Fig. 1. Absorption intensity of H₂O，CO₂ and NO

Fig. 2. Absorption intensity spectra of H₂O，NO and CO₂ near wavenumber of 1 909 cm^-1

Fig. 3. Schematic diagram of three-stage temperature control structure of laser

Fig. 4. Structure schematic of temperature control chamber

Fig. 5. Temperature contour of each component of temperature control structure

Fig. 6. Simulation results of Y_2f/X_2f concentration inversion model

Fig. 7. Sensitivity curve of Y2f/X2f as a function of i2/i0

Fig. 8. Sensitivity curve of Y2f/X2f as function of modulation index

Fig. 9. Temperature variation curve of laser

Fig. 10. Real time data of temperature control experiment（NTC voltage division inversion）

Fig. 11. Real time data of temperature control experiment（NTC voltage division inversion）

Fig. 12. Allan variance of inverted concentrations at different temperature fluctuations with second harmonic peaks

Fig. 13. Experiment data of 300×10^-6 concentration NO gas

Fig. 14. Y2f/X2f signal waveform at different concentrations

Fig. 15. Peak-to-Peak values of Y2f/X2f signal waveform at different concentrations

Fig. 16. Allan variance of system Y2f/X2f signal

Fig. 17. 2f/1f signal waveform at different concentrations

Fig. 18. Allan variance of system 2f/1f signal