Unstable confocal resonator with external flat semi-transparent mirror for additional feedback

Abstract

Main characteristics (mode structure of power loss, divergence of resonant radiation) of a three-mirror laser cavity with additional feedback are theoretically studied in the Fresnel approximation on the basis of the Fox-Lee integral equation. The resonator consists of two spherical mirrors, which form an unstable confocal resonator, and a flat semi-transparent mirror, which is installed outside the reference unstable resonator perpendicular to its optical axis. The flat coupling mirror returns part of the radiation back inside the resonator. The level of additional feedback is determined by the reflection coefficient of the flat mirror and the position of the mirror along the optical axis of the resonator. Variation of these parameters makes it possible to control the transparency of the three-mirror resonator over a wide range, up to values close to zero. Results of this paper can be used in the development of high-power lasers with wide aperture (Fresnel numbers of ∼100) on the basis of low-gain active media.