Fluoride glass


Fluoride glass is a class of non-oxide optical glasses composed of fluorides of various metals. Due to their low viscosity, it is very difficult to completely avoid the occurrence of any crystallization while processing it through the glass transition.
Thus, although heavy metal fluoride glasses exhibit very low optical attenuation, they are not only difficult to manufacture, but are quite fragile, and have poor resistance to moisture and other environmental attacks.
Fluoride glasses' best attribute is that they lack the absorption band associated with the hydroxyl group which is present in nearly all oxide-based glasses.

Properties

HMFGs were initially slated for optical fiber applications, because the intrinsic losses of a mid-IR fiber could, in principle, be lower than those of silica fibers, which are only transparent to wavelengths shorter than ~2 μm.
Such low losses were never realized in practice, and the fragility and high cost of fluoride fibers made them less than ideal as primary candidates. Later, the utility of fluoride fibers for various other applications was discovered.
These applications include mid-IR spectroscopy, fiber-optic sensors, thermometry, and imaging. Also, fluoride fibers can be used for guided lightwave transmission in media such as YAG lasers at 2.9 μm, as required for medical applications.
An example of a heavy metal fluoride glass is the ZBLAN glass group, composed of zirconium, barium, lanthanum, aluminum, and sodium fluorides. These materials' main technological application is as optical waveguides in planar and fibre form. They are advantageous especially in mid infrared range.

Production

Some fluoride glasses are difficult to produce on Earth due to their rapid crystallization. The crystallization is slower in microgravity conditions due to reduced convection effects.
Fluoride glass may also mean an oxide optical glass doped with zirconium tetrafluoride.
A mix of fluoride glass and phosphate glass is fluorophosphate glass.
Fluoride glasses based on zirconium fluoride are a class of fluorozirconate glasses. ZBLAN glasses belong to this group as well.
Fluoride glasses based on aluminium fluoride are a class of fluoroaluminate glasses.
Doping praseodymium in fluoride glass allows it to be used as a single mode fiber optical amplifier.
Optical elements made of calcium fluoride, namely of fluorite crystals, are used in some telephoto lenses, to correct chromatic aberration. They are however being replaced with various low-dispersion glasses, which have higher refractive indexes, better dimensional stability, and lower fragility.