Fig. 1. Schematic diagram of gas photoacoustic detection technology

Fig. 2. Absorption lines of CO₂. (a) 2004 nm; (b) 4300 nm

Fig. 3. Reflectance of gold-plated absorption cell. (a) 250-2000 nm; (b) 2000-16000 nm

Fig. 4. 3D structure diagrams of photoacoustic cell. (a) Front view; (b) left view; (c) right view

Fig. 5. Finite element simulation model. (a) Light beam excitation mode; (b) mesh generation;

Fig. 6. Sound pressure distribution and sound frequency characteristic curve of the first-order longitudinal resonance mode of photoacoustic cell. (a) Sound pressure distribution; (b) sound frequency characteristic curve

Fig. 7. Emission spectrum of LED

Fig. 8. Current-optical power curves of LED

Fig. 9. LED drive circuit. (a) Schematic diagram; (b) physical drawing

Fig. 10. Schematic diagram of photoacoustic detection setup

Fig. 11. Flow chart of photoacoustic detection

Fig. 12. Frequency response curve of photoacoustic signal

Fig. 13. Photoacoustic signals of different concentrations of CO₂

Fig. 14. Calibration curve of concentration and photoacoustic signal amplitude

Fig. 15. Stability and sensitivity analysis. (a) Noise distribution of long-time detection;

Fig. 16. Schematic diagram of CO₂ detection by laser

Fig. 17. Photoacoustic signals of different concentrations of CO₂

Fig. 18. Calibration curve of relationship between concentration and photoacoustic signal amplitude

Fig. 19. Stability and sensitivity analysis. (a) Noise distribution of long-time detection; (b) Allan deviation curve