Sensitivity of a three-mirror cavity to thermal and nonlinear lensing: Gaussian-beam analysis

We consider a compact three-mirror cavity consisting of a flat output coupl er, a curved folding mirror, and an active medium with one facet cut at the Brewster angle and the other facet coated for unit reflectivity. We examin e the sensitivity to thermal lensing and to self-focusing in the active med ium of the Gaussian beam that is circulating in that cavity. We use a simpl e thin-lens model; the astigmatism of the beam that is circulating in the c avity and the nonlinear coupling between the field distributions along the two orthogonal axes are taken into account. We find configurations in which beam ellipticity is compensated for at either end of the cavity in the pre sence of thermal lensing. We have derived an analytical criterion that pred icts the sensitivity of the beam size to nonlinear lensing. The ability of the cavity to favor self-mode locking is found to be sensitive to the stren gth of thermal lensing. In the absence of thermal lensing, cavities operate d as telescopic systems (C = 0) or self-imaging systems (B = 0) are most ap propriate for achieving self-mode locking, with nonlinear mode selection ac complished through saturation of the spatially varying laser gain. We ident ify conditions for which self-mode locking can be produced by variable-refl ectivity output couplers with either maximum or minimum reflectivity at the center of the coupler. We use our model to estimate the nonlinear gain pro duced in laser cavities equipped with such output couplers. We identify a c avity configuration for which nonlinear lensing can simultaneously produce mode locking and correction of beam ellipticity at the output coupler. (C) 2000 Optical Society of America OCIS codes: 140.0140, 140.3410, 140.3580, 1 40.3590, 140.4050, 140.6810.