Patent ID: 8443630

Claim:
A method for manufacturing a primary preform for optical fibres using an internal vapour deposition process, comprising the steps of: i) providing a hollow glass substrate tube having a supply side and a discharge side, ii) surrounding at least part of the hollow glass substrate tube by a furnace, iii) supplying a constant gas flow, doped or undoped, of glass-forming gases to the interior of the hollow glass substrate tube via the supply side thereof, iv) creating a reaction zone with conditions such that deposition of glass will take place on the interior of the hollow glass tube, and v) moving the reaction zone back and forth in a longitudinal direction over the hollow glass substrate tube between a reversal point located near the supply side and a reversal point located near the discharge side of the hollow glass substrate tube, wherein during at least part of step v) a pulsed additional amount of a gas comprising a fluorine-containing compound is supplied in addition to the constant gas flow to the interior of the hollow glass substrate tube via the supply side thereof when the reaction zone is located near or at a reversal point.