City of Light: The Story of Fiber Optics

136 CITY OF LIGHT
University of Waterloo. Kapron had been studying laser glass, but Maurer
asked him to calculate the ideal dimensions for core and cladding of singlemode
fiber. 17
Keck and Schultz started working with Zimar on rods and tubes drilled
from chunks of fused silica, hoping to improve the rod-in-tube process.
However, collapsing the tube trapped bubbles, cracks, and abrasive grit remaining
from mechanical polishing at the critical interface between core and
cladding. Those irregularities scattered light. ‘‘Polishing is simply a process of
going to finer and finer abrasives to make the cracks as fine as possible. If
you get them small enough, you don’t see them, but the light will,’’ says
Keck. 18
Next they tried flame polishing. The rod was easy; they just ran it through
a hot flame. Reaching inside the tube was much harder, because the central
hole was only about a quarter-inch (six millimeters) across. Keck first tried
the infrared beam from a carbon dioxide laser, which glass absorbs strongly.
He bounced the beam off a mirror at one end of the tube and directed it at
the inside wall with a second mirror that moved along the tube. The focused
beam melted the surface, but the moving mirror made screw-thread patterns
inside the tube. He eventually turned to sliding a small torch through the
tube to flame polish the inside surface smoothly.
Meanwhile, Schultz and Keck decided to try a different approach to making
single-mode fibers. They didn’t need much material for the core, so it
seemed simpler to deposit a thin layer of core glass inside a thick tube. Flame
hydrolysis could deposit fluffy white fused-silica soot. Heating the tube
should melt the soot to form a clear inner layer that would become the core
when drawn into a fiber. That, they reasoned, should make a cleaner interface.
The trick was to deposit the glass soot uniformly. Schultz had set up his
burners in a fume hood for ventilation and pollution control, so they had to
custom-build a lathe to roll up to the torches. Keck mounted a stainless-steel
tube inside a giant ball bearing with a two-inch (five-centimeter) central hole,
and mounted a 1-inch (2.5-centimeter) fused silica tube inside it. He added a
belt drive to turn the tubes and hauled the whole assembly in the elevator
from his first-floor laboratory to Schultz’s lab on the fifth floor.
Schultz carefully aligned a burner to point along the hole in the tube. They
started the lathe rotating the tube slowly, then fired up the burner to spray
titania-doped soot for the fiber core. The lathe spun perfectly, but all the soot
collected at the end of the tube. The soot couldn’t go down the 1 ⁄4-inch (6millimeter)
hole because no air was flowing through it.
They needed to get air flowing down the tube to carry the soot and coat
the inside uniformly. The frustrated pair looked around Schultz’s lab, and
their eyes fell on an old General Electric canister vacuum sitting in a corner,
which Schultz used to clean up the mess inevitable in a glass lab. Inspiration
struck. They shut down the lathe and burner, cleaned off the tube, and remounted
it. This time they hooked the vacuum to the end of the tube to suck
air and soot through the little hole. When they fired up the burner and re-