Study on the Methods for Generating Optical PAM-4 Signal With the Standard Silicon Mach–Zehnder Optical Modulator

We propose an analytical model to evaluate three different methods for generating the optical four-level pulse-amplitude-modulation (PAM-4) signal with a standard silicon Mach-Zehnder (M-Z) optical modulator. With this model, we further use the uniformity of the modulation intervals as the metric to find the optimum operating points to achieve optical PAM-4 signals with uniform intervals for all three methods. We find that to achieve a uniform optical PAM-4 signal, the operating point for the single-arm-driving configuration needs to be farther away from the quadrature point compared with the other driving configurations. We characterize the optical PAM-4 signals generated by the three methods and verify the theoretical analysis using a fabricated 39.2 GHz silicon M-Z optical modulator. A 50 Gbaud optical PAM-4 modulation with open eye is successfully demonstrated using the silicon modulator.