Fluoride glass and optical fiber fabrication

Fluoride glasses have emerged as a class of special glasses with unique properties and applications. It includes several families according to the main glass former. Major groups are fluorozirconates, fluoroaluminates, and fluoroindates. They are oxygen free and they must be processed in a very dry environment. Chemical composition is adjusted to minimize devitrification. Their softening temperature lies between 250 °C and 400 °C and their viscosity at liquidus temperature is very low. They are continuously transparent from the UV to the mid infrared spectrum. Their phonon energy is small by comparison to oxide glasses, which makes them attractive host for active rare earths. The major interest of fluoride glasses is based on optical fibers. They are currently drawn from preforms. Depending on applications, they are multimode, singlemode or polarization maintaining. Astronomy programs use them for coupling telescopes. Optical losses are lower than 20 dB/km in the 1–3.5 μm spectral range with a minimum attenuation of 1 dB/km at 2.55 μm. They may be used over tens of meters. While the development of ultra-low optical losses (< 0.1 dB/km) is still a matter of technology, various applications take place for optical transmission, remote spectroscopy, laser power handling and infrared imaging. Rare earth doped fibers are used in mid-IR fiber lasers and also for visible lasers. Reliable supercontinuum sources are available up to 4–5 μm. UV supercontinuum has been demonstrated with photonic crystal fibers.