A joint effort by researchers at the University of Nijmegen in the Netherlands and at Sandia National Laboratories' Combustion Research Center in Livermore, Calif., has resulted in a continuous-wave optical parametric oscillator (OPO) whose high power and broad tunability in the mid-infrared make it an ideal source for spectroscopic and chemical-sensing applications. Pumped by 11 W from an Nd:YAG laser, the CW optical parametric oscillator generated output from 3.7 to 4.7 µm. They tuned the oscillator's output from 3.7 to 4.7 µm, while the output power varied from 1.2 W to 120 mW.The OPO was arranged in a four-mirror, "bow-tie" ring resonator, with a periodically poled LiNbO3 (PPLN) crystal between two spherical mirrors that focused the light into it (see figure). The OPO resonated only at the signal wavelength (1400 to 1500 nm); the OPO mirrors were transmissive at the pump and idler wavelengths, 1.06 µm and 3.7 to 4.7 µm, respectively. An 11-W, single-mode Nd:YAG laser whose 5-kHz bandwidth could be continuously tuned over a 24-GHz range pumped the OPO.When the scientists pumped it with the maximum 11 W from the Nd:YAG laser, they observed an (idler) output of 1.2 W at 3.9 µm. This output decreased to 120 mW as they tuned the OPO to 4.7 µm. The decrease resulted both from a quantum effect (as the idler is tuned to longer wavelengths, its share of the energy of each pump photon shrinks) -- but, more importantly, to increased absorption of the idler by the PPLN crystal. Estimated to be as great as 35 percent per centimeter, this absorption is a significant issue for an OPO operating in the 4- to 5-µm region. In fact, the absorption apparently causes thermal focusing in the PPLN crystal that accounts for the discrepancy between these experimental results and a previously published theory. The theory predicts both the threshold pump power and outputs at above-threshold pumping levels.The investigators found good agreement between their results and the theory for the threshold values, but discovered that their OPO produced significantly less output than predicted when operating at higher powers. They attribute the discrepancy to thermal focusing in the PPLN, an effect not taken into account in the theory. Thermal focusing misaligns the OPO resonator and likely diminishes the spatial overlap between the pump, signal and idler in the crystal.The scientists inserted an uncoated, 400-µm-thick etalon into the OPO resonator to enhance its single-mode operation, but in doing so, they reduced the output power by about 50 percent. By tilting the etalon, they tuned the OPO's frequency by hopping the resonant signal from one longitudinal mode to the next while the pump wavelength remained constant. They also were able to tune the OPO continuously by tuning the pump laser across its 24-GHz range. In this case, the resonant signal continued oscillating in the same longitudinal mode while the pump and idler wavelengths varied.