High-average power KTP ring OPO

A KTiOPO₄ (KTP) ring OPO operating at an average power level of 26.3 W was demonstrated. To our knowledge this is the highest average power that a KTP OPO has been operated at to date. The OPO used three 1 sq. cm aperture KTP crystals cut for non-critical phase matching (NCPM) at 1.064 μm. The crystals were 10×10×20 mm in size, cut from a large crystal, which was grown from a modified self-flux by pulling on an x-oriented seed. The pump laser operated at 100 pps repetition rate at an average power of just over 71 watts, and pump power densities above 60 MW/sq. cm. All of the elements were AR coated for 1.06 μm and 1.576 μm. Signal wavelength conversion efficiency of 37% was achieved in this configuration. Idler output was not measured, but no thermo-lensing was observed due to the enhanced transmission (41%) of our crystals at the idler wavelength of 3277 nm. Some crystal damage was observed over time, apparently due to pump laser instability. The results are especially interesting since past experience had shown that KTP elements fail rapidly under these high average power conditions. Future work will compare the performance of a single long KTP OPO element with that of multiple short elements, in the same resonator.