Improved beam quality from a high energy optical parametric oscillator using crystals with orthogonal critical planes.

We demonstrate with simulations and experiments that an optical parametric oscillator using two different crystals with orthogonal walk-off planes can generate a symmetric, high-quality beam even if the resonator has a high Fresnel number. In the experiments we used KTA and BBO crystals to convert 5 ns pulses at 1.06 microm to 1.7 microm pulses with more than 10 mJ energy and beam quality M(2) approximately 2.