Doubly clad fiber
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In fiber optics, a doubly clad fiber is a single-mode optical fiber that has two claddings. This is also called depressed-inner-cladding fiber, and W-profile fiber (from the fact that a symmetrical plot of its refractive index profile superficially resembles the letter W).
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. A doubly clad fiber has the advantage of very low microbending losses. It also has two zero-dispersion points, and low dispersion over a much wider wavelength range than a singly clad fiber.
This type of fiber is often used for fiber lasers and optical amplifiers. In this case, the core is doped and acts as the gain medium, and the inner cladding acts to carry the pump light that maintains the population inversion in the core.