Abstract: A hollow silica bar is formed on a mandrel by an external deposition method using a plasma torch. The mandrel is removed and on the inner face of the bar is deposited a vitreous coating to be used for the subsequent formation of the core of the fibers. Deposition is obtained by means of a plasma column, formed in the bar as a result of an ultra-high frequency coupler able to inject a travelling surface wave into the plasma column.