Method of manufacturing an optical fiber preform by collapsing a tube onto a rod

Method of making an optical fiber preform, comprising the steps of:

• centrally positioning a rod within a tube such that a gap exists between an outer surface of the rod and an inner wall of the tube and mechanically forming a seal between one end of said tube and said rod;
• thermally collapsing a first portion of a first section of the tube, said first section being located at an end of said tube opposite from the mechanically formed seal, by traversing a heat source along said first portion of the tube at a predetermined collapse speed, in order to sealingly close said first portion of said first section of the tube onto said rod;
• applying a force for biasing the tube inwardly towards the rod;
• moving the heat source to a second section of the tube, located in the proximity of the mechanically formed seal, and back to said first section, to allow removal of residual volatile compounds adhering to the inner wall of the tube or onto the rod, by traversing said heat source at a first predetermined traverse speed, said first predetermined traverse speed being such as to avoid any collapsing of the tube onto the rod between the first and second sections and to avoid any thermal cracking of the preform;
• thermally collapsing a remaining portion of said first section of the tube, by traversing the heat source along said remaining portion at substantially said predetermined collapse speed;
• moving the heat source to the second section of the tube by traversing the heat source at a second predetermined traverse speed, said second predetermined traverse speed being such as to avoid any collapsing of the tube onto the rod between the first and second sections and to avoid any thermal cracking of the preform;
• and
• thermally collapsing the second section of the tube onto the rod by traversing said heat source along said second section of the tube at substantially said predetermined collapse speed.