a method and apparatus for manufacturing optical fibers provided with hermetic coatings , wherein the bare fiber made from a heated preform advances through a stream of reactive gaseous medium which flows at a constant speed in the direction of advance of the fiber , which is then accelerated , and which is finally removed remote from the fiber by a stream of inert gas flowing in the opposite direction from the direction of the stream of reactive gaseous medium . application is to manufacturing optical fibers for telecommunications .