Fig. 1. Sensing waveguide structure. (a) Schematic diagram of sensing waveguide and system; (b) cross-section structure of trapezoidal waveguide; (c) cross-section structure of slot waveguide

Fig. 2. Scanning electron microscope image of waveguide cross-section structure

Fig. 3. Diagram of waveguide gas sensor system based on WMS

Fig. 4. Simulated absorption spectrum of pure CH₄

Fig. 5. CH₄ sensing results. (a) 2f signal under CH₄ volume fraction of 4×10^-1; (b) 2f signal amplitude, fitting result of the 2f signal amplitude, and simulated 2f signal amplitude under different CH₄ volume fractions

Fig. 6. Optimization process of slot waveguide. (a) Variation of γ and TE mode effective refractive index of waveguide with core layer height; (b) variation of γ and TE mode effective refractive index of waveguide with ridge width; (c) variation of γ and TE mode effective refractive index of waveguide with slot width

Fig. 7. Simulation performance results of slot waveguide. (a) Variation of 2f signal amplitude with L under different α_WG conditions (inset: optical field distribution of slot waveguides); (b) variation of C_min with L under different α_WG conditions

Fig. 8. Influence of environmental pressure on the 2f signal amplitude and C_min

Fig. 9. Influence of fabrication errors on slot waveguide sensor performance. (a) Relative error of γ and C_min with core layer height; (b) relative error of γ and C_min with ridge width; (c) relative error of γ and C_min with slot width