Absorption measurement of methane gas with a broadband light source and interferometric signal processing

The relationship between methane concentration and second-harmonic output of a particular optical methane-detection system is derived. The system uses a broadband source and a comb filter with its transmission peaks matched to the R-branch absorption lines of methane. An optimal modulation index is obtained, and a noise-limited detection resolution of 0.02% lower explosive level (3-s time constant) is estimated.