City of Light: The Story of Fiber Optics

138 CITY OF LIGHT
pulling about 160 feet (50 meters) of fiber a minute. The experiments taught
Corning important practical lessons. Heat and humidity made small fiber, 50
micrometers or 2 mils thick, stick to the tractor drives. Fiber 250 micrometers
or 10 mils thick grew brittle and broke in a few weeks, with loose ends
springing up on the reels. Corning settled on an intermediate diameter, 125
micrometers or 5 mils, which remains the standard size for telecommunications
fiber.
Other challenges appeared. The transparency of titanium oxides depends
on their chemical form. When one titanium atom combines with a pair of
oxygen atoms to form titanium dioxide, TiO 2, the molecule absorbs almost no
light. However, the high temperatures needed to melt silica drove off some
oxygen atoms, leaving some pairs of titanium atoms to share three instead of
four oxygen atoms. That compound absorbs light, increasing fiber absorption
to a decibel per meter or worse—just as bad as the compound glasses Corning
was trying to avoid. Another Corning scientist, Roy Harrington, suggested
they could replace the missing oxygen by heating the fiber to 800 to 1000� C
in oxygen, returning titanium to its more transparent state. Unfortunately,
the cycle of heating and cooling tended to make the randomly oriented atoms
in glass align themselves into crystals at the fiber surface. The crystallization
caused tiny cracks to grow at the surface, making the fibers very fragile. 21
The Corning team found a delicate but not perfect balance. By early 1970,
they had found a titanium doping level that seemed right and had learned
how to make good preforms. Progress was slow because it took them about
three months to make a preform, measure its properties, draw it into fiber,
analyze the results, and use the lessons from those experiments to make another
preform.
That Eureka Moment
Experiments inevitably have frustrating moments. One came in early 1970,
when a preform slipped against the furnace wall soon after fiber drawing
started. The accident ruined the preform, but Keck decided to test the 20
meters (65 feet) of fiber that had been pulled. After heat-treating, he aimed
a helium-neon laser down the fiber. At first he couldn’t detect any loss, but
more careful measurements showed attenuation probably was close to the
magic figure of 20 decibels per kilometer. 22
That was a big improvement. Keck had been estimating the loss of fibers
only about three meters (10 feet) long by measuring changes in light transmission
as he broke small pieces off the end. The light intensity had changed
every time he broke a 10-centimeter (4-inch) segment off the old fiber, but
with the new fiber the change was too small to measure accurately with the
best instruments he had. With the preform ruined, he couldn’t draw any more
fiber from it. Nor did he have enough data to claim spectacular results.
The only thing for Schultz and Keck to do was to make another preform
using the same formula and hope the fiber was as good. Zimar pulled a kil-