Multichannel free-space intrachip optical interconnections: combining plastic micro-optical modules and VCSEL-based OE-FPGA

Abstract

We fabricated and replicated in semiconductor compatible plastics a multi-channel free-space optical interconnection module designed to establish intra-chip interconnections on an Opto-Electronic Field Programmable Gate Array (OE-FPGA). The micro-optical component is an assembly of a refractive lenslet-array and a high-quality microprism. Both components were prototyped using deep lithography with protons and were monolithically integrated using a vacuum casting replication technique. The resulting 16 channel module shows optical transfer efficiencies of 50% and interchannel cross-talks as low as -22 dB. These characteristics are sufficient to establish multi-channel intra-chip interconnects with OE-FPGA's.The OE-FPGA we used was designed within a European co-founded MEL-ARI consortium, working towards a manufacturable solution for optical interconnects between CMOS IC's. The optoelectronic chip combines fully functional FPGA digital logic with the drivers, receivers and flip-chipped optoelectronic components. It features 2 optical inputs an 2 optical outputs per FPGA cell, amounting to 256 photonic I/O links based on multi-mode 980 nm VCSELs and InGaAs detectors.With a careful alignment of the micro-optical free-space module above the OE-VLSI chip, we demonstrated for the first time to our knowledge a multi-channel free-space intra-chip optical interconnection. Data-communication was achieved with 4 simultaneous channels working at 10Mb/s. The bitrate was limited by the chiptester. Notwithstanding the use of non-aggressive 0.6 µm CMOS technology the FPGA will provide an 80 Mbit/s information rate per channel using manchester encoded links. The whole chip therefore has in principle a peak aggregate signalling rate of approximately 20 GBit/s. This first demonstration paves the way for a practical solution to solve the electronic intra-chip interconnect bottleneck with low-cost mass-producible chip-compatible plastic micro-optical modules.

The benefits of ultrashort optical pulses in optically interconnected systems

“…It can be separated into three components: transmitter latency, propagation delay, and receiver latency. In most cases, the propagation delay is the largest component, but receiver latency can be comparable in magnitude for short links such as on-chip interconnects [17]. By simulating the receiver designs used in this work, we have shown [18] that the use of short pulse signaling can substantially reduce the latency of the receiver types used for optical interconnection.…”

Optical pump-probe measurements of the latency of silicon CMOS optical interconnects

“…Thus, for a 30-mm on-chip line (i.e., roughly the edge dimension of a typical CMOS processor), such a wire will have a latency of about 800 ps. Using a recently demonstrated on-chip optical approach [5], a hypothetical optical link on the same chip would have an effective propagation velocity of roughly 0.33 c (which accounts for the refractive index of the transmission medium and a doubling of the path length because of the three-dimensional optical assembly). The resulting optical link latency is then the 300 ps required for signal propagation, plus the E/O/E conversion latency ( 500 ps as measured here).…”

Increasing the functionality of free-space micro-optical intrachip modules with DOEs: towards reconfigurable photonic interconnects

“…This project has mainly been working towards a manufacturable solution for optical interconnects between CMOS chips [3] . Moreover the concept was extended at the Applied Physics and Photonics Department of the Vrije Universiteit Brussel, and multichannel free-space on-chip optical interconnects have been demonstrated for the first time [4] . This FS on-chip optical interconnect demonstrator featured a combination of a custom designed OIIC opto-electronic chip and an in-house designed and fabricated micro-optical FS interconnect module.…”