Mode degeneracy and the light-trapping mode in lossy waveguides

Abstract

In some waveguides, mutually opposing power flows are formed in the core and cladding. They give rise to forward and backward modes whose degeneracy results in the light-trapping mode. When the guiding modes become lossy, this mode degeneracy or formation of the light-trapping mode is forbidden if the loss is described as spatial attenuation but is allowed if the loss is described as temporal decay. We show that this is because the guiding modes have only one temporal direction of decay (only forward), while they have two spatial directions of attenuation (both forward and backward).