JPS6121175B

PURPOSE:To obtain the titled multi-or single-mode preform, having stepwise refractive index distribution, extremely easily, by rotating a longitudinally separable mold, and pouring the molten fluoride glass for clad and the one for core successively into the cylindrical hollow part of the mold. CONSTITUTION:The outer frame 1 of the mold is composed of e.g. three pieces of longitudinally separable semimetallic or metallic members 2, 2' and 2'' which are tightly bound together by the ring 3 and closed with the bottom plate 4 at the bottom to form a hollow part 5. The mold 6 is heated at about 300 deg.C, fixed to the center axis of the rotating table 8 with chucks 7, 7' and rotated with the motor 10. The molten fluoride glass for clad is poured from a gold crucible into the hollow part 5. The mold is removed from the table, and the unsolidified glass at the central part of the hollow is poured out of the mold to leave a glass tube. The tube is fixed to the table, rotated, and without delay, a molten fluoride glass for core having high refractive index is poured into the cavity, and annealed for a long time. After annealing, the mold is cooled in the furnace to normal temperature, and the outer frame is removed from the mold to obtain the objective preform. As an alternative method, the mold is rotated at high speed to form the molten glass for clad in the form of tube by the centrifugal force, and the molten core glass is poured in the tube.