RTU 2015 Question Paper Semester VIII Electronics and Communication Engineering Optical Communication

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o B. Tech. VIII Sem. (Main/Back) Exam., April, 2015
sEI Electronics & Communication Engineering
6 8EC3 Optical Communication

Time: 3 Hours Maximum Marks: 80
Min. Passing Marks: 24
Instructions to Candidate s :
Attempt any five questions, selecting one questi.on from each unit. All questions
carry equal marks. Schematic diagrams must be shown wherever necessary. Any
data youfeel missing suitably be assumed and stated clearly.
Units of quantities used/calculated must be stated clearly.
Use of following supporting material is permitted during examination.
1. NIL 2.NIL

Q.1 (i) Describe the techniques employed and the fiber structures utilized to provide-[10]

(a) Dispersion - shifted single mode fiber
(b) Dispersion - flattened single mode fiber
(ii) A multimode graded index fiber exhibits total pulse broadening of 0.1 ps over a
distance of 15 km. Estimate- t6l

(a) The maximum possible bandwidth on the link assuming no inter-symbol
interference.

(b) The pulse dispersion per unit length.

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B
Q.l (a) Briefly explain the various factors contributing to dispersion. t6l

(b) Explain Material dispersion and Waveguide dispersion' t61

(c) when the mean optical power launched into an 8 km length of fiber
is 120pw,

the mean optical power at the fiber output is 3pw. Determine
- t4l

(i) Overall signal attenuation
(ii) The signal attenuation per kilometer for the fiber'

UNIT - II
quantum efficiency of
Q.2 (a) Explain the quantum efficiency of LED and external
t81
Laser.

(b) A double heterojunction in GaAsP LED emitting at a peak wavelength of 1310
nm has radiative and non-radiative recombination times-of 30 and 100 ns,
respectively. The drive current is 40 mA' Calculate - t8l

(i) Internal quantum efficiencY

(ii) Optical power generated internally'
OR

length (GaAlAs)
e.2 (a) Discuss the Buried - hetrostructure laser diodes for short wave
and long wave length (In GaAsP)' t8l

(b) Write Laser diode rate equation. t8l

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UNIT _ III

Q.3 (a) A photo diode has a quantum efficiency of 657o when photons of energy
1.5x10-re J are incident upon it. tgl

(i) At what wavelength is the photodiode operating?

(ii) Calculate the incident optical power required to obtain a photocurrent of
2.51tA when the photo diode is operating as above.

(b) Draw the block diagram of optical receiver and explain the fundamental concept
of coherent light wave system. t8l

OR
Q.3 (a) Discuss the following in brief. lt21
(i) Detector response time

(ii) Avalanchemultiplicationnoise.
(b) If the photodiode capacitance is 3PF, the amplifier capacitance is 4 PF, the load
resistor is lKQ and the amplifier resistance is lMC), find the circuit bandwidth.[4]

UNIT - IV
Q.4 (a) Draw the schematic diagram of a light souSce coupled to an optical fiber and
calculate the maximum optical power coupled into fiber. t10l
(b) A GaAs optical source with a refractive index of 3.6 is coupled to a silica fiber
that has a reflective index of 1.48. If the fiber end and the source are in close
physical contact, calculate power loss 'L' in decibels.
t6l

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OR

Q.4 (a) Write some principal requirements of a good connector design and explain the
different optical fiber connectors. t8l

(b) Explain the losses caused by longitudinal, lateral and angular displacement in
splicing of fibers. t8l

Q.5 Explain with a neat diagram-
(i) Time domain intermodal dispersion measurement technique t6l
(ii) Frequency domain intermodal dispersion measurement tsl
(iii) Chromatic dispersion measurement Isl

OR

Q.5 (a) Di'scuss the cut back technique of attenuation measurement with diagram. Also
state the merits and demerits of it. tSl

(b) Briefly explain the measurement of numerical aperture of optical fibers. t8l

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