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

A substrate tube is inserted using glazing of tubular porous silicon dioxide green body present in a core glass layer which is produced. A first doping material increasing the refractive index of quartz glass is added to the partial region of the silicon dioxide green body, before glazing. Process for producing a preform for an optical fiber for optical communications included preparing a quartz glass substrate tube which has doping in different radial direction regions, introducing of core glass into the substrate tube from synthetic quartz glass and coating of substrate tube with a cover glass-tube.