City of Light: The Story of Fiber Optics

EPILOGUE: THE BOOM, THE BUBBLE, AND THE BUST 241
They noted that a fiber with a doped core could be used as an optical
amplifier. All they needed was to remove the mirrors and fire a weak pulse
of light at the right wavelength down the fiber. That input would stimulate
excited neodymium atoms to emit light at exactly the same wavelength, amplifying
the signal. Initially the idea didn’t seem promising. Neodymium atoms
emitted at 1.06 micrometers, far from the best transmission windows of glass
fibers. Stimulated emission from neodymium also didn’t seem strong enough.
To be useful for communications, an optical amplifier had to multiply the
power of an input signal by a factor of 1000, or 30 decibels, which Payne
didn’t think was possible in a few meters of fiber. Instead, he played with
lasers, where power built up as light bounced back and forth between the
mirrors at the opposite ends of the fiber.
Other rare earth elements also had useful optical properties, so the Southampton
group also added them to the cores of fibers. 7 They soon made lasers
from fibers containing another rare earth called erbium, which emits strongly
at 1.53 micrometers. The wavelength was interesting because it is very close
to where silica fibers are most transparent. Experiments showed the erbium
laser wavelength could be varied across a range of 25 nanometers. 8
The physics of erbium didn’t look promising, but the Southampton group
studied it closely. Only slowly did they realize that the nature of their fiber
laser experiments was changing the rules. They excited the erbium atoms by
focusing the blue-green beam from an argon gas laser into the core of a singlemode
fiber. The power from the argon laser aimed into the fiber was modest,
but concentrating it in the tiny core excited most of the erbium atoms, eliminating
the loss they had expected from unexcited erbium. ‘‘It took us 26
publications on fiber lasers before we realized that, if we took the mirrors off
and looked at what the gain was, we’d have a huge gain of 30 decibels’’
needed for an amplifier, Payne recalled. 9
Their first experiments in late 1986 lived up to expectations. They recorded
peak amplification of 26 decibels during pulses from the argon laser. Critically,
they also saw low noise, which is essential for practical optical amplification. 10
Later they boosted the peak gain to 28 decibels by exciting the erbium atoms
with red light from a different laser. They also made another critical discovery:
erbium atoms could amplify light by a factor of ten (10 decibels) across a 25nanometer
range of wavelengths. 11
The first experiment suggested that optical amplifiers could extend the
reach of systems transmitting one signal per optical fiber. The second experiment
held out another hope—that erbium could multiply the bandwidth of
a single fiber by amplifying signals at two or more different wavelengths.
The idea is called wavelength-division multiplexing, and the old Bell System
had tried it in its ill-fated Northeast Corridor system. The idea of sending
signals simultaneously at two different wavelengths wasn’t bad; the problem
had been the technology Bell used. Every seven kilometers the optical signals
had to be converted to electrical form and amplified, then used to drive a new
laser transmitter. That required separating the two signals by wavelength and
directing each one to a separate receiver and transmitter.