Characterization of an Nd:YAG-pumped β-BaB2O4 optical parametric oscillator in collinear and noncollinear phase-matched configurations

We present the characterization of type I (e→o+o) phase matching in beta-barium borate (BBO) optical parametric oscillators pumped by the third harmonic of a Q-switched Nd:YAG laser in a collinear phase-matched geometry and three separate noncollinear configurations. Using a 12 mm long crystal, we show that the optical conversion efficiency increases from 10.7% in the collinear case to 40% with a noncollinearity of 90.8 mrad, and a corresponding fall in the operational threshold from 274 to 188 mJ·cm^-2. We discuss the constraints imposed upon the spectral and spatial coherence of the pump beam, and show that the spatial restriction is eased in the noncollinear regime. Included in the paper is a theoretical model of noncollinear phase matching which predicts the tuning responses of each OPO accurately. Several of the oscillators were capable of tuning ranges from 450 nm to 1.68 μm using a single mirror set. We discuss in some detail the observed increases in conversion efficiency and linewidth, and the fall in operational threshold with noncollinearity, with particular attention to the pump beam walkoff