City of Light: The Story of Fiber Optics

A CRITICAL INSIGHT 49
internal reflection requires a virtually perfect surface—clean, smooth, and
touching nothing. The surfaces of bare glass or plastic fibers were far from
perfect after they were assembled into a bundle. The fibers touched, and everywhere
they touched light could pass from fiber to fiber. The fibers also rubbed
each other, forming surface scratches that scattered light out of the fibers.
Dirt and fingerprints on the surface also let light leak out.
The cladding offered a simple and elegant way around those problems
because it protected the surface where total internal reflection occurred. At a
stroke, it kept light from leaking between the light-guiding cores and protected
the vital reflecting surface from scratches and fingerprints. The trade-offs were
added complexity and a slight reduction in how much light the fiber could
collect. 9 Van Heel probably was annoyed that he hadn’t thought of the idea,
but that didn’t stop him from embracing the crucial conceptual breakthrough
that launched modern fiber optics.
An Insight from Vision
The idea of cladding did not come instantly to Brian O’Brien. It grew from
his study of an esoteric visual effect discovered in the 1930s. The visual center
of the eye responds more strongly to light if it comes through the center of
the pupil than if it enters at an angle near its edge. 10 Specialists were puzzled
until O’Brien found the reason lay in the structure of the light-sensing cells
in the retina, a layer of cells at the back of the eyeball.
The retina contains two types of light-sensing cells, cones concentrated
near the center that give us color vision, and rods spread over a larger area
that provide night vision. Both are long and thin, with one end exposed to
light and the other attached to nerve fibers. Light must travel through the
whole cell to reach the sensing elements at the back. As far back as 1844, a
German physiologist had suggested that total internal reflection might guide
light along rods and cones, perhaps inspired by Colladon’s demonstration. 11
A century later, O’Brien recognized that was exactly what was happening.
Light arriving from the center of the pupil was guided to the back of the cones
more efficiently than light coming from the sides of the pupil. This made light
coming from the center of the pupil look brighter.
To test his theory, O’Brien made a model cone cell from plastic foam. His
model was 20,000 times larger than a cell, so he tested it with microwaves
20,000 times longer than visible light. That’s how physicists think. Both visible
light and microwaves are electromagnetic waves; they should behave
similarly if you multiply all the dimensions by the same factor. The log-sized
blocks of lightweight plastic drew some surprised stares at Rochester, but the
experiments validated O’Brien’s model. 12 It was an elegant example of scientific
deduction, and earned O’Brien the Optical Society’s highest award. 13
O’Brien had not stopped with the microwave experiments. He had seen
transparent plastic rods guide light for illumination, and wondered what
would happen in thin transparent fibers. He tested clear glass fibers from the