Method of fabricating optical fiber preforms

In a method of fabricating an optical fiber preform by inserting a silicate glass rod into a silicate glass tube and collapsing the tube by heating the resulting assembly, the improvement which comprises, before the tube is collapsed, flowing a glass surface treating agent and oxygen gas through the clearance between the rod and the tube while the assembly is heated at a high temperature to produce substances which effect removal of foreign material and cure of surface defects on the surfaces of said rod and tube;

• said glass surface treating agent:
• (a) satisfying the following criteria:
• (i) the hydrogen content thereof being not more than about 1% by weight, and (ii) the substances produced therefrom in the presence of oxygen gas at the high temperature having a boiling point or sublimation point of not more than the temperature required for collapsing the tube so that the substances are substantially removed from the clearance between the surfaces of the rod and the tube before or during collapsing of the tube, (b) being employed in an amount of at least 0.1 part by volume per 100 parts by volume of oxygen gas, and (c) containing at least one element selected from the group consisting of elements capable of forming a glass network forming oxide, elements capable of forming a glass modifying oxide, halogens and sulfur.