Fig. 1. Spectral line simulation and selection. (a) Simulated absorption lines of CH₄, CO₂, and H₂O based on HITRAN2016 database and the wavenumber of CO₂ absorption spectra versus laser driving current; (b) Simulated absorption lines of CH₄, CO₂, and H₂O based on HITRAN2016 database and the wavenumber of CH₄ absorption spectra versus laser driving current

Fig. 2. Schematic of dual gas sensor. (a) Design cavity and integrated optical path; (b) experimental setup structure diagram of OA-ICOS

Fig. 3. Time-division multiplexing of dual lasers. (a) Dual laser scanning signal based on time division multiplexing method; (b) actual absorption spectra for CO₂ and CH₄ in different systems

Fig. 4. OA-ICOS absorption spectra of CH₄ with volume fraction of 2.3×10^-6 at different power RF noise. (a) CH₄ absorption spectral signals after averaging 50 times; (b) absorption spectra and residual

Fig. 5. Measurement results and analysis of CO₂ under different conditions. (a) Left images are the continuous measurement results of CO₂ using an OA-ICOS system without RF noise source and with -20 dBm RF noise source for approximately 2 h, and right images are histograms of the volume fraction distributions; (b) Allan variance plots

Fig. 6. Measurement results and analysis of CH₄ under different conditions. (a) Left images are the continuous measurement results of CH₄ using an OA-ICOS system without RF noise source and with -20 dBm RF noise source for approximately 2 h, and right panels are histograms of volume fraction distributions; (b) Allan variance plot

Fig. 7. Continuous monitoring of CO₂ and CH₄ volume fractions in actual atmosphere in April 2023