Method for measuring twist of optical fiber and method for manufacturing optical fiber

Abstract

This method for measuring twist of an optical fiber is for measuring twist of an optical fiber during fiber drawing, and includes: a step for irradiating an optical fiber traveling between an optical fiber base material and a roller positioned directly below with light from an irradiation unit; a step for detecting the intensity distribution of light transmitted through the optical fiber; and a step for calculating twist of the optical fiber on the basis of the detected intensity distribution. The optical fiber has a cladding and a refractive index change part which is disposed in the cladding away from the central axis of the optical fiber and has a refractive index that is different from the refractive index of the cladding. In the irradiation step, the optical fiber is irradiated with light so that the refractive index change part is positioned in a fan-shaped region having a central angle of 90 degrees with the central axis as an apex and a straight line passing through the irradiation unit and the central axis as the central line in a cross section that is orthogonal to the central axis.

Description

Method for Measuring Twist of Optical Fiber and Method for Manufacturing Optical Fiber

[0001] The present disclosure relates to a method for measuring the twist of an optical fiber and a method for manufacturing an optical fiber. This application claims priority based on Japanese Application No. 2024-197662 filed on November 12, 2024, and incorporates all the descriptions described in the said Japanese application.

[0002] Patent Document 1 describes a method for measuring the twist of an optical fiber during wire drawing. In this method, the twist of the optical fiber is measured using a plurality of measuring devices spaced apart in the running direction of the optical fiber.

[0003] International Publication No. 2023 / 095916

[0004] The method for measuring the twist of an optical fiber according to the present disclosure is a method for measuring the twist of an optical fiber during wire drawing, including a step of irradiating light from an irradiation unit to the optical fiber running between the optical fiber base material and the bottom roller, a step of detecting the intensity distribution of the light transmitted through the optical fiber, and a step of calculating the twist of the optical fiber based on the detected intensity distribution. The optical fiber has a cladding and a refractive index change portion disposed in the cladding away from the central axis of the optical fiber and having a refractive index different from that of the cladding. In the irradiation step, in a cross section perpendicular to the central axis, light is irradiated so that the refractive index change portion is located within a sector region with a central angle of 90 degrees having the central axis as the vertex and the straight line passing through the irradiation unit and the central axis as the center line.

[0005] Figure 1 is a schematic diagram of a manufacturing apparatus used in the optical fiber manufacturing method according to the embodiment. Figure 2 is a flowchart of the optical fiber manufacturing method according to the embodiment. Figure 3 is a flowchart of the measurement process. Figure 4 is a diagram comparing a cross-sectional view of the optical fiber base material with a cross-sectional view of the optical fiber at the measurement position. Figure 5 is a diagram for explaining the irradiation process according to the embodiment. Figure 6 is a diagram for explaining the irradiation process according to a comparative example. Figure 7 is an explanatory diagram of the twisting process, a perspective view of the roller directly below. Figure 8 is an explanatory diagram of the twisting process, a view of the roller directly below from above. Figure 9 is an explanatory diagram of the twisting process according to the first modified example. Figure 10 is an explanatory diagram of the adjustment process according to the second modified example. Figure 11 is a diagram for explaining the irradiation process according to the third modified example. Figure 12 is a diagram for explaining the irradiation process according to the fourth modified example. Figure 13 is a diagram for explaining the irradiation process according to the fifth modified example. Figure 14 is a diagram for explaining the irradiation process according to the sixth modified example.

[0006] In the method described in Patent Document 1, it was sometimes not possible to measure the twist of the optical fiber during drawing with sufficient sensitivity.

[0007] The purpose of this disclosure is to provide a method for measuring the twist of an optical fiber and a method for manufacturing an optical fiber that can sensitively measure the twist of an optical fiber during drawing.

[0008] According to this disclosure, it is possible to provide a method for measuring the twist of an optical fiber and a method for manufacturing an optical fiber that can sensitively measure the twist of an optical fiber during drawing.

[0009] First, embodiments of the present disclosure will be listed and described. (1) A method for measuring the twist of an optical fiber according to one aspect of the present disclosure is a method for measuring the twist of an optical fiber during drawing, comprising the steps of: irradiating an optical fiber running between an optical fiber base material and a roller directly below it with light from an irradiation unit; detecting the intensity distribution of light transmitted through the optical fiber; and calculating the twist of the optical fiber based on the detected intensity distribution, wherein the optical fiber has a cladding and a refractive index changing unit that is located within the cladding away from the central axis of the optical fiber and has a refractive index different from that of the cladding, and in the irradiation step, light is irradiated such that the refractive index changing unit is located in a sector-shaped region with a central angle of 90 degrees, with the central axis as the vertex and a straight line passing through the irradiation unit and the central axis as the center line, in a cross section perpendicular to the central axis.

[0010] In the above method for measuring the twist of an optical fiber, light is irradiated so that the refractive index change portion of the optical fiber is located within the above-mentioned region. This increases the amount of change in the intensity distribution corresponding to the change in the rotation angle around the central axis of the optical fiber. As a result, the twist of the optical fiber can be measured with high sensitivity.

[0011] (2) In (1) above, the optical fiber is a polarization-maintaining optical fiber, and the refractive index changing section may be a stress-applying section.

[0012] (3) In (1) above, the optical fiber is a multicore optical fiber, and the refractive index changing portion may be the core.

[0013] (4) In (1) above, the optical fiber is a multicore optical fiber, and the refractive index changing part may be a marker.

[0014] (5) Any of the above steps (1) to (4) may include a step of adjusting the rotation angle around the central axis of the optical fiber base material so that the refractive index changing portion is located within the region. In this case, the refractive index changing portion can be easily adjusted to be located within the region by adjusting the position of the optical fiber base material.

[0015] (6) Any of (1) to (4) above may include a step of adjusting the arrangement of the measuring device, including the irradiation unit, around the central axis such that the refractive index changing part is located within the region in a cross section perpendicular to the central axis. In this case, the refractive index changing part can be easily adjusted to be located within the region by adjusting the arrangement of the measuring device.

[0016] (7) In (5) or (6) above, the adjustment step may be performed before or during the start of drawing. By adjusting before the start of drawing, the twist of the optical fiber can be measured with high sensitivity from the start of drawing. By adjusting during drawing, the twist that changes during drawing can be measured with high accuracy, and the twist can be adjusted with high accuracy.

[0017] (8) In (6) above, the irradiation unit is arranged on a rotating stage that rotates around a central axis, and the adjustment process may be performed by rotating the rotating stage. In this case, the arrangement of the irradiation unit can be easily adjusted.

[0018] (9) In any of (6) to (8) above, the measuring device is one of a plurality of measuring devices arranged along the direction of travel of the optical fiber, and the adjustment process may be performed with the orientations of the plurality of measuring devices aligned with each other. In this case, the difference in the rotation angles measured by the plurality of measuring devices becomes the amount of twist, and the twist of the optical fiber can be easily measured.

[0019] (10) A method for manufacturing an optical fiber according to one aspect of the present disclosure includes the steps of: melting an optical fiber base material by heating and drawing an optical fiber; measuring the twist of the optical fiber during drawing; and applying a twist to the optical fiber during drawing to adjust the measured twist, wherein the measuring step includes irradiating an optical fiber running between the optical fiber base material and a roller directly below it with light from an irradiation unit; detecting the intensity distribution of the light transmitted through the optical fiber; and calculating the twist of the optical fiber based on the detected intensity distribution, wherein the optical fiber has a cladding and a refractive index changing section located within the cladding away from the central axis of the optical fiber and having a refractive index different from that of the cladding, and in the irradiation step, light is irradiated such that the refractive index changing section is located in a sector-shaped region with a central angle of 90 degrees, with the central axis as the vertex and a straight line passing through the irradiation unit and the central axis as the center line, in a cross section perpendicular to the central axis.

[0020] In the above optical fiber manufacturing method, during the measurement process, light is irradiated so that the refractive index change portion of the optical fiber is located within the above-mentioned region, so the change in intensity distribution corresponding to the change in rotation angle around the central axis of the optical fiber becomes large. As a result, the twist of the optical fiber can be measured with high sensitivity. Since a twist that adjusts the measured twist is applied to the optical fiber during drawing, optical fibers with adjusted twist can be manufactured efficiently.

[0021] [Details of Embodiments of the Disclosure] Specific examples of the method for measuring the torsion of an optical fiber and the method for manufacturing an optical fiber according to the Disclosure will be described below with reference to the drawings. The Disclosure is not limited to these examples, and is intended to include all changes within the meaning and scope of the claims, as indicated by the claims. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant descriptions are omitted.

[0022] Figure 1 is a schematic diagram of a manufacturing apparatus used in the optical fiber manufacturing method according to the embodiment. As shown in Figure 1, the manufacturing apparatus 1 comprises a drawing furnace 2, a cooling device 3, a measuring device 4, a coating device 5, a curing device 6, a direct roller 7, a capstan 8, a winding drum 9, and a control unit 10.

[0023] The optical fiber 20 manufactured by the manufacturing apparatus 1 has, for example, a circular cross-section. The optical fiber 20 has a cladding 24 (see Figure 4) and a refractive index changing section. The refractive index changing section has a refractive index different from that of the cladding 24. The optical fiber 20 has a structure in which the refractive index changing section has a degree of freedom of position around the central axis C (see Figure 4) of the optical fiber 20. The refractive index changing section is located within the cladding 24, away from the central axis C. The cross-sectional shape of the optical fiber 20 is not circular; it may be non-circular.

[0024] In the following description, a polarization-maintaining optical fiber 20 will be described as having one core 22 (see Figure 4), a plurality of (two in this embodiment) stress-applying sections 23 (see Figure 4), and a cladding 24 (see Figure 4). The core 22 includes a central axis C. The core center coincides with the central axis C. The two stress-applying sections 23 face each other across the central axis C (see Figure 4). The stress-applying sections 23 and the cladding 24 have different refractive indices.

[0025] As described above, in this example, the refractive index changing section is the stress-applying section 23. The two stress-applying sections 23 are positioned point-symmetrically with respect to the central axis C. The cladding 24 surrounds the core 22 and the multiple stress-applying sections 23. In this embodiment, the optical fiber 20 includes a coating resin (not shown) that covers the outer surface of the cladding 24. The optical fiber before the coating resin is applied is a glass fiber 21.

[0026] The glass fiber 21 is obtained by drawing the optical fiber preform 31. The optical fiber preform 31 has a core portion 32 (see Figure 4) which becomes the core 22, a stress-applying portion 33 (see Figure 4) which becomes the stress-applying portion 23, and a cladding portion 34 (see Figure 4) which becomes the cladding 24. The core portion 32 has a central axis C0. The cross-sectional structure of the optical fiber preform 31 is similar to the cross-sectional structure of the glass fiber 21.

[0027] Figure 2 is a flowchart illustrating a method for manufacturing an optical fiber according to an embodiment. As shown in Figure 2, the method for manufacturing an optical fiber 20 according to an embodiment includes an adjustment step S0, a wire drawing step S1, a cooling step S2, a measurement step S3, a coating step S4, a curing step S5, a twisting step S6, a take-up step S7, and a winding step S8. Steps S0 to S8 are shown in order when focusing on a certain point in the longitudinal direction of the optical fiber 20. In other words, a certain point in the longitudinal direction of the optical fiber 20 is manufactured by sequentially going through steps S0 to S8. The method for manufacturing an optical fiber 20 according to an embodiment is a method for manufacturing an optical fiber 20 using an optical fiber twist measurement method that measures the twist of the optical fiber 20 online while it is being drawn. The optical fiber twist measurement method includes a measurement step S3.

[0028] The adjustment process S0 is a process of adjusting the rotation angle around the central axis C0 (see Figure 4) of the optical fiber base material 31. The adjustment process S0 will be described later.

[0029] The drawing process S1 is a process in which the optical fiber base material 31 is heated and melted in the drawing furnace 2 and the optical fiber 20 is drawn. From the lower end of the optical fiber base material 31 that has been melted in the drawing furnace 2, the glass fiber 21 is drawn vertically downward.

[0030] Cooling step S2 is a step in which the drawn glass fibers 21 are cooled. In cooling step S2, the glass fibers 21 are cooled by a cooling device 3. The cooling device 3 is located downstream of the drawing furnace 2 in the direction A in which the glass fibers 21 travel. The direction A coincides with the axial direction of the glass fibers 21.

[0031] Measurement step S3 is a step of measuring the twist of the optical fiber 20 while it is being drawn. In measurement step S3, the twist of the optical fiber 20 around the central axis C is measured online by the measuring device 4 and the control unit 10. The method for manufacturing the optical fiber 20 includes measurement step S3 in order to use the method for measuring the twist of the optical fiber 20.

[0032] The measuring device 4 includes an irradiation unit 11, a detection unit 12, polarizing plates 13 and 18, differential interference prisms 14 and 17, a capacitor 15, and an objective lens 16. The measuring device 4 is a torsion monitoring device for the optical fiber 20.

[0033] Figure 3 is a flowchart of the measurement process. Figure 3 is also a flowchart showing the method for measuring the twist of an optical fiber according to the embodiment. As shown in Figure 3, the measurement process S3 includes an irradiation process S11, a detection process S12, an acquisition process S13, and a calculation process S14. Processes S11 to S14 are performed in this order.

[0034] The irradiation step S11 is a step of irradiating light from the irradiation unit 11 onto the glass fiber 21 or optical fiber 20 running between the optical fiber base material 31 and the roller directly below 7. More specifically, the irradiation unit 11 irradiates light from the irradiation unit 11 onto the glass fiber 21 or optical fiber 20 at the measurement position P between the optical fiber base material 31 and the roller directly below 7. If the measurement position P is upstream of the coating device 5, the target of the light irradiation is the glass fiber 21. If the measurement position P is downstream of the coating device 5, the target of the light irradiation is the optical fiber 20. Hereinafter, "glass fiber 21 or optical fiber 20" will be referred to as "optical fiber 20, etc.". In this embodiment, the measurement position P is between the optical fiber base material 31 and the coating device 5. More specifically, the measurement position P is between the cooling device 3 and the coating device 5.

[0035] The distance d from the lower end of the optical fiber preform 31 to the measurement position P is, for example, 0.5 m or more. The lower end of the optical fiber preform 31 is the heating center position of the drawing furnace. Distance d is the vertical distance. If distance d is less than 0.5 m, when the rotation angle is adjusted at the optical fiber preform 31, the distance from the optical fiber preform 31 to the measurement position P is short, and the rotation angle of the twist is small, so the accuracy of twist detection may decrease. Distance d may be 1 m or more, or 2 m or more. The longer the distance d, the less the deviation in the orientation of the optical fiber preform 31 affects the measurement accuracy. It is preferable to set distance d to a length such that a twist of 90° or more does not occur in the optical fiber 20, etc. Distance d is, for example, 15 m or less.

[0036] The distance d is set, for example, by referring to the results of measuring the twist of the optical fiber 20 offline after manufacturing the optical fiber 20 in the manufacturing apparatus 1 beforehand. The optical fiber 20 is twisted when it comes into contact with the roller 7 directly below it, provided in the manufacturing apparatus 1. The manufacturing apparatus 1 has a substantially constant twisting tendency. The manufacturing apparatus 1 may be pre-adjusted to control (i.e., reduce) the twist. For example, the orientation of the roller 7 directly below it may be adjusted.

[0037] The irradiation unit 11 irradiates light onto the side surface of the optical fiber 20, etc., located at the measurement position P. The irradiation unit 11 irradiates light onto the side surface of the optical fiber 20, etc., in a direction perpendicular to the travel direction A.

[0038] In irradiation step S11, the light emitted from the irradiation unit 11 is polarized by the polarizing plate 13, and the polarized light is irradiated onto the optical fiber 20, etc. The polarizing plate 13 is provided between the irradiation unit 11 and the optical fiber 20, etc. The polarizing plate 13 is positioned on the optical path of the light emitted from the irradiation unit 11.

[0039] In irradiation step S11, the optical fiber 20 or the like may be irradiated with light that has passed through the differential interference prism 14. In irradiation step S11, the optical fiber 20 or the like may be irradiated with light that has passed through the capacitor 15. The polarizing plate 13, the differential interference prism 14, and the capacitor 15 can be arranged in this order.

[0040] The detection step S12 is a step in which the detection unit 12 detects the intensity distribution of light transmitted through the optical fiber 20, etc., at the measurement position P. Light irradiated onto the optical fiber 20, etc., is refracted at the refractive index change section. Therefore, the intensity distribution of the transmitted light reflects the position and shape of the refractive index change section. The internal structure of the optical fiber 20, etc., can be investigated based on the intensity distribution of the transmitted light. The irradiation unit 11 and the detection unit 12 are arranged on either side of the measurement position P.

[0041] The intensity distribution is the distribution of the light intensity (luminance) detected by the detection unit 12 along the depth direction D of the optical fiber 20 or the like. The depth direction D of the optical fiber 20 or the like is a direction orthogonal to the traveling direction A. The depth direction D is also a direction orthogonal to the direction in which light passes through the optical fiber 20 or the like at the measurement position P. As the detection unit 12, for example, a line sensor in which pixels for detecting light are arranged linearly or an area sensor in which pixels are arranged two-dimensionally is used. In the case of a line sensor, for example, by extracting the signals of one pixel row arranged in the depth direction D, the intensity distribution along the depth direction D can be obtained.

[0042] In the detection step S12, the light transmitted through the optical fiber 20 or the like is polarized by the polarizing plate 18. Therefore, the detection unit 12 detects the intensity distribution of the polarized light. The polarizing plate 18 is disposed between the optical fiber 20 or the like and the detection unit 12. The polarizing plate 18 is disposed on the optical path of the transmitted light incident on the detection unit 12.

[0043] In the detection step S12, for example, the light transmitted through the optical fiber 20 or the like is expanded in the depth direction D by the objective lens 16. Therefore, the detection unit 12 detects the intensity distribution of the light expanded in the depth direction D. The objective lens 16 is, for example, disposed between the optical fiber 20 or the like and the detection unit 12. The objective lens 16 is disposed on the optical path of the transmitted light incident on the detection unit 12.

[0044] In the detection step S12, the light transmitted through the optical fiber 20 or the like at the measurement position P is transmitted through the objective lens 16, the differential interference prism 17, and the polarizing plate 18. The polarized light separated by the differential interference prism 14 in the irradiation step S11 is synthesized by the differential interference prism 17 in the same optical path in the detection step S12. There is an optical path difference in the light separated when passing through the optical fiber 20 or the like. Therefore, differential interference occurs in the synthesized light. The detection unit 12 detects the intensity distribution of the differentially interfered light.

[0045] The acquisition step S13 is a step in which the rotation angle θ (see Figure 4) of the optical fiber 20, etc., around the central axis C at the measurement position P is acquired based on the intensity distribution detected in the detection step S12. The rotation angle θ indicates the orientation or azimuth angle of the optical fiber 20, etc., in a cross section perpendicular to the central axis C of the optical fiber 20, etc. The rotation angle θ is defined, for example, as the angle between a straight line L1 passing through the central axis C of the optical fiber 20, etc., and the centroid of the stress application unit 23, and a straight line L2 parallel to the optical path of the light irradiated from the irradiation unit 11, in a cross section perpendicular to the central axis C of the optical fiber 20, etc. The straight line L2 is, for example, a straight line passing through the irradiation unit 11 and the central axis C. The rotation angle θ of the optical fiber 20, etc., can be defined in any way as long as the orientation or azimuth angle of the optical fiber 20, etc., can be determined. The acquisition step S13 is performed, for example, by the control unit 10.

[0046] The control unit 10 may be formed as a computer system including, for example, a processor such as a CPU (Central Processing Unit), memory such as RAM (Random Access Memory) and ROM (Read Only Memory), input / output devices such as a touch panel, mouse, keyboard, and display, and communication devices such as a network card. The control unit 10 functions by operating each piece of hardware under the control of the processor based on a computer program stored in memory.

[0047] Calculation step S14 is a step in which the twist of the optical fiber 20, etc. is calculated based on the rotation angle θ0 (not shown) of the optical fiber base material 31 around the central axis C0 and the rotation angle θ of the optical fiber 20, etc. obtained in acquisition step S13. The rotation angle θ0 is the orientation or azimuth angle of the optical fiber base material 31 in a cross section perpendicular to the central axis C0 of the optical fiber base material 31. The rotation angle θ0 is defined, for example, as the angle between a straight line L3 corresponding to a straight line L1 and a straight line L4 corresponding to a straight line L2. Straight line L3 is a straight line that passes through the central axis C0 of the optical fiber base material 31 and the centroid of the stress application part 33 in a cross section perpendicular to the central axis C0 of the optical fiber base material 31. Straight line L4 is a straight line parallel to straight line L2.

[0048] In the calculation process S14, for example, the difference θ - θ0 between the rotation angle θ and the rotation angle θ0 is calculated as the twist of the optical fiber 20 or the like. The optical fiber base material 31 is arranged such that, for example, the rotation angle θ0 becomes 0 deg. Thereby, the rotation angle θ can be regarded as the twist of the optical fiber 20 or the like as it is. Based on the distance d and the difference θ - θ0, the twist per unit length of the optical fiber 20 or the like can be grasped. The unit of the twist per unit length is, for example, deg / mm.

[0049] FIG. 4 is a diagram showing a comparison between a cross-sectional view of the optical fiber base material and a cross-sectional view of the optical fiber at the measurement position. In FIG. 4, the optical fiber base material 31 and the optical fiber 20 or the like are shown reduced or enlarged so that their diameters are the same length. When there is no twist in the optical fiber 20 or the like, the position of the stress application portion 23 of the optical fiber 20 or the like at the measurement position P (see FIG. 1) does not change from the position of the stress application portion 33 of the optical fiber base material 31. When there is twist in the optical fiber 20 or the like, the position of the stress application portion 23 of the optical fiber 20 or the like at the measurement position P changes from the position of the stress application portion 33 of the optical fiber base material 31.

[0050] FIG. 5 is a diagram for explaining the irradiation process according to the embodiment. (a) of FIG. 5 is a diagram showing a cross-section orthogonal to the central axis C at the measurement position P. (b) of FIG. 5 is the intensity distribution of the transmitted light detected by the detection unit 12. In the example of FIG. 5, as shown in the cross-section of (a) of FIG. 5, the stress application portion 23 is located in two regions X of the optical fiber 20 or the like. The region X is a sector region with a central angle of 90 degrees having the central axis C as the vertex and the straight line passing through the irradiation unit 11 and the central axis C as the center line. The optical axis B is a straight line passing through the irradiation unit 11 and the central axis C.

[0051] When there are a plurality of stress application portions 23, for example, 1 / 5 or more of the stress application portions 23 are located within the region X. That is, the fact that the stress application portion 23 is located within the region X means that the rotation angle θ defined by the angle formed by the straight line L1 and the straight line L2 (corresponding to the optical axis B) described in FIG. 4 is -45 degrees or more and +45 degrees or less, and 1 / 5 or more of the stress application portions are located within this range. All of the stress application portions 23 may be located within the region X.

[0052] Region X is defined by a straight line passing through the central axis C and making a +45-degree angle (45 degrees clockwise in Figure 5) with respect to the optical axis B, a straight line passing through the central axis C and making a -45-degree angle (45 degrees counterclockwise in Figure 5) with respect to the optical axis B, and the outer edge of the optical fiber 20 or the like connecting these two straight lines. The stress-applying section 23 only needs to be located within at least one of the regions X, and may be located within both regions X.

[0053] Figure 6 is a diagram illustrating the irradiation process according to a comparative example. Figure 6(a) is a cross-section perpendicular to the central axis C at the measurement position P. Figure 6(b) is the intensity distribution of the transmitted light detected by the detection unit 12. In the example of Figure 6, as shown in the cross-section of Figure 6(a), the stress application unit 23 is located outside region X of the optical fiber 20, etc.

[0054] As can be seen by comparing Figures 5 and 6, even if the amount of change in the rotation angle around the central axis C of the optical fiber 20 is the same, the amount of change in the intensity distribution is greater when the stress application unit 23 is located within region X than when the stress application unit 23 is located outside region X. In the irradiation step S11 according to this embodiment, light is irradiated so that the stress application unit 23 is located within region X in a cross section perpendicular to the central axis C, so that the twist of the optical fiber 20 during drawing can be measured with high sensitivity.

[0055] The adjustment step S0 described above is a step of adjusting the rotation angle of the optical fiber base material 31 around the central axis C0 so that the stress application part 23 is located within the region X in a cross section perpendicular to the central axis C. The adjustment step S0 is performed before the start of line drawing. The adjustment step S0 is performed, for example, by manufacturing the optical fiber 20 in the manufacturing apparatus 1 in advance and referring to the results of measuring the twist of the optical fiber 20 offline. The adjustment step S0 may also be performed while checking the intensity distribution with the detection unit 12 during line drawing.

[0056] The coating step S4 is a step in which a coating resin is applied to the surface of the glass fiber 21 using a coating device 5. The coating device 5 is located downstream of the cooling device 3 in the travel direction A. The coating device 5 is a metal jig having a hole in its center through which the glass fiber 21 passes. The coating device 5 holds a liquid coating resin and applies the coating resin to the surface of the glass fiber 21 passing through the hole. The coating device 5 is a die. In this embodiment, the coating resin is an ultraviolet-curing resin.

[0057] The curing step S5 is a step in which the coating resin on the surface of the glass fiber 21 is cured by the curing device 6. The curing device 6 is located downstream of the coating device 5 in the travel direction A. The curing device 6 cures the coating resin applied to the surface of the glass fiber 21. This forms an optical fiber 20 with the coating resin. In this embodiment, the curing device 6 is an ultraviolet irradiation device, and the coating resin is cured by irradiation with ultraviolet light.

[0058] The twisting step S6 is a step in which a twist is applied to the optical fiber 20 during drawing, for example, by the direct-below roller 7, to adjust the twist measured in the measurement step S3. The direct-below roller 7 is a guide roller positioned directly below the optical fiber base material 31 in the vertical direction. The direct-below roller 7 is provided downstream of the curing device 6 in the travel direction A. The direct-below roller 7 is positioned between the coating device 5 and the capstan 8 in the travel direction A. In this embodiment, the direct-below roller 7 is positioned between the curing device 6 and the capstan 8 in the travel direction A. The direct-below roller 7 contacts the optical fiber 20 and changes the travel direction A of the optical fiber 20 from the vertical direction to a direction different from the vertical direction. The twisting step S6 is a step in which a twist is applied to the optical fiber 20 during drawing, for example, to counteract the twist measured in the measurement step S3.

[0059] The control unit 10 performs feedback control on the roller directly below 7 based on the calculation results. Specifically, the control unit 10 transmits a control signal corresponding to the calculation results to the roller directly below 7. The feedback control will be described later.

[0060] Figures 7 and 8 are explanatory diagrams of the torsion imparting process. Figure 7 shows a perspective view of the lower roller 7. Figure 8 shows a view of the lower roller 7 from the upstream side (curing apparatus 6 side) in the travel direction A. As shown in Figures 7 and 8, the lower roller 7 is formed to be movable in the axial direction AX of the lower roller 7. The lower roller 7 is positioned so that the axial direction AX is perpendicular to the vertical direction. The lower roller 7 may be formed so that the angle of the rotation axis of the lower roller 7 with respect to the travel direction A of the optical fiber 20 (for example, the angle of the axial direction AX with respect to the depth direction D when viewed from the vertical direction) can be changed.

[0061] In the diagram below the arrow in Figure 8, the optical fiber 20 contacts the lower (upstream) side of the roller 7, is wound around the roller 7, and is positioned in the groove on the upper (downstream) side of the roller 7. In this way, if the position where the optical fiber 20 contacts the roller 7 on the lower (upstream) side is not in the groove, the optical fiber 20 can change its direction of travel so that it fits into the groove while still in contact with the roller 7. At this time, a rotational force R is applied to the optical fiber 20, causing it to twist.

[0062] The twist of the optical fiber 20 is transmitted to the molten portion of the optical fiber base material 31. This imparts a twist to the glass fiber 21 drawn from the molten portion. Unlike twists imparted after cooling and solidification, twists imparted before cooling and solidification do not return to their original state after manufacturing and become residual twists. To adjust the residual twist, it is necessary to measure the twist during drawing and adjust the amount of twist imparted. For example, the twist of the optical fiber 20 can be adjusted by changing the position of the optical fiber 20 entering the roller 7 directly below it.

[0063] The lower roller 7 may move in the axial direction AX and in the direction AY perpendicular to the axial direction AX. Alternatively, the lower roller 7 may change in the direction perpendicular to the axial direction AX and the direction AY. By adjusting the angle of the rotation axis of the lower roller 7 with respect to the running direction A of the optical fiber 20 (for example, the angle of the axial direction AX with respect to the depth direction D when viewed from the vertical direction), the position where the optical fiber 20 first contacts the lower roller 7 (the position on the lower roller 7) may be changed, thereby reducing twisting of the optical fiber 20.

[0064] The direction and magnitude of the rotational force R applied to the optical fiber 20 by the lower roller 7 can be adjusted, for example, by the axial position AX of the lower roller 7. For example, as shown in Figure 8, in the lower diagram, the position where the optical fiber 20 first contacts the lower roller 7 (upstream position) is not a groove, and the optical fiber 20 rotates and twists on the lower roller 7 as it is guided into the groove of the lower roller 7. As shown in the upper diagram of Figure 8, if the lower roller 7 is moved so that the position where the optical fiber 20 first contacts the lower roller 7 (upstream position) is a groove, then the position where the optical fiber 20 contacts the downstream side of the lower roller 7 is also a groove, and therefore the optical fiber 20 does not twist.

[0065] The control unit 10 transmits a control signal based on the calculation result to the roller directly below 7, and adjusts the axial position AX of the roller directly below 7 to adjust the twist of the optical fiber 20. In other words, feedback control of the twist to the roller directly below 7 is performed.

[0066] The take-up process S7 is the process of taking up the optical fiber 20 with the capstan 8. The capstan 8 is located downstream of the lower roller 7 in the travel direction A.

[0067] The winding process S8 is a process in which the optical fiber 20 is wound up by the winding drum 9. The winding drum 9 is located downstream of the capstan 8 in the travel direction A.

[0068] As described above, in the torsion measurement method according to the embodiment, in measurement step S3, light is irradiated so that the stress-applying part 23, such as the optical fiber 20, is located within region X. Therefore, the amount of change in the intensity distribution corresponding to the amount of change in the rotation angle around the central axis C of the optical fiber 20 becomes large. As a result, the torsion of the optical fiber 20 can be measured with high sensitivity.

[0069] In this twist measurement method, the twist of the optical fiber 20 is calculated using the rotation angle θ of the optical fiber base material 31, in addition to the rotation angle θ of the optical fiber base material 31 obtained by measurement at the measurement position P. Compared to methods that do not use the rotation angle θ0 of the optical fiber base material 31, the number of required measuring devices 4 can be reduced. Therefore, the twist of the optical fiber 20 during drawing can be measured efficiently.

[0070] In the method for manufacturing the optical fiber 20 according to this embodiment, the twist of the optical fiber 20 can be measured with high sensitivity in the measurement step S3, and a twist is applied to the optical fiber 20 during drawing to adjust the measured twist, so that an optical fiber 20 with adjusted twist can be manufactured efficiently.

[0071] In optical fiber 20, twisting is reduced, thus reducing connection loss. For example, in multicore optical fiber, even if the core axis is aligned at one end in the longitudinal direction, the core axis may be misaligned at the other end. This also solves the problem of the core axis being misaligned due to changes in the longitudinal position of the optical fiber 20 caused by polishing of the fiber end face. Similarly, in polarization-maintaining optical fiber, this solves the problem of the polarization plane at the input end and the polarization plane at the output end being misaligned.

[0072] While embodiments and modifications have been described above, this disclosure is not necessarily limited to the embodiments and modifications described above, and various modifications are possible without departing from its essence.

[0073] Figure 9 is an explanatory diagram of the torsion-imparting process according to the first modified example. In the torsion-imparting process S6 according to the first modified example, the optical fiber 20 is twisted by adjusting the angle of a pair of guide rollers 30, rather than the direct-down roller 7 (see Figure 1). The pair of guide rollers 30 are positioned between the coating device 5 and the direct-down roller 7 in the running direction A. In this embodiment, the pair of guide rollers 30 are positioned between the curing device 6 and the direct-down roller 7 in the running direction A, but they may also be positioned between the direct-down roller 7 and the capstan 8.

[0074] The pair of guide rollers 30 face each other with the optical fiber 20 in between. The pair of guide rollers 30 are formed to be able to tilt in opposite directions with respect to the travel direction A. When the pair of guide rollers 30 tilt, a rotational force R about the central axis C is applied to the optical fiber 20. As a result, twisting is applied to the optical fiber 20, etc.

[0075] The direction and magnitude of the rotational force R applied to the optical fiber 20 by the pair of guide rollers 30 can be adjusted by the angle of the guide rollers 30, that is, the angle between the plane perpendicular to the axial direction of the guide rollers 30 and the travel direction A. Therefore, the control unit 10 transmits a control signal based on the calculation result to the pair of guide rollers 30 and performs feedback control to the pair of guide rollers 30 by adjusting the angle of the pair of guide rollers 30.

[0076] Figure 10 is an explanatory diagram of the adjustment process according to the second modified example. The adjustment process S0 according to the second modified example is a process of adjusting the arrangement of the measuring device 4 around the central axis C so that the stress application unit 23 is located within region X in a cross section perpendicular to the central axis C. The adjustment process S0 according to the second modified example may also be performed before the start of line drawing or during line drawing. As shown in Figure 10, the manufacturing apparatus 1 is equipped with a rotating stage 40 that rotates around the central axis C. The measuring device 4 is arranged on the rotating stage 40. The adjustment process S0 is performed by rotating the rotating stage 40. In Figure 10, the components of the measuring device 4 other than the irradiation unit 11 and the detection unit 12 are omitted. The optical fiber 20, etc., during line drawing is traveling approximately at the center of the rotating stage 40.

[0077] The manufacturing apparatus 1 may include a plurality of measuring devices 4 arranged spaced apart from each other along the traveling direction A of the optical fiber 20, etc. In other words, the measuring device 4 described above may be one of a plurality of measuring devices 4 arranged along the traveling direction A. In this case, the plurality of measuring devices 4 may be arranged so as to irradiate light from the same direction to a plurality of measurement positions P that are spaced apart from each other in the traveling direction A. That is, the plurality of measuring devices 4 may be arranged so that the optical paths (optical axes B) of the light irradiated from a plurality of irradiation units 11 overlap or completely coincide with each other when viewed from the traveling direction A. By using a plurality of measuring devices 4, the accuracy of torsion measurement is improved. In particular, when measuring torsion between two points, such as the optical fiber base material 31 and a measurement position P, if, for example, another measurement position P is provided between those two points, the torsion can be grasped more accurately even if a twist of one or more turns occurs between those two points.

[0078] If the manufacturing apparatus 1 is equipped with such multiple measuring devices 4, the twist of the optical fiber 20 may be measured based on the intensity distribution detected by the multiple measuring devices 4, without using the rotation angle θ0 around the central axis C0 of the optical fiber preform 31, as in the invention described in Patent Document 1. The adjustment step S0 may be performed with the orientations of the multiple measuring devices 4 aligned with each other. As an example, multiple measuring devices 4 may be placed on a single rotating stage 40. In this case, multiple measuring devices 4 are stacked with their orientations aligned with each other. Since the difference in rotation angles measured by the multiple measuring devices 4 becomes the amount of twist, the twist of the optical fiber can be easily measured. As another example, the manufacturing apparatus 1 may be equipped with multiple rotating stages 40, with one measuring device 4 placed on each rotating stage 40. In this case, the positions of the multiple rotating stages 40 may be adjusted to align the orientations of the multiple measuring devices 4.

[0079] The orientations of the multiple measuring devices 4 may differ from one another. However, in this case, it is necessary to correct the relative angles of the multiple measuring devices 4 in order to calculate the amount of torsion. The relative positions of the multiple rotating stages 40 may not be fixed, and the arrangement of the multiple measuring devices 4 may be adjusted individually.

[0080] The optical fiber 20 may be a multicore optical fiber having a plurality of cores 22. Figure 11 is a diagram illustrating the irradiation process according to a third modified example. As shown in Figure 11, in the third modified example, the optical fiber 20 etc. includes four cores 22 and a cladding 24 surrounding the cores 22. All four cores 22 are away from the central axis C. In this example, the refractive index change portion is the core 22. In the irradiation process S11, light is irradiated such that at least one core 22 is located within region X in a cross section perpendicular to the central axis C.

[0081] Figure 12 is a diagram illustrating the irradiation process according to the fourth modified example. As shown in Figure 12, in the fourth modified example, the optical fiber 20, etc., includes seven cores 22 and a cladding 24 surrounding the cores 22. Six of the cores 22 are located away from the central axis C. One core 22 is positioned so that its core center coincides with the central axis C. In this example, the refractive index changing portion consists of the six cores 22 located away from the central axis C. In the irradiation process S11, light is irradiated such that at least one of the six cores 22 located away from the central axis C is located within region X in a cross section perpendicular to the central axis C.

[0082] Figure 13 is a diagram illustrating the irradiation process according to the fifth modified example. As shown in Figure 13, in the fifth modified example, the optical fiber 20, etc., includes eight cores 22 and a cladding 24 surrounding the cores 22. All eight cores 22 are located away from the central axis C. In this example, the refractive index change portion is the core 22. In the irradiation process S11, light is irradiated such that at least one core 22 is located within a region X in a cross section perpendicular to the central axis C. In this example, multiple cores 22 are arranged within one region X.

[0083] Figure 14 is a diagram illustrating the irradiation process according to the sixth modified example. As shown in Figure 14, in the sixth modified example, the optical fiber 20, etc., includes four cores 22, one marker 25, and a cladding 24 surrounding the cores 22 and the marker 25. The marker 25 is located away from the central axis C. The marker 25 is positioned to disrupt the symmetry (line symmetry, rotational symmetry, etc.) of the multiple cores 22. In this example, the refractive index change portion is the marker 25 and the cores 22. In the irradiation process S11, light is irradiated such that the marker 25 and the cores 22 are located within region X in a cross section perpendicular to the central axis C.

[0084] The coating resin of the optical fiber 20 may consist of two or more layers. In this case, for example, the manufacturing apparatus 1 may include two or more sets of coating apparatuses 5 and curing apparatuses 6, and the coating and curing steps may be repeated two or more times in the manufacturing method of the optical fiber 20.

[0085] The above embodiments and modifications may be combined as appropriate. It should be understood that at least one configuration or feature described in each embodiment and example can be combined with other embodiments and examples, or modified in various ways.

[0086] 1...Manufacturing equipment 2...Drawing furnace 3...Cooling equipment 4...Measuring equipment 5...Coating equipment 6...Curing equipment 7...Bottom roller 8...Capstan 9...Winding drum 10...Control unit 11...Irradiation unit 12...Detection unit 13...Polarizing plate 14...Differential interference prism 15...Capacitor 16...Objective lens 17...Differential interference prism 18...Polarizing plate 20...Optical fiber 21...Glass fiber 22...Core 23...Stress application unit 24...Cladding 25...Marker 30...Guide roller 31...Optical fiber base material 32...Core unit 33...Stress application unit 34...Cladding unit 40...Rotating stage A...Travel direction AX...Axial direction AY...Direction perpendicular to the axial direction B...Optical axis C...Central axis C0...Central axis d...Distance D...Depth direction L1...Straight line L2...Straight line L3...Straight line L4...Straight line P...Measurement position R...Rotational force S0...Adjustment process S1...Wire drawing process S2...Cooling process S3...Measurement process S4...Coating process S5...Curing process S6...Twisting process S7...Take-up process S8...Winding process S11...Irradiation process S12...Detection process S13...Acquisition process S14...Calculation process X...Area θ...Rotation angle

Claims

1. A method for measuring the twist of an optical fiber during drawing, comprising: irradiating an optical fiber running between an optical fiber base material and a roller directly below it with light from an irradiation unit; detecting the intensity distribution of light transmitted through the optical fiber; and calculating the twist of the optical fiber based on the detected intensity distribution, wherein the optical fiber comprises a cladding and a refractive index changing section located within the cladding away from the central axis of the optical fiber and having a refractive index different from that of the cladding, and in the irradiation step, light is irradiated such that the refractive index changing section is located within a sector-shaped region with a central angle of 90 degrees, with the central axis as its vertex and a straight line passing through the irradiation unit and the central axis as its centerline, in a cross section perpendicular to the central axis.

2. The optical fiber is a polarization-maintaining optical fiber, and the refractive index changing portion is a stress-applying portion, the method for measuring the twist of an optical fiber according to claim 1.

3. The optical fiber is a multicore optical fiber, and the refractive index changing portion is a core, the method for measuring the twist of an optical fiber according to claim 1.

4. The method for measuring the twist of an optical fiber according to claim 1, wherein the optical fiber is a multicore optical fiber, and the refractive index changing portion is a marker.

5. A method for measuring the twist of an optical fiber according to any one of claims 1 to 4, comprising the step of adjusting the rotation angle around the central axis of the optical fiber base material so that the refractive index change portion is located within the region.

6. A method for measuring the twist of an optical fiber according to any one of claims 1 to 4, comprising the step of adjusting the arrangement of a measuring device including the irradiation unit around the central axis such that the refractive index changing portion is located within the region in a cross section perpendicular to the central axis.

7. The method for measuring the twist of an optical fiber according to claim 5 or claim 6, wherein the adjustment step is performed before or during the start of drawing.

8. The method for measuring the twist of an optical fiber according to claim 6, wherein the irradiation unit is arranged on a rotating stage that rotates around the central axis, and the adjustment step is performed by rotating the rotating stage.

9. The method for measuring the twist of an optical fiber according to any one of claims 6 to 8, wherein the measuring device is one of a plurality of measuring devices arranged along the direction of travel of the optical fiber, and the adjustment step is performed with the orientations of the plurality of measuring devices aligned with each other.

10. A method for manufacturing an optical fiber, comprising: a step of melting an optical fiber base material by heating and drawing an optical fiber; a step of measuring the twist of the optical fiber during drawing; and a step of applying a twist to the optical fiber during drawing to adjust the measured twist, wherein the measuring step comprises: irradiating the optical fiber running between the optical fiber base material and a roller directly below it with light from an irradiation unit; detecting the intensity distribution of the light transmitted through the optical fiber; and calculating the twist of the optical fiber based on the detected intensity distribution, wherein the optical fiber comprises a cladding and a refractive index changing section disposed within the cladding away from the central axis of the optical fiber and having a refractive index different from that of the cladding, and in the irradiation step, light is irradiated such that the refractive index changing section is located in a sector-shaped region with a central angle of 90 degrees, with the central axis as the vertex and a straight line passing through the irradiation unit and the central axis as the center line, in a cross section perpendicular to the central axis.

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