GB1577323A - Optical guides - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO OPTICAL GUIDES (71) We, BICC LIMITED, a British Company, of 21 Bloomsbury Street, London WCIB 3QN, 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 guides for the transmission of the ultra-violet, visible and infra-red regions of the electromagnetic spectrum, which regions, for convenience, will hereinafter all be included in the generic term "light" and especially, but not exclusively, to optical waveguides for use in the communications field adapted for transmission of light having a wavelength within the range 0 8 to 13 micrometers.

For the purpose of transmitting light in an optical transmission system it has been proposed to employ optical guides in the form of optical cables including one or more than one optical fibre. Such optical fibres may be of glass or other transparent material whose refractive index is arranged to decrease over at least a part of the radius of the fibre in a direction towards the outer surface of the fibre or they may be of composite form comprising a core of transparent material clad throughout its length with another transparent material of lower refractive index which, by total internal refraction of light being transmitted along the fibre, confines at least a major proportion of the light within the core. A composite optical fibre is generally, but not necessarily, made of two glasses of different refractive indices, the glass forming the core having a higher refractive index than the glass forming the cladding; the refractive index of the glass of the core may gradually decrease towards the outer surface of the core over at least a part of the distance between the central axis of the core and its outer surface. In an alternative form of composite fibre the core may be a transparent liquid having a higher refractive index than that of the cladding.

All such optical fibres generally, but not necessarily, have diameters lying in the range 100 to 150 llm. They are usually of substantially circular cross-section but, in some circumstances, they may be of non-circular cross-section.

According to the present invention we provide an improved optical guide in the form of an optical cable comprising an elongate central core consisting of at least one reinforcing member; at least two optical fibres and at least two elongate spacing elements, each separately formed with respect to the central core and of substantially greater diameter than that of each optical fibre, arranged together in at least one layer in which optical fibres and spacing elements extend helically about the core with each optical fibre positioned between, and housed so loosely in a separate elongate space bounded in part by, two spacing elements that each optical fibre is capable of limited relative movement relative to the other component parts of the cable when the cable is flexed; and, surrounding the assembled body so formed and in contact with each of the spacing elements of the layer or of the outer or outermost layer, a covering layer.

Each optical fibre may be of a length substantially greater than that of the two spacing elements between which it is positioned but preferably the optical fibres and the spacing elements are of equal or approximately equal lengths.

Preferably each optical fibre has a coating of a buffer material and in this case preferably each elongate spacing element is of a material that is substantially harder than the buffer material of the optical fibres and each elongate spacing element has a diameter that is substantially greater than the overall diameter of each buffer-coated optical fibre.

If desired, two or more optical fibres may be embedded in a single elongate body of buffer material, an optical fibre unit so formed being housed loosely in an elongate space bounded in part by two spacing elements, the optical fibre unit having an overall diameter less than the diameter of each of the spacing elements.

The separately formed elongate spacing elements may each be of non-circular transverse cross-section but preferably each spacing element is of circular cross-section and the spacing elements are of substantially the same diameter.

Preferred materials that are suitable for the buffer coating of each optical fibre or bundle of optical fibres include thermoplastic rubber and polyurethane. Where either of these buffer materials is employed, preferably each spacing element is made of high density polyethylene.

If desired, one or more than one of the spacing elements may have embedded throughout its length an electrically conductive wire or strand thereby constituting an electrically insulated conductor.

The covering layer may be of any material that can be applied to the assembled body in such a way that it does not tend to enter the elongate spaces between adjacent spacing elements and so reduce the extent to which the optical fibres and/or optical fibre units can move when the cable is flexed. The covering layer may be an extruded tube of rubber or plastics material which is drawn down on to the assembled body in such a way that the material of the extruded tube does not flow into the elongate spaces.

Preferably, however, the covering layer is formed by helically lapping around the assembled body, or applying longitudinally to and folding transversely around the assembled body, at least one tape which may be of paper or other fibrous material, of plastics material, or of a metal or metal alloy; where the tape or outermost tape is of metal or metal alloy, at least its outer surface may have a coating of plastics material. Where a tape or tapes is or are lapped helically about the assembled body, the direction of lay of the tape or of at least one of the tapes preferably is of opposite hand to that of the spacing elements and optical fibres and/or optical fibre units. Where the covering layer is at least one helically or longitudinally applied tape, preferably an outer protective sheath of rubber or plastics material surrounds the taped assembled body. Where the tape or outermost tape is of metal or metal alloy and has a coating of plastics material on its outer surface, preferably the sheath is bonded to this coating. The sheath may have an external transverse cross-section of circular or non-circular form.

Preferably the elongate central core comprises a single wire or strand of metal or metal alloy having an overall layer of rubber or plastics material, which material will be substantially harder than the buffer coating of the optical fibres, when present. High density polyethylene is preferred. Although the wire or strand of the central core is preferably of steel or other metal or metal alloy of high tensile strength, in some circumstances it may be of copper or other metal or metal alloy of high electrical conductivity.

The optical cable preferably includes at least four elongate spacing elements and at least four optical fibres but it will be appreciated that the number of spacing elements and optical fibres incorporated in the cable will depend upon the particular use for which a cable is required. The number of spacing elements and the number of optical fibres need not be the same and, in some circumstances where fewer optical fibres are required, one or more of the elongate spaces may house a preformed dummy filler or preformed dummy fillers.

The invention will be further illustrated by a description, by way of example, of a preferred optical cable with reference to the accompanying diagrammatic drawing which shows a transverse cross-sectional view, not to scale, of the cable.

Referring to the drawing, the cable comprises an elongate central core 1 consisting of a steel wire 2 having a diameter of 1 0 mm and a layer 3 of high density polyethylene having a radial thickness of 1-3 mm; the central core has an overall diameter of 3-6 mm. Arranged helically about the central core 1 are six spacing elements 4 of high density polyethylene, each having a diameter of 15 mm, and six buffer-coated optical fibres 5 of overall diameter 1-0 mm, each buffer coated optical fibre being arranged between two spacing elements. Each buffercoated optical fibre 5 consists of an optical fibre 6 of diameter 120 um and a buffer coating 7 of thermoplastics rubber of radial thickness 0 44 mm. Surrounding the assembled body is a covering layer 8 formed by helically lapping a paper tape about the assembled body, the direction of lay of the paper tape being of opposite hand to that of the spacing elements 4 and buffer-coated optical fibres 5. An extruded protective sheath 9 of high density polyethylene having an external diameter of 9.5 mm overlies the covering layer 8.

WHAT WE CLAIM IS: 1. An optical cable comprising an elongate central core consisting of at least one reinforcing member; at least two optical fibres and at least two elongate spacing elements, each separately formed with respect to the central core and each of substantially greater diameter than that of each optical fibre, arranged together in at least one layer in which optical fibres and spacing elements extend helically about the core with each optical fibre positioned between, and housed so loosely in a separate elongate space bounded in part by, two spacing elements that each optical fibre is capable of limited relative movement relative to the other com

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

Claims (16)

**WARNING** start of CLMS field may overlap end of DESC **. The separately formed elongate spacing elements may each be of non-circular transverse cross-section but preferably each spacing element is of circular cross-section and the spacing elements are of substantially the same diameter. Preferred materials that are suitable for the buffer coating of each optical fibre or bundle of optical fibres include thermoplastic rubber and polyurethane. Where either of these buffer materials is employed, preferably each spacing element is made of high density polyethylene. If desired, one or more than one of the spacing elements may have embedded throughout its length an electrically conductive wire or strand thereby constituting an electrically insulated conductor. The covering layer may be of any material that can be applied to the assembled body in such a way that it does not tend to enter the elongate spaces between adjacent spacing elements and so reduce the extent to which the optical fibres and/or optical fibre units can move when the cable is flexed. The covering layer may be an extruded tube of rubber or plastics material which is drawn down on to the assembled body in such a way that the material of the extruded tube does not flow into the elongate spaces. Preferably, however, the covering layer is formed by helically lapping around the assembled body, or applying longitudinally to and folding transversely around the assembled body, at least one tape which may be of paper or other fibrous material, of plastics material, or of a metal or metal alloy; where the tape or outermost tape is of metal or metal alloy, at least its outer surface may have a coating of plastics material. Where a tape or tapes is or are lapped helically about the assembled body, the direction of lay of the tape or of at least one of the tapes preferably is of opposite hand to that of the spacing elements and optical fibres and/or optical fibre units. Where the covering layer is at least one helically or longitudinally applied tape, preferably an outer protective sheath of rubber or plastics material surrounds the taped assembled body. Where the tape or outermost tape is of metal or metal alloy and has a coating of plastics material on its outer surface, preferably the sheath is bonded to this coating. The sheath may have an external transverse cross-section of circular or non-circular form. Preferably the elongate central core comprises a single wire or strand of metal or metal alloy having an overall layer of rubber or plastics material, which material will be substantially harder than the buffer coating of the optical fibres, when present. High density polyethylene is preferred. Although the wire or strand of the central core is preferably of steel or other metal or metal alloy of high tensile strength, in some circumstances it may be of copper or other metal or metal alloy of high electrical conductivity. The optical cable preferably includes at least four elongate spacing elements and at least four optical fibres but it will be appreciated that the number of spacing elements and optical fibres incorporated in the cable will depend upon the particular use for which a cable is required. The number of spacing elements and the number of optical fibres need not be the same and, in some circumstances where fewer optical fibres are required, one or more of the elongate spaces may house a preformed dummy filler or preformed dummy fillers. The invention will be further illustrated by a description, by way of example, of a preferred optical cable with reference to the accompanying diagrammatic drawing which shows a transverse cross-sectional view, not to scale, of the cable. Referring to the drawing, the cable comprises an elongate central core 1 consisting of a steel wire 2 having a diameter of 1 0 mm and a layer 3 of high density polyethylene having a radial thickness of 1-3 mm; the central core has an overall diameter of 3-6 mm. Arranged helically about the central core 1 are six spacing elements 4 of high density polyethylene, each having a diameter of 15 mm, and six buffer-coated optical fibres 5 of overall diameter 1-0 mm, each buffer coated optical fibre being arranged between two spacing elements. Each buffercoated optical fibre 5 consists of an optical fibre 6 of diameter 120 um and a buffer coating 7 of thermoplastics rubber of radial thickness 0 44 mm. Surrounding the assembled body is a covering layer 8 formed by helically lapping a paper tape about the assembled body, the direction of lay of the paper tape being of opposite hand to that of the spacing elements 4 and buffer-coated optical fibres 5. An extruded protective sheath 9 of high density polyethylene having an external diameter of 9.5 mm overlies the covering layer 8. WHAT WE CLAIM IS:

1. An optical cable comprising an elongate central core consisting of at least one reinforcing member; at least two optical fibres and at least two elongate spacing elements, each separately formed with respect to the central core and each of substantially greater diameter than that of each optical fibre, arranged together in at least one layer in which optical fibres and spacing elements extend helically about the core with each optical fibre positioned between, and housed so loosely in a separate elongate space bounded in part by, two spacing elements that each optical fibre is capable of limited relative movement relative to the other com

ponent parts of the cable when the cable is flexed; and, surrounding the assembled body so formed and in contact with each of the spacing elements of the layer or of the outer or outermost layer, a covering layer.

2. An optical cable comprising an elongate central core consisting of at least one reinforcing member; at least two optical fibres each having a coating of a buffer material and at least two elongate spacing elements, each separately formed with respect to the central core and of a material that is substantially harder than the buffer material of the optical fibres and each of substantially greater diameter than the overall diameter of each buffer-coated optical fibre, the buffer-coated optical fibres and spacing elements being arranged together in at least one layer in which optical fibres and spacing elements extend helically about the core with each optical fibre positioned between, and housed so loosely in a separate elongate space bounded in part by, two spacing elements that each optical fibre is capable of limited relative movement relative to the other component parts of the cable when the cable is flexed; and, surrounding the assembled body so formed and in contact with each of the spacing elements of the layer or of the outer or outermost layer, a covering layer.

3. An optical cable as claimed in Claim 1 or 2, wherein at least one of the elongate spaces houses loosely an optical fibre unit comprising two or more optical fibres embedded in a single elongate body of buffer material, the optical fibre unit having an overall diameter less than that of each of the spacing elements.

4. An optical cable as claimed in Claim 2 or 3, wherein the buffer material is a thermoplastic rubber or polyurethane.

5. An optical cable as claimed in any one of the preceding Claims, wherein the optical fibres and the spacing elements are of equal or approximately equal lengths.

6. An optical cable as claimed in any one of the preceding Claims, wherein each spacing element is of circular cross-section and the spacing elements are of substantially the same diameter.

7. An optical cable as claimed in any one of the preceding Claims, wherein each spacing element is made of high density polyethylene.

8. An optical cable as claimed in any one of the preceding Claims, wherein one or more than one of the spacing elements has embedded throughout its length an electrically conductive wire or strand.

9. An optical cable as claimed in any one of the preceding Claims, wherein the elongate central core comprises a single wire or strand of metal or metal alloy having an overall layer of rubber or plastics material.

10. An optical cable as claimed in Claim 9, wherein the wire or strand is of a metal or metal alloy of high electrical conductivity.

11. An optical cable as claimed in any one of the preceding Claims, wherein the covering layer comprise at least one tape which is helically lapped around, or is applied longitudinally to and transversely folded around, the assembled body.

12. An optical cable as claimed in Claim 11 in which the or each tape is lapped helically about the assembled body, wherein the direction of lay of the tape or of at least one of the tapes is of opposite hand to that of the spacing elements and optical fibres and/ or optical fibre units.

13. An optical cable as claimed in Claim 11 or 12, wherein an outer protective sheath of rubber or plastics material surrounds the taped assembled body.

14. An optical cable as claimed in Claim 13, wherein the tape or outermost tape is of metal or metal alloy and has, on its outer surface, a coating of plastics material to which the outer protective sheath is bonded.

15. An optical cable as claimed in any one of Claims 1 to 10, wherein the covering layer is an extruded tube of rubber or plastics material.

16. An optical cable substantially as hereinbefore described with reference to and as shown in the accompanying drawing.