SOL-GEL-DERIVED SILICA PREFORMS FOR OPTICAL FIBERS

Sol-gel fabrication of high-silica glass preforms for ultralow-loss optical fibers is reviewed, mainly on the basis of the works of the author. Inherent difficulties in the sol-gel process such as bubble formation during fiber drawing, incorporation of dopants, and formation of a refractive index profile were investigated. Bubbling occurred when the pore-closing temperature of dry gels was lower than a suitable value. Increasing the particle size in a sol solution was remarkably useful in suppressing reboil from dehydrated silica glasses. Using gel-derived fluorine-doped silica glass tubes, a refractive index profile was formed by the out-diffusion technique. Gel-derived optical fibers of 0.43 dB/km optical loss have been successfully fabricated.