Optical fiber

DETAILED DESCRIPTION What is claimed is: 1.
A method of making an optical waveguide comprising: making a porous pure silica cylindrical rod preform by depositing particulate silica onto a support body; cooling and then removing the preform rod from the support body; making a porous pure silica hollow cylindrical preform by depositing particulate silica onto a second support body; cooling the hollow cylindrical preform; and removing the support body; drying the preforms; heating the hollow cylindrical preform in a fluorine-containing atmosphere to cause diffusion of fluorine thereinto; putting the cylindrical rod preform into the hollow cylindrical preform; heating the preforms to cause consolidation of the porous silica and to cause collapse of the hollow preform onto the rod preform to form a composite preform; heating the composite preform locally to a drawing temperature; and drawing an optical waveguide from the composite preform, such waveguide having a core part comprising silica derived from the rod preform and a cladding part comprising fluorine doped silica derived from the hollow preform.
2.
A method as claimed in claim 1 in which the hollow cylindrical preform is made by depositing particulate silica obtained from the flame hydrolysis of a silicon halide onto the curved surface of a mandrel and subsequently removing the hollow cylindrical preform from the mandrel upon cooling the preform.
3.
A method as claimed in claim 1 in which the rod preform is made by axial deposition of particulate silica obtained from the flame hydrolysis of a silicon halide onto the end surface of a seed rod.
4.
A method as claimed in claim 1 in which the steps of deposition of particulate silica comprise chemical vapor deposition of silicon tetrachloride.
5.
A method as claimed in claim 1 in which the fluorine is derived from a fluoride gas fed to he hollow cylindrical preform in conjunction with helium gas.
6.
A method as claimed in claim 5, in which the fluoride gas is one of the group consisting of sulphur hexafluoride and silicon tetrafluoride