City of Light: The Story of Fiber Optics

A Change of Mode
A DEMONSTRATION FOR THE QUEEN 167
As Miller’s group looked closely at fiber-optic systems, they began worrying
about getting light into the fibers. Kao’s proposal for transmitting light in a
single mode was theoretically elegant, but the light-carrying core of a singlemode
fiber was microscopic. Corning’s fibers had cores only three micrometers
across—0.003 millimeter. If they were much larger, they would carry light
in more than one mode. That was far larger than unclad single-mode fibers
would have been, but it was uncomfortably small for telecommunications
engineers. There was no obvious way to increase core size much; the upper
limit for single-mode transmission depends on the light wavelength and the
difference in refractive index between core and cladding. Moving to the 850nanometer
wavelength of gallium arsenide diode lasers would only increase
core diameter to four micrometers; the index difference had already been cut
to one percent.
Bell Labs took a hard look at the trade-offs and decided single-mode transmission
had to go.
A crucial issue was aligning the tiny fiber cores with each other or with
light sources, and in 1970 that seemed impossible. Their positions has to be
adjusted to within a micrometer—a task extremely difficult in a well-equipped
optical laboratory, and totally inconceivable for a technician in a manhole or
on a telephone pole. The light-emitting zones of semiconductor lasers were
several times wider than single-mode cores, and Miller wasn’t optimistic about
their prospects. 47 Bell considered light-emitting diodes (LEDs) to be much better
light sources. Their output was feebler than semiconductor lasers, and
they emitted from an area much larger, but they could operate for years. In
fact, Crawford Hill had already made a special LED with a hole etched into
its top so that a fiber could be inserted to collect light efficiently. 48 The things
had to be made by hand, but they survived almost indefinitely and delivered
a usable amount of light to a fiber.
Another concern was that single-mode fiber seemed hard to make. Corning
was keeping mum while its patent was pending, and obvious alternative techniques—the
rod-in-tube and double-crucible processes—were poorly suited
for drawing single-mode fibers.
What made large-core fibers appear more practical was the invention of
the graded-index fiber at Tohoku University. By nearly equalizing the speeds
of the many modes in the fiber, the refractive index gradient avoided the
biggest problem of large-core fibers—large pulse spreading that severely limited
signal speed. Alec Reeves spotted the idea and called graded-index fiber
‘‘the most promising practical wide band optical waveguide that can be foreseen
now.’’ 49 The Japanese were making graded-index fibers, although they
weren’t as clear as Corning’s, and Miller had also thought about graded-index
waveguides, which may have made him more receptive.
Graded-index fibers could not completely eliminate pulse spreading, but
Miller wasn’t worried. He still expected millimeter waveguides or hollow optical
waveguides to carry long-haul, high-speed traffic. He wanted fibers to