Faster Optical Switching Through Chemistry

Specially designed molecules could lead to all-optical data switches that could make the Internet far faster.

New molecules produced at Georgia Tech could enable engineers to build all-optical data routers, ultimately leading to transmission speeds as high as two terabits–or 2,000 gigabits–per second. Today’s fastest commercial routers switch data at 40 gigabits per second.

Modern fiber-optic networks are limited in speed because the light that carries data has to be converted into electrical signals when it reaches an Internet router. This step lets the router determine the signal’s destination and forward the data accordingly. Keeping data all-optical would significantly speed up transmission of large amounts of data, such as detailed medical images, telepresence applications, high-speed image recognition, and high-definition video.

To address this problem, engineers have built devices that can switch optical signals by manipulating mirrors or bubbles to redirect the light beams. The Georgia Tech team, in contrast, designed molecules that could theoretically switch optical signals in just a few femtoseconds, versus the microseconds needed by systems that use physical mechanisms to redirect the light.

The project was a collaboration among the labs of Georgia Tech chemistry professors Seth Marder, who led the synthesis phase of the project; Jean-Luc Brédas, a theoretical chemist; and Joseph Perry, the physical chemist who characterized the molecules. The team started its design process by looking at a class of organic molecules called polymethine dyes. These brightly colored molecules have unusual properties that allow researchers to change the refractive index of the material by shining a laser on it–and hence shift the phase of any light waves traveling through it. This gives them a way to control the modulation of light using only optical systems–no electricity needed.

Researchers had looked into using organic molecules for optical switching about 15 or 20 years ago because of their very fast response to electric and optical fields, says Larry Dalton, a chemist and electrical engineer who develops optical materials at the University of Washington. In fact, the intrinsic response time of organic molecules is between 10 and 100 terahertz, meaning that if the right material is found, data might be processed at those astonishing speed. However, no one was able to create organic materials that could shift the phase far enough without absorbing too much of the light wavelengths used in telecommunications systems. The dye created by the Georgia Tech team “is the first that allows you to change the index of refraction without light being lost,” Dalton says. “You have the potential to move forward with practical applications now”–including improved methods of optically encoding data and all-optical computing, as well as ultrafast optical switching.

“People had largely given up on doing this with organic materials,” Marder says. Through a combination of theoretical design and trial and error, the Georgia Tech scientists were able to create a molecule that had the properties they needed.

So far, the researchers have only measured the molecule’s optical properties in a liquid solution. “The hard work comes now in taking these molecules and putting them into a material and making the switch,” Marder says. The Georgia Tech chemists are already working on that task. While the dye itself is “not the easiest thing to make” and the material will ultimately be expensive, Marder says, any device will probably use only very small quantities.

Perry says that while engineers might be able to push existing electro-optical technology to provide transmission speeds of up to 100 gigabits per second, all-optical processing could theoretically allow for speeds as high as two terabits per second, allowing download of high-definition movies in minutes rather than hours. While they may not be able to hit those speeds, he says that if everything goes well, the group may have a device that can switch data at hundreds of gigabits per second in about five years.

A photonics company contacted the Georgia Tech team only a day or two after the research was first published, and the academics plan to begin discussions with company representatives this week.

Even if they can develop working optical routers as quickly as they hope, Perry notes, it will take much longer for the technology to have any appreciable influence on the speed of consumers’ Internet links, largely because telecom companies tend to upgrade their systems incrementally. He expects that the first customers for any all-optical switch will be companies like Google that run large server farms.

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