Patent Description:
Nowadays, fiber optic cables are extensively used in access networks to provide connectivity to user premises. In an optical access network, such as a FTTx network (e.g., FTTH or FTTP), the majority of cables are aerial fiber optic cables, which are laid suspended between two poles or between a pole and a house. As known, aerial cables are subject to high tension loads due to a number of factors including the cable weight and the environmental conditions (ice, wind, etc.). As a result of wind and/or ice, in particular, the fiber optic cable is subject to dynamic loading conditions. These conditions cause the cable length to increase due to the increase in cable sag when the loading is applied, while the cable is reverted back to its original configuration when the loading ceases.

In aerial cable installations, typically, the optical fibers are terminated at each end of a span inside a joint closure. Therefore, the so-called "central loose tube cables" (briefly, CLT cables) are often used, as the optical fibers must be individually extracted from the cable and spliced. In a CLT cable, all the optical fibers of the cable are loosely arranged within a single buffer tube, which is in turn surrounded by a sheath. Besides, in a FTTH network, the cables typically have a low fibre count (e.g. up to <NUM> optical fibers), so that the choice of the CLT design make these cables cost effective.

With such type of cables, however, when the cable length increases due to ice and/or wind, the excess of the optical fiber(s) located within the joint closure is pulled into the span. When the loading ceases, the optical fiber(s) are unable to revert back to their original configuration due to the resistance of the gel within the loose tube. This situation would cause an increment in the optical fiber losses within the span due to micro-bending of the optical fiber(s).

To prevent the pulling of the optical fiber(s) into the span, a cable anchoring device can be used to secure the optical fibers against movement. For instance, the cable anchoring device may comprise a couple of bollards around which the cable may be wound to block the optical fiber(s). The device may be positioned close to the joint closure at each end of the span, which means that it may be positioned on a pole, in an underground chamber, in a street-side cabinet or on the wall of a house, at the customer premises.

<CIT> discloses a method of protecting a weak point along a cable, the cable being subject to stress, comprising the steps of (i) selecting a section of the cable between a source of the strain and up to and including the weak point, (ii) using the section of the cable to form a cable configuration comprising a pair of coils, by - putting a twist in the cable by winding the cable in a first direction to form a first coil, and - taking the twist out of the cable by winding the cable in a second direction to form a second coil, and (iii) securing under tension the cable configuration formed.

<CIT> discloses a method of installing an optical cable, which includes the steps of exposing a portion of the buffer tube over a predetermined length and forming the exposed portion of the buffer tube into a coupling coil.

<CIT> discloses an optical fiber unit for air-blown installations comprising a plurality of optical fiber sub-units and a central member, wherein the optical fiber sub-units are stranded around the central member; wherein each of the optical fiber sub-units comprises a number of optical fibres, an inner layer which is radially outer to the optical fibres and an outer layer which is radially outer to the inner layer, wherein the outer layer comprises particulate material which is partially embedded into the outer layer; and wherein the optical fiber unit further comprises a binder for keeping the stranded optical fiber sub-units in a proper arrangement. The relatively high surface roughness of the outer layer reduces the friction between the optical fiber unit and the duct during the blowing procedure and increases the ability of the optical fiber unit to be entrained by the air blowing.

<CIT> discloses an optical fiber cable core including a buffer tube containing at least one optical fiber and reinforced by at least two substantially radially incompressible longitudinal strength members, each strength member having surface portions radially outermost with respect to the tube axis which are at or protrude from the exterior surface of the buffer tube. If the strength members protrude from the exterior surface of the buffer tube, less than <NUM>%, and preferably less than <NUM>%, of the outer surface of the strength members protrudes from the exterior surface of the buffer tube. The positions of the strength members can be readily determined, can be visible and can be easily removed from the buffer tube prior to slitting the buffer tube to achieve midspan access. The core can include a buffer tube having an inner tubular portion, an outer tubular portion including the strength members, and a release agent between the tubular portions to enable easy separation of the tubular portions so that conventional slitting tools can be used to gain midspan access.

The inventors noticed that using a cable anchoring device comprising the bollards as described above (such as the device shown in Figures 7A, 7B and <NUM> of <CIT>) is not feasible when the aerial cable installation comprises more than one central loose tube cable branching off from the joint closure. In that case, indeed, using a number of such devices to anchor each cable with a respective pair of bollards would result in an excessively cumbersome arrangement.

In view of the above, the Applicant has tackled the problem of providing an aerial fiber optic cable for terrestrial networks, in particular, but not exclusively, FTTx, e.g., FTTH or FTTP, networks, which allows avoiding the fiber(s) to be pulled into the span in the presence of a load on the fiber optic cable while avoiding any cumbersome arrangement of cable anchoring devices. It is further to be noticed that avoiding using the cable anchoring devices described above would also result in a cheaper installation.

The inventors surprisingly found that the problem above may be solved in a particularly effective manner by using a fiber optic cable comprising an optical fiber unit in which the optical fibers are embedded into a resin and are firmly locked within the optical fiber unit. In other words, the optical fibers are tightly buffered in the optical fiber unit thanks to the presence of the resin among them. As a result, the movement of the optical fibers within the optical fiber unit is advantageously prevented. In particular, this also prevents the fibers from being pulled out of the optical fiber unit into the span in the presence of a load on the cable, without using any other device such as the bollards described above.

Moreover, the optical fiber unit of the fiber optic cable of the present invention comprises an outer layer with particulate material at an external boundary thereof (i.e., at the boundary of the outer layer which is radially farthest from the center of the cable), the particulate material forming, at the external boundary of the outer layer, a rough external surface which provides a grab surface for one or more strength members stranded around the optical fiber unit and/or a sheath (namely, a surface configured to grab one or more strength members stranded around the optical fiber unit and/or a sheath), so as to lock the optical fiber unit within the cable and prevent it from being pulled out of the cable. In other words, the outer layer of the optical fiber unit within the fiber optic cable of the present invention provides an unexpected high friction when enclosed by additional elements such as the one or more strength members and/or the sheath.

In the following description and in the claims, the expression "aerial fiber optic cable" will refer to a fiber optic cable used in aerial cable installations, namely suspended above the ground to form a span between two end points located on two poles, or a pole and a house, or two buildings or the like.

In one aspect, the present invention relates to an aerial fiber optic cable comprising an optical fiber unit, one or more strength members and a sheath, wherein the optical fiber unit comprises a number of optical fibers, an inner layer which is radially outer to the optical fibers and embeds the optical fibers and an outer layer which is radially outer to the inner layer, the outer layer comprising particulate material which is partially embedded into the outer layer, wherein the one or more strength members are arranged in a radial outer position with respect to said optical fiber unit, and wherein the sheath is extruded over the optical fiber unit and the one or more strength members.

In embodiments of the present invention, the optical fibers are thightly buffered in the optical fiber unit by the material of the inner layer, which fills the space among the optical fibers and prevents the movement of the optical fibers in the optical fiber unit.

In embodiments of the present invention, the outer layer is in direct contact with at least one of the strength members or the sheath. In particular, according to these embodiments of the present invention, the particulate material at least partially projects from the outer surface of the outer layer and hence it provides the outer layer with a rough external surface. This external surface is in direct contact with at least one of the strength members or the sheath. The rough external surface of the outer layer is configured to grab at least one of the strength members or the sheath.

According to an embodiment of the present invention, the aerial fiber optic cable comprises two strength members arranged within the thickness of the sheath. According to this embodiment, the two strength members are arranged at diametrically opposed positions embedded within the thickness of the sheath. Moreover, each strength member comprises a number of stranded metallic wires.

According to these embodiments of the present invention, the sheath has a substantially uniform outer diameter along its perimeter, said diameter being lower than <NUM>.

According to further embodiments, the aerial fiber optic cable comprises two strength members arranged at diametrically opposed positions with respect to the optical fiber unit and parallel thereto. According to these embodiments, the two strength members are glass reinforced plastic (GRP) strength members and have a diameter between <NUM> and <NUM>.

According to these embodiments, the aerial fiber optic cable is a flat fiber optic cable whose width is from <NUM> to <NUM> and thickness from <NUM> to <NUM>.

According to even further embodiments, the aerial fiber optic cable comprises a number of strength members arranged on a circumference radially outer to the outer layer of the optical fiber unit.

In particular, according to these embodiments, the strength members comprise multiple layers of aramid yarns with an overall thickness from <NUM> to <NUM>.

In the aerial fiber optic cables of the present invention the sheath is a polymeric sheath made of LSOH (Low Smoke Zero Halogen) or HDPE (High-Density Polyethylene) or glass filled polypropylene.

In another aspect, the present invention relates to a method of manufacturing an aerial fiber optic cable, the method comprising:.

In a further aspect, the present invention relates to a use of an optical fiber unit in an aerial fiber optic cable, the optical fiber unit comprising a number of optical fibers, an inner layer which is radially outer to the optical fibers and embeds the optical fibers and an outer layer which is radially outer to the inner layer, the outer layer comprising particulate material which is partially embedded into the outer layer, and wherein said aerial fiber optic cable comprises one or more strength members arranged in a radial outer position with respect to the optical fiber unit, and a sheath extruded over the optical fiber unit and the one or more strength members.

Further characteristics and advantages will become more apparent by reading the following detailed description of an embodiment given as an example with reference to the accompanying drawings, wherein:.

In the present description and claims, unless otherwise specified, all the numbers and values should be intended as preceded by the term "about". Also, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.

The fiber optic cable of the present invention is an aerial fiber optic cable for FFTx (e.g., FTTH or FTTP) applications. <FIG> schematically shows an exemplary arrangement of a fiber optic cable <NUM> to form a span <NUM> of an optical access network. The span of <FIG> extends from a pole <NUM> to a house <NUM>. At each end of the span <NUM>, a respective joint box or closure 5a, 5b is provided as mounted on the pole <NUM> and on the wall of the house <NUM>. The continuous line illustrates the situation according to which the fiber optic cable <NUM> is under normal loading conditions (due to, e.g., its weight), while the dashed line illustrates the situation in which the fiber optic cable <NUM> is under wind and/or ice loading conditions causing the length of the span to increase.

The fiber optic cable according to the present invention comprises one or more optical fiber units, a number of strength members stranded around the optical fiber unit(s) and a sheath, which is extruded over the optical fiber unit(s) and the strength members.

In the embodiments schematically shown in the Figures, the aerial fiber optic cable comprises, for sake of example, a single optical fiber unit. The optical fiber unit has an outer layer comprising particulate material partially embedded therein at an external boundary thereof (i.e., at the boundary of the outer layer which is farthest from the center of the optical fiber unit). The optical fiber unit, in particular, its outer layer, is hence provided with a rough external surface. According to the claimed invention, the optical fiber unit, the external surface of the outer layer, is in direct contact with at least one of the strength members and the sheath and is configured to grab the at least one of the strength members and the sheath to lock the optical fiber unit within the cable.

<FIG> shows a fiber optic cable 1a according to a first embodiment of the present invention.

The optical fiber unit <NUM> is schematically shown in greater detail in <FIG>. In particular, <FIG> represents a schematic cross-section of the optical fiber unit <NUM> according to the present invention.

The optical fiber unit <NUM> comprises a number of optical fibers <NUM>, an inner layer <NUM>, which is radially outer to the optical fibers <NUM> and embeds the optical fibers <NUM>, and an outer layer <NUM> which is radially outer to the inner layer <NUM>. The outer layer <NUM> comprises particulate material <NUM> which is partially embedded into the outer layer <NUM> at an external boundary thereof.

The term "optical fiber" is meant to indicate an optical glass core surrounded by a glass cladding and a coating system comprising one or two layers of cured resins, for example acrylate resins, optionally provided with a coloured ink layer. The optical fibers may be single mode or multimode optical fibers with a nominal diameter between about <NUM> and <NUM>. Relative to each other, the optical fibers may have a length difference of less than about <NUM> %.

The optical fiber unit <NUM> of the example of <FIG> and <FIG> comprises six optical fibers <NUM>. In other examples (not shown), there are provided more than six optical fibers. In other examples (not shown), there are provided less than six optical fibers. In other examples (not shown), the optical fiber unit comprises a single optical fiber. The weight of the optical fiber unit <NUM> may range between <NUM>/m with <NUM> optical fibers to <NUM>/m with twelve optical fibers.

Particulate material <NUM> can be selected among beads of glass, of ceramic, of polytetrafluoroethylene (PTFE) or of high-density polyethylene (HDPE). The beads can be either hollow or solid. The beads may have a diameter of from <NUM> to <NUM>. The beads at least partially project from the outer surface of the outer layer <NUM> of the optical fiber unit <NUM>.

The particle material coverage - i.e., the number of beads per unit surface area of the product - in the optical fiber unit <NUM> can be of from <NUM> to <NUM> beads/mm<NUM>.

The embedding is the amount of sinking of the particles into the outer layer <NUM>, expressed as percentage of the particle dimension which is embedded into the outer layer. According to examples of the invention, the embedding of the particle material is of from <NUM>% to <NUM>%, in particular from <NUM>% to <NUM>%.

As said above, inner layer <NUM> is arranged radially outer to the optical fibers <NUM> and embeds them. In other words, the optical fibers <NUM> are tightly buffered in the optical fiber unit by the material of the inner layer <NUM>, which fills the space among the optical fibers <NUM> and prevents the movement of the optical fibers <NUM> in the optical fiber unit. Profitably, the inner layer <NUM> comprises a layer of cured resin such as, for instance, acrylate resin. The diameter of inner layer <NUM> could be in a range between <NUM> and <NUM>, in particular between <NUM> and <NUM>. Example of a material for the inner layer <NUM> is Cabelite <NUM>-<NUM>-39A from DSM Desotech.

Outer layer <NUM> is radially outer to the inner layer <NUM>. Profitably, outer layer <NUM> comprises a layer of cured resin such as, for instance, acrylate resin. The outer diameter of outer layer <NUM> could be in a range between <NUM> and <NUM>, in particular between <NUM> and <NUM>. Outer layer <NUM> is generally harder than inner layer <NUM>. According to embodiments of the present invention, the modulus of elasticity of the outer layer <NUM> is higher than the modulus of elasticity of the inner layer <NUM>. Example of a material for the outer layer <NUM> is Cabelite <NUM>-<NUM>-<NUM> from DSM Desotech.

According to the first embodiment of the present invention, the exemplary fiber optic cable 1a of <FIG> comprises two strength members 11a arranged within the thickness of the sheath 12a. In the particular embodiment of <FIG>, the two strength members 11a are arranged at diametrically opposed positions, embedded within the thickness of the sheath 12a. For instance, each strength member 11a comprises a number of (three, in the embodiment of <FIG>) stranded metallic wires, e.g., coated steel wires, or stranded non-metallic yarns, e.g. aramid yarns. For example, the strength member 11a comprises a number of stranded steel wires having a diameter of about <NUM> each.

In some embodiments (as, for example, in the embodiment shown in <FIG>) the strength members 11a are arranged substantially parallel to the optical fiber unit <NUM>.

The sheath 12a is a polymeric sheath which is extruded over the optical fiber unit <NUM> and the strength members 11a. The sheath 12a is made of, for example, LSOH (Low Smoke Zero Halogen) or HDPE (High-Density Polyethylene) or glass filled polypropylene. The sheath 12a has a substantially uniform thickness along its perimeter. In particular, the sheath 12a may have a substantially uniform outer diameter along its perimeter. The outer diameter of the sheath 12a is for instance lower than <NUM>, in some embodiments lower than <NUM>, in further embodiments between <NUM> and <NUM>.

According to the embodiment shown in <FIG>, the sheath 12a comprises two grooves (or notches) <NUM> placed at substantially diametrically opposed positions. The grooves <NUM> have, for example, a blunt profile. Furthermore, the groove depth can be, for example, substantially equal to <NUM>. This way, even when the cable <NUM> is bent or twisted e.g., during installation, the presence of the grooves <NUM> does not impair the sheath integrity. As shown in <FIG>, at least one of the grooves <NUM> is optionally provided with an identification stripe <NUM> and/or ink-jet printed identification codes allowing identification of the cable <NUM>. The bisector of the grooves <NUM> may be arranged on a longitudinal plane perpendicular to the longitudinal plane containing the strength members 11a.

The fiber optic cable 1a also comprises two optional ripcords <NUM> arranged between the optical fiber unit <NUM> and the sheath 12a. The ripcords <NUM> may be aligned with the grooves <NUM>. Each ripcord <NUM> is made, for example, of non-metallic yarns, e.g., aramid yarns or polyester yarns.

<FIG> shows a fiber optic cable 1b according to a second embodiment of the present invention. According to this embodiment, the fiber optic cable 1b is a flat fiber optic cable.

The fiber optic cable 1b comprises an optical fiber unit <NUM> with optical fibers <NUM>, as already described with reference to the first embodiment of the present invention.

Moreover, the exemplary fiber optic cable 1b comprises two strength members 11b arranged at diametrically opposed positions with respect to the optical fiber unit <NUM> and parallel to it. The two strength members 11b may be glass reinforced plastic (GRP) strength members. In some examples, the strength members 11b are made of GRP and have a diameter between <NUM> and <NUM>.

Furthermore, the fiber optic cable 1b preferably comprises a ripcord <NUM>.

The fiber optic cable 1b then comprises a polymeric sheath 12b. The sheath 12b is extruded over the optical fiber unit <NUM>, the strength members 11b and the ripcord <NUM> and it is made of, for example, LSOH (Low Smoke Zero Halogen) or HDPE (High-Density Polyethylene) or glass filled polypropylene. In this case, the sheath 12b does not embed the strength members 11b. In particular, the sheath 12b surrounds the optical fiber unit <NUM>, the strength members 11b and the ripcord <NUM>. Furthermore, it may have a substantially uniform thickness along its perimeter. According to this embodiment, the fiber optic cable 1b may have width from <NUM> to <NUM> and thickness from <NUM> to <NUM>. In some embodiments, the fiber optic cable 1b has a width between <NUM> and <NUM> and a thickness between <NUM> and <NUM>.

<FIG> shows a fiber optic cable 1c according to a third embodiment of the present invention.

The fiber optic cable 1c comprises an optical fiber unit <NUM> with optical fibers <NUM>, as already described with reference to the first embodiment of the present invention.

Moreover, the exemplary fiber optic cable 1c comprises a number of strength members 11c arranged on a circumference radially outer to the outer layer <NUM> of the optical fiber unit <NUM>. The strength members 11c are stranded around the optical fiber unit <NUM>. In particular, the strength members 11c are filamentary aramid yarns stranded around the optical fiber unit <NUM> and possibly arranged in multiple layers. In some embodiments, the overall thickness of the multiple layers of filamentary aramid yarns can be comprised between <NUM> to <NUM>. In some examples, the strength members 11c comprise multiple layers of <NUM> dtex aramid yarns.

The fiber optic cable 1c then comprises a polymeric sheath 12c. The sheath 12c is extruded over the optical fiber unit <NUM> and the strength members 14c in a radially outer position thereto, and it is made of, for example, LSOH (Low Smoke Zero Halogen) or HDPE (High-Density Polyethylene) or glass filled polypropylene. The sheath 12c has a substantially uniform thickness along its perimeter. In particular, the sheath 12c may have a substantially uniform outer diameter along its perimeter. The outer diameter of the sheath 12c is e.g., lower than <NUM>, in some embodiments lower than <NUM>, in further embodiments between <NUM> and <NUM>.

According to the embodiments of the present invention as described herein above, when the strength members and/or the sheath are placed in direct contact with the optical fiber unit, they are grabbed by the rough surface provided by the particulate material partially embedded in the outer layer of the optical fiber unit. Indeed, as mentioned above, a high static friction is provided between the particulate material and the strength members and/or the sheath in direct contact with the rough surface. This effectively locks the optical fiber unit and the strength members and/or the sheath together. The optical fiber unit is then effectively locked within the cable due to the interaction of the rough surface and the strength members.

The advantage of using the optical fiber unit with the particulate material in the outer layer rather than using, e.g., a glue between the optical fiber unit and the strength members is that it is easier to remove the sheath and strength members when the fiber optic cable is to be punt into a joint.

It is to be noticed that the description above has been made with reference to an exemplary cable comprising a single optical fiber unit. This is not limiting since two or more optical fiber units of the same type as described above may be used in aerial fiber optic cables having similar features (e.g. strength members, polymeric sheath) as those detailed above.

The present invention also provides a method of manufacturing an aerial fiber optic cable, which comprises the following steps:.

As apparent from the description above, the present invention provides for blocking the optical fiber(s) in an aerial fiber optic cable by avoiding to use CLT cables and by using instead optical fiber unit(s) in which the fibers are immersed in a resin, which effectively keeps them firmly in position within the cable. As a result, the cable of the present invention allows avoiding any movement of the fibers that are locked within the optical fiber unit. Moreover, also the optical fiber unit is locked within the cable thanks to the interaction between the rough surface of the optical fiber unit and the strength members.

Claim 1:
An aerial fiber optic cable (1a; 1b; 1c) comprising an optical fiber unit (<NUM>), one or more strength members (11a; 11b; 11c) and a sheath (12a; 12b; 12c), wherein
said optical fiber unit (<NUM>) comprises a number of optical fibers (<NUM>), an inner layer (<NUM>), which is radially outer to the optical fibers and embeds the optical fibers (<NUM>), and an outer layer (<NUM>) which is radially outer to the inner layer (<NUM>), the outer layer (<NUM>) comprising particulate material (<NUM>) which is partially embedded into the outer layer (<NUM>),
said one or more strength members (11a; 11b; 11c) are arranged in a radial outer position with respect to said optical fiber unit (<NUM>), said sheath (12a; 12b; 12c) is extruded over said optical fiber unit (<NUM>) and said one or more strength members (11a; 11b; 11c), and characterised in that the particulate material (<NUM>) provides the outer layer (<NUM>) with a rough external surface, and said external surface is in direct contact with at least one of the strength members (11a; 11b; 11c) or the sheath (12a; 12b; 12c) and provides a grab surface for at least one of the strength members (11a; 11b; 11c) or the sheath (12a; 12b; 12c).