Patent Description:
Optical fiber cables for use in certain environments, particularly indoor or plenum spaces, are evaluated and rated for resistance to burning and smoke generation, among other factors. New regulations, such as the Construction Products Regulation (CPR) in Europe, include increased scrutiny of burn performance to qualify under the new ratings. For example, the CPR guidelines require that cables are rated on a scale from d0 to d2. One of the factors in determining the burn rating is a flaming droplet test. D0 indicates that when the cable was subjected to a controlled burn, no flaming droplets were produced. D1 indicates the presence of flaming droplets which extinguished within <NUM> seconds, and d2, which is the lowest rating, covers all other outcomes.

Minimizing flaming droplets has not been a high priority in past cable qualification. However, because of this new rating regime, there is additional focus on achieving good flaming droplet performance.

<CIT> discloses a fiber optic cable has one or more optical fibers, an inner tube surrounding the optical fibers, a strength member, an inner jacket surrounding the inner tube and strength member, and an outer jacket surrounding the inner jacket without being adhered to the inner jacket such that the outer jacket is easily strippable from the inner jacket. <CIT>, <CIT> and <CIT> disclose additional prior art.

The invention provides an optical communication cable according to claim <NUM>.

Additional features and advantages will be set forth in the detailed description that follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings.

The accompanying drawings are included to provide a further understanding and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the description serve to explain principles and the operation of the various embodiments.

A conventional optical fiber cable generally includes a jacket that surrounds a number of core elements. The core elements may most commonly include, for example, optical fibers or optical fiber ribbons, strength elements such as a central strength member or strength yarns, buffer tubes or sleeves that segregate groups of fibers into subunits within the core, and/or water blocking technologies such as gels, SAP powders or tapes. When a cable is subjected to a burn test, flaming droplets originate primarily from the jacket layer. Different factors have been observed to contribute to the rate of flame droplet generation, such as the jacket compound material and, with respect in particular to tube-based designs, the amount of free space in the interior of the cable. The more free-space there is in the core of the cable, the more apt a burn test is to produce flaming droplets.

However, the answer to reducing flaming droplets is not simply to change the jacket material or remove the free space from the interior of the cable. Doing so would in many cases substantially impact other mechanical factors, such as flexibility and crush resistance, for example. Accordingly, there is a need for another design attribute that can be used in certain cable designs to reduce flaming droplets when the cable is subjected to burning.

In accordance with aspects of the present disclosure, <FIG> illustrates an optical communication cable <NUM> in which a relatively small portion of the internal cross-sectional area of the cable jacket (or other surrounding cable layer) is occupied by the optical fibers of unstranded optical fiber subunits. Applicant has determined that ranges of internal cross-sectional areas occupied by the optical fibers of the subunits discussed herein provide sufficient free space to allow for the subunits to shift and assume low stress positions during cable bending, and thus, provides good attenuation performance, even though the subunits are unstranded. However, the free space that is good for attenuation performance creates the need to address flaming droplet production during a burn.

As shown in <FIG>, the optical communication cable <NUM> includes an outer cable layer or jacket, shown as cable jacket <NUM>, having an inner surface <NUM> that defines an inner passage or cavity, shown as central bore <NUM>. As will be generally understood, inner surface <NUM> of jacket <NUM> defines the central bore <NUM> within which the various cable components discussed below may be located. In accordance with aspects of the present disclosure, an outside diameter of the cable <NUM> may be approximately <NUM> millimeters and the jacket <NUM> may have a wall thickness of approximately <NUM> millimeters such that an inner diameter of the inner surface <NUM> is approximately <NUM> millimeters. Strength members <NUM>, such as fiberglass yarns or GRP rods, for example, may be embedded in the jacket <NUM> and/or provided in the central bore <NUM> to provide added tensile strength to the cable <NUM>.

According to claim <NUM>, cable <NUM> include a plurality of optical fiber subunits <NUM>. As shown in <FIG>, each subunit <NUM> include a plurality of optical fibers, shown as a stack of optical fiber ribbons <NUM>. Each optical fiber ribbon <NUM> in the stack has a plurality of optical fibers supported in an aligned array via a polymer ribbon matrix material. Each subunit <NUM> also includes a subunit jacket <NUM> that defines a subunit passage, and the plurality of optical fiber ribbons <NUM> of each subunit are located within the subunit jacket <NUM>. In various embodiments, subunit jacket <NUM> may be a relatively thin layer of continuous and contiguous polymer material (e.g., contiguous circumferentially and longitudinally for a longitudinal length of at least <NUM> along the length of the cable) that surrounds the optical fiber ribbons <NUM> closely to contain ribbons <NUM> with limited compression force such that the plurality of ribbons <NUM> act as a unitary subunit while ribbon to ribbon friction is sufficiently reduced to allow some longitudinal movement of ribbons during bending to prevent buckling.

In various embodiments, subunit jacket <NUM> is formed from solid or unfoamed polymer material and may have a thickness of between <NUM> and <NUM>. In specific embodiments, subunit jacket <NUM> is formed from solid or unfoamed polymer material that has a thickness between <NUM> and <NUM>. In other embodiments, subunit jacket <NUM> may be formed from a foamed material (e.g., a foamed polymer material) and may have a thickness up to <NUM>. In accordance with yet other aspects of the present invention, the subunit jacket <NUM> may be a multilayered jacket comprising more than one layer of solid polymer material and/or a combination of solid and foamed materials.

In various embodiments, subunit jacket <NUM> is a thin jacket of extruded material that cools to provide a limited inwardly directed force on to ribbons <NUM>. The inwardly directed force provided by this embodiment of subunit jacket <NUM> acts to prevent/resist unraveling of the stranded ribbons <NUM> while allowing sufficient movement of the individual ribbons in the stack during bending. Thus, in this manner (and unlike loose buffer tubes) subunit jacket <NUM> generally conforms to the shape or outer perimeter of the group or stack of optical fiber ribbons <NUM> within each subunit. In specific embodiments, subunit jacket <NUM> is formed from an extrudable polymer material having a thickness as discussed above and a modulus of elasticity of less than <NUM> MPa at room temperature (e.g., <NUM> degrees F). A subunit jacket formed from a material of this thickness and modulus is capable of holding together the subunit, while conforming to the shape of the internal optical fiber ribbons, while also providing a subunit jacket that can be easily opened manually by a user (e.g., peelable) to access the optical fiber ribbons for splicing, connection, etc..

In accordance with aspects of the present invention, a wide variety of optical fiber elements may be contained in each subunit <NUM> of the cables discussed herein. Moreover, the subunit jackets <NUM> may be conventional buffer tubes, which have a thicker, more protective wall surrounding the plurality of fibers. In accordance with yet other aspects of the present disclosure, which are not encompassed by the claimed invention, each subunit <NUM> may include a plurality of loose, individual optical fibers surrounded by the subunit jacket <NUM>. In other embodiments, each subunit <NUM> may include flexible or rollable optical fiber ribbons, which are different from standard optical ribbons in that the rollable ribbons do not typically have a solid matrix surrounding the entire array of fibers forming the ribbon. Rather, the individual fibers in each rollable ribbon may be bonded at intermittent or spaced intervals such that individual or groups of fiber may be folded or rolled toward other fibers in the array of fibers. In various embodiments, the cables discussed herein may include more than two subunits <NUM>.

Referring to <FIG>, in specific embodiments, cable <NUM> may include only two subunits <NUM>, and in a specific embodiment, each subunit <NUM> includes a stack of optical fiber ribbons <NUM> such that the subunit includes at least <NUM> individual fibers. Each optical fiber ribbon <NUM> includes a plurality of fibers, which may include <NUM> fibers, <NUM> fibers, <NUM> fibers, <NUM> fibers, <NUM> fibers, <NUM> fibers, <NUM> fibers, etc. Each subunit can include any number of ribbons and any combination of ribbons of different optical fiber counts.

To further facilitate good attenuation performance, cable <NUM> may be designed to provide sufficient space for subunits <NUM> and/or the plurality of fibers to reposition and assume low stress positions within cable jacket <NUM> as cable <NUM> is bent within various installations. A relatively low portion of the space within cable jacket <NUM> may be occupied by ribbons <NUM> and/or subunits <NUM>. The inner surface <NUM> of outer cable jacket <NUM> defines a cable jacket internal cross-sectional area and less than <NUM>% of the cable jacket internal cross-sectional area is occupied by the cross-sectional area of optical fiber ribbons <NUM>. Thus, when cable <NUM> goes around sheaves, rollers and other bends during installation, or when it is coiled for slack storage, subunits <NUM> wanting to migrate toward the neutral axis of the bend have the free space available to move in that manner to attain their lowest energy position. This enables good attenuation performance for cable <NUM>.

Referring to the specific embodiment shown in <FIG>, cable <NUM> may be configured as a cable suitable for indoor use. In specific embodiments, cable <NUM> may be a flame-retardant indoor cable or a flame retardant indoor/outdoor cable. In such embodiments, a flame-retardant scaffolding structure <NUM> is configured into the central bore <NUM> of the cable <NUM>. The scaffolding structure <NUM> is placed into the interior of the cable <NUM> in a manner that the structure <NUM> is adjacent to and provides support to the jacket <NUM>. In this manner, as the cable <NUM> is subjected to a burn, as the jacket <NUM> melts, the jacket material bonds to the scaffolding structure <NUM> rather than sloughing off the cable to become a flaming droplet. The scaffolding structure <NUM> may be preferably comprised of a suitable material that has a much higher melting point than the jacket material <NUM> in order to maintain structural support during a burn. For example, the scaffolding structure <NUM> may be a substrate material having a fiberglass backing or fiberglass elements embedded throughout, such as fiberglass tapes and/or mica tapes with a fiberglass backing.

<FIG> illustrates the results of burn testing of similar cables for new cable designs made with and without the scaffolding structure <NUM> shown in <FIG>. Two burns were conducted for each type of cable design, the first design (see the first two rows in the table of <FIG> and the third row showing the average of the first two rows) having the same features and dimensions as cable <NUM> shown in <FIG>, except without the scaffolding structure <NUM>. The second design subjected to two burns (see the fourth and fifth rows in the table of <FIG> and the sixth row showing the average of the fourth and fifth rows) were the cables <NUM> shown in <FIG>. The presence of the scaffolding structure <NUM> did not appear to have a significant impact on the main burn performance characteristics such as flame spread (FS) and heat release (see, e.g., peak heat release rate (PHRR) and total heat release (THR)). However, although the scaffolding structure <NUM> does not have a notable impact on aspects such as flame spread and heat performance. there is a significant improvement in flaming droplet performance. Both cable burns with the cables <NUM> performed significantly well on flaming droplet test such that each would have received the best d0 rating under the CPR regulations, indicating no flaming droplets.

In various embodiments, subunit jacket <NUM> may be formed from a variety of extruded polymer materials. In various embodiments, subunit jacket <NUM> is made of a peelable plasticized PVC material tightly extruded to surround each ribbons <NUM> in each subunit <NUM>. Subunit jacket <NUM> may be a single extruded layer of plasticized PVC that is both thin ( e.g., a thickness of between <NUM> and <NUM>, specifically, <NUM> and. <NUM>, and more specifically about. <NUM>) and comprised of a soft material that easily separates by manually pinching the sheath material. In various embodiments, the elastic modulus of subunit jacket <NUM> at room temperature is less than <NUM> MPa and rises to only approximately <NUM> MPa at cold temperatures (e.g., -<NUM>). Various aspects of cable <NUM>, including bundle jackets <NUM> and the stranding of ribbons <NUM> within each subunit <NUM>, may be formed via the methods and materials disclosed in <CIT>.

In various embodiments, subunits <NUM> can include a wide variety of optical fibers including multi-mode fibers, single mode fibers, bend insensitive fibers, etc. In various embodiments, cable jacket <NUM> and subunit jacket <NUM> may be a variety of materials used in cable manufacturing, such as polyethylene, polyvinyl chloride (PVC), polyvinylidene difluoride (PVDF), nylon, polypropylene, polyester or polycarbonate and their copolymers. In addition, the material of cable jacket <NUM> and subunit jacket <NUM> may include small quantities of other materials or fillers that provide different properties to the material of cable jacket <NUM>. For example, the material of cable jacket <NUM> and/or subunit jacket <NUM> may include materials that provide for coloring, UV/light blocking (e.g., carbon black), fire resistance as discussed above, etc..

The optical fibers discussed herein may be flexible, transparent optical fibers made of glass or plastic. The fibers may function as a waveguide to transmit light between the two ends of the optical fiber. Optical fibers may include a transparent core surrounded by a transparent cladding material with a lower index of refraction. Light may be kept in the core by total internal reflection. Glass optical fibers may comprise silica, but some other materials such as fluorozirconate, fluoroaluminate and chalcogenide glasses, as well as crystalline materials such as sapphire, may be used. The light may be guided down the core of the optical fibers by an optical cladding with a lower refractive index that traps light in the core through total internal reflection. The cladding may be coated by a buffer and/or another coating(s) that protects it from moisture and/or physical damage. These coatings may be UV-cured urethane acrylate composite materials applied to the outside of the optical fiber during the drawing process. The coatings may protect the strands of glass fiber.

Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is in no way intended that any particular order be inferred. In addition, as used herein, the article "a" is intended to include one or more than one component or element, and is not intended to be construed as meaning only one.

Claim 1:
An optical communication cable (<NUM>) comprising:
a jacket (<NUM>) having an interior surface that defines a cable jacket internal cross-sectional area;
a plurality of optical fibers, wherein the plurality of optical fibers defines a plurality of optical fiber ribbons (<NUM>), wherein the plurality of optical fiber ribbons (<NUM>) is arranged in a plurality of ribbon stacks that define a plurality of subunits (<NUM>), and wherein each subunit (<NUM>) has a subunit jacket (<NUM>), wherein the plurality of optical fiber subunits (<NUM>) are unstranded, wherein less than <NUM>% of the cable jacket (<NUM>) internal cross-sectional area is occupied by the cross-sectional area of the plurality of optical fiber ribbons (<NUM>); and
a scaffolding structure (<NUM>) provided adjacent to and supporting the jacket (<NUM>) such that when the jacket (<NUM>) is subjected to a burn and melts, the melted jacket material bonds to the scaffolding structure (<NUM>) rather than sloughing off.