Fiber optic cable thermal protection device and method

[0001] The present invention generally relates to devices and methods for protecting fiber optic cables from damage or loss of performance due to changes in temperature.

[0002] Fiber optic cables generally include a fiber optic strand and one or more outerjackets. When the cable experiences temperature changes, one or more of the jackets can contract or expend at a different rate from the inner cable members, including the fiber strand. There is a need for devices and methods that reduce the effects of differential thermal expansion and contraction characteristics of the fiber optic cable.

[0003] In telecommunications networks installed in the field, it is known to have a large multi-fiber cable, such as an outside plant (OSP) cable, including multiple sub-units or cables within an outer jacket, with a strength member extending along with the sub-units. Each of these sub-units may include one or more optical fiber strands within an outer jacket. As the larger cable is extended throughout the area to which service is desired, at a plurality of fiber access enclosures, fiber strands in one or more of the sub-units with the larger cable may be extracted and spliced, or otherwise connected to a customer drop cable. The remaining sub-units within the larger cable continue on the route of the larger cable to be split off at a further fiber access enclosure. Those sub-units, which are passed through an enclosure with connecting to customer or other cables, are referred to as “expressed” sub-units.

[0004] Fiber access enclosures are often surface or pedestal mounted enclosures, subject to extremes of temperature due to the external environmental conditions. The materials in the sub-units, such as the fiber strands, the external jackets and potentially water blocking gel within the jacket about the fiber strands, may all react differently to exposure to these temperature extremes. For example, the jackets may contract more in cold temperatures than the relatively stable fiber strands. Differential contraction of the jacket with respect to the fiber strands can cause micro-bending of the fiber strands with the sub-units, leading to unacceptable attenuation losses with the fiber strands.

[0005] It is desirable to provide for the differential thermal expansion and contraction characteristics of the elements of the sub-units to be accommodated to reduce the level of attenuation induced by temperature extremes.

[0006] The present invention relates generally to a device and method to provide thermal protection to a fiber optic cable. More specifically, the present invention relates to a fiber optic cable with a cable outer jacket and at least one sub-unit within the outer jacket. The sub-unit includes at least one optical fiber and a sub-unit outer jacket. At least one sub-unit is removed from the cable outer jacket. A circumferential cut through the sub-unit outer jacket is included in the sub-unit removed from the cable outer jacket. A tube is positioned about the sub-unit extending on either side of the circumferential cut and a carrier positioned about the sub-unit on both ends of the tube.

[0007] The present invention also relates to a system for providing thermal protection to a fiber optic cable including a frame with a cable entry and a cable exit. A fiber optic cable extends from the cable entry to the cable exit and forms a loop. The cable includes a cable outer jacket and at least one sub-unit, and the at least one sub-unit is removed from the cable outer jacket along the loop. Each of the at least one sub-units includes an outer jacket and at least one optical fiber, and a circumferential cut through the outer jacket along the loop. A tube extends along a portion of the at least one sub-unit about the circumferential cut, and a carrier is positioned about the sub-unit about the tube.

[0008] The present invention further relates to a method of protecting a fiber optic cable including providing an optical fiber cable with a cable outer jacket and at least one sub-unit, each sub-unit including an outer jacket and at least one optical fiber. The optical fiber cable is extended into a loop. At least one sub-unit is removed from the cable outer jacket along the loop. A circumferential cut is made through the outer jacket of the at least one sub-unit along the loop. A tube is positioned about the at least one sub-unit over the circumferential cut, and the sub-unit and the tube about the sub-unit are placed within a carrier.

[0009] The present invention relates still further to a kit for providing thermal protection to a telecommunications cable including a carrier and a plurality of tubes. The carrier includes a base with opposing endplates. The endplates each include a plurality of slots for receiving the telecommunications cable. The tubes include a split from one end to an opposite end and are sized to be received about the telecommunications cable and between the endplates of the carrier.

[0021] Reference will now be made in detail to the exemplary aspects of the present invention that are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

[0022] Fiber optic distribution and access termination enclosures are well known and provide an environmentally protected field installation to connect distribution optical fibers with customer drop cables. Inside these enclosures are typically frames to which different pieces of equipment for routing, splicing and connecting cables. FIG. 1 shows a frame 10 which may be mounted to a base 11 positioned in ground 9 and also mounted within an enclosure 7, shown schematically in FIG. 1. A multi-strand optical fiber distribution cable 12 passes through a first cable access 14 and a second cable access 16 for routing through several cable routing features 34 of frame 10. Adjacent first cable access 14 is a first cable clamp 18, and adjacent second cable access 16 is a second cable clamp 20. Each of the cable clamps 18 and 20 include a strength member clamp 22 and an outer jacket clamp 24 and provide a secure anchor for cable 12 at its entry and exit from frame 10.

[0023] Cable 12 includes an outer jacket 26 and a plurality of sub-units 28 (additional details of cable 12 are described below in FIGS. 2 and 3). Sub-units 28 are broken out from outer jacket 26 and a loop 32 of sub-units 28 extends between cable clamps 18 and 20. Adjacent each cable clamp 18 and 20 is a cable thermal expansion protection device 30. Another device 30 is positioned along loop 32 at approximately the mid-point of the loop. Within frame 10, one or more of the sub-units 28 may be split out of cable 12 above cable clamp 18 to be spliced or connected to customer drop cables, or to branch out from the enclosure to service customers who are not conveniently located to cable 12. The remaining sub-units 28 are passed or expressed through cable clamp 20 and out cable access 16, to transit to other enclosures or equipment further downstream in the network. Loop 32 includes those expressed sub-units 28.

[0024] FIG. 2 shows a cross-section of cable 12, with outer jacket 26 about sub-units 28. Cable 12 is shown as an outside plant (OSP) cable although other types and styles of multi-strand optical fiber cables may also be used. A central strength member 36 extends through cable 12. To extend cable 12 through frame 10, cable 12 may be clamped at outer jacket clamp 24 of one of the cable clamps 18 or 20. A sufficient length of cable 12 is drawn up to form the desired length of loop 32 and cable 12 is clamped at outer jacket clamp 24 of the other cable clamp. Outer jacket 26 of cable 12 is removed between outer jacket clamps 24 to expose sub-units 28 and central strength member 36. Strength member 36 may be extended to and clamped by each of the strength member clamps 22 and trimmed so that it does not extend along loop 32. Any sub-units 28 being split out for splicing, connecting or branching within frame 10 are also removed from loop 32 and the remaining sub-units 28 are the expressed sub-units.

[0025] Frame 10 and the enclosure in which it is mounted may be installed at a location exposed to the environment, including temperature extremes. Within each sub-unit 28, as shown in FIG. 3, may be an outer jacket 38 and a plurality of sheathed optical fibers 40. Filling the space about fibers 40 may be a water blocking gel to prevent water infiltration and migration within sub-unit 28. Fibers 40 and outer jacket 38 may have different coefficients of thermal expansion, so that each will elongate or contract different amounts in response to the temperature extremes. Fibers 40 may typically be more thermally stable than jacket 38. Thus, to accommodate elongation of jacket 38, fibers 40 are loosely and slidably positioned within sub-unit 28. This may provide enough excess fiber length to account for the additional elongation of jacket 38 without excessively tensioning fibers 40. However, when jacket 38 contracts, the excess fiber length within sub-unit 28 is increased to the extent that micro-bending may occur, resulting in undesirable and potentially unacceptable attenuation losses to signals transmitted through the fiber.

[0026] Cable thermal protection device 30 permits the differential in thermal expansion and contraction of the elements of sub-units 28 to be compensated for to avoid excessive attenuation losses within fibers 40. Device 30 includes a carrier 44 through which the expressed sub-units extend through and each of the expressed sub-units is sheathed in a tube 46 within carrier 44. Carrier 44 includes a pair of opposing endplates 48 and 50 between which tubes 46 are captured. Carrier 44 also includes a base 52 that is mounted adjacent frame 10. A strap connector such as tie-wrap 54 extends about tubes 46 and sub-units 28, and through a loop 56 formed in base 52 to hold sub-units 28 and tubes 46 between endplates 48 and 50.

[0027] Tube 46 is illustrated in FIG. 5, and may include a split 58 extending the length of tube 46. At one end of tube 46, a chamfer 60 is formed on either side of split 58 to ease entry of sub-unit 28 into tube 46.

[0028] FIG. 6 shows tube 46 with an expressed sub-unit 28. Within tube 46, a circumferential or ring cut 62 is made through outer jacket 38 of expressed sub-unit 28, creating two intermediate ends 64 and 66 of outerjacket 38 on either side of ring cut 62. This circumferential cut may be made with known products, such as the Cable Stripper, part number 45-463, made by IDEAL, and utilizing existing field practices. The cut does not extend through any portion of fiber 40. As fiber 40 is slidable within outerjacket 38, the cut permits outer jacket 38 and fibers 40 to contract independent of each other. As shown in FIG. 6, outer jacket 38 has contracted, pulling intermediate ends 64 and 66 away from each other. Fibers 40, which contract less, are free to slide within outer jacket 38 and leave a portion of fibers 40 exposed between the intermediate ends. This allows outer jacket 38 to contract in response to environmental conditions without causing undesirable attenuation within fibers 40. Where outer jacket 38 has pulled back along cut 62, fibers 40 are left without protection from flexing or damage that is normally provided by outer jacket 38. This exposure may leave fibers susceptible to damage or unacceptably sharp bends. Tube 46 extends along outer jacket 38 across cut 62 to provide the support to fibers 40 exposed by cut 62.

[0029] It is also known that some materials used to make outer jacket 38 will adopt a permanent set, or contracted length, after repeated exposure to temperature changes. Cut 62 allows these permanent sets to be compensated for as well. Cut 62 will also expose water blocking gel 42 within each sub-unit 28, potentially allowing the gel to escape from outerjacket 38. A close fit between tube 46 and outer jacket 38 may serve to reduce the amount of leakage of gel 42. However, as loop 32 is formed in a sealed enclosure within which frame 10 is mounted, so that the chance of water infiltration is relatively minor and the escape of a portion of gel 42 is acceptable.

[0030] Several devices 30 may be mounted along loop 32 within frame 10, although the number of devices required may vary with the length and thermal characteristics of loop 32. Loop 32 may be short enough, or the difference in thermal coefficients of expansion may be small enough that only a single device 30 is required. Added length of loop 32, or greater differences in the thermal coefficients of expansion of outer jacket 38 and fibers 40 may call for additional devices 30 along loop 32.

[0031] Loop 32 is preferably sized so that loop 32 can by folded back on itself to lie within frame 10 so that an enclosure can be positioned about frame 10. In such a folded over configuration, the mid-loop mounted device 30 would lie adjacent one of the lower mounted cable routing features 34 on frame 10. It is anticipated that a ribbon cable or other forms of multi-strand fiber optic distribution cables may used with the thermal protection device shown herein.

[0032] FIGS. 7 to 10 show carrier 44 with endplates 48 and 50 and base 52. Endplates 48 and 50 each include a plurality of slots 68 for receiving sub-units 28. Slots 68 are deep enough to hold at least one sub-unit 28, and may be configured to hold more than one, depending on the number of sub-units being expressed through a particular frame 10. Slots 68 are also narrow enough to prevent passage of tube 46 along the length of any of the sub-units 28 within a slot 68. This will prevent any unwanted sliding or migration of tubes 46 from about cuts 62 in sub-units 28. Each slot 68 may include one or a pair of tapered entries 70 to aid in the positioning of sub-units within the slot. However, it may desirable for slots 68 to be configured with a uniform width, for example, if slots 68 are configured to hold two sub-units, or if endplates 48 and 50 were made smaller and the overall depth of any slot 68 was closer to the size of a sub-unit, or for other reasons. Loop 56 in base 52 defines an opening 72 to receive tie-wrap 54 or some other form of holddown or strap connector to secure sub-units to device 30. A pair of fastener openings 74 is included in base 52 to aid in mounting of device 30 to frame 10. It is also anticipated that an adhesive may be used to mount device 30.

[0033] Referring now to FIGS. 5 and 9, carrier 44 defines a width W between endplates 48 and 50. This width W is preferably closely matched and slightly greater than a length L of tube 46. Length L should be sufficiently long to cover any expected separation between intermediate ends 64 and 66 and extend from cut 62 along a portion of outerjacket 38 adjacent cut 62 to provide the desired support to any exposed fibers 40 of sub-unit 28. This desired length L may change due to the thermal characteristics of sub-units 28 and length of loop 32 but may not exceed width W. When the desired length L approaches width W, an additional thermal protective device 30 should be included in loop 32. For the anticipated length of loop 32 shown and described above, it is anticipated that three devices 30 will be desirable to provide adequate thermal and other protection to the fibers within sub-units 28. It is anticipated that the device 30 positioned mid-loop along loop 32 will not be fastened or mounted to frame 10 but will be attached only to sub-units 28, providing improved future access to frame 10 as loop 32 can be moved away from frame 10, as shown in FIG. 1.

[0034] The embodiments of the inventions disclosed herein have been discussed for the purpose of familiarizing the reader with novel aspects of the present invention. Although preferred embodiments have been shown and described, many changes, modifications, and substitutions may be made by one having skill in the art without unnecessarily departing from the spirit and scope of the present invention. Having described preferred aspects and embodiments of the present invention, modifications and equivalents of the disclosed concepts may readily occur to one skilled in the art. However, it is intended that such modifications and equivalents be included within the scope of the claims which are appended hereto.