GB2169095A - Optical cables - Google Patents

To provide greater strength than plastic and be lighter than metal, a protective strength member (6,7) for an optical cable is formed of fibre- reinforced resin such as that formed by the "Pultrusion" process. The fibres may be either polymeric high-strength fibres such as Kevlar fibres, or mineral fibres such as carbon, boron or glass. <IMAGE>

SPECIFICATION Optical cables This invention relates to optical cables and more particularly to protective strength members for such cables.

Optical cables generally consist of at least one optical fibre enclosed within a protective sheath and having a strength member either at the axis of the cable or around the outside.

An armour sheath or pressure-resistant sheath is usually also provided around the sheath protecting the fibre.

The strength members and armour sheaths are generally formed of metallic members with the protective sheath being formed of plastic.

It is an object of the present invention to provide a protective strength member for an optical cable which is considerably stronger than plastic and yet is lighter than metal.

Accordingly, the invention provides an optical cable having at least one optical fibre and incorporating a hollow protective strength member formed of fibre-reinforced resin.

The resin may be either a thermosetting resin or a polymer matrix and the fibres may be either polymeric high-strength fibres such as Kevlar fibres or mineral fibres such as carbon, boron or glass fibres.

Such fibres are well known for their high tensile strength and embedding them in a resin matrix provides the material with compressive strength.

One example of such material is made of glass fibres embedded in thermosetting resin and extruded to the desired shape by a process known as Pultrusion.

The hollow protective strength member may be a tube made of the fibre-reinforced resin in which one or more optical fibres are loosely accommodated. Several such tubes may then be stranded together to form a composite cable structure.

Alternatively, a known type of optical cable may be loosely enclosed within a tube of fibre-reinforced resin to protect it, or the tube may be used as a tight-fitting sheath within a cable structure.

A further possibility is to have the protective strength member composed of several rods of fibre-reinforced resin stranded together helically to form a hollow tube-like member which can be used to loosely enclose a known type of cable. Alternatively the rods may be tightly helically wound around other sheaths within the cable structure.

As will be appreciated, any arrangement of an optical cable may be constructed in accordance with the invention provided that the optical fibres are enclosed within a hollow member made of this material so as to protect the fibres.

The invention will now be more fully described, by way of example, with reference Figs. 1 to 4 of the drawings of which: Figure 1 shows a cable structure incorporating several protective strength members in the form of fibre-reinforced resin tubes stranded together.

Figure 2 shows an alternative form of optical cable having a strength member in the form of a fibre-reinforced resin tube.

Figure 3 shows a further form of cable having a fibre reinforced tube used as a tight fitting sheath within a cable structure, and Figure 4 shows a cable with a protective strength member composed of several rods of fibre-reinforced resin.

Referring to Fig. 1 this shows a cable incorporating a plurality of hollow protective strength members 1 each in the form of a tube 2 of glass fibre-reinforced thermosetting resin conveniently formed by pultrusion, and each of which loosely accommodates one or more optical fibres 3. The tubes 2 in this case seven, may be stranded together to form an optical fibre cable unit 4 as shown accommodated within a protective outer sheath 5. The sheath 5 may also be made of a fibre-reinforced resin.

Referring to Fig. 2 this illustrates an alternative form of cable utilising one or more known optical cable structures, one of which is shown at 6 loosely accommodated in a hollow protective strength member 5 of fibre reinforced resin. The constructional details of the optical cable structure 6 are not relevant to the present invention and will not be described.

Referring to Fig. 3, an outer protective strength member 5, in this case, in the form of a tube 7 of fibre-reinforced resin tightly surrounds one or more, in this case three, optical fibre cable units 4 of the kind described with reference to Fig. 1 but, it will of course be appreciated that the outer protective strength member 5 need not necessarily take the form of a tube. For example the strength member may take the form of one homogeneous mass, not shown, or any other shape as desired.

Referring to Fig. 4 one or more optical fibres 3 are loosely accommodated within a protective strength member 8 which consists of a plurality of rods 9 of fibre reinforced resin arcuate cross section joined together to form a hollow tube. The protective strength member 8 may as shown, tightly sheath a known type of extruded plastics tube 10 which houses the optical fibres 3.

1. An optical cable having at least one optical fibre and incorporating a hollow protective strength member formed of fibre-reinforced resin.

2. An optical cable according to Claim 1, wherein the resin is a thermosetting resin.

3. An optical cable according to Claim 1,

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

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Optical cables This invention relates to optical cables and more particularly to protective strength members for such cables. Optical cables generally consist of at least one optical fibre enclosed within a protective sheath and having a strength member either at the axis of the cable or around the outside. An armour sheath or pressure-resistant sheath is usually also provided around the sheath protecting the fibre. The strength members and armour sheaths are generally formed of metallic members with the protective sheath being formed of plastic. It is an object of the present invention to provide a protective strength member for an optical cable which is considerably stronger than plastic and yet is lighter than metal. Accordingly, the invention provides an optical cable having at least one optical fibre and incorporating a hollow protective strength member formed of fibre-reinforced resin. The resin may be either a thermosetting resin or a polymer matrix and the fibres may be either polymeric high-strength fibres such as Kevlar fibres or mineral fibres such as carbon, boron or glass fibres. Such fibres are well known for their high tensile strength and embedding them in a resin matrix provides the material with compressive strength. One example of such material is made of glass fibres embedded in thermosetting resin and extruded to the desired shape by a process known as Pultrusion. The hollow protective strength member may be a tube made of the fibre-reinforced resin in which one or more optical fibres are loosely accommodated. Several such tubes may then be stranded together to form a composite cable structure. Alternatively, a known type of optical cable may be loosely enclosed within a tube of fibre-reinforced resin to protect it, or the tube may be used as a tight-fitting sheath within a cable structure. A further possibility is to have the protective strength member composed of several rods of fibre-reinforced resin stranded together helically to form a hollow tube-like member which can be used to loosely enclose a known type of cable. Alternatively the rods may be tightly helically wound around other sheaths within the cable structure. As will be appreciated, any arrangement of an optical cable may be constructed in accordance with the invention provided that the optical fibres are enclosed within a hollow member made of this material so as to protect the fibres. The invention will now be more fully described, by way of example, with reference Figs. 1 to 4 of the drawings of which: Figure 1 shows a cable structure incorporating several protective strength members in the form of fibre-reinforced resin tubes stranded together. Figure 2 shows an alternative form of optical cable having a strength member in the form of a fibre-reinforced resin tube. Figure 3 shows a further form of cable having a fibre reinforced tube used as a tight fitting sheath within a cable structure, and Figure 4 shows a cable with a protective strength member composed of several rods of fibre-reinforced resin. Referring to Fig. 1 this shows a cable incorporating a plurality of hollow protective strength members 1 each in the form of a tube 2 of glass fibre-reinforced thermosetting resin conveniently formed by pultrusion, and each of which loosely accommodates one or more optical fibres 3. The tubes 2 in this case seven, may be stranded together to form an optical fibre cable unit 4 as shown accommodated within a protective outer sheath 5. The sheath 5 may also be made of a fibre-reinforced resin. Referring to Fig. 2 this illustrates an alternative form of cable utilising one or more known optical cable structures, one of which is shown at 6 loosely accommodated in a hollow protective strength member 5 of fibre reinforced resin. The constructional details of the optical cable structure 6 are not relevant to the present invention and will not be described. Referring to Fig. 3, an outer protective strength member 5, in this case, in the form of a tube 7 of fibre-reinforced resin tightly surrounds one or more, in this case three, optical fibre cable units 4 of the kind described with reference to Fig. 1 but, it will of course be appreciated that the outer protective strength member 5 need not necessarily take the form of a tube. For example the strength member may take the form of one homogeneous mass, not shown, or any other shape as desired. Referring to Fig. 4 one or more optical fibres 3 are loosely accommodated within a protective strength member 8 which consists of a plurality of rods 9 of fibre reinforced resin arcuate cross section joined together to form a hollow tube. The protective strength member 8 may as shown, tightly sheath a known type of extruded plastics tube 10 which houses the optical fibres 3. CLAIMS