A 3.5-mW continuous-wave difference-frequency source around 3 ¼m for sub-Doppler molecular spectroscopy

We present the development of a mid-infrared tunable coherent radiation source, based on difference-frequency generation (DFG). The signal and pump beams coming from two cw rare-earth-doped amplifiers are mixed into a periodically-poled LiNbO3 crystal to produce radiation in the 2.9-3.5 ¼m spectral range. For the first time to our knowledge, an output power of a few mW has been achieved. We show that such a high power can be exploited for high-resolution investigation of molecular absorption spectra. Saturation Lamb-dips have been observed for a number of ro-vibrational transitions belonging to the CH4 ½3 fundamental band.