Optical fibre fabrication

Claim 1

In a rod-in-tube method of manufacturing an optical fibre including the steps of positioning a glass rod preform within a glass sleeve tube which is disposed with its longitudinal axis vertical; feeding the glass rod preform and the glass sleeve tube into a heating zone of a furnace in which the glass sleeve tube collapses onto the outer surface of the glass rod preform to form a single glass element; and drawing the single glass element down whereby to provide optical fibre with a core formed from the glass rod preform and cladding on the core formed from the glass sleeve tube; the improvement comprising the additioning step of mounting the glass rod preform, during the said positioning step, within the glass sleeve tube in such a manner that the glass rod preform pivots freely about a point at the top and in the center of the glass sleeve tube; and the additional step of feeding the glass rod preform and the glass sleeve tube into the heating zone at the different rates, the glass rod preform feed rate being greater than the glass sleeve tube feed rate whereby to achieve a draw-on profile which is bulged out and convex below the point where the glass sleeve collapses onto the glass rod preform whereby to achieve forces tending to cause self-centering of the glass rod preform in the glass sleeve tube when the glass sleeve tube collapses onto the glass rod preform, said self-centering being permitted to occur by virtue of said pivotal mounting of the glass rod preform within the glass sleeve tube.