Fig. 1. Simulation results. (a) Signal with noises; (b) signal-to-noise ratios at different denoising numbers; (c) interference rejection ratio at different denoising numbers; (d) peak signal amplitude at different denoising numbers

Fig. 2. Denoised signals obtained at different EMD denoising numbers. (a) Three; (b) four

Fig. 3. Schematic of experimental platform of multi-pass White cell with the combination of DAS and WMS-2f

Fig. 4. Raw signals and DAS signals denoised by different denoising methods. (a) Multi-averaging method; (b) low pass filtering method; (c) multi-EMD method

Fig. 5. Experimental results using different denoising methods. (a) Direct absorption signals denoised by multi-EMD method for CO2 with different volume fractions; (b) linearity of signal denoised by EMD; (c) linearity of signal denoised by low pass filtering; (d) linearity of signal denoised by multi-averaging

Fig. 6. Experimental results of second harmonic signal.(a) Second harmonic signals before and after denoising for CO2 with a volume fraction of 6‰; (b) denoised CO2 signals after multi-EMD; (c) linearity of second harmonic signal