Optical Fiber

One half of the 2009 Nobel Prize in Physics goes to Charles Kao for his contributions to the development of low-loss optical fiber.

Although free-space transmission had been proposed by Alexander Graham Bell (and such optical techniques as smoke signals and mirrors were used for communication even earlier) optical signals did not become technologically important until the 1970s. The laser provided the needed bright light source, but sending light beams through the air or evacuated tubes never became widespread. Optical fiber, by avoiding the natural spreading of the beams and by letting them be routed like electrical signals in a wire, made widespread optical communication, including undersea transmission, possible.

Many researchers contributed to the development of practical fiber. Early studies had demonstrated the principles of total internal reflection that allowed light to be guided down gently curving paths, and the usefulness of an outer cladding to keep light from leaking out into the surroundings. But in the late 1960s, the attenuation in optical fibers would have prevented signals from being sent more than a few tens of meters.

Charles Kao helped to elucidate intrinsic loss mechanisms in silica (SiO₂) fibers. Inherent density fluctuations in the fiber cause Rayleigh scattering, which increases for higher-frequency light (which is why the sky looks blue and the setting sun looks red). On the other side, low-frequency light is directly absorbed by atomic vibrations in the material. The best transmission occurs for intermediate frequencies where each of these processes is relatively unimportant, which for silica is in the near infrared region of the spectrum. If impurities could be removed from silica fibers, Kao showed, this material could be much clearer.

A major advance came from researchers at Corning, who in 1970 made very clear fibers using chemical-vapor deposition from very pure ingredients. This technique lowered the loss to a few dB per kilometer, making long-distance transmission feasible. Bell Labs later developed a modified deposition process that further reduced the loss from tiny amounts of residual hydrogen in the fibers. Previous prizes, like the Draper Prize, have often included the Corning and Bell Labs contributions to making fiber communication practical. Modern fibers have losses of around 0.2dB per kilometer, meaning that a very useful 1% of the original light power will travel 500km down the fiber, an astonishing degree of clarity for a solid material.

Researchers have explored many variations on the silica fiber over the years. For example, a single crystal core might reduce density fluctuations, while avoiding a material without oxygen would have fewer high-energy vibrations and less absorption. Cheaper materials like plastic can make useful fibers for carrying light over a few meters. But for long-distance transmission, no material has displaced the silica that was championed by Charles Kao.