Optical parametric oscillators with high pulse energy and beam quality

A new method for obtaining high beam quality from high pulse energy optical parametric oscillators (OPOs) is demonstrated. By using different nonlinear crystals that have walk-off in orthogonal directions but are type 2 phase matched for the same interaction, the strong beam asymmetry that is common in critically type 2 phase matched OPOs is removed. Experimentally, this was demonstrated by adding BBO crystals to a type 2 phase matched KTA OPO, where the beam quality improved from M2 ≈ 2 x 12 in the KTA OPO to M2 ≈ 2 x 2 from the KTA-BBO OPO.