An improved resonator and optical cavity is adapted for use with a chemical laser that has a nozzle upstream of the resonator that emits a gain medium in a flow direction, and a pressure-recovery system downstream of the resonator. The optical resonator comprises first and second optical elements that are spaced apart from one another along an optical axis. Each of the optical elements has a selected geometry and a selected optical transmissivity to permit transmissive outcoupling of a beam of laser radiation from the resonator, the outcoupled beam being transmitted about the optical axis. The transmissivity and geometry of each optical element is selected to define an unstable region between the optical elements and around the optical axis, and a stable region in a region surrounding the unstable region and spaced apart from the optical axis.