A 108 Gb/s PAM-4 VCSEL-Based Direct-Drive Optical Engine for Co-Packaged Optics Applications

This article presents a multimode vertical-cavity surface-emitting laser (VCSEL)-based direct-drive optical engine for co-packaged optics (CPOs) applications that employs pulse amplitude modulation (PAM)-4 to enhance shoreline bandwidth (BW) density and features a compact direct optical wiring (DOW) approach for on-package fiber termination. On the transmit side, a VCSEL driver (VCDRV) comprising a passive complementary-drive complex-zero continuous-time linear equalizer (CZ-CTLE) is introduced to compensate for the VCSEL’s complex-pole response while offering the high linearity required for PAM-4 signaling. The receiver integrates a high-linearity transimpedance amplifier front-end (TIA-FE) capable of handling large input current swings, followed by an active CZ-CTLE to equalize the complex-pole pair response of the low-gain shunt-feedback transimpedance amplifier (TIA). The transceiver prototype, fabricated in a 22-nm FinFET process and integrated on-package with optical devices and fiber-termination, achieves a PAM-4 data rate of 108 Gb/s with a record energy efficiency (EE) of 0.9pJ/b—representing over $1.7\times $ improvement in data rate compared to previously reported VCSEL-based transceivers. The optical transmitter supports 128 Gb/s PAM-4 operation, demonstrating a $1.5\times $ data rate improvement and $1.6\times $ better EE than the fastest reported integrated VCDRV. Compared to PAM-4 VCDRVs in the prior art, this work achieves $2\times $ higher data rate, $\gt 1.8\times $ better optical modulation amplitude (OMA), and $3\times $ better EE.