Illuminating Silicon
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The second device the group is working on is the modulator, which enables light to carry data. When laser light enters a conventional modulator, the modulator rapidly turns it on and off, encoding the 1s and 0s of binary data onto the beam. Modulators are usually made of expensive materials, such as lithium niobate, that easily alter light passing through them if a voltage is applied. Since silicon doesn’t readily alter light, Paniccia had to turn to a different design, which takes advantage of the material’s ability to guide light through channels. His modulator uses an interferometer, a device that creates interference between waves of light. Light enters one end of the modulator and is split into two beams. An electrical device alters each beam’s phase–basically, knocking the two light waves out of sync. Then the beams, with their slightly altered phases, recombine. The result is a beam that flickers on and off, representing digital information.
This past July, Paniccia announced that his group had made a silicon modulator that can operate at a record-breaking 40 gigabits per second–as fast as the best modulators currently used in the telecom industry. There’s still work to be done on the design, to optimize the device’s performance. But Paniccia thinks mass production is viable.
The last part of the silicon-photonics puzzle is a working detector that can receive light from a laser and modulator. Again, Paniccia is attempting to overcome a basic limitation of silicon: it doesn’t absorb light very efficiently. He and his team have been experimentally adding atoms of germanium to silicon to change its photonic properties so that it can absorb light at telecom wavelengths. They’ve built detectors that operate at 20 gigabits per second, but that figure is constantly improving as the researchers vary the way the germanium is added and tinker with the design of the electrical contacts. Paniccia expects to have a 40-gigabit-per-second detector operating by the fall.
Paniccia refers to the next stage of development as the “valley of death,” because unforeseen problems can crop up as a technology moves from the lab to the market. But he and his coworkers are optimistic. Paniccia points to the guts of a computer that is using a combination of lasers, modulators, and detectors made of traditional optical materials–each device is about the size of a deck of cards and can cost hundreds of dollars–to transfer data around the motherboard. He hopes to replace those devices with photonic chips mass-produced on the same scale as the microprocessors.
If silicon photonic chips are built into computers, says Paniccia, a lot will need to change, including fundamental functions such as the way the computer boots up and the way the microprocessor accesses memory. “No one’s looking at these problems yet, because there hasn’t been a reason to,” he says. But now that the various elements of silicon photonics are becoming a reality, that might be about to change. “Silicon photonics is making us rethink a lot of things,” he says.
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