City of Light: The Story of Fiber Optics

‘‘THE ONLY THING LEFT IS OPTICAL FIBERS’’ 107
tral dielectric could not conduct electricity, and that (for light) its refractive
index be smaller than that of the central dielectric. 13 Karbowiak realized, as
Brian O’Brien had a decade earlier, that the surrounding material did not
have to be air. Any kind of cladding or coating would make a minuscule
optical waveguide easier to handle. In theory, an infinitely thick cladding
should behave the same way as one just a few wavelengths thick, so the
cladding could be as thick as the designer wanted—thick enough to ease
handling but thin enough to remain flexible.
More subtle, and it turned out more important, surrounding the waveguide
with another material changed the diameter needed for single-mode operation.
The critical number is the difference in refractive index between the
waveguide (or fiber core) and the surrounding cladding. The larger the difference,
the smaller the core must be to transmit only a single mode. For
glass in air, the difference is 0.5, so an unclad glass fiber must be no larger
than 0.1 to 0.2 micrometer to transmit light in a single mode. Anything
larger operates multimode. However, the smaller the difference between the
refractive indexes of the core and the surrounding material, the larger the
diameter for single-mode transmission. Apply a cladding with a refractive
index just one percent lower than the core, and the core or central waveguide
layer can transmit single-mode light even if it is several micrometers thick.
That’s still small, but it’s getting into the realm of feasibility, especially because
the surrounding cladding can be many times thicker.
Increasing the size of the waveguide offered a crucial benefit for optical
communications. Directing light into a fiber core is like threading a needle;
the bigger the target, the easier it is. In the 1960s, no one knew how to focus
light onto a spot much smaller than its wavelength (it’s still very difficult).
You couldn’t get a useful amount of light into a bare fiber waveguide 0.1 to
0.2 micrometers wide. However, with great care you could aim a laser beam
into a core several micrometers across in a clad fiber. Thus, adding a cladding
put single-mode optical communications into the realm of possibility.
Eli Snitzer had formulated the same rules earlier at American Optical, but
he had come to single-mode fibers from a different approach. Imaging fibers
typically have large cores surrounded by thin cladding layers, so that they
can transmit the brightest image possible. Karbowiak had a different goal—
transmitting light in a single mode for communications—and he envisioned
a different structure, with a tiny core surrounded by a thick cladding.
While the cladding solved some problems, it added another: Light had to
travel in the transparent material, instead of in the air. (In fact, cladding
changes the properties of a single-mode waveguide such that most light travels
in its core, rather than along the surface.) This threatened to raise transmission
loss tremendously. Even the clearest solids available in the early
1960s absorbed too much light for a practical optical waveguide, Karbowiak
concluded in an internal report. Yet other possibilities looked even worse. ‘‘It
would be unwise to dismiss any of the proposed means of [optical] communications
as too impractical or too costly,’’ he told a London meeting on laser
applications in September 1964. ‘‘Nonetheless ...ofalltheguides known to-