Multiple solution-doping in optical fibre fabrication I – Aluminium doping

This work is part of a study of solution-doping in Modified Chemical Vapour Deposition, as used in fabrication of specialty silica optical fibres. The present paper concentrates on aluminium solution-doping and the effects of introducing additional heat-treatment stages into the process. The behaviours of the aluminium salt when heat-treated independently and after deposition in the silica soot have been studied by X-ray diffractometry, thermo-gravimetric analysis with mass spectrometry, electron microscopy and microanalysis. It has been shown that a procedure involving multiple cycles of solution-soaking and heat-treatment of the soot layer increases the level of aluminium doping obtainable. Efficiency of aluminium incorporation has been measured in the soaked soot, the sintered glass layer and in the final collapsed preform. An interpretive model of the multiple cycle soak/heat-treatment process is suggested.