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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.7\times $</tex-math> </inline-formula> improvement in data rate compared to previously reported VCSEL-based transceivers. The optical transmitter supports 128 Gb/s PAM-4 operation, demonstrating a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.5\times $</tex-math> </inline-formula> data rate improvement and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.6\times $</tex-math> </inline-formula> better EE than the fastest reported integrated VCDRV. Compared to PAM-4 VCDRVs in the prior art, this work achieves <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times $</tex-math> </inline-formula> higher data rate, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\gt 1.8\times $</tex-math> </inline-formula> better optical modulation amplitude (OMA), and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times $</tex-math> </inline-formula> better EE.