Plasma deposition process for producing an optical preform with a cladding glass layer of fluorine-doped quartz glass

A plasma deposition process for producing an optical preform having a cladding glass layer of fluorine-doped quartz glass, said method comprising:

• forming SiO2 particles with a plasma torch in the presence of fluorine, and
• depositing said SiO2 particles layer by layer on an outer surface of a cylindrical substrate body of quartz glass rotating about a longitudinal axis thereof, and
• vitrifying said SiO2 particles so as to form the cladding glass layer,
• the plasma torch and the substrate body being moved relative to each other such that the plasma torch reversingly moves between first and second turning points along the substrate body,
• wherein each of said deposited layers is vitrified by the plasma torch directly into the fluorine-doped quartz glass;
• and
• wherein a thermal element thermally acts on an area of the first turning point when the plasma torch is positioned in an area of the second turning point.