Patent ID: 8667816

Claim:
A method for manufacturing a primary preform for optical fibres, using an internal vapour deposition of a PCVD-type, comprising: supplying a gas flow of doped and/or undoped glass-forming gases to an interior of a hollow substrate tube having a supply side and a discharge side via the supply side; and depositing glass layers on a portion of the interior of the substrate tube that is surrounded by a furnace such that glass layer deposition is effected as a result of the presence of a reaction zone of a plasma type; wherein: the gas flow of doped and/or undoped glass-forming gases being supplied to the interior of the substrate tube comprises a main gas flow and a secondary gas flow, a flow rate of the main gas flow is higher than a flow rate of the secondary gas flow, the secondary gas flow comprises oxygen and a fluorine-containing compound free from hydrogen atoms, and of the gas flow of doped and/or undoped glass-forming gases being supplied to the interior of the substrate tube, only the secondary gas flow is subjected to a pretreatment step of a pre plasma type prior to being supplied to entry in the interior of the hollow glass substrate tube to minimize the incorporation of OH- groups in the deposited glass.