A coupled-mode formalism for elastic waveguides

A coupled-mode formulation is presented for the problem of elastic wave propagation in a waveguide which bends in an arbitrary manner and whose elastic properties vary as a function of axial distance. The method employs `quasimodes' which are adapted to the local nature of the guide. The method is particularly useful when the bends are of large radius of curvature and when the spatial frequency of the guide variations is small compared with the differences in wavenumber of the guided modes involved.