Abstract: The invention provides a method for preparing a preform for producing by drawing an optical waveguide comprising a core region and a sheath region whose refractive index varies in the radial direction, so that the index of the former is greater than those of the second. The method consists in starting from a bar of a first composition for forming the core region and in forming, by means of a plasma torch, on its surface, a deposit of a plurality of layers whose composition varies in the radial direction, is characterized in that glass powder is fed to said torch transversely to the jet of said torch; in that said bar is drawn after each layer is deposited to bring its diameter to a value close or equal to the initial diameter; and in that the chemical composition of the glass powder is changed after each layer is deposited. The preform obtained by the present method makes it possible to produce by drawing optical waveguides for applications in the field of information transmission.