Fig. 1. Energy conversion process of gas photoacoustic effect

Fig. 2. Current-wavenumber relationship of DFB laser at 18 ℃ and spectral distributions of CO, H₂O, and CO₂ in this wavelength range

Fig. 3. Schematic diagram of structure based on photoacoustic spectroscopy

Fig. 4. Photoacoustic signal of CO gas under different modulation frequencies

Fig. 5. Photoacoustic signal of CO gas under different modulation amplitudes

Fig. 6. Second harmonic signal under different relative humidity of CO

Fig. 7. Second harmonic signals of 0.15% CO-N₂ gas in dry and wet conditions

Fig. 8. Second harmonic signal of CO with different concentrations

Fig. 9. Linear relationship between second harmonic signal amplitude and CO concentration

Fig. 10. Comparison between measured value and inversion value of CO concentration

Fig. 11. Second harmonic signal of CO gas in air. (a) Photoacoustic spectral signal of CO gas in air; (b) noise in N₂ atmosphere

Fig. 12. Comparison of detected Allan variances of CO in air under dry and wet conditions