Multirod unstable resonators for high-power solid-state lasers

The properties of positive-branch and negative-branch unstable resonators with variable reflectivity mirrors and several variable internal lenses were investigated both theoretically and experimentally. Design rules for optimized unstable resonators for one or more active elements are derived on the basis of the ABCD matrix formalism. Experiments were performed with a pulsed Nd:YAG system consisting of three 6 in. × 3/8 in. (15.24 cm × 0.95 cm) rods. This system provided a maximum output power of 550 W per rod when a symmetric flat-flat resonator was used. Unstable resonators achieved up to 75% of this maximum value with beam-parameter products between 2 and 10 mm mrad. The beam quality becomes worse as more active elements are used inside the resonator. This deterioration of focusability is caused by spherical aberration in combination with differences of refractive power for r and Φ polarizations.