Patent Publication Number: US-2023137341-A1

Title: Method for producing coated optical fiber and coated optical fiber production apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a method for producing a coated optical fiber, and a coated optical fiber production apparatus used for the same. 
     BACKGROUND ART 
     Optical fibers usually have a form in which a coating film on the circumferential side surface of an optical fiber wire is formed. The coating film of the optical fiber is required to ensure, for example, optical transmission characteristics, mechanical characteristics, weather resistance, or the like of the optical fiber. The technique relating to the coating film forming on the optical fiber is disclosed in, for example, Patent Document 1 below. 
     CITATION LIST 
     Patent Document 
     
         
         Patent Document 1: Japanese Unexamined Patent Publication No. 2006-3720 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     To form the coating film of the optical fiber, for example, a coating die is used having the following configuration. The coating die includes an inner space filled with a coating material in liquid state, a minute-diameter fiber insertion path, and a minute-diameter fiber drawing path, where the paths communicate with the inner space and are opposed to each other with such space interposed therebetween. In this coating die, an optical fiber wire is sequentially passed through the fiber insertion path, the inner space (filled with the coating material in liquid state), and the fiber drawing path, thereby coating the circumferential side surface of the optical fiber wire with the coating material. The coated film is then, for example, dried to form a coating film on the optical fiber. Such formation of the coating film in this manner conventionally causes variations in film thickness along a circumferential direction of the optical fiber. Specifically, the film thickness is uneven in the cross section of the fiber, and this unevenness is irregular along the fiber extending direction. In the optical fiber wire that passes through the fiber drawing path in the coating die together with the coating material, deviation (slight position fluctuations) occurs in its radial position in the fiber drawing path, and this positional deviation may cause the above-mentioned variations in film thickness. Large variations in film thickness along the circumferential direction of the optical fiber are not preferable in view of ensuring various characteristics such as desired optical transmission characteristics and mechanical characteristics of the optical fiber. 
     The present invention is to provide a method for producing a coated optical fiber suitable for suppressing variations in film thickness along the fiber circumferential direction, and a coated optical fiber production apparatus used for the same. 
     Means for Solving the Problem 
     The present invention [1] includes a method for producing a coated optical fiber using a coating die including a liquid retaining chamber that receives a coating material in liquid state; an insertion hole portion that communicates with the liquid retaining chamber; and a coating hole portion that communicates with the liquid retaining chamber, the coating hole portion being opposed to the insertion hole portion via the liquid retaining chamber and extending in a direction away from the liquid retaining chamber, the method including coating a circumferential side surface of an optical fiber with the coating material by, in the coating die, passing the optical fiber sequentially through the insertion hole portion, the liquid retaining chamber, and the coating hole portion while the coating material in the liquid retaining chamber is supplied to the coating hole portion, in which a viscosity μ (Pa·s) of the coating material in the liquid retaining chamber and a length L (mm) of the coating hole portion in an extending direction satisfy a relationship of μL≥1.5. 
     In this production method, as mentioned above, in the state where the viscosity μ (Pa·s) of the coating material in the liquid retaining chamber, and the length L (mm) of the coating hole portion in the extending direction satisfy the conditional equation of μL≥1.5, while the coating material in the liquid retaining chamber is supplied to the coating hole portion, the optical fiber is passed sequentially through the insertion hole portion, the liquid retaining chamber, and the coating hole portion, thereby coating the circumferential side surface of the optical fiber with the coating material. This configuration is suitable for suppressing occurrence of a deviation (slight position fluctuations) of the optical fiber in its radial position in the coating hole portion, the optical fiber that passes through the coating hole portion together with the coating material, and is therefore suitable for suppressing variations in thickness of the film along the fiber circumferential direction, the film formed on the circumferential side surface of the fiber. 
     The present invention [2] includes the method for producing a coated optical fiber described in [1], in which the viscosity μ is 0.3 Pa·s or more. 
     This configuration is suitable for securing fluid resistance that the optical fiber passing through the coating hole portion receives from the coating material passing through the coating hole portion together with the optical fiber, to suppress the above-described positional deviation of the optical fiber, and is therefore suitable for suppressing variations in film thickness along the fiber circumferential direction. 
     The present invention [3] includes the method for producing a coated optical fiber described in [1] or [2], in which the length L is 1.5 mm or more. 
     This configuration is suitable for securing the length of the optical fiber passing through the coating hole portion, the length for receiving fluid resistance from the coating material passing through the coating hole portion together with the optical fiber, to suppress the above-described positional deviation of the optical fiber, and is therefore suitable for suppressing variations in film thickness along the fiber circumferential direction. 
     The present invention [4] includes the method for producing a coated optical fiber described in any one of the above-described [1] to [3], in which the coating material is an ultraviolet curing resin. 
     This configuration is suitable for achieving high adhesion to the fiber and high durability of the coating film formed on the circumferential side surface of the optical fiber, and is also suitable for achieving high productivity of the produced coated optical fiber. 
     The present invention [ 5 ] includes a coated optical fiber production apparatus for performing the method for producing a coated optical fiber described in any one of the above-described [1] to [4], the apparatus including a coating die having a liquid retaining chamber for receiving a coating material in liquid state; an insertion hole portion that communicates with the liquid retaining chamber, and a coating hole portion that communicates with the liquid retaining chamber, the coating hole portion being opposed to the insertion hole portion via the liquid retaining chamber and extending in a direction away from the liquid retaining chamber. 
     This coated optical fiber production apparatus can suitably perform the above-described method for producing an optical fiber. 
     The present invention [6] includes the coated optical fiber production apparatus described in [5], in which the coating hole portion has a length L in an extending direction of 1.5 mm or more. 
     This configuration is suitable for securing the length of the optical fiber passing through the coating hole portion, the length for receiving fluid resistance from the coating material passing through the coating hole portion together with the optical fiber, to suppress the above-described positional deviation of the optical fiber during use of the apparatus, and is therefore suitable for suppressing variations in film thickness along the fiber circumferential direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a configurational diagram of a coated optical fiber production apparatus for performing a method for producing an optical fiber according to an embodiment of the present invention. 
         FIG.  2    is a perspective view of a coating die provided in the coated optical fiber production apparatus shown in  FIG.  1   . 
         FIG.  3    is a cross-sectional view of the coating die shown in  FIG.  2   , taken along line III-III. 
         FIG.  4    is a partially enlarged cross-sectional view of the coating die shown in  FIGS.  2  and  3   . 
         FIG.  5    illustrates coating in the coating die according to the present invention. 
         FIG.  6    is a cross-sectional view of an example of an optical fiber having a coating film. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
       FIG.  1    shows a coated optical fiber production apparatus X according to an embodiment of the present invention. The coated optical fiber production apparatus X is an apparatus to be used for the method for producing a coated optical fiber according to an embodiment of the present invention, and includes a feeding unit  11 , a coating die Y, a curing unit  12 , a capstan  13 , and a winding unit  14  sequentially in an optical fiber traveling direction.  FIG.  1    also shows an optical fiber F traveling from the feeding unit  11  to the winding unit  14  in the coated optical fiber production apparatus X (the optical fiber traveling direction is indicated by arrows). 
     The feeding unit  11  is a part that supplies the optical fiber F to be film-formed into a process in the apparatus and is, for example, a reel (optical fiber reel) with the optical fiber F wound thereon and rotatably provided around a predetermined axis. The optical fiber F may be an optical fiber wire or an optical fiber core wire. The optical fiber F is a plastic optical fiber or a glass optical fiber, and in the present embodiment, a plastic optical fiber is preferably used. 
     The optical fiber F as a plastic optical fiber includes a core having a relatively high refractive index and serving as an optical transmission line itself, and a clad having a relatively low refractive index and positioned around the core to extend along the core. Examples of a constituent material of the core include an acrylic polymer such as polymethyl methacrylate, and a fluorine-containing polymer such as fluorine-containing polyimide. To the constituent material of the core may be added a low-molecular material (dopant) for adjusting the refractive index to a high value. Examples of a constituent material of the clad include an acrylic polymer such as polymethyl methacrylate, a polycarbonate, and a fluorine-containing polymer such as fluorine-containing polyimide. 
     The optical fiber F has a thickness, that is, a cross-sectional diameter of, for example, 100 to 1000 μm when it is an optical fiber wire, and for example, 25 to 500 μm when it is an optical fiber core wire. 
     The coating die Y is a die for coating a surface, specifically, a circumferential side surface of the optical fiber F with a coating material, and has a liquid retaining chamber  21 , a coating material supply port  22 , an insertion hole portion  23 , and a coating hole portion  24 . The coating die Y is also constituted in that the temperature inside the die can be controlled by a temperature control mechanism (not illustrated). 
     The liquid retaining chamber  21  is a chamber, or space, in the coating die Y for receiving a coating material in liquid state. In the present embodiment, the liquid retaining chamber  21  includes a cylindrical space  21   a  and a truncated conical space  21   b . The bottom face of the truncated conical space  21   b  has a smaller diameter than that of the cylindrical space  21   a , and the cylindrical space  21   a  and the truncated conical space  21   b  are continuously arranged to have a common axis of rotational symmetry Ax (that is, the liquid retaining chamber  21  is a space shaped to have the axis of rotational symmetry Ax). The cylindrical space  21   a  has a diameter D 1  of, for example, 3 to 50 mm, and a height D 2  of, for example, 5 to 100 mm. The truncated conical space  21   b  has a diameter D 3  of, for example, 2 to 50 mm at the bottom face thereof, and a height D 4  of, for example, 1 to 20 mm. An opening angle θ formed between opposed generating lines, conical generatrices, in the truncated conical space  21   b  is, for example, from 5 to 90 degrees. The coating die Y is installed in such a posture that in the liquid retaining chamber  21 , the cylindrical space  21   a  is located at a relatively upper position and the truncated conical space  21   b  is located at a relatively lower position (preferably, in such a posture that the axis of rotational symmetry Ax of the liquid retaining chamber  21  extends in a vertical direction). 
     The coating material supply port  22  is a flow channel for supplying a coating material in liquid state from outside of the coating die Y to the liquid retaining chamber  21 , and communicates with the cylindrical space  21   a  of the liquid retaining chamber  21 . The coating material supply port  22  extends in a radial direction of the cylindrical space  21   a , and one end thereof is open in an outer surface of a side wall of the coating die Y, while the other end thereof is open in an inner surface of a side wall of the cylindrical space  21   a . The coating material supply port  22  has a diameter of, for example, 1 to 20 mm. The coating material supply port  22  is in connection with a tank (not illustrated) for storing the coating material in liquid state with a predetermined tube member (not illustrated) such as a flexible tube interposed therebetween. To the tank or the tube member is attached a coating material supply means (not illustrated) such as a pump capable of delivering the coating material in the tank toward the liquid retaining chamber  21  of the coating die Y. In the present embodiment, this coating material supply means is constituted such that a supply pressure of the coating material can be controlled. 
     Examples of the coating material include an ultraviolet curing resin composition and a thermosetting resin composition. From a viewpoint of achieving high adhesion to the fiber and high durability of the film formed on the circumferential side surface of the optical fiber, an ultraviolet curing resin composition is preferably used as the coating material. Examples of the resin in the resin composition include urethane acrylate resin, polyester acrylate resin, epoxy acrylate resin, polyol acrylate resin, epoxy resin, silicone resin, nylon resin, and polyamide resin. 
     The coating material may contain a coloring component such as pigment and dye. Examples of the pigment include black pigment such as carbon black; white pigment such as titanium oxide and zinc oxide; red pigment such as iron oxide; yellow pigment such as yellow lead or chrome yellow, and zinc chromate; blue pigment such as ultramarine blue and cobalt blue; and green pigment such as chromium oxide. 
     The insertion hole portion  23  is a hole for inserting the optical fiber F toward the liquid retaining chamber  21  from outside of the coating die Y, and communicates with the cylindrical space  21   a  of the liquid retaining chamber  21 . The insertion hole portion  23  extends along the axis of rotational symmetry Ax, and one end thereof is open in an outer surface of an upper wall of the coating die Y, while the other end thereof is open in an inner surface of an upper wall of the cylindrical space  21   a . The diameter of the insertion hole portion  23  is set according to the diameter of the optical fiber F and is, for example, 5 to 500 μm larger than that of the optical fiber F. The insertion hole portion  23  also has a length in the extending direction of, for example, 1 to 50 mm. 
     The coating hole portion  24  is a hole for letting the optical fiber F through from the liquid retaining chamber  21  to outside of the coating die Y, and communicates with the truncated conical space  21   b  of the liquid retaining chamber  21 . The coating hole portion  24  extends along the axis of rotational symmetry Ax, and one end thereof is open at the top of the truncated conical space  21   b , while the other end thereof is open in an outer surface of a lower wall of the coating die Y. The coating hole portion  24  is opposed to the insertion hole portion  23  with the liquid retaining chamber  21  interposed therebetween, and extends in a direction away from the liquid retaining chamber  21 . The coating hole portion  24  extends from the liquid retaining chamber  21  to the opposite side of the insertion hole portion  23 . 
     The diameter of the coating hole portion  24  is set according to the sum of the diameter of the optical fiber F and the thickness of a coating film to be formed, and is, for example, 10 to 2000 μm larger than that of the optical fiber F. 
     The coating hole portion  24  has a length L (shown in  FIG.  3   ) in the extending direction of preferably 1.5 mm or more, more preferably 2 mm or more, more preferably 3 mm or more, more preferably 3.5 mm or more, more preferably 4 mm or more. The length L is, for example, 20 mm or less, preferably 15 mm or less. 
     The length L of the coating hole portion  24  in the extending direction is, for example, 30 mm or less, preferably 20 mm or less. This configuration is preferable from viewpoints of accuracy of machining of the coating hole portion  24  in the production process of the coating die Y, ease of cleaning of the coating hole portion  24  during non-use of the coating die Y, and ease of work for allowing the optical fiber F to pass through the coating hole portion  24  of the coating die Y at the time of starting the method for producing a coated optical fiber in the coated optical fiber production apparatus X. 
     The curing unit  12  is a part that cures the coating material applied to the optical fiber F that has passed through the coating die Y. When an ultraviolet curing resin composition is used as the coating material, the curing unit  12  is, for example, an ultraviolet irradiation device such as an UV lamp. When a thermosetting resin composition is used as the coating material, the curing unit  12  is, for example, a heating furnace. 
     The capstan  13  is a part that draws the optical fiber F from the feeding unit  11  by rotary drive to control a linear speed, that is, a traveling speed of the optical fiber F. 
     The winding unit  14  is a part that winds up the optical fiber F. 
     In the present embodiment, guide rollers R 1  and R 2  are provided for guiding the optical fiber F between the feeding unit  11  and the coating die Y, a guide roller R 3  is provided for guiding the optical fiber F between the coating die Y and the capstan  13 , and guide rollers R 4  and R 5  are provided for guiding the optical fiber F between the capstan  13  and the winding unit  14 . The number of guide rollers and their arrangement locations are appropriately determined according to the sizes, arrangement locations, and the like of the feeding unit  11 , the coating die Y, the curing unit  12 , the capstan  13 , and the winding unit  14 . 
     The method for producing a coated optical fiber according to an embodiment of the present invention is performed using the coated optical fiber production apparatus X having the coating die Y and the like as described above. Specific details are as follows. 
     In this production method, the optical fiber F travels from the feeding unit  11  to the winding unit  14  while the optical fiber F is fed from the feeding unit  11  and wound up by the winding unit  14 . The traveling speed is controlled by the capstan  13  that draws the optical fiber F by the rotary drive and is set to, for example, 10 to 200 m/min. 
     The coating die Y is controlled to have a predetermined temperature by the above-described temperature control mechanism, and the coating material C in liquid state is supplied from the above-described tank to the coating die Y through the above-described tube member. By operating the above-described coating material supply means, the coating material C from the tank is supplied to the liquid retaining chamber  21  through the coating material supply port  22 . In the present embodiment, the coating material C is supplied under pressure to the liquid retaining chamber  21 . The coating material C that has been supplied to the liquid retaining chamber  21  is then supplied to the coating hole portion  24  immediately below the liquid retaining chamber  21  and communicating therewith. 
     In this production method, in the coating die Y, while the coating material C in the liquid retaining chamber  21  is supplied to the coating hole portion  24 , the optical fiber F is sequentially passed through the insertion hole portion  23 , the liquid retaining chamber  21 , and the coating hole portion  24  as shown in  FIG.  5   , thereby coating the circumferential side surface of the optical fiber F with the coating material C. This coating is performed under a condition that a viscosity μ (Pa·s) of the coating material C in the liquid retaining chamber  21 , and the length L (mm) of the coating hole portion  24  in the extending direction satisfy the following equation (1). That is, while the state where the following equation (1) is satisfied is maintained, continuous coating is performed on the optical fiber F with the coating die Y. In this production method, the μL value is 1.5 or more, preferably 2 or more, more preferably 2.5 or more. The p L value is, for example, 5 or less. 
       μ L≥ 1.5  (1)
 
     The viscosity μ of the coating material C in the liquid retaining chamber  21  is preferably 0.3 Pa·s or more, more preferably 0.5 Pa·s or more, more preferably 1 Pa·s or more. The viscosity μ is, for example, 3 Pa·s or less. The viscosity μ of the coating material C in the liquid retaining chamber  21  can be adjusted, for example, by controlling the temperature of the coating die Y. 
     A pressure difference ΔP between a liquid pressure of the coating material C in the liquid retaining chamber  21  and a pressure outside the coating die Y is preferably 0.3 MPa or less, more preferably 0.2 MPa or less, more preferably 0.15 MPa or less, more preferably 0.1 MPa or less. The pressure difference ΔP is, for example, 0.001 MPa or more. The pressure difference ΔP can be adjusted by controlling the supply pressure of the coating material C to the coating die Y by the above-described coating material supply means of the coating material C. 
     The optical fiber F that has passed through the coating die Y to be coated with the coating material C then passes through the curing unit  12 . In the curing unit  12 , the coating material C on the optical fiber F is cured. When an ultraviolet curing resin composition is used as the coating material C, the curing unit  12  is an ultraviolet irradiation device, and the coating material C is cured by ultraviolet irradiation. When a thermosetting resin composition is used as the coating material C, the curing unit  12  is a heating furnace, and the coating material C is cured by heating. By passing the optical fiber F through the curing unit  12 , a film of the coating material C is formed on the circumferential side surface of the optical fiber F. 
     After passing through the curing unit  12 , the optical fiber F goes through the capstan  13  and is then wound up by the winding unit  14 . 
     In the manner described above, the optical fiber F having its circumferential side surface coated with the cured coating material C, that is a coated optical fiber, is produced.  FIG.  6    shows a cross section of the optical fiber F having the film of the coating material C formed on its circumferential side surface. The film has a thickness of, for example, 1 to 2000 μm. 
     In this production method, as described above, in the state where the viscosity μ (Pa·s) of the coating material C in the liquid retaining chamber  21 , and the length L (mm) of the coating hole portion  24  in the extending direction satisfy the above-mentioned conditional equation (1), while the coating material C in the liquid retaining chamber  21  is supplied to the coating hole portion  24 , the optical fiber F is sequentially passed through the insertion hole portion  23 , the liquid retaining chamber  21 , and the coating hole portion  24 , thereby coating the circumferential side surface of the optical fiber F with the coating material C. The μL value is 1.5 or more, preferably 2 or more, more preferably 2.5 or more. 
     The present inventors have found that this configuration is suitable for suppressing variations in thickness of the film along a fiber circumferential direction, where the film is formed by coating the circumferential side surface of the optical fiber F with the coating material C. It is considered that the configuration of the production method such that the p L value is 1.5 or more is suitable for suppressing the occurrence of deviation (slight position fluctuations) of the optical fiber F in its radial position in the coating hole portion  24 , the optical fiber F that passes through the coating hole portion  24  together with the coating material C. As the positional deviation is suppressed, the variations in thickness of the film of the coating material C formed on the circumferential side surface of the optical fiber F along the fiber circumferential direction are suppressed. 
     In the production method, the viscosity μ of the coating material C in the liquid retaining chamber  21  is preferably 0.3 Pa·s or more, more preferably 0.5 Pa·s or more, more preferably 1 Pa·s or more, as described above. This configuration is suitable for securing fluid resistance that the optical fiber F passing through the coating hole portion  24  receives from the coating material C passing through the coating hole portion  24  together with the optical fiber F, to suppress the above-described positional deviation of the optical fiber F, and is therefore suitable for suppressing variations in film thickness along the fiber circumferential direction. 
     The pressure difference ΔP in the production method is preferably 0.3 MPa or less, more preferably 0.2 MPa or less, more preferably 0.15 MPa or less, more preferably 0.1 MPa or less, as described above. This configuration is suitable for suppressing pulsations of the coating material C passing through the coating hole portion  24  together with the optical fiber F, during passage of the coating material C in the coating hole portion  24 , and is therefore suitable for suppressing the variations in film thickness of the optical fiber. 
     The coating hole portion  24  in the coating die Y has a length L in the extending direction of preferably 1.5 mm or more, more preferably 2 mm or more, more preferably 3 mm or more, more preferably 3.5 mm or more, more preferably 4 mm or more, as described above. This configuration is suitable for securing the length of the optical fiber F passing through the coating hole portion  24 , the length for receiving fluid resistance from the coating material C passing through the coating hole portion  24  together with the optical fiber F, to suppress the above-described positional deviation of the optical fiber F, and is therefore suitable for suppressing variations in film thickness of the optical fiber. 
     EXAMPLE 
     Example 1 
     &lt;Formation of Film&gt; 
     Using the coated optical fiber production apparatus X having the configuration shown in  FIG.  1   , a film of the coating material C was formed on the circumferential side surface of the optical fiber F. Specifically, the optical fiber F was traveled from the feeding unit  11  to the winding unit  14  (at a traveling speed of 10 m/min), and in the coating die Y, while the coating material C in the liquid retaining chamber  21  was supplied to the coating hole portion  24 , the optical fiber F was sequentially passed through the insertion hole portion  23 , the liquid retaining chamber  21 , and the coating hole portion  24 , thereby coating the circumferential side surface of the optical fiber F with the coating material C, and the coating material C of the coated optical fiber was cured by ultraviolet irradiation using an ultraviolet irradiation device. As the optical fiber F, an optical fiber wire having a core, a clad around the core, and an overclad around the clad was used (the core and the clad was made of acrylic polymer, and the overclad was made of polycarbonate). The optical fiber wire had an average outer diameter of 470 μm. As the coating material C in liquid state, a coating material (trade name “BESTCURE FA013”, ultraviolet curing resin composition, manufactured by T&amp;K TOKA Corporation) having a temperature of about 40° C. was supplied under pressure to the liquid retaining chamber  21  of the coating die Y and to the coating hole portion  24  communicating therewith. The pressure difference ΔP between the liquid pressure of the coating material C in the liquid retaining chamber  21  and the pressure outside the coating die Y was 0.01 MPa. In this Example, the viscosity μ of the coating material C in the liquid retaining chamber  21  was 1.314 Pa·s by controlling the temperature of the coating die Y to 28° C.±0.1° C. The coating hole portion  24  of the coating die Y used in this Example had a diameter of 510 μm and a length L in the extending direction of 2 mm. 
     &lt;Evaluation of Variations in Film Thickness&gt; 
     First, a cross section of the resulting optical fiber for observation was formed at 18 points (at 40 cm intervals) in the extending direction of the optical fiber. To form the cross section of the optical fiber for observation, a cut surface of the optical fiber was polished using an abrasive paper until an interface between the optical fiber wire and the film around it was clearly identified by a microscope. Then, the cross section for observation was observed using a digital microscope VHX-5000 (manufactured by KEYENCE) and an amount of eccentricity (μm) of the center position of the optical fiber wire in the cross section for observation was measured by a measurement function provided in such microscope. The amount of eccentricity is a distance (amount of deviation between a first center position and a second center position) between a center position (first center position) of the optical fiber wire itself in the cross section for observation and a center position (second center position) of the entire optical fiber (including the optical fiber wire and the film). Average values of the amounts of eccentricity at 18 points in the optical fibers are shown in Table 1 as the amount of eccentricity (μm) in this Example. When the amount of eccentricity was 2 μm or less, the variations in film thickness of the optical fiber were evaluated as “excellent”; when the amount of eccentricity was more than 2 μm and 3μ or less, the variations were evaluated as “good”; and when the amount of eccentricity was more than 3 μm, the variations were evaluated as “bad”. This result is also shown in Table 1. 
     Example 2 
     A film of the coating material C was formed on the circumferential side surface of the optical fiber in the same manner as in Example 1, except that the viscosity μ of the coating material C in the liquid retaining chamber  21  was adjusted to 0.590 Pa·s by controlling the temperature of the coating die Y to 40° C.±0.1° C. while the temperature of the coating material C to be supplied to the coating die Y was about 40° C., and the length L in the extending direction of the coating hole portion  24  was 4 mm instead of 2 mm. The above-described amount of eccentricity was then measured to evaluate variations in film thickness. The evaluation result is shown in Table 1 (the same applies to those in Examples and Comparative Examples to be described later). 
     Example 3 
     A film of the coating material C was formed on the circumferential side surface of the optical fiber in the same manner as in Example 1, except that the length L in the extending direction of the coating hole portion  24  was 4 mm instead of 2 mm. The above-described amount of eccentricity was then measured to evaluate variations in film thickness. 
     Example 4 
     A film of the coating material C was formed on the circumferential side surface of the optical fiber in the same manner as in Example 1, except that the length L in the extending direction of the coating hole portion  24  was 7 mm instead of 2 mm. The above-described amount of eccentricity was then measured to evaluate variations in film thickness. 
     Example 5 
     A film of the coating material C was formed on the circumferential side surface of the optical fiber in the same manner as in Example 1, except that the viscosity μ of the coating material C in the liquid retaining chamber  21  was adjusted to 0.590 Pa·s by controlling the temperature of the coating die Y to 40° C.±0.1° C. while the temperature of the coating material C to be supplied to the coating die Y was about 40° C., and the length L in the extending direction of the coating hole portion  24  was 7 mm instead of 2 mm. The above-described amount of eccentricity was then measured to evaluate variations in film thickness. 
     Comparative Example 1 
     A film of the coating material C was formed on the circumferential side surface of the optical fiber in the same manner as in Example 1, except that the viscosity μ of the coating material C in the liquid retaining chamber  21  was adjusted to 0.207 Pa·s by controlling the temperature of the coating die Y to 60° C.±0.1° C. while the temperature of the coating material C to be supplied to the coating die Y was about 60° C. The above-described amount of eccentricity was then measured to evaluate variations in film thickness. 
     Comparative Example 2 
     A film of the coating material C was formed on the circumferential side surface of the optical fiber in the same manner as in Example 1, except that the viscosity μ of the coating material C in the liquid retaining chamber  21  was adjusted to 0.590 Pa·s by controlling the temperature of the coating die Y to 40° C.±0.1° C. while the temperature of the coating material C to be supplied to the coating die Y was about 40° C. The above-described amount of eccentricity was then measured to evaluate variations in film thickness. 
     [Evaluation] 
     For example, as seen in the difference in the amount of eccentricity in Example 1 and Comparative Examples 1 and 2, it was found that the larger the above-mentioned viscosity μ was, the smaller the variations in film thickness along the fiber circumferential direction tended to be. For example, as seen in the difference in the amount of eccentricity in Examples 2 and 5, it was found that the larger the above-mentioned length L was, the smaller the variations in film thickness along the fiber circumferential direction tended to be. Then, in Examples 1 to 5 in which the p L value was 1.5 or more, the amount of eccentricity was suppressed to 3 μm or less while in Comparative Examples 1 and 2 in which the μL value was not 1.5 or more, the amount of eccentricity was large than 3 μm. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Amount of 
                   
               
               
                   
                 Viscosity μ 
                 Length L 
                   
                 eccentricity 
               
               
                   
                 (Pa · s) 
                 (mm) 
                 μL 
                 (μm) 
                 Evaluation 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 1.314 
                 2 
                 2.628 
                 1.89 
                 Excellent 
               
               
                 Example 2 
                 0.590 
                 4 
                 2.36 
                 2.63 
                 Good 
               
               
                 Example 3 
                 1.314 
                 4 
                 5.256 
                 1.97 
                 Excellent 
               
               
                 Example 4 
                 1.314 
                 7 
                 9.198 
                 1.04 
                 Excellent 
               
               
                 Example 5 
                 0.590 
                 7 
                 4.13 
                 2.16 
                 Good 
               
               
                 Comparative 
                 0.207 
                 2 
                 0.414 
                 3.82 
                 Bad 
               
               
                 Example 1 
               
               
                 Comparative 
                 0.590 
                 2 
                 1.18 
                 3.59 
                 Bad 
               
               
                 Example 2 
               
               
                   
               
            
           
         
       
     
     INDUSTRIAL APPLICABILITY 
     The present invention can be employed for producing an optical fiber having a film formed on its surface. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         X coated optical fiber production apparatus 
         F optical fiber 
         C coating material 
           11  feeding unit 
           12  curing unit 
           13  capstan 
           14  winding unit 
         Y coating die 
           21  liquid retaining chamber 
           22  coating material supply port 
           23  insertion hole portion 
           24  coating hole portion