Fig. 1. Schematic diagram and physical diagram of hammer tuning fork

Fig. 2. Frequency response curve of the hammer-shaped tuning fork at atmospheric pressure

Fig. 3. Schematic of the C₂H₂ QEPAS sensor

Fig. 4. The picture of the acoustic detection module

Fig. 5. QEPAS signals obtained at different laser focal point locations

Fig. 6. Optimization of the length of the resonator tubes

Fig. 7. C₂H₂-QEPAS signal amplitude with different modulation depth

Fig. 8. The second harmonic signal of 30 ppm C₂H₂ and the noise signal obtained in nitrogen background