Delson Torikai

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 diAerent deposition conditions and their X-ray diAraction (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 aAected 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 ( 850∞C). The GeO2 is eliminated at the center of preform in reductive atmosphereOH2=O2 > 2U 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. ” 2000 Elsevier Science B.V. All rights reserved.

Effect of H2/O2 ratio on the GeO2 concentration profile in SiO2:GeO2 glass preforms prepared by vapor-phase axial deposition

GeO 2 radial concentration in silica glass preforms, prepared by vapor-phase axial deposition (VAD) method, was analyzed by X-ray fluorescence (XRF) measurements. The results were used to evaluate the effect of the H 2 /O 2 ratio used during the deposition process in the formation of the GeO 2 concentration profile. Considering four different H 2 /O 2 ratios, GeO 2 distribution was observed to decrease monotonically with the increasing radius for H 2 /O 2 ≤1.5, and the acute shape around the core center of the GeO 2 concentration profile, decreased with increasing H 2 /O 2 ratio. When the ratio H 2 /O 2 =2.5, the central doping of GeO 2 was minimal, and a constant distribution was obtained along glass preform radii. The results seem to indicate that the control of spatial distribution in the concentration of GeO 2 deposited is favorable for the ratio H 2 /O 2 ≤1.5.

VITREOUS SILICA PROCESSING BY VAPOR PHASE DEPOSITION FOR OPTICAL FIBER PREFORM

Abstract With the fast development of information technologies, the demand for high performance and low cost, for example in the production of optical fiber, is continuously increasing, specifically in the case of Brazil that depends on improved silica glass tube in the Modified Chemical Vapor Deposition process. In the case of Vapor-Phase Deposition (VPD) technique, deposition efficiency is very high with lower production cost in comparison with other processes. In the present work, we report some results on high purity vitreous silica processing by VPD technique, such as: efficiency as high as 80% for a burner with circular cross-section, deposition of soot with neutral-like network and randomly aggregated structure, good consolidation process by heat treatment in electrical furnace, at temperature up to 1400°C and controlled atmosphere and observation of a layer of bubbles arrested at the interface of two deposited spiral soot layers.

Comparison of high-purity H2/O2 and LPG/O2 flame-fused silica glasses from sol-gel silica powder

Abstract High-quality flame-fused silica glasses were obtained from high-purity starting sol-gel silica powders, under H 2 /O 2 and liquid propane gas (LPG)/O 2 flames. Neither metallic nor carbon contamination was observed during the fusion process, even by using domestic LPG. The flame-fused sol-gels showed lower impurity levels than silica glasses from natural quartz and of the same order as the commercial synthetic silica glasses (chemical vapor deposition process). By fusion with an LPG/O 2 flame, the aggregation of OH radicals during the fusion process was verified to be about 100 ppm lower with viscosity two times higher than that of silica glasses fused by H 2 /O 2 . The viscosity of this flame-fused sol-gel silica is lower than the flame-fused silica from rock crystal, but is one order of magnitude higher than that of the synthetic silica glass.

Effect of Heat Treatment on the High-Purity Silica Glass Prepared from Amorphous and Crystalline Powders by Small Angle X-Ray Scattering

High-purity silica glass prepared from high-purity amorphous and crystalline powders by flame fusion and sintering at high temperatures in vacuum, was heat-treated at 1250°C for several periods of time and its structure was studied using the small-angle X-ray scattering technique. As a result, the structure of silica glass was dependent on its thermal history. The structure of as-fused silica glass samples prepared by flame fusion appeared to be quite similar, even though raw materials with different degrees of homogeneity were used. However, the structure of silica glass behaved differently after heat treatment. This different response of the structure of silica glass to heat treatment was analyzed in terms of impurity content and viscosity.

ETCH CHANNEL ELIMINATION IN SYNTHETIC QUARTZ'BY ELECTRODIFFUSION PROCESS

Abstract The generation of channels in synthetic quartz by chemical etching is an undesirable effect for technological applications. These etch channels can be suppressed by electrodiffusion (swepping) processing of specific ions by the application of a high electric field at temperatures below 570°C. In the present paper we report the study on the operational parameters of the electrodiffusion process for the suppression of etch channels opened by the action of hydrofluoric acid in Z bars of synthetic quartz. The efficiency of inhibit etch channel formation was evaluated by measuring the density of such channels in samples treated in HF 48% solution for 24 h.

DEPENDENCE OF FLAME CHARACTERISTICS ON THE BUBBLE GENERATION AND HYDROXYL CONTENT IN SILICA GLASS

Abstract High quality silica glass was prepared by flame fusion Verneuil technique using two types of silica powders: purified natural quartz and synthetic sol-gel silica. Two different types of flames, GC2/O2 and LPG/O2, with various conditions of mixture ratios were used. Bubble generation and hydroxyl incorporation were analyzed in these as-fused materials. Silica glass prepared with purified natural quartz powder showed large concentration of bubbles in the most recently fused region. The bubbles were generated from the fluid and depending on the fusion condition it was possible to obtain bubble-free silica glass. A strong dependence of hydroxyl concentration on the flame mixture ratio was also observed.

Micron order structural inhomogeneities in vitreous silica observed by etching technique

Abstract Etching of various vitreous silica samples over a long period (∼ 200 min) with hydrofluoric acid revealed a high contrast configuration of micron to sub-millimeter order boundaries similar to the grain boundaries of crystalline materials. In order to better understand this new result, various parameters, such as the vitreous silica preparation processes, the type of basic insume, the grain size of silica powder, the surface roughness, and the OH-content, were studied. They should no correlation with the configuration of “amorphous-grain”. A possibility of an anistropic micro-strain configuration induced by heterogeneous distribution of defects and impurities in the vitreous silica structure is suggested to explain the appearance of such grain boundary-like contrast.

Paramagnetic centers in SiO2 glass

Abstract Electron Paramagnetic Resonance (EPR) of ten samples of SiO2 glass was studied at X-band and at room temperature. Spectra of the iron impurity in all these samples were observed even in those with Fe content below 0.06 ppm. The lines found with g-factors 6.5 and 3.7 were assigned to Fe2+ and with g = 2.1 to Fe3+. Sample prepared with addition of nicle at 50 ppm gave a single line at g = 2.219 due to Ni2+ ions in a cluster form. As the resonance of Fe3+ ions also might be interpreted as due to cluster formation, it is suggested that these transition metal ions are some part of the sample segregated in the surface of some grain.

Small-angle X-ray scattering of heat-treated flame-fused silica glass from amorphous and crystalline powders

High-quality as-fused silica glass, heat treated at 1523 K and prepared by the flame-fusion process using natural crystalline quartz and high-purity amorphous silica glass powders, was investigated by small-angle X-ray scattering (SAXS). The X-ray scattering intensity amplitude from the amorphous structure after heat treatment was analysed in terms of viscosity, density and metallic impurity content. It is shown that SAXS scattering of the amorphous sample is sensitive to the structural change induced by annealing.