Title of Invention

COATED OPTICAL FIBER

Abstract

The present invention relates to a coated optical fiber having at least one coating formed around the outside of a glass optical fiber, wherein the Storage modulus E of the primary coating in contact with said glass optical fiber is within the range of 0.01 kg/mm2 to 2.0 kg/mm2 at 25° C and 110 Hz, and the adhesion between said glass optical fiber and said primary coating is within the range of 10 g/cm to 200 g/cm.

Full Text

DESCRIPTION COATED OPTICAL FIBER Technical Field The present invention relates to a coated optical fiber, an optical fiber ribbon, and an optical fiber unit. Background Art An optical fiber is obtained by drawing an optical fiber preform having at least a core and a cladding. Immediately after drawing, a coating is formed around the outside of the glass optical fiber in order to protect and reinforce the fiber and to give it flexibility and for other purposes. It is known to form at least two layered coatings, namely, a relatively soft (low Young's modulus) primary coating which is in contact with the outer circumference of the glass optical fiber and which has a buffering function, and a hard (high Young's modulus) secondary coating formed on the outermost side and having a protective function. For example, a coated optical fiber is proposed in Japanese Patent Application Laid open No. 9"5587, in which, by setting the extraction force (the force necessary to pull a glass optical fiber out of the coated optical fiber which is fixed from the outside), which force serves as the adhesion between the glass optical fiber and the primary coating material forming the primary coating, at 90 g/mm to 180 g/mm, an adequate adhesion between the glass optical fiber \ and the primary coating material is obtained, and partial delamination will not occur at the interface between the glass optical fiber and the primary coating material even when they are soaked in water for a long period. However, when the extraction force corresponding to the adhesion between the glass optical fiber and the primary coating material is set at 90 g/mm to 180 g/mm as described in the above publication, delamination sometimes occurs at the interface between the glass and the coating material in the coated optical fiber during or after the production of the coated optical fiber- for example, during the period from drawing to cabling of the optical fiber or during winding onto another bobbin. It is thought that minute foreign materials such as chips of the optical fiber, adhering onto a pulley, on which the coated optical fiber travels, due to static electricity or the like cause the coated optical fiber to undergo large local deformation, and as a result, delamination occurs at the interface between the glass optical fiber and the primary coating. An object of the present invention is to provide a coated optical fiber in which such delamination during the period from drawing to cabling is prevented. A tapelike (ribbonlike) cable, which is formed by placing a plurality of coated optical fibers in parallel in a plane and by forming a unitary jacket around the coated optical fibers so as to coat them therein, is referred to as an "optical fiber ribbon". When connecting optical fiber ribbons, it is necessary to collectively remove the layers' the primary coating formed around the outside of the glass optical fiber, and the unitary jacket. This operation is referred to as "collective stripping of coatings". In this case, it is undesirable that the coatings such as the primary coating or secondary or other coatings remain on the surface of the glass optical fiber. Another object of the present invention is to provide a coated optical fiber which has an improved delamination resistance during drawing and rewinding and whose collective strippability of coatings is not degraded when it is used as a component of an optical fiber ribbon. A further object of the present invention is to provide a coated optical fiber which is assembled into a certain type of optical fiber unit or inserted into a loose tube and is subjected to stripping of individual coated fiber when connection is performed and in which the delamination resistance is increased and stripping of individual coated fiber is easy. A still further object of the present invention is to provide an optical fiber ribbon in which the delamination resistance is increased and the collective stripping of coatings is easy or an optical fiber unit in which the delamination resistance is increased and stripping of individual coated fiber is easy. Disclosure of Invention In order to achieve the above objects, the present invention relates to-(l) A coated optical fiber having at least one coating formed around the outside of a glass optical fiber, wherein the storage modulus E' of the primary coating in contact with the glass optical fiber is within the range of 0.01 kg/mm2 to 2.0 kg/mm2 at 25°C and 110 Hz, and the adhe sion between the glass optical fiber and the primary coating is within the range of 10 g/cm to 200 g/cm. (2) A coated optical fiber according to (l) above, wherein the storage modulus E' is within the range of 0.01 kg/mm2 to 0.5 kg/ mm2 and the adhesion is within the range of 10 g/cm to 100 g/cm. (3) A coated optical fiber according to (l) above, wherein the storage modulus E' is within the range of 0.01 kg/mm2 to 2.0 kg/mm2 and the adhesion is within the range of 100 g/cm to 200 g/cm. (4) A coated optical fiber according to any of (l) to (3) above, wherein the primary coating is made of ultraviolet-curable resin. (5) An optical fiber ribbon wherein a plurality of coated optical fibers according to any of (l), (2), and (4) above are arranged in parallel and are collectively coated with a unitary resin jacket. (6) An optical fiber unit wherein a plurality of coated optical fibers according to any of (l), (3), and (4) above are arranged and are collectively coated with a unitary resin jacket. Brief Description of the Drawings Figure 1 is a graph showing the correlation among delamination, the storage modulus E' of a primary coating, and the adhesion (between glass and the primary coating), and the collective strippability of coatings of an optical fiber ribbon. Figure 2 is a schematic sectional view showing an example of a coated optical fiber according to the present invention. Figure 3 is a schematic sectional view showing the structure of an optical fiber ribbon using the coated optical fiber of the present invention shown in Fig. 1. Figure 4 is a graph showing the correlation among delamination, the storage modulus E' of the primary coating, and the adhesion (between glass and the primary coating), and the coating strippability. Figure 5 is a schematic sectional view showing an example of an optical fiber unit using the coated optical fiber of the present invention shown in Fig. 1. Best Mode for Carrying out the Invention The present inventors have found that regarding delamination the storage modulus E' of a primary coating and the adhesion between a glass optical fiber and the primary coating is correlated with the frequency of delamination, and thus achieved the present invention. Figure 2 is a schematic sectional view of an example of the present invention. A primary coating 2, which has a storage modulus E' of 0.01 kg/mm2 to 0.5 kg/mm2 at 25°C and at a frequency of 110 Hz, is formed around the outside of a glass optical fiber 1 having at least a core and a cladding, the adhesion between the primary coating 2 and the glass optical fiber 1 is 10 g/cm to 100 g/cm, and a secondary coating 3 is formed around the outside of the primary coating 2, thereby constructing a coated optical fiber 4. In the present invention, the storage modulus E' is obtained by performing a calculation acωrding to the following Equation (l), based on an out-of-phase sinusoidal stress S = S0 exp[i(ω+δ)] which is detected at one end of a sample when a sinusoidal strain r == r0 exp(iωt) is applied as mechanical vibration to the other end- where E* represents the ωmplex visωelasticity, E' represents the storage modulus, E* represents the loss visωelasticity, S represents the stress, S0 represents the amplitude of the stress, r represents the strain, r0 represents the amplitude of the strain, ω represents the angular velocity, t represents the time, and 6 represents the phase shift angle. In Eq. (l), δ is a function of ω. In a quasi-static state, that is, when ω is 0, 6 is 0, E" is 0, and the storage modulus E' ωrresponds to the so-called Young's modulus. A description will be given to explain why the storage modulus E' of the primary ωating resin serves as an index in the present invention. In drawing of an optical fiber, a coated optical fiber is wound onto a bobbin via some pulleys. When static electricity or the like causes foreign materials to adhere to portions of the pulleys where the optical fiber travels, delamination sometimes occurs at the interface between the glass and the ωating. Since the ωated optical fiber is produced at high speed, the pulleys are also rotated at high speed, and strain is rapidly applied to the ωating, that is, at a high strain rate. ωnventionally, the Young's modulus of the ωating is measured based on "the stress applied to extend the ωating by 2.5% at a low strain rate of approximately 1 mm/min". The Young's modulus found from the gradient of a line which links a stress ωrresponding to a strain of 0 and a stress ωrresponding to a strain of 2.5% is referred to as "2.5% secant Young's modulus". From the viewpoint of measurement, this Young's modulus is used instead of the Young's modulus at the origin where the strain is 0. However, the present inventors have ωnsidered that it is more appropriate to use the storage modulus E' as the index, which is the real part of the elastic modulus in a high strain rate region, as opposed to using the Young's modulus measured at a low strain rate which has been used in this technical field, such as the 2.5% secant Young's modulus, because the resin used for the primary ωating or the like exhibits visωelastic behavior, and have performed investigations. As a result, the present inventors found that there was a ωrrelation between the storage modulus E' at 25°C and 110 Hz and the frequency of delamination, which can be used for an index of elastic modulus in the high strain rate region. In the present invention, the adhesion is expressed as the force necessary to separate a primary ωating from a glass plate by 50 mm in a 180°-direction at a pulling rate of 200 mm/min after the primary ωating is formed on the surface of the glass plate. A more precise definition thereof will be provided in the following examples. Figure 1 is a graph which, based on the results of the following examples of the present invention, shows the proper ranges of the storage modulus E' (25°C, 110 Hz) of a primary ωating of an optical fiber ribbon acωrding to the present invention, and the adhesion between a glass optical fiber and the primary ωating. As shown in Fig. 1, when the storage modulus E' of the primary ωating at 25°C and 110 Hz is within the range of 0.01 kg/mm2 to 0.5 kg/mm2 and the adhesion is within the range of 10 g/cm to 100 g/cm, delamination does not occur at the interface between the glass optical fiber and the primary ωating, and productivity and reliability of the ωated optical fiber are improved. When the storage modulus E' of the primary ωating is less than 0.01 kg/mm2, the primary ωating is prone to internal rupture (voids) during handling. Since the voids increase the transmission loss, for example, in a low-temperature environment at 40°C, the formation of voids must be prevented. In ωntrast, when the storage modulus E' exceeds 0.5 kg/mm2 in the ωated optical fiber of the optical fiber ribbon, delamination is prone to occur at the interface between the glass optical fiber and the primary ωating. It is supposed that, when the elastic modulus at a high strain rate is too high, a large stress is produced at the interface between the glass optical fiber and the primary ωating when the ωated optical fiber is deformed, causing the likelihood of delamination to occur. A particularly preferable storage modulus E' is within the range of 0.02 kg/mm2 to 0.3 kg/mm2 at 25°C and 110 Hz. Delamination at the interface can be prevented by increasing the adhesion between the glass optical fiber and the primary ωating. When the adhesion is excessively strong, however, the ωllective stripping of ωatings is difficult when ωnnecting the optical fiber ribbon to other glass optical fibers. Therefore, it is preferable that the adhesion be less than or equal to 100 g/cm. In ωntrast, when the adhesion is less than 10 g/cm, delamination at the interface between the glass optical fiber and the primary ωating is likely to occur. A particularly preferable adhesion is within the range of 15 g/cm to 75 g/cm. Not all of the optical fibers are formed into a ribbon, but the optical fibers may be assembled into a tight unit or may be inserted in a loose tube. In these cases, it is only necessary to take stripping of individual ωated fiber into ωnsideration. In such cases, both the storage modulus E' and the adhesion between the glass optical fiber and the primary ωating can be made stronger than those in the above-described ωated optical fiber for the fiber ribbon, and this can increase the delamination resistance. Figure 4 is a graph which shows, based on the results of the following examples of the present invention, a region in which the storage modulus E' of a primary ωating of a ωated optical fiber and the adhesion between a glass optical fiber and the primary ωating are proper and stripping of individual ωated fiber is possible in a case in which it is unnecessary to take ωllective stripping of ωatings into ωnsideration. As shown in Fig. 4, even when the storage modulus E' of the primary ωating at 25'C and 110 Hz is within the range of 0.01 kg/mm2 to 2.0 kg/mm2 and the adhesion between the glass and the primary ωating is within the range of 100 g/cm to 200 g/cm, delamination does not occur at the interface between the glass and the primary ωating, and therefore, productivity and reliability of the ωated optical fiber are increased. When the storage modulus E' at 25°C and 110 Hz is less than 0.01 kg/mm2, internal rupture (voids) is likely to occur in the primary ωating during handling, and the transmission loss tends to increase at low temperature. When the storage modulus E' exceeds 2.0 kg/mm2, delamination is likely to occur at the interface between the glass optical fiber and the primary ωating. While it is satisfactory for a minimum of the adhesion between the glass optical fiber and the primary ωating to be 10 g/cm, as shown in Fig. 1, it is preferable that the adhesion be more than or equal to 100 g/cm in a case where only the strippability of stripping of individual ωated fiber is to be ωnsidered, since the delamination resistance is increased. When the adhesion exceeds 200 g/cm, it is difficult to perform stripping of individual ωated fiber when ωnnecting the optical fiber to another optical fiber. In the present invention, the composition, structure, and production method of the glass optical fiber itself are not limited specifically, and may ωnform to those known in this technical field. It is only necessary for the ωating material of the primary ωating in the present invention to satisfy the storage modulus E' and the adhesion of the present invention. While the ωating material is, for example, a (metha)acrylate resin, such as urethane (metha)acrylate resin, polybutadiene (metha)acrylate resin, polyether (metha)acrylate resin, polyester (metha)acrylate resin, or epoxy (metha)acrylate resin: an unsaturated polyester; a cation-polymerized epoxy resinJ an allylic compound resin; or an ultraviolet-curable resin ωnsisting of a mixture of the resins, it is not limited thereto. These resins may be a composite resin ωntaining various components, and, for example, various kinds of reactive monomers, a polymerization initiator, and various kinds of additives, such as a chain-transfer agent, an anti'oxidizing agent, a photostabilizer, a plasticizer, a silane ωupling agent, a polymerization-inhibitor, a sensitizing agent, and a slip additive, may be added thereto as necessary. In the present invention, resin and composite resin are generically referred to as "resin". It is necessary to select, as the primary ωating in the present invention, a material having a storage modulus E' and an adhesion to the glass optical fiber which are within the ranges specified in the invention, as described above. The storage modulus E' of the primary ωating can be adjusted by the molecular weight of polyether in an oligomer which makes up the skeleton of the resin serving as the wating material, and the the of reactive diluent monomer. That is, the storage modulus E' can be increased, for example, by reducing the molecular weight of polyether, increasing the blending amount of a polyfunctional monomer, or selecting a monomer with high rigidity. Furthermore, the storage modulus E' can be increased by arranging for the composition to cotain many benzene rings with high rigidity, or the like. The adhesion between the glass optical fiber and the primary wating can be adjusted by changing the cotent of an adhesive monomer in the wating material serving as the primary wating, and the additive rate (including O) of the silane ωupling agent. The adhesive monomer is, for example, isobornyl acrylate, acrylamide, N-vinylpyrrolidone, or acryloyl morpholine. While the secodary wating formed around the outside of the primary wating, and subsequent watings are not specifically limited in the present invention, watings, which have a Young's modulus higher than that of the primary wating, for example, approximately 50 kg/mm2 to 150 kg/mm2, and which function as a protective layer are preferable. While a resin similar to the primary wating may be used as the material of the watings, a ωlored optical fiber may be formed by, for example, adding a ωloring agent to the outermost layer. There are no limitations to the method of forming the watings, such as the primary wating and the secodary wating, immediately after drawing the optical fiber, and the method may coform to a method well known in this V 1 technical field. For example, in the case of an energy curable resin which is curable with, for example, heat or light, a wating is formed by curing the wating material by radiation of the ωrresponding energy. The use of an ultraviolet-curable resin has the advantage of shorter curing time. In the case in which the wated optical fiber of the present invention is also provided with a unitary jacket so as to form an optical fiber ribbon, delamination on the production line is reduced, and ωllective strippability of watings thereof is enhanced as compared with coventional products. Figure 3 is a schematic sectional view showing a example of an optical fiber ribbon 8 formed by placing a plurality of ωlored fibers 6, in each of which a ωlored layer 5 is formed around the outside of the wated optical fiber 4 shown in Fig. 2, in parallel, and ωllectively wating the ωlored fibers 6 with a unitary jacket 7. In general, for example, an ultraviolet-curable resin, which is similar to the primary wating and the secodary wating and has a Young's modulus of, for example, approximately 50 kg/mm2 to 150 kg/mm2, is used as the unitary jacket material of the optical fiber ribbon. The jacket may be formed by a well-known method. Figure 5 shows a cocrete example of a structure of an optical fiber unit 11 formed by bundling a plurality of ωlored fibers 6, in which a ωlored layer 5 is formed around the outside of each of the wated optical fibers 4, around a center tension member 12, and ωllectively wating the ωlored fibers 6 with unitary jackets 9 and 10. Various structures other than those illustrated above are well known. The material of the unitary jackets may be a resin similar to that of the above-described optical fiber ribbon. The jackets may be formed by a well-known method. Examples While examples of the present invention will be described below in detail, the present invention is not limited thereto. (Examples 1 to 4, comparative Examples 1 to 4, and comparative Examples 1' to 40 When producing a two"layers wated optical fiber in which a primary wating was formed around the outside of a quartz glass optical fiber having an outer diameter of 125 μm so as to obtain an outer diameter of 200μm, and a secodary wating was formed therearound so as to obtain an outer diameter of 240 μm, a relatively soft photo-curable urethane acrylate resin was used as the material of the primary wating, and a relatively hard photo-curable urethane acrylate resin was used as the secodary wating. In order to adjust the adhesion, resins were prepared as the material of the primary wating, in which a polar monomer (for example, acrylamide, N* vinylpyrrolidone, or acryloyl morpholine) or a silane-ωupling agent was cotained at different rates. First, tests were coducted to measure the storage modulus E' of the primary wating materials and the adhesion thereof to glass which was the same material as that of the glass optical fiber. 1 Regarding the measurement of the storage modulus E', each of the primary wating materials was formed into, the shape of a sheet and was irradiated with 1000 mJ/cm2 of ultraviolet light [the amount of light was measured by using a UV meter UV-MIO (spectral sensitivity UV-35) manufactured by Ohku Seisakusho] using a mercury lamp (a metal halide lamp M015-L312 manufactured by Eye Graphics) in a nitrogen atmosphere, thereby obtaining a sheet having a thickness of 200 μm. A sample having a width of 4 mm, a length of 20 mm, and a thickness of 200 μm was cut from the sheet, and the storage modulus E' of the sample was measured at a frequency of 110 Hz with a vibration displacement of 0.016 mm by using Rheo-Vibron (manufactured by Orientic ωrporation) as an elastometer. The adhesion was measured by the following procedure- (1) By soaking a quartz glass plate (200 mm. by 150 mm) in sulfuric acid for more than five minutes, the surface thereof was cleaned. (2) A resin liquid for forming the primary wating was applied onto the cleaned quartz glass plate, was irradiated with 100 mJ/cm2 of ultraviolet light by using the same mercury lamp as described above, and was thereby cured. As a result, a sample was obtained in which the cured resin layer had a thickness of 200 μm, a width of 50 mm, and a length of 170 mm. (3) Obtained samples were left in an atmosphere at 25°C and at 50% RH for one week. (4) Next, the resin layer of each sample was peeled off the quartz glass plate by 50 mm at a pulling rate of 200 mm/min in the 180°-direction. The maximum force (g/cm per unit width) required in this case was defined as the adhesion between the glass optical fiber made of the same material and the primary wating resin, and was regarded as the adhesion between the glass optical fiber and the primary wating. A primary wating was formed around the outside of a quartz glass optical fiber having an outer diameter of 125 μm so as to obtain an outer diameter of 200 μm, and a secodary wating was formed therearound so as to obtain an outer diameter of 240 μm. Primary watings were formed so as to have various combinations of the adhesion to the glass optical fiber and the storage modulus E' shown, in Table I acωrding to the above test results, and wated optical fibers were produced at a drawing rate of 200 m/min using one UV furnace F-10, manufactured by Fusion UV Systems, Inc. for curing both the primary wating and the secodary wating (Examples 1 to 4, comparative Examples 1 to 4, and comparative Examples V to 4'). Delamination and Void (Internal Rupture of wating) Tests and Evaluations After the production, the wated optical fiber was unreeled from the take-up bobbin, and was soaked in a matching oil the refractive index of which is adjusted to be equal to that of the wating (if it is not soaked in the matching oil, the observation of the interface between the glass optical fiber and the wating is impossible), and was observed at 50x magnification by an optical microsωpe from the direction of the side face of the wated optical fiber. It was determined that the cases in which voids and delamination did not occur at all were acceptable. Evaluations of ωllective strippability of watings A fiber ribbon was produced by using the obtained wated optical fiber, and the watings of the fiber ribbon were manually and ωllectively stripped by a heating remover JR-4A (trade name, manufactured by Sumitomo Electric Industries, Ltd.). It was determined that the cases in which the watings ωuld be removed from the surface of the glass optical fiber at a heating temperature of 90°C were acceptable. When the adhesion between the glass optical fiber and the wating was too strong, the wating ωuld not be stripped from the glass optical fiber. The above results are shown in Table I and Fig. 1. In Fig. 1, the horizontal axis represents the storage modulus E' (kg/mm2), the vertical axis represents the adhesion (g/cm) between the glass and the primary wating, "O" represents proper adhesion (no delamination, no void, and good ωllective strippahility of watings) in the examples of the present invention, "x" shows that delamination occurred, "-f" represents that voids occurred, and "" shows that ωllective stripping of watings of the fiber ribbon failed. In the figure, the diagonally shaded area represents a region where the storage modulus E' and the adhesion are proper. It is shown that delamination and voids do not occur and the ωllective strippability of watings is high when the storage modulus E' and the adhesion are within the range of the present invention. (Examples 5 to 9 and comparative Examples 5 and 6) In a manner similar to that of Example 1 described above, wated optical fibers were produced in which the storage modulus E' and the adhesion between the glass and a primary wating were different, as shown in Table H (Examples 5 to 9, and comparative Examples 5 and 6). The obtained wated optical fibers were tested for delamination and voids, in a manner similar to that of Example 1, and further, the wating strippabilities thereof were evaluated as follows. The results are shown in Table II and Fig. 4. Evaluations of wating Strippability" The wating of the wated optical fiber was stripped by using a wating stripper "No-Nick NN203" (trade name, manufactured by Clauss Inc.). It was determined that stripping of individual wated fiber was difficult in the cases in which the wating ωuld not be stripped off the glass optical fiber, and the cases are marked with "x" in Table II. In Table 11, "o" means that signal stripping of individual wated fiber is easy In Fig. 4, the horizontal axis represents the storage modulus E' (kg/mm2), the vertical axis represents the adhesion (g/cm) between the glass and the primary wating, "O" represents proper adhesion (no delamination, no void, and good wating strippability) in the examples of the present invention, "x" shows that delaminatioti occurred, and "" shows that stripping of individual wated fiber of the wated optical fiber failed. In the figure, the diagonally shaded area represents a region where the storage modulus E' and the adhesion are proper. It is shown that delamination and voids do not occur and the wating strippability is high when the storage modulus E' and the adhesion are within the range of the present invention. Industrial Applicability As described above, acωrding to the present invention, the delamination resistance of the wated optical fiber was increased, the problem of delamination during drawing and winding processes can be solved, and the occurrence of voids is prevented inside the wating. This enhances the productivity and quality of the wated optical fiber. In the optical fiber ribbon using the wated optical fibers of the present invention, ωllective strippability of watings is enhanced, and the working efficiency in, for example, the conection to other optical fibers can be improved. Since the optical fiber unit using the wated optical fiber of the present invention has a high delamination resistance and allows easy stripping of individual wated fiber, the working efficiency in, for example, the conection to another optical fiber can be improved. WE CLAIM: 1. A coated optical fiber having at least one coating formed around the outside of a glass optical fiber, wherein the storage modulus E of the primary coating in contact with said glass optical fiber is within the range of 0.01 kg/mm2 to 2.0 kg/mm2 at 25oC and 110 Hz, and the adhesion between said glass optical fiber and said primary coating is within the range of 10 g/cm to 200 g/cm. 2. The coated optical fiber as claimed in claim 1, wherein the storage modulus E' is within the range of 0.01 kg/mm to 0.5 kg/mm and the adhesion is within the range of 10 g/cm to 100 g/cm. 3. The coated optical fiber as claimed in claim 2, wherein said primary coating is made of ultraviolet-curable resin. 4. The optical fiber ribbon wherein a plurality of coated optical fibers as claimed in claim 2 or 3, are arranged in parallel and are collectively coated with a unitary resin jacket. 5. The coated optical fiber as claimed in claim 1, wherein the storage modulus E is within the range of 0.01 kg/mm to 2.0 kg/mm and the adhesion is within the range of 100 g/cm to 200 g/cm. 6. The coated optical fiber as claimed in claim 6, wherein said primary coating is made of ultraviolet-curable resin. 7. The optical fiber ribbon wherein a plurality of coated optical fibers as claimed in claim 5 or 7, are arranged and are collectively coated with a unitary resin jacket. 8. The coated optical fiber as claimed in claim 1, wherein said primary coating is made of ultraviolet-curable resin. Dated this 3rd day of May 2002

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