City of Light: The Story of Fiber Optics

TRYING TO SELL A DREAM 127
their own double-crucible apparatus. 49 They confirmed that impurity absorption
was the big problem, and it proved as hard to reduce at Bell Labs as
elsewhere. In the spring of 1970, the lowest total loss they measured in fibers
was about 0.7 decibel per meter, still far too high for communications. 50
The main thrust at Bell Labs remained the millimeter waveguide. Field
trials were scheduled for 1973, Kompfner reported in March, 1970. He considered
laser communications to be another technological generation in the
future, but he said some 100 Bell Labs engineers were already working on
it, mostly on underground systems using confocal waveguides or gas lenses. 51
Fiber remained a tiny effort.
A New Type of Fiber in Japan
Meanwhile, the Japanese had tackled another problem: getting light into fibers.
Squeezing a laser beam into the core of a single-mode fiber required
alignment accuracy of about a micrometer. In 1967, that was an extremely
difficult task for a specialist in a fully equipped optics laboratory; it seemed
an inconceivable task for a technician in a manhole or on a telephone pole.
That worried Shojiro Kawakami at Tohoku University, and in the spring of
1967 he suggested the problem could be eased by switching to a new type
of graded-index fiber.
Standard large-core fibers were not attractive for communications because
they suffered from pulse spreading. Whether you consider the light as rays
bouncing around in the core or as modes confined by a waveguide, light
could follow many paths through a large-core fiber. Each path took a different
time to travel, so the further the light pulse traveled, the more it spread. The
more the pulses spread, the longer you had to wait between pulses to keep
them from interfering from each other. That reduced transmission capacity.
Graded-index fibers work differently because the refractive index varies
with the distance from the center of the fiber. The original reason for the
design was to avoid losses at the core-cladding boundary. However, Kawakami
realized that careful choice of the refractive-index gradient also could
do something else because the speed of light in glass depends on its refractive
index. The higher the refractive index, the slower light travels. Thus, light in
the high-index center of the fiber lags behind light farther out, where the
refractive index is lower. The more the light travels in the outer core, the
higher its average velocity, offsetting the delay it suffers from having to travel
a greater distance. Kawakami spent a week or two grinding through the
numbers to convince himself that this change in speed could compensate for
pulse spreading. Complete compensation was impossible, but proper tailoring
of the refractive index gradient could reduce pulse spreading by a factor of
100 to 1000. He was thrilled; the effect meant that graded-index fiber would
have almost as much transmission capacity as single-mode fiber over distances
of a few miles. 52