Fig. 1. Resonator model based on differential photoacoustic spectroscopy

Fig. 2. Structural diagram of experimental device system

Fig. 3. Frequency response curves

Fig. 4. Background noise of two resonators without excited photoacoustic signal and noise characteristic after difference. (a) Background noise of channel-1; (b) background noise of channel-2; (c) noise characteristic after difference

Fig. 5. Relationship between amplitude of photoacoustic signal and demodulation phase

Fig. 6. Second-harmonic PA signals produced by water vapor absorption in two channels of acoustic-resonance cavity when demodulation phase is 25°

Fig. 7. Experimentally measured results with and without difference calculation. (a) Second harmonic signals; (b) amplitudes of signals measured continuously

Fig. 8. Allan deviation analysis results of single-channel and differential detections