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

Abstract

This method for measuring the twist of an optical fiber involves measuring the twist of the optical fiber during drawing, the method comprising: a step for adjusting a preform rotation angle about the central axis of an optical fiber preform; a step for irradiating the optical fiber with light at a measurement position between the optical fiber preform and a directly-underneath roller; a step for detecting the intensity distribution of the light transmitted through the optical fiber at the measurement position; a step for acquiring an optical fiber rotation angle about the central axis of the optical fiber at the measurement position on the basis of the detected intensity distribution; and a step for calculating the twist of the optical fiber on the basis of the acquired optical fiber rotation angle and the preform rotation angle.

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. 2024-197664 filed on November 12, 2024, and incorporates all the descriptions described in the above Japanese application.

[0002] Patent Document 1 describes a method for measuring the twist of an optical fiber being drawn. 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 being drawn, including a step of adjusting the rotation angle of the base material around the central axis of the optical fiber base material, a step of irradiating light to the optical fiber at a measurement position between the optical fiber base material and the directly below roller, a step of detecting the intensity distribution of the light transmitted through the optical fiber at the measurement position, a step of obtaining the rotation angle of the optical fiber around the central axis of the optical fiber at the measurement position based on the detected intensity distribution, and a step of calculating the twist of the optical fiber based on the obtained rotation angle of the optical fiber and the rotation angle of the base material.

[0005] FIG. 1 is a schematic diagram of a manufacturing apparatus used in the method for manufacturing an optical fiber according to an embodiment. FIG. 2 is a flowchart showing the method for manufacturing an optical fiber according to an embodiment. FIG. 3 is a flowchart showing the measurement step. FIG. 4 is a diagram showing a comparison between a cross-sectional view of an optical fiber base material and a cross-sectional view of an optical fiber at a measurement position. FIG. 5 is an explanatory diagram of the twist application step and is a perspective view of the directly below roller. FIG. 6 is an explanatory diagram of the twist application step and is a view of the directly below roller seen from above. FIG. 7 is an explanatory diagram of the twist application step according to a modification. FIG. 8 is a cross-sectional view of a multi-core optical fiber.

[0006] In the method described in Patent Document 1, the measurement accuracy improves as the number of measuring devices increases. However, when the number of measuring devices increases, the capital investment and maintenance burden also increase.

[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 efficiently measure the twist of an optical fiber while drawing it, while maintaining measurement accuracy.

[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 efficiently 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: adjusting the rotation angle of the optical fiber base material about the central axis of the optical fiber base material; irradiating the optical fiber at a measurement position between the optical fiber base material and a roller directly below it with light; detecting the intensity distribution of the light transmitted through the optical fiber at the measurement position; obtaining the rotation angle of the optical fiber about the central axis of the optical fiber at the measurement position based on the detected intensity distribution; and calculating the twist of the optical fiber based on the obtained optical fiber rotation angle and the base material rotation angle.

[0010] In the above method for measuring the twist of an optical fiber, the twist is calculated using the rotation angle of the optical fiber around its central axis, obtained by measurement at the measurement position, as well as the rotation angle of the optical fiber matrix around its central axis. Compared to methods that do not use the rotation angle of the optical fiber matrix around its central axis, the number of measuring devices required can be reduced. Therefore, the twist of an optical fiber during drawing can be measured efficiently.

[0011] (2) In (1) above, the vertical distance between the heating center position of the wire drawing furnace and the measurement position may be 0.5 m or more. In this case, the accuracy of twist detection can be improved.

[0012] (3) In (1) or (2) above, the measurement position may be between the optical fiber base material and the coating device that applies resin to the optical fiber. In this case, the measurement can be performed on the glass fiber in a state where no coating resin is applied, thereby improving the measurement accuracy.

[0013] (4) In any of (1) to (3) above, the step of irradiating with light may be performed using multiple irradiation units, and the step of detection may be performed using multiple detection units corresponding to the multiple irradiation units.

[0014] (5) 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 imparting a twist to the optical fiber during drawing by adjusting a roller directly below or adjusting the optical fiber base material to adjust the measured twist, wherein the measuring step includes the steps of: adjusting the rotation angle of the base material around the central axis of the optical fiber base material; irradiating the optical fiber at a measurement position between the optical fiber base material and the roller directly below with light; detecting the intensity distribution of the light transmitted through the optical fiber at the measurement position; obtaining the rotation angle of the optical fiber around the central axis of the optical fiber at the measurement position based on the detected intensity distribution; and calculating the twist of the optical fiber based on the obtained rotation angle of the optical fiber and the rotation angle of the base material.

[0015] In the optical fiber manufacturing method described above, the twist of the optical fiber is calculated using the rotation angle of the optical fiber around its central axis, obtained by measurement at the measurement position, as well as the rotation angle of the optical fiber matrix around its central axis. Compared to methods that do not use the rotation angle of the optical fiber matrix around its central axis, the number of measuring devices required can be reduced. Therefore, the twist of the optical fiber during drawing can be measured efficiently. Since a twist is added to the optical fiber during drawing to adjust for the measured twist, optical fibers with reduced twist can be manufactured efficiently.

[0016] [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.

[0017] 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.

[0018] The optical fiber 20 manufactured by the manufacturing apparatus 1 has, for example, a circular cross-section and a refractive index changing section inside. The optical fiber 20 has a structure in which the refractive index changing section has a degree of freedom in position around the central axis. Examples of such optical fibers 20 include multicore optical fibers and polarization-maintaining optical fibers. The cross-sectional shape of the optical fiber 20 does not have to be circular; it may be non-circular.

[0019] The following description uses a polarization-maintaining optical fiber 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) as an example. The core 22 includes a 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. The stress-applying sections 23 are refractive index changing sections. The cladding 24 surrounds the core 22 and the plurality of stress-applying sections 23. In this embodiment, the optical fiber 20 includes a coating resin (not shown) covering the outer surface of the cladding 24. The optical fiber before the coating resin is applied is a glass fiber 21.

[0020] 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.

[0021] Figure 2 is a flowchart showing 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 a drawing step S1, a measurement step S3, and a twisting step S6. The method for manufacturing an optical fiber 20 according to an embodiment may further include a cooling step S2, a coating step S4, and a curing step S5. 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] The measuring device 4 includes an illumination 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. The measuring device 4 may have a plurality of corresponding illumination units 11 and a plurality of detection units 12. Note that in Figure 1, one illumination unit 11 and one detection unit 12 are shown, and the remaining illumination units 11 and detection units 12 are omitted.

[0026] 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 a base material rotation angle adjustment process S10, an irradiation process S11, a detection process S12, an acquisition process S13, and a calculation process S14. Processes S10 to S14 are performed in this order. Note that the base material rotation angle adjustment process S10 may be performed before starting the line drawing process S1.

[0027] The base material rotation angle adjustment step S10 is a step of adjusting the rotation angle of the optical fiber base material 31. As will be described later, the position of the optical fiber base material 31 is adjusted so that the rotation angle θ0 (not shown) of the optical fiber base material 31 around the central axis C0 of the optical fiber base material 31 is within a specified range. This step may be performed before the wire drawing step S1 or during the wire drawing.

[0028] The irradiation step S11 is a step of irradiating the glass fiber 21 or optical fiber 20 at a measurement position P between the optical fiber base material 31 and the roller directly below 7 with light from the irradiation unit 11. When the measurement position P is upstream of the coating device 5, the target of the light irradiation is the glass fiber 21. When 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.". As an example, 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.

[0029] The distance d between the lower end of the optical fiber base material 31 and the measurement position P may be, for example, 0.5 m or more. The lower end of the optical fiber base material 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, the distance from the optical fiber base material 31 to the measurement position P is short, and if the rotation angle of the twist is small, 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 base material 31 affects the measurement accuracy. Distance d is set to a length such that the optical fiber 20, etc., does not twist by, for example, one rotation or more. Distance d is, for example, 15 m or less. In this embodiment, distance d is set to a length such that the optical fiber 20, etc., does not twist by 1 / 2 rotation or more.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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. Irradiation step S11 is performed using a plurality of irradiation units 11.

[0034] 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.

[0035] The intensity distribution is the distribution of light intensity (luminance) detected by the detection unit 12 along the depth direction D of the optical fiber 20, etc. The depth direction D of the optical fiber 20, etc. is perpendicular to the travel direction A. The depth direction D is also perpendicular to the direction in which light passes through the optical fiber 20, etc. at the measurement position P. The detection process S12 is performed using a plurality of detection units 12 corresponding to a plurality of illumination units 11. As the plurality of detection units 12, for example, a line sensor in which light-detecting pixels are arranged in a line, or an area sensor in which pixels are arranged in two dimensions can be used. In the case of a line sensor, for example, the intensity distribution along the depth direction D can be obtained by extracting the signal of a single pixel row arranged in the depth direction D.

[0036] In detection step S12, the light transmitted through the optical fiber 20, etc., 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 positioned between the optical fiber 20, etc., and the detection unit 12. The polarizing plate 18 is positioned on the optical path of the transmitted light incident on the detection unit 12.

[0037] In detection step S12, for example, light transmitted through the optical fiber 20 is magnified in the depth direction D by the objective lens 16. Therefore, the detection unit 12 detects the intensity distribution of the light magnified in the depth direction D. The objective lens 16 is positioned between, for example, the optical fiber 20 and the detection unit 12. The objective lens 16 is positioned on the optical path of the transmitted light incident on the detection unit 12.

[0038] In detection step S12, light that has passed through the optical fiber 20, etc., at measurement position P is transmitted through the objective lens 16, differential interference prism 17, and polarizer 18. The polarizations separated by the differential interference prism 14 in irradiation step S11 are combined into the same path by the differential interference prism 17 in detection step S12. There is an optical path difference in the light that is separated when it passes through the optical fiber 20, etc. Therefore, differential interference occurs in the combined light. The detection unit 12 detects the intensity distribution of the differentially interfered light.

[0039] 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 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.

[0040] 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.

[0041] The calculation step S14 is a step in which the twist of the optical fiber 20, etc. is calculated based on the rotation angle θ0 of the optical fiber base material 31 around the central axis C0 set in the base material rotation angle adjustment step S10 and the rotation angle θ of the optical fiber 20, etc. obtained in the 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, by 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.

[0042] In calculation step S14, for example, the difference θ-θ0 between the rotation angle θ and the rotation angle θ0 is calculated as the torsion of the optical fiber 20, etc. The optical fiber base material 31 is arranged such that, for example, the rotation angle θ0 is 0 degrees. This allows the rotation angle θ to be directly considered as the torsion of the optical fiber 20, etc. Based on the distance d and the difference θ-θ0, the torsion per unit length of the optical fiber 20, etc. can be determined. The unit of torsion per unit length is, for example, degrees / mm.

[0043] Figure 4 is a diagram comparing a cross-sectional view of the optical fiber preform with a cross-sectional view of the optical fiber at the measurement position. In Figure 4, the optical fiber preform 31 and the optical fiber 20 are shown scaled down or enlarged so that their diameters are the same length. If there is no twisting in the optical fiber 20, the position of the stress application portion 23 of the optical fiber 20 at the measurement position P (see Figure 1) does not change from the position of the stress application portion 33 of the optical fiber preform 31. If there is twisting in the optical fiber 20, the position of the stress application portion 23 of the optical fiber 20 at the measurement position P changes from the position of the stress application portion 33 of the optical fiber preform 31.

[0044] The coating step S4 is a step of coating the surface of the glass fiber 21 with a coating resin by the coating device 5. The coating device 5 is provided, for example, downstream of the cooling device 3 in the traveling direction A. The coating device 5 is a metal jig having a hole for passing the glass fiber 21 through the central portion. The coating device 5 holds the liquid coating resin and coats the surface of the glass fiber 21 passing through the hole with the coating resin. The coating device 5 is a die. In the present embodiment, the coating resin is an ultraviolet curable resin.

[0045] The curing step S5 is a step of curing the coating resin on the surface of the glass fiber 21 by the curing device 6. The curing device 6 is provided downstream of the coating device 5 in the traveling direction A. The curing device 6 cures the coating resin applied to the surface of the glass fiber 21. Thereby, the optical fiber 20 provided with the coating resin is formed. In the present embodiment, the curing device 6 is an ultraviolet irradiation device, and cures the coating resin by irradiating ultraviolet rays.

[0046] The twist application step S6 is a step of applying, for example, by the directly-below roller 7, a twist for adjusting the twist measured in the measurement step S3 to the optical fiber 20 being drawn. The directly-below roller 7 is a guide roller disposed directly below the optical fiber base material 31 in the vertical direction. The directly-below roller 7 is provided downstream of the curing device 6 in the traveling direction A. The directly-below roller 7 is disposed between the coating device 5 and the capstan 8 in the traveling direction A. In the present embodiment, the directly-below roller 7 is disposed between the curing device 6 and the capstan 8 in the traveling direction A. The directly-below roller 7 abuts on the optical fiber 20 and converts the traveling direction A of the optical fiber 20 from the vertical direction to a direction different from the vertical direction. The twist application step S6 is, for example, a step of applying to the optical fiber 20 being drawn a twist that cancels out the twist measured in the measurement step S3.

[0047] The control unit 10 performs feedback control on the directly-below roller 7 based on the calculation result. Specifically, the control unit 10 transmits a control signal corresponding to the calculation result to the directly-below roller 7. The feedback control will be described later.

[0048] FIG. 5 and FIG. 6 are explanatory views of the twisting application step. FIG. 5 shows a perspective view of the directly-below roller 7. FIG. 6 shows a view of the directly-below roller 7 as seen from the upstream (e.g., the curing device 6) in the traveling direction A. As shown in FIGS. 5 and 6, the directly-below roller 7 is formed so as to be movable in the axial direction AX of the directly-below roller 7. The directly-below roller 7 is arranged such that the axial direction AX is orthogonal to the vertical direction. The directly-below roller 7 may be formed so as to be able to change the angle of the rotation axis of the directly-below roller 7 with respect to the traveling direction A of the optical fiber 20 (e.g., the angle of the axial direction AX with respect to the depth direction D when viewed from the vertical direction).

[0049] FIG. 6 shows the state before the elapse of a predetermined time and the state after the elapse of the predetermined time. In the figure below the arrow in FIG. 6 showing the state before the elapse of the predetermined time, the optical fiber 20 contacts the lower portion (upstream portion) of the directly-below roller 7, is wound around the directly-below roller 7, and is located in the groove portion at the upper portion (downstream portion) of the directly-below roller 7. Thus, if the position where the optical fiber 20 contacts the directly-below roller 7 in the lower portion (upstream portion) of the directly-below roller 7 is not the groove portion, the traveling direction of the optical fiber 20 can be changed so as to fit into the groove portion while contacting the directly-below roller 7. At this time, a rotational force R for rotating the optical fiber 20 is applied to the optical fiber, and the optical fiber 20 is twisted.

[0050] The twist of the optical fiber 20 is transmitted to the molten portion of the optical fiber base material 31. Thereby, twist is applied to the glass fiber 21 drawn from the molten portion. The twist applied before cooling and solidifying, unlike the twist applied after cooling and solidifying, does not return to its original state after manufacturing and becomes residual twist. In order to adjust the residual twist, it is necessary to measure the twist during drawing and adjust the application of the twist. For example, in the directly-below roller 7, the twist of the optical fiber 20 can be adjusted by changing the position of the optical fiber 20 entering the directly-below roller 7.

[0051] 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), 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.

[0052] 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, in the diagram below the arrow in Figure 6, which shows the state before a predetermined time has elapsed, the position where the optical fiber 20 first contacts the lower roller 7 (the upstream position) is not the 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. In the diagram above the arrow in Figure 6, which shows the state after a predetermined time has elapsed, if the lower roller 7 is moved and the position where the optical fiber 20 first contacts the lower roller 7 (the upstream position) is the groove, then the position where the optical fiber 20 contacts the downstream part of the lower roller 7 is also the groove, and therefore the optical fiber 20 does not twist.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] Figure 7 is an explanatory diagram of the twisting process according to a modified example. In the twisting process S6 according to the modified example, twisting is applied to the optical fiber 20 by adjusting the angle of a pair of guide rollers 30, rather than the direct roller 7 (see Figure 1). The pair of guide rollers 30 are positioned between the coating device 5 and the direct 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 roller 7 in the running direction A, but they may also be positioned between the direct roller 7 and the capstan 8.

[0057] 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.

[0058] 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.

[0059] As described above, in the twist measurement method according to the embodiment, the twist of the optical fiber 20 is calculated using the rotation angle θ0 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 a method that does not use the rotation angle θ0 of the optical fiber base material 31, the number of measuring devices 4 required can be reduced. Therefore, the twist of the optical fiber 20 during drawing can be measured efficiently.

[0060] In the method for manufacturing the optical fiber 20 according to this embodiment, in the measurement step S3, the twist of the optical fiber 20 is calculated using the rotation angle θ0 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 a method that does not use the rotation angle θ0 of the optical fiber base material 31, the number of measuring devices 4 required can be reduced. Therefore, the twist of the optical fiber 20 during drawing can be measured efficiently. Since a twist is applied to the optical fiber 20 during drawing to adjust the measured twist, an optical fiber 20 with adjusted twist can be manufactured efficiently. Alternatively, the optical fiber base material 31 may be rotated to apply a twist to the optical fiber 20 in order to adjust the measured twist.

[0061] Although the above explanation used a polarization-maintaining optical fiber as an example, the optical fiber 20 may also be a multicore optical fiber. In this case, as shown in Figure 8, the optical fiber 20 includes multiple (two in this case) cores 22 and a cladding 24 surrounding the cores 22. The cores 22 are the refractive index change sections and correspond to the stress application section 23 (see Figure 4) in the above example. In the case of a multicore optical fiber, there may be markers (not shown) in the cladding 24. The markers are also sections with a different refractive index relative to the cladding 24, just like the cores 22 and the stress application section 23. Therefore, the twisting of the optical fiber 20 can be observed as the twisting of the markers.

[0062] In optical fiber 20, the twist is adjusted, which reduces 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 solves the problem of the core axis being misaligned due to changes in the longitudinal position of optical fiber 20 caused by polishing of the fiber end face. Similarly, in polarization-maintaining optical fiber, it solves the problem of the polarization plane at the input end and the polarization plane at the output end being misaligned.

[0063] 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.

[0064] The manufacturing apparatus 1 may be equipped with a plurality of measuring devices 4. In this case, the plurality of measuring devices 4 may be arranged to irradiate light from the same direction to a plurality of measurement positions P that are spaced apart from each other in the travel direction A. That is, the plurality of measuring devices 4 may be arranged so that the optical paths of the light irradiated from the plurality of irradiation units 11 overlap or completely coincide with each other when viewed from the travel 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, the optical fiber base material 31 and the measurement position P, if an additional measurement position P is provided between those two points, the torsion can be accurately grasped even if a twist of one or more turns occurs between those two points.

[0065] 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.

[0066] 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.

[0067] 1...Manufacturing equipment 2...Drawing furnace 3...Cooling equipment 4...Measurement equipment 5...Coating equipment 6...Curing equipment 7...Direct 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 30...Guide roller 31...Optical fiber base material 32...Core unit 33...Stress application unit 34...Cladding unit A...Travel direction AX...Axial direction AY...Direction perpendicular to the axial direction C...Central axis C0...Central axis D...Depth direction d...Distance L1...Straight line L2...Straight line L3...Straight line L4...Straight line P...Measurement position R...Rotational force S1...Drawing process S2... Cooling process S3... Measuring process S4... Coating process S5... Curing process S6... Twisting process S7... Picking up process S8... Winding process S10... Base material rotation angle adjustment process S11... Irradiation process S12... Detection process S13... Acquisition process S14... Calculation process θ... Rotation angle

Claims

1. A method for measuring the twist of an optical fiber during drawing, comprising: adjusting the rotation angle of the optical fiber base material around the central axis; irradiating the optical fiber at a measurement position between the optical fiber base material and a roller directly below it with light; detecting the intensity distribution of the light transmitted through the optical fiber at the measurement position; obtaining the rotation angle of the optical fiber around the central axis of the optical fiber at the measurement position based on the detected intensity distribution; and calculating the twist of the optical fiber based on the obtained rotation angle of the optical fiber and the rotation angle of the base material.

2. The vertical distance between the heating center position of the drawing furnace and the measurement position is 0.5 m or more, the method for measuring the twist of an optical fiber according to claim 1.

3. The method for measuring the twist of an optical fiber according to claim 1 or claim 2, wherein the measurement position is located between the optical fiber base material and a coating device for applying resin to the optical fiber.

4. The method for measuring the twist of an optical fiber according to any one of claims 1 to 3, wherein the step of irradiating with light is performed using a plurality of irradiation units, and the step of detection is performed using a plurality of detection units corresponding to the plurality of irradiation units.

5. A method for manufacturing an optical fiber, comprising: heating and melting an optical fiber preform to draw an optical fiber; measuring the twist of the optical fiber during drawing; and adjusting the roller directly below or adjusting the optical fiber preform to adjust the measured twist, wherein the measuring step includes: adjusting the rotation angle of the preform about the central axis of the optical fiber preform; irradiating the optical fiber with light at a measurement position between the optical fiber preform and the roller directly below; detecting the intensity distribution of the light transmitted through the optical fiber at the measurement position; obtaining the rotation angle of the optical fiber about the central axis of the optical fiber at the measurement position based on the detected intensity distribution; and calculating the twist of the optical fiber based on the obtained rotation angle of the optical fiber and the rotation angle of the preform.

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