The core refractive index and a relative refractive index difference of a multimode step-index fiber are specified as 1.5 and 2%, respectively. At operating wavelength of 1300 nm, the approximate number of propagating modes is 1000. The diameter of the fiber core is :

Question

Question

This question was previously asked in

NIELIT Scientific Assistant ECE 5 Dec 2021 Official Paper

Answer 1

Concept:

The number of propagating modes in a multimode step-index fiber is approximately given by,

Where V is the normalized frequency (V-number), and is given by,

NA is the numerical aperture and is defined as,

Where, n₁ = core refractive index, Δ = relative refractive index difference, λ = operating wavelength, a = core radius

Calculation:

Given:

n₁ = 1.5, Δ = 2% = 0.02, λ = 1300 nm = 1.3 × 10^-6 m, M = 1000

Calculate NA:

Now,

From,

Core diameter = 2a ≈

Correct Option:

The closest answer is: 3) 62 µm