Abstract:
Buffered optical fibers and methods of fabricating them are presented. A representative buffered optical fiber includes an optical fiber through which optical signals can be transmitted and an inner layer comprising an ultra-violet (UV) curable acrylate material that surrounds the optical fiber and protects the core of the optical fiber from microbending forces.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is generally related to optical fiber layers and, more particularly, to one or more optical fiber layers that buffer the optical fiber and enhance microbend resistance and low-temperature performance of the optical fiber. 
     BACKGROUND OF THE INVENTION 
     Optical fibers are now in widespread use as communication media. Conventional optical fibers typically include a glassy core and one or more coating layers surrounding the core. Surrounding the coating layers is at least one further layer of material, commonly referred to as a buffer or outer layer, which protects the fiber from damage and which provides the appropriate amount of stiffness to the fiber. The outer layer usually is mechanically stripped away from the fiber when the fiber is connected to an optical fiber connector. Normally, the outer layer is composed of a thermoplastic polymeric material, which is extruded directly over the coated optical fiber. Common materials used to form outer layers include polyvinyl chloride (PVC), nylon, and polyesters, fluropolymers, etc. 
     In many cases, it is necessary that the thermoplastic outer layer be removable without disturbing the optical fiber coatings. This is facilitated by the use of an inner layer, which allows the removal the outer buffer material without removing the coating. The inner layer also facilitates better temperature performance at low temperatures by serving as a compliant layer between the hard thermoplastic buffer material and the optical fiber. 
     Conventional dual-layered tight buffered optical fibers have an inner layer made of polyethylene/ethylene-ethyl acrylate (PE/EEA) copolymer. However, this material has several disadvantages (U.S. Pat. No. 5,684,910). One disadvantage of using PE/EEA is that, being a thermoplastic, the viscosity of the PE/EEA copolymer decreases when the outer layer is applied thereby causing the PE/EEA copolymer to become much less viscous and more fluid-like. The impact of this is that any volatilization coming from the optical fiber coating, whether it is moisture or low molecular weight components, may cause the formation of bubbles in the inner layer. Bubbles, depending on size and frequency, will cause attenuation to increase in the optical fiber, which is typically seen at low temperatures (e.g., −20° C.). If the bubbles are severe, an increase in attenuation may occur at room temperature (˜21° C.). 
     An additional problem caused by the material is that the attenuation of the optical fiber cable increases at temperatures below −20° C. The increased attenuation at temperatures below −20° C. can be attributed to the increase in the elastic modulus of the EEA. Therefore, EEA does not satisfy the need for inner layers having minimal variation in elastic modulus over the temperature range of −40° C. to 80° C. 
     The critical nature of the inner layer becomes even more apparent when applied to newer, higher bandwidth fibers (e.g., 50 micron multi-mode fibers with reduced differential modal dispersion). These fibers and others have higher bandwidths but generally are more microbend sensitive. 
     Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies. 
     SUMMARY OF THE INVENTION 
     Briefly described, embodiments of the present invention provide for buffered optical fibers and methods for fabricating them. A representative buffered optical fiber in accordance with the present invention includes an optical fiber through which optical signals can be transmitted and a layer comprising an ultra-violet (UV) curable acrylate material that surrounds the optical fiber and protects the core of the optical fiber from microbending forces. 
     A representative method for fabricating the buffered optical fiber includes: advancing a fiber core through a coating head oriented in a vertical position; the coating head placing an inner layer on the optical fiber, the inner layer being an ultra-violet (UV) curable acrylate material; advancing the optical fiber having the inner layer thereon through a UV oven oriented in a vertical position, the UV oven curing the UV curable acrylate material; and advancing the optical fiber having the cured inner layer thereon into a horizontal processing system using a transition sheave. 
     Other features and advantages of the present invention will become apparent to one skilled in the art upon examination of the following drawings and detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
     FIG. 1 illustrates a representative buffered optical fiber having an inner layer made of an ultra-violet (UV) curable acrylate material and an outer thermoplastic buffer material. 
     FIG. 2 illustrates a representative buffered optical fiber having an optical fiber glass core, a primary layer, a secondary layer or coating, an inner layer made of the UV curable acrylate material, and a thermoplastic outer buffer coating or layer. 
     FIG. 3 illustrates a perspective view of the optical fiber cable shown in FIG.  2 . 
     FIG. 4 illustrates a schematic diagram of the process for fabricating the optical fibers shown in FIGS. 1 and 2. 
     FIG. 5 illustrates a flow diagram of a representative method for fabricating the buffered optical fiber shown in FIGS.  1  and  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Broadly speaking, embodiments of the present invention are directed to buffered (e.g., tight or semi-tight) optical fibers, which can then be made into cables or cordage, having increased microbend resistance and improved low-temperature performance. The buffered optical fiber can be a single-fiber buffered optical fiber, a multiple-fiber buffered optical fiber bundle or array, or a buffered ribbon, for example. 
     The buffered optical fibers of the present invention can include, for example, an inner layer and at least one coating layer constructed of an ultra-violet (UV) curable acrylate material whose modulus and thickness can be varied to optimize performance. 
     Constructing the inner layer and/or the coating layer of UV curable acrylate material enables the optical fiber cable to have increased microbend resistance, which enables the optical fiber to resist the lateral forces encountered during the manufacture of the optical fiber cable or cordage (interconnect cable) without increasing attenuation of the optical fiber(s) in the cable. The resultant cable or cordage is also better able to resist lateral forces encountered during the installation of the cable in the service environment, thereby avoiding increases in attenuation. In addition, the resultant cable and/or cordage has better performance at low temperatures, which, because of the thermal expansion coefficient of the materials making up the cable and/or cordage, cause the cable and/or cordage to contract and induce microbending in the buffered fiber. 
     UV curable acrylates are cured (i.e., cross-linked) materials that do not flow substantially during the extrusion process in the way that thermoplastics do, which are used as inner layer materials in conventional arrangements. Thus, using cured materials enables the formation of bubbles in the inner layer to be avoided. In addition, the UV curable acrylates exhibit minimal variation in elastic modulus over the temperature range of about −40° C. to about 85° C. This allows the inner layer to remain compliant over a much broader temperature range, and thereby prevents attenuation loss at low temperatures. 
     Now, having described optical fibers of the present invention in general, FIGS. 1 through 5 will be described in order to demonstrate some potential embodiments of optical buffered fibers of the present invention and the associated methods of fabrication thereof. While embodiments of buffered optical fibers are described in connection with FIGS. 1 through 5 and the corresponding text, there is no intent to limit embodiments of the optical fibers to these descriptions. To the contrary, the intent is to cover all alternatives, modifications, and equivalents included within the spirit and scope of the present invention. 
     With reference to the figures, FIG. 1 illustrates a buffered optical fiber  10  having an outer or buffer layer  12 , an inner layer  14 , and an optical fiber  16 . The optical fiber  16  may include a glassy core and one or more coating layers surrounding the core (not depicted). In the embodiment illustrated in FIG. 1, the coating layers surrounding the glassy core can be made of materials known in the art. 
     The inner layer  14  can be made from an UV curable acrylate material. Preferably, the inner layer  14  is made of UV curable acrylate materials such as, for example, UV curable urethane acrylate, a UV curable silicon acrylate, and/or a UV curable siloxane acrylate material. 
     When the inner layer  14  is made of UV curable acrylate materials, the microbend resistance and low-temperature performance of the buffered optical fiber  10  will be improved. Increased microbend resistance allows the buffered optical fiber  10  to resist lateral forces encountered during cable manufacture and installation such that minimal optical loss occurs. Improved low-temperature performance of the buffered optical fiber  10  allows an optical fiber cable made up of one or more buffered optical fibers  10  to be constructed such that minimal optical loss occurs. 
     In addition, the inner layer  14  comprised of the UV-curable acrylate material can have one or more of the following characteristics. First, the inner layer  14  can have a glass transition temperature less than about −10° C. Buffered optical fibers  10  having inner layers  14  with low glass transition temperatures have increased optical performance at low temperatures. Second, the inner layer  14  can have a secant tensile modulus at 2.5% elongation (tensile modulus modulus at 2.5%) of about 0.5 megapascals to about 10 megapascals, about 0.8 megapascals to about 2.5 megapascals, or preferably, about 0.9 megapascals to about 1.7 megapascals. Buffered optical fibers  10  having an inner layer  14  with low tensile modulus have increased low-temperature optical performance and increased microbend resistance, and the strip force is kept within an acceptable range. 
     Third, the inner layer  14  has a gel fraction of greater than about 70%, about 70% to about 95%, or preferably, about 85% to about 95%. Decreased outgassing during processing can be achieved by fabricating optical fiber cables  10  having inner layers  14  with high gel fractions, thereby decreasing interfacial voids. 
     Fourth, the inner layer  14  has a viscosity of about 2,000 to about 10,000 megapascals per second, or preferably, about 3,300 to about 6,200 megapascals per second. Increased processing line speed can be achieved for optical fiber cables  10  having inner layers  14  with low viscosities. In addition, buffered optical fibers  10  having low viscosity inner layers  14  coat the optical fiber in a uniform manner. 
     The inner layer  14  can have a thickness of about 10 μm to about 200 μm, about 20 μm to about 125 μm, or preferably, about 35 μm to about 95 μm. 
     The outer layer  12  can be a high-modulus material such as, for example, polyvinyl chloride (PVC), polyamide (nylon), polypropylene, polyesters (e.g., PBT), and fluoropolymers (e.g., PVDF or FEP). In addition, the outer layer  12  may include one or more layers. Preferably, the outer layer  12  has a thickness of about 200 μm to about 350 μm. 
     FIG. 2 illustrates a cross sectional view of a buffered optical fiber  30 , and FIG. 3 illustrates a perspective view of the buffered optical fiber  30 . In accordance with this embodiment, the buffered optical fiber  30  includes an outer layer  12 , an inner layer  14 , an optical fiber  32 , a primary layer coating  36 , and a secondary layer  34 . The primary layer coating  36  surrounds the fiber core  38  and the secondary layer  34  surrounds the primary layer. The fiber core  38  is a conduit for transmitting energy (e.g., light) and can be made of materials such as glass or plastic. 
     The primary layer coating  36  can include an UV-curable acrylate material. Preferably, the primary layer coating  36  is made of UV-curable acrylate materials such as, for example, a UV curable urethane acrylate, a UV curable silicon acrylate, and/or a UV curable siloxane acrylate material. When the primary layer coating  36  is made of UV-curable acrylate materials, the microbend resistance of the buffered optical fiber  30  is increased. As stated above, increased microbend resistance allows the buffered optical fiber  30  to resist lateral forces encountered during cable manufacture and installation such that minimal optical loss is detected. 
     In addition, the primary layer coating  36  can have one or more of the following characteristics. First, the primary layer coating  36  can have a glass transition temperature less than −10° C. Optical fiber cables  30  having a primary layer coating  36  with low glass transition temperatures have increased optical performance at low temperatures. 
     Second, the primary layer coating  36  can have a 2.5% secant tensile modulus (tensile modulus at 2.5% elongation) of about 0.5 megapascals to about 10 megapascals, about 0.8 megapascals to about 2.5 megapascals, or preferably, about 0.9 megapascals to about 1.7 megapascals. Buffered optical fibers  30  having a primary layer coating  36  with low tensile modulus increases the low-temperature optical performance and increases the microbend resistance, while keeping the strip force in an acceptable range. 
     Third, the primary layer coating  36  has a gel fraction of greater than about 85%, about 85% to about 95%, or preferably, about 90% to about 95%. Decreased outgassing during processing can be achieved by fabricating buffered optical fibers  30  having primary layer coating  36  with high gel fractions, thereby decreasing interfacial voids. 
     Fourth, the primary layer coating  36  has a viscosity of about 2,000 to about 10,000 megapascals per second, or preferably, about 3,300 to about 6,200 megapascals per second. Increased processing line speed can be achieved for optical fibers  32  having respective primary layers  36  with low viscosities. In addition, low viscosity inner layers  14  coat the optical fiber  32  in a uniform manner. 
     The primary layer coating  36  can have a thickness of about of about 20 μm to about 50 μm, and preferably a thickness of about 35 μm to about 45 μm. 
     The secondary layer  34  can be fabricated from materials such as a UV curable urethane acrylate, a UV curable silicon acrylate, and/or a UV curable siloxane acrylate material, for example. The thickness of the secondary layer  34  can range from about 10 μm to about 40 μm, and the thickness can be adjusted to yield an optical fiber  32  having with an outer diameter of, for example, about 250 μm. The thickness of the primary layer coating  36  and of the secondary layer  34  can be adjusted to achieve an appropriate or desired microbend resistance and low-temperature performance. For example, if the primary layer coating  36  and the secondary layer  34  each have a thickness of about 40 μm and 22 μm, respectively, improved microbend resistance and low-temperature performance will be achieved. 
     The inner layer  14  and the outer buffer layer  12  have been described above with reference to FIG.  1 . Therefore, no further discussion about these two layers will be provided herein. It should be noted that buffered optical fiber  30  having the primary layer coating  36  and the inner layer  14  fabricated from UV-curable acrylate materials can achieve appropriate microbend resistances. The resultant cable or cordage is also better able to resist lateral forces encountered during the installation of the cable in the service environment, thereby preventing increases in attenuation. In addition, the resultant cable and/or cordage is better able to perform at low temperatures. As stated above, because of the thermal expansion coefficient of the materials making up the cable and/or cordage can cause the cable and/or cordage to contract and induce microbending in the buffered fiber. This is avoided by using the aforementioned materials in accordance with the present invention. 
     FIG. 4 illustrates a schematic diagram of a representative apparatus  40  for fabricating the buffered optical fiber  10  and/or the buffered optical fiber  30  of the present invention. The apparatus  40  includes a vertical processing system  45  and a horizontal processing system  50 . The fiber core  16  and/or fiber core  38  (hereinafter fiber  54 ) is located on a spool  52 . The fiber  54  is advanced through a coating head  56  oriented in a vertical position which places the inner layer on the fiber  57 . 
     After the inner layer is placed on the fiber  54 , the coated fiber  57  is advanced through ultra-violet ovens  58 , which cure the inner layer. The orientation of the coating head  56  in the vertical position allows for geometric control of the layer and greater line speeds than if the coating head  56  were in the horizontally oriented. However, it should be noted that the optical fiber cables  10  and  30  can be fabricated while having the coating head  56  and/or the ultra-violet ovens  58  oriented in the horizontal position. 
     Thereafter, a transition sheave  60  directs the cured inner-layer-coated fiber  57  into the horizontal processing system  50 . The cured inner-layer-coated fiber  57  is advanced through a cross-head extruder  62 , which places a thermoplastic material on the inner-layer-coated fiber. Water troughs  64 , cool and harden the thermoplastic coating. The fiber is then taken up on a take-up roll  66 . 
     FIG. 5 illustrates a representative flow diagram of the process  70  of fabricating buffered optical fiber  10  and/or buffered optical fiber  30 . Initially, the optical fiber  54  is fed into the vertical processing system  45 , as shown in block  72 . Then, the optical fiber  54  is advanced through the coating head  56  that is vertically oriented, where the inner layer is coated onto the optical fiber, thereby forming the inner-layer-coated fiber  57 , as shown in block  74 . Subsequently, the inner-layer-coated fiber  57  is advanced through the ultra-violet ovens  58  to cure the layer, as shown in block  76 . 
     Next, the cured inner-layer-coated fiber  57  is advanced through the horizontal processing system  50 , as shown in block  78 . The cured inner-layer-coated fiber  57  is then advanced into a thermoplastic extrusion cross-head  62 , which coats the fiber with a thermoplastic material, and then through the water troughs  64 , which cool and harden the thermoplastic material as shown in block  80 . Thereafter, the buffered optical fiber  10  and/or  30  is taken up on a take-up roll  66 . 
     It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely examples of implementations of the present invention, and are set forth herein to provide a clear understanding of the principles of the present invention. Many variations and modifications may be made to the above-described embodiments of the present invention without departing from the scope and principles of the invention. For example, the inner layer coating step discussed above could be accomplished in a horizontal orientation and the buffer extrusion step could be accomplished in a vertical orientation. All such modifications and variations are within the scope of this disclosure and the present invention.