Cladding in optical fibers is one or more layers of materials 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.[1] Light propagation in the cladding is suppressed in typical fiber. Improving transmission through fibers by applying a cladding was discovered in 1953 by Dutch scientist Bram van Heel. Some fibers can support cladding modes in which light propagates in the cladding as well as the core.

## History

The fact that transmission through fibers could be improved by applying a cladding was discovered in 1953 by Dutch scientist Bram van Heel, who used it to demonstrate image transmission through a bundle of optical fibers.[2] Early cladding materials were oils, waxes, and polymers. Lawrence E. Curtiss at the University of Michigan developed the first glass cladding in 1956, by inserting a glass rod into a tube of glass with a lower refractive index, fusing the two together, and drawing the composite structure into an optical fiber.[2]

## Modes

A cladding mode is a mode that is confined to the cladding of an optical fiber by virtue of the fact that the cladding has a higher refractive index than the surrounding medium, 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. An exception to this is double-clad fiber, which is designed to support a mode in its inner cladding, as well as one in its core.

## Effect on numerical aperture

The numerical aperture of a multimode optical fiber is a function of the indices of refraction of the cladding and the core:

${\displaystyle \mathrm {NA} ={\sqrt {n_{\mathrm {core} }^{2}-n_{\mathrm {clad} }^{2}}}}$

Most glass fibers have a cladding that raises the total outer diameter to 125 microns.[3]

## References

This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).

1. ^ The Basics of Fiber Optic Cable
2. ^ a b Hecht, Jeff (2004). City of Light: The Story of Fiber Optics (revised ed.). Oxford University. p. 55–70. ISBN 9780195162554.
3. ^ "The FOA Reference For Fiber Optics - Optical Fiber". www.thefoa.org. Retrieved 2016-04-10.