Characterization of Nonlinear Distortion in Intensity Modulation and Direct Detection Systems

Intensity modulation and direct detection (IM-DD) systems are widely used in short-reach optical networks. Thus, developing a practical and accurate method to characterize nonlinear distortion and to estimate system performance is essential, as this is the foundation of IM-DD system design. The amount of nonlinear distortion can be characterized using the noise-to-power ratio (NPR). Although measuring the NPR using a notch requires only a spectrum analyzer, this method cannot be applied to a nonlinear system with non-Gaussian stimuli in general. We employ the method of measuring the NPR using a notch to characterize the nonlinear distortion in directly modulated distributed feedback (DM-DFB) laser-based IM-DD systems. Nonlinear distortion in these IM-DD systems can be attributed to two specific mechanisms: the imperfect power-current characteristic of the DM-DFB laser and the interaction among the modulation, chromatic dispersion (CD), and detection. Experimental results demonstrated that the nonlinear distortion can be accurately characterized with a mean absolute error of 0.47 dB, even when the stimuli are pulse amplitude modulation. The system performance subjected to nonlinearity could be estimated by the equivalent additive noise model. Besides, the method of measuring the NPR using a notch is also effective when the Mach-Zehnder modulator-based IM-DD system has nonnegligible CD. However, it failed in the case of negligible CD. Considering that this method is not always effective, we analyzed the attributes of various nonlinear systems and found that the above method is applicable only to systems dominated by second-order nonlinearity.