City of Light: The Story of Fiber Optics

242 CITY OF LIGHT
The erbium amplifier offered a way to get around that awkwardness. In
principle a single device could amplify signals at multiple wavelengths without
converting them into electronic format. The question was whether the
technology could be tamed.
Competition and Pump Bands
Southampton didn’t have the field to itself. Other groups were already studying
fiber lasers, and it was only a small step to fiber amplifiers. French physicist
Emmanuel Desurvire led the charge at Bell Labs. He arrived in 1986,
fresh from a stint at Stanford University, and built his own erbium-fiber amplifier.
Although Southampton had built one first, Desurvire made the detailed
measurements that he needed to develop a theoretical model that showed
how to optimize its length. 12
The Southampton and Bell results lured others into the field, but big practical
problems remained. At the top of the list was the need for a better way
to power the amplifier. The early experiments required massive and costly
lasers that needed tens of kilowatts of electricity, a continuous flow of cooling
water, and tender loving care from laser specialists. They wouldn’t have a
prayer outside of a laser lab, but they were the only lasers that emitted the
right wavelengths.
What developers wanted was a compact and efficient semiconductor diode
laser able to excite erbium atoms. The problem was to match a wavelength
that excited erbium with one emitted by a semiconductor laser. British Telecom
Research Labs found several other wavelengths that could excite erbium
atoms. 13 Southampton found that none were ideal. Only 980 nanometers
looked like it would excite erbium efficiently, 14 and that wasn’t available from
a standard semiconductor laser. But the prospects were good enough that
Payne turned his attention to that wavelength.
Meanwhile, Eli Snitzer popped up with a different idea at Polaroid, where
Will Hicks had lured him to develop advanced fiber systems. Erbium absorbed
light as well as emitted it near 1530 nanometers. Everyone else thought the
emission would cancel the absorption, but Snitzer found an important difference.
Absorption is much stronger at shorter wavelengths, so erbium atoms
can absorb light at those wavelengths faster than they can emit it. Nobody
had tried to use this effect before, but that didn’t scare Snitzer. In his first
experiments, he showed that a laboratory laser emitting at 1490 or 1500
nanometers could power an erbium amplifier. 15 Desurvire refined the idea,
and soon demonstrated an impressive 37 decibels of gain in an erbium fiber
amplifier pumped at 1480 nanometers. 16
The big appeal of 1480 nanometers was that semiconductor lasers could
be developed easily for that wavelength, but neither Snitzer and Desurvire
had them. Desurvire couldn’t interest Bell Labs specialists in developing semiconductor
lasers for that wavelength. 17 However, within a year after Snitzer’s