WAVE PROPAGATION IN GRADIENT FIBRES WITH CURVATURE.

The dominant mode of a monomode optical fiber with imperfections, e. g. random curvature, is attenuated owing to power conversion into the radiation field at the imperfections. In order to avoid analytical difficulties with a continuous mode spectrum a large diameter metal shield, which encloses the fiber consisting of a uniform or a gradient index core with a lower index cladding, is used to represent surface wave modes as well as the radiation field by discrete sets of modes. The refractive index of the gradient fiber is assumed to have a square law profile. The equations for the electromagnetic field and the characteristic equation for the whole structure are derived. Design criteria are given for the gradient fiber to have lower attenuation than the uniform core fiber. The bending loss can be lowest for a structure involving a refractive index gradient in the core and a step decrement to a lower cladding index.