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Nonlinear distortion is critical for optical communication systems with a high baud rate and a high-order modulation format. Thus, a simple and accurate method to measure the nonlinear distortion is highly desired. Although simple notch, which directly removes the certain frequency components of nonlinear system input and then measures the re-growth components of nonlinear system output, is straightforward to measure nonlinear distortion, it is only applicable to the Gaussian input. However, pulse amplitude modulation (PAM) and quadrature amplitude modulation (QAM), which are widely used in communication systems, are not Gaussian signals. In this study, we propose a deep-depth probability-maintained notch generation method for the PAM to accurately characterize the nonlinear distortion based on the signal spectrum measurement. Employing the proposed time order perturbation, the notch depth for PAM2/4/8 achieves 20/24/28 dB, which allows for a large dynamic range nonlinear distortion measurement for various modulation formats. The experiment demonstrates that the nonlinear noise power ratio could be measured within a root mean square error of 0.6 dB for the uniform PAM2/4/8 as well as the probabilistic shaped PAM4/8. The proposed method is also applicable to square QAM modulation formats because a square QAM is composed of two independent PAMs.

非线性失真对于高波特率、高阶调制格式的光通信系统而言至关重要。因此，业界亟需一种简便且精准的非线性失真测量方法。尽管直接移除非线性系统输入的特定频率分量后，再测量非线性系统输出中的再生分量的简易陷波法，在测量非线性失真时操作简便，但该方法仅适用于高斯输入信号。然而，通信系统中广泛应用的脉冲幅度调制（Pulse Amplitude Modulation, PAM）与正交幅度调制（Quadrature Amplitude Modulation, QAM）均不属于高斯信号。本研究基于信号频谱测量，提出一种面向PAM信号的深度概率保持型陷波生成方法，以精准表征非线性失真。通过采用所提出的时序扰动方案，PAM2/4/8的陷波深度分别可达20/24/28 dB，可为多种调制格式下的大动态范围非线性失真测量提供支撑。实验结果表明，对于均匀分布的PAM2/4/8以及概率整形PAM4/8，非线性噪声功率比的测量均方根误差（Root Mean Square Error, RMSE）可控制在0.6 dB以内。由于方形正交幅度调制由两路独立的PAM信号构成，所提方法同样适用于方形QAM调制格式。