Fig. 1. Schematic diagram of the multi-pass cavity

Fig. 2. Multi-pass cavity Raman detection system^[26]

Fig. 3. The basic setup of a PDH system

Fig. 4. PDH error signal

Fig. 5. Raman spectrum showing H₂ Q(1) branch^[35]

Fig. 6. Schematic diagram of optical feedback frequency locking process based on external resonant cavity

Fig. 7. The relationship between the laser coupled frequency and the free-running frequency, where ω₀ is the center frequency of the cavity mode

Fig. 8. F-P cavity enhanced Raman detection system based on optical feedback^[40]

Fig. 9. Schematic diagram of Raman laser cavity for gas Raman enhancement

Fig. 10. Basic principle of fiber-enhanced gas Raman spectroscopy

Fig. 11. Silver-plated capillary Raman detection system^[46]

Fig. 12. Hollow fiber Raman detection system^[53]

Fig. 13. Schematic diagram of the combination of optical fiber and F-P cavity for gas Raman signal enhancement