Crystalline and amorphous phases of GeO2 in VAD silica–germania soot preform

In germanium doped optical fiber, produced by vapor-phase axial deposition (VAD) technique, the refractive index profile is controlled by the radial distribution profile of germanium. At the porous soot state, GeO2 may be deposited as amorphous and/or crystalline phases in a SiO2–GeO2 system. To correlate the quantitative deposition between amorphous and crystalline phases of GeO2, several silica–germania soot preforms were prepared under different deposition conditions and their X-ray diffraction (XRD) patterns and X-ray fluorescence spectra were analyzed. The presence of GeO2 crystalline (hexagonal) phase was detected in all soot samples, and its concentration was affected by deposition conditions, mainly the soot surface temperature and H2/O2 ratio in the flame. The concentration of the crystalline phase is minimum at the center of the silica–germania soot preform and increases along the radial direction. The greater concentrations at the outer diameter of the silica–germania soot preform are due to the lower deposition temperature (<700°C) in comparison to the one at the center (>850°C). The GeO2 is eliminated at the center of preform in reductive atmosphere (H2/O2>2) and higher temperature (T ∼ 1000°C), independently of SiCl4/GeCl4 ratio. These results are important to understand the main parameters of the doping process, to produce high quality materials with a given germanium concentration profile.