Fig. 1. The schematic of the TDLAS with multi-pass cell

Fig. 2. Description of cavity transmission and diagram of spot distributions of regular spherical mirrors

Fig. 3. The schematic of the principle of the cavity ring-down spectroscopy（CRDS）

Fig. 4. The simulated absorption spectrum of 5% H₂O and 500 ppm CO₂ isotope gases based on HITRAN database

Fig. 5. The simulated absorption spectrum of vapor isotope gases

Fig. 6. Temperature control box structure diagram and internal temperature distributions simulation results

Fig. 7. Flow chart of temperature and pressure control and fuzzy adaptive PID cycle control algorithm

Fig. 8. The continuous measurement results of temperature and pressure

Fig. 9. The experimental system based on TDLAS technology and its appearance diagram

Fig. 10. Simulation results of CH₄ absorption spectra at different pressures near 1 658 nm

Fig. 11. The measurement results of CH₄ isotope and Allan variance analysis of system stability

Fig. 12. The experimental system based on ICOS technology and its appearance diagram

Fig. 13. The direct absorption signals of ¹⁶O¹²C¹⁶O，¹⁶O¹³C¹⁶O and ¹⁶O¹²C¹⁸O

Fig. 14. The experimental system based on CRDS technology and its appearance diagram

Fig. 15. The photograph of cross-mode spot with infrared camera

Fig. 16. CRDS measurement results

Fig. 17. ICOS measurement results and isotopic abundance values

Fig. 18. Methane absorption spectra and isotopic abundance values

Fig. 19. Water isotope measurement results

Fig. 20. Comparison of isotopic values of water vapor with IRMS and Picarro instrument