Fig. 1. Wavelet threshold denoising process

Fig. 2. Flow chart of EMD-DFA-wavelet adaptive threshold denoising method

Fig. 3. Second harmonic signals. (a) Original second harmonic signal; (b) second harmonic signal with noise

Fig. 4. IMFs of signals with noise. (a) IMF1; (b) IMF2; (c) IMF3; (d) IMF4

Fig. 5. EMD-DFA-wavelet adaptive threshold noise reduction

Fig. 6. Comparison of original signal and EMD-DFA-wavelet adaptive threshold denoising signals

Fig. 7. Noise reduction effect diagram of different algorithms. (a) Use EMD-DFA to reduce noise; (b) use wavelet soft threshold to reduce noise; (c) use wavelet hard threshold to reduce noise; (d) use wavelet adaptive threshold to reduce noise

Fig. 8. System structure diagram

Fig. 9. Relationship between second harmonic amplitude and CO₂ concentration before denoising

Fig. 10. Relationship between second harmonic signal and CO₂ concentration after EMD-DFA-wavelet adaptive threshold noise reduction processing

Fig. 11. Linear fitting of relationship between amplitude of second harmonic and concentration of CO₂