Fig. 1. Wavelet transform decomposition flow chart

Fig. 2. Principle of Gabor transform for signal denoising

Fig. 3. Simulated NH₃ spectral signal.（a）Original second harmonic (2f) signal;（b）Gaussian noise;（c）noised second harmonic signal

Fig. 4. SNR as a function of parameters in different methods. (a) SNR as a function of decomposition level in wavelet transform; (b) SNR as a function of threshold value in Gabor transform

Fig. 5. RMSE after denoising. (a) Wavelet transform denoising; (b) Gabor transform denoising

Fig. 6. Second harmonic （2f） signal after denoising. (a) Wavelet transformation denoising; (b) Gabor transformation denoising

Fig. 7. System for measuring low concentration NH₃

Fig. 8. Noisy second harmonic signals

Fig. 9. Second harmonic signals of different concentrations of NH₃ after denoising. （a） Wavelet transform denoising；（b） Gabor transform denoising

Fig. 10. Linear relationship between peak height of second harmonic signal of different concentrations of NH₃ and measured concentration. （a） Wavelet transform; （b） Gabor transform