Question

A single mode fiber with radius of 4.2um with core refractive index =1.48 and that of cladding=1.475, the cutoff wavelength is given by

Answer 1

Explanation:

Concept:

The CUTOFF WAVELENGTH of a single-mode fiber is the wavelength above which the fiber propagates only the fundamental mode.

Below cut-off, the fiber will transmit more than one mode. An optical fiber that is single-mode at a particular wavelength may have two or more modes at wavelengths lower than the cutoff wavelength. It is given by:

λc = Theoretical Cut-Off Wavelength

a = radius of the core

n1 = Refractive index of the core

Where Vc = Normalized Frequency, which is typically 2.4 for single-mode fiber.

Δ = Relative Refractive Index Difference (Between the core and the cladding)

Calculation:

Given n1 = 1.48, n2 =1.475 and a = 4.2 μm

Putting on respective values in Equation-(1), we get;

λc = 1334 nm