Fig. 1. Schematic diagram of ARROW model

Fig. 2. Absorption spectrum of methane

Fig. 3. Fiber cross section and relationship between fiber wavelength and loss. (a) Fiber cross section; (b) relationship between fiber wavelength and loss

Fig. 4. Fiber coupling device. (a) Schematic diagram; (b) physical diagram

Fig. 5. Structural diagram of detection device

Fig. 6. Second harmonic signal

Fig. 7. Concentration calibration curve

Fig. 8. Measurement accuracy of device. (a) Distribution of methane concentration; (b) histogram of concentration frequency distribution

Fig. 9. Amplitude change curve of second harmonic signal

Fig. 10. Allan variance curve