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Silicon Lasers Get Up to Speed

A new silicon-based laser emits the short, high-frequency light pulses that are necessary for today’s telecommunications networks.
August 29, 2007

Researchers at the University of California, Santa Barbara (UCSB), have designed a silicon-based laser that emits ultrashort pulses of light at high frequencies–two characteristics that are crucial if silicon-based lasers are to become practical. Eventually, the researchers hope that the new laser could replace other, more expensive lasers in optical communication networks. It could even lead to faster computers that shuttle data around using light instead of electricity.

Scintillating silicon: This image illustrates the design for a new hybrid silicon laser. The gray and orange base consists of two layers of silicon sandwiching a layer of silicon dioxide. Indium phosphide, the light-emitting material, is bonded to the top of the silicon with a thin layer of glass glue. Emitted photons bounce back and forth in a channel etched into the top layer of silicon, until they emerge as laser light.

Modern telecommunications networks use three distinct gadgets–lasers, modulators, and detectors–to produce, encode, and detect light. Currently, all three are made of nonsilicon semiconductors, such as indium phosphide, that are difficult to mass-produce; as a consequence, they tend to be expensive and bulky. But if they could instead be made from silicon, they could be integrated on individual chips, says John Bowers, professor of electrical and computer engineering at UCSB. Devices that currently cost hundreds of dollars each could then be made in bulk for pennies, and the cost of bandwidth would plummet. The one snag in the plan is that it’s hard to make silicon produce light.

In September 2006, however, the UCSB team and Intel announced a new hybrid laser that, although it still used indium phosphide, was built on a silicon base. (See “Bringing Light to Silicon.”) The manufacture of the device began with a wafer that consisted of a layer of silicon dioxide sandwiched between two layers of silicon. In the top layer of silicon, the researchers etched a channel, called a waveguide, within which light bounced back and forth. To the top of the wafer, they bonded strips of indium phosphide, using a layer of glass glue only 25 atoms thick. Adding this additional layer, says Bowers, isn’t much different from adding layers of other materials to silicon, something that’s regularly done in today’s manufacturing process.

To turn the laser on, the researchers applied electrical current to metal contacts on top of the indium phosphide. Indium phosphide is a naturally light-emitting material, so the strips of it on top of the wafer produced photons that got trapped in the channel below, bouncing back and forth along the length of the silicon waveguide. In certain materials, that bouncing is enough to amplify normal light into laser light, but not in silicon. So the device was designed to let a small amount of light, called the evanescent tail, sneak back into the indium phosphide, where it was amplified. The benefit of this design is that it avoids the costly fabrication of an indium-phosphide waveguide.

For the new laser, which is described in a recent issue of Optics Express, the researchers made their design slightly more complex. “We needed to turn it into a device with multiple sections,” explains Alexander Fang, a graduate student who worked on the project. He says that he had to make sure the lengths of the cavities were precise, and that regions that amplified light and absorbed light were electrically isolated from each other.

The result is a laser that emits picosecond pulses of light at a frequency of 40 gigahertz. “This thing puts out short pulses of light, which is what you need for high-speed communication,” says Bowers. “If you pulse at 40 gigahertz and combine that with a modulator [which puts information onto the light], then you have a light source.”

The work “represents some nice progress toward proving a laser source on a silicon wafer,” says Ivan Kaminow, professor of electrical engineering and computer science at the University of California, Berkeley, but he cautions that silicon photonics still has a long way to go. “Silicon is not an optimum photonic material,” he says. “The hybrid approach is a compromise and, as such, is far from optimum performance.” For instance, the hybrid laser can’t operate at the same high temperatures that silicon circuits do.

Bowers agrees that there is still work to be done, and improving the device’s temperature threshold is on the list. “This is pretty far-out research,” he says. “Our goal last year was just to make a good laser on silicon, and now we’re expanding that not just to do lasers, but photonic integrated-circuit technology.” He suspects that silicon photonic devices based on his group’s approach could appear in products as early as 2012.

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