City of Light: The Story of Fiber Optics

‘‘THE ONLY THING LEFT IS OPTICAL FIBERS’’ 113
only on the ends of fibers. A second is scattering of light by atoms in the
glass, which sends it in some direction other than down the fiber. A third is
absorption of light by atoms in the material.
Early on, Eaglesfield asked about scattering and came back with an encouraging
estimate that it was less than five decibels per kilometer for quartz. 29
Later Kao found a formula for light scattering derived several years earlier by
Robert D. Maurer of the Corning Glass Works. 30 When he and Hockham
plugged in the numbers, they got an estimate even more to their liking—one
decibel per kilometer at a wavelength of one micrometer. That implied scattering
should not be a big problem for communications.
That left the issue of light absorption. Kao recalls, ‘‘I was seeking the
answer to the question, ‘What are the loss mechanisms and can these mechanisms
be totally removed?’ It appeared that no one had really asked this
question before.’’ 31 Where others had asked for the best existing glass, Kao
sought the fundamental limit. The experts didn’t have a ready answer. When
he pressed them, they blamed most absorption on impurities.
The stuff we call glass is a mixture of things. The basic raw ingredient is
sand, the debris left after the weather wears down rocks until only the hardest
crystals remain—grains of quartz, which chemically is silicon dioxide, also
called silica. To melt sand at reasonable temperatures, glass makers add soda,
potash, and lime. They add other compounds to make special glasses for purposes
from optical instruments to fine crystal ware. Add cobalt and the glass
turns a rich dark blue; other metals give other tints.
Traditional glasses are not chemically pure, but they are adequate for their
usual jobs. Small dashes of impurities don’t absorb enough light to notice in the
thickness of a sheet of window glass or a camera lens. However, the absorption
becomes noticeable if the light has to go a long distance through a fiber. Iron,
copper, and some other elements soak up light, darkening the glass.
How clear would glass be if you removed all the impurities? Many experts
were only guardedly optimistic. They weren’t sure because they hadn’t measured
absorption in extremely pure glasses. They weren’t sure how pure glass
could be made. They simply didn’t have the answers to Kao’s questions. 32 Yet
they also had no showstoppers, and Kao heard some encouraging words.
Professor Harold Rawson of the Sheffield Institute of Glass Technology said
he was convinced that removing impurities could reduce absorption below
the target level of 20 decibels per kilometer. 33 If all went well, that meant
fiber optic communications might be possible.
Putting the Pieces Together
Encouraged, Kao and Hockham drew a few fibers and tested them at STL.
The fibers were lossy, but those with cores smaller than four micrometers
transmitted the red light from a helium-neon laser in a single mode. They
experimented with semiconductor lasers and white light. They tested Hockham’s
microwave guides and analyzed the results. They convinced themselves