City of Light: The Story of Fiber Optics

TRYING TO SELL A DREAM 125
A key step was to get rid of the impurities, but how to do that was far
from obvious. Kao had measured fused silica, which most major laboratories—STL,
the British Post Office, Bell Labs, and the Japanese—considered an
impractical material. Its melting point is over 1600�C (2900�F), 41 far higher
than other glasses. Virtually no one had furnaces hot enough to soften it for
drawing into fibers. More troublesome optically, its refractive index is 1.46,
the lowest of any standard glass. That was a big problem because an optical
fiber requires a cladding with refractive index lower than the core, but nobody
knew how to make glass with lower index than fused silica. STL engineers
groped in vain for ideas, even considering cladding pure silica with ice. 42 The
difficulties were so large and so obvious that virtually everyone crossed pure
fused silica off their list of potential fiber materials.
The consensus was that the best way to make fibers was to purify other
glasses, in which other oxide compounds such as phosphates, borates, soda,
and lime are added to impure silica. The glass industry had generations of
experience with multicomponent glasses. Blending other materials into silica
reduced its melting point to reasonable temperatures and gave control over
the refractive index. Purity was a problem, but progress was being made in
purifying raw materials. It seemed just a straightforward matter of slogging
slowly forward to purer and clearer glass.
Dyott’s group at the Post Office refined their methods of pulling fibers from
preforms made by sealing a core-glass rod inside a tube, but they weren’t
satisfied. They had problems keeping rod and tube surfaces clean, and wanted
to pull fibers continuously, without replacing preforms. Dyott started looking
at a process used to make other glass fibers, pulling them from a hole in the
bottom of a crucible filled with thick molten glass. It yielded a continuous
fiber as long as fresh material was fed into the crucible, but a simple crucible
could not make the clad fiber needed for communications. The technical director
of a glass company outside London 43 suggested Dyott try a double
crucible, with core glass melted in an inner crucible which sat in the middle
of an outer crucible filled with molten cladding glass. The core glass emerged
from a central hole at the bottom of the inner crucible; cladding glass emerged
from a concentric ring around that hole.
Dyott liked the idea, a variation on a scheme invented in the 1930s to
make insulating glass fibers. 44 He didn’t know that Will Hicks had briefly
experimented with a similar approach a decade earlier, 45 but that probably
would not have mattered. Dyott thought industrial production would require
a process that could draw fibers continuously. If the core and cladding glass
were both molten as they emerged from the nozzle, but cooled quickly enough
that they didn’t mix, he expected them to form a smooth core-cladding
interface.
While the double-crucible concept was simple, if was far from obvious how
to calculate exactly how the process worked. That didn’t stop Dyott, who like
Reeves devised intuitive models to test new ideas. He decided he could test
the double-crucible process with sugar. Although the choice sounded unlikely,