OPTICAL CABLES

Abstract: In order to provide additional gas flow space in an opti­ cal cable to be used in gas pressurised systems, the central strength member (3) is coated with a plastic sheath (4) having longitudinal grooves (5) in its surface. Optical units comprising one or more optical fibres (1) loosely enclosed within a tube (2) are then stranded around the coated strength member (3).

Description: Optical Cables This invention relates to optical cables of the type having at least one optical unit wound helically around a central strength member and surrounded by an outer protective sheath, the or each said optical unit comprising one or more optical fibres loosely enclosed within a tube. Such cables will be referred to hereinafter as being of the type specified". Cables of the type specified are particularly suited for installation in gas pressurised systems. However in such systems the distance between repeaters is such that a low pneumatic resistance is essential. It is thus an object of the present invention to provide additional space within the cable for gas flow along the length of the cable, over and above that provided by the interstices between the stranded optical units. Accordingly, the invention provides an optical cable of the type specified wherein the central strength member is coated with a plastic sheath having longitudinal grooves in its surface which provide paths for the passage of gas. The longitudinal grooves may extend parallel to the cable axis, or may follow a helical or undulating path along the sheath, and it will be understood that grooves of these different forms are included within the term "longitudinal grooves1,. Preferably, the plastic sheath is extruded around the central strength member. The cable may also include at least one electrical conductor also wound helically around the coated central strength member. The cable may include a plurality of optical units and electrical conductors in terwound with one another. These conductors may be plastic insulated copper conductors which may be singles or formed into pairs or quads. Further sheaths may be provided around the outer protective sheath. One embodiment of an optical cable according to the invention will now be more fully described by way of example with reference to the drawing. The optical cable comprises a plurality of optical fibres 1 loosely enclosed within plastic tube 2 to form optical units. In this embodiment three fibres 1 are shown within each such tube 2 of which nine are shown. The tubes 2 are helically wound around a central strength member 3 which has been coated with a plastics extrudate 4 having longitudinal grooves 5 running along its length. These grooves 5 allow more gas to flow along the cable than can flow along the interstices between the tubes 2. The tubes 2 stranded around the central strength member 3 are finally enclosed within at least one protective sheath 6. Although only one such sheath is shown, several sheaths having different properties may of course be utilised, as desired. One or more of the optical units may be replaced by electric conductors which may be in,he form of insulated copper conductors of any convenient form, preferably having substantially the same external diameter as the optical units. The longitudinal grooves may either be straight or extend in a helical fashion around the extrudate. In the latter case they. may be of opposite hand to the helically wound optical units. Also in some cases the grooves may follow an undulating path.

Claims: CLAIMS 1. An optical cable having one or more optical units wound helically around a central strength member and surrounded by an outer protective sheath' wherein the central strength member is coated with a plastic sheath having longitudinal grooves in its surface which provide paths for the passage of gas. 2. An optical cable according to Claim 1 including one or more electrical conductors also wound helically around the central strength member. 3. An optical cable according to Claim 2 including a plurality of optical untis and a plurality of electrical conductors interwound with each other. 4. An optical cable according to any preceding Claim wherein the grooved sheath has been extruded around the central strength member. 5. An optical cable according to any preceding Claim wherein the longitudinal grooves extend parallel to the cable axis. 6. An optical cable according to any preceding Claim wherein the longitudinal grooves extend helically around the sheath. 7. An optical cable according to any preceding Claim wherein the longitudinal grooves follow an undulating path along the sheath.

Inventors: PEACOCK, ALAN JAMES
Applicants: TELEPHONE CABLES LIMITED
Application year: 1985
Main classification: G02B6
IPC codes: G02B6