3.5-μm high-resolution gas sensing employing a LiNbO3 QPM-DFG waveguide module

Diode laser technology coupled with a wavelength-conversion unit to produce mid-infrared narrow bandwidth laser light applicable to trace-gas detection and with the potential for high-resolution spectroscopy is described. Quasi-phase-matched difference-frequency generation (QPM-DFG) in a compact and fibre-coupled periodically poled lithium niobate (PPLN) waveguide module mixing 1063 and 1525-nm radiations has been adopted for generating 34 mu W of 3.5-mu m wavelength laser light. Optical detection methods, including sensitive wavelength modulation spectroscopy and a rapid wavelength chirp technique, have been employed with a single-pass cell to investigate methane and formaldehyde absorption profiles around 2855 cm(-1), as proof of principle experiments for high sensitivity and resolution spectroscopy on atmospherically important molecules.