City of Light: The Story of Fiber Optics

232 CITY OF LIGHT
part of Lucent Technologies after the second breakup of AT&T, are top competitors
at the forefront of fiber technology.
The Fiber-Optic Performance Olympics
Bell is usually in the thick of the ‘‘hero experiments’’ that seek to demonstrate
ever more spectacular feats of sending more information over greater distances.
Teams of specialists boast of their achievements at the two big annual
meetings, the winter Conference on Optical Fiber Communications in America
and the fall European Conference on Optical Communications. They work
long hours fine-tuning their experiments before dashing off to catch their
flights, then report their latest and greatest results at special sessions of papers
that came in after the normal conference deadline.
There are no formal rules and no referees to declare the winners, but the
fiber-optic performance Olympics test the cutting edge of the technology. The
usual entrants come from the major industrial labs, including Bell Labs and
its spin-offs in America, NTT and some of the big electronics companies in
Japan, and British Telecom Labs in England. In the 1980s, hero experiments
laid the groundwork for long-distance systems on land and under the sea,
which now carry billions of bits per second through each fiber. They tested
ways of reducing the pulse dispersion that limited transmission speed at 1.55
micrometers. They stretched the spacing between repeaters and eventually
showed that optical amplifiers could replace repeaters.
They pushed the speed and distance frontiers further in the early 1990s.
For Bell Labs, a major goal was to develop better technology for a new generation
of submarine cables. Neal Bergano at Crawford Hill and Linn Mollenauer
at Holmdel took different approaches, using different types of light pulses.
Bergano used the standard pulses that spread gradually with distance, at the
mercy of the pulse dispersion inherent in fibers. Mollenauer took more exotic
pulses, called solitons or solitary waves, which do not spread in the same
way.
Soliton transmission is among the boldest ideas to emerge from Bell Labs.
The allure of the soliton is its peculiar stability, first seen in water waves that
retained their shapes for long distances along nineteenth-century canals. Theoretical
physicists explained the anomaly as a kind of balancing act, possible
only in certain materials when certain conditions were met. In 1973, Akira
Hasegawa, a Japanese theorist working at Bell Labs, found that optical fibers
could carry soliton light pulses at wavelengths longer than 1.2 micrometers.
However, high fiber attenuation quickly dimmed them, making the idea seem
impractical.
That changed when fiber developers opened the long-wavelength window,
and Mollenauer soon produced solitons in fibers. That renewed Hasegawa’s
interest, and he calculated that solitons could travel thousands of miles if the
pulses were amplified. Mollenauer started a new round of experiments but