Crystallization in large volume samples of fluoride glass.

The attainment of long distance ultra-low-loss optical transmission (~10 sup (-2) dB/km at 2-2.5microns) in heavy metal fluoride glass fibers is currently precluded by scattering losses induced by crystallization which occurs during the glass preparation or subsequent fiber fabrication. In this study we investigated the crystallization behavior of a ZrF sub 4 - BaF sub 2 - LaF sub 3 - AlF sub 3 - NaF glass, currently viewed as the principal candidate for fluoride fiber fabrication. Samples with comparable dimensions to "state of the art" preforms (~10mm dia x 150mm long) were selected as the most appropriate for study. The techniques of large volume, scanning calorimetry and thermal gradient annealing were found to be most suitable for our investigations and preliminary results indicate that serious problems of phase instability and crystallization occur at temperatures very close to the glass transition and well within the "safe working range" previously assigned for this glass.