Patent ID: 8499585

Claim:
A method for manufacturing an optical preform by means of an internal vapour deposition process, comprising the following steps: supplying glass-forming precursors at a supply side of a substrate tube; depositing glass layers in the interior of the substrate tube by using an energy source, which energy source is moved along the length of the substrate tube between a point of reversal at the supply side and a point of reversal at the discharge side; and discharging constituents that have not been deposited on the interior of the substrate tube from the interior of the substrate tube at the discharge side thereof, wherein an insertion tube is positioned in the interior of the substrate tube, at the discharge side thereof, before the glass-forming precursors are supplied to the interior of the substrate tube, the external diameter and the shape of the insertion tube substantially corresponding to the internal diameter and the shape of the substrate tube and the insertion tube extends outside the substrate tube, wherein any low-quality quartz is deposited on the interior of the insertion tube.