A 2.4pJ/b 100Gb/s 3D-integrated PAM-4 Optical Transmitter with Segmented SiP MOSCAP Modulators and a 2-Channel 28nm CMOS Driver

Data centers continue to require interconnects with higher bandwidth densities and energy efficiencies. Silicon photonics (SiP)-based solutions have gained interest for implementing low-cost and power efficient 100+Gb/s/A optical transceivers. While microring modulators (MRMs) have small footprints and high electro-optical bandwidth (EOBW), they suffer from an inherent tradeoff between bandwidth and optical phase efficiency, high sensitivity to process and temperature variations, and non-linear electro-optic characteristics [1 – 2]. Travelling-wave Mach-Zehnder Modulators (TW-MZMs) require power-hungry drivers to compensate microwave losses and occupy large areas on chip [3 – 4]. Metal-oxide-silicon-capacitor (MOSCAP)-based phase modulators can significantly scale the area and power of the optical transmitter (OTX) owing to their superior optical efficiency (voltage-length product at n phase shift of V π L < 1Vmm) and compact footprint (< 1mm) [5]. MOSCAP modulators, however, impose large capacitive parasitics (~3fF/μm), which could limit the electro-optical bandwidth (EOBW) significantly. State-of-the-art wireline transmitters cannot meet the requirements of MOSCAP modulators due to their 50Ω-terminated design and limited output voltage swing [6]. This paper presents a 3D-integrated 100Gb/s PAM-4 OTX with electronic pre-distortion (PD) and BW extension techniques to compensate for MOSCAP modulator BW limitations.