Reduction of impurities in fluoride glass fibers

In order to realize a low-loss fluoride Optical fiber, impurity reduction is indispensable. The methods of recrystallization, extraction, chemical vapor-phase deposition, distillation, and sublimation were tried to purify the starting materials for a fluoride fiber. These purification methods were evaluated by measuring transmission loss of the fibers prepared by using the purified materials. It was found that recrystallization was useful for reducing impurities in ZrF
4
to the level of below 1 ppm. The extraction technique could be also applicable to purification of BaF
2
and GdF
3
. It was concluded that the sublimation was the most probable purification in order to prepare fluoride materials with less than 0.1-ppm impurities. As a result, a low-loss fiber with 8.5 dB/km at 2.12 μm was obtained by using the starting materials purified by sublimation.