A new hybridized process for preparing optical fibers has been developed. The process consists of overcladding a VAD-derived core rod with commercial silica powder by cold isostatic pressing (CIP) technique. Single-mode optical fibers were fabricated by the process which used the core rod having a ratio of core to cladding radius of 3 and silica powder of average particle diameter of 10μm. The losses were 0.36dB/km at 1.3μm and 0.23dB/km at 1.55μm. The points for the process are following; (1) Silica powder was granulated by spray dryer from slurry containing poly-vinyl-alcohol (PVA) to be packed uniformly into a rubber mold for CIP forming. (2) To prevent the core rod from breaking in the compression, a rubber mold with a structure in which both ends of the core rod were pressed by hydrostatic pressure was used. This CIP process provided the hybridized soot preform which consisted of the core rod and a porous body around it from silica powder in one step. (3) It was very important that 10μm silica powder was used because it was easy to purify into high-purity glass. (4) To remove PVA which was added in granulating, heat treatment was carried out at 500°C in an atmosphere of dry air. The heat-treated porous body containing 25ppm carbon was consolidated into the overcladding glass for low-loss optical fibers. Further, a relation between the pore diameter and purification of the porous body was studied. As a result, pores with more than 0.2μm diameter were presumed to be required for making high-purity glass by chlorine purification.
