I read this morning a sort of primer on optical
transmission for dummies like me.
noj
New pipelines promise unprecedented speed
page 2: Optics earn a larger role
Traditionally, the only optics were the laser and a length of glass, with an
optoelectronic detector in the middle. They only carried signals from
point to point.
The emerging optical network is spreading optics outward from the heart
of the network to perform more and more system functions. The optical
amplifier was the first step on that road.
In many ways, the idea of optical
amplification was obvious. A laser amplifies
light, and the amplified signal is precisely in
phase with the input -- exactly what's needed
for communication. Converting an optical
signal to electronic form for amplification is
cumbersome in comparison, and its
complexity adds to costs and risks of failure. Yet, repeaters also can
regenerate the original signal, stripping away noise that accumulates
during transmission.
More importantly, nobody found the right material for optical amplifiers
until the late 1980s, when David Payne, a fiber researcher at the
University of Southampton in England, doped the light-carrying cores of
optical fibers with an element called erbium. Erbium was not only a very
good optical amplifier, it worked the infrared wavelengths where glass
fibers are clearest.
Developing practical fiber amplifiers took a few years, but they
expanded the optical domain in the heart of the network by stretching the
distance signals could travel as light before they saw another electron.
Wavelength-division multiplexing
Erbium-fiber amplifiers, in turn, opened another door. They could
amplify signals across a range of wavelengths -- initially from 1,530 to
1,565 nanometers, and now up to about 1,620 nanometers. If the input
fiber carried signals on two wavelength channels, erbium could amplify
both without scrambling them.
Research engineers had been playing with multi-wavelength systems for
years, but repeaters had been showstoppers. Every wavelength had to
be separated from the others and run through a separate repeater,
sending costs skyward.
Wavelength-division multiplexing had a powerful allure because it
multiplied the number of channels a fiber could carry. Many wavelength
channels can share a fiber without scrambling each other, like the many
radio stations and television channels that share the broadcast radio
spectrum. Developers tried optical amplifiers for four wavelengths, and
they worked.
Eight followed, then 16. Channel counts rose as engineers sliced the
erbium-amplifier spectrum into smaller slices. Plain wavelength-division
multiplexing (WDM) quickly became "dense-WDM," or DWDM.
The International Telecommunications Union (ITU) sliced the
erbium-fiber range into a grid of standard wavelengths about 0.8
nanometers apart. (ITU engineers actually specified the grid in equivalent
frequency units, like specifying a microwave as having a 10-gigahertz
frequency instead of a 3-centimeter wavelength.)
That didn't stop system developers from paring the spectrum even
thinner, into 0.4-nanometer slivers, packing 80 slots into the main
erbium-amplifier band. They have now opened a second
erbium-amplifier band, stretching from about 1,570 to 1,610
nanometers, which offers another 80 wavelength slots.
It's a whole new way to organize telecommunications. Start by stacking
bits together at faster and faster speeds, until you can't make the
electronics go any faster. Then assign each high-speed signal to a
separate wavelength channel.
Optics become the way to organize the signals feeding into the new,
fatter DWDM pipes, the highest denomination in the bandwidth
sweepstakes.
Practical limits do exist. The more bits an optical channel carries, the
bigger the slice of the spectrum it demands. In today's systems,
2.5-gigabit signals can fit into 0.4-nanometer (50-gigahertz) slots, but
10-gigabit signals typically get 0.8-nanometer slots.
Nortel's new system squeezes 10-gigabit signals into 0.4-nanometer slots
by sending alternating wavelength channels in opposite directions to
minimize cross talk. "Hero experiment" teams can transmit 40 and 80
gigabits per optical channel in the lab, but those signals require larger
slices of the spectrum and can't travel as far.
The usable spectrum is expanding; fibers can carry high-speed signals
from about 1,260 to 1,650 nanometers. Transmission distances are
limited to tens of miles outside the erbium-amplifier range, but new
amplifiers in development may change that.
"Fiber itself can hold 50 terahertz bandwidth, so you can
imagine 50 terabits per second of information," says Glass of Bell Labs.
Put half the people in the world at each end of a fiber-optic cable
operating at that speed, hand everybody a telephone, and it would take
only a half-dozen fibers to let everybody talk at once.
How close we can come to that theoretical limit remains to be seen.
Surging Internet growth
Without the Internet, the ultimate fiber bandwidth would be little more
important than how many angels could dance on the head of a pin.
Telephone-traffic growth is healthy but can be measured in percentage
points per year, while the explosive growth of Internet traffic is forcing
carriers to unprecedented expansion.
"Some of our customers are doubling every four
months," says Vivian Hudson, vice president of high-capacity optical
networks for Nortel Networks. "Not just one, but several of our very
large customers."
The number of Internet users continues to grow, but their bandwidth
usage is growing even faster. Internet bandwidth is an addictive drug; like
closet space, you can never have enough. Home computer links have
risen from 300 bits per second on early dial-up modems to around a
megabit on cable modems and DSL.
Fancy Web graphics, music downloads, streaming audio and video, and
bloated software fill the new bandwidth faster than household clutter
crams closets. Few people step back down the ladder voluntarily.
Telecommunications carriers had planned for growth, laying cables
containing extra-"dark" fibers to provide extra future capacity. Extra
fibers are cheap compared to installing new cables, and costly
transmitters and receivers aren't added until they're needed.
But the carriers had planned on telephone-style growth, not the Internet
explosion, and their reserve quickly eroded. Last year, a KMI Corp.
analysis of Federal Communications Commission data showed that
Sprint had lit 85 percent of its long-distance fibers, while AT&T had lit
50 percent. |
