By the mid-1960's, worldwide efforts were ongoing to develop long-distance lightwave transmission capability. In particular, the British Post Office sought an optical waveguide with an attenuation of 20 db/km, the approximate transmission efficiency of the copper wire commonly used in telephone communications.
Corning's work on optical waveguides began in 1966, when it was contacted by the British Post Office. Drs. Robert D. Maurer and Peter C. Schultz, working at Corning, developed the world's first 20 db/km optical waveguide fiber by early 1970. That achievement was due, in part, to the development of a fiber with a pure fused silica cladding and a fused silica core containing approximately three percent by weight of titania as the dopant in the core. It was also due to the careful selection of the core diameter and the RI differential between the core and the cladding.
Dopants are chemicals added to another material (here, fused silica) to alter one or more of its properties (here, the RI). The effect of the titania was to increase the RI of the core.
Bell Laboratories confirmed the attenuation measurements of Corning's fibers and considered Corning's achievement an important breakthrough, making long-distance optical telecommunications possible. Dr. Maurer first publicly reported the achievement of a 20 db/km optical waveguide fiber at the Conference on Trunk Telecommunications by Guided Waves held in London, England. That announcement created enormous interest and was the subject of many articles in both technical and general publications. The inventors' advancement in technology won them accolades from various societies and institutes, for which they were presented with many prestigious awards and honors. In addition, the invention of the '915 patent has achieved impressive commercial success on a worldwide basis. The district court determined that "[t]he 915 patent clearly covers a basic, pioneering invention." 671 F. Supp. at 1377, 5 USPQ2d at 1551.
The '915 patent discloses a fused silica optical waveguide fiber capable of limiting the transmitted light to preselected modes for use in optical communication systems. Specifically, such a fiber is disclosed as having a doped fused silica core and a fused silica cladding (doping optional), wherein the RI of the core is greater than that of the cladding. Prior to the filing date of the application for the '915 patent, the inventors had experimented with dopants which increased the RI of fused silica, e.g. titania, and the '915 specification mentions only such positive dopant materials. At the time the application was filed, the inventors did not know of specific dopants that would decrease the RI of fused silica, although it had been known in the art since 1954 that the introduction of fluorine decreases the RI of certain multicomponent glasses.
Corning's titania-doped fibers required heat treatment to reduce attenuation to an acceptable level. An undesirable result of that treatment was a lowering of the mechanical strength of the fibers. Consequently, Corning sought to develop a low attenuation fiber which did not require heat treatment. In 1972, Drs. Maurer and Schultz found a solution in doping a fused silica core with germania, which also had the advantage of transmitting more light than using titania.
Corning recognized that when optical waveguide fibers were produced by flame hydrolysis, they contained hydroxyl ions. The residual hydroxyl ions absorbed light at certain wavelengths used in optical communications and, if they remained, would increase the attenuation of the fiber at those wavelengths. Working at Corning, Dr. Robert D. DeLuca invented a process to overcome this inherent problem by introducing a chlorine-containing drying atmosphere into the furnace during the "consolidation" phase.