Fig. 1. Four kinds of light beam excitation models. (a) Single excitation-pass; (b) signal excitation-reflection; (c) multiple excitation-pass; (d) multiple excitation-reflection

Fig. 2. Section diagram of multiple excitation-pass model

Fig. 3. Section diagram and equivalent optical path diagram of multiple excitation-reflection model.(a) Section diagram;(b) equivalent optical path diagram

Fig. 4. Schematic diagram of multiple reflection excitation mode

Fig. 5. Finite element simulation model of photoacoustic cell. (a) Geometric model; (b) light beam energy distribution

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

Fig. 7. Three simulated excitation light beams. (a) Excitation light beam with power density of 0.1 W/kg; (b) excitation light beam with power density of 0.2 W/kg; (c) two excitation light beams with power density of 0.1 W/kg

Fig. 8. Resonance modes and sound frequency characteristic curves under three simulated excitation light beams. (a) Resonance mode under excitation light beam in Fig. 7(a); (b) resonance mode under excitation light beam in Fig. 7(b); (c) resonance mode under excitation light beam in Fig. 7(c); (d) sound frequency characteristic curves

Fig. 9. Absorption line of CO₂ in wavelength range of 2002－2006 nm

Fig. 10. Physical diagram of photoacoustic cell

Fig. 11. Schematic diagram of photoacoustic detection device

Fig. 12. Measured sound frequency characteristic curve at end of acoustic resonance tube

Fig. 13. Second harmonic signals of CO₂ under five light beam excitation modes

Fig. 14. Concentration calibration curve of photoacoustic detection device

Fig. 15. Allan variance curve under long-time detection

Table 1. Equivalent absorption paths of different light beam excitation modes

Table 2. SNRs under five light beam excitation modes

Table 3. LoD and NNEA under five beam excitation modes