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 d istribution profile of germanium. At the porous soot state, GeO2 may be dep osited as amorphous and/or crystalline phases in a SiO2-GeO2 system. To cor relate the quantitative deposition between amorphous and crystalline phases of GeO2, several silica-germania soot preforms were prepared under differe nt deposition conditions and their X-ray diffraction (XRD) patterns and X-r ay fluorescence spectra were analyzed. The presence of GeO2 crystalline (he xagonal) phase was detected in all soot samples, and its concentration was affected by deposition conditions, mainly the soot surface temperature and H-2/O-2 ratio in the name. The concentration of the crystalline phase is mi nimum 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 temperatur e (<700 degrees C) in comparison to the one at the center (>850 degrees C). The GeO2 is eliminated at the center of preform in reductive atmosphere (H -2/O-2 > 2) and higher temperature (T similar to 1000 degrees C), independe ntly of SiCl4/GeCl4 ratio. These results are important to understand the ma in parameters of the doping process, to produce high quality materials with a given germanium concentration profile. (C) 2000 Elsevier Science B.V. Al l rights reserved.