CASTING MOLD FOR PRODUCING PREFORM FOR OPTICAL FIBER AND PRODUCTION OF PREFORM FOR OPTICAL FIBER BY USING SAID CASTING MOLD

摘要

PURPOSE:To easily produce a preform for an optical fiber having high quality by casting a clad glass melt then core glass melt successively into the hollow part of a casting mold whose bottom part is formed to a large diameter. CONSTITUTION:The diameter of the hollow part 1A of the casting mold 1 having the hollow part 1A for forming the preform for the optical fiber is made larger in the bottom part than at the open end to provide a clad glass melt pool 5. The clad glass melt 2 obtd. by heating and melting the clad glass in a gold crucible 3 is cast into the hollow part 1A of the casting mold 1. The core glass melt 4 is poured and cast from the gold crucible 3 into the central cavity formed when the melt 2 solidifies from the casting mold 1 surface. The two melts 2, 4 are slowly cooled to obtain the preform for the optical fiber. The above-mentioned preform decreases the absorption loss of an OH impurity and has the high quantity to obviate the crystal deposition at the boundary face between the core and the clad.