Fig. 1. Schematic diagram of photoacoustic spectrum sensor system

Fig. 2. Software simulated beam results (a) and the spot distribution of 640 nm laser in the mirror (b)

Fig. 3. Relationship between photoacoustic signal intensity and experimental conditions. (a) Modulation frequency ; (b) pressure; (c) gas flow; (d) modulation amplitude

Fig. 4. 2f signal of 42 times reflection (red line) and 2f signal of single reflection (black line) (a) and the relationship between photoacoustic signal and CO₂ mass concentration (b), the red straight line represents linear fitting

Fig. 5. 2f signal at 196.42 mg/m^-3 CO₂ standard gas (a) and long term monitoring of gas mass concentration in the photoacoustic cavity of 500 mg/m^-3 CO₂ standard gas (b), the illustration is the amplification of 1 h signal