Geometric modes of an unstable ring resonator with 90° beam rotation

Abstract

An unstable ring resonator design which produces 90 deg beam rotation in a single pass (UR90 or HiQ) has been developed in order to solve the problem of obtaining an unobscured, near diffraction-limited output beam from low-gain laser media requiring saturation of a large transverse gain volume. A computer code was used to model the bare-cavity geometric modes of the UR90 by ray tracing through an equivalent thin lens resonator. Mode footprints of the collimated forward mode and expanding reverse mode showed only partial overlap, indicating that proper aperturing, especially at the gain generator, can aid in reverse mode suppression.