Modes and Losses of a Four-Mirror Ring Resonator (Letters)

An investigation is reported on modes and losses of a square ring resonator that consists of the numerical solution of the appropriate integral equation for a number of values of the mirrors´ aperture and spacing. It turns out that two sets of modes can exist: circulating modes and modes between two facing mirrors. The circulating modes are of the Fabry-Perot type, and their losses can be evaluated on the basis of the losses of suitably equivalent Fabry-Perot resonators.