Performance of Nd : YAG lasers in coupled generalized self-filtering and positive-branch unstable resonators

A new optical resonator based on the combination of a generalized self-filt ering unstable resonator (GSFUR) and a positive-branch unstable resonator ( PBUR) in a three-mirror scheme is reported. It is shown both theoretically and experimentally that a nearly diffraction-limited Gaussian-output laser beam with a large mode volume can be obtained with this cavity design. The laser cavity is particularly interesting for use in high-threshold pumped g ain media and eliminates some disadvantages of the SFUR and GSFUR designs. This resonator, with an effective magnification of -6.16, was applied to a pulsed Nd:YAG laser in free-running and in Q-switched modes of operation. T he output energy was similar to 70 mJ, 5.5 times greater than when a single GSFUR design was used. The output beam had a pulse duration of similar to 30 ns in the Q-switched mode of operation and a beam divergence of 0.26 mra d. The required relations for the GSFUR-PBUR optical design and the output energy were derived and verified experimentally. (C) 1999 Optical Society o f America. OCIS codes: 140.0140, 140.3410, 140.3530, 140.3540, 140.3580, 14 0.4780.