Fig. 1. Absorption spectra of methane gas in the 1650 nm region

Fig. 2. Physical map. (a) Laser CH₄ sensor module; (b) laser CH₄ portable instrument

Fig. 3. Laser CO detection data via 2.3 μm absorption spectral analysis^[47]

Fig. 4. Absorption peaks of CO, CH₄, and CO₂ gases in the 1566 nm wavelength region

Fig. 5. Absorption peaks of CO and CH₄ gas in the 2.33 μm wavelength region

Fig. 6. Absorption spectra of CO gas with 4.6 μm wavelength region

Fig. 7. Fiber wind velocity sensor. (a) Schematic of fiber hot-wire wind velocity sensor; (b) wavelength of hot-wire sensor changes with temperature and time^[62]

Fig. 8. Photo of fiber wind velocity sensor installation at coal mine site

Fig. 9. Measurement data of fiber wind velocity sensor at the coal mine site

Fig. 10. Data of successful hot zone detection and positioning of coal mine goaf in Northeast China in March 2019