GB2046471A - Tube containing optic fibre(s) and thixotropic fluid - Google Patents

Abstract

In a fibre optic element, which may constitute a cable or a unit for incorporation in a cable, consisting of one or more straight resin-coated optical fibres loosely disposed in a tube of synthetic polymeric material, the tube is filled with a thixotropic fluid. A preferred fluid is a colloidal suspension of finely powdered silica containing silanol groups, in a silicone oil. The fluid can be introduced into the tube simultaneously with the fibre or fibres, during the extrusion of the tube: apparatus for use in carrying out this procedure is described.

Description

SPECIFICATION Fibre optic elements and their manufacture This invention relates to fibre optic elements of the type consisting essentially of one or more straight optical fibre waveguides, each having an adherent protective coating of synthetic resin, loosely disposed within a tube of synthetic polymeric material and lying substantially parallel to the tube axis. The invention also relates to a method of and apparatus for manufacturing such elements, and to optical fibre cables including one or more of such elements.

The fibre-containing tube may be the sheath, or an inner layer of the sheath, of an optical fibre cable, the fibre or fibres constituting the cable core and being loosely disposed in the bore of the sheath. Alternatively, the element consisting of one or more optical fibres within a said tube may constitute a unit for incorporation in a cable including a plurality of such units, or including one or more of such units together with one or more electrical conductors.

The term "straight", as used herein with reference to an optical fibre, is to be understood to have its usual meaning, namely without curve or bend and extending uniformly in the same direction, and in particular to mean that the fibre is free from twist.

The term "loosely", as used herein with reference to the disposition of the fibre or fibres within the said tube, is to be understood to mean that the bore of the tube is of sufficiently large cross-section to permit freedom of movement of the fibre, or of each individual fibre, within the bore in both radial and axial directions. Such freedom of movement of optical fibre waveguides within a containing tube is advantageous in that strains introduced into the fibres by bending and manipulation of the cable or element are thereby minimised, and hence optical losses in operation, resulting from such strains, are reduced.

However, some optical losses can arise from other causes, in such loosely contained fibres: thus, where several fibres are contained within a single tube, micro-bending losses may result from contact of the fibres with one another; in addition, transient increases in optical loss can occur as a result of external forces, for example when a sharp blow is delivered to the cable or tube, especially in the case of fibres having a protective coating of a hard resin.

Another possible cause of increased optical loss is the entry of water into the tube or cable bore, and subsequent freezing of the water around the fibres.

It is an object of the present invention to provide an improvement in a fibre optic element of the type referred to, whereby optical losses resulting from causes such as those described in the preceding paragraph can be reduced or eliminated.

According to the invention, in a fibre optical element consisting essentially of one or more straight optical fibre waveguides, each having an adherent protective coating of synthetic resin, loose ly disposed within a tube of synthetic polymeric material and lying substantially parallel to the tube axis, the said tube contains a thixotropic fluid surrounding the fibre or each fibre and filling the space between the fibre or fibres and the tube wall.

Athixotropicfluid is advantageous as a filling medium in an optical fibre-containing tube in view of its relatively high viscosity when subjected to a shearing force. Thus, the viscosity of the unstressed fluid is sufficiently high to prevent it from flowing out of the open ends of a tube of narrow bore during manipulation of the tube in the manufacture of a cable, or when the tube is cut for splicing or termination of the fibre or fibres, or for any other purpose, while at the same time permitting adequate freedom of movement of the fibre or fibres immersed in the fluid relative to one another (in cases where there is a plurality of fibres) and to the wall of the tube: preferably, to ensure that the fluid is retained in the tube, the diameter of the tube bore is not greater than 4 millimetres, but the upper limit of the tube bore diameter in this respect may vary for thixotropic fluids of different normal viscosities. The viscous fluid filling acts as a cushion minimising the effect on the fibre of any external forces applied to the cable or tube, and the possibility of stress being transmitted to the fibres is further reduced by the reduction of the viscosity of the fluid under applied stress, which enables the fibres to move rapidly in the fluid to take up positions of minimum strain. In addition, the reduction in viscosity under stress facilitates the introduction of the fluid into the tube, during manufacture of the element, by pumping under relatively light pressure.

The fluid filling prevents the entry of water into the tube following damage during installation or service, for example, thus eliminating the risk of increased optical loss from this cause. The filling may also be beneficial in minimising or preventing possible effects of vibration on the optical transmission of the fibre or fibres. Furthermore, when several fibres are introduced into the fluid-filled tube, the fluid surrounds the individual fibres and thus tends to minimise the extent of mechanical contact between the fibres. A relatively large number of fibres can be incorporated in a cable by means of a plurality of elements each consisting of a tube of narrow bore, for example up to 2 millimetres diameter, containing a small number of fibres: this arrangement is preferred to the use of a single element comprising a tube of wider bore containing all the required fibres.

The thixotropic fluid employed is suitably a colloidal solution or suspension. A preferred type of material is a silicone oil containing a colloidal suspension of silica of high purity and preferably incorporating silanol (Si-O-H) groups in its chemical constitution: the silicone oil has the advantage that its viscosity undergoes little change with variations in temperature, and the colloidal silica increases the viscosity of the liquid silicone and imparts thixotropic properties of the fluid. A colloidal suspension of this type has the added advantage that, after its viscosity has been reduced by the application of a shearing stress, it very rapidly reverts to its normal viscosity when the stress is removed.

In the manufacture of a fibre optic element in accordance with the invention, the polymer tube is usually formed by extrusion, and the fluid filling can be introduced into the tube during the extrusion process, provided that the fluid employed is capable of withstanding the extrusion temperature without vaporising, decomposing or undergoing any detrimental chemical changes. Preferably the fluid and the fibre or fibres are introduced simultaneously into the tube during the extrusion thereof: this is conveniently achieved by passing the fibre or fibres into the bore of the extruded tube through a feed tube, suitably of steel or polytetrafluoroethylene, located in a central duct in the extruder head, and at the same time pumping the filling fluid into the said feed tube through a side tube attached thereto.The introduction of the fluid into the extruded tube simultaneously with the fibres is advantageous in that, in addition to tending to prevent the fibres from coming into contact with one another within the extruded tube, the fluid prevents the fibres from sticking to the extrudate, which can occur while the latter is in a soft condition on emerging from the extruder head: it is therefore unnecessary for the fibre feed tube to extend beyond the outlet of the extruder head into the bore of the extruded tube, which extension would otherwise be desirable to prevent sticking of the fibres to the soft tube wall.

If desired, one or more elongate reinforcement members, such as metal wires or resin-impregnated aromatic polyamide yarns, may be embedded in the wall of the polymer tube, for increasing the tensile modulus of the tube and increasing its resistance to buckling, crushing and longitudinal shrinkage, and thus providing further protection for the optical fibre or fibres against mechanical stress. Such reinforcement members can also be incorporated in the tube wall during the extrusion of the tube, by feeding said members into the extrudate through ducts suitably located in the extruder head.

A single fibre optic element in accordance with the invention may be employed for forming an optical fibre cable, with or without an outer sheath of synthetic plastic material, and with or without reinforcing members as aforesaid in the wall of the element tube and/or in the outer sheath. Alternatively, a cable may be formed of a plurality of such elements, or of one or more of the fibre optic elements and one or more electrical conductor elements, contained within an outer sheath, all the elements being either laid up straight and parallel to one another or, preferably, helically stranded, and in either case the elements may be arranged around a central strength member, in the form of a metal wire or resin-impregnated aromatic polyamide yarn.

One specific form of fibre optic element in accordance with the invention will now be described by way of example, and a method of manufacturing the element will be described, also by way of example, with reference to the accompanying diagrammatic drawing which shows, in sectional elevation, the apparatus employed for said manufacture.

The fibre optic element of the example consists of an extruded tube of amorphous nylon having internal and external diameters of 1.5 mm and 2.0 mm respectively, containing a single resin-coated optical fibre, and filled with a thixotropic fluid consisting of a colloidal suspension of 3.5% by weight of"Aerosil" (Registered Trade Mark) in "Dow Corning 200" (Registered Trade Mark) silicone oil. "Aerosil" is a very finely powdered pure silica of amorphous structure which has a thixotropic filler action due to the presence of silanol groups on the particle surfaces: the grade employed in making up the fluid used in this example consists of particles 7 nanometres in diameter.The silicone oil is a dimethyl siloxane polymer, the grade employed having a viscosity of 20 centistokes at 25"C. The optical fibre employed consists of a silica fibre waveguide 120 microns in diameter, with a dopant distribution imparting a graded refractive index profile to the waveguide, and with a multilayer coating of polyurethane resin containing fillers consisting of carbon powder in the inner iayers and titanium oxide powder and colouring matter in the outer layers, the overall diameter of the coated fibre being 150 microns.

It is to be understood that the element of the above example is not limited to the specific form of optical fibre described: for example the fibre may be of the stepped refractive index profile type comprising a core of silica or doped silica or other vitreous material, and a cladding of silica, doped silica, or polymeric material, and the protective coating may be formed of a resin other than polyurethane, for example any other suitable type of hard resin, or a soft resin such as a silicone, or of two or more different resins, or the fibre may have a buffer coating of silicone covered with a tightly fitting extruded jacket of harder resin.

The fibre optical element of the example is manufactured by introducing the optical fibre and the thixotropic fluid simultaneously into the extruded nylon tube, using the apparatus shown in the drawing. The apparatus consists essentially of an extruder head comprising a die component 1 and a point component 2 having a central duct 3 and a straight nozzle 4, the two components being separated by an annular channel through which the hot extrudate material 5, heated to 200"C in the case of amorphous nylon, is passed under pressure, and a steel feed tube 6 which is inserted through the duct 3, terminating at the tip of the nozzle 4, and is fitted with a side tube 7; the length of the feed tube 6 to the rear of the side tube 7 (that is to say remote from the extruder head) is suitably about 20 cm.In operation, while the extrusion process is carried out to produce the tube 8, the nozzle 4 ensuring the formation of a tube with the requisite bore diameter, an optical fibre 9 is fed through the tube 6 into the bore of the extruded tube 8 and at the same time the thixotropic fluid 10 is pumped through the side tube 7, under a pressure of 35 to 40 kN/m2, into the tube 6 and thence into the extruded tube 8, loss of fluid from the rear end of the tube 6 being prevented by the comparatively high impedance to fluid flow afforded by the length of the tube 6 to the rear of the side tube. The directions of travel of the fibre, the fluid, and the completed assembly of fluid-filled tube and fibre, are indicated by the arrows. These operations are carried out continuously until an element of the required length which may be up to several kilometres, has been produced; the element may be wound upon a drum as it is formed.

It will be understood that an element containing a plurality of optical fibres may be manufactured in the manner described above, the required number of fibres being fed simultaneously through the steel tube 6 into the bore of the extruded tube, and if necessary an extruder head adapted to produce an extruded tube of wider bore being used.

Claims (14)

1. A fibre optic element consisting essentially of one or more straight optical fibre waveguides, each having an adherent protective coating of synthetic resin, loosely disposed within a tube of synthetic polymeric material and lying substantially parallel to the tube axis, wherein the said tube contains a thixotropic fluid surrounding the fibre or each fibre and filling the space between the fibre or fibres and the tube wall.

2. Afibre optic element according to Claim 1, wherein the said tube has a bore diameter not greater than 4 millimetres.

3. A fibre optic element according to Claim 1 or 2, wherein the said fluid is a colloidal solution or suspension.

4. Afibre optic element according to Claim 3, wherein the said fluid consists of a colloidal suspension of silica in a silicone oil.

5. A fibre optic element according to Claim 4, wherein the silica incorporates silanol groups in its chemical constitution.

6. Afibre optic element according to any preceding Claim, wherein one or more elongate reinforcement members is or are embedded in the wall of the said tube.

7. A method of manufacturing a fibre optic element according to any preceding Claim, wherein the said tube is formed by extrusion, and the thixotropicfluid is introduced into the tube during the extrusion process.

8. A method according to Claim 7, wherein the tube is extruded by means of an extruder head having a central duct in which a fibre feed tube is located, which feed tube has a side tube attached thereto, and during the extrusion process an optical fibre or fibres is or are passed into the bore of the extruded tube through the said feed tube and simultaneously the thixotropic fluid is pumped into the said feed tube through the side tube and is thus introduced into the extruded tube.

9. A method according to Claim 7 or 8, wherein one or more elongate reinforcement members is or are incorporated in the wall of the extruded tube by feeding said member or members into the extrudate through a duct or ducts located in the extruder head.

10. Afibre optic element according to Claim 1, substantially as hereinbefore described in the specific example.

11. A method of manufacturing a fibre optic element according to Claim 1, carried out substantially as hereinbefore described in the specific example.

12. An optical fibre cable including or consisting of one or more fibre optic elements according to any of the preceding Claims 1 to 6 and 10, with or without one or more electrical conductor elements.

13. An optical fibre cable according to Claim 12, including a plurality of said fibre optic elements, or one or more said fibre optic elements and one or more electrical conductor elements, all of which elements are helically stranded around a central strength member.

14. Apparatus for the manufacture of a fibre optic element according to any of the preceding Claims 1 to 6 and 10, substantially as hereinbefore described with reference to the accompanying drawing.