Linking with Light

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 farther 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. The speed picks up again, though, out on the optical highways of telecommunications networks.

But the copper bottleneck is under attack. Within just a few years, some researchers say, many of the copper connections in computers will yield to high-speed optical interconnects. Basically, an electrical signal from the processor would modulate a miniature laser beam, which would shine through an optical fiber or through air on a photodetector, which would pass the signal on to the electronics. And while more expensive than electronics, soon only optical technologies will keep up with the demands of ever-more-powerful microprocessors, writes IEEE Spectrum contributing editor Neil Savage.

After all, the rate at which a copper wire can transmit bits of information is at the mercy of its resistance, capacitance, and inductance--factors highly dependent on the wire's geometry, especially its length. A longer thinner wire means a lower bit rate. You can pump in more power, but that adds noise and heat, or make the wires fatter, but then you'll run out of space.

Photons have different limitations, but none that matters inside a computer, or even across a room. Already, optical setups are in the works that will speed transport of data between circuit boards within a computer. Farther down the road are systems for increasing the bandwidth between two microprocessor chips, or among stacks of chips for massively parallel supercomputers.

All the pieces are in place--cheap lasers, sensitive detectors, and the methods to transmit from one to the other. Now it's just a question of when the optical interconnects will perform well enough, and their product cost will fall low enough, for them to replace copper wires.