m=3,4.
Show that the total broadening of a light pulse T, due to intermodal dispersion in a
multimode step index fiber may be given by:
T = [L.(NA)2]/[2n1.c];
here ‘L’ is fiber length, ‘NA’ is numerical aperture, ‘n1’ the refractive index of core, ‘c’ the
velocity of light in vacuum.
3(a) GaAs has a bandgap energy of 1.43 eV at 300 K. Determine the wavelength above which (6)
an intrinsic photodetector fabricated from this material will cease to operate.
1
Define quantum efficiency and responsivity in reference to a photodiode. List the main
drawback and advantages of APDs.
(b) Using suitable diagrams discuss the effect of temperature in case of Laser diode and LED (6)
devices. Also distinguish between DFB and DBR lasers.
(c) Draw and explain the characteristic emission pattern of different types of Laser diodes (6)
and LEDs.
A Laser diode has a GaAs active region with a bandgap energy of 1.43 eV. Estimate the
wavelength of optical emission from the device and determine its linewidth in Hertz
when the measured spectral width is 0.1nm..
4(a) Explain the working principle of (i) Long period grating based filter (ii) Multilayer (6)
dielectric thin film filters
(b) Ellaborate on operation of Mach-Zehnder based MUX/DEMUX. (6)
In a Mach- Zehnder based interferometric DEMUX, the wavelength in the region of 0.7-
1.8microns is incident at input port no. 1 with no input applied at port no. 2. If the effective
refractive index of the waveguide is 1.12, and the path difference between interferometric
arms is 5.01microns, calculate the wave-lengths separated out at port no. 1. What is to be
done if another set of wavelengths are required as output at the ports.
(c) How does the stark effect and thermalization phenomenon important in the context of (6)
EDFA. With the help of suitable diagrams, dwell on the principle of operation of Erbium
Doped Fiber Amplifiers (EDFA).
5(a) List various advantages of SONET technology over PDH. Ellaborate on the SONET (6)
frame structure.
(b) Ellaborate on the functional elements and salient features of (i) Optical line terminals (6)
(ii) Optical Add/Drop Multiplexers (iii) Optical Cross connects.
(c) List and explain various WDM network architectures. List their limitations. What are (6)
the constraints to be followed in RWA.

2022 May ETC Optical Communication

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2022141

IV BE Examination May- 2022

Time: 3 Hours Max. marks:60

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