In fiber optics, a cladding mode is a mode

Transverse mode

A transverse mode of a beam of electromagnetic radiation is a particular electromagnetic field pattern of radiation measured in a plane perpendicular to the propagation direction of the beam...

 that is confined to the cladding

Cladding (fiber optics)

Cladding is one or more layers of material of lower refractive index, in intimate contact with a core material of higher refractive index. The cladding causes light to be confined to the core of the fiber by total internal reflection at the boundary between the two. Light propagation in the...

 of an optical fiber

Optical fiber

An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...

 by virtue of the fact that the cladding has a higher refractive index

Refractive index

In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 than the surrounding medium

Medium (optics)

An optical medium is material through which electromagnetic waves propagate. It is a form of transmission medium. The permittivity and permeability of the medium define how electromagnetic waves propagate in it...

, which is either air or the primary polymer overcoat. These modes are generally undesired.

Modern fibers have a primary polymer overcoat with a refractive index that is slightly higher than that of the cladding, so that light propagating in the cladding is rapidly attenuated and disappears after only a few centimeters of propagation

Wave propagation

Wave propagation is any of the ways in which waves travel.With respect to the direction of the oscillation relative to the propagation direction, we can distinguish between longitudinal wave and transverse waves....

. An exception to this is double-clad fiber

Double-clad fiber

Double-clad fiber is a class of optical fiber with a structure consisting of three layers of optical material instead of the usual two. The inner-most layer is called the core. It is surrounded by the inner cladding, which is surrounded by the outer cladding...

, which is designed to support a mode in its inner cladding, as well as one in its core.