Patent Publication Number: US-11022549-B2

Title: Optical fiber inspecting device, optical fiber manufacturing apparatus, method for inspecting optical fiber, and method for manufacturing optical fiber

Description:
TECHNICAL FIELD 
     The present invention relates to an optical fiber inspecting device, an optical fiber manufacturing apparatus, a method for inspecting an optical fiber, and a method for manufacturing an optical fiber. This application claims the benefit of priority from Japanese Patent Application No. 2016-059943, filed on Mar. 24, 2016; the entire contents of which are incorporated herein by reference. 
     BACKGROUND ART 
     Patent Literature 1 discloses a method and device for measuring the diameter and/or degree of eccentricity of the coating layer of a coated optical fiber. With the method and device, a coated optical fiber is irradiated with the luminous flux emitted from a light source along a direction generally perpendicular to the axial direction of the coated optical fiber, and the luminous flux is received by a photosensor that faces the light source and is disposed generally perpendicularly to the axial direction of the coated optical fiber. The diameter and/or degree of eccentricity of a primary coating layer of the coated optical fiber is determined by analyzing an image formed by the photosensor. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Publication No. 2001-215169 
     SUMMARY OF INVENTION 
     An optical fiber inspecting device of the present disclosure comprises: a light-emitting unit that irradiates an optical fiber with a light beam, the optical fiber including a glass fiber and a coating resin and moving in an axial direction; and a light-receiving unit that receives light scattered in the optical fiber, and converts the scattered light to an electrical signal. An optical axis of the light-receiving unit passes through an irradiation position where the light beam strikes the optical fiber, and the light beam and the optical axis of the light-receiving unit diagonally intersect each other, thereby preventing the light beam from directly entering the light-receiving unit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows the configuration of an optical fiber manufacturing apparatus according to one embodiment. 
         FIG. 2A  is a cross-sectional view showing the configuration of a typical optical fiber. 
         FIG. 2B  is a cross-sectional view showing the configuration of an optical fiber according to one embodiment. 
         FIG. 3  is a diagram showing a process for manufacturing a typical optical fiber. 
         FIG. 4  is a diagram showing a process for manufacturing an optical fiber according to one embodiment. 
         FIG. 5  shows the configuration of an air bubble sensor according to one embodiment. 
         FIG. 6A  is a diagram for explaining the acts of the air bubble sensor. 
         FIG. 6B  is a diagram for explaining the acts of the air bubble sensor. 
         FIG. 7A  is a diagram for explaining a problem that arises when a secondary resin layer is colored. 
         FIG. 7B  is a diagram for explaining a problem that arises when the secondary resin layer is colored. 
         FIG. 8  schematically shows the configuration of an air bubble sensor according to one modification and shows the air bubble sensor seen along the axial direction of the optical fiber. 
         FIG. 9  schematically shows the configuration of an air bubble sensor according to one modification and shows the air bubble sensor seen along a direction perpendicular to the axial direction of the optical fiber. 
         FIG. 10  is a diagram showing signal waveforms which result from air bubbles etc. and are generated in the respective light-receiving units by irradiation of the respective irradiation positions with light beams. 
         FIG. 11A  is a diagram showing the irradiation of the optical fiber with a light beam in the case where one pair of a light-emitting unit and a light-receiving unit is used. 
         FIG. 11B  is a diagram showing the irradiation of the optical fiber with a light beam in the case where two pairs of light-emitting units and light-receiving units are used. 
         FIG. 11C  is a diagram showing the irradiation of the optical fiber with a light beam in the case where three pairs of light-emitting units and light-receiving units are used. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Problems to be Solved by the Disclosure 
     To manufacture an optical fiber, a glass fiber containing a core and a cladding is first drawn from a glass preform, and a coating resin is applied to the outer surface of the glass fiber and is then cured. If air bubbles or voids (hereinafter referred to as air bubbles etc.) are formed in the glass fiber or coating resin in such a step, the optical-transmission properties of the optical fiber deteriorate. For this reason, whether air bubbles etc. are formed in the optical fiber is inspected in some cases. This inspection can be conducted in the middle of the production line of the optical fiber which moves in the drawing direction (axial direction). 
     In the device and method described in Patent Literature 1, a luminous flux is emitted along a direction perpendicular to the axial direction of the optical fiber, and the luminous flux that has passed through the optical fiber is detected. However, light scattering caused by air bubbles etc. is inconsiderable as compared with the light intensity of a luminous flux; thus, it is difficult to detect air bubbles etc. accurately with the device and method. 
     Advantageous Effects of Disclosure 
     According to this disclosure, the air bubbles etc. formed in a glass fiber or coating resin are accurately detectable. 
     Description of Embodiments of Invention 
     First, the details of embodiments of the present invention will be listed and described. An optical fiber inspecting device according to one embodiment of the present invention comprises: a first light-emitting unit that irradiates an optical fiber with a first light beam, the optical fiber including a glass fiber and a coating resin and moving in an axial direction; and a first light-receiving unit that receives scattered light resulting from the first light beam scattered in the optical fiber, and converts the scattered light to an electrical signal. An optical axis of the first light-receiving unit passes through an irradiation position where the first light beam strikes the optical fiber, and the first light beam and the optical axis of the first light-receiving unit diagonally intersect each other, thereby preventing the first light beam from directly entering the first light-receiving unit. 
     In this optical fiber inspecting device, the optical fiber is first irradiated with the first light beam. If no air bubbles etc. are formed in the glass fiber or coating resin of the optical fiber, the first light beam is not scattered and passes through the optical fiber. At the time, the first light beam and the optical axis of the first light-receiving unit diagonally intersect each other in the optical fiber, and therefore the first light beam is prevented from directly entering the first light-receiving unit, so that the first light-receiving unit barely detects light. In contrast, if there are air bubbles etc. in the glass fiber or the coating resin of the optical fiber, the first light beam is scattered and the scattered light enters the first light-receiving unit. Accordingly, in this optical fiber inspecting device, compared with, for example, the configuration shown in Patent Literature 1, the rate of change in the amount of light incident on the light-receiving unit increases when air bubbles etc. are formed in the glass fiber or coating resin, allowing the air bubbles etc. to be accurately detected. 
     The above-described optical fiber inspecting device further may comprise: a second light-emitting unit that irradiates the optical fiber with a second light beam; and a second light-receiving unit that receives scattered light resulting from the second light beam scattered in the optical fiber, and converts the scattered light to an electrical signal. An optical axis of the second light-receiving unit may pass through an irradiation position where the second light beam strikes the optical fiber, and the second light beam and the optical axis of the second light-receiving unit may diagonally intersect each other, thereby preventing the second light beam from directly entering the second light-receiving unit. A position irradiated with the first light beam from the first light-emitting unit and a position irradiated with the second light beam from the second light-emitting unit may be different each other along the axial direction of the optical fiber. Thus, the entry of scattered light resulting from any of the light beams traveling from the light-emitting units to a light-receiving unit not paired with the light-emitting unit (crosstalk) can be inhibited, and the signal waveform based on air bubbles etc. can be accurately generated in each light-receiving unit. 
     The above-described optical fiber inspecting device may further comprise: a third light-emitting unit that irradiates the optical fiber with a third light beam; and a third light-receiving unit that receives scattered light resulting from the third light beam scattered in the optical fiber, and converts the scattered light to an electrical signal. An optical axis of the third light-receiving unit may pass through an irradiation position where the third light beam strikes the optical fiber, and the third light beam and the optical axis of the third light-receiving unit may diagonally intersect each other, thereby preventing the third light beam from directly entering the third light-receiving unit. Output wavelengths of the first light beam and the third light beam may be different each other, and each of the first light-receiving unit and the third light-receiving unit may include a wavelength filter transmitting an output wavelength of the corresponding light-emitting unit and blocking an output wavelength of the other light-emitting unit. Thus, the detection of scattered light or stray light resulting from any of the light beams traveling from the light-emitting units in the light-receiving unit in a pair with a wavelength different from that of the pair that the corresponding light-emitting unit belongs to (crosstalk) can be inhibited, and the signal waveform based on air bubbles etc. can be accurately generated in each light-receiving unit. 
     In the above-described optical fiber inspecting device may further comprise: a fourth light-emitting unit that irradiates the optical fiber with a fourth light beam; and a fourth light-receiving unit that receives scattered light resulting from the fourth light beam scattered in the optical fiber, and converts the scattered light to an electrical signal. An optical axis of the fourth light-receiving unit may pass through an irradiation position where the fourth light beam strikes the optical fiber, and the fourth light beam and the optical axis of the fourth light-receiving unit may diagonally intersect each other, thereby preventing the fourth light beam from directly entering the fourth light-receiving unit. A position of the first light-emitting unit and a position of the fourth light-emitting unit with respect to the optical fiber along a circumferential direction may be different each other. Thus, reliable detection of air bubbles etc. can be achieved independently of the positions where the air bubbles etc. are formed in a face perpendicular to the axial direction of the optical fiber. This optical fiber inspecting device may comprise: a fifth light-emitting unit that irradiates the optical fiber with a fifth light beam; and a fifth light-receiving unit that receives scattered light resulting from the fifth light beam scattered in the optical fiber, and converts the scattered light to an electrical signal. An optical axis of the fifth light-receiving unit may pass through an irradiation position where the fifth light beam strikes the optical fiber, and the fifth light beam and the optical axis of the fifth light-receiving unit may diagonally intersect each other, thereby preventing the light beam from directly entering the fifth light-receiving unit. A position of the first light-emitting unit, a position of the fourth light-emitting unit, and a position of the fifth light-emitting unit with respect to the optical fiber along a circumferential direction may be different each other. In this case, reliable detection of air bubbles etc. can be achieved in a face perpendicular to the axial direction of the optical fiber. 
     The above-described optical fiber inspecting device may further comprise a computing unit that synthesizes signal waveforms of the electrical signals from the plurality of light-receiving units. If there are air bubbles etc. in the glass fiber or the coating resin, the amount of incident light due to the air bubbles etc. changes in the plurality of light-receiving units. Accordingly, the noise component due to stray light and the like is equalized by synthesizing the signal waveforms of the electrical signals from the plurality of light-receiving units. Accordingly, the S/N ratio can be improved and air bubbles etc. can be detected more accurately. 
     In the above-described optical fiber inspecting device, each light beam and the optical axis of the light-receiving unit corresponding to that light beam may intersect each other at an angle in a range of 131° to 135°. According to the knowledge of the inventors, since the light beam and the optical axis of the light-receiving unit form such an angle, the direct incidence of the light beam on the light-receiving unit can be avoided and the efficiency of incidence of scattered light through air bubbles etc. can be made approximate to the maximum value. 
     In the above-described optical fiber inspecting device, an output wavelength of light emitted from each light-emitting unit may correspond to non-visible light. In particular, an output wavelength of light emitted from each light-emitting unit may correspond to infrared light (e.g., a wavelength in a range of 1000 to 2000 nm) or ultraviolet light (e.g., a wavelength in a range of 250 to 400 nm). Thus, even when the coating resin contains a pigment, the S/N ratio can be increased and air bubbles etc. can be accurately detected. 
     An optical fiber manufacturing apparatus according to one embodiment of the present invention comprises: a drawing furnace that draws a glass fiber from a glass preform; a resin coating unit that coats the glass fiber with a primary resin and a secondary resin; a resin curing unit that cures the primary resin and the secondary resin; and the above-described optical fiber inspecting device (with the light-emitting units having an output wavelength of non-visible light) that inspects an optical fiber extending from the resin curing unit. In this optical fiber manufacturing apparatus, the air bubbles etc. formed in the glass fiber or coating resin can be accurately detected. 
     The present invention relates to a method for inspecting an optical fiber, as another embodiment. This optical fiber inspecting method comprises: a step of irradiating the optical fiber with the first light beam by using the first light-emitting unit of any of the above-described optical fiber inspecting devices, the optical fiber including a glass fiber and a coating resin and moving in an axial direction; a step of receiving scattered light resulting from the first light beam scattered in the optical fiber, and converting the received scattered light to an electrical signal, by using the first light-receiving unit of the optical fiber inspecting device; and a step of measuring presence of air bubbles or an internal existing rate of air bubbles in the optical fiber by comparing the electrical signal indicating the amount of incident light on the first light-receiving unit or a rate of change in the amount of the incident light with a predetermined threshold. The present invention further relates to a method for manufacturing an optical fiber, as another embodiment. This optical fiber manufacturing method comprises: a step of drawing a glass fiber from a glass preform; a step of coating the glass fiber with a primary resin and a secondary resin; a step of curing the primary resin and the secondary resin; and a step of inspecting an optical fiber where the resin is cured and the fiber is extended, by using any of the above-described optical fiber inspecting devices; and a step of winding up the optical fiber. According to these methods, air bubbles etc. formed in the glass fiber or coating resin can be accurately detected or an optical fiber can be manufactured with the accurate detection. 
     Details of Embodiments of Invention 
     Examples of an optical fiber inspecting device, an optical fiber manufacturing apparatus, a method for inspecting an optical fiber, and a method for manufacturing an optical fiber according to embodiments of the present invention will now be described with reference to the accompanying drawings. The scope of the invention should not be limited by these examples and should be defined by claims, and equivalents and all modifications of claims should be included in the scope of the invention. In the description below, when drawings are explained, the same components are denoted by the same reference numeral and overlapping description will be omitted. 
       FIG. 1  shows the configuration of an optical fiber manufacturing apparatus  1 A according to this embodiment. As shown in  FIG. 1 , the optical fiber manufacturing apparatus  1 A is a device for manufacturing an optical fiber F which includes a glass fiber F 11  containing a core and a cladding, and a coating resin. The optical fiber manufacturing apparatus  1 A includes, in sequence along the flow path of the glass fiber F 11  and the optical fiber F, a drawing furnace  11 , a forced cooling device  12 , an outer diameter measuring device  13 , a first resin coating unit  14 , a second resin coating unit  15 , an uneven thickness measuring device  16 , a UV furnace  17 , an outer diameter measuring device  18 , an air bubble sensor (optical fiber inspecting device)  19 , guide rollers  20 , a capstan  21 , and a wind-up bobbin  22 . 
     In the optical fiber manufacturing apparatus  1 A, the direction in which the optical fiber F travels in early stages is set to the vertical direction, and in the stages located downstream from the guide roller  20  below the air bubble sensor  19 , the direction in which the optical fiber F travels is set to the horizontal direction or a slanting direction. The drawing furnace  11  draws a preform(glass preform)  10  mainly composed of quartz glass, thereby forming the glass fiber F 11  containing a core and a cladding. The drawing furnace  11  includes a heater disposed on both sides of (or surrounding) the preform  10  set in the drawing furnace  11 . The preform  10  has an end that is heated with the heater, is fused, and is drawn to be the glass fiber F 11 . The drawn glass fiber F 11  moves along a predetermined travelling direction. 
     The forced cooling device  12  cools the drawn glass fiber F 11 . The forced cooling device  12  has a length long enough to adequately cool the glass fiber F 11 , along the predetermined travelling direction. The forced cooling device  12  has, for example, an intake port and an exhaust port, which are not shown in the drawing, for cooling the glass fiber F 11 , and cools the glass fiber F 11  by introducing cooling gas from the intake port and the exhaust port. 
     The outer diameter measuring device  13  measures the outer diameter of the glass fiber F 11  after cooling. For example, the outer diameter measuring device  13  irradiates the glass fiber F 11  with a luminous flux and picks up an image of the luminous flux that has passed through the glass fiber F 11 , thereby measuring the outer diameter of the glass fiber F 11 . 
     The resin coating units  14  and  15  coat the glass fiber F 11  with a resin. Two kinds of liquid resins which are curable with ultraviolet rays are held in the resin coating units  14  and  15 , and the glass fiber F 11  passes through the resin in the resin coating units  14  and  15 , thereby allowing an inner-layer resin (primary resin  14 A) and an outer-layer resin (secondary resin  15 A) to be coated on the surface of the glass fiber F 11  in this order. 
     The uneven thickness measuring device  16  measures the center deviation of the glass fiber F 11  with respect to the optical fiber F. For example, the uneven thickness measuring device  16  irradiates the optical fiber F with a luminous flux and picks up an image of the luminous flux that has passed through the optical fiber F, thereby measuring the center deviation. 
     The UV furnace  17  is a resin curing unit that irradiates the two kinds of resins (the primary resin and secondary resin) coated on the surface of the glass fiber F 11  with ultraviolet rays and thus cures them. The glass fiber F 11  with two kinds of resins on its surface passes through the UV furnace  17 , forming the optical fiber F which has the glass fiber F 11  and a coating layer consisting of two layers. 
     The outer diameter measuring device  18  measures the outer diameter of the optical fiber F which is prepared by coating the glass fiber F 11  with the resins. The outer diameter is measured by the same method as that for the outer diameter measuring device  13 . 
     The air bubble sensor  19  is an optical fiber inspecting device in this embodiment that inspects the optical fiber F extending from a UV furnace  17 D and detects air bubbles and voids (hereinafter referred to as air bubbles etc.) formed in the glass fiber F 11  or coating resin. As described below, the air bubble sensor  19  irradiates the optical fiber F with a light beam and detects the light scattered by the air bubbles etc., thereby detecting the existence of air bubbles etc. (e.g., the presence of air bubbles or the internal existing rate of air bubbles). 
     The guide roller  20  guides the optical fiber F so that the optical fiber F can move along a predetermined travelling direction. The travelling direction of the optical fiber F is changed by the guide rollers  20 , and the optical fiber F is received by the capstan  21  and is then sent to the wind-up bobbin  22 . The wind-up bobbin  22  winds up the completed optical fiber F. 
     In this embodiment, the secondary resin  15 A may be colored with an agent including a pigment or dye. In that case, unlike a typical optical fiber, the manufactured optical fiber F does not include a colored layer on a secondary resin layer.  FIG. 2A  is a cross-sectional view showing the configuration of an optical fiber FA including a colored layer  36 . As shown in  FIG. 2A , the typical optical fiber FA includes a glass fiber F 11  containing a core  31  and a cladding  32 , and a coating resin  33 A provided around the glass fiber F 11 . The coating resin  33 A consists of a primary resin layer  34 , a secondary resin layer  35 A, and the colored layer  36 . In contrast, as shown in  FIG. 2B , the optical fiber F in which the secondary resin layer is colored includes the glass fiber F 11  and a coating resin  33 B provided around the glass fiber F 11 . The coating resin  33 B consists of the primary resin layer  34  and a secondary resin layer  35 B which is colored. 
       FIG. 3  is a diagram showing a process for manufacturing the optical fiber FA including a colored layer. As shown in the drawing, a glass preform is drawn to form the glass fiber F 11  (Step S 11 ) which is then coated with the primary resin and the secondary resin (Step S 12 ) and then coated with an ink resin for coloring (Step S 13 ). Meanwhile,  FIG. 4  is a diagram showing a process for manufacturing the optical fiber F in which the secondary layer is colored. In the manufacturing process shown in the drawing, the coloring step S 13  is omitted unlike the process chart of  FIG. 3 . Thus, the number of manufacturing steps can be reduced by coloring the secondary resin layer to omit the colored layer. 
       FIG. 5  shows the configuration of the air bubble sensor  19  of this embodiment. As shown in  FIG. 5 , the air bubble sensor  19  includes a light-emitting unit  23 , a light-receiving unit  24 , an amplifier circuit  25 , and a signal-processing unit  26 . The light-emitting unit  23  irradiates the optical fiber F, which moves in the axial direction, with a light beam B. The output wavelength of the light-emitting unit  23  corresponds to non-visible light, more appropriately, infrared light (e.g., near-infrared light) or ultraviolet light. In one example, the output wavelength of light emitted from the light-emitting unit  23  may be in a range of 1.0 to 2.0 μm or in a range of 1.3 to 1.6 μm. When the output wavelength is in a range of 1.0 to 1.6 μm, the light-emitting unit  23  can be composed of, for example, a laser diode. When the output wavelength is around 2.0 μm, the light-emitting unit  23  can be composed of, for example, a thulium-added fiber laser. 
     The light-receiving unit  24  receives light L scattered in the optical fiber F and converts the scattered light L into an electrical signal. This scattering is caused by air bubbles etc. formed in the glass fiber F 11  or coating resin of the optical fiber F. The light-receiving unit  24  includes a photosensor  24   a , a lens  24   b , and a housing  24   c  for accommodating them. The photosensor  24   a  is disposed on the principal axis of the lens  24   b , i.e., the axis of rotation symmetry, and constitutes an optical axis AX of the light-receiving unit  24 . This optical axis AX passes an irradiation position P where the light beam B strikes the optical fiber F. The light L that has entered the light-receiving unit  24  is condensed by the lens  24   b  and condenses to the photosensor  24   a . The photosensor  24   a  is, for example, a photodiode composed of Si, Ge, or InGaAs, for example. A wave filter  27  may be installed between the lens  24   b  and the photosensor  24   a.    
     The light beam B diagonally intersects the optical axis AX of the light-receiving unit  24 . This prevents the light beam B from directly entering the light-receiving unit  24  (i.e., the light beam B from entering the lens  24   b ). In one example, an angle θ between the light beam B and the optical axis AX of the light-receiving unit  24  is 131° to 135°. In another example, the angle between the light beam B and the optical fiber F is 41° to 45°, and the optical fiber F and the optical axis AX are perpendicular to each other. The housing  24   c  of the light-receiving unit  24  may have a hood  24   d  for adequate avoidance of incidence of the light beam B or other stray light. The hood  24   d  is a hollow circular truncated cone which covers the periphery of the optical axis AX, and leads only the incident light from the opening formed at or around its top, to the lens  24   b . The apical angle of the hood  24   d  in a cross-section including the optical axis AX is determined according to the numerical aperture of the lens  24   b.    
     The advantageous effects obtained through the air bubble sensor  19  and the optical fiber manufacturing apparatus  1 A of this embodiment described above will be described with reference to  FIGS. 6A and 6B . In this air bubble sensor  19 , the optical fiber F is first irradiated with the light beam B. As shown in  FIG. 6A , if there are no air bubbles etc. in the glass fiber F 11  or coating resin  33 B of the optical fiber F, the light beam B passes through the optical fiber F without being scattered. At the time, as shown in  FIG. 5 , the light beam B and the optical axis AX of the light-receiving unit  24  diagonally intersect each other in the optical fiber F, and the light beam B is prevented from directly entering the light-receiving unit  24 , so that the light-receiving unit  24  barely detects light. In contrast, as shown in  FIG. 6B , if there are air bubbles etc. D in the glass fiber F 11  or the coating resin  33 B, the light beam B is scattered and the scattered light L enters the light-receiving unit  24 . Accordingly, in this air bubble sensor  19 , unlike the configuration described, for example, in Patent Literature 1, the rate of change in the amount of light that is incident on the light-receiving unit  24  when the air bubbles etc. D are formed in the glass fiber F 11  or the coating resin  33 B increases, so that the air bubbles etc. D can be accurately detected. To be specific, for example, comparison is performed between the amount of scattered light L incident on the light-receiving unit  24  or the rate of change in it (e.g., an electrical signal) and a predetermined threshold set up in advance, by using the optical fiber manufacturing apparatus  1 A (the air bubble sensor  19 ); thus, the presence of the air bubbles etc. D, the internal existing rate of the air bubbles etc. D, and the like can be accurately detected. Note that the internal existing rate of the air bubbles etc. D (the amount of air bubbles) can be measured based on the number of times of determination of air bubbles per unit time (e.g., the number of times when the threshold is exceeded). 
     As described above, the angle between the light beam B and the optical axis AX of the light-receiving unit  24  may be 131° to 135° in the optical fiber F. According to the knowledge of the inventors, since the light beam B and the optical axis AX of the light-receiving unit  24  form such an angle, the incidence of the light beam B on the light-receiving unit  24  is avoided and the efficiency of incidence of scattered light through air bubbles etc. can be made approximate to the maximum value, thereby improving the S/N ratio. 
     The problems that arise in the case where the secondary resin layer  35 B is colored will now be described with reference to  FIGS. 7A and 7B . In the case where the secondary resin layer  35 B is colored with a pigment, upon irradiation with the light beam B, the light beam B may strike a pigment C contained in the secondary resin layer  35 B and may be scattered as shown in  FIG. 7A . Scattered light L 2  then partially enters the light-receiving unit  24 . For this reason, as shown in  FIG. 7B , even in the case where the air bubbles etc. D are formed in the glass fiber F 11  or the coating resin  33 B, the scattered light L 2  from the secondary resin layer  35 B enters the light-receiving unit  24  concurrently with the scattered light L resulting from the air bubbles etc. D, so that the S/N ratio deteriorates, which may make it difficult to detect the air bubbles etc. D. 
     In the case where the secondary resin layer  35 B is colored with a dye, upon irradiation with the light beam B, the light beam B may strike and be absorbed in the dye. For this reason, the light beam B and the scattered light L attenuate in the secondary resin layer  35 B; thus, the detection of the air bubbles etc. D may be difficult even when the air bubbles etc. D are formed. 
     To solve these problems, it is preferable that, like in this embodiment, the output wavelength of the light-emitting unit  23  corresponds to non-visible light, particularly infrared light or ultraviolet light. This is because light of the wavelength of the infrared region easily passes through a resin and is hardly scattered as compared with visible light. Measurement of the transmission spectrum for each color of a resin layer (film) showed that the transmittance was mostly low in the range of wavelength of visible light. For example, it is shown that at the wavelength of 690 nm, the transmittance of a black resin layer is as small as about 80%, and the transmittance of the resin layer of other colors is about 40%, which is still smaller. In contrast, in the wavelength band (especially the wavelength band of 1000 nm or more) of non-visible light, especially infrared light, high transmittance was obtained in any color. This is based on the fact that the light scattering (Mie scattering or Rayleigh scattering) due to a pigment becomes inconsiderable as the wavelength increases from the pigment particle size. Accordingly, since the output wavelength of light emitted from the light-emitting unit  23  is non-visible light (especially infrared light), the scattered light from the colored secondary resin layer  35 B can be reduced and the air bubbles etc. D can be accurately detected. Moreover, in the case of light with a very short wavelength called ultraviolet light, the absorption by the dye is negligible; thus, the same advantageous effects as the above can be provided. 
     (Modification) 
       FIGS. 8 and 9  schematically show the configuration of an air bubble sensor  29  according to one modification of the above-described embodiment.  FIG. 8  shows the air bubble sensor  29  of this modification seen along the axial direction of the optical fiber F.  FIG. 9  shows the air bubble sensor  29  of this modification seen along a direction perpendicular to the axial direction of the optical fiber F. 
     As shown in  FIGS. 8 and 9 , the air bubble sensor  29  of this modification includes the light-emitting units  23  and the light-receiving units  24  in two or more pairs. As an example,  FIGS. 8 and 9  show the light-emitting units  23  and the light-receiving units  24  in three pairs. Further, as shown in  FIG. 9 , the air bubble sensor  29  may further include a computing unit  30  which synthesizes the waveforms of electrical signals from a plurality of light-receiving units  24 . 
     As shown in  FIG. 9 , in the air bubble sensor  29 , the three light-emitting units  23  respectively emit light beams B 1  to B 3  toward irradiation positions P 1  to P 3  arranged in the axial direction of the optical fiber F. In other words, the irradiation positions P 1  to P 3  of the optical fiber F related to the plurality of light-emitting units  23  are located in different positions along the axial direction of the optical fiber F. 
     As shown in  FIG. 8 , in the air bubble sensor  29 , the three light-emitting units  23  are located in different positions along the circumferential direction with respect to the optical fiber F. As an example,  FIG. 8  shows a mode in which the three light-emitting units  23  are arranged at equal intervals (120° intervals) in the circumferential direction of the optical fiber F. 
     In the air bubble sensor  29 , the output wavelengths of at least two light-emitting units  23  are different. Each light-receiving unit  24  has a wavelength filter for transmitting the output wavelength of the corresponding light-emitting unit  23  and blocking the output wavelength(s) of the other light-emitting unit(s)  23 . As an example, the output wavelengths of the three light-emitting units  23  are different each other. 
     The graphs of (a) portion, (b) portion and (c) portion of  FIG. 10  respectively show the signal waveforms which result from the air bubbles etc. and are generated in the respective light-receiving units  24  by irradiation of the respective irradiation positions P 1  to P 3  with the light beams B. The graph of (d) portion of  FIG. 10  shows their composite waveform. Since, among the irradiation positions P 1  to P 3 , the irradiation position P 1  is located most upstream, when air bubbles etc. exist in the optical fiber F, a signal waveform S 1  (intensity S) resulting from scattered light first appears in the electrical signal (the graph of (a) portion of  FIG. 10 ) from the light-receiving unit  24  located in the irradiation position P 1 . Afterwards, when the air bubbles etc. move to the irradiation position P 2 , a signal waveform S 2  (intensity S) resulting from scattered light appears in the electrical signal (the graph of (b) portion of  FIG. 10 ) from the light-receiving unit  24  located in the irradiation position P 2 . Finally, when the air bubbles etc. move to the irradiation position P 3 , a signal waveform S 3  (intensity S) resulting from scattered light appears in the electrical signal (the graph of (c) portion of  FIG. 10 ) from the light-receiving unit  24  located in the irradiation position P 3 . As described above, a signal-waveform delay based on a difference between the irradiation positions occurs in each electrical signal. In view of the above, in the case of the composite waveform shown in the graph of (d) portion of  FIG. 10 , signal waveforms are synthesized after such a delay is corrected. The graph of (d) portion of  FIG. 10  shows the results of synthesis made according to the time of the signal waveform S 3  shown in the graph of (c) portion of  FIG. 10 . 
     In correcting a delay through the computing unit  30  for synthesis of the signal waveforms, the delay time is inversely proportional to the flow rate of the optical fiber F. Accordingly, a signal related to a rate, for example, the voltage value proportional to the rotational speed of the capstan or the like can be input to the computing unit  30 , and the computing unit  30  can calculate the delay time with reference to that signal and synthesize 
     The advantageous effects obtained in this modification are as follows. The air bubble sensor  29  according to this modification includes the light-emitting units  23  and light-receiving units  24  in a plurality of pairs, and further includes the computing unit  30  for synthesis of the signal waveforms of the electrical signals from a plurality of the light-receiving units  24 . In such a configuration, if air bubbles etc. are formed in the optical fiber F, the amount of incident light resulting from the air bubbles etc. changes in the plurality of light-receiving units  24  (see the graphs of (a) portion, (b) portion, and (c) portion of  FIG. 10 ). Accordingly, the signal change resulting from the air bubbles etc. is increased by synthesizing the signal waveforms of the electrical signals from the plurality of light-receiving units  24  (see the graph of (d) portion of  FIG. 10 ) (intensity 3×S). Meanwhile, a noise component (intensity N) resulting from the stray light and the like contained in light incident on each light-receiving unit  24  is contained in the electrical signal. Such a noise component, which exists at random in time, is equalized by synthesis and exhibits the same intensity N after the synthesis. Accordingly, in this modification, the S/N ratio can be improved and air bubbles etc. can be detected more accurately. 
     In the air bubble sensor  29 , the positions P 1  to P 3  which are arranged along the axial direction of the optical fiber F and to be irradiated with the light beams B from the plurality of light-emitting units  23  may be different each other. Thus, the entry of scattered light resulting from any of the light beams B 1  to B 3  traveling from the light-emitting units  23  to the light-receiving unit  24  not paired with the light-emitting unit  23  (crosstalk) can be inhibited, and the signal waveform based on air bubbles etc. can be accurately generated in each light-receiving unit  24 . 
     In this air bubble sensor  29 , the output wavelengths of at least two light-emitting units  23  may be different each other, and each light-receiving unit  24  may have a wavelength filter for transmitting the output wavelength of the corresponding light-emitting unit  23  and blocking the output wavelength(s) of the other light-emitting unit(s)  23 . Thus, the entry of scattered light or stray light resulting from any of the light beams B 1  to B 3  traveling from the light-emitting units  23  to the light-receiving unit  24  in a pair with a wavelength different from that of the pair that the corresponding light-emitting unit  23  belongs to (crosstalk) can be inhibited, and the signal waveform based on air bubbles etc. can be accurately generated in each light-receiving unit  24 . 
     In the air bubble sensor  29 , the positions of the plurality of light-emitting units  23  in the circumferential direction and with respect to the optical fiber F may be different each other. When the light beams B are emitted from a single position along the circumferential direction, depending on the positions where air bubbles etc. are formed in a face perpendicular to the axial direction of the optical fiber F, the light beams B may not adequately strike the air bubbles etc. and detection of the air bubbles etc. may be difficult. In contrast, when the light beams B are emitted from a plurality of positions along the circumferential direction, reliable detection of air bubbles etc. can be achieved independently of the positions where the air bubbles etc. are formed in a face perpendicular to the axial direction of the optical fiber F. 
     It is preferable that the number of pairs of the light-emitting unit  23  and the light-receiving unit  24  be three or more. In this case, almost all the air bubbles can be detected over the perimeter of the optical fiber F.  FIG. 11A  is a diagram showing the irradiation of the optical fiber F with the light beam B in the case where one pair of the light-emitting unit  23  and the light-receiving unit  24  is used.  FIG. 11B  is a diagram showing the irradiation of the optical fiber F with the light beams B in the case where two pairs of the light-emitting unit  23  and the light-receiving unit  24  are used.  FIG. 11C  is a diagram showing the irradiation of the optical fiber F with the light beams B in the case where three pairs of the light-emitting unit  23  and the light-receiving unit  24  are used. As shown in  FIG. 11A , in the case where only one pair of the light-emitting unit  23  and the light-receiving unit  24  is used, regions D 1  where the optical fiber F is not irradiated with the light beam B are produced by refraction of the light beam B. As shown in  FIG. 11B , even in the case where two pairs of the light-emitting unit  23  and the light-receiving unit  24  are used, although being small, regions D 2  where the optical fiber F is not irradiated with a light beams B are produced. In contrast, as shown in  FIG. 11C , in the case where three or more pairs of the light-emitting unit  23  and the light-receiving unit  24  are used, no region is produced where the optical fiber F is not irradiated with the light beams B. Accordingly, when the number of pairs of the light-emitting unit  23  and the light-receiving unit  24  is three or more, air bubbles etc. can be detected more reliably. 
     The optical fiber inspecting device and the optical fiber manufacturing apparatus according to the present invention are not limited to the above-described embodiment, and various other modifications can be made. For instance, the above-described embodiment and modification may be combined with each other according to the intended use and effects. In addition, although an optical fiber in which the secondary resin layer is colored and which does not have a colored layer on the secondary resin layer is a target to be inspected in the above-described embodiment, the present invention is also applicable to an optical fiber having a colored layer on the secondary resin layer. 
     REFERENCE SIGNS LIST 
       1 A . . . optical fiber manufacturing apparatus,  10  . . . preform,  11  . . . drawing furnace,  12  . . . forced cooling device,  13  . . . outer diameter measuring device,  14 ,  15  . . . resin coating unit,  14 A . . . primary resin,  15 A . . . secondary resin,  16  . . . uneven thickness measuring device,  17  . . . UV furnace,  18  . . . outer diameter measuring device,  19 ,  29  . . . air bubble sensor(optical fiber inspecting device),  20  . . . guide roller,  21  . . . capstan,  22  . . . wind-up bobbin,  23  . . . light-emitting unit,  24  . . . light-receiving unit,  24   a  . . . photosensor,  24   b  . . . lens,  24   c  . . . housing,  24   d  . . . hood,  25  . . . amplifier circuit,  26  . . . signal-processing unit,  30  . . . computing unit,  31  . . . core,  32  . . . cladding,  33 A,  33 B . . . coating resin,  34  . . . primary resin layer,  35 A,  35 B . . . secondary resin layer,  36  . . . colored layer, AX . . . optical axis, B, B 1  to B 3  . . . light beam, D . . . air bubbles etc., F, FA . . . optical fiber, F 11  . . . glass fiber, L . . . scattered light, P, P 1  to P 3  . . . irradiation position.