Fig. 1. Absorption line of CH₄ molecule at 1 492.5 - 1 694.5 nm

Fig. 2. Experimental configuration

Fig. 3. 2f signals’ amplitude of 10 different concentrations of CH₄

Fig. 4. Fitting curve of the 2f signal's amplitude and the CH₄ concentration

Fig. 5. 2f signal's amplitude (SA) at a CH₄ concentration of 250 ppmv

Fig. 6. Noise fluctuation signal (NA) when CH₄ concentration is 0 ppmv

Fig. 7. 10 hours of stability observation experimental data

Fig. 8. Allan deviation analysis of the detection system