150-mJ 1550-nm KTA OPO with good beam quality and high efficiency

We've demonstrated a nanosecond KTA OPO utilizing a high Fresnel-number quasi-monolithic image-rotating nonplanar-ring optical cavity to efficiently generate 1550 nm light with beam quality M-2 similar to 4. The OPO was pumped at 1064 run and injection-seeded at 1550 nm and was tested using either one or two 10x 10x 17 mm(3) KTA crystals. Total measured conversion efficiencies were as high as 45% and 55% respectively, with corresponding 1550 nm energies of approximately 135 mJ and 170 mJ. While energy and efficiency were high, agreement with numerical models that included walkoff, diffraction, and geometry of the nonplanar-ring, was poor near the oscillation threshold. Single-crystal oscillation revealed different thresholds for each KTA crystal. When tested by observing unphasematched 2omega generation, each crystal appears to contain a single ferroelectric domain, suggesting that refractive index inhomogeneity, or some other type of defect, prevents perfect phasematching.