DAC-Less PAM-4 Slow-Light Silicon Photonic Modulator Providing High Efficiency and Stability

We report a slow-light silicon modulator that enables high-speed PAM operation without using an electrical digital-to-analogue converter (DAC). Bragg grating resonators, integrated into each arm of a Mach-Zehnder modulator, enhance the phase modulation through the slow light effect. The optical 4-level PAM signal is generated by driving directly the segmented phase shifter design with two binary signals. This modulator presents an ultra-compact footprint (LSL-MZM = 570 mu m), low energy consumption (73 fJ/bit), large electro-optic bandwidth (>40 GHz). Up to 90 Gb/s is achieved over an nm-range spectral operation bandwidth (Delta lambda = 2 nm). Compared to other low-energy resonator-based modulators, such as micro-rings, this operating bandwidth confers higher stability with a potential operating temperature range of Delta T = 50 degrees C. We further examine the robustness of the proposed design to fabrication variations by measurements of spectral properties across the wafer. This modulator is of particular interest for applications, such as short-range data communications that require multiple compact and energy-efficient modulators on a single chip.