Numerical analysis of the spatial behaviour of nanosecond optical parametric oscillators of beta-barium borate

We report on a numerical analysis of the temporal and spatial beam properties of nanosecond optical parametric oscillators (OPOs). The analysis is performed for a 355-nm-pumped critically phase-matched OPO of beta-barium borate. The calculations provide detailed information on the dependence of the OPO beam quality (measured by the quality factor M-2) On pump energy. An important result is the strong increase of the M-2 value for pump energies exceeding 1-2 times the energy at threshold. Furthermore, a temporal analysis of single OPO pulses indicates that the ML value strongly increases during the first few nanoseconds of the OPO oscillation. This increase is understood by considering the temporal dynamics of the spatial profiles of the OPO signal beam and the depleted pump radiation.