A single Mode optical Fiber with four claddings. The Refractive Index of each Cladding is lower than the core. With respect to one another, their relative refractive indices are, in order of distance from the core: lowest, highest, lower, higher.
A quadruply clad fiber has the advantage of very low Macrobending losses. It also has two zero-dispersion points, and moderately low Dispersion over a wider WavelengthRange than a singly clad fiber or a doubly clad fiber.
A quadruply clad fiber is a Single Mode fiber that has four claddings. As with doubly clad fibers, the Refractive Index of the cladding layers are lower than that of the core. The four layers of cladding are arranged SO that the lowest refractive index Layer is closest to the Core and then alternates as it moves out. Therefore the arrangement is lowest, highest, lower, higher as you radiate out from the core. Quadruply clad fibers have a very low macrobending loss.