Dual-beam antiphase method to improve the WMS measurement limit in long-distance methane detection

Wavelength modulation spectroscopy (WMS) with second harmonic detection is an extremely effective technique to detect gases in site applications. However, the significant levels of nonlinear effects in a system give rise to high background signals that either limit detection sensitivity or distort the harmonic signals. This paper outlines the theory of WMS-involved background signals and focuses on the elimination of undesirable effects in the background. A real-time, long-distance methane sensor using a tunable diode laser near 1653.7 nm is developed to continuously monitor methane by using a variable optical attenuator to suppress the background. Trace methane detection experiments verify that the minimum detection limit of the system can be increased by 47.5 times compared to the traditional WMS method.