Thermal lensing and power scaling of a AgGaSe2 OPO

The 2-μm pumped AgGaSe2 OPO is currently limited to outputs below 1 W and pulse recognition frequencies of a few kilohertz, apparently because of thermal lensing. Conflicting values for K and dn/dT and the lack of accurate 2-μm absorption data have severely hampered predictions of thermal focusing from material properties. We present results of polarized 2-μm laser calorimetry and extracavity thermal lens measurements on AgGaSe2 samples grown at two laboratories over the past 5 years. We show that the 2-μm absorption is due primarily to an e-polarized extrinsic band. Even for our best material, the thermal focal length can be comparable to the crystal length (in a typical OPO configuration) at pump powers of only a few watts. We find no astigmatism due to crystalline anisotropy, and we demonstrate the possibility of intracavity optical compensation. We have also achieved an order of magnitude reduction in thermal focusing by cooling the crystal to 77 K.