Patent Application
20230280559
The present invention relates to fibre units for use in fibre optic cables. A single fibre unit may be used as a fibre optic cable, for example adapted for installation in a duct by blowing. A plurality of fibre units may be formed into a larger cable. The invention further relates to methods of manufacturing such cables and methods of installation thereof. Such cables allow a selected fibre unit to be retracted from a section of the cable, and rerouted to an individual user without the need to create a splice joint.
Optical fibre transmission lines can be installed through the ground, through ducts, and through service spaces within buildings by a variety of methods, including direct burying (trenching), pulling through ducts, pushing through ducts, blowing through ducts, and combinations of these. Fibre to the home (FTTH) is the generic term for broadband network architecture that uses optical fibre technology to carry data to a residential dwelling from a broadband service provider via a telecommunications cabinet located near the residential dwelling. More generally, not only homes, but office premises are increasingly connected by optical fibres to the wider telecommunications network.
One type of optical fibre cable is a blown fibre unit of the type disclosed in published international patent application WO2004015475A2. The known blown fibre unit comprises two or more optical fibres embedded in a solid resin material to form a coated fibre bundle covered by an extruded polymer sheath of low-friction high-density polyethylene (HDPE). Such fibre units have been designed, and used for many years, for installation by blowing with air or other compressed fluid. Fibre units of this type are known to blow hundreds and even thousands of metres, in micro-ducts having a compatible low-friction high-density polyethylene (HDPE) lining. However, they can also be installed by pulling and/or pushing, depending on the distance and the route involved.
The known blown fibre unit has been commercially very successful, extending fibre optic communications in a cost-effective manner to streets and homes, as well as commercial premises. Aside from the cost of the product itself, the speed and ease of installation become ever more important. Various enhancements to the form of the sheath, and modifications of the HDPE material have been applied to increase performance under a wide range of use cases and environmental conditions.
Another type of cable is known, which comprises multiple fibre units contained loosely within an extruded tube. Once installed in the ground, or on or within a building, the extruded tube can be opened at any point along its length to access the individual fibre units, which extend loosely inside. A selected fibre unit can be accessed, retracted, and rerouted to drop directly to a home / business where fibre provision is required. Several commercial cables of this type are available, including one branded RTRYVA™ from the present applicant. They may be referred to as “pullback cable”, “retractable fibre cable”, or “mid span”/“mid span access” cable, depending on the manufacturer and user preference. The term “pullback cable” will be used in the following description, as a convenient term for this type of product, and with the existing RTRYVA™ product as a specific known example. Pullback cable offers a number of advantages over traditional cabling solutions because several times more fibre drops can be made from an existing duct compared to traditional cables. Fibre units within pullback cable can contain multiple fibres, varying from 2 to 12 fibres per fibre unit. High speed installation and connectivity can be attained with no specialist training, and without breaking or splicing the fibres, where they branch from the pullback cable to the customer premises. GRP strength members are incorporated in the extruded tube to offer additional strength and longevity, without the need for bulky strength members in the individual fibre units.
Drop tubes can have a pre-installed draw string to aid fibre installation to the home. Expensive installation equipment, such as fibre blowing is not required.
Despite these benefits of pullback cable, the use is restricted, or made inefficient, by the limited length of fibre unit that can be withdrawn in one section. Where the premises is located more than a few tens of metres from the route of the pullback cable, steps of withdrawing the selected fibre unit, and redirecting it to the customer premises, must be performed in multiple stages, opening the extruded tube within the ground or other environment several times, and repositioning operatives several times to reach customer premises in stages.
Accordingly, the inventors have recognised that, in many situations, the potential benefits of pullback cabling are not realised. The inventors have further recognised that the length of fibre unit withdrawn or installed in one step is limited by the materials and loose tube construction of the fibre units in a conventional pullback cable. Unfortunately, the use of other types of fibre unit, such as fibre units with low friction HDPE sheaths, that are known for installation by blowing, cannot readily be substituted into known types of pullback cables, due to the manufacturing process.
A first aspect of the present invention provides a fibre optic cable comprising at least one fibre unit, wherein said fibre unit comprises two or more optical fibres embedded in a solid resin material to form a coated fibre bundle and an extruded polymer sheath covering the coated fibre bundle, wherein the extruded polymer sheath of each said fibre unit comprises a mixture of polybutylene terephthalate polymer (PBT) and at least one friction reducing additive.
The PBT polymer excluding additives may comprise at least 95% by weight, at least 90% by weight or at least 80% by weight of the extruded sheath.
Embodiments of the invention are disclosed in which the friction reducing additive comprises a polydimethylsiloxane material, PDMS, in a carrier material. These materials are available for example from Dow Corning in the form of masterbatch additives for blending with the base polymer of the sheath in an extrusion machine.
The amount of friction reducing additive may be between 1% and 5%, optionally between 2% and 4% by weight of the material of the extruded sheath.
In some examples, said PDMS is an ultra-high molecular weight PDMS and said carrier material is a polyacrylate material, for example a copolymer of ethylene and methyl acrylate, EMA.
In other examples, said PDMS is an ultra-high molecular weight PDMS and said carrier material is a polyolefin, such as low-density polyethylene (LPDE). The additive may comprise at least 40%, for example 50% by weight ultra-high molecular weight PDMS dispersed in said low-density polyethylene (LPDE).
The inventors have found that between 2% and 4%, more particularly between 2.5 and 3.5% of a commercially available LDPE-based PDMS additive affords a substantial reduction in friction, with no attendant problems in extrusion. This performance was apparently better than using a polyacrylate based additive specifically marketed for blending with PBT. The overall siloxane content of the sheath material may be at least 1%, for example 1.5% or more (including any friction reducing material that is blended already with the PBT base polymer in a commercial product).
The solid resin material may comprise a UV-cured resin such as an acrylate material.
The solid resin material may have a tensile modulus greater than 100 MPa, optionally in the range 250-700 MPa, optionally around 300 MPa.
The invention further provides a fibre optic cable comprising a plurality of said fibre units extending in parallel with one another and being arranged within an extruded polymer tube, the fibre units being free to slide relative to one another and relative to the tube such that a selected fibre unit can be accessed and re-directed by forming an opening in a wall of the tube and withdrawing a length of the selected fibre unit through the opening.
The inventors have recognised that the known blown fibre unit having a low-friction PE sheath, if it were to be used as a fibre unit in a pullback cable, might greatly extend the range of distances that can be covered by a single withdrawal and installation step. If such a known fibre unit were to be used in the existing extruded tube, however, it is not likely to survive the manufacturing process of the pullback cable, without fusing at some point to the hot extruded tube. The inventors have recognised that, by changing the sheath material applied over the resin-coated fibre bundle to be based on PBT polymer instead of PE, the benefits of the fibre unit with resin core may be obtained to some extent, while avoiding the problem of fusing to the hot extruded tube. Reasons for this may include the dissimilar chemical and crystalline character of the materials, as well as the higher melting point of PBT compared with the extruded PE. In such an embodiment, the thickness of the PBT sheath on each fibre unit may be between 0.05 mm and 0.25 mm, optionally between 0.15 mm and 0.25 mm.
In some embodiments, an inner surface of the extruded polymer tube of the fibre optic cable is formed with projections that are effective to reduce an area of contact between material of the tube and the fibre units. The projections may be extruded in the form of longitudinal ribs.
The extruded polymer tube may be extruded with one or more strength members integrated in a wall of the tube during extrusion.
The lining of the extruded polymer tube may comprise primarily polyethylene, typically HDPE. This may for example be the same material as in the known pullback cable, while the choice of PBT for the fibre unit sheath allows manufacture of the pullback cable without fusing.
The lining of the extruded polymer tube may further comprise one or more additives including a friction reducing material.
The extruded polymer tube may comprise a co-extrusion of said lining material within a main tubular body of a different polymer to the lining. The main tubular body may be of polyethylene. The main tubular body may be extruded of medium density polyethylene MDPE.
The extruded polymer tube may be extruded with one or more strength members integrated in a main wall of the tube during extrusion.
The strength member may be a fibre-reinforced resin rod.
The extruded polymer tube may be further provided with external markings by which a user can avoid the strength member(s) when making said opening.
The extruded sheath of each of said fibre units may be provided with colour and/or other markings by which a selected fibre unit is distinguishable from all the other fibre units in the tube.
Performance of pullback cables according to the present invention can be verified by one or more of the following tests.
When said fibre optic cable is laid out in a generally straight route, a length of 100 m of a selected fibre unit may be withdrawn through an opening in the extruded tube at a speed greater than 1.4 m/s, without a pulling force exceeding the weight of a mass W, defined as the mass per kilometre length of the selected fibre unit.
A length of 100 m of a selected fibre unit may be withdrawn through an opening in the extruded tube at a speed of 1.4 m/s, without a pulling force exceeding a specified fraction of the weight of said mass W, for example 3W/4 or W/2 or W/3.
When said fibre optic cable is laid out in a generally straight route, a length of 100 m of a selected fibre unit may reliably be withdrawn through an opening in the extruded tube at a speed of 1.4 m/s, without a pulling force exceeding 5 N multiplied by the number of optical fibres in the selected fibre unit.
When said fibre optic cable is laid out in a generally straight route, said length of 100 m of a selected fibre unit may reliably be withdrawn through an opening in the extruded tube at a speed of 1.4 m/s, without a pulling force exceeding 2.5 N multiplied by the number of optical fibres in the selected fibre unit.
When said fibre optic cable is laid out in a generally straight route, a length of 200 m of a selected fibre unit may reliably be withdrawn through an opening in the extruded tube at a speed of 1.4 m/s, without a pulling force exceeding 5 N multiplied by the number of optical fibres in the selected fibre unit.
A coefficient of friction μ between the sheath of one of said fibre units and the lining of the extruded tube may be 0.2 or less, when measured by a capstan friction test of the general type described herein and illustrated in
A coefficient of friction μ between the sheaths of said fibre units may be 0.2 or less, when measured by a capstan friction test of the general type described herein and illustrated in
The invention in the first aspect further provides a fibre optic cable comprising a single fibre unit whose outermost layer is said PBT sheath, and which is adapted to be installed in a duct by blowing.
The inventors have found that a fibre unit with very good blowing performance and mechanical properties can be achieved by changing the low friction HDPE sheath of the known blown fibre unit for a sheath made of PBT with friction reducing additive. With additives of the type mentioned above, blowing performance exceeding that of the known blown fibre unit has been obtained in tests. The PBT sheath material is substantially harder and stronger than the HDPE material, and can be made thinner than the known HDPE sheath, if desired.
In one such embodiment, the thickness of the PBT sheath on the fibre unit is between 0.05 mm and 0.2 mm, optionally between 0.08 mm and 0.15 mm, optionally less than 0.130 mm.
In some embodiments, the number of optical fibres including any mechanical fibre is up to four and an outer diameter (OD) of the fibre unit is less than 1.2 mm, optionally less than 1.1 mm. The OD may increase with the number of fibres, for example so that fibre units having up to 6, 8, 12 or 24 fibres may have OD less than 1.3, 1.5, 1.6 and 2.1 mm, respectively.
In other embodiments, said fibre optic cable is further adapted to be installed by pushing as well as by blowing, and an outer diameter of the fibre unit is in the range of 1.5 to 2.5 mm, for example in the range 1.9 to 2.2 millimetres, for example 2.0 to 2.1 mm. In some such examples, said coated fibre bundle includes one or more strength members, for example an FRP strength member, embedded together with said optical fibres within said resin material.
In some embodiments, at least one of said optical fibres is terminated at at least one end with a blowable optical ferrule prior to installation in a duct.
Pullback cables and blown fibre units are only some examples of the applications of the invention in the first aspect. A fibre unit with a slightly thicker PBT sheath may be adapted for use as a cable for pushing and/or pulling installation methods.
The invention in a second aspect provides a method of manufacturing a fibre unit for use as a fibre optic cable or for use in the manufacture of a fibre optic cable, the method comprising:
Features of the first aspect may equally apply to this further aspect of the invention. For example, a lining of the extruded polymer tube may comprise primarily polyethylene, typically HDPE. For example, the coated fibre bundle may include one or more strength members embedded together with the optical fibres.
There is also provided a method of manufacturing a fibre optic cable comprising a plurality of fibre units extending in parallel with one another within an extruded polymer tube, the method comprising:
The lining of the extruded polymer tube may further comprise one or more additives including a friction reducing material.
The extruded sheath of each said fibre unit may comprise a mixture of PBT polymer and one or more additives including a friction reducing material. The friction reducing material may be additional to friction reducing material included in a commercial PBT grade.
The solid resin material may comprise a UV-cured resin such as an acrylate material.
The solid resin material may have a tensile modulus greater than 100 MPa, optionally greater than 300 MPa.
In step (d) the extruded tube may be formed by co-extrusion of said lining material within a main tubular body of a different polymer to the lining.
The lining of the extruded polymer tube may for example comprise primarily polyethylene, HDPE.
The lining of the extruded polymer tube may further comprise one or more additives including a friction reducing material.
The main tubular body may be of polyethylene.
The main tubular body may be extruded of medium density polyethylene MDPE.
In step (b) said extruded tube may be extruded with one or more strength members integrated therein.
The strength member may be a fibre-reinforced resin rod.
In step (b) said extruded tube may be further co-extruded with stripes by which a user can identify the circumferential location(s) of the strength member(s) when making said opening.
The extruded sheath of each of said fibre units may be provided with colour and/or other markings by which a selected fibre unit is distinguishable from all the other fibre units in the tube.
A vacuum tank may be provided downstream of said extrusion die to control shrinkage of the extruded tube during initial cooling.
These and other features of the invention will be understood from consideration of the examples described below and the dependent claims, illustrated with the appended drawings.
Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which:
As mentioned in the introduction, the present application discloses a particular form and material composition of a fibre unit, and different types of fibre optic cable in which such fibre units may be applied. The fibre unit comprises two or more optical fibres embedded in a solid resin material to form a coated fibre bundle and an extruded polymer sheath covering the coated fibre bundle, wherein the extruded polymer sheath of each said fibre unit comprises a mixture of polybutylene terephthalate polymer, PBT, and at least one friction reducing additive. Purely by way of example, locations of this fibre unit will be described, including a pullback cable. The fibre unit, either singly or in combination with other fibre units, can be applied in a variety of other cable types, where its properties of robustness and low surface friction may be beneficial.
Returning to the construction of the pullback cable 100, shown in
In contrast to the products disclosed herein, the fibre units 102 of known pullback cables have a conventional “loose tube” design, so that the fibres 106 within each unit tube 108 are also free to slide, the unit tube 108 being filled by a compound such as a water-blocking gel. The optical fibres 106 are generally so-called primary coated optical fibres, in which the glass core and glass cladding layers are coated with layers of resin immediately upon formation, to provide buffering and to protect the surface against damage. The number of optical fibres 106 within each unit tube 108 may vary, for example ranging from 2 to 12. All of the fibre units 102 in the illustrated example comprise two optical fibres 106, but some or all of the fibre units 102 in another example product may contain four fibres, or a different number. The number of fibres 102 within each fibre unit 102 may vary between products, and even within the same product, some tubes 108 may contain different numbers of fibres 106, to provide flexibility of application.
Similarly, the number of fibre units in the pullback cable, and hence the number of optical fibres, may vary, with typical numbers being 12, 24 or 48 fibre units. Higher numbers such as 96 fibre units are possible if desired. To produce the fibre units 102, the appropriate number of primary-coated optical fibres 106, each with appropriate colour coding, are passed through an extrusion die, which forms the unit tube 108 around the optical fibres. The different fibre units 102 are made with different colours of extruded unit tube 108, so that they may be identified in the finished pullback cable. Then, to produce the pullback cable 100, the appropriate number of fibre units 102 are bundled together and passed through an extrusion die which forms the extruded tube 104. Depending on whether the cable 100 is for exterior or interior use, the polymer of the extruded tube 104 may vary. In an example for exterior use, polyethylene, for example high-density polyethylene (HDPE) or medium-density polyethylene (MDPE) may be selected. An inner surface 110 of the tube wall may be coated with a low friction coating. In some known examples, a thin lining of HDPE with friction reducing additives (slip agents) and antistatic additives is used to form a thin lining, by coextrusion with a main body of the wall. For interior use (within premises) the polyethylene of the tube main body may be substituted by a flame resistant, zero halogen polymer, as is well known.
Also included in the wall of the extruded tube are strength members 112, typically glass fibre reinforced plastic (GFRP, FRP or GRP for short) rods, and typically at diametrically opposite positions on the circumference of the tube 104. The tube wall is provided with stripes or other external markings 114, so that the locations of the strength members 112 can be identified. This allows the strength members to be avoided when making the opening 120. In a known example, stripes of different coloured polymer are co-extruded with the main wall body to provide the external markings 114.
Referring now to
Referring to
Referring then to
Referring then to
Because the strength members 112 are provided in the extruded tube 104, and there are no separate strength members in the fibre units 102, the overall design can be very compact, compared with what would be required to accommodate the same number of fibre units as individual cables. The diameter of the extruded tube, and hence the overall diameter of the pullback cable itself, may be on the order of 15 to 20 mm. For example, the cable size may be designated 15/9, meaning an outer diameter of 15 mm combined with an inner diameter of 9 mm. Note that the bore of the tube 104 is slightly oval, so that the strength members 112 and stripes 114 can be accommodated in thicker portions of the wall.
Referring now to
In the situation shown at
Similarly, re-installation of the selected fibre unit into the branching duct 310 is performed in the following steps: step S3 to install the selected fibre unit from opening C3 along a first section of the branching duct 310 and gather it in a third coil shown at 308″ via opening C4; step S4 installing the remaining length from the coil shown at 308″ through the last section of the branching duct 310 to the customer access point 306. It will be appreciated that the effort in the operation, and the risk of damaging fibres and fibre units in the process, is doubled. Moreover, when one considers that customer drops of 200 or 300 m are commonplace, and 500 m is not unknown, the number of intermediate openings and withdrawal steps can become very great indeed. The practical and economic benefits of the pullback cable concept become reduced, and eventually lost completely.
The tube wall is provided with stripes or other external markings 514, so that the locations of the strength members 512 can be identified. This allows the strength members 512 to be avoided when making an opening. In the illustrated example, stripes of different coloured polymer are co-extruded with the main wall body to provide the external markings 514. Other means of providing external markings 514 can be used.
The optical fibres 506 are again so-called primary coated optical fibres, in which a glass body 526 (typically comprising a core and cladding layer, or a graded index core) is coated with two or three layers of resin 528, to provide buffering and to protect the surface against damage. The diameter of the glass core is commonly on the order of 100 μm, for example 125 μm. The diameter of the primary coated optical fibre 506 is typically 250 μm.
The modified pullback cable 500 differs from the known cable 100 in that the individual fibre units 502 are no longer in the form of a loose tube of PBT, containing fibres and a gel. As shown in the enlarged detail of
The selected resin has a relatively high glass transition temperature, so that it is not rubbery, but rather solid as it encases the fibres 506 and locks them into a unitary structure. The elastic modulus of the resin material 520 is greater than 100 MPa, for example in the range 300 to 900 MPa. For the purposes of installation and operation, resin material 520 has a hardness (modulus) and tensile strength such that the individual optical fibres 506 are locked in a bundle, and substantially prevented from moving relative to one another, and/or relative to the resin material 520. This coated fibre bundle therefore has a unitary structure and stiffness very different from the loose individual fibres contained within the conventional fibre unit 102 of the known pullback cable 100. On the other hand, the resin material 520 is not so hard and strong that it cannot be broken away from the fibres 506, when access to the individual fibres 506 is required for termination and/or splicing.
The coated fibre bundle in turn is surrounded by an extruded polymer sheath 524. This type of fibre unit 502 has a structure similar in many respects to a cable assembly of the type disclosed in published international patent application WO2004015475A2. Such fibre units have been designed, and used for many years, for installation by blowing with air or other compressed fluid. Fibre units of this type are known to blow hundreds and even thousands of metres, in microducts having a compatible low-friction lining. However, they can also be installed by pulling and/or pushing, depending on the distance and the route involved. The outer sheath 524 is extruded onto the optical fibre bundle during manufacture of the fibre unit, which occurs in advance of manufacture of the pullback cable. The outer sheath in the known fibre unit for blowing is made of HDPE, with a friction reducing additive and optionally antistatic additives, colour etc. The outer sheath 524 protects the bundle and facilitates sliding of the bundle through the tube 504. By suitable control of the extrusion process, and selection of materials, the extruded outer sheath 524 can be prevented from bonding to the coated fibre bundle. This allows it to be ring-cut and removed by sliding over the outer surface 522 of the resin material, when stripping the fibre unit to access the individual fibres. If desired, the inner periphery of the extruded sheath 524 can be made longer than the outer periphery of the surface 522, so that the sheath slides freely at all times relative to the bundle, but this is not essential.
In contrast to the known blown fibre units, however, the material of the extruded outer sheath 524 of each fibre unit in the modified pullback cable of
The stripping of the outer sheath of the fibre unit may be by the same sliding action as in the known blown fibre units. However, in some embodiments of the present invention, the PBT sheath fits tightly onto the resin bundle. In that case, there is no free sliding, and a longitudinal cut and peeling technique may be employed to remove a required length of sheath.
The dimensions of the coated fibre bundle and the fibre unit as a whole depend of course on the number of optical fibres contained therein. The components of the fibre unit 502 in
The inventors have recognised that fibre units adapted for installation by blowing have certain properties that would make them attractive for withdrawal by pulling from a pullback cable. For example, the coefficient of friction of the HDPE extruded sheath of the air blown fibre units compares favourably with that of the PBT unit tubes 102 currently used. Similarly, the withdrawn lengths might be expected to install easily in a branching duct, whether by pushing or pulling for short and medium distances, or blowing over longer distances. Unfortunately, the inventors have also recognised that merely substituting such fibre units for the fibre units 102 in the known pullback cable 100 would not be practicable. The reason for this is that the fibre units 102 must survive the process of extrusion of the extruded tube 104, while remaining free to slide in the finished product, and without suffering damage. As illustrated schematically in the detail
By adopting the structure of the known blown fibre units, but selecting the outer sheath material to be PBT-based, the modified pullback cable 500 can be manufactured without this risk of fusing, and without varying the materials of the extruded tube 504. As mentioned in the introduction, PBT is chemically different to PE, and also has a higher melting/processing temperature, by typically 40 to 50° C. Accordingly, in the modified cable 500 at least a lining of the extruded polymer tube 504 of the pullback cable 500 can be formed using HDPE, optionally with friction reducing additives, the same as in the commercially available pullback cable.
As illustrated in
Depending whether the cable is for exterior or interior use, the polymer of the extruded tube 504 may vary. In an example for outdoors use, polyethylene, for example high-density polyethylene (HDPE) or medium-density polyethylene (MDPE) may be selected. For indoor use (within buildings) the polyethylene tube body may be substituted by a flame resistant, zero halogen polymer. Commercially-available grades of polymer for indoors use include Casico FR6083 (from Borealis Group), Eccoh 5995 (from PolyOne Corporation), Megolon® HF8110, and Megolon® S300 (from Mexichem Speciality Compounds).
In an alternative embodiment of the modified pullback cable, the lining of the extruded tube may be simply the inner surface of the main body.
The manufacturing method and general structure of the product are readily adapted from the method of manufacturing the known pullback cable 100 described and illustrated above. In simple terms, for the manufacture of the pullback cable 500, the appropriate number of fibre units 502 with the extruded PBT-based sheath 524 are bundled together and passed through an extrusion die which forms the extruded tube 504 with the lining 510.
In advance of manufacturing the pullback cable 500, a desired number of fibre units 502, each containing the appropriate number of primary-coated optical fibres 506, are manufactured by a method such as that described in WO2004015475A2, modified by the use of the PBT-based material for the extruded sheath 524. Processing conditions for the PBT material (extrusion temperature, pressure etc.) will be substantially as for PBT loose tube extrusion, which is rather different from the settings of temperature and pressure for extrusion of the HDPE sheath on the known blown fibre unit. Additionally, as discussed further below, additional friction reducing additive may be included in the extrusion of the PBT sheath. The different fibre units 502 for the pullback cable are made with different colours of extruded sheath 524, and/or other markings so that they may be identified individually, when an opening is made in the finished pullback cable. Each fibre unit will be received, coiled on a reel or drum of suitable diameter, or coiled in pans. Payoff reels allow supply of cable with a designated back-tension.
For an example pullback cable 500 having 48 fibre units, four payoff banks 602 are provided, each delivering 12 individual fibre units 502 into the process. The payoff banks 602 deliver each fibre unit with a suitably controlled back tension, for example of a few hundred grams force. The individual fibre units are gathered into a guide plate 604 which, although illustrated here in a one-dimensional cross-section, is designed to guide the fibre units 502 into a desired two-dimensional array, for presentation to an extrusion head 606. A succession of guide plates may be provided, in practice, although only one is shown. Also shown are payoffs 608 for the strength members 512. As illustrated, these strength members also pass through dedicated openings in the guide plate 604, while they may be provided with dedicated guides in practice. To ensure good mechanical cohesion between the strength members and the surrounding polymer, a coating of heat-activated adhesive may be provided on the strength members when they are supplied.
The extrusion head 606 is shown only as a block in the middle of the drawing
A liner extruder 624 processes the polymer of the liner, for example HDPE with friction reducing and antistatic additives, and delivers it at high pressure to the extrusion head 606 to form the lining 510 of the extruded tube 504. The pressure of the liner extruder may be higher for the reason that the annular opening for the liner material is narrower, and a higher pressure is required to match the speed of extrusion of the liner to that of the main body. If very different materials are used for the liner and the main body, processing temperatures are chosen so that each material is not overheating the other, either within the extrusion head or when they come into contact. A stripe extruder 626 delivers polymer of a similar composition to the main body extruder, but with different colouring, into the extrusion head 606, to form the external markings 514 of the extruded tube 504.
As illustrated in the detail of the extrusion die 610, the 48 fibre units 502 are drawn together as a bundle through a central opening 632 in extrusion die 610, while extruding polymer tube 504 through annular channels in the die around the bundle. Dedicated tooling 634 delivers the GRP strength members 512 into the extrusion die 610 to become surrounded by the melted polymer which will form the main body of the extruded tube wall. The melted and pressurised main body polymer from main body extruder 622 enters extrusion die through channels 642. The melted and pressurised lining polymer from liner extruder 624 enters extrusion die through channels 644. The melted and pressurised marking polymer from stripe extruder 626 enters the extrusion die through channels 646 which extend only over the part of the circumference to be marked. In this way, the lining and main body of the tube 504 are extruded around the bundle of fibre units 502, while incorporating the strength members 512 and external markings 514 into the wall of the tube. As mentioned, a coating of adhesive may be provided on the strength members 512 to ensure they become structurally integrated with the tube wall. This adhesive, which is a dry and solid coating when the strength members are supplied, is activated by the heat of the melted main body material.
Downstream of extrusion head 606, a series of cooling tanks 650, 652 are provided, followed by a printing station 654. A tractor unit 656 of caterpillar or similar design applies the tension to draw all the elements of the cable 500 from the payoff banks 602, through the extrusion head and onto a take-up unit 656. In this way, the apparatus draws the extruded tube 504 and the bundle of fibre units through the extrusion die while process parameters of all the illustrated units are controlled to draw and cool the polymer tube to have finished interior and exterior dimensions such that the fibre units remain loose within the extruded tube 504.
Detail of the cooling tanks and control systems can be adapted from known cabling production apparatus, such as used for production of cables generally, and in particular for production of the pullback cable 100 which is already commercially available from various manufacturers. The requirement is to produce the pullback cable 500 in such a form that a selected fibre unit can be accessed and re-directed reliably by forming an opening in a wall of the tube and withdrawing a length of the selected fibre unit through the opening.
In an example apparatus, a first cooling tank 650 is a vacuum tank, for example between five and 10 m long. The application of a (partial) vacuum outside the extruded tube 504 helps the tube to keep its form and avoid collapse onto the bundle of fibre units 502. The second cooling tank 652 may be a longer tank, with water spray cooling, for example over 15 or more metres in length.
As mentioned above, a key limitation with known pullback cables is the difficulty in withdrawing a sufficient length of a selected fibre unit, without exceeding tensile performance limits of the fibre unit. To measure the force required for withdrawal, a set up similar to that illustrated schematically in
For practical purposes, withdrawal should be possible at a reasonable pace, without exceeding the tensile performance specification. A walking pace, for example 1 m/s or 1.4 m/s may be specified, as indicated by velocity v in the diagram. It is a matter of choice, whether the test is performed using an automated and calibrated carriage as a pulling device, or whether simply pulling by a human operator walking is accurate enough. For accuracy, tests are repeated multiple times, to ensure that a given performance can be reliably achieved in the field. The term “reliably” in this context may be understood to mean that any and all of the 24, 28, 96 or whatever number of fibre units in the pullback cable can be selected and withdrawn without exceeding the specified force.
Accordingly, if the tensile performance of an individual optical fibre is specified as, for example 10 N force (roughly 1 kg weight), and if a 50% safety margin is applied, the tensile performance Fmax for the fibre unit comprising two, four, six, eight or twelve fibres can be specified simply as 10, 20, 30, 40 or 60 N, respectively.
Another force unit that may be used in measuring tensile performance of cables is the “W” unit, being the weight of a one-kilometre length of the cable product in question. Supposing that a fibre unit has a mass of 1.0 g/m, which may be typical for a 2-fibre or 4-fibre unit of the type used in the present disclosure. That corresponds to 1 kg/km, giving a force W=9.81 N. The parameter W for a 12-fibre unit weighing 2 g/m (i.e. 2 kg/km) represents a force W=19.6 N, and so on. The parameter W can therefore be used to obtain expressions of tensile force such as “1W” or “W/3”, which adapt automatically to different products. The tensile performance Fmax can then be expressed as multiples or fractions of the parameter W for a give fibre unit, such as W or 3W/4 and the like.
Different embodiments are disclosed, depending on the composition of the PBT sheath. The extruded sheath may comprise a commercially-available PBT material designed for loose tube optical fibre applications. The extruded sheath may comprise a commercially available PBT material such as a grade of BASF Ultradur® 6550. Samples described herein have been made using BASF Ultradur® B 6550 LN in particular. Other grades of PBT may be used with suitable adaptation. The preferred grade will combine desirable properties for processing, finished product performance and cost. Certain grades may allow a thinner sheath, or easier processing, but at greater cost. For example, BASF Ultradur® B6550LNX is a high viscosity extrusion grade for microtubes in fibre optical cable applications, offering potentially thinner sheath. PBT is of course available from manufacturers other than BASF.
In a first comparative example of pullback cable 500 the sheath 524 of the fibre unit is made using BASF Ultradur® B 6550 LN polymer without additional friction reducing additives. Thirty 4-fibre units were included within the extruded tube. Pull back tests by the method of
A second example was made where the sheath 524 of each fibre unit 502 comprised a mixture of polybutylene terephthalate PBT and additional friction reducing and/or antistatic additives. As before, the PBT material was BASF Ultradur® B 6550 LN. This PBT material is designed for loose tube optical fibre applications, and is believed already to contain a certain amount of friction reducing material (“lubricant” in the manufacturer's terminology). As mentioned above though, some embodiments according to the present disclosure are made with additional friction reducing additive. The additional friction reducing additive may comprise a silicon-based lubricant, for example a siloxane such as polydimethylsiloxane-based additive, for example a polyacrylate dimethyl siloxane. A polyacrylate dimethyl siloxane used in the second example is Dow Corning® HMB-1103 Masterbatch, which is available commercially as a “tribology modifier for polar engineered plastics such as polyamide (PA) and polyoxymethylene (POM)”. The amount of polyacrylate dimethyl siloxane may be between 1% and 5% by weight of the material of the extruded sheath, for example 2 or 3%. The amount to be included was determined during set-up tests of the extrusion process of the fibre units. The percentage can be increased in steps starting from 1%, say, until one finds that increasing the amount of additive adds to cost without adding to performance, or causes excessive flowing of the melt during the extrusion process. Below we describe examples with alternative PDMS-based additives.
The
As seen in the table, every selected fibre unit pulled easily from the cable over the full length of 265 m without exceeding the permitted maximum force. There was no need to perform the test at shorter increments.
Summarising these results, we see that the modified pullback cable, in which fibre units based on bundles of fibres embedded in a resin core are sheathed in a PBT material, allows selected fibre units to be pulled over a length of at least 100 m. In the second example, with additional friction reducing material, fibre units could be pulled over a length in excess of 200 m, in fact in excess of 250 m.
By way of contrast, results of pullback tests using a conventional pullback cable 100 are illustrated schematically in
As will be appreciated, unless the distance from each customer access point to the pullback cable route is less than 30 m, using the modified pullback cable 500 will allow the same premises to be connected with far fewer cuts and withdrawal steps, resulting in a much faster and cheaper installation overall, and with less disruption of the ground. Referring to the example of
In further experiments, it has been shown that the modified fibre units with PBT sheath can be pushed substantial distances, for example 30 m. Pushing distances are further enhanced in the second example with additional friction reducing additive. In this example, pushing into a drop tube can be performed up to 50 m, and over 90 m has been achieved in 4-fibre and 12-fibre designs. Pulling into a drop tube has been performed up to 100 m. These distances cannot be matched by conventional PBT loose tube fibre unit. As discussed further below, the fibre units with PBT sheath can also be suitable for installation by blowing, potentially allowing even longer distances.
Optical performance of the fibre units under temperature cycling is more than satisfactory in tests.
Ease of stripping of the sheath from a fibre unit to access the individual fibres is also an important characteristic for a practical product. In tests the fibre units with PBT+ sheath have been stripped quickly and without damage in lengths of 3 m. Since the PBT+ sheath may be tougher and/or tighter on the fibre bundle than the HDPE+ sheath of the known blown fibre units, a different stripping method may be preferred to the “sliding” method. Stripping may be performed using a tool to carefully cut longitudinally along the length of the sheath. A Miller MSAT16 stripper from Ripley Tools is a suitable tool. Short lengths of product were stripped using the MSAT 16 stripper. In testing, different settings were checked by carrying out short tests on sample product to establish the optimum setting. Once the optimum setting was found, 10×3 m samples were stripped and checked for any damage to the acrylate and bundle. Care was taken to pull the strippers over the product in a straight line, and at a steady pace.
Using the modified pullback cable 500, the benefits of the pullback cable principle can be extended to a much wider range of applications. Because the strength members 112 are provided in the extruded tube 104, and there are no separate strength members in the fibre units 102, the overall design can be very compact, compared with what would be required to accommodate the same number of fibre units as individual cables. The diameter of the extruded tube, and hence the overall diameter of the pullback cable itself, may be on the order of 15 to 20 mm. For example, the cable size may be designated “15/9”, meaning an outer diameter of 15 mm combined with an inner diameter of 9 mm. Note that the bore of the tube 104 is slightly oval, so that the strength members 112 and stripes 14 can be accommodated in thicker portions of the wall. Away from these thicker portions, it can be deduced that the wall thickness, including any lining, is 3 mm. Another example may have a size 16/10, meaning an outer diameter of 16 mm combined with an inner diameter of 10 mm. Again, the wall thickness away from the thickened portions is 3 mm. Another example may have a size 20/16, with a wall thickness of 2 mm.
The ratio of the forces T1 and T2, according to a mathematical model of the capstan test, is determined by the wrap angle θ and the coefficient of friction μ, in accordance with the formula of Equation 1.
Therefore, when T1, T2 and θ are known from the experiment, the coefficient of friction μ can be determined for a given combination of fibre unit and tube lining using Equation 2.
Depending on the setup, it may be considered to use a modified formula. For example, it is known that the above formula for the simple capstan model can be modified into a “V-belt” model, in which the moving element sits between two fixed sides having an angle α between them. This angle α becomes a further parameter taken into account in the modified formula:
The situation illustrated in
Table 3 presents results of tests on a number of samples including the known pullback cable 100 and the new pullback cable 500, as described above. Six tests are performed, each one using four or five different samples to obtain a statistical average. Test A corresponds to the known pullback cable 100, having two fibres (2 fu) contained loosely in PBT unit tubes, within a duct lined with a liner comprising HDPE mixed with antifriction and antistatic additives (designated “HDPE+” in the table). The first type of friction test (
Comparing the results of Tests A to D in the Table 3, we see that the mean coefficient of friction between the PBT fibre unit and tube lining (Test A, μ=0.248) in the known pullback cable is significantly greater than any of the other samples. When a HDPE+ blown fibre unit with a ribbed sheath is used, the coefficient of friction is much lower (Test B, μ=0.125), but the problems of fusing would be expected in manufacture. When a blown fibre unit with a ribbed PP+ sheath is used (Test C), the coefficient of friction is between that of Test A and Test B, with significant variance. On the other hand, when the PBT sheath with additional friction reducing material is used, according to the present disclosure, the mean coefficient of friction μ measured over a number of samples is lower than any of the other examples (Test D, μ=0.115), less than 0.2, and in fact less than 0.15.
Moving to the second type of test, illustrated in
As will be seen from the table, the coefficient of friction between fibre units having the HDPE+ sheath is much lower than that in the known cable 100 having PBT unit tubes. The coefficient of friction μ=0.18, measured by the method of
In conclusion, and bearing in mind that Tests A and E represent the known product, while Test D represents the product made according to the present disclosure, the present disclosure provides a pullback cable which can be manufactured by extrusion of the extruded tube around a plurality of PBT-sheathed fibre units, and with friction coefficients lower than those in the known pullback cable. Combined with the superior strength of the modified fibre units, in which the fibres are embedded in a solid resin material, the length of fibre unit that can be retrieved without damage is greatly increased, as demonstrated in
As mentioned above, the requirement of the lining of the extruded tube in the modified pullback cable according to the present disclosure is that it should not damage and/or adhere to the extruded sheath of individual fibre units, even through the process of extrusion of the extruded tube 504 around the bundle of pre-manufactured fibre units. PBT, with or without additives, has been mentioned as a material suitable for the extruded sheath 524 of the fibre units, which will not be damaged by the extrusion of an HDPE-based extruded tube 504. As an alternative to HDPE, a lining of the extruded polymer tube may comprise other polymers, for example primarily polypropylene or primarily nylon. Grade 11 or 12 nylon may be suitable, for example. Nylon has the benefit of hardness and low friction, but will typically be more expensive than polypropylene, and both are typically more expensive than HDPE. If the lining of the extruded tube is a different material than the main body, extra care may be required to avoid delamination of the lining from the body of the extruded tube 504. Such considerations are reduced, if the material of the lining and the tube body are the same, or are grades or blends of the same type of polymer, for example polyethylene.
Other features and advantages of the pullback cable 1100 the same as described above for pullback cable 500. The same alternatives and modifications also apply. Only the differences from pullback cable 500 will now be described in a little detail.
The modified pullback cable 1100 differs from the pullback cable 500, illustrated in
The inclusion of this ribbed profile reduces a contact surface area between a fibre unit 1102 and the lining 1110 of the tube 1104, during manufacture and use. The reduced surface contact during use of the product provides for easier retrieval/pullback of fibre units 1102 from the cable 1100. During manufacture, reduced contact surface area reduces the risk of these surfaces sticking together when the tube 1104 is extruded over the fibre units, and may therefore permit a large number of fibre units to be included within the same diameter of tube 1104, without manufacturing problems.
Compared with the example 502, extruded polymer sheath 1224 in the fibre unit 1202 provides a ribbed or undulating profile. The ribbed or undulating profile reduces the contact surface area between a fibre unit 1202 and the lining 1210 of the tube. This is illustrated in
In designing and manufacturing a pullback cable, the ribbed fibre unit 1202 can be used in combination with a tube 504 having smooth-lining, or a tube 1104 having a ribbed lining. Similarly, the tube 1104 having the ribbed inner surface can be used in combination with a ribbed fibre unit 1202 or a fibre unit with a smooth or other-textured surface.
As mentioned in the introduction, the polymer of the extruded polymer sheath 524/1224 may include various additives, such as for friction reducing, colouring, UV protection, antistatic etc. While conventional PBT material for loose tube fibre units may include some friction reducing component, additional friction reducing material is be included in the sheaths of the fibre units of this modified pullback cable. The additional friction reducing additive may comprise a polydimethylsiloxane material, PDMS, in a carrier material. The carrier material in particular examples is a polyacrylate material, for example a copolymer of ethylene and methyl acrylate, EMA. In other examples the carrier is a polyolefin, such as low-density polyethylene (LPDE). The additive may be called a polyacrylate dimethyl siloxane. More generally, the additive may comprise a silicon-based material including a polyether modified polydimethylsiloxane material such as a polyether modified hydroxy functional polydimethylsiloxane material. Alternatively, or in addition, forms of carbon including carbon nanotubes, erucamide and/or oleamide materials may be used for improving slip and reducing friction. As is known, different additives can take different amounts of time to migrate to the surface and deliver their benefits of lowering friction. The polymer may also include cross-linked material and/or fillers.
The density of the sheath material will depend on the materials blended into it, as well on processing conditions.
According to other embodiments, cross-linking may optionally be applied to the body of the extruded tube 504/1104, and optionally in the lining.
In addition to friction reducing properties, it has been mentioned already that the selection and proportion of additives has an influence on the extrusion process. That is to say, the additives alter the behaviour of the molten material during extrusion, as well as the bulk and surface properties of the finished product. The quantity of additive used may be limited to avoid excess flowing of the melt, even if a greater proportion of additive might be beneficial for frictional properties in the finished product.
The inventors have found that a further class of siloxane-based additives different to the above-mentioned polyacrylate dimethyl siloxane can be used to obtain friction reduction in the PBT sheath of fibre units, without causing problems in extrusion. An example of this class is Dow Corning® MB 50-002 Masterbatch, which is available commercially as a formulation containing 50% of an ultra-high molecular weight (UHMVV) siloxane polymer dispersed in low-density polyethylene (LDPE). It is designed to be used as an additive in polyethylene compatible systems to impart benefits such as processing improvements and modification of surface characteristics, according to the manufacturer's datasheet. The MB50-002 additive is promoted for (non-polar) plastics such as polyethylene and is based on an LDPE carrier. Conventionally, incompatibility between the PBT and LDPE components would be expected to prevent mixing, leading for example to tearing of the sheath. Surprisingly such effects are found to be absent and the additive blends well. One explanation for this may be that the LDPE becomes “momentarily polar” due to oxidisation at the point where the thin tubular film exits the extrusion tip and die. This oxidation creates carboxyl groups, having the effect of making the PE of the masterbatch compatible, in that moment, with the polar polymer such as PBT.
Whatever the cause, the superior performance of the LDPE-based additive is a surprising discovery, since the polyacrylate dimethyl siloxane additive HMB-1103 is the one promoted by the manufacturer for use in polar plastics, including PBT. The same may be expected for PDMS additives promoted by other manufacturers.
As for the previous example, the amount of LDPE additives additive to be included can be determined during set-up tests of the extrusion process of the fibre units. The percentage can be increased in steps starting from 1%, say, until one finds that increasing the amount of additive adds to cost without adding to performance, or causes excessive flowing of the melt during the extrusion process. The amount of additive may be between 1% and 5% by weight of the material of the extruded sheath, for example between 2 and 4%, more particularly between 2.5 and 3.5%. A value of 3% has been found suitable, bringing further enhancement in friction performance, without the extrusion problems that would be encountered using the polyacrylate dimethyl siloxane additive. The masterbatch MB50-002 has a loading of PDMS of 50%, which may be high compared with the (unknown) percentage in the HMB-1103. Based on the value of 50% and the inclusion of 3% of the additive as a whole, it will be seen that the overall siloxane content of the sheath material is around 1.5%, i.e. greater than 1%.
As for the earlier examples, the PBT polymer sheath in these examples may also be fully or partially cross-linked, for example to improve dimensional stability and/or high temperature performance. Other additives such as fillers, colouring, anti-static and the like may also be included.
In addition to the benefits relating to its use in pullback cables of the type described above, fibre units according to the invention have been found to perform very well as a blown fibre unit, matching or exceeding in some cases the performance of the fibre units known from WO2004015475A2, mentioned above. The different mechanical properties of PBT compared with HDPE, such as higher tensile modulus and yield strength, raise the possibility to reduce dimensions, and/or to implement different mechanical designs in the application of the cables.
As explained already above, such a resin material 1320 has a hardness (modulus) and tensile strength such that the individual optical fibres 1306 are locked in a bundle, and substantially prevented from moving relative to one another, and/or relative to the resin material 1320. On the other hand, the resin material 1320 is not so hard and strong that it cannot be broken away from the fibres 1306, when access to the individual fibres 1306 is required for termination and/or splicing.
The coated fibre bundle in turn is surrounded by an extruded polymer sheath 1324. This type of fibre unit 1302 has a structure similar in many respects to a cable assembly of the type disclosed in published international patent application WO2004015475A2. Compared with the HDPE sheath of the known low fibre unit, which already sets the standard for compactness and blowability, a PBT sheath has been found to offer yet further unexpected benefits in terms of low friction and compact size. While the HDPE sheath of the known blown fibre units is relatively thin and hard, relative to other designs available at the time, the PBT sheath according to the present disclosure may be significantly harder (stiffer) and/or significantly thinner than the sheath of the known fibre units.
For example, the HDPE sheath material may have a tensile modulus on the order of 1000 MPa (for example in the range 700 to 1300 MPa), while the PBT material has a tensile modulus on the order of 2500 MPa, for example 2600 MPa. Even allowing for some reduction in the modulus caused by the inclusion of a small percentage of friction-reducing additive in LDPE or polyacrylate carrier, the modulus of the PBT sheath material will be in excess of 2000 MPa, 2200 MPa and 2400 MPa. Likewise, the tensile strength (or tensile stress at yield) of PBT material can be significantly higher than that of HDPE. For example, tensile yield stress of HDPE is typically in the mid-20 s MPa, while the tensile yield stress of PBT can be greater than 40 MPa, typically 50 MPa or more.
A single such fibre unit, without being encased in any other structure, is found to be suitable for use as a fibre optic cable suitable for installation in microducts by means of blowing. As is known for the known blown fibre unit (WO2004015475A), the embedding of the optical fibres in a relatively solid resin provides a stiffness to the structure of the fibre unit, independent of the stiffness of the outer sheath. With the increased strength, hardness and stiffness of the PBT material relative to HDPE, a fibre unit better suited to pushing and pulling can be provided. Additionally, a fibre well suited to installation by blowing can be provided. The thickness and detailed composition of the PBT sheath can be adjusted and optimised for one particular installation method. To favour blowing, a thinner sheath can be provided, which is nevertheless a robust protection for the fibres contained within, and does not interfere with blowing performance. On the other hand, (as mentioned already above) a single design of fibre unit can have adequate performance in pushing, pulling and blowing. This is particularly useful in the case of a pullback cable, where a wide range of distances and topographies may exist between the pullback cable and the premises access points, between installations and even within the same installation.
Comparing the three designs shown in
The fibre unit 1302 at
Now considering fibre unit 1302′ at
It will be understood that the above are not the only designs of fibre optic cable that are possible within the scope of the present disclosure. A fourth example is described below, with reference to
Referring also to
The leading end 1418 of the fibre unit 1410, which includes a ferrule connector 1424, leads the installation of the optical fibre or fibres through the duct 1420. The leading end 1418 passes through the duct 1420 and is fed from the reel 1412 until the ferrule connector 1424 and a length of the optical fibre cable assembly 1410 exits the duct 1420 within the telecommunications cabinet (see
Particular forms of pre-terminated fibre optic cable assembly and methods of installation are disclosed in our earlier patent application WO2018146470A1 (Attorney's reference 11050PWO). The fibre units disclosed herein can be used as part of those assemblies and methods. An alternative form of pre-terminated optical fibre cable assembly and its use are described in another patent application GB21#####.# having the same filing date as the present application (attorney's reference 12009PGB).
Similarly to the fibre units included in a pullback cable, the fibre unit designed primarily for blowing may also be adapted for pushing and/or pulling, when the need arises. An alternative or supplementary installation process illustrated in
Particularly in the form shown in
The first type of test, friction tests similar to those illustrated in
The fibre unit tested was the fibre unit 1302′ of
Coefficients of friction were calculated using the capstan Equation 2.
The results were as shown in the following Tables 4A (ribbed micro-duct) and 4B (smooth micro-duct).
Without ascribing any significance to the absolute values of these results, what is clear from the tests is that the PBT-sheathed fibre unit of the present disclosure has a significantly lower coefficient of friction than the conventional HDPE-sheathed fibre unit. Moreover, the combination of a PBT-sheathed fibre unit and ribbed lining of the micro-duct provides the lowest friction of the four situations. Accordingly, in conjunction with a commercially available ribbed micro-duct, the PBT-sheathed fibre unit may be expected to perform even better in blowing the blowing method of installation. Of course, reduced friction would also indicate better performance in both pushing and pulling methods as well.
Having said that, blowing performance in a real application depends on many variables as well as the coefficient of friction. Various different testing regimes of blowing performance are known and used in the industry, including standard tests and custom tests for individual manufacturers and/or customers.
A long-established test, and one which is generally very challenging for blown fibre products, is the 500 m drum test.
For this test, 500 metres of a commercially available tube with outside diameter 5 mm and internal diameter 3.5 mm with smooth low-friction HDPE lining was wound onto a drum with barrel diameter of 500 mm. A length of fibre unit 1302′ with outer diameter 1.05 mm was made according to the example of
The results are shown in Table 5. The fibre unit was installed successfully through the entire length in under 20 minutes. The cable travelled at a constant speed of 30 m/min. The air pressure and driving torque of the blowing machine were adjusted in the usual manner.
indicates data missing or illegible when filed
Further blowing tests were performed with the route shown schematically in
Blowing was performed using lengths of fibre unit 1302′ with outer diameter 1.05 mm made according to the example of
Tests using this route were done with the fibre unit 1302′ soon after manufacture. It is known that performance can change over time, for example due to temperature induced coil set. To test for this, the tests were then repeated with fibre unit 1302′ which had been subject to temperature cycling, specifically 2 cycles 12 hours prior to the blowing trial between −10 degrees Celsius and +50 degrees Celsius. Tests using this route were done with two different compressors, different to the one used in the drum test.
Results are shown in Table 6A and 6B (different compressors; fibre unit before temperature cycling) and Table 7A and 7B (different compressors; fibre unit after temperature cycling).
indicates data missing or illegible when filed
indicates data missing or illegible when filed
indicates data missing or illegible when filed
indicates data missing or illegible when filed
These blowing tests have shown that the new fibre unit having a PBT outer sheath with PDMS additive and blowable optical ferrule can perform extremely well in blowing, requiring a very modest air pressure, especially bearing in mind the very convoluted route that has been laid out to simulate more challenging real-world installations. The temperature cycling did not adversely affect the blowing performance of the fibre units. The ability to install using lower air pressures has significant benefits in allowing the use of more lightweight and lower cost equipment. It can also be seen that the second compressor in the trial outperformed the first compressor considerably, with more than two minutes faster installation time.
In a known product of this design, with an HDPE-based sheath, such a cable has been found to have good blowing performance and excellent pushing performance. For example, with a ferrule sub-assembly pre-fitted on the end, a 2-fibre example has been pushed over 90 m through buried micro-duct of 7/3.5 mm dimensions with no difficulty. The lower friction of the PBT-based sheath may be expected to perform even better. The higher tensile stiffness and strength of the PBT-based sheath may also be expected to provide excellent pulling characteristics, but the vast majority of the product tensile strength is in the optical fibres and the FRP strength member(s), so that increased sheath strength may not be significant.
The strength member in this example is shown with a diameter of approximately 0.5 mm. An outer diameter of the fibre unit 1910 may be greater than that of the examples of
In versions with more fibres, the additional strength member may be unnecessary to provide adequate stiffness for pushing. For example, a coated fibre bundle of 12 optical fibres is suitable for blowing and for pushing, without having the additional strength member 1926. A 12-fibre example with PBT-based sheath material and 1.8 mm outer diameter Ds has been pushed 100 m through a micro-duct of 6/3.2 mm size.
The above embodiments of the invention can be modified, and/or combined as required for a given commercial application. For example, where the installer needs to use a pullback cable, with a long drop to be installed by pushing, nanocable units 1910 of the type shown in
It goes without saying that all of the above examples also achieve satisfactory optical performance under a range of environmental and mechanical conditions. The optical fibres used in the examples were single mode fibres compliant with G.657.A2 (ITU-T).
While specific embodiments of the present invention have been described above, it will be appreciated that departures from the described embodiments may still fall within the scope of the present invention, defined by the appended claims and their equivalents.
The present disclosure further includes the following numbered clauses and other statements, based on the claims of the priority application GB 2013892.1.
Clause 1. A fibre optic cable comprising a plurality of retractable fibre units extending in parallel with one another within an extruded polymer tube, the fibre units being free to slide relative to one another and to the tube such that a selected fibre unit can be accessed and re-directed by forming an opening in a wall of the tube and withdrawing a length of the selected fibre unit through the opening, wherein each of said fibre units comprises two or more optical fibres embedded in a solid resin material to form a coated fibre bundle and an extruded polymer sheath covering the coated fibre bundle, wherein the extruded polymer sheath of each said fibre unit comprises primarily polybutylene terephthalate, PBT polymer.
Clause 2. A fibre optic cable according to clause 1 wherein the extruded sheath of each said fibre unit comprises a mixture of PBT polymer and one or more additives including at least one friction reducing material.
Clause 3. A fibre optic cable according to clause 2 wherein said PBT polymer excluding additives comprises at least 95% by weight, at least 90% by weight or at least 80% by weight of the extruded sheath.
Clause 4. A fibre optic cable according to clause 2 or 3 wherein said friction reducing material(s) include a polydimethylsiloxane (PDMS).
The PDMS may be an ultra-high molecular weight PDMS. The carrier material may be for example a polyacrylate, for example a copolymer of ethylene and methyl acrylate (EMA). The carrier material may be for example a polyolefin, such as low-density polyethylene (LPDE). These materials are available for example from Dow Corning in the form of masterbatch additives for leather with the base polymer of the sheath in an extrusion machine.
Clause 5. A fibre optic cable according to clause 2, 3 or 4 wherein the amount of friction reducing additive is between 1% and 5%, optionally between 2% and 4% by weight of the material of the extruded sheath.
The amount of additional friction reducing additive, for example polyacrylate dimethyl siloxane, may be between 1% and 5% by weight of the material of the extruded sheath. The inventors have found that between 2% and 4%, more particularly between 2.5 and 3.5% of a commercially available LDPE-based PDMS additive affords a substantial reduction in friction, with no attendant problems in extrusion. This performance was apparently better than using a polyacrylate based additive specifically markets for blending with PBT.
Clause 6. A fibre optic cable according to any preceding clause wherein an inner surface of the extruded polymer tube of the fibre optic cable has been formed with projections effective to reduce an area of contact between material of the tube and the fibre units.
Clause 7. A fibre optic cable according to any preceding clause wherein the extruded polymer tube comprises a co-extrusion of a lining material within a main tubular body of a different polymer to the lining.
Clause 8. A fibre optic cable according to any preceding clause wherein said extruded polymer tube is extruded with one or more strength members integrated in a main wall of the tube during extrusion.
Clause 9. A fibre optic cable according to any preceding clause wherein, when said fibre optic cable is laid out in a generally straight route, a length of 100 m of a selected fibre unit can be withdrawn through an opening in the extruded tube at a speed greater than 1.4 m/s, without a pulling force exceeding the weight of a mass W, defined as the mass per kilometre length of the selected fibre unit, optionally without exceeding three quarters of the weight of said mass W, or optionally one half or one third of the weight of said mass W.
Clause 10. A fibre optic cable according to any preceding clause wherein, when said fibre optic cable is laid out in a generally straight route, a length of 100 m of a selected fibre unit can reliably be withdrawn through an opening in the extruded tube at a speed of 1.4 m/s, without a pulling force exceeding 5 N multiplied by the number of optical fibres in the selected fibre unit, optionally 2.5 N multiplied by the number of optical fibres in the selected fibre unit.
Clause 11. A fibre optic cable according to any preceding clause wherein, when said fibre optic cable is laid out in a generally straight route, a length of 200 m of a selected fibre unit can be withdrawn through an opening in the extruded tube at a speed of 1.4 m/s, without a pulling force exceeding 5 N multiplied by the number of optical fibres in the selected fibre unit.
Clause 12. A method of manufacturing a fibre optic cable comprising a plurality of fibre units extending in parallel with one another within an extruded polymer tube, the method comprising:
Clause 13. A method according to clause 12 wherein the extruded sheath of each said fibre unit comprises a mixture of PBT polymer and one or more additives including at least one friction reducing material.
Clause 14. A method according to clause 13 wherein said PBT polymer excluding additives comprises at least 95% by weight, at least 90% by weight or at least 80% by weight of the extruded sheath.
Clause 15. A method according to clause 13 or 14 wherein said friction reducing material(s) include a polydimethylsiloxane, for example a polyacrylate dimethyl siloxane.
Clause 16. A method according to clause 13, 14 or 15 wherein the amount of friction reducing material(s) is between 1% and 5%, optionally between 2% and 4% by weight of the material of the extruded sheath.
Clause 17. A method according to clause 13, 14 or 15 wherein the material of said extruded polymer tube comprises a commercially available PBT loose tube material having friction reducing material therein and one or more additional friction reducing materials.
Clause 18. A method according to any of clauses 12 to 17 wherein a lining of the extruded polymer tube comprises primarily high density polyethylene, HDPE.
Clause 19. A method according to any of clauses 12 to 18 wherein the lining of the extruded polymer tube comprises one or more additives including a friction reducing material.
Clause 20. A method according to any of clauses 12 to 19 wherein an inner surface of the extruded polymer tube of the fibre optic cable is formed with projections effective to reduce an area of contact between material of the tube and the fibre units.
Clause 21. A method according to any of clauses 12 to 20 wherein the solid resin material has a tensile modulus greater than 100 MPa, optionally greater than 300 MPa.
Clause 22. A method according to any of clauses 12 to 21 wherein in step (b) the extruded tube is formed by co-extrusion of a lining material within a main tubular body of a different material to the lining.
Clause 23. A method according to any of clauses 12 to 22 wherein in step (b) said extruded tube is extruded with one or more strength members integrated therein.
Clause 24. A method of providing fibre optic connections from a distribution point to a plurality of customer access points, the method comprising:
Clause 25. A method according to clause 24 wherein for at least one selected fibre unit the length of fibre unit withdrawn through the opening exceeds 100 m.
Clause 26. A method according to clause 24 wherein for at least one selected fibre unit the length of fibre unit installed through the branching duct exceeds 50 m.
Clause 27. A method according to any of clauses 24 to 26 wherein for at least one customer access point in step (d) the selected fibre unit is installed through the branching duct by pushing.
Clause 28. A method according to any of clauses 24 to 27 wherein for at least one customer access point in step (d) the selected fibre unit is installed through the branching duct by blowing.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013892.1 | Sep 2020 | GB | national |
| 2111598.5 | Aug 2021 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/073976 | 8/31/2021 | WO |
