Hybrid ray-mode analysis of high-frequency wave coupling into large waveguides and cavities

When a high-frequency wave impinges upon a large open-ended waveguide or cavity (here illustrated in plane parallel geometry), the fields coupled into the interior can be analyzed either by tracking multiple-reflected and edge-diffracted rays or by converting the initial field into waveguide or cavity modes. Both descriptions are inconvenient and physically obscure because (1) they require many rays or many modes and (2) they image neither the beamlike character of the initial aperture-limited field nor its diffusion after undergoing multiple internal reflections. These defects are overcome by a self-consistent hybrid ray-mode formulation wherein those modes, whose angular spectra cluster around the beam propagation angle, are converted compactly into beam and certain edge-diffracted ray fields. Mode or ray fields with spectra outside this cluster are found to be negligible.