Optical Communications Questions and Answers – Single – Mode Fibers

This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on “Single-Mode Fibers”.

1. An optical fiber has core-index of 1.480 and a cladding index of 1.478. What should be the core size for single mode operation at 1310nm?
a) 7.31μm
b) 8.71μm
c) 5.26μm
d) 6.50μm
View Answer

Answer: d
Explanation: Normalized frequency V<=2.405 is the value at which the lowest order Bessel function J=0. Core size(radius) Normalized frequency V less than equal to 2.405 is the value at which the lowest order Bessel function J=0.

2. An optical fiber has a core radius 2μm and a numerical aperture of 0.1. Will this fiber operate at single mode at 600 nm?
a) Yes
b) No
View Answer

Answer: a
Explanation: V= 2πa.NA/λ. Calculating this equation, we get the value of V. V is the normalised frequency and should be below 2.405 in order to operate the fiber at single mode. Here, V=2.094, is less than 2.405. Thus, this optical fiber exhibit single mode operation.

3. What is needed to predict the performance characteristics of single mode fibers?
a) The intermodal delay effect
b) Geometric distribution of light in a propagating mode
c) Fractional power flow in the cladding of fiber
d) Normalized frequency
View Answer

Answer: b
Explanation: A mode field diameter (MFD) is a fundamental parameter of single mode fibers. It tells us about the geometric distribution of light. MFD is analogous to core diameter in multimode fibers, except in single mode fibers not all the light that propagates is carried in the core.
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4. Which equation is used to calculate MFD?
a) Maxwell’s equations
b) Peterman equations
c) Allen Cahn equations
d) Boltzmann’s equations
View Answer

Answer: b
Explanation: Mode field diameter is an important parameter for single mode fibers because it is used to predict fiber properties such as splice loss, bending loss. The standard technique is to first measure the far-field intensity distribution and then calculating mode field diameter using Peterman equations.

5. A single mode fiber has mode field diameter 10.2μm and V=2.20. What is the core diameter of this fiber?
a) 11.1μm
b) 13.2μm
c) 7.6μm
d) 10.1μm
View Answer

Answer: d
Explanation: For a single mode fiber, MFD=2w0. Here, core radius For a single mode fiber, MFD=2w0 in given circuit Solving this equation, we get a=5.05μm. Core-diameter=2a=10.1μm.
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6. The difference between the modes’ refractive indices is called as ___________
a) Polarization
b) Cutoff
c) Fiber birefringence
d) Fiber splicing
View Answer

Answer: c
Explanation: There are two propagation modes in single mode fibers. These two modes are similar but their polarization planes are orthogonal. In actual fibers, there are imperfections such as variations in refractive index profiles. These modes propagate with different phase velocities and their difference is given by Bf =ny – nx. Here, ny and nx are refractive indices of two modes.

7. A single mode fiber has a beat length of 4cm at 1200nm. What is birefringence?
a) 2*10-5
b) 1.2*10-5
c) 3*10-5
d) 2
View Answer

Answer: c
Explanation: Bf=ny– nx = λ/Lp. Here, λ=wavelength and Lp = beat length. Solving this equation, we will get the answer.
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8. How many propagation modes are present in single mode fibers?
a) One
b) Two
c) Three
d) Five
View Answer

Answer: b
Explanation: For a given optical fiber, the number of modes depends on the dimensions of the cable and the variations of the indices of refraction of both core and cladding across the cross section. Thus, for a single mode fiber, there are two independent, degenerate propagation modes with their polarization planes orthogonal.

9. Numerical aperture is constant in case of step index fiber.
a) True
b) False
View Answer

Answer: a
Explanation: Numerical aperture is a measure of acceptance angle of a fiber. It also gives the light gathering capacity of the fiber. For a single mode fiber, core is of constant refractive index. There is no variation with respect to core. Thus, Numerical aperture is constant for single mode fibers.
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10. Plastic fibers are less widely used than glass fibers.
a) True
b) False
View Answer

Answer: a
Explanation: The majority of the fibers are made up of glass consisting of silica. Plastic fibers are used for short distance transmissions unlike glass fibers which can also be used for long haul applications. Also, plastic fibers have higher attenuation than glass fibers.

Sanfoundry Global Education & Learning Series – Optical Communications.

To practice all areas of Optical Communications, here is complete set of 1000+ Multiple Choice Questions and Answers.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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