A 0.9pJ/b 108Gb/s PAM-4 VCSEL-Based Direct-Drive Optical Engine

The surge in high-throughput applications necessitates advancements in data transmission. Increasingly complex, power-hungry equalization and digital signal processing (DSP) techniques limit electrical interconnect scalability and reach. Pluggable optical modules extend the reach, but at the cost of scalability/power challenges, due to their integrated re-timer (or DSP) which cleans up the received signal before sending it to the optical engine (OE). By directly connecting computing (XPU) systems to OEs, emerging co-packaged optics (CPO) [1]–[2] can address these challenges by eliminating the re-timer. Multi-mode vertical-cavity surface-emitting lasers (VCSEls) continue to improve their performance and enable high-BW connectivity over a few tens of meters [1]–[2] in conjunction with high bandwidth (BW) circuit techniques. Higher order pulse-amplitude modulation such as PAM-4 doubles the BW. This paper presents a PAM-4 VCSEl -based direct-drive OE targeting CPO applications [1]–[2]. The XPU IC is not implemented in this work. The direct-drive OE integrates VCSEl driver (VCDRV) and transimpedance amplifier front end (TIAFE) $\text{lCs}$, with their VCSEl and photodiode (PO) counterparts which are fiber terminated with optical waveguides or direct optical wiring (DOW) technology [3]. Several circuit techniques are introduced to enable the $> 100\text{Gb}/\mathrm{s}$ PAM-4 direct-drive OE: (1) a high-linearity coupled-inductor-based compact complex-zero continuous-time linear equalizer (CZ-CTlE) to equalize a complex-pole pair in VCSEl optical response; (2) a high-linearity differential TIAFE to handle high input optical modulation amplitudes (OMAs); and (3) an active complex-zero CTlE to equalize for the complex-pole pair in the shunt-feedback TIA response.