doped zone 12 a on a preform 10 a while fig2 x shows four recesses 14 b bordering four respective doped areas 12 b on another preform 10 b it should be noted that the apparent symmetry of the doped zones in these figures is generally not required to implement the invention in one variant a plurality of recesses 14 c coaxial to the axis 13 c of the preform 10 c are implemented said recesses 14 c being arranged symmetrically with respect to said axis 13 c of the preform 10 c this variant is illustrated by fig2 d which shows a preform 10 c in accordance with the invention comprising six recesses 14 c forming arcs of circles which are concentric and symmetrical with respect to the axis 13 c of the preform 10 c said recesses 14 c being separated by a nanoparticle free layer of material 16 c in this example of a preform the cylindrical symmetry of the doped zones 12 c with respect to the axis of the preform 10 c makes it possible to obtain a fiber with the same properties fig3 b illustrates the cross section of an optical fiber 18 e which includes a glass structure 16 e comprising nanoparticles doped zones 12 e and a central conventionally doped zone 13 f in order to enable guiding germanium phosphorus etc this fiber may be obtained for example by a step of forming fiber out of the preform 10 b in fig2 c using the method of the present invention it is also possible to manufacture photonic crystal fibers pcf that include holes obtained by forming fiber out of the performs obtained through a glass assembly method to manufacture such pcf fibers.