Continuous-wave 4.3-mum intracavity difference frequency generation in an optical parametric oscillator

We have achieved 150 mW of cw output at 4.3mum, using difference frequency mixing in a singly resonant optical parametric oscillator (OPO). We pumped the OPO cavity, which contains periodically poled LiNbO(3) (PPLN), with a 14-W 1.06-mum Nd:YAG laser to generate a signal at 1.7 mum and an idler at 2.8 mum. Mixing of the two waves at the same crystal temperature and grating spacing yielded emission in the mid IR. This technique avoids the mid-IR absorption-high-threshold problem, which has limited the cw performance of PPLN OPO's at wavelengths beyond 4 mum. Provided that tunability is not required, this method is a simple alternative to multiple-crystal configurations.