We show that there is a limit to the total number of bits per second, B, of information that can flow in a simple digital electrical interconnection that is set only by the ratio of the length l of the interconnection to the total cross-sectional dimension p A of the interconnect wiring—the “aspect ratio ” of the interconnection. This limit is largely independent of the details of the design of the electrical lines. The limit is approximately B BoA/l2 bits/s, with Bo 1015 (bit/s) for high-performance strip lines and cables, 1016 for small on-chip lines, and 1017 –1018 for equalized lines. Because the limit is scale-invariant, neither growing nor shrinking the system substantially changes the limit. Exceeding this limit requires techniques such as repeatering, coding, and multilevel modulation. Such a limit will become a problem as machines approach Tb/s information bandwidths. The limit will particularly affect architectures in which one processor must talk reasonably directly with many others. We argue that optical interconnects can solve this problem since they avoid the resistive loss physics that gives this limit. © 1997 Academic Press 1.

an optimal foundation

A rebuilding of the world’s information infrastructure is taking place to give instantaneous availability of data, voice and video. This revolution of the Information Age is being gated more by the introduction of new materials and components, than by the design of systems, software and networks. Electrons transmitted through metal wires have an information carrying capacity limited by the resistance and capacitance of the cable and the terminating electronic circuits. Photons transmitted through fiber are capacity limited only by the dispersion of the medium. Each network node that requires transduction from photonics to electronics limits the performance and affordability of the network. The key frontier is the large scale integration and manufacturing of photonic components to enable the distribution of high bit rate optical streams to the individual information appliance. Microphotonics is the platform for large scale, planar integration of optical signal processing capability.

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 are prototyped using deep lithography with protons and are monolithically integrated using a vacuum casting replication technique. The resulting 16-channel module shows optical transfer efficiencies of 46 % and inter-channel 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 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 a multi-channel free-space intrachip optical interconnection. Data-communication was achieved with 4 simultaneous channels working at 10Mb/s. The bit rate 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.

Abstract—This paper presents a new optical interconnect system for intra-chip communications based on free-space optics. It provides all-to-all direct communications using dedicated lasers and photodetectors, hence avoiding packet switching while offering ultra-low latency and scalable bandwidth. A technology demonstration prototype is built on a circuit board using fabricated germanium photodetectors, micro-lenses, commercial vertical-cavity surface-emitting lasers, and micro-mirrors. Transmission loss in an optical link of 10-mm distance and crosstalk between two adjacent links are measured as 5 dB and-26 dB, respectively. The measured small-signal bandwidth of the link is 10 GHz. I.

Optics may allow interconnects to continue to scale to match the processing ability of future electronic chips, though very-low-energy optoelectronic devices and novel compact optics will be needed. By David A. B. Miller, Fellow IEEE ABSTRACT | We examine the current performance and future demands of interconnects to and on silicon chips. We compare electrical and optical interconnects and project the requirements for optoelectronic and optical devices if optics is to solve the major problems of interconnects for future highperformance silicon chips. Optics has potential benefits in interconnect density, energy, and timing. The necessity of low interconnect energy imposes low limits especially on the energy of the optical output devices, with a 10 fJ/bit device energy target emerging. Some optical modulators and radical laser approaches may meet this requirement. Low (e.g., a few femtofarads or less) photodetector capacitance is important. Very compact wavelength splitters are essential for connecting the information to fibers. Dense waveguides are necessary onchip or on boards for guided wave optical approaches, especially if very high clock rates or dense wavelength-division multiplexing (WDM) is to be avoided. Free-space optics potentially can handle the necessary bandwidths even without fast clocks or WDM. With such technology, however, optics may enable the continued scaling of interconnect capacity required by future chips.