Wednesday, January 17, 2007

A Nano Solution to Increasing Bandwidth

MIT researchers develop microphotonic devices for communications, clearing the way for higher-performance optical networks.

By Kevin Bullis

Researchers at MIT have developed a new device that will improve communications. Fiber optics cause random polarizations of light that can lead to weakened or garbled signals. The new structure makes all incoming light one polarization before the data is processed. Credit: Tymon Barwicz

Optical fibers can quickly transmit huge amounts of data. But the technology for sorting and sending photons lags far behind the microelectronics that generate and process the data, putting a crimp on bandwidths. Now researchers at MIT's Research Laboratory of Electronics report in the current issue of Nature Photonics that they have developed a method for overcoming a fundamental problem in using photonics in communications, paving the way to cheaper, more complex, and higher-performance optical networks.

In the past few years, scientists and engineers have made great strides in miniaturizing photonic devices and integrating them onto a single chip--advances that allow for cheaper manufacturing, smaller sizes, and higher performance. Along the way they've developed techniques for working with materials common to the semiconductor industry, which is a step toward integrating photonics and electronics on the same chip. And these researchers have made structures with phenomenal precision, in some cases down to distances smaller than those that separate atoms.

Even with these successes, however, a major obstacle remained. Light delivered via cylindrical fiber optics breaks into different polarizations, or orientations of light waves. In devices at the microscale, the outputs change depending on if the waves are oriented vertically or horizontally so they're suited to processing only certain polarizations, which can lead to weakened signals. If researchers are limited to using horizontally polarized light, for example, they end up throwing away vertically polarized light and lose half the signal strength. That's a problem particularly when sending signals over long distances, such as between continents.

One approach to this problem is to run light through more than one device, each specifically designed to process one polarization. But researchers at MIT's Research Laboratory of Electronics took a different approach. Rather than building separate devices for different light polarizations, they invented a device for converting vertically polarized light into horizontally polarized light. First, the device splits light into its horizontally and vertically polarized components, directing these into separate channels. Then it gradually rotates the vertically polarized light to make it horizontal. At this point, the light in both channels has the same polarization. This makes it possible to use identical devices to process that light. As a result, all of the light is processed in the same way, allowing clear, strong signals.

The current advance pertains only to those photonic applications that involve light with multiple polarizations--mainly communications applications that involve fiber optics. There hasn't been much economic pressure in the past couple of years to develop technology for these applications because of a glut in bandwidth, but now communications demands are increasing again, says Erich Ippen, professor of electrical engineering and physics at MIT and one of the researchers on the project.

"When you integrate things like this, the complexity and the performance of the kinds of filtering we can do are a little more advanced than the methods that are used today," Ippen says. And that, he says, will make it possible to meet the demands of next-generation telecommunications.