# Optical Fiber Polarization MCQ || Polarization in Optical Fiber Questions and Answers

1. Which one of the following optical phenomena gives evidence that light waves are transverse?

1. Rectilinear propagation of light
2. Polarization of light
3. Diffraction of light
4. Interference of light

Answer.2. Polarization of light

Explanation:-

• Light is a transverse, electromagnetic wave that can be seen by the typical human.
• The wave nature of light was first illustrated through experiments on diffraction and interference.
• Like all electromagnetic waves, light can travel through a vacuum.
• The transverse nature of light can be demonstrated through polarization.
• The phenomenon of the restriction of the vibrations of light waves in a particular plane perpendicular to the direction of the propagation of wave motion is called polarization.
• Polarization explains the wave nature of light, as light waves are polarized in a particular plane.
• The longitudinal waves cannot be polarized. So, the transverse waves can only be polarized. Hence, the polarization of light proves the transverse nature of light.

2. The phenomenon of polarization shows that light has _____ nature.

1. Particle
2. Transverse
3. Longitudinal
4. Dual

Answer.2. Transverse

Explanation:-

Polarization is a process by which unpolarised light is transformed into polarized light. Polarisation tells about the transverse wave nature of light, as the light wave is polarised in a particular plane. The longitudinal waves cannot be polarised. Only transverse waves can be polarized. So, by the phenomenon of polarization, light shows transverse nature.

3. The phenomenon of polarization CANNOT occur in ________

1. Radio waves
2. x-rays
3. Acoustic waves
4. Visible light waves

Answer.3. Acoustic waves

Explanation:-

• Transverse waves can be polarized because their vibrations can potentially occur in all directions perpendicular to the direction of travel. It is, therefore, possible to confine the vibrations to a single plane. Transverse waves: The wave in which the movement of the particles is at right angles to the motion of the energy is called a transverse wave. Light is an example of a transverse wave.
• Longitudinal wave: The wave in which the movement of the particles is parallel to the motion of the energy is called a longitudinal wave. A sound wave is an example of a longitudinal wave. Longitudinal waves cannot be plane polarised because the direction of vibration and direction of propagation is the same.
• Radio waves, x-rays, and visible light waves come under the EM waves category. Hence they can be polarized.
• Sound waves (acoustic waves) are longitudinal waves hence cannot be polarized.

4.  _______ fiber does not maintain the polarization state of the light input.

1. Cylindrical Optical Fiber
2. Plastic Optical Fiber
3. Coaxial Fiber
4. Glass Fiber

Answer.1. Cylindrical Optical Fiber

Explanation:-

Cylindrical optical fibers do not generally maintain the polarization state of the light input for more than a few meters, and hence, for many applications involving optical fiber transmission, some form of intensity modulation of the optical source is utilized.

The optical signal is thus detected by a photodiode which is insensitive to optical polarization or phase of the lightwave within the fiber.

5. Transverse nature of light is examined by:

1. Dispersion
2. Scattering
3. Diffraction
4. Polarization

Answer.4. Polarization

Explanation:-

Polarised light: The light having vibrations only along a single straight line perpendicular to the direction of propagation of light is said to be polarised. Its vibrations are one-sided, therefore, there is a lack of symmetry about the direction of propagation of light.

Only transverse waves can be plane polarised because their vibration is perpendicular to the direction of energy travel you can therefore have this vibration at different orientations. Longitudinal waves cannot be plane polarised because the direction of vibration and direction of propagation is the same.

6. The optical source used for detection of optical signal is _________

1. IR sensors
2. Photodiodes
3. Zener diodes
4. Transistors

Answer.2. Photodiodes

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.

7. Light can be polarized by

1. Reflection
2. Refraction
3. Scattering
4. 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.

8. An optical fiber behaves as a birefringence medium due to differences in _______

1. Effective R-I and core geometry
2. Core-cladding symmetry
3. Transmission/propagation time of waves
4. 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.
• 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.

9. Polarization modal noise can _________ the performance of communication system.

1. Degrade
2. Improve
3. Reduce
4. 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.

10. 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. 1 × 10-5
2. 3.5 × 10-5
3. 2 × 10-5
4. 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

11. When two components are equally excited at the fiber input, then for polarization-maintaining fibers δΓg should be around ________

1. 1.5 ns/km
2. 1 ns/km
3. 1.2 ns/km
4. 2 ns/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.

12.  Beat length of a single-mode optical fiber is 0.6cm. Calculate the difference between propagation constants for the orthogonal modes.

1. 69.8
2. 99.86
3. 73.2
4. 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

13. 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.

1. 0.7 seconds
2. 0.6 seconds
3. 0.23 seconds
4. 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.

14. The cross polarizing effect may be minimized when the period of the perturbations is less than ______

1. Bandwidth
2. Cutoff Period
3. Cutoff Frequency
4. 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:

1. High (large) birefringence: the maximization of the fiber birefringence,  may be achieved by reducing the beat length  to around 1 mm or less
2. Low (small) birefringence: the minimization of the polarization coupling perturbations with a period of T.

15. High birefringence fiber and Low birefringence fiber are the part of

1. Polarization Maintaining Fiber
2. Non-Polarization Maintaining Fiber
3. Single Polarization Fiber
4. Two-Polarization Fiber

Answer.1. Polarization Maintaining Fiber

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.

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