Ray methods for modal fields in general closed and open propagation environments

The author presents a systematic methodology for using modal solutions in nonseparable geometries. Ray parametrization is used as a technique for constructing approximate modal wave fields to treat guiding and ducting environments with varying transverse and longitudinal properties. Ideal modes are decoupled from, and propagate independently of, other modes in the set. When the channel does not conform with separability in one of the basic coordinate systems, ideal modes defined for separable conditions become coupled. If the departure from separability is weak, one may define adiabatic modes (AMs) that adapt smoothly, without coupling to other AMs, to the slow nonseparable longitudinal variations, retaining their distinct spectral identity throughout. Each AM fails in its own cutoff region, across which its wavefield changes from being propagating to becoming evanescent, and it must there be extended into an intrinsic mode (IM) which has greater spectral content