20-Hz operation of an eye-safe cascade Raman laser with a Ba(NO3)2 crystal

Operation of a 1.598-mum eye-safe third-Stokes Raman laser with a Ba(NO3)(2) crystal pumped by a 1.064-mum Nd:YAG laser is described. We observed a substantial decrease in the output energy during the first 50 s of the continuous operation at 20 Hz. The energy drop is similar to 76% of the initial third-Stokes output. We confirmed negative thermal lensing and thermally induced birefringence in the crystal. With a concave cavity mirror at a matched curvature to the thermal lensing, we obtained an output energy of 11 mJ at 20 Hz. TEM00 output was also obtained with a smaller pump-beam diameter with a highest conversion efficiency of 15.5% for a pumping power of only 45 MW/cm(2) (0.9 J/cm(2)).