Cladding

A layer of glass or other material surrounding the core of a fibre, forming the conduit that carries the light through the fibre. It has a refractive index slightly lower than the core.
The low-refractive-index material that surrounds the core of an optical fiber to contain core light while protecting against surface contaminant scattering.
Material that surrounds the core of an optical fiber. Its lower index of refraction, compared to that of the core, causes the transmitted light to travel down the core.