Method of silica optical fiber preform production
View Patent ↗A method is disclosed for the manufacture of optical fiber preforms using plasma enhanced chemical vapor deposition (PECVD). The invention consists of a cylindrical reactor in which material such as flourine-doped silica glass is deposited on a cylindrical silica rod. A furnace for regulating reactor temperature encases the reactor. A microwave generator coupled with a resonator and an H 10 waveguide delivers microwave energy to the reactor, producing simultaneously symmetrical excitations in the E 010 mode and a plasma surface wave in E 01 mode located at the surface of the rod. A microwave plasma is scanned along the length of the rod through a slit in the reactor to deposit a homogeneous film of a desired thickness. The benefits of the present invention over the prior art include increased absorption of delivered power, and the ability to uniformly deposit films such as flourine-doped silica on rods with diameters of up to 30–35 mm and thus produce optical fiber preforms with diameters greater than 40 mm.
1. A method of manufacturing optical fiber preforms comprising the steps of:
a. arranging a rod coaxially within a hollow cylindrical reactor, creating a cavity between said rod and said reactor;
b. reducing pressure in said cavity to a predetermined level;
c. enclosing said rod and said reactor coaxially within a hollow cylindrical furnace, and heating said rod and said reactor to a specified temperature;
d. introducing a process gas mixture in said cavity, wherein said mixture is comprised of materials to be deposited;
e. generating a plasma in said cavity by simultaneously exciting an E 010 electric field mode wave and an E 01 electric field type plasma surface wave in said cavity by means of a microwave system, wherein said plasma surface wave is located at an interface between said rod and said plasma;
f. wherein an electric field is at a maximum intensity at a radial distance from the central axis of said cavity equal to a distance where said rod-plasma interface is located; and wherein said intensity exponentially diminishes as the radius increase to said reactor and as the radius decreases toward said axis; and
g. moving said plasma along said cavity in a direction parallel to the axis of said cavity, thereby uniformly depositing said materials on said starting body.
2. A method of manufacturing optical fiber preforms according to claim 1 , wherein said microwave system comprises a resonator, a waveguide, and a microwave source.
3. A method of manufacturing optical fiber preforms according to claim 1 , wherein said starting body is a solid silica rod.
4. A method of manufacturing optical fiber preforms according to claim 1 , wherein said E 010 mode and plasma surface wave E 01 are created simultaneously in said microwave source by symmetrical and homogeneous excitation from said microwave source through a system of coupled inputs of energy.
5. A method of manufacturing optical fiber preforms according to claim 1 , wherein said material is deposited on said rod to form a reflective cladding.
6. A method of manufacturing optical fiber preforms according to claim 5 , wherein said material is fluorine doped silica.
7. A method of manufacturing optical fiber preforms according to claim 1 , wherein said material is deposited on said rod to form a protective layer.
8. A method of manufacturing optical fiber preforms according to claim 7 , wherein said protective layer is deposited by the method in claim 1 after a reflective cladding is deposited.
9. A method of manufacturing optical fiber preforms according to claim 7 , wherein said process gas mixture consists of SiH 4 , NH 3 , and NO.
10. A method of manufacturing optical fiber performs according to claim 7 wherein said protective layer is made from a process gas mixture selected from the group consisting of (N 2 +O 2 +SiCl 4 ) and (N 2 +air+SiCl 4 ).
11. A method of manufacturing optical fiber preforms according to claim 10 wherein said protective layer is a film of SiO x N y .