Parametric amplification and oscillation with walkoff-compensating crystals

We measure and model parametric gain and oscillation for two crystals arranged for walkoff compensation. We show how the orientation of the crystals determines the relative sign of the nonlinear mixing coefficient in the two crystals. This sign dramatically influences small signal gain and oscillator performance, and we show how to determine the correct crystal orientation from parametric-gain measurements. The performance of two-crystal oscillators is examined with particular attention to beam tilts, conversion efficiency, and beam quality. We find reduced efficiency and increased oscillation threshold when the coefficients have opposite signs in a two-crystal ring oscillator. Sign reversal seems to have little influence on spectral purity or far-field beam profiles when the oscillator is seeded.