Patent Publication Number: US-2004052487-A1

Title: Foamed optical fiber coating and method of manufacture

Description:
TECHNICAL FIELD  
       [0001] The present invention is generally related to optical fiber coatings and, more particularly, is related to foamed optical fiber coatings and methods of manufacture.  
       BACKGROUND OF THE INVENTION  
       [0002] Optical fibers are implemented in various applications, for example but not limited to, single and multiple fiber cables, ribbonized optical fiber cables, etc. During the manufacturing and deployment of fiber cables and the like, various forces and stresses are applied to the cladding and core structures of the optical fibers that make up the units within the cables.  
       [0003] An optical fiber comprises a core region imbedded within a cladding region, the composite of which may be described as a very thin thread or strand of light-transmitting medium. Typically, the optical medium is a substantially pure silica (SiO 2 ) glass, sometimes including small quantities of dopants, such as germania (GeO 2 ), added to the silica to alter the refractive index. While such materials have extremely high inherent strength they are easily damaged. More particularly, the surface of an optical fiber is fragile and susceptible to damage resulting in micron size flaws; therefore it is desirable to protect the fiber during manufacturing or use. Coating material is typically applied to the optical fiber as a liquid during the fiber draw operation, in order that the exposed surface of the fiber be protected immediately. It is preferable that the liquid coating material solidifies rapidly in order to allow the manufacturing process to continue at high speeds.  
       [0004] Optical fiber performance properties most affected by the coating material are strength and transmission loss caused by microbending. Because the optical fibers are thin and flexible, they are readily bent when subjected to mechanical stresses, such as those encountered during placement in a cable or when the cabled fiber is exposed to varying temperature environments or mechanical handling operations. If the stresses placed on the optical fiber result in a random bending distortion of the optical fiber core axis with periodic components in a critical range, light rays, or modes, propagating through the fiber may escape from the core. These microbending losses can be very large. Therefore, the optical fiber should be isolated from mechanical disturbances that cause microbending. The properties of the optical fiber coating material play a major role in providing this isolation.  
       [0005] Two layers of coating materials are typically applied to the optical fiber core. An inner layer, commonly referred to as the primary coating, is applied directly to the surface of the optical fiber cladding. An outer layer, commonly referred to as the secondary coating, is applied directly over the primary coating. It is preferable that the primary coating has a relatively low modulus and that the secondary coating has a relatively high modulus. The primary coating and the secondary coating are applied to the optical fiber either simultaneously or separately during the fiber drawing manufacturing operation.  
       [0006] The primary coating material and the secondary coating material are cured from the outside progressing inwardly toward the cladding surface of the optical fiber. The primary coating material and the secondary coating material typically comprise ultraviolet light curable materials, each material being characterized by a photoactive region. A photoactive region is that region of the light spectrum, which upon the absorption of curing light causes the coating material to change from a liquid phase to a solid phase.  
       [0007] With such materials, the modulus of the primary coating material can be affected by changing the inherent formulation chemistry or by adjusting operational curing parameters. For example, the modulus of the material can be reduced by reducing the degree of cross-linking, and hence, increasing the “cushioning” afforded the optical fiber, but continuing to reduce the cross-link density will result in reducing the robustness of the coating material itself. Modulus is typically altered by modifying the chemistry of the primary coating material. It is preferable that the “cushioning” afforded the optical fiber be increased without compromising the chemical make-up or further reducing the cross-link density, and therefore the robustness, of the material itself.  
       [0008] Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.  
       SUMMARY OF THE INVENTION  
       [0009] Preferred embodiments of the present invention provide an optical fiber product and a method of manufacture. Briefly described, in architecture, one embodiment of the apparatus can be implemented as follows. An optical fiber product comprises an optical fiber through which optical signals can be transmitted. A primary coating layer comprising a foam material surrounds the optical fiber. A secondary coating layer surrounds the primary layer. The coating layers protect the optical fiber and resist microbending forces.  
       [0010] Preferred embodiments of the present invention can also be viewed as providing methods of manufacturing an optical fiber. In this regard, one embodiment of such a method, among others, can be broadly summarized by the following steps: providing an optical fiber through which optical signals can be transmitted; providing a primary coating material; providing an additive for combining with the primary coating material; and providing a secondary coating material.  
       [0011] The method further comprises: combining the additive with the primary coating material; applying the primary coating material to the optical fiber such that the primary coating surrounds the optical fiber; and applying the secondary coating material to the optical fiber surrounded by the primary coating material. At least one of the primary coating material and the secondary coating material is adapted to protect the optical fiber core and resist microbending forces.  
       [0012] Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0013] 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.  
     [0014]FIG. 1 illustrates a perspective view of an optical fiber of the present invention.  
     [0015]FIG. 2 illustrates a cross-section view of the optical fiber illustrated in FIG. 1.  
     [0016]FIG. 3 illustrates a schematic of a manufacturing process of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0017]FIGS. 1 and 2 illustrate one preferred embodiment of an optical fiber product  10  of the present invention. The optical fiber product  10  can be implemented in a single fiber optical cable, multiple fiber optical cable, ribbon cable, etc. As illustrated, the optical fiber product  10  comprises an optical fiber  12 , a primary coating layer  14 , and a secondary coating layer  16 . The optical fiber  12  preferably comprises a core region through which optical signals can be transmitted, and a cladding region. As one example, the optical fiber  12  can comprise a thin strand of light-transmitting silica. The silica may optionally include dopants capable of altering the refractive index of the silica.  
     [0018] The primary coating layer  14  preferably comprises a substantially foam-like material comprising a primary coating material  18  having a plurality of bubbles  20  dispersed throughout. The primary coating material  18  preferably produces enhanced microbend characteristics, thereby enabling the optical fiber product  10  to resist lateral forces encountered during the manufacturing and installation of the optical fiber product  10 . Such materials include, but are not limited to, ultraviolet light curable acrylates, and materials exhibiting an increasing cross-link density as they cure. Voids, or bubbles  20 , added to the primary coating material  18  result in reduction of the primary layer&#39;s modulus without affecting the cross-link density or the curing mechanism of the material  18  itself. For example, a preferred material may exhibit a cross-link density corresponding to a solid modulus of around 100 psi and a foam modulus of 50 psi. The foamed primary layer may range from 25 to 35 μm in thickness. The primary coating layer polymer can be an ultraviolet curable system with an unfoamed modulus of 50 to 500 psi. The bubbles  20 , or voids, can be introduced to the primary coating material  18  by various methods.  
     [0019] In one embodiment, bubbles  20  are formed of various gases and can be introduced into the primary layer material  18  in various manners. A gas, such as nitrogen, oxygen, or the like, can be introduced to the primary coating material  18 , resulting in a foaming of the material  18 . When the foamed material cures or cross-links the chemical make-up or the strength, or integrity, of the material  18  itself is unaffected. The bubbles  20  combined with the primary material  18  create a foam material having reduced modulus, as compared to a primary coating layer  14  comprising a substantially solid primary layer material, without decreasing cross-link density of the resulting polymer. Therefore, the robustness of the primary layer  14  remains relatively high. The bubbles  20  combined with the primary coating material  18  also result in the need for less volume of primary material  18  required to create a primary layer  14  than that needed to manufacture a primary layer  14  comprising solid primary layer material  18 . Simply put, volume comprised of voids, or bubbles  20 , is volume that does not need to be filled by solid material. For example, a 30% foam or void content will allow a 30% reduction in primary coating material used in the manufacture of the optical fiber product.  
     [0020] In another embodiment, bubbles  20  can also be formed in the primary coating material  18  by the introduction of a chemical blowing agent into the primary coating material  18 . Various chemical agents can be added to the primary coating material  18 , for example, such as AZO compounds like azodicarbonamide (also ABFA or azobisformamide) or other such similarly-behaving compounds which decompose to form gasses.  
     [0021] It is preferable that the gas-producing additive does not react with the primary coating material  18  since such reaction between the additive and the primary material  18  could result in changes in the cross-link density, or even premature and uncontrolled foaming. The creation of voids, or bubbles  20 , in the primary layer material  18  occurs as a result of the decomposition of the chemical blowing agent generating gases such as nitrogen. The gas-producing reaction can occur at the introduction of the additive to the primary coating material  18 , during the ultraviolet curing of the primary coating material  18 , or at any suitable point during the manufacturing of the optical fiber product  10 .  
     [0022] Bubbles  20  can also be created through the process of high pressure or super critical fluid (SCF) gas injection of a nominally gaseous material such as carbon dioxide (CO 2 ). Other gasses or gas mixtures may otherwise be appropriate additives for this purpose depending on the end product characteristics sought. In this particular example, the carbon dioxide is preferably under very high pressure as to be in a liquid state prior to its injection into and blending with the liquid primary coating material. It is preferable that the carbon dioxide blending operation with the coating prepolymer result in little or no effect on properties of the resulting primary coating material  18 , such as to decrease the cross-link density. The pressurized carbon dioxide is introduced to the primary coating material  18  and dissolves or disperses uniformly within the prepolymer. The high pressure carbon dioxide mixture expands upon expulsion of the coating material from the die, creating bubbles or voids  20  in the primary coating material  18 , or a foam-like material.  
     [0023] A nucleating agent can also be added to the primary coating material  18 , where either a gas is introduced to the material or where a chemical reaction is used to generate the bubbles  20 , such as with a chemical blowing agent. A preferred nucleating agent encourages uniformity in formation of the bubbles  20  with respect to both the size of the bubbles  20  themselves as well as the distribution of the bubbles  20  throughout the primary coating material  18 . A preferred nucleating agent comprises an inorganic material, such as, for example, silica or titanium dioxide. It is preferable for the nucleating agent to have no effect on material properties of the primary coating material  18 , such as, for example, the cross-link density, the strength of the glass, or the like.  
     [0024] A secondary layer  16  comprises a secondary coating material  19 . It is preferable that the secondary layer  16  is substantially rigid in order to provide mechanical protection for the fiber product. The secondary layer  19  preferably comprises an ultraviolet curable polymer, or any suitable material for forming a substantially stiff exterior coating of modulus ranging from about 50,000 psi to about 150,000 psi. It is preferable that the secondary coating material be applied onto the optical fiber  12  as surrounded by the primary layer material  14 , such as a primary layer material  14  disclosed above. The secondary coating material  19  hardens to form the substantially rigid exterior secondary layer  16  upon ultraviolet curing or at any time during the manufacturing process. It should be noted that the present invention is not limited to two layers, but that any suitable number of layers can be applied around the primary layer  14 .  
     [0025] A preferred embodiment of the present invention also includes methods for manufacturing an optical fiber  10 . FIG. 3 illustrates an embodiment of a manufacturing method  30  of the present invention. The method  30  generally comprises a coating applicator  32 , a primary coating material  18 , a secondary coating material  19  and at least one additive  34 , responsible for producing the foamed structure. The coating applicator  32  applies layers of coating to a bare optical fiber, such as the optical fiber  12  of the optical fiber product  10  of the present invention. The coating applicator  32  receives the optical fiber  12 , the primary layer material  18 , the secondary layer material  19  and preferably at least one additive  34 . The additive  34  may also be pre-incorporated into the coating material in advance, so as to avoid the need to blend or mix the material on-line with the processing of the optical fiber product. It should be noted that the coating applicator  32  can apply the desired number of coatings onto the optical fiber  12 , and is not limited to application of two coatings, as illustrated herein. The primary coating and secondary coating as applied to the optical fiber  12  are cured, solidified, or hardened.  
     [0026] In one embodiment, the primary layer  14  and a secondary layer  16  are applied to the optical fiber  12  in a wet-on-wet coating application process. In a wet-on-wet application process the primary coating material  18  is applied to the optical fiber  12  first. Before the primary coating material  18  is cured, solidified, or hardened, the secondary coating material  19  is applied around the primary coating material  18 . After both the primary coating material  18  and the secondary coating material  19  are applied, both are cured, solidified, or hardened to form the coating layers of the optical fiber product  10 .  
     [0027] In another embodiment, the primary layer  14  and the secondary layer  16  are applied to the optical fiber  12  in a wet-on-dry coating application process. In a wet-on-dry application process the primary coating material  18  is applied to the optical fiber  12  and cured, solidified, or hardened. After the primary coating material  18  is cured, the secondary coating material  19  is applied around the primary layer  14 . The secondary coating material  18  is then cured, solidified, or hardened. It should be noted that although the wet-on-wet and wet-on-dry application processes are disclosed here, any suitable application process may be implemented.  
     [0028] The primary coating material  18  is preferably combined with an additive  34 , as disclosed above. The mixture of primary coating material  18  and additive  34  is communicated to the coating applicator  32  through a primary coating material line  36 . The mixture is applied to the optical fiber  12 . The primary coating material  18  preferably comprises an ultraviolet curable material imparting a resistance to microbending to the optical fiber. Examples of such materials include but are not limited to ultraviolet curable acrylates, and the like.  
     [0029] The additive  34  can comprise any suitable material for creating bubbles  20  in the primary coating material  18 , or foaming the primary coating material  18 . Such materials can include but are not limited to, gas, such as nitrogen, air, carbon dioxide in a gaseous or super critical state, or the like. The additive  34  can also comprise a chemical capable of reacting to produce bubbles or voids throughout the material  18 . The additive  34  can be introduced to the primary coating material  18  as the material  18  is introduced to the coating applicator  32  through the primary coating material line  36  during the manufacturing process (as illustrated in FIG. 3). Alternatively, the additive  34  can be added to the primary coating material  18  before the manufacturing process begins.  
     [0030] The mixture of primary coating material  18  and additive  34  expands as the bubbles  20  are created in the primary coating material  18  to create a “foam-like” material. The primary layer material  18  comprises the solid portions of the “foam-like” material. The bubbles  20 , resulting from the presence of the additive  34  in the primary material  18 , comprise the voids of the “foam-like” material. The primary coating material  18 -additive  34  combination expands as the additive  34  is introduced or after introduction of the additive  34 . Expansion can also occur as the primary coating material  18 -additive  34  combination is communicated to the coating applicator  32  through the primary coating material line  36 , in the coating applicator  32 , or after the primary coating material  18 -additive  34  mixture is applied to the optical fiber  12 . It is preferable, however, that a substantial portion of the expansion occurs prior to the application of subsequent layers, for example but not limited to the secondary layer  16 , around the primary layer  14 . The primary layer material  18 -additive  34  combination can also expand to a “foam-like” material during ultraviolet curing. The expanded primary coating material  18  as combined with the additive  34  and expanded to a “foam-like” material having a plurality of bubbles  20  dispersed throughout the material  18  forms the primary layer  14 .  
     [0031] In another method, an optional nucleating agent is added to the primary coating material  18 . The nucleating agent is preferably present in the primary coating material  18  prior to the application process. For example, the primary coating material  18  can be provided by a supplier of such materials containing the nucleating agent therein or the nucleating agent can be added to the primary coating material before the manufacturing process begins. It is at least preferable that the nucleating agent is present in the primary coating when the bubbles  20  are formed in the primary layer  14 . The combined primary layer material  18 -additive  34 -nucleating agent mixture expands to form a substantially “foam-like” material resulting in the primary layer  14  as previously described. Expansion of the mixture can occur during various steps of the manufacturing process.  
     [0032] Secondary coating material  19  is communicated to the coating applicator  32  through a secondary coating material line  38 . The secondary coating material  19  is applied to the optical fiber  12  as coated by the primary layer material  18 -additive  34  combination. The secondary coating material  19  is preferably introduced to the optical fiber product  10  after the primary coating material  18 -additive  34  combination is applied to the optical fiber  12 . The secondary coating material  19  is introduced around the materials comprising the primary coating  18 . As disclosed above, the application process can be a wet-on-wet application process, a wet-on-dry application process, or any suitable application process.  
     [0033] It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.