Patent ID: 7930903

Claim:
A method for manufacturing a preform for optical fibers by a vapour deposition process, the method comprising the steps: i) providing a hollow glass 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 effecting the deposition of glass layers on the inner surface of the hollow substrate tube, where deposition is to 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, 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, while deposition of soot takes place at the supply side of the substrate tube, and iv) consolidating the substrate tube obtained in step iii) into the preform, where the position of the reversal point near the supply side of the substrate tube is shifted along the longitudinal axis of the substrate tube in step iii), such that the location where the soot deposition associated with one phase takes place is axially spaced from the location where the soot deposition associated with the other phase(s) takes place.