The invention provides a low cost method of manufacturing high capacity preforms by chemical vapor deposition. More particularly, there is described a method of manufacturing an optical fiber preform, which method comprises the steps of providing a substrate tube of silica doped with sufficient chlorine to obtain an OH concentration of less than 100 ppb and doped with sufficient fluorine proportional to the chlorine doping to obtain a refractive index that is lower than that of a natural silica, depositing inner cladding and an optical core inside the substrate tube, collapsing the substrate tube to form a primary preform, and depositing outer cladding of said natural silica on the resulting primary preform. The invention is applicable to manufacturing optical fibers.