City of Light: The Story of Fiber Optics

90 CITY OF LIGHT
Fibers as Dielectric Waveguides
To a theorist, an optical fiber is a dielectric waveguide for light. The process
classical optics sees as total internal reflection is the optical equivalent of the
process that guides microwaves along the inside of a thick plastic rod. Brian
O’Brien probably was the first to recognize an optical fiber as a waveguide,
but he never settled down to document the idea. In practice, it didn’t matter
much as long as the fiber core was much bigger than the wavelength of light.
The traditional optical view of total internal reflection works, and the concept
is simpler.
Differences arise when fiber cores are shrunk close to the wavelength of
light, restricting the number of modes, or paths the light can follow through
the core. That didn’t happen until the late 1950s, when Will Hicks wondered
how fine he could stretch optical fibers in a fused bundle. It was a natural
experiment to try, and one with practical import because the core size limits
resolution of a fiber bundle (bundled fibers use very thin claddings). The
smaller the cores, the finer the details you can see. As Hicks shrank the cores,
he saw a strange phenomenon: Geometric patterns and different colors began
to appear in individual fiber cores. He eventually decided it must be a waveguide
effect but didn’t settle down to document it before he quit American
Optical.
The topic was still a hot one at American Optical in early 1959, when
Steve MacNeille, Walt Siegmund, and Lewis Hyde interviewed Elias Snitzer
over lunch. Siegmund pulled out a photograph and asked the young physicist
if he could explain it. Snitzer asked what it was and grew excited after Siegmund
said it was very fine fibers. ‘‘That’s waveguide modes in the visible
region of the spectrum. I don’t believe anybody’s ever seen that before!’’ His
response passed the test, helping convince MacNeille to hire him despite a
blot on his record. The Lowell Institute of Technology had recently fired him
because he refused to cooperate with an investigation of left-wing student
politics by the House un-American Activities Committee. 47
Snitzer recognized the phenomenon because he had worked on microwave
systems. It appears when core diameters approach the wavelength of light,
leaving only a few modes, which form curved geometric patterns inside the
fiber cores. The patterns and colors varied from fiber to fiber because slight
differences in core shape and diameter gave them different mode patterns.
Shrink the fibers far enough, and only one mode remains. Once he started
work at American Optical, Snitzer teamed with Hicks to experiment on fiber
modes, analyze their structure in detail, and report the results. 48
American Optical was not in the communication business, but Snitzer and
Hicks knew microwave waveguides were used in telecommunications. Before
they published anything, they applied for a patent, suggesting that the ability
to transmit light in ‘‘separate well-defined and readily detectable and distinguishable
electromagnetic modes ...maybeused advantageously in various
ways’’ to transmit ‘‘data, information, signals and the like.’’ 49 They didn’t
worry too much about how far their fibers transmitted light; their main con-