Wahid Rahman

Bio/CV: 

Coherent Silicon Photonic Links for Datacenter Applications

With the growing appetite for compute-intensive AI applications, data centers continue to push for high-bandwidth compute solutions. Such systems demand high-speed I/O. Traditional data center interconnects, such as Ethernet, have continued their march towards exponentially higher data rates to meet this demand. This protocol is realized with servers using optical fibers to communicate between one switch ASIC chip to another. The bottleneck here is "the last electrical mile". Historically, a switch ASIC transmits electrical signals across long, lossy SERDES channels to reach the front-plate server edge. There, an optical module converts the electrical signal to an optical one that can be sent and received through optical fibers. As Ethernet switches double in capacity approximately every three years, this last electrical mile must advance in lockstep. This, however, is proving increasingly difficult: electrical baud rates and modulation schemes can only stall the imminent transistor and Shannon limits for so long. While 100Gb/s per lane solutions currently exist and 200Gb/s per lane has shown initial promise, developing practical long-reach electrical lanes beyond this remains questionable. To avoid these lossy electrical channels in the first place, we propose a co-packaged optics solution that targets short-reach Ethernet data center applications using monolithically-integrated silicon photonics on a CMOS process. Specifically, we are investigating a coherent optical solution with an end goal of 448Gb/s per wavelength using dual-polarization QAM16 (DP-QAM16). Our solution uses a CMOS silicon photonics chip that connects through short, low-loss traces to a switch ASIC co-packaged on an organic substrate. Our electro-photonic integrated circuit, or EPIC, interfaces with the switch's SERDES directly - no retiming is required. The EPIC receives four lanes of 112Gb/s PAM4 electrical signals and, using on-chip CMOS amplifiers, drives integrated Mach-Zehnder modulators (MZMs) in a dual-polarization QAM16 fashion before sending
it out through the optical fiber. The same local laser that sources the EPIC MZMs is forwarded with the data link to the EPIC receive side, facilitating coherent demodulation of optical 448Gb/s DP-QAM16 back to 4x112Gb/s electrical PAM4. These electrical signals are then transmitted directly to the receive side of switch ASIC, creating a successful link. Our research proposes a paradigm shift in three key ways. First, we depart from the traditional pluggable optics solutions to a co-packaged approach, effectively eliminating the last electrical mile. Next, we maximize the capacity per wavelength using coherent optical modulation.

Lastly, we simplify coherent transmission and recovery by using a laser-forwarded approach. In doing so, we target a high-bandwidth, energy-
efficient co-packaged optics solution that can meet the growing data center interconnect demand for the next decade and beyond.

Research interests: 

Expected Graduation Date:

May, 2025

Role: