AN IMPROVED OPTICAL CABLE

Abstract: In a optical cable comprising at least one separate optical fibre and/or optical bundle, the optical fibre or bundle (4) is loosely housed in, or surrounded by, an elon­ gate compartment (11) bounded by a fluid-impermeable wall or fluid-impermeable walls (6, 8). The elongate com­ partment (11) extends throughout the cable length and contains a fluid, e. g. air above or below atmospheric pressure, at such a pressure that detection of a change in pressure of the fluid will serve to indicate that a fluid-­ impermeable wall (6, 8) has been ruptured, e. g. by so­ meone illegally tapping the cable. The cable may also incorporate at least one electric conductor (5, 9) exten­ ding along the cable length radially outward of the op­ tical fibre or bundle and, when a detectable current is flowing in the conductor (5, 9), constituting part of an elec­ tric alarm system.

Description: AN TMPROVED OPTICAL CABLE This invention relates to optical cables 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 cables for use in the communlcation field adapted for trars- mission of light having a wavelength within the range 0.5 to 1.3 micrometres. Optical cables for this purpose comprise at least one separate optical fibre and/or at least one optical bundle as hereinafter defined and examples of such optical cables are described and claimed in the Complete Specifications of our British Patents Nos. 1 479 426 and 1 479 427 and of our co-pending Patent Application No. 30474/77 (Serial No. By the expression "optical bundle" is meant a group of optical fibres or a group of fibres including at least one optical fibre and at least one non-op'tical re ..',-zin- fibre or other reinforcing elongate member. Each optical fibre of the optical bundle may be used independently as a separate light guide, each with its own modulated light source and detector, or a plurality of optical fibres of a bundle may be'used together as a single light guide, with a single light source. It is in the interests of users of optical cables forming part of a communications system that the system should be protected against illegal tapping of transmitted signals and it is an object of the present invention to provide an improved optical cable of such a form that any tapping of the cable can readily be detected. According to the invention, in the improved optical cable the separate optical fibre or fibres and/or the optical bundle or bundles is or are loosely housed in, or surrounded by, an elongate compartment bounded by a fluid-impermeable wall or fluid-impermeable walls, which elongate compartment extends throughout the length of the cable and contains a fluid at such a pressure that detection of a change in pressure of said fluid will serve to indicate that the fluid-impermeable wall or one of the fluid-impermeable walls has been ruptured. The optical cable has the important advantage that there is no visible indication to a person wishing to tap the cable illegally that the cable incorporates an alarm system for alerting that the cable has been opened up. The fluid may be air or any other suitable gas at a pressure above or below atmospheric pressure or it may be a liquid, e.g. water. When the optical fibre or fibres and/ or optical bundle or bundles is or are loosely housed in the elongate compartment containing a fluid under said pressure, the fluid will be a gas or liquid to which the or each optical fibre is inert and which will have no deleterious effect on the transmission characteristics of the cable. In its simplest form, the elongate compartment of the optical cable is the bore of a tube in which the or each separate optical fibre and/or optical bundle is loosely housed, the wall of the tube being of fluid-impermeable material and the bore of the tube containing fluid under said pressure. Preferably, the optical cable also includes, radially outwardly of the or each optical fibre and/or optical bundle, at least one electric conductor which extends throughout the length of the cable and along which a detectable current can be caused to flow, an interruption in the current flow indicating that the conductor has been cut. The or each electric conductor is preferably embedded in or positioned radially inwardly of the cable sheath, or of the fluid-impermeable wall which is radially outwardly of the or each optical fibre and/or optical bundle, so that there is no visible indication that the cable includes an electric alarm system. In this case, preferably the conductor is a circumferentially continuous conductor that surrounds the separate optical fibre or fibres and/or optical bundle or bundles and that is embedded in the cable sheath or said fluid-impermeable wall. In one preferred embodiment, the optical cable includes inner and outer electric conductors radially spaced apart and electrically insulated from one anQther2 ;te elon- gate compartment is a bore in, and extending throughout the inner conductor and/or a comDartment extending through- ¯ ¯ out the length of the cable between the inner and outer conductors and surrounding the inner conductor; and the or each separate optical fibre and/or optical bundle is loosely housed in the bore of the inner conductor and fluid under said pressure is contained in said bore orinthe compartment between the inner and outer conductors, or the or each separate optical fibre and/or optical bundle is loosely housed in the compartment between the inner and outer conductors and fluid under said pressure is contained in said compartment. Where the optical cable includes inner and outer conductors, or has one or more than one pair of conductors, radially outwardly of the or each optical fibre and/or optical bundle, a train of pulses can be sent down the cable using the inner and outer conductors or the or each pair of conductors. Any person attempting to tap the cable illegally will not only have to rupture the fluid-impermeable wall and thereby cause a change in pressure of the fluid in the elongate compartment but will have to manipulate the conductors and introduce objects between them. Such interference with the conductors would be detectable as impedance irregularities. The inner and outer conductors may take any convenient form. Where fluid under said pressure is contained in an elongate compartment between the inner and outer conductors, the means by which the inner and outer conductors are radially spaced apart and electrically insulated from one another will be of such a form that the elongate compart- ment is continuous throughout the length of the cable. Such spacing means may ccmprise at least one elongate spacing member which extends helically around or substantiall:: parallel to the inner conductor or longitudinally spaced discs each having a hole or holes for longitudinal flow of fluid. The invention also includes a transmission system comprising one or more than one improved optical cable as hereinbefore described and, coupled to the cable or cables, monitoring means for detecting a change in pressure of the fluid in the elongate compartment. Where the or each optical cable of the system includes at least one electric conductor, the transmission system may also include monitoring means for detecting an interruption in current flow or irregularities in impedance. The invention will be further illustrated bv a description, by way of example, of a preferred optical cable with reference to the accompanying drawing which shows a transverse crcss-sectional view of the cable drawn cn an enlarged scale. The optical cable ccmprises a tube 1 of e@@@ruded polyethylene having a bore 2 and, embedded in the ruse, two strain embers 3 of arcmatic polyamide. An optical fibre 4 is loosely housed in the bore 2. A circumferentially continuous inner conductor 5 of plain annealed copper wire braid overlies tube 1 and is surrounded by an extruded layer 6 of polyethylene constituting an inner fluidimpermeable wall. A spacing thread 7 of polyethylene is helically lapped around the fluid-impermeable wall 6 and an extruded layer 8 of polyethylene overlies the helically lapped spacing thread 7 in such a way that it is concentric to the inner conductor 5 and constitutes an outer fluidimpermeable wall. Overlying the outer fluid-impermeable wall 8 is a circumferentially continuous outer conductor 9 of plain annealed copper wire braid. The outer conductor 9 is surrounded by an extruded sheath 10 of polyvinylchloride. The elongate compartment 11 bounded by the inner and outer fluid-impermeable walls 6 and 8 contains air at a pressure of 70 kN/m2.

Claims: What we claim is: 1. An optical cable comprising at least one separate optical fibre and/or at least one optical bundle as hereinbefore defined, characterised in that the separate optical fibre or fibres and/or the optical bundle or bundles is or are loosely housed in, or surrounded by, an elongate compartment bounded by a fluid-impermeable wall or fluid-impermeable walls, which elongate compartment extends throughout the length of the cable and contains a fluid at such a pressure that detection of a change in pressure of said fluid will serve to indicate that the fluid-impermeable wall or one of the fluid-impermeable walls has been ruptured. 2. An optical cable as claimed in Claim 1, characterised in that the fluid is air or other gas at a pressure above or below atmospheric pressure. 3. An optical cable as claimed in Claim 1, characterised in that the fluid is water or other liquid. 4. An optical cable as claimed in any of the preceding Claims, characterised in that the elongate compartment is the bore of a tube in which the or each separate optical fibre and/or optical bundle is loosely housed, the wall of the tube being of fluid-impermeable material and the bore of the tube containing fluid under said pressure. 5. An optical cable as claimed in any one Or the preceding Claims, characterised in that the optical cable also includes, radially outwardly of the or each optical fibre and/or optical bundle, at least one electric conductor which extends throughout the length of the cable and along hitch a detectable current can be caused to flow, an inner eruption in the current flow indicating that the conductor has been cut. An optical cable as claimed in Claim a, caracterised in that the or each electric conductor is embedded in or positioned radially inwardly of the cable sheath, or of the flud-imnermeable wall which is radially ou'.':ardly of the or each optical fibre and/or optical bundle, so that there is no visible indication that the cable includes an electric alarm system. 7. An optical cable as claimed in Claim 6, naracterised in that the electric conductor is a circumferentially contin- uous conductor that surrounds the separate optical fibre or fibres and/or optical bundle or bundles and tat is embedded in the cable sheath or said fluid-impermeable wall. S. An optical cable as claimed in 'any one of Claims 5 to 7, characterised in that the optical cable includes inner and outer electric conductors radially spacea apart and electrically insulated from one another; in tat the elongate compartment is a bore in, and extending throughout the length of, the inner conductor andbr a compartment extending throughout the length of the cable between the inner and outer conductors and surrounding the inner conductor; and in that the or each separate optical fibre and/or optical bundle is loosely housed in the bore of the inner conductor and fluid nder said pressure is contained in said bore or in the compartment between the inner and outer conductors. 9. An optical cable as claimed in any one of Claims 5 to 7, characterised in that the optical cable includes inner and outer electric conductors radially spaced apart and electrically insulated from one another; in that the elongate compartment is a compartment extending throughout the length of the cable between the inner and outer conductors and surrounding the inner conductor; and in that the or each separate optical fibre and/or optical bundle is loosely housed in the compartment between the inner and outer conductors and fluid under said pressure is contained in said compartment. 10. An optical cable as claimed in Claim 9, wherein the inner and outer conductors are radially spaced apart by spacing means comprising at least one elongate member which extends helically around or substantially parallel to the inner conductor W bylongitudinally spaced discs each having a,hole or holes for longitudinal flow of fluid.

Inventors: GOLDBERG, JOHN LESLIE, SADLER, ALAN ARTHUR
Applicants: BICC PUBLIC LIMITED COMPANY
Application year: 1979
Main classification: G02B5
IPC codes: G02B6