STUDIES OF THE CHEMICAL-KINETICS ON THE VAPOR-PHASE AXIAL DEPOSITION METHOD

In order to prepare silica-based graded index fibers having a very wide transmission bandwidth by the vapor phase axial deposition (VAD) method, the deposition rates of the oxides of Si or Ge as a function of the molar ratio of the halide to water vapor, temperature and the molar ratio of an additional gas to the halide were examined. The evaluated apparent activation energies for the deposition reaction of SiCl4, GeCl4, BBr3 and POCl3 with water vapor were 30, 30, 5 and 50 kcal/mol, respectively. On the other hand, the estimated activation energies of SiCl4 and GeCl4 reactants with oxygen were 100 and 60 kcal/mol, respectively. It has been found that the reaction of GeCl4, which plays an important role in making the refractive index profile of an optical fiber preform, is suppressed by H-2 and Cl-2 in both the oxidation and hydrolysis reactions. This effect was applied to the control of the refractive index profile in the VAD soot process, The achieved attenuation and 6 dB bandwidth were 2.5 dB/km at 0.85 mu m and 1.4 GHz km, respectively.