Optical detection of molecular species at sub-ppt concentration levels

We have designed and realized a difference-frequency-generated coherent source, with 3850 - 4540 nm tuning range and TEM00 spatial mode. Phase locking of the near-IR "pump" laser to the "signal" one involved in the non-linear mixing process, through a Ti:sapphire-based optical frequency comb, also gives the generated "idler" radiation narrow linewidth and Cs-standard traceability. The relatively high power of this source (30 mW at 4510 nm), greatly enhanced by a high-finesse Fabry-Perot cavity (F >; 11000), enables saturation spectroscopy at pressures even higher than 10 mbar.