Fig. 1. Structural diagram of QEPAS system based on MI ^[38]

Fig. 2. Structural diagram of CEPAS system^[40]

Fig. 3. Schematic diagram of CEPAS system based on dual cantilever beam^[33]

Fig. 4. Schematic diagram of CEPAS system based on silicon cantilever beam^[41]

Fig. 5. Structural diagram of all-optical ellipsoidal resonance photoacoustic spectroscopy system^[26]

Fig. 6. Schematic diagram of fiber-optic FPI acoustic sensor based on composite nanomembranes^[44]

Fig. 7. Schematic diagram of all-optical photoacoustic spectroscopy gas detection system based composite nanomembranes^[44]

Fig. 8. Schematic diagram of all-optical photoacoustic spectroscopy system for trace C₂H₂ detection^[35]

Fig. 9. Schematic diagram of dual-channel QEPAS system based on FPI^[49]

Fig. 10. Schematic diagram of FPI acoustic sensor based on stainless steel cantilever beam^[51]

Fig. 11. Schematic diagram of all-optical high-sensitivity resonant CEPAS system^[52]

Fig. 12. Schematic diagram of miniaturized photoacoustic gas sensor based on polymer film^[58]

Fig. 13. Schematic diagram of miniaturized photoacoustic gas sensor based on cantilever beam^[59]

Fig. 14. Schematic diagram of all-optical high-sensitivity miniaturized photoacoustic spectroscopy CH₄ detection system based on differential silicon cantilever optical microphones^[60]

Fig. 15. Schematic diagram of fiber-tip photoacoustic gas sensor^[61]

Fig. 16. Schematic diagram of multi-component fiber photoacoustic gas sensing system^[62]

Fig. 17. Schematic diagram of all-optical photoacoustic spectroscopy system for CH₄ detection^[63]

Fig. 18. Schematic diagram of all-optical high-sensitivity miniaturized dual-resonance CEPAS system ^[64]

Fig. 19. Structural diagram of designed fiber-optic photoacoustic sensing system for dissolved gas in oil^[66]

Fig. 20. Schematic diagram of iPAS system and test setups^[68]. (a) Schematic diagram of iPAS system; (b) enlarged view of iPAS sensor; (c) setup for calibration test in gas medium; (d) setup for calibration test in transformer oil; (e) setup for simulating discharge fault monitoring

Fig. 21. Schematic diagrams of three-dimensional assembly structure and cross-sectional structure of fiber-optic photoacoustic sensor^[69]. (a) Schematic diagram of three-dimensional assembly structure; (b) schematic diagram of cross-sectional structure

Fig. 22. Schematic diagram of FPI-based all-optical photoacoustic spectroscopy system^[70]

Fig. 23. Structural diagram of trace NH₃ detection system based on near-infrared CEPAS^[71]