Method of manufacturing large capacity preforms by MCVD
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.
1. A method of manufacturing an optical fiber preform, the method comprising the following steps:
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 relative 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.
2. A method according to claim 1 , in which the substrate tube is doped to have a mass concentration of fluorine lying in the range 0.25 XCl<XF<4 XCl.
3. A method according to claim 1 , in which the resulting preform presents a b:a ratio greater than 2.
4. A method according to claim 1 , in which the natural silica is an alpha quartz.
5. A method according to claim 1 , in which the refractive index of said doped substrate tube is less than that of the natural silica by an amount lying in the range 0.25×10 −3 to 1.5×10 −3 .
6. A method according to claim 1 , in which the outer cladding is deposited by plasma-assisted deposition.
7. A method of manufacturing an optical fiber, said method including the step of hot fiber drawing a preform obtained using the method of claim 1 .