Fluoride glasses belong to the non-oxide glasses, made from compounds of fluorine with zirconium, for example (fluorozirconate glass), or with aluminum (fluoroaluminate glasses). They can also contain various other heavy metals like lead. There are also indium fluoride glasses (fluoroindate glasses) with improved infrared transmission at longer wavelengths.
Fluoride glasses are notoriously difficult to produce with high optical quality. They have a high tendency for partial crystallization during the glass transition, and the resulting microcrystallites can deteriorate the transparency. Further, they are difficult to use, since they are mechanically quite weak (fragile) and chemically not resistant to moisture, for example.
Fluoride and fluorophosphate glasses can be used as high-index low-dispersion glasses, e.g. for the construction of achromatic optics with particularly high performance. Due to the high cost of fluoride optics, however, its use for such purposes is limited.
A special property of heavy-metal fluoride glasses is their good transmission in the infrared spectral region. (This results from the low photon energies, which reduce the rates of multi-phonon transitions.) Therefore, mid-infrared fluoride fibers were once thought to be a good candidate for long-distance optical fiber communications: the intrinsic scattering losses could in principle be lower than those of silica fibers because of the longer operation wavelengths which would be possible with fluoride fibers. However, silica fibers have firmly occupied this application area for various reasons like fabrication cost, mechanical strength, available light sources and fiber amplifiers.
Among the fluorozirconate glasses for fibers, ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) is the most common. Such fibers can be doped with a number of rare earth ions for application in fiber lasers and amplifiers. Due to the high phonon energies, the lifetimes of various metastable electronic levels can be strongly increased compared with silica fibers, allowing the realization of certain lasers and amplifiers operating in the mid infrared. Also, there are fluoride-based upconversion lasers in the form bulk lasers or fiber lasers. Such fluoride glasses can be doped with thulium (Tm3+) or praseodymium (Pr3+), for example.
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