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 (L
_SL-MZM
= 570 μm), 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 (Δλ = 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 ΔT = 50 °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.