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 $1.5\mu m$.

MONOMODE FIBRE

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.

MULTIMODE FIBRE

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

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