Full opinion text
OPINION AND ORDER WILLIAM C. CONNER, District Judge: These are two consolidated civil actions involving issues of infringement and validity of three patents relating to optical waveguides of the type now widely used for telecommunications, such as long-distance telephone transmissions. The actions were tried by the Court without a jury commencing June 1, 1987. This Opinion incorporates the Court’s findings of fact and conclusions of law pursuant to Rule 52(a), F.R.Civ.P. NATURE OF THE ACTION AND THE PARTIES In these two actions, Corning Glass Works (“Corning”) charges infringement by Sumitomo Electric Research Triangle, Inc. (“SERT”), Sumitomo Electric U.S.A., Inc. (“SEUSA”) and Sumitomo Electric Industries, Ltd. (“SEI”) of U.S. patents 3,659,915 (“the ’915 patent”), 3,884,550 (“the ’550 patent”) and 3,933,454 (“the ’454 patent”). The ’915 and ’550 patents are product patents covering the structure and composition of optical waveguide fibers. The ’454 patent covers a process for producing such fibers. Corning, a New York corporation with its headquarters in Corning, New York, owns the three patents in suit as assignee of the inventors. SEI is a Japanese corporation engaged, inter alia, in the manufacture and sale of optical waveguide fiber. SERT and SEUSA are wholly-owned subsidiaries of SEI. SERT, a North Carolina corporation having its principal place of business at Research Triangle Park, North Carolina, also manufactures and sells optical waveguide fiber. SEUSA, a New York corporation having its principal place of business in New York City, sells optical waveguide fiber manufactured by SEI and SERT. SERT, SEUSA and SEI are hereinafter referred to collectively as “Sumitomo.” The first complaint in these consolidated actions was filed by SERT on August 16, 1984 in the United States District Court for the Middle District of North Carolina. On April 19, 1985, that action was transferred to this District. By stipulation, SEI has been added as a plaintiff in that action. That complaint sought, inter alia, a declaratory judgment that Coming’s ’915 and ’454 patents are invalid, unenforceable and not infringed by SERT. In its answer, Corning counterclaimed for willful infringement of those patents by SERT. On December 19, 1984, Corning filed in this Court the second complaint in these actions, seeking damages and an injunction against SEUSA and SEI for their allegedly willful infringement of the ’915, ’550 and ’454 patents. Five different types of optical waveguide fibers made, used or sold by Sumitomo in the United States are in issue in this litigation. Three of these are single-mode optical waveguide fibers — designated S-l (known as type D within Sumitomo), S-2 (type D’ within Sumitomo) and S-3 (type Z within Sumitomo) — and two are multimode graded-index fibers — designated M-l (type A within Sumitomo) and M-2 (type C’ within Sumitomo). Each of these includes a core of circular cross-section and an outer cladding, both formed of fused silica, with the refractive index difference between the core and cladding controlled through the addition of dopant material to the core and/or the cladding. SEI’s type S-l, S-2, S-3, M-l and M-2 fibers are manufactured by SEI at its plant in Yokohama, Japan and exported to the United States. SERT has made types S-2, S-3 and M-2 optical waveguide fiber at its plant in Research Triangle Park, North Carolina. SEUSA sells all of these fibers in the United States. CORNING’S ’915 PATENT Coming’s '915 patent, entitled “Fused Silica Optical Waveguide,” was issued May 2, 1972 on an application filed May 11, 1970 by Drs. Robert D. Maurer and Peter C. Schultz. It contains eight claims; however, only claims 1 and 2 are asserted in these actions. Background of the Invention Light is a form of electromagnetic radiation, a narrow segment of the continuum extending from radio waves at the low-frequency (long wavelength) end of the electromagnetic spectrum through gamma rays near the upper end. Only those light waves in the even narrower wavelength range from about 400 nanometers (0.4 microns) to about 750 nanometers (0.75 microns) are visible to the eye. Although visible light propagates through air with very little loss, in transparent solids like glass, it is transmitted with less efficiency than invisible infrared radiation in the range of about 750 to 1600 nanometers (0.75 to 1.6 microns). In silica glass, for example, there are two infrared wavelength “windows,” centered at 1300 and 1550 nanometers, in which the attenuation or loss in transmission is particularly low. These wavelengths are accordingly used for transmission through silica, as in the optical fibers with which we are here concerned. It has long been known that light can be guided through any transparent medium which is surrounded by another medium of lower refractive index, i.e., that light will follow the path of the medium of higher refractive index. Because air has a lower refractive index than glass, an unclad glass fiber surrounded by air will act as a conduit for light waves. However, such “air-clad” glass fibers are very inefficient as optical waveguides because scratches, imperfections or foreign materials on the surface of the fiber cause the light to be scattered instead of being refracted properly into the fiber. Thus, in the 1950s, the idea emerged of cladding an optical glass fiber with a different glass having a lower index of refraction. These early glass-clad, glass-core fibers were generally referred to as “fiber optics.” Such an optical fiber acts as a waveguide for light because light rays that enter the cladding from the core at less than a critical angle relative to the axis of the fiber are refracted back into the core and thus “bounce” back and forth in a zig-zag, somewhat sinusoidal path along the length of the fiber. Light rays which enter the cladding at angles greater than the critical angle pass through the cladding and are lost. If the diameter of the core is sufficiently large (e.g., ten or more times the wavelength of the light being transmitted), light rays may enter the core over a fairly wide range of angles and still be propagated along the fiber provided, of course, they enter at less than the critical angle. Those rays or “modes” entering at shallower angles relative to the axis of the fiber will “bounce” back and forth across the core fewer times than those which are at steeper angles and thus will arrive sooner at the receiving end of the fiber. If the fiber is being used to transmit information, for example in the form of binary data pulses, this difference in transit times will cause the pulses to be dispersed or blurred at the receiving end. It follows that restricting the number of modes in the transmitted light increases intelligibility of the information transmission, the optimum being achieved when only a single mode is transmitted. This is accomplished by limiting the diameter of the core and carefully controlling the differential between the refractive indices of the core and cladding. The critical angle, conventionally expressed as the “numerical aperture,” of an optical waveguide fiber depends not only upon the diameter of the core but also its refractive index, which in turn depends upon its composition. The light rays or “modes” which enter the core at less than the critical angle and are refracted back from cladding into the core and thus proceed along the core, penetrate only the innermost portion of the cladding at each “bounce” or refraction so that 90 to 95 per cent of their travel path is in the core. In most optical waveguide fibers, the composition of the cladding, and consequently its index of refraction, are constant throughout the thickness of the cladding. This is not necessarily true of the core of multi-mode fibers in which the core is of relatively large diameter in relation to the wavelength of light being transmitted. In such fibers, the core may advantageously be made with an index of refraction varying across its diameter, being highest at the center of the core and decreasing gradually toward the outer surface. Such fibers are called “graded-index” optical waveguides. Prior to 1970, optical fibers had been used to transmit light for illumination or as elements of an optical image, e.g., in endoscopic probes. Such conventional optical fibers or “fiber optics” were capable of transmitting light of practical intensity only for very short distances, such as several meters, because of the poor transmission efficiency (high attenuation) of the fibers then available. Optical waveguide fibers and conventional optical fibers are designed very differently. In conventional optical fibers used for illumination, the diameter of the core is very large, relatively, to increase the amount of light captured from the input light source, and the refractive index difference between the core and the cladding is very great to confine most of the light energy to the core. The light transmission' efficiency of the cladding is therefore relatively unimportant. In contrast, in an optical waveguide fiber, the core must be of relatively much smaller diameter, for example, only on the order of five to ten microns for single-mode transmission. Also, in an optical waveguide fiber, since a greater portion of the light path is through the cladding, the transmission efficiency of the cladding is very important. In use, a light source, either a laser or a light emitting ■ diode, is coupled with the optical waveguide fiber so that the light generated in response to an electrical signal is directed into the core at the transmitting end. The light which travels along the fiber is captured by a detector at the receiving end, which converts it back to an electrical signal. When optical waveguide fibers are used for telephone communications, for example, a single optical waveguide fiber of an overall diameter of 125 microns (5 one-thousandths of an inch) can carry over 1,000 simultaneous voice transmissions and can replace a conventional copper cable of a diameter of greater than 5 centimeters (2 inches). In the early conventional fiber optics, mixed silicate' glasses — typically containing 50% to 70% silica, along with other oxides— were generally used, although non-silicate glasses, such as germanate glasses, were also tried. Most of these glasses could be worked at temperatures of 1500°C. or lower and were well suited for drawing into fibers. Their high attenuation, however, made them hopelessly unsuitable for use in a lightwave communication system. ' Attenuation, or light loss, is expressed in units of decibels per kilometer (dB/km). Decibels are computed on a logarithmic scale, as 10 times the power to the base 10 of the ratio between the input and output light energy. Typical attenuations for these conventional fiber optics were on the order of 1000 decibels per kilometer (1000 dB/km). This means that over a distance of one kilometer, the input energy would be ten to the one hundredth power times as great as the output energy. Dr. Theodore Maiman, while working at the Hughes Research Laboratories, developed the first practical laser in 1960. The laser generates monochromatic, coherent light, with all of the energy at one precise wavelength, and with all of the lightwaves in phase. This energy can be almost perfectly focused to a fine spot the size of the wavelength of the light. The light can be propagated in an almost parallel beam, with very little dispersion. The invention of the laser sparked interest in lightwave communication systems as an ideal source of light for information transmission. Still, the attenuation of the fiber optics then available was many trillions of times too great for practical application. By the mid-1960s, efforts were under way around the world to develop a long-distance lightwave transmission capability. This effort was backed in part by the British Post Office, whose goal was an optical waveguide with an attenuation of 20 dB/km, meaning that over a distance of one kilometer, it must deliver at the receiving end at least one percent of the input light energy — the approximate transmission efficiency of the copper wire commonly used in telephone communications. In January 1966, Mr. Stewart Miller of Bell Laboratories wrote: “Today there are probably more physicists and engineers working on the problem of adapting the laser for use in communication than on any other single project in the field of laser applications.” At Bell Laboratories, about 50 engineers and physicists were involved in laser research, and 6 to 10 were involved in research on transmission media. Dr. Charles Kao of ITT's Standard Telecommunications Laboratories (“STL”) in England, began working on optical communications in about 1963, as did Thompson CSF in France. In July 1966, Kao and Hockham of STL published a paper discussing the feasibility of a long-distance optical waveguide fiber. The paper identified some of the problems to be overcome in attaining a practical optical waveguide fiber, e.g., absorption losses due to impurities such as iron in the glass and scattering losses caused by core/cladding interface imperfections. However, this paper offered no practical solutions to the problems it posed. Subsequently, a number of companies conducted research in an attempt to overcome the problems identified in that article. This was done under the sponsorship of the British Post Office and included companies such as Barr and Stroud, Pilkington Brothers (a major glass-making company in England), and British Titan Products. In November 1970, STL was still carrying out work on the use of double-crucible techniques for melting sodium silicate glasses for the production of optical waveguides. However, the results were far from successful. The minutes of the British Low Loss Optical Fiber Committee Meeting of February 24, 1971 state that “the fiber work seems sadly to have stuck at a loss of about 100 dB/km with the exception of the Corning work.” Bell Laboratories meanwhile was working on other types of light-transmitting media including a gaseous lens system. In this system, light was transmitted down a hollow tube filled with an inert gas. Heating the wall of the tube caused a radial temperature gradient, correspondingly altering the refractive index of the gas to form a converging lens. An experimental test of the system was carried out over a length of about 100 meters with these gaseous lenses spaced about one meter apart. Bell also experimented with hard lenses mounted inside a gas-tight pipe and aligned by servo-mechanisms. This system was tested in an underground conduit system over a distance of approximately one-third mile. These systems of light-focusing gas and glass lenses worked, but were too costly and difficult to maintain for practical application. Bell Laboratories also did work on optical fiber waveguides, because it was clear that a glass fiber waveguide would be the practical ideal if the losses could be reduced sufficiently. Bell contacted glass companies, including American Optical and Bausch & Lomb and asked if they were willing to collaborate in an effort to make acceptable glass fibers, but both declined. In the mid-1960s, Bell obtained some sample glass fibers from a third company, De-Belle & Richardson. These fibers were made of multicomponent glass and had high attenuations, so Bell set up its own program to attempt to make acceptable glass fibers. During the 1960s, Sumitomo also engaged in research on gas lens systems and on liquid cores for optical waveguide fibers. Dr. Kapany of Sumitomo was simultaneously engaged in research on glass-clad glass-core optical waveguides and by 1970 had produced fibers with attenuations in the range of hundreds of decibels per kilometer (with losses at least a hundred thousand times as great as the goal of 20 dB/km). Efforts to reduce the attenuation of these fibers were unsuccessful. By late 1970, the 20dB/km standard appeared remote and perhaps impossible to attain. The Invention of the ’915 Patent Coming's work on optical waveguides began in 1966, when it was contacted by the British Post Office. Responsibility for the basic research fell to Dr. Maurer, who decided to investigate the properties of and to evaluate various Corning glasses for possible waveguide use. At the time, Corning manufactured, among many other glasses, a pure fused silica (SÍO2) glass. This glass was made by a flame hydrolysis process such as is described in Hyde U.S. patent 2,272,342. Corning also manufactured a silica glass doped with about 7% by weight of titania (Ti02). This glass, disclosed in Nordberg U.S. patent 2,326,059, had a very low thermal expansion coefficient and was used for structural applications such as reflecting telescope mirrors, where superi- or dimensional stability was required, but not for light transmission. Dr. Maurer was initially skeptical about the possibility of pure silica and titania-doped silica glasses for use in optical waveguide fibers for a number of reasons, including the fact that they required higher working temperature than conventional silicate glasses then in use in fiber optics (2000°C. v. 1000°C.). There was also little knowledge of their optical properties, such as light scattering and absorption. Moreover, the use of doped silica seemed contraindicated because doping fused silica would have been expected to increase its attenuation to an unacceptable degree. Indeed, in view of the Kao and Hockham paper teaching the need to reduce impurities in glasses such as fused silica to one part per million, the deliberate addition of thousands of parts per million of a dopant material to pure fused silica would have appeared counterproductive. Despite these negative indications, Dr. Maurer on March 1, 1967 directed the making and testing of a fiber with a titania-doped fused silica core and a pure fused silica cladding. During 1967, two such fibers were made by the rod-in-tube method, in which a rod of titania-doped fused silica glass was placed inside a tube of pure fused silica cladding glass for drawing into a fiber. Dr. Schultz joined Corning in August 1967, and shortly thereafter became involved in Coming’s optical waveguide fiber research. He was principally responsible for identifying possible doped fused silica glasses, containing both titania and other dopants, and the fabrication of optical waveguide fiber preforms containing such doped silica glasses. He also co-developed a method of making such fibers by flame hydrolysis deposition of doped fused silica “soot” on the inside of a pure fused silica tube, which was subsequently drawn into an optical waveguide fiber in which the doped silica soot was sintered to form the core, and the pure fused silica tube formed the cladding. In February 1967, Dr. Schultz attempted to produce an optical waveguide fiber by depositing flame hydrolysis-produced pure fused silica soot on the outer surface of a titania-doped fused silica core rod. This effort continued until April 1968, when he attempted to coat the inner surface of a pure fused silica tube with a titania-doped fused silica soot produced by flame hydrolysis. Work with these materials continued, and, on August 1, 1968, a titania-doped fused silica fiber drawn from a preform produced by Dr. Schultz was measured to have an attenuation of approximately 250 dB/km. This marked the production of the first doped fused silica optical waveguide fiber with losses sufficiently low for short-range communication uses and constituted an actual reduction to practice of the invention of the '915 patent. Throughout the following year and a half, Corning experienced steady and significant improvement, leading to the fabrication by early 1970 of the world’s first 20 dB/km optical waveguide fiber — a fiber with a pure fused silica cladding and a doped fused silica core containing approximately 3% by weight titania. On May 11, 1970, the application was filed for the ’915 patent on the invention of a silica-based optical waveguide fiber containing fused silica to which a dopant had been added. In the fall of 1970, Dr. Maurer took one of these fibers to Bell Laboratories and asked Bell to confirm Coming’s loss measurements. Bell Laboratories did so and determined the loss to be about 16 or 17 dB per kilometer. Bell Laboratories considered Coming’s achievement an important breakthrough which made long-distance optical telecommunications possible. Corning did not tell Bell how the fibers had been made and Bell and others were unable to duplicate them. Dr. Maurer first publicly reported the achievement of a 20 dB/km optical waveguide fiber, the goal originally set by the British Post Office, at the Conference on Trunk Telecommunications by Guided Waves, held in London, England, from September 29 to October 2, 1970. This announcement created enormous interest and was the subject of many articles in both technical publications and general interest media. For example, the July 5, 1971 issue of Electronics carried an article entitled “Fiber Optics Sharpens Focus on Laser Communications,” stating (at p. 47): As recently as last fall, attenuation all but eliminated fiber optics from consideration as a transmission medium. Writing in the proceedings of the IEEE last October, Nilo Lindgren of Technology Communication Inc., New York, asserted: “At the present time, the glass used in fiber optics is very lossy, amounting to a decibel per meter at the very best. In actuality, with present glasses, the losses would amount to thousands of decibels per mile, which makes the material clearly unsuitable for long-distance transmission.” But by the next month, Robert D. Maurer, manager of the Applied Physics Research Group at Corning Glass Works, Corning, New York, reported two 30-me-ter sections of fiber optic waveguide with a total attenuation of 20 dB/km at the 6,328-angstrom wavelength. Several fibers were loaned to researchers at the British Post Office, London, and supplied to Bell Laboratories, Murray Hill, New Jersey. They confirmed Maurer’s measurements. This was the breakthrough many communications systems designers were waiting for. Another article, entitled “Communicating on a Beam of Light,” published in the March 1973 issue of Fortune, reported the development as follows: A few laboratories here and abroad maintained an interest in trying to improve the fibers, but for five years the movement was so slow and the required degree of perfection so elusive that the task seemed hopeless. Breaching the 20-decibel barrier Late in 1970, Corning Glass announced the laboratory development of an optical fiber in which the light loss was reduced to 20 decibels per kilometer or less. At this critical level glass fibers could begin to be considered competitive with metal wires, cables, and microwave relay. Similar articles appeared in other publications, including the November 1973 issue of The Radio and Television Engineer, Dr. Kapany, who is commonly referred to as the “father of fiber optics,” and who testified at the trial as an expert for Sumitomo, acknowledged that Drs. Maurer and Schultz provided the first optical waveguide fiber that was practical for use in long-distance communications, and that this was a step forward in the art which made possible something commercially desirable that had been unattainable theretofore. The invention caused many prestigious awards and honors to be accorded the inventors: In 1976, Dr. Maurer received the George W. Morey Award of the American Ceramic Society. In 1977, Dr. Schultz was the recipient of the first International Glass Science Weyl Award. Dr. Maurer was awarded the Prize for Industrial Physics of the American Institute of Physics and the Morris N. Liebmann Award of the Institute of Electrical and Electronic Engineers both in 1978. Dr. Maurer was also awarded the L.M. Ericsson International Prize for Telecommunications in 1979. Dr. Schultz was a recipient of the 1981 International Society for Optical Engineering Technology Achievement Award. In addition, Drs. Maurer and Schultz were co-recipients in 1983 of the Engineering Materials Achievement Award of the American Society for Metals. In 1986, Dr. Maurer was the recipient of the I.R.I. Achievement Award, bestowed by the Industrial Research Institute for his “contributions to the understanding and discovery of materials and techniques for the fabrication of glass-fiber waveguides for optical communications.” The citation went on to state: In less than four years from the start of the research, he was able to produce fiber with optical losses low enough to be considered acceptable for wide use in telecommunications. This pioneering research made possible the optical communications revolution. And, early this year, Dr. Maurer became the first recipient of the John Tyndall Award of the Lasers and Electro-Optics Society of IEEE and the Optical Society of America. The invention of the ’915 patent has achieved impressive commercial success, literally creating a worldwide multimillion dollar optical waveguide fiber industry. By 1986, Coming’s own annual sales of such fibers had grown to over [* * *] kilometers. The respect accorded this basic invention is further reflected by the number of licensees under the ’915 patent and its foreign counterparts. These licensees include, for example, ITT (now CIT Alcatel), SpecTran Corporation and Northern Telecom. Through the end of 1986, Corning has received in excess of [* * *] in royalties and other payments on its optical waveguide fiber patents. [* * *] by ITT, in settlement of litigation. The United States Government, after litigation, entered into an agreement with Corning setting a rate of compensation to be paid to Corning for the Government’s procurement and/or use of optical waveguide fiber covered by Corning patents, including the ’915 patent. The initial compensation rate is 6.5%, declining to 5%. The Government paid $650,000 when the agreement was signed in 1983. After two years of litigation, another manufacturer of optical waveguide fiber and cable, Valtec, consented in 1984 to entry of a judgment that the ’915 patent is valid, enforceable, and has been infringed by Valtec. Finally, the ’915 patent was held valid and infringed by Sumitomo optical waveguide fiber, in a proceeding before the United States International Trade Commission. The '915 patent was found to be a pioneer patent. The ’915 patent clearly covers a basic, pioneering invention. The ’915 Patent and its Disclosure 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 a fiber having a fused silica core and a fused silica cladding, to either or both of which a dopant or dopants have been added to make the index of refraction of the core greater than that of the cladding by a predetermined percentage. Fused silica is made by the process disclosed in Hyde U.S. patent 2,272,342 to yield a vitreous silica containing no impurities in an amount greater than 0.1% by weight except for hydrogen-oxygen groups, which may be present in amounts up to 5% by weight. The dopant or do-pants which are intentionally added are not considered impurities. Prior to the filing date of the application for the ’915 patent, the inventors had only experimented with dopant materials which increased the refractive index of fused silica. Thus, the specification of the ’915 patent only specifically mentioned such dopant materials, although the concept of the invention is clearly broad enough to include the use of dopants which decrease the index of refraction of silica to achieve the necessary differential between core and cladding. The inventors simply did not know of specific dopants that would decrease the refractive index of fused silica at the time the application was filed. It has been known in the art at least since 1954 that the introduction of fluorine decreases the index of refraction of certain multicomponent glasses. No teaching in the specification of the ’915 patent excludes the use in the cladding of dopant materials which negatively alter the refractive index of fused silica. Nor is there any suggestion in the specification that such dopant materials would not perform substantially the same function, in substantially the same way, to obtain the same result of a precise refractive index difference between core and cladding as that obtained through the use of dopant materials which positively alter the refractive index of the core. The ’915 patent discusses two methods by which doped fused silica optical waveguide fibers can be made, namely, the rod-in-tube method and the inside vapor deposition method described in U.S. patent 3,711,-262, which is specifically incorporated by reference in the ’915 patent. Flame hydrolysis is one inside vapor deposition method described in U.S. patent 3,711,262. The flame hydrolysis method for making optical waveguide fiber described in the specific examples of the ’915 patent and U.S. patent 3,711,262 is the method utilized by the inventors for reducing to practice the invention of the ’915 patent and for the subsequent production of optical waveguide fibers with even lower loss. The ’915 patent contains teachings adequate to enable one of ordinary skill in the art to produce a doped fused silica optical waveguide fiber, as was recognized by Sumitomo’s reference to the ’915 patent at column 1, lines 16 through 20, of its own U.S. patent 3,877,-912, filed October 9, 1973. Prior to May 1970, Corning also made fibers containing alumina-doped fused silica and zirconia-doped fused silica, but the results were not as good as those achieved with titania. The first fiber with which the inventions had achieved a 20 dB/km loss contained approximately 3% by weight titania in the core. However, they had encountered difficulties with the 3% titania core composition, because of variations in composition inherent in the production process. Attempts to dope with such low concentrations of titania sometimes led to a failure to obtain sufficient dopant in the fused silica core to create the refractive index differential necessary for the fiber to function as a waveguide. Coming’s most consistent and reproducible results in the production of optical waveguide fibers prior to May 1970 were achieved primarily with a core of fused silica doped with 5.25% titania. In teaching others how to practice their invention, the inventors believed that it was best to disclose this most consistently performing formulation as the preferred embodiment. The specification of the '915 patent lists examples of materials which the inventors believed to be suitable dopants. The examples specifically identified include titanium oxide, tantalum oxide, tin oxide, niobium oxide, zirconium oxide, ytterbium oxide, lanthanum oxide, aluminum oxide, cesium and rubidium. That this listing was not intended to be exclusive is apparent from the use of language such as that appearing at lines 23 and 24 of Column 4: “Suitable dopants having minimum diffusion properties include, for example....” The materials specifically listed in column 4 included all those actually used by the inventors prior to the filing of the application for the ’915 patent. Much of the work of Dr. Schultz prior to 1970, in addition to that directed specifically to the making of optical waveguide fibers, involved the exploration of glass systems based on fused silica. This work involved the making of fused silica glasses with various dopants and the analysis and testing of the glasses formed. This work, combined with the experience gained in the making of optical waveguide fibers, formed the basis for the selection of the materials specifically listed in the '915 patent. All of the suggested dopant materials listed in column 4 of the ’915 patent are suitable for use in optical waveguide fibers. Germanium oxide, or germania (Ge02) is not expressly mentioned as a dopant in column 4 of the ’915 patent, only because it had not been tested prior to the filing date of the application for the ’915 patent. Dr. Schultz had produced a germania-doped fused silica in bulk form by a direct vitrification process, in which the germania-silica soot was deposited into a highly heated furnace, causing the germania to volatilize. Thus only a small amount of germania (less than one tenth of one percent by weight) remained in the silica — an amount too small to alter the refractive index of fused silica sufficiently to serve as the core of an optical waveguide fiber with a pure fused silica cladding. Germania is a dopant material which positively alters the refractive index of fused silica without unacceptable absorption or scattering of light. The ’915 patent and U.S. patent 3,711,262, incorporated by reference therein, teach the deposition of doped fused silica into an unheated tube at room temperature, followed by a sintering step at temperatures below those at which germania will volatilize. The ’915 patent and U.S. patent 3,711,262 therefore teach a method for practical production of a fused silica doped with useful amounts of germa-nia. Thus, if either the specific example of the ’915 patent, at column 4, line 60 through column 5, line 7, or the specific example of the incorporated U.S. patent 3,711,262, at column 7, lines 16 through 54, were followed, substituting germania for the titania used in each, each would produce a usable optical waveguide fiber. Indeed, in 1972, when Dr. Schultz first began experimenting with the preparation of silica fibers doped with germania, such fibers demonstrated their usefulness as optical waveguides. And, by mid-1972, a doped fused silica optical waveguide fiber containing approximately 9% germania by weight in the core had been made with an attenuation of only 4 dB/km. Germania is a do-pant material within the scope of the ’915 patent, as is germania in combination with phosphorus pentoxide. With regard to the amount of dopant to be used with fused silica in an optical waveguide fiber, the ’915 patent specification states that: To make certain that doped fused silica possesses optical and physical characteristics almost identical to those of pure fused silica, doping materials should not exceed 15 percent by weight. The patent does not teach that an optical fiber containing more than 15% by weight of dopant material will not function as a waveguide, but only that there are advantages to be gained from limiting dopant levels to 15% or lower. That teaching was based upon the experimental observations of the inventors and was intended to assist in obtaining optimum results. Prior to the filing date of the application for the ’915 patent, the inventors had produced no commercially useful fiber containing more than 15% by weight of dopant material. Only one fiber with more than 15% by weight of dopant had been made — a fused silica fiber doped with approximately 37% by weight of alumina (AI2O3). While this fiber did exhibit single-mode light transmission, its loss was measured to be approximately 450 dB/km, far worse than the attenuation of the fiber by which the invention was first reduced to practice in August 1968. Moreover, work performed by Dr. Schultz on bulk alumina-doped silica glasses suggested that glass containing high amounts of alumina would not be useful for optical waveguide applications. This was consistent with the thinking in the art at the time. U.K. patent application 2,029,400, filed in 1979 by SEI and Nippon Telegraph and Telephone Public Corporation and listing SEI employee Hoshikawa as a co-inventor, discusses the advantages of maintaining germania dopant levels below about 15% by weight. For example, page 1, lines 75 through 78, states that: Since scattering loss increases in proportion to the amount of the additives, the amount of additives such as Ge02, P205 and B203 in the core must be small in order to reduce scattering loss. It is also noted on page 1, at lines 98 through 100, that too high an amount of germania in the core leads to a narrower transmission bandwidth. Moreover, SEI notes on page 2, lines 12 through 30, that high concentrations of germania increase the probability of generating bubbles in the fiber during drawing: Various experiments have shown, Ge02 amount should be less than 15wt%, in order to ensure that the generation of bubbles is negligible and low transmission loss is obtained. The Prosecution of the ’915 Patent During the prosecution of the application for the ’915 patent, the examiner cited three references, Flam et al. U.S. patent 3,542,536, Koester et al. U.S. patent 3,445,-785 and Seitz U.S. patent 3,553,013. Only the Flam patent was applied against the ’915 application. During prosecution, Corning distinguished the invention of the ’915 patent from the Flam optical waveguide, wherein the refractive index of the base material is altered by neutron irradiation, which causes structural dislocation of the atoms or molecules of the base material. Coming contrasted the invention of the ’915 patent as “chemical doping.” Allowance of the claims of the ’915 patent followed two telephone interviews between the examiner and Coming’s patent attorney. As a result of those interviews, two changes were made in claim 1. The first, which appears in two places in the claim, calls for the doped fused silica to be “fused silica to which a dopant material on at least an elemental basis has been added.” This change, specifying that a chemical material in a form no smaller than an element be added to the fused silica base material, was made to distinguish the Flam patent, which discloses irradiation by subatomic particles. The second change made to claim 1 was to add a limitation on the amount of dopant material in the core to no more than 15% by weight. Why this addition came to be made is unclear because it is not called for by the examiner’s prior arguments for rejection, to wit, that the Flam patent disclosed an optical waveguide formed by “doping” by irradiation. There is no indication in the prosecution history of the ’915 patent that Corning in any way represented the 15% limitation in claim 1 to be critical. The Art Relied Upon By Sumitomo (1) The United Kingdom ’101 Patent U.S. Patent Specification 1,113,101 (the “U.K. ’101 patent”), published May 8, 1968, discloses luminescent glasses. It does not disclose or suggest an optical waveguide. Photoluminescent fibers made in accordance with the U.K. ’101 patent would absorb radiation and convert the energy to luminescence. Of course, light absorption by the core is precisely what must be avoided in an optical waveguide fiber. However, Sumitomo contends that the fibers of the U.K. '101 patent do not absorb visible light but ultraviolet radiation, and therefore would be capable of light transmission. However, there is no evidence establishing the attenuation of such fibers. Sumitomo attempts to minimize the significance of the fact that luminescence was the prime object of the U.K. ’101 patent by pointing out that the glass of the ’915 patent will also luminesce if excited at certain wavelengths. But these wavelengths are well outside the range of the light being transmitted. Although the U.K. ’101 patent teaches that luminescent fibers made in accordance with the patent may be enclosed within a sheath of fused silica, the patent does not teach the use of a dopant for the purpose of creating a controlled refractive index differential between core and cladding. Sumitomo asserts that the inclusion of 5-5000 parts per million of rare earth elements in silica glass, as taught by the U.K. ’101 patent would in fact produce sufficient refractive index differential to cause the fiber to function as an optical waveguide. However, this was in no way suggested by the patent, and was entirely foreign to, and possibly inconsistent with, its objective of a luminescent fiber. Sumitomo further contends that the structural elements set forth in claim 26 of the U.K. ’101 patent are identical with those recited in claim 1 of the ’915 patent, which Sumitomo argues Corning effectively admitted when it disclaimed the structure of claim 26 in order to obtain allowance of Coming’s U.K. application corresponding to the ’915 patent. Thus, Sumito-mo reasons, the ’915 invention constitutes nothing more than the discovery of a new use for an old product. But there is no persuasive evidence that any fiber disclosed by the U.K. ’101 patent has ever been or could be used for practical telecommunications. Despite claim 26, the U.K. ’101 patent did not teach the art the solution to the problem which was solved by the ’915 invention, nor was it even addressed to that problem. (2) The Kao Paper The article “Dielectric-Fibre Surface Waveguides for Optical Frequencies,” by Kao and Hockham (the “Kao paper”), published in July 1966 in the Proceedings of the IEEE, Vol. 113, No. 7, suggested a dielectric fiber with a refractive index higher than its surroundings as a possible medium for optical communication. The Kao paper teaches that the light losses in a dielectric waveguide fiber are caused by absorption and scattering in the fiber and the surrounding medium, and that such losses must be low. Although the Kao paper concluded that cladded glass fibers were possibly usable as optical waveguides, it expressly recognized that the feasibility of waveguides depended on the availability of suitable low-loss materials, and that this was a “crucial” and “difficult” materials problem which was yet to be solved. (3) The Flam Patent Flam et al. U.S. patent 3,542,536 (the “Flam patent”) for a “Method of Forming Optical Waveguide by Irradiation of Dielectric Material,” issued November 24, 1970, on an application filed September 1, 1967 was cited and considered by the examiner during the prosecution of the ’915 patent. The effective date of the Flam patent as a reference is September 1,1967, subsequent to the date of conception of the ’915 patent. Flam relates to a method of fabricating an optical waveguide by irradiating a block of a solid dielectric material, such as silica, with a beam of protons to change the distribution of silicon and oxygen and thereby form a region with a different refractive index. This in no way suggests the invention of the ’915 patent in which a dopant — a physical substance — is added to fused silica to alter its refractive index. There is no evidence which suggests that the method of the Flam patent could be used in fabrication of practical optical waveguides kilometers in length. (4)The Nordberg Patent Nordberg U.S. patent 2,326,059 (the “Nordberg patent”) for “Glass Having an Expansion Coefficient Lower Than That of Silica,” was issued on August 3, 1943, on an application filed April 22, 1939. Nord-berg disclosed glasses having low expansion coefficients and had as its primary object the provision of a glass having a coefficient of expansion less than that of fused silica. Another object was to produce an “opal” (non-transparent) glass. The Nordberg patent was disclosed to the U.S. Patent Office by Corning in connection with the prosecution of the ’915 patent. The ’915 patent, in column 4, lines 4-12 specifically incorporates by reference the copending U.S. patent application Serial No. 36,267, which later issued as U.S. patent 3,711,262. The Nordberg patent is referred to at column 5, lines 60-65 of U.S. patent 3,711,262. Nordberg does not relate to optical waveguides nor to any other kind of fiber. It merely discloses titania-doped fused silica and the method of making such a glass, for use as a structural material with a very low coefficient of expansion. There was no suggestion of any advantages in optical transmission. A skilled art worker in the optical waveguide field in the late 1960s who read Nord-berg’s patent would be led away from using his material in optical waveguides because he stated that a secondary object of his invention was to produce an opal glass. The opacity of such glass results from scattering of light, which renders the glass unsuitable for light transmission, as taught in the Kao paper. While the Nordberg patent teaches that titania-doped fused silica glass may be “transparent,” the fact that it may also be opaque would cause one skilled in the art to question whether such glass possessed the virtually perfect transparency which the Kao paper taught was necessary for long-distance optical transmission. (5) The Mattmuller Patent Mattmuller U.S. patent 3,334,982 (the “Mattmuller patent”) for the “Manufacture of Silica Glass,” was issued August 8,1967, on an application filed January 30,1962. It discloses a method for the manufacture of silica glass by the decomposition of silicon halide vapors in the flame of an oxyhydro-gen blowpipe and the direct vitrification of the resulting silica. The Mattmuller patent does not relate to or discuss optical fibers, nor does the patent indicate that the material produced by the process of the patent is at all usable for its optical properties. Although it was referred to in the invention disclosure prepared by Dr. Schultz for the invention of the ’915 patent, the Mattmuller patent was referred only for its disclosure of the vapor deposition method. A worker of ordinary skill in the optical waveguide field in the 1960s would have been discouraged from utilizing the fused silica glasses of the Mattmuller patent for an optical waveguide fiber because such materials were highly refractory, requiring extremely high temperatures for working. Although the Mattmuller patent speaks of achieving “transparent” glasses, it does not teach that it is possible to achieve the extremely high transparency necessary for long-distance telecommunications. (6) The Hyde Patent Hyde U.S. patent 2,272,342 (the “Hyde patent”) for a “Method of Making a Transparent Article of Silica,” was issued February 10,1942, on an application filed August 27, 1934. It had as its object the production of articles of fused silica at relatively low temperatures and, if desired, of a high degree of purity. It discloses the “flame hydrolysis” method for producing a “soot” which is sintered to form fused silica. It does not teach that this method can be used to form optical waveguides for long-distance telecommunications. The Hyde patent was disclosed to the U.S. Patent Office by Coming in connection with the prosecution of the ’915 patent. The '915 patent, in column 4, lines 4-12 specifically incorporates by reference co-pending U.S. patent application Serial No. 36,267, which later issued as U.S. patent 3,711,262. The Hyde patent was referred to at column 5, lines 60-64 of U.S. patent 3,711,262. (7) The Koester Patent Koester U.S. patent 3,445,785 (the “Koes-ter patent”) for “Laser Systems and the Like Employing Solid Laser Components and Light-Absorbing Claddings,” was issued May 20, 1969, on an application filed August 5, 1963. It relates to laser components comprising selectively absorbing claddings which assertedly provide enhanced laser operating efficiencies. It does not discuss optical fibers of any kind. Indeed, the highly absorbing dopants it discloses would be altogether unsuitable for use in optical waveguides. The Koester patent was cited and considered by the examiner during the prosecution of the ’915 patent. (8) The Seitz Patent Seitz U.S. patent 3,533,013 (the “Seitz patent”) for an “Optical Maser Having Means for Concentrating the Pumping Light Energy in the Central Portion Thereof,” was issued October 6, 1970, on an application filed March 23, 1967. It discloses a laser design wherein a doped laser material is enclosed within an “exteriorly silvered generator,” which focuses the laser pumping energy along the central laser axis. The patent does not relate to optical fibers of any kind. The effective date of the Seitz patent as a reference is March 23, 1967, which is subsequent to the conception date of the '915 patent; nevertheless, the Seitz patent was cited and considered by the examiner during the prosecution of the ’915 patent. (9) The United Kingdom ’535 Patent U.K. patent 1,160,535 (the “U.K. ’535 patent”) for “Dielectric Fibers” was published on August 6, 1969. It describes fiber optics formed of silicate glasses, i.e., multicomponent glasses containing between 20% and 65% silica, together with numerous other materials; these materials are altogether different from doped fused silica. Moreover, the effective date of the U.K. ’535 patent as a reference is August 6,1969 which is subsequent to the dates of conception and actual reduction to practice of the invention of the ’915 patent. Fiber optic glasses of the type disclosed in the U.K. ’535 patent were widely known as fiber materials; these were the “high quality optical glasses” referred to in the Kao paper as being among the “best transparent materials known,” at that time. It was the high attenuation of such conventional fiber optics that forced those seeking a useful optical waveguide to search elsewhere. (10) The United Kingdom ’509 Patent U.K. patent 1,108,509 (“the U.K. ’509 patent”) was published April 3, 1968. It describes multi-component germanate glasses containing 35-62% germania. Sum-itomo does not assert that any such glasses could be used in optical waveguide fibers for effective telecommunications, but relies upon the U.K. ’509 patent only as teaching the use of certain of the oxides referred to in the ’915 patent specification as dopants to increase the refractive index of a fiber core. However, the ’915 patent does not claim doping per se, but the controlled doping of a pure fused silica glass to create the desired refractive index differential. The U.K. ’509 patent teaches nothing about the doping of pure fused silica. (11) The Schultz patent Schultz U.S. Patent 3,320,114 (the “Schultz patent”) for “method for Lowering Index of Refraction of Glass Surfaces” was issued May 16, 1967. It is directed to a method of making an optical fiber by treating the exterior of a silica fiber with a dopant which lowers the refractive index of the outer portion of the fiber. Schultz’s criterion of success was whether the fiber was capable of transmitting light at all. There is no evidence that any such fiber was ever tested to determine whether its attenuation was sufficiently low to permit its use for practical telecommunications, and no evidence suggesting that such a fiber could be so used. And, of course, the fibers of Schulz are structurally different from those with distinct core and cladding layers as called for in the ’915 patent claims. (12) The United Kingdom ’953 Patent United Kingdom patent 1,152,953 (the “U.K. ’953 patent”), published May 21, 1969, discloses an optical fiber with a fused silica core and a synthetic plastic cladding of lower refractive index. Again, there is no evidence that any such fiber could be used for practical telecommunications. And not only does it differ structurally from the silica-clad silica-core fiber claimed in the ’915 patent, it actually teaches away from that construction, stating at page 1, lines 60-68: It is also impossible to fuse on to a quartz glass core a glass of lower refractive index, firstly because a glass of sufficient low refractive index, apart from the fluorine glasses which are completely unsuitable for use for this purpose, formerly did not exist and secondly because the fusing would not be possible on account of the great difference in the coefficients of expansion. (13) Contract Proposal RIO/37 This contract proposal, authored by Dr. H.T. Roettgers and Dr. Kao of STL, was submitted to certain British Government groups in December 1967. Sumitomo asserts that “between 50 and 100 persons had access to the document, all of whom were knowledgeable in the art.” Apparently Sumitomo refers to the members of the Signals Research and Development Establishment of the British Government and to the Low Loss Optical Fibre Committee of the British Post Office. However, there is no evidence respecting the extent to which it was available to members of the general public. Sumito-mo’s statement that Corning must have been informed of the proposal because it had been in contact with the British Post Office since 1966 is pure speculation and such knowledge was denied by the Corning personnel who would have been informed about it. In any event, the proposal is nothing more than a suggestion of research to be undertaken at STL in an attempt to develop a low-loss optical waveguide fiber. Although a coated fiber was one of three possibilities STL proposed to investigate, there was no disclosure of any specific fiber, but merely a suggestion that a quartz (silica) fiber might be coated with one of “a fair number of materials, namely fluorinated compounds whose refractive indexes can be considerably smaller than quartz.” The proposed project was apparently carried out by STL, but there is no evidence that STL succeeded in developing optical waveguide fibers capable of practical long-distance telecommunications. Of $ Hf. * & * person of ordinary skill in the waveguide the late 1960s was a person having a degree in materials science, ceramics, physics or a similar field and familiar with the concepts of light transmission, material scattering and turbidity and the effect of composition on refractive index. He was also familiar with the phenomenon of glass transition and with devitrification, phase separation and cooling stresses in glasses. The invention claimed in claims 1 and 2 of the ’915 patent was not obvious to such persons at the time in view of any of the patents or publications relied on by Sumito-mo, considered singly or in any combination. Sumitomo’s Other Affirmative Defenses (1) Adequacy of Disclosure Sumitomo contends that the ’915 fails to contain sufficient disclosure to enable one skilled in the art to practice the invention, as required by 35 U.S.C. § 112. Specifically Sumitomo asserts that there is “no teaching in the ’915 patent regarding structural parameters such as core diameter, refractive index difference or mode control of waveguides.” On the contrary, the patent specification at column 4 line 60 to column 5, line 7 gives a specific example of a waveguide having a core diameter of approximately 3 microns and an overall diameter of approximately 100 microns, with refractive indices of 1.466 for the core and 1.4584 for the cladding, and describes in detail an inside vapor deposition process of making such a fiber. This disclosure was sufficient to permit one skilled in the art to practice the invention without undue experimentation. See Minerals Separation Ltd. v. Hyde, 242 U.S. 261, 270-71, 37 S.Ct. 82, 86, 61 L.Ed. 286 (1916); Lindemann Maschinenfabrik v. American Hoist & Derrick, 730 F.2d 1452, 1463 (Fed.Cir.1984). (2) Disclosure of Best Mode Sumitomo asserts that in Coming’s experimental work prior to the filing of the application for the ’915 patent, the only fiber (“1-94”) which exhibited an attenuation below 20 dB/km contained only 3% titania dopant in the core, whereas fibers doped with 5.25% titania, as disclosed in the ’915 patent, had an “attenuation of about 80 dB/km.” Thus, Sumitomo urges, the ’915 patent fails to satisfy the requirement of 35 U.S.C. § 112 that the inventors disclose the best mode known to them for the practice of the invention. Sumitomo has ignored the results of other Corning tests indicating that the inventors were unable to duplicate the loss attenuation of fiber 1-94 with other fibers doped with 3% titania, but were able, with reasonable consistency, to achieve attenua-tions much lower than 80 dB/km with fibers doped with 5.25% titania by weight. Their choice of the 5.25% doping level as the preferred example in the ’915 patent specification was therefore reasonable in the circumstances. If the inventors had chosen a 3%-doped fiber instead, Sumitomo would doubtless be complaining that the example was based on a “freak” experiment which the inventors had never been able to duplicate up to the time the application was filed. The fact that up to that time the inventors had not achieved 20 dB/km attenuation with fibers doped with 5.25% titania is of no significance. The ’915 patent does not discuss any specific attenuation level as characterizing the invention. (3) Criticality of the 15% Doping Limit Sumitomo further contends that the '915 patent is invalid or unenforceable because the 15% limitation in the amount of dopant in the core, which was added to claim 1 by amendment, is not critical, as shown by the ’550 patent issued to the same inventors. It is undisputed that at the time the application for the ’915 patent was filed, the inventors believed that fibers containing more than 15% dopant in the core were not practical as, indeed, they specifically stated at column 3, lines 56-59 of the specification. The fact that this assumption later proved to be incorrect, at least insofar as germania dopant is concerned, does not invalidate the ’915 patent or render it unenforceable. The patent does not teach that the 15% limitation is critical, and no such representation was made to the Patent Office during prosecution of the patent application, nor was the limitation relied on in distinguishing any prior art. Claim 1 was patentable without the limitation, but nothing prevents inventors from claiming less than they were entitled to claim. In comparable circumstances, in W.L. Gore & Associates, Inc. v. Garlock, Inc., 721 F.2d 1540, 1556 (Fed.Cir.1983), the Court of Appeals for the Federal Circuit stated: Garlock’s appeal argument that the ’390 claims are invalid because the recited minimum matrix tensile strengths are not “critical” is without merit. A claim to a new product is not legally required to include critical limitations. In re Miller, 441 F.2d 689, 169 USPQ 597, 602 (CCPA 1971). The '390 claims are not drawn to optimization of ingredients or ranges within broad prior art teachings, but to new porous PTFE products of particular characteristics. Corning is not attempting to recapture the coverage which was surrendered by this amendment by contending that the ’915 patent is infringed by fibers containing more than 15% dopant by weight in the core. Sumitomo thus stands only to benefit from the unnecessary restriction of the ’915 patent coverage. (4) Disclosure of Pertinent Art Sumitomo further contends that the ’915 patent is unenforceable because the inventors failed to disclose to the Patent Office pertinent prior art known to them, including the Nordberg, Hyde and Mattmuller U.S. patents, and a number of their own internal disclosure memoranda. However, as previously mentioned, both Nordberg and Hyde were indirectly called to the attention of the Patent Office by being referred to in U.S. Patent 3,711,262, the application for which was incorporated by reference in the ’915 patent. The remaining items of prior art which Sumitomo charges were withheld from the Patent Office are no more relevant than the art that was cited and considered by the Patent Office during prosecution of the application for the ’915 patent. Infringement by Sumitomo’s Optical Waveguide Fibers Corning charges infringement of only Claims 1 and 2 of the ’915 patent, which read as follows: 1. An optical waveguide comprising a cladding layer formed of a material selected from the group consisting of pure fused silica and fused silica to which a dopant material on at least an elemental basis has been added, and a core formed of fused silica to which a dopant material on at least an elemental basis has been added to a degree in excess of that of the cladding layer so that the index of refraction thereof is of a value greater than the index of refraction of said cladding layer, said core being formed of at least 85 percent by weight of fused silica and an effective am