Abstract:
A method of manufacturing an optical fiber of the invention includes: preparing one or more direction changers; drawing the bare optical fiber from an optical fiber preform; providing a coated layer on a periphery of the bare optical fiber; obtaining an optical fiber by curing the coated layer; changing the direction of the bare optical fiber at the position between the bare-optical-fiber formation position and the coated-layer provision position; detecting the position of the bare optical fiber in at least one of the direction changers; and adjusting the introduction flow rate of the fluid into the direction changer based on positional information obtained by the detection.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from Japanese Patent Application No. 2015-024688 filed on Feb. 10, 2015, the contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a method of manufacturing an optical fiber, an optical fiber manufacturing apparatus, and a control apparatus that controls the manufacturing apparatus. 
         [0004]    2. Description of the Related Art 
         [0005]    Generally, in manufacture of an optical fiber, a method is employed which vertically and downwardly draws an optical fiber from an optical fiber preform along a linear pathway. 
         [0006]    The overall height of the system thereof is limited and becomes a factor that affects the productivity of the manufacturing method. Because the height of the system becomes a main factor that limits the productivity, it is necessary to ensure the distance of a bare optical fiber in the system which is required to sufficiently cool the bare optical fiber obtained by drawing an optical fiber preform. 
         [0007]    The above-described limitation can be eased by construction of new facilities such as new buildings; however, a huge cost is necessary in order to ease the limitation, if improvement in productivity is required in the future, it will be necessary to construct new facilities at great expense. 
         [0008]    As a method of easing the limitation, a method of using a direction changing device that includes a noncontact retention mechanism and changes the direction in which a drawn fiber is drawn is known. 
         [0009]    This noncontact retention mechanism is a mechanism that contactlessly retains an object by using a pressure of fluid such as air, and a direction changer that is provided with this mechanism can change the direction, in which a bare optical fiber (bare fiber) is drawn, without being in contact with the bare optical fiber. 
         [0010]    By using this direction changer, the direction of a bare optical fiber which is fiber-drawn a optical fiber preform along a first pathway can be changed into a direction along a second pathway (for example, refer to Japanese Patent No. 5571958 and Japanese Unexamined Patent Application, First Publication No. S62-003037). 
         [0011]    Japanese Patent No. 5571958 discloses a method of manufacturing an optical fiber, which uses a direction changing device that changes the direction of a drawn fiber. The instrument has a groove into which an optical fiber is to be introduced, and the groove has an opening firmed therein. In this method, a gas that is introduced into the instrument is discharged through one flow inlet port, and a direction of an optical fiber is changed in a state where the optical fiber floats due to the pressure of the gas. 
         [0012]    Japanese Unexamined Patent Application, First Publication No. S62-003037 discloses a direction changer, the direction changer has a guide groove that guides a bare optical fiber into, and gas outlet nozzles are formed on the bottom surface and both side surfaces of the guide groove (refer to Example and  FIGS. 3 and 4 ). in this manufacturing method using the direction changer, a direction of an optical fiber is changed due to the pressure of the gas blown from four outlet nozzles in a state where the optical fiber floats. 
         [0013]    The amount of flotation of the bare optical fiber is determined by the balance between: a pressure of the gas blown in a groove; and a drawing tension applied to a bare optical fiber. Therefore, in the case where the gas pressure and the drawing tension are constant, the amount of flotation of the bare optical fiber s also constant, and a stable fiber drawing is thereby possible. 
         [0014]    However, in an actual manufacturing process, a drawing tension varies due to variation in an outer diameter of an optical fiber preform, variation in the drawing speed of a bare optical fiber, or the like, as a result, the amount of flotation of the bare optical fiber varies in some cases. 
         [0015]    In the ease where the drawing tension is low, as the amount of flotation the bare optical fiber increases, a stability in a flotation state is degraded, and there is a concern that bare optical fiber is in contact with the inner surface of the groove of the direction changing device. In the case where the bare optical fiber comes into contact with the direction changing device, the bare optical fiber is damaged, and there is a possibility that the strength of the bare optical fiber is degraded. On the other hand, in the case where the drawing tension is high, as the amount of flotation of the bare optical fiber decreases, there is a concern that bare optical fiber is in contact with the inner surface of the groove of the direction changing device. In the case where the bare optical fiber comes into contact with the direction changing device, the bare optical fiber is damaged, and there is a possibility that the strength of the bare optical fiber is degraded. 
         [0016]    Furthermore, in both cases where the drawing tension is high and low, as the position at which the bare optical fiber is introduced into a coating unit varies due to variation in the flotation position, and there is a concern that deviation thickness of the coating occurs. 
       SUMMARY OF THE INVENTION 
       [0017]    Some aspects of the invention were conceived in view of the above-described circumstances and have an object thereof to provide a method of manufacturing an optical fiber, an optical fiber manufacturing apparatus, and a control apparatus that controls the manufacturing apparatus, which can stabilize a flotation position of a bare optical fiber in a direction changer. 
         [0018]    A first aspect of the invention provides a method of manufacturing an optical fiber, including: preparing one or more direction changers, each direction changer including a guide groove, an internal space into which fluid is introduced from an outside, and an outlet nozzle, the guide groove being configured to guide a bare optical fiber, the outlet nozzle being formed in the guide groove and being configured to blow off the fluid from the internal space and thereby cause the bare optical fiber to float inside the guide groove; drawing the bare optical fiber from an optical fiber preform, thereby forming the bare optical fiber (drawing step); providing a coated layer made of a resin on a periphery of the bare optical fiber (coating step); obtaining an optical fiber by curing the coated layer (curing step); changing the direction of the bare optical fiber at the position between the position at which the bare optical fiber is formed (the position in the drawing step, a bare-optical-fiber formation position) and the position at which the coated layer is provided on the periphery of the bare optical fiber (the position in the coating step, a coated-layer provision position) by use of the direction changer; detecting the position of the bare optical fiber in at least one of the direction changers; and adjusting the introduction flow rate of the fluid into the direction changer based on positional information obtained by the detection. 
         [0019]    In the first aspect of the invention, it is preferable that the position of the bare optical fiber in the direction changer serving as part of the plurality of direction changers be detected and that introduction flow rates of the fluid into all of the direction changers be controlled based on positional information obtained by the detection. 
         [0020]    In the first aspect of the invention, it is preferable that each of positions of the bare optical fiber in the plurality of direction changers be detected and that introduction flow rates of the fluid into the plurality of direction changers be individually controlled based on positional information obtained by the detection. 
         [0021]    A second aspect of the invention provides a method of manufacturing an optical fiber, including: preparing one or more direction changers, each direction changer including a guide groove, an internal space into which fluid is introduced from an outside, and an outlet nozzle, the guide groove being configured to guide a bare optical fiber, the outlet nozzle being formed in the guide groove and being configured to blow off the fluid from the internal space and thereby cause the bare optical fiber to float inside the guide groove; previously obtaining a relationship between the position of the bare optical fiber in at least one of the direction changers, the introduction flow rate of the fluid into the direction changer, and the drawing tension that is to be applied to the optical fiber; drawing the bare opt cal fiber from an optical fiber preform, thereby forming the ha e optical fiber (drawing step); providing a coated layer made of a resin on a periphery of the bare optical fiber (coating step); obtaining an optical fiber by curing the coated layer (curing step); changing the direction of the bare optical fiber at the position between the position at which the bare optical fiber is formed (the position in the drawing step, a bare-optical-fiber formation position) and the position at which the coated layer is provided on the periphery of the bare optical fiber (the position in the coating step, a coated-layer provision position) by use of the direction changer; and the introduction flow rate of the fluid into the direction changer is controlled based on the obtained relationship. 
         [0022]    In the second aspect of the invention, it is preferable that a relationship between the position of the bare optical fiber in the direction changer serving as part of the plurality of direction changers, the introduction flow rate of the fluid into the direction changer, and the drawing tension that is to be applied to the optical fiber be previously obtained and that introduction flow rates of the fluid into all of the direction changers be controlled based on the obtained relationship. 
         [0023]    In the second aspect of the invention, it is preferable that relationships among positions of the bare optical fiber in the respective plurality of direction changers, introduction flow rates of the fluid into the respective direction changers, and the drawing tensions that are to be applied to the optical fiber be previously obtained and that introduction flow rates of the fluid into the plurality of direction changers be individually controlled based on the obtained relationships. 
         [0024]    A third aspect of the invention provides a control apparatus used in an optical fiber manufacturing apparatus, the manufacturing apparatus including: a drawing unit that forms a bare optical fiber by drawing the bare optical fiber from an optical fiber preform; a coating unit that provides a coated layer made of a resin on a periphery of the bare optical fiber; and a curing unit that cures the coated layer, the control apparatus including: one or more direction changers that change the direction of the bare optical fiber at the position between the drawing unit and the coating unit, each direction changer including a guide groove, an internal space into which fluid is introduced from an outside, and an outlet nozzle, the guide groove being configured to guide a bare optical fiber, the outlet nozzle being formed in the guide groove and being configured to blow off the fluid from the internal space and thereby cause the bare optical fiber to float inside the guide groove; a detector that detects the position of the bare optical fiber in the direction changer; and a controller that controls the introduction flow rate of the fluid into the direction changer based on positional information associated with the bare optical fiber which is measured in the detector, the controller detecting the position of the bare optical fiber in at least one of the direction changers and thereby controlling the introduction flow rate of the fluid into the direction changer based on the positional information obtained by the detection. 
         [0025]    In the third aspect of the invention, it is preferable that the position of the bare optical fiber in the direction changer serving as part of the plurality of direction changers is detected, and introduction flow rates of the fluid into all of the direction changers be controlled based on positional information obtained by the detection. 
         [0026]    In the third aspect of the invention, it is preferable that each of positions of the bare optical fiber in the plurality of direction changers be detected and that introduction flow rates of the fluid into the plurality of direction changers be individually controlled based on positional information obtained by the detection. 
         [0027]    A fourth aspect of the invention provides a control apparatus used in an optical fiber manufacturing apparatus, the manufacturing apparatus including: a drawing unit that forms a bare optical fiber by drawing the bare optical fiber from an optical fiber preform; a coating unit that provides a coated layer made of a resin on a periphery of the bare optical fiber; and a curing unit that cures the coated layer, the control apparatus including: one or more direction changers that change the direction of the bare optical fiber at the position between the drawing unit and the coating unit, each direction changer including a guide groove, an internal space into which fluid is introduced from an outside, and an outlet nozzle, the guide groove being configured to guide a bare optical fiber, the outlet nozzle being formed in the guide groove and being configured to blow off the fluid from the internal space and thereby cause the bare optical fiber to float inside the guide groove; a detector that detects the position of the bare optical fiber in the direction changer; and a controller that controls the introduction flow rate of the fluid into the direction changer based on positional information associated with the hare optical fiber which is measured in the detector, the controller controlling the introduction flow rate of the fluid into the direction changer based on a relationship between the position of the bare optical fiber in at least one of the direction changers, the introduction flow rate of the fluid into the direction changer, and the drawing tension that is to be applied to the optical fiber. 
         [0028]    In the fourth aspect of the invention, it is preferable that the controller control introduction flow rates of the fluid into all of the direction changers based on a relationship between the position of the bare optical fiber in the direction changer serving as part of the plurality of direction changers, the introduction flow rate of the fluid into the direction changer, and the drawing tension that is to be applied to the optical fiber. 
         [0029]    In the fourth aspect of the invention, it is preferable that the controller individually control introduction flow rates of the fluid into the plurality of direction changers based on relationships among positions of the bare optical fiber in the respective plurality of direction changers, introduction flow rates of the fluid into the respective direction changers, and the drawing tensions that are to be applied to the optical fiber. 
         [0030]    A fifth aspect of the invention provides an optical fiber manufacturing apparatus including: a control apparatus according to the third aspect or the fourth aspect; a drawing unit that forms a bare optical fiber by drawing the bare optical fiber from an optical fiber preform; a coating unit that provide a coated layer made of a resin on a periphery of the bare optical fiber; and a curing unit that cures the coated layer. 
       EFFECTS OF THE INVENTION 
       [0031]    According to the aspects of the invention, as a result of controlling the introduction flow rate of the fluid into the direction changer based on positional information associated with the bare optical fiber, the flow velocity of the fluid discharged to the guide groove from the outlet nozzle in the direction changer is adjusted, and it is possible to adjust the amount of flotation of the bare optical fiber. 
         [0032]    Consequently, the flotation position of the bare optical fiber in the direction changer can be stabilized, the flotation of the bare optical fiber can be prevented from being unstable due to excess or lack of flotation, and it is possible to avoid the bare optical fiber from being in contact with the inner side surface of the guide groove. Accordingly, as a result of using the direction changer, it is possible to manufacture the optical fiber with a high level of yield without damage to the bare optical fiber. Additionally, since the availability of the manufacturing apparatus is increasing, it is possible to improve productivity of the optical fiber, and the cost of manufacturing the optical fiber can be reduced. 
         [0033]    Furthermore, since the flotation position of the bare optical fiber is stabilized in the direction changer, the fiber inlet position at which the bare optical fiber enters the coating unit is constant. Therefore, deviation in thickness of the coating is prevented, and it is possible to manufacture an optical fiber having a stable quality. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  is a schematic diagram showing the configuration of an optical fiber manufacturing apparatus according to a first embodiment of the invention. 
           [0035]      FIG. 2  is a cross-sectional schematic view showing the structure of a direction changer of the manufacturing apparatus shown in  FIG. 1 . 
           [0036]      FIG. 3  is a front view showing an example of the direction changer. 
           [0037]      FIG. 4  is a front showing a modified example of the direction changer shown  FIG. 3 . 
           [0038]      FIG. 5  is a schematic diagram showing a modified example of an optical fiber manufacturing apparatus according to a first embodiment. 
           [0039]      FIG. 6  is a schematic diagram showing the configuration of an optical fiber manufacturing apparatus according to a second embodiment of the invention. 
           [0040]      FIG. 7  is a schematic diagram showing a modified example of an optical fiber manufacturing apparatus according to a second embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]      FIG. 1  is a schematic diagram showing the configuration of a manufacturing apparatus  1 A which serves as an optical fiber manufacturing apparatus according to a first embodiment of the invention. 
         [0042]    The manufacturing apparatus  1 A includes a drawing unit  10 , direction changers  20  ( 20 A,  20 B), a coating unit  30 , a curing unit  40 , a detector  50 , a controller  60 , a flow controller  70 , a pulley  80 , a pick-up unit  90 , and winding unit  100 . 
         [0043]    The direction changers  20 , the detector  50 , the controller  60 , and the flow controllers  70  ( 70 A,  70 B) constitute a control apparatus  101 . 
         [0044]    The drawing unit  10  is provided with a heating furnace  11 , an optical fiber preform  2  is heated by the heating furnace  11 , and a bare optical fiber  3  is formed by drawing the heated preform. 
         [0045]    The direction changers  20  change a direction of the bare optical fiber  3 . As shown in  FIG. 1 , two direction changers  20  are used in the manufacturing apparatus  1 A. The direction changers  20  are each referred to as a first direction changer  20 A and a second direction changer  20 B in order from the upstream side to the downstream side in the fiber drawing direction. 
         [0046]    The first direction changer  20 A changes, by 90 degrees, the direction of the bare optical fiber  3  that is drawn from the optical fiber preform  2  in the downward vertical direction (first pathway L 1 ), as a result, the direction of the hare optical fiber  3  is in the horizontal direction (second pathway L 2 ). 
         [0047]    A plane including the first pathway L 1  and the second pathway L 2  is referred to as P 1 . The X-direction is the direction extending in the second pathway L 2  in the plane P 1  and the Y-direction is the direction perpendicular to the plane P 1 . 
         [0048]    The second direction changer  20 B changes the direction of the bare optical fiber  3  by 90 degrees, as a result, the direction of the bare optical fiber  3  is in the downward vertical direction (third pathway L 3 ). 
         [0049]    The coating unit  30  is configured to coat (coating) the periphery of the bare optical fiber  3  with a coating material such as a urethane acrylate-based resin, thereby forms a coated layer, and an optical fiber production intermediate  4  is obtained. 
         [0050]    The resin coating is, for example, a bilayer coating such that a material used to form a first coated layer having a lower Young&#39;s modulus is applied to the inside thereof and a material used to form a second coated layer having a higher Young&#39;s modulus is applied to the outside thereof. The material used to form the resin coating is, for example, an ultraviolet curable resin. 
         [0051]    The coating unit  30  may be configured to independently apply the first coated layer and the second coated layer or may be configured to simultaneously apply the first coated layer and the second coated layer. 
         [0052]    The curing unit  40  is provided with one or more UV lamps  40   a,  is configured to cure the coated layer of the optical fiber production intermediate  4  and thereby form an optical fiber  5 . For example, the curing unit  40  includes UV lamps  40   a  which form a plurality of pairs thereof and are provided to sandwich spaces through which the optical fiber production intermediate  4  passes. 
         [0053]    The detector  50  is provided in the downstream side of the second direction changer  20 B in the fiber drawing direction and detects the position of the bare optical fiber  3  in the third pathway L 3 . As the detector  50 , for example, a position sensor using a laser can be used. 
         [0054]    When the amount of flotation of the bare optical fiber  3  in the second direction changer  20 B increases or decreases, the position of the bare optical fiber  3  in the X-direction in the third pathway L 3  varies. For this reason, the detector  50  can detect the amount of flotation of the bare optical fiber  3  in the second direction changer  20 B based on positional information of the bare optical fiber  3 . 
         [0055]    The detector  50  outputs a detection signal to the controller  60  based on information associated with the position of the bare optical fiber  3 . 
         [0056]    The flow controller  70  can adjust the flow rates of the fluid which are to be introduced into the first direction changer  20 A and the second direction changer  20 B. The flow controller  70  can be provided in, for example, an introduction passage (e.g., introduction passage  26  shown in  FIG. 3 ) which introduces fluid into the first direction changer  20 A and the second direction changer  20 B. As the flow controller  70 , a mass-flow controller (MFC) or the like can be used. 
         [0057]    Two flow controllers  70  are used in the manufacturing apparatus  1 A shown in  FIG. 1 . Of the two flow controllers  70 , the flow controller  70  that adjusts the flow rate of the fluid introduced into the first direction changer  20 A is referred to as a first flow controller  70 A, and the flow controller  70  that adjusts the flow ate of the fluid introduced into the second direction changer  20 B is referred to as a second flow controller  70 B. 
         [0058]    The controller  60  controls the flow rates of the fluid which are introduced into the first direction changer  20 A and the second direction changer  20 B based on a detection signal output from the detector  50  by use of the first flow controller  70 A and the second flow controller  70 B and thereby can adjust the amounts of flotation of the bare optical fiber  3  in the first direction changer  20 A and the second direction changer  20 B. 
         [0059]    The direction of the optical fiber  5  is changed by the pulley  80 , and the optical fiber  5  is picked up by the pick-up unit  90  and is wound around the winding unit  100 . 
         [0060]    The pick-up unit  90  is, for example, a pick-up capstan, and the pick-up unit determines the drawing speed. The drawing speed is greater than or equal to, for example, 1500 m/min. 
         [0061]    The winding unit  100  is a bobbin that winds the optical fiber  5  therearound. 
         [0062]    The outer diameter of the optical fiber preform  2  is greater than or equal to for example, 100 mm, and the length of the optical fiber  5  manufactured from the optical fiber preform  2  is, for example, several thousands km. 
         [0063]    Hereinbelow, configurations of a direction changer  20  will be described. 
         [0064]    As shown in  FIG. 3 , a direction changer  201  is a first example of the direction changer  20  and can change a direction of the bare optical fiber  3  by 90 degrees. 
         [0065]    The direction changer  201  is formed in a quarter-circular shape (arc) in plan view and has an outer peripheral face  20   a  and a guide groove  21 . The guide groove  21  is formed on and along the entire periphery of the outer peripheral face  20   a.  The center of the direction changer  201  coincides with the Y-direction, and the direction changer  201  is provided in the attitude in which the radial direction D 1  (refer to  FIG. 2 ) is in the direction along the plane P 1  (refer to  FIG. 1 ). Here, the direction along the outer peripheral face  20   a  formed in a circular-arc shape in plan view is referred to as a circumferential direction. 
         [0066]    An outlet nozzle  22  is formed at the bottom of the guide groove  21  and along the guide groove  21 . The outlet nozzle blows fluid (e.g., air) into the guide groove, and the fluid (e.g., air) causes the bare optical fiber  3 , which is arranged along and introduced into the guide groove  21 , to float. The outlet nozzle  22  is formed over the entire guide groove  21 . 
         [0067]    A first end  22   a  (one of the outlet nozzle  22  reaches a first end  21   a  (one end) of the guide groove  21 , and a second end  22   b  (the other end) of the outlet nozzle  22  reaches a second end  21   b  (the other end) of the guide groove  21 . 
         [0068]    As shown in  FIG. 2 , the direction changer  201  is configured to be able to discharge fluid (for example, air) of the inner space (fluid reservoir  25 ) into the guide groove  21  through the outlet nozzle  22 . The inner space is ensured in the direction changer  201 . 
         [0069]    The direction changer  201  can be configured to introduce the fluid into the fluid reservoir  25  from the outside and discharge the fluid into the guide groove  21  through the outlet nozzle  22 . 
         [0070]    As shown in  FIG. 3 , it is preferable that an introduction portion  27 , which is connected to the introduction passage  26  that introduces fluid from the outside to the fluid reservoir  25 , be formed on the direction changer  201 . The introduction portion  27  is, for example, an introduction port into which fluid is introduced. 
         [0071]    It is preferable that the guide groove  21  be formed to be inclined with respect to the radial direction Dl so that the space (length in the Y-direction) between the inner side surfaces  21   c  thereof gradually increases in the radial-outer direction. It is preferable that the inclination angles θ 1  of the two inner side surfaces  21   c  with respect to the radial direction D 1  are equal to each other. 
         [0072]    In the direction changer  201  shown in  FIG. 3 , the bare optical fiber  3  enters the first end  21   a  of the guide groove  21  that is formed in a quarter-circular shape (arc) and exits from the second end  21   b,  and the direction of the bare optical fiber is thereby changed by 90 degrees. A fiber inlet portion  23  to which the bare optical fiber  3  enters is a portion including the first end  21   a  of the guide groove  21 , and a fiber outlet portion  24  from which the e bare optical fiber  3  exits is a portion including the second end  21   b  of the guide groove  21 . 
         [0073]    A direction changer  202  shown in  FIG. 4  is a modified example of the direction changer  201  and is formed in a three-quarter-circular shape (arc) in plan view. Hereinbelow, identical reference numerals are used for the elements which are identical to those of the above description, and the explanations thereof are simplified here. 
         [0074]    The direction changer  202  is configured to include: a body part  29   a  having the same structure as that of the direction changer  201  shown in  FIG. 3 ; and auxiliary parts  29   b,  and  29   c,  each of which has the same structure as that of the body part  29   a  and which are consecutively connected to the incoming side and the outgoing side of the body part  29   a.  Regarding the direction changer  202 , the bare optical fiber  3  enters the guide groove  21  of the body part  29   a  through the fiber inlet portion  23 ′, the direction of the bare optical fiber is changed by 90 degrees by the body part  29   a,  thereafter, the bare optical fiber exits from the body part through the fiber outlet portion  24 ′. Therefore, the basic function of the direction changer  202  is the same as that of the direction changer  201 . 
         [0075]    The direction changers  201  and  202  can change the direction of the bare optical fiber  3  by 90 degrees, and therefore can be used as the first direction changer  20 A and the second direction changer  20 B as shown in  FIG. 1 . 
         [0076]    Next, a method according to a first embodiment of the invention will be described which manufactures an optical fiber in the case of using, for example, the manufacturing apparatus  1 A. 
       (Drawing Step) 
       [0077]    As shown in  FIG. 1 , her the drawing unit  10 , the optical fiber preform  2  is heated and drawn, and the bare optical fiber  3  is formed. 
       (Direction Change by Use of Direction Changer) 
       [0078]    The direction of the bare optical fiber  3  that is drawn from the optical fiber preform  2  in the downward vertical direction (first pathway is changed by 90 degrees by the first direction changer  20 A, as a result, the direction of the bare optical fiber  3  is in the horizontal direction (second pathway L 2 ). 
         [0079]    The direction of the bare optical fiber  3  is changed by 90 degrees by the second direction changer  20 B and is in the downward vertical direction (third pathway L 1 ). 
         [0080]    As shown in  FIG. 2 , as a result of discharging the fluid (for example, air) in the fluid reservoir  25  into the guide groove  21  through the outlet nozzle  22  in the first direction changer  20 A and the second direction changer  20 B, it is possible to cause the bare optical fiber  3  to float. Particularly, since a difference in pressure between a deep portion  21   d  of the guide groove  21  and a shallow portion  21   e  thereof increases due to the discharged air, a force in the radial-outer direction of the direction changer is applied to the bare optical fiber  3 , and the hare optical fiber  3  floats. 
         [0081]    The amount of flotation of the bare optical fiber is determined by the balance between: a flow velocity of fluid which is discharged from the outlet nozzle  22  into the guide groove  21 ; and a drawing tension applied to the bare optical fiber  3 . 
         [0082]    In the case where a drawing tension of the bare optical fiber  3  is constant, an amount of flotation of the bare optical fiber  3  depends on a flow velocity of fluid; that is, as a flow velocity of fluid is higher, the amount of flotation increases; in contrast, as a flow velocity of fluid is slower, the amount of flotation decreases. On the other hand, in the case where a flow velocity of fluid is constant, as a drawing tension is higher, the amount of flotation decreases; as a drawing tension smaller, the amount of flotation increases. 
         [0083]    As shown in  FIG. 1 , as a method of controlling the flow velocity of fluid in the first direction changer  20 A and the second direction changer  20 B, for example, two methods are applicable. Here, a first control method will be described. A second control method will be described later. 
         [0084]    The first control method is a method of controlling the introduction flow rate of the fluid in the first direction changer  20 A and the second direction changer  20 B based on the positional information on the bare optical fiber  3  which is obtained by the detector  50 . 
         [0085]    As the amount of flotation of the bare optical fiber  3  in the second direction changer  20 B increases or decreases, the position of the bare optical fiber  3  in the X-direction in the third pathway L 3  varies. This control method utilizes such variation in the position of the bare optical fiber  3 . 
         [0086]    The detector  50  outputs a detection signal to the controller  60  based on the positional information associated with the hare optical fiber  3  in the third pathway L 3 . This detection signal is, for example, a signal corresponding to the position of the bare optical fiber  3  inside the guide groove  21  in the second direction changer  20 B. 
         [0087]    The controller  60  controls introduction flow rates of the fluid which are introduced into the first direction changer  20 A and the second direction charger  20 B based on the detection signal, by use of the first flow controller  70 A and the second flow controller  70 B. Because of this, the flow velocity of the fluid discharged to the guide groove  21  from the outlet nozzle  22  in the first direction changer  20 A and the second direction changer  20 B is controlled, and it is possible to adjust the amount of flotation of the bare optical fiber  3 . 
         [0088]    In the case where, for example, as the amount of flotation of the bare optical fiber  3  decreases, the bare optical fiber  3  in the third pathway L 3  is displaced in the direction such that the bare optical fiber approaches the second direction changer  20 B, the controller  60  increases the introduction flow rates of the fluid which are introduced into the first direction changer  20 A and the second direction changer  20 B. As a result, the flow velocity of the fluid discharged from the outlet nozzle  22  to the guide groove  21  increases in the first direction changer  20 A and the second direction changer  20 B, the amount of flotation of the bare optical fiber  3  is recovered. 
         [0089]    On the other hand, in the case where, as the amount of flotation of the bare optical fiber  3  increases, the bare optical fiber  3  in the third pathway L 3  is displaced in a direction such that distance between the bare optical fiber and the second direction changer  20 B increases, the controller  60  reduces the introduction flow rates of the fluid which are introduced into the first direction changer  20 A and the second direction changer  20 B. Consequently, the flow velocity of the fluid discharged from the outlet nozzle  22  to the guide groove  21  decreases in the first direction changer  20 A and the second direction changer  20 B, an increase in the amount of flotation of the bare optical fiber  3  is limited. 
         [0090]    As a control method, feedback control such as PID control is preferable. Accordingly, it is possible to carry out control of the flow ate of the fluid with a high degree of responsiveness. 
         [0091]    Particularly, in the ease where the detector (first detector) that is used in the first direction changer  20 A is provided between the first direction changer  20 A and the second direction changer  20 B, the introduction flow rate of the fluid which is introduced into the first direction changer  20 A is controlled based on the detection signal output from the first detector by use of the first flow controller  70 A. Accordingly, in the first direction changer  20 A, the flow velocity of the fluid discharged from the outlet nozzle  22  to the guide groove  21  is controlled, and it is possible to adjust the amount of flotation of the bare optical fiber  3 . 
         [0092]    In this case, the introduction flow rate of the fluid which is introduced into the second direction changer  20 B is controlled based on the detection signal output from the detector  50  (second detector) by use of the second flow controller  70 B. Accordingly, in the second direction changer  20 B, the flow velocity of the fluid discharged from the outlet nozzle  22  to the guide groove  21  is controlled, and it is possible to adjust the amount of flotation of the bare optical fiber  3 . 
       (Coating Step) 
       [0093]    In the coating unit  30 , the optical fiber production intermediate  4  is obtained by forming a coated layer by applying a coating material such as a urethane acrylate based resin onto the periphery of the b optical fiber  3 . 
       (Curing Step) 
       [0094]    In the curing unit  40 , the coated layer of the optical fiber production intermediate  4  is cured by irradiating the coating material with UV light by the UV lamp  40   a  or the like, and the optical fiber  5  is thereby formed. 
         [0095]    The direction of the optical fiber  5  is changed by the pulley  80 , and the optical fiber  5  is picked up by the pick-up unit  90  and is wound around the winding unit  100 . 
         [0096]    According to the manufacturing method, as a result of controlling the introduction flow rates of the fluid which are introduced into the first direction changer  20 A and the second direction changer  20 B based on the positional information associated with the bare optical fiber  3 , the flow velocity of the fluid discharged from the outlet nozzle  22  to the guide groove  21  is adjusted in the first direction changer  20 A and the second direction changer  20 B, and it is possible to adjust the amount of flotation of the bare optical fiber  3 . 
         [0097]    For this reason, it is possible to stabilize the flotation position of the bare optical fiber  3  in the first direction changer  20 A and the second direction changer  20 B, and it is possible to avoid the bare optical fiber  3  from coming into contact with the inner side surface  21   c  of the guide groove  21  due to excess or lack of flotation. Accordingly, as a result of using the first direction changer  20 A and the second direction changer  20 B, it is possible to manufacture the optical fiber  5  with a high level of yield without damage to the bare optical fiber  3 . Additionally, since the availability of the manufacturing apparatus  1 A is increasing, it is possible to improve productivity of the optical fiber, and the cost of manufacturing the optical fiber can be reduced. 
         [0098]    Furthermore, since the flotation positions of the bare optical fiber  3  are stabilized in the first direction changer  20 A and the second direction changer  20 B, the fiber inlet position at which the bare optical fiber  3  enters the coating unit  30  is constant. Therefore, deviation in thickness of the coating is prevented, and it is possible to manufacture an optical fiber  5  having a stable quality. 
         [0099]    According to the manufacturing method, since amounts of fluid are controlled in all of direction changers  20  (the first direction changer  20 A and the second direction changer  20 B), it is possible to accurately control the amount of flotation. 
         [0100]      FIG. 5  is a schematic diagram showing a manufacturing apparatus  1 A′ serving as a modified example of the manufacturing apparatus  1 A shown in  FIG. 1 . 
         [0101]    The manufacturing apparatus  1 A′ is different from the manufacturing apparatus  1 A shown in  FIG. 1  in that the detector  50  is provided with a first detector  50 A and a second detector  50 B. The direction changer  20 , the detectors  50  ( 50 A,  50 B), the controller  60 , and the flow controllers  70  ( 70 A,  70 B) constitute the control apparatus  101 ′. 
         [0102]    The first detector  50 A provided at the downstream side of the first direction changer  20 A in the fiber drawing direction and detects the position of the bare optical fiber  3  in the second pathway L 2 . The second detector  50 B is provided at the downstream side of the second direction changer  20 B in the fiber drawing direction and detects the position of the bare optical fiber  3  in the third pathway L 3 . As the first detector  50 A and the second detector  50 B, for example, a laser position sensor can be used. 
         [0103]    When the amount of flotation of the bare optical fiber  3  in the first direction changer  20 A increases or decreases, the position of the bare optical fiber  3  in the Z-direction (the direction perpendicular to the X-direction and in the Y-direction) in the second pathway L 2  varies. For this reason, the first detect  50 A can detect the amount of flotation of the bare optical fiber  3  in the first direction changer  20 A based on positional information of the bare optical fiber  3 . 
         [0104]    When the amount of flotation of the bare optical fiber  3  in the second direction changer  20 B increases or decreases, the position of the bare optical fiber  3  in the X-direction in the third pathway L 3  varies. Accordingly, the second detector  50 B can detect the amount of flotation of the bare optical fiber  3  in the second direction changer  20 B based on positional information of the bare optical fiber  3 . 
         [0105]    The first detector  50 A and the second detector  50 B output a detection signal to the controller  60  based on information associated with the position of the bare optical fiber  3 . 
         [0106]    The controller  60  uses the first flow controller  70 A, controls the introduction flow rate of the fluid which is introduced into the first direction changer  20 A based on the detection signal output from the first detector  50 A, and can adjust the amount of flotation of the bare optical fiber  3  in the first direction changer  20 A. 
         [0107]    The controller  60  uses the second flow controller  70 B, controls the introduction flow rate of the fluid which is introduced into the second direction changer  20 B based on the detection signal output from the second detector  50 B, and can adjust the amount of flotation of the bare optical fiber  3  in the second direction changer  20 B. 
         [0108]    According to the manufacturing method, since the amount of fluid is individually controlled in each of the first direction changer  20 A and the second direction changer  20 B, it is possible to reliably stabilize the flotation position of the bare optical fiber  3  in each of the first direction changer  20 A and the second direction changer  20 B. 
         [0109]      FIG. 6  is a schematic diagram showing the configuration of a manufacturing apparatus  1 B which serves as an optical fiber manufacturing apparatus according to a second embodiment of the invention. 
         [0110]    The manufacturing apparatus  1 B can carry out the aforementioned second control method. As an example of using the manufacturing apparatus  1 B, a method of manufacturing an optical fiber according to the second embodiment of the invention will be described. 
         [0111]    The manufacturing apparatus  1 B includes a controller  160  instead of the controller  60 . In the manufacturing apparatus  1 B, it is possible to measure a drawing tension of the optical fiber  5  by use of a load sensor (not shown in the Figure) provided on the pulley  80 . The load sensor outputs a measurement signal to the controller  160  based on a measurement value of the drawing tension of the optical fiber  5 . 
         [0112]    The detector  50  outputs a detection signal to the controller  160  based on information associated with the position of the bare optical fiber  3 . 
         [0113]    The direction changer  20 , the detector  50 , the controller  160 , and the flow controllers  70  ( 70 A,  70 B) constitute a control apparatus  111 . The other configurations of the manufacturing apparatus  1 B are the same as those of the manufacturing apparatus  1 A shown in  FIG. 1 . 
         [0114]    The drawing step, the coating step, and the curing step of the manufacturing method according to the second embodiment are the same as those of the first embodiment. 
         [0115]    In the manufacturing method according to the second embodiment, as a method of controlling introduction flow rates of the fluid in the first direction changer  20 A and the second direction changer  20 B, a second control method is adopted. In the second control method, data associated with introduction flow rates of fluid such that the amount of flotation of the bare optical fiber  3  in the second direction changer  20 B is in a proper range is previously obtained. Particularly, such data is obtained for each of drawing tensions (for each of predetermined drawing tensions). The second control method utilizes this data. 
         [0116]    That is, the relationship among the position of the bare optical fiber in the second direction changer  20 B and the introduction flow rate of the fluid which is introduced into the second direction changer  20 B is previously obtained for each of drawing tensions. Based on the relationships (particularly, relationship among the position of the bare optical fiber  3 , the introduction flow rate of the fluid, and the drawing tension), the introduction flow rates of the fluid which are introduced into the first direction changer  20 A and the second direction changer  20 B are controlled by use of the first flow controller  70 A and the second flow controller  70 B. 
         [0117]    The detector  50  obtains the positional information associated with the bare optical fiber  3  in the second direction changer  20 B. 
         [0118]    In other cases, after the relationship between the position of the bare optical fiber  3  and the introduction flow rate of the fluid which is introduced into the second direction changer  20 B is obtained, the detector  50  may be removed. 
         [0119]    Particularly, a load sensor (not shown in the figure) provided on the pulley  80  outputs a detection signal obtained based on the drawing tension of the optical fiber  5  to the controller  160 . 
         [0120]    The controller  160  uses the first flow controller  70 A and the second flow controller  70 B and controls the introduction flow rates of the fluid which are introduced into the first direction changer  20 A and the second direction changer  20 B based on the measurement signal output from the load sensor. Because of this, the flow velocity of the fluid discharged to the guide groove  21  from the outlet nozzle  22  in the first direction changer  20 A and the second direction changer  20 B is adjusted, and it is possible to adjust the amount of flotation of the bare optical fiber  3 . 
         [0121]    As a control method, proportional control is preferable. 
         [0122]    As described above, amounts of flotation of the bare optical fiber  3  in the first direction changer  20 A and the second direction changer  20 B depend on a drawing tension. Particularly, as a drawing tension is higher, the flotation of the bare optical fiber  3  decreases; as a drawing tension is smaller, the flotation of the bare optical fiber  3  increases. 
         [0123]    Consequently, as the controller  160  controls the introduction flow rates of the fluid which are introduced into the first direction changer  20 A and the second direction changer  20 B based on the measurement value of the drawing tension of the optical fiber  5  by use of the first flow controller  70 A and the second flow controller  70 B, the flow velocity of the fluid discharged from the outlet nozzle  22  to the guide groove  21  is adjusted in the first direction changer  20 A and the second direction changer  20 B, and it is possible to adjust the flotation of the bare optical fiber  3 . 
         [0124]    In the coating unit  30 , the optical fiber production intermediate  4  is obtained by forming a coated layer by coating the periphery of the bare optical fiber  3  with a coating material. 
         [0125]    Next, in the curing unit  40 , the coated layer of the optical fiber production intermediate  4  is cured by irradiating the coating material with UV light by the UV lamp  40   a  or the like, and the optical fiber  5  is thereby formed. 
         [0126]    The direction of the optical fiber  5  is changed by the pulley  80 , and the optical fiber  5  is picked up by the pick-up unit  90  and is wound around the winding unit  100 . 
         [0127]    According to the manufacturing method, as a result of controlling the introduction flow rates of the fluid which are introduced into the first direction changer  20 A and the second direction changer  20 B based on the measurement value of the drawing tension of the optical fiber  5 , the flow velocity of the fluid discharged from the outlet nozzle  22  to the guide groove  21  is adjusted in the first direction changer  20 A and the second direction changer  20 B, and it is possible to adjust the amount of flotation of the bare optical fiber  3 . 
         [0128]    For this reason, it is possible to stabilize the flotation position of the bare optical fiber  3  in the first direction changer  20 A and the second direction changer  20 B, and it is possible to avoid the bare optical fiber  3  from coming into contact with the inner side surface  21   c  of the guide groove  21  due to excess lack of flotation. Accordingly, as a result of using the first direction changer  20 A and the second direction changer  20 B, it is possible to manufacture the optical fiber  5  with a high level of yield without damage to the bare optical fiber  3 . Additionally, since the availability of the manufacturing apparatus  1 A is increasing, it is possible to improve productivity of the optical fiber, and the cost of manufacturing the optical fiber can be reduced. 
         [0129]    Moreover, since the flotation positions of the bare optical fiber  3  are stabilized in the first direction changer  20 A and the second direction changer  20 B, the fiber in position at which the bare optical fiber  3  enters the coating unit  30  is constant. Therefore, deviation in thickness of the coating is prevented, and it is possible to manufacture an optical fiber  5  having a stable quality. 
         [0130]    According to the manufacturing method, since amounts of fluid are controlled in all of direction changers  20  (the first direction changer  20 A and the second direction changer  20 B), it is possible to accurately control the amount of flotation. 
         [0131]      FIG. 7  is a schematic diagram showing a manufacturing apparatus  1 B′ serving as a modified example of the manufacturing apparatus  1 B shown in  FIG. 6 . 
         [0132]    The manufacturing apparatus  1 B′ is different from the manufacturing apparatus  1 B shown in  FIG. 6  in that the detector  50  is provided with a first detector  50 A and a second detector  50 B. 
         [0133]    The detectors  50  ( 50 A,  50 B) output a detection signal to the controller  160  based on information associated with the position of the bare optical fiber  3 . 
         [0134]    The direction changer  20 , the detectors  50  ( 50 A,  50 B), the controller  160 , and the flow controllers  70  ( 70 A,  70 B) constitute the control apparatus  111 ′. 
         [0135]    In manufacture of the optical fiber  5  by use of the manufacturing apparatus  1 B′, data associated with introduction flow rates of fluid such that amounts of flotation of the bare optical fiber  3  in the first direction changer  20 A and the second direction changer  20 B are each in a proper range is previously obtained. Particularly, such data is obtained for each of drawing tensions (for each of predetermined drawing tensions). The manufacturing apparatus utilizes this data. 
         [0136]    Particularly, as the relationship between the position of the bare optical fiber  3  in the first direction changer  20 A and the introduction flow rate of the fluid which is introduced into the first direction changer  20 A is previously searched for each of drawing tensions, the relationship between the position of the bare optical fiber  3 , the introduction flow rate of the fluid, and the drawing tension is preliminarily obtained. The positional information associated with the bare optical fiber  3  in the first direction changer  20 A is obtained by the first detector  50 A. 
         [0137]    Similarly, as the relationship between the position of the bare optical fiber  3  in the second direction changer  20 B and the introduction flow rate of the fluid which is introduced into the second direction changer  20 B is previously searched for each of drawing tensions, the relationship between the position of the bare optical fiber  3 , the introduction flow rate of the fluid, and the drawing tension is preliminarily obtained. The second detector  50 B obtains the positional information associated with the bare optical fiber  3  in the second direction changer  20 B. 
         [0138]    In other cases, after the relationship between the drawing tension, the positions of the bare optical fiber  3  in the first direction changer  20 A and the second direction changer  20 B, and the introduction flow rates of the fluid which are introduced into the first direction changer  20 A and the second direction changer  20 B is obtained, the detectors  50  ( 50 A,  50 B) may be removed. 
         [0139]    The first detector  50 A and the second detector  50 B output a detection signal obtained based on the positional information associated with the bare optical fiber  3  to the controller  160 . 
         [0140]    The controller  160  controls the introduction flow rate of the fluid which is introduced into the first direction changer  20 A based on the relationship between the position of the bare optical fiber  3 , the introduction flow rate of the fluid, and the drawing tension which are associated with the first direction changer  20 A, by use of the first flow controller  70 A. Accordingly, it is possible to adjust the amount of flotation of the bare optical fiber  3  in the first direction changer  20 A. 
         [0141]    Similarly, the controller  160  controls the introduction flow rate of the fluid which is introduced into the second direction changer  20 B based on the relationship between the position of the bare optical fiber  3 , the introduction flog rate of the fluid, and the drawing tension which are associated with the second direction changer  20 B, by use of the second flow controller  70 B. Accordingly, it is possible to adjust the amount of flotation of the bare optical fiber  3  in the second direction changer  20 B. 
         [0142]    According o the manufacturing method, since the amount of fluid is individually controlled in each of the first direction changer  20 A and the second direction changer  20 B, it is possible to reliably stabilize the flotation position of the bare optical fiber  3 . 
       EXAMPLES 
     Example 1 
       [0143]    The manufacturing apparatus  1 A shown in  FIG. 1  was prepared. As the first direction changer  20 A and the second direction changer  20 B, the direction changer  201  shown in  FIG. 3  was used. 
         [0144]    As shown in  FIG. 2 , an angle θ 1  formed between the inner side surface  21   c  of the guide groove  21  and the radial direction D 1  was 0.5 degrees. The width of the bottom of the guide groove  21  was 50 μm. 
         [0145]    A flotation turning radius of the hare optical fiber  3  was approximately 62.5 mm. 
         [0146]    Fluid which is to be introduced into the direction changers  20 A and  20 B is air, and the temperature thereof was at a room temperature (approximately 24° C.). 
         [0147]    The introduction flow rate of air into each of the first direction changer  20 A and the second direction changer  20 B was 100 liters per minute. 
         [0148]    The first direction changer  20 A was provided at the position at which the temperature of the bare optical fiber  3  becomes approximately 1000° C. The second direction changer  20 B was provided at the position away from the first direction changer  20 A at 1 m in the downstream side of the fiber drawing direction. 
         [0149]    The bare optical fiber  3  (outer diameter of 125 μm) was obtained by drawing the optical fiber from the optical fiber preform  2  in the drawing unit  10 . The drawing speed was 30 m/second, and the drawing tension was 150 gf. 
         [0150]    The direction of the bare optical fiber  3  that is drawn from the optical fiber preform  2  in the downward vertical direction (first pathway L 1 ) is changed by the first direction changer  20 A to be in the horizontal direction (second pathway L 2 ). Subsequently, the direction of the bare optical fiber  3  is changed by the second direction changer  20 B to be in the downward vertical direction (third pathway L 3 ). 
         [0151]    In the coating unit  30 , the bare optical fiber  3  was subjected to a coating step using an ultraviolet curable resin, the UV lamp  40   a  irradiated the resin with ultraviolet light in the curing unit  40  and the by a coated layer was cured, and the optical fiber  5  was obtained. 
         [0152]    The optical fiber  5  passed through the pulley  80  and the pick-up unit  90  and was wound around the winding unit  100 . 
         [0153]    The detector  50  output a detection signal to the controller  60  based on the positional information associated with the bare optical fiber  3  in the third pathway L 3 , and the controller  60  controlled the introduction flow rates of the fluid which are introduced into the first direction changer  20 A and the second direction changer  20 B based on the detection signal, by use of the first flow controller  70 A and the second flow controller  70 B. Consequently, the flow velocity of the fluid discharged from the outlet nozzle  22  to the guide groove  21  was controlled in the first direction changer  20 A and the second direction changer  20 B. As a control method, PID control was used. 
         [0154]    Particularly, the center value of the flotation turning radius of the bare optical fiber  3  (for example, shown in  FIGS. 3 and 4 ) was 62.5 mm. The control was carried out such that: in the case where the flotation turning radius decreases less than the center value, the introduction flow rates of the fluid which are introduced into the first direction changer  20 A and the second direction changer  20 B increase; in the case where the flotation turning radius increases more than the value, the introduction rates of the fluid which are introduced into the first direction changer  20 A and the second direction changer  20 B decrease. 
         [0155]    The optical fiber  5  of 10000 km was manufactured by the above-described manufacturing method. In the manufacture of the optical fiber  5 , a drawing velocity of the optical fiber  5  was 30±1 m/second and a drawing tension was 150±25 gf. 
         [0156]    Moreover, since the flotation turning radius of the bare optical fiber  3  in the first direction changer  20 A and the second direction changer  20 B was 62.5±0.05 mm, the flotation of the bare optical fiber  3  was stabilized. 
         [0157]    After the optical fiber  5  is manufactured by the manufacturing method, a proof test was carried out with respect to the optical fiber. As a result, it was determined that the optical fiber  5  can be manufactured with a high level of yield without damage to the bare optical fiber  3  by use of the first direction changer  20 A and the second direction changer  20 B. 
       Example 2 
       [0158]    The manufacturing apparatus  1 B shown in  6  was prepared. 
         [0159]    A drawing tension of the optical fiber  5  was measured by use of a load sensor (not shown in the figure) provided on the pulley  80 , and a measurement signal is output to the controller  160  based on the measurement value. The controller  160  controlled the introduction flow rates of the fluid which are introduced into the first direction changer  20 A and the second direction changer  20 B by use of the first flow controller  70 A and the second flow controller  70 B based on the measurement signal. As a control method, proportional control was adopted. In this Example, the optical fiber  5  was manufactured under the control condition described above, and the condition other than the above-described condition was the same as that of Example 1. 
         [0160]    The specific control method is as follows. 
         [0161]    Firstly, data associated with the amount of fluid which causes the amount of flotation of the bare optical fiber  3  to be constant was previously obtained for each drawing tension. 
         [0162]    The introduction flow rates of the fluid which are introduced into the first direction changer  20 A and the second direction changer  20 B when the floatation turning radius of the bare optical fiber  3  is 62.5 mm at the drawing tension of 125 gf was 75 L/min. The introduction flow rates of the fluid when the flotation turning radius of the bare optical fiber  3  is 62.5 mm at the drawing tension of 175 gf was 125 L/min. 
         [0163]    Based on the results, regarding the first direction changer  20 A and the second direction changer  20 B, a proportionality factor of 1 l/min/gf was set thereto, and the fluid introduction flow rates were each controlled therein. Particularly, the fluid introduction flow rate was controlled to be 100/min by the controller in the case where the drawing tension is 150 gf. Furthermore, the control was carried out so that the fluid introduction flow rate varies a ±1 L/min when the drawing tension varies at ±1 gf. Specifically, the control was carried out so that: when the drawing tension increases at 1 gf, the fluid introduction flow rate increases at 1 L/min; and when the drawing tension is reduced at 1 gf, of the fluid introduction flow rate is reduced at 1 L/min. 
         [0164]    The optical fiber  5  of 10000 km was manufactured by the above-described manufacturing method. In the manufacture of the optical fiber  5 , a drawing velocity of the optical fiber  5  was 30±1 m/second and a drawing tension was 150±25 gf. 
         [0165]    Moreover, the flotation of the bare optical fiber  3  in the first direction changer  20 A and the second direction changer  20 B was stabilized. 
         [0166]    After the optical fiber  5  is manufactured by the manufacturing method, a proof test was carried out with respect to the optical fiber. As a result, it was determined that the optical fiber  5  can be manufactured with a high level of yield without damage to the bare optical fiber  3  by use of the first direction changer  20 A and the second direction changer  20 B. 
       Comparative Example 1 
       [0167]    In this Comparative Example, an optical fiber  5  of 10000 km was manufactured under the same condition as that of Example 1 except that the fluid introduction flow rates which are introduced into the first direction changer  20 A and the second direction changer  20 B are set as a constant value, that is, 100 L/min. 
         [0168]    In the manufacture of an optical fiber  5 , a drawing velocity of the optical fiber  5  was 30±1 m/second, and a drawing tension was 150±25 gf; however, variations in the amounts of flotation of the bare optical fiber  3  in the first direction changer  20 A and the second direction changer  20 B were large. 
         [0169]    After the optical fiber  5  is manufactured by the manufacturing method, a proof test was carried out with respect to the optical fiber. As a result, it was determined that the wire breakage occurs, it is thought that contact of the bare optical fiber  3  into an inner side surface of the guide groove causes the wire breakage, and the Comparative Example does not contribute to a high level of yield in manufacture of the optical fiber. 
         [0170]    In the above-description, a method of manufacturing an optical fiber of the invention and an optical fiber manufacturing apparatus thereof are described; however, technical scope of the invention is not limited to the above embodiments, and various modifications may be made without departing from the scope of the invention. 
         [0171]    For example, the number of direction changers used in the method of manufacturing the optical fiber according to the embodiment of the invention may be one or more. Each of the manufacturing apparatus  1 A and  1 B shown in  FIGS. 1 and 6  uses two direction changers  20 ; however, the number of direction changers  20  may be 1 or an arbitrary number, for example, 3 or more.