Fig. 1. Schematic diagram of experimental setup of optical feedback cavity ringdown system

Fig. 2. Measured spectra used to determine FSR values

Fig. 3. Transmitted signals of cavity

Fig. 4. （a） Decay rate of empty cavity and （b） Allan variance

Fig. 5. Laser scanning signals （a） and （c） and acquired transmission signals （b） and （d）

Fig. 6. Measured spectrum and fitting results （a） and fitting residuals （b） of CH₄

Fig. 7. 1.5×10^-6 CH₄ standard gas measurement results