University of Rochester

Optical Fiber Keeps Ultrafast Light Pulses Intact

November 17, 1994

Engineers from Corning Inc. and the University of Rochester have built and tested a new type of optical fiber that one day may carry much more information than today's fibers. They discussed their findings at the recent annual meeting of the Optical Society of America.

Andrew Stentz, a graduate student at the University, built a soliton laser and used its one-picosecond light pulses just fractions of a millimeter long to test the unique dispersion- decreasing fiber designed and manufactured by Corning. Stentz worked closely with Alan Evans, senior research scientist at Corning, and Robert Boyd, professor at the University's Institute of Optics.

Fiber optics technology is enveloping the world like a giant spider web, linking offices and homes via glass fibers that carry unprecedented amounts of information even though the fibers are thinner than the width of a typical human hair.

To understand optical communication, think of forest rangers using mirrors to flash signals from hilltop to hilltop to communicate. A ranger with good eyes might be able to detect about 60 such flashes per second before they blur together and appear as one continuous flash. Scientists use optical fibers in a similar fashion, sending billions of light signals a second to encode everything from telephone conversations to video offerings. The faster that scientists can send the signals without blurring, the more information a fiber can carry.

But different colors of light travel at different speeds, and if the pulses aren't placed far enough apart, they can blur together, destroying information. This is especially true if the pulses are being sent over long distances, such as across the ocean. This blurring, caused by dispersion, limits the amount of signals and information that scientists can send through fibers.

"Imagine a group of messengers running from one city to the next," says Stentz. "If the messengers run at different speeds, then it won't be long before the slowest runners in the first group fall behind and mix with the fastest runners in the second group. Pretty soon the messages are all mixed up."

Scientists are trying a variety of techniques to keep the messages straight. For the past several years several companies, including AT&T and Nippon Telegraph, have tried to reduce the blurring by using solitons, a special type of light wave that doesn't change shape as it travels. As a soliton travels down the fiber, the soliton's intensity level changes the fiber's index of refraction, keeping the pulse intact.

But even ultrashort solitons can have dispersion problems, because their energy drops as they travel down fibers. If scientists periodically boost their energy, the solitons can become unstable.

So Corning built a special fiber to compensate for these energy fluctuations, matching the soliton's decreasing energy with a fiber whose dispersion decreases proportionally.

To test Corning's fiber, Stentz built a soliton laser based on amplifiers doped with the rare element erbium. The laser generated one-picosecond pulses (a picosecond is a millionth of a millionth of a second), dozens of times faster than those used in today's fiber systems, and sent them through a 40-kilometer stretch of fiber spooled in the laboratory. The solitons emerged intact. When Stentz sent solitons from the same laser through conventional fibers, the signals quickly degraded.

"This is a first step to see if these fibers can be used to improve soliton transmission," says Stentz. "We have shown that you can send ultrafast solitons down this fiber."

The group's work is funded by Corning Inc., the New York State Science and Technology Foundation, and the U.S. Army. tr




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