City of Light: The Story of Fiber Optics

A VISION OF THE FUTURE 89
of the light that entered a one-mile length would emerge from the other end.
(In metric terms, loss was 1.6 decibels per kilometer, and 70 percent would
emerge from a one-kilometer length.) That was about 10 times better than
the theoretical loss of Wheeler-style light pipes under the same conditions. 42
Eaglesfield estimated his optical pipeline could carry light around very gradual
bends, with a radius of about half a mile (0.8 kilometer).
To test the idea, STL assembled 35 four-foot (1.2-meter) segments, coated
inside with epoxy resin to provide a smooth base for the silver film. After
considerable trouble joining the pipes, STL technicians stretched them along
eight concrete posts sunk deep into the ground to give a sound footing. The
assembly was within 1/16 inch (1.6 millimeters) of being perfectly straight,
but the results were disappointing. Eaglesfield had predicted 97 percent of the
light should emerge after a 276-foot (84-meter) round trip, but the measured
amount was under 9 percent, corresponding to a loss of more than 200
decibels a mile. 43 That meant that only 10 �20 of the input light would have
emerged from a mile-long pipe. Eaglesfield complained that the measurements
didn’t do his idea justice, and a few years later Czech engineers did somewhat
better. 44 However, Reeves held out little hope and went looking for other
ideas.
One came from Ramsay, who suggested arranging a series of lenses along
the inside of a pipe such that each one focused an image of the previous lens
onto the next lens (figure 7-3). Such a ‘‘confocal lens waveguide’’ could relay
light along the pipe such that none was lost by hitting the sides. 45
The little team also began looking at optical analogs of a less common
kind of microwave waveguide made out of a nonconductive material like glass
or plastic. The edges of such ‘‘dielectric’’ materials also can guide electromagnetic
waves. The materials absorb microwaves, so thick rods that guide
microwaves inside themselves do not work well. However, in the late 1940s,
engineers at RCA Laboratories in Princeton found they could do much better
with plastic rods thinner than about a quarter of the microwave wavelength.
46 In that case, most of the microwaves travel along the outside of the
waveguide, not inside where the material can absorb it. That means that thin
plastic waveguides have very low loss, but only if they are perfectly straight;
like many other waveguides, they radiate energy at bends. This was a serious
practical problem at microwave frequencies, but it didn’t keep STL engineers
from considering making dielectric waveguides of transparent materials for
optical communications.
Figure 7-3: A confocal waveguide guided light from lens to lens without hitting
the walls of the pipe.