Abstract:
The purpose of the present invention is to provide, by a configuration or method different from conventional art, a coated optical fiber enabling reduced interface delamination between a glass fiber and a primary coating layer when the coated optical fiber is immersed in water, and a reduction of transmission loss increase. A coated optical fiber according to one embodiment of the present invention is provided with a glass fiber, a primary coating layer coated on the glass fiber, a secondary coating layer coated on the primary coating layer, and a colored layer coated on the secondary coating layer. The coated optical fiber is configured so that small water bubbles are generated substantially evenly within the primary coating layer when the coated optical fiber is immersed for 200 days in warm water of 60° C.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of International Application No. PCT/JP2012/008037, filed Dec. 17, 2012, which claims the benefit of Japanese Patent Application No. 2012-033088, filed Feb. 17, 2012. The contents of the aforementioned applications are incorporated herein by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a coated optical fiber, an optical fiber ribbon, and an optical cable having excellent water resistance. 
       BACKGROUND ART 
       [0003]    With the recent demand for large-capacity communication, optical fibers have been installed in wide areas. An example of a coated optical fiber used for such an optical fiber includes: a glass fiber; a primary coating layer coating an outer periphery of the glass fiber; a secondary coating layer coating an outer periphery of the primary coating layer; and a colored layer coating an outer periphery of the secondary coating layer. 
         [0004]    When a coated optical fiber having such a structure is immersed in water, particularly, in hot water for a long period, the following problem is known to occur in some cases. Specifically, water bubbles are generated in the primary coating layer, and peeling occurs at an interface between the glass fiber and the primary coating layer (the interface being referred to also as a glass/primary interface), so that the transmission loss increases as compared with that before the immersion in water. 
         [0005]    To solve this problem, various attempts have been made conventionally. For example, Patent Document 1 discloses that suppression of generation of large water bubbles during immersion in hot water and reduction of the increase in transmission loss are achieved by using, as a primary coating layer, a material having a peeling force adjusted to a predetermined value. Here, the peeling force is measured as follows. First, a glass plate is coated with the material used as the primary coating layer of the coated optical fiber and immersed in hot water. Then, the peeling force for peeling the material from the glass plate at an angle of 90 degrees is measured. 
         [0006]    Meanwhile, Patent Document 2 discloses that suppression of generation of large water bubbles during immersion in hot water and reduction of the increase in transmission loss are achieved by adjusting the glass transition temperatures of a primary coating layer and a secondary coating layer of a coated optical fiber to predetermined values, respectively. 
       CITATION LIST 
     Patent Documents 
       [0007]    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-54138 
         [0008]    Patent Document 2: Japanese Patent Application Laid-Open No. 2008-40369 
         [0009]    Patent Document 3: Japanese Patent Application Laid-Open No. 2010-217800 Non Patent Document 
         [0010]    Non Patent Document 1: Charles Aloisio et al., “Optical Fiber Coating Delamination Using Model Coating Materials”, Proceedings of the 51st IWCS, 2002, pp. 738-747 
       SUMMARY OF INVENTION 
       [0011]    Patent Document 1 discloses that the generation of large water bubbles during immersion in hot water can be suppressed by using a monomer such as polypropylene glycol or poly tetramethylene glycol as an oligomer skeleton constituent component in a material for the primary coating layer. Meanwhile, Patent Document 2 discloses that the generation of large water bubbles during immersion in hot water can be suppressed by making the Tg (glass transition temperature) of the primary coating layer lower than the Tg of the secondary coating layer and making the thickness of the primary coating layer not larger than the thickness of the secondary coating layer. As described above, suppression of generation of water bubbles has been conventionally required to suppress the peeling at the glass/primary interface and to reduce the increase in transmission loss. 
         [0012]    As a result of intensive studies, the present inventors have newly found that a non-uniform (uneven) distribution of the water bubbles formed in the primary coating layer during immersion in water is a cause of the peeling at the interface between the glass fiber and the primary coating layer and the increase in transmission loss. For this reason, the present invention employs an approach different from the conventional one in which the formation of water bubbles in the primary coating layer is suppressed during immersion in water. Specifically, in the approach employed in the present invention, the water resistance is improved by rather allowing water bubbles to be formed evenly in the primary coating layer during immersion in water. 
         [0013]    An object of the present invention is to provide a coated optical fiber having such an excellent water resistance that the peeling at the interface between the glass fiber and the primary coating layer can be suppressed and the increase in transmission loss can be reduced by allowing water bubbles to be formed evenly in the primary coating layer during immersion in water. 
         [0014]    An aspect of the present invention is a coated optical fiber comprising: an optical fiber including a glass fiber, a primary coating layer coating an outer periphery of the glass fiber, and a secondary coating layer coating an outer periphery of the primary coating layer; and a tertiary coating layer coating an outer periphery of the optical fiber and having an elastic modulus of 100 MPa or higher, wherein when the coated optical fiber is immersed in hot water at 60° C. for 200 days, water bubbles are formed substantially evenly inside the primary coating layer, a number of the water bubbles present per unit area in the primary coating layer on a cross-section taken along a longitudinal direction of the coated optical fiber is one or more, the unit area being defined with a thickness of the primary coating layer taken as one unit, and the water bubbles have diameters of 10 μm or less. 
         [0015]    The coated optical fiber according to the present invention makes it possible to relax a stress by water bubbles because the water bubbles are formed substantially evenly inside the primary coating layer during immersion in water. Consequently, the peeling at the interface between the glass fiber and the primary coating layer can be suppressed and the increase in transmission loss can be reduced. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1  is a schematic diagram of a coated optical fiber according to an embodiment of the present invention. 
           [0017]      FIG. 2  is a schematic diagram of an optical fiber ribbon according to an embodiment of the present invention. 
           [0018]      FIG. 3  is a schematic diagram of an optical fiber cable according to an embodiment of the present invention. 
           [0019]      FIG. 4  is a graph showing the relationship between the load and the time taken until a glass fiber comes out of a primary coating layer. 
           [0020]      FIG. 5  is a schematic diagram of a load testing apparatus used for conducting a load test. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]    Hereinafter, embodiments of the present invention are described with reference to the drawings. However, the present invention is not limited to these embodiments. Note that, in the drawings described below, elements having the same functions are denoted by the same reference numerals, and overlapping descriptions thereof are omitted in some cases. 
       Embodiments 
       [0022]      FIG. 1  shows a schematic diagram of a coated optical fiber  100  according to the present embodiment. Specifically, the coated optical fiber  100  includes: an optical fiber  100   a  including a glass fiber  101 , a primary coating layer  102  coating an outer periphery of the glass fiber  101 , a secondary coating layer  103  coating an outer periphery of the primary coating layer  102 ; and a colored layer  104  coating an outer periphery of the optical fiber  100   a.    
         [0023]    Each of the primary coating layer  102 , the secondary coating layer  103 , and the colored layer  104  is made from a coating material adjusted to have predetermined characteristics. As the coating material for each of the layers, an ultraviolet ray curable resin is used, in general. The ultraviolet ray curable resin contains at least an oligomer, a diluent monomer, and a photo-polymerization initiator, and further contains additives such as a silane coupling agent and a chain transfer agent for adjusting various properties. 
         [0024]    Here, it is desirable that the coating materials of the layers be adjusted so that the primary coating layer  102  can have a low elastic modulus and the secondary coating layer  103  can have a high elastic modulus. In such a case, the primary coating layer  102  serves as a soft buffer layer, and the secondary coating layer  103  functions as a hard protective layer. Hence, increase in transmission loss can be suppressed even when an external force is applied to the optical fiber  100   a . Specifically, in order to keep functions as an optical fiber, the elastic modulus of the primary coating layer  102  is preferably 0.2 to 3 MPa, the elastic modulus of the secondary coating layer  103  is preferably 500 to 1200 MPa, and the elastic modulus of the colored layer  104  is preferably 100 to 2000 MPa. 
         [0025]    The coating material for the colored layer  104  is colored with a dye or a pigment for visibility. However, the colored layer  104  does not necessarily have to be colored, and a non-colored tertiary coating layer for strength retention and the like is also acceptable. 
         [0026]    Recently, coated optical fibers of a type in which the primary coating layer  102  or the secondary coating layer  103  is colored with a dye or a pigment and the colored layer  104  is not provided for cost reduction are gaining popularity. In a case of coated optical fibers having such a two-layer structure, a ribbon coating layer used for forming an optical fiber ribbon by arranging the coated optical fibers in parallel to each other and coating the coated optical fibers together acts as the tertiary coating layer. 
         [0027]    When a hard coating layer having an elastic modulus of 100 MPa or higher, such as the colored layer or the ribbon coating layer, exists on the outer periphery of the optical fiber  100   a , the movement of the secondary coating layer is restrained, and the stress acting on an interface between the glass fiber and the primary coating layer increases, so that interface peeling becomes more likely to occur. Moreover, when a coated optical fiber is immersed in hot water for a long period, water may be accumulated in a peeled portion between the glass fiber and the primary coating layer. For this reason, particularly when a hard coating layer such as the colored layer or the ribbon coating layer exists outside the secondary coating layer, problems of the peeling at the interface between the glass fiber and the primary coating layer and the increase in transmission loss tend to arise particularly. Accordingly, a structure in which the tertiary coating layer having an elastic modulus of 100 MPa or higher coats the outer periphery of the secondary coating layer is a prerequisite for the present invention. 
         [0028]    Note that soluble components in the coating layers are dissolved into water accumulated in the peeled portion between the glass fiber and the primary coating layer to form an aqueous solution. If a colored layer is present outside the secondary coating layer, the colored layer acts as a semipermeable membrane. An osmotic pressure is generated because of the difference in concentration between the aqueous solution in the interlayer peeling portion and water outside the colored layer, and water moves to the interlayer peeling portion because of the osmotic pressure. As a result, the interlayer peeling may grow. 
         [0029]    Hence, particularly the transmission loss may increase remarkably in a coated optical fiber having a colored layer. 
         [0030]    In this embodiment, an ordinary value is employed for the size of each of the layers. Specifically, the glass fiber  101  has a diameter of 90 to 150 μm and preferably about 125 μm, each of the primary coating layer  102  and the secondary coating layer  103  has a thickness of 10 to 60 μm and preferably 20 to 50 μm, and the colored layer  104  has a thickness of 5 to 20 μm. The sizes of these layers are not limited to these values, and can be changed to any values. 
         [0031]    Multiple coated optical fibers  100  may be integrated into a tape shape.  FIG. 2  shows a schematic diagram of an optical fiber ribbon  200  having such a structure. The optical fiber ribbon  200  has a structure in which the outside of four coated optical fibers  100  arranged in parallel to each other is coated with an ribbon coating layer  201 . The ribbon coating layer  201  preferably has an elastic modulus of 100 to 2000 MPa from the viewpoints of strength retention and the like. The size of the optical fiber ribbon  200  is about 320 μm in thickness and 1.1 mm in width. The size of the optical fiber ribbon  200  and the number of the coated optical fibers  100  are not limited to these values, and may be changed to any values. 
         [0032]    Note that the optical fibers  100   a  may be used instead of the coated optical fibers  100  to form the optical fiber ribbon  200 . In such a case, the optical fiber ribbon  200  has a structure in which the outside of the secondary coating layers  103  of the optical fibers  100   a  is coated with an ribbon coating layer  201  serving as a tertiary coating layer. 
         [0033]    Moreover, an optical fiber cable may be formed by housing multiple optical fiber ribbons  200  in a slot.  FIG. 3  shows a schematic diagram of an optical fiber cable  300  having such a structure. The optical fiber cable  300  is, for example, an SZ cable including 40 fibers. The optical fiber cable  300  includes a spacer  301  having five SZ slots  302 . Two optical fiber ribbons  200  are housed in each of the SZ slots  302 . A pressing and wrapping tape  303  is wound around the spacer  301 , and the outside of the pressing and wrapping tape  303  is covered with a sheath  304 . In addition, a tension member  305  is provided at a center in a cross section of the spacer  301 . Tracer marks  306  are provided on an outer periphery of the spacer  301 . A ripcord  307  is provided on a part of an outer periphery of the pressing and wrapping tape  303 . 
         [0034]    The SZ slots  302  are not limited to the five-grooved type, and the number of the groves can be selected as appropriate. Moreover, the number of the optical fiber ribbons  200  in each SZ slot  302  is not limited to two, but can be selected as appropriate. In addition, S slots may be used instead of the SZ slots. 
         [0035]    (Method for Measuring Elastic Modulus) 
         [0036]    As an index of the hardness of each of the coating layers, the elastic modulus (also referred to as Young&#39;s modulus) is used. The elastic modulus is measured for each of the coating layer after an optical fiber is fabricated. A well-known approach may be employed as a specific approach for measuring the elastic modulus. 
         [0037]    (Method for Measuring Limit-Adhesion Strength) 
         [0038]    A limit-adhesion strength is an index indicative of the adhesion strength of an interface between a glass fiber and a primary coating layer in a hot and humid environment. The limit-adhesion strength is defined as follows. Note that the definition of the limit-adhesion strength is based on the description in Patent Document 3. 
         [0039]    A coated optical fiber whose coating layers are nicked over the entire periphery at a position 10 mm away from an end is prepared while only the glass fiber is left intact. Then, the coating layer in a region ranging from the end of the coated optical fiber to a position 10 mm away from the end is fixed to sandpaper by an adhesive agent. The sandpaper is fixed in an atmosphere of a temperature of 60° C. and a humidity of 98% RH with the end of the coated optical fiber located on the upper side. Then, a weight is attached to the other end of the coated optical fiber. Thus, a constant load is applied between the glass fiber and the primary coating layer in the portion extending above the nick over a length of 10 mm. In this state, the time taken until the glass fiber comes out of the primary coating layer is measured. Then, the load applied to the glass fiber, i.e., the mass of the weight is gradually changed, and the relationship between the load and the time taken until the glass fiber comes out is found. 
         [0040]      FIG. 4  shows an example of a typical measurement result of the limit-adhesion strength.  FIG. 4  is a graph shown as an example to help the definition of the limit-adhesion strength to be understood. Hence, it should be noted that  FIG. 4  does not show a result of a specific example of this embodiment carried out and that specific numeric values on the vertical axis and the horizontal axis are omitted. Based on  FIG. 4 , as the load applied between the glass fiber and the primary coating layer decreases, the time taken until the glass fiber comes out of the primary coating layer increases. However, when the load reaches a load C or less, the time taken until the glass fiber comes out suddenly increases and the slope becomes gentle. An inflection point D exists at which the time taken until the glass fiber comes out suddenly increases and the slope becomes gentle as described above. The load C at the inflection point D at which the slope becomes gentle is defined as the limit-adhesion strength. Patent Document 3 shows that the higher the limit-adhesion strength is, the more the increase in transmission loss is reduced after immersion in hot water. 
         [0041]    (Methods for Measuring D50 and T50) 
         [0042]    In the present description, peeling at the glass/primary interface is referred to as delamination, and tearing inside the primary coating layer or void formation in the primary coating layer is referred to as tearing. Susceptibility to delamination and susceptibility to tearing are presumably associated with susceptibility to peeling at the glass/primary interface and susceptibility to water bubble formation. As quantitative indices of the susceptibility to delamination and the susceptibility to tearing, D50 and T50 are defined as follows, respectively. 
         [0043]      FIG. 5  shows a schematic diagram of a load testing apparatus  900  as an example of an apparatus for measuring the delamination and the tearing. The load testing apparatus  900  includes a stage  901  on which a coated optical fiber  100  to be measured is to be mounted, a load unit  902  being provided away from the stage  901  and capable of adjusting the value of the load, a rod  903  fixed to the load unit  902  on the stage  901  side, and a driving unit  904  capable of moving the load unit  902  and the rod  903  in a normal line direction A of the stage  901 . The rod  903  is provided to be perpendicular to the longitudinal direction of the coated optical fiber  100  in a state of being mounted on the stage  901  and in parallel to a surface of the stage  901 . 
         [0044]    When the driving unit  904  releases the load unit  902 , the load unit  902  and the rod  903  move by their own weights toward the stage  901 . The rod  903  and the stage  901  sandwiches the coated optical fiber  100 , and the load unit  902  applies a load to a point B on the coated optical fiber  100 . After that, after a predetermined time has elapsed, the driving unit  904  moves the load unit  902  and the rod  903  in the direction away from the coated optical fiber  100 , and the load unit  902  does not apply a load to the coated optical fiber  100  anymore. 
         [0045]    Note that the start and the end of the application of the load may be controlled manually by a user or may be conducted automatically by providing a control unit. The measuring apparatus is not limited to that having the apparatus structure of  FIG. 5 , as long as a predetermined load can be applied for a predetermined time to a point B of the coated optical fiber  100  in a direction perpendicular to the longitudinal direction of the coated optical fiber. 
         [0046]    For the measurement, a coated optical fiber  100  to be measured is fixed to the load testing apparatus  900 . Then, a predetermined load is applied to a point for 4.5 seconds, and further the load is applied repeatedly to 30 points at intervals of approximately 6 mm. After that, the presence or absence of delamination or tearing is observed for each of the points with an optical microscope. Moreover, the load is increased, and application of the load to 30 points different from those in the prior observation and observation of the points are repeated every time the load is increased. Based on this, the value of the load and the number of points where the delamination or tear occurs are plotted against each other. Note that the points to which the load is applied is not limited to 30. 
         [0047]    A smallest load with which delamination or tearing is observed at 50% or more of all the points to which the load is applied is defined as D50 or T50. In other words, in the plot, a smallest load with which delamination is observed at 50% (15 points) or more of the 30 points to which the load is applied is defined as D50. Meanwhile, in the plot, a smallest load with which tearing is observed at 50% (15 points) or more of the 30 points to which the load is applied is defined as D50. 
         [0048]    It can be said that a smaller D50 indicates that a smaller load is required to cause delamination, i.e., a smaller D50 indicates a higher susceptibility to delamination. Likewise, it can be said that a smaller T50 indicates that a smaller load is required to cause tearing, i.e., a smaller T50 indicates a higher susceptibility to tearing. Hence, D50 and T50 can be used as indices of susceptibilities to delamination and tearing, respectively. 
         [0049]    Note that these measuring methods are based on the methods described in Non Patent Document 1. 
         [0050]    (Sample Adjustment) 
         [0051]    Conventionally, the generation of water bubbles in the primary coating layer during immersion in water itself has been thought to be a cause of the increase in transmission loss as shown in Patent Documents 1 and 2. Meanwhile, the present inventors have conducted intensive study and consequently have newly found that a non-uniform distribution of water bubbles formed in the primary coating layer during immersion in water is a cause of the peeling at the interface between the glass fiber and the primary coating layer and the increase in transmission loss. 
         [0052]    It is conceivable that when water bubbles are not formed or unevenly formed in the primary coating layer during immersion in water, the peeling easily occurs because a stress from the secondary coating layer is transmitted directly to the glass/primary interface or is transmitted in a concentrated manner on some regions. On the other hand, it is conceivable that when water bubbles are formed evenly in the primary coating layer during immersion in water, the stress from the secondary coating layer is relaxed by the uniformly distributed water bubbles, so that the peeling at the glass/primary interface is suppressed. 
         [0053]    Hence, when the materials of the coated optical fiber are adjusted so that water bubbles can be formed evenly in the primary coating layer, a force is applied evenly to the glass/primary interface, and consequently the suppression of the peeling and the reduction of transmission loss can be achieved without reduction of the number of water bubbles generated during immersion in water. 
         [0054]    In order that water bubbles can be formed evenly in the primary coating layer during immersion in hot water, the material of each coating layer is adjusted as follows. 
         [0055]    The present inventors have found that water bubbles are formed evenly in the primary coating layer during immersion in hot water by making the tearing more likely to occur than the delamination, so that the increase in transmission loss can be reduced. When the tearing is more likely to occur than the delamination, in other words, when the D50 is greater than the T50, a stress from the secondary coating layer causes formation of water bubbles inside the primary coating layer before delamination during immersion in hot water. Presumably, as a result of this, the stress is relaxed by the water bubbles, so that the increase in transmission loss can be reduced. 
         [0056]    To make the delamination less likely to occur (i.e., to increase the D50), an external force transmitted to the glass/primary interface is relaxed by decreasing the elastic modulus of the primary coating layer and increasing the elastic modulus of the secondary coating layer. Alternatively, the adhesion strength between the surface of the glass fiber and the primary coating layer is increased by adding an additive such as a silane coupling agent, for example, to the material of the primary coating layer. 
         [0057]    To make the tearing more likely to occur (i.e., to decrease the T50), the elastic modulus of the primary coating layer is decreased, i.e., the crosslink density of the primary coating layer is decreased. 
         [0058]    Moreover, to distribute water bubbles evenly in the primary coating layer, it is possible to prevent water bubbles from being formed locally by using a hydrophobic oligomer such as polytetramethylene glycol (PTMG) as a material for the primary coating layer and to adjust water absorption by adding a hydrophilic monomer such as acrylamide. 
         [0059]    Accordingly, a coated optical fiber in which water bubbles are formed evenly in the primary coating layer during immersion in hot water can be obtained by conducting, in an integrated manner, the adjustment of the materials for the coating layers so that the D50 and the T50 can take predetermined values and the adjustment of the mixing ratio of the hydrophobic substance and the hydrophilic substance in the material for the primary coating layer so that water bubbles can be distributed uniformly. 
       Examples 
       [0060]    Examples 1 to 4 and Comparative Examples 1 to 3 were prepared which were optical fiber ribbons having the structure shown in  FIG. 2  and among which properties of the coating layers were varied. The coated optical fibers were taken out from the optical fiber ribbons, and the above-described elastic modulus, D50, T50, and limit-adhesion strength were measured. In addition, the transmission loss before immersion in hot water was measured by using the optical fiber ribbons. 
         [0061]    In addition, Examples and Comparative Examples were immersed in hot water at 60° C. for 200 days, and then the water bubbles were observed and the transmission loss was measured. Table 1 shows the results of the observation and measurement carried out. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Example 
                 Comparative Example 
               
             
          
           
               
                 Number 
                 1 
                 2 
                 3 
                 4 
                 1 
                 2 
                 3 
               
               
                   
               
             
          
           
               
                 Elastic modulus of primary 
                 0.4 
                 0.5 
                 0.7 
                 0.4 
                 0.5 
                 0.3 
                 0.5 
               
               
                 coating layer (MPa) 
               
               
                 Elastic modulus of 
                 700 
                 800 
                 1000 
                 1000 
                 600 
                 800 
                 600 
               
               
                 secondary coating layer 
               
               
                 (MPa) 
               
               
                 Elastic modulus of tertiary 
                 800 
                 800 
                 800 
                 800 
                 800 
                 800 
                 800 
               
               
                 coating layer (MPa) 
               
               
                 D50 (g) 
                 550 
                 550 
                 550 
                 1000 
                 200 
                 550 
                 550 
               
               
                 T50 (g) 
                 200 
                 300 
                 450 
                 800 
                 200 
                 550 
                 150 
               
               
                 Limit-adhesion strength (N) 
                 0.8 
                 0.8 
                 1.0 
                 0.6 
                 0.5 
                 0.5 
                 1.0 
               
               
                 Presence or absence of 
                 Absent 
                 Absent 
                 Absent 
                 Absent 
                 Present 
                 Absent 
                 Absent 
               
               
                 delamination 
               
               
                 Maximum number of fine 
                 &gt;10 
                 &gt;10 
                 8 
                 &gt;10 
                 0 
                 0 
                 &gt;10 
               
               
                 bubbles (number of bubbles) 
               
               
                 Minimum number of fine 
                 1 
                 5 
                 3 
                 7 
                 0 
                 0 
                 1 
               
               
                 bubbles (number of bubbles) 
               
               
                 Maximum diameter of fine 
                 10 
                 7 
                 6 
                 3 
                 — 
                 — 
                 &gt;10 
               
               
                 bubbles (μm) 
               
               
                 Increase in transmission 
                 0.095 
                 0.03 
                 0.06 
                 0.05 
                 0.2 
                 0.12 
                 0.15 
               
               
                 loss (db/km) 
               
               
                 Evaluation 
                 B 
                 A 
                 A 
                 A 
                 C 
                 C 
                 C 
               
               
                   
               
             
          
         
       
     
         [0062]    Examples and Comparative Examples immersed in hot water at 60° C. for 200 days were each observed with an optical microscope for the presence or absence of generation of delamination at the glass/primary interface, the number of water bubbles (fine bubbles), and the sizes of the water bubbles. 
         [0063]    Specifically, after the immersion in hot water, an approximately 10-cm piece was cut out of the coated optical fiber. Then, three points (3 cm, 6 cm, and 9 cm) away from an end of the coated optical fiber at intervals of approximately 3 cm were observed. Further, the coated optical fiber was rotated by 90 degrees in the circumferential direction of the cross-section, and the three points were observed again. In each of the six observation points described above, the presence or absence of generation of the delamination and the number and the sizes of the water bubbles were observed and measured in a direction perpendicular to the longitudinal direction of the coated optical fiber in an area of 30 μm in the thickness direction and 30 μm in the longitudinal direction in the primary coating layer (since the thickness of the primary coating layer is 30 μm, this area is referred to as a unit area defined with the thickness of the primary coating layer being taken as one unit). This measurement makes it possible to determine whether or not the water bubbles are evenly formed not only in the longitudinal direction of the coated optical fiber but also in the circumferential direction of the cross-section. 
         [0064]    As for the presence or absence of the delamination, the delamination was determined to be present if one or more delaminations at the glass/primary interface occurred at any of the six observation points, and the delamination was determined to be absent if no delamination occurred. 
         [0065]    The maximum number of fine bubbles represents the largest number of water bubbles among those observed at the six observation points. Meanwhile, the minimum number of fine bubbles represents the smallest number of water bubbles among those observed at the six observation points. In a case where the number of water bubbles was greater than 10, the maximum number is expressed as “&gt;10”. 
         [0066]    The maximum diameter of fine bubbles represents the diameter of the largest water bubble in the six observation points. Note that when the water bubble had an elliptical shape, the longest distance passing through the center of the water bubble was employed as the diameter of the water bubble. When the diameter of the water bubble was larger than 10 μm, the maximum diameter is expressed as “&gt;10”. 
         [0067]    The transmission loss of each of Examples and Comparative Examples was measured before and after the immersion in hot water. The increase in transmission loss was an increment in transmission loss between that measured by using the coated optical fiber after the immersion in hot water at 60° C. for 200 days and that measured by using the coated optical fiber before the immersion in hot water. For the measurement of the transmission losses, light with a wavelength of 1.55 μm was used. 
         [0068]    In general, an increase in transmission loss of less than 0.1 dB/km is regarded as being acceptable in a practical sense. For this reason, if the increase was 0.1 dB/km or more, the water resistance was evaluated to be insufficient (this evaluation was represented by C); if the increase was less than 0.1 dB/km, the water resistance was evaluated to be sufficient (this evaluation was represented by B); and further if the increase was less than 0.07 dB/km, the water resistance was evaluated to be excellent (this evaluation was represented by A). 
         [0069]    From the results shown in Table 1, it can be seen that the increase in transmission loss was reduced more in the cases (Examples 1 to 4) where one or more water bubbles were generated than in the cases (Comparative Examples 1 and 2) where water bubbles were not generated. Moreover, in the case (Comparative Example 1) where water bubbles were not formed, generation of delamination was also observed. 
         [0070]    In addition, among the cases where one or more water bubbles were generated, the increase in transmission loss was reduced more in the cases (Examples 1 to 4) where the maximum diameter of the water bubbles was 10 μm or smaller than in the case (Comparative Example 3) where the maximum diameter of the water bubbles was larger than 10 μm. 
         [0071]    From the fact that at least one water bubble was generated at all the points observed, it can be said that the water bubbles were formed substantially evenly in the primary coating layer. Hence, it is conceivable that the stress from the secondary coating layer to the glass/primary interface was relaxed by the generation of small water bubbles substantially evenly in the primary coating layer during the immersion in hot water at 60° C. for 200 days, so that the increase in transmission loss was reduced. Specifically, the increase in transmission loss was reduced when the water bubbles were formed substantially evenly in the primary coating layer, the sizes of the water bubbles were 10 μm or less in diameter, and the number of the water bubbles present per unit area on the cross-section taken along the longitudinal direction of the primary coating layer was one or more. By taking the thickness of the primary coating layer as one unit, the unit area is defined as an area equal to the square of the thickness of the primary coating layer. 
         [0072]    In addition, from the results shown in Table 1, it can be seen that the increase in transmission loss was reduced in the cases (Examples 1 to 4) where the D50 was larger than the T50. On the other hand, the increase in transmission loss increased in the case (Comparative Example 3) where the T50 was smaller than 200 g. This is presumably because too small a T50 made the water bubbles more likely to be formed in the primary coating layer, and the water bubbles aggregated to form larger water bubbles, so that the stress was applied unevenly to the glass/primary interface, resulting in the increase in transmission loss. For this reason, it is more preferable that the D50 be 500 g or higher and the T50 be 200 g or higher.