The present invention relates to a method for manufacturing a bar-shaped preform, which method comprises the following steps: i) supplying one or more reactive gases, at least one comprising fluorine which may or may not be doped, to the interior of a substrate tube, ii) carrying out a deposition reaction of gases from step i) in said susbstrate tube so as to obtain one or more glass layers on the interior of the substrate tube, and iii) contracting the substrate tube from ii) while heating said tube so as to obtain the massive, bar-shaped preform, characterized in that the amount of fluorine compound that is supplied to the interior of the substrate tube in accordance with step I) is increased by at least 10% at the end of the deposition of glass layers on the inner side of the substrate in accordance with step ii), after which the substrate tube is subjected to the heat treatment under conditions such that diffusion of the additionally incorporated fluorine from the deposited glass layers will occur.
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