Plastic micro-optical modules for VCSEL-based free-space intra-chip interconnections: demonstrator testbeds with OE-FPGAs

We fabricated and replicated in semiconductor compatible plastics a multi-channel free-space optical interconnection module designed to establish infra-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 inter-channel cross-talks as low as -22 dB. These characteristics are sufficient to establish multi-channel infra-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 and 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 multi-channel free-space intra-chip optical interconnections. 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 mum 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. Furthermore we investigated the possibilities of a more advanced interconnection module prototyped by combining an in-house fabricated baseplate with microlenses and a commercially available micro 3D glass prism. With this approach the channel count is no longer limited by the thickness of the prism we can fabricate with deep lithography with protons. To conclude we report on the integration of this glass prism and our baseplate and on the first results obtained with this interconnection module.