Broadly Tunable Laser Spectrometer for Trace Gas Detection Based on Critically Phase-Matched Difference-Frequency Generation in Bulk LiNbO/sub 3/

The selective and sensitive detection of trace gases in multicomponent gas mixtures by means of absorption spectroscopy requires narrowband continuously tunable laser sources. Using a cw External Cavity Diode Laser (ECDL), tunable between 795 and 825 nm as pump wave, and a diode-pumped monolithic cw Nd:YAG laser at 1064 nm as signal wave, we produce narrowband (a few MHz) mid infrared laser light between 3.16 and 3.67 μm by Difference-Frequency Generation (DFG) in bulk LiNbO3. Many compounds of interest (CH4, H2CO, …) show characteristic strong absorptions in this fundamental wavelength region. A multipass cell employed as a gas chamber with an effective path length of 36 m is used in combination with sensitive IR detectors for recording absorption spectra. True multicomponent capability of such a laser spectrometer requires a broad, narrowband, continuous tunability in the mid-IR.