The present invention relates to glass compositions based on BeF.sub.2 and particularly to ThF.sub.4 --BeF.sub.2 glasses exhibiting improved melting properties and an extended melting range, rendering them particularly suitable for infrared or u.v.-transmitting optical devices.
In recent years there has been considerable interest in fluoroberyllate glasses, primarily for applications in laser technology. Very low refractive index, dispersion power, and non-linear coefficients of refractive index are among the properties that are unique to this family of glasses. In addition, fluoroberyllate glasses possess better ultraviolet transmission than fused silica and are good hosts for fluorescing elements such as the rare earths. The primary drawbacks to these glasses are the toxicity of beryllium and the poor water durability of BeF.sub.2 -based compositions.
Besides laser applications, the low dispersion characteristics and long range infrared transparency of these glasses make them good candidates for ultra-low-loss optical waveguides. In a cladded optical waveguide structure the importance of core durability is minimal if the cladding can be formed of a durable glass.
Many possible variations of BeF.sub.2 compositions appear in the literature, but nearly all of these require relatively large amounts of a second or third component to achieve the necessary softening of the glass. Rawson, "Inorganic Glass-Forming Systems", Academic, New York (1967) pages 236-247, provides a good background description of the known BeF.sub.2 glass-forming systems, while U.S. Pat. No. 3,650,779 describes alkali-fluoroberyllate glass used as a host matrix for xenon.
In U.S. Pat. No. 3,958,970, MgF.sub.2 --PbF.sub.2 --BeF.sub.2 glasses are used to make fluorescing materials via additions of ytterbium fluoride, erbium fluoride and thulium fluoride. U.S. Pat. No. 4,142,986 discloses NaF--AlF.sub.3 --BeF.sub.2 glasses containing NaPO.sub.3 and doped with NdF.sub.3 to provide an infrared laser glass, while similarly doped glasses based in the BeF.sub.2 --AlF.sub.3 composition system are described in published French application No. 2,445,820.
The list of additives to BeF.sub.2 -based glasses has included, in addition to the modifying alkali and alkaline earth metal fluorides, lanthanide series elements, cerium, and thorium. U.S. Pat. Nos. 2,466,507 and 2,511,224 disclose complex optical glasses of this type.
As noted by Rawson, supra, BeF.sub.2 forms a glass by itself and has a structure which is analogous to fused silica. However, the melting characteristics of pure BeF.sub.2 are not well suited to the preparation of homogeneous glasses containing minor additions of modifying constituents, because of the high viscosity and short working range of BeF.sub.2 glass. In order to effectively mix BeF.sub.2 melts, particularly crucible melts, viscosities below 100 poises are needed. To achieve these viscosities in pure BeF.sub.2 melts ordinarily requires heating to temperatures above 970.degree. C., and at these temperatures the vapor pressure of BeF.sub.2 becomes appreciable. It is this characteristic which has necessitated the development of the complex BeF.sub.2 -containing glass compositions known in the prior art.
Phase equilibrium studies of the systems BeF.sub.2 --ThF.sub.4 and LiF--BeF.sub.2 --ThF.sub.4 are reported by R. E. Thoma et al. in J. Phys. Chem., 64 (July 1960) 865-870. However, Thoma et al. characterized the BeF.sub.2 melts as high in viscosity. No information is provided as to the clarity of the melts prepared for the study, nor as to any of the other material properties of the system.
In any glass composition field, it is advantageous to be able to obtain desired property modifications with as small a composition excursion as possible. In BeF.sub.2 glasses, small additions of alkali can be tolerated without destroying clarity; however, other additions such as PbF.sub.2 have a strong destabilizing effect and do not produce useful glasses. Both systems have been described by D. M. Ray et al., J. Am. Cer. Soc., 37(7) 300-305 (1954).
This consideration is particularly critical for optical applications such as optical fibers wherein substantial modification of optical constants such as refractive index must be achieved with very small quantities of additives, in order to avoid large shifts in physical properties and, most importantly, phase separation of any kind. Optical fibers provide extremely large optical path lengths wherein even the slightest crystallization or phase separation destroys utility.
A principal object of the present invention is to simplify the production of optical devices comprising BeF.sub.2 -based glasses through the use of glass compositions of improved stability and reduced viscosity.
It is a further object of the invention to adapt BeF.sub.2 -based glass compositions comprising only simple additions of a single viscosity-modifying constituent to the fabrication of BeF.sub.2 glass optical devices.
Other objects and advantages of the invention will become apparent from the following description.