Fig. 1. (a) Absorption spectra of CH4 and H2O; (b) relationship between the emission wavenumber of laser and the drive current at the working temperature of 30.95 ℃

Fig. 2. Schematic of the mid-infrared CH4 sensor based on ICL

Fig. 3. Function diagram of the laptop platform based on LabVIEW

Fig. 4. Experimental measured normalized absorption signal and its Lorentzian fitting signal for CH4 sample with the volume fraction of 2.1×10-6. The figure also shows the relation between the laser emission wavenumber, initial output power and the drive current

Fig. 5. Dynamic measurement results of CH4 gas samples with different concentrations by using CH4 sensor

Fig. 6. (a) Long time measurement of CH4 concentration for a CH4 gas sample with the volume fraction of 2.1×10-6; (b) Allan deviation curve

Fig. 7. Measurement results of CH4 concentration monitoring in outdoor atmosphere for 84 h continuously