Optical fiber winding device and optical fiber evaluation method

Abstract

This optical fiber winding device (10) comprises: a winding unit (40) that winds an optical fiber (11), which is a coupling-type multicore optical fiber, onto a winding member 41 that can rotate around a rotation center axis (41a); and a torsion imparting unit (50) that imparts torsion to the optical fiber (11) wound by the winding unit (40). The winding member (41) has a winding surface (42) onto which the optical fiber (11) is wound, and the winding surface (42) includes a curved surface (43). The curvature of the curved surface (43) with respect to the rotation center axis (41a) changes along at least one among the circumferential direction and the axial direction of the rotation center axis (41a).

Description

Optical fiber winding device and optical fiber evaluation method

[0001] The present disclosure relates to an optical fiber winding device and an optical fiber evaluation method.

[0002] As one of the transmission technologies for next-generation optical transmission systems that will support large-capacity communications, space division multiplexing (SDM) technology using coupled multi-core fiber (C-MCF) is being investigated. C-MCF is one of various types of multi-core optical fibers and is an optical fiber that tolerates crosstalk (mode coupling) between cores. C-MCF allows for a shorter spacing between adjacent cores, allowing for high-density packaging of multiple cores without increasing the outer diameter of the cladding.

[0003] In optical transmission systems using C-MCF, it is essential to introduce MIMO DSP (Multiple-Input Multiple-Output Digital Signal Processing) technology to compensate for crosstalk at the receiving side. The circuit scale of the MIMO DSP depends on the degree of spatial mode dispersion (SMD) between cores.

[0004] Non-Patent Document 1 shows that the SMD of C-MCF can be reduced by applying bending and twisting to the optical fiber. Non-Patent Document 2 shows that the SMD can be reduced by intentionally applying bending and twisting to the optical fiber using the tension of the bundle tape wound around a two-core C-MCF.

[0005] T. Sakamoto, T. Mori, M. Wada, T. Yamamoto, F. Yamamoto and K. Nakajima, "Fiber Twisting- and Bending-Induced Adiabatic / Nonadiabatic Super-Mode Transition in Coupled Multicore Fiber," in Journal of Lightwave Technology, vol. 34, no. 4, pp. 1228-1237, 15 Feb.15, 2016, doi: 10.1109 / JLT.2015.2502260.Y. Yamada et al., "Design of High-Density Cable Parameters for Controlling Spatial-Mode Dispersion of Randomly Coupled Multi-Core Fibers," in Journal of Lightwave Technology, vol. 39, no. 4, pp. 1179-1185, 15 Feb.15, 2021, doi: 10.1109 / JLT.2020.3045761.

[0006] In the past, to evaluate the SMD of optical fibers, a prototype optical cable was used to simulate conditions in which the optical fiber would be bent and twisted, and measurements were carried out under these conditions. Alternatively, as another method of evaluating SMD, the optical fiber was wound around a bobbin and measurements were carried out in that state.

[0007] The former requires the separate fabrication of an optical fiber cable for evaluation. On the other hand, the latter requires the optical fiber to be wound around a bobbin with a fixed radius of curvature, making it impossible to reproduce the bending and twisting of the optical fiber within the optical cable that occurs during the manufacturing and installation of the optical fiber. In other words, it is difficult to grasp the SMD of the optical fiber that occurs within the optical cable in its stand-alone state. Therefore, with either method, evaluating the SMD of an optical fiber tends to be time-consuming and costly.

[0008] An object of the present disclosure is to provide an optical fiber winding device capable of simulating bending and twisting of an optical fiber that occurs when a coupled multi-core optical fiber is housed in an optical cable, and an optical fiber evaluation method using the optical fiber winding device.

[0009] An optical fiber winding device according to a first aspect of the present disclosure includes a winding unit that winds an optical fiber, which is a coupled multi-core optical fiber, onto a winding member rotatable about a rotation center axis, and a twist imparting unit that imparts a twist to the optical fiber wound by the winding unit. The winding member has a winding surface around which the optical fiber is wound, the winding surface including a curved surface. The curvature of the curved surface relative to the rotation center axis varies along at least one of a circumferential direction and an axial direction of the rotation center axis.

[0010] An optical fiber evaluation method according to a second aspect of the present disclosure includes winding a first optical fiber, a second optical fiber that is a coupled multi-core optical fiber, and a third optical fiber, sequentially in layers, around a winding member of the optical fiber winding device according to the first aspect, irradiating laser light onto one of a plurality of cores at one end of the second optical fiber to sweep the wavelength, measuring the intensity of the wavelength-swept laser light from one of the plurality of cores at the other end of the second optical fiber, and performing a Fourier transform on the measured intensity to calculate its dispersion.

[0011] According to the present disclosure, it is possible to provide an optical fiber winding device capable of simulating bending and twisting of an optical fiber that occurs when a coupled multi-core optical fiber is housed in an optical cable, and an optical fiber evaluation method using the optical fiber winding device.

[0012] FIG. 1 is a configuration diagram of an optical fiber winding device according to an embodiment of the present disclosure. FIG. 2 is a front and side view of an example of a second bobbin (winding member) according to an embodiment. FIG. 3 is a diagram for explaining the major and minor radii of an ellipse. FIG. 4 is a diagram showing changes in curvature when the major radius is 500 mm and the minor radius is 100 mm. FIG. 5 is a diagram for explaining the relationship between combinations of the major and minor radii of an ellipse and combinations of the radii of the inscribed circle and the radii of the circumscribed surface. FIG. 6A is a diagram showing changes in the relative positions of a first pulley and a second pulley. FIG. 6B is a diagram showing changes in the relative positions of the first pulley and the second pulley. FIG. 7A is a diagram showing changes in the relative positions of the first pulley and the second pulley. FIG. 7B is a diagram showing changes in the relative positions of the first pulley and the second pulley. FIG. 8 is a diagram showing an example cross section of a winding surface and changes in its curvature. FIG. 9 is a front and side view of another example of a second bobbin (winding member). Fig. 10A is a diagram for explaining a twist imparting unit according to a first modified example. Fig. 10B is a diagram for explaining a twist imparting unit according to the first modified example. Fig. 11 is a diagram for explaining a twist imparting unit 50 according to a second modified example. Fig. 12 is a diagram for explaining a twist imparting unit 50 according to a third modified example. Fig. 13 is a diagram showing a winding state of an optical fiber in the optical fiber evaluation method according to the present embodiment. Fig. 14 is a diagram showing an example of a configuration for carrying out the optical fiber evaluation method.

[0013] An optical fiber winding device 10 according to an embodiment of the present disclosure will be described below. Note that common parts in the various figures are given the same reference numerals, and redundant explanations will be omitted. For ease of explanation, the "optical fiber winding device" will be simply referred to as the "winding device."

[0014] 1 is a diagram showing an example of the configuration of a winding device (optical fiber winding device) 10 according to this embodiment. The optical fiber winding device 10 according to this embodiment is a device that winds an optical fiber 11 while imparting a twist at a desired angle. Furthermore, during this winding, the optical fiber winding device 10 imparts a bend with a desired curvature to the optical fiber 11. For ease of explanation, hereinafter, the "optical fiber winding device" will be simply referred to as the "winding device."

[0015] The optical fiber 11 wound by the winding device 10 is a so-called coupled multi-core optical fiber. Therefore, the optical fiber 11 has multiple cores (not shown) in one cladding (not shown). The number of cores in the optical fiber 11 is, for example, 2, 4, 8, or 12. The multiple cores are arranged at positions with a periodicity such as a line, a ring, a square lattice, or a triangular lattice, depending on the number of cores. Furthermore, the multiple cores may be arranged so as to have a predetermined rotational symmetry around the central axis of the optical fiber 11.

[0016] The core spacing Λ, which is the minimum spacing between cores, is set to a value that allows mode coupling between the cores. The core spacing Λ is set to a value in the range of 10 μm to 30 μm, for example. However, the core spacing Λ is not limited to this range of values. The optical fiber 11 has a step-index, graded-index, or trench-assisted refractive index profile. Alternatively, the optical fiber 11 may have any other well-known refractive index profile.

[0017] 1, the winding device 10 includes a payout section 20, a guide section 30, and a winding section 40. The optical fiber 11 is paid out (sent out) from the payout section 20, passes through the guide section 30, and is wound by the winding section 40.

[0018] The unwinding unit 20 has a first bobbin 21. The optical fiber 11 is wound in advance around the first bobbin 21. When the optical fiber 11 is wound by the winding unit 40, the first bobbin 21 rotates, and the optical fiber 11 is unwound (sent out) from the first bobbin 21.

[0019] The guide unit 30 applies an appropriate tension to the optical fiber 11 between the payout unit 20 and the take-up unit 40 to prevent excessive slack. The guide unit 30 includes, for example, a pair of pulleys 31. The pair of pulleys 31 are connected to a known biasing device (not shown) and are arranged along the take-up direction (in other words, the longitudinal direction) of the optical fiber 11. A groove 32 that guides the optical fiber 11 is formed on the outer circumferential surface of each pulley 31. The groove 32 has, for example, a V-shaped cross section (see FIG. 6A ).

[0020] For ease of explanation, the pulley 31 and its groove 32 located upstream in the winding direction of the pair of pulleys 31, 31 will be referred to as the first pulley 31a and the first groove 32a, respectively. Similarly, the pulley 31 and its groove 32 located downstream in the winding direction will be referred to as the second pulley 31b and the second groove 32b, respectively.

[0021] As will be described later, the guide unit 30 may function as a twist imparting unit 50 that imparts a twist to the optical fiber 11. In this case, one of the first pulley 31a and the second pulley 31b (a pair of pulleys 31, 31) moves parallel to the other pulley so as to impart a twist to the optical fiber 11 (see FIGS. 7A and 7B ). The angle of the twist imparted to the optical fiber 11, i.e., the angle of rotation of the optical fiber 11 around the central axis of the optical fiber 11, can be set by controlling the relative positional deviation between the first groove portion 32a and the second groove portion 32b as viewed in the winding direction.

[0022] The winding unit 40 has a second bobbin 41 as a winding member that can rotate around a rotation center axis 41 a. The second bobbin 41 rotates around the rotation center axis 41 a by a driving device (not shown) such as a motor, thereby winding the optical fiber 11 onto the second bobbin 41.

[0023] FIG. 2 is a front and side view of an example of the second bobbin 41. As shown in FIG. 2, the second bobbin 41 has a winding surface 42 around which the optical fiber 11 is wound. The winding surface 42 is formed in a cylindrical shape centered on the rotation axis 41a. The winding surface 42 also includes at least a curved surface 43. The curvature of the curved surface 43, based on the rotation axis 41a, changes along the circumferential direction of the rotation axis 41a. For example, the curved surface 43 changes continuously along the circumferential direction of the rotation axis 41a. In this case, as shown in FIG. 2, the cylindrical winding surface 42 (curved surface 43) has a cross section of an ellipse 44 that is perpendicular to the rotation axis 41a.

[0024] 3 is a diagram illustrating the major radius a and the minor radius b of an ellipse 44. As shown in Fig. 3, when the winding surface 42 (curved surface 43) forms a cross section of the ellipse 44, the maximum value of the curvature ρ is equal to the curvature of an inscribed circle 45 inscribed at an intersection 44a between the ellipse 44 and the major axis Ea of the ellipse 44. The minimum value of the curvature ρ is equal to the curvature of a circumscribed circle 46 circumscribed at an intersection 44b between the ellipse 44 and the minor axis Eb of the ellipse 44.

[0025] The major axis a is the length from the center O of the ellipse 44 to the intersection 44a of the major axis Ea and the ellipse 44, i.e., 1 / 2 of the length of the major axis Ea. The minor axis b is the length from the center O of the ellipse 44 to the intersection 44b of the minor axis Eb and the ellipse 44, i.e., 1 / 2 of the length of the minor axis Eb. The radius Ra of the inscribed circle 45 is b. ２ The radius Rb of the circumscribing circle 46 is given by a ２ / b.

[0026] 4 is a diagram showing, as an example, how the curvature ρ changes when the major radius a is 500 mm and the minor radius b is 100 mm. As shown in this figure, the cross section of the winding surface 42 (curved surface 43) is an ellipse 44, so that the curvature ρ can be continuously changed according to the number of windings on the second bobbin 41.

[0027] According to Non-Patent Document 2, the curvature of the optical fiber in the optical cable is 0.5 to 20 m. －１ Therefore, the optical fiber in the optical cable is bent at an effective bending radius R of approximately 50 mm to 2000 mm. Therefore, the major radius a and the minor radius b according to this embodiment are set so that the curvature ρ is at least 0.5 mm. －１ 20m from －１ For example, if a = 500 mm and b = 100 mm, the radius Ra of the inscribed circle 45 is 20 mm, and the radius Rb of the circumscribed circle 46 is 2500 mm, and the optical fiber 11 can be bent continuously with bending radii ranging from 20 mm to 2500 mm (see FIG. 4). That is, the above-mentioned effective bending radius R of the cable can be reproduced by winding the optical fiber 11 around the winding surface 42 (curved surface 43).

[0028] The effective bending radius R is expressed by the following formula (1). where r and P are the radius of the helix formed by the optical fiber and the distance (pitch) of the helix traveled per one rotation, respectively. That is, the effective bending radius R is the radius of curvature when the optical fiber is bent into a helix.

[0029] The range of the bending radius of the optical fiber 11 when wound around the second bobbin 41 (winding surface 42) can be set by the combination of the major radius a and the minor radius b of the ellipse 44. Fig. 5 is a diagram for explaining the relationship between the combination of the major radius a and the minor radius b of the ellipse 44 and the combination of the radius Ra of the inscribed circle 45 and the radius Rb of the circumscribed circle 46. The horizontal axis of the graph represents the major radius a, and the vertical axis of the graph represents the radii of the inscribed circle 45 and the circumscribed circle 46. As an example, the graph in Fig. 5 shows how the radius Ra of the inscribed circle 45 and the radius Rb of the circumscribed circle 46 change when the minor radius b is set to 20 mm, 50 mm, 100 mm, and 200 mm, respectively, and the major radius a is changed.

[0030] According to the graph of FIG. 5, when simulating an optical cable with an effective bending radius R ranging from 50 mm to 3200 mm, it is sufficient to set the major radius a to 800 mm and the minor radius b to 200 mm, for example.

[0031] Next, the operation of the winding device 10 will be described. In the following description, an example will be given in which the guide unit 30 functions as the twist imparting unit 50. Figures 6A to 7B are diagrams showing changes in the relative positions of the first pulley 31a and the second pulley 31b of the guide unit 30.

[0032] In an initial state, the optical fiber 11 is wound beforehand around the first bobbin 21 of the unwinding unit 20. The end of the optical fiber 11 unwound from the first bobbin 21 is held by the second bobbin 41 of the winding unit 40 via the guide unit 30 (see FIG. 1). As shown in FIG. 6A, the first pulley 31a and the second pulley 31b are arranged in a line along the winding direction of the optical fiber 11. Furthermore, as shown in FIG. 6B, when viewed from the winding direction, the position of the bottom 33a of the first groove 32a, in which the optical fiber 11 is located, coincides with the position of the bottom 33b of the second groove 32b. Therefore, no twisting of the optical fiber 11 occurs.

[0033] The guide unit 30 applies a predetermined tension to the optical fiber 11 to prevent the optical fiber 11 from loosening. Then, the second bobbin 41 rotates to start winding the optical fiber 11. As described above, the curved surface 43 of the winding surface 42 changes along the circumferential direction. Therefore, the optical fiber 11 wound around the second bobbin 41 is bent in accordance with the curvature ρ of the curved surface 43.

[0034] While the second bobbin 41 is winding the optical fiber 11, or when the second bobbin 41 temporarily stops winding the optical fiber 11, the guide unit 30 serving as the twist imparting unit 50 imparts a twist to the optical fiber 11. For example, as shown in Fig. 7A , from a state in which the positions of the first groove portion 32a and the second groove portion 32b are aligned with each other when viewed from the winding direction, the second pulley 31b moves parallel to the first pulley 31a in a direction perpendicular to the winding direction.

[0035] In this case, the optical fiber 11 remains at the bottom 33a in the first groove 32a, while in the second groove 32b, it moves from the bottom 33b along the inner surface 34b. However, a predetermined tension is applied to the optical fiber 11 by the first pulley 31a and the second pulley 31b. Therefore, a force returning the optical fiber 11 in the second groove 32b to the bottom 33b of the second groove 32b is applied. As a result, while the optical fiber 11 moves downstream in the winding direction, it moves to the bottom 33b while being twisted by friction with the inner surface 34b of the second groove 32b. This imparts a twist to the optical fiber 11.

[0036] When twisting the optical fiber 11, the first pulley 31a may move in parallel with the second pulley 31b. In this case, the same twist can be imparted to the optical fiber 11.

[0037] In this way, the optical fiber 11 wound around the second bobbin 41 is given a bend according to the curvature ρ of the curved surface 43 and a twist due to the operation of the twist imparting unit 50 (guide unit 30). The curvature ρ imparted to the optical fiber 11 changes at least within the range of curvatures that can occur when the optical fiber 11 is housed in an optical cable. That is, according to this embodiment, it is possible to simulate the bend and twist of the optical fiber that occur when the coupled multi-core optical fiber is housed in an optical cable.

[0038] As long as the curved surface 43 has multiple curvatures on the same cross section, the cross-sectional shape of the second bobbin 41 is not limited to the ellipse described above. The cross-sectional shape of the curved surface 43 may be defined by combining curves such as a hyperbola, a parabola, a cycloid, and a cardioid.

[0039] FIG. 8 shows an example of a cross section of the winding surface 42 and the change in its curvature ρ. As shown in FIG. 8, the winding surface 42 of the second bobbin 41 may further include a plurality of flat surfaces 47 in addition to the curved surface 43. That is, the curvature ρ of the curved surface 43 may vary intermittently along the circumferential direction of the rotation center axis 41a. In this case, the curved surfaces 43 and the flat surfaces 47 are alternately arranged along the circumferential direction, and the curved surfaces 43 are located at the vertices of the approximately polygonal cross section. Furthermore, the curved surfaces 43 located at each vertex have different curvatures, e.g., 1 / r1, 1 / r2, or 1 / r3. Therefore, as shown in FIG. 8, the curvature ρ imparted to the optical fiber 11 varies intermittently when the optical fiber 11 is wound around the second bobbin 41.

[0040] FIG. 9 shows a front and side view of another example of the second bobbin 41. As shown in FIG. 9, the second bobbin 41 may have a truncated cone shape with the rotation axis 41a as the central axis. That is, the curvature ρ of the curved surface 43 may vary along the axial direction of the rotation axis 41a. In this case, the winding surface 42 forms the outer peripheral surface of the truncated cone (conical surface). Furthermore, the cross section perpendicular to the rotation axis 41a is a circle, and the curvature of the circle varies along the axial direction from at least the minimum value to the maximum value of the desired curvature. When winding the optical fiber 11, the second bobbin 41 rotates around the rotation axis 41a while moving parallel to the rotation axis 41a.

[0041] The curvature ρ of the curved surface 43 may vary along both the circumferential direction and the axial direction of the rotation center axis 41 a. For example, the major axis a and the minor axis b of the ellipse 44 may vary along the axial direction of the rotation center axis 41 a.

[0042] In the above-described embodiment, the pair of pulleys 31, 31 of the guide unit 30 also serves as the twist imparting unit 50. However, as described below, the twist imparting unit 50 may be provided separately from the pair of pulleys 31, 31. In this case, intentional translation of the pulley 31 is not performed to rotate the optical fiber 11.

[0043] 10A and 10B are diagrams illustrating a twist imparting unit 50 according to a first modified example. As shown in Fig. 10A, the twist imparting unit 50 includes a rotating disk 51 and a receiving portion 52. The rotating disk 51 and the receiving portion 52 are installed, for example, at position P1 (see Fig. 1) between the pair of pulleys 31. The rotating disk 51 contacts the outer peripheral surface of the optical fiber 11 to rotate the optical fiber 11. The twist imparting unit 50 also includes the receiving portion 52 located on the opposite side of the rotating disk 51 with the optical fiber 11 sandwiched therebetween.

[0044] The rotating disk 51 is provided with a speed reduction mechanism 53 such as a worm gear. A driving device 54 such as a motor for rotating the rotating disk 51 is connected to the speed reduction mechanism 53. The speed reduction mechanism 53 reduces the number of rotations transmitted from the driving device. This makes it possible to finely adjust the rotation angle of the rotating disk 51. The receiving portion 52 rotatably receives the optical fiber 11 pressed by the rotating disk 51. The receiving portion 52 may be a pair of rollers 55, 55 arranged parallel to the optical fiber 11 (see FIG. 10A), or may be a base 56 having a groove 56a formed from a low-friction material such as fluororesin (see FIG. 10B).

[0045] The rotating disk 51 comes into contact with the outer peripheral surface of the optical fiber 11 and presses the optical fiber 11 against the receiving portion 52. The rotating disk 51 is then rotated by a driving device, causing the optical fiber 11 to rotate. As a result, a twist is imparted to the optical fiber 11.

[0046] 11 is a diagram illustrating a twist imparting unit 50 according to the second modification. The twist imparting unit 50 according to the second modification is disposed at position P2 (see FIG. 1 ) between the guide unit 30 and the second bobbin 41 in the winding direction of the optical fiber 11. In this case, the twist imparting unit 50 includes a gripping unit 57 that grips the optical fiber 11 and an arm unit 58 that supports the gripping unit 57. With the gripping unit 57 gripping the optical fiber 11, the driving device 54 rotates the gripping unit 57 around the central axis of the optical fiber 11 via the arm unit 58. This rotation imparts a twist to the optical fiber 11 between the twist imparting unit 50 and the second bobbin 41. The optical fiber 11 is then wound around the second bobbin 41 while still in a twisted state.

[0047] FIG. 12 is a diagram illustrating a twist imparting unit 50 according to a third modified example. As shown in FIG. 12, the twist imparting unit 50 according to the third modified example rotates the second bobbin 41 around an axis perpendicular to the rotation center axis 41a. For example, the second bobbin 41 has a shaft 41b whose central axis is the rotation center axis 41a. The driving device 54 is connected to this shaft 41b and rotates the shaft 41b around an axis perpendicular to the rotation center axis 41a. In the third modified example, the second bobbin 41 rotates with the optical fiber 11 wound around the second bobbin 41. This allows a twist to be imparted to the optical fiber 11.

[0048] Next, an optical fiber evaluation method according to this embodiment will be described. Fig. 13 is a diagram showing the winding state of an optical fiber in the optical fiber evaluation method according to this embodiment. Fig. 14 is a diagram showing an example of a configuration for implementing the optical fiber evaluation method. The optical fiber evaluation method according to this embodiment uses the above-mentioned winding device 10 to measure the spatial mode dispersion (SMD) occurring in the optical fiber. In this measurement, three optical fibers, a first optical fiber F1, a second optical fiber F2, and a third optical fiber F3, are used.

[0049] The second optical fiber F2 is a coupled multi-core optical fiber to be measured. Meanwhile, the first optical fiber F1 and the third optical fiber F3 are so-called dummy optical fibers. The first optical fiber F1 and the third optical fiber F3 may each have the same cladding diameter and coating diameter as the second optical fiber F2. In this case, the reproducibility of the lateral pressure between optical fibers generated within the cable can be improved.

[0050] 13, the first optical fiber F1, the second optical fiber F2, and the third optical fiber F3 are wound in succession in one layer around the second bobbin 41. The second optical fiber F2 is sandwiched between the first optical fiber F1 and the third optical fiber F3.

[0051] When winding the first optical fiber F1 around the second bobbin 41, the tension of the first optical fiber F1 may be adjusted to increase the reproducibility of the above-mentioned lateral pressure. The tension of the first optical fiber F1 can be adjusted by adjusting the position of the guide unit 30 or the second bobbin 41. In a similar manner, the tension of the third optical fiber F3 may be adjusted when winding the third optical fiber F3 around the second bobbin 41.

[0052] 14, a single-mode optical fiber (hereinafter referred to as SMF) 61 connected to the tunable laser 60 is fusion-spliced ​​to one end of the second optical fiber F2 wound around the second bobbin 41. Before this fusion-splicing, the core of the SMF 61 is aligned with one of the multiple cores of the second optical fiber F2.

[0053] Furthermore, the SMF 63 connected to the optical power meter 62 is fusion-spliced ​​to the other end of the second optical fiber F2. During this fusion-splicing, the core of the SMF 63 is also aligned with one of the multiple cores of the second optical fiber F2.

[0054] Next, the wavelength of the laser beam is swept by being input from the wavelength tunable laser 60 to the second optical fiber F2 via the SMF 61. The optical power meter 62 measures the intensity of the wavelength-swept laser beam via the SMF 63.

[0055] Thereafter, the intensity measured by the optical power meter 62 is subjected to a Fourier transform to calculate its dispersion, thereby obtaining the SMD of the second optical fiber F2.

[0056] The core of the second optical fiber F2 into which the laser light is incident and the core of the second optical fiber F2 measured by the optical power meter 62 may be the same or different. The second optical fiber F2 is a coupled multi-core optical fiber. Therefore, random mode coupling occurs between cores in the optical fiber, and the laser light can be measured from a core other than the core into which the laser light is incident, and the SMD can be calculated from the intensity of the measured laser light.

[0057] The second optical fiber F2 is subjected to lateral pressure due to contact with the first optical fiber F1 and the third optical fiber F3. Furthermore, the second optical fiber F2 is given bending and twisting that occur when housed in an optical cable. That is, in the evaluation method according to this embodiment, the SMD of the second optical fiber F2 can be measured in a state simulating the state in which the second optical fiber F2 is housed in an optical cable. Therefore, it is possible to measure the SMD of the coupled multi-core optical fiber that occurs when housed in an optical cable without actually housing the coupled multi-core optical fiber in an optical cable, in other words, without separately fabricating a prototype optical cable. Therefore, it is possible to reduce the time and cost required for SMD evaluation.

[0058] The evaluation method according to this embodiment can simulate a state in which a coupled multi-core optical fiber is housed in an optical cable, so the phenomenon to be evaluated is not limited to SMD, and other phenomena that require this state to be assumed can also be evaluated.

[0059] REFERENCE SIGNS LIST 10 Optical fiber winding device 11 Optical fiber 20 Unwinding section 21 First bobbin 30 Guide section 31 Pulley 40 Winding section 41 Second bobbin 41a Rotational center axis 42 Winding surface 43 Curved surface 44 Ellipse 45 Inscribed circle 46 Circumscribed circle 47 Plane 50 Twist imparting section

Claims

1. An optical fiber winding device comprising: a winding unit that winds an optical fiber, which is a coupled multi-core optical fiber, onto a winding member that can rotate around a central rotation axis; and a twist imparting unit that imparts a twist to the optical fiber wound by the winding unit, wherein the winding member includes a winding surface around which the optical fiber is wound, the winding surface including a curved surface, and the curvature of the curved surface relative to the central rotation axis varies along at least one of the circumferential direction and the axial direction of the central rotation axis.

2. The optical fiber winding device according to claim 1, wherein the curvature changes continuously along the circumferential direction.

3. The optical fiber winding device according to claim 2, wherein the curved surface of the winding member is formed into a cylindrical shape having an elliptical cross section perpendicular to the central axis of rotation.

4. The optical fiber winding device according to claim 3, wherein the major axis of the ellipse is 500 mm or more, and the minor axis of the ellipse is 1000 mm or more.

5. The optical fiber winding device according to claim 1, wherein the curvature varies intermittently along the circumferential direction.

6. An optical fiber winding device according to any one of claims 1 to 4, wherein the twist imparting section includes a pair of pulleys arranged along the winding direction of the optical fiber and having grooves that guide the optical fiber, and one of the pair of pulleys moves parallel to the other pulley so as to impart a twist to the optical fiber.

7. An optical fiber winding device according to any one of claims 1 to 4, wherein the twist imparting unit includes a rotating disk that comes into contact with the outer peripheral surface of the optical fiber to rotate the optical fiber.

8. An optical fiber evaluation method comprising: winding a first optical fiber, a second optical fiber which is a coupled multi-core optical fiber, and a third optical fiber in sequence in one layer around a winding member of an optical fiber winding device according to any one of claims 1 to 7; irradiating a laser beam onto one of a plurality of cores at one end of the second optical fiber to sweep the wavelength; measuring the intensity of the wavelength-swept laser beam from one of the plurality of cores at the other end of the second optical fiber; and performing a Fourier transform on the measured intensity to calculate its dispersion.

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