Some Glass!

Scientists at Lucent Technologies’ Bell Labs have calculated the maximum
amount of information that can be transmitted over optical fiber, demonstrating
that fiber optics technology will result in robust, long-term and scalable
communications networks.

The Bell Labs team determined that it was 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.

"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. "As we light more optical fiber, and
couple this with all-optical switches, we will continue to improve the speed and
quality of metropolitan and backbone networks, helping to enrich the end-user

While current commercial optical systems can transmit just under two terabits
of information per second and laboratory experiments have demonstrated
transmission rates of 10 Tbps, it has been difficult to theoretically calculate
how much information can be transmitted over a glass fiber. This is 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

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 could be conveyed from a transmitter to a receiver. They found
that if a signal was sent with too little power, it would be overcome by the
noise in the system. On the other hand, too powerful a signal would interfere
with other signals.

With wavelengths and values typically used in communication networks, the
scientists determined that it was 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 fiber," 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’s still uncertain
when optical systems will be able to approach the theoretical limits."


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