GB1581554A - Manufacture of optical fibre cables - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THE MANUFACTURE OF OPTICAL FIBRE CABLES (71) We, TELEPHONE CABLES LIMITED, of Chequers Lane, Dagenham, Essex RM9 6QA, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the manufacture of optical fibre cables, that is to say cables of the type including a core consisting of one or more optical fibre waveguides along which communications signals can be transmitted in the form of visible light or infra red or ultra violet radiation, and a protective sheath surrounding the core.

The sheath of an optical fibre cable is suitably formed of a thermoplastic material of low coefficient of friction, such as polyethylene, and the invention is concerned with a method of manufacturing such a cable wherein an optical fibre waveguide, or a bundle of optical fibre waveguides, constituting the cable core, is fed into the bore of a tubular sheath of thermoplastic material during the extrusion of the sheath.

This procedure may be carried out by means of an extruder with a point and die head, the point component having a central duct through which the optical fibre or fibre bundle is fed into the sheath extruded from an annular channel between the point and die components. The assembly of fibre or fibre bundle and sur rounding extruded sheath is pulled continuously from the extruder head, if desired first through a tank of water wherein the sheath is cooled, and thence to a drum on which the assembly, usually constituting the finished cable, is wound. The invention is also concerned with an improvement in the apparatus employed for carrying out this process.

It has been proposed to cover an optical fibre bundle with a sleeve of braided or woven textile material, within the cable sheath, but we have now found that some advantages are gained by omitting this sleeve, and covering the fibre or fibre bundle only with a thermoplastic sheath of sufficiently wide bore to permit freedom of movement of the fibre or individual fibres, within it in both radial and axial directions: such freedom of movement assists in maintaining the optical losses of the fibre or fibres in operation at a low value. However, in the manufacture of a cable of this form, a difficulty arises in that the fibres adhere to the hot, softened thermoplasic sheath emerging from the extruder head, so that in the finished cable the fibres follow a "wavy" path, due to shrinkage of the sheath on cooling, this distortion of the fibres resulting in high microbending losses in the fibres. The desired freedom of movemnt of the fibre or fibres is also prevented or restricted by adhesion to the sheath, and this again can result in high optical losses in operation.

It is an object of the present invention to provide an improvement in the process for manufacturing an optical fibre cable of the form described above, consisting of an optical fibre waveguide or a bundle of optical fibre waveguides surrounded only by a loosely fitting thermoplastic sheath, with no inner sleeve, whereby the abovementioned difficulty can be overcome.

According to the invention, in a method of manufacturing an optical fibre cable including a core consisting of an optical fibre waveguide or a bundle of optical fibre waveguides, and surrounding said core, a tubular sheath of thermoplastic material having a bore of sufficiently large cross- section to permit freedom of movement of the fibre or the individual fibre of the core within it, in both radial and axial directions, which method includes the steps of extruding hot softened thermoplastic material from an annular channel between a die component and a point component of ,an extruder die-head to form a said sheath, feeding a said core through a central duct in the said point component and thence into the bore of the extruded sheath, and continuously pulling the assembly of sheath and core from the extruder die-head, wherein before said core is introduced into said duct an inner tube the bore of which is of suitable cross-section for accommodating said core with a loose or sliding fit, is inserted through said duct so as to lie within the whole length of the duct and to extend beyond the point of the extruder die-head to a position at which the extruded sheath will have cooled sufficiently to prevent adhesion of the fibre or fibres of the core thereto, then the core is pushed through the bore of the said inner tube so as to project beyond the so extending end thereof and to enter the bore of the extruded sheath at said position, and the sheath is caused to grip the end or ends of the fibre or fibres at a point beyond said position in the path of travel of the sheath from the extruder die-head, to enable the fibre or fibres to be pulled from the extruder die-head with the sheath, the fibre or fibres subsequently being released from said grip.

The use of the inner tube for initially supporting the optical fibre core, in accordance with the invention, is advantageous in that it prevents the fibre or fibres from coming into contact with the extruded sheath until the latter has cooled to such an extent that the fibre or fibres will not adhere thereto. The gripping of the end or ends of the fibre or fibres by the sheath enables the whole length of the fibre or fibre bundle to be pulled from the extruder die-head together with the sheath, after passing through the inner tube, which is retained in situ.

In the apparatus for use in carrying out the above-described method of manufacturing an optical fibre cable, which comprises an extruder die-head including a die component, a point component with a central duct therein, an annular extrusion channel between said die component and point component, and a back plate, and a said inner tube, the inner tube is of sufficient length to extend through the back plate of the extruder head so that the outer end of said tube, into which the optical fibre core is introduced, is ready accessible. To facilitate insertion of the inner tube into the point component duct, the latter may be extended to the back plate in the form of a metal tube integral with the said component.

The inner tube may be formed of somewhat flexible material with low friction with respect to the optical fibre or fibres and the material of which the point component and said metal tube extension are formed: for example, polytetrafluoroethylene is a suitable material for the tube. Alternatively, the inner tube may be formed of a rigid material such as stainless steel. If desired, a composite tube, consisting of a stainless steel tube fitting closely within a polytetrafluoroethylene tube, may be employed: this gives the combined advantages of rigidity and low friction. When the cable sheath is formed of a translucent material such as polyethylene, as is preferred, and the inner tube is also formed of translucent material, the inner tube is advantageously coloured so as to be visible through the sheath, so that its position can be observed.

Since the assembly of sheath and optical fibre core is usually wound on to a drum as it is pulled from the extruder die-head, gripping of the fibre end or ends by the sheath can be achieved during the winding operation, by virtue of the tensioning effect produced thereby, which causes the fibre or fibres to lie upon and be held against the part of the sheath wall adjacent to the drum: release of the fibre or fibres occurs when the cable is unwound from the drum to be laid in an operating location.

However, if this method of gripping the fibre end or ends is relied upon, it is necessary, after the commencement of the extrusion process, to push the fibre or fibres through the inner tube and the initial length of the extruded sheath until the fibre end or ends is or are caught in the sheath around the drum. It is therefore preferable to pinch the sheath on the fibre end or ends shortly after the latter emerge from the inner tube. Any initial length of the sheath which has been pulled before the insertion of the fibre or fibres into the sheath is subsequently cut off, and if the latter method of gripping the fibre end or ends has been used the pinched portion of the sheath is also cut off to effect release of the fibre or fibres.

The optical fibre waveguide or wave- guides employed in the manufacture of the cable may be of either the single mode or multi-mode type, and may be of any desired composition and structure. Thus the fibre or fibres may be formed wholly of glass or vitreous silica containing one or more additives for modifying the refractive index of the vitreous material, distributed in the fibre in such a manner as to give either a step refractive index profile or a graded refractive index profile, in known manner.

Alternatively, the fibre or fibres may consist of a vitreous core and a cladding layer of synthetic plastic material having a refractive index lower than that of the core material. Preferably the fibre, or the in dividual fibres of a bundle, is or are coated with one or more protective layers of a synthetic plastic material or materials, at least the first coating layer being applied immediately after the fibre is drawn, in order to preserve the initial high strength of the fibre and to protect it from damage.

Advantageously at least part of the thickness of such coating contains an inorganic filler material to improve the modulus of elasticity of the coating and so increase the breaking load of the coated fibre; suitable filler materials are, for example, carbon powder, silica flour, and titania powder, and where a white filler material is used this may be combined with colouring matter for colour coding of the individual fibres of a bundle.

If desired, suitable reinforcing material may be incorporated in the extruded cable sheath, for increasing the tensile strength of the sheath. A preferred formed of reinforcement consists of a plurality of elongate members of high tensile strength, suitably metal wires, embedded in the wall of the sheath and disposed around and parallel to the optical fibre or fibres. Such members can be incorporated in the sheath during the extrusion thereof, by feeding the wires (or similar members) through ducts, surrounding the central duct, in the point component of the extruder diehead. Alternatively, particulate reinforcing material, for example small glass beads, may be mixed with the thermoplastic material before the latter is extruded to form the sheath.

A specific method, in accordance with the invention, for the manufacture of an optical fibre cable consisting of a bundle of optical fibre waveguides surrounded by a loosely fitting sheath of thermoplastic material reinforced with steel wires, will now be described by way of example, with reference to the diagrammatic drawing accompanying the Provisional Specification, in which Figure 1 shows, in part-sectional elevation, apparatus including parts of an extruder die-head, employed for the method, with the components of the cable at an early stage of the extrusion process, and Figure 2 is a cross-section of a cable produced by the said method.

The parts of the extruder die-head shown in Figure 1 of the drawing consist of a conventional die component 1, a specially designed point component 2, separated from the die component by an annular channel 3, and a back plate 4. The point component has an elongate nozzle 5, through which the central duct 6 extends, and is formed with a plurality of additional ducts 7, arranged at regular intervals in a circle around the central duct, and terminating in holes in a conical collar 8 located around the central duct. The back plate 4 has a central hole 9 and a plurality of smaller surrounding holes 10, respec tively corresponding to and aligned with the central duct 6 and surrounding ducts 7 in the point component, and a stainless steel tube 11 extends backwards from the central duct 6 of the point component, through the central hole 9 in the back plate. A cold water tank 12 is provided adjacent to the outlet 13 of the die-head, leaving a small air gap for initial cooling of the extrudate, the water tank being of a sufficient length to further cool the extruded sheath to room temperature in the time taken for the extrudate to travel through the tank.

In carrying out the sheath-forming process, steel wires 14 are fed continuously through the holes 10 in the back plate and the ducts 7 in the point component, while at the same time thermoplastic material supplied to the channel 3 is extruded around the wires, the nozzle 5 ensuring that the extruded tubular sheath 15 has a bore of diameter somewhat larger than that of the optical fibre bundle which is to constitute the cable core. Shortly after commencement of the extrusion process a tube 16 of polytetrafluoroethylene is inserted through the tube 11 and central duct 6 of the point component, the tube 16 being of sufficient length to extend from some distance beyond the outer side of the back plate 4 to a position in the water tank at which the extruded sheath will have cooled sufficiently to prevent adhesion of the optical fibre waveguides thereto. A bundle of optical fibres 17 is then introduced into the outer end of the tube 16 and pushed manually through this tube until the ends of the fibres have emerged from the end thereof within the extruded sheath. When the fibre bundle protrudes sufficiently far from the said end of the tube 16, the moving sheath 15 is pinched on to the fibre ends, suitably manually by means of pliers, so that the sheath grips the fibre ends firmly and, as the sheath continues to travel, the fibres are pulled through the polytetrafluoroethylene tube 16. Thereafter the assembly of fibre bundle 17, wires 14 and sheath 15 is continuously pulled and wound on to a drum (the pullling means and drum are of conventional form and are not shown in the drawing).

In a specific example of the apparatus and procedure described above with refer ence to Figures 1, the bore diameter of the die-head outlet 13 and the external diameter of the nozzle 5 are such that the sheath 15 produced has a radial thickness of 3 mm and a bore of 2 to 3 mm diameter; eight reinforcing wires 14, each of dia meter 0.5 mm, are located approximately in the centre of the radial thickness of the sheath. The cable core consists of a bundle of seven optical fibre waveguides, each composed of a 120 micrometre diameter fibre of vitreous silica doped with phosphorus pentoxide to give a graded refractive index profile, coated with a first layer of carbon-loaded polyurethane resin and three further layers of the same resin containing titania powder with a dye or other colouring material; the total diameter of each coated fibre is approximately 150 micrometres, and the bundle has a diameter of approximatley 0.5 mm.

The thermoplastic material employed for forming the sheath, in the specific example, is polyethylene, which is extruded at a temperature of 1900 to 2000 C, and which is required to be cooled to approximately 100"C before being brought into contact with the optical fibres, to ensure that adhesion between the fibres and the sheath is avoided. The cable is pulled from the ex truded die-head at the rate of 10 metres per minute, and at this rate of travel a water tank 20 metres long is required for cooling the extruded sheath to room tem perature; the temperature is reduced to 100"C in approximately half a minute, in cold water: hence the inner polytetraluoroethylene tube 16 is arranged to extend into the water tank for a distance of 5 metres, that is to say to the point at which the sheath temperature is reduced to approxi mately 100"C. The tube 16 employed to accommodate the specific fibre bundle described above has an external diameter of 1.2 mm and an internal diameter of 0.75 mm; the tube may suitably be coloured orange, in order to be visible through the translucent polyethylene sheath so that the position of the tube end within the sheath can be correctly located.

In a modification of the apparatus described above with reference to Figure 1 of the drawing, the polytetrafluoroethylene tube 16 may be replaced by a stainless steel tube, or a stainless steel tube may be employed in addition, fitting closely within the polytetrafluoroethylene tube.

The completed cable, manufactured in accordance with the specific example, is shown in cross-section in Figure 2, in which the components of the cable are indicated by the same reference numerals as those employed for the said components in Figure 1.

WHAT WE CLAIM IS: - 1. A method of manufacturing an optical fibre cable including a core consisting of one or more optical fibre waveguides and, surrounding said core, a tubular sheath of thermoplastic material having a bore of sufficiently large cross-section to permit freedom of movement of the fibre or the individual fibres of the core within it, in both radial and axial directions, which method includes the steps of extruding hot softened thermoplastic material from an annular channel between a die component and a point component of an extruder die-head to form said sheath, feeding the said core through a central duct in the said point component and thence into the bore of the extruded sheath, and continuously pulling the assembly of sheath and core from the extruder diehead, wherein before said core is introduced into said duct an inner tube, the bore of which is of suitable cross-section for accommodating said core with a loose or sliding fit, is inserted through said duct so as to lie within the whole length of the duct and to extend beyond the point of the extruder die-head to a position at which the extruded sheath will have cooled sufficiently to prevent adhesion of the fibre or fibres of the core thereto, then the core is pushed through the bore of the said inner tube so as to project beyond the so extending end thereof and to enter the bore of the extruded sheath at said position, and the sheath is caused to grip the end or ends of the fibre or fibres at a point beyond said position in the path of travel of the sheath from the extruder die-head, to enable the fibre or fibres to be pulled from the extruder die-head with the sheath, the fibre or fibres subsequently being released from said grip.

2. Apparatus for use in manufacturing an optical fibre cable by the method according to Claim 1, which comprises an extruder die-head including a die component, a point component with a central duct therein, an annular extrusion channel between said die and point components, and a back plate, and a said inner tube, wherein the said inner tube is of sufficient length to extend through the back plate of the extruder die-head.

3. Apparatus according to Claim 2, wherein the central duct in the point component is extended to the back plate in the form of a metal tube integral with the point component.

4. Apparatus according to Claim 2 or 3, wherein the said inner tube is formed of polytetrafluoroethylene, or of stainless steel, or is a composite tube consisting of a stainless steel tube fitting closely within a polytetrafluoroethylene tube.

5. A method according to Claim 1, wherein the assembly of sheath and optical fibre core is wound on to a drum as it is pulled from the extruded die-head, and wherein gripping of the end or ends of the fibre or fibres by the sheath is effected during the winding operation by virtue of

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

Claims (10)

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

   meter 0.5 mm, are located approximately in the centre of the radial thickness of the sheath. The cable core consists of a bundle of seven optical fibre waveguides, each composed of a 120 micrometre diameter fibre of vitreous silica doped with phosphorus pentoxide to give a graded refractive index profile, coated with a first layer of carbon-loaded polyurethane resin and three further layers of the same resin containing titania powder with a dye or other colouring material; the total diameter of each coated fibre is approximately

   150 micrometres, and the bundle has a diameter of approximatley 0.5 mm.

   The thermoplastic material employed for forming the sheath, in the specific example, is polyethylene, which is extruded at a temperature of 1900 to 2000 C, and which is required to be cooled to approximately 100"C before being brought into contact with the optical fibres, to ensure that adhesion between the fibres and the sheath is avoided. The cable is pulled from the ex truded die-head at the rate of 10 metres per minute, and at this rate of travel a water tank 20 metres long is required for cooling the extruded sheath to room tem perature; the temperature is reduced to 100"C in approximately half a minute, in cold water: hence the inner polytetraluoroethylene tube 16 is arranged to extend into the water tank for a distance of 5 metres, that is to say to the point at which the sheath temperature is reduced to approxi mately 100"C. The tube 16 employed to accommodate the specific fibre bundle described above has an external diameter of 1.2 mm and an internal diameter of 0.75 mm; the tube may suitably be coloured orange, in order to be visible through the translucent polyethylene sheath so that the position of the tube end within the sheath can be correctly located.

   In a modification of the apparatus described above with reference to Figure 1 of the drawing, the polytetrafluoroethylene tube 16 may be replaced by a stainless steel tube, or a stainless steel tube may be employed in addition, fitting closely within the polytetrafluoroethylene tube.

   The completed cable, manufactured in accordance with the specific example, is shown in cross-section in Figure 2, in which the components of the cable are indicated by the same reference numerals as those employed for the said components in Figure 1.

   WHAT WE CLAIM IS: - 1. A method of manufacturing an optical fibre cable including a core consisting of one or more optical fibre waveguides and, surrounding said core, a tubular sheath of thermoplastic material having a bore of sufficiently large cross-section to permit freedom of movement of the fibre or the individual fibres of the core within it, in both radial and axial directions, which method includes the steps of extruding hot softened thermoplastic material from an annular channel between a die component and a point component of an extruder die-head to form said sheath, feeding the said core through a central duct in the said point component and thence into the bore of the extruded sheath, and continuously pulling the assembly of sheath and core from the extruder diehead, wherein before said core is introduced into said duct an inner tube, the bore of which is of suitable cross-section for accommodating said core with a loose or sliding fit, is inserted through said duct so as to lie within the whole length of the duct and to extend beyond the point of the extruder die-head to a position at which the extruded sheath will have cooled sufficiently to prevent adhesion of the fibre or fibres of the core thereto, then the core is pushed through the bore of the said inner tube so as to project beyond the so extending end thereof and to enter the bore of the extruded sheath at said position, and the sheath is caused to grip the end or ends of the fibre or fibres at a point beyond said position in the path of travel of the sheath from the extruder die-head, to enable the fibre or fibres to be pulled from the extruder die-head with the sheath, the fibre or fibres subsequently being released from said grip.
2. 2. Apparatus for use in manufacturing an optical fibre cable by the method according to Claim 1, which comprises an extruder die-head including a die component, a point component with a central duct therein, an annular extrusion channel between said die and point components, and a back plate, and a said inner tube, wherein the said inner tube is of sufficient length to extend through the back plate of the extruder die-head.
3. 3. Apparatus according to Claim 2, wherein the central duct in the point component is extended to the back plate in the form of a metal tube integral with the point component.
4. 4. Apparatus according to Claim 2 or 3, wherein the said inner tube is formed of polytetrafluoroethylene, or of stainless steel, or is a composite tube consisting of a stainless steel tube fitting closely within a polytetrafluoroethylene tube.
5. 5. A method according to Claim 1, wherein the assembly of sheath and optical fibre core is wound on to a drum as it is pulled from the extruded die-head, and wherein gripping of the end or ends of the fibre or fibres by the sheath is effected during the winding operation by virtue of

   the tensioning effect produced thereby, the fibre or fibres being subsequently released on unwinding of the cable from the drum.
6. 6. A method according to Claim 1, wherein gripping of the end or ends of the fibre or fibres by the sheath is effected by pinching the sheath on to the said fibre end or ends shortly after the latter emerge from the said inner tube, the pinched portion of the sheath being subsequently cut off to release the fibre or fibres.
7. 7. A method according to Claim 1, 5 or 6, wherein elongate reinforcing members are incorporated in the cable sheath, and disposed around and parallel to the cable core, by feeding said members through ducts surrounding the central duct in the point component of the extruder diehead, during the extrusion of the sheath.
8. 8. A method according to Claim 1, for the manufacture of an optical fibre cable, substantially as hereinbefore described by way of example and with reference to the drawing accompanying the Provisional Specification.
9. 9. Apparatus according to Claim 2, for the manufacture of an optical fibre cable, substantially as shown in, and as hereinbefore described with reference to, Figure 1 of the drawing accompanying the Provisional Specification.
10. 10. An optical fibre cable manufactured by a method according to Claim 1, 5, 6, 7 or 8.