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Modelling of thermal effects within a 2-μm pumped ZGP optical parametric oscillator operating in the mid-infrared

Optical parametric oscillators (OPOs) using zinc germanium phosphide (ZGP) crystals as the active non-linear medium are important devices for wavelength conversion into the 3 to 5 μm mid-infrared waveband. However, the presence of optical absorption within ZGP at the pump wavelength can lead to detrimental thermo-optic effects (thermal lensing and dephasing) when operated under high average power conditions. In order to characterise the strength of thermal effects within ZGP OPOs a theoretical model is under development based on the commercially available software package GLAD. Pump, signal and idler beams are represented by transverse arrays of complex amplitudes and propagated according to diffraction and kinetics algorithms. The ZGP crystal is modelled as a series of crystal slices, using a split-step technique, with the effects of non-linear conversion, absorption and thermal effects applied to each step in turn. We report modelling predictions obtained to date for the strength of the thermal lens induced in a ZGP crystal on exposure to a 5 Watt Q-switch pulsed high-repetition rate (10 kHz) wavelength doubled Nd:YLF laser at 2.094 μm. Predicted steady-state thermal focal lengths and time constants are compared to experimental results measured for two ZGP crystals, with high and low pump absorption levels. GLAD model predictions for a singly-resonant ZGP OPO in the absence of thermal effects are also compared to predictions from the widely available software package SNLO.