City of Light: The Story of Fiber Optics

98 CITY OF LIGHT
the potential of carrying many separate beams. Gloge calculated a 20centimeter
(8-inch) guide could carry some 300 separate beams. 33
Unfortunately, there were a plethora of practical problems. Temperature
variations, ground vibrations, and mechanical instabilities perturbed the
beam even when the waveguide was buried to isolate it from the environment,
so low loss was very hard to maintain. Bends are inevitable on any
communications route, but they presented serious problems because bending
the waveguide required moving the lenses much closer together than straight
segments. The more lenses, the more their reflection losses accumulated, even
when reduced to about 0.5% per surface. The bends had to be gradual; it
took about 3 ⁄4 kilometer (2500 feet) of lensed waveguide to turn 90 degrees,
and even then only 1/1000th of the light got through, a loss of 30 decibels.
Engineers hoped to reduce that loss by replacing lenses with pairs of focusing
mirrors like those used in some periscopes, but the mirrors posed other practical
problems. 34
Gas Lenses
Meanwhile, Kompfner went looking for help on the problem of surface reflection.
His typical approach was to scatter ideas among other Bell Labs scientists,
trying to catalyze them to make innovations. He mentioned the problem
to managers at the Murray Hill lab, whose mission was basic research. One
of them mentioned the idea to a young scientist working for him as they
stood talking in the hall. Remembering how mirages form, Dwight Berreman
suggested making lenses of air. 35
Temperature gradients in air bend light to form mirages because of a simple
principle of physics. Heat a gas and it expands, thinning out; cool it, and
it becomes denser. The refractive index of air changes with its density, so
heating and cooling change its refractive power. You can see the effects if
you look over a stretch of asphalt road on a hot sunny day, or across the
hood of a hot car. Rising pockets of hot and cool air bend light from distant
objects back and forth, so they look rippled. Berreman thought the same effect
could make air in a tube focus light like a lens. Heating the walls of the tube
would warm gas near the walls, making it expand, while the gas in the middle
stayed cool. The resulting gradient in density would cause a gradient in refractive
power, focusing light toward the middle of the tube like a lens. Unlike
a lens, it would have no surface to cause reflection losses (figure 8-1).
Berreman’s department was not working on optical communications, but
Murray Hill let its scientists play with new ideas. He built a couple of short
gas lenses, satisfied himself that they worked, and wrote up the results before
going on to other projects. 36
At Crawford Hill, Stew Miller embraced gas lenses wholeheartedly, as a
welcome way to circumvent troublesome surface reflection. In fact, Miller
liked them considerably more than Berreman did after giving his idea a more
careful second look. Berreman found that slight fluctuations tended to make