Cladding 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. Some fibers can support cladding modes in which light propagates in the cladding as well as the core. (From Federal Standard 1037C and from MIL-STD-188)

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

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