Method of purifying silica and depositing on an optical fiber preform

A method of depositing natural or synthetic silica on an optical fiber preform, in which a substantially cylindrical preform (1) extending in a longitudinal direction (L) is set into rotation (7) about its axis in front of a plasma or flame (4) coming from a heat energy supply source (3) which moves back and forth substantially parallel to the longitudinal direction (L) of the preform (1), and in which a feed duct (9) feeds the plasma or the flame with grains of natural or synthetic silica (11), characterized in that the same feed duct (9) feeds the plasma or flame with a fluorine or chlorine compound mixed with a carrier gas (15), wherein the flourine or chlorine compound is combined with the grains of natural or synthetic silica in the feed duct by means of an additional feed duct intersecting the wall of feed duct (9) prior to the flourine or chlorine compound and the grains of natural or synthetic silica exiting the feed duct (9), the feed conditions of said duct (9) being adjusted to cause alkali or alkaline-earth elements contained in the grains (11) of natural or synthetic silica to react with the fluorine or the chlorine of the fluorine or chlorine compound.