Fig. 1. Absorption spectra of CH₄ and H₂O at temperature of 298 K, pressure of 101.3 kPa, and effective optical path of 26.4 m. (a) CH₄ absorption spectrum in 2870－3180 cm^－1; (b) absorption spectra in 3025－3070 cm^－1; (c) absorption lines at 3038.5 cm^－1

Fig. 2. Block diagram of CH₄ sensor system

Fig. 3. Interface of LabVIEW data processing platform

Fig. 4. Emission wavenumbers of laser at different temperatures and currents with emission spectra of ICL under different driven currents at 16 ℃ shown in inset

Fig. 5. Modulation depth optimization

Fig. 6. Calibration and fitting of sensor system. (a) 2f signal amplitudes recorded at different CH₄ concentrations with 2f spectral signals of five CH₄ samples shown in inset; (b) CH₄ concentration versus 2f signal amplitude

Fig. 7. Allan deviation analysis of sensor. (a) CH₄ concentration measured by passing N₂ into gas cell; (b) Allan deviation versus averaging time τ

Fig. 8. Response time measurement of sensor

Fig. 9. Vehicle-borne movement measurement experiment for sensor. (a) Experimental photograph; (b) measured concentration versus observation time; (c) spatial distribution of measured concentrations

Fig. 10. Location experiment of CH₄ leakage source. (a) Experimental photograph; (b) measured concentration versus observation time; (c) location result of leakage source