High energy, near diffraction limited output from optical parametric oscillators using unstable resonators

We describe a computationally efficient numerical simulation model of optical parametric oscillators (OPOs) appropriate for the simulation of high energy pulsed OPOs using unstable resonators. The model calculations are time dependent and include the effects of diffraction and pump depletion in the parametric amplification process. The computational time required to solve the model equations for unstable resonator OPOs is reduced by applying a coordinate transformation to the paraxial three-wave mixing equations. An essential characteristic of this transformation is that it allows for the extraction of the geometric curvature from the intracavity fields resulting in the propagation of approximately collimated fields. We demonstrate the utility of this new model by presenting the results of a detailed numerical study of a high energy, non-critically phase matched KTA OPO using unstable resonators. Our results predict that an optimally designed confocal unstable resonator can produce near diffraction limited output at high pulse energies.