Buffer tubes for mid-span storage

1 BUFFER TUBES FOR MID-SPAN STORAGE

CROSS-REFERENCE TO PRIORITY APPLICATIONS

This application claims the benefit of the following commonly assigned applications: U.S. Provisional Application No. 61/096,545 (filed Sep. 12, 2008); U.S. Provisional Application No. 61/096,750 (filed Sep. 12, 2008); U.S. Provisional Application No. 61/113,146 (filed Nov. 10, 2008); and U.S. Provisional Application No. 61/139,228 (Dec. 19, 2008). Each of the foregoing patent applications is hereby incorporated by reference in its entirety.

FIELD OF INVENTION

The present invention relates to optical-fiber cables and buffer tubes. The present invention, for instance, embraces buffer tubes that are particularly suitable for mid-span deployments.

BACKGROUND OF THE INVENTION

As compared with traditional wire-based networks, optical fiber communication networks are capable of transmitting significantly more infonnation at significantly higher speeds. Optical fibers, therefore, are being increasingly employed in communication networks.

To expand total transmission throughput, optical-fiber network providers are attempting to place ever more optical fibers in ever-smaller spaces. Packing fibers into tight spaces, however, can cause undesirable attenuation. Indeed, there is an inherent trade-off between increased fiber density and signal attenuation.

Many optical-fiber cables designed for installation in microducts (e.g., via blowing) achieve high fiber counts and relatively small cable diameters. For example, commonly assigned U.S. Pat. No. 6,912,347 (Rossi et al.), which is hereby incorporated by reference in its entirety, achieves optical-fiber cables with high fiber counts and small cable diameters.

Such optical-fiber cables can achieve higher fiber densities, but the constituent buffer tubes have unsatisfactory mid-span storage perfonnance as positioned in pedestals, cabinets, or other optical-fiber enclosures. By way of illustration, after installation in a microduct, an optical-fiber cable typically experiences temperature cycles during use. These temperature cycles can lead to signal attenuation. Thus, a fiber-optic cable that is less susceptible to attenuation is more suitable for such installations (e.g., installations requiring mid-span storage).

Similarly, U.S. Patent Publication No. 2007/0274647 A1 (Pizzomo et al.), now U.S. Pat. No. 7,373,057, each ofwhich is hereby incorporated by reference in its entirety, discloses an optical-fiber cable structure suitable for microduct installation. This publication requires the use of bend-insensitive fibers to reduce cable size.

Despite efforts to achieve liigh-fiber-density optical-fiber cables that can be installed in microducts, a need continues to exist for improved optical-fiber cables and buffer tubes that not only possess desirable optical fiber densities, but also are capable of satisfactory mid-span storage.

SUMMARY OF THE INVENTION

In general, as buffer-tube filling coeflicients increase so too does the problem of attenuation during mid-span storage. At

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higher filling coeflicients, buffer tubes must have better dimensional stability (e.g., reduced post-extrusion shrinkage) to provide satisfactory mid-span perfonnance.

In this regard, post-extrusion shrinkage (PES) is often a contributing factor to attenuation experienced during midspan storage of optical-fiber buffer tubes and cables. In particular, post-extrusion shrinkage, which can occur when a buffer tube is subjected to elevated temperatures, may cause unwanted increases in excess fiber length (EFL).

Accordingly, it is an object of the present invention to provide optical-fiber buffer tubes possessing a higher filling coeflicient while ensuring that the optical fibers enclosed therein demonstrate improved attenuation perfonnance when subjected to temperature variations (e.g., between -40° C. and 70° C.).

It is another object of the present invention to provide optical-fiber buffer tubes that include standard single-mode fibers (SSMF) and that have a higher filling coeflicient.

It is yet another object of the present invention to provide optical-fiber buffer tubes that include standard single-mode fibers (SSMF) configured in a way that promotes low attenuation.

It is yet another object of the present invention to provide optical-fiber buffer tubes that include standard single-mode fibers (SSMF) and that are capable of satisfactory mid-span storage over a wide range of temperatures.

It is yet another object of the present invention to provide optical-fiber buffer tubes that meet the mid-span standard found in Bulletin 1753F-601 (PE-90) from the United States Department of Agriculture Rural Electrification Admir1istration.

It is yet another object of the present invention to provide optical-fiber buffer tubes that meet the mid-span standard defined by Telcordia Technologies generic requirements for optical-fiber cables as set forth in GR-20-CORE (Issue 2, July 1998; Issue 3, May 2008; Mid-Span Buffer Tube Performance of Stranded Cable—6.5.11).

It is yet another object of the present invention to provide optical-fiber buffer tubes that exhibit low post-extrusion shrinkage.

It is yet another object of the present invention to provide optical-fiber buffer tubes fonned of a polyolefin, such as a nucleated polypropylene-polyethylene copolymer.

The foregoing, as well as other objectives and advantages of the invention and the manner in which the same are accomplished, is further specified within the following detailed description and its accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically illustrates shrinkage values observed for a nucleated polypropylene-ethylene copolymer material as compared with polyvinyl chloride (PVC) and polyethylene materials.

FIG. 2 graphically illustrates coeflicient of thennal expansion (CTE) values for a nucleated polypropylene copolymer as compared with polyvinyl chloride (PVC) and polyethylene.

FIG. 3 graphically illustrates total shrinkage measurements for a nucleated polypropylene copolymer and an alternative material when cycled from about room temperature to about -40° C. to about +85° C. to about -40° C.

FIG. 4 schematically depicts a cross-sectional view of an exemplary embodiment of an optical-fiber cable employing buffer tubes according to the present invention.

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FIG. 5 schematically depicts a perspective, cross-sectional view of an exemplary embodiment of an optical-fiber cable employing buffer tubes according to the present invention.

DETAILED DESCRIPTION

As noted, the invention embraces an optical-fiber buffer tube having a higher fiber count and yet suitable for deployments requiring mid-span access.

An exemplary optical-fiber cable includes one or more (e.g., six or so) buffer tubes according to the present invention positioned within a cable jacket (e.g., a polymeric sheath). At least one of the buffer tubes includes a plurality of optical fibers (e.g., 12-24 optical fibers). More typically, each of the buffer tubes positioned within the cable jacket include optical fibers (e.g., six buffer tubes each enclosing 24 optical fibers).

The optical fibers employed in the buffer tubes according to the present invention are typically conventional standard single-mode fibers (SSMF) possessing diameters of between about 235 microns and 265 microns (i.e., the combined diameter of the glass fiber and its coatings). That said, it is within the scope of the invention to employ optical fibers having smaller diameters (e.g., 200 microns or so). Suitable singlemode optical fibers that are compliant with the ITU-T G.652.D standard are commercially available, for instance, from Draka Comteq (Claremont, N.C.). The respective ITU-T G.652 standards are hereby incorporated by reference in their entirety.

Optical-fiber cables in accordance with the present invention meet or exceed certain Telcordia Technologies generic requirements for optical-fiber cables as set forth in GR-20CORE (Issue 2, July 1998; Issue 3, May 2008), such as low-temperature and high-temperature cable bend (6.5.3), impact resistance (6.5.4), compression (6.5.5), tensile strength of cable (6.5.6), cable twist (6.5.7), cable cyclic flexing (6.5.8), mid-span buffer tube performance of stranded cable (6.5.11), temperature cycling (6.6.3), cable aging (6.6.4), cable freezing (6.6.5), and water penetration (6.6.7). These GR-20-CORE generic requirements (i.e., Issue 2, July 1998, and Issue 3, May 2008, respectively) are hereby incorporated by reference in their entirety.

Moreover, the optical-fiber cables and buffer tubes according to the present invention possess outstanding performance when subjected to extreme temperature variations. In this regard, the present optical-fiber buffer tubes demonstrate exceptional resistance to attenuation as determined by temperature cycle testing. For example, under testing conditions modified from the U.S. Department of Agriculture’s Bulletin 1753F-601 (PE-90) (Rural Electrification Administration), the present optical-fiber buffer tubes demonstrate mean increases in optical-fiber attenuation of less than 0.05 dB. Furthennore, each optical fiber that is enclosed within a buffer tube typically demonstrates increased optical-fiber attenuation of less than 0.1 dB.

To achieve low optical-fiber attenuation that satisfies midspan temperature cycle testing, it may be beneficial to employ buffer tubes with a low filling coefficient (i.e., the buffer tubes within the cabling should not be overfilled). Alternatively, it may be beneficial to employ low-shrink buffer tubes having higher filling coeflicients.

Though prior fiber-optic cabling has employed buffer tubes having relatively low filling coefficients, the resulting fiberoptic cables have possessed relatively low cable fiber densities. Conversely, efforts to increase fiber densities within optical-fiber cables have heretofore required the use of buffer tubes with higher filling coeflicients (i.e., increasing the fiber

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count within a buffer tube of a given cross-sectional area), resulting in buffer-tubes with poor mid-span performance.

The optical-fiber cables of the present invention improve upon such past efforts by achieving relatively high cable filling coefficients (and cable fiber densities), while employing buffer tubes with exceptional mid-span perfonnance. In other words, the optical fibers employed in buffer tubes in accordance with the present invention demonstrate exceptional performance when subjected to mid- span temperature cycling testing.

As used herein, the term “cable filling coeflicient” of an optical-fiber cable refers to the ratio of the sum of the crosssectional areas of all of the optical fibers within the opticalfiber cable versus the inner cross-sectional area of the opticalfiber cable (i.e., defined by the inner boundary of the protective outer jacket). As used herein and unless otherwise noted, the term “cable filling coeflicient” employs the inner cross-sectional area of the optical-fiber cable.

Conversely, the term “outer cable filling coeflicient” specifically refers to the ratio of the sum of the cross-sectional areas of all of the optical fibers within the optical-fiber cable versus the outer cross-sectional area of the optical-fiber cable (i.e., defined by the outer boundary of the protective outer jacket).

Optical-fiber cables of the present invention have relatively high cable fill ratios (i.e., cable filling coeflicients). Opticalfiber cables having higher fill ratios are desirable because they increase the amount of information that can be passed through a cable while decreasing the amount of space that the optical-fiber cable requires for installation.

As used herein, the tenn “cable fiber density” of an opticalfiber cable refers to the ratio of the total number of optical fibers within the optical-fiber cable versus the cross-sectional area of the optical-fiber cable as defined by the outer boundary of the protective outer jacket. Optical-fiber cables of the present invention have relatively high cable fiber densities. Optical-fiber cables having higher cable fiber densities are desired, because such high-fiber-density cables have an increased number of optical fibers and/or require less space for installation.

As used herein, the term “buffer-tube filling coeflicient” refers to the ratio of the total cross-sectional area of the fibers within a buffer tube versus the inner cross-sectional area of that buffer tube (i.e., defined by the inner boundary of the buffer tube). Optical-fiber cables of the present invention include buffer tubes typically having a buffer-tube filling coeflicient between about 0.2 and 0.6, such as between about 0.3 and 0.4 (e.g., about 0.33).

Altematively, to the extent that non-circular buffer tubes are used, the longest inner cross-sectional width of the buffer tube can be used to define the diameter of a theoretical circularized buffer tube cross-sectional area (e.g., J'c(Dma,j2)2). As used herein, the tenn “circularized buffer-tube filling coeflicient” refers to the ratio of the total cross-sectional area of the optical fibers enclosed within buffer tubes versus the sum of the theoretical circularized cross-sectional areas of the buffer tubes containing those optical fibers.

The circularized buffer-tube filling coeflicient is one convenient way to characterize the cross-sectional area of an irregularly shaped buffer tube. Those having ordinary skill in the art will appreciate that for all but circular buffer tubes, the buffer-tube filling coeflicient and the circularized buffer-tube filling coeflicient are unequal.

Additionally, as used herein, the term “cumulative buffertube filling coeflicient” refers to the ratio of the total crosssectional area of the optical fibers enclosed within buffer

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tubes versus the sum of the inner cross-sectional areas of the buffer tubes containing those optical fibers.

Generally, optical-fiber cables with higher buffer-tube filling coefficients are more susceptible to attenuation over the length of the optical-fiber cable. Cables containing buffer tubes having a lower buffer-tube filling coeflicient are typically less susceptible to attenuation when subjected to temperature cycling. That said, the present invention embraces low-shrink buffer tubes having higher buffer-tube filling coeflicients. Therefore, the optical-fiber buffer tubes of the present invention are less susceptible to attenuation and are suitable for mid-span storage.

The present optical-fiber buffer tubes are suitable for midspan storage even in extreme temperatures. In this regard, fiber-optic cables suitable for mid-span storage are typically subjected to a mid-span temperature cycle test, which assures certain minimum performance specifications for fiber-optic cables. As noted, one such test can be found in Bulletin 1753F-601 (PE-90) from the United States Department of Agriculture Rural Electrification Administration, which is hereby incorporated by reference in its entirety, an excerpt of which is provided as Appendix I in priority U.S. Patent Application Nos. 61/096,750 and 61/113,146.

As used herein and unless otherwise specified, reference to the “mid-span test,” the “mid-span temperature cycle test,” or the “temperature cycle test” refers the testing procedures set forth in the USDA Rural Electrification Administration midspan standard, which is outlined as follows:

According to the USDA Rural Electrification Administration mid-span standard, buried and underground loose tube single-mode cables intended for mid-span applications with tube storage should meet the following mid-span test without exhibiting an increase in fiber attenuation greater than 0.1 dB and a maximum average increase over all fibers of 0.05 dB.

Initially, the test section of the optical-fiber cable is installed in a commercially available pedestal or enclosure (or in a device that mimics their perfonnance) as follows: A length of the protective outer jacket, equal to the mid-span length (e.g. 20 feet), is removed from the middle of the test specimen to allow access to the buffer tubes. All binders, tapes, strength members, etc. are removed. The buffer tubes are left intact. The cable ends defining the ends of the midspan length are properly secured in the enclosure (i.e., as they would be secured within an enclosure in regular commercial use). The strength members are secured with an end stop type clamp and the protective outer jacket is clamped to prevent slippage. A minimum of 6.096 meters (20 feet) of cable extends from the entry and exit ports of the enclosure (i.e., 20 feet of the cable remain outside of the enclosure), so that optical measurements may be taken. Typically, the buffer tubes are wound in a coil with a minimum width of three (3) inches and minimum length of 12 inches. The exposed buffer tubes are loosely constrained during the test.

The enclosure, with installed cable, is placed in an enviromnental chamber for temperature cycling. It is acceptable for some or all of the two 20-foot (6.096 meters) cable segments (i.e., the cable segments that remain outside of the enclosure) to extend outside the enviromnental chamber.

Lids, pedestal enclosures, or closure covers should be removed if possible to allow for temperature equilibrium of the buffer tubes.

The attenuation of the optical fibers is measured at 1550110 nanometers. The supplier of the optical-fiber cable must certify that the perfonnance of lower specified wavelengths complies with the mid-span perfonnance requirements.

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After measuring the attenuation of the optical fibers, the cable is tested per the FOTP-3 temperature-cycling standard. Temperature cycling, measurements, and data reporting must conform to the FOTP-3 standard. The test is conducted for at least five complete cycles. The following detailed test conditions are applied (i.e., using the enviromnental chamber) to the enclosure containing the optical-fiber cable: (A) loose tube single-mode optical cable sample shall be tested; (B) an 8-inch to 12-inch diameter optical buried distribution pedestal or a device that mimics their perfonnance shall be tested; (C) mid-span opening for installation of loose tube singlemode optical cable in pedestal shall be 6.096 meters (20 feet); (D) three hours soak time (i.e., exposure time); (E) Test Condition C-2, minimum —40° C. (—40° F.) and maximum 70° C. (158° F.); (F) a statistically representative amount of transmitting fibers in all express buffer tubes passing through the pedestal and stored shall be measured; and (G) the buffer tubes in the enclosure or pedestal shall not be handled or moved during temperature cycling or attenuation measurements.

Fiber cable attenuation measured through the exposed buffer tubes during the last cycle at —40° C. (—40° F.) and +70° C. (158° F.) should not exceed a maximum increase of 0.1 dB and should not exceed a 0.05 dB average across all tested fibers from the initial baseline measurements. At the conclusion of the temperature cycling, the maximum attenuation increase at 23° C. from the initial baseline measurement should not exceed 0.05 dB in order to allow for measurement noise that may be encountered during the test. The cable should also be inspected at room temperature at the conclusion of all measurements. The cable should not show visible evidence of fracture of the buffer tubes nor show any degradation of the exposed cable assemblies.

Additionally, optical-fiber buffer tubes according to the present invention have undergone the mid-span temperature cycle test described (above) and fulfilled the minimum performance specification of the USDA Rural Electrification Administration mid-span standard. Additionally, these buffer-tube embodiments were subjected to harsher conditions than required by the USDA Rural Electrification Administration mid- span standard. For example, buffer-tube embodiments of the present invention were soaked at 70° C. for 14 hours, which is longer than the three hours required by the testing conditions set forth in the aforementioned USDA bulletin. Typically, under these harsher conditions only one temperature cycle is perfonned.

In this regard, it has been observed that attenuation for the initial temperature cycle tends to be higher than for subsequent temperature cycles. This counterintuitive observation means that testing over one cycle yields higher tested attenuation levels than testing over multiple temperature cycles (e.g., five or more as set forth in the USDA Rural Electrification Administration mid-span standard).

This modified mid-span standard is hereinafter referred to as the “modified USDA Rural Electrification Administration mid-span standard.” A longer soak time (i.e., exposure time) may alter the defonnation of the buffer tubes because of post-extrusion shrinkage differences at this temperature (i.e., the buffer tubes may shrink in length because of the amorphous orientation generated during the extrusion process and/ or crystallization).

Buffer-tube embodiments of the present invention also passed a mid-span temperature cycle test with conditions similar to FOTP-3 with the exception that the soak time at —40° C. was reduced from three hours to one hour. This change of conditions probably did not affect the results of the test because the change in dimensions of the buffer tubes at