Time Lens Speeds Optical Data

An energy-efficient silicon device compresses light to make ultrafast signals.

Researchers at Cornell University have developed a simple silicon device for speeding up optical data. The device incorporates a silicon chip called a "time lens," lengths of optical fiber, and a laser. It splits up a data stream encoded at 10 gigabits per second, puts it back together, and outputs the same data at 270 gigabits per second. Speeding up optical data transmission usually requires a lot of energy and bulky, expensive optics. The new system is energy efficient and is integrated on a compact silicon chip. It could be used to move vast quantities of data at fast speeds over the Internet or on optical chips inside computers.

Time lens: This silicon chip, called a time lens, is patterned with waveguides that split optical signals and combine them with laser light to speed data rates. Credit: Alexander Gaeta

Most of today's telecommunications data is encoded at a rate of 10 gigabits per second. As engineers have tried to expand to greater bandwidths, they've come up against a problem. "As you get to very high data rates, there are no easy ways of encoding the data," says Alexander Gaeta, professor of applied and engineering physics at Cornell University, who developed the silicon device with Michal Lipson, associate professor of electrical and computer engineering. Their work is described online in the journal Nature Photonics.

The new device could also be a critical step in the development of practical optical chips. As electronics speed up, "power consumption is becoming a more constraining issue, especially at the chip level," says Keren Bergman, professor of electrical engineering at Columbia University, who was not involved with the research. "You can't have your laptop run faster without it getting hotter" and consuming more energy, says Bergman. Electronics have an upper limit of about 100 gigahertz. Optical chips could make computers run faster without generating waste heat, but because of the nature of light--photons don't like to interact--it takes a lot of energy to create speedy optical signals.

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The new ultrafast modulator gets around this problem because it can compress data encoded with conventional equipment to ultrahigh speeds. The Cornell device is called a "time telescope." While an ordinary lens changes the spatial form of a light wave, a time lens stretches it out or compresses it over time. Brian Kolner, now a professor of applied science and electrical and computer engineering at the University of California, Davis, laid the theoretical groundwork for the time lens in 1988 while working at Hewlett-Packard. He made one in the early 1990s, but it required an expensive crystal modulator that took a lot of energy. The Cornell work, Kolner says, is "a sensible engineering step forward to reduce the proofs of principle to a useful practice."