OPTICAL FIBERS EMERGE

Although hair-thin strands of glass were already known to carry light over short distances and were already being used in industry and medicine to take light to otherwise inaccessible places, the light typically lost up to 99% of its strength when passing along as little as 30 feet of fiber.

In 1966, Charles Kao and George Hockham at Standard Telecommunications Laboratories in England asserted that fibers of much greater transparency lay within reach. In a landmark theoretical paper, they showed that the high losses characteristic of existing fibers theoretically resulted from minute impurities in the glass--primarily water and metals--rather than from intrinsic limits of the glass itself. They forecasted that light loss in fibers could be decreased dramatically from 1,000 decibels to less than 20 decibels per kilometer. Given that improvement, amplifiers to boost the light signal could be spaced at intervals of miles rather than yards--comparable with the spacing of repeaters that amplified weak signals along conventional telephone lines.

Like the work of Townes and Schawlow a decade earlier, the Kao-Hockham paper spurred a number of researchers to produce such low-loss fibers. The breakthrough came in 1970 at Corning Glass Works when Donald Keck, Peter Schultz, and Robert Maurer successfully prepared an optical fiber hundreds of yards long with the crystal clarity that Kao and Hockham had proposed. Shortly thereafter, Panish and Hayashi at Bell Laboratories demonstrated a semiconductor laser that could operate continuously at room temperature, and John MacChesney and coworkers, also at Bell Laboratories, independently developed fiber preparation methods.

These activities marked a turning point. The means now existed to take fiber-optic communication out of the physics laboratory and into the realm of mainstream engineering. In the course of the next decade, as research continued, optical fibers gained increasingly in transparency. By 1980, the best fibers were so transparent that a signal might pass through 150 miles of fiber before becoming too weak to detect. If the seas of the world were that clear, one could sail across the deepest parts of the Pacific and see the ocean floor as easily as the bottom of a swimming pool.

But optical fibers with this degree of transparency could not be prepared using traditional methods. The breakthrough came with the realization that pure silica glass, free of all light-absorbing metal impurities, could only be prepared directly from vapor components--thereby avoiding the contamination that inevitably accrued from the conventional use of melt-containing crucibles. Progress now centered on selecting the right balance of vapor components and on optimizing their reactions. The developing technology rested heavily on a knowledge of chemical thermodynamics, a science refined by three generations of chemists from its original espousal by Willard Gibbs in the 19th century.