City of Light: The Story of Fiber Optics

TRYING TO SELL A DREAM 121
was forthcoming, then they would be very interested in looking into the fiber
waveguide as a possible optical wave guiding medium,’’ 20 Kao wrote when
he returned home. Without solid evidence of more transparent glass, Bell
would stay with gas lenses and confocal waveguides. Yet Kao felt a blessing
from Bell Labs was critical. When a younger engineer asked why he wanted
to invite a tough competitor into the field, Kao replied, ‘‘The thing will only
take off if we get them into it.’’ 21
Kao and Roberts visited Spitz and Werts at CSF, 22 and Kao toured German
labs. Many listened to his sales pitch, but initially few bought it.
An Invitation to Japan
In late 1966, Kao’s fiber-optic campaign yielded him invitations to speak at
Tohoku University and Nippon Telegraph and Telephone in Japan. Japanese
engineers had also been thinking about fiber communications and wanted to
hear what Kao had to say.
Engineering professor Zen-ichi Kiyasu grew interested in optical communications
after leaving NTT and joining the university faculty, but he could
not see much future for hollow optical waveguides. In 1964, he told another
Tohoku professor, Jun-ichi Nishizawa, that he had not heard any proposals
for optical communications that would be reliable enough for practical use.
A couple of days later, Nishizawa suggested using optical fibers, evidently
inspired by fiber-optic endoscopes. 23
The fibers used in endoscopes have large cores and thin claddings, so they
transmit many modes, like millimeter waveguides. The two professors quickly
realized that was a problem, but instead of turning to single-mode fibers, they
invented a new way to guide light along a fiber. Imaging fibers rely on total
internal reflection at a sharp boundary between two materials. Specialists call
them ‘‘step-index’’ fibers because the refractive index changes abruptly at the
boundary between the light-guiding core and the cladding. Kiyasu and Nishizawa
proposed grading the refractive index so that it changes gradually from
core into cladding. Instead of reflecting light abruptly from a sharp boundary,
a graded-index fiber bends it back gradually. You can visualize the light rays
as following a wavy path, rather than the zigzag path defined by total internal
reflection in a large-core step index fiber. The Japanese hoped this would
reduce losses caused by imperfections in the core-cladding boundary, a problem
with some early fibers. Nishizawa filed for a patent in November 1964
and was later surprised to learn that Stew Miller at Bell Labs had filed for a
patent on a similar idea in February. 24
High glass loss stalled the Tohoku group, who like most communications
engineers knew little about glass. The Japanese listened carefully to Kao and
took him to the Japanese equivalent of Bell Labs, the NTT Electrical Communication
Laboratory, which gave Kao a sample of its own experimental
single-mode fiber. 25