[A Methane Gas Sensor Based on Mid-Infrared Quantum Cascaded Laser and Multipass Gas Cell]

According to the principle of mid-infrared absorption spectrum, the fundamental absorption characteristics at the wavelength of 7.5 μm of methane (CH4) molecule was used to design a mid-infrared quantum cascaded laser (QCL) and multi-pass gas cell (MPC)-based methane gas sensor. This sensor uses a thermoelectrically cooled, pulse mode QCL whose central wavelength is 7.5 μm. The QCL wavelength was scanned over CH4 absorption line (1 332.8 cm-1)through adjusting the injection current under the condition of room temperature. Meanwhile, a compact MPC (40 cm long and 800 mL sampling volume) was utilized to achieve an effective optical path length of 16 meters. Additionally, a reference gas cell was occupied and joined a spatial filtering optical structure to meet the requirement of MPC in incidence beam, effectively improved the beam quality, reduced the noise which is caused by the fluctuation of QCL and improved the detection sensitivity of this instrument under the guidance of differential optical absorption spectroscopy method. It indicated that the stability of this instrument is good by means of multiple measurements to the methane gas with different concentration, a detection limit of 1 μmol·mol-1 will be obtained when the signal-to-noise ratio equals 1.