Frequency doubling with KNbO(3) in an external cavity.

Potassium niobate is employed in an external resonator to generate single-frequency tunable radiation near 430 nm. For excitation with 1.35 W of power from a cw titanium-sapphire laser, 0.65 W of blue light is produced. A simple model has been developed to account for thermal lensing in the nonlinear crystal. Owing to the large second-order susceptibility of potassium niobate (KNbO 3), nonlinear mixing in KNbO 3 has been found to be an attractive source of blue light.'-7 For example, a cw power of 40 mW at 429 nm has been achieved by frequency doubling of a semiconductor diode laser, 6 while 150 mW of power at 467 nm has been generated by nonlinear mixing of diode and Nd:YAG lasers in KNbO 3. 7 Although much of the research in this area has employed external buildup cavities to enhance the conversion efficiency , the absolute blue power has been limited by the available pump power. In this regard, the titanium sapphire (Ti:A1 2 0 3) laser is a suitable source since single-frequency output power in excess of 2 W is readily achieved. Indeed, the Ti:A1 2 0 3 laser has recently been used for external-cavity doubling with LiIO 3 to produce 40 mW of broadly tunable blue output light for 600 mW of infrared input. 8 For doubling schemes with KNbO 3 within the Ti:A1 2 0 3 laser cavity, 100-150 mW of blue light power-near 430 nm has been obtained in unpublished experiments in our laboratory and elsewhere. 3 Motivated by these developments as well as by a variety of possible applications in optical physics, we have undertaken an investigation of frequency doubling with KNbO 3 in an external buildup cavity driven by the light from a cw Ti:A1 2 0 3 laser. We have made measurements of conversion efficiency over a range of operating conditions and have compared these results in absolute terms with those of a simple theoretical model. In a cw mode of operation , 0.65 W of blue light near 430 nm has been generated for 1.35 W of fundamental input. Because thermal lensing significantly affects the conversion efficiency and cavity servo performance at these power levels, we have also employed a transient mode of operation with the cavity length swept through resonance to obtain peak power of 1.0 W of blue light for 2.0 W of infrared excitation. For our experiment, a single-frequency Ti:A1 2 …