Method and induction furnace for drawing large diameter preforms to optical fibres

A drawing furnace for drawing an optical preform, said furnace comprising:

• a furnace body having an upper end and a lower end and comprising at least a susceptor, an induction coil and an insulating material disposed between said susceptor and said induction coil;
• a muffle connected to the upper end of said furnace body, said muffle comprising a mechanical seal for avoiding inlet of ambient air into the furnace, said muffle being adapted to surround the optical preform before the optical preform is moved into said furnace body;
• a bottom portion connected to the lower end of said furnace and wherein said bottom portion comprising at least a lower portion with a decreasing cross-sectional area from the top to the bottom of the bottom portion in a plane perpendicular to the longitudinal axis;
• and
• a distributor body having a substantially annular distribution chamber, a distribution ring, and an outlet in fluid communication with an interior of the muffle, the distributor body configured to receive conditioning gas substantially tangentially with respect to the substantially annular distribution chamber, the distributor body including (i) at least one downwardly angled channel operable to forcedly direct a first portion of the conditioning gas into the muffle in a downward direction towards said furnace body and (ii) at least one upwardly angled path to direct a second portion of the conditioning gas to an upper portion of the substantially annular distribution chamber to create a buffer of conditioning gas having a pressure higher than a pressure outside the drawing furnace.