# Optical Fiber Communication MCQ

Optical Fiber Communication MCQ, Multiple Choice Questions on Optical Fiber Communication, Fiber Optics MCQ, GATE questions on Optical Fiber Communication, Objective Questions with answers on Fiber Optics, Objective Questions with answers on Optical Fiber Communication

## Optical Fiber Communication MCQ

1. A glass fiber has refractive indices n1 of 1.5 and n2 of 1. Assuming c = 3 x 108 m/s the multipath time dispersion will be

• (a) 2.5 ns/m
• (b) 2.5 µs/m
• (c) 85 ns/m
• (d) 5 µs/m

Explanation: We know that multipath time dispersion, MTD = \frac{\Delta t}{z}= \frac{n_1}{n_2}.\frac{\Delta n}{c}.

\therefore MTD = \frac{1.5}{1}. \frac{0.5}{3 \times 10^8}\, m/sec \therefore MTD = 2.5 \, ns/m

2. In optical communications, the losses in optical fibers can be caused by

1. impurities
2. microbending
3. attenuation in glass
4. stepped index operations

Which of these statements is correct?

• (a) 1, 2 and 3
• (b) 1, 3 and 4
• (c) 1, 2 and 4
• (d) 2, 3 and 4

Answer: (a) 1, 2 and 3

Explanation: Losses in OFC are caused due to:

1. Impurities
2. Micro-bending
3. Attenuation in glass

3. A certain optical fiber has a refractive index of clad n1 = 1.4 and that of the core n2 = 1.05. Its numerical aperture will be

• (a) 0.8575
• (b) 0.9260
• (c) 0.3500
• (d) 0.1585

Explanation: n1 = 1.40 , n2 = 1.05

Numerical aperture, \mathrm{NA=\sqrt{n_1^2 - n_2^2}}

\Rightarrow \mathrm{NA=0.9260}

4. Velocity of light traveling in an optical fiber is

• (a) Equal to c
• (b) Greater than c by a few percent
• (c)Less than c by a few percent
• (d) Much greater than c, approaching the magnitude of c2

Answer: (b) Greater than c by a few percent

5. What is the responsivity of a photodiode having a quantum efficiency 65% with photons of energy 1.5 X 10-19 J incident upon it?

• (a) 0.832 AW-1
• (b) 0.715 AW-1
• (c) 0.694 AW-1
• (d) 0.452 AW-1

Explanation:

Responsivity, \mathrm{R = \frac{Output\ Photocurrent}{incident\ optical\ power} = \frac{\eta_e}{\eta_f}}

\Rightarrow \mathrm{R = \frac{0.65 \times 1.6 \times 10^{-19}}{1.5 \times 10^{-19}} = 0.694 AW^{-1} }

6. The normalized frequency of a step-index fiber is 28 at 1300 nm wavelength. What is the total number (approximately) of guided modes that can be supported by the fiber?

• (a) 50
• (b) 200
• (c) 400
• (d) 800

Explanation: For a step-index fiber, \mathrm{M_s \cong \frac{V^2}{2} }

where, Ms = total number of guided mode, V = normalized frequency of fiber

\mathrm{M_s \cong \frac{(28)^2}{2}= 392 }

7. Which one of the following is not a correct reason for biasing the LASER near but below the threshold in the off state for a laser drive circuit?

• (a) It reduces the switch-on delay
• (b) It allows easy compensation for changes in ambient
• (c) It eliminates the spontaneous emission of light in the off state
• (d) It reduces the junction heating caused by the digital drive current

Answer: (c) It eliminates the spontaneous emission of light in the off state

8. The modal noise in multimode fibers cannot be reduced by

• (a) use of a broad spectrum source
• (b) using fibers having large numerical apertures
• (c) phase correlation between the modes
• (d) use of a single-mode fiber

Answer: (c) phase correlation between the modes

Explanation: Phase correlation between modes is a condition to rise modal noise.

9. Which of the following are the advantages of a fiber optic link over a conventional copper wire link?

1. A fiber optic link has greater bandwidth.
2. A fiber optic link has a lower cost.
4. A fiber optic link is easy to split.

Select the correct answer using the code given below:

• (a) 1 and 2
• (b) 1 and 3
• (c) 2 and 3
• (d) 1 and 4

10. Light travels along the optical fibers by which mechanism?

• (a) Refraction
• (b) Reflection
• (c) Scattering
• (d) Total internal reflection

Explanation: The principle of optical fiber communication is “Total Internal Reflection”.

11. Which one of the following is correct. statement? The graded index optical fiber cable can be considered to have

• (a) smaller numerical aperture than step index cable
• (b) several layers of cladding and each layer having a smaller index number relative to its distance from the core
• (c) several layers of cladding and each layer having a higher index number relative to its distance from the core

Answer: (b) several layers of cladding and each layer having a smaller index number relative to its distance from the core

12. In an optical fiber, the light beam propagates due to which one of the following?

• (a) Simple reflection of light at a boundary between two media
• (b) Refraction of light in the medium fiber
• (c) Total internal reflection at the boundary of the fiber
• (d) Scattering of light in the medium

Answer: (c) Total internal reflection at the boundary of the fiber

13. Dispersion in an optical fiber used in a communication link is of which type?

• (a) Angular dispersion
• (b) Modal dispersion
• (c) Chromatic dispersion
• (d) Dispersion arising due to structural irregularities in the fiber

14. In optical communication, wavelength 1550 nm is used nowadays. What are the possible reasons?

1. Dispersion is very low
2. Loss is very low
3. WDM and optical amplification are feasible

Select the correct answer from the codes given below:

• (a) 1, 2 and 3
• (b) 2 only
• (c) 1 only
• (d) 1 and 3 only

Answer: (d) 1 and 3 only

15. Why are semiconductor lasers important for communication engineering?

1. They possess large bandwidth
2. They operate at low voltage
3. They are suitable for coupling to fibers at wavelengths, where the attenuation is very small
4. Their integration with other components is easy

Select the correct answer using the code given below.

• (a) 1 only
• (b) 1 and 2 only
• (c) 2 and 3 only
• (d) 1, 2, 3 and 4

Answer: (d) 1, 2, 3 and 4

16. A Ruby Maser is considered to be superior to an Ammonia Maser because of

• (a) higher power output
• (b) low signal-to-noise ratio
• (c) greater bandwidth
• (d) low amplification factor and low noise levels

17. Losses in optical fibers can be caused by which of the following?

1. Impurities
2. Microbending
3. Stepped index operation

Select the correct answer using the code given below.

• (a) 1 and 3
• (b) 2 and 3
• (c) 1 and 2
• (d) 3 only

18. A single-mode fiber does not suffer from which type of dispersion?

• (a) Waveguide dispersion
• (b) Material dispersion
• (c) Intermodal dispersion
• (d) Polarization mode dispersion

Explanation: Intermodal dispersion arises in multimode fiber.

19. Which of the following introduces mode partition noise?

• (a) Coaxial line
• (b) Waveguide
• (c) Fiber transmission line
• (d) Both coaxial line and waveguide

Explanation: Mode partition noise exists in the fiber line. Mode partition noise is intensity fluctuation in longitudinal modes of LASER.

20. Which of the following does not cause losses in optical fiber cables?

• (a) Impurities
• (b) Microbending
• (c) Attenuation in glass
• (d) Stepped index operation

21. On which bands, do the optical fibers operate?

1. Ultraviolet band
2. Ultra high-frequency band
3. Visible light band
4. Infrared band

Select the correct answer from the codes given below :

• (a) 1 only
• (b) 1 and 2 only
• (c) 1, 2 and 3
• (d) 1, 3 and 4

Answer: (d) 1, 3 and 4

22. Which one of the following photo-detector does not provide gain?

• (a) Photo-transistor
• (b) Photoconductor
• (c) Avalanche photodiode
• (d) p-i-n photodiode

23. Population inversion in the semiconductor laser diode is achieved by

• (a) Lightly doping p and n sides.
• (b) Introducing trap centers on the p and n sides
• (c) Heavily doping p and n sides
• (d) Reverse biasing the junction

Answer: (b) Introducing trap centers on the p and n sides

24. Consider the following advantages of optical fiber- cables:

1. Small diameter
2. Immunity to cross-talk and electromagnetic interference
3. Laser and LED modulation methods lend themselves ideally to digital operation

Which of these advantages is correct?

• (a) 1 and 2 only
• (b) 2 and 3 only
• (c) 3 and 1 only
• (d) 1, 2 and 3

Answer: (d) 1, 2 and 3

25. Polarization mode dispersion (PMD) is mainly observed in

• (a) Multiple step-index fiber
• (b) Single-mode fiber
• (d) Plastic fiber

Explanation: PMD is a form of dispersion that affects high-speed signals in longer-range systems. It is a major type of dispersion that becomes significant in a system that has already minimized other forms of dispersion and is operating at a Gigabit rate. PMD arises from the fact that even a single mode can have two polarization states. A typical PMD value is 0.5 psec/km. It is significant in the case of single-mode fiber.

26. An optical fiber has a refractive index core

• (a) high and low refractive index cladding
• (b) low and high refractive index cladding
• (c) uniform surrounded by variable index cladding
• (d) variable with refractive index increases from low at the center to high at the junction with cladding

Explanation: In an optical fiber refractive index of the core is high in comparison to the refractive index of cladding so that total internal reflection can take place.

27. Optical fibers are preferred as communication links for Lasers because they

• (a) prevent interference by other Lasers
• (b) ensure that the beam does not spread
• (c) prevent atmospheric interference
• (d) ensure amplification of the signal

Explanation: Single-mode optical fibers are generally preferred as communications links for Lasers because it prevents atmospheric interference in transmission by lasers.

28. The cladding which surrounds the fiber core

• (a) is used to protect the fiber
• (b) is used to reduce optical interference
• (c) helps to guide the light in the core
• (d) ensures that the refractive index remains unaltered

Answer: (c) helps to guide the light in the core

29. If n1 and n2 are refractive indices of the core and cladding respectively, the maximum acceptance angle at the air-core interface should be

• (a) \mathrm{ \tan^{-1} \frac{n_2}{n_1} }
• (b) \mathrm{ \sin^{-1} \sqrt{n_2^2 - n_1^2} }
• (c) \mathrm{ \sin^{-1} \sqrt{n_1^2 - n_2^2} }
• (d) \mathrm{ \tan^{-1} \frac{n_1}{n_2} }

Answer: (c) \mathrm{ \sin^{-1} \sqrt{n_1^2 - n_2^2} } Hello friends, my name is Trupal Bhavsar, I am the Writer and Founder of this blog. I am Electronics Engineer(2014 pass out), Currently working as Junior Telecom Officer(B.S.N.L.) also I do Project Development, PCB designing and Teaching of Electronics Subjects.

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