GB1568178A

GB1568178A - Optical fibre cables

Info

Publication number: GB1568178A
Application number: GB781077A
Authority: GB
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1978-02-22
Filing date: 1978-02-22
Publication date: 1980-05-29

Description

(54) IMPROVEMENTS IN OR RELATING TO OPTICAL FIBRE CABLES (71) We, THE GENERAL ELECTRIC COMPANY LIMITED, of 1 Stanhope Gate, London W1A 1EH, 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 optical fibre cables, that is to say cables of the type consisting of a core composed essentially of one or more optical fibre waveguides along which communications signals can be transmitted in the form of radiation in the infra-red, visible or ultra violet region of the electromagnetic spectrum, with a protective sheath surrounding said core. The invention also relates to methods of manufacturing cables of the form described.

The sheath of an optical fibre cable is suitably formed of one or more layers of synthetic plastic material, which may be extruded as a tube or tubes. However, in view of the fragile nature of the optical fibre waveguides, which are usually wholly or mainly composed of glass or vitreous silica, it is desirable to incorporate in the plastic cable sheath some reinforcing material which will increase the tensile modulus of the cable at low strain, and also increase its resistance to buckling and to crushing, hence increasing the load bearing capability of the cable without appreciably reducing its flexibility. It has been proposed to employ a plurality of elongate tensile members, incorporated in the plastic matrix of the cable sheath and disposed around and parallel to the optical fibre or fibres constituting the cable core. A particularly suitable reinforcement of this type consists of steel wires of appropriate diameter for permitting flexibility of the cable whilst imparting the desired strength and load bearing characteristics thereto; where the optical fibres are composed essentially of silica, steel wires are additionally advantageous in having a coefficient of thermal expansion sufficiently low to ensure that the optical fibres will not be subjected to undesirable strains due to differential thermal expansion and contraction of the fibres and the wires.

In some cases, however, the presence of electrically conducting members, such as steel wires, in an optical fibre cable is undesirable. Thus in some circumstances radio interference or high voltages induced in metallic members in the cables could be conducted to ancillary equipment connected to the cables, for example photodiodes and associated circuitry. In such cases there is a requirement for nonconducting reinforcement of the cable sheaths.

It is an object of the present invention to provide an optical fibre cable which has incorporated in the sheath a form of reinforcement which is non-conducting and also has a high tensile modulus. It is a further object of the invention to provide such a cable wherein the material of the sheath matrix, and the form of construction incorporating said reinforcement, are such that the resulting composite sheath structure has good overall load-bearing characteristics including high resistance to crushing and buckling forces.

According to the invention, an optical fibre cable includes a core composed essentially of one or more straight optical fibre waveguides lying parallel to the cable axis and, surrounding the core, a tubular sheath of synthetic plastic material in the wall of which are embedded a plurality of straight elongate reinforcement members extending throughout the length of the cable and disposed around and parallel to the optical fibre or fibres, each of which reinforcement members consists of an assemblage of aromatic polyamide fibres, with or without an impregnant synthetic resin, and with or without a surrounding sleeve of thermoplastic material.

It is to be understood that the term "embedded", as used herein with reference to the reinforcement members, means that the said members are wholly, or substantially wholly, contained within, but not necessarily adherent to, the sheath wall.

The term "straight" means that there are not marked undulations relative to or twistings round the cable axis.

The said assemblages are preferably in the form of yarn composed of aromatic polyamide fibres, one or more straight lengths of yarn being employed to form each reinforcement member. A suitable type of yarn is that sold under the trade name "Kevlar". Thus one preferred form of reinforcement member consists of one or more unimpregnated straight aromatic polyamide yarns, and another preferred form of reinforcement member consists of a semi-rigid wire formed of several straight aromatic polyamide yarns laid parallel to one another and bonded together with resin to form a cohesive structure; in the case of either of these forms of unsleeved reinforcement member, the said members are wholly contained within the sheath wall. A third preferred form of reinforcement member includes a sleeve of thermoplastic material within which one or more yarns, in either the unimpregnated or resin-impregnated state, is or are contained, in sliding contact with the sleeve. Such sleeved reinforcement members may, if desired, be disposed in the region of the sheath wall adjacent to the interior surface thereof, a minor portion of the sleeve of each said member being exposed at the said surface. It is to be understood that the above statement, that the term "embedded" can mean that the reinforcement members are "substantially wholly" contained within the sheath wall, refers only to this arrangement of the sleeved reinforcement members.

The reinforcement members, khether in the form of unimpregnated yarns or resinimpregnated wires, and whether sleeved or not, may suitably be from 2 to 12 in number, and are preferably substantially equally spaced apart in the sheath wall and subsantially equidistant from the optical fibre or fibres.

The reinforcement members, whether in corporated in the cable sheath during the extrusion of the latter around the optical fibre or fibres, the whole of the sheath preferably being formed around the optical fibre or fibres and the reinforcement members in a single continuous extrusion step.

Where sleeved reinforcement members are employed, the thermoplastic sleeves may, at least partially, adhere to the hot ex trudate, and thus become incorporated in the sheath.

Suitable resins for forming the wires composed of resin-bonded polyamide yarns, described above, include, for example, epoxy resins, polyurethane resins, silicone resins, and polyester resins; if desired, the resin may incorporate a filler material for imparting increased load-bearing strength to the bonded structure. In the producing the resin-bonded wires, the aromatic polyamide yarn assemblages are impregnated with liquid resin, under vacuum, and the resin is then cured in known manner.

The aromatic polyamide yarn reinforcing member possess high tensile strength and high tensile modules, of comparable magnitude to the corresponding properties of steel wire: for example, reinforcing members formed of "Kevlar 49" yarn have a tensile modulus of 110 GPa (giga pascals), the tensile modulus of stainless steel wire being, typically, 140 GPa. Such members are thus advantageous for use as non-conducting reinforcement in optical fibre cables, in particular having considerable advantages, in respect of strength, in comparison with other synthetic plastics materials. Furthermore, when employed for reinforcing suitable matrix material, for example consisting of polyethylene, a melt processable polytetrafluoroethylene copolymer, polyurethane resin, polyamide-acrylic resin copolynier or polyester resin, the aromatic polyamide yarn members can impart additional resistance to crushing and buckling forces acting on the composite sheath.

The core of a cable in accordance with the invention may consist of a single optical fibre waveguide or of a bundle of any desired number of such waveguide. The waveguide or waveguides may be of either the single mode or multimode type, and may be of any desired composition and structure. Thus the 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 so as to give either a step refractive index profile or a graded refractive index profile across the crosssection of the fibre, in known manner.

Alternatively, the 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 fibres are individually coated with one or more protective layers of a synthetic resin material or materials, at least part of the thickness of such protective coating advantageous containing an in organic filler material for increasing tile modulus of elasticity of the coating and this increasing the breaking load of the coated fibre.

If desired, the optical fibre or bundle of fibres may be surrounded by a sleeve consisting of, for example, braided or woven textile material, formed of natural or synthetic fibres, or consisting of a tube of thermoplastic material, such a sleeve constituting part of the cable core. Alternatively, the coated optical fibre or fibres may lie loosely in the bore of the tubular thermoplastic sheath, without an intervening sleeve, the sheath bore being of sufficently large diameter to permit freedom of movement of the fibre or the individual fibres within it in both radial and axial directions.

Some specific optical fibre cables in accordance with the invention, and methods by which they can be manufactured, will now be described in the following examples, with reference to the accompanying diagrammatic drawings, in which: Figure 1 shows a first form of cable, in elevation, with part of the sheath cut away to disclose the internal structure of the cable, Figure 2 is a cross-section drawn on the line II-II in Figure 1, Figures 3 and 4 similarly show a second form of cable, in elevation and crosssection respectively, and Figures 5 and 6 are similar views of a third form of cable.

Example 1.

The cable of this example is of the form shown in Figures 1 and 2 of the drawings, in which the cable core consists of an unsleeved bundle of optical fibre waveguides 1, lying loosely in the bore of the cable sheath 2. The sheath is an extruded tube of polyethylene, having embedded in its wall eight semi-rigid wires 3 of circular cross-section, each wire being composed of a number of straight "Kevlar" yarns bonded together with polyurethane resin or epoxy resin, the wires being arranged in a circle around the tube bore and equally spaced apart, substantially in the centre of the sheath wall thickness (only two of the wires 3 are shown in Figure 1, in the interests of clarity).

In the manufacture of the cable of this example, the reinforcing wires 3 are incorporated in the sheath 2 during the extrusion of the sheath around the bundle of optical fibres: thus the tubular sheath is extruded from an annular channel in an extruder die-head, around a central duct in the die-head through which the fibre bundle is fed into the bore of the extruded tube, and the reinforcing wires are fed simultaneously through steel tubes arranged within the central duct, around the fibre bundle, and extending into the die tip, so that the wires are surrounded by the extruded material emerging from the die tip.

Preferably the optical fibre bundle is fed through a steel or polytetrafluoroethylene tube supported centrally in the central duct of the extruder die-head and extending for some distance into the bore of the extruded sheath.

In a specific example of a cable of the form described above with reference to Figures 1 and 2, the core is constituted by seven optical fibre waveguides, each of which consists of a 120 micron diameter fibre of vitreous silica doped with phosphorus pentoxide to give a graded refractive index profile, coated with a first layer of polyurethane resin containing carbon powder as a filler, and three further layers of the same resin containing titania powder and a dye or other colouring matter, for colour coding of the fibres. The total diameter of each coated fibre is approximately 150 microns, and the fibre bundle has an overall diameter of approximately 0.5 mm. The polyethylene sheath has a radial wall thickness of 3 mm and a bore diameter of 2 to 3 mm, and the diameter of each of the reinforcing members is 0.5 mm.

Example 2.

The cable of this example, which is shown in Figures 3 and 4 of the drawings, consists of an optical fibre bundle core 4, surrounded by a sheath 5 composed either of a polyamide-acrylic resin copolymer or of a melt-processable polyester resin, in which sheath are embedded six unimpregnated "Kevlar" yarns, 6 (only two of which are shown in Figure 3), disposed parallel to the optical fibres and somewhat nearer to the inner surface than to the outer surface of the sheath wall. This form of cable is manufactured by a single extrusion step, as in the case of the cable of Example 1, the whole of the sheath being extruded as a tube while the optical fibre bundle is fed into the bore of the extruded tube as described in Example 1, and the "Kevlar" yarns are simultaneously fed through steel tubes arranged around a central tube of polytetrafluoroethylene or steel through which the optical fibre bundle is introduced, in the central duct of the extruder die-head, so that the "Kevlar" yarns are introduced into the radial thickness of the extrudate but do not adhere thereto. Both of the resins specified above for use for forming the cable sheath in this case are advantageous in that they can withstand temperatures up to about 125 C.

In a specific example of the cable of the form described with reference to Figures 3 and 4, the core consists of two optical fibre waveguides of the same form as those described in Example 1. The bore of the sheath is 1 mm in diameter, the radial thickness of the sheath wall is 3 mm, and each of the reinforcing member 6 consists of a single 1420 denier "Kevlar" yarn.

Example 3.

In the cable shown in Figures 5 and 6 of the drawings, the core consists of a single optical fibre waveguide 7 loosely surrounded by a tubular polyethylene sleeve 8. The cable sheath 9 is formed of a melt-processable polyester resin and in cludes six reinforcing members in the form of sleeved "Kevlar" yarns 10, the sleeves 11 being composed of the same polyester resin as the sheath, and being disposed adjacent to the sheath bore. In Figure 5, parts of the sheath 9 and sleeves 8 and 11 have been cut away to show, respectively, the sleeves, and the optical fibre and "Kevlar" yarns: only two of the reinforcing members are shown in Figure 5. This cable is also manufactured by a single extrusion process, during the course of which the sleeves 1-1 are partially fused with the extrudate and are embedded therein with a minor portion of each said sleeve remaining exposed in the sheath bore; the fibre sleeve 8 is in loose sliding contact with these portions of the sleeves 11.

In a specific example of the form of cable shown in Figures 5 and 6, the optical fibre is of the same form as those de scribed in Example 1, all the sleeves 8 and 11 are of 1.2 mm external diameter, and the wall thickness of the sheath 9 is 3 mm.

It will be appreciated that the cable of Example 3 may, if desired, be modified by incorporating a plurality of optical fibres instead of the single fibre 7, the diameter of the sleeve 8 being increased if necessary to accommodate the fibres with the desired freedom of movement within said sleeve.

The optical fibre cables of all the forms described in the above examples have high load-bearing characteristics, crush resistance, buckling resistance and bending strength.

WHAT WE CLAIM IS: 1. An optical fibre cable including a core composed essentially of one or more straight optical fibre waveguides lying parallel to the cable axis and, surrounding the core, a tubular sheath of synthetic plastics material in the wall of which are embedded (as hereinbefore defined) a plurality of straight elongate reinforcement members extending throughout the length of the cable and disposed around and parallel to the optical fibre or fibres, each of which reinforcement members consists of an assemblage of aromatic polyamide fibres, with or without an impregnant synthetic resin, and with or without a surrounding sleeve of thermoplastic material.

2. An optical fibre cable according to Claim 1, wherein each of said reforcement members consists of one or more straight yarns composed of aromatic polyamide fibres, unimpregnated with resin.

3. An optical fibre cable according to Claim 1, wherein each of said reinforcement members consists of a semi-rigid wire formed of several straight yarns composed of aromatic polyamide fibres, said yarns being laid parallel to one another and bonded together with an impregnant resin to form a cohesive structure.

4. An optical fibre cable according to Claim 3, wherein the resin employed for bonding said yarns together to form the reinforcing wires consists of an epoxy resin, or a polyurethane resin, or a silicone resin, or a polyester resin 5. An optical fibre cable according to Claims 3 or 4, wherein the said resin incorporates a filler material for imparting increased load-bearing strength to the reinforcement members.

6. An optical fibre cable according to Claim 1, wherein each of said reinforcement members consists of one or more straight yarns composed of aromatic polyamide fibres, either unimpregnated or impregnated with resin, surrounded by a sleeve of thermoplastic material with which said yarn or yarns is or are in sliding contact.

7. An optical fibre cable according to Claim 6, wherein said reinforcement members are disposed in the region of the sheath wall adjacent to the interior surface thereof, a portion of the sleeve of each said member being exposed at the said surface.

8. An optical fibre cable according to any preceding Claim, wherein the number of said reinforcement members embedded in the sheath is from 2 to 12.

9. An optical fibre cable according to any preceding Claim, wherein the said reinforcement members are substantially equally spaced apart in the sheath wall, and are substantially equidistant from the opical fibre or fibres.

10. An optical fibre cable according to any preceding Claim, wherein the sheath is formed of polyethylene, or a melt-processable polytetrafluoroethylene copolymer, or polyurethane resin, or polyamide-acrylic resin copolymer, or polyester resin.

11. An optical fibre cable according to any preceding Claim, wherein the core consists of one or more optical fibre waveguides individually coated with synthetic resin and surrounded by a sleeve of braided or woven textile material or of thermoplastic material.

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Claims

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as those described in Example 1. The bore of the sheath is 1 mm in diameter, the radial thickness of the sheath wall is 3 mm, and each of the reinforcing member 6 consists of a single 1420 denier "Kevlar" yarn.

Example 3.

In the cable shown in Figures 5 and 6 of the drawings, the core consists of a single optical fibre waveguide 7 loosely surrounded by a tubular polyethylene sleeve 8. The cable sheath 9 is formed of a melt-processable polyester resin and in cludes six reinforcing members in the form of sleeved "Kevlar" yarns 10, the sleeves

11 being composed of the same polyester resin as the sheath, and being disposed adjacent to the sheath bore. In Figure 5, parts of the sheath 9 and sleeves 8 and

11 have been cut away to show, respectively, the sleeves, and the optical fibre and "Kevlar" yarns: only two of the reinforcing members are shown in Figure 5. This cable is also manufactured by a single extrusion process, during the course of which the sleeves 1-1 are partially fused with the extrudate and are embedded therein with a minor portion of each said sleeve remaining exposed in the sheath bore; the fibre sleeve 8 is in loose sliding contact with these portions of the sleeves 11.

In a specific example of the form of cable shown in Figures 5 and 6, the optical fibre is of the same form as those de scribed in Example 1, all the sleeves 8 and

11 are of 1.2 mm external diameter, and the wall thickness of the sheath 9 is 3 mm.

It will be appreciated that the cable of Example 3 may, if desired, be modified by incorporating a plurality of optical fibres instead of the single fibre 7, the diameter of the sleeve 8 being increased if necessary to accommodate the fibres with the desired freedom of movement within said sleeve.

The optical fibre cables of all the forms described in the above examples have high load-bearing characteristics, crush resistance, buckling resistance and bending strength.

WHAT WE CLAIM IS: 1. An optical fibre cable including a core composed essentially of one or more straight optical fibre waveguides lying parallel to the cable axis and, surrounding the core, a tubular sheath of synthetic plastics material in the wall of which are embedded (as hereinbefore defined) a plurality of straight elongate reinforcement members extending throughout the length of the cable and disposed around and parallel to the optical fibre or fibres, each of which reinforcement members consists of an assemblage of aromatic polyamide fibres, with or without an impregnant synthetic resin, and with or without a surrounding sleeve of thermoplastic material.
2. An optical fibre cable according to Claim 1, wherein each of said reforcement members consists of one or more straight yarns composed of aromatic polyamide fibres, unimpregnated with resin.
3. An optical fibre cable according to Claim 1, wherein each of said reinforcement members consists of a semi-rigid wire formed of several straight yarns composed of aromatic polyamide fibres, said yarns being laid parallel to one another and bonded together with an impregnant resin to form a cohesive structure.
4. An optical fibre cable according to Claim 3, wherein the resin employed for bonding said yarns together to form the reinforcing wires consists of an epoxy resin, or a polyurethane resin, or a silicone resin, or a polyester resin
5. An optical fibre cable according to Claims 3 or 4, wherein the said resin incorporates a filler material for imparting increased load-bearing strength to the reinforcement members.
6. An optical fibre cable according to Claim 1, wherein each of said reinforcement members consists of one or more straight yarns composed of aromatic polyamide fibres, either unimpregnated or impregnated with resin, surrounded by a sleeve of thermoplastic material with which said yarn or yarns is or are in sliding contact.
7. An optical fibre cable according to Claim 6, wherein said reinforcement members are disposed in the region of the sheath wall adjacent to the interior surface thereof, a portion of the sleeve of each said member being exposed at the said surface.
8. An optical fibre cable according to any preceding Claim, wherein the number of said reinforcement members embedded in the sheath is from 2 to 12.
9. An optical fibre cable according to any preceding Claim, wherein the said reinforcement members are substantially equally spaced apart in the sheath wall, and are substantially equidistant from the opical fibre or fibres.
10. An optical fibre cable according to any preceding Claim, wherein the sheath is formed of polyethylene, or a melt-processable polytetrafluoroethylene copolymer, or polyurethane resin, or polyamide-acrylic resin copolymer, or polyester resin.
11. An optical fibre cable according to any preceding Claim, wherein the core consists of one or more optical fibre waveguides individually coated with synthetic resin and surrounded by a sleeve of braided or woven textile material or of thermoplastic material.
12. An optical fibre cable according to

any of the preceding Claims 1 to 10, wherein the core consists of one or more optical fibre waveguides individually coated with synthetic resin and lying loosely in the bore of the sheath, the sheath bore being of sufficient large diameter to permit freedom of movement of the coated fibre or the individual coated fibres within it in both radial and axial directions.
13. A method of manufacturing an optical fibre cable according to any preceding Claim, which includes the step of extruding the whole of the sheath around the optical fibre or fibres and the said reinforcement members in a single continuous extrusion process.
14. An optical fibre cable according to Claim 1, substantially as hereinbefore described in Example 1 and with reference to Figures 1 and 2 of the accompanying drawings.
15. An optical fibre cable according to Claim 1, substantially as hereinbefore described in Example 2 and wwith reference to Figures 3 and 4 of the accompanying drawings.
16. An optical fibre cable according to Claim 1, substantially as hereinbefore described in Example 2 and with reference Figures 5 and 6 of the accompanying drawings.