Fig. 1. Schematic diagram of O₂ sensor experimental system based on QEPAS technology

Fig. 2. Spectral lines of water and oxygen at 13054.83‒13123.36 cm^-1

Fig. 3. Normalized frequency response curve of quartz tuning fork

Fig. 4. Optimization of modulation amplitude. (a) QEPAS 2f signal at different modulation amplitudes; (b) relationship between QEPAS 2f signal and modulation amplitude

Fig. 5. O₂ photoacoustic signal versus volume fraction of water vapor

Fig. 6. Linear response curves of volume fraction of QEPAS system. (a) QEPAS 2f signals for different volume fractions of oxygen; (b) relationship between QEPAS 2f amplitude and oxygen volume fraction

Fig. 7. Oxygen QEPAS 2f signal with volume fraction of 21%

Fig. 8. Long-term stability analysis of system. (a) Oxygen 2f signal amplitude monitored continuously for 8 h; (b) Allan variance of detection system