Process for producing a preform for an optical fibre and a substrate tube suitable for use in the process

Starting from a known method for producing a preform for an optical fiber for optical communications technology by providing a quartz glass substrate tube which has in radial direction regions with different dopants, introducing a core glass of synthetic quartz glass into the substrate tube, and overcladding the substrate tube with a cladding glass tube is to improve the productivity of the process for the production of complex refractive index profiles, and use a suitable substrate tube are provided, wherein the preform producing less core glass material, be it at an internal deposition or in a core glass rod for rod-in- tube technique, is required. Regarding the method the invention proposes to use a product obtained by vitrification of a tubular, porous SiO¶2¶ blank substrate tube, which is provided with a core glass layer which is produced by a first radial portion of the SiO¶2¶ blank prior to vitrification a refractive index-increasing of silica first dopant is added. The substrate tube according to the invention comprises areas of different doping, which is a core glass layer having a refractive index of at least 1.459 comprises, in the radial direction.