Ring and 90° roof open resonators

An investisation is described on modes and losses of a rectangular four-mirror ring resonator and on the behavior of such modes in the transition region between the ring and the 90° roof mirror resonators. The modes of either the rectangalar ring of the 90° roof resonators are of the Fabry-Perot type. When passing from roof to rectangular ring, the zeroth mode of the roof degenerates into the first odd mode, which represents the lowest order mode of the ring. Experimental checks confirm the theoretical results.