Fig. 1. Experimental configuration

Fig. 2. Simulation and physical picture of the absorption cell

Fig. 3. Direct absorption measurement results

Fig. 4. Fitting curve of configured concentration to calculated concentration

Fig. 5. The second harmonic signal of methane

Fig. 6. Relationship between peak second harmonic and methane concentration

Fig. 7. Second harmonic signal's amplitude at a methane concentration of 100 ppm

Fig. 8. Sensitivity analysis of the detection system