Scientists calculate the theoretical limits of fibre-optic communications

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18 July 2001 News

Bell Labs' scientists claim to have calculated the maximum amount of information that can be transmitted over optical fibre. The Bell Labs team have determined that it is theoretically possible to send approximately 100 terabits of information, or roughly 20 billion one-page e-mails, simultaneously per strand of fibre.

"As networks continue to make communication faster, smaller, cheaper and smarter in the next decade, there will be an even greater emphasis on fibre-optics technology," said Rod Alferness, Senior Vice President of Optical Networking Research at Bell Labs. "As we light more optical fibre, 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, it has been difficult to theoretically calculate how much information can be transmitted over a glass fibre because the physical properties of glass make light transmitted over fibre susceptible to scrambling in a very complicated fashion. For example, the speed of a light signal travelling through fibre depends on the intensity of the light and is not a constant as it would be in free space - a 'nonlinear' response. These nonlinear effects cause part of a signal travelling through the fibre to turn into noise, making calculating the exact amount of information that can be sent over a fibre becomes a thorny challenge.

The scientists used an analogy from quantum physics, together with ideas from information theory. They looked at telecommunication systems that use wavelength division multiplexing, 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, they determined that it is theoretically possible to send 100 terabits of data per second without excessive noise or interference.

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

See www.bell-labs.com

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Scientists calculate the theoretical limits of fibre-optic communications

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