A tubular porous silica preform 12 is made by depositing particulate silica on a mandrel 10 from which the preform is subsequently removed. A second solid cylindrical porous silica preform 28 is made by depositing particulate silica on an end surface of a support body. The porous preforms 12, 28 are then dried and the tubular preform 12 is heated in a fluorine containing atmosphere to introduce a fluorine dopant into the silica. The rod preform 28 is then placed inside the tubular preform 12 and the two preforms are further dried in a chlorine atmosphere to remove residual moisture. The dual preforms are then heated in a zone sintering furnace to consolidate and collapse the porous material of the rod and tubular preforms into a transparent fused silica rod. The composite preform obtained is heated and dielectric optical waveguide drawn from it, the waveguide having a pure silica or doped silica core derived from the rod preform 28 and a fluorine doped silica cladding derived from the tubular preform 12.