City of Light: The Story of Fiber Optics

168 CITY OF LIGHT
link switching centers in urban and suburban areas. That meant they had to
carry a hundred to a thousand voice lines, 10 to 100 million bits per second,
over 2 to 20 kilometers (1.2 to 12 miles). LEDs could handle the lower end
of that range, but faster semiconductor lasers would be needed for the high
end. On the technological horizon, Miller envisioned multimode graded-index
fibers carrying 300 million bits per second between repeaters six to eight
kilometers (four to five miles) apart—four to five times farther than coaxial
cables. 50 The ever-optimistic Reeves thought the ultimate speed limit might
be 10 billion bits per second over a few kilometers. 51
Bell turned its attention to improving graded-index fibers. Two top theorists,
Gloge and Henry Marcatili, refined the design by calculating a new
refractive-index gradient that in theory increased transmission capacity a
thousand times above large-core fibers with a sharp ‘‘step’’ boundary between
core and cladding. In practice, the improvement was much less, ranging from
20 (in production) to 75 (in the laboratory). 52 However, that seemed good
enough because the theorists expected other pulse-spreading effects to limit
transmission capacity, even for single-mode fibers.
The most important of these is the same phenomenon that makes a prism
spread white light into a spectrum—material dispersion. The refractive index
of glass varies slightly with wavelength, so some colors travel faster than
others. Semiconductor lasers are not purely monochromatic, and although
the effect is small, it builds up with distance. Material dispersion limited the
transmission capacity of single-mode fibers at the 800 to 850 nanometer
wavelength of gallium arsenide semiconductor lasers to only two to three
times that of graded-index fibers. That wasn’t enough of an advantage to
offset the other problems of single-mode fibers. 53 Indeed, for short links between
switching offices, even LED sources looked viable, although their
broader range of wavelengths made material dispersion 10 to 50 times worse
than for semiconductor lasers.
Most of the rest of the world came to the same conclusion. Some staunch
single-mode advocates remained at the Post Office, but to others lightcoupling
problems made single-mode seem ‘‘an unattainable dream.’’ 54 STL
also was slow to move away from single-mode fibers, wary of multimode
transmission problems in millimeter waveguides and seeking the highest possible
capacity for its parent company’s submarine cable business. 55
The Corning Connection
Corning knew glass and it had a healthy head start on making low-loss fibers,
but it didn’t know much about communications. Chuck Lucy could see enticing
possibilities. ‘‘It really seemed like it had the possibility for infinite bandwidth
and zero loss,’’ he recalls. ‘‘If you had that, there had to be a market
somewhere.’’ 56 However, merely supplanting millimeter waveguides in highend,
long-distance systems would amount to what another glass company