City of Light: The Story of Fiber Optics

124 CITY OF LIGHT
It’s a big challenge to measure a little attenuation because it’s a small
difference between two large numbers. One percent accuracy may sound fine,
but it’s useless if you want to measure the difference between 99 and 99.5
microwatts. Doing better is a challenge for the best of laboratory wizards, and
that was not Charles Kao’s specialty.
Today, engineers measure attenuation in the clearest fibers by passing light
through one kilometer, or ten, or a hundred. That builds up loss until it’s
high enough to measure easily. Kao didn’t have that luxury; no one knew
how to draw good fibers from the clearest glasses. The best he could do was
work with samples of bulk glass about 30 centimeters (a foot) long.
Nobody had ever tried to measure such low losses before, so he had to
develop a new technique. His first study showed glass could be made clearer
than standard imaging fibers, but could not measure loss as low as 20 decibels
per kilometer. 36 That required him and Mervin W. Jones to devise an even
more sensitive instrument, which compared light passing through two glass
rods, one 20 centimeters (8 inches) longer than the other. Accurate comparison
is a demanding task, but it let them cancel the effects of surface reflection
and concentrate on loss in the bulk glass.
Kao sought the purest glass available for his measurements. Ordinary optical
glass would not do because the raw materials that go into it are riddled
with impurities. Instead, he studied fused silica, a synthetic glass that is essentially
pure silicon dioxide (SiO 2), with less than one part per million of the
troublesome iron impurities. 37 At first their results looked too good to be true.
The samples seemed to be perfectly clear, with no attenuation at all. They
knew that meant that the real attenuation was somewhere within their margin
of error, which allowed loss between 4 and �4 decibels per kilometer.
They spent months analyzing the experiments, to make sure they had everything
right, and to be sure their calculations did not yield negative loss (which
would imply their measurements were wrong because the glass would have
to generate light). In 1969 they finally reported loss of five decibels per kilometer,
38 with the lower limit of their error margin close to zero. 39
Difficult and elegant, those measurements opened the eyes of skeptics
around the world. Before them, Kao had only a handful of believers because
ultraclear glass existed only on paper. The measurements demonstrated he
was right; extremely pure glass could be clearer than anyone else had imagined.
‘‘Kao gave everybody a jolt,’’ recalls Dave Pearson of Bell Labs. ‘‘That
was the first practical measurement which said, hey, you’re not just whistling
Dixie.’’ 40
The Search for Clear Fibers
The measurements were a milestone far from the finish line. They showed
that pure glass could be extremely clear; they did not show how to make
ultratransparent optical fibers.