Abstract: A rod-in-tube method of manufacturing optical fibre where the rod and the tube are fed into the heating zone of a furnace at different rates, the rod feed rate being greater than the tube feed rate. The rod is mounted relative to the tube such as to permit self-centering of the rod in the tube when the tube collapses onto the rod in the heating zone. The drawn fibre may be monitored by means of an in-line fibre core to O/D monitor and feed-back employed to drive the tube and rod feed mechanisms accordingly thereby providing precise control of the core to O/D ratio of the fibre. Very long fibre lengths may be achieved by feeding the rod into the heating zone at a greater rate than a very large mass tube billet and consecutively feeding further rods into the tube billet as the preceding rod is used up.

Abstract: In the manufacture of silica optical fibers a tube having an internal coating of a volatile dopant is locally heated by a hot zone repeatedly traversed along its length to promote shrinkage and finally collapse of the tube bore. During the shrinkage prior to the final collapse traversal a gas flow of oxygen and a halide of the volatile dopant is maintained through the tube in order firstly to provide a small overpressure to ensure circularity in the shrinkage and secondly to compensate for the tendency for dopant to be lost by volatilization.