Fig. 1. Simulated absorption spectra of CH₄, H₂O and CO₂ near 6046.9 cm^-1 based on HITRAN database

Fig. 2. Off-axis integral cavity output spectral methane detection system. (a) System structure diagram; (b) system physical map; (c) F-P cavity model based on cage-like structure

Fig. 3. Detection data of the system on CH₄. (a) Absorption spectra of standard gas with volume fraction of 5×10^-6and atmosphere in reference cavity, and atmospheric absorption spectrum in open cavity; (b) Voigt fitting result of atmospheric absorption spectrum in open cavity; (c) Voigt fitting residual error

Fig. 4. Measured volume fraction of atmospheric CH₄ for 90 min. (a) Volume fraction of CH₄; (b) Allan deviation results

Fig. 5. Results of different data processing methods. (a) Raw data; (b) 10th averaged data; (c) Kalman filtered data; (d) Allan variance curve after filtering

Fig. 6. Simulated CH₄ leakage experiment. (a) Experimental photograph; (b) measurement results