M.Sc. (Semester IV) (CBCS) Examination Nov/Dec-2018
Physics (Applied Electronics)
FIBER OPTIC COMMUNICATIONS
Time: 2½ Hours Max. Marks: 70
Instructions: Q.1 and Q.2 is compulsory.
Any three questions from Q.3 to Q.7.
All questions carry equal marks.
Assume suitable data if necessary.
Q.1 Choose the correct alternatives:- 06
Laser is optical source.
Non-coherent Coherent
Both a and b None of these
Multipath dispersion does not exist in a
Single mode fiber Multi mode fiber
Plastic optical fiber Low loss fiber
The requirement of detector is
High fidelity Small size
Low bias voltage All of the above
Multimode graded index fibers tend to have core diameters
than multimode step index fibers.
Smaller Greater
Varying Constant
The transmission distance up to which a fiber optic link will work well is
dependent greatly on three fiber parameters, namely the numerical
aperture and attenuation.
Dispersion Cladding size
Material used for cladding Core size
The is directly proportional to the quantum efficiency at a
particular wavelength.
Attenuation Responsivity
Dispersion Quantum efficiency
Q.1 State True or False:- 08
Multipath dispersion does not exist in a Single mode fiber.
The ray passes through the axis of the fiber core is called meridional ray.
Numerical aperture of an optical fiber represents the cone outside which
the light is incident on fiber end.
Edge emitting LED has lower temperature dependence then surface
emitting LED.
Light signal through optical fiber attenuates due to radiation losses only.
Stimulated Brillouin scattering may be regarded as the modulation of
light through thermal molecular vibrations within the fiber.
The Dome LED has a higher external power efficiency than the planar
LED.
LASER works on absorption principle only.
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SLR-VQ-467
Q.2 Write short notes:-
Advantages and disadvantages of fiber communication system.
Photo conductive detectors.
Fiber transmission characteristics.
14
05
05
04
Q.3 Compare dispersion in different types of optical fibers.
A step index fiber in air has a numerical aperture of 0.16, a core refractive
index of 1.45 core diameter of 60 Determine the normalized frequency
for the fiber when light at wavelength of 0.9 is transmitted. Further
estimate the number of guided modes propagating in the fiber.
08
06
Q.4 The velocity of light in the core of a step index fiber is 2.01 X 108ms-1,
critical angle at the core cladding interface is 80°. Determine the numerical
aperture acceptance angle for the fiber in air, assuming it has a core
diameter suitable for consideration for ray analysis.
Explain about fiber coupler used in fiber optics communication systems.
08
06
Q.5 The numerical input/output mean optical power ratio in a 1Km length of optical
fiber is found to be 2.5. Calculate the received mean optical power when a
mean optical power of 1mw is launched into a 5km length of the fiber.
Explain the experimental setup for the near field scanning measurement of the
refractive index profile.
08
06
Q.6 A p-n photodiode has a quantum efficiency of 50% at the wavelength of 0.9
calculate:
Its responsivity at
The received optical power if the mean photocurrent is 10-6A.
The corresponding number of received photons at this wavelength.
08
With diagram explain vapor phase deposition technique. 06
Q.7 Define quantum efficiency and responsivity of a photodetector. Derive
expression for responsivity of an intrinsic photo detector in terms of efficiency
and wavelength of incident radiation of device.
08
Explain applications of fiber optic in public network, military industrial and
computer applications.