Patent Publication Number: US-2023150870-A1

Title: Coating condition detection method, coating condition detection device, and optical fiber manufacturing method

Description:
TECHNICAL FIELD 
     The present disclosure relates to a coating condition detection method, a coating condition detection device, and an optical fiber manufacturing method. 
     This application claims the priority of Japanese Patent Application No. 2020-013921 filed on Jan. 30, 2020, which is incorporated herein by reference in its entirety. 
     BACKGROUND ART 
     As a method for measuring a degree of eccentricity of a coated fiber obtained as a result of coating a glass fiber (bared fiber) with resin during preform drawing, Patent Documents 1 to 4 disclose an optical fiber eccentricity measurement device and measurement method for causing a laser light source to emit a laser beam to a side surface of the coated fiber to detect a grayscale image formed by forward scattered light (transmitted light) of the laser beam, and measuring a degree of eccentricity of the glass fiber in the coated fiber on the basis of a degree of unevenness in thickness of a resin layer. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Document 1: Japanese Patent Application Laid-Open No. H4-315939 
         Patent Document 2: Japanese Patent Application Laid-Open No. H4-319642 
         Patent Document 3: Japanese Patent Application Laid-Open No. H5-107046 
         Patent Document 4: Japanese Patent Application Laid-Open No. H5-087681 
       
    
     SUMMARY OF INVENTION 
     A coating condition detection method according to an embodiment of the present disclosure is a method for detecting a coating condition of resin with which an optical fiber is coated (coated fiber) in a circumferential direction around a fiber axis using light released from the optical fiber. Under the coating condition detection method according to one aspect, an imaging optical system is prepared, and intensity of light at each point of an image formed by imaging optical system on an imaging plane (light receiving surface) is detected with the intensity of light associated with information on a corresponding position on an object plane. Specifically, the imaging optical system thus prepared includes a reflection mirror disposed on an optical path between the imaging plane and the object plane conjugate with the imaging plane. The reflection mirror has a guide hole through which the coated fiber passes. Further, on the imaging plane side, the imaging optical system is caused to form, on the imaging plane, an image of light released from a portion of the coated fiber that has passed through the guide hole of the reflection mirror, the portion intersecting the object plane, to detect intensity of light at each position on the imaging plane with the intensity of light associated with information on a corresponding position on the object plane. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating various examples of an imaging optical system applicable to a coating condition detection device according to an embodiment of the present disclosure, the imaging optical system including a reflection mirror having a flat surface. 
         FIG.  2    is a diagram illustrating various examples of an imaging optical system applicable to the coating condition detection device according to the embodiment of the present disclosure, the imaging optical system including a reflection mirror having a curved surface. 
         FIG.  3    is a diagram illustrating an example of an optical fiber manufacturing device (drawing device) for implementing an optical fiber manufacturing method according to the embodiment of the present disclosure. 
         FIG.  4    is a diagram illustrating an example applied to a resin coating device configured to further coat, with resin, an outer peripheral surface of a coated fiber obtained after a preform is drawn. 
         FIG.  5    is a diagram for describing a mechanism of how light is released from a drawn optical fiber. 
         FIG.  6    is a diagram illustrating an example where a coating condition detector to which an imaging optical system  2 B illustrated in  FIG.  2    is applied is applied to the optical fiber manufacturing device illustrated in  FIG.  3   . 
         FIG.  7    is a diagram for describing an example of control operation of a controller (data processor) of a coating condition detection device according to the embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Technical Problem 
     As a result of examining the above-described conventional techniques, the inventors have found the following problems. That is, when a ratio between a coating outer diameter (an outer diameter of the coated fiber) and a glass diameter (an outer diameter of the bared fiber) is small (for example, 1.7 or less), the grayscale image formed by monitor light for detecting a degree of unevenness in thickness of the coated resin layer cannot be visually recognized or is hardly visually recognized. Further, in order to detect the degree of unevenness in thickness of the resin layer in the circumferential direction around the fiber axis from the side surface of the coated fiber, it is necessary to prepare a plurality of detection optical systems (a laser light source and a light receiving device), which makes the alignment work of the detection optical systems complicated, makes the device larger in size, and makes the device structure complicated, in addition to an increase in cost of manufacturing the device. 
     The present disclosure has been made to solve the above-described problems, and it is therefore an object of the present disclosure to provide a coating condition detection method, a coating condition detection device, and an optical fiber manufacturing method that allow, even when a ratio between a coating outer diameter and a glass diameter is small, a coating condition of a resin layer of a coated fiber to be detected with a simple device structure as compared with the related art. 
     Advantageous Effects of Invention 
     The coating condition detection method and the like of the present disclosure allow, when the ratio of the coating outer diameter to the glass diameter is small, the coating condition of the resin layer of the coated fiber to be detected with a simple device structure as compared with the related art. 
     Description of Embodiment of Present Disclosure 
     First, details of the embodiment of the present disclosure will be individually listed and described. 
     (1) A coating condition detection method according to the embodiment of the present disclosure is a method for detecting a coating condition of resin with which an optical fiber is coated (coated fiber obtained as a result of coating a bared fiber with resin) in a circumferential direction around a fiber axis using light released from the coated fiber. Under the coating condition detection method according to one aspect, an imaging optical system is prepared, and intensity of light at each point of an image formed by imaging optical system on an imaging plane (light receiving surface) is detected with the intensity of light associated with information on a corresponding position on an object plane. Specifically, the imaging optical system thus prepared includes a reflection mirror disposed on an optical path between the imaging plane and the object plane conjugate with the imaging plane. The reflection mirror has a guide hole through which the coated fiber passes. Further, on the imaging plane side, the imaging optical system is caused to form, on the imaging plane, an image of light released from a portion of the coated fiber that has passed through the guide hole of the reflection mirror, the portion intersecting the object plane, to detect intensity of light at each position on the imaging plane with the intensity of light associated with information on a corresponding position on the object plane. 
     Note that the “light released from the coated fiber” is a light component released to the outside of the optical fiber after propagating in the optical fiber among light components emitted to the optical fiber during manufacturing, and examples of such a light component include UV light for curing the coating of the optical fiber. 
     Note that the imaging optical system may be configured to form an intermediate imaging plane on the optical path between the object plane and the imaging plane. Such a configuration allows an increase in contrast of a grayscale image formed on the imaging plane by disposing a diaphragm on the intermediate imaging plane. 
     The above-described configuration allows the coating condition of the resin layer around the fiber axis to be efficiently detected with a simple device structure. In other words, the above-described configuration allows the coating condition in the circumferential direction around the fiber axis to be detected without depending on a ratio between a coating outer diameter (an outer diameter of the coated fiber) and a glass diameter (an outer diameter of the bared fiber). Note that, the “coating condition of the resin” refers herein to fluctuations in thickness in the circumferential direction of the resin layer provided on the outer periphery of the bared fiber (a degree of unevenness in thickness of the resin layer or a degree of eccentricity of the bared fiber in the coated fiber), a degree of mixture of air bubbles in the resin layer, and a degree of separation along the interface between the bared fiber and the resin layer, and the like. Further, the coated fiber that is a detection target is a coated fiber obtained as a result of coating a glass fiber (bared fiber) with a resin layer, and the resin layer includes a single layer (primary coating) or a plurality of layers (continuous layers of the primary coating, the secondary coating, and the like) provided on the bared fiber during preform drawing. The resin layer also includes colored resin with which the coated fiber is coated while the coated fiber wound around the drum during preform drawing is rewound around another drum. 
     (2) According to one aspect of the present disclosure, a two-dimensional image visually representing the coating condition of the resin on the basis of the intensity of light detected and the information on the corresponding position may be outputted onto a monitor. This allows the condition of the cross section of the coated fiber that is a detection target to be visually confirmed. 
     (3) According to one aspect of the present disclosure, the two-dimensional image may include at least one of a grayscale image showing a cross section of the coated fiber that is a detection target, a light intensity distribution shown along each of two orthogonal axes on the imaging plane, the two orthogonal axes being orthogonal to each other at an intersection of an axis on the imaging plane corresponding to the fiber axis and the imaging plane, or a light intensity distribution in a circumferential direction around the axis on the imaging plane corresponding to the fiber axis. In particular, performing numerical analyses on an image once captured by an image capturing device such as a camera allows the coating condition (fluctuations in thickness in the circumferential direction of the resin layer, a degree of mixture of air bubbles in the resin layer, or a degree of separation along the interface between the bared fiber and the resin layer) to be quantitatively or dynamically determined. Further, giving detection data to a measuring instrument enables a process control (it is possible to generate control information used for controlling the operation of each unit of a manufacturing device or the like on the basis of the detection data thus given). 
     (4) According to one aspect of the present disclosure, the reflection mirror may include an off-axis parabolic mirror, and in this case, the off-axis parabolic mirror has a hole serving as the guide hole. When the off-axis parabolic mirror is used as the reflection mirror, the imaging optical system is disposed so as to cause the coated fiber to pass through a focal point of the off-axis parabolic mirror after passing through the hole of the off-axis parabolic mirror. The off-axis parabolic mirror reflects light from the focal point as collimated light. Therefore, the use of the off-axis parabolic mirror as the reflection mirror allows a reduction in the number of lens elements constituting the imaging optical system (simplification of the structure of the imaging optical system). 
     (5) According to one aspect of the present disclosure, the reflection mirror may include an ellipsoid mirror, and in this case, the ellipsoid mirror has a hole serving as the guide hole. When the ellipsoid mirror is used as the reflection mirror, the imaging optical system is disposed so as to cause the coated fiber to pass through one focal point of the ellipsoid mirror after passing through the hole of the ellipsoid mirror and to cause an other focal point of the ellipsoid mirror to be positioned on the imaging plane or an optical path between the ellipsoid mirror and the imaging plane. The ellipsoid mirror concentrates light from the one focal point on the other focal point (the two focal points are conjugate with each other). Therefore, the use of the ellipsoid mirror as the reflection mirror allows the imaging optical system to be constituted of only the ellipsoid mirror. Further, even a combination of the ellipsoid mirror and a lens can constitute an imaging optical system having a simple structure (simplification of the structure of the imaging optical system). 
     (6) According to one aspect of the present disclosure, the light released from the coated fiber may include resin curing light emitted to the resin in a space on a side of the reflection mirror remote from the object plane. That is, when the coating condition detection method is applied to a coated fiber manufacturing device (drawing device), disposing the above-described imaging optical system on the downstream side of a resin coating device allows a light source for resin curing to be used as a light source for coating condition detection. 
     (7) According to one aspect of the present disclosure, the light released from the optical fiber may include light from an external light source other than the resin curing light emitted to the optical fiber in the space on the side of the reflection mirror remote from the object plane. As described above, preparing the external light source separately from an ultraviolet light source of the resin coating device increases the degree of freedom in arrangement of the imaging optical system. Further, installing the external light source can make the grayscale image formed by the light released from the coating of the coated fiber clearer (increase the S/N ratio of the grayscale image formed on the imaging plane). 
     (8) A coating condition detection device according to the embodiment of the present disclosure is a device for implementing the above-described coating condition detection method, and the coating condition detection device is structured to detect the coating condition of resin with which an optical fiber is coated (coated fiber) in the circumferential direction around a fiber axis using light released from the coated fiber. Specifically, according to one aspect, the coating condition detection device includes a light receiving device and an imaging optical system. The imaging optical system includes a reflection mirror disposed on an optical path between an imaging plane to be projected onto a light receiving surface of the light receiving device and an object plane conjugate with the imaging plane. The reflection mirror having a guide hole through which the coated fiber passes. Further, the light receiving device detects intensity of light at each point on the imaging plane where an image of the light released from a portion of the coated fiber that has passed through the guide hole of the reflection mirror is formed by the imaging optical system, the portion intersecting the object plane, with the intensity of light associated with information on a corresponding position on the object plane. This configuration allows the above-described coating condition detection method to be implemented. 
     (9) According to one aspect of the present disclosure, the coating condition detection device may further include a controller configured to output, onto a monitor, a two-dimensional image visually representing the coating condition of the resin on the basis of the intensity of light detected by the light receiving device and the information on the corresponding position. This allows the condition of the cross section of the coated fiber that is a detection target to be visually confirmed. Further, the two-dimensional image may include at least one of a grayscale image corresponding to the cross section of the coated fiber that is a detection target, a light intensity distribution shown along each of two orthogonal axes on the imaging plane, the two orthogonal axes being orthogonal to each other at an intersection of the fiber axis and the imaging plane, or a light intensity distribution in the circumferential direction around the fiber axis. For example, performing numerical analyses on an image once captured by an image capturing device such as a camera allows the coating condition (fluctuations in thickness in the circumferential direction of the resin layer, a degree of mixture of air bubbles in the resin layer, or a degree of separation along the interface between the bared fiber and the resin layer) to be quantitatively or dynamically determined. Further, giving detection data to a measuring instrument enables a process control (it is possible to generate control information used for controlling the operation of each unit of a manufacturing device or the like on the basis of the detection data thus given). 
     (10) According to one aspect of the present disclosure, the reflection mirror may include an off-axis parabolic mirror, and in this case, the off-axis parabolic mirror has a hole serving as the guide hole. When the off-axis parabolic mirror is used as the reflection mirror, the imaging optical system is disposed so as to cause the coated fiber to pass through a focal point of the off-axis parabolic mirror after passing through the hole of the off-axis parabolic mirror. This can make the imaging optical system simple in structure as described above. 
     (11) According to one aspect of the present disclosure, the reflection mirror may include an ellipsoid mirror, and in this case, the ellipsoid mirror has a hole serving as the guide hole. When the ellipsoid mirror is used as the reflection mirror, the imaging optical system is disposed so as to cause the coated fiber to pass through one focal point of the ellipsoid mirror after passing through the hole of the ellipsoid mirror and to cause an other focal point of the ellipsoid mirror to be positioned on the imaging plane or an optical path between the ellipsoid mirror and the imaging plane. This can also make the imaging optical system simple in structure as described above. 
     (12) According to one aspect of the present disclosure, the coating condition detection device may include, in a space on a side of the reflection mirror remote from the object plane, a light source configured to emit, to the optical fiber including the resin, light that can propagate in the optical fiber. When the coating condition detection device is applied to the optical fiber manufacturing device, disposing the coating condition detection device on the downstream side of the resin coating device allows light from the ultraviolet light source of the resin coating device to be used as detection light (released light), for example. Further, the use of an external light source in addition to the light source (the ultraviolet light source for resin curing) of the resin coating device can effectively increase the S/N ratio of the grayscale image formed on the imaging plane. 
     (13) An optical fiber manufacturing method according to the embodiment of the present disclosure is a method for winding a bared fiber with the bared fiber coated with resin, the bared fiber being obtained as a result of drawing an optical fiber preform. In particular, under the optical fiber manufacturing method according to one aspect, a coating condition detection device having the above-described structure (the coating condition detection device of the present disclosure) is disposed on the downstream side of a resin coating device including a die configured to coat the bared fiber with the resin, and a resin coating condition is changed on the basis of a detection result obtained from the coating condition detection device. Note that examples of the resin coating condition include regulation of a flow rate of a flushing gas (CO 2  regulation), prevention of air bubbles from mixing into the resin layer (specifically, temperature control on a cooling device disposed on the upstream side of the resin coating device), and the like, in addition to a change in posture of the die (elimination of eccentricity of the bared fiber in the coated fiber). 
     As described above, each of the aspects listed in “Description of embodiment of present disclosure” is applicable to all remaining aspects or all combinations of the remaining aspects. 
     Details of Embodiment of Present Disclosure 
     Specific examples of an optical fiber coating condition detection method, an optical fiber coating condition detection device, and an optical fiber manufacturing method according to the present disclosure will be described in detail below with reference to the accompanying drawings. It should be noted that the present disclosure is not limited to such examples, and is intended to be defined by the claims and to include all modifications within the scope of the claims and their equivalents. Further, in a description of the drawings, the same components are denoted by the same reference numerals, and a redundant description will be omitted. 
     First, a representative structure of an imaging optical system for implementing a coating condition detection method according to the embodiment of the present disclosure and a coating condition detection device according to the embodiment of the present disclosure (a device for implementing the coating condition detection method according to the embodiment of the present disclosure) will be described with reference to  FIGS.  1  and  2   . Note that  FIG.  1    is a diagram illustrating various examples of an imaging optical system applicable to the coating condition detection method and the coating condition detection device according to the embodiment of the present disclosure, the imaging optical system including a reflection mirror having a flat surface. Note that  FIG.  2    is a diagram illustrating various examples of an imaging optical system applicable to the coating condition detection method and the coating condition detection device according to the embodiment of the present disclosure, the imaging optical system including a reflection mirror having a curved surface. 
     An imaging optical system  1 A illustrated in  FIG.  1    is an imaging optical system having the simplest structure using a flat reflection mirror, and includes an imaging lens  10  and a flat reflection mirror  20 . A light receiving surface of a light receiving device that receives part of light released from a coated fiber is installed at an imaging plane IP, and an object plane OP and the imaging plane IP are conjugate with each other via the imaging lens  10 . That is, an intersection C 1  of an optical axis AX of the imaging optical system  1 A and the object plane OP and an intersection C 2  of the optical axis AX and the imaging plane IP are conjugate points. The flat reflection mirror  20  is disposed on an optical path between the object plane OP and the imaging lens  10 , and light released from the object plane OP is reflected off the flat reflection mirror  20  and concentrated onto the imaging plane IP. Further, the flat reflection mirror  20  is provided on one side of a guide member  30 . Furthermore, the flat reflection mirror  20  and the guide member  30  have a guide hole  30   a  through which the coated fiber passes, and the flat reflection mirror  20  has an output-side opening  21  of the guide hole  30   a . The coated fiber passes through from an input-side opening  31  of the guide hole  30   a  to the output-side opening  21  provided in the flat reflection mirror  20 . Further, the guide member  30  is held by a support member  32  in order to fix a positional relationship between the coated fiber and the flat reflection mirror  20 . 
     For example, when the imaging optical system  1 A is disposed so as to cause the coated fiber that has passed through the guide hole  30   a  of the guide member  30  connecting the input-side opening  31  and the output-side opening  21  to intersect the object plane OP at the conjugate point C 1 , the imaging plane IP is formed at a distance from the coated fiber by a predetermined distance, and light released from a portion of the coated fiber located at the conjugate point C 1  forms an image at the conjugate point C 2  on the imaging plane IP. As a result, a two-dimensional grayscale image showing the cross section of the coated fiber at the conjugate point C 1  is formed on the imaging plane IP. 
     An imaging optical system  1 B illustrated in  FIG.  1    includes two collimator lenses  11 ,  12  and the flat reflection mirror  20 . Further, it is possible to adjust a length (optical path length) of an optical axis AX of the imaging optical system  1 B by changing a distance L between the collimator lenses  11 ,  12 . The position of the object plane OP is determined by a position of a focal point of the collimator lens  11 , and the position of the imaging plane IP is determined by a position of a focal point of the collimator lens  12 . The object plane OP and the imaging plane IP are conjugate with each other via the collimator lenses  11 ,  12 . That is, the intersection C 1  of the optical axis AX of the imaging optical system  1 B and the object plane OP and the intersection C 2  of the optical axis AX and the imaging plane IP are conjugate points. The flat reflection mirror  20  is disposed on the optical path between the object plane OP and the collimator lens  11 , and light released from the object plane OP is reflected off the flat reflection mirror and concentrated onto the imaging plane IP. Further, as with the imaging optical system  1 A, the flat reflection mirror  20  is provided on one side of the guide member  30  whose position relative to the coated fiber is fixed by the support member  32 . The guide member  30  is provided with the guide hole  30   a  that connects the output-side opening  21  and the input-side opening  31  provided in the flat reflection mirror  20 . 
     Also in the imaging optical system  1 B, when the imaging optical system  1 B is disposed so as to cause the coated fiber that has passed through the guide hole  30   a  of the guide member  30  connecting the input-side opening  31  and the output-side opening  21  to intersect the object plane OP at the conjugate point C 1 , light released from a portion of the coated fiber located at the conjugate point C 1  forms an image at the conjugate point C 2  on the imaging plane IP. As a result, a two-dimensional grayscale image showing the cross section of the coated fiber at the conjugate point C 1  is formed on the imaging plane IP. 
     An imaging optical system  1 C illustrated in  FIG.  1    includes two imaging lenses  13 ,  14 , the flat reflection mirror  20 , and a diaphragm  15  disposed between the two imaging lenses  13 ,  14 . In the imaging optical system  1 C, the diaphragm  15  is disposed on an imaging plane formed on an optical path between the imaging lens  13  and the imaging lens  14 , and the object plane OP, the plane on which the diaphragm  15  is disposed, and the imaging plane IP are conjugate with each other. That is, the intersection C 1  of the optical axis AX of the imaging optical system  1 C and the object plane OP, and the intersection C 2  of the optical axis AX and the imaging plane IP are conjugate points. The C 2  and an intersection C 3  of the optical axis AX and the imaging plane on which the diaphragm  15  is disposed are conjugate points. The flat reflection mirror  20  is disposed on an optical path between the object plane OP and the imaging lens  13 , and light released from the object plane OP is reflected off the flat reflection mirror  20  and concentrated onto the imaging plane IP. Further, as with the imaging optical system  1 A, the flat reflection mirror  20  is provided on one side of the guide member  30  whose position relative to the coated fiber is fixed by the support member  32 . The guide member  30  is provided with the guide hole  30   a  that connects the output-side opening  21  and the input-side opening  31  provided in the flat reflection mirror  20 . 
     Also in the imaging optical system  1 C, when the imaging optical system  1 C is disposed so as to cause the coated fiber that has passed through the guide hole  30   a  of the guide member  30  connecting the input-side opening  31  and the output-side opening  21  to intersect the object plane OP at the conjugate point C 1 , light released from a portion of the coated fiber located at the conjugate point C 1  forms an image at the conjugate point C 2  on the imaging plane IP. As a result, a two-dimensional grayscale image showing the cross section of the coated fiber at the conjugate point C 1  is formed on the imaging plane IP. Note that when the coated fiber that has passed through the flat reflection mirror  20  intersects the object plane OP at a position that is not coincident with the conjugate point C 1 , the position of the diaphragm  15  on the imaging plane (plane orthogonal to the optical axis AX) is adjusted. 
     The imaging optical system  1 A to the imaging optical system  1 C described above are each an imaging optical system including the flat reflection mirror  20 , but the use of a special curved mirror instead of the flat reflection mirror  20  can make the imaging optical system simpler in structure. For example, an imaging optical system  2 A illustrated in  FIG.  2    is obtained as a result of applying a reflection mirror (hereinafter, referred to as an “ellipsoid mirror”)  50  having a curved surface coincident with a part of a spheroid surface  52  to the imaging optical system  1 A illustrated in  FIG.  1    or the imaging optical system  1 C illustrated in  FIG.  1   . Further, an imaging optical system  2 B illustrated in  FIG.  2    is obtained as a result of applying an off-axis parabolic mirror  60  having a curved surface coincident with a part of a paraboloid of revolution  62  to the imaging optical system  1 B illustrated in  FIG.  1   . 
     Specifically, in the imaging optical system  2 A to which the ellipsoid mirror  50  is applied instead of the flat reflection mirror  20  and the imaging lens  10  of the imaging optical system  1 A illustrated in  FIG.  1   , the position of the object plane OP is determined by the position of one of the two focal points of the ellipsoid mirror  50  (two focal points of the spheroid), and the position of the imaging plane IP is determined by the position of the other focal point (the two focal points of the ellipsoid mirror  50  are conjugate points C 1 , C 2 ). This allows the imaging optical system  2 A to work in the same manner as the imaging optical system  1 A even without a lens element on the optical axis AX. 
     Further, in the imaging optical system  2 A to which the ellipsoid mirror  50  is applied instead of the flat reflection mirror  20  and the imaging lens  13  of the imaging optical system  1 C illustrated in  FIG.  1   , the position of the object plane OP is determined by the position of one of the two focal points of the ellipsoid mirror  50 , and the position of the other focal point coincides with the position of the imaging plane on which the diaphragm  15  is disposed (two focal points of the ellipsoid mirror  50  are conjugate points C 1 , C 3 ). This allows the imaging optical system  2 A to work in the same manner as the imaging optical system  1 C with the number of lenses arranged on the optical axis AX reduced. 
     In the imaging optical system  2 A, light released from the object plane OP is reflected off the ellipsoid mirror  50  and concentrated onto the imaging plane IP. Further, as with the imaging optical system  1 A and the imaging optical system  1 C, the ellipsoid mirror  50  is provided on one side of a guide member  40  whose position relative to the coated fiber is fixed by a support member  42 . The guide member  40  is provided with a guide hole  40   a  that connects an output-side opening  51  provided in the ellipsoid mirror  50  and an input-side opening  41 . 
     Further, in the imaging optical system  2 B to which the off-axis parabolic mirror  60  is applied instead of the flat reflection mirror  20  and the collimator lens  11  of the imaging optical system  1 B illustrated in  FIG.  1   , the position of the object plane OP is determined by the position of the focal point of the off-axis parabolic mirror  60 , and the position of the imaging plane IP is determined by the position of the focal point of the collimator lens  12 . In this case, the focal point of off-axis parabolic mirror  60  is conjugate with the intersection C 2  of the optical axis AX and the imaging plane IP (the mirror focal point on the object plane OP is the conjugate point C 1  conjugate with the intersection C 2  on the imaging plane IP). Therefore, in the imaging optical system  2 B, light from the focal point of the off-axis parabolic mirror  60  is collimated by the off-axis parabolic mirror  60 , and an image is formed on the imaging plane IP by the collimator lens  12 . 
     In the imaging optical system  2 B, light released from the object plane OP is reflected off the off-axis parabolic mirror  60  and concentrated onto the imaging plane IP. Further, as with the imaging optical system  1 A and the like, the off-axis parabolic mirror  60  is provided on one side of the guide member  40  whose position relative to the coated fiber is fixed by the support member  42 . The guide member  40  is provided with the guide hole  40   a  that connects an output-side opening  61  provided in the off-axis parabolic mirror  60  and the input-side opening  41 . 
       FIG.  3    is a diagram illustrating an example of an optical fiber manufacturing device (drawing device) for implementing the optical fiber manufacturing method according to the embodiment of the present disclosure. Specifically, the optical fiber manufacturing device illustrated  FIG.  3    includes a heater  150  for heating one end of an optical fiber preform  100 , a drum  200  for winding a coated fiber  120  obtained as a result of coating, with resin, a bared fiber (glass fiber)  110  obtained as a result of drawing the optical fiber preform  100 , a resin coating device  300 , and a coating condition detection device  500 . The optical fiber preform  100  includes a core portion  100   a  and a cladding portion  100   b . Note that the core portion  100   a  is a region to be a core  110   a  of the bared fiber  110  obtained as a result of drawing the preform, and the cladding portion  100   b  is a region that surrounds the core portion  100   a  and is to be a cladding  110   b  of the bared fiber  110 . When the drum  200  rotates in a direction indicated by an arrow S 1 , the coated fiber  120  is wound around the drum  200 . The resin coating device  300  is a device that is disposed between the optical fiber preform  100  and the drum  200  and that coats the outer peripheral surface of the running bared fiber  110  with resin. The bared fiber  110  is coated with resin by the resin coating device  300  to become the coated fiber  120 . The resin coating device  300  includes a die  310  for coating the outer peripheral surface of the bared fiber  110  with an ultraviolet-curable resin, a posture control device  320  for adjusting the posture of the die  310 , and an ultraviolet irradiation device  330 . The coating condition detection device  500  detects a two-dimensional grayscale image showing the cross section of the coated fiber  120  using ultraviolet rays emitted from the ultraviolet irradiation device  330  of the resin coating device  300  located on the upstream side of the coating condition detection device  500 . 
     Note that, although not illustrated, a cooling device for forcibly cooling the bared fiber  110  is disposed between the optical fiber preform  100  and the resin coating device  300 . Although  FIG.  3    illustrates an example where the single-stage resin coating device  300  is provided, a plurality of stages of resin coating devices may be arranged in a longitudinal direction of the coated fiber  120  to be wound around the drum. Further, the resin used in the resin coating device  300  need not necessarily be the ultraviolet-curable resin. 
       FIG.  4    is a diagram illustrating an example applied to a resin coating device configured to further coat, with resin (for example, colored resin), the outer peripheral surface of the coated fiber obtained after the preform is drawn. The example illustrated in  FIG.  4    is a device that rewinds a coated fiber  120  from the drum (drum around which the coated fiber  120  manufactured by the optical fiber manufacturing device illustrated in  FIG.  3    is wound)  200  around a drum  210  while coloring the coated fiber. A resin coating device  400  coats, with the colored resin, the outer peripheral surface of the coated fiber  120  to be rewound around the drum  210  rotating in a direction indicated by an arrow S 3  from the drum  200  rotating in a direction indicated by an arrow S 2 . The coating condition detection device of the present disclosure is disposed on the downstream side of the resin coating device  400 . 
     The resin coating device  400  illustrated in  FIG.  4    is the same in structure as the resin coating device  300  illustrated in  FIG.  3   , and a colored coated fiber  130  is obtained as a result of coating the coated fiber  120  with the colored resin by the resin coating device  400 . That is, the resin coating device  400  includes a die  410  for coating the outer peripheral surface of the coated fiber  120  with an ultraviolet-curable resin (colored resin), a posture control device  420  for adjusting the posture of the die  410 , and an ultraviolet irradiation device  430 . Further, the coating condition detection device  500  illustrated in  FIG.  4    also detects a two-dimensional grayscale image showing the cross section of the colored coated fiber  130  using ultraviolet rays emitted from the ultraviolet irradiation device  430  of the resin coating device  400  located on the upstream side of the coating condition detection device  500 . 
       FIG.  5    is a diagram for describing a mechanism of allowing the coating condition detection device  500  of the present disclosure to work on the downstream side of the resin coating device  300  or on the downstream side of the resin coating device  400 , that is, a mechanism of how light is released from the coated fiber as illustrated in  FIGS.  3  and  4   . Although  FIG.  5    illustrates an internal structure of the resin coating device  300  of the optical fiber manufacturing device illustrated in  FIG.  3   , the resin coating device  400  illustrated in  FIG.  4    also allows the coated fiber to release ultraviolet rays by the same mechanism. 
     The bared fiber  110  obtained after the preform is drawn includes core  110   a  and cladding  110   b  provided on the outer peripheral surface of the core  110   a . When the bared fiber  110  moves in a direction indicated by an arrow S 4  (in  FIG.  3   , a direction from the optical fiber preform  100  toward the drum  200 ), the bared fiber  110  passes through the resin coating device  300 . First, the bared fiber  110  that has entered the resin coating device  300  passes through the die  310  into which the resin (ultraviolet-curable resin) is introduced so as to have its outer peripheral surface coated with resin  110   c . Subsequently, the bared fiber  110  having the resin  110   c  passes through the ultraviolet irradiation device  330 . The ultraviolet irradiation device  330  includes a housing  331  in which an ultraviolet light source  333  is disposed. The housing  331  has an input-side opening  332   a  and an output-side opening  332   b  provided for introducing the bared fiber  110  having the resin  110   c . The bared fiber  110  having the resin  110   c  is irradiated with ultraviolet rays UV outputted from the ultraviolet light source  333  while moving from the input-side opening  332   a  to the output-side opening  332   b . Note that the ultraviolet rays UV are partially reflected off the bared fiber  110  having the resin  110   c , but partially enter the bared fiber  110  and propagate in any direction (scattered light) in the bared fiber  110  having the resin  110   c . As described above, the coated fiber  120  is obtained as a result of irradiating the resin  110   c  with the ultraviolet rays UV. 
     The ultraviolet rays UV emitted in the housing  331  are confined within the coated fiber  120  (after the resin  110   c  is cured) coining out from the output-side opening  332   b  of the housing  331  in the direction indicated by the arrow S 4 . Therefore, the ultraviolet rays UV are released from the surface of the coated fiber  120  that has moved to the downstream side of the resin coating device  300 .  FIG.  5    illustrates a positional relation between a portion from which the ultraviolet rays UV are released, and the object plane OP and the reflection plane (the flat reflection mirror  20 , the ellipsoid mirror  50 , and off-axis parabolic mirror  60 ). 
     As described above, with a light source such as the ultraviolet rays UV having a wavelength that can pass through the coated fiber  120  already installed on the upstream side of the coating condition detection device  500  of the present disclosure, the coating condition detection device  500  can detect, via the reflection plane, light (the ultraviolet rays UV in the example illustrated in  FIG.  5   ) released from the portion of the coated fiber  120  that has passed through the reflection plane, the portion intersecting the object plane OP. 
       FIG.  6    is a diagram illustrating a specific example where a coating condition detector (the coating condition detection device  500  of the present disclosure) to which the imaging optical system  2 B illustrated in  FIG.  2    is applied is applied to the optical fiber manufacturing device illustrated in  FIG.  3   . Note that an imaging optical system other than the imaging optical system  2 B illustrated in  FIG.  2    may be applied. 
     As described with reference to  FIG.  5   , the configuration where the coating condition detection device  500  of the present disclosure is disposed on the downstream side of the resin coating device  300  (or the resin coating device  400 ) eliminates the need of preparing an external light source, but a configuration where the resin coating device  300  located on the upstream side and the coating condition detection device  500  located on the downstream side are at a distance from each other may be unable to detect a sufficient amount of released light. In such a case, as illustrated in  FIG.  6   , an external light source  350  that emits light having a wavelength that can pass through the coated fiber  120  may be provided near the guide member  40  having the off-axis parabolic mirror  60  provided on one side of the guide member  40 . 
     As illustrated in  FIG.  6   , the off-axis parabolic mirror  60  is provided on the one side of guide member  40 , and the guide member  40  is provided with the guide hole  40   a  that connects the output-side opening  61  provided in the off-axis parabolic mirror  60  and the input-side opening  41 . The coated fiber  120  having the resin  110   c  is wound around the drum  200  after passing through the guide hole  40   a  in a direction indicated by an arrow S 5 . The support member  42  fixes the position of the guide member  40  relative to the coated fiber  120  so as to cause the coated fiber  120  that has passed through the guide hole  40   a  to pass through the focal point of the off-axis parabolic mirror  60 . Note that the off-axis parabolic mirror  60  coincides with a part of the paraboloid of revolution  62 , so that the focal point of the off-axis parabolic mirror  60  is located on the object plane OP. Further, this focal point becomes the conjugate point C 1  conjugate with the intersection C 2  of the optical axis AX and the imaging plane IP (which coincides with a light receiving surface  610  of a light receiving device  600  on the optical axis AX). Here, “coincides with the light receiving surface” does not necessarily mean an exact coincidence, and a slight difference of about 0.1 μm is allowed. 
     Light released from a portion of the coated fiber  120  that has passed through the output-side opening  61  of the off-axis parabolic mirror  60  located near the focal point of the off-axis parabolic mirror  60  (the position where the object plane OP and the coated fiber  120  intersect each other) is partially collimated by and reflected off the off-axis parabolic mirror  60 . The collimated reflected light travels from the off-axis parabolic mirror  60  toward the collimator lens  12 , and is concentrated by the collimator lens  12  onto the conjugate point C 2  on the imaging plane IP. The coating condition detection device  500  includes a controller  700 , and the controller  700  controls a rendering unit  720  in order to output, onto a monitor, a two-dimensional image visually representing the coating condition of the resin  110   c  of the coated fiber  120  on the basis of intensity of light detected by the light receiving device  600  and information on a corresponding position (see  FIG.  7   ). 
     Specifically, as illustrated in  FIG.  7   , the rendering unit  720  generates, from the grayscale image showing the cross section of the coated fiber  120  that is a detection target, a monitor screen  810  representing two-dimensionally a light intensity distribution shown on each of two orthogonal axes Ix, Iy on the imaging plane IP, the orthogonal axes Ix, Iy being orthogonal to each other at the intersection of the axis on the imaging plane IP corresponding to the fiber axis and the imaging plane IP. The rendering unit  720  can also generate a monitor screen  820  representing two-dimensionally a light intensity distribution in a circumferential direction around the axis on the imaging plane IP corresponding to the fiber axis, and generates at least one of the monitor screens  810 ,  820 . 
     The controller  700  can perform various types of control in addition to rendering control (generation of the two-dimensional image visually representing the coating condition of the resin  110   c  of the coated fiber  120 ) on the rendering unit  720 . For example, performing numerical analyses on an image once captured by an image capturing device such as a camera allows the coating condition of the resin  110   c  to be quantitatively or dynamically determined. Note that examples of a detectable coating condition of the resin  110   c  include a degree of unevenness in thickness of the resin (resin layer)  110   c  (a degree of eccentricity of the bared fiber  110  in the coated fiber  120 ), a degree of mixture of air bubbles in the resin  110   c , a degree of separation along the interface between the bared fiber  110  and the resin  110   c , and the like. Further, giving detection data to a measuring instrument enables a process control. That is, it is possible to generate a control signal (control information)  710  used for controlling the operation of each unit of the manufacturing device or the like on the basis of the detection data thus given to change the resin coating condition. 
     Note that, in order to change the resin coating condition, for example, the controller  700  outputs the control signal  710  to the posture control device  320  (or the posture control device  420  illustrated in  FIG.  4   ) to change the posture of the die  310  illustrated in  FIG.  3    and the like. Specifically, the posture control performed by the controller  700  on the resin coating device  300  includes (1) moving the die  310  along a plane orthogonal to the travel direction of the coated fiber  120  (an x-y plane defined by the x axis and the y axis orthogonal to each other), (2) tilting the die  310  in a direction indicated by an arrow γ x  about the x axis, (3) tilting the die  310  in a direction indicated by an arrow γ y  about the y axis, and the like. Note that such a posture control is performed on the resin coating device  400  illustrated in  FIG.  4    in the same manner. 
     In addition to the posture control, the controller  700  can further regulate a flow rate of a flushing gas (for example, CO 2  gas) applied to an inlet for the bared fiber provided in the resin coating device  300  to prevent air bubbles from mixing into the resin  110   c . The controller  700  can further output the control signal  710  used for changing a temperature to the cooling device disposed on the upstream side of the resin coating device  300  to prevent air bubbles from mixing into the resin  110   c.    
     REFERENCE SIGNS LIST 
     
         
         
           
               1 A,  1 B,  1 C,  2 A,  2 B imaging optical system 
               10 ,  13 ,  14  imaging lens 
               11 ,  12  collimator lens 
               15  diaphragm 
               20  flat reflection mirror 
               21 ,  51 ,  61 ,  332   b  output-side opening 
               30 ,  40  guide member 
               30   a ,  40   a  guide hole 
               31 ,  41 ,  332   a  input-side opening 
               32 ,  42  support member 
               50  ellipsoid mirror 
               52  spheroid surface 
               60  off-axis parabolic mirror 
               62  paraboloid of revolution 
               100  optical fiber preform 
               100   a  core portion 
               100   b  cladding portion 
               110  bared fiber 
               110   a  core 
               110   b  cladding 
               110   c  resin 
               120  coated fiber 
               130  colored coated fiber 
               150  heater 
               200 ,  210  drum 
               300 ,  400  resin coating device 
               310 ,  410  die 
               320 ,  420  posture control device 
               330 ,  430  ultraviolet irradiation device 
               331  housing 
               333  ultraviolet light source 
               500  coating condition detection device 
               600  light receiving device 
               610  light receiving surface 
               700  controller 
               710  control signal 
               720  rendering unit 
               810 ,  820  monitor screen 
             OP object plane 
             IP imaging plane 
             AX optical axis 
             UV ultraviolet ray 
             C 1 , C 2 , C 3  conjugate point 
             S 1 , S 2 , S 3 , S 4 , S 5  arrow (direction)