Nanoscale characterization of silica soots and aluminium solution doping in optical fibre fabrication

Visual and microanalytical studies have been made of the nanostructures and mechanisms involved in aluminium solution doping of silica soot generated by Modified Chemical Vapour Deposition used in the fabrication of silica optical fibres. Three aspects were studied – the nature of the soot particles, the structures in thermophoretically deposited soot layers and the way aluminium bonds to the surface of the soot particles. Soot particles were shown to be non-crystalline using both X-ray diffraction and electron diffraction. Particle size distributions, particle morphologies and deposited soot layer structures were characterised by electron microscopy and microprobe analysis. Atomic distributions of aluminium on surfaces of soot particles soaked in aluminium chloride solution were studied by high resolution transmission electron microscopy. The surface of solution doped particles was shown to have high aluminium content and nanoscale crystalline regions were identified amongst the mainly amorphous material. Soot layer structures and porosity were examined by electron and optical microscopy and distribution of the aluminium impregnation was mapped across the thickness of the deposited soot layer. Heterogeneous porosity distributions were observed and the effect of different types of pores, holes and cavities in the silica soot layer are discussed with regard to efficiency of deposition and solution doping.