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.
Solve any three questions from Q.3 to Q.7.
All questions carry equal marks.
Q.1

Choose the correct option:
06
A step index multimode fiber with N.A. 0.2 supports approximately 1000 modes at 850 nm wavelength. What is core diameter?
15.20 um
30.25 um
40.80 um
60.50 um
To achieve optical amplification the condition N2 N1 (density of atoms in energy levels E1 and E2) known as
Amplification
Polarization
Population inversion
Attenuation
The requirement of detector is
High fidelity
Small size
Low bias voltage
All of 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

State True or False:
08
Multipath dispersion does not exist in a Single mode fiber.
The ray possesses trough the axis of the fiber core is called meridional ray.
Impact ionization phenomenon occurs in PN photodiode.
The cutback or differential method use to measure refractive index profile.
Light signal trough optical fiber attenuates due to radiation losses only.
Stimulated Brillouin scattering may be regarded as the modulation of light through thermal molecular vibration within the fiber.
Edge emitting LED has greater temperature dependence then surface emitting LED.
LASER works on absorption principle only.

Q.2
Write short notes:
a)Phase velocity and group velocity of optical fiber
05
b)Working principle of LASER
05
c)Principle of photodiode.
04
Q.3
a)With the help of diagram, explain step index and graded index fibers.
08
b)Explain a typical experimental arrangement for the measurement of spectral loss in optical fiber using the cutback technique.
06
Q.4
a)Define quantum efficiency and responsivity of a photodetector. Derive expression for responsivity of an intrinsic photodetector in terms of efficiency and wavelength of incident radiation of device.
08
b)Explain characteristics of injection LASER.
06
Q.5
a)Explain the effect of dispersion in different types of optical fiber with neat diagram.
08
b)Explain the experimental setup for the near field scanning measurement of the refractive index profile.
06
Q.6
a)A p-n photodiode has aquqntum efficiency of 50% at the wavelength of 0.9μm. Calculate:
Its responsivity at 0.9μm
The received optical power if the mean photocurrent is 10-6 A
The corresponding number of received photons at this wavelength.
08
b)With diagram, explain vapour phase deposition techniques.
06
Q.7
a)The velocity of the light in the core of a step index fiber is 2.01X108 critical angle at the core cladding interface in 80o. Determine the numerical aperture acceptance angle for the fiber in air, assuming it has a core diameter suitable for consideration for ray analysis.
08
b)Explain advantages of Fiber optic communication.
06