Abstract: A process of fabricating an optical fiber preform, preferably a multimode optical fiber preform, having a maximum refractive index along an axis of the fiber and a lower refractive index at its periphery. The process includes the following steps: depositing successive layers onto the interior of the tube with the refractive index increasing from the first, larger diameter layer to the central layer, each layer being formed from gases which react with each other inside the tube, and varying the proportions of the gases from one layer to another to vary the index. The gases are introduced into the tube at a velocity such that the radial index profile is homogenized in the longitudinal direction. This velocity is advantageously maintained substantially the same for the fabrication of all the layers. This process produces preforms having a homogeneous index profile in the longitudinal direction.