Sri vidya college of Engineering & Technology OED – QUESTION Bank – UNIT - 4
UNIT – IV
OPTOELECTRONIC MODULATOR
Part-A
1. Compare Analog Modulation Digital Modulation NOV/DEC 2016
S.No Analog Modulation Digital Modulation
1 Both message and carrier waves Message signal will be in continuous
are continuous. form and carrier will be digital
2 Requires higher SNR Requires low SNR
3 Good for only low frequency and Good for high frequency and high
low bandwidth signals. bandwidth signals
4 High current levels are needed for Lower current levels is needed for
modulating higher bandwidth modulating higher bandwidth signals
signals
2. Define electro-optic modulators and electro-optic effect.
Electro-optic modulators is an optical device in which a signal controlling
element displays electro-optic effect to modulate a beam of light. The modulation can
be done by changing phase, frequency, amplitude, or polarization of the modulated
beam. Electro-optic effect refers to change in refractive index of the material resulting
from application of a d.c. or low frequency electric field.
3. Define the term birefringence.
Birefringence refers, for a linearly polarized wave that is propagating in z-
direction, itspolarization vector will depend on the direction of electric field. Due to
this effect, the amplification of electric field in one direction will not be same in
opposite direction.
4. What are magneto-optic devices? APR/MAY 2017
These are devices which work under magneto-optic effect. A magneto-optic
effect is aphenomena in which an electromagnetic wave propagates through a medium
that has been altered by the presence of a quasistatic magnetic field. In such a
material, which is also called gyrotropic or gyromagnetic, left- and right-rotating
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elliptical polarizations can propagate at different speeds, leading to a number of
important phenomena
5 . What are acoustoptic devices?
Acoustoptic devices are devices which work under acoustoptic effect.
Acoustoptic effect refers to there will be a change in material permittivity ‘e’ due to
mechanical strain ‘a’.
6. What do you mean by SEED?
This is a device exhibiting nonlinear absorption or reflection of an optical
signal, photonic switching, bistability, and optically induced oscillations. It is a
combination of a detector self biasing a n electro-absorption modulator. The working
is based upon multiple quantum well (MQW)-III V technology.
7. What are acoustoptic modulators?
These devices will vary the acoustic wave properties such as amplitude, phase,
frequency, or polarization to modulate acoustic wave. These properties can be varied
by making the optical wave travelled through acoustic field.
8. What are the limitations of acoustoptic modulators?
The limitations of acoustoptic modulators are,
The design is complex and should be carefully designed
Switching speed is limited
Light cannot be fully switched ON and OFF
9. Define acoustoptic filter.
The principle of operation of acoustoptic filter is based upon the wavelength of
diffracted light. Wavelength depends on frequency. By tuning the frequency of
acoustic wave, desired wavelength of optical wave can be diffracted.
10. Give the merits of PCM?
The non –linearities of the light source do not affect the quality of the
transmitted signal.
Even though the transmitted signals are highly degraded or attenuated, the
original analog signals may be obtained without any error. this is free from noise and
temperature effects.
11. What are the demerits of PCM? { MAY-12]
There is a small quantization error.
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Sri vidya college of Engineering & Technology OED – QUESTION Bank – UNIT - 4
There is a possibility of formation of noise during coding or decoding the
signal.
12. What are the types of microelectrodes?
• Metallic microelectrode
• Nonmetallic or micropipet.
13. What is the necessity of cladding for an optical fiber?
To provide proper light guidance inside the core To avoid leakage of light
from the fiber
To give mechanical strength for the fiber.
14. Define relative refractive index difference .
∆=n1 2 - n22/2n12=n1- n2/n1
Thus the relative refractive index difference is the ratio between the refractive index
difference .
15.What arev skew rays? { MAY-13]
Skew rays are the rays following the helical path around the fiber axis when
they travel through the fiber and they would not cross the fiber axis at any time.
16.What are meridional rays? { DEC-12]
Meridional rays are the rays following ZIG-Zag path when they travel through
fiber and for every reflection it will cross the fiber axis.
17.What are microbending losses?
These occur due to bends in the fiber axis these microbending losses occur
during cabling and stresses acting on the fiber.these produce mode coupling losses
also.
18.Define cutoff wavelength of the fiber.
The cut off wavelength is defined as the minimum value of wavelength that can
be transmitted through the fiber .the wavelengths greater than the cutoff wavelength
can be transmitted.
Λcutoff=2πa(N.A)/V
19.What is material dispersion?
Material dispersion arises due to variation of refractive index of core with
respect to wavelength of light this occur when d2 n1/dλ2≠0
20.What is wave guide dispersion? { MAY-13]
Wave guide dispersion is due to finite frequency bandwidth and the
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dependence of mode group velocity on the frequency of light.
21. Why do we have smaller dispersion in graded index fibers?
Due to shaping the refractive index profile in the parabolic manner and by self
focusing effect, the dispersion is small.
22.What is dark current noise? { NOV/DEC-13]
Dark current noise is due to the flow of current through the bias circuit even
though there is no incident light.
PART –B
1. Explain the concept of external modulation and compare with direct
modulation.
Tabulation
Explanation
Diagram.
2. Briefly explain about the Analog and Digital Modulation.
Explanation
Diagram.
Types
Waveform
3.i) Explain the concept of birefringence in Uniaxial crystal with necessary
diagrams. [May/June-2013]
Diagram
Construction
Explanation
3.ii) Explain with neat diagram, the construction of electro optic effect based
External modulator. Also deduce the expression of modulated light.
[Nov/Dec - 2015]
Diagram
Construction
Derivation
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4. Discuss in detail the principle and operation of a photonic switch based on self
electro optic Device. And Explain the concept of Bipolar controller
Modulator. [Nov/Dec - 2015]
Principle
Operation
Diagram
Equation
Graph
5. Explain the Electro-Optic Modulator [NOV/DEC2016 & APR/MAY 2017]
Definition
Construction
Diagram
Working
Derivation
6. i) Explain the Magneto-Optic Devices:
Definition
Construction
Faraday Effect:
Diagram
Equation
6. ii) Explain about Acoustic optic effect: [Nov/Dec 2013]
Definition
Construction
Schematic illustration of acousto-optic modulation.
Equation
Diagram
OPTO ELECTRONIC DEVICES - QUESTION BANK- UNIT IV ECE DEPT
