Rays and modes for plane wave coupling into a large open-ended circular waveguide

Abstract

Generalizing an earlier study of high-frequency plane wave coupling into a wide semi-infinite parallel-plane waveguide, this investigation is concerned with coupling into a large semi-infinite circular waveguide. Emphasis is placed on the interplay between ray fields and mode fields, on the excitation mechanism of each, and on their properties when the field incidence angle is strongly oblique, thereby introducing marked azimuthal asymmetries. The tools of analysis are the geometrical theory of diffraction (GTD), physical optics (PO) and hybrid ray-mode theory. The results reveal that coupling from edge-diffracted rays into a waveguide mode occurs when such rays emanating from the flash points on the edge line up with rays in the modal congruence. However, the diffracted ray caustics between initial reflections do not coincide with the cylindrical caustic of the mode, and only after many reflections is there synchronism between the edge-diffracted and modal rays. Complete synchronism throughout obtains for each angular harmonic constituent of the field, for which the incident wave is conical. Under these conditions, one may also distinguish a multiple reflected truncated conical geometric optical beam, the analog of the geometric optical sheet beam in plane parallel geometry. A variety of other ray and modal characteristics are elucidated as well from different perspectives, thereby granting insight into the ray-mode phenomenology in the circular wave-guide environment. Such insight is expected to be useful when one attempts to deal with nonideal but strongly guiding configurations.