Mid-infrared optical parametric oscillators based on periodically-poled lithium niobate
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A study has been conducted of mid-infrared optical parametric oscillators (OPOs) based on periodically poled lithium niobate (PPLN) as the nonlinear optical material. Experiments have been performed with a number of 1.06 µm pump sources and OPO resonator configurations, in both pulsed and cw mode of operation. The OPO idler wavelength could be tuned across the 3.1-3.8 µm region, by using 8 different PPLN grating periods on the same crystal. In nearly all cases, the measured pump-to-idler conversion efficiency was in the range of 7-9 %, and the idler output power was in the range of 1-1.4 W. A notable exception was the result obtained with a monolithic OPO resonator with very low signal and idler feedback, and a short pump pulse (<10 ns). In this case, an output power of almost 1 W was obtained at only 5.3 W pump power, corresponding to 18 % conversion efficiency. This was the maximum pump power that could be used without risking optical damage. The reason for the high efficiency in this case is probably reduced back-conversion due to the low signal and idler feedback. Using the totally available 18 W pump power to pump 3 different PPLN OPOs, there is potential for achieving 3 W of mid-infrared output at 3 different mid-infrared wavelengths with this configuration. The conversion efficiency obtained in cw operation was 8-9 %, which is more than a factor of 2 below the best results reported in the literature. It is suspected that the conversion efficiency was limited by thermal lens effects caused by a small residual absorption at the pump and signal wavelengths in the crystals used in the cw experiments.