Fiber Optics' Speed Limit Found - Lucent

Scientists from Lucent Technologies' Bell Labs have calculated the maximum amount of information that can be transmitted over optical fiber--it is "theoretically possible" to send approximately 100 terabits of information, or roughly 20 billion one-page e-mails, simultaneously per strand of fiber.

As demand for services like high-speed Internet access continues to grow and bandwidth-hungry applications like video-on-demand become increasingly popular, optical fiber will be able to keep up with the demand for these services and those yet to be imagined, Lucent officials said. The Bell Labs team's scientific results appear in today's issue of the British journal Nature.

"As networks continue to make communication faster, smaller, cheaper and smarter in the next decade, there will be an even greater emphasis on fiber optics technology," said Rod Alferness, senior vice president of optical networking research at Bell Labs, in a statement. "As we light more optical fiber, and couple this with all-optical switches like the Lucent WaveStar LambdaRouter, we will continue to improve the speed and quality of metropolitan and backbone networks, helping to enrich the end-user experience."

While current commercial optical systems can transmit just under two terabits of information per second and laboratory experiments have demonstrated transmission rates of 10 terabits per second, Lucent said it has been difficult to theoretically calculate how much information can be transmitted over a glass fiber because the physical properties of glass make light transmitted over fiber susceptible to scrambling in a very complicated fashion. For example, the speed of a light signal traveling through fiber depends on the intensity of the light and is not a constant as it would be in free space; physicists refer to this behavior as a "non-linear" response.

These non-linear effects cause part of a signal traveling through the fiber to turn into noise. As a result, calculating the exact amount of information that can be sent over a fiber becomes a thorny challenge.

The Bell Labs scientists were able to simplify the challenge by using an analogy from quantum physics, together with ideas from information theory. They looked at telecommunication systems that use wavelength division multiplexing -- a technique by which lightwaves of different colors are simultaneously transmitted over the same fiber, allowing more information to be sent -- and estimated how much information can be conveyed from a transmitter to a receiver. They found that if a signal is sent with too little power, the signal will be overcome by the noise in the system. On the other hand, sending a signal that is too powerful will interfere with other signals. With wavelengths and values typically used in communication networks, the scientists determined that it is theoretically possible to send 100 terabits of data per second without excessive noise or interference.

In an accompanying commentary in the same issue of Nature, Joseph Kahn of the University of California at Berkeley and Keang-Po Ho of the Chinese University of Hong Kong describe the Bell Labs research, led by physicist Partha Mitra, as "a useful step towards working out the limits to the spectral efficiency of optical fibers."

"This paper highlights the fundamental understanding of the ultimate capacity of fiber," said Alastair Glass, chief technical officer of Lucent's Optical Networking Group, in the statement. "It says that we are still a long way from the fundamental limits in current commercial systems, and it's still uncertain when optical systems will be able to approach the theoretical limits."