Novel Optical Parametric Oscillator Design for High-Power, High-Resolution, Frequency-Stable, CW Infrared Spectroscopic Applications

We present results for novel design and implementation of a high-power, high-resolution continuous-wave optical parametric oscillator (OPO) emitting mid-infrared radiation tunable from 3-4 μm, with output powers in excess of 2.5 W. Our home-built "JILA OPO" design is based on a periodically poled lithium niobate fanout crystal inside a four-mirror ring cavity singly resonant at the signal wavelength (1.4-1.7 μm). The 50 mm-long fanout crystal is pumped by a single-mode, narrow-line width continuous-wave 1064 nm fiber laser, which achieves an oscillation threshold of 2 W. For long-term frequency stabilization and control, we actively lock the OPO to an optical transfer cavity (OTC), which is in turn locked to a polarization-stabilized HeNe laser to achieve root-mean-square noise of ≤2.3(2) MHz within 10 ms and absolute OPO idler frequency drifts of <1 MHz/hour. The high output power and narrow line width of the OPO are demonstrated via saturated absorption spectroscopy on the ν₃ P(7) transition of CH₄ (40-300 mTorr) in a retroreflecting double-pass cell, for which narrow Lamb dips with a full width at half-maximum (fwhm) of 3.1(1) MHz are readily observed. Technical aspects of our monolithic laser block construction, thermal cooling, optical transfer cavity, electronic servo loop control, mirror selection, and beam size considerations for optimal low pumping threshold, long-term frequency stability, and maximal conversion efficiency are presented, with further details available on request.