Fig. 1. Diagram of telemetry device

Fig. 2. Optical power (DC) signal and spectrum (2f) signal of detection device

Fig. 3. Laser beam divergence angle under different driving currents. (a) In W direction; (b) in V direction

Fig. 4. Changes in the received optical power

Fig. 5. Variation of spot center position. (a) At 44 cm; (b) at 167 cm

Fig. 6. Zoom lens in ZEMAX simulation. (a) Structure of zoom lens; (b) deformation model of zoom lens surface affected by gravity

Fig. 7. Concentration calibration. (a) 2f spectra of methane at different concentrations; (b) relationship between peak-peak value of 2f signal and integral concentration

Fig. 8. Stability test. (a) CH₄ concentration reading; (b) Allan variance curve

Fig. 9. Telemetry device and results. (a) Test for methane leaking in corridor; (b) optical devices of telemetry device; (c) test spectrum for methane concentration of 4.95×10^-6