1.  Field of the Invention
This invention relates to a new transparent oxyfluoride glass ceramic composition and to a method for making said composition.  The glass composition of the present invention is subject to a wide range of applications but is especially suited for optical devices, including but not limited to planar and fiber configurations thereof, providing optical amplification in the 1.3 micron and 1.55 micron telecommunication windows, lasers, 3-D displays, and various wavelength up-conversion applications. 
2.  Description of Related Art
Traditional oxide glasses conventionally used in optical amplifier applications, for example, derive their light amplifying properties via stimulated emission of rare earth (RE) element ions doped in the glass.  Both the fluorescence lifetime of the dopant ions and the dopant density largely affect the light amplification properties of the glass.  The maximum useable concentration of rare earth ions in a glass host, however, is limited by concentration quenching.  Concentration quenching is due to the nonradiative decay of excited rare earth ions which are too close to their neighboring unexcited rare earth ions.  Excited rare earth ions will transfer energy to closely neighboring nonexcited rare earth ions through dipole-dipole coupling, resulting in phonon, instead of photon, transitions.  Increasing the concentration of rare earth ions in the glass host causes clustering of the rare earth ions with neighboring rare earth ions which reduces the fluorescence lifetime of the rare earth energy levels.  Excited levels can also be quenched by phonon coupling to the host matrix.  For example, the .sup.1 G.sub.4 level of Pr.sup.3+ is severely quenched in high phonon energy glasses like silicates; but in low phonon energy hosts such as halide and chalcogenide glasses and crystals, emission from the .sup.1 G.sub.4 level of Pr.sup.3+ can be used for 1.3 .mu.m amplification. 
Fluoride glasses and crystals are desirable hosts for optically active ions because of their broad transparency, low phonon energy, and high rare earth solubility.  They are, however, difficult to process because they require a controlled atmosphere for batching and melting.  Moreover, fluoride glasses are difficult to draw into fiber and cannot be used for large optics due to instability of the glass.  Single crystal fluorides produce narrow fluorescence line widths, but cannot be fiberized and are costly to produce. 
Conventional oxyfluoride glass ceramics offer economic and performance alternatives with substantial improvements over pure oxide and fluoride glasses.  For example, they can be melted in ambient atmospheres and processed by conventional glass forming methods.  They provide the durability and mechanical properties of oxide glasses while offering a fluoride environment for rare earth dopants.  In addition, there exists the possibility of orienting the crystals by plastic deformation of the body after ceramming for polarizing effects.  However, limited rare earth solubility and clustering continue to be problematic in current oxyfluoride glass ceramic compositions. 
The inventor has thus recognized a need for a glass composition that provides the stability and workability of traditional oxide glass compositions, the performance characteristics offered by fluoride glass compositions, and which accommodates high rare earth doping concentrations.