Method and device for manufacturing an optical preform by means of an internal vapour deposition process, and a corresponding substrate tube assembly

A method for manufacturing a precursor for a primary preform by an internal vapor deposition process comprising:

• providing a hollow substrate tube having a supply side and a discharge side;
• supplying glass-forming precursors to the hollow substrate tube on the supply side thereof and moving an energy source along a length of the hollow substrate tube thereby generating deposition conditions in the hollow substrate tube;
• and
• depositing a plurality of glass layers on an inner surface of the substrate tube to form the precursor for a primary preform, wherein:
• an elongation tube is mounted over, and connected, in a substantially air-tight manner, to the hollow substrate tube at the discharge side thereof such that the hollow substrate tube extends into an interior of the elongation tube, thereby defining an overlapping region between the hollow substrate tube and the elongation tube;
• an internal diameter of the elongation tube at the overlapping region is at least 0.5 millimeters larger than an external diameter of the hollow substrate tube at the overlapping region;
• and
• the elongation tube further comprises a hollow insertion tube positioned inside its inner diameter such that the hollow insertion tube is aligned with and adjacent to the hollow substrate tube, the hollow insertion tube having an internal diameter greater than or equal to an internal diameter of the hollow substrate tube.