City of Light: The Story of Fiber Optics

48 CITY OF LIGHT
Figure 5-1: Total internal reflection at the boundary between core and cladding
guides light along a clad optical fiber (bottom); light can leak from unclad fibers
where they touch other objects (top).
recognizable images. In fact, over 1000 reflections are likely in a single meter
(39 inches) of fiber. 7
‘‘That’s not the way to do it, Bram,’’ O’Brien said, surprised van Heel had
not recognized the limits of metal coatings. The trick was to use total internal
reflection in a way subtly different than everyone else had since Daniel Colladon
demonstrated light guiding a century earlier. Total internal reflection
works as long as the surrounding material has a lower refractive index than
the light guide. Those who followed Colladon thought automatically of the
outside material as air, but other materials also have refractive indexes lower
than glass or plastic. Coating a bare fiber with such a low-index material (see
figure 5-1) would maintain total internal reflection while protecting the optical
surface. The secret to making fibers transmit light well, O’Brien said, was
to clad them. 8
Van Heel immediately realized O’Brien had a good idea, although neither
knew quite how good it was or how hard it would be to implement. Total