Fig. 1. Light distribution of different reflection mode
(a): Concentric cavity; (b): Near concentric cavity;(c): Fold concentric cavity; (d): Fold near concentric cavity The different colors represent the different light energy 100%≥red≥66.6%, 66.6%>green≥33.3%, 33.3%>blue≥25%

Fig. 2. Schematic diagram of the fold near concentric cavity enhanced Raman signal experimental setup

Fig. 3. Photograph of the experimental apparatus
(a): Fold near concentric mode; (b): Fold concentric mode

Fig. 4. Experimental results comparison between different working mode of concentric cavity, near concentric cavity, fold concentric cavity, fold near concentric cavity and single pass

Fig. 5. Raman spectra of O₂ and CO₂ based on the different cavity mode and single pass mode