An optical fiber preform is formed by plasma chemical vapor deposition utilizing a plasma torch that includes a chamber into which a plasma starting gas, such as argon, is introduced and converted to a plasma by application of high frequency electromagnetic energy. A suitable entry port is provided in the torch chamber to allow dopant material in solid form to be introduced directly into the plasma for vaporization or ionization. The dopant material passes into a lower temperature region where gaseous silica precursor gases are introduced to undergo reaction in the presence of the vaporized dopant to form appropriately doped silica particles for deposition onto a substrate. Solid dopants of low volitility or vapor pressure may be used as dopants which could not otherwise be used in vapor deposition processes. The absence of hydrogen in the reaction dynamics results in a preform having greatly reduced hydroxyl ion content. If desired, a high-purity starting glass, rather than the precursor gases, can be introduced directly into the plasma to form the appropriately doped material.