Optical fiber, flat-type optical fiber strand, optical fiber tape core wire, and method for manufacturing optical fiber tape core wire

Abstract

An optical fiber (10) comprises a glass fiber (13) and a resin coating layer (16). The glass fiber (13) includes a central axis (G), at least one core (11), and cladding (12) covering the core (11). The glass fiber extends along the central axis (G). The resin coating layer (16) is in contact with the glass fiber (13) and surrounds the glass fiber (13). The glass fiber (13) has a circular shape in a cross-section orthogonal to the central axis (G) and has directionality with respect to the rotational direction around the central axis (G). The outer peripheral surface of the resin coating layer (16) is provided with a reference surface (17a) that serves as a reference for the directionality of the glass fiber (13) with respect to the rotational direction.

Description

Optical fiber, flat optical fiber, optical fiber ribbon, and method for manufacturing optical fiber ribbon

[0001] This disclosure relates to an optical fiber, a flat optical fiber, an optical fiber ribbon, and a method for manufacturing an optical fiber ribbon. This application claims priority to Japanese Application No. 2024-097494, filed on June 17, 2024, and incorporates by reference all the contents of said Japanese application.

[0002] Patent Document 1 discloses an optical fiber ribbon in which a structure (e.g., a flat surface) for adjusting the directionality of a plurality of cores in the rotation direction is provided on the outer peripheral surface of each optical fiber. Patent Documents 2 and 3 disclose providing a marker in an optical fiber ribbon in which two or more glass fibers are provided.

[0003] US Patent Application Publication No. 2013 / 0322835 US Patent Application Publication No. 2016 / 0223774 US Patent Application Publication No. 2017 / 0269322

[0004] An optical fiber according to an embodiment of the present disclosure includes a glass fiber and a resin coating layer. The glass fiber includes a central axis, at least one core, and a cladding covering the core, and extends along the central axis. The resin coating layer contacts the glass fiber and surrounds the glass fiber. The glass fiber has a circular shape in a cross section perpendicular to the central axis and has directionality relative to a rotation direction based on the central axis. A reference plane is provided on the outer peripheral surface of the resin coating layer, which serves as a reference for the directionality of the glass fiber relative to the rotation direction.

[0005] FIG. 1 is a cross-sectional view of an optical fiber according to a first embodiment. FIG. 2A is a view showing a state of rotation adjustment when the optical fiber shown in FIG. 1 is pressed against the conveying surface of a guide roller. FIG. 2B is a view showing a state of rotation adjustment when an optical fiber having a general shape is pressed against the conveying surface of a guide roller. FIG. 3A is a view of an example of transmitted light when inspection light is incident on the optical fiber shown in FIG. 1 to observe the arrangement position of a core. FIG. 3B is a view showing transmitted light when inspection light is incident on an optical fiber having a general shape to observe the arrangement position of a core. FIG. 4 is a schematic diagram showing a method of manufacturing the optical fiber shown in FIG. 1. FIG. 5A is a cross-sectional view showing a first modified example of the optical fiber according to the first embodiment. FIG. 5B is a cross-sectional view showing a second modified example of the optical fiber according to the first embodiment. FIG. 6A is a cross-sectional view showing a third modified example of the optical fiber according to the first embodiment. FIG. 6B is a cross-sectional view showing a fourth modified example of the optical fiber according to the first embodiment. FIG. 7 is a cross-sectional view of an optical fiber ribbon according to a second embodiment. FIG. 8 is a cross-sectional view of a flattened optical fiber according to a third embodiment. FIG. 9 is a schematic diagram showing a method of manufacturing the flattened optical fiber shown in FIG. 8. Fig. 10 is a cross-sectional view showing a first modified example of the flat optical fiber according to the third embodiment, and Fig. 11 is a cross-sectional view of the optical fiber ribbon according to the fourth embodiment.

[0006] In Patent Document 1, an optical fiber ribbon is formed using a multicore fiber in which multiple cores (four cores arranged horizontally) are provided in each glass fiber. Multicore fibers and polarization-maintaining fibers have directionality relative to the rotation direction based on the central axis. With such glass fibers, rotational alignment is required to align each core when fusion splicing or connecting an optical connector. The optical fiber described in Patent Document 1 has a flat or concave lower surface on the outer circumferential surface, which functions as a reference for directionality relative to the rotation direction. While this allows the core placement position to be adjusted, the special cross-sectional shape of the glass fiber places limitations on core placement. Furthermore, manufacturing methods such as fiber drawing can be complicated. Therefore, further improvements are desired.

[0007] The present disclosure aims to provide an optical fiber, an optical fiber flat wire, an optical fiber ribbon, and a method for manufacturing an optical fiber ribbon that can improve the alignment accuracy of an optical fiber that has directionality in the rotation direction based on a central axis while increasing the degree of freedom in core placement, and to eliminate the need for rotational alignment when performing fusion splicing or optical connector connection, or to simplify connection by reducing the rotational alignment angle range to, for example, 10 degrees or less.

[0008] According to the present disclosure, it is possible to improve the alignment accuracy of an optical fiber that has directionality with respect to the rotation direction about its central axis, while increasing the degree of freedom in core arrangement.

[0009] First, the contents of the embodiments of the present disclosure will be listed and described. [1] An optical fiber according to one embodiment includes a glass fiber and a resin coating layer. The glass fiber includes a central axis, at least one core, and a cladding covering the core, and extends along the central axis. The resin coating layer contacts the glass fiber and surrounds the glass fiber. The glass fiber has a circular shape in a cross section perpendicular to the central axis and has directionality with respect to a rotation direction based on the central axis. A reference plane is provided on the outer peripheral surface of the resin coating layer, which serves as a reference for the directionality of the glass fiber with respect to the rotation direction.

[0010] In the optical fiber according to the above [1], a reference surface is provided on the outer peripheral surface of the resin coating layer, which serves as a reference for the directionality of the glass fiber relative to the rotation direction. This allows for improved alignment accuracy of the optical fiber, which has directionality relative to the rotation direction based on the central axis. Furthermore, in this optical fiber, the glass fiber has a circular cross-sectional shape, which allows for greater freedom in core placement and makes it easier to reduce splice loss. Furthermore, the circular cross-sectional shape of the glass fiber makes it easier to achieve the circularity of the glass fiber (clad), thereby improving the placement accuracy of the core position within the clad. Furthermore, because the glass fiber is not dome-shaped as in the prior art, more cores can be placed within a clad of the same outer diameter.

[0011] [2] In the optical fiber of [1] above, the reference surface may include a flat surface. In this case, it is possible to improve the alignment accuracy of the optical fiber, which has directionality with respect to the rotation direction around the central axis, with a simple structure.

[0012] [3] In the optical fiber of [2] above, the flat surface may have a width of at least 40 μm or more in the fiber width direction intersecting with the central axis. By having a width of 40 μm or more on the reference surface, it is possible to improve the alignment accuracy of the optical fiber, which has directionality with respect to the rotation direction based on the central axis. Note that the flat surface on the reference surface may have a width of 100 μm or more in the fiber width direction.

[0013] [4] In the optical fiber of any one of [1] to [3] above, the outer surface of the resin coating layer may be provided with an observation surface that allows observation of the orientation of the glass fiber relative to the rotation direction. In this case, the incident light or the outgoing light can be observed through the observation surface through which the light irradiated to the optical fiber enters or exits. This improves the detection accuracy of the orientation of the glass fiber relative to the rotation direction (e.g., the position of the core in the rotation direction).

[0014] [5] In the optical fiber of [4] above, the observation surface may include a flat surface. In this case, the incident or exiting light is less likely to be deformed at the observation surface, allowing for more reliable observation of the transmitted light. This further improves the accuracy of detecting the directionality relative to the rotation direction of the glass fiber. Note that a "flat surface" ideally refers to a surface with an infinite radius of curvature, but any radius of curvature that can be considered flat relative to the diameter of the optical fiber is acceptable, and some convex or concave shapes are acceptable.

[0015] [6] In the optical fiber of [4] or [5], the at least one core may be a plurality of cores, and at least two of the plurality of cores may be arranged along an axis perpendicular to the observation surface. In this case, the accuracy of detecting the arrangement position of the cores in the rotation direction can be improved.

[0016] [7] In any one of the optical fibers [1] to [6], the resin coating layer may have a rectangular shape in a cross section perpendicular to the central axis. In this case, a reference surface or an observation surface can be easily formed on the outer peripheral surface of the resin coating layer. Note that the rectangular shape here also includes a substantially rectangular shape with curved corners (R portions).

[0017] [8] In the optical fiber of any of [1] to [7] above, the at least one core may include at least 10 or more cores. Since the glass fiber of this optical fiber has a circular cross section, it is easy to arrange more cores in the glass fiber (clad). By providing a multicore fiber with such a large number of cores, the transmission capacity of the optical fiber can be significantly improved. The number of cores in the optical fiber may be 12 or more, or 16 or more.

[0018] [9] In the optical fiber of any one of [1] to [8] above, a marker may be provided on the resin coating layer. In this case, the marker can identify the direction of the optical fiber, and the optical fiber can be easily tape-formed.

[0019]

[10] A flat optical fiber according to one embodiment includes a first glass fiber, a second glass fiber, a first resin coating layer, a second resin coating layer, and an outer coating layer. The first glass fiber includes a first central axis, at least one first core, and a first cladding covering the first core, and extends along the first central axis. The second glass fiber includes a second central axis, at least one second core, and a second cladding covering the second core, and extends along the second central axis. The first resin coating layer surrounds the first glass fiber in contact with the first glass fiber. The second resin coating layer surrounds the second glass fiber in contact with the second glass fiber. The outer coating layer connects the first resin coating layer and the second resin coating layer so that the first glass fiber and the second glass fiber are arranged in parallel. The first glass fiber has a circular shape in a cross section perpendicular to the first central axis and has directionality relative to a rotational direction based on the first central axis. The outer peripheral surfaces of the first resin coating layer and the second resin coating layer are provided with reference planes that serve as a reference for the orientation of the first glass fiber and the second glass fiber relative to the rotation direction. In this flat optical fiber, the outer peripheral surface of the outer coating layer is provided with a tape reference plane that serves as a reference for the orientation of at least the first glass fiber relative to the rotation direction.

[0020] In the flat optical fiber according to the above

[10] , a tape reference surface is provided on the outer peripheral surface of the outer coating layer, which serves as a reference for the orientation of the first glass fiber relative to the rotation direction. This improves the alignment accuracy of the first glass fiber, which has orientation relative to the rotation direction based on the first central axis. Furthermore, in this flat optical fiber, the first glass fiber has a circular cross-sectional shape, which increases the degree of freedom in core placement and makes it easier to reduce splice loss. Furthermore, the circular cross-sectional shape of the first glass fiber makes it easier to achieve the circularity of the first glass fiber (clad), thereby improving the placement accuracy of the core position within the clad. Furthermore, since the glass fiber is not dome-shaped as in the prior art, more cores can be placed within a clad of the same outer diameter.

[0021]

[11] In the flat optical fiber of

[10] , the ribbon reference surface may include a flat surface. In this case, it is possible to improve the alignment accuracy of the first glass fiber, which has directionality with respect to the rotation direction about the first central axis, by a simple means.

[0022]

[12] In the flat optical fiber of either

[10] or

[11] above, the second glass fiber may have a circular shape in a cross section perpendicular to the second central axis and may have directionality with respect to a rotation direction based on the second central axis. The ribbon reference plane may serve as a reference for the directionality of the second glass fiber with respect to the rotation direction. In this case, it is possible to improve the alignment accuracy of not only the first glass fiber but also the second glass fiber, which has directionality with respect to a rotation direction based on the second central axis. Furthermore, since the cross section of the second glass fiber is circular, it is possible to increase the degree of freedom in core arrangement, etc.

[0023]

[13] In the flat optical fiber according to any one of

[10] to

[12] above, an observation surface may be provided on the outer peripheral surface of the outer coating layer, allowing observation of the orientation of the first glass fiber relative to the rotation direction. In this case, the incident light or the outgoing light can be observed through the observation surface through which the light irradiated onto the glass fiber enters or exits. This improves the detection accuracy of the orientation of the glass fiber relative to the rotation direction (e.g., the position of the core in the rotation direction).

[0024]

[14] In the flat optical fiber of

[13] , the observation surface may include a flat surface. In this case, the incident or outgoing light is less likely to be deformed at the observation surface, so that the transmitted light can be observed more reliably. This further improves the accuracy of detecting the directionality with respect to the rotation direction of the glass fiber.

[0025]

[15] In the flat optical fiber of

[13] or

[14] , the at least one first core may include a plurality of first cores, and at least two of the plurality of first cores may be arranged along an axis perpendicular to the observation surface. In this case, the detection accuracy of the core arrangement position in the rotation direction can be improved. Note that the number of first cores may be 10 or more, 12 or more, or 16 or more.

[0026]

[16] In the flat optical fiber of any one of

[10] to

[15] above, the outer coating layer may have a rectangular shape in a cross section perpendicular to the first central axis. In this case, a reference surface or an observation surface can be easily formed on the outer peripheral surface of the outer coating layer. Note that the rectangular shape here also includes a substantially rectangular shape with curved surfaces (R portions) or the like at the corners. The same applies hereinafter.

[0027]

[17] In the flat optical fiber of any one of

[10] to

[16] above, at least one of the first resin coating layer and the second resin coating layer may have a rectangular shape in a cross section perpendicular to the first central axis. In this case, rotation of each optical fiber can be easily suppressed during the process of collecting the optical fibers in pairs, etc.

[0028]

[18] An optical fiber ribbon according to one embodiment includes a plurality of optical fibers, each of which is a plurality of optical fibers according to any one of [1] to [9] above and arranged in parallel with one another, and a connecting portion connecting the plurality of optical fibers to one another. In this case, the alignment accuracy of the optical fibers having directionality with respect to the rotation direction based on the central axis can be improved in the optical fiber ribbon. Furthermore, in this optical fiber ribbon, the cross-sectional shape of the glass fiber is circular, thereby increasing the degree of freedom in core arrangement.

[0029]

[19] An optical fiber ribbon according to one embodiment includes a plurality of flat optical fiber wires, each of which is a plurality of flat optical fiber wires according to any one of

[10] to

[17] above and arranged in parallel with one another, and a connecting portion connecting the plurality of flat optical fiber wires to one another. In this case, the alignment accuracy of the glass fiber, which has directionality with respect to the rotation direction based on the central axis, can be improved in the optical fiber ribbon. Furthermore, in this optical fiber ribbon, the cross-sectional shape of the glass fiber is circular, which increases the degree of freedom in core arrangement.

[0030]

[20] A method for manufacturing an optical fiber ribbon according to one embodiment includes the steps of providing a plurality of optical fibers, each of which is any one of the optical fibers described above in [1] to [9], aligning the plurality of optical fibers so that they are arranged in parallel with one another, and connecting the plurality of optical fibers to one another over their entire length or intermittently with a connecting resin. In the aligning step, the reference surfaces of the plurality of optical fibers are pressed against an optical fiber transport member to adjust the directionality of the glass fibers relative to the rotational direction, and align the plurality of optical fibers. In this case, an optical fiber ribbon can be easily manufactured with improved alignment accuracy of optical fibers having directionality relative to the rotational direction about the central axis. Furthermore, in this method for manufacturing an optical fiber ribbon, the cross-sectional shape of the glass fibers used is circular, thereby enabling the manufacture of an optical fiber ribbon with increased flexibility in core arrangement.

[0031]

[21] A method for manufacturing an optical fiber ribbon according to one embodiment includes the steps of providing a plurality of flat optical fiber wires, each of which is any one of the above

[10] to

[17] , aligning the plurality of flat optical fiber wires so that they are arranged in parallel with one another, and connecting the plurality of flat optical fiber wires to one another over their entire length or intermittently with a connecting resin. In the aligning step, the ribbon reference surfaces of the plurality of flat optical fiber wires are pressed against an optical fiber transport member to adjust the orientation of the first glass fiber relative to the rotational direction, and align the plurality of flat optical fiber wires. In this case, an optical fiber ribbon with improved alignment accuracy of the optical fiber having orientation relative to the rotational direction about the central axis can be easily manufactured. Furthermore, in this method for manufacturing an optical fiber ribbon, since the cross-sectional shape of the first glass fiber is circular, an optical fiber ribbon with increased flexibility in core arrangement can be manufactured.

[0032] Specific examples of optical fibers, flat optical fibers, optical fiber ribbons, and methods for manufacturing optical fiber ribbons according to embodiments of the present disclosure will be described below with reference to the drawings. In the following description, identical elements or elements having identical functions will be designated by the same reference numerals, and duplicated descriptions will be omitted. Note that the present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0033] First Embodiment An example of an optical fiber according to a first embodiment will be described with reference to FIG. 1 . FIG. 1 is a cross-sectional view of the optical fiber according to the first embodiment. As shown in FIG. 1 , an optical fiber 10 includes a glass fiber 13 including a central axis G, at least one core 11, and a cladding 12 covering the core 11, and extending along the central axis G, and a resin coating layer 16 that contacts the glass fiber 13 and surrounds the glass fiber 13. The optical fiber 10 may be, for example, a multicore fiber. In this case, the optical fiber 10 may include a glass fiber 13 including a plurality of cores 11 and a cladding 12 covering the plurality of cores 11, a primary resin layer 14 that coats the outer periphery of the glass fiber 13, and a secondary resin layer 15 that coats the outer periphery of the primary resin layer 14. The primary resin layer 14 and the secondary resin layer 15 may be a resin coating layer 16 that contacts the glass fiber 13 and surrounds the glass fiber 13. Such an optical fiber 10 extends along the central axis G.

[0034] In the optical fiber 10, a plurality of cores 11 (for example, three cores 11 in the example of FIG. 1 ) are arranged in the cladding 12 at a predetermined interval and arrangement angle. As a result, the optical fiber 10 has a fiber structure that has directionality with respect to the rotation direction based on the central axis G. Note that "having directionality with respect to the rotation direction" means that the cross-sectional structure (for example, the position of the cores of a multi-core fiber) changes during one rotation. The number of cores 11 included in the optical fiber 10 is not limited to three as shown in FIG. 1 , but may be seven, ten or more. Furthermore, the optical fiber 10 (glass fiber 13) is not limited to an MCF, and may be a PMF, HAF, or HCF that requires alignment in the rotation direction. When the optical fiber 10 is an optical fiber such as a PMF, the number of cores 11 may be at least one.

[0035] The core 11 is made of pure silica (SiO ２The cladding 12 is made of silica glass or silica glass doped with germanium dioxide or fluorine. The cladding 12 has a refractive index lower than that of the core 11. The cladding 12 is made of, for example, pure silica glass or silica glass doped with fluorine. A trench having a refractive index lower than that of the cladding 12 may be provided between each core 11 and the cladding 12.

[0036] The primary resin layer 14 coats the outer periphery of the clad 12 of the glass fiber 13. Specifically, the primary resin layer 14 is in contact with the outer periphery of the clad 12 and coats the entire circumferential surface of the clad 12. The secondary resin layer 15 further coats the outer periphery of the primary resin layer 14. Specifically, the secondary resin layer 15 is in contact with the outer periphery of the primary resin layer 14 and coats the entire circumferential surface of the primary resin layer 14. As shown in FIG. 1 , the secondary resin layer 15 according to this embodiment has a rectangular shape (e.g., a square shape) in a cross section perpendicular to the central axis G. Details will be described later. Although not shown, a colored layer may be provided on the outer periphery of the secondary resin layer 15, or the secondary resin layer 15 may be colored, in order to identify the optical fiber.

[0037] The primary resin layer 14 is formed by curing, with ultraviolet light, an ultraviolet-curable resin composition containing a photopolymerizable compound, a photopolymerization initiator, and a silane coupling agent. Examples of the photopolymerizable compound include urethane (meth)acrylate and epoxy (meth)acrylate. The primary resin layer 14 has a lower modulus of elasticity (Young's modulus) than the secondary resin layer 15, and is softer than the secondary resin layer 15. For example, the Young's modulus of the primary resin layer 14 at 23°C is 0.1 MPa or more and 5 MPa or less. This provides the optical fiber 10 with lateral pressure resistance, preventing an increase in transmission loss in the optical fiber 10 even when lateral pressure is applied.

[0038] The secondary resin layer 15 is formed by curing a resin composition containing urethane (meth)acrylate, a monomer, and a photopolymerization initiator with ultraviolet light. The secondary resin layer 15 has a higher elasticity (Young's modulus) than the primary resin layer 14, and is harder than the primary resin layer 14. For example, the Young's modulus of the secondary resin layer 15 at 23°C is 1200 MPa or more and 2800 MPa or less. The thickness of each of the primary resin layer 14 and the secondary resin layer 15 is, for example, 5 μm or more and 50 μm or less. The thickness of the secondary resin layer 15 here refers to the thickness of the thinnest portion.

[0039] In the optical fiber 10 described above, if the outer diameter of the glass fiber 13 is 125 μm, the vertical and horizontal widths of the resin coating layer 16 (secondary resin layer 15) may be 250 μm. If the outer diameter of the glass fiber 13 is 180 μm, the vertical and horizontal widths of the secondary resin layer 15 may be 250 μm. For example, if the outer diameter of the glass fiber 13 is as large as 180 μm, the microbending resistance is improved. Therefore, the thickness of the primary resin layer 14 can be set to approximately 20 μm and the thickness of the secondary resin layer 15 to approximately 15 μm, thereby thinning the coating and making the overall outer diameter the same as that of a standard fiber (corresponding to the vertical and horizontal widths). In this way, by setting the vertical and horizontal widths of the optical fiber to 250 μm, the same as conventional products, and setting the center pitch between optical fibers to 250 μm, compatibility with existing products can be maintained.

[0040] Here, the structure of the resin coating layer 16 (secondary resin layer 15) of the optical fiber 10 will be described in detail with reference to Figures 1 to 3B. Figure 2A is a diagram showing the state of rotation adjustment when the optical fiber 10 is pressed against the conveying surface of a guide roller. Figure 2B is a diagram showing the state of rotation adjustment when an optical fiber having a general shape (circular) is pressed against the conveying surface of a guide roller. Figure 3A is a diagram showing an example of the state when inspection light L is incident on the optical fiber 10 to observe the position of the core 11. Figure 3B is a diagram showing the state when inspection light L is incident on an optical fiber having a general shape to observe the position of the core 11.

[0041] In the optical fiber 10 according to this embodiment, as shown in FIG. 1 , the resin coating layer 16 has a rectangular shape in a cross section perpendicular to the central axis G. A flat lower surface 17a, which is a part of this rectangular shape, functions as a reference surface that serves as a reference for the direction of orientation in the rotation direction with respect to the central axis G of the glass fiber 13. That is, the optical fiber 10 is fabricated so that the positions of the cores 11 in the rotation direction of the glass fiber 13 have a predetermined positional relationship with respect to the lower surface 17a, which is the reference surface. Therefore, as shown in FIG. 2A , when the optical fiber 10 is drawn and guided by the guide roller 107, the direction of the glass fiber 13 in the rotation direction can be adjusted by pressing the lower surface 17a, which is the reference surface of the optical fiber 10, against the guide surface 107a of the guide roller 107 with pressure (indicated by the arrow in the figure). 2B , when a general optical fiber 110 having a circular outer shape of the resin coating layer 116 (secondary resin layer 115) is drawn and guided by the guide roller 107, even if an attempt is made to adjust the directionality of the glass fiber 13 relative to the rotational direction while applying pressure (indicated by the arrow in the figure), it becomes difficult to adjust the directionality relative to the rotational direction of the glass fiber 13 because the outer peripheral surface is circular and easily rotates. Note that, even when the circular optical fiber 110 is in contact with the hypotenuse of the V-groove guide roller, it is difficult to adjust the directionality because the outer peripheral surface of the optical fiber 110 is circular and easily rotates, as in the case of FIG. 2B .

[0042] In other words, when the optical fiber 110 has a round outer shape, vibrations may occur during travel on the guide rollers or reeling, causing the outer periphery of the optical fiber 110 to rotate slightly due to contact vibration friction with the wall surfaces of the V-groove of the guide rollers. In contrast, the above-mentioned rectangular optical fiber 10 (e.g., rectangular shape) makes it easier to control minute rotations of 20 to 30 degrees or less. This allows for effective centering of the precision array angle.

[0043] As described above, in the optical fiber 10 according to this embodiment, the outer peripheral surface of the resin coating layer 16 is provided with a reference surface (flat lower surface 17a) that serves as a reference for the directionality of the glass fiber 13 relative to the rotation direction. This improves the alignment accuracy of the optical fiber 10, which has directionality relative to the rotation direction based on the central axis G. The width of the lower surface 17a, which serves as the reference surface, in the fiber width direction T1 may be 40 μm or more, 100 μm or more, 125 μm or more, or 150 μm or more. Increasing the width of the lower surface 17a further improves the alignment accuracy of the optical fiber 10. Note that in this optical fiber 10, the glass fiber 13 has a circular cross-sectional shape, which increases the degree of freedom in core placement. The circular cross-sectional shape of the glass fiber 13 facilitates achieving the circularity of the glass fiber 13 (clad 12), thereby improving the placement accuracy of the core position within the clad 12 and reducing splice loss. Unlike conventional techniques, the glass fiber is not dome-shaped, so more cores can be placed within a clad of the same outer diameter.

[0044] Furthermore, the optical fiber 10 according to this embodiment has a flat upper surface 17b, which is another part of the rectangular shape, as shown in FIG. 1 . When light is incident from the lower surface 17a and emitted from the upper surface 17b, the upper surface 17b functions as an observation surface that allows observation of the directionality relative to the rotation direction around the central axis G of the glass fiber 13. The upper surface 17b, which functions as the observation surface, faces the lower surface 17a, which functions as the reference surface, along the opposing axis S1. As a result, as shown in FIG. 3A , when observing the arrangement positions of the cores 11 of the glass fiber 13 in the rotation direction for inspection, inspection light L is incident from below (e.g., the lower surface 17a) and transmitted light L1 passes through the observation surface, allowing the arrangement of the cores 11 to be observed. Note that the multiple cores 11 are arranged along an axis perpendicular to the observation surface. Because this observation surface (upper surface 17b) is flat, the transmitted light L1 is less likely to be deformed. Therefore, in the optical fiber 10, the detection accuracy of the directionality relative to the rotation direction of the glass fiber 13 (e.g., the position of the core 11 in the rotation direction) can be improved. On the other hand, as shown in FIG. 3B , when the inspection light L is incident on a general optical fiber 110 having a circular resin coating layer 116 (secondary resin layer 115) and the transmitted light L2 is emitted from the observation surface, the transmitted light L2 tends to be deformed. For this reason, in the optical fiber 110, it is difficult to improve the detection accuracy of the directionality relative to the rotation direction of the glass fiber 13. Alternatively, the reference surface may be used as the observation surface, or the side surfaces 18 a and 18 b may be used as the observation surfaces. Using the side surfaces 18 a and 18 b as the observation surfaces (light incidence and emission surfaces) makes it easier to measure the core angle because the roller itself does not become an obstacle and the position of the core 11 can be observed in an area with little vibration near the roller.

[0045] As described above, in the optical fiber 10 according to this embodiment, the outer peripheral surface of the resin coating layer 16 is provided with an observation surface (e.g., a flat upper surface 17b or side surfaces 18a, 18b) that allows observation of the directionality of the glass fiber 13 relative to the rotation direction. Therefore, transmitted light L1 of the inspection light L irradiated onto the optical fiber 10 can be reliably observed through the observation surface (e.g., the flat upper surface 17b or side surfaces 18a, 18b). This improves the detection accuracy of the directionality of the glass fiber 13 relative to the rotation direction (e.g., the position of the core 11 in the rotation direction). The width of the upper surface 17b or the side surfaces 18a, 18b that function as the observation surface in the fiber width direction T1 is sufficient as long as it is wide enough to allow observation, but may be 40 μm or more, or may be 100 μm or more.

[0046] Next, a method for producing an optical fiber 10 having a rectangular outer shape will be described with reference to FIG. 4 . FIG. 4 is a diagram schematically illustrating a method for producing the optical fiber 10. As shown in FIG. 4 , the drawing apparatus 100 includes a furnace 101, a primary resin applicator 102, a secondary resin applicator 103, a primary resin curing device 104 a, a secondary resin curing device 104 b, a measuring device 105, and a direct-below roller 106. The drawing apparatus 100 further includes a guide roller 107 (see FIGS. 2A and 2B ) downstream of the direct-below roller 106 in the conveying direction. If necessary, a forming device for forming the optical fiber into a rectangular shape may be provided downstream of the secondary resin applicator 103.

[0047] In the manufacturing method using the drawing apparatus 100, a glass preform P is placed in a furnace 101, and the tip of the glass preform P is heated to a melting temperature. The glass preform P has core portions corresponding to the cores 11 and clad portions corresponding to the clads 12 and covering the multiple core portions. When the tip of the glass preform P is heated and melted to a state where it can be drawn, drawing begins, and a portion corresponding to the glass fiber 13 is drawn out of the furnace 101. The outer diameter of this glass fiber 13 may then be measured through a glass outer diameter measuring device (not shown). Based on the measured outer diameter of the glass fiber 13, the heating temperature of the furnace 101, the drawing speed, and the like are adjusted.

[0048] The drawn glass fiber 13 then enters a primary resin applicator 102, where a primary resin is applied to the glass fiber 13. The primary resin covers the glass fiber 13, which has a circular cross section, with a uniform thickness. When the glass fiber 13 is covered with the primary resin (corresponding to the primary resin layer 14), the fiber has a circular cross section. The primary resin is then cured by ultraviolet light, heat, or both in a primary resin curing device 104a. The glass fiber 13 then enters a secondary resin applicator 103, where a secondary resin is applied to the primary resin. The secondary resin covers the primary resin, which has a circular cross section. However, because the secondary resin (corresponding to the secondary resin layer 15) has a rectangular outer shape (e.g., a square-shaped corner), the thickness of the secondary resin varies depending on the location. To form a secondary resin having such a rectangular outer shape, the shape of the die used to apply the resin, the melt viscosity, surface tension, application temperature, etc. of the resin used, can be adjusted. A forming device for forming a rectangular shape may also be provided.

[0049] Next, after the secondary resin is applied to the glass fiber 13, the glass fiber 13 enters a secondary resin curing device 104b, where the coated resin is cured by ultraviolet light, heat, or both. By applying and curing the resin in this manner, a resin coating layer 16 including a primary resin layer 14 and a secondary resin layer 15 is formed. Various known methods can be used to form the resin coating layer. Note that the primary resin and secondary resin may be cured together.

[0050] Next, once the resin coating layer 16 has been formed, the optical fiber 10 is placed in a measuring device 105, where the shape and core position are measured. In measuring the shape, for example, the outer shape (the shape itself and the length and width, etc.) of the optical fiber 10 are measured. In measuring the core position, the position of each core 11 relative to the lower surface 17a, which is the reference surface, is measured, as shown in Fig. 3A. The position here includes the directionality in the rotational direction with respect to the central axis G of the optical fiber 10.

[0051] Thereafter, when the optical fiber 10 that has passed through the rollers 106 is rotated and conveyed by the guide rollers 107, the optical fiber 10 is rotationally aligned and wound up on a roll (not shown) based on the information on the shape and the core arrangement position measured by the measuring device 105. In this way, the above-mentioned optical fiber 10 is produced.

[0052] Modified examples of the optical fiber 10 according to the first embodiment will now be described with reference to Figures 5A, 5B, 6A, and 6B. Figure 5A is a cross-sectional view showing a first modified example of the optical fiber 10, and Figure 5B is a cross-sectional view showing a second modified example of the optical fiber 10. Figure 6A is a cross-sectional view showing a third modified example of the optical fiber 10, and the observation planes in this case are the edges on both sides of the reference plane (side surfaces 18a, 18b). Figure 6B is a cross-sectional view showing a fourth modified example of the optical fiber 10.

[0053] The optical fiber 10A according to a first modified example shown in FIG. 5A includes a glass fiber 13A including a plurality of cores 11 and a cladding 12, a primary resin layer 14 that coats the outer periphery of the glass fiber 13A, and a secondary resin layer 15 that coats the outer periphery of the primary resin layer 14. The optical fiber 10A has a large number of cores 11, i.e., 16. In the optical fibers 10, 10A according to this embodiment, the glass fibers 13, 13A (cladding 12) have a circular cross section, which allows for a high degree of freedom in core arrangement, allowing for a larger number of cores 11 to be arranged. For example, the optical fiber 10A according to the first modified example has a number of cores 11 of 16, which is greater than or equal to 10. Even the optical fiber 10A having such a large number of cores 11 can achieve the various effects described above.

[0054] The optical fiber 10B according to the second modification shown in FIG. 5B includes, similarly to the first modification, a glass fiber 13B including a plurality of cores 11 and a cladding 12, a primary resin layer 14 coating the outer periphery of the glass fiber 13B, and a secondary resin layer 15 coating the outer periphery of the primary resin layer 14. In the optical fiber 10B, the cores 11 of the first modification are arranged rotated 45° relative to the central axis G. This core arrangement in the optical fiber 10B allows the distance between the outermost cores 11a and 11b to be increased, and the imaginary line H connecting these cores 11a and 11b passes through the central axis G. Therefore, according to the second modification, when the position of the cores 11 is observed by irradiating the inspection light L from the lower surface 17a, the detection accuracy of the position of the cores 11 can be further improved. Note that in the optical fiber 10B according to the second modification, the outermost pair of cores 11a and 11b may be hollow holes. In this case, the difference in refractive index is increased, thereby further improving detection accuracy.

[0055] The optical fiber 10C according to the third modification shown in FIG. 6A includes a glass fiber 13 including a plurality of cores 11 and cladding 12, a primary resin layer 14 coating the outer periphery of the glass fiber 13, and a secondary resin layer 15C coating the outer periphery of the primary resin layer 14. In the optical fiber 10C, a portion of the lower surface 17C, which serves as a reference surface for the directionality of the glass fiber 13 relative to the rotation direction, is formed as a concave surface 19a. Even in this structure, the concave surface 19a is deformed flat when pressed against the guide roller, thereby functioning as the reference surface described above. Each of the protrusions 19b formed on both edges of the concave surface 19a may include a flat surface. The optical fiber 10C having a secondary resin layer 15C with such a shape can still achieve the various effects described above.

[0056] 6B includes a glass fiber 13 including a plurality of cores 11 and cladding 12, a primary resin layer 14D that coats the outer periphery of the glass fiber 13, and a secondary resin layer 15D that coats the outer periphery of the primary resin layer 14D. In the optical fiber 10D, not only the secondary resin layer 15D but also the primary resin layer 14D has a rectangular cross-sectional shape. In this case, the primary resin layer 14D and the secondary resin layer 15D have similar shapes, which makes it easier to improve the accuracy of forming the outer shape of the secondary resin layer 15D, including the lower surface 17a that serves as a reference surface.

[0057] Furthermore, in the optical fibers 10, 10A to 10D according to this embodiment, markers may be provided in the secondary resin layers 15, 15C, and 15D. The markers may be provided near the center of a side of the rectangular cross section, or may be provided off-center. The markers can be identified, for example, by utilizing a reaction in which the coating discolors in the irradiated area when the coating (secondary resin layer 15, 15C, and 15D) is irradiated with a laser. Unlike ink application, the thickness of the marker does not increase, and the discoloration of a specific area inside the coating provides the visibility required for the marker function. Of course, the markers may also be formed by ink application, etc.

[0058] Second Embodiment An example of an optical fiber ribbon according to a second embodiment will be described with reference to FIG. 7 . FIG. 7 is a cross-sectional view of the optical fiber ribbon according to the second embodiment. As shown in FIG. 7 , an optical fiber ribbon 20 includes a plurality of optical fibers 10 and connecting portions 25 connecting the optical fibers 10. The optical fibers 10 in the second embodiment can be any of the optical fibers 10, 10A to 10D, according to the first embodiment and its modifications. Although the optical fiber ribbon 20 shown in FIG. 7 includes eight optical fibers 10 connected together, this is not limiting and two or more optical fibers 10 may be connected together. Each of the optical fibers 10 extends along its central axis G, is arranged in a horizontal direction perpendicular to (intersecting) the central axis G, and is connected (fixed) to each other by connecting portions 25. The connecting portions 25 are formed from a tape-forming resin. Examples of the tape-forming resin include an ultraviolet-curable resin.

[0059] Here, a method for manufacturing the optical fiber ribbon 20 will be described. In this manufacturing method, a plurality of optical fibers 10 according to the first embodiment are first prepared (provided). Then, the plurality of optical fibers 10 are aligned so that they are arranged parallel to one another in the width direction. This alignment can be performed using a tape forming device or the like. During alignment, inspection light L for inspecting the core position may be irradiated onto the lower surface 17a of each optical fiber 10 to obtain transmitted light L1, and the directionality in the rotational direction based on the central axis G of each optical fiber 10 may be adjusted. This adjustment may keep the deviation in directionality (deviation in core arrangement angle) between the optical fibers 10 within a predetermined range. The difference in arrangement angle between the optical fibers 10 may be, for example, 5 degrees or less, 2 degrees or less, or 1 degree or less.

[0060] After that, when the adjustment of the directionality of the optical fibers 10 in the rotation direction is completed, the optical fibers 10 are connected to each other with a tape-forming resin (connecting resin) in a tape-forming device. The tape-forming resin is applied, for example, to the vicinity of the corners (R portions) of the optical fibers 10, and then cured by irradiating with ultraviolet light to form connecting portions 25. The connecting portions 25 may be formed over the entire length of the optical fibers 10, or may be formed intermittently along the longitudinal direction of the optical fibers 10.

[0061] In this manufacturing method, when aligning the optical fibers 10, the lower surfaces 17a, which are the reference surfaces of the optical fibers 10, are pressed against an optical fiber transport member (for example, a guide roller 107 (see FIG. 2B)), while adjusting the directionality of the glass fibers 13 with respect to the rotation direction based on the central axis G of the glass fibers 13, and aligning the optical fibers 10. Since such rectangular optical fibers 10 are used to form the tape, it is possible to easily manufacture an optical fiber ribbon 20 with improved alignment accuracy of the optical fibers 10, which have directionality with respect to the rotation direction based on the central axis G. Note that in the optical fiber ribbon 20, the cross-sectional shape of the glass fibers 13 of the optical fibers 10, which are components, is circular, and therefore, similar to the first embodiment, it is possible to increase the degree of freedom in core arrangement and the number of cores to be arranged.

[0062] In this manufacturing method, if the position of the core in the rotation direction of each optical fiber 10 is sufficiently accurate relative to the reference surface (lower surface 17a), the optical fibers 10 may be aligned and connected together using only the lower surface 17, which is the reference surface, as an index when tape-making. In this case, the process of producing the optical fiber ribbon 20 does not require the step of irradiating the inspection light L for inspection described above, and the manufacturing process can be simplified.

[0063] [Third Embodiment] An example of a flat optical fiber wire according to a third embodiment will be described with reference to Fig. 8 . Fig. 8 is a cross-sectional view of a flat optical fiber wire (hereinafter referred to as "flat fiber") according to the third embodiment. As shown in Fig. 8 , the flat fiber 30 includes a plurality of optical fibers 10 and an outer coating layer 35 connecting the optical fibers 10. The flat fiber 30 is a member having a flat cross-sectional shape including a minor axis (opposing axis S2) and a major axis (fiber width direction T2). The optical fibers 10 included in the flat fiber 30 can be any of the optical fibers 10, 10A to 10D according to the first embodiment and its modifications. Furthermore, the number of optical fibers 10 included in the flat fiber 30 is not limited to two, but may be two or more, such as three or four.

[0064] In the flat wire 30, a pair of (two) optical fibers 10 (glass fibers 13) are arranged in parallel in the horizontal direction and connected to each other so that the difference in the arrangement angles of the cores 11 of the two optical fibers 10 (glass fibers 13) on the left and right sides in the rotational direction around the central axis G is a certain value or less. In the flat wire 30, as will be described later, both built-in optical fibers 10 are drawn at the same time (simultaneously) using the same drawing device, which makes it easy to reduce the difference in the arrangement angles between them. Specifically, the difference in the arrangement angles of the optical fibers 10 included in the flat wire 30 may be, for example, 5 degrees or less, 2 degrees or less, or 1 degree or less. If the difference in the arrangement angles is 1 degree or less, when connecting the flat wires 30 to each other with a connecting portion 45 (tape-forming resin) in manufacturing an optical fiber ribbon according to a fourth embodiment described later (see FIG. 11 ), angle adjustment between the flat wires 30 is unnecessary or simplified, thereby improving manufacturing efficiency.

[0065] The outer coating layer 35 is a member for connecting and fixing the optical fibers 10 to each other. The outer coating layer 35 is disposed between the optical fibers 10 or on the outside of the optical fibers 10 to connect them. The outer coating layer 35 covers the entire circumferential surface of the optical fiber 10. The outer coating layer 35 is formed so as to extend continuously along the longitudinal direction of the optical fiber 10. Such an outer coating layer 35 is formed, for example, by curing an ultraviolet-curable resin, which is a tape-formed resin, with ultraviolet light or the like. The tape-formed resin has a lower elasticity (Young's modulus) than the secondary resin layer 15, and is softer than the secondary resin layer 15.

[0066] The structure of the outer coating layer 35 of the flat fiber 30 will now be described in detail. The outer coating layer 35 has a rectangular shape (e.g., a rectangular shape) in a cross section perpendicular to the central axis G. A flat lower surface 37, which is part of this rectangular shape, functions as a tape reference surface that serves as a reference for the orientation in the rotation direction with respect to the central axis G of each glass fiber 13. That is, the flat fiber 30 is fabricated so that the position of each core 11 of the glass fiber 13 in the rotation direction has a predetermined positional relationship with the lower surface 37, which is the tape reference surface. Therefore, when the flat fiber 30 is guided by guide rollers to be tape-formed, the orientation of the flat fiber 30 in the rotation direction can be adjusted by pressing the lower surface 37, which is the tape reference surface of the flat fiber 30, against the guide surface of the guide roller. The width D2 of the lower surface 37 in the fiber width direction T2 is longer than the distance D1 between the central axes G (first and second central axes) of the optical fiber 10. The width D2 of the lower surface 37 may be, for example, 100 μm or more, 250 μm or more, or 300 μm or more.

[0067] In the flat wire 30 according to this embodiment, a tape reference surface (flat lower surface 37) that serves as a reference for the directionality of the glass fiber 13 relative to the rotation direction is provided on the outer peripheral surface of the outer coating layer 35. This allows the flat wire 30 to have improved alignment accuracy for the optical fiber 10, which has directionality relative to the rotation direction based on the central axis G. Note that in the optical fiber 10 of the flat wire 30, the glass fiber 13 has a circular cross-sectional shape, which increases the degree of freedom in core placement. The circular cross-sectional shape of the glass fiber 13 makes it easier to achieve the circularity of the glass fiber 13 (clad 12), and increases the placement accuracy of the core position within the clad 12.

[0068] Furthermore, in the flat wire 30 according to this embodiment, the outer peripheral surface of the outer coating layer 35 is provided with a flat upper surface 38, which is another part of the rectangular shape. The upper surface 38 functions, for example, as an observation surface that allows observation of the orientation of the glass fiber 13 in the rotation direction relative to the central axis G of the glass fiber 13. The upper surface 38 is provided to face the lower surface 37, which serves as a reference surface, at the opposing axis S2. This allows the position of the cores 11 of each optical fiber 10 to be observed for inspection by irradiating inspection light L from below (e.g., the lower surface 37) and transmitting light L1 through the observation surface. The multiple cores 11 are arranged along an axis perpendicular to the observation surface. Because this observation surface (upper surface 38) is flat, the transmitted light L1 is less likely to deform. Therefore, in the flat wire 30, the detection accuracy of the orientation of the glass fiber 13 in the rotation direction (e.g., the position of the cores 11 in the rotation direction) can be improved. The width of each upper surface 38 functioning as an observation surface in the wire width direction T2 is sufficient to allow observation, and may be, for example, 40 μm or more, or 100 μm or more. The lower surface 37 may also be used as the observation surface.

[0069] Next, a method for producing the flat optical fiber 30 will be described with reference to FIGS. 8 and 9 . FIG. 9 is a diagram schematically illustrating a method for producing the flat optical fiber 30. The drawing apparatus 100A shown in FIG. 9 is configured to simultaneously draw two optical fibers 10 and to ensure that the core positions of the two optical fibers 10 are within a certain range. The drawing apparatus 100A includes a pair of furnaces 101, a pair of primary resin applicators 102, a pair of secondary resin applicators 103, a pair of primary resin curing devices 104a, a pair of secondary resin curing devices 104b, a pair of measuring devices 105, a pair of directly below rollers 106, a two-core concentrator 108a, a curing device 108b, an inspection device 109a, and a capstan 109b. If necessary, a forming device for forming the optical fiber into a rectangular shape may be provided downstream of each secondary resin applicator 103. In the manufacturing method using this wire drawing apparatus 100A, a pair of glass preforms P are placed in the corresponding furnaces 101, and the tip portions of the glass preforms P are heated to a predetermined melting temperature. The glass preforms P are the same as the glass preforms used in the first embodiment.

[0070] Next, when the tip of each glass preform P is heated and melted to a state where it can be drawn, drawing begins, and a portion corresponding to the glass fiber 13 is drawn from the furnace 101. The outer diameter of each glass fiber 13 may then be measured using a glass diameter measuring device. Based on the measured outer diameter of the glass fiber 13, the heating temperature of the furnace 101, the drawing speed, and the like are adjusted. The drawn glass fiber 13 then enters a twist detection device (not shown), which detects the twist around the central axis G of the glass fiber 13, which corresponds to the arrangement angle of the cores 11. Since this is a stage before the glass fiber 13 is coated with resin, the twist can be easily detected. Based on the detected twist value, the angle of the below-mentioned roller 106 is adjusted so that the difference in the arrangement angles of both glass fibers 13 in the rotation direction is equal to or less than a certain value. Specifically, the twist of the glass fiber 13 is adjusted by rotating the rotation axis of the below-mentioned roller 106 around the vertical axis or tilting it from the horizontal direction. This adjustment makes the difference in the arrangement angles of both glass fibers 13 5 degrees or less. The rotation angle and tilt angle of the immediately below roller 106 may be adjusted so that the difference in the arrangement angles is 2 degrees or less, or the rotation angle and tilt angle of the immediately below roller 106 may be finely adjusted so that the difference in the arrangement angles is 1 degree or less.

[0071] Next, as in the first embodiment, each of the angle-adjusted glass fibers 13 enters the corresponding primary resin applicator 102, where a primary resin is applied to the outer periphery of the glass fiber 13. The primary resin is then cured by ultraviolet light, heat, or both in the primary resin curing device 104a. The glass fiber 13 then enters the secondary resin applicator 103, where a secondary resin is applied to the outer periphery of the primary resin. The secondary resin (corresponding to the secondary resin layer 15) has a rectangular outer shape (e.g., a square). To form a secondary resin having such a rectangular outer shape, the shape of the die used to apply the resin, the melt viscosity of the resin used, and the application temperature, etc., are adjusted. After the secondary resin is applied to the glass fiber 13, each glass fiber 13 enters the corresponding secondary resin curing device 104b, where the coated resin is cured by ultraviolet light, heat, or both. This resin application and curing results in a resin coating layer 16 including the primary resin layer 14 and the secondary resin layer 15.

[0072] Next, once the resin coating layer 16 has been formed, the optical fiber 10 is placed in a measuring device 105, where the shape and core position are measured. In measuring the shape, for example, the outer shape (the shape itself and the length and width, etc.) of the optical fiber 10 are measured. In measuring the core position, the position of each core 11 relative to the lower surface 17a, which is the reference surface, is measured, as shown in Fig. 3A. The position here includes the directionality in the rotational direction with respect to the central axis G of the optical fiber 10.

[0073] Next, the optical fiber 10 with the resin coated on the outer periphery of the glass fiber 13 advances toward the dual-fiber concentrator 108a. As it advances toward the dual-fiber concentrator 108a, both optical fibers 10 move toward each other and are arranged adjacent to each other in the horizontal direction. In the dual-fiber concentrator 108a, both optical fibers 10 are arranged in parallel so that the difference in the arrangement angles of the cores 11 is equal to or less than a certain value. In this state, a resin (e.g., a tape-formed resin) used for the outer coating layer 35 is applied. The tape-formed resin may be the same material as the secondary resin. Then, in the curing device 108b, the resin is cured by ultraviolet light, heat, or both, to form the flat fiber 30 shown in FIG. 8 . In the dual-fiber concentrator 108a, both optical fibers 10 may be connected in parallel such that their outer peripheries are in contact with each other, or they may be connected in parallel such that a gap is provided between the outer peripheries of the both optical fibers 10 (i.e., the optical fibers are spaced apart).

[0074] When the flat wire 30 is produced, an inspection device 109a inspects whether the difference in the arrangement angles of the cores 11 of both optical fibers 10 in the flat wire 30 is equal to or less than a predetermined value. If necessary, a rotation angle control signal is sent to a device controlling controlled objects such as the directly below roller 106, and the inclination angle of the directly below roller 106, etc. is adjusted under control of the control device. The inspection device 109a may also inspect other configurations of the flat wire 30. After the inspection is completed, the flat wire 30 passes through a capstan 109b and is taken up by a take-up roller (not shown). In this way, in this method for producing the flat wire 30, the difference in the arrangement angles of the cores of the two optical fibers 10 is adjusted before being taken up by the take-up roller, so that twists occurring during winding occur in the same manner in both optical fibers 10. Therefore, when an optical fiber ribbon is produced using the flat wire 30, the work can be performed without worrying about the difference in the arrangement angles between the optical fibers in the flat wire 30. Furthermore, because the optical fibers 10 are drawn at the same timing in the drawing apparatus 100A with the optical fibers 10 aligned at the same angle, the tendency for deviation in the arrangement angles of the cores 11 of both optical fibers 10 tends to be the same, and it is easy to adjust the difference in the arrangement angles of the cores 11 of the optical fibers 10 when producing the flat fiber 30. Note that when producing the flat fiber 30, both optical fibers 10 may be produced separately and then tape-formed.

[0075] As described above, in the flat wire 30 according to this embodiment, a tape reference surface (lower surface 37) that serves as a reference for the directionality of the glass fibers 13 relative to the rotation direction is provided on the outer peripheral surface of the outer coating layer 35. This makes it possible to improve the alignment accuracy of the glass fibers 13, which have directionality relative to the rotation direction based on the central axis G. Furthermore, in the flat wire 30, the glass fibers 13 have a circular cross-sectional shape, which increases the degree of freedom in core arrangement. Note that, since the cross-sectional shape of the glass fibers 13 is circular, it is easy to achieve the circularity of each glass fiber 13 (clad 12), and the arrangement accuracy of the core position within the clad 12 can be improved.

[0076] Here, a modified example of the flat wire 30 according to the third embodiment will be described with reference to Fig. 10. Fig. 10 is a cross-sectional view showing a modified example of the flat wire 30 according to the third embodiment.

[0077] The flat wire 30A according to a modified example shown in Fig. 10 has a plurality of optical fibers 10 and an outer coating layer 35A connecting the optical fibers 10. In the flat wire 30A, a portion of the lower surface 37A, which serves as a reference surface for the directionality of the glass fibers 13 relative to the rotation direction, is formed as a concave surface 39a. Even in this structure, the concave surface 39a is deformed flat when pressed against the guide roller, so it functions as the reference surface described above. Each of the protrusions 39b formed on both edges of the concave surface 39a may include a flat surface. Even a flat wire 30A having an outer coating layer 35A with such a shape can achieve the various effects described above.

[0078] [Fourth Embodiment] An example of an optical fiber ribbon according to a fourth embodiment will be described with reference to FIG. 11 . FIG. 11 is a cross-sectional view of the optical fiber ribbon according to the fourth embodiment. As shown in FIG. 11 , an optical fiber ribbon 40 includes a plurality of flat wires 30 and a connecting portion 45 connecting the flat wires 30. In the fourth embodiment, any of the flat wires 30 and 30A according to the third embodiment and its modifications can be used. Furthermore, any of the optical fibers 10, 10A to 10D according to the first embodiment and its modifications can be used as the optical fibers 10 included in the flat wire. In the optical fiber ribbon 40 shown in FIG. 11 , four flat wires 30 (eight optical fibers 10) are connected together, but this is not limiting and two or more flat wires 30 may be connected together. Each of the flat wires 30 extends along the central axis G, is arranged horizontally in a direction perpendicular to (intersecting) the central axis G, and is connected (fixed) to each other by a connecting portion 45. The connecting portion 45 is formed from a tape-forming resin. The tape resin may be, for example, an ultraviolet curable resin.

[0079] Here, a method for manufacturing the optical fiber ribbon 40 will be described. In this manufacturing method, a plurality of flat wires 30 according to the third embodiment are first prepared (provided). Then, the plurality of flat wires 30 are aligned so that they are arranged parallel to one another in the width direction. This alignment can be performed using a tape forming device or the like. During alignment, an inspection light L for inspecting the core position is irradiated onto the lower surface 37 of each flat wire 30 to obtain transmitted light L1, and the directionality in the rotational direction around the central axis G of each optical fiber 10 is adjusted. This adjustment ensures that the deviation in directionality (deviation in core arrangement angle) between the optical fibers 10 or between the flat wires 30 is within a predetermined range. The difference in arrangement angle between the optical fibers 10 may be, for example, 5 degrees or less, 2 degrees or less, or 1 degree or less.

[0080] After that, when the adjustment of the directionality in the rotation direction of the optical fiber 10 is completed, the flat wires 30 are connected to each other with a tape-forming resin (connecting resin) in a tape-forming device. The tape-forming resin is applied, for example, to the vicinity of the corners (R portions) of the flat wires 30, and then cured by irradiating with ultraviolet light to form connecting portions 45. The connecting portions 45 may be formed over the entire length of the flat wires 30, or may be formed intermittently along the longitudinal direction of the flat wires 30.

[0081] In this manufacturing method, when aligning the flat wires 30, the lower surfaces 37, which are the reference surfaces of the respective flat wires 30, are pressed against an optical fiber transport member (e.g., a guide roller) to adjust the directionality of the glass fibers 13 relative to the rotation direction based on the central axis G of the glass fibers 13, and align the flat wires 30. Since the tape is made using such rectangular flat wires 30, it is possible to easily manufacture an optical fiber ribbon 40 with improved alignment accuracy of the optical fibers 10, which have directionality relative to the rotation direction based on the central axis G.

[0082] In this manufacturing method, if the position of the core in the rotation direction of each optical fiber 10 is sufficiently accurate relative to the reference surface (lower surface 37), the flat fiber 30 may be aligned and connected together using only the lower surface 37, which is the reference surface, as an index when tape-making. In this case, the process of producing the optical fiber ribbon 40 does not require the step of irradiating the inspection light L for inspection described above, and the manufacturing process can be simplified.

[0083] As described above, the optical fiber ribbon 40 according to the fourth embodiment can improve the alignment accuracy of the glass fibers 13, which have directionality with respect to the rotation direction based on the central axis G. In the optical fiber ribbon 40, the glass fibers 13 have a circular cross-sectional shape, which increases the degree of freedom in core arrangement.

[0084] Although not limited to this, by drawing each glass fiber 13 at the same time using the same drawing apparatus 100A for each of the flat strands 30 in the optical fiber ribbon 40, it is easy to make the arrangement angles of the glass fibers 13 forming each flat strand 30 similar. Furthermore, when the flat strands 30 including each glass fiber 13 are formed in one go by drawing in this manner, the flat strands 30 can be formed before twisting occurs during winding after drawing, which also makes it easy to make the arrangement angles of the glass fibers 13 similar. Furthermore, when the flat strands 30 including each glass fiber 13 are formed in one go by drawing in this manner, the arrangement angles can be adjusted before the outer peripheries of the glass fibers 13 are coated with resin, making alignment easier, which also makes it easy to make the arrangement angles of the glass fibers 13 similar.

[0085] The optical fiber, flat optical fiber, and optical fiber ribbon, as well as the manufacturing methods thereof, according to the embodiments of the present disclosure have been described in detail above. However, the present invention is not limited to the above embodiments and can be applied to various other embodiments and modifications. For example, the flat optical fiber 30 described above includes a pair of independent optical fibers 10, but the optical fiber may be bunched. In a bunched fiber, a pair of glass fibers are solid-state bonded to each other at their side surfaces. Specifically, the claddings are solid-state bonded to each other at their opposing ends to form an integrated fiber. In such a bunched fiber, the integrated fibers are collectively covered with a primary resin and a secondary resin. In this case, the cross-sectional shape of the secondary resin layer is rectangular, as shown in FIG. 8 .

[0086] The connecting portions 25, 45 of the optical fiber ribbons 20, 40 and the outer coating layer 35 of the flat fiber 30 are symmetrical in the vertical direction, but are not limited to this. That is, they may be asymmetrical in the vertical direction. In this case, the top and bottom of the optical fiber ribbons 20, 40 and the flat fiber 30 can be easily distinguished. For such distinction, a marker may be formed by irradiating a part of the connecting resin with a laser. Furthermore, as described above, in the optical fibers 10, 10A to 10D, a marker may be formed in a part of the secondary resin layer 15, 15C, 15D by irradiating a laser.

[0087] DESCRIPTION OF SYMBOLS 10, 10A, 10B, 10C, 10D, 110... Optical fiber 11, 11a, 11b... Core 12... Cladding 13, 13A, 13B... Glass fiber 14, 14D... Primary resin layer 15, 15C, 15D, 115... Secondary resin layer 16, 116... Resin coating layer 17a, 17C... Lower surface (reference surface) 17b... Upper surface (observation surface) 18a, 18b... Side surface 19a... Concave surface 19b... Protrusion 20, 40... Optical fiber ribbon 25, 45... Joint portion 30, 30A... Flat wire (flat optical fiber wire) 35, 35A... Outer coating layer 37, 37A... Lower surface (tape reference surface) 38... Upper surface (observation surface) 39a... Concave surface 39b... Protrusion 100, 100A... Fiber drawing device 101... Furnace 102... Primary resin application device 103... Secondary resin application device 104a... Primary resin hardening device 104b... Secondary resin hardening device 105... Measuring device 106... Directly below roller 107... Guide roller 108a... Two-core concentrator 108b... Hardening device 109a... Inspection device 109b... Capstan D1... Distance D2... Width G... Center axis H... Virtual line L... Inspection light L1, L2... Transmitted light P... Glass preform S1, S2... Opposing axes T1... Fiber width direction T2... Wire width direction

Claims

1. An optical fiber comprising: a glass fiber including a central axis, at least one core, and a cladding covering the core, and extending along the central axis; and a resin coating layer in contact with and surrounding the glass fiber, wherein the glass fiber has a circular shape in a cross section perpendicular to the central axis and has directionality relative to a rotation direction based on the central axis, and a reference plane is provided on the outer surface of the resin coating layer as a reference for the directionality of the glass fiber relative to the rotation direction.

2. The optical fiber according to claim 1, wherein the reference surface comprises a flat surface.

3. The optical fiber according to claim 2, wherein the flat surface has a width of at least 40 μm in the fiber width direction intersecting with the central axis.

4. An optical fiber according to any one of claims 1 to 3, wherein the outer surface of the resin coating layer is provided with an observation surface that enables observation of the orientation of the glass fiber relative to the rotation direction.

5. The optical fiber according to claim 4, wherein the observation surface comprises a flat surface.

6. The optical fiber according to claim 4 or claim 5, wherein the at least one core comprises a plurality of cores, and at least two of the plurality of cores are arranged along an axis perpendicular to the observation surface.

7. The optical fiber according to any one of claims 1 to 6, wherein the resin coating layer has a rectangular shape in a cross section perpendicular to the central axis.

8. The optical fiber according to any one of claims 1 to 7, wherein the at least one core comprises at least 10 or more cores.

9. The optical fiber according to any one of claims 1 to 8, wherein a marker is provided on the resin coating layer.

10. A glass fiber comprising: a first glass fiber having a first central axis, at least one first core, and a first cladding covering the first core, and extending along the first central axis; a second glass fiber having a second central axis, at least one second core, and a second cladding covering the second core, and extending along the second central axis; a first resin coating layer in contact with the first glass fiber and surrounding the first glass fiber; a second resin coating layer in contact with the second glass fiber and surrounding the second glass fiber; and an outer coating layer connecting the first resin coating layer and the second resin coating layer so that the first glass fiber and the second glass fiber are arranged in parallel, wherein the first glass fiber has a circular shape in a cross section perpendicular to the first central axis and has directionality with respect to a rotation direction based on the first central axis; and a reference plane serving as a reference for the directionality of the first glass fiber and the second glass fiber with respect to the rotation direction is provided on the outer peripheral surfaces of the first resin coating layer and the second resin coating layer, a tape reference surface that serves as a reference for the direction of at least the first glass fiber relative to the rotation direction is provided on an outer peripheral surface of the outer coating layer.

11. The flat optical fiber according to claim 10, wherein the tape reference surface includes a flat surface.

12. An optical fiber flat wire as set forth in claim 10 or 11, wherein the second glass fiber has a circular shape in a cross section perpendicular to the second central axis and has directionality relative to the rotation direction based on the second central axis, and the tape reference surface serves as a reference for the directionality of the second glass fiber relative to the rotation direction.

13. The flat optical fiber according to any one of claims 10 to 12, wherein the outer peripheral surface of the outer coating layer is provided with an observation surface that enables observation of the orientation of the first glass fiber relative to the rotation direction.

14. The flat optical fiber according to claim 13, wherein the observation surface includes a flat surface.

15. The flat optical fiber according to claim 13 or 14, wherein the at least one first core comprises a plurality of first cores, and at least two of the plurality of first cores are arranged along an axis perpendicular to the observation surface.

16. The flat optical fiber according to any one of claims 10 to 15, wherein the outer coating layer has a rectangular shape in a cross section perpendicular to the first central axis.

17. An optical fiber flat wire according to any one of claims 10 to 16, wherein at least one of the first resin coating layer and the second resin coating layer has a rectangular shape in a cross section perpendicular to the first central axis.

18. An optical fiber ribbon comprising: a plurality of optical fibers, each of which is a plurality of optical fibers according to any one of claims 1 to 9 and which are arranged in parallel with one another; and a connecting portion that connects the plurality of optical fibers to one another.

19. An optical fiber ribbon comprising: a plurality of flat optical fiber wires, each of which is a plurality of flat optical fiber wires according to any one of claims 10 to 17 and which are arranged in parallel with each other; and a connecting portion that connects the plurality of flat optical fiber wires to each other.

20. A method for manufacturing an optical fiber ribbon, comprising the steps of: providing a plurality of optical fibers, each of which is defined in any one of claims 1 to 9; aligning the plurality of optical fibers so that they are arranged in parallel with each other; and connecting the plurality of optical fibers to each other over their entire length or intermittently with a connecting resin, wherein in the aligning step, the reference surface of each of the plurality of optical fibers is pressed against an optical fiber transport member to adjust the orientation relative to the rotational direction of the glass fiber and align the plurality of optical fibers.

21. A method for manufacturing an optical fiber ribbon, comprising the steps of: providing a plurality of flat optical fiber strands, each of which is defined in any one of claims 10 to 17; aligning the plurality of flat optical fiber strands so that they are arranged in parallel with each other; and connecting the plurality of flat optical fiber strands to each other over their entire length or intermittently with a connecting resin, wherein in the aligning step, the tape reference surface of each of the plurality of flat optical fiber strands is pressed against an optical fiber transport member to adjust the direction relative to the rotational direction of the first glass fiber and align the plurality of flat optical fiber strands.

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