Dispersion in Optical Fibres Questions and Answers

Q: Assertion (A) : Graded index optical fibers are used for reasonably high band width application.Reason (R) : Graded index fibers are designed to have low time dispersion.

The answer is both (A) and (R) are true and (R) is the correct explanation of (A).In a step-index multimode fiber, rays travelling at steep angles cover a longer path than axial rays and arrive later, spreading each pulse (modal or intermodal dispersion) and limiting bandwidth. A graded-index fiber has a refractive index that falls gradually from the core axis outward, so the longer outer paths are travelled faster. Arrival times equalise and time dispersion is low.Low dispersion means pulses can be sent closer together, which is what gives the reasonably high bandwidth, so R is exactly the reason for A.

Q: Which is the major cause of dispersion in multimode step index optical fiber ?

The major cause is intermodal dispersion.A multimode step index fiber carries many modes, which travel along paths of different lengths: a ray along the axis covers less distance than one zigzagging at the critical angle. Since all travel in the same core material at the same speed, they arrive at different times, and a short input pulse spreads out. This intermodal (modal) delay is far larger than any other dispersion in such a fiber.Material (chromatic) and waveguide dispersion arise from the variation of speed with wavelength. They dominate in single-mode fiber, where intermodal dispersion is absent, but they are small compared with modal dispersion in multimode step index fiber.

Direction for [ Question No: 1 To 1 ] :

Assertion-Reason type questions : The following items consist of two statements, one labelled as 'Assertion (A)', and the other labelled the 'Reason (R)'. You are to examine these two statements and decide if the Assertion (A) and the Reason (R) are individually true and if so, whether the reason is a correct explanation of the Assertion. Select your answer to these items using the codes given below and mark your answer sheet accordingly.

Codes :
(A) Both (A) and (R) are true and (R) is the correct explanation of (A)
(B) Both (A) and (R) are true but (R) is not the correct explanation of (A)
(C) (A) is true but (R) is false
(D) (A) is false and (R) is true

Assertion (A) : Graded index optical fibers are used for reasonably high band width application.

Reason (R) : Graded index fibers are designed to have low time dispersion.

A.Both (A) and (R) are true and (R) is the correct explanation of (A)
B.Both (A) and (R) are true but (R) is not the correct explanation of (A)
C.(A) is true but (R) is false
D.(A) is false and (R) is true

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: A

The answer is both (A) and (R) are true and (R) is the correct explanation of (A).

In a step-index multimode fiber, rays travelling at steep angles cover a longer path than axial rays and arrive later, spreading each pulse (modal or intermodal dispersion) and limiting bandwidth. A graded-index fiber has a refractive index that falls gradually from the core axis outward, so the longer outer paths are travelled faster. Arrival times equalise and time dispersion is low.

Low dispersion means pulses can be sent closer together, which is what gives the reasonably high bandwidth, so R is exactly the reason for A.

Discuss in Forum

Which is the major cause of dispersion in multimode step index optical fiber ?

A.Intermodal
B.Material
C.Wave guide
D.Chromatic

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: A

The major cause is intermodal dispersion.

A multimode step index fiber carries many modes, which travel along paths of different lengths: a ray along the axis covers less distance than one zigzagging at the critical angle. Since all travel in the same core material at the same speed, they arrive at different times, and a short input pulse spreads out. This intermodal (modal) delay is far larger than any other dispersion in such a fiber.

Material (chromatic) and waveguide dispersion arise from the variation of speed with wavelength. They dominate in single-mode fiber, where intermodal dispersion is absent, but they are small compared with modal dispersion in multimode step index fiber.

Discuss in Forum

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Dispersion in Optical Fibres Questions and Answers

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