Patent ID: 8863557

Claim:
A method for manufacturing a primary preform for an optical fibre, using a plasma chemical internal vapour deposition process, wherein doped or undoped glass-forming precursors are supplied to the interior of a hollow glass substrate tube, a reaction zone in the form of a plasma is moved back and forth along the length of the hollow glass substrate tube between a point of reversal near a supply side and a point of reversal near a discharge side of the hollow substrate tube, wherein the substrate tube is positioned in a furnace and wherein such conditions are created in the reaction zone that one or more glass layer packages made up of at least two separate glass layers are deposited on the interior of the substrate tube, the method comprising the following step: defining deposition conditions as a function of a position of the reaction zone, seen in a longitudinal direction of the hollow glass substrate tube, for deposition of at least one glass layer, with the deposition conditions thus defined differing from each other within the deposition of the glass layer, wherein: within a particular glass layer the refractive index value and/or the cross-sectional area of one glass layer obtained by deposition differs from the refractive index value and/or the cross-sectional area of another glass layer obtained by deposition, the defining of deposition conditions comprises setting one or more process parameters selected from the group consisting of: flow of the additional gas to be metered at the supply side, velocity of the reaction zone, intensity of the plasma of the reaction zone, and length of the reaction zone, and the number of glass layers of each glass layer package is set so that the following condition is met: N ≤ 0.1 * λ * Q final d * Q fibre where N=the number of glass layers in a glass package [-] λ=the minimum wavelength used of the optical fiber [μm] d=the thickness of a glass layer in a glass layer package of a primary preform [μm] Q final =the diameter of the final preform produced on the basis of the primary preform [mm] Q fibre =the diameter of the optical fibre [mm].