This application claims the benefit of U.S. Provisional Application No. 61/174,059, filed Apr. 30, 2009, the entire contents of which are hereby incorporated by reference.
This application is related to U.S. App. No. 12/261,645, filed Oct. 30, 2008, issued as U.S. Pat. No. 7,702,203 on Apr. 20, 2010, to U.S. Prov. App. 61/168,005, filed Apr. 9, 2009, and to U.S. App. No. 12/748,925, filed Mar. 29, 2010, the entire contents of which are hereby incorporated by reference.
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The present disclosure relates to optical fiber assemblies, and in particular relates to fiber optic assemblies having dielectric armor.
Fiber optic cables and assemblies should preserve optical performance when deployed in the intended environment while also satisfying any other requirements for the environment. Indoor cables for riser and/or plenum spaces, for example, may require certain flame-retardant ratings as well as mechanical requirements. Mechanical characteristics such as crush performance, permissible bend radii, and temperature performance in part determine how installation and use of the cable in the installation space affect optical performance of the cable.
Certain conventional indoor riser applications use a fiber optic cable disposed within a metallic interlocking armor layer. “BX armor” or “Type AC” cables utilize such armors. BX armor is wound spirally about the fiber optic cable so that the edges of the adjacent wraps of armor mechanically interlock to form an armor layer. Interlocking armors are robust but expensive to install. In particular, the metallic armor must be grounded in order to meet electrical safety standards. FIG. 1 shows several prior art examples of interlocking armored cables 10 having a metallic (typically aluminum) armor layer 12. The metallic armor layer 12 must be grounded, for example, in order to comply with the National Electrical Code (NFPA 120) safety standard. Additionally, the metallic armor 12 can be plastically deformed (i.e., permanently deformed) under crush loads, which can pinch the cable and cause permanently elevated levels of optical attenuation that remain after the crush load is released.
Manufacturers have attempted to design dielectric armor cables to overcome the drawbacks of conventional metallic armor constructions. U.S. Pat. No. 7,064,276 discloses a dielectric armor cable having two synthetic resin layers where the hard resin layer has a continuous spiral groove cut completely through the hard resin layer along the length of the armor. The hard adjoining edge portions of the spiral groove abut to inhibit bending below a certain radius. However, one skilled in the art would recognize this design does not provide the craft with all of the desired features. Moreover, it can be difficult for the craft to recognize the cable of the '276 patent as an armored cable layered because it has a smooth outer surface, whereas conventional metal armored cables as depicted by FIG. 1 are easily identified by the craft.
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The disclosure is directed to armored fiber optic assemblies having a dielectric armor and methods for manufacturing cables having dielectric armor. The dielectric armor can have an armor profile resembling conventional metal armored cables. The dielectric armor provides crush and impact resistance to the optical fibers and/or fiber optic assembl(ies) therein. After being subjected to crush loads, the dielectric armor recovers to substantially recover or to wholly recover its original shape. The dielectric armor is also advantageous in that it provides desired mechanical performance without requiring the time and expense of grounding during installation.
According to another aspect of the present embodiments, the armored fiber optic assemblies can have suitable flame and/or smoke ratings for spaces such as plenum and riser applications.
According to yet another aspect of the present embodiments, an exterior cable jacket can be omitted from the armored fiber optic assemblies to reduce manufacturing time. The cost of materials used to form the assemblies is also reduced.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
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The accompanying drawings are included to provide further understanding of the invention. The drawings illustrate the various example embodiments of the invention and, together with the description, serve to explain the principals and operations of the invention.
FIG. 1 is a perspective view of three different prior art interlocking armor cables.
FIG. 2 is a side cut-away view of a first example embodiment of a fiber optic assembly having a dielectric armor.
FIG. 3 is a perspective view of the armored fiber optic assembly of FIG. 2 showing a partial longitudinal cross-section of the dielectric armor.
FIG. 4 is a partial cross-section of the armored fiber optic assembly of FIG. 2 taken along the line 4-4 showing only the armor and the core fiber optic assembly jacket.
FIG. 5 illustrates a test apparatus for applying crush loads to fiber optic assemblies.
FIG. 6 is a close-up view of the armored fiber optic assembly of FIG. 2 showing a partial longitudinal cross-section of the dielectric armor superimposed on a grid for reference of the shape of the armor.
FIG. 7 is an enlarged view of a portion of the dielectric armor further showing various dimensions associated therewith.
FIG. 8 is an enlarged perspective view of a portion of a generic armored profile showing the geometry of used for finite-element modeling of the dielectric armor.
FIG. 9 is a schematic diagram of an explanatory extrusion system for making dielectric armor.
FIG. 10 is a schematic cross-sectional view of the crosshead of the extrusion system of FIG. 9.
FIG. 11 is a schematic side view illustrating another method of forming dielectric armor.
FIG. 12 is a partial, cross-sectional view of another explanatory example of a crosshead wherein the profiling feature is within the crosshead die.
FIG. 13 is a side view of an example extrusion system wherein the profiling feature is located external to the crosshead and impresses the profile into the dielectric armor.
FIG. 14 is a perspective view of an example roller-type deforming member that is used to impress the armor profile into the dielectric armor.
FIG. 15 is a front view illustrating the use of two roller-type deforming members to impress the armor profile into the dielectric armor.
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Reference is now made in detail to exemplary embodiments of the invention, examples of 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 parts.
FIG. 2 is a side cut-away view of an armored fiber optic assembly 20 having at least one optical fiber disposed within a dielectric armor 50. FIG. 3 is an enlarged perspective view of the armored fiber optic assembly 20 with a portion of the dielectric armor 50 cut away. The dielectric armor 50 is non-conductive and has an outer surface 52 having an armor profile 54 generally formed in a spiral along a longitudinal axis. An inner surface 56 of the armor 50 may also have an armor profile. As used herein, “armor profile” means that the surface has an undulating surface along its length that looks similar to conventional metal armors (e.g., a undulating shape along the length of the armor). The armor profile could, for example, also be formed by a series of spaced rings connected by depressed regions. The dielectric armor 50 is advantageous in that it both provides crush resistance and recovers to assume its original shape when subjected to crush loads. The dielectric armor 50 may also meet specified flame and/or smoke ratings, and does not require grounding.
The dielectric armor 50 is illustrated as comprising a single layer of dielectric material, with the outer surface 52 of the armor 50 being the outer surface of the assembly 20. The cost in manufacturing and materials for the assembly are accordingly low in that an exterior jacket is not required to cover the armor 50. The armor 50 may be made from a rigid or a semi-rigid material. As used herein, “rigid material” means the material has a Shore D hardness of about 65 or greater. A “semi-rigid material” has a Shore D hardness of about 55 to 64. A “non-rigid material” has a Shore D hardness of less than about 54. In some cases, materials of Shore D hardness less than 54 may be used to address issues with brittleness, flexibility, and other factors.
Still referring to FIG. 2, an inner or core fiber optic assembly 80 is housed within and protected by the dielectric armor 50. In the illustrated embodiment, the fiber optic assembly 80 is a fiber optic cable having a cable jacket 90 with an outer surface 92 and a plurality of tight-buffered optical fibers 94 extending longitudinally through the assembly 20 within the cable jacket 90. Strength elements 98, such as aramid fibers, also extend longitudinally through the interior of the cable jacket 90. The cable jacket 90 has a generally circular annular cross-section with a smooth, arcuate outer surface 92. The outer surface 92 is adjacent to the inner surface 56 of the armor 50 and contacts the inner surface 56 at various points along the length of the assembly 20. In one embodiment, the cable jacket 90 can be omitted.
By way of example, the core fiber optic assembly 80 may be a stranded tube cable, a monotube cable, a micromodule cable, a slotted core cable, loose fibers, tube assemblies, or the like. Additionally, fiber optic assemblies according to the present embodiments can include any suitable components such as water-blocking and/or water-swelling components, flame-retardant components such as tapes, coatings, or other suitable components. The fiber optic assembly 80 may have any suitable fiber count such as 6, 12 or 24-fiber MIC® cables available from Corning Cable Systems of Hickory, N.C..
In the illustrated embodiment, the armor 50 has a “continuous annular cross-section”. As used herein, “continuous annular cross-section” means there are no spiral grooves, openings, cuts or slits that extend entirely through (i.e., from the inner surface 56 to the outer surface 52) the armor 50. The illustrated armor 50 also comprises a single, continuous and uniform layer.
FIG. 4 is a partial cross-sectional view of the armored fiber optic assembly 20 of FIG. 2 taken along the line 4-4. In FIG. 4, the optical fibers 94 and the strength members 98 are omitted so that certain dimensions of the assembly 20 can be illustrated. For simplicity in illustration, the dielectric armor 50 is depicted with a uniform circular cross-section that does not reflect the spiral undulations of the armor profile.
As shown in FIG. 4, the core fiber optic assembly 80 has an outer radius RC and the dielectric armor 50 has an average inner radius RI. The inner surface 56 of the exemplary dielectric armor 50 also has an undulating armor profile, so there will be no true radius of the inner portion of the armor. The assembly 20 can include a free space 100 disposed between the outer surface of the fiber optic assembly 80 and the inner surface of the dielectric armor 50 generally represented by a separation ΔR. While the separation ΔR between the cable jacket 90 outer surface 92 and the armor 50 inner surface 56 is shown as uniform around the jacket circumference, it will in fact vary along the length of the fiber optic assembly 20, and the cable jacket 90 and the armor 50 will actually contact one another at numerous points. An average or median separation ΔR can therefore be calculated as ΔR=RI−RC. The presence of the free space 100 improves optical performance during crush events and the like as discussed below. By way of example, the average free space separation ΔR is typically about 2 millimeters or less, but free space separation ΔR values larger than 2 millimeters are possible. In one embodiment, the free space separation ΔR is between 0.1-1.0 millimeters. In a second embodiment, the free space separation ΔR is in the range of 0.2-0.8 millimeter. In the illustrated embodiment, the free space separation is about 0.5 millimeter.
Mechanical characteristics used in designing the armored fiber optic assembly 20 include minimum bend radius, impact resistance, crush-resistance, tensile strength, durability of the dielectric armor, susceptibility to plastic deformation, the ability to recover from crush loads, etc. Material characteristics such as the hardness, modulus, etc. along with geometry influence the desired characteristics/optical performance for the armored fiber optic assembly 20. For instance, the armor 50 should have a suitable modulus of elasticity. By way of example, a modulus of elasticity at 1% strain for the armor 50 is about 1200 MPa or greater.
A fiber optic assembly as illustrated in FIG. 2 has an overall average outside diameter of about 9.2 mm, allowing for some ovality in the cross-section, an average armor 50 thickness in the range of about 1.1-1.3 mm, a cable jacket 90 thickness of about 0.8 mm, an assembly 80 outside diameter of about 6.3 mm, and a median separation ΔR in the range of about 0.25-0.5 mm. The core assembly cable jacket 90 is made from NAP 16881, and the armor 50 is made from Teknor Apex flame retarded rigid PVC available under the designation FG RE 8015B. The fiber optic assembly 80 includes 12 optical fibers of flame retarded tight-buffered fibers. The armored fiber optic assembly 20 has a weight of about 73.3 kilogram per kilometer, with the fiber optic assembly 80 accounting for about 32.2 kilogram per kilometer, and the armor 50 of the armor 50 accounting for about 41.1 kilogram per kilometer.
A fiber optic assembly as illustrated in FIG. 2 has an overall average outside diameter of about 10.6 mm, allowing for some ovality in the cross-section, an average armor 50 thickness in the range of about 2.2-2.5 mm, a cable jacket 90 thickness of about 0.5 mm, a core assembly 80 outside diameter of about 5.2 mm, and a median separation ΔR in the range of about 0.25-0.5 mm. The core assembly cable jacket 90 is made from NAP 16881, and the armor 50 is made from flame retarded semi-rigid PVC available from AlphaGary Corporation under the designation GW 2052 S. The fiber optic assembly 80 includes 12 optical fibers of flame retarded tight-buffered fibers. The armored fiber optic assembly 20 has a weight of about 99 kilogram per kilometer, with the fiber optic assembly 80 accounting for about 23.1 kilogram per kilometer, and the armor 50 of the armor 50 accounting for about 75.9 kilogram per kilometer.
One mechanical property provided by the dielectric armor 50 is its resistance to crush under loads. FIG. 5 illustrates the fiber optic assembly 20 under crush load testing in a test apparatus 200. The test apparatus 200 includes two rigid plates 202, 204 of 10 centimeter length LP in FIG. 5. The plates 202, 204 are configured to exert compressive loads at a mid-span section of a cable. Edges of the plates 202, 204 can be rounded so that the plates do not cut into the surface of the assembly 20. The test apparatus 200 can be used to test, for example, the ability of the fiber optic assembly 20 to recover its original shape after being subjected to crush loads. While the load required to deflect the dielectric armor 50 generally is lower than metallic BX-type armors, the deformation is not as severe, and most or all of the attenuation in optical signals conveyed by the assembly 20 is relieved after removing the test load. By contrast, metallic armors deform plastically, so that they may recover little, if at all, after removing a test load. The elastic properties of the rigid dielectric material of the armor 50 allow the assembly 20 to recover generally to its original shape after crush or impact.
For PVC materials, such as Teknor Apex materials FG RE 8015A, 8015B and 8015D, and AlphaGary GW 2052 S, the elastic region along the stress/strain curve defines where the dielectric armor will return to its original shape. The elastic deformation region of the dielectric armor 50 is defined on a stress/strain curve generated from a flexural modulus test. If the elastic region is exceeded, the dielectric armor 50 yields (or plastically deforms) 180 degrees apart and may recover to an oval shape. According to one aspect of the present embodiments, the dielectric armor cable 50 has superior resistance to crush loads. According to a further aspect, even if a crush rating, such as ICEA S-83-596-2001, is exceeded, the fiber optic assembly 20 significantly or substantially wholly recovers its original shape after removal of the crush load. ICEA S-83-596-2001 covers fiber optic communications cables intended for use in buildings. Cables according to the present embodiments can also be designed to recover after testing under ICEA S-104-696, which covers fiber optic communications cables intended for indoor and outdoor use, and testing under ICEA S-87-640, which covers fiber optic communications cables intended for outdoor use.
Crush testing may cause unacceptable optical attenuation in the optical fibers 94. According to another aspect of the present embodiments, under the described test conditions, assuming none of the optical fibers 94 are damaged, attenuation caused by the crush load is relieved when the crush load is removed. By contrast, if a BX cable crush/impact rating is exceeded and the armor plastically deforms, the cable typically remains pinched resulting in a permanent attenuation step in the cable.
Fiber optic assemblies may be subjected to crush testing under extremely high loads in an apparatus as generally depicted in FIG. 5. The test procedure involves applying a crush test load (Newtons) over an axial length LP of 10 centimeters. Several different Locations along the length of the two assemblies are crush tested. The opposed plates 202, 204 apply crush loads in the “z” or “crush” direction, which is aligned with an initial, pre-crush outside diameter at each location of the assembly. Average, pre-crush outside diameters of the tested cable assemblies may be used for comparison purposes because the assemblies may have some degree of ovality in cross-section. The test begins by advancing the plates 202, 204 together in the z-direction to apply an initial crush load to the assemblies. The initial load compresses the assembly to an initial crush height. The height of the compressed assembly is assumed to be the spacing of the plates 202, 204 during crush testing. The initial crush load is applied to generally align peaks on the assembly armor profile between the plates 202, 204. The crush load is then increased to a Maximum Force (Newtons). The Maximum Force may correspond to any desired peak value. At Maximum Force, the assembly is pressed between the plates 202, 204 at a Plate Spacing at Maximum Crush (mm). The assembly is held at that load for 10 minutes. Percent of Outside Diameter at Crush percentages can be calculated as the Plate Spacing at Maximum Crush values divided by the pre-crush Outside Diameter of the assembly. This calculation indicates the degree to which the assembly is crushed from its pre-crush state. The test crush load is then released and the assembly allowed to recover for five minutes. The Cable Dimension After Recovery, now reduced in height from the original Outside Diameter, is measured in the crush or z-direction. Percent of Outside Diameter After Recovery percentages can be calculated to reflect the Cable Dimension After Recovery values divided by the pre-crush Outside Diameter of the assembly. According to one aspect of the present embodiments, rigid PVCs such as those used to form the armor 50 give substantial recovery to assemblies even after severe compression.
Those skilled in the art will appreciate the difficulty in satisfying the required mechanical, low-smoke, and/or flame-retardant characteristics etc. for armored fiber optic assemblies. The NFPA 262 plenum burn rating is especially stringent. The large combustible polymer mass of the armored fiber optic assemblies renders it difficult to meet both mechanical and flame/smoke requirements. Preferred mechanical and burn characteristics for the armor 50 are listed below in Table A.
Preferred Properties for Rigid Armor Layer