In plasma deposition processes for producing an optical preform according to the POD method, a cladding glass layer composed of fluorine-doped quartz glass is produced by means of a plasma torch on a cylindrical substrate body composed of quartz glass, said substrate body rotating about the longitudinal axis thereof. In this case, the plasma torch performs a reversing relative movement between two turning points (A; B) along the substrate body. In order, proceeding therefrom, to achieve a high fluorine doping in conjunction with a dopant distribution that is as uniform as possible axially, the invention proposes that a heat element has a heating effect on the region of one turning point (A; B) when the plasma torch is situated in the region of the other turning point (B; A).