Question

What is monomode and multimode fibre? Explain the term V-number, Calculate the number of modes of a step index optical fibre of diameter 40 𝝁𝒎 will transmit as its core and cladding refractive indices are 1.5 and 1.46 respectively. Wavelength of light used is 1.5 𝝁𝒎.

Answer 1

Monomode:
1. It supports only 1 mode of propagation 2. It has very small core diameter of the order 5 to 10μm 3. Transmission losses are very small 4. It has higher bandwidth 5. It requires laser diode as source of light 6. It is used for long distance. 7. It is by default step index fibre 8. Mostly it is made up of glass
Multimode:
1. It supports large on of modes of propagation 2. It has larger core diameter of the order 50 to 150μm 3. Transmission losses are more 4. It has lower bandwidth 5. It can work with LED also 6. It is used for long distance communication 7. It can be step index or graded index fibre 8. It is made preferably from plastic
The normalized Frequency parameter or V-number gives the upper limit of the number of Modes that can be transmitted in a multimode optical fiber. It depends on the core diameter, dc, the NA, and the wavelength.

The normalized Frequency parameter or V-number gives the upper limit of the number of Modes that can be transmitted in a multimode optical fiber. It depends on the core diameter, dc, the NA, and the wavelength.
`V=(2pia)/λ xxsqrt(n_1^2-n_2^2)`

`V=(2pia)/λxxNA`

where, a=core radius

λ=wavelength in vacuum
n1=maximum refractive index of core
n2=refractive index of cladding

Given: - d=40 𝜇𝑚
n₁=1.5

n₂=1.46

λ=1.5 𝜇𝑚

Formula: -`NA=sqrt(n_1^2-n_2^2)`

`M=1/2((pid)/λxxNA)^2`

M=415