Abstract: An apparatus for producing preforms for the production of optical fibers by plasma coating and drawing. A plasma stream produced with the aid of a radio frequency generator is caused to successively pass through three zones in the first of which a mixture of fluidized powders of two materials of appropriate composition is injected and completely vaporized. In the second zone an oxidizing gas having a mixing action is introduced into the deeply cooled stream. The materials and gas react with each other to form microseeds. The powders contain high purity silicon and at least one doping element of the group comprising Al, Ge, B. Ti and Zr. In the third zone an inert gas is introduced and deposition of substance is effected on substrates suitably arranged with respect to the path followed by the plasma stream.

Abstract: The apparatus comprises a plasma torch capable of an output speed of at least 100 m/s, a fine powder distributor, and a substrate holder which can be heated electrically and cooled by water, which is at an adjustable distance from the torch, and which is movable in two transverse directions at adjustable speeds. The method consists in starting from ceramic having a low roughness CLA and from powder having a narrow granulometric distribution over an average value of at most 10 .mu.m, and projecting the melted powder onto the substrates kept at a regulated temperature and subjected to the said transverse movements.