The core of the internet and long-haul telecom links made the switch to fiber optics long ago. A single fiber strand can now carry up to one trillion bits of data per second, enough to transmit a phone call from every resident of New York City simultaneously. In theory, you could push fiber up to 150 trillion bits per second—a rate that would deliver the text of all the books in the U.S. Library of Congress in about a second.

Unlike electronic data, optical signals can travel tens of kilometers without distortion or attenuation. You can also pack dozens of channels of high-speed data onto a single fiber, separating the channels by wavelength, a technique called wavelength-division multiplexing. Today, 40 separate signals, each running at 10 gigabits per second, can be squeezed onto a hair-thin fiber.

Inside your PC, though, it's copper all the way. In fact, more than 99 percent of the world's interconnects reside in and around PCs and servers, where optical communication is nowhere to be found.

Why? Because photonic components are expensive. Today's devices are specialized components made from indium phosphide, lithium niobate, and other exotic materials that can't be integrated onto silicon chips. That makes their assembly much more complex than the assembly of ordinary electronics, because the paths that the light travels must be painstakingly aligned to micrometer precision. In a sense, the photonics industry is where the electronics industry was a half century ago, before the breakthrough of the integrated circuit.

The only way for photonics to move into the mass market is to introduce integration, high-volume manufacturing, and low-cost assembly—that is, to "siliconize" photonics. By that we mean integrating several different optical devices onto one silicon chip, rather than separately assembling each from exotic materials. In our lab, we have been developing all the photonic devices needed for optical communications, using the same complementary metal oxide semiconductor (CMOS) manufacturing techniques that the world's chip makers now use to fabricate tens of millions of microprocessors and memory chips each year.

To understand how optical data might one day travel through silicon in your computer, it helps to know how it travels over optical fiber today. First, a computer sends regular electrical data to an optical transmitter, where the signal is converted into pulses of light. The transmitter contains a laser and an electrical driver, which uses the source data to modulate the laser beam, turning it on and off to generate 1s and 0s.

Imprinted with the data, the beam travels through the glass fiber, encountering switches at various junctures that route the data to different destinations. If the data must travel more than about 100 kilometers, an optical amplifier boosts the signal. At the destination, a photodetector reads and converts the data encoded in the photons back into electrical data.

Similar techniques could someday allow us to collapse the dozens of copper conductors that currently carry data between processors and memory chips into a single photonic link. To do that cheaply, though, you need to figure out how to render the optical components—the laser, the modulator, the photodetector, and so on—in silicon [see illustration, "Moving Data With Light"].

Until very recently, silicon had not been considered a good candidate for optical communications. Just three years ago, an article in this magazine carried the memorable quote, "If God wanted ordinary silicon to efficiently emit light, he would not have given us gallium arsenide." [See IEEE Spectrum, "Linking With Light," August 2002.]

Such skepticism is not unfounded. It's hard to make silicon emit light efficiently. In contrast, materials such as indium phosphide readily emit light; applying a voltage temporarily elevates the energy of electrons in the material's crystal lattice from one energy level, known as the valence band, to a higher one, called the conduction band. When an electron returns to the valence band, it releases energy in the form of a photon. In silicon, though, the electrons tend to release their energy as heat, in the form of lattice vibrations, rather than light.

What's more, silicon lacks a strong electro-optic effect—a measure of how fast light travels through it in the presence of an electrical field. That characteristic means it's not very good at modulating a laser beam. Finally, silicon is poor at photodetection—converting photons into electrons—at the infrared wavelengths commonly used for optical communications.

Despite silicon's shortcomings, researchers have been studying silicon photonics for more than 20 years, starting with Richard Soref's pioneering work in the mid-1980s at the Air Force Research Laboratory. Since then, there have been a host of silicon photonics breakthroughs at Cornell University, the Massachusetts Institute of Technology, the University of California at Los Angeles, the University of Catania in Sicily, the University of Surrey, IBM, Intel, STMicroelectronics, and elsewhere [for a discussion of the photonics work in Catania, see " Light From Silicon," in this issue]. To date, though, none of the silicon-based devices developed can manipulate and control light as well as existing commercial optical devices made from such nontraditional semiconductors as indium phosphide and gallium nitride.

To siliconize photonics, you need six basic building blocks.

• An inexpensive light source.

• Devices that route, split, and direct light on the silicon chip.

• A modulator to encode or modulate data into the optical signal.

• A photodetector to convert the optical signal back into electrical bits.

• Low-cost, high-volume assembly methods.

• Supporting electronics for intelligence and photonics control.