GB2106663A - Optical fibre cable - Google Patents

Abstract

An optic fibre is coated with a high melting point synthetic polymer (PVDF) or an equivalent copolymer and subsequent sheath layer, the later being extruded at a lower temperature than the melting point of the coating.

Description

SPECIFICATION Optical fibre cable This invention relates to a cable for the transmission of information wherein the information carrier is an optical fibre and a protective sheath for the delicate fibre is provided.

According to one aspect of the present invention there is provided a communications cable comprising an optical fibre coated with the high melting point synthetic polymer PVDF and at least one subsequent layer of sheath material, this subsequent layer being extruded at a lower temperature than the melting point of the coating.

The high melting point synthetic polymer herein referred to as PVDF is polyvinylidene di fluoride having the formula:- (CH2CF2)n One suitable PVDF polymer is that manufactured by Solway 8 Cie and sold under the Trade Mark SOLEF. The expression PVDF in this specification is to be interpreted to cover any copolymer resins with equivalent properties.

PVDF has a high melting point, typically 240"C, good heat resistance and thermal stability which allows a cable based on the coated fibre subsequently to be provided by extrusion with outer protective layers of other lower melting point plastics, polyolefins and polyvinyls typically, without the fibre being disturbed by the heat of the extrusion process.

Optical fibres being thin strands of quartz or glass are fragile and easily snapped on bending, consequently any optical fibre based cable must provide resistance to bending. However cable-like flexibility is required in order to install and wind onto a drum and it has been difficult to reconcile these two requirements of strength and cable-like flexibility.

Thus in an embodiment a cable in accordance with the invention comprises a single optical fibre coated, by extrusion, with PVDF, a multi-strand helical case of a substantially inextensible fibrous material surrounding the coated fibre with radial clearance, a polyethylene coating on the fibrous case and a plasticised PVC outer sheath. The fibre is a loose fit within the PVDF so that it can be extracted and replaced.

The polyethylene coating is preferably slit into an advancing helix. The helix is held in place around the fibrous case by virtue of its shape alone, the coils of the helix are not directly fixed to one another or to the remainder of the sheath. Therefore, the cable as a whole has extreme flexibility to the point wherein adjacent coils of the helix abut one another, severely restricted flexing then only being allowed by compression of abutting helix coils.

A particular embodiment of the invention will now be described by way of example an-d with reference to the accompanying drawing which is a longitudinal section through an optical fibre cable in accordance with the invention.

The core 1 of the cable iliustrated is an optical fibre loosely fitting within an extruded coating of PVDF 2. The core 1 extends with radial clearance within a sheath comprising a helical multi-strand wrapping 3 of tough fibres such as those sold under the Trade Mark KEVLAR. Around wrapping 3 an advancing helix 4 of relatively thick polyethylene extends. This helix 4 is made by extruding a continuous polyethylene coating and thereafter advancing the coating in contact with a rotating blade thus making a helical slit. The rate of rotation of the blade and the rate of extrusion of the polyethylene coating determine the angle at which the helix is cut, and the spacing between the coils is determined by the tension being applied to the coating.

An outer casing of polyvinylchloride 5, is then provided by extrusion.

In use bending of the cable is permitted by its inherent flexibility until adjacent coils of the helix abut one another. Further bending is then only permitted by the mutual compression of the relatively inelastic polyethylene coils.

1. A communications cable comprising an optical fibre coated with the high melting point synthetic polymer PVDF, as defined in the specification, and at least one subsequent layer of sheath material, this subsequent layer being extruded at a lower temperature than the melting point of the coating.

2. A cable as claimed in Claim 1 wherein one sheath layer is a multi-strand helical wrapping of a substantially inextensible fibrous material surrounding the coated fibre, and a second sheath layer is a loose fitting helix of a polymeric material coiled around the fibrous wrapping.

3. A cable as claimed in Claim 2 wherein the cable is encased in an outer sheath of a polymeric material.

4. A cable as claimed in Claim 2 wherein the optical fibre is a loose fit within the coating of PVDF.

5. A cable substantially as herein described with reference to and as shown in the accompanying drawing.

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

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Optical fibre cable This invention relates to a cable for the transmission of information wherein the information carrier is an optical fibre and a protective sheath for the delicate fibre is provided. According to one aspect of the present invention there is provided a communications cable comprising an optical fibre coated with the high melting point synthetic polymer PVDF and at least one subsequent layer of sheath material, this subsequent layer being extruded at a lower temperature than the melting point of the coating. The high melting point synthetic polymer herein referred to as PVDF is polyvinylidene di fluoride having the formula:- (CH2CF2)n One suitable PVDF polymer is that manufactured by Solway 8 Cie and sold under the Trade Mark SOLEF. The expression PVDF in this specification is to be interpreted to cover any copolymer resins with equivalent properties. PVDF has a high melting point, typically 240"C, good heat resistance and thermal stability which allows a cable based on the coated fibre subsequently to be provided by extrusion with outer protective layers of other lower melting point plastics, polyolefins and polyvinyls typically, without the fibre being disturbed by the heat of the extrusion process. Optical fibres being thin strands of quartz or glass are fragile and easily snapped on bending, consequently any optical fibre based cable must provide resistance to bending. However cable-like flexibility is required in order to install and wind onto a drum and it has been difficult to reconcile these two requirements of strength and cable-like flexibility. Thus in an embodiment a cable in accordance with the invention comprises a single optical fibre coated, by extrusion, with PVDF, a multi-strand helical case of a substantially inextensible fibrous material surrounding the coated fibre with radial clearance, a polyethylene coating on the fibrous case and a plasticised PVC outer sheath. The fibre is a loose fit within the PVDF so that it can be extracted and replaced. The polyethylene coating is preferably slit into an advancing helix. The helix is held in place around the fibrous case by virtue of its shape alone, the coils of the helix are not directly fixed to one another or to the remainder of the sheath. Therefore, the cable as a whole has extreme flexibility to the point wherein adjacent coils of the helix abut one another, severely restricted flexing then only being allowed by compression of abutting helix coils. A particular embodiment of the invention will now be described by way of example an-d with reference to the accompanying drawing which is a longitudinal section through an optical fibre cable in accordance with the invention. The core 1 of the cable iliustrated is an optical fibre loosely fitting within an extruded coating of PVDF 2. The core 1 extends with radial clearance within a sheath comprising a helical multi-strand wrapping 3 of tough fibres such as those sold under the Trade Mark KEVLAR. Around wrapping 3 an advancing helix 4 of relatively thick polyethylene extends. This helix 4 is made by extruding a continuous polyethylene coating and thereafter advancing the coating in contact with a rotating blade thus making a helical slit. The rate of rotation of the blade and the rate of extrusion of the polyethylene coating determine the angle at which the helix is cut, and the spacing between the coils is determined by the tension being applied to the coating. An outer casing of polyvinylchloride 5, is then provided by extrusion. In use bending of the cable is permitted by its inherent flexibility until adjacent coils of the helix abut one another. Further bending is then only permitted by the mutual compression of the relatively inelastic polyethylene coils. CLAIMS

1. A communications cable comprising an optical fibre coated with the high melting point synthetic polymer PVDF, as defined in the specification, and at least one subsequent layer of sheath material, this subsequent layer being extruded at a lower temperature than the melting point of the coating.

2. A cable as claimed in Claim 1 wherein one sheath layer is a multi-strand helical wrapping of a substantially inextensible fibrous material surrounding the coated fibre, and a second sheath layer is a loose fitting helix of a polymeric material coiled around the fibrous wrapping.

3. A cable as claimed in Claim 2 wherein the cable is encased in an outer sheath of a polymeric material.

4. A cable as claimed in Claim 2 wherein the optical fibre is a loose fit within the coating of PVDF.

5. A cable substantially as herein described with reference to and as shown in the accompanying drawing.