Abstract:
A bend-insensitive glass fiber with a novel coating system yields exceptionally low losses. The coating system features (i) a softer primary coating with excellent low-temperature characteristics to protect against micro-bending in any environment and in the toughest physical situations and, optionally, (ii) a colored secondary coating possessing enhanced color strength and vividness. The secondary coating provides improved ribbon characteristics for structures that are robust, yet easily entered (i.e., separated and stripped). The optional dual coating is specifically balanced for superior heat stripping in fiber ribbons, with virtually no residue left behind on the glass. This facilitates fast splicing and terminations. The improved coating system provides optical fibers that offer significant advantages for deployment in most, if not all, fiber-to-the-premises (FTTx) systems.

Description:
CROSS-REFERENCE TO PRIORITY APPLICATIONS 
     This U.S. nonprovisional application hereby claims the benefit of U.S. patent application Ser. No. 60/986,737 for a Microbend-Resistant Optical Fiber (filed Nov. 9, 2007), U.S. patent application Ser. No. 61/041,484 (filed Apr. 1, 2008) for a Microbend-Resistant Optical Fiber, and U.S. patent application Ser. No. 61/112,595 for a Microbend-Resistant Optical Fiber (filed Nov. 7, 2008), each of which is incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention embraces optical fibers possessing an improved coating system that reduces stress-induced microbending. The present invention further embraces the deployment of such optical fibers in various structures, such as buffer tubes and cables. 
     BACKGROUND OF THE INVENTION 
     Fiber to the premises/business/home (i.e., FTTx) provides broadband data transfer technology to the individual end-user. FTTx installations, which are being increasingly deployed throughout the world, are making use of innovative, reduced-cost system designs to promote the spread of the technology. For example, fiber may be delivered in the last link by way of a microcable. Air-blown fibers provide another efficient model for delivering the link to the end-use terminus. There continues to be industry-wide focus on modes of deployment that overcome economic obstacles that impede fiber-based broadband solutions for data transmission to businesses and residences. 
     Cost-effectiveness is important, of course, for achieving successful FTTx systems. Reduced size for cables, drops, and structures for blowing are often critical, too. Installation of conduits suitable for traditional cable designs is often prohibitive in existing infrastructure. Thus, existing small ducts or tight pathways have to be used for new fiber installations. Low-cost and reduced-size requirements are driving in a direction that reduces protection for the optical fibers (i.e., away from conventionally robust, more bulky cable designs). 
     Glass designs are now available that offer reduced sensitivity to small bending radius (i.e., decreased added attenuation due to the phenomenon known as macrobending). These include trench-assisted core design or void-assisted fibers. Glass designs with lower mode field diameter are less sensitive to macrobending effects, but are not compatible with the G.652 SMF standard. Single-mode optical fibers that are compliant with the ITU-T G.652.D requirements are commercially available, for instance, from Drake Comteq (Claremont, N.C.). 
     Microbending is another phenomenon that induces added loss in fiber signal strength. Microbending is induced when small stresses are applied along the length of an optical fiber, perturbing the optical path through microscopically small deflections in the core. 
     In this regard, U.S. Pat. No. 7,272,289 (Bickham et al.), which is hereby incorporated by reference in its entirety, proposes an optical fiber having low macrobend and microbend losses. U.S. Pat. No. 7,272,289 broadly discloses an optical fiber possessing (i) a primary coating having a Young&#39;s modulus of less than 1.0 MPa and a glass transition temperature of less than −25° C. and (ii) a secondary coating having a Young&#39;s modulus of greater than 1,200 MPa. 
     Nonetheless, better protection against microbending is still needed to help ensure successful deployment in more FTTx applications. To this end, it is necessary to discover and implement new coating systems that better address the demands FTTx installations place on fiber and cable structures in a way that is commercially practical (i.e., cost-effective). 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an optical fiber having an improved coating system that provides improved protection against stress-induced microbending. 
     It is another object to provide an improved coating system that can be readily mated with bend-insensitive optical fiber, as well as G.652-compliant fiber. 
     It is yet another object to provide an improved optical fiber coating system including a primary coating that possesses a low modulus to provide enhanced cushioning against lateral and axial stresses induced by external forces. 
     It is yet another object to provide an improved optical fiber coating system including a primary coating that possesses an exceptionally low glass transition temperature (T g ) that reduces temperature-induced stresses in unusually cold environments. 
     It is yet another object to provide an improved optical fiber coating system including a primary coating that possesses an improved curing rate. 
     It is yet another object to provide an improved optical fiber coating system including an ink-free secondary coating that has improved brightness and visibility. 
     It is yet another object to provide an improved optical fiber coating system that can be applied at commercial processing speeds (e.g., forming the primary coating at rates of at least about 20 meters per second). 
     It is yet another object to provide an optical fiber possessing coatings that are readily stripped. 
     It is yet another object to provide an optical fiber having enhanced performance characteristics for use in FTTx installations in which conventional, robust cable designs are impractical. 
     It is yet another object to provide an optical fiber that synergistically combines a bend-insensitive glass fiber (e.g., Draka Comteq&#39;s single-mode glass fibers available under the trade name BendBright XS ®) with the coating according to the present invention (e.g., Draka Comteq&#39;s ColorLock XS  brand coating system). 
     It is yet another object to provide an optical fiber that can be advantageously deployed in buffer tubes and/or fiber optic cables. 
     It is yet another object to provide an optical fiber that requires less external protection (e.g., enclosed within thinner buffer tubes and/or cable jacketing). 
     It is yet another object to provide a bend-insensitive optical fiber possessing a reduced diameter (e.g., having thinner coating layers). 
     It is yet another object to provide an optical fiber that can be installed in a way that employs small-radius bends. 
     It is yet another object to provide an optical fiber that facilitates direct installation onto buildings or other structures (e.g., stapled or otherwise secured to structural surfaces). 
     The foregoing, as well as other objectives and advantages of the invention, and the manner in which the same are accomplished, are further specified within the following detailed description and its accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts microbend testing results demonstrating that exceptionally low microbending losses are achieved, in accordance with the present invention, by pairing a bend-insensitive glass fiber with a low-modulus primary coating. 
         FIG. 2  schematically depicts the relationship between the in situ modulus of a primary coating and added loss for a multimode optical fiber. 
         FIG. 3  depicts the dynamic mechanical properties of a typical commercial primary coating (i.e., a conventional primary coating). 
         FIG. 4  depicts the dynamic mechanical properties of an exemplary primary coating used in producing optical fibers according to the present invention. 
         FIG. 5  depicts microbend testing results for optical fibers that include a conventional primary coating and for optical fibers that include an exemplary primary coating according to the present invention. 
         FIG. 6  depicts microbend testing results (under rigorous temperature-cycle testing conditions) for optical fibers that include a conventional primary coating and for optical fibers that include an exemplary primary coating according to the present invention. 
         FIG. 7  depicts microbend testing results (under modified temperature-cycle testing conditions) for optical fibers that include a conventional primary coating and for optical fibers that include an exemplary primary coating according to the present invention. 
         FIG. 8  depicts microbend testing results demonstrating that exceptionally low microbending losses are achieved, in accordance with the present invention, by pairing a bend-insensitive glass fiber with a low-modulus primary coating. 
         FIG. 9  depicts microbend testing results (under rigorous temperature-cycle testing conditions) for conventional optical fibers and for optical fibers that, in accordance with the present invention, combine a bend-insensitive glass fiber with a low-modulus primary coating. 
         FIG. 10  depicts microbend testing results (under modified temperature-cycle testing conditions) for conventional optical fibers and for optical fibers that, in accordance with the present invention, combine a bend-insensitive glass fiber with a low-modulus primary coating. 
         FIG. 11  depicts attenuation (added loss) as a function of MAC number (i.e., mode field diameter divided by cutoff wavelength) for various exemplary optical fibers. 
         FIG. 12  depicts, on a logarithmic scale, microbend sensitivity as a function of MAC number (i.e., mode field diameter divided by cutoff wavelength) for various exemplary optical fibers. 
     
    
    
     DETAILED DESCRIPTION 
     In one aspect, the present invention embraces optical fibers possessing an improved coating system that reduces stress-induced microbending, even in exceptionally cold environments required for FTTx deployments. The coating system according to the present invention includes a primary coating that combines low in situ modulus (e.g., less than about 0.5 MPa as measured on the fiber) and low glass transition temperature (T g ) (e.g., less than about −50° C.) to reduce stresses caused by external force and temperature. In addition, the coating system can be processed at high production speeds (e.g., 15-20 m/sec or more). 
     The present invention achieves a microbend-resistant optical fiber, particularly a single-mode optical fiber, by employing as its primary coating a UV-curable, urethane acrylate composition. In this regard, the primary coating includes between about 40 and 80 weight percent of polyether-urethane acrylate oligomer as well as photoinitiator, such as LUCIRIN® TPO, which is commercially available from BASF. In addition, the primary coating includes one or more oligomers and one or more monomer diluents (e.g., isobornyl acrylate), which may be included, for instance, to reduce viscosity and thereby promote processing. A suitable composition for the primary coating according to the present invention is a UV-curable urethane acrylate product provided by DSM Desotech (Elgin, Illinois) under the trade name DeSolite® DP 1011. 
     In this regard, this application incorporates entirely by reference the following commonly assigned patent applications: U.S. Patent Application No. 60/986,737 for a Microbend-Resistant Optical Fiber (Overton), U.S. Patent Application No. 61/041,484 (Overton) for a Microbend-Resistant Optical Fiber, U.S. Patent Application No. 61/112,595 for a Microbend-Resistant Optical Fiber (Overton), and International Patent Application No. PCT/U.S.08/82927 Microbend-Resistant Optical Fiber (Overton). 
     Suitable glass fibers for use in the present invention include glass fibers such as those disclosed in U.S. Pat. No. 4,838,643 for a Single Mode Bend Insensitive Fiber for Use in Fiber Optic Guidance Applications (Hodges et al.); U.S. Patent Application Publication No. US 2007/0127878 Al and its related U.S. patent application Ser. No. 11/556,895 for a Single Mode Optical Fiber (de Montmorillon et al.); U.S. Patent Application Publication No. US 2007/0280615 Al and its related U.S. patent application Ser. No. 11/697,994 for a Single-Mode Optical Fiber (de Montmorillon et al.); U.S. Pat. No. 7,356,234 and its related U.S. patent application Ser. No. 11/743,365 for Chromatic Dispersion Compensating Fiber (de Montmorillon et al.); U.S. Patent Application Publication No. US 2008/0152288 Al and its related U.S. patent application Ser. No. 11/999,333 for an Optical Fiber (Flammer et al.); U.S. patent application Ser. No. 61/101,337 for a Single Mode Optical Fiber (de Montmorillon et al.); U.S. patent application Ser. No. 61/112,006 for a Bend-Insensitive Single-Mode Optical Fiber (de Montmorillon et al.); and U.S. patent application Ser. No. 61/112,374 for a Bend-Insensitive Single Mode Optical Fiber (de Montmorillon et al.). Each of these commonly assigned patent documents is hereby incorporated by reference in its entirety. One exemplary glass fiber, for instance, possesses a step-index core having a refractive index that is between about 0.003 and 0.006 higher than the refractive index of its adjacent silica cladding. 
     Exemplary single-mode glass fibers for use in the present invention are commercially available from Draka Comteq (Claremont, N.C.) under the trade name BendBright®, which is compliant with the ITU-T G.652.D requirements, and the trade name BendBright XS ®, which is compliant with the ITU-T G.657.A/B and ITU-T G.652.D requirements. 
     In particular and as set forth herein, it has been unexpectedly discovered that the pairing of a bend-insensitive glass fiber (e.g., Draka Comteq&#39;s single-mode glass fibers available under the trade name BendBright XS ®) and a primary coating having very low modulus (e.g., DSM Desotech&#39;s UV-curable urethane acrylate product provided under the trade name DeSolite® DP 1011) achieves optical fibers having exceptionally low losses (e.g., reductions in microbend sensitivity of at least 10× (e.g., 40× to 100× or more) as compared with a single-mode fiber employing a conventional coating system). Draka Comteq&#39;s bend-resistant, single-mode glass fiber available under the trade name BendBright XS ® employs a trench-assisted design that reduces microbending losses. 
       FIG. 1  depicts this outstanding result by comparing the aforementioned exemplary single-mode fiber according to the present invention with various single-mode fibers employing conventional coating systems. In this regard,  FIG. 1  presents spectral attenuation data by measuring initial spectral attenuation on the optical fiber on a shipping spool, thereby obtaining the peaks and valleys typical of the attenuation across the full spectrum of wavelengths between the limits shown. The optical fiber is then wound onto a sandpaper-covered, fixed-diameter drum (i.e., measurement spool) as described by the IEC fixed-diameter sandpaper drum test (i.e., IEC TR62221, Method B), and another spectral attenuation curve is obtained. 
     The IEC fixed-diameter sandpaper drum test (i.e., IEC TR62221, Method B) provides a microbending stress situation that affects single-mode fibers even at room temperature. The sandpaper, of course, provides a rough surface that subjects the optical fiber to thousands, if not millions, of stress points. With respect to the test data presented in  FIG. 1 , a 300-mm diameter fiber spool was wrapped with adhesive-backed, 40-micron grade sandpaper (i.e., approximately equivalent to 300-grit sandpaper) to create a rough surface. Then, 400-meter fiber samples were wound at about 2,940 mN (i.e., a tension of 300 gf on a 300-mm diameter cylinder), and spectral attenuation was measured at 23° C. 
     The curves presented in  FIG. 1  represent the difference between the initial spectral curve and the curve when the fiber is on the sandpaper drum, thereby providing the added loss due to microbending stresses. 
     * * * 
     Those having ordinary skill in the art will recognize cable designs are now employing smaller diameter buffer tubes and less expensive materials in an effort to reduce costs. Consequently, when deployed in such cable designs, single-mode optical fibers are less protected and thus more susceptible to stress-induced microbending. As noted, the present invention provides an improved coating system that better protects optical fibers against stresses caused by external mechanical deformations and by temperature-induced, mechanical property changes to the coatings. 
     As noted, conventional solutions for protecting optical fibers involved using large-diameter buffer tubes, buffer tubes made of high-modulus materials that resist deformation and stresses upon the fiber, and stronger, thicker cable jackets to resist deformations that might pinch or otherwise squeeze the optical fibers. These solutions, however, are not only costly, but also fail to address the temperature-induced stresses caused by changes to the protective coatings. In other words, conventional primary coatings possess high modulus at temperatures below their respective glass transition temperatures. 
     As disclosed herein, the optical fiber according to the present invention includes a primary coating possessing lower modulus and lower glass transition temperature than possessed by conventional single-mode fiber primary coatings. Even so, the improved primary coating formulation nonetheless facilitates commercial production of the present optical fiber at excellent processing speeds (e.g., 1,000 m/min or more). In this regard, the primary coating employed in the optical fibers of the present invention possesses fast curing rates—reaching 50 percent of full cure at a UV dose of about 0.3 J/cm 2 , 80 percent of full cure at a UV dose of about 0.5 J/cm 2 , and 90 percent of full cure at a UV dose of about 1.0 J/cm 2  as measured on a standard 75-micron film at 20° C. and atmospheric pressure (i.e., 760 torr) (i.e., standard temperature and pressure—STP). 
       FIG. 2  schematically depicts the observed relationship between the in situ modulus of a primary coating and the attenuation (added loss) of the optical fiber, here a 50-micron graded-index multimode fiber. The primary coating modulus is measured as cured on the glass fiber and the added loss is measured using a fixed-diameter sandpaper drum procedure in accordance with the IEC TR62221 microbending-sensitivity technical report and standard test procedures (e.g., IEC TR62221, Method B, Ed. 1), which are hereby incorporated by reference in their entirety. 
     As will be appreciated by those having ordinary skill in the art, prior, commercially available single-mode fibers typically include a Young&#39;s modulus of 100-150 psi measured in situ (i.e., on the fiber). The optical fiber according to the present invention possesses a primary coating having reduced modulus as compared with such commercially available primary coatings. Employing a lower modulus primary coating provides better cushioning around the glass fiber. 
     Although lower modulus of the in situ primary coating can be achieved by selectively undercuring, the present invention achieves in situ primary coating having lower modulus even approaching full cure (i.e., near full cure). In this regard, the modulus of the in situ primary coating according to the present invention is less than about 0.65 MPa (e.g., less than about 95 psi), typically less than about 0.5 MPa, and more typically less than 0.4 MPa (e.g., between about 0.3 MPa and 0.4 MPa or between about 40 psi and 60 psi). It has been determined that an in situ primary coating having a modulus of less than about 0.5 MPa significantly reduces bend sensitivity of the glass fiber. On the other hand, the modulus of the in situ primary coating according to the present invention is typically greater than about 0.2 MPa (e.g., 0.25 MPa or more). 
     To achieve its reduced modulus as compared with conventional optical fiber coatings, the present primary coating possesses a lower crosslink density, specifically a reduced concentration of the reactive acrylate groups. Those having ordinary skill in the art will appreciate that acrylate groups crosslink via free radical polymerization during photoinitiation (e.g., UV-induced curing during drawing operations). The reaction kinetics dictate reduced cure rates during processing. This is commercially undesirable, of course, and so the present invention implements processing modifications to provide satisfactory cure rate for the low-modulus primary coating. 
     There are at least two components of the curing process that retard the rate of polymerization of the primary coating. First, the combination of (i) high curing temperatures induced by exposure to a high-intensity, UV environment and (ii) the exothermic polymerization reaction slows the observed curing rate of the primary coating. Second, close proximity of stacked UV lamps, in effect, creates rapidly superposed, repeated photoinitiation periods. The reaction rate of acrylate groups under this configuration is likewise retarded—a somewhat counterintuitive result. With respect to the latter, disposing (i.e., positioning) UV lamps to increase the period between consecutive UV exposures significantly increases the degree of coating cure as compared with other conventional processes employing the same draw speed and UV dose. In this way, it is possible to process the reduced-modulus, primary coating according to the present invention in a way that achieves near-complete curing at fast fiber draw speeds, which are required for a commercially viable process. An exemplary method and apparatus for curing a coated fiber is disclosed in commonly assigned U.S. Pat. No. 7,322,122, which is hereby incorporated by reference in its entirety. 
     The temperature dependence of the modulus is an important consideration to ensure that the primary coating provides enhanced microbending protection in FTTx applications. A primary coating having low modulus only at room temperature would be inadequate because deployment in the field will expose the optical fiber to microbend-inducing stresses at extreme environmental temperatures (e.g., −40° C. and below). Therefore, a suitable primary coating according to the present invention possesses an exceptionally low glass transition temperature so that the primary coating remains soft and protective in extremely cold environmental conditions. 
     Example 1 Comparison of Mechanical Properties 
       FIGS. 3 and 4 , respectively, depict dynamic mechanical properties of a typical commercial primary coating (i.e., the conventional primary coating) and an exemplary primary coating used in making the optical fibers according to the present invention. The conventional primary coating was a UV-curable urethane acrylate provided by DSM Desotech (Elgin, Ill.) under the trade name DeSolite® DP1007. The exemplary primary coating according to the present invention (i.e., employed to form optical fibers of the present invention) was a UV-curable urethane acrylate provided by DSM Desotech (Elgin, Ill.) under the trade name DeSolite® DP 1011. 
     The data for the conventional primary coating were obtained on a Dynamic Mechanical Analyzer (DMA) at an oscillatory stress rate of 1 Hz. In doing so, the strain was maintained within the linear region of stress-strain behavior. The sample of conventional primary coating was cured on polyester to form a standard 75-micron film. A UV dose of 1 J/cm 2  was applied using a mercury-halide bulb operating at a 300 W/in output. This UV exposure was sufficient to ensure that the coating was on the plateau of the dose-modulus curve. 
     Referring to  FIG. 3 , the data show the equilibrium modulus to be approximately 1.5 MPa as measured on a 75-micron film. On a glass fiber (i.e., in situ), this conventional primary coating typically cures well to a modulus of about 0.8 MPa, a level indicative of many single-mode fiber primary coatings in the industry. Those having ordinary skill in the art will appreciate that modulus measurements of softer primary coatings tend to be lower on a glass fiber (i.e., in situ) as compared with on a 75-micron film. 
     The glass transition temperature of the conventional primary coating is estimated by the peak in tan δ to be approximately −30° C. Thus, the conventional primary coating (and similar formulations) will behave like a glassy polymer at extremely low temperatures (e.g., less than −40° C., particularly less than −50° C.). (Although stress induced by strain is time dependent at low temperatures, estimated glass transition temperature is a useful comparative property.) 
     A sample of the exemplary primary coating according to the present invention was likewise cured on polyester to form a comparable 75-micron film. As before, a UV dose of 1 J/cm 2  was applied to the primary coating using a mercury-halide bulb operating at a 300 W/in output. As noted,  FIG. 4  depicts dynamic mechanical properties of the exemplary primary coating according to the present invention. 
     The exemplary primary coating according to the present invention exhibited an equilibrium modulus at just under 1 MPa in the cured film. The in situ modulus (i.e., measured on the glass fiber), was between about 0.3 MPa and 0.4 MPa. This is significantly lower than the respective modulus measurements for the conventional primary coating. 
     The glass transition temperature of the exemplary primary coating according to the present invention is estimated by the peak in tan δ at less than about −50° C. (e.g., about −60° C.). This is at least about 20° C. below the glass transition temperature of the comparative, conventional primary coating. Accordingly, primary coatings according to the present invention provide much more rapid stress relaxation during temperature excursions. 
     * * * 
     As set forth in Examples 2 and 3 (below), two different methods were used to evaluate the respective microbend sensitivities of glass fibers coated with (i) a typical commercial primary coating (i.e., the conventional primary coating) and (ii) an exemplary primary coating according to the present invention. As with Example 1 (above), the conventional primary coating was a UV-curable urethane acrylate provided by DSM Desotech (Elgin, Ill.) under the trade name DeSolite® DP 1007, and the exemplary primary coating according to the present invention (i.e., employed to form optical fibers of the present invention) was a UV-curable urethane acrylate provided by DSM Desotech (Elgin, Ill.) under the trade name DeSolite® DP 1011. 
     Each test method provided aggravated lateral stress conditions. Moreover, after measuring the effect on attenuation at room temperature, the test structures were temperature cycled to determine the additional loss induced by such temperature excursions. 
     Example 2 Comparison of Microbending Sensitivity 
     The first test method employed was a basket-weave, temperature cycling procedure known by those having ordinary skill in the art. According to this test procedure, optical fiber was wound at about 490 mN (i.e., a tension of 50 gf on a 300-mm diameter quartz cylinder with a 9-mm “lay”). Fifty layers were wound on the quartz drum to create numerous fiber-to-fiber crossovers. The testing procedure for Example 2 was an adaptation of IEC TR62221, Method D, which, as noted, is incorporated by reference in its entirety. 
     Those having ordinary skill in the art will appreciate that, at room temperature, such fiber crossovers can sometimes cause added loss (i.e., if the optical fiber is very sensitive) but that typically little or no added loss is observed. Consequently, the drum (with wound fiber) was temperature cycled twice from about room temperature through (i) −40° C., (ii) −60° C., (iii) +70° C., and (iv) +23° C. (i.e., near room temperature) while making loss measurements at 1550 nanometers. In both temperature cycles, fiber attenuation was measured after one hour at each test temperature. 
       FIG. 5  depicts exemplary results for single-mode glass fibers coated with, respectively, a conventional primary coating (i.e., DeSolite® DP1007) and an exemplary primary coating according to the present invention (i.e., DeSolite® DP 1011). The respective fiber specimens were chosen to match the coating geometry, mode field diameter, and cutoff wavelength. Accordingly, the respective optical fibers employed different formulations of colored secondary coatings. 
     In summary, the conventional primary coating and the exemplary primary coating according to the present invention each provided good protection against microbending stresses at 23° C. Moreover, at −40° C., the optical fiber having the conventional primary coating demonstrated only a small added loss. (It would appear that at −40° C., the conventional primary coating provided adequate protection against microbending by stress relaxing in a reasonable timeframe, even though this was near its glass transition temperature.) By way of comparison, the optical fiber according to the present invention demonstrated essentially no added loss at −40° C. (i.e., better performance). 
     At −60° C., however, the optical fiber having the conventional primary coating demonstrated significant added loss. (This temperature extreme was well below the glass transition temperature of the conventional primary coating.) By way of comparison, the optical fiber according to the present invention demonstrated essentially no added loss at −60° C., which is close to the glass transition temperature of this embodiment of the primary coating according to the present invention. 
     Example 3 Comparison of Microbending Sensitivity 
     The second test method employed more aggressive environments (i.e., conditions) in order to evaluate the respective microbend sensitivities of (i) an optical fiber possessing a typical commercial primary coating (i.e., the conventional primary coating) and (ii) an optical fiber possessing an exemplary primary coating according to the present invention. 
     In particular, the second method modified the IEC fixed-diameter sandpaper drum test (i.e., IEC TR62221, Method B), which, as noted, is incorporated by reference in its entirety, to provide a microbending stress situation sufficiently harsh to affect single-mode fibers even at room temperature (i.e., a rougher drum surface than that used to measure the data depicted in  FIG. 1 ). To do this, a 300-mm diameter quartz drum was wrapped with adhesive-backed, 220-grit sandpaper (i.e., approximately equivalent to 66-micron-grade sandpaper) to create a rough surface. 
     In an initial test condition, each of the respective fiber samples was wound in a single layer at about 980 mN (i.e., a tension of 100 gf on a 300-mm diameter quartz cylinder). In a modified test condition, three (3) each of the respective fiber samples was wound in a single layer at about 1,470 mN (i.e., a tension of 150 gf on a 300-mm diameter quartz cylinder). Thus, as compared with the first test condition, the second test condition increased the winding tension by 50 percent. 
     Using matched fiber samples (as with the basket weave/temperature cycling test of Example 2) fiber attenuation was measured after winding at room temperature (i.e., 23° C.) for each test condition. Then, the drum (with 400 meters of wound fiber) was temperature cycled from about room temperature through (i) −40° C., (ii) −60° C., and (iii) +23° C. (i.e., near room temperature) while making loss measurements at 1550 nanometers using an optical time domain reflectometer (OTDR). 
     The several samples of each kind of optical fiber were initially measured at 23° C. on the original spools (i.e., before winding on the roughened drum surface to establish baseline spectral attenuation) then were subjected to the foregoing rigorous testing conditions for one hour at each temperature. Fiber attenuation was measured after one hour (as in Example 2) at each test temperature. 
       FIG. 6 , a line chart, and  FIG. 7 , a box plot, depict exemplary results under these more rigorous testing conditions for single-mode optical fibers that include a conventional primary coating (i.e., DeSolite® DP1007 UV-curable urethane acrylate) and for single-mode optical fibers that include an exemplary primary coating according to the present invention (i.e., DeSolite® DP1011 UV-curable urethane acrylate). 
       FIG. 6 , for instance, shows that, as compared with conventional optical fibers, exemplary optical fibers according to the present invention possess reduced microbend sensitivity (i.e., a reduction of about 40-60 percent). 
     Likewise,  FIG. 7  shows that, as compared with conventional optical fibers, exemplary optical fibers according to the present invention possess substantially reduced microbend sensitivity at a higher winding tension (i.e., 150 gf on a 300-mm diameter quartz cylinder).  FIG. 7  thus illustrates that the exemplary primary coating according to the present invention (i.e., DeSolite® DP1011 UV-curable urethane acrylate) promotes both significantly reduced and significantly more uniform microbending performance. 
     In accordance with the foregoing, it has been found that, as compared with a conventional coating system, the present coating system provides significant microbending improvement when used in combination with a conventional single-mode glass fiber. 
     It has been further found that pairing a bend-insensitive glass fiber (e.g., Draka Comteq&#39;s single-mode glass fibers available under the trade name BendBright XS ®) and a primary coating having very low modulus (e.g., DSM Desotech&#39;s UV-curable urethane acrylate product provided under the trade name DeSolite® DP 1011) achieves optical fibers having exceptionally low losses. Additional testing was performed, therefore, to demonstrate the dramatic and unexpected reductions in microbend sensitivity provided in accordance with the present invention. 
     Example 4 Comparison of Microbending Sensitivity 
     The respective microbend sensitivities were measured for exemplary optical fibers, including (i) a conventional single-mode glass fiber with a conventional commercial coating, (ii) a bend-insensitive glass fiber with a conventional commercial coating, and (iii) a bend-insensitive glass fiber (e.g., Draka Comteq&#39;s single-mode glass fibers available under the trade name BendBright XS ®) with the coating according to the present invention (e.g., Draka Comteq&#39;s ColorLock XS  brand coating system). 
       FIG. 8  demonstrates that the optical fiber according to the present invention, namely including a bend-insensitive glass fiber (e.g., Draka Comteq&#39;s single-mode glass fibers available under the trade name BendBright XS ®) and a primary coating having very low modulus (e.g., DSM Desotech&#39;s UV-curable urethane acrylate product provided under the trade name DeSolite® DP 1011), provides exceptionally low attenuation losses as compared with other optical fibers. Moreover, this bend-resistant optical fiber exhibits small wavelength dependence within the transmission window between 1400 nanometers and 1700 nanometers, and is essentially unaffected by the microbend-inducing test conditions across the test spectrum. 
       FIG. 8  presents exemplary spectral attenuation data obtained adhering to IEC TR62221, Method B (fixed-diameter drum). In accordance with IEC TR62221, Method B, initial spectral attenuation was measured on a 440-meter sample of optical fiber wound on a shipping spool (i.e., obtaining the peaks and valleys typical of the attenuation across the full spectrum of wavelengths between the limits shown). The optical fiber was then wound at about 3 N onto a 300-mm diameter measurement spool wrapped with adhesive-backed, 40-micron grade sandpaper (i.e., approximately equivalent to 300-grit sandpaper), and another spectral attenuation curve was obtained. 
     Like the curves presented in  FIG. 1 , the curves depicted in  FIG. 8  represent, at 23° C., the difference between the initial spectral curve and the curve when the fiber is on the sandpaper drum of fixed diameter, thereby providing the added loss due to microbending stresses (i.e., delta-attenuation across the spectral range). 
     Example 5 Comparison of Microbending Sensitivity 
     The respective microbend sensitivities were measured under rigorous test conditions for exemplary optical fibers, including (i) a conventional single-mode glass fiber with a conventional commercial coating and (ii) a bend-insensitive glass fiber (e.g., Draka Comteq&#39;s single-mode glass fibers available under the trade name BendBright XS ®) with the coating according to the present invention (e.g., Draka Comteq&#39;s ColorLock XS  brand coating system). 
       FIG. 9  demonstrates that, even under extremely harsh conditions, the optical fiber according to the present invention, namely including a bend-insensitive glass fiber (e.g., Draka Comteq&#39;s single-mode glass fibers available under the trade name BendBright XS ®) and a primary coating having very low modulus (e.g., DSM Desotech&#39;s UV-curable urethane acrylate product provided under the trade name DeSolite® DP 1011), provides surprisingly low attenuation losses as compared with other optical fibers. 
     The testing procedure for Example 5 was an adaptation of IEC TR62221, Method B, which, as noted, is incorporated by reference in its entirety. For this modified IEC fixed-diameter sandpaper drum test, a 300-mm diameter quartz drum was wrapped with adhesive-backed, 180-grit sandpaper (i.e., approximately equivalent to 78-micron-grade sandpaper) to create an even rougher surface than that described in Example 3 (above). Then, 440-meter fiber samples were wound in a single layer at about 1,470 mN (i.e., a controlled back tension of 150 gf on the 300-mm diameter quartz cylinder using a Delachaux optical fiber winding apparatus), and spectral attenuation was measured. 
       FIG. 9  presents exemplary temperature-cycle data for three specimens of standard single-mode fiber (i.e., a conventional single-mode glass fiber with a conventional commercial coating) and three specimens of optical fiber according to the present invention (i.e., a bend-insensitive glass fiber with improved coating according to the present invention). As noted, 440 meters of optical fiber is wound onto the aforementioned sandpaper-covered, fixed-diameter drum. One hour after winding, fiber attenuation was measured at room temperature (i.e., 23° C.) using an optical time domain reflectometer (OTDR). Then, the drum (with 440 meters of wound fiber) was temperature cycled from about room temperature through (i) −40° C. and (ii) −60° C. in a temperature-controlled chamber. Fiber attenuation at 1550 nanometers was measured by an OTDR after one hour of equilibration at both −40° C. and −60° C. 
     Microbending sensitivity (S m ) may be described as αR/T, wherein α is the attenuation increase on the drum (dB/km), R is the radius of the fixed drum (mm), and T is the winding tension applied to the fiber (N). See e.g., IEC TR62221 Technical Report (Microbending Sensitivity). In addition to the parameters α, R, and T, however, the microbending-sensitivity metric obtained from the fixed-diameter sandpaper drum test is dependent on the coarseness of the sandpaper employed on the measurement drum. 
     Table 1 (below) presents the microbending-sensitivity metric obtained from the attenuation data (at a wavelength of 1550 nanometers) depicted in  FIG. 9  (i.e., employing 180-grit sandpaper). Table 1 shows that, as compared with a conventional standard single-mode fiber, the optical fiber according to the present invention provides microbending sensitivity that is about 2×-10× lower at 23° C. and about 2×-5× lower at −40° C.: 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 (Microbend Sensitivity) 
               
             
          
           
               
                   
                   
                 23° C. 
                 −40° C. 
                 −60° C. 
               
               
                   
                 Optical Fiber 
                 (dB/km)/ 
                 (dB/km)/ 
                 (dB/km)/ 
               
               
                   
                 (Coating Color) 
                 (N/mm) 
                 (N/mm) 
                 (N/mm) 
               
               
                   
               
             
          
           
               
                   
                 Conventional SMF 
                 139.9 
                 220.6 
                 331.8 
               
               
                   
                 (blue) 
                   
                   
                   
               
               
                   
                 Conventional SMF 
                 261.0 
                 329.7 
                 417.9 
               
               
                   
                 (red) 
                   
                   
                   
               
               
                   
                 Conventional SMF 
                 104.3 
                 161.9 
                 228.0 
               
               
                   
                 (aqua) 
                   
                   
                   
               
               
                   
                 BendBright xs  ® w/ 
                 35.8 
                 76.5 
                 163.4 
               
               
                   
                 ColorLock xs  (slate) 
                   
                   
                   
               
               
                   
                 BendBright xs  ® w/ 
                 30.1 
                 70.6 
                 144.2 
               
               
                   
                 ColorLock xs  (red) 
                   
                   
                   
               
               
                   
                 BendBright xs  ® w/ 
                 42.7 
                 84.7 
                 166.4 
               
               
                   
                 ColorLock xs  (aqua) 
               
               
                   
               
             
          
         
       
     
     Example 6 Comparison of Microbending Sensitivity 
     The respective microbend sensitivities were further measured for exemplary optical fibers, including (i) a conventional single-mode glass fiber with a conventional commercial coating and (ii) a bend-insensitive glass fiber (e.g., Draka Comteq&#39;s single-mode glass fibers available under the trade name BendBright XS ®) with the coating according to the present invention (e.g., Draka Comteq&#39;s ColorLock XS  brand coating system). 
     The testing procedure for Example 6 was an adaptation of IEC TR62221, Method B, which, as noted, is incorporated by reference in its entirety. For this modified IEC fixed-diameter sandpaper drum test, a 300-mm diameter quartz drum was wrapped with adhesive-backed, 220-grit sandpaper (i.e., approximately equivalent to 66-micron-grade sandpaper) to create a rough surface like that described in Example 3. Each of the fiber samples was wound in a single layer at about 1,470 mN (i.e., a tension of 150 gf on a 300-mm diameter quartz cylinder). As compared with the test conditions of Example 5, the test conditions of Example 6 employed finer grade sandpaper (i.e., 220-grit rather than 180-grit). 
     As in Example 3, using matched fiber samples, fiber attenuation was measured after winding at room temperature (i.e., 23° C.). Then, the drum (with about 400 meters of wound fiber) was temperature cycled from about room temperature through (i) −40° C., (ii) −60° C., and (iii)+23° C. (i.e., near room temperature) while making loss measurements at 1550 nanometers using an optical time domain reflectometer (OTDR). 
     Three (3) samples of each kind of optical fiber were initially measured at 23° C. on the original spools (i.e., before winding on the roughened drum surface to establish baseline spectral attenuation) and then were subjected to the foregoing rigorous testing conditions for one hour at each temperature. Fiber attenuation was measured after one hour at each temperature. 
       FIG. 10  depicts exemplary results for single-mode optical fibers that include a conventional primary coating (i.e., DeSolite® DP1007 UV-curable urethane acrylate) and for bend-insensitive glass fibers (e.g., Draka Comteq&#39;s single-mode glass fibers available under the trade name BendBright XS ®) that include a primary coating having very low modulus (i.e., DSM Desotech&#39;s UV-curable urethane acrylate product provided under the trade name DeSolite® DP 1011). 
       FIG. 10  demonstrates that the optical fiber according to the present invention, namely Draka Comteq&#39;s single-mode glass fibers available under the trade name BendBright XS ® with a primary coating having very low modulus (e.g., DSM Desotech&#39;s UV-curable urethane acrylate product provided under the trade name DeSolite® DP 1011), provides exceptionally low attenuation losses as compared with standard single-mode optical fibers (SSMF). 
     In addition,  FIGS. 11 and 12  depict attenuation and microbend sensitivity, respectively, at a wavelength of 1550 nanometers as a function of MAC number (i.e., mode field diameter divided by cutoff wavelength) for various exemplary optical fibers in accordance with the standard IEC fixed-diameter sandpaper drum test (i.e., IEC TR62221, Method B). The respective attenuation data depicted in  FIG. 11  (added loss) and  FIG. 12  (microbend sensitivity) were obtained at 23° C. under the test conditions previously described with respect to  FIG. 1  (i.e., 400-meter fiber samples were wound at about 2,940 mN (i.e., a tension of 300 gf) on a 300-mm diameter fiber spool wrapped with adhesive-backed, 40-micron grade sandpaper). 
       FIG. 11  shows that Draka Comteq&#39;s bend-resistant, single-mode glass fiber available under the trade name BendBright XS ® in combination with Draka Comteq&#39;s ColorLock XS  brand coating system provides outstanding performance with respect to added loss. 
       FIG. 12  shows that Draka Comteq&#39;s bend-resistant, single-mode glass fiber available under the trade name BendBright XS ® in combination with Draka Comteq&#39;s ColorLock XS  brand coating system provides superior microbend sensitivity (i.e., microbend sensitivity of 0.01 to 0.03 (dB/km)/(gf/mm)). 
     The optical fibers according to the present invention typically further include a tough secondary coating to protect the primary coating and glass fiber from damage during handling and installation. For example, the secondary coating might have a modulus of between about 800 and 1,000 MPa (e.g., about 900 MPa) as measured on a standard 75-micron film. As disclosed herein, this secondary coating may be inked as a color code or, preferably, may be color-inclusive to provide identification without the need for a separate inking process. 
     In one embodiment according to the present invention, the secondary coating, which surrounds the primary coating to thereby protect the fiber structure, features an inclusive coloring system (i.e., not requiring an extra layer of ink to be added for color coding). The colors, which conform to Munsell standards for optical fiber color-coding, are enhanced for brightness and visibility under dim lighting (e.g., in deep shade or in confined spaces, such as manholes) and are easily distinguished against both light and dark backgrounds. 
     Furthermore, the secondary coating features a surface that provides an excellent interface with ribbon matrix material so that the matrix separates easily from the colored fiber in a way that does not sacrifice robustness. The mechanical properties of the colored secondary coating are balanced with those of the primary coating so that, in heat stripping, the coating/matrix composite separates cleanly from the glass fibers. 
     Employing Draka Comteq&#39;s bend-resistant, single-mode glass fiber available under the trade name BendBright XS®  with the present dual-coating system, which includes a low-modulus primary coating, has been found to reduce microbending sensitivity by between about one to two orders of magnitude relative to standard single-mode fiber (SSMF) at the key transmission frequencies of 1550 nanometers and 1625 nanometers. As noted, such optical fiber not only provides outstanding resistance to microbending and macrobending, but also complies with the ITU-T G.657.A/B and ITU-T G.652.D requirements. Table 2(below) depicts optical-fiber attributes of an exemplary bend-insensitive optical fiber in accordance with the present invention. 
     
       
         
               
             
               
               
               
             
               
               
               
             
               
               
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 (Exemplary Optical-Fiber Attributes) 
               
             
          
           
               
                 Attribute 
                 Detail 
                 Value 
               
               
                   
               
             
          
           
               
                 Mode Field  
                 Wavelength (nm) 
                 1310 
               
               
                 Diameter 
                 Range of Nominal 
                 8.5-9.3 
               
               
                   
                 Values (μm) 
                   
               
               
                 Cladding Diameter 
                 Nominal (μm) 
                 125 
               
               
                   
                 Tolerance (μm) 
                 ±0.7 
               
               
                 Core Concentricity  
                 Maximum (μm) 
                 0.5 
               
               
                 Error 
                   
                   
               
               
                 Cladding  
                 Maximum (%) 
                 0.7 
               
               
                 Non-Circularity 
                   
                   
               
               
                 Cable Cut-Off  
                 Maximum (nm) 
                 1260 
               
               
                 Wavelength 
                   
                   
               
             
          
           
               
                 Macrobending  
                 Radius (mm) 
                 15 
                 10 
                 7.5 
               
               
                 Loss 
                 Number of Turns 
                 10 
                 1 
                 1 
               
               
                   
                 Maximum @ 1550 nm (dB) 
                 0.03 
                 0.1 
                 0.5 
               
               
                   
                 Maximum @ 1625 nm (dB) 
                 0.1 
                 0.2 
                 1.0 
               
             
          
           
               
                 Proof Stress 
                 Minimum (GPa) 
                 0.7 
               
               
                 Chromatic  
                 λ 0 min  (nm) 
                 1300 
               
               
                 Dispersion 
                 λ 0 max  (nm) 
                 1324 
               
               
                 Coefficient 
                 S 0 max  (ps/(nm 2  · km)) 
                 ≦0.092 
               
               
                   
               
             
          
         
       
     
     In particular, Draka Comteq&#39;s bend-resistant, single-mode glass fiber available under the trade name BendBright XS ® (e.g., enhanced with Draka Comteq&#39;s ColorLock XS  brand coating system) provides resistance to macrobending required for sustained bends having a radius as low as five (5) millimeters with an estimated failure probability of less than two (2) breaks per million full circle bends (i.e., 360°) over 30 years in a properly protected environment. These bend-resistant optical fibers facilitate the rapid deployment of small, flexible cables for the delivery of fiber to the premises/business/home (i.e., FTTx) by virtue of the optical fiber&#39;s ability to sustain a loss-free transmission through small-radius bends. Cables employing such bend-resistant optical fibers may be routed around sharp bends, stapled to building frame, coiled, and otherwise employed in demanding environments while retaining clear and strong signal transmission. 
     * * * 
     The bend-insensitive optical fibers according to the present invention facilitate the reduction in overall optical-fiber diameter. As will be appreciated by those having ordinary skill in the art, a reduced-diameter optical fiber is cost-effective, requiring less raw material. Moreover, a reduced-diameter optical fiber requires less deployment space (e.g., within a buffer tube and/or fiber optic cable), thereby facilitating increased fiber count and/or reduced cable size. 
     Those having ordinary skill in the art will recognize that an optical fiber with a primary coating (and an optional secondary coating and/or ink layer) typically has an outer diameter of between about 235 microns and about 265 microns (μm). The component glass fiber itself (i.e., the glass core and surrounding cladding layers) typically has a diameter of about 125 microns, such that the total coating thickness is typically between about 55 microns and 70 microns. 
     With respect to the optical fiber according to the present invention, the component glass fiber typically has an outer diameter of about 125 microns. With respect to the optical fiber&#39;s surrounding coating layers, the primary coating typically has an outer diameter of between about 175 microns and about 195 microns (i.e., a primary coating thickness of between about 25 microns and 35 microns) and the secondary coating typically has an outer diameter of between about 235 microns and about 265 microns (i.e., a secondary coating thickness of between about 20 microns and 45 microns). Optionally, the optical fiber according to the present invention may include an outermost ink layer, which is typically between two and ten microns. 
     In an alternative embodiment, an optical fiber according to the present invention may possess a reduced diameter (e.g., an outermost diameter between about 150 microns and 230 microns). In this alternative optical fiber configuration, the thickness of the primary coating and/or secondary coating is reduced, while the diameter of the component glass fiber is maintained at about 125 microns. By way of example, in such embodiments the primary coating layer may have an outer diameter of between about 135 microns and about 175 microns (e.g., about 160 microns), and the secondary coating layer may have an outer diameter of between about 150 microns and about 230 microns (e.g., more than about 165 microns, such as 190-210 microns or so). In other words, the total diameter of the optical fiber is reduced to less than about 230 microns (e.g., about 200 microns). 
     * * * 
     As noted, the optical fiber according to the present invention includes one or more coating layers (e.g., a primary coating and a secondary coating). At least one of the coating layers—typically the secondary coating—may be colored and/or possess other markings to help identify individual fibers. Alternatively, a tertiary ink layer may surround the primary and secondary coatings. 
     As discussed previously, combining (i) a coating system according to the present invention with (ii) a glass fiber having a refractive index profile that itself provides bend resistance (e.g., low macrobending sensitivity) has been found to provide unexpectedly superior reductions in microbend sensitivity. Indeed, bend-insensitive glass fibers are especially suitable for use with the coating system of the present invention (e.g., Draka Comteq&#39;s ColorLock XS  brand coating system). 
     The optical fiber according to the present invention may be deployed in various structures, such as those exemplary structures disclosed hereinafter. 
     For example, one or more of the present optical fibers may be enclosed within a buffer tube. For instance, optical fiber may be deployed in either a single fiber loose buffer tube or a multi-fiber loose buffer tube. With respect to the latter, multiple optical fibers may be bundled or stranded within a buffer tube or other structure. In this regard, within a multi-fiber loose buffer tube, fiber sub-bundles may be separated with binders (e.g., each fiber sub-bundle is enveloped in a binder). Moreover, fan-out tubing may be installed at the termination of such loose buffer tubes to directly terminate loose buffered optical fibers with field-installed connectors. 
     In other embodiments, the buffer tube may tightly surround the outermost optical fiber coating (i.e., tight buffered fiber) or otherwise surround the outermost optical fiber coating or ink layer to provide an exemplary radial clearance of between about 50 and 100 microns (i.e., a semi-tight buffered fiber). 
     With respect to the former tight buffered fiber, the buffering may be formed by coating the optical fiber with a curable composition (e.g., a UV-curable material) or a thermoplastic material. The outer diameter of tight buffer tubes, regardless of whether the buffer tube is formed from a curable or non-curable material, is typically less about 1,000 microns (e.g., either about 500 microns or about 900 microns). 
     With respect to the latter semi-tight buffered fiber, a lubricant may be included between the optical fiber and the buffer tube (e.g., to provide a gliding layer). 
     As will be known by those having ordinary skill in the art, an exemplary buffer tube enclosing optical fibers as disclosed herein may be formed of polyolefins (e.g., polyethylene or polypropylene), including fluorinated polyolefins, polyesters (e.g., polybutylene terephthalate), polyamides (e.g., nylon), as well as other polymeric materials and blends. In general, a buffer tube may be formed of one or more layers. The layers may be homogeneous or include mixtures or blends of various materials within each layer. 
     In this context, the buffer tube may be extruded (e.g., an extruded polymeric material) or pultruded (e.g., a pultruded, fiber-reinforced plastic). By way of example, the buffer tube may include a material to provide high temperature and chemical resistance (e.g., an aromatic material or polysulfone material). 
     Although buffer tubes typically have a circular cross section, buffer tubes alternatively may have an irregular or non-circular shape (e.g., an oval or a trapezoidal cross-section). 
     Alternatively, one or more of the present optical fibers may simply be surrounded by an outer protective sheath or encapsulated within a sealed metal tube. In either structure, no intermediate buffer tube is necessarily required. 
     Multiple optical fibers as disclosed herein may be sandwiched, encapsulated, and/or edge bonded to form an optical fiber ribbon. Optical fiber ribbons can be divisible into subunits (e.g., a twelve-fiber ribbon that is splittable into six-fiber subunits). Moreover, a plurality of such optical fiber ribbons may be aggregated to form a ribbon stack, which can have various sizes and shapes. 
     For example, it is possible to form a rectangular ribbon stack or a ribbon stack in which the uppermost and lowermost optical fiber ribbons have fewer optical fibers than those toward the center of the stack. This construction may be useful to increase the density of optical elements (e.g., optical fibers) within the buffer tube and/or cable. 
     In general, it is desirable to increase the filling of transmission elements in buffer tubes or cables, subject to other constraints (e.g., cable or mid-span attenuation). The optical elements themselves may be designed for increased packing density. For example, the optical fiber may possess modified properties, such as improved refractive-index profile, core or cladding dimensions, or primary coating thickness and/or modulus, to improve microbending and macrobending characteristics. 
     By way of example, a rectangular ribbon stack may be formed with or without a central twist (i.e., a “primary twist”). Those having ordinary skill in the art will appreciate that a ribbon stack is typically manufactured with rotational twist to allow the tube or cable to bend without placing excessive mechanical stress on the optical fibers during winding, installation, and use. In a structural variation, a twisted (or untwisted) rectangular ribbon stack may be further formed into a coil-like configuration (e.g., a helix) or a wave-like configuration (e.g., a sinusoid). In other words, the ribbon stack may possess regular “secondary” deformations. 
     As will be known to those having ordinary skill in the art, such optical fiber ribbons may be positioned within a buffer tube or other surrounding structure, such as a buffer-tube-free cable. Subject to certain restraints (e.g., attenuation) it is desirable to increase the density of elements such as optical fibers or optical fiber ribbons within buffer tubes and/or optical fiber cables. 
     A plurality of buffer tubes containing optical fibers (e.g., loose or ribbonized fibers) may be positioned externally adjacent to and stranded around a central strength member. This stranding can be accomplished in one direction, helically, known as “S” or “Z” stranding, or Reverse Oscillated Lay stranding, known as “S-Z” stranding. Stranding about the central strength member reduces optical fiber strain when cable strain occurs during installation and use. 
     Those having ordinary skill in the art will understand the benefit of minimizing fiber strain for both tensile cable strain and longitudinal compressive cable strain during installation or operating conditions. 
     With respect to tensile cable strain, which may occur during installation, the cable will become longer while the optical fibers can migrate closer to the cable&#39;s neutral axis to reduce, if not eliminate, the strain being translated to the optical fibers. With respect to longitudinal compressive strain, which may occur at low operating temperatures due to shrinkage of the cable components, the optical fibers will migrate farther away from the cable&#39;s neutral axis to reduce, if not eliminate, the compressive strain being translated to the optical fibers. 
     In a variation, two or more substantially concentric layers of buffer tubes may be positioned around a central strength member. In a further variation, multiple stranding elements (e.g., multiple buffer tubes stranded around a strength member) may themselves be stranded around each other or around a primary central strength member. 
     Alternatively, a plurality of buffer tubes containing optical fibers (e.g., loose or ribbonized fibers) may be simply placed externally adjacent to the central strength member (i.e., the buffer tubes are not intentionally stranded or arranged around the central strength member in a particular manner and run substantially parallel to the central strength member). 
     Alternatively still, the present optical fibers may be positioned with a central buffer tube (i.e., the central buffer tube cable has a central buffer tube rather than a central strength member). Such a central buffer tube cable may position strength members elsewhere. For instance, metallic or non-metallic (e.g., GRP) strength members may be positioned within the cable sheath itself, and/or one or more layers of high-strength yarns (e.g., aramid or non-aramid yarns) may be positioned parallel to or wrapped (e.g., contrahelically) around the central buffer tube (i.e., within the cable&#39;s interior space). Likewise, strength members can be included within the buffer tube&#39;s casing. 
     In other embodiments, the optical fibers may be placed within a slotted core cable. In a slotted core cable, optical fibers, individually or as a fiber ribbon, may be placed within pre-shaped helical grooves (i.e., channels) on the surface of a central strength member, thereby forming a slotted core unit. The slotted core unit may be enclosed by a buffer tube. One or more of such slotted core units may be placed within a slotted core cable. For example, a plurality of slotted core units may be helically stranded around a central strength member. 
     Alternatively, the optical fibers may also be stranded in a maxitube cable design, whereby the optical fibers are stranded around themselves within a large multi-fiber loose buffer tube rather than around a central strength member. In other words, the large multi-fiber loose buffer tube is centrally positioned within the maxitube cable. For example, such maxitube cables may be deployed in optical ground wires (OPGW). 
     In another cabling embodiment, multiple buffer tubes may be stranded around themselves without the presence of a central member. These stranded buffer tubes may be surrounded by a protective tube. The protective tube may serve as the outer casing of the fiber optic cable or may be further surrounded by an outer sheath. The protective tube may tightly or loosely surround the stranded buffer tubes. 
     As will be known to those having ordinary skill in the art, additional elements may be included within a cable core. For example, copper cables or other active, transmission elements may be stranded or otherwise bundled within the cable sheath. Passive elements may also be placed within the cable core, such as between the interior walls of the buffer tubes and the enclosed optical fibers. Alternatively and by way of example, passive elements may be placed outside the buffer tubes between the respective exterior walls of the buffer tubes and the interior wall of the cable jacket, or, within the interior space of a buffer-tube-free cable. 
     For example, yarns, nonwovens, fabrics (e.g., tapes), foams, or other materials containing water-swellable material and/or coated with water-swellable materials (e.g., including super absorbent polymers (SAPs), such as SAP powder) may be employed to provide water blocking and/or to couple the optical fibers to the surrounding buffer tube and/or cable jacketing (e.g., via adhesion, friction, and/or compression). Exemplary water-swellable elements are disclosed in commonly assigned U.S. Patent Application Publication No. US 2007/0019915 A1 and its related U.S. patent application Ser. No. 11/424,112 for a Water-Swellable Tape, Adhesive-Backed for Coupling When Used Inside a Buffer Tube (Overton et al.), each of which is hereby incorporated by reference in its entirety. 
     Moreover, an adhesive (e.g., a hot-melt adhesive or curable adhesive, such as a silicone acrylate cross-linked by exposure to actinic radiation) may be provided on one or more passive elements (e.g., water-swellable material) to bond the elements to the buffer tube. An adhesive material may also be used to bond the water-swellable element to optical fibers within the buffer tube. Exemplary arrangements of such elements are disclosed in commonly assigned U.S. Patent Application Publication No. US 2008/0145010 A1 for a Gel-Free Buffer Tube with Adhesively Coupled Optical Element (Overton et al.), which is hereby incorporated by reference in its entirety. 
     The buffer tubes (or buffer-tube-free cables) may also contain a thixotropic composition (e.g., grease or grease-like gels) between the optical fibers and the interior walls of the buffer tubes. For example, filling the free space inside a buffer tube with water-blocking, petroleum-based filling grease helps to block the ingress of water. Further, the thixotropic filling grease mechanically (i.e., viscously) couples the optical fibers to the surrounding buffer tube. 
     Such thixotropic filling greases are relatively heavy and messy, thereby hindering connection and splicing operations. Thus, the present optical fibers may be deployed in dry cable structures (i.e., grease-free buffer tubes). 
     Exemplary buffer tube structures that are free from thixotropic filling greases are disclosed in commonly assigned U.S. patent application Ser. No. 12/146,588 for a Coupling Composition for Optical Fiber Cables, filed Jun. 26, 2008, (Parris et al.), which is hereby incorporated by reference in its entirety. Such buffer tubes employ coupling compositions formed from a blend of high-molecular weight elastomeric polymers (e.g., about 35 weight percent or less) and oils (e.g., about 65 weight percent or more) that flow at low temperatures. Unlike thixotropic filling greases, the coupling composition (e.g., employed as a cohesive gel or foam) is typically dry and, therefore, less messy during splicing. 
     As will be understood by those having ordinary skill in the art, a cable enclosing optical fibers as disclosed herein may have a sheath formed from various materials in various designs. Cable sheathing may be formed from polymeric materials such as, for example, polyethylene, polypropylene, polyvinyl chloride (PVC), polyamides (e.g., nylon), polyester (e.g., PBT), fluorinated plastics (e.g., perfluorethylene propylene, polyvinyl fluoride, or polyvinylidene difluoride), and ethylene vinyl acetate. The sheath and/or buffer tube materials may also contain other additives, such as nucleating agents, flame-retardants, smoke-retardants, antioxidants, UV absorbers, and/or plasticizers. 
     The cable sheathing may be a single jacket formed from a dielectric material (e.g., non-conducting polymers), with or without supplemental structural components that may be used to improve the protection (e.g., from rodents) and strength provided by the cable sheath. For example, one or more layers of metallic (e.g., steel) tape along with one or more dielectric jackets may form the cable sheathing. Metallic or fiberglass reinforcing rods (e.g., GRP) may also be incorporated into the sheath. In addition, aramid, fiberglass, or polyester yarns may be employed under the various sheath materials (e.g., between the cable sheath and the cable core), and/or ripcords may be positioned, for example, within the cable sheath. 
     Similar to buffer tubes, optical fiber cable sheaths typically have a circular cross section, but cable sheaths alternatively may have an irregular or non-circular shape (e.g., an oval, trapezoidal, or flat cross-section). 
     By way of example, the optical fiber according to the present invention may be incorporated into single-fiber drop cables, such as those employed for Multiple Dwelling Unit (MDU) applications. In such deployments, the cable jacketing must exhibit crush resistance, abrasion resistance, puncture resistance, thermal stability, and fire resistance as required by building codes. An exemplary material for such cable jackets is thermally stable, flame-retardant polyurethane (PUR), which mechanically protects the optical fibers yet is sufficiently flexible to facilitate easy MDU installations. Alternatively, a flame-retardant polyolefin or polyvinyl chloride sheath may be used. 
     In general and as will be known to those having ordinary skill in the art, a strength member is typically in the form of a rod or braided/helically wound wires or fibers, though other configurations will be within the knowledge of those having ordinary skill in the art. 
     Optical fiber cables containing optical fibers as disclosed may be variously deployed, including as drop cables, distribution cables, feeder cables, trunk cables, and stub cables, each of which may have varying operational requirements (e.g., temperature range, crush resistance, UV resistance, and minimum bend radius). 
     Such optical fiber cables may be installed within ducts, microducts, plenums, or risers. By way of example, an optical fiber cable may be installed in an existing duct or microduct by pulling or blowing (e.g., using compressed air). An exemplary cable installation method is disclosed in commonly assigned U.S. Patent Application Publication No. 2007/0263960 for a Communication Cable Assembly and Installation Method (Lock et al.), and U.S. patent application Ser. No. 12/200,095 for a Modified Pre-Ferrulized Communication Cable Assembly and Installation Method, filed Aug. 28, 2008, (Griffioen et al.), each of which is incorporated by reference in its entirety. 
     As noted, buffer tubes containing optical fibers (e.g., loose or ribbonized fibers) may be stranded (e.g., around a central strength member). In such configurations, an optical fiber cable&#39;s protective outer sheath may have a textured outer surface that periodically varies lengthwise along the cable in a manner that replicates the stranded shape of the underlying buffer tubes. The textured profile of the protective outer sheath can improve the blowing performance of the optical fiber cable. The textured surface reduces the contact surface between the cable and the duct or microduct and increases the friction between the blowing medium (e.g., air) and the cable. The protective outer sheath may be made of a low coefficient-of-friction material, which can facilitate blown installation. Moreover, the protective outer sheath can be provided with a lubricant to further facilitate blown installation. 
     In general, to achieve satisfactory long-distance blowing performance (e.g., between about 3,000 to 5,000 feet or more), the outer cable diameter of an optical fiber cable should be no more than about seventy to eighty percent of the duct&#39;s or microducts inner diameter. 
     Compressed air may also be used to install optical fibers according to the present invention in an air blown fiber system. In an air blown fiber system, a network of unfilled cables or microducts is installed prior to the installation of optical fibers. Optical fibers may subsequently be blown into the installed cables as necessary to support the network&#39;s varying requirements. 
     Moreover, the optical fiber cables may be directly buried in the ground or, as an aerial cable, suspended from a pole or pylon. An aerial cable may be self-supporting or secured or lashed to a support (e.g., messenger wire or another cable). Exemplary aerial fiber optic cables include overhead ground wires (OPGW), all-dielectric self-supporting cables (ADSS), all dielectric lash cables (AD-Lash), and figure-eight cables, each of which is well understood by those having ordinary skill in the art. (Figure-eight cables and other designs can be directly buried or installed into ducts, and may optionally include a toning element, such as a metallic wire, so that they can be found with a metal detector. 
     In addition, although the optical fibers may be further protected by an outer cable sheath, the optical fiber itself may be further reinforced so that the optical fiber may be included within a breakout cable, which allows for the individual routing of individual optical fibers. 
     To effectively employ the present optical fibers in a transmission system, connections are required at various points in the network. Optical fiber connections are typically made by fusion splicing, mechanical splicing, or mechanical connectors. 
     The mating ends of connectors can be installed to the fiber ends either in the field (e.g., at the network location) or in a factory prior to installation into the network. The ends of the connectors are mated in the field in order to connect the fibers together or connect the fibers to the passive or active components. For example, certain optical fiber cable assemblies (e.g., furcation assemblies) can separate and convey individual optical fibers from a multiple optical fiber cable to connectors in a protective manner. 
     The deployment of such optical fiber cables may include supplemental equipment. For instance, an amplifier may be included to improve optical signals. Dispersion compensating modules may be installed to reduce the effects of chromatic dispersion and polarization mode dispersion. Splice boxes, pedestals, and distribution frames, which may be protected by an enclosure, may likewise be included. Additional elements include, for example, remote terminal switches, optical network units, optical splitters, and central office switches. 
     A cable containing optical fibers according to the present invention may be deployed for use in a communication system (e.g., networking or telecommunications). A communication system may include fiber optic cable architecture such as fiber-to-the-node (FTTN), fiber-to-the-telecommunications enclosure (FTTE), fiber-to-the-curb (FTTC), fiber-to-the-building (FTTB), and fiber-to-the-home (FTTH), as well as long-haul or metro architecture. Moreover, an optical module or a storage box that includes a housing may receive a wound portion of the optical fiber disclosed herein. By way of example, the optical fiber may be wound with a bending radius of less than about 15 millimeters (e.g., 10 millimeters or less, such as about 5 millimeters) in the optical module or the storage box. 
     Moreover, optical fibers according to the present invention may be used in other applications, including, without limitation, fiber optic sensors or illumination applications (e.g., lighting). 
     In another aspect, the optical fibers according to the present invention may be enclosed by buffer tubes formed from a hardened polymer material (e.g., polysulfone). 
     Those having ordinary skill in the art will appreciate that hardened buffer tubes subject conventional optical fibers to excessive risk of microbending. In contrast and as noted, the present bend-insensitive optical fibers provide exceptional microbending resistance, and so can be satisfactory deployed in hardened buffer tubes. 
     By way of example, the hardened buffer tube may have an outer diameter between about one and two millimeters. An exemplary hardened buffer tube may possess an inner diameter of about 300 microns, thus forming a single-fiber, semi-tight buffer tube (e.g., a hardened buffer tube having an outer diameter of 1.0 millimeters and an inner diameter of about 300 microns). 
     In a particular embodiment, a bend-insensitive optical fiber according to the present invention may be enclosed by a hardened buffer tube formed from polysulfone, such as by extrusion or pultrusion. This kind of hardened buffer tube provides superior resistance to lateral stresses that could otherwise cause microbending or macrobending of the enclosed optical fiber. The hardened buffer tube is capable of withstanding high temperatures (e.g., 200° C.) and exposure to corrosive chemicals (e.g., gasoline). Similar to more complex structures, the present hardened buffer tube offers protection against lateral stresses, high temperatures, and corrosive chemicals yet is less expensive and simpler to manufacture. 
     * * * 
     This application incorporates entirely by reference the following commonly assigned patent, patent application publications, and patent applications: U.S. Pat. No. 5,574,816 for Polypropylene-Polyethylene Copolymer Buffer Tubes for Optical Fiber Cables and Method for Making the Same, and its related patent application Ser. No. 08/377,366; U.S. Pat. No. 5,717,805 for Stress Concentrations in an Optical Fiber Ribbon to Facilitate Separation of Ribbon Matrix Material; U.S. Pat. No. 5,761,362 for Polypropylene-Polyethylene Copolymer Buffer Tubes for Optical Fiber Cables and Method for Making the Same; U.S. Pat. No. 5,911,023 for Polyolefin Materials Suitable for Optical Fiber Cable Components; U.S. Pat. No. 5,982,968 for Stress Concentrations in an Optical Fiber Ribbon to Facilitate Separation of Ribbon Matrix Material; U.S. Pat. No. 6,035,087 for Optical Unit for Fiber Optic Cables; U.S. Pat. No. 6,066,397 for Polypropylene Filler Rods for Optical Fiber Communications Cables; U.S. Pat. No. 6,175,677 for Optical Fiber Multi-Ribbon and Method for Making the Same; U.S. Pat. No. 6,085,009 for Water Blocking Gels Compatible with Polyolefin Optical Fiber Cable Buffer Tubes and Cables Made Therewith; U.S. Pat. No. 6,215,931 for Flexible Thermoplastic Polyolefin Elastomers for Buffering Transmission Elements in a Telecommunications Cable; U.S. Pat. No. 6,134,363 for Method for Accessing Optical Fibers in the Midspan Region of an Optical Fiber Cable; U.S. Pat. No. 6,381,390 for Color-Coded Optical Fiber Ribbon and Die for Making the Same; U.S. Pat. No. 6,181,857 for Method for Accessing Optical Fibers Contained in a Sheath; U.S. Pat. No. 6,314,224 for Thick-Walled Cable Jacket with Non-Circular Cavity Cross Section; U.S. Pat. No. 6,334,016 for Optical Fiber Ribbon Matrix Material Having Optimal Handling Characteristics; U.S. Pat. No. 6,321,012 for Optical Fiber Having Water Swellable Material for Identifying Grouping of Fiber Groups; U.S. Pat. No. 6,321,014 for Method for Manufacturing Optical Fiber Ribbon; U.S. Pat. No. 6,210,802 for Polypropylene Filler Rods for Optical Fiber Communications Cables; U.S. Pat. No. 6,493,491 for Optical Drop Cable for Aerial Installation; U.S. Pat. No. 7,346,244 for Coated Central Strength Member for Fiber Optic Cables with Reduced Shrinkage; U.S. Pat. No. 6,658,184 for Protective Skin for Optical Fibers; U.S. Pat. No. 6,603,908 for Buffer Tube that Results in Easy Access to and Low Attenuation of Fibers Disposed Within Buffer Tube; U.S. Pat. No. 7,045,010 for Applicator for High-Speed Gel Buffering of Flextube Optical Fiber Bundles; U.S. Pat. No. 6,749,446 for Optical Fiber Cable with Cushion Members Protecting Optical Fiber Ribbon Stack; U.S. Pat. No. 6,922,515 for Method and Apparatus to Reduce Variation of Excess Fiber Length in Buffer Tubes of Fiber Optic Cables; U.S. Pat. No. 6,618,538 for Method and Apparatus to Reduce Variation of Excess Fiber Length in Buffer Tubes of Fiber Optic Cables; U.S. Pat. No. 7,322,122 for Method and Apparatus for Curing a Fiber Having at Least Two Fiber Coating Curing Stages; U.S. Pat. No. 6,912,347 for Optimized Fiber Optic Cable Suitable for Microduct Blown Installation; U.S. Patent No. 6,941,049 for Fiber Optic Cable Having No Rigid Strength Members and a Reduced Coefficient of Thermal Expansion; U.S. Pat. No. 7,162,128 for Use of Buffer Tube Coupling Coil to Prevent Fiber Retraction; U.S. Patent Application Publication No. US 2007/0019915 A1 for a Water-Swellable Tape, Adhesive-Backed for Coupling When Used Inside a Buffer Tube (Overton et al.); International Patent Application Publication No. 2007/013923 for Grease-Free Buffer Optical Fiber Buffer Tube Construction Utilizing a Water-Swellable, Texturized Yarn (Overton et al.); European Patent Application Publication No. 1,921,478 A1, for a Telecommunication Optical Fiber Cable (Tatat et al.); U.S. Patent Application Publication No. US 2007/0183726 A1 for a Optical Fiber Cable Suited for Blown Installation or Pushing Installation in Microducts of Small Diameter (Nothofer et al.); U.S. Patent Application Publication No. US 2008/0037942 A1 for an Optical Fiber Telecommunications Cable (Tatat); U.S. Patent Application Publication No. US 2008/0145010 A1 for a Gel-Free Buffer Tube with Adhesively Coupled Optical Element (Overton et al.); U.S. Patent Application Publication No. US 2008/0181564 A1 for a Fiber Optic Cable Having a Water-Swellable Element (Overton); U.S. patent application Ser. No. 12/101/528 for a Method for Accessing Optical Fibers within a Telecommunication Cable, filed Apr. 11, 2008, (Lavenne et al.); U.S. patent application Ser. No. 12/146,526 for a Optical Fiber Cable Having a Deform able Coupling Element, filed Jun. 26, 2008, (Parris et al.); U.S. patent application Ser. No. 12/146,535 for an Optical Fiber Cable Having Raised Coupling Supports, filed Jun. 26, 2008, (Parris); U.S. patent application Ser. No. 12/146,588 for a Coupling Composition for Optical Fiber Cables, filed Jun. 26, 2008, (Parris et al.); U.S. Patent Application No. 61/096,545 for a Optical Fiber Cable Assembly, filed Sep. 12, 2008, (Barker et al.); U.S. Patent Application No. 61/096,750 for a High-Fiber-Density Optical Fiber Cable, filed Sep. 12, 2008, (Lovie, et al.). 
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     In the specification and figures, typical embodiments of the invention have been disclosed. The present invention is not limited to such exemplary embodiments. Unless otherwise noted, specific terms have been used in a generic and descriptive sense and not for purposes of limitation.