Intrinsic mode theory of tapered optical waveguides

The concept of an `intrinsic¿ mode has recently been devised as the basis for a systematic theory of wave propagation in slowly varying waveguides. An intrinsic mode is a self-consistent plane-wave spectrum which is approximated, to lowest order in the slow scale, by a more conventional adiabatic mode. However, where the adiabatic mode breaks down (at its transition from bound to cutoff behaviour) the intrinsic mode is uniform, and correctly predicts the radiation which occurs into the substrate or cladding. The object of this paper is to summarise the relevant theory and to present numerical computations of the intrinsic modes for a model problem consisting of a dielectric wedge with plane boundaries.