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This note describes an analysis based on geometrical optics which determines the conditions for stable operation of devices employing lasers. The same procedure may also be applied to the investigation of mode selection in toroidal ruby lasers by periodic variations in the reflectivity of the toroid surface. Investigation of optical resonators other than the two-mirror resonator with plane or spherical reflectors, has shown that the geometrical optics of cavities with more than two reflectors can be treated in a manner similar to that of Pierce and Boyd and Kogelnik.
P.O. Clark (Tue,) studied this question.