Process of making an optical fibre preform

The production of a fibre optic preform is by the Furnace Chemical Vapour Deposition of layers. The fibre optic has a core, incorporating at least one central part, an internal sleeve and an annulus, and an outer sleeve, the fibre optic being of the Dispersion Shifted Fibre or the Dispersion Compensating fibre type. The layers of the preform corresponding to the internal sleeve and to the annulus of the fibre optic have a phosphorus content of between 0 and 0.1% by wt. The phosphorus content is preferably between 0.03 and 0.1% by wt.