City of Light: The Story of Fiber Optics

110 CITY OF LIGHT
combination of work and school was demanding, but the salary beat starving
as a full-time graduate student. 23 When Karbowiak left, Kao was finishing his
thesis for millimeter waveguide pioneer Harold Barlow. He also was busy at
home helping raise two small children; his wife had continued working after
they were born, a rarity in the early 1960s. While the 26-year-old Hockham
raced motorcycles, the slightly older Kao had little time for outside recreation.
Both young engineers had mastered waveguide theory; their other skills
were complementary. Kao had a good physical intuition and a knack for
assembling components into working systems. Hockham’s gift was mathematical
analysis of how waveguides radiate energy, an arcane art of the utmost
importance for transmission lines and antennas. Antennas are supposed
to radiate energy; transmission lines are not. Hockham’s job was to make
sure waveguides didn’t act like antennas. 24
Before Karbowiak left for Australia, Hockham tested a larger polyethylene
film model scaled to work with eight-millimeter microwaves. Karbowiak found
the results ‘‘most encouraging, showing small attenuation, good field confinement,
and ability to negotiate bends and twists.’’ 25
Optical thin-film waveguides proved more troublesome. Light waves are
over 10,000 times shorter than eight-millimeter microwaves, so Kao and
Hockham had to shrink the guide dramatically. To make films thin enough,
they dissolved plastic in a solvent which evaporated readily at room temperature,
then dropped the solution gently onto water. The solvent evaporated,
leaving behind an extremely thin film that they had to gently lift off the water.
After a series of experiments, they finally made films less than half a micrometer
thick, so thin that they were iridescent, like an oil slick, because light
waves interfered inside them.
Karbowiak’s theory said the delicate films should carry light in just one
mode. Kao and Hockham played with the films, aiming the red beam from a
helium-neon laser into the thin guide. When they put a film guide on a curved
support, light spread over the walls of their laboratory, leaking prodigiously
from the bent waveguide. ‘‘It was a spectacular sight, and we took a photo
to record this event,’’ recalls Kao. 26 ‘‘And that was the end of the surface
waveguide, because there was no way you could use it.’’ 27 He and Hockham
had done their job, testing their former superior’s idea. With Karbowiak gone,
no one remained to advocate and refine the thin-film guide. They turned to
the idea Kao considered more promising—clad optical fibers (figure 9-2).
Seeking Clearer Fibers
Theory clearly showed that a cladding would keep light from leaking out at
bends in a single-mode waveguide, solving the problem that killed the thinfilm
guide. That left the question of material transparency.
Kao and Hockham began analyzing requirements for optical waveguides
months before Karbowiak left. They targeted needs of the British Post Office
that were quite different than AT&T’s plans for high-capacity systems to cross