Method of manufacturing an optical fibre glass preform

A method of manufacturing an optical fibre comprising:

• producing a core rod having a core rod diameter;
• inserting the core rod into a glass fluorine-doped intermediate cladding tube so as to form a core assembly having a length, the intermediate cladding tube having an inner diameter and an outer diameter, wherein the inner diameter is larger than the core rod diameter, the radial difference between the inner diameter and the core rod diameter defines an annular gap, and the intermediate cladding tube has a radial thickness of 1 mm to 10 mm;
• arranging the core assembly in such a way that the core assembly extends vertically;
• applying a negative pressure inside the annular gap;
• forming a core preform by heating the core assembly to collapse the intermediate cladding tube around the core rod while maintaining the negative pressure, wherein heating comprises moving a heater outside the intermediate cladding tube at a traverse speed of 20 mm/min to 40 mm/min and along an axial direction at least upwardly of the intermediate cladding tube initially from a position at one end of the core assembly to traverse the length of the same, and in a plurality of passes along opposite axial directions;
• forming an overcladding region surrounding the core preform so as to form an optical fibre preform;
• and
• drawing the optical fibre preform to fabricate an optical fibre.