Optical waveguide manufacture

Abstract

Optical waveguide having a fused silica core and a fluorine doped silica cladding is made by heating a cylinder of silica in fluorine, the cylinder initially having an outer annular particulate or porous region which is dried by the fluorine and into which fluorine diffuses to lower the refractive index of the silica when fused. The cylinder is made by depositing particulate silica onto a mandrel, drying the silica in chlorine, and heating the silica to densify it. Further particulate silica is deposited and is then heated in a fluorine-containing gas to dry, fluorine diffuse, and sinter the porous outer part of the silica. The resulting tubular silica preform is heated to collapse the tubular preform into a rod from which optical waveguide is drawn, the waveguide having a fluorine doped silica cladding. In one alternative the silica cylinder is made by depositing onto a mandrel a first layer of relatively dense silica followed by a second layer of less dense particulate silica so that fluorine diffuses preferentially into the lower density material. In another alternative, the silica cylinder is made by depositing particulate silica directly onto a high purity fused silica rod without using a mandrel, the deposited silica then being dried and subjected to fluorine diffusion.