71. The optical source used for detection of optical signal is _________
IR sensors
Photodiodes
Zener diodes
Transistors
Answer.4. Polarization
Explanation:-
The photodiode is used for detecting optical signals. While reverse bias is applied, observing the change in the current with the light intensity will help in detecting the optical signals.
The photodiode is a diode that generates a potential difference when exposed to the light
72. Light can be polarized by
Reflection
Refraction
Scattering
All the above
Answer.4. All the above
Explanation:-
Lightwave that is vibrating in more than one plane is referred to as unpolarized light. Light emitted by the sun, by a lamp in the classroom, or by a candle flame is unpolarized light.
It is possible to transform unpolarized light into polarized light. Polarized light waves are light waves in which the vibrations occur in a single plane. The process of transforming unpolarized light into polarized light is known as polarization. There are a variety of methods of polarizing light. The four methods discussed on this page are:
Polarization by Transmission
Polarization by Reflection
Polarization by Refraction
Polarization by Scattering
Polarization by Selective absorption.
Polarization by Double refraction.
73. An optical fiber behaves as a birefringence medium due to differences in ___________
Effective R-I and core geometry
Core-cladding symmetry
Transmission/propagation time of waves
Refractive indices of glass and silica
Answer.1. Effective R-I and core geometry
Explanation:-
Single-mode fibers with nominal circular symmetry about the core axis allow the propagation of two nearly degenerate modes with orthogonal polarizations.
Hence, in an optical fiber with an ideal optically circularly symmetric core both polarization modes propagate with identical velocities.
Manufactured optical fibers, however, exhibit some birefringence resulting from differences in the core geometry (i.e., ellipticity) resulting from variations in the internal and external stresses, and fiber bending.
An optical fiber behaves as a birefringence medium due to differences in effective R-I and core geometry.
The fiber, therefore, behaves as a birefringent medium due to the difference in the effective refractive indices, and hence, phase velocities, for these two orthogonally polarized modes. The modes
In an optical fiber with an ideal optically circulatory symmetric core, both polarization modes propagate with the same velocities. These fibers have variations in internal and external stress; fiber bending and so exhibit some birefringence.
74. Polarization modal noise can _________ the performance of communication system.
Degrade
Improve
Reduce
Attenuate
Answer.1. Degrade
Explanation:-
Polarization modal noise is generally of larger amplitude than modal noise obtained within multimode fibers.
It can therefore significantly degrade the performance of a communication system such that high-quality analog transmission may prove impossible.
With digital transmission, it is usually necessary to increase the system channel loss margin.
It is therefore important to minimize the use of elements with polarization-dependent insertion losses (e.g., beam splitters, polarization-selective power dividers, couplers to single-polarization optical components, bends in high-birefringence fibers) on single-mode optical fiber links.
75. The beat length in a single-mode optical fiber is 8 cm when light from a laser with a peak wavelength of 0.8 μm is launched into it. Estimate the modal birefringence.
1 × 10-5
3.5 × 10-5
2 × 10-5
4 × 10-5
Answer.1. 1 × 10-5
Explanation:-
Modal Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light.
Calculate
Modal birefringence is given by-
BF = λ/LB
Where
λ = peak wavelength = 8 µm = 0.8 × 10-6 m
LB = beat length = 8 cm = .08 m
= 0.8 × 10-6/0.08
BF= 1 × 10-5
76. When two components are equally excited at the fiber input, then for polarization-maintaining fibers δΓg should be around ___________
1.5ns/km
1 ns/km
1.2ns/km
2ns/km
Answer.2. 1 ns/km
Explanation:-
When two components are equally excited at the fiber input, then for polarization-maintaining fibers δΓg should be around 1 ns/km.
When both polarization components are equally excited (θ = 45°), they remain bound together.
The differential group delay δΓg is related to polarization mode dispersion (PM of fiber. This linear relationship to fiber length however applies only to short fiber-lengths in which birefringence are uniform.
77. Beat length of a single-mode optical fiber is 0.6cm. Calculate the difference between propagation constants for the orthogonal modes.
69.8
99.86
73.2
104.66
Answer.4. 104.66
Explanation:-
The difference between the propagation constant for two orthogonal modes is given by
βx – βy = 2π/LB
Where
βx & βy are propagation constants for slow & fast modes resp.
LB = beat length = 0.6cm = 0.06 m
= 2 × 3.14/0.06
βx – βy= 104.66
78. A polarization-maintaining fiber operates at a wavelength of 1.2μm and has a modal birefringence of 1.8 × 10-3. Calculate the period of perturbation.
0.7 seconds
0.6 seconds
0.23 seconds
0.5 seconds
Answer.2. 0.6 seconds
Explanation:-
The period of perturbation in an optical fiber is given by-
T = λ/BF
Where
λ is operating wavelength = 1.2μm = 1.2 × 10−6m
BF = Birefringence = 1.8 × 10-3
T = 1.2 × 10−6m/1.8 × 10-3
T = 0.6 seconds
T = period of perturbation.
79. The cross polarizing effect may be minimized when the period of the perturbations is less than ______
Bandwidth
Cutoff Period
Cutoff Frequency
None of the above
Answer.3. Cutoff Frequency
Explanation:-
The cross polarizing effect may be minimized when the period of the perturbations is less than a cutoff period T(around 1 mm). Hence polarization-maintaining fibers may be designed by either:
High (large) birefringence: the maximization of the fiber birefringence, may be achieved by reducing the beat length to around 1 mm or less
Low (small) birefringence: the minimization of the polarization coupling perturbations with a period of T.
80. High birefringence fiber and Low birefringence fiber are the part of
Polarization Maintaining Fiber
Non-Polarization Maintaining Fiber
Single Polarization Fiber
Two-Polarization Fiber
Answer.3. Cutoff Frequency
Explanation:-
Polarized light occurs when these two components differ in phase or amplitude.
Polarization is also of concern when a single-mode fiber is coupled to a modulator or other waveguide device that can require the light to be linearly polarized for efficient operation.
Hence, there are several reasons why it may be desirable to use fibers that will permit light to pass through while retaining its state of polarization.
Such polarization-maintaining (PM) fibers can be classified into two major groups: ~ namely, high-birefringence (HB) and low-birefringence (LB) fibers.
