Solution-doping of optical fibre preforms

In a method for doping the core of a silica-based optical fibre preform, soot layers intended to form the preform core and containedaed inside a reaction tube (1) supported by a lathe (3), are impregnated with a soln. of a precursor of a first dopant intended to raise the refractive index of the layers without giving rise to the central dip of said index. The first precursor is in the form of a soln. in an anhydrous organic solvent. After layer deposition, the reaction tube (1) is disconnected from the reactant inlet conduits as well as from the exhaust conduits for the volatile products of reaction. The exhaust end (U) is sealed whilst maintaining an inert, anhydrous gas flow in the reaction tube during sealing. Soln. is introduced into the reaction tube without removing it from the lathe so that the soln. covers the whole surface of the deposited layers. Once impregnation is over, the soln. is made to slowly flow t at uniform rate from the reaction tube still mounted in the lathe.