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 signal.