Doubly clad fiber

In telecommunication, a doubly clad fiber is a single-mode optical fiber that has two claddings.

Note 1: Each cladding has a refractive index that is lower than that of the core. Of the two claddings, inner and outer, the inner cladding has the lower refractive index. Note 2 : A doubly clad fiber has the advantage of very low macrobending losses. It also has two zero-dispersion points, and low dispersion over a much wider wavelength range than a singly clad fiber. Synonyms depressed-cladding fiber, depressed-inner-cladding fiber, W-profile fiber (from the fact that a symmetrical plot of its refractive index profile superficially resembles the letter W).

Source: from Federal Standard 1037C