Fig. 1. Absorption spectra of CH₄ in different ranges. (a) 5800-6800 cm^-1; (b) 6040-6050 cm^-1

Fig. 2. Overall structure design drawing of telemetry system

Fig. 3. Structure diagrams of methane telemetry system. (a) Optical telemetry components; (b) FPGA+ARM laser modulation, signal acquisition, processing, and demodulation embedded integrated module; (c) connection diagram of telemetry component and embedded integrated module

Fig. 4. Internal structure diagram of FPGA

Fig. 5. Flowchart of FPGA harmonic signal demodulation

Fig. 6. Experimental results of 1f, 2f, and 2f/1f signal value and detection distance

Fig. 7. System measurement of in range of 0-26000×10^-6 m concentration

Fig. 8. Harmonic signal diagrams of different concentrations. (a) 1f signal diagram; (b) 2f signal diagram

Fig. 9. Change of 2f/1f peak value in the range of 0-2000×10^-6 m

Fig. 10. 2f/1f signal and concentration fitting curve

Fig. 11. Measurement error of different concentration

Fig. 12. Measurement of detection lower limit.(a) 2f signal diagram of 1500×10^-6 m methane concentration; (b) noise diagram of 0×10^-6 m methane concentration system