This article describes optical fibers, their structures, and the propagation of light in optical fibers.

The optical fiber is a thin flexible waveguide which guides visible light or invisible (near infrared) radiation to and from the light source and detector.
A schematic of the optical fiber is shown in figure below.

The characteristics of optical fiber determine suitability for different applications in optical devices including telecommunication. The dimension of the core and cladding of a
fiber determine many of its optical characteristics. optical fiber determine suitability for different applications in optical devices including telecommunication. The dimension of the core and cladding of a fiber determine many of its optical characteristics. The dimension also determines some physical characteristics, Schematics of the cross section for three conventional optical fibers are shown in Figure below.

The core transmits the light waves and the cladding keeps the light waves within the core through total internal reflection (TIR). In physics, mode is a complex mathematical and physical concept describing the propagation of electromagnetic waves (EM). But for the present purpose “mode” distinguishes the nature of paths along which the light is propagated through a fiber.

For a given wavelength, λ, the total number of modes that can be carried by an optical fiber depends on a dimensionless parameter, known as the “V” number or the normalized frequency parameter of the fiber,

where, *r* is the radius of the fiber core, n1 and n2 are the refractive indices of core and cladding respectively.

When V < 2.405, the lowest order spatial mode can be guided in the coherence properties of light, and the fiber is classified as single-mode.

For V > 2.405, the fiber can guide more than one spatial mode, and the fiber is classified as multimode.