Frequency division multiplexing and wavelength stabilized 2f/1f wavelength modulation spectroscopy for simultaneous trace CH4 and CO2 detection

Abstract

A near-infrared (NIR) dual-gas sensor has been developed for simultaneous detection of atmospheric trace methane (CH₄) and carbon dioxide (CO₂). To realize high sensitivity and high precision, wavelength modulation spectroscopy with 2f/1f (WMS-2f/1f) detection method was adopted for eliminating laser light intensity fluctuation, and laser wavelength locking strategy based on a self-developed proportion integration differentiation (PID) algorithm was used for suppressing laser wavelength shifting effect. Two fiber-coupled DFB diode lasers with central wavelengths near 1653.7 nm and 1579.6 nm are applied for simultaneously measuring CH₄ and CO₂ spectra, respectively, and frequency division multiplexing (FDM) technique is employed to resolve the potential crosstalk effect. Real-time measurement of ambient atmospheric trace CH₄ and CO₂ was performed to demonstrate the long-term stability of the sensor system. Allan deviation analysis indicates that detection sensitivity of 0.1 ppm for CH₄ and 2.27 ppm for CO₂ was achieved with a 1 s average time, which can be further improved to 18 ppb and 0.3 ppm with the optimal integration time of 462 s and 392 s, respectively.

Keywords: Frequency-division multiplexing; Multi-gas sensor; Normalized wavelength modulation spectroscopy; Wavelength stabilization.