Apparatus and method for bend radius control of fiber optic cable assemblies

A cable assembly, for example, a pulling grip for pulling a trunk cable assembly having a plurality of cable legs may include at least one pliable core for receiving the cable legs, the cable legs being wrapped at least one time around the at least one pliable core causing distal ends of the cable legs to be a distance from a furcation point, the distance being shorter than the length of the cable legs, the cable assembly further providing protection from exceeding a minimum bend radius and enabling a relatively short pulling grip.

BACKGROUND

1. Technical Field

The disclosure relates generally to fiber optic cable assemblies and more particularly to bend radius control of fiber optic cable assemblies within a pulling grip, including associated apparatuses and methods.

2. Field of the Disclosure

So called pre-connectorized trunk assemblies require protection for the connectors and legs while in packaging and while being pulled through cable conduits or ducts, or over sheaves. Cable legs require some means to secure them while in packaging or in pulling grips to ensure the legs do not get bent to a radius smaller than a recommended minimum bending radius. The length of pulling grips can be minimized by folding legs over in some controlled fashion.

Connectors in pulling grips are currently bundled together or the legs are made to different lengths, with a slightly staggered formation. By staggering the connector placement, the diameter of pulling grips can be made smaller enabling trunks to be pulled in smaller ducts or enabling the use of more trunks per duct. What is needed is a fiber optic assembly with a pulling grip that minimizes pulling grip length and diameter while protecting the cable legs and connectors inside.

No admission is made that any reference cited herein constitutes prior art. Applicant expressly reserves the right to challenge the accuracy and pertinency of any cited documents.

SUMMARY

One embodiment of the disclosure relates to a cable assembly, for example, a trunk assembly, the assembly comprising at least one fiber optic trunk cable, the at least one fiber optic trunk cable including a jacket and at least one optical fiber extending beyond the jacket; at least one cable leg having a first length, the at least one cable leg being associated with the fiber optic trunk cable and containing the at least one optical fiber; at least one connector assembly, the at least one connector assembly in communication with the at least one optical fiber and being operatively connected to an end the at least one cable leg; and at least one pliable core having a second length, the at least one pliable core associated with the at least one cable leg, the cable leg being wrapped at least one time around the at least one pliable core, the second length being shorter than the first length.

An additional embodiment of the disclosure relates to a method of making a cable assembly, the method comprising the steps of: providing a furcated optical cable assembly, the furcated optical cable assembly including at least one trunk cable, at least one furcation point and at least one cable leg having at least one optical connector on an end; providing a coupler; providing a convoluted sleeve; providing an expandable mesh; providing a pulling sock; providing at least one pliable core; wrapping the at least one cable leg around the at least one pliable core, causing the at least one optical connector to be a distance from the furcation point; installing the coupler about the furcation point; inserting the wrapped pliable core into the convoluted sleeve; mating the convoluted sleeve to the coupler; sliding the expandable mesh over the convoluted sleeve; securing the expandable mesh to the coupler and the convoluted sleeve; and installing the pulling sock about the coupler, the sleeve, the expandable mesh, and the wrapped pliable core to make the cable assembly.

DETAILED DESCRIPTION

Reference is now made in detail to various embodiments of the disclosure, which are illustrated in the accompanying drawings. Whenever possible, identical or similar reference numerals are used throughout the drawings to refer to identical or similar elements. It should be understood that the embodiments disclosed are merely exemplary with each one incorporating certain benefits of the disclosure. Various modifications and alterations may be made to the following exemplary embodiments within the scope of the disclosure, and aspects of the embodiments may be mixed in different ways to achieve yet further embodiments. Accordingly, the true scope of the disclosure is to be understood from the entirety of the disclosure in view of, but not limited to, the embodiments described.

FIG. 1is a partially exploded view of a pre-connectorized trunk assembly10. Trunk assembly10may include, in exemplary embodiments, a fiber optic cable assembly11, and pulling grip components, for example, for pulling assembly10along ducts or around sheaves. Cable assembly11may include in exemplary embodiments, at least one fiber optic cable20, at least one furcation body30on an end of cable20and at least one cable leg60disposed opposite cable20. Cable leg60may have a connector on an opposite end, for example, at least on multifiber connector70. Cable leg60and an associated connector70comprise at least one connector assembly80. Cable assembly11may include a plurality of connector assemblies80, for example, 4, 6, 8, 12 or more.

In an exemplary embodiment trunk assembly10may include at least one pliable core50. Pliable core50may include a foam, an elastomer or the like for wrapping cable assembly80, for example, multiple times in a coiling or helical fashion, causing a shorter length than an overall length of cable assembly50. In exemplary embodiments a plurality of cable assemblies80may be wrapped in the same direction around pliable core50in such a way as to stagger each of connectors70to reduce an overall size of the assembly.

A bendable tube like structure, for example, a corrugated pulling grip sleeve90may be applied over the connector assemblies80wrapped around pliable core50. Sleeve90, in exemplary embodiments, may be resistant to crushing forces yet may bend under bending forces. Sleeve90may interface with a pulling grip housing40, in exemplary embodiments surrounding furcation body30. A strength member sleeve100, for example, a woven carbon fiber or similar mesh material may be applied and secured about both pulling grip housing40and sleeve90. Sleeve100may, in exemplary embodiments, be resistant to pulling forces yet not add dimensionally to the overall size of assembly10. Sleeve90may be secured to housing40, for example, by a heat shrink, a compression band, a cable tie or tie-wrap, or some other suitable fastener. Housing40may include separate parts, for example, a first housing portion42and a second housing portion44that may be placed about furcation body30and have complimentary internal geometry to accommodate the external geometry of body30such that forces applied in at least one axial direction do not permit housing40to slip from body30.

FIG. 2is a cross section of an example cable20having a diameter D20. Each cable20includes its own outer jacket22of thickness T22and that defines an interior27that contains a plurality of cable legs60. Cable legs60each include a buffer tube62that defines an interior64containing at least one optical waveguide such as at least one optical fiber66. Buffer tube62has a thickness D62. Exemplary cable legs60are not stranded within the cables20, although some degree of stranding may be used for certain applications. For example, the cable legs60can be twisted in helical fashion with respect to one another, in particular when a plurality of or all of the cable legs60are arranged in such a way that they are rotated with a specified lay length. In the present disclosure, any stranding of cable legs60(except for that at the furcation point FP, discussed below) is generally considered to be loose, e.g., so that the optical fibers66are free to move within their respective buffer tubes62.

With continuing reference toFIG. 2, a strain-relief element24may be disposed in cable interior27adjacent jacket22and surrounding cable legs60. Strain-relief element24may include, for example, a layer of yarn or yarns (e.g. aramid yarn) for absorbing tensile loads. Strain-relief element24is shown with a non-uniform thickness because the locations of the cable legs60may cause the strain-relief element to compress at various locations along the length of the cable20.

FIG. 3is a cross section of cable leg60having a diameter D60. In an example, buffer tubes62are made of a polymer and are formed as a polymeric sheath. Buffer tubes62have a thickness D62. In exemplary embodiments a plurality of, for example, twelve (12), optical fibers66may be included in interior64of cable leg60.

The furcation assemblies and methods of the disclosure are discussed herein in connection with cable20by way of illustration. Cable20can be constructed of selected materials of selected thicknesses such that it has riser or plenum burn ratings according to desired specifications. Cable legs60can also be constructed so that they are relatively robust, such that they are suitable for field use, while also providing a desired degree of accessibility. For example, cable legs60can be constructed with relatively thick buffer tubes62, e.g., on the order of D62=0.2 millimeters (mm) or more, so that the exposed cable legs that form part of the fiber optic cable furcation assembly (discussed below) provide sufficient protection for the optical fibers66contained therein

Cable jacket22and buffer tubes62can also be formed from, for example, fire-retardant materials to obtain a desired plenum burn rating. For example, highly-filled PVC of a specified thickness can be used to form buffer tubes62. One well-known plenum burn standard is the National Fire Protection Act Standards (NFPA) 262 burn test. NFPA 262 prescribes the methodology to measure flame travel distance and optical density of smoke for insulated, jacketed, or both, electrical wires and cables and fiber optic cables that are to be installed in plenums and other spaces used to transport environmental air without being enclosed in raceways. Cables20may be constructed to be low skew within cable legs60so that they are suitable for use in parallel optic transmission systems. Skew is generally defined as the difference in the time of flight of optical signals for the fibers within a module and has units of picoseconds per meter (ps/m).

FIG. 4is a side view of an end portion of an example cable20having an end21and showing cable legs60and strain relief members24contained within cable interior27.FIG. 4also shows a location120a distance L1from end21where cable jacket22may be cut to form a furcation point (discussed below). The furcation point is formed by removing an end portion of jacket22to expose end portions of cable legs60in anticipation of connecting the optical fibers66(not shown inFIG. 4) carried by the cable legs to one or more multifiber connectors.

FIG. 5Ais a side view of cable20and shows the cable after jacket22has been cut at location120and an end portion of the jacket removed, thereby forming a jacket end23. This exposes end portions of cable legs60and an end portion of strain-relief members24. Six cable legs60are shown inFIG. 5Aby way of illustration.FIG. 5Bis a close-up perspective view of a cable20similar to that ofFIG. 5A, wherein the cable is shown carrying four cable legs60by way of example, with the four cable legs extending from jacket end23. Exposed cable legs60are denoted60E and have ends61E. InFIG. 5B, strain-relief members24are shown in the form of yarn by way of illustration.

FIG. 6Ais similar toFIG. 5A, except that the exposed cable legs60E are now stranded over a stranded section130having a lay length L2.FIG. 6Bis a close-up view of cable20ofFIG. 6Aat jacket end23and shows stranded region130and lay length L2. In an exemplary embodiment, exposed cable legs60E may be unidirectionally and helically stranded in a manner that does not violate the minimum bend radius RMINof optical fibers66within the buffer tubes62, but that provides sufficient contact between the optical fibers and their respective buffer tubes to substantially immobilizing the optical fibers within the buffer tubes. The cable jacket end23and stranded section130of stranded cable legs60E generally define the furcation point FP. In an example, stranded section130is located immediately adjacent cable jacket end23, or is relatively close thereto, and the furcation point may further include 20 mm of the end of the cable jacket. In an example, exposed cable legs60E are stranded by hand to form stranded section130.

The bending radius R of the cable legs60when helically stranded in a single direction may be calculated using the equation:

Where P is the pitch or lay length, and D′ is the pitch circle diameter. In an example, exposed cable legs60E are helically wound in a single direction with at least three helical wraps (turns) to provide sufficient contact between optical fibers66and their buffer tubes62to substantially immobilize the optical fibers. In example embodiments, the pitch/lay length P=L2as denoted in the pertinent Figures is in the range of 10 mm≦L2≦100 mm, or preferably 15 mm≦L2≦50 mm, or more preferably in the range of 15 mm≦L2≦20 mm.

By way of example, for exposed cable legs60E stranded with a pitch/lay length P=L2=17 mm with a pitch circle diameter D′=16 mm (which is an example buffer tube diameter), the bend radius R is about 37 mm, which is substantially larger than the minimum bend radius RMINfor most optical fibers66. Consequently, this amount of bending would not lead to significant bend-induced attenuation. Example optical fibers66include multi-mode, bend-insensitive optical fibers, such as the CLEAR CURVE® optical fiber, available from Corning, Inc., Corning, N.Y.

FIG. 7is a side view of cable20after exposed cable legs60E are stranded as discussed above, and shows a maintaining member140arranged on (e.g., applied to) the stranded exposed cable legs60E at furcation point FP over at least a portion of stranded section130to maintain the cable legs in their stranded configuration. In an example, maintaining member140extends the entire lay length L2of stranded section130. Examples of maintaining member140include tape, strapping, shrink tubing, shrink-wrap, binder, yarn, epoxy, urethane sealant, adhesive material, and combinations thereof.

FIG. 8is similar toFIG. 7and illustrates an example where a furcation body150is optionally added at furcation point FP, i.e., is disposed on (e.g., secured to, fixed to, etc.) at least a portion of maintaining member140. Furcation body150adds protection to the stranded, exposed cable legs60E at furcation point FP and can also serve to further secure the exposed cable legs in their stranded configuration. Furcation body150can also facilitate handling of the final cable assembly by providing a gripping/handling location for use by field personnel. In an example, furcation body150extends beyond stranded section, as shown inFIG. 8.

FIG. 9is a schematic diagram of the cable20after furcation body150is added and wherein the optical fibers66carried in the exposed cable legs60E are in the process of being connected to respective multifiber connectors70to form a connectorized cable assembly11. One multifiber connector70is shown as awaiting connection to the exposed optical fibers66of one of the exposed cable legs60E. An example multifiber connector70is an MTP connector. Also in an example, the optical fibers66in a given buffer tube62are connected to a corresponding multifiber connector70.

FIG. 10is side view of a completed connectorized cable assembly11comprising cable20having stranded section130and multifiber connectors70connected to optical fibers66at the ends of the exposed cable legs60E.

Alternatives to the embodiment ofFIG. 1could be, for example, the use of non-normal helical routings (a normal helix being around a cylindrical core, generally with a fixed pitch). Variations would include, for example, changes in diameter of a representative pliable core and the pitch on a single helix and the use of opposing helix (or helix variants). By way of example,FIG. 11is a side view of such an exemplary pre-connectorized trunk assembly13having connectorized cable legs wrapped around pliable core50in different orientations H1and H2.

In the present embodiments, the minimum bend radius is not exceeded due to the physical presence of the foam core which is sufficiently sized to be larger than the minimum bend radius. The minimum bend radius of a helix is the radius of the core. The bend radius is greatly increased as the pitch is increased. The curvature k may be:

Providing a radius of curvature that may be 1/k:

From inspection, when P=0, the radius of curvature is r, and for any other value of P, the radius of curvature is larger.

Leg Staggering—By wrapping the legs around a core and increasing the pitch (spacing of legs) along the length, a stagger in the connector ends can be achieved. With staggered connectors, the end diameter is potentially much smaller than a bundle of connectors.

The arc length of a helix may be represented by the equation:

Where A is the arc length of the helix, r is the radius, and P is the pitch and N is the number of rotations. For the embodiments of the disclosure the radius of the core, the number of rotations and the leg length may be constant, but the pitch can be varied per leg so that the arc length traveled by each leg is different causing the ends to land in staggered locations. Because the equation is non-linear, it is not possible to solve directly for the difference in pitch lengths for a difference in arc lengths implicitly, but iterative solutions may be used.

By way of example, the following algorithm may be followed to achieve a connector stagger (d(L-A)) spacing of 2 inches for a radius(r)=0.75 inches, number of rotations(N)=4 turns, leg length(L)=36 inches. Pitch(P) of 1 may be tried. A, L-A, and d(L-A) and P may then varied from leg to leg to get d(L-A), the spacing between leg ends, near the desired 2 inches.

In the exemplary embodiments a generally cylindrically shaped solid or hollow cores may be used. However, many variations in core shape, size, and designed in features are numerous and may be contemplated, for example, conical taper, hourglass shape, pockets (recesses) or slits designed in to hold connectors, paths cut in to allow for easy routing of legs, “U” shaped design, for example, folding the cylinder into a “U” shape so the connectors end up near the furcations, meaning a larger “diameter” grip, but shorter length, and the use of multiple cores for furcated legs. In one embodiment and by way of example, as seen inFIG. 12in a side view, another pre-connectorized trunk assembly15may have connectorized cable legs60wrapped around an alternate55pliable core and having respective multifiber connectors70tucked into at least one receiving slot56of alternate core55.

Other variations of the pulling grip disclosed herein are also possible. For instance, the pulling grip housing may omit the locking feature with the pulling grip sleeve and provide the anti-rotation feature in other ways. By way of example,FIG. 13depicts a perspective exploded view of a pulling grip40that is similar to other embodiments having a pulling grip sleeve90and a pulling grip housing40. Pulling grip housing40may include a first housing portion42that is configured to mate with a second housing portion44. An internal cavity46may be formed inside the pulling grip housing40above for receiving the furcation plug30as disclosed in one or more of the embodiments described. By way of example, internal cavity46may include one or more notches or recesses that are tailored for fitting with the profile of the furcation body of the fiber optic assembly for transferring pulling forces thereto. In other embodiments, the first housing portion42may be hingedly attached or connected together in a suitable fashion to the second housing portion44to use fewer parts and/or reduce the risk of misplacing a portion of the pulling grip housing.

In exemplary embodiments, pulling grip housing40may not have a locking feature with pulling grip sleeve90. However, pulling grip10still provides an anti-rotation feature for the fiber optic assembly11being pulling in by the craft. Specifically, the anti-rotation feature is provided by the friction fit between the outer portion of the pulling grip housing40and a portion of the inner surface of the pulling grip sleeve90. The friction fit between the pulling grip housing40and the inner surface of pulling grip sleeve90advantageously inhibits twisting of fiber optic assembly11within the sleeve during installation. Pulling grip housing40is also shown with a ribbed construction (not numbered), which advantageously reduces the amount of material compared with a similarly sized part.

Pulling grip sleeve90may be a corrugated tube (e.g., with ridges) for providing flexibility and crush resistance, but other types of pulling grip sleeves are possible. For instance, the use of a smooth wall tube is possible. Other variations for the pulling grip housing, the fiber optic assembly, and the like may be incorporated into this embodiment as disclosed.

In exemplary embodiments, pulling grip40may include a pulling sock110placed over the pulling grip sleeve90, allowing attachment of a fish tape or line to the pulling sock loop114for pulling the fiber optic assembly into place. Simply stated, the distal end of pulling sock110may be necked down such as with a conical portion118for engaging with the a portion of the pulling grip housing and/or pulling grip sleeve so that the pulling force is transferred to the furcation body/strength members or other suitable portion of the fiber optic assembly11. In use, an end portion of fiber optic assembly11such as the furcation body is placed within the pulling grip housing40such as discussed in one of the several embodiments above. Next, the connectorized end of the fiber optic assembly is inserted into the pulling grip sleeve90so that a portion of the pulling grip housing40fits within the pulling grip sleeve90. Unlike conventional pulling grips, the pulling grip housings disclosed only fit over a small portion of fiber optic assembly11. In other words, the pulling grip housing does not fit over the connectors of fiber optic assembly11, thereby allowing a flexible pulling grip. Additionally, the connectorized end of fiber optic assembly11may include a protective layer such as a plastic wrap or the like and to aid the insertion into the pulling grip sleeve. Thereafter, the assembly is placed within the pulling sock110so that the conical portion118of the pulling sock110engages pulling grip housing40and then is properly secured thereabout. Consequently, the craft can route and attach a fish tape or line to loop114of pulling sock110for installation. Furthermore, any of the embodiments discussed herein can be assembled and packaged on a reel in the factory for quick and easy deployment in the field.

As illustrated inFIG. 14, pulling sock110may also be provided and placed over the pulling grip sleeve18to further facilitate pulling of the pulling grip10. Pulling sock110may include a front conical portion118that is shaped essentially like the front portion of the pulling grip housing40when the first housing portion22is mated to the second housing portion24. The pulling sock110contains an opening112to allow the fiber optic cable20to pass therethrough while retaining the pulling grip housing20inside the pulling sock110. A pulling sock loop114may be disposed on a second end116of the pulling sock110to facilitate pulling of the pulling grip sleeve18and thus the fiber optic cable20. The pulling sock110may include a zipper117or other attachment means such that it can be opened and disposed laterally around the pulling grip sleeve18and the pulling grip housing20. The pulling sock110may be constructed out of any material, including but not limited to a polymer, metal, filament, and provided in any form, including but not limited to a solid material, mesh, and composite.