Cladding mode

In telecommunication, a cladding mode is an undesired 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, i.e., air or primary polymer overcoat.

Note: Modern fibers have a primary polymer overcoat with a refractive index that is slightly higher, rather than lower, than that of the cladding, in order to strip off cladding modes after only a few centimeters of propagation.

Source: from Federal Standard 1037C