APPARATUS FOR PRODUCING POROUS PREFORMS FOR OPTICAL FIBERS

Abstract: An apparatus for the production of a porous preform of SiO₂ used in the manufacture of optical fiber wherein damage to the preform caused by falling clusters of SiO₂ or GeO₂ formed on the wall of the deposition chamber is eliminated. In accordance with the invention, a conductive shielding layer is provided inside the wall defining the deposition chamber. This shielding layer cancels an electrostatic field produced on the wall by charging, therefore preventing the adherence of fine particles thereto.

Description: BACKGROUND OF THE INVENTION The present invention relates primarily to an improved VAD (Vapor-phase Axial Deposition) apparatus used for producing porous preforms for optical fiber production. The invention may be applied as well, however, to a so-called "outside vapor-phase oxidation" method, the basic principles of which are the same as employed with the VAD apparatus. A conventional VAD apparatus (see e.g. US-A-4 435 199) is depicted schematically in Fig. 1. In Fig. 1, reference numeral 3 indicates an oxyhydrogen burner employing a mixture of oxygen and hydrogen as a combustion gas. The burner 3 also includes passages through which gases such as SiCl₄, GeCl₄, Ar, etc., are passed to the flame 4. Consequently, there are produced fine particles of SiO₂, GeO₂, etc. The fine particles of SiO₂ or GeO₂ are deposited on a target material 2, which may be a quartz bar, to grow a porous preform 1 thereon. Reference numeral 5 indicates a wall (sometimes called a "muffle") which defines a deposition chamber around the preform 1. The wall 5 can be made of ordinary glass, quartz glass, or the like. A discharge hole 6 is formed in one side of wall 5, and the stem of the target material 2 extends upwardly through another passage formed at the upper end of the chamber. The conventional apparatus of Fig. 1 suffers a serious drawback. That is, because the wall 5 is subject to electrostatic charging and the particles themselves are charged, clusters of particles tend to collect on the wall 5. Some of these clusters near the top of the chamber, when they become sufficiently large, drop off the wall 5 and strike the growing preform 1 causing nonuniformities therein. Others of the following clusters may fall into the flame 4, resulting in nonuniform scattering and again nonuniformities in the growing porous preform 1. Such a preform may be unsuitable for use in the production of optical fibers. SUMMARY OF THE INVENTION Accordingly, an object of the invention is to provide a VAD apparatus in which the above-discussed drawbacks have been eliminated. More specifically, it is an object of the present invention to provide a VAD apparatus in which the adherence of fine SiO₂ or GeO₂ particles to the wall of a deposition chamber of the apparatus is eliminated. In accordance with the above and other objects, the invention provides a VAD apparatus comprising at least one wall defining a deposition chamber; burner means for producing fine particles for deposition of a porous preform; and means for electrostatically shielding the at least one wall from the porous preform. The shielding means is preferably a layer of electrically conductive material provided inside the at least one wall surrounding the preform and the flame of the burner means. With the VAD apparatus of the present invention, an electrostatic field produced by a charged wall of the deposition chamber is canceled, thereby preventing fine particles of SiO₂ or GeO₂ from depositing on the wall. Accordingly, a porous preform produced with the VAD apparatus of the present invention is much more uniform than can be attained with the conventional apparatus. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a schematic view of a conventional VAD apparatus; and Fig. 2 shows a schematic view of a VAD apparatus constructed in accordance with the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Fig. 2, a preferred embodiment of a VAD apparatus of the present invention will be described. In Fig. 2, reference numerals employed commonly in Fig. 1 denote like elements, and a further detailed description of those elements will consequently be omitted. In accordance with the invention, a shielding layer 7 is provided adjacent the inner surface of the wall 5, conforming as closely as possible to the shape thereof. The shielding layer 7 should surround the porous preform 1 and the flame 4. A mesh-type shielding layer may be employed. Of course, it is necessary to provide apertures in the shielding layer 7 for passage therethrough of the oxyhydrogen burner 3 and the target material 2. The shielding layer 7 cancels out any electrostatic field arising due to charges present on the wall 5. Therefore, fine particles of SiO₂ or GeO₂ are prevented from depositing on the inner surface of the wall 5. Accordingly, damage to the preform 1, as unavoidably occurred in the conventional VAD apparatus, is positively eliminated. That is, there is no danger of damage to the preform 1 by falling clusters of particles striking the preform 1 or particles scattered nonuniformally upon falling into the flame 4. Hence, porous preforms of very high quality and having a desired shape can easily be produced with the use of the invention. Although, in the above-described embodiment, the wall 5 and the shielding layer 7 have a spherical form, other shapes may be used as well, for instance, a cylindrical shape. For the material of the shielding layer, aluminum is preferred, although various other materials can be used as well. This completes the description of the preferred embodiments of the invention. Although preferred embodiments have been described, it is believed that numerous modifications and alterations thereto would be apparent to one of ordinary skill in the art without departing from the scope of the claims.

Claims: An apparatus for producing a porous preform for use in producing optical fiber, comprising at least one wall (5) defining a deposition chamber, said wall having a passage formed therein for receiving a target material upon which a porous preform is to be grown, an aperture for receiving an oxyhydrogen burner (3), a discharge outlet, and an oxyhydrogen burner (3) for producing fine particles of SiO₂, said burner being disposed in said aperture characterised in that shielding means (7) are disposed inside said at least one wall (5) for canceling an electrostatic field produced by a charge on said at least one wall. The apparatus of claim 1, wherein said shielding means (7) comprises a layer of conductive material conforming in shape to an inner shape of said at least one wall. The apparatus of claim 2, wherein said conductive layer comprises a conductive mesh. The apparatus of claim 3, wherein said conductive layer is made of aluminum.

Inventors: ENDO, SHIGEKI C/O YOKOHAMA WORKS, KAKUZEN, HIDEO C/O YOKOHAMA WORKS, KISANUKI, SHIGERU C/O YOKOHAMA WORKS, YOSHIDA, MASAAKI C/O YOKOHAMA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1985
Main classification: C03B37
IPC codes: C03B37, G02B6, C03B19