Efficient high-gain two-crystal optical parametric oscillator

A high-gain optical parametric oscillator containing two crystals in series with 90% signal outcoupling after the first crystal and 100% idler outcoupling after both crystals is found to give stable highly efficient operation over a wide dynamic range of pump intensity. Most of the small-signal gain results from the longer second crystal, but most of the energy conversion at saturation occurs in the shorter first crystal. Power loading in the crystals is predominately due to the incident pump, and is small in the second crystal after saturation. Theoretical description of the device is presented in terms of a plane-wave model, as well as a numerical model which incorporates transverse effects in one transverse dimension. It is found that the device can exhibit a period-two limit cycle when the two crystals are tuned to the same frequencies. Steady-state operation is obtained by tuning the crystals to slightly different frequencies, without significantly affecting the average efficiency. Experiments using KTP crystals synchronously pumped at 1.064 μm by a Q-switched mode-locked Nd:YAG laser are proposed. For degenerate down-conversion, a calculation including transverse effects shows that 88.8% of the incident power is converted to 2.128 μm in a single beam outcoupled after the first crystal. An additional 5.7% emerges as idler power after the second crystal