Abstract:
In a coated optical fiber tape comprises a plurality of optical fibers each having a coating and being aligned parallel in the same plane, a common coating layer covering the array of the optical fibers, and a peelable cured coating layer being provided between each of the coatings on the optical fibers and the common coating layer to prevent the latter from being bonded or urged to said coatings on the optical fibers, the improvement wherein the peelable cured coating layer has a compression modulus of elasticity of at least 5 kg/mm 2  but not exceeding 300 kg/mm 2 .

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a novel optical fiber tape that can be used advantageously in such applications as high density lightwave communications. More particularly, the present invention relates to a coated optical fiber tape furnished with a coating layer that allows easy handling in such operations as splicing and which exhibits superior resistance to lateral pressures. 
     With the recent advances in the technology of information communications, there has been a growing need for transmitting signals at a higher speed. One of the embodiments of such high-speed signal transmission technologies is the commercial application of lightwave communications. Besides being capable of realizing highspeed transmission by virtue of the nature of light itself, optical signal transmission not only exploits the light-weightness of optical fibers serving as lightguides but also offers distinct advantages in communications technol-ogy such as immunity to magnetic and electric fields. Notwithstanding these capabilities, conventional optical fibers as lightguides are yet to fully satisfy the requirement for transmitting a larger volume of signals and researchers are responding to this need by making various improvements of signal transmitting optical fibers. 
     Coated optical fiber tape is one of the applications of the techniques so far proposed under these circumstances. Coated optical fiber tape consists of an array of optical fibers serving as lightguides which are covered with a common coating layer to form a unitary assembly that has the potential to realize high-density signal transmission by simple handling. 
     FIG. 5 is a cross section showing the composition of a typical coated optical fiber tape. Light-transmitting optical fibers 1 individually furnished with a coating 2 are arranged side by side in tape form and provided with a common coating layer 4. In the prior art, the individual fiber coatings 2 and the common coating layer 4 are typically formed of an ultraviolet (uv) curable polyurethane acrylate resin. 
     In commercial operations, such coated optical fiber tape has to be spliced to other parts such as single coated optical fibers. In such instances, if the coated optical fibers to be spliced remain in tape form, problems will occur in terms of such aspects as handling and splicing loss. To solve these problems, the common coating layer 4 has to be stripped, followed by end preparation of individual fibers 1. 
     With a view to meeting this need, it has been proposed that a peelable cured coating layer be provided between the coatings 2 on the individual fibers 1 and the common coating layer 4 so as to prevent the common coating layer 4 from being bonded or urged to the fiber coatings 2. Coated optical fiber tapes having this construction are described in, for example, Japanese Utility Model Application No. 126584/1986. 
     One major problem with this type of coated optical fiber tapes is that the physical properties of the material of which the peelable cured coating layer is made have not been fully considered. As a consequence, installation of such a peelable cured coating layer often causes deterioration of the characteristics, in particular, lateral pressure resistance, of the coated optical fiber tape, and this leads to an undesired phenomenon such as increased transmission loss when a plurality of fiber tapes are assembled into a cable. 
     SUMMARY OF THE INVENTION 
     An object, therefore, of the present invention is to provide a coated optical fiber tape having improved lateral pressure resisting characteristics. This tape has been accomplished on the basis of both the theoretical and experimental reviews made by the present inventors in order to establish the fact that when the peelable cured coating layer was made of a material having compression moduli of elasticity within a certain range, the lateral pressure resisting characteristics of the fiber tape would deteriorate significantly. 
     In order to solve the aforementioned problem of the prior art on the basis of this finding, the present invention provides a coated optical fiber tape in which a peelable cured coating layer having a compression modulus of elasticity of at least 5 kg/mm 2  but not exceeding 300 kg/mm 2  is provided between the coatings on the individual fibers and the common coating layer surrounding the array of coated optical fibers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a coated optical fiber tape according to an embodiment of thepresent invention; 
     FIG. 2 is a graph showing the relationship between the compression modulus of a peelable cured coating layer and the relative value of stress developing in glass fibers; 
     FIG. 3a and 3b illustrate a method for lateral pressure test to be conducted on a coated optical fiber tape; 
     FIG. 4 is a graph showing the results of a lateral pressure test conducted on the coated optical fiber tapes fabricated in Examples 1-7 of the present invention; and 
     FIG. 5 is a cross-sectional view of a prior art coated optical fiber tape. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     When lateral pressure is imposed on a coated optical fiber tape, the applied force is transmitted through the various coating layers to the optical glass fibers, thereby causing microbending which would increase the transmission loss occurring in the affected areas. Theoretically, the increase in transmission loss that occurs in the microbending portions of optical fibers under lateral pressure can be evaluated by determining the stresses that develop in the glass fibers when an external force is applied to the coated optical fiber tape (see, for example, Collected Papers for 1984 of the Light &amp; Electric Wave Section, No. 2256, The Institute of Electronics and Communication Engineers of Japan). 
     The present inventors particularly noted the following two facts: first, no review has been made about the physical properties of a peelable cured coating layer incorporated in a coated optical fiber tape between the coatings on individual optical fibers and the common coating layer surrounding the array of such optical fibers; secondly, the compression modulus of elasticity of the peelable cured coating layer is closely related to the lateral pressure resisting properties of the fiber tape. On the basis of these findings, the present inventors investigated the relationship between the compression modulus of elasticity of the peelable cured coating layer in a coated optical fiber tape and the stresses developing in the glass fibers by the finite element method employing the stress analyzing program MSC/NASTRAN. The results of this analysis are shown in FIG. 2. The dimensions and physical properties of the individual components of the fiber under test that provided the basis for the calculations made to obtain the results shown in FIG. 2 were as follows: 
     
         ______________________________________Diameter of glass fiber 0.125   mmCompression modulus of glass fiber                   7000    kg/mm.sup.2Thickness of the first layer on glass                   0.0375  mmfiberCompression modulus of the first layer                   0.15    kg/mm.sup.2on glass fiberThickness of the second layer on glass                   0.02    mmfiberCompression modulus of the second layer                   70      kg/mm.sup.2on glass fiberMinimum thickness of the common coating                   0.06    mmlayerCompression modulus of the common                   70      kg/mm.sup.2coating layer______________________________________ 
    
     FIG. 2 shows that the stress developing in the glass fibers is largely dependent on the compression modulus of the peelable cured coating. At a compression modulus of 0.5 kg/mm 2 , a maximum stress occurs in glass fibers, and at compression moduli either above or below 0.5 kg/mm 2 , lower stresses will develop. Therefore, in order to ensure that the provision of a peelable cured coating layer will not cause more than 20% increase in the stress developing in glass fibers, one needs to employ a peelable cured coating layer having a compression modulus of elasticity of at least 5 kg/mm 2  or not exceeding 0.04 kg/mm 2 . It should be noted here that results similar to those shown in FIG. 2 were obtained even when different values were selected for the dimensions and physical properties of the individual components of the coated optical fiber tape. 
     The peelable cured coating layer for use in the present invention may be formed of any resin that has photocuring (e.g., uv curable) or thermosetting properties to be readily moldable and which will not be urged or bonded to either the common coating layer or coatings on the individual optical fibers. 
     A typical example of such a resin that is suitable for use as the material of the peelable cured coating layer is a release agent that is chiefly made of a uv curable silicone or fluorine resin. The silicone or fluorine resin is an organic compound having silicon (Si) or fluorine (F) atoms in the molecule and which is curable by the action of heat or light to yield a cured product having good release properties. 
     The following examples are provided for the purpose of further illustrating the present invention but are in no way to be taken as limiting. 
     EXAMPLES 1-7 
     FIG. 1 is a cross section showing the basic composition of the coated optical fiber tapes fabricated in Examples 1-7 in accordance with the present invention. As shown, each of the fiber tapes consists of optical fibers 1, a two-layered coating 2 on each fiber that is composed of the inner first layer and the outer second layer, a peelable cured coating layer 3 on each coated fiber, and a common coating layer 4. 
     In Examples 1-7, coated optical fiber tapes having the composition shown in FIG. 1 were fabricated, with their dimensions and physical properties being shown in the following table. The parameters listed in the table are: the outside diameter, core diameter, refractive index profile and differential refractive index of the glass fiber making each of the optical fibers 1; the thicknesses and compression moduli of elasticity of the first and second layers making up each of the fiber coatings 2; the thickness and compression modulus of elasticity of each peelable cured coating layer 3; the minimum thickness and compression modulus of elasticity of the common coating layer 4; and the cross-sectional dimensions of the coated optical fiber tape. A test was conducted to evaluate the lateral pressure characteristics of these seven fiber tapes. 
     The first and second layers of the fiber coatings 2 and the common coating layer 4 as well, were formed of a uv curable urethane acrylate resin, whereas the peelable cured coating layers were formed of a uv curable silicone acrylate resins. 
     
                                           TABLE__________________________________________________________________________Parameter    Example 1            Example 2                   Example 3                           Example 4                                  Example 5                                          Example 6                                                  Example__________________________________________________________________________                                                  7Glass Fiber:Fiber outer    125 μm            125 μm                   126 μm                           125 μm                                  125 μm                                          125 μm                                                  125 μmdiameterCore Dia.    51 μm            50 μm                   51 μm                           50 μm                                  50 μm                                          50 μm                                                  50 μmRefractive index    graded  graded graded  graded graded  graded  gradedprofile  index   index  index   index  index   index   indexDifferential index    1.0%    1.0%   1.0%    1.0%   1.0%    1.0%    1.0%First Layer:Thickness    38 μm            38 μm                   38 μm                           38 μm                                  38 μm                                          38 μm                                                  38 μmCompression    0.14 Kg/mm.sup.2            0.14 Kg/mm.sup.2                   0.14 Kg/mm.sup.2                           0.14 Kg/mm.sup.2                                  0.14 Kg/mm.sup.2                                          0.14 Kg/mm.sup.2                                                  0.14 Kg/mm.sup.2                                                  9modulusSecond Layer:Thickness    25 μm            25 μm                   25 μm                           25 μm                                  25 μm                                          25 μm                                                  25 μmCompression    55 Kg/mm.sup.2              55 Kg/mm.sup.2                   55 Kg/mm.sup.2                             55 Kg/mm.sup.2                                  55 Kg/mm.sup.2                                          55 Kg/mm.sup.2                                                  55 Kg/mm.sup.2modulusPeelable CuredCoating Layer:Thickness     5 μm             5 μm                    5 μm                            5 μm                                  --       5 μm                                                   5 μmCompression    0.2 Kg/mm.sup.2            0.74 Kg/mm.sup.2                   5.8 Kg/mm.sup.2                           10.3 Kg/mm.sup.2                                  --      110 Kg/mm.sup.2                                                  203 Kg/mm.sup.2modulusCommonCoating Layer:Minimum  60 μm            60 μm                   60 μm                           60 μm                                  60 μm                                          60 μm                                                  60 μmThicknessCompression    55 Kg/mm.sup.2              55 Kg/mm.sup.2                   55 Kg/mm.sup.2                             55 Kg/mm.sup.2                                  55 Kg/mm.sup.2                                          55 Kg/mm.sup.2                                                  55 Kg/mm.sup.2modulusCross-sectional    0.40 × 1.43            0.40 × 1.43                   0.40 × 1.43                           0.40 × 1.43                                  0.39 × 1.38                                          0.40 × 1.43                                                  0.40 × 1.43Dimensions of    mm.sup.2            mm.sup.2                   mm.sup.2                           mm.sup.2                                  mm.sup.2                                          mm.sup.2                                                  mm.sup.2Fiber Tape__________________________________________________________________________ 
    
     The lateral pressure characteristics of the fiber tapes fabricated in Examples 1-7 were evaluated by the test method illustrated in FIGS. 3a and 3b. As shown in FIG. 3a, the test apparatus was composed of a light source 5, an excitation dummy 6, a coated optical fiber tape 7, a pair of flat metal plates (50×50 mm 2 ) 8 between which the fiber tape was to be held and given lateral pressure, and a power meter 9. FIG. 3b shows the essential part of a cross section of the metal plates 8 between which a fiber tape 7 is held and pressurized. 
     The results of the lateral pressure test conducted on the fiber tapes of Examples 1-7 are shown graphically in FIG. 4, in which curves I-VI show the data for the fiber tapes of Examples 1-7 , respectively. The data shown in FIG. 4 were in agreement with the results of theoretical considerations. 
     As described on the foregoing pages, the coated optical fiber tape of the present invention has individual optical fibers coated with a peelable cured layer having a compression modulus of elasticity of at least 5 kg/mm 2  but not exceeding 300 kg/mm 2 . This limitation on the compression modulus of the peelable cured coating layer is effective in improving the lateral pressure resisting properties of the fiber tape, thereby preventing the occurrence of increased transmission loss due to external force.