Multiple solution-doping in optical fibre fabrication II – Rare-earth and aluminium co-doping

This work is part of a study of solution-doping in Modified Chemical Vapour Deposition, as used for fabrication of specialty silica optical fibres. It has been shown in an earlier paper that multiple soakings of a porous silica soot layer with intermediate heat-treatment stages between soakings can be used to introduce high levels of aluminium into an optical fibre core. The present paper demonstrates that the same technique can be applied to doping a fibre core with the rare-earth erbium and to co-doping aluminium and erbium simultaneously. Dopant levels have been tracked through the various stages, from soaked soot layer to the sintered glass layer to the collapsed preform. Experimental data are presented for both erbium soaking and co-doping with aluminium and erbium. The role of aluminium in preventing rare-earth segregation is confirmed.