Fig. 1. Schematic diagram of conversion absorption spectroscopy technology. (a) Laser induced fluorescence-(LIF); (b) resonance enhanced multiphoton ionization-(REMPI); (c) photoacoustic spectroscopy (PAS)

Fig. 2. Classification of optical path absorption spectroscopy and conversion absorption spectroscopy

Fig. 3. Absorption optical paths length in different optical path absorption spectroscopy technologies: non-dispersive (NDIR), differential optical absorption spectroscopy (DOAS), tunable diode laser absorption spectroscopy (TDLAS), integrated cavity output spectroscopy (ICOS), off-axis ICOS (OA-ICOS), broadband cavity-enhanced absorption spectroscopy (BB-CEAS), optical feedback CEAS (OF-CEAS), and cavity ring-down spectroscopy (CRDS)

Fig. 4. (a) Schematic diagram of direct absorption TDLAS technology; (b) experimental setup diagram of multi-channel absorption cell TDLAS technology^[16]

Fig. 5. Schematic diagram of (a) active and (b) passive differential optical absorption spectroscopy

Fig. 6. (a) Schematic diagram of White cavity^[32]. (b) Three-dimensional structure diagram of the planar optical multipass cell and (c) its simulation model^[34]

Fig. 7. Schematic diagram of wavelength modulation in the off-axis integral cavity output spectroscopy^[35]

Fig. 8. Schematic diagram of the broadband cavity-enhanced absorption spectroscopy device

Fig. 9. Schematic diagram of OA-CEAS technology

Fig. 10. Schematic diagram of optical feedback cavity enhanced absorption spectroscopy

Fig. 11. Schematic diagram of measurement principle of the decay absorption spectroscopy technique

Fig. 12. (a) Schematic diagram of triangular cavity transverse mode matching; (b) schematic diagram of longitudinal mode matching

Fig. 13. Measurement diagram of linear cavity ring-down spectroscopy

Fig. 14. Measurement diagrams of cavity ring-down spectroscopy with (a) triangular cavity and (b) butterfly cavity

Fig. 15. Schematic diagram of fiber loop ring-down spectroscopy

Fig. 16. High-precision alignment scheme based on Hermite-Gaussian mode excitation characteristics in a triangular cavity^[120]

Fig. 17. Structural diagram of the triangular cavity Hermite-Gaussian mode resonance coupling experimental setup^[121]