Advanced numerical simulation models for second-order nonlinear interactions

A general model for nonlinear optical frequency conversion devices based on second-order parametric processes is presented. The main emphasis is on optical parametric oscillators (OPOs). First, the model allows propagation in any direction in uniaxial or biaxial crystals, and diffraction and walk-off are included. Alternative numerical methods for solving the equations for the nonlinear interaction in the birefringent crystal are compared. Second, techniques for modeling temporal walk-off are considered. This is important in devices operating with short pulses or wide spectra. Third, initiation of parametric oscillation from spontaneous emission noise is modeled by adding random noise to the signals. The random nature of the noise initiation process leads to pulse to pulse fluctuations in energy, spectrum, and transverse beam shape. The fluctuations in transverse shape are small for narrow pump beams, but for wide pump beams they can be significant. Finally, thermal effects are considered. In devices with high average power, even a small absorption of one of the interacting beams can cause a temperature gradient in the nonlinear crystal. This temperature gradient leads to thermal lensing and spatially varying phase matching.