M.Sc. (Semester IV) (CBCS) Examination Mar/Apr-2018
Physics (Applied Electronics)
FIBER OPTIC COMMUNICATIONS
Time: 2½ Hours
Max. Marks: 70
Instructions: Q.1 and Q.2 are compulsory. Attempt any three questions from Q. 3 to 7. All questions carry equal marks. Assume suitable data in necessary.
Q.1

Select the correct alternative:
06
To achieve optical amplification the condition N2 N1 (density of atoms in energy levels E1 and E2) is known as
Amplification
Polarization
Population inversion
Attenuation
Laser is optical source.
Non-coherent
Coherent
Both and
None of these
The total number of guided modes for a step index fiber is approximately given by expression, where V is normalized frequency as
M V2
M V2
M V
M=V2
Microscopic meandering of the fiber core axis is known as
Bending
Microbending
Dispersion
None of these
Multipath dispersion does not exist in a
Single mode fiber
Multi mode fiber
Plastic optical fiber
The internal quantum efficiency of LED decreases with
Increase in temp
Decrease in temp
Increase in pressure
Decrease in pressure
Q.1

State true or false:
08
Multimode graded index fibers exhibit for less intermodal dispersion than multimode step index fibers due to their multipath reflection.
Linear scattering may also occur at in homogeneities which are comparable in size with the guided wavelengths.
The LED can operate at higher current density than the injection laser.
Numerical aperture of an optical fiber represents the cone outside which the light is incident on fiber end
The refractive index profile of the fiber cladding plays an important role in characterizing the properties of optical fibers.
Microscopic meandering of the fiber core axis is known as Dispersion.
In order to allow operation at longer wavelengths where the light penetrates more deeply into the semiconductor material, a wider depletion region is necessary.
Stimulated Brillouin scattering may be regarded as the modulation of light through thermal molecular vibrations within the fiber.
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SLR-UR-579
Q.2
Write short notes:
14
Linear and non scattering losses
05
Attenuation measurement technique using the loss at a single wavelength
05
Fiber connectors
04
Q.3
Define quantum efficiency and responisivity of a photo detector. Derive expression for responsivity of an intrinsic photo detector in terms of efficiency and wavelength of incident radiation of device.
08
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 10-6 A.
06
Q.4
Define relative refractive index difference for an optical fiber show how it may be related to Numerical aperture.
08
Explain requirements of optical sources.
06
Q.5
Explain a typical experimental arrangement for the measurement of dispersion loss with the swept frequency measurement method.
08
An 8 km optical fiber link without repeaters uses multimode graded index fiber which has a bandwidth length product of 400MhzKm.Estimate:
The total pulse broadening on the link
The rms pulse broadening on the link
06
Q.6
What are the different applications of fiber optics, explain in detail?
08
The velocity of light in the core of a step index fiber is 2.01X108 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.
06
Q.7
What are couplers? Explain different types of couplers used in fiber optical communication.
08
Explain the structure of surface emitting LED.
06