Model and Design of Silicon Photonic Carrier-Depletion Mach-Zehnder Modulators for 400Gb/s and Beyond PAM & QAM Applications

Silicon photonic (SiPho) Mach-Zehnder modulator (MZM) working in carrier depletion mode has been demonstrated for its high speed linear response capabilities, promising high bitrate optical transceivers with PAM or QAM modulation formats in Data Center interconnect and communication applications. However, a number of key elements, such as PN junction, phase shifter (PS), traveling waveguide (TW), and termination, have impacts on performances including bandwidth (BW), V-pi, optical loss, extinction ratio, etc. Therefore accurate models are needed for design optimization including phase and impedance matches and various trade-offs, which are critical in different high bitrate applications. Modelling high speed SiPho TW modulators is challenging with traditional methods. The high speed response of TW needs electromagnetic (EM) model; but PN junction requires distributed circuit model to align with characterization test. In this paper, we developed a hybrid model with an innovated segmental method, which allows us to combine EM and circuit models to accurately represent TW and PN distributed characterizations for SiPho TW MZM modulator. By using this model, the impacts of critical design parameters are studied. We fabricated MZMs with design optimizations using the model with commercial processes in open foundries. The test results agreed well with the simulation data. 6dB EO BW of 42-56GHz and 43-61GHz without notches and roll-off are achieved at -3V PN bias with 3.5-2mm long PS, by adjusting PN doping levels to achieve V-pi* L of 12V*mm and 16V*mm, respectively. These MZMs have great potentials for 50-100Gbaud PAM and QAM optical transceiver applications at 400Gb/s and beyond.