Patent Publication Number: US-2021171394-A1

Title: Method and apparatus for manufacturing optical fiber cable

Description:
TECHNICAL FIELD 
     The present invention relates to a method of producing an optical fiber wire. The present invention also relates to an apparatus for producing an optical fiber wire. 
     BACKGROUND 
     An optical fiber wire includes (1) an optical fiber bare wire made of glass and (2) a coating made of a resin and covering a side surface of the optical fiber wire. The coating serves to reduce a lateral pressure on the optical fiber bare wire to thereby improve resistance to external damage. In production of an optical fiber wire, it is common to apply an ultraviolet curable resin to a side surface of an optical fiber bare wire and then cure the ultraviolet curable resin by irradiation with ultraviolet light to form a coating. 
     It is known that production of an optical fiber wire involves a plurality of irradiation steps involving applying ultraviolet light. For example, Patent Literature 1 discloses a technique of carrying out a first irradiation step to cure a surface layer of an ultraviolet curable resin and then carrying out a second irradiation step to cure an inner layer. Patent Literature 2 discloses a technique of carrying out a first irradiation step to partially cure an ultraviolet curable resin of an optical fiber wire, causing the optical fiber wire to pass through a cooling pipe through which a cooling gas flows and thereby cooling the optical fiber wire, and then carrying out a second irradiation step. 
     However, according to conventional methods of producing an optical fiber wire, if curing of an inner layer of a coating of the optical fiber wire is insufficient, the optical fiber wire may suffer cracking of the coating after the optical fiber wire is produced. 
     PATENT LITERATURE 
     [Patent Literature 1]
     Japanese Patent Application Publication, Tokukai, No. 2014-77918 (Publication Date: May 1, 2014)   

     [Patent Literature 2]
     Japanese Patent Application Publication, Tokukaihei, No. 10-297942 (Publication Date: Nov. 10, 1998)   

     SUMMARY 
     One or more embodiments of the present invention provide a production method and a production apparatus for an optical fiber wire that is less likely to suffer cracking of a coating after the optical fiber wire is produced. 
     A method of producing an optical fiber wire in accordance with one or more embodiments of the present invention is a method of producing an optical fiber wire that includes a coating constituted by an ultraviolet curable resin, the method including: a first irradiation step including applying ultraviolet light to each point on the optical fiber wire in which at least a portion of the ultraviolet curable resin is in an uncured state, the portion constituting a surface layer of the coating; and a second irradiation step including applying ultraviolet light to each point on the optical fiber wire which is obtained from the first irradiation step and in which at least the portion constituting the surface layer of the coating is in a cured state, wherein a temperature of the optical fiber wire immediately before the second irradiation step is not lower than 50° C. and not higher than 300° C. 
     An apparatus for producing an optical fiber wire in accordance with one or more embodiments of the present invention is an apparatus for producing an optical fiber wire that includes a coating constituted by an ultraviolet curable resin, the apparatus including: a first irradiation section configured to apply ultraviolet light to each point on the optical fiber wire in which at least a portion of the ultraviolet curable resin is in an uncured state, the portion constituting a surface layer of the coating; and a second irradiation section configured to apply ultraviolet light to each point on the optical fiber wire which is obtained by application of the ultraviolet light by the first irradiation section and in which at least the portion constituting the surface layer of the coating is in a cured state, wherein a temperature of the optical fiber wire immediately before receiving the ultraviolet light from the second irradiation section is not lower than 50° C. and not higher than 300° C. 
     According to one or more embodiments of the present invention, it is possible to provide a production method and a production apparatus for an optical fiber wire that is less likely to suffer cracking of a coating after the optical fiber wire is produced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a cross section of an optical fiber wire produced in one or more embodiments of the present invention. 
         FIG. 2  is a block diagram illustrating a configuration of an apparatus for producing an optical fiber wire in accordance with one or more embodiments of the present invention. 
         FIG. 3  is a chart showing examples of spectra of ultraviolet light emitted from a UV lamp of a primary irradiation section and ultraviolet light emitted from a UV LED of a secondary irradiation section in accordance with one or more embodiments of the present invention. 
         FIG. 4  is a cross-sectional view of a first irradiation unit included in the primary irradiation section in accordance with one or more embodiments of the present invention. 
         FIG. 5  is a cross-sectional view of a second irradiation unit included in the secondary irradiation section in accordance with one or more embodiments of the present invention. 
         FIG. 6  is a graph showing a relationship between the temperature of the optical fiber wire immediately before entry into the secondary irradiation section and the degree of cure of a primary coating after production, in accordance with one or more embodiments of the present invention. 
         FIG. 7  is a flowchart showing a method of producing an optical fiber wire in accordance with one or more embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description will discuss production apparatuses and production methods for an optical fiber in accordance with embodiments of the present invention. Note that configurations and steps each having the same function in the embodiments are given the same reference numerals, and descriptions on such configurations and steps will not be repeated. 
     [Configuration of Optical Fiber Wire] 
     First, the following description discusses, with reference to  FIG. 1 , an optical fiber wire  10  produced by production apparatuses and production methods for an optical fiber in accordance with embodiments (described later) of the present invention.  FIG. 1  is a cross-sectional view illustrating a cross section (which is perpendicular to optical axis) of the optical fiber wire  10 . 
     The optical fiber wire  10  includes: an optical fiber bare wire  11  in the form of a circular rod; and a coating  12  that covers the side surface of the optical fiber bare wire  11 . 
     The optical fiber bare wire  11  is constituted by: a core  11   a  in the form of a circular rod; and a cladding  11   b  that covers the side surface of the core  11   a  and that is in the form of a circular tube. The core  11   a  and the cladding  11   b  are each made of quartz glass. Note, however, that the quartz glass constituting the cladding  11   b  is lower in refractive index than the quartz glass constituting the core  11   a . Such a difference in refractive index between the core  11   a  and the cladding  11   b  is formed by, for example: adding a dopant for increasing refractive index (e.g., germanium) to the quartz glass constituting the core  11   a ; or adding a dopant for reducing refractive index (e.g., fluorine) to the quartz glass constituting the cladding  11   b . A reason why an arrangement in which the cladding  11   b  is lower in refractive index than the core  11   a  is employed is that this arrangement imparts the function of confining light within the core  11   a  to the optical fiber bare wire  11 . 
     The coating  12  is constituted by: a primary coating  12   a  that covers the side surface (outer side surface of the cladding  11   b ) of the optical fiber bare wire  11  and that is in the form of a circular tube; and a secondary coating  12   b  that covers the outer side surface of the primary coating  12   a  and that is in the form of a circular tube. The primary coating  12   a  and the secondary coating  12   b  are each made of an ultraviolet curable resin. Note, however, that the ultraviolet curable resin constituting the primary coating  12   a  is lower in Young&#39;s modulus than the ultraviolet curable resin constituting the secondary coating  12   b . Such a difference in Young&#39;s modulus between the primary coating  12   a  and the secondary coating  12   b  is formed by, for example: causing the degree of polymerization of the ultraviolet curable resin constituting the primary coating  12   a  to be different from that of the ultraviolet curable resin constituting the secondary coating  12   b . A reason why an arrangement in which the Young&#39;s modulus of the secondary coating  12   b  is relatively high and the Young&#39;s modulus of the primary coating  12   a  is relatively low is employed is that this arrangement allows resistance to external damage to be improved by the secondary coating  12   b  which is hard and allows a shock absorbency to be improved by the primary coating  12   a  which is soft. 
     The ultraviolet curable resin constituting the primary coating  12   a  and the ultraviolet curable resin constituting the secondary coating  12   b  each contain a photopolymerization initiator. These ultraviolet curable resins start curing upon irradiation with ultraviolet light having a wavelength falling within the range of absorption wavelengths of the photopolymerization initiator. Note that the higher the temperature during carrying out curing is, the easier it is for the curing of the ultraviolet curable resin constituting the secondary coating  12   b  to advance and the more difficult it is for the curing of the ultraviolet curable resin constituting the primary coating  12   a  to advance. The lower the temperature during carrying out curing is, the more difficult it is for the curing of the ultraviolet curable resin constituting the secondary coating  12   b  to advance and the easier it is for the curing of the ultraviolet curable resin constituting the primary coating  12   a  to advance. 
     (Configuration of Apparatus for Producing Optical Fiber) 
     The following discusses a configuration of a production apparatus  1  in accordance with one or more embodiments with reference to  FIG. 2 .  FIG. 2  is a block diagram illustrating a configuration of the production apparatus  1 . 
     The production apparatus  1  is an apparatus for producing the optical fiber wire  10  (see  FIG. 1 ), and includes a drawing section  101 , a cooling section  102 , a bare wire outer diameter measuring section  103 , a coating section  104 , a wire outer diameter measuring section  105 , a primary irradiation section  106 , a haul-off section  107 , a secondary irradiation section  108 , and a take-up section  109 . These constituents are arranged in the order named along the path of advancement of the optical fiber wire  10 . The production apparatus  1  also includes a control section  110  which controls the coating section  104  and the haul-off section  107  with reference to monitor signals obtained from the bare wire outer diameter measuring section  103  and the wire outer diameter measuring section  105 . The production apparatus  1  also includes a plurality of pulleys  111 _ 1  through  111 _ 6 . The path of advancement of the optical fiber wire  10  is defined by the pulleys  111 _ 1  through  111 _ 6 . 
     Note that the primary irradiation section  106  constitutes an example of a first irradiation section in the present invention. The secondary irradiation section  108  constitutes an example of a second irradiation section in the present invention. 
     The drawing section  101  is a means to draw a preform which will constitute the optical fiber bare wire  11 . In one or more embodiments, a furnace is used as the drawing section  101 . The preform is heated and melted by the furnace. The melted preform is drawn by its own weight. Melting and extending a preform in this manner is referred to as “drawing”. The preform drawn by the drawing section  101  is sent into the cooling section  102  provided below the drawing section  101 . 
     The cooling section  102  is a means to cool the preform which has been drawn. In one or more embodiments, a cooling cylinder is used as the cooling section  102 . The preform which has been drawn is cooled and cured by a cooling gas flowing in the cooling cylinder. With this, the optical fiber bare wire  11  is obtained. The optical fiber bare wire  11 , obtained in the cooling section  102 , passes through the bare wire outer diameter measuring section  103  by which the outer diameter of the optical fiber bare wire  11  is measured, and then is sent into the coating section  104  provided below the cooling section  102 . 
     The coating section  104  is a means to apply uncured ultraviolet curable resins, which will constitute the coating  12 , to the side surface of the optical fiber bare wire  11 . In one or more embodiments, a two-stage coating die, which consists of two coating dies stacked together, is used as the coating section  104 . To the side surface of the optical fiber bare wire  11 , an uncured ultraviolet curable resin which will constitute the primary coating  12   a  is applied by one of the two coating dies located upstream of the other. To the outer side surface of the primary coating  12   a , an uncured ultraviolet curable resin which will constitute the secondary coating  12   b  is applied by the other of the two coating dies located downstream. With this, an optical fiber wire  10  whose primary coating  12   a  and secondary coating  12   b  are both in an uncured state is obtained. The optical fiber wire  10  in this state is hereinafter referred to as an optical fiber wire  10 α. The optical fiber wire  10 α obtained in the coating section  104  passes through the wire outer diameter measuring section  105  for measurement of the outer diameter thereof, and then is sent into the primary irradiation section  106  provided below the coating section  104 . 
     Note that the thickness of the ultraviolet curable resins applied by the coating section  104  can be changed, and is controlled by the control section  110  in accordance with the outer diameter of the optical fiber wire  10 α measured by the wire outer diameter measuring section  105 . In a case where the outer diameter of the optical fiber wire  10 α is less than a predetermined value, the control section  110  controls the coating section  104  to increase the thickness of the ultraviolet curable resins applied. On the contrary, in a case where the outer diameter of the optical fiber wire  10 α is greater than the predetermined value, the control section  110  controls the coating section  104  to reduce the thickness of the ultraviolet curable resins applied. This makes it possible to make the outer diameter of the resulting optical fiber wire  10  closer to a predetermined value. 
     The primary irradiation section  106  is a means to irradiate the optical fiber wire  10 α with ultraviolet light (to apply ultraviolet light to the optical fiber wire  10 α) with use of one or more UV lamps (one or more ultraviolet lamps) in a low-oxygen atmosphere. In one or more embodiments, n (n is a natural number of 1 or more) UV lamp units  106 _ 1  through  106 _ n , each including a UV lamp as a light source, are used as the primary irradiation section  106 . A configuration of each of the UV lamp units  106 _ i  (i is a natural number of 1 or more and n or less) will be described later in detail with reference to another drawing. Note that, although  FIG. 2  shows an example in which n=3, the number of the UV lamp units  106 _ i  included in the primary irradiation section  106  may be any number. 
     Upon irradiation with ultraviolet light from the UV lamps of the primary irradiation section  106 , the ultraviolet curable resins that will constitute the coating  12  cure in a manner such that outer portions thereof cure first. Through this irradiation with ultraviolet light from the UV lamps of the primary irradiation section  106 , mainly the ultraviolet curable resin constituting the secondary coating  12   b  cures. Note, however, that, as of the end of the ultraviolet irradiation using the UV lamps of the primary irradiation section  106 , it is only necessary that at least a portion constituting the surface layer of the secondary coating  12   b  has sufficiently cured, and the other portions of the ultraviolet curable resins may remain in an uncured or semi-cured state. The optical fiber wire  10  in this state is hereinafter referred to as an optical fiber wire  10 β. The optical fiber wire  10 β obtained in the primary irradiation section  106  passes over the pulley  111 _ 1 , and then is sent to the haul-off section  107 . The pulley  111 _ 1  serves as a turn pulley that changes the direction of advancement of the optical fiber wire  10 β from a first direction (which is parallel to the direction of gravitational force, downward direction in  FIG. 2 ) to a second direction (which is perpendicular to the direction of gravitational force, rightward direction in  FIG. 2 ). 
     The haul-off section  107  is a means to pull on the optical fiber wire  10 β at a specific pulling speed. As used herein, the term “pulling speed” refers to the length of the optical fiber wire  10 β pulled on by the haul-off section  107  per unit time. In one or more embodiments, a capstan is used as the haul-off section  107 . The optical fiber wire  10 β, pulled on by the haul-off section  107 , passes over the pulleys  111 _ 2  through  111 _ 6  and then is sent to the secondary irradiation section  108  provided lateral to the haul-off section  107 . Note, here, that the pulley  111 _ 5  is a dancer pulley that is capable of being displaced parallel to the first direction (displaced in upward and downward directions in  FIG. 2 ). When this pulley  111 _ 5  is biased in the first direction (downward direction in  FIG. 2 ), tension is applied to the optical fiber wire  10 β. 
     Note that the pulling speed at which the haul-off section  107  pulls on the optical fiber wire  10 β can be changed, and is controlled by the control section  110  in accordance with the outer diameter of the optical fiber bare wire  11  measured by the bare wire outer diameter measuring section  103 . In a case where the outer diameter of the optical fiber bare wire  11  is less than a predetermined value, the control section  110  controls the haul-off section  107  to reduce the pulling speed. On the contrary, in a case where the outer diameter of the optical fiber bare wire  11  is greater than the predetermined value, the control section  110  controls the haul-off section  107  to increase the pulling speed. This makes it possible to make the outer diameter of the resulting optical fiber bare wire  11  closer to the predetermined value. 
     The secondary irradiation section  108  is a means to irradiate the optical fiber wire  10 β with ultraviolet light (to apply ultraviolet light to the optical fiber wire  10 β) with use of one or more UV LEDs (one or more ultraviolet light emitting diodes). In one or more embodiments, m (m is a natural number of 1 or more) UV LED units  108 _ 1  through  108 _ m , each including a UV LED as a light source, are used as the secondary irradiation section  108 . A configuration of each of the UV LED units  108 _ j  (j is a natural number of 1 or more and m or less) will be described later in detail with reference to another drawing. Note that, although  FIG. 2  shows an example in which m=2, the number of the UV LED units  108 _ j  included in the secondary irradiation section  108  may be any number. 
     Of the ultraviolet curable resins that will constitute the coating  12 , a portion which has not sufficiently cured even after the irradiation with ultraviolet light from the UV lamps of the primary irradiation section  106  is allowed to cure to its full extent by irradiation with ultraviolet light from the UV LEDs of the secondary irradiation section  108 . Through the irradiation with ultraviolet light from the UV LEDs of the secondary irradiation section  108 , mainly the ultraviolet curable resin constituting the primary coating  12   a  cures. With this, the optical fiber wire  10  is obtained. The optical fiber wire  10  obtained in the secondary irradiation section  108  is sent to the take-up section  109 . 
     The take-up section  109  is a means to take up the optical fiber wire  10 . In one or more embodiments, a take-up drum  109   a  having a rotation axis parallel to the second direction and a pulley  109   b  capable of being displaced parallel to the second direction are used as the take-up section  109 . The take-up drum  109   a  is rotated while the pulley  109   b  is translated back and forth along the second direction, and thereby the optical fiber wire  10  is wound around the take-up drum  109   a  in a uniform manner. 
     As has been described, in the production apparatus  1 , UV lamps are used as light sources of the primary irradiation section  106 , whereas UV LEDs are used as light sources of the secondary irradiation section  108 . A reason therefor is as follows. 
     UV LEDs consume less electric power than UV lamps. Furthermore, UV LEDs are less likely to generate heat, and therefore enable simplification of cooling equipment and, in turn, make it possible to further reduce power consumption during operation. In addition, UV LEDs are advantageous in that it is possible to reduce the deterioration of ultraviolet curable resin that would occur under high temperature environment. The use of UV LEDs as light sources of the primary irradiation section  106 , however, entails the following situation. 
     Specifically, as shown in  FIG. 3 , ultraviolet light emitted by a UV LED has a narrower spectrum width than ultraviolet light emitted by a UV lamp. Therefore, the peak wavelength of the UV LED is likely to be different from the absorption wavelengths of the photopolymerization initiator contained in the secondary coating  12   b . In addition, the secondary coating  12   b  tends to cure more rapidly when the temperature of the fiber during curing is higher. Therefore, if UV LEDs are used in the primary irradiation section  106 , the primary irradiation section  106  will probably be incapable of allowing the portion, which constitutes the surface layer of the secondary coating  12   b , of the ultraviolet curable resin to sufficiently cure. This leads to a situation in that, when the optical fiber wire  10 β makes contact with the pulley  111 _ 1 , the surface of the secondary coating  12   b  adheres to the pulley  111 _ 1  and comes off. 
     In view of this, the production apparatus  1  employs an arrangement in which UV lamps are used as light sources of the primary irradiation section  106 , and thereby avoids such a situation. 
     The primary irradiation section  106  of the production apparatus  1  further employs the following arrangement. 
     Specifically, the primary irradiation section  106  irradiates the optical fiber wire  10 α with ultraviolet light with use of the UV lamps in a low-oxygen atmosphere containing oxygen at a concentration of not more than 2%. This is to prevent hindering of the curing of the ultraviolet curable resin that would otherwise be caused by oxygen. In particular, the primary irradiation section  106  is configured such that an inert gas, containing oxygen at a concentration of not more than 2%, flows through a quartz pipe which is passed through by the optical fiber wire  10 α irradiated with ultraviolet light emitted from the UV lamps. 
     The primary irradiation section  106  is configured such that each point on the optical fiber wire  10 α is irradiated with ultraviolet light from the UV lamps for not less than 0.01 seconds. This time period, which is “irradiation time”, is the time for at least a portion, which constitutes the surface layer of the secondary coating  12   b , of the ultraviolet curable resin to sufficiently cure. Note that the irradiation time is the time from when a point on the optical fiber wire  10 α enters an area irradiated with ultraviolet light from the primary irradiation section  106  (such an area may be hereinafter referred to as “irradiation area”) to when the point on the optical fiber wire  10 α goes out of the irradiation area. For example, assume that the drawing speed is 3000 m/min. In this case, in order to ensure irradiation time of 0.01 seconds, it is only necessary to employ an arrangement in which the length of the irradiation area, in which the irradiation by the primary irradiation section  106  is carried out in a low-oxygen atmosphere, is not less than 0.6 m. 
     The primary irradiation section  106  is further configured such that each point on the optical fiber wire  10 α is irradiated with ultraviolet light from the UV lamps for not more than 0.07 seconds. This is the irradiation time that prevents deterioration of the ultraviolet curable resin that would be caused by a high temperature environment resulting from use of the UV lamps, while ensuring sufficient curing of at least a portion, which constitutes the surface layer of the secondary coating  12   b , of the ultraviolet curable resin. For example, assume that the drawing speed is 1000 m/min. In this case, in order to ensure irradiation time of not more than 0.07 seconds, it is only necessary to employ an arrangement in which the length of the irradiation area, in which irradiation by the primary irradiation section  106  is carried out in a low-oxygen atmosphere, is not more than 1.2 m. 
     The secondary irradiation section  108  of the production apparatus  1  may further employ the following arrangement. 
     Specifically, the UV LEDs included in the secondary irradiation section  108  may be UV LEDs that emit ultraviolet light whose wavelength is included in the absorption wavelengths of the photopolymerization initiator contained in the ultraviolet curable resin constituting the primary coating  12   a . A reason therefor is as follows. In one or more embodiments, it is likely that, in the optical fiber wire  10 β which has passed through the primary irradiation section  106 , the ultraviolet curable resin constituting the secondary coating  12   a  has cured to some extent. It is inferred from this that, among the ultraviolet curable resins that will constitute the coating  12  of the optical fiber wire  10 β, an insufficiently cured portion is mainly the ultraviolet curable resin constituting the primary coating  12   a.    
     (Configurations of UV Lamp Unit and UV LED Unit) 
     The following description will discuss configurations of the UV lamp units  106 _ i  included in the primary irradiation section  106 , with reference to  FIG. 4 .  FIG. 4  is a cross-sectional view of one of the UV lamp units  106 _ i.    
     The UV lamp unit  106 _ i  includes: a housing  106   a ; a quartz pipe  106   b  that penetrates through the housing  106   a ; a UV lamp  106   c  contained in the housing  106   a ; and a reflecting plate  106   d  that is contained in the housing  106   a  and that surrounds the quartz pipe  106   b  and the UV lamp  106   c . The UV lamp  106   c  is, for example, a metal halide lamp. Ultraviolet light emitted from the UV lamp  106   c  is directly applied to the optical fiber wire  10 α advancing through the quartz pipe  106   b  or is reflected by the reflecting plate  106   d  and then is applied to the optical fiber wire  10   a.    
     Note that the housing  106   a  has: a gas inlet  106   a   1  through which cooling gas is supplied into the housing  106   a ; and a gas outlet  106   a   2  through which the cooling gas is discharged from the housing  106   a . The UV lamp  106   c  contained in the housing  106   a  is cooled by the cooling gas. 
     The UV lamp unit  106 _ i  further includes: an upper cap  106   e  for accommodating the top end of the quartz pipe  106   b  projecting upward from the housing  106   a ; and a lower cap  106   f  for accommodating the bottom end of the quartz pipe  106   b  projecting downward from the housing  106   a . The upper cap  106   e  has a gas inlet  106   e   1  through which an inert gas containing oxygen at low concentration is supplied into the upper cap  106   e . The lower cap  106   f  has a gas outlet  106   f   1  through which the inert gas is discharged from the lower cap  106   f . The inert gas is, for example, nitrogen, argon, or helium. The interior of the upper cap  106   e , the interior of the quartz pipe  106   b , and the interior of the lower cap  106   f  are filled with the inert gas. That is, the optical fiber wire  10 α, which advances through the quartz pipe  106   b , is irradiated with ultraviolet light in a low-oxygen atmosphere. 
     In one or more embodiments, such UV lamp units  106 _ 1  through  3  are arranged in sequence. The total length of the areas irradiated with ultraviolet light from the UV lamp units  106 _ i  is such a length that irradiation time of not less than 0.01 seconds and not more than 0.07 seconds is achieved. This length can change with changes in pulling speed. 
     The following description will discuss configurations of the UV LED units  108 _ j  included in the secondary irradiation section  108 , with reference to  FIG. 5 .  FIG. 5  is a cross-sectional view of one of the UV LED units  108 _ j.    
     The UV LED unit  108 _ j  includes: a housing  108   a ; a quartz pipe  108   b  that penetrates through the housing  108   a ; a UV LED bar  108   c  contained in the housing  108   a ; and a reflecting plate  108   d  that is contained in the housing  108   a  and that surrounds the quartz pipe  108   b  so as to face the UV LED bar  108   c . The UV LED bar  108   c  is an ultraviolet light source including a plurality of UV LEDs  108   c   1  through  108   c   5  arranged in a straight line. Ultraviolet light emitted from the UV LED bar  108   c  is directly applied to the optical fiber wire  10 β advancing through the quartz pipe  108   b  or is reflected by the reflecting plate  108   d  and then is applied to the optical fiber wire  10 β. 
     (Temperature of Optical Fiber Wire  10 β Immediately Before Entry into Secondary Irradiation Section  108 ) 
     The temperature of the optical fiber wire  10 β immediately before entry into the secondary irradiation section  108  may not be lower than 50° C. or higher than 300° C. To achieve this, one or more embodiments employ an arrangement in which the length of the path of advancement of the optical fiber wire  10 β, from the primary irradiation section  106  to the secondary irradiation section  108 , is long enough so that the optical fiber wire  10 β is allowed to naturally cool to have a temperature not lower than 50° C. and not higher than 300° C. immediately before entry into the secondary irradiation section  108 . Note that the rate of temperature decrease, resulting from natural cooling, of the optical fiber wire  10 β is, for example, not less than 400° C. per second and not more than 1400° C. per second. However, the rate of temperature decrease, resulting from natural cooling, of the optical fiber wire  10 β changes with changes in drawing speed. Therefore, the length of the path of advancement of the optical fiber wire  10 β from the primary irradiation section  106  to the secondary irradiation section  108  is also set to a value corresponding to the drawing speed. 
     The following description will discuss a reason why the temperature of the optical fiber wire  10 β immediately before entry into the secondary irradiation section  108  may not be lower than 50° C. or higher than 300° C., with reference to  FIG. 6 .  FIG. 6  is a graph showing a relationship between the temperature of the optical fiber wire  10 β immediately before entry into the secondary irradiation section  108  and the degree of cure of the primary coating  12   a  constituting the produced optical fiber wire  10 . The indicator used here to indicate the degree of cure of the primary coating  12   a  is gel fraction. The graph shown in  FIG. 6  indicates that, when the temperature of the optical fiber wire  10 β immediately before entry into the secondary irradiation section  108  is not lower than 50° C. and not higher than 300° C., the gel fraction of the primary coating  12   a  is not less than 85%. 
     If the optical fiber wire  10  after production receives a lateral pressure, the coating  12  may crack. According to the findings made by the inventors of the present invention, (1) when the gel fraction of the primary coating  12   a  is less than 80%, several tens of percentage of optical fiber wires  10  have cracking in their coating  12 , (2) when the gel fraction of the primary coating  12   a  is not less than 80% and less than 85%, several percentage of optical fiber wires  10  have cracking in their coating  12 , and (3) when the gel fraction of the primary coating  12   a  is not less than 85%, no optical fiber wires  10  have cracking in their coating  12 . As such, in cases where the temperature of the optical fiber wire  10 β immediately before entry into the secondary irradiation section  108  is not lower than 50° C. and not higher than 300° C., the gel fraction of the primary coating  12   a  is not less than 85%, resulting in the prevention of cracking that would otherwise occur in the coating  12 . 
     The temperature of the optical fiber wire  10 β immediately before entry into the secondary irradiation section  108  may not be lower than 63° C. or higher than 100° C. In such cases, the gel fraction of the primary coating  12   a  further increases, resulting in the presentation of cracking that would otherwise occur in the coating  12 , with more certainty. 
     (Temperature of Optical Fiber Wire  10 β Immediately after Passage Through Primary Irradiation Section  106 ) 
     The temperature of the optical fiber wire  10 β immediately after the passage through the primary irradiation section  106  may not be higher than 300° C. This is because, provided that the temperature of the optical fiber wire  10 β immediately after the passage through the primary irradiation section  106  is not higher than 300° C., the temperature of the optical fiber wire  10 β immediately before entry into the secondary irradiation section  108  will definitely be not higher than 300° C. 
     Note that the rate of temperature increase of the optical fiber wire  10 α in the primary irradiation section  106  is not less than 3000° C./sec. and not more than 24000° C./sec. For example, in a case where the rate of temperature increase of the optical fiber wire  10 α in the primary irradiation section  106  is 3000° C./sec., the drawing speed is set so that the time taken for the optical fiber wire  10 α to pass through the primary irradiation section  106  is 0.1 seconds or shorter. This makes it possible to keep the temperature of the optical fiber wire  10 β, immediately after the passage through the primary irradiation section  106 , not higher than 300° C. Alternatively, in a case where the rate of temperature increase of the optical fiber wire  10 α in the primary irradiation section  106  is 24000° C./sec., the drawing speed is set so that the time taken for the optical fiber wire  10 α to pass through the primary irradiation section  106  is 0.0125 seconds or shorter. This makes it possible to keep the temperature of the optical fiber wire  10 β, immediately after the passage through the primary irradiation section  106 , not higher than 300° C. 
     (Production Method for Optical Fiber Wire) 
     The following description will discuss a production method  51  for an optical fiber wire  10  in accordance with one or more embodiments of the present invention, with reference to  FIG. 7 .  FIG. 7  is a flowchart showing the production method  51  for an optical fiber wire  10 . The production method  51  is a method for producing the optical fiber wire  10  (see  FIG. 1 ), and includes the following steps S 101  through S 109 . 
     Step S 101 : The drawing section  101  draws a preform which will constitute the optical fiber bare wire  11 . 
     Step S 102 : The cooling section  102  cools the preform drawn in step S 101 . This provides the optical fiber bare wire  11 . 
     Step S 103 : The bare wire outer diameter measuring section  103  measures the outer diameter of the optical fiber bare wire  11  obtained in step S 102 , and supplies, to the control section  110 , a monitor signal indicative of the measured outer diameter. 
     Step S 104  (coating step): The coating section  104  applies, to the side surface of the optical fiber bare wire  11  whose outer diameter has been measured in step S 103 , uncured ultraviolet curable resins that will constitute the coating  12 . Specifically, the coating section  104  carries out both of the following operations: (i) the operation of applying, to the outer side surface of the optical fiber bare wire  11 , an uncured ultraviolet curable resin which will constitute the primary coating  12   a ; and (ii) the operation of applying, to the outer side surface of the primary coating  12   a , an uncured ultraviolet curable resin which will constitute the secondary coating  12   b . This provides the optical fiber wire  10   a.    
     Note that the thickness of the ultraviolet curable resins applied in step S 104  is adjusted by control by the control section  110  in accordance with the outer diameter of the optical fiber wire  10 α measured in step S 105  (described below). 
     Step S 105 : The wire outer diameter measuring section  105  measures the outer diameter of the optical fiber wire  10 α obtained in step S 104 , and supplies, to the control section  110 , a monitor signal indicative of the measured outer diameter. 
     Step S 106  (first irradiation step): The primary irradiation section  106  irradiates the optical fiber wire  10 α, obtained in step S 105 , with ultraviolet light with use of the one or more UV lamps. With this, the ultraviolet curable resin that will constitute the secondary coating  12   b  mainly cures, and the optical fiber wire  10 β is obtained. In this step, at least a portion, which constitutes the surface layer of the secondary coating  12   b , of the ultraviolet curable resin is allowed to sufficiently cure. The temperature of the optical fiber wire  10 β obtained here is not higher than 300° C. 
     Step S 107 : The haul-off section  107  pulls on the optical fiber wire  10 β, obtained in step S 106 , at a specific pulling speed. 
     Note that the pulling speed at which the optical fiber wire  10 β is pulled on in step S 107  is adjusted by control by the control section  110  in accordance with the outer diameter of the optical fiber bare wire  11  measured in the foregoing step S 103 . 
     Step S 108  (second irradiation step): The secondary irradiation section  108  irradiates the optical fiber wire  10 β, pulled on in step S 107 , with ultraviolet light with use of the one or more UV LEDs. With this, the ultraviolet curable resin that will constitute the primary coating  12   a  mainly cures, and the optical fiber wire  10  is obtained. Note that the temperature of the optical fiber wire  10 β immediately before step S 108  is carried out is not lower than 50° C. and not higher than 300° C., because the optical fiber wire  10 β is allowed to naturally cool after step S 106 . 
     Step S 109 : The take-up section  109  takes up the optical fiber wire  10 , obtained in step S 108 , around the take-up drum  109   a . This provides the optical fiber wire  10  wound around the take-up drum  109   a.    
     Note that, in the foregoing step S 106 , the ultraviolet irradiation by the primary irradiation section  106  using the UV lamps is carried out for not less than 0.01 seconds in a low-oxygen atmosphere containing oxygen at a concentration of not more than 2%, as described earlier. 
     As has been described, according to one or more embodiments, each point on an optical fiber wire, in which at least a portion, containing the surface layer, of the ultraviolet curable resin constituting the coating is in an uncured state, is irradiated with ultraviolet light from UV lamps for not less than 0.01 seconds in a low-oxygen atmosphere containing oxygen at a concentration of not more than 2%. Then, according to one or more embodiments, each point on the optical fiber wire is irradiated with ultraviolet light from UV LEDs. 
     Note, here, that the absorption wavelengths of the photopolymerization initiator contained in the secondary coating  12   b  are highly likely to be included in the wavelength range of the ultraviolet light emitted from the UV lamps, which has a wide spectrum width. Also note that the secondary coating  12   b  tends to cure more rapidly when the temperature of the fiber during curing is higher. 
     Therefore, according to one or more embodiments, it is possible to allow at least the surface layer of the secondary coating  12   b  to sufficiently cure by irradiation in an early stage of the production process of the optical fiber wire  10 . This makes it possible, according to one or more embodiments, to produce the optical fiber wire  10  that is less likely to lose surface quality than conventional techniques, with use of the production apparatus  1  configured to apply the primary coating  12   a  and the secondary coating  12   b  in a single step. 
     (Variations) 
     The discussions in the above-described embodiments are based on the assumption that, in each of the UV lamp units  106 _ 1  through  106 _ 3  included in the primary irradiation section  106 , irradiation is carried out in a low-oxygen atmosphere containing oxygen at a concentration of not more than 2%. Note, however, that, in one or more UV lamp units  106 _ i  located in the downstream portion of the primary irradiation section  106 , the ultraviolet irradiation does not need to be carried out in a low-oxygen atmosphere. Specifically, provided that irradiation time of not less than 0.01 seconds is achieved by one or more UV lamp units  106 _ i  located in the upstream portion of the primary irradiation section  106 , ultraviolet irradiation by the other one or more UV lamp units  106 _ i  located in the downstream portion of the primary irradiation section  106  may be carried out in air. In other words, an inert gas containing oxygen at low concentration does not need to be passed through such one or more UV lamp units  106 _ i  located in the downstream portion of the primary irradiation section  106 . 
     A reason therefor is as follows: provided that the surface layer of the ultraviolet curable resin constituting the secondary coating  12   b  is allowed to sufficiently cure by one or more of the UV lamp units  106 _ i  located in the upstream portion of the primary irradiation section  106 , it is no longer necessary to prevent hindering of curing caused by oxygen in the other one or more UV lamp units  106 _ i , because the rest of the ultraviolet curable resin is not exposed. 
     When such an arrangement is employed, the former part of the first irradiation step in accordance with the present invention is carried out by one or more UV lamp units  106 _ i  that are located in the upstream portion of the primary irradiation section  106  and that carry out ultraviolet irradiation in a low-oxygen atmosphere. Next, the latter part of the first irradiation step in accordance with the present invention is carried out by the other one or more UV lamp units  106 _ i  that are located in the downstream portion of the primary irradiation section  106  and that carry out ultraviolet irradiation in air. Then, the second irradiation step in accordance with the present invention is carried out by the secondary irradiation section  108 . 
     In the above-described embodiments, an example has been discussed in which the coating  12  of the optical fiber wire  10  is composed of the following two layers: the primary coating  12   a ; and the secondary coating  12   b . Note, however, that the present invention can be applied to cases where the coating  12  is composed of a single layer. In such cases, the following arrangement may be employed in one or more embodiments: the coating section  104  is configured to apply, to the optical fiber bare wire  11 , an ultraviolet curable resin that forms a single-layer coating  12 . 
     According to one or more embodiments, cooling of the optical fiber wire  10 β in an area extending from the primary irradiation section  106  to the secondary irradiation section  108  is achieved by natural cooling. Note, however, that the present invention is not limited as such. Specifically, cooling of the optical fiber wire  10 β in the area extending from the primary irradiation section  106  to the secondary irradiation section  108  can be achieved by forced cooling. In a case where such an arrangement is employed, a cooling section for forced cooling is provided in the area extending from the primary irradiation section  106  to the secondary irradiation section  108 . The cooling section cools the optical fiber wire  10 β so that the temperature of the optical fiber wire  10 β immediately before entry into the secondary irradiation section  108  is not lower than 50° C. and not higher than 300° C. Note that the cooling section is constituted by, for example, a cooling cylinder through which a cooling gas flows. 
     According to one or more embodiments, ultraviolet irradiation in the primary irradiation section  106  is carried out by UV lamps, whereas ultraviolet irradiation in the secondary irradiation section  108  is carried out by UV LEDs. Note, however, that the present invention is not limited as such. Specifically, the ultraviolet irradiation in the primary irradiation section  106  can be carried out by UV LEDs, and the ultraviolet irradiation in the secondary irradiation section  108  can be carried out by UV lamps. 
     One or more embodiments of the present invention can also be expressed as follows. 
     A method of producing an optical fiber wire ( 10 ) in accordance with one or more embodiments of the present invention is a method of producing an optical fiber wire ( 10 ) that includes a coating ( 12 ,  12   a ,  12   b ) constituted by an ultraviolet curable resin, the method including: a first irradiation step including applying ultraviolet light to each point on the optical fiber wire ( 10 α) in which at least a portion of the ultraviolet curable resin is in an uncured state, the portion constituting a surface layer of the coating ( 12   b ); and a second irradiation step including applying ultraviolet light to each point on the optical fiber wire ( 10 β) which is obtained from the first irradiation step and in which at least the portion constituting the surface layer of the coating ( 12   b ) is in a cured state, wherein a temperature of the optical fiber wire ( 10 β) immediately before the second irradiation step is not lower than 50° C. and not higher than 300° C. 
     An apparatus for producing an optical fiber wire ( 10 ) in accordance with one or more embodiments of the present invention is an apparatus for producing an optical fiber wire ( 10 ) that includes a coating ( 12 ,  12   a ,  12   b ) constituted by an ultraviolet curable resin, the apparatus including: a first irradiation section ( 106 ) configured to apply ultraviolet light to each point on the optical fiber wire ( 10 α) in which at least a portion of the ultraviolet curable resin is in an uncured state, the portion constituting a surface layer of the coating ( 12   b ); and a second irradiation section ( 108 ) configured to apply ultraviolet light to each point on the optical fiber wire ( 10 β) which is obtained by application of the ultraviolet light by the first irradiation section ( 106 ) and in which at least the portion constituting the surface layer of the coating ( 12   b ) is in a cured state, wherein a temperature of the optical fiber wire ( 10 β) immediately before receiving the ultraviolet light from the second irradiation section ( 108 ) is not lower than 50° C. and not higher than 300° C. 
     According to the configuration, the optical fiber wire, in which at least the surface layer of its coating has cured in the first irradiation step (first irradiation section), has a temperature not lower than 50° C. and not higher than 300° C. immediately before entry into the second irradiation step (second irradiation section). By applying ultraviolet light to the optical fiber wire which entered the second irradiation step (second irradiation section) at a temperature falling within the above range, it is possible to allow an internal portion of the coating other than the surface layer to sufficiently cure as compared to conventional techniques. This makes it possible to reduce the frequency of cracking in the coating even if a lateral pressure acts on the coating after the production. 
     A method of producing an optical fiber wire ( 10 ) in accordance with one or more embodiments of the present invention may be arranged such that the temperature of each point on the optical fiber wire ( 10 β) immediately after the first irradiation step is not higher than 300° C. 
     According to the configuration, it is possible to further ensure that the temperature of the optical fiber wire immediately before entry into the second irradiation step is not lower than 50° C. and not higher than 300° C. 
     A method of producing an optical fiber wire ( 10 ) in accordance with one or more embodiments of the present invention may be arranged such that the first irradiation step is carried out in a low-oxygen atmosphere containing oxygen at a concentration of not more than 2%. 
     The configuration makes it possible to prevent hindering of the curing of the ultraviolet curable resin that would otherwise be caused by oxygen. 
     A method of producing an optical fiber wire ( 10 ) in accordance with one or more embodiments of the present invention may be arranged such that a length of a path of advancement of the optical fiber wire ( 10 β), from a place where the first irradiation step is carried out to a place where the second irradiation step is carried out, is set so that the optical fiber wire ( 10 β) is allowed to naturally cool to have a temperature not lower than 50° C. and not higher than 300° C. immediately before the second irradiation step. 
     The configuration makes it possible to provide the foregoing effects without having to add a feature such as a cooling section. 
     Supplementary Note 
     Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  production apparatus 
               10  optical fiber wire 
               11   a  core 
               11   b  cladding 
               12   a  primary coating 
               12   b  secondary coating 
               11  optical fiber bare wire 
               12  coating 
               101  drawing section 
               102  cooling section 
               103  bare wire outer diameter measuring section 
               104  coating section 
               105  wire outer diameter measuring section 
               106  primary irradiation section 
               107  haul-off section 
               108  secondary irradiation section 
               109  take-up section 
               110  control section 
               111 _ 1  through  111 _ 6  pulley 
               106   a ,  108   a  housing 
               106   b ,  108   b  quartz pipe 
               106   c  UV lamp 
               108   c  UV LED bar 
               106   a   1 ,  106   e   1  gas inlet 
               106   a   2 ,  106   f   2  gas outlet 
               106   d ,  108   d  reflecting plate