Optical cable and manufacturing method

Abstract

An optical cable including a load bearing core includes a longitudinally and radially extending slot housing at least one optical fibre, wherein the slot has a width providing a low clearance for the optical fibre(s) housed therein and preventing two optical fibres being stuck to one another; and the slot has a depth equal to or lower than a radius of the core.

Claims

1. An optical cable comprising a slotted load bearing core and a sheath arranged in radial external position with respect to the slotted load bearing core, wherein the slotted load bearing core comprises a longitudinally and radially extending slot housing an optical fibre; wherein the slot has an opening and a width ranging from about 1.1 to 1.5 times a diameter of the optical fibre thereby providing a low clearance for the optical fibre housed therein and preventing the optical fibre being stuck to a second optical fibre when present; wherein the slot has a depth equal to or less than a radius of the slotted load bearing core; wherein the slotted load bearing core has a diameter of 3 mm at most; wherein either the sheath is disposed directly on the slotted load bearing core or the optical cable comprises an adhesive layer interposed between and in contact with the slotted load bearing core and the sheath; and wherein the sheath directly closes the slot opening.

2. The optical according to claim 1, wherein the sheath is an extruded sheath.

3. The optical cable according to claim 1, wherein the slotted loading bearing core has a substantially circular cross-section.

4. The optical cable according to claim 1, wherein the slotted loading bearing core has a diameter at least four times the slot width.

5. The optical cable according to claim 1, wherein the slotted loading bearing core is made of fibre reinforced plastic.

6. The optical cable according to claim 1, wherein the slotted loading bearing core is made of a material having an elastic modulus of at least 40 GPa.

7. The optical cable according to claim 1, wherein the slotted loading bearing core has a diameter greater than 1.9 mm.

8. The optical cable according to claim 1, wherein the cable has a diameter ranging from 2.5 mm to 5 mm.

9. The optical cable according to claim 1, wherein two optical fibres are housed in the slot.

10. The optical cable according to claim 1, wherein the slot width is less than 1.3 times the optical fibre diameter.

11. The optical cable according to claim 1, wherein the slot contains water swellable material.

12. The optical according to claim 1, wherein the sheath is configured so that an indicium of the position of the slot in the slotted loading bearing core is present.

13. The optical cable according to claim 12, wherein the indicium is a flattened longitudinally extended area of an outer surface of the sheath in correspondence with the slot longitudinal extension.

14. The optical cable according to claim 12, wherein the indicium is a colored line.

15. A method of manufacturing an optical cable, comprising: providing a slotted load bearing core, wherein the slotted load bearing core comprises a longitudinally and radially extending slot configured for housing an optical fibre, and wherein the slot has an opening and a width ranging from about 1.1 to 1.5 times a diameter of the at least one optical fibre thereby providing a low clearance for the optical fibre housed therein and preventing the optical fibre being stuck to a second optical fibre when present, and wherein the slot has a depth equal to or less than a radius of the core, and wherein the core has a diameter of 3 mm at most; housing the optical fibre in the slot; applying a sheath either directly over the slotted load bearing core or with an adhesive layer interposed between and in contact with the slotted load bearing core and the sheath; and winding the cable onto a coil while orienting the opening of the slot radially outwardly with respect to the coil.

16. The method according to claim 15, wherein the sheath is applied by extrusion.

17. The method according to claim 15, further comprising the step of providing a water swellable material in the slot.

18. The method according to claim 15, comprising the step of providing an adhesive layer interposed between the slotted load bearing core and the sheath.

19. The method according to claim 15, further comprising the step of providing an indicium of the position of the slot in the slotted load bearing core, wherein the indicium is a flattened longitudinally extended area of an outer surface of the sheath in correspondence with the slot longitudinal extension or a colored line.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The present invention will become more clear from the following detailed description, given by way of example and not of limitation, with reference to the accompanying figures, wherein:

(2) FIGS. 1a and 1b are a cross-section and an axonometric view, respectively, of an optical cable according to a first embodiment of the present invention;

(3) FIGS. 2a and 2b are a cross-section and an axonometric view, respectively, of an optical cable according to a second embodiment of the present invention;

(4) FIGS. 3a and 3b are a cross-section and an axonometric view, respectively, of an optical cable according to a third embodiment of the present invention;

(5) FIGS. 4a and 4b are a cross-section and an axonometric view, respectively, of an optical cable according to a fourth embodiment of the present invention; and

(6) FIGS. 5a and 5b show two cable cores according to embodiments of the invention with slots of different width.

DESCRIPTION OF EXAMPLES

(7) FIGS. 1a and 1b schematically show an optical cable according to a first embodiment of the present invention.

(8) Cable 10 comprises a load bearing core 11 with a slot 12 where one or more optical fibres 15a, 15b are housed. Cable 10 also preferably comprises a sheath or jacket 13 arranged in an outer position with respect to core 11.

(9) Load bearing core 11 has preferably a substantially curved-bottom cross-section. Core 11 is preferably made of a fibre reinforced composite material or FRP based, for example, on any of glass fibres, carbon fibres, aramid fibres, poly (p-phenylene-2,6-benzobisoxazole) (PBO) fibres or the like embedded in a polymeric resin. In examples, core 11 is essentially made of glass-reinforced plastic, GRP, with modulus of elasticity of 50 GPa.

(10) The core 11 can be produced by pultrusion, by UV curing or any other known technique. Pultrusion results in a more regular product.

(11) Core 11 has preferably a circular cross-section. However, it can have a cross-section different from circular, for instance it can be oval.

(12) In the embodiment of FIGS. 1a, 1b, one single optical fibre 15a is housed in the slot 12. Another optical fibre 15b can be housed in the slot 12 in stacked configuration with respect to the optical fibre 15a. Optical fibre(s) 15 can be independent fibres, fibres in ribbon or bundled fibres.

(13) In the embodiment of FIGS. 1a, 1b the core 11 has a diameter of 1.95 mm.

(14) In the embodiment of FIGS. 1a, 1b the slot 12 has a depth D of 0.975 mm.

(15) Preferably, the slot 12 is open at the outer surface of the core 11 and the opening is bevelled for a more comfortable introduction of the optical fibre(s) 15a, 15b. Preferably, the opening is rounded with a radius of between 0.08 mm and 0.15 mm. In one example, the radius of rounded opening sides is 0.01 mm.

(16) In order to avoid longitudinal water propagation, the slot 12 can contain water swellable material (not shown). A layer of water swellable material can be provided to cover, at least partially, the slot 12 surface. In addition or as an alternative, one or more water swellable yarns can be housed into the slot 12 together with the optical fibre(s) 15a, 15b, preferably in radial external position with respect to the fibre(s).

(17) Preferably, a ratio between the diameter of the core and the slot width W is higher than 4. This is of particular advantage for aerial installation where a significant cross section of the core can limit the cable elongation when installed at a determinate span under wind and ice loads.

(18) A polymeric sheath 13 can be arranged radially outer of the core 11. The sheath 13 can be extruded directly on the load bearing core 11. The sheath 13 may comprise polyethylene (PE), crosslinked PE, poly(vinyl chloride) (PVC), thermoplastic or thermoset compounds, preferably emitting limited smoke and not containing halogen, aliphatic polyamides, Nylon, Silicone, Rubber, or the like.

(19) Preferred thickness of the sheath 13 can be of from 0.45 to 0.50 mm. However, thickness of the jacket 13 can be higher or lower than the above range.

(20) Preferably, the outer surface of the core 11 is coated, at least partially, with a hot melt adhesive layer 18. Hot melt can be based, for example, on ethylene ethyl acrylate (EEA) or ethylene vinyl acetate (EVA).

(21) In the present embodiment, the sheath 13 comprises a flat portion (or flattened longitudinally extended area) 17 which extends in a longitudinal direction. Flat portion 17 is extended radially external with respect to a plane passing through the middle plane of the slot 12. Therefore, the slot aperture results arranged substantially below the flat portion and can be easily identified. Therefore, it becomes easy to locate the slot position. This has a number of advantages, including an easy management (for instance easy coiling) of the cable during manufacturing thereof or an easy positioning of dead-end clamps. Further or different indicia could be provided for locating the opening of the slot from the outer surface of the sheath, such as an indentation or an ink line.

(22) The flat portion 17 can have a width between 0.5 mm and 0.9 mm. In one example, flat portion 17 is about 0.77 mm width thus providing a reduction of the cable diameter of 0.05 mm in the flat area.

(23) A thin polymeric layer (based on, for instance, EEA or EVA) can be provided over the outer surface of the load bearing core 11 to improve adhesion of the sheath 13.

(24) As said above, outer sheath 13 can be made of thermoplastic or thermoset compounds that emit limited smoke and substantially no halogen when exposed to high temperature sources. Preferably, the sheath outer surface has a limited coefficient of friction (0.08-0.15) due to the material which is made of or because of the addition, for example by spraying, of a suitable anti-friction agent.

(25) The cable 10 according to the invention can be manufactured, preferably in a continuous process, starting from the load bearing core 11 which is obtained by extrusion or pultrusion. During the manufacturing process optical fibre(s) 15a (15b) are inserted into the slot 12 and the sheath 13 is applied by extrusion. In the case the position of the slot is marked by a flat portion (17), the sheath is preferably manufactured by tube extrusion.

(26) Preferably, the cable manufacturing process includes a step of winding the cable in a coil while orienting the opening of the longitudinal slot radially outwardly with respect to the coil wherein a pulley of controlled diameter is use for coiling the cable. This results in a cable with a suitable excess fibre length (EFL).

(27) As excess fibre length is meant the value given by the following formula

(28) $\mathrm{EFL}\left(\%\right)=\frac{L_f-L_t}{L_t}.Math.100$

(29) wherein

(30) L.sub.f is the length of an optical fibre and

(31) L.sub.t is the length of the cable housing the fibre(s).

(32) In particular, the amount of EFL can depend on the diameter of the pulley where the cable is passing through. For example, a pulley having a diameter of 200 mm can provide the optical cable with an EFL of 1.2%; a pulley having a diameter of 100 mm can provide the optical cable with an EFL of 2.4%; a pulley having a diameter of 50 mm can provide the optical cable with an EFL of 4.7%; a pulley having a diameter of 40 mm can provide the optical cable with an EFL of 5.8%.

(33) The optical cable according to the invention is self-supporting and it can be profitably used as drop cable for span of at least 10 m, advantageously of 50-150 m. While providing a good protection of optical fibres the small diameter of the cable of the invention allows a better aerial mechanical performance because it offers a reduced resistance to wind compared with other drop cables existing on the market.

(34) The cable can be installed either by pulling or by pushing into a conduit also without a probe guide.

(35) A further advantage is that it can be easily mounted with connectors. This is because the load bearing core performs as bend stiffener protecting the fibre from much reduced diameter bends.

(36) FIGS. 2a and 2b show a second embodiment of the cable according to the present invention. The cable has been designated by reference number 20. The same or similar parts of the cable of the second example have been designated by reference numbers similar to the reference numbers of the first example wherein the first digit is 2 and not 1.

(37) The cable 20 of FIGS. 2a, 2b is generally similar to the cable of FIG. 1 and the detailed description will not be repeated. The difference is that the cable 20 further comprises two reinforcing elements 24. Preferably, the centre of each reinforcing element lies in a plane perpendicular to the depth D of the slot 22. Preferably, each of the reinforcing elements 24 is at the same distance from the centre of the load bearing core 21.

(38) Reinforcing elements 24 can be in the form of a couple of rods or yarns. They can comprise aramid yarns or glass fibres.

(39) Reinforcing elements 24 can have a diameter of from 0.3 to 1 mm.

(40) Profitably, a thin polymeric layer (for instance including EEA or EVA) can be provided over the reinforcing elements 24 to improve adhesion with core material.

(41) FIGS. 3a and 3b show a third embodiment of the cable according to the present invention. The cable has been designated by reference number 30. The same or similar parts of the cable of the third example have been designated by reference numbers similar to the reference numbers of the first example wherein the first digit is 3 and not 1.

(42) The cable 30 of FIGS. 3a, 3b is generally similar to the cable of FIG. 1 and the detailed description will not be repeated. The difference is that the cable 30 further two reinforcing elements 34. Preferably, the centre of each reinforcing element 34 lies in a plane perpendicular to the depth D of the slot 32. Preferably, each of the reinforcing elements 34 is at the same distance from the centre of the load bearing core 31.

(43) Reinforcing elements 34 can be in the form of a couple of rods or yarns. They can comprise aramid yarns or glass fibres.

(44) Reinforcing elements 34 can have a diameter of from 0.3 to 1 mm.

(45) Profitably, a thin polymeric layer 34 (for instance including EEA or EVA) can be provided over the reinforcing elements 34 to improve adhesion with sheath material.

(46) FIGS. 4a and 4b show a fourth embodiment of a cable according to the present invention. The cable has been designated by reference number 40. The same or similar parts of the cable of the third example have been designated by reference numbers similar to the reference numbers of the first example wherein the first digit is 4 and not 1.

(47) The cable 40 of FIGS. 4a and 4b is generally similar to the cable of FIG. 1 and the detailed description will not be repeated. The difference is that the cable 40 according to the fourth example further comprises a longitudinally extended reinforcing layer 44 between the load bearing core 41 and the sheath 43. Alternatively, layer 44 can be helically wound. Preferably, the cylinder element 44 has a thickness of 0.3 to 1 mm.

(48) Profitably, a thin polymeric layer (for instance including EEA or EVA) can be provided over the inner and/or outer surface of the reinforcing element 44 to improve adhesion with core material and/or sheath material.

(49) FIG. 5a shows a core 51 with a slot 52 having a width W, housing an optical fibre 55 having a diameter Y. W is 1.1 times Y so as to provide a low clearance housing of the fiber in the slot without causing interference when the optical fibre 55 is inserted in the slot 52.

(50) In FIG. 5b W is 1.5 times Y, so as to prevent two optical fibres 55a and 55b from being stuck one another. The possibility of sticking two optical fibres co-housed in the core slot can also depend on the friction coefficient of the optical fibre coating, of the core material and, if any, of the water swellable material covering, at least partially, the slot surface, and the suitable width W should be selected accordingly.