Fig. 1. Schematic diagram of principle of cavity ring-down^[1]

Fig. 2. Schematic diagram of multi-beam interference theory^[15]

Fig. 3. Schematic diagram of principle of optical feedback (OF) frequency-locking technology^[29]

Fig. 4. Schematic diagram of frequency-locking based on Pound-Drever-Hall

Fig. 5. Schematic diagram of current modulation current components

Fig. 6. Schematic diagram of current modulation matching point feedback

Fig. 7. Outgoing cavity curves at different t_s of incident light on [(a), (b)] and light off [(c), (d)]^[40]

Fig. 8. Schematic diagram of CRDS system device based on double lock^[44]

Fig. 9. Comparison of ring-down signals before and after Kalman filtering^[44]. (a) Ring down time in cavity; (b) result after Kalman filtering

Fig. 10. Spatial concentration distributions and matching points of (a), (c) methane and (b), (d) ethane at suspected leaks in residential areas in field test. Star indicates location of leak^[45]

Fig. 11. Schematic diagram of human exhaled gas detection^[56]

Fig. 12. Typical spectrum of human breath samples around 2983 cm^-1[58]

Fig. 13. Changes in plasma ¹⁴C concentration after human injection of YH12852 receptor agonist^[59]. (a) Linear mean PK model (N=4); (b) semi-logarithmic PK model; (c) proportion of ¹⁴C in urea after different inoculation time

Fig. 14. Schematic diagram of experimental setup for double-cavity frequency-locking measurement of ¹⁴C^[64]