Pulsed optical parametric oscillators with intracavity optical parametric amplification: a critical study

It is known that the idler conversion efficiency of optical parametric oscillators (OPOs) can be increased by adding a second nonlinear crystal in the cavity. This crystal is pumped by the signal and acts as an optical parametric amplifier (OPA) for the idler. However, this technique unavoidably increases the oscillation threshold because of additional losses and increased build-up time due to cavity lengthening. In this paper, we investigate both theoretically and experimentally the benefits and drawbacks of this so called OPO-OPA configuration versus the singly resonant OPO (SRO) configuration. Calculations are found to be in agreement with an experimental study of a SRO and an OPO-OPA operating near 3.4 mu m both pumped by a 90-mJ 27-ns Nd:YAG laser. Our study reveals that the OPO-OPA needs to be driven at least two times above threshold to produce more idler energy than the SRO. In addition, near 3 mu m the OPO-OPA is particularly efficient given that the difference frequency wave generated in the second crystal is also output coupled.