Spectral properties of a millimeter-wavelength open cavity with a complex phase filter

The small-perturbation method is used to analyze the influence of thin-dielectric-film phase filters of complex configuration on the mode structure and field distribution of the natural oscillations of an open cavity with large-aperture spherical mirrors. It is shown experimentally that the filter eliminates the degeneration of the axisymmetric mode, which is manifested in the relative shift of the natural frequencies of modes participating in the degeneration. The field distributions of splitting modes correspond to natural oscillations of the open cavity, described by Hermito-Gauss functions.