Fig. 1. Schematics of integrated cavity output spectroscopy and transmittance. (a) On-axis alignment; (b) off-axis alignment

Fig. 2. Schematic of CH₄ and CO₂ detection system based on off-axis integrated cavity output spectroscopy(LC: laser collimator; OC: optical cavity; HE: heat exchanger; TR: thermal resistor; TB: thermostatic box; DA: photoelectric detector and amplifier; PV: proportional valves; OS: optical switch; LD: laser diodes and drivers; PS: pressure sensor; TPU: temperature and pressure controlling unit; IPC: industrial personal computer)

Fig. 3. Temperature in the optical cavity measured for 24 h

Fig. 4. Pressure in the optical cavity measured for 24 hours

Fig. 5. Spectral data of calibration experiments. (a) CH₄; (b) CO₂

Fig. 6. Linear relation between absorption coefficients calculated from spectral data and the concentration of measured gases. (a) CH₄; (b) CO₂

Fig. 7. Absorption coefficient fitted to Voigt profiles and the fitted residuals. (a) CH₄; (b) CO₂

Fig. 8. The launch of the M-ICOS system

Fig. 9. Variations of depth, gas pressure, and volume fractions of CH₄ and CO₂ with time