Optical parametric oscillators (OPO) provide light sources of wide range of spectrum which are not available with existing lasers. Although OPOs using potassium niobate ($KNbO_3$) have been studied since 1981, no detailed investigation of beam properties have been reported in association with pump energy levels.
A type I phase-matched OPO was constructed by using $KNbO_3$. A $KNbO_3$ sample with 9.5 mm length was prepared in the direction of $\theta$=22˚, $\theta$=0˚. The $KNbO_3$ OPO was pumped by a pulsed laser operated with 9 ns pulsewidth and 10 Hz repetition rate at 532 nm wavelength. The OPO cavity was formed by two mirrors which have reflectances of R＞99% and R=80%, respectively. The angle tuning curves of the signal and idler radiation were obtained by rotating the sample continuously in the spectral regions of 767∼919 nm and 1262∼1711 nm, respectively, which gives excellent agreement with theoretical calculations.
The pump threshold intensity was measured to be 18.2 MW/㎠. The total energy of signal and idler waves was 1.76 mJ at the pump pulse energy of 13 mJ, which corresponds to a conversion efficiency of 13.5%. The spatial beam profiles at different pump energy levels were monitored using a CCD camera, and depletion and walk-off of the pump pulse were also investigated.
