Patent ID: 8168267

Claim:
A method for manufacturing a preform for optical fibres by means of a vapour deposition process, wherein the method comprises the steps of: i) providing a hollow glass quartz substrate tube having a supply side and a discharge side; ii) supplying doped or undoped glass-forming gases to the interior of the hollow substrate tube via the supply side thereof; iii) creating temperature and plasma conditions in the interior of the hollow substrate tube for depositing glass layers on the inner surface of the hollow substrate tube, wherein deposition can be regarded as comprising a number of separate phases, each phase having an initial refractive index value and a final refractive index value and comprising the deposition of a number of glass layers on the inside of the hollow substrate tube during a certain time period, the plasma being moved back and forth along the longitudinal axis of the hollow substrate tube between a reversal point near the supply side and a reversal point near the discharge side of the hollow substrate tube; iv) depositing glass layers in a deposition phase having an initial refractive index value and a final refractive index value on the inner surface of the hollow substrate tube; v) interrupting the deposition phase by carrying out at least one intermediate step comprising supplying an etching gas to the supply side of the hollow substrate tube while the supply of the doped or undoped glass-forming gases is stopped; and vi) depositing glass layers in a next deposition phase having an initial refractive index value and a final refractive index value on the inner surface of the hollow substrate tube after termination of the at least one intermediate step.