PREFORM FOR FLUORIDE GLASS FIBER AND ITS PRODUCTION

PURPOSE:To suppress degradation of mechanical strength and generation of a scattering loss by infiltration of moisture into a glass rod contg. 15-25% NaF by coating said glass rod with Ge-Se glass contg. 10-20% Ge and removing the moisture on the rod surface by the plasma in a vacuum vessel. CONSTITUTION:A core part and clad part are constituted of the glass having the compsn. contg. ZrF4, BaF2, NaF, LaF3, and AlF3 and 15-25mol% NaF. Since this glass is drawn at 315-340 deg.C, the coating material is constituted of the Ge-Se glass contg. 10-20mol% Ge is order to maintain the viscosity of the coating material for the glass at 10 -10 Poise. Gaseous halogen 30 introduced into the vacuum vessel 25 is subjected to high-frequency heating 29 and the moisture sticking to the glass rod 26 surface is removed by the generated radicals so that the Ge-Se glass evaporated in a crucible 32 is uniformly deposited by evaporation on the rod 26 surface. The infiltration of the moisture to the glass rod is, therefore, completely prevented, by which the scattering loss at the time of drawing and the degradation in the mechanical strength and absorption loss of the fiber are suppressed.