Highly sensitive measurement of trace methane gas using mid-infrared tunable diode laser absorption spectroscopy method

To monitor the concentration of harmful gas methane (CH4) in the tunnel in real time, a high-sensitivity measurement method for CH4 trace gas based on tunable diode laser absorption spectroscopy (TDLAS) is designed in this paper. An interband cascade laser with the emission wavelength of 3.29 mu m was used to target noninterference CH4 lines at 3038.5 cm(-1). To diminish the background noise of the sensor system, wavelength modulation spectroscopy combined with a second harmonic measurement technique was used in this work. Finally, 1.83 parts per billion in volume (ppbv) minimum measurement limit (MML) was obtained with a 4 s integration time based on an Allan-Werle variance analysis, and the MML can be further improvedto similar to 0.4 ppbv by increasing the integration time up to 104 s. Indoor and outdoor environmental data of CH4 acquired using the TDLAS sensor system are also reported.