Fluorine incorporation in silica glass by the MCVD process: Study of fluorine incorporation zone, evaluation of optical properties and structure of the glass

A theoretical approach was made to find out a complete fluorine incorporation zone on a ternary diagram which serves as a useful graphical representation to select the flows of the supplied reagents for incorporation of the suitable amount of fluorine into cladding glass of optical fiber preform made by the MCVD process using CCl(2)F(2) as a source of fluorine under oxygen abundance, oxygen deficiency and intermediate oxygen state conditions. The possible mechanism for incorporation of fluorine into cladding glass of optical fiber is also evaluated on the basis of the thermodynamical data. The fluorine incorporation mechanism in silica glass by the MCVD process is found to be dependent on the CCl(2)F(2)/SiCl(4) ratio in the input gas mixture. Fluorine doping is found to be effective for removing the strained Si-O-Si bonds, which govern the optical transparency in deep ultra-violet (DUV) and vacuum Ultra-violet (VUV) regions. The maximum refractive index depression of -0.5 x 10(-3) is obtained with incorporation of fluorine into silica cladding glass by the MCVD process using CCl(2)F(2) as a dopant precursor with suitable flow of SiCl(4) vapor along with O(2) through backward deposition pass. The structure of fluorine doped silica glass preform samples containing 1.70-1.79 mol% fluorine incorporated by the MCVD process based on the analyses of (19)F MAS spectra done by high-resolution (19)F NMR spectroscopy reveal the presence of two distinct types of fluorine environments. The majority of the fluorine environments are formed in SiO(1.5)F polyhedral and less abundant species is observed to be highly unusual, yielding a fivefold coordinated silicon of the type SiO(2)F polyhedral which become increased with increasing the fluorine content. (C) 2008 Elsevier B.V. All rights reserved