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A s silicon electronic processors achieve greater integration density and faster clock speeds, data communication requirements become more demanding. Electrical bandwidth is inherently limited by capacitive and inductive effects at high frequency, electromagnetic interference, and pin-density requirements at the chip and board level. All these things seriously inhibit conventional interconnection technologies. Tomorrow's systems will need high-bandwidth and dense communication paths at various levels: • Intercabinet connections among individual computer systems. • Backplane-level connections among boards. • Board-level connections among multichip modules (MCMs) on boards. • Chip-to-chip connections on a board or MCM. • Gate-to-gate connections within a chip. Commercial high-performance computers are now beginning to use optical interconnections at the intercabinet level. These connections usually consist of optical fiber ribbons, with each fiber carrying signals at 1 to 2 Gbits per second over distances of 200 to 300 meters. The aggregate bandwidth is as much as 30 Gbits per second. Some manufacturers of specialized high-performance systems are also investigating optical technologies at lower levels of the hierarchy. For example, Cray Research is studying the use of polymer optical wave guides to distribute the optical clock in its T-90 computer. 1 Mercury Computer Systems is also investigating optical backplane technologies, which it believes will be essential for some of its high-performance systems. 2 Compared to VLSI fabrication, interconnection technologies have advanced slowly. Conventional technologies will have difficulty meeting the future requirements for communication among processors and between a processor and memory. The Semiconductor Industry Association predicts that in the year 2007, systems will require as many as 5,000 I/Os, with off-chip clock rates of about 500 MHz. 3 However, if we integrate suitable optoelectronic devices with silicon electronics, we can use optical communication channels to transfer data on and off chips. Optics can effectively communicate data to the chip surface in a massively parallel fashion and at high speed. Optical connections are not limited to wire bonds around the edges of the chip die. Nor do they suffer from electromagnetic interference and high frequency losses. In addition, they can cross in free space without experiencing signal crosstalk among channels. And once the signal is in the optical domain, the driving power is independent of interconnection distance, given no attenuation losses. At the University of Tokyo, we have been working on some technologies to overcome the interconnection bottlenecks experienced by high-speed parallel processing systems. In particular, we have developed an optically interconnected architecture for high-speed computation, image processing, and robotic vision systems. Our architecture utilizes the programmability of mature electronic technology while taking advantage of the density and parallelism of high-speed optical interco...

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