Question: What is monomode and multimode fibre? Explain the term V-number.

Calculate the number of modes a step index optical fibre of diameter $40\mu m$ will transmit as its core and cladding refractive indices are 1.5 and 1.46 respectively. Wavelength of light used is $1.5\mu m$.

Subject : Applied Physics 2

Topic : Fibre Optics

Difficulty : High

ap2 • 322 views
modified 8 months ago by gravatar for Ankit Pandey Ankit Pandey100 written 10 months ago by gravatar for vermavarsha432 vermavarsha4320


It supports only one mode of propagation. It has very small core diameter requires laser diode as source of light. Transmission losses are very small . They have higher width. used for very long distance applications. They are by default step index they are mostly made up of glass.


It supports large number of modes of propagation they have larger core diameter they can work with LED also.transmission losses are more they have lower bandwidth used for short to medium distance applications. They can be step index or graded index they are made preferably from plastic.

V number

An optical fibre is characterized by an important parameter called V number or normalized frequency It is given by

enter image description here

'a' is the radius of core λ is the wavelength of light NA is the numerical aperture

enter image description here

written 8 months ago by gravatar for vermavarsha432 vermavarsha4320
Please log in to add an answer.