Dynamic Correction of Thermal Focusing in Nd:YAG Confocal Unstable Resonators by Use of a Variable Radius Mirror

Excellent beam quality and divergence stability over a wide pump power range was demonstrated in a Q-switched, Nd:YAG, positive branch confocal unstable resonator by using a one degree-of-freedom, adaptive optic. Unlike single-element flexible-membrane mirrors, the variable radius mirror (VRM) consisted of a lens and mirror, whose separation determined the VRM's effective radius of curvature. This simple method enabled low cost and efficient thermal focusing compensation. The VRM was demonstrated by producing a 300-mJ Q-switch or 1-J free-running at a beam quality factor M(2) that varied between 1.2 and 1.8 as the average output power varied between 0 and 33 W.