City of Light: The Story of Fiber Optics

188 CITY OF LIGHT
Opening the 1.3-micrometer window made single-mode fibers look much
better. The lower loss—about 0.5 decibel per kilometer—meant that signals
could travel tens of kilometers. The low material dispersion promised capacity
many times higher than at 850 nanometers. Moreover, core size increases
with wavelength, to nine micrometers at the longer wavelength, compared
to a mere four micrometers at 850 nanometers. That eased alignment tolerances,
which had been improving with splice and connector technology.
The single-mode revival spread rapidly. Having finished its trials of gradedindex
fiber, the British Post Office turned to single-mode. 60 Corning, committed
to a strategy of staying at the forefront of the new technology, shifted Bob
Olshansky to single-mode, and he discovered it was easier to design and make
than graded-index fiber. 61
The Japanese stepped up single-mode research after they opened the 1.3micrometer
window. By late 1977, NTT was making low-loss single-mode
fibers. 62 The Ibaraki lab pushed to remove the last traces of water, paying off
at the end of 1978 with single-mode fiber showing a dip at 1.55 micrometers
where loss was lower than anything anyone had ever seen before. They had
made the clearest glass in the world, with attenuation only 0.2 decibels per
kilometer, just a little higher than the theoretical lower limit on scattering.
NTT knew they couldn’t do much better, and called it ‘‘ultimate low-loss’’
fiber. 63
The lower the loss, the more enticing single-mode fibers became. Pulse
spreading increases with distance; it’s a hundred times larger over 100 kilometers
of fiber than over one kilometer. Good graded-index fibers could carry
a hundred million bits per second for 10 kilometers, but only 20 million bits
over 50 kilometers—and at 1.3 micrometers, 50 kilometers (30 miles) became
a reasonable transmission distance. Single-mode fibers could easily carry a
billion bits 50 kilometers, leaving graded-index fibers in the dust.
In America, single-mode fibers caught the eye of Will Hicks, recovering
from a descent into alcoholism that followed his sale of Mosaic Fabrications.
Never satisfied with other people’s explanations, he calculated the properties
of single-mode fiber for himself and found its transmission capacity went far
beyond the billion bits a second that impressed others. He stubbornly ignored
people who insisted single-mode wouldn’t work and started evolving his own
vision of future fiber-optic systems.
Hicks knew from his early experiments with fiber bundles that light could
leak between fiber cores. Electromagnetic theory explained the process, and
Hicks realized it could be applied to switching light into and out of fibers,
something important for practical communications. He also realized that one
fiber could simultaneously carry signals at many wavelengths, an idea called
wavelength-division multiplexing. Others could see the possibility in theory.
Glass transmits the whole visible spectrum as well as some infrared light,
while the air simultaneously carries radio and television signals at many different
frequencies. A single fiber could carry many different wavelengths, but
getting many separate signals into the same fiber and separating them at the
other end were extremely challenging problems. At best, most specialists