CHARACTERIZATION OF AN ND-YAG-PUMPED BETA-BAB2O4 OPTICAL PARAMETRIC OSCILLATOR IN COLLINEAR AND NONCOLLINEAR PHASE-MATCHED CONFIGURATIONS

We present the characterization of type I (e --> o+o) phase matching i n beta-barium berate (BBO) optical parametric oscillators pumped by th e third harmonic of a Q-switched Nd:YAG laser in a collinear phase-mat ched geometry and three separate noncollinear configurations. Using a 12 mm long crystal, we show that the optical conversion efficiency inc reases from 10.7% in the collinear case to 40% with a noncollinearity of 90.8 mrad, and a corresponding fall in the operational threshold fr om 274 to 188 mJ.cm(-2). We discuss the constraints imposed upon the s pectral and spatial coherence of the pump beam, and show that the spat ial restriction is eased in the noncollinear regime. Included in the p aper is a theoretical model of noncollinear phase matching which predi cts the tuning responses of each OPO accurately. Several of the oscill ators were capable of tuning ranges from 450 nm to 1.68 mu m using a s ingle mirror set. We discuss in some detail the observed increases in conversion efficiency and linewidth, and the fall in operational thres hold with noncollinearity, with particular attention to the pump beam walkoff.