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Lousy Emitter
Optical fibers constitute the backbones of long-distance telecommunications networks and are largely responsible for the speed of the Internet. But optical components don’t come cheap. Optically sending and receiving data requires a laser that creates a light beam; a “modulator” that chops that beam into on/off bursts that represent digital 1s and 0s; “waveguides” that pipe the light through chips; and photodetectors that capture the light and convert it back into an electronic signal. Currently, these devices are not made out of silicon and cost thousands of dollars to put into place. Telecom providers can afford those prices, but making the technology feasible for moving data within a computer means reducing prices by orders of magnitude.

Silicon may be the answer. “Silicon to us, it’s maybe not a religious experience, but it’s pretty close,” Gelsinger says. “Silicon has proven cost effective, scalable, durable, manufacturable and has all sorts of other wonderful characteristics.” Photonic parts made of silicon would make optics more affordable and broaden potential uses. “Today, optics is a niche technology. Tomorrow it’s the mainstream of every chip that we build,” Gelsinger says.

Until about a year ago, it looked as if silicon would never play a significant role in optics. “Silicon is not intrinsically the best optical material,” explains Reed. Among its most obvious deficits is that it’s a lousy light-emitter. When the electrons in silicon are excited, instead of releasing photons they cause the silicon’s crystal lattice to vibrate. The result is heat, not light. By contrast, semiconductors such as gallium arsenide and indium phosphide emit light when electrically excited. So while researchers have been fascinated by the prospects of an “optical chip” for years, the consensus was that silicon was not the right material to build it with.

Then, in the late 1990s, researchers reported a series of encouraging, albeit preliminary, advances in silicon optics (see “Upstream,” Technology Review, June 2001). At Intel, the progress made by Paniccia’s team convinced executives to ramp up the company’s silicon-photonics program. Intel’s first breakthrough came in February 2004, when Paniccia reported in the journal Nature that his group had made a silicon modulator capable of converting a steady stream of light from a laser into rapid pulses of digital 1s and 0s at a rate of one billion hertz, or one gigahertz, a 50-fold advance over the previous experimentally demonstrated record for silicon. “But it still wasn’t anywhere near fast enough,” Reed says. Then this spring, Intel researchers led by materials scientist Ling Liao reported a silicon modulator that runs at 10 gigahertz, roughly on par with other optical modulators.

But the crucial silicon-photonic component was still the laser. Last September, four separate groups, including Paniccia’s, reported silicon lasers that fire staccato pulses of light. Because silicon does a poor job of converting electrical charges into light, all these silicon lasers relied on external lasers as energy sources. Like all chip-based lasers, the silicon lasers work by converting energy – in this case, photons from another light source – into a burst of photons with essentially the same wavelength and phase. The Intel researchers exploited a long-known principle called the Raman effect, in which photons gain energy from collisions with vibrating atoms.

Pulsed lasers aren’t great for transmitting data, though. Optics engineers much prefer continuous lasers, which they can slice and dice with modulators to create data signals. But all of the groups struggled with the same problem. As they increased the amount of continuous laser light they fed into the silicon chips, the likelihood that pairs of incoming photons would strike a single silicon atom at the same time also increased. When that happened, the silicon atoms kicked electrons out of their atomic orbits, and those mobile charges voraciously gobbled up photons. The incoming laser had to be pulsed to give the electrons the millionths of seconds they needed to give up their excess energy and relax back to their resting states.

Paniccia’s team came up with an answer that was both brilliant and, for those familiar with silicon technology, conceptually simple. Etched into the Intel laser chip was a silicon waveguide channel in which light bounced back and forth, gaining in intensity. The researchers implanted electrodes on both sides of the channel. When they turned on a voltage between the electrodes, it created an electric field that herded the negatively charged electrons toward the positively charged electrode, effectively sweeping them out of the way. As a result, the photons were able to build up unhampered, until they produced a continuous laser beam.

Last winter, three days before Christmas, Paniccia’s colleagues Haisheng Rong and Richard Jones saw the first sign that the strategy was working: a line on the display of an optical- spectrum analyzer showing that the infrared photons produced by the laser were coming out in a steady stream.

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