针对利用可调谐半导体激光器吸收光谱学(TDLAS)技术测量气体浓度过程中二次谐波谱线存在的外界噪声干扰问题,提出一种基于变分模态分解和小波阈值函数复合算法的二次谐波降噪方法。首先对二次谐波含噪信号进行分解,得到有用固有模态函数(IMF)并进行重构,再对重构信号进行小波阈值函数降噪处理。讨论了变分模态分解中最佳平衡参数的选取,得出最佳平衡参数与含噪信号中噪声成正比的结论。通过改变小波变换的阈值函数改变高频小波系数,以更好地抑制噪声。对实际测量曲线的降噪结果表明,所提出的降噪方法可以在信噪比较低的情况下有效抑制噪声,提取有用的二次谐波信号。

A second harmonic de-noising method based on variational mode decomposition and wavelet threshold function is proposed to solve the problem of external noise interference in second harmonic spectra during gas concentration measurement by tunable diode laser absorption spectroscopy (TDLAS). In this paper, we decompose the noisy second harmonic signal to get the useful intrinsic mode functions (IMFs) and reconstruct them. Then, we conduct the de-noising process for the reconstructed signal with the wavelet threshold function. The selection of the optimal balance parameter in the variational mode decomposition is discussed, and the proportional relationship of the optimal balance parameter with the noise in the noisy signal is obtained. Better noise suppression is achieved by changing the threshold function of wavelet transform and thereby altering the high-frequency wavelet coefficients. The de-noising results of actual measurement curves show that the proposed de-noising method can effectively suppress the noise and extract the useful second harmonic signal in the case of a poor signal-to-noise ratio.