Fig. 1. Absorption spectrum of CO₂, CH₄ and H₂O

Fig. 2. Diagram of the experimental device

Fig. 3. (a) Simulation chart of cantilever beam (The lower right is the physical diagram of the cantilever beam); (b) The relationship between the first-order resonance frequency and the thickness and length of the cantilever

Fig. 4. Diagram of the internal structure of the cantilever microphone （The lower right is the physical diagram of the cantilever microphone）

Fig. 5. (a) Diagram of the experimental device for program control; (b) Comparison diagram before and after Q-point stabilization program control

Fig. 6. Frequency response of a photoacoustic measurement device

Fig. 7. (a) The photoacoustic measurement device tests the 2f-WMS signal at different concentrations of CH₄; (b) Response of photoacoustic device to the CH₄ concentration

Fig. 8. (a) Background noise of photoacoustic device within 1 000 s; (b) Allan variance results