The movement of data in a computer is almost the converse of the movement of traffic in a city. Downtown, in the congested core of the microprocessor, the bits fly at an extraordinary rate. But further out, on the broad avenues of copper that link one processor to another and one circuit board to the next, things slow down to a comparative crawl. A Pentium 4 introduced this spring operates at 2.4 GHz, but the data travels on a bus operating at only 400 MHz. The speed picks up again, though, out on the highways of the world's optical-fiber telecommunications networks. Obviously, the closer engineers can bring the optical superhighway to the microprocessor, the fewer copper bottlenecks can occur, as if you could pull out of your driveway straight onto the Autobahn.
So some researchers say that, within just a few years, many of the copper connections in computers will yield to high-speed optical interconnects, in which photons, rather than electrons, will pass signals from board to board, or chip to chip, or even from one part of a chip to another [see illustration, above].
The idea is simple in principle, and parallels telecommunications systems. An electrical signal from the processor would modulate a miniature laser beam, which would shine through the air or a waveguide to a photodetector, which would in turn pass the signal on to the electronics. Though at the moment it is more expensive to communicate with light than with electric current, the day is coming when only optical technologies will be able to keep up with the demands of ever-more-powerful microprocessors, just as they are now the only reasonable way to move the world's Internet traffic across the kilometers.
"We're already projecting that for certain system requirements data rates are going to be high enough and the link length long enough that we're going to have to use optics," says Modest Oprysko, senior manager of communications technologies at IBM Corp.'s Thomas J. Watson Research Center (Yorktown Heights, N.Y.).
A wire's bit rate is at the mercy of its parasitic resistance, capacitance, and inductance. At low frequencies, the series resistance and shunt capacitance of a circuit board trace dominate its behavior, determining the transition (rise and fall) times and thereby limiting its data rate. At higher frequencies, like those on circuit boards today, the wire's series inductance becomes more important than its resistance as an impeding factor, but with the same end result—a limit on the rate at which the trace can transmit pulses.
All these parasitic factors depend heavily on the geometry of the wire, especially its length. Resistance, for instance, is proportional to the wire's length and inversely proportional to its cross-sectional area. Because of this dependence on geometry, a simple wire's ultimate bit rate turns out to be proportional to its cross section, but falls with the square of its length. So thinner—and especially longer—means a lower bit rate. On a chip, the story has some caveats, but the conclusion is the same.
Transition-time limitations can be fought by driving the lines harder, but that's not a very good solution. It adds noise, increases power requirements, and aggravates already serious thermal management problems. Alternatively, the wires can be made fatter, but then you'll run out of space. Photons don't suffer from these limitations; their biggest problems are absorption and attenuation, neither of which is an issue over the distances inside a computer, or even across a room.
In telecommunications, transmission slows as it passes from the all-optical long-haul network to the lower-bandwidth metropolitan-area network, and finally crawls into the home along copper wires. Just as telecom companies want to increase total bandwidth by moving all-optical transmission closer to your house, the designers of optical interconnects want to get as much bandwidth as possible as close as possible to the microprocessor. Already, optics connect computer systems across distances of less than 300 meters, and backplane setups are in the works that will speed up transport of data from one board to another within a computer. Farther down the road are systems for increasing the bandwidth between two microprocessors, or among stacks of chips for massively parallel computing. All the pieces are in place—cheap lasers, sensitive detectors, and the methods needed to transmit from one to the other. Now it's just a question of when optical interconnects will perform well enough, and their production cost fall low enough, for them to replace copper wires.