Perturbation theory of a doubly clad optical fiber with a low-index inner cladding

A doubly clad optical fiber which is composed of the core, inner cladding, and outer cladding is called a -type fiber when the core has the largest index of refraction of the three and the inner cladding has the lowest. A -type fiber is known to have several advantages over a conventional singly clad (SC) fiber. This paper presents simple, closed form approximations of a -type fiber with respect to cutoff, the attenuation constant in the leaky wave region and a rapid change of the group velocity near cutoff, etc. In the approach to be described, a -type fiber is thought of as a combined system of an SC fiber and an outer perturbing medium.