Chlorination and vaporization of GeO2 component in SiO2:GeO2 porous preform in dehydration process by VAD method and spontaneous formation of SiO2 cladding layer during the dehydration by selective vaporization of GeO2

Abstract In order to prepare silica-based graded index fibers of uniform transmission quality and low OH content by the vapor phase axial deposition (VAD) method, the halogenation rates of various types of GeO 2 as a function of the molar ratio of oxygen to chlorine and of temperature were measured. As a result, the behavior of both the hexagonal GeO 2 and the solid solution GeO 2 in the dehydration and consolidation processes has become clear, that is, the solid solution GeO 2 having SiOGe bonds which is formed in the high temperature region is very stable in the dehydration and consolidation processes. On the other hand, the crystalline hexagonal GeO 2 which is formed in the low temperature region is unstable for chlorination reaction in the dehydration process and for volatilization in the consolidation process. This effect has been applied for OH content, and bandwidth control of fibers and VAD fibers of low OH content with losses less than 1 dB/km at 1.39 μm without any cladding deposition process and also those of highly uniform bandwidth fluctuations within ± 100 MHz km in the longitudinal direction of the fibers were obtained.