基于可调谐二极管吸收光谱技术（TDLAS）的痕量气体检测中，由于波长调制光谱（WMS）中光强也被调制，因此会伴随着剩余振幅调制（RAM）的产生，对检测信号的线型和系统噪声产生一定程度的影响。在傅里叶分析的基础上对任意调制度的二次谐波信号进行了建模分析，给出了光强幅度调制引起吸收谱线畸变的理论解释，并分析了光强调制的线性和非线性部分以及调制度对谱线畸变的影响情况，给出了二次谐波不对称及RAM噪声产生的根本原因。以NH3为例在考虑光强幅度调制的情况下进行基于TDLAS的二次谐波检测实验，给出了调制度对二次谐波线型及噪声水平的影响，并对理论分析进行了实验验证。

Since the light intensity is modulated in the wavelength modulation spectroscopy (WMS) in the trace gas detection based on tunable diode laser absorption spectroscopy (TDLAS), the output signal can be accompanied by the residual amplitude modulation (RAM), which can affect the line shape of the detection signal and the noise to a certain extent. Theoretical model and analysis of the second harmonic signal of arbitrary large modulation amplitude based on Fourier analysis are presented, and the theoretical explanations about the distortion of the absorption lines are given. The effect of linear intensity modulation and non-linear modulation on the spectral distortion is analyzed, and the basic reasons of the asymmetry of the second harmonic and the noise are given. Considering the intensity amplitude modulation, the experiment of the the second harmonic detection of NH3 based on the TDLAS is conducted and the effect of the modulation amplitude on the distortion of absorption lines is evaluated. The experimental results confirm the theoretical analysis.