Characteristics of modes in a crossed-flute optical cavity

A numerical analysis is made of azimuthally uniform modes in a crossed-flute cavity with central apertures in the mirrors. The complex frequencies and the mode field distributions are calculated by the Galerkin method in the quasi-optical approximation. The characteristics both of the principal mode with an annular field distribution and of the nearest higher-order modes are considered as a function of the cavity parameters. It is shown that modes with low radiation losses may exist in the presence of relatively large aperture at the mirror centres.