Application of a diode-laser-based CW tunable IR source to methane detection in air

This work was done to develop a diode-laser-based technique for sensitive atmospheric trace detection gases such as CH₄, CO, N₂O, and NO. Spectroscopic detection of methane in the fundamental, and overtone stretch vibration bands using tunable infrared lasers has been reported. The ν₃ band of methane near 3.2 μm includes its strongest known molecular transition and therefore is better suited for sensitive detection. The band is accessible by either conventional spectroscopy or with Ar⁺-dye laser difference-frequency generation, the carbon monoxide overtone laser, the helium-neon laser near 3.39 μm, lead-salt diode lasers, and color-center lasers. However, each one of these mid-infrared laser sources suffers from its own specific practical drawbacks such as large physical size, lack of portability, high cost, high power consumption, poor tunability, or the need for cryogenic cooling. In the work, detection of the methane in natural air (1.8 ppmv) was performed using diode-laser-pumped cavity-enhanced CW tunable difference-frequency generation (DFG) near 3.2 μm