Intrinsic chirp analysis of a single dual-drive silicon PAM-4 optical modulator

We propose an analytical model to investigate the intrinsic frequency chirp with the method of 4-level pulse-amplitude-modulation (PAM-4) optical modulation by driving a single dual-drive silicon optical modulator. With the analytical model, we calculate the chirp parameters of this PAM-4 generation numerically. The intensity and phase of output signal changes with the PAM levels' switching, which results in the frequency chirp. We find that the modulator operating at different quadrature points (Q_- and Q₊) will have different frequency chirp behavior. The Q_- modulator has mainly negative chirp parameters while Q₊ modulator has mainly positive chirp parameters. We characterize the eye diagrams and BER of the optical PAM-4 signals generated by the Q_- and Q₊ modulator at the modulation rates of 25 Gbaud and 32 Gbaud, respectively, for both back-to-back and 2 km transmission in single mode fiber. The BER results show the Q_- modulator has a small power penalty attribute to the positive fiber dispersion, which is suited for the short-distance transmission in data center application.