Cladding (fiber optics)

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