GB2148874A - Optical fibre fabrication by the rod-in-tube method - Google Patents

Abstract

In a rod-in-tube method of manufacturing optical fibre (7) a rod preform (4) is supported by a clamp (5), which allows it to hang vertically, and a tube sleeve (1) is supported by a clamp (2), which allows the tube sleeve to pivot about the rod preform (4). Feedback from a rod-in-tube concentricity monitor (8) monitoring the concentricity in a plane (A) above the entrance to a furnace (3) is used to pivot the tube sleeve in order to optimise the concentricity at the heating and draw-down zone (6). <IMAGE>

Description

SPECIFICATION Optical fibre fabrication This invention relates to the fabrication of optical fibres and in particular to silica-based optical fibre fabrication employing a rodtube method of manufacture.

In the type of rod-in-tube method of manufacture with which the invention is particularly, but not exclusively, concerned, a rod preform comprising glass forming the core material of the fibre and provided with a core-cladding structure, is placed inside a sleeve tube of glass which serves to increase the cross-sectional area of the composite.

The rod preform and sleeve tube assembly are then lowered progressively into a furnace. In the hot zone the sleeve tube collapses onto the rod preform forming a single entity which is then drawn down into a fibre. There are many factors which influence the quality of the resultant fibre including assembly advance rate, fibre draw rate, furnace temperature, furnace hot zone profile and start rod and tube geometry. The rodtube method serves to extend thetbre yield of a rod preform by adding to it the mass of the sleeve tube. To achieve.the correct core to O/D (outside diameter) ratio in the final fibre, the optical material in the rod preform is conventionally grown to be oversize, so that a sleeve tube of a given cross-sectional area will reduce it to the correct geometry in the final fibre.

According to the present invention there is provided a method of manufacturing optical fibre in which a rod-in-tube assembly is disposed substantially vertically for downwards advancement into the heating zone of a furnace, including the steps of forming a rod-in-tube assembly by clamping the uppermost end of a rod whereby the rod hangs vertically, clamping the uppermost end of a tube, with the rod disposed within the tube, such that the tube is pivotable with respect to the rod, and during advancement of the assembly into the furnace monitoring the concentricity of the rod in the tube at a position adjacent to the furnace entrance and in response thereto pivoting the tube whereby to optimise the concentricity at the heating zone.

An embodiment of the invention will now be described with reference to the accompanying schematic drawing of a rod-in-tube collapse and draw process.

For a conventional rod-in-tube collapse and draw process a rod preform is prealigned within a sleeve tube and the two elements are fixed rigidly with respect to one another at one end, the upper end during collapse and draw, by glass blowing techniques or by a mechanical double chuck arrangement. No matter how accurately this prealignment is achieved, if the tube and/or preform isitare bowed, there will be "frozen-in" non-concentricity.

over parts ofthe overall length of the assembly.

This can lead to core to.O/D non-concentricity in corresponding parts of the final drawn fibre.

For the collapse and draw rod-in-tube process now proposed a sleeve tube is held at one end, the uppermost end during collapse and draw, in a clamp device 2 so that it extends vertically downwards and can be advanced through a furnace 3. A rod preform 4 is held at one end, the uppermost end during collapse and draw, in a clamp device 5 which is such as to permit the rod preform to hang vertically, that is be suspended like a plumb bob, within the sleeve tube. For this purpose the clamp device 5 may comprise a gimbal pivot mechanism.

The clamp device, or tube chuck 2 is such as to pivot about the longitudinal axis of the rod such that the tube is pivotal. Thus the tube 1 can be aligned to be concentric . with the preform in the drawdown zone 6. This can be carried out continuously throughout a pull (drawing of fibre 7) thus compensating for bowed tubes or preforms. For this purpose a concentricity monitor 8, shown only schematically, is mounted at a position in a plane A adjacent the entrance. to the furnace 3, which monitor may detect the concentricity on two orthogonal axes, and feedback from the concentricity monitor is employed to tilt the tube chuck 2 to make the tube concentric with the suspended rod preform in the vicinity of the drawdown zone A.

The distance between the draw-down zone 6, in the plane 3, and the plane A is kept as small as possible in order to minimise the error to concentricity in the hot zone of the furnace. During fibre drawing the rod preform 4 and the sleeve tube 1 are advanced towards the furnace at the same rate by a common drive means (not shown) which may act on support means for the clamps 2 and 5 in order to prevent additional contact with the rod and sleeve and ensure that the amount of fibre drawn from the rod and sleeve is maximised.

1. A method of manufacturing optical fibre in which a rod-in-tube assembly is disposed substantially vertically for downwards advancement into the heating zone of a furnace, including the steps of forming a rod-in-tube assembly by clamping the uppermost end of a rod whereby the rod hangs vertically, clamping the uppermost end of a tube, with the rod disposed within the tube, such that the tube is pivotable with respect to the rod, and during advancement of the assembly into the furnace monitoring the concentricity of the rod in the tube at a position adjacent to the furnace entrance and in response thereto pivoting the tube whereby to optimise the concentricity at the heating zone.

2. A method as claimed in claim 1, wherein the rod is clamped by means of a gimbal pivot mechanism.

3. A method as claimed in claim 1, wherein the rod is a rod preform comprising glass forming the core material of the fibre and provided with a corecladding structure.

4. A method of manufacturing optical fibre substantially as herein described with reference to and as illustrated in the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Optical fibre fabrication This invention relates to the fabrication of optical fibres and in particular to silica-based optical fibre fabrication employing a rodtube method of manufacture. In the type of rod-in-tube method of manufacture with which the invention is particularly, but not exclusively, concerned, a rod preform comprising glass forming the core material of the fibre and provided with a core-cladding structure, is placed inside a sleeve tube of glass which serves to increase the cross-sectional area of the composite. The rod preform and sleeve tube assembly are then lowered progressively into a furnace. In the hot zone the sleeve tube collapses onto the rod preform forming a single entity which is then drawn down into a fibre. There are many factors which influence the quality of the resultant fibre including assembly advance rate, fibre draw rate, furnace temperature, furnace hot zone profile and start rod and tube geometry. The rodtube method serves to extend thetbre yield of a rod preform by adding to it the mass of the sleeve tube. To achieve.the correct core to O/D (outside diameter) ratio in the final fibre, the optical material in the rod preform is conventionally grown to be oversize, so that a sleeve tube of a given cross-sectional area will reduce it to the correct geometry in the final fibre. According to the present invention there is provided a method of manufacturing optical fibre in which a rod-in-tube assembly is disposed substantially vertically for downwards advancement into the heating zone of a furnace, including the steps of forming a rod-in-tube assembly by clamping the uppermost end of a rod whereby the rod hangs vertically, clamping the uppermost end of a tube, with the rod disposed within the tube, such that the tube is pivotable with respect to the rod, and during advancement of the assembly into the furnace monitoring the concentricity of the rod in the tube at a position adjacent to the furnace entrance and in response thereto pivoting the tube whereby to optimise the concentricity at the heating zone. An embodiment of the invention will now be described with reference to the accompanying schematic drawing of a rod-in-tube collapse and draw process. For a conventional rod-in-tube collapse and draw process a rod preform is prealigned within a sleeve tube and the two elements are fixed rigidly with respect to one another at one end, the upper end during collapse and draw, by glass blowing techniques or by a mechanical double chuck arrangement. No matter how accurately this prealignment is achieved, if the tube and/or preform isitare bowed, there will be "frozen-in" non-concentricity. over parts ofthe overall length of the assembly. This can lead to core to.O/D non-concentricity in corresponding parts of the final drawn fibre. For the collapse and draw rod-in-tube process now proposed a sleeve tube is held at one end, the uppermost end during collapse and draw, in a clamp device 2 so that it extends vertically downwards and can be advanced through a furnace 3. A rod preform 4 is held at one end, the uppermost end during collapse and draw, in a clamp device 5 which is such as to permit the rod preform to hang vertically, that is be suspended like a plumb bob, within the sleeve tube. For this purpose the clamp device 5 may comprise a gimbal pivot mechanism. The clamp device, or tube chuck 2 is such as to pivot about the longitudinal axis of the rod such that the tube is pivotal. Thus the tube 1 can be aligned to be concentric . with the preform in the drawdown zone 6. This can be carried out continuously throughout a pull (drawing of fibre 7) thus compensating for bowed tubes or preforms. For this purpose a concentricity monitor 8, shown only schematically, is mounted at a position in a plane A adjacent the entrance. to the furnace 3, which monitor may detect the concentricity on two orthogonal axes, and feedback from the concentricity monitor is employed to tilt the tube chuck 2 to make the tube concentric with the suspended rod preform in the vicinity of the drawdown zone A. The distance between the draw-down zone 6, in the plane 3, and the plane A is kept as small as possible in order to minimise the error to concentricity in the hot zone of the furnace. During fibre drawing the rod preform 4 and the sleeve tube 1 are advanced towards the furnace at the same rate by a common drive means (not shown) which may act on support means for the clamps 2 and 5 in order to prevent additional contact with the rod and sleeve and ensure that the amount of fibre drawn from the rod and sleeve is maximised. CLAIMS

1. A method of manufacturing optical fibre in which a rod-in-tube assembly is disposed substantially vertically for downwards advancement into the heating zone of a furnace, including the steps of forming a rod-in-tube assembly by clamping the uppermost end of a rod whereby the rod hangs vertically, clamping the uppermost end of a tube, with the rod disposed within the tube, such that the tube is pivotable with respect to the rod, and during advancement of the assembly into the furnace monitoring the concentricity of the rod in the tube at a position adjacent to the furnace entrance and in response thereto pivoting the tube whereby to optimise the concentricity at the heating zone.

2. A method as claimed in claim 1, wherein the rod is clamped by means of a gimbal pivot mechanism.

3. A method as claimed in claim 1, wherein the rod is a rod preform comprising glass forming the core material of the fibre and provided with a corecladding structure.

4. A method of manufacturing optical fibre substantially as herein described with reference to and as illustrated in the accompanying drawing.

5. An optical fibre made by a method as claimed in any one of the preceding plaims.