Influence of metallic side walls on the properties of an open resonator

The properties of a Fabry-Perot resonator encircled by a metallic shell with diffractional extraction of radiation are studied experimentally with reference to the development of high-power cyclotron-resonance masers and free-electron lasers in the submillimeter range. It is shown that the presence of conducting side walls leads to the excitation of oscillation modes characteristic of open resonators; the Q factor increases by five-to-ten times in this case. The introduction of coupling apertures of significant size facilitates the selection of a number of lower oscillation modes. A diffraction grating on one of the mirrors acts as a polarization filter and can be useful in the experimental selection of the optimal feedback coefficient.