GB2079742A

GB2079742A - Optical fibre preform manufacture

Info

Publication number: GB2079742A
Application number: GB8118476A
Authority: GB
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1980-07-19
Filing date: 1981-06-16
Publication date: 1982-01-27
Also published as: ATA316281A; BE889655A; DE3027450A1; CH652112A5; AT380867B; FR2486927B1; DE3027450C2; GB2079742B; FR2486927A1

Abstract

Internal CVD is assisted by an electric field downstream of hot zone 3 and moving conjointly therewith. The field may be generated by (a) static electricity produced by the friction of brushes 4 (Figure 1) or of a dry gas jet on the outside of the tube 1, or (b) electrodes 7, 8 (Figure 3). <IMAGE>

Description

SPECIFICATION Vapour deposited optical fibre This invention relates to the formation of a coating on the bore of a glass substrate tube to form an internally coated tube for optical fibre waveguide manufacture. In particular it relates to the formation of such a coating by a method in which the coating is formed as a deposition product of a chemical vapour reaction between vapour phase reagents caused to flow down the substrate tube, and caused to react in a localised hot zone traversed along the tube, the deposition product being collected on the internal wall of tube downstream of the hot zone. One example of such a coating process is described in International Patent Application No. WO 80/00440, and in that example collection of the deposition product on the wall of the tube is assisted by thermophoresis.

According to the present invention there is provided a method of forming a coating on the bore of a glass substrate tube for optical fibre waveguide manufacture, wherein vapour phase reagents are caused to flow through the tube while it is rotated about its axis and caused to react to form a deposit in the localised region of a hot zone traversed along the tube, which deposit is collected on the inner wall of the tube downstream of the hot zone to form a glassy coating, wherein a local electric field is produced in the tube downstream of the hot zone and moved synchronously therewith, which electric field assists said collection of the deposit.

There follows a description of the manufacture of an optical fibre waveguide by methods embodying the invention in preferred forms. The description refers to the accompanying drawings in which Figures 1,2, and 3 are schematic diagrams of alternative forms of deposition apparatus.

Referring to Figure 1, a glass substrate tube 1 whose inner wall is to be coated is held horizontally in a lathe (not shown) and is rotated about its axis during the coating process. A heat source 2, for example an oxy-hydrogen burner, produces a localised hot zone 3 in the tube 1. The heat source is mounted on a support 5 which allows itto be smoothly traversed in the axial direction of the tube so that the hot zone 3 can be traversed up and down the tube. Reagents, which may be in the gaseous or vapour state, for the chemical vapour reaction used to provide the required deposit on the bore of the tube are caused to flow through the tube in the direction of the arrow 1 A. These reagents react in the heated zone 3 and produce glass forming reaction products which are collected and fused together to form a glassy coating downstream of the hot zone 3.

Rotation of the tube and controlled translation of the burner ensure that a coating of uniform thickness is laid down.

With the apparatus as therefor described it is found that a significant proportion of the glass forming reaction product which should go to form the coating may be lost, that is it fails to be collected upon the bore of the tube but gets discharged from the tube 1 amongst the spent reagent gases. Deposition efficiency is improved by providing a local electric field downstream of the hot zone 3.

In the apparatus of Figure 1 this electric field is provided by friction-producing means 4 arranged to rub the outer wall of the tube 1 as it is rotated and thereby produce static electricity. The acceleration of the glass-forming reaction products in direction to the inner wall of the tube owing to the resulting electric field is due to the fact that a dielectric body within an inhomogenous electric field is given a force in direction to the location of higher field intensity. From the friction-producing means it is possible to use brushes or other elements made of a suitable material. The friction-producing means 4 is arranged at a fixed spacing downstream of the heated zone. For this purpose it is conveniently mounted on the support 5 on which the heat source 2 is mounted to both.Accordingly, the produced electric field is moved along the longitudinal axis of the substrate tube in synchronism with the hot zone 3.

In the apparatus depicted in Figure 2 the local electric field is similarly built up by producing static electricity on the substrate tube 1, but in this case the substrate tube 1 is not rubbed by physical frictionproducing means, but by a dry gas stream, which is blown at it within a restricted region from a jet 6.

This jet 6 is again mounted in common with the heat force on a joint support 5 so that the electric field produced by way of friction within a region downstream of the hot zone 3, is traversed synchronously with that hot zone along the longitudinal axis of the substrate tube 1. The gas for the dry gas stream flowing out of the jet 6 is conveniently an inert gas, such as nitrogen.

A further possibility for producing the electric field assisting the deposition of glass-forming material, is shown in Figure 3. Here, the electric field is produced with the aid of two electrodes 7 and 8 which are connected to the poles of a source of DC voltage source 9. The one electrode 8 is of rod-shaped design and projects into the substrate tube 1 from the end from which the exhaust gases emerge. The other electrode 7 is arranged outside the substrate tube 1 downstream of the heat source 2 and, for example, is of annular design, so that in the space between the two electrodes there exists an almost radial symmetrical electric field.The inner electrode 8 is connected to the negative pole, and the outer electrode 7 is connected to the positive pole of the source of voltage 9.ln order that the electric field, as in the hitherto described examples of embodiment, can be moved in synchronism with the hot zone 3, both electrodes together with their holding elements 10 and 11, are arranged rigidly with respect to the heat source 2. For this purpose holding elements 10, 11 are firmly connected to the heat source 2 by means of a rod 12. The support common to both the heat source 2 and the holding elements 10 and 11 is not shown in the drawings.

The electric field as built up by the electrodes 7 and 8, now acts in the way of an electrofilter of the type known per se.

In the direct proximity of the inner electrode 8 there exists a high electric field intensity under the influence of which glass-forming particles are charged negatively. The thus charged particles move in direction towards the positive electrode 7 and are thus deposited on the inner wall of the tube.

Once the required deposition has been completed, the temperature of the hot zone is increased so as to soften the glass tube to the extent that its bore collapses under the effects of surface tension. The heat source is traversed and the process continued until the bore has been completely eliminated from the main body of the tube so as to produce an optical fibre waveguide preform with a solid cross-section.

The preform is removed from the lathe and is subsequently mounted in a pulling tower in which it is drawn into optical fibre waveguide.

Claims

1. A method of forming a coating on the bore of a glass substrate tube for optical fibre waveguide manufacture, wherein vapour phase reagents are caused to flow through the tube while it is rotated about its axis and caused to react to form a deposit in the localised region of a hot zone traversed along the tube, which deposit is collected on the inner wall ofthetube downstream of the hot zone to form a glassy coating, wherein a local electric field is produced in the tube downstream of the hot zone and moved synchronously therewith, which electric field assists said collection of the deposit.

2. A method as claimed in claim 1, wherein said electric field is built up by way of producing static electricity on said substrate tube.

3. A method as claimed in claim 2, wherein said static electricity is established by a physical rubbing of the outer wall of the tube.

4. A process as claimed in claim 2, wherein said static electricity is established by directing a dry gas stream against the outer wall ofthetube.

5. A method as claimed in claim 1, wherein said electric field is produced between two electrodes one arranged inside, and the other electrode arranged outside said substrate tube in such a way that between said electrodes there exists a radially directed electric field.

6. A method as claimed in claim 5, wherein said internal electrode is of rod-shaped, and said external electrode is of ring-shaped design.

7. A method of forming a coating on the bore of glass substrate tube for optical fibre waveguide manufacture, which method is substantially as hereinbefore described with reference to Figure 1, 2 or 3 of the accompanying drawings.

8. A coated tube coated by the method claimed in any preceding claim.

9. Asolid cross-section optical fibrewaveguide preform made by collapsing the bore of a coated tube as claimed in claim 8.

10. An optical fibre waveguide drawn from a preform as claimed in claim 9.