Silicon Photonic Mach–Zehnder Modulator Architectures for on Chip PAM-4 Signal Generation

Four level pulse amplitude modulation (PAM-4) has become the modulation format of choice to replace on-off keying (OOK) for the 400 Gb/s short reach optical communications systems. In this paper, we investigate the possible modifications to conventional Mach-Zehnder modulator structures to improve the system performance. We present three different silicon photonic Mach-Zehnder modulator architectures for generating PAM-4 in the optical domain using OOK electrical driving signals. We investigate the transfer function and linearity of each modulator and experimentally compare their PAM-4 generation and transmission performance with and without use of digital signal processing (DSP). We achieve the highest reported PAM-4 generation and transmission without the use of DSP. The power consumption of each modulator is presented, and we experimentally show that multielectrode Mach-Zehnder modulators provide a clear advantage at higher symbol rates compared to conventional Mach-Zehnder modulators.