Fig. 1. Schematic of the laser methane remote sensor system based on WMS and WD

Fig. 2. Absorption signal and second harmonic signal simulated by MATLAB for methane sample with concentration of 100×10^-6. (a) Ideal absorption signal; (b) ideal second harmonic signal; (c) polluted absorption signal; (d) polluted second harmonic signal; (e) denoised absorption signal; (f) denoised second harmonic signal

Fig. 3. Absorption signal of CH₄ and the second harmonic signal denoised by the EMD-based WD. (a) Absorption signal; (b) the second harmonic signal

Fig. 4. Under a concentration level of 200×10^-6 m, the extracted second harmonic signal using the lock-in amplifier program. (a) Extracted second harmonic without WD; (b) extracted second harmonic with WD

Fig. 5. Relation curve of amplitude of the second harmonic signal are extracted by using an orthogonal lock-in amplifier program(the integral concentration levels of methane is (0--200)×10^-6 m with an increase of 50×10^-6 m for each sample). (a) Calibration result without WD; (b) the calibration result with WD

Fig. 6. Experimental calibration results and fitting curve. (a) Without WD; (b) with WD

Fig. 7. In nitrogen atmosphere, the measured Allan deviation curves. (a) Without WD; (b) with WD

Fig. 8. Experimental results of dynamic response of the remote sensor system with WD