A fibre optical preform is produced using a recharged plasma. The process comprises the axial modulation of at least one pass of the recharged plasma for improvement of the opto-geometric properties of the preformer. By preference, the modulation is placed during latter passes of the recharged plasma and is effected by modulation of the flow rate of recharging grains. The dia. of the preformer is evaluated (6) and a recharging control device (8) regulates the flow rate of the grains (11,5). The preform and the fibre optic produced by the process are also claimed.