Mid-infrared generation and spectroscopy with a PPLN ridge waveguide

We have utilised a periodically poled LiNbO3 crystal with a waveguide structure to produce up to 146 mu W of mid-IR radiation around 2976 cm(-1) by difference frequency mixing of 1064 nm pump radiation with 1558 nm signal radiation. A conversion efficiency of 45% W-1 is achieved within a crystal of length 50 mm, and temperature tuning curves are reported. The resultant idler radiation is exploited for high resolution absorption studies of both methane and methanethiol. The absorption cross-sections for methanethiol in the region of 2979 cm(-1) are measured and found to be similar to 10(-19) cm(2), the relevance of these results to breath and headspace analysis of clinical samples containing volatile sulphur compounds is briefly discussed.