City of Light: The Story of Fiber Optics

EPILOGUE: THE BOOM, THE BUBBLE, AND THE BUST 243
experiment the NTT Transmission Systems Laboratory had a 40-milliwatt
semiconductor laser pumping an erbium fiber amplifier. 18 Later that year, NTT
reached amplifier gain of 46.5 decibels. 19 Semiconductor lasers were harder
to make at 980 nanometers, but the old RCA Laboratories, by then spun off
as the David Sarnoff Research Center, soon succeeded. 20
Promise and Problems
The rapid-fire development of erbium-fiber amplifiers dazzled even the developers
themselves. By 1989, they had reached the threshold of success, optical
amplifiers that could be powered with a semiconductor laser. That was an
improvement over the cumbersome old repeaters that had to convert optical
signals into electronic form for amplification, but it was not a revolution.
The revolution would come if optical amplifiers could handle more than
one signal at once. That could multiply the fiber’s bandwidth by the number
of signals. Send signals through a system at 20 separate wavelengths, and
you multiplied its capacity twenty-fold. Payne had already shown that erbium
could amplify a range of wavelengths, but that might not be good enough.
Trying to amplify two or more signals at once runs the risk of crosstalk. Noise
from one signal might mix with the other signal, scrambling them together
in an unintelligible mess.
To assess the possibilities, Desurvire performed a crucial experiment. He
sent signals at two separate wavelengths through the same erbium amplifier,
and carefully measured their properties. He modulated both signals at two
billion bits per second, and tested them at different pairs of wavelengths. He
found very low levels of crosstalk, indicating the two channels had minimal
effects on each other. 21 Bell Labs was still pumping its amplifiers with bulky
argon lasers, but Desurvire said that detail shouldn’t matter. His test had
shown that erbium amplifiers could handle multiple wavelengths at once.
The next step for the technology was in the fiber-optic performance Olympics
informally called ‘‘hero experiments.’’ Teams of top engineers filled their
labs with costly equipment and set out to see how far and how fast they could
send signals through optical fibers. Viewed from afar it seemed an eccentric
and costly competition, with rules continually redefined. Whenever Bell Labs
jumped into the lead, they issued press releases heralding a record-setting
demonstration. Yet the hero experiments were serious probes of the fiberoptic
frontier. Not all the new ideas proved practical, even those that set
records, but many hero experiments foreshadowed the cutting edge of commercial
systems a few years later.
The state of the art in 1989 was amplification of one wavelength at a time.
A group from Bellcore managed to transmit separate signals at 16 different
wavelengths through a single fiber, but they didn’t use optical amplifiers, and
each channel carried only a modest 155 million bits per second. 22 The following
year engineers from KDD, the Japanese overseas telephone company,