Theoretical study of unstable resonators for diode-pumped alkali laser using wave optics simulation

Unstable optical resonators employing graded reflectivity mirrors (GRMs) for diode-pumped alkali laser (DPAL) are theoretically studied. In this study, a wave optics resonator model was coupled with a simplified gas flow model. The platform modeled was a 100-W class Cs DPAL proposed by us. As the pump beams are obliquely incident to the active medium, a wide gain region appears compared with that of the axially pumping scheme. Although an unstable resonator was expected to be suitable to provide a high-quality output beam, it was found to be inapplicable for our relatively small-scale DPAL apparatus. Instead, we demonstrated that the application of a GRM unstable resonator provide a nearly diffraction-limited beam (M2 < 2) with minimal penalty for the output power. The effect of the gas flow on the beam quality is also discussed.