Patent ID: 8402792

Claim:
A method for manufacturing a primary preform for optical fibres using an internal vapor 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 doped or undoped glass-forming gases to the interior of the hollow glass substrate tube via the supply side thereof, iv) creating a reaction zone in which conditions are created 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 along the length of 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), the gas flow comprises a first concentration of fluorine-containing compound when the reaction zone is moving in the direction of the discharge side, wherein the gas flow comprises a second concentration of fluorine-containing compound upon movement of the reaction zone in the direction of the supply side following movement thereof in the direction of the discharge side, wherein the first concentration of fluorine-containing compound is different from the second concentration of fluorine-containing compound during at least part of step v), wherein the deposition of glass layers on the interior of the substrate tube takes place continuously during deposition and during the concentration changes of the fluorine-containing compound dependent on the direction of the movement of the reaction zone, and wherein a proportion R, viz. the quotient of the first concentration of fluorine-containing compound and the second concentration of fluorine-containing compound in the gas flow, is set so that the proportion R is within the range 1<R<100.