GB2129580A

GB2129580A - Optical fibre cable

Info

Publication number: GB2129580A
Application number: GB08327344A
Authority: GB
Inventors: John Carswell Smith
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1982-10-28
Filing date: 1983-10-12
Publication date: 1984-05-16
Also published as: SE8305942D0; ES275331Y; BE898103A; AU2051283A; GB2129580B; ES275331U; GB8327344D0

Abstract

An optical fibre cable unit (1)has a central strength member (11), for providing increased tensile strength, a helically-wrapped corrugated tape (15) having a plurality of longitudinal grooves (17) and a plurality of optical fibres 21 laid into the longitudinal grooves. An outer layer (25) is provided to hold the fibres in place. The depth of the grooves is sufficient to receive one or more optical fibres which may be coated for protection. If a high fibre count cable is desired, it is possible to combine a plurality of cable units to form a composite cable. <IMAGE>

Description

SPECIFICATION Optical fibre cable This invention relates to an optical fibre cable. More particularly, the invention relates to an optical fibre cable of the kind having a central strength member and one or more optical fibres helically laid about the central strength member in a protected manner.

Optical fibres have become a desirable information-transmitting medium due to their broad bandwidth capacity and small physical size and weight as compared with metal electrical conductors. A number of characteristics of optical fibres including their susceptibility to breakage and their bending and stress losses pose serious problems in their use. It is, therefore, necessary to find suitable means to protect the fibres.

One approach to the above problems has been to start with a plurality of optical fibres and to form them into linear arrays packaged in ribbon-like structures. This approach is described in detail in U.S. Patent No. 4,129,468, issued December 12, 1978. The requirements imposed on such optical fibre ribbons include the need to provide mechanical strippability for ease of cable termination and splicing, the need for small size, the need for resistance to breakage when subjected to tensile stress, the need for individual fibre identification within the ribbon, and the need to protect the ribbon from distorting forces which may cause deterioration of the optical signal.

After forming these ribbons, they are used to form optical cables. It has been asserted that these ribbons provide adequate protection for the optical fibres when used for information transmission.

Various processes for manufacturing such an optical cable are described in U.S. Patents 3,937,559; 4,110,001; 4,138,193 and 4,146,302. As appears in each of these patents, it is still necessary to form the ribbon-like structure prior to the cabling operation.

Indeed, it is specifically stated in each of these patents that forming of the ribbon-like structure reduces the risk of injury to the optical fibres during the cabling operation. The drawback to such a requirement is particularly significant when a cable having only a few fibres is required. it is an unduly burdensome procedure first to produce ribbons and then to make a cable to carry the ribbon. The cabling of the optical fibre ribbons is a complicated and expensive procedure, even after the ribbons have been assembled. Cabling of these ribbons requires the use of planetary stranders at least for the laying of strength members into the required extruded sheath. Such equipment is expensive and hence contributes significantly to the overall cost of the cabling operation.

It has been proposed to avoid the requirement for a multistep cabling process by incorporating one or more optical fibres into a cable as the cable was formed. Such procedures are described in U.S.

Patent Nos. 4,155,963; 4,154,049; and 4,205,899.

These patents disclose the extrusion of a profiled central member having grooves into which optical fibres are laid, followed by closure of the groove, thereby enclosing the fibre in a longitudinally extending chamber. U.S. Patent No. 4,199,224 is similar but adds a separate central strength member.

These methods permit optical fibres to be laid into the open channels as the cable is manufactured, hence eliminating the necessity of first manufacturing an optical fibre ribbon. The methods disclosed in these patents require expensive machinery and a large amount of floor space for cabling of optical fibre. As a result of the expenses associated with the machines and dedicated floor space, these are expensive cabling techniques.

Another approach for the cabling of optical fibres is disclosed in U.S. Patent No. 4,153,332 issued May 8, 1979. This patent specification explains that a cable may be formed by winding unitary elements having an adhering sheath on a supporting core.

When this structure is bent on a mandrel having a small bending radius, the unitary element is compressed in the inner portion facing the mandrel and is stretched in the outer portion. This occurs because the friction between the element and the core about which it is wound prevents the element from sliding significantly with respect to the core. The optical fibre or fibres contained in the unitary element are subjected to compressive and tension stresses. To overcome this drawback, it is recommended that the fibres be centered in a tubular sheath. This patent specification discloses that, at the present state of the art, no processes are known for producing unitary optical fibre elements which permit a perfect centering of the optical fibre with respect to the sheath. This is especially true when the sheath has a diameter much greater than that of the fibre.When the fibre is not centered, it is well known to those skilled in the art that the tension or compressive stress on the fibre, when the unitary element is subjected to flexing, is proportional to the distance of the optical fibre from the neutral axis of the unitary element and inversely proportional to the bending radius of the element. To compensate for this, the fibres may be made longer than the length of the corresponding surrounding sheath by either winding the fibres around a central core within the sheath or by imparting a helical bend to the fibres and laying them loosely within the sheath. The patent goes on to assert that the provision of a tubular sheath into which the optical fibre is inserted provides a protective structure which shields the surface of the fibre from radial compressive force and from contact with corrosive substances.

Another approach which has been proposed for providing a protective cable structure for optical fibres is disclosed in U.S. Patent No. 4,235,511 issued November25, 1980. This patent describes a cabling technique wherein a central strength member is surrounded by elements which define chambers running the length of the strength member and which are covered in order to enclose an optical fibre laid into the strength member and which are covered in order to enclose an optical fibre laid into the chamber. The patent discloses folded splicing tape helically wrapped about the central strength member for formation of the chambers.

All of the above described cables suffer from one or more of the problems including high expense in cable manufacture, susceptibility to fibre breakage during cabling or cable laying operations, and poor optical transmission characteristics.

According to the invention it its broadest aspect there is provided an optical fibre cable comprising a central strength member, a corrugated tape helically wrapped about the strength member, the corrugated tape forming a plurality of longitudinal grooves, optical fibres in the plurality of longitudinal grooves, optical fibres in the grooves and an outer protective member extending around the corrugated tape.

The depth of the grooves is sufficient to receive one or more optical fibres which may be coated for protection. In the event that a high fibre count cable is desired, it is possible to combine plurality of cable units to form a composite cable.

An embodiment ofthe invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of an optical fibre cable unit embodying the invention.

Figure 2 is a perspective with partial cutaway showing the cable unit structure.

Figure 3 is a cross-section of a composite cable including a plurality of optical fibre cable units.

Referring to the drawings, Figures 1 and 2 show a cable 1. The cable 1 is formed with a core including a central strength member 11 surrounded by an insulation or cushioning layer 13. A corrugated tape 15 is helically wrapped about the core in such a way that helical grooves 17 are formed. The width and lay length of the corrugated tape 15 are such that the longitudinal edges overlap and the grooves of successive helical laps are interlocking at an overlap region 19. In this manner, the corrugated tape 15 is firmly held in place along the entire length of the cable structure. The grooves 17 contain one or more optical fibres 21 and are of such a depth that the optical fibres are loosely contained therein without any portion of the fibre extending radially beyond the outermost edge of the groove.A heat barrier film 23 may be provided about the corrugated tape 15 to serve multiple functions of protecting the cable structure from short exposures to high temperatures and to prevent the optical fibres from being displaced from their respective grooves. Filling compound 101 may be provided in some or all of the grooves 17 and may, if desired, be provided between the strength member and the corrugated tape.

The strength member 11 is preferably made of a material having high tensile strength such as solid steel wire or stranded steel wire. However, any material having high tensile strength such as solid alloy wire, stranded alloy wire, glass yarn, alumina yarn, aramid yarn, graphite yarn or carbon yarn may suitably be used for the axial strength member. In some applications, it will be desirable to have a cable which is relatively rigid, in which case metal strength members might be preferred; while in those applications where flexibility is desired, the more flexible yarns would be preferred.

The insulation or cushioning layer 13 is preferably of an organic polymer. In an alternative embodiment, the strength member may be impregnated with the organic polymer. Preferred organic polymers include polyethylene and epoxy.

The corrugated tape 15 may be either polymeric, such as a polyester tape, or metallic, such as an aluminum tape. That known as MYLAR RTM is a preferred tape material.

As noted, the interstices betwen the insulating layer 13, the corrugated tape 15 and the fibre 21 may be filled with a filling compound 101. Examples of such a filling compound are petroleum jelly or amorphous polypropylene. Any other suitable filling compound could be used. Alternatively, these interstices may be left unfilled and, if desired, these interstices could be filled with a pressurised fluid. If a filling compound is provided, the heat barrier film 23 applied over the optical fibres additionally prevents the filling compound from dripping.

A jacket 25 can be used for further protection and can be of any polymeric substance, polyethylene being preferred. If desired, a metallic water barrier tape 27 can be applied between the heat barrier film 23 and the jacket 25 or alternatively, it can be applied over the jacket 25. If the metallic water barrier tape 27 is applied over the jacket 25, an additional polymeric jacket may be applied over the water barrier tape.

As noted, the width and lay length of the corrugatd tape 15 is such that each helical lap partially overlays the prior lap. Figure 1a illustrates a portion of corrugated tape 15 wherein a successive lay 1 2a overlaps a previously formed lay 1 2b in the overlap region 19. In Figure 1a, three grooves are shown in heavy lines indicating that these grooves are overlaying grooves in the first wrapped layer 1 2b. This positive interlocking of successive helical laps ensures that the tape cannot slide relative to adjacent helical laps. As a result, the optical fibres 21 lying in the grooves are maintained in better helical continuitry than would be obtainable if the grooves could move relative to one another.

Referring to Figure 3, a composite cable structure is illustrated wherein three optical fibre cable units 1 are enclosed in a single outer protective sheath 49.

The interstices 48 may be filled with a filling compound 101 or left empty if a pressurised cable is desired.

Claims

1. An optical fibre cable comprising a central strength member, a corrugated tape helically wrapped about the strength member, the corrugated tape forming a plurality of longitudinal grooves, optical fibres in the grooves and an outer protective member extending around the corrugated tape.

2. An optical fibre cable as claimed in claim 1 wherein the corrugated tape is helically laid in such a way that the longitudinal grooves of successive helical laps are interlocking.

3. An optical fibre cable as claimed in claim 1 wherein the strength member is of a material selected from the group consisting of solid steel wire, stranded steel wire, solid alloy wire, stranded alloy wire, glass yarn, alumina yarn, aramid yarn, graphite yarn and carbon yarn, and extruded plastic.

4. An optical fibre cable as claimed in claim 3 wherein the strength member is at least partly covered by an organic polymer.

5. An optical fibre cable as claimed in claim 1 wherein the corrugated tape is polymeric.

6. An optical fibre cable as claimed in claim 1 wherein the said corrugated tape is metallic.

7. An optical fibre cable as claimed in claim 1 further including a filling compound.

8. An optical fibre cable as claimed in claim 1 further including a heat barrier.

9. An optical fibre cable as claimed in claim 1 further including a polymeric jacket.

10. An optical fibre cable as claimed in claim 1 or 8 further including a metallic water barrier.

11. An optical fibre cable as claimed in claim 10 further including a polymeric jacket surrounding the metallic water barrier.

12. An optical fibre cable as claimed in claim 1 wherein the grooves extend helically about the strength member.

13. An optical fibre cable substantially as described with reference to the accompanying drawings.