A Sub-1pJ/bit Laser Power Independent 32Gb/s Silicon Photonic EAM Driver in 65nm CMOS

Recent Germanium electro-absorption modulators (EAM) integrated into silicon photonic platforms promise energy-efficient electrooptic modulation. The Franz Keldysh absorption effect with a sub-ps response enables their high-speed modulation. Furthermore, the compact EAM footprint with a small capacitive load allows a lumped CMOS-based driver. However, the resulting large photocurrent due to absorption in EAM poses a challenge at moderate to high transmitter (TX) laser power, which hasn’t been adequately addressed in the prior art. This work demonstrates a novel push-pull 2.4V
pp
NRZ CMOS driver at 32 Gb/s speed. The driver features a segmented design with a tunable class-AB style stage to source/sink the large photocurrent. The driver is designed in the 65nm LP CMOS technology operating with dual supply voltages of 2.4V and 1.2V, offering seamless operation from 0dBm to 17dBm TX laser power with energy efficiency of 0.7-0.8 pJ/bit for nominal TX laser power settings (0-8dBm) and achieving an extinction ratio (ER) of 4.14dB. The compact layout only occupies 9736µm
²
area.