Degenerate oscillations in a quasioptical open resonator with rectangular large-aperture mirrors

Abstract

The results of this study suggest the following. 1. The spectrum of a precisely fabricated and adjusted open resonator with large-aperture mirrors (2a ≫ λ anda ∼ d) consists of axisymmetric modes when excitation is concentrated along the resonator axis. 2. Alinear combination of modes with rectangular even symmetry corresponds to an axisymmetric mode when the inequality m+n+1=2p+l+1 is satisfied. 3. The degeneracy of modes is effectively removed by insertion of a phase filter into space of the open resonator, or by the use of mirrors with slightly different radii of curvature in mutually orthogonal directions (astigmatic mirrors). 4. Using the well-known method of a test probe for analyzing the spectral content of oscillations in an open resonator [4] requires a certain caution, viz.: it must be remembered that reliable quantitative results will be obtained only for the lowest-order TEM00q mode and for nondegenerate modes with rectangular symmetry. 5. It is expedient to use an open resonator with a “pure” spectrum of natural modes in millimeter-wave devices such as radiospectrometer cells and resonance spaces in diffraction radiation generators, also as active cells in spectrum analyzers of spatial field structures of any configuration.