Patent Description:
An optical fiber cable generally comprises two or more optical fibers enclosed within a jacket. A distribution cable is a type of optical fiber cable that is used to distribute optical signals from a central office to buildings, homes, and other sites (i.e., so-called "FTTx"), as well as to distribute optical signals to wireless network cell sites. It is common to branch and splice a distribution cable at various points along its length ("mid-span") into lower fiber count cables and drop cables.

In a loose tube distribution cable, multiple fibers may be organized into subunits by grouping them within sub-jackets or buffer tubes or by loosely wrapping groups of fibers in threads or yarns for ease of identification. Buffer tubes, like the cable jacket, are commonly made of a relatively stiff, hard material to help protect the fibers. However, buffer tubes commonly have a lower coefficient of thermal expansion than that of the cable jacket to help shield the fibers against detrimental effects of thermal expansion and contraction of the cable jacket.

The fibers in a distribution cable may be ribbonized. The term optical fiber "ribbon" refers to two or more parallel optical fibers that are joined together along their lengths. A material commonly referred to as a matrix adheres the fibers together. Ribbonization offers the benefit of mass fusion splicing. In a "flat" (also referred to as "encapsulated") type of optical fiber ribbon, the fibers may be fully or partially encapsulated within the matrix material along the entire length of the ribbon. The rigidity of conventional flat optical fiber ribbons presents challenges to achieving high fiber packing density in cables. Flat ribbons have other disadvantages in cables, such as preferential bending, and corner fibers exposed to stresses. So-called "rollable" or "partially bonded" optical fiber ribbons have been developed to achieve high fiber packing density and avoid some of the other disadvantages of flat ribbons. In a rollable or partially bonded ribbon, the matrix material is intermittently distributed along the fibers, providing sufficient flexibility to roll up each individual ribbon about an axis parallel to the fibers or otherwise compact the ribbon into a fiber bundle with a roughly cylindrical shape.

To provide high tensile strength needed to meet installation load standards, a semi-rigid reinforcing member, such as a fiberglass-epoxy or aramid-epoxy composite rod or a solid steel wire, may be located centrally within the cable. Buffer tubes may be arrayed around the central reinforcing member.

Another type of optical fiber cable is known as slotted core. A slotted core cable features a plastic (e.g., polyethylene) core having a radial array of arms, such that the spaces between adjacent arms define slots in which the ribbons are retained. A semi-rigid reinforcing member may be located centrally within the core.

Providing compact, high packing density optical fiber distribution cables that meet installation load requirements, facilitate mid-span access, and provide other advantages over prior distribution cables presents challenges, which may be addressed by the present invention in the manner described below. Document <CIT> describes an optical fiber cable, disclosing a "press-wrapping layer"for retaining each fiber bundle within one of the slots of the slotted core. Document <CIT> relates to a slotless type optical fiber cable in which a plurality of optical fibers or ribbons in which the plurality of optical fibers are arranged are covered with a cable jacket. Document <CIT> relates to an optical fiber ribbon having an intermittent fixing structure in which adjacent optical fibers are intermittently connected together via connecting portions, and relates to an optical fiber cable housing the optical fiber ribbon. Document <CIT> relates to fiber optic cables having partitions that create buffer cells for housing fiber optic ribbons.

Cables, methods, features, and advantages will be or become apparent to one of skill in the art upon examination of the following figures and detailed description.

The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention.

As illustrated in <FIG> (not to scale), in an illustrative embodiment of the invention, an optical fiber cable <NUM> includes a cable jacket <NUM>, rigid tensile reinforcement member <NUM>, and two or more "rollable" (also referred to as "partially bonded") ribbons <NUM>. Although for purposes of illustration (<FIG>) optical fiber cable <NUM> may have exactly <NUM> partially bonded ribbons <NUM>, a cable in accordance with the present disclosure may have any other number of such ribbons.

Each of ribbons <NUM> comprises individual optical fibers <NUM> (not shown in <FIG> for purposes of clarity), as described below in further detail with regard to <FIG>. Although for purposes of illustration (<FIG>) each ribbon <NUM> may have exactly <NUM> optical fibers <NUM>, in other embodiments each such ribbon may have any number of such fibers. Accordingly, in the embodiment illustrated in <FIG> optical fiber cable <NUM> has a total of exactly <NUM> optical fibers <NUM>. Optical fibers <NUM> may have a standard size, such as an overall diameter of <NUM> or <NUM>.

Rigid tensile reinforcement member <NUM> is centrally located within cable jacket <NUM>, i.e., approximately along a longitudinal axis of optical fiber cable <NUM>. Rigid tensile reinforcement member <NUM> may be made of, for example, a fiberglass-epoxy or aramid-epoxy composite rod or a solid steel wire. Rigid tensile reinforcement member <NUM> is "rigid" (i.e., has high tensile modulus) with respect to other elements of optical fiber cable <NUM> and thus substantially provides the tensile reinforcement to optical fiber cable <NUM> necessary to meet installation load standards.

Ribbons <NUM> may be stranded around rigid tensile reinforcement member <NUM>. The stranding may be of the "S-Z" type, where the twist direction alternates between clockwise and counter-clockwise, reversing after a certain number of twists. Alternatively, the stranding may be helical. Stranding ribbons <NUM> around rigid tensile reinforcement member <NUM> rather than laying ribbons <NUM> parallel to rigid tensile reinforcement member <NUM> provides the advantage of allowing optical fiber cable <NUM> to be pulled at higher tensions than some prior (e.g., central tube) cables while reducing fiber strain for a given amount of cable strain. It also allows for bending strain to be averaged across the optical fibers in tight slack storage coils often used in distribution cable applications. Stranding ribbons with an S-Z twist also has the advantage of providing extra ribbon length at the reversal points between S and Z twisted sections, making mid-span access easier in distribution cables.

The optical fiber cable <NUM> includes at least one cushioning layer <NUM> between ribbons <NUM> and cable jacket <NUM>. Cushioning layer <NUM> may be a coating (i.e., adhered to cable jacket <NUM> or otherwise tightly fitted), it may be a tube (i.e., more loosely fitted within cable jacket <NUM>), or it may take the form of a tape wrapped around ribbons <NUM>. In addition, optical fiber cable <NUM> includes a cushioning layer <NUM> between ribbons <NUM> and rigid tensile reinforcement member <NUM>. Cushioning layer <NUM> may be a coating (i.e., adhered to rigid tensile reinforcement member <NUM> or otherwise tightly fitted), it may be a tube (i.e., more loosely fitted around rigid tensile reinforcement member <NUM>), or it may take the form of a tape wrapped around ribbons <NUM>. Cushioning layers <NUM> and <NUM> may be foamed (chemically or physically).

Cushioning layers <NUM> and <NUM> serve to protect ribbons <NUM>. Cushioning layers <NUM> and <NUM> help prevent microbending or macrobending loss, provide crush resistance, and may also help keep ribbons <NUM> in place. The outer layer (not separately shown) may also serve as a "warning track" indication to an installer during an installation process that fibers are nearing the surface, and can be aid the installer in setting blade depth during the stripping process to avoid nicking fibers. Accordingly, cushioning layers <NUM> and <NUM> may be made of a material that is relatively soft and flexible compared with the materials of which cable jacket <NUM> and rigid tensile reinforcement member <NUM> are made. For example, cushioning layers <NUM> and <NUM> may be include one or more of: linear low-density polyethylene (LLDPE), ethylene-vinyl acetate (E-VA) copolymer, polyvinyl chloride (PVC), ethylene rubber, propylene rubber, and thermoplastic elastomer (TPE), including thermoplastic urethane (TPU) elastomers. Such materials and other suitable materials may be characterized generally by a Young's modulus in the range of <NUM>-<NUM> MPa and a either a Shore D hardness less than <NUM> or a Shore A hardness less than <NUM>. However, materials that would tend to leave a (e.g., sticky) residue on the fibers are not suitable.

Note that optical fiber cable <NUM> does not include any buffer tubes or a central tube for containing ribbons <NUM>. Rather, as illustrated in <FIG> with regard to an exemplary embodiment, ribbons <NUM> are located within a space <NUM> between cushioning layers <NUM> and <NUM>. Ribbons <NUM> are adjacent cushioning layers <NUM> and <NUM> and thus cushioned between them. The elasticity or low Young's modulus of cushioning layers <NUM> and <NUM> may be contrasted with the rigidity or high Young's modulus of conventional buffer tubes or central tubes made of high-density polyethylene, polypropylene, etc..

Alternately, cushioning layers <NUM> and <NUM> may take the form of a tape, including tapes made from one or more of nonwoven polyester, nonwoven polypropylene, or extruded materials including one or more of: linear low-density polyethylene (LLDPE), ethylene-vinyl acetate (E-VA) copolymer, polyvinyl chloride (PVC), ethylene rubber, propylene rubber, and thermoplastic elastomer (TPE), including thermoplastic urethane (TPU) elastomers. The tape may include material that has been foamed (chemically of physically).

One or both of cushioning layers <NUM> and <NUM> may include a water-swellable material, such as a super-absorbent polymer, that provides a waterblocking function. One or both of cushioning layers <NUM> and <NUM> may include flame retardant material, such as metal hydrates (e.g., magnesium dihydrate, aluminum trihydrate, etc.). The water-swellable and/or flame retardant material may be compounded with the above-described materials (e.g., LLDP, E-VA, PVC, etc.) of which cushioning layers <NUM> and <NUM> may be made, so as to provide a homogeneous cushioning layer <NUM> or <NUM>. Alternatively, the water-swellable material and/or flame retardant material may be a coating.

A binding thread <NUM> may be twisted (e.g., helically with respect to the length or extent of optical fiber cable <NUM>) around a bundle of one or more ribbons <NUM>. Binding threads <NUM> may be color coded to aid identification of ribbons <NUM> or bundles of two or more ribbons <NUM>. However, other methods of fiber and ribbon identification, such as printing of various shapes, numbers, or letters, on the ribbons and fibers may also be used. Binding thread <NUM> may include (e.g., be coated with or impregnated with) a water-swellable material. In the embodiment illustrated in <FIG>, a binding thread <NUM> is twisted around each of ribbons <NUM> (as generally indicated in broken line around each of ribbons <NUM>). Nevertheless, in other embodiments (not shown) one or more binding threads may be configured around a bundle of two or more ribbons. Also, although in the illustrated embodiment binding thread <NUM> serves as a bundling structure around a bundle of one or more ribbons <NUM>, in other embodiments (not shown) such a bundling structure may comprise alternative or additional elements, such as a nonwoven or paper tape (e.g., between the binding thread and ribbon). Such other bundling structures may include a water-swellable material.

One or more ripcords <NUM> may be provided under cable jacket <NUM> to aid removing a portion of cable jacket <NUM> for splicing or other mid-span access. Also, as ribbons <NUM> are not contained within any buffer tubes or a central tube, a cable in accordance with the present disclosure provides benefits that include facilitating mid-span access to ribbons <NUM>.

An exemplary structure of each of ribbons <NUM> is shown in <FIG>. Ribbon <NUM> comprises two or more optical fibers <NUM> joined to each other intermittently along their lengths with patches of adhesive, commonly referred to as a matrix material <NUM>. The pattern of matrix material <NUM> shown in <FIG> or other characteristics of ribbon <NUM> described herein are intended only as examples, and one of ordinary skill in the art will recognize that other types of rollable or partially bonded optical fiber ribbon are suitable.

As well understood by one of ordinary skill in the art, while ribbon <NUM> has the ribbon shape shown in <FIG> when laid flat with its optical fibers <NUM> arrayed parallel to each other, optical fibers <NUM> can also roll into or otherwise assume a compact bundle or roughly cylindrical shape. That is, the intermittent rather than continuous distribution of matrix material <NUM> provides ribbon <NUM> with sufficient flexibility to be rolled about an axis substantially parallel to the fibers. The terms "rollable" and "partially bonded" are understood by one of ordinary skill in the art in the context of optical fiber ribbons to specifically refer to a ribbon having this characteristic, provided by the intermittent rather than continuous distribution of matrix material <NUM>. A "rollable" ribbon may be contrasted with what is commonly referred to in the art as a "flat" or "encapsulated" ribbon, in which matrix material is distributed continuously along the length of the fibers. In a flat ribbon, the fibers may be fully encapsulated within the matrix material. The rigidity of encapsulated optical fiber ribbons presents challenges to achieving high fiber packing density in cables. The development of rollable ribbons has led to higher fiber packing density in cables.

Including optical fibers <NUM> in the form of rollable or partially bonded ribbons <NUM>, combined with the absence of buffer tubes or a central tube promotes high packing density. For purposes of this disclosure, packing density is defined as the ratio between the total cross-sectional area of the optical fibers <NUM> and the cross-sectional area of the cable jacket <NUM> interior. Also, as noted above, the absence of buffer tubes or a central tube facilitates mid-span access to ribbons <NUM>.

Claim 1:
An optical fiber cable (<NUM>), comprising:
a cable jacket (<NUM>);
a rigid tensile reinforcement member (<NUM>) centered within the cable jacket (<NUM>);
a plurality of partially bonded optical fiber ribbons (<NUM>) around the rigid tensile reinforcement member (<NUM>), wherein the optical fiber cable (<NUM>) includes no buffer tubes; and
at least one cushioning layer (<NUM>, <NUM>), including a first cushioning layer having a Young's modulus in a range of <NUM>-<NUM> megapascal between the plurality of partially bonded optical fiber ribbons and the cable jacket (<NUM>), the at least one cushioning layer further including a second cushioning layer having a Young's modulus in a range of <NUM>-<NUM> megapascal between the plurality of partially bonded optical fiber ribbons and the rigid tensile reinforcement member, wherein the plurality of partially bonded optical fiber ribbons are retained in a space between the first and second cushioning layers and are adjacent the first and second cushioning layers.