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