Multicore optical fiber and core identification method

Abstract

A multi-core optical fiber (1A) comprises a plurality of cores (10) extending along a fiber axis (C), a cladding (20) that includes the plurality of cores (10) therein and has rotational symmetry about the fiber axis (C) except for the portion thereof in which the plurality of cores (10) are provided, and a covering (30) that surrounds the cladding (20) and has rotational symmetry about the fiber axis (C). The multi-core optical fiber (1A) has non-inversion symmetry with respect to the fiber axis (C) in lateral observation images (2, 3) from at least one direction from among two directions orthogonal to the fiber axis (C).

Description

Technical Field

[0001] The present disclosure relates to a multi-core optical fiber and a core identification method. This application claims priority based on Japanese Application No. 2021-063042 filed on April 1, 2021, and incorporates all the descriptions set forth in the above-mentioned Japanese application.

Background Art

[0002] When the interference occurring between signals propagating through each core, that is, crosstalk, is sufficiently small, each core can be regarded as an independent transmission path, and a multi-core optical fiber having such cores is classified as a non-coupled fiber. In a non-coupled fiber, since a plurality of cores in the same fiber behave as independent transmission paths, a configuration for identifying the cores at both ends of the fiber is required. Patent Document 1 describes a multi-core optical fiber provided with a notch or a dummy core as a marker. Patent Document 2 describes a multi-core optical fiber in which the clad symmetry is impaired by separating the core group position by a predetermined distance from the clad center.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

[0004] A multicore optical fiber according to one embodiment of the present disclosure comprises a plurality of cores extending along the fiber axis, a cladding that encloses the plurality of cores and has rotational symmetry with respect to the fiber axis except for the portion where the plurality of cores are provided, and a covering that surrounds the cladding and has rotational symmetry with respect to the fiber axis. The multicore optical fiber has non-inversion symmetry with respect to the fiber axis in a lateral observation image from at least one of two directions orthogonal to the fiber axis.

[0005] A multicore optical fiber according to another embodiment of the present disclosure comprises a plurality of cores extending along the fiber axis; a low refractive index portion provided in a circle having a circumference centered on the fiber axis and passing through the center of the core closest to the fiber axis among the plurality of cores, in a cross section perpendicular to the fiber axis; a cladding that encloses the plurality of cores and the low refractive index portion and has rotational symmetry with respect to the fiber axis except for the portion where the plurality of cores and the low refractive index portion are provided; and a covering that surrounds the cladding and has rotational symmetry with respect to the fiber axis. The multicore optical fiber has non-inversion symmetry with respect to the fiber axis in a lateral observation image from at least one of two directions perpendicular to the fiber axis. [Brief explanation of the drawing]

[0006] [Figure 1] Figure 1 is a cross-sectional view of a two-core optical fiber according to a comparative example. [Figure 2] Figure 2 is a cross-sectional view of a two-core optical fiber according to the first embodiment. [Figure 3] Figure 3 shows a lateral observation image and brightness distribution of a two-core optical fiber according to the first embodiment. [Figure 4] Figure 4 is a cross-sectional view of a two-core optical fiber according to the second embodiment. [Figure 5] Figure 5 is a cross-sectional view of a two-core optical fiber according to the third embodiment. [Figure 6] Figure 6 is a cross-sectional view of a two-core optical fiber according to the fourth embodiment. [Figure 7]Figure 7 is a cross-sectional view of a two-core optical fiber according to the fifth embodiment. [Figure 8] Figure 8 is a cross-sectional view of a two-core optical fiber according to the sixth embodiment. [Modes for carrying out the invention]

[0007] [Issues this disclosure aims to address] In the invention described in Patent Document 1, the marker not only complicates the manufacturing of the fiber matrix but may also affect the propagation characteristics of the signal propagating through the core. In the invention described in Patent Document 2, although the core can be identified by observing the cross-section of the fiber, there are cases where observing the cross-section of the fiber is difficult, such as during fiber fusion.

[0008] Therefore, the object of this disclosure is to provide a multicore optical fiber and a core identification method that can identify the core without relying on markers and cross-sectional observation.

[0009] [Effects of this disclosure] This disclosure provides a multicore optical fiber and a core identification method that can identify the core without relying on markers or cross-sectional observation.

[0010] [Description of Embodiments in this Disclosure] First, embodiments of the present disclosure will be listed and described. A multicore optical fiber according to one embodiment comprises a plurality of cores extending along the fiber axis, a cladding that encloses the plurality of cores and has rotational symmetry with respect to the fiber axis except for the portion where the plurality of cores are provided, and a covering that surrounds the cladding and has rotational symmetry with respect to the fiber axis. The multicore optical fiber has non-inversion symmetry with respect to the fiber axis in a lateral observation image from at least one of two directions orthogonal to the fiber axis.

[0011] In one embodiment of a multicore optical fiber, the core can be identified by the non-inversion symmetry of the lateral observation image.

[0012] In a cross-section perpendicular to the fiber axis, the centroids of multiple core groups may be spaced apart from the fiber axis. Here, the "centroid of a core group" is also the point whose coordinates are the average of the position coordinates of the centers of each core on the cross-section.

[0013] Multiple cores may have the same shape as each other in the lateral observation image. Even in this case, the cores can be identified by non-inversion symmetry based on the arrangement of the cores.

[0014] At least one of the multiple cores may have a different shape from the other cores in the lateral observation image. In this case, the cores can be identified by non-inversion symmetry based on their shapes.

[0015] The multicore optical fiber described above may have the following configuration: the multicore optical fiber further comprises a low refractive index portion having a refractive index lower than that of the cladding. In a cross-section perpendicular to the fiber axis, the low refractive index portion is positioned to coincide with the fiber axis. The cladding has rotational symmetry with respect to the fiber axis, except for the portion where the multiple cores and low refractive index portion are provided. In this case, crosstalk between signals propagating in the two cores flanking the low refractive index portion can be suppressed. As a result, the inter-core distance required to ensure a certain crosstalk characteristic can be reduced, thereby reducing connection losses caused by rotational angle deviations around the fiber axis during fiber connection.

[0016] A multi-core optical fiber according to another embodiment includes a plurality of cores extending along a fiber axis, a low refractive index portion provided within a circle having a circumference centered on the fiber axis and passing through the center of the core closest to the fiber axis among the plurality of cores in a cross-section orthogonal to the fiber axis, a cladding that encloses the plurality of cores and the low refractive index portion and has rotational symmetry with respect to the fiber axis except for the portion where the plurality of cores and the low refractive index portion are provided, and a coating that surrounds the cladding and has rotational symmetry with respect to the fiber axis. The multi-core optical fiber has non-inversion symmetry with respect to the fiber axis in a side view image from at least one of two directions orthogonal to the fiber axis.

[0017] Even in a multi-core optical fiber according to another embodiment, the cores can be identified based on the non-inversion symmetry of the side view image.

[0018] In a cross-section orthogonal to the fiber axis, the low refractive index portion may have an elliptical shape with an asphericity higher than 0% and 10% or less. The asphericity is defined as a value representing the difference between the diameter of the circumscribed circle and the diameter of the inscribed circle of an object as a percentage of the average diameter. In this case, by flattening the shape of the low refractive index portion, while keeping the area of the low refractive index portion constant, the distance between the core and the low refractive index portion can be narrowed compared to the case of a perfect circle. Thereby, the crosstalk suppression effect can be enhanced.

[0019] The asphericity may be 6% or more and 10% or less. In this case, the distance between the core and the low refractive index portion can be further narrowed. Thereby, the crosstalk suppression effect can be further enhanced.

[0020] The plurality of cores may be two cores.

[0021] A core identification method according to an embodiment includes acquiring side view images from two directions orthogonal to the fiber axis of the multi-core optical fiber, and identifying a plurality of cores based on the non-symmetry with respect to the fiber axis in the side view images. Invert symmetry.

[0022] In the core identification method according to the above embodiment, the non- Invert The core can be identified by its symmetry.

[0023] The two directions mentioned above may be orthogonal to each other. In this case, the lateral view image may be non-linear. Invert It can detect symmetry with the utmost sensitivity.

[0024] [Details of the embodiments of this disclosure] Specific examples of the multicore optical fibers of this disclosure will be described below with reference to the drawings. However, the present invention is not limited to these examples, and is intended to include all modifications within the meaning and scope of the claims, as defined by the claims. In the description of the drawings, identical elements are denoted by the same reference numerals, and redundant descriptions are omitted.

[0025] Figure 1 is a cross-sectional view of a two-core optical fiber according to a comparative example. As shown in Figure 1, the two-core optical fiber 100 according to the comparative example comprises two cores 10, a cladding 20, and a covering 30.

[0026] The two cores 10 extend along the fiber axis C. The two cores 10 have the same shape as each other. In a cross-section perpendicular to the fiber axis C (hereinafter simply referred to as "cross-section"), the cores 10 have a circular shape. In the cross-section, the line segment L connecting the centers 10c of the cores 10 passes through the fiber axis C, and the line segment L is divided in two by the fiber axis C. That is, in the cross-section, the fiber axis C is the midpoint of the line segment L, and the distance from the center 10c of the two cores 10 to the fiber axis C is equal to each other. The cores 10 are made of silica glass containing a halogen such as chlorine. The glass forming the two cores 10 has the same composition as each other. The glass forming the two cores 10 may have different compositions as well.

[0027] The cladding 20 is a common cladding that encloses (surrounds) the two cores 10. The center 20c of the cladding 20 coincides with the fiber axis C. The cladding 20 has rotational symmetry with respect to the fiber axis C, except for the portion where the two cores 10 are provided. The cladding 20 has an optical cladding 21 and a physical cladding 22. The optical cladding 21 encloses the two cores 10 and is provided in contact with the outer circumferential surfaces of the two cores 10. The physical cladding 22 encloses the optical cladding 21 and is provided in contact with the outer circumferential surface of the optical cladding 21.

[0028] The optical cladding 21 is made of, for example, fluorine-containing silica glass. The refractive index of the optical cladding 21 is lower than that of the core 10. The physical cladding 22 is made of, for example, fluorine-containing silica glass. The refractive index of the physical cladding 22 is higher than that of the optical cladding 21 and lower than that of the core 10.

[0029] The coating 30 is provided on the outer circumferential surface of the cladding 20. The coating 30 encloses (surrounds) the cladding 20 and is provided in contact with the outer circumferential surface of the cladding 20. The coating 30 has rotational symmetry with respect to the fiber axis C. The coating 30 is made of resin. An example of the resin constituting the coating 30 is a urethane acrylate-based UV-curable resin.

[0030] The two-core optical fiber 100 does not have a notch or dummy core that functions as a marker. Furthermore, the two cores 10 have the same shape and are positioned in a cross-sectional area that is rotationally symmetrical with respect to the fiber axis C. Therefore, it is not possible to distinguish between the two cores 10.

[0031] Figure 2 is a cross-sectional view of a two-core optical fiber according to the first embodiment. As shown in Figure 2, the two-core optical fiber 1A according to the first embodiment differs from the two-core optical fiber 100 in the arrangement of the two cores 10, and is otherwise identical. The two-core optical fiber 1A comprises two cores 10 having the same shape as each other, a cladding 20 including an optical cladding 21 and a physical cladding 22, and a covering 30. The cladding 20 does not have markers, and the cladding 20 has rotational symmetry with respect to the fiber axis C, except for the portion where the two cores 10 are provided. The covering 30 has rotational symmetry with respect to the fiber axis C. The outer surface of the covering 30 does not have notches, and the outer surface of the covering 30 has rotational symmetry with respect to the fiber axis C.

[0032] In the two-core optical fiber 1A, the two cores 10 are positioned parallel to the line segment L such that the distance between the midpoint M of the line segment L and the fiber axis C is Δx in the cross-section. That is, the midpoint M of the line segment L is separated from the fiber axis C (center 20c of the cladding 20) by a distance Δx in the direction parallel to the line segment L.

[0033] Next, with reference to Figure 3, a core identification method for the two-core optical fiber 1A according to this embodiment will be described. In the core identification method, lateral observation images are acquired from two directions orthogonal to the fiber axis C, and the two cores 10 are identified based on the non-inversion symmetry with respect to the fiber axis C in the lateral observation images. Specifically, the core identification method includes the steps of acquiring lateral observation images, evaluating the inversion symmetry of the acquired lateral observation images, and identifying the cores 10 using the lateral observation images that are evaluated as not having symmetry.

[0034] In the process of acquiring a lateral observation image, the two observation directions are different from each other. For example, the two directions are orthogonal to each other. The lateral observation image is, for example, a transmitted image of a 2-core optical fiber 1A, and is acquired by irradiating the 2-core optical fiber 1A with parallel light parallel to the observation direction.

[0035] Figure 3 shows a lateral observation image and brightness distribution of a two-core optical fiber according to the first embodiment. In Figure 3, the coating 30 (see Figure 2) is not shown. Figure 3 shows lateral observation images 2 and 3 from two directions perpendicular to the fiber axis C. Here, lateral observation images 2 and 3 are images obtained by lateral observation from mutually orthogonal directions. The observation direction of lateral observation image 2 is parallel to line segment L. The observation direction of lateral observation image 3 is perpendicular to line segment L.

[0036] Figure 3 also shows the brightness (luminance) distribution corresponding to each of the lateral observation images 2 and 3. In each brightness distribution, the horizontal axis (axis perpendicular to the observation direction) indicates the radial position, and the vertical axis (axis parallel to the observation direction) indicates brightness. In the brightness distribution corresponding to lateral observation image 2, the horizontal axis is perpendicular to line segment L. In the brightness distribution corresponding to lateral observation image 3, the horizontal axis is parallel to line segment L. The brightness of the part corresponding to core 10 is higher than the brightness of the part corresponding to cladding 20.

[0037] In the lateral observation image 2, the observation direction is parallel to the line segment L, so the two cores 10 are observed overlapping each other. Since the line segment L passes through the fiber axis C, in the lateral observation image 2, both cores 10 are aligned with the fiber axis C. The two-core optical fiber 1A exhibits inversion symmetry with respect to the fiber axis C in the lateral observation image 2.

[0038] In the lateral observation image 3, the observation direction is perpendicular to the line segment L, so the two cores 10 are observed spaced apart from each other. Since the two cores 10 have the same shape, they also have the same shape (same width) in the lateral observation image 3, and are indistinguishable by their individual shapes. As described above, in the two-core optical fiber 1A, the two cores 10 are positioned parallel to the line segment L. For this reason, the two-core optical fiber 1A does not have inversion symmetry with respect to the fiber axis C in the lateral observation image 3.

[0039] In the symmetry evaluation process, for example, the lateral observation image is inverted by rotating the 2-core optical fiber 1A 180 degrees around the fiber axis C, and the orthogonality (degree of overlap) of the brightness distribution before and after inversion is evaluated. The orthogonality is evaluated, for example, by taking the dot product of the brightness distribution before and after inversion, and the evaluation is based on the obtained dot product. The dot product is higher the higher the degree of agreement. For example, if the dot product is above a predetermined threshold, it is judged to be inversion symmetry, and if it is below the threshold, it is judged to be asymmetric. The threshold is set, for example, to 60% of the maximum value of the dot product. The dot product is at its maximum value when the brightness distribution is the same.

[0040] The two-core optical fiber 1A exhibits non-inversion symmetry with respect to the fiber axis C in a lateral observation image 3 taken from at least one of two directions orthogonal to the fiber axis C. The non-inversion symmetry of the two-core optical fiber 1A in the lateral observation image 3 is an asymmetry that breaks the inversion symmetry with respect to the fiber axis C. In the identification step, the core 10 is identified using the lateral observation image 3, which is evaluated as not having inversion symmetry. According to the lateral observation image 3, two cores 10 that cannot be distinguished by their individual shapes can be identified. Furthermore, it is more preferable that the cross-sectional structure has symmetry with respect to mirror image inversion, as in the two-core optical fiber 1A shown in Figure 2. This makes the end face structures of the two ends of the optical fiber congruent, and both ends can have the same connectivity.

[0041] As explained above, the 2-core optical fiber 1A has an asymmetry that breaks the inversion symmetry with respect to the fiber axis C in the lateral observation image 3 from at least one of the two directions orthogonal to the fiber axis C. Furthermore, the core identification method acquires lateral observation images 2 and 3 of the 2-core optical fiber 1A from the two directions orthogonal to the fiber axis C, and identifies the two cores 10 based on the non-inversion symmetry of the 2-core optical fiber 1A with respect to the lateral observation image 3.

[0042] By adding a core 10 identification function to a multicore optical fiber fusion splicer, it becomes possible to identify the core 10 without acquiring a cross-sectional image of the 2-core optical fiber 1A. Furthermore, because alignment around the fiber axis C of the 2-core optical fiber 1A can be performed, fusion loss between multicore optical fibers can be suppressed.

[0043] Figure 4 is a cross-sectional view of a two-core optical fiber according to the second embodiment. As shown in Figure 4, the two-core optical fiber 1B according to the second embodiment differs from the two-core optical fiber 100 in the arrangement of the two cores 10, and is otherwise identical to the two-core optical fiber 100. That is, the two-core optical fiber 1B comprises two cores 10 having the same shape as each other, a cladding 20 including an optical cladding 21 and a physical cladding 22, and a covering 30. The cladding 20 has rotational symmetry with respect to the fiber axis C, except for the portion where the two cores 10 are provided. The covering 30 has rotational symmetry with respect to the fiber axis C.

[0044] In the two-core optical fiber 1B, the two cores 10 are positioned perpendicular to the line segment L such that the distance between the midpoint M of the line segment L and the fiber axis C is Δy in the cross-section. That is, the midpoint M of the line segment L is separated from the fiber axis C (center 20c of the cladding 20) by a distance Δy in the direction perpendicular to the line segment L.

[0045] Although not shown in the diagram, the 2-core optical fiber 1B does not have non-inversion symmetry with respect to the fiber axis C in the lateral observation image 3 (see Figure 3), where the observation direction is perpendicular to the line segment L. Therefore, the two cores 10 cannot be distinguished using only the lateral observation image 3. The 2-core optical fiber 1B has non-inversion symmetry with respect to the fiber axis C in the lateral observation image 2 (see Figure 3), where the observation direction is parallel to the line segment L. However, in the lateral observation image 2, since the observation direction is parallel to the line segment L, the two cores 10 are observed overlapping each other. Therefore, the two cores 10 cannot be distinguished using only the lateral observation image 2. In the 2-core optical fiber 1B, the two cores 10 can be distinguished using both the lateral observation images 2 and 3.

[0046] As explained above, the 2-core optical fiber 1B has an asymmetry that breaks the inversion symmetry with respect to the fiber axis C in the lateral observation image 2 from at least one of the two directions orthogonal to the fiber axis C. Furthermore, the core identification method acquires lateral observation images 2 and 3 of the 2-core optical fiber 1B from the two directions orthogonal to the fiber axis C, and identifies the two cores 10 based on the non-inversion symmetry of the 2-core optical fiber 1B with respect to the lateral observation image 2 and the lateral observation image 3. Therefore, it can be said that the 2-core optical fiber 1B and the core identification method can also distinguish the two cores 10 due to asymmetry.

[0047] Figure 5 is a cross-sectional view of a two-core optical fiber according to the third embodiment. As shown in Figure 5, the two-core optical fiber 1C according to the third embodiment differs from the two-core optical fiber 100, in that the core diameters (diameters) of the two cores 10 are different from each other, while being identical to the two-core optical fiber 100 in other respects. That is, the two-core optical fiber 1C comprises a cladding 20 including an optical cladding 21 and a physical cladding 22, and a covering 30. The cladding 20 has rotational symmetry with respect to the fiber axis C, except for the portion where the two cores 10 are provided. The covering 30 has rotational symmetry with respect to the fiber axis C.

[0048] The core diameter of one core 10 is equivalent to the core diameters of the two cores 10 of the two-core optical fiber 100. The core diameter of the other core 10 is larger than the core diameter of the first core 10. The positions of the centers 10c of the two cores 10 are the same as the positions of the centers 10c of the two cores 10 of the two-core optical fiber 100. Therefore, in cross-section, the shortest distance between one core 10 and the fiber axis C is longer than the shortest distance between the other core 10 and the fiber axis C. The two cores 10 are similar to each other. Note that the shortest distance between a point and a core is defined as the minimum value of the set of distances between all points contained in the core and that point.

[0049] Although not shown in the diagram, in the lateral observation image 3 (see Figure 3) where the observation direction is perpendicular to the line segment L, the two cores 10 have different shapes (different widths). The 2-core optical fiber 1C does not have inversion symmetry with respect to the fiber axis C in the lateral observation image 3. Therefore, in the 2-core optical fiber 1C, the cores 10 can be distinguished by the lateral observation image 3. In the lateral observation image 2 (see Figure 3) where the observation direction is parallel to the line segment L, the two cores 10 are observed overlapping each other. For this reason, the two cores 10 cannot be distinguished by the lateral observation image 2 alone.

[0050] As explained above, the 2-core optical fiber 1C has an asymmetry that breaks the inversion symmetry with respect to the fiber axis C in the lateral observation image 3 from at least one of the two directions orthogonal to the fiber axis C. Furthermore, the core identification method acquires lateral observation images 2 and 3 of the 2-core optical fiber 1C from the two directions orthogonal to the fiber axis C, and identifies the two cores 10 based on the non-inversion symmetry of the 2-core optical fiber 1C with respect to the lateral observation image 3. Therefore, it can be said that the 2-core optical fiber 1C and the core identification method can also distinguish the two cores 10 due to non-inversion symmetry.

[0051] Figure 6 is a cross-sectional view of a two-core optical fiber according to the fourth embodiment. As shown in Figure 6, the two-core optical fiber 1D according to the fourth embodiment differs from the two-core optical fiber 1B in that it includes a low refractive index section 40, and is otherwise identical to the two-core optical fiber 1B. The low refractive index section 40 is located within the optical cladding 21. The low refractive index section 40 has a refractive index lower than that of the optical cladding 21. The low refractive index section 40 is made of, for example, silica glass containing fluorine. The diameter of the low refractive index section 40 is, for example, smaller than the core diameter of the core 10.

[0052] The low refractive index portion 40 is located between the two cores 10 in cross-section. Specifically, the low refractive index portion 40 is positioned to coincide with the fiber axis C in cross-section. The low refractive index portion 40 has a circular shape in cross-section, with its center aligned with the fiber axis C. The cladding 20 has rotational symmetry with respect to the fiber axis C, except for the portion where the two cores 10 and the low refractive index portion 40 are located.

[0053] Although not shown in the diagram, similar to the 2-core optical fiber 1B, the 2-core optical fiber 1D does not have non-inversion symmetry with respect to the fiber axis C in the lateral observation image 3 (see Figure 3), but does have non-inversion symmetry with respect to the fiber axis C in the lateral observation image 2 (see Figure 3). In the lateral observation image 2, since the observation direction is parallel to the line segment L, the two cores 10 are observed overlapping each other. Therefore, in the 2-core optical fiber 1D, the two cores 10 can be identified using both the lateral observation images 2 and 3.

[0054] As explained above, the 2-core optical fiber 1D has an asymmetry that breaks the inversion symmetry with respect to the fiber axis C in the lateral observation image 2 from at least one of the two directions orthogonal to the fiber axis C. Furthermore, the core identification method acquires lateral observation images 2 and 3 of the 2-core optical fiber 1D from the two directions orthogonal to the fiber axis C, and identifies the two cores 10 based on the non-inversion symmetry of the 2-core optical fiber 1D with respect to lateral observation image 2 and lateral observation image 3. Therefore, it can be said that the 2-core optical fiber 1D and the core identification method can also distinguish the two cores 10 due to non-inversion symmetry.

[0055] The 2-core optical fiber 1D is equipped with a low refractive index section 40. Therefore, crosstalk between signals propagating through the two cores 10 flanking the low refractive index section can be suppressed. As a result, the distance between cores required to ensure a certain crosstalk characteristic can be reduced, thereby reducing connection loss caused by rotational angle misalignment around the fiber axis C during fiber connection. The smaller the distance between the fiber axis C and the center 10c of the core 10, the smaller the positional misalignment of the center 10c with respect to a certain rotational angle around the fiber axis C. Therefore, as described above, connection loss caused by angular misalignment can be reduced. Since the low refractive index section 40 is positioned to overlap with the fiber axis C, it does not function as a marker for identifying the core 10.

[0056] Figure 7 is a cross-sectional view of a two-core optical fiber according to the fifth embodiment. As shown in Figure 7, the two-core optical fiber 1E according to the fifth embodiment differs from the two-core optical fiber 1D in terms of the shape of the low refractive index portion 40, and is otherwise identical to the two-core optical fiber 1D. The low refractive index portion 40 has an elliptical shape in cross-section, with a noncircularity higher than 0% and 10% or less, and is arranged so that its major axis is parallel to the line segment L. The length of the major axis of the low refractive index portion 40 is, for example, equivalent to the core diameter of the core 10.

[0057] In the 2-core optical fiber 1E, the lateral observation image 2 from at least one of the two directions orthogonal to the fiber axis C exhibits an asymmetry that breaks the inversion symmetry with respect to the fiber axis C. Furthermore, the core identification method acquires lateral observation images 2 and 3 of the 2-core optical fiber 1E from two directions orthogonal to the fiber axis C, and identifies the two cores 10 based on the non-inversion symmetry of the 2-core optical fiber 1E with respect to lateral observation image 2 and lateral observation image 3. Therefore, it can be said that the 2-core optical fiber 1E and the core identification method can also distinguish the two cores 10 due to non-inversion symmetry.

[0058] In the 2-core optical fiber 1E, by making the cross-sectional shape of the low refractive index portion 40 flattened, the distance between the core 10 and the low refractive index portion 40 can be narrowed compared to the case of a perfect circle, while keeping the cross-sectional area of ​​the low refractive index portion 40 constant. This enhances the crosstalk suppression effect between signals transmitted between the two cores 10 flanking the low refractive index portion. The non-circularity may be between 6% and 10%, in which case the crosstalk suppression effect can be further enhanced. Since the cross-sectional area of ​​the low refractive index portion 40 is suppressed compared to the case of a perfect circle, manufacturing costs are also reduced. Furthermore, the influence of the low refractive index portion 40 on the optical properties of the 2-core optical fiber 1E is also suppressed.

[0059] Figure 8 is a cross-sectional view of a two-core optical fiber according to the sixth embodiment. As shown in Figure 8, the two-core optical fiber 1F according to the sixth embodiment differs from the two-core optical fiber 100 in that it includes a low refractive index portion 40, and is otherwise identical to the two-core optical fiber 100. The low refractive index portion 40 is located within the optical cladding 21. The low refractive index portion 40 has a refractive index lower than that of the optical cladding 21. The low refractive index portion 40 is made of, for example, silica glass containing fluorine. The diameter of the low refractive index portion 40 is, for example, shorter than the core diameter of the core 10. The low refractive index portion 40 has a circular cross-section, but may also have an elliptical cross-section. In this case, the low refractive index portion 40 may have an elliptical shape with a non-circularity higher than 0% and 10% or less, and may be arranged so that its major axis is parallel to the line segment L. The non-circularity may be 6% or more and 10% or less.

[0060] The low refractive index portion 40 is located between the two cores 10 in cross-section. Specifically, the low refractive index portion 40 has a circumference centered on the fiber axis C and passing through the center of the core 10 closer to the fiber axis C. R It is located inside. In a 2-core optical fiber 1F, the distance from the two cores 10 to the fiber axis C is equal to each other. Therefore, circle R The fiber axis C is at the center, and passes through the centers 10c of the two cores 10.

[0061] The low refractive index portion 40 is spaced apart from the fiber axis C in cross-section. The low refractive index portion 40 is spaced apart from the fiber axis C in both the direction parallel to line segment L and the direction perpendicular to line segment L in cross-section. The cladding 20 has rotational symmetry with respect to the fiber axis C, except for the portion where the two cores 10 and the low refractive index portion 40 are provided.

[0062] Although not shown in the diagram, in the 2-core optical fiber 1F, the low refractive index portion 40 is positioned at a different location from the fiber axis C in each of the lateral observation images 2 and 3 (see Figure 3). Therefore, even though the two cores 10 are positioned in inversion-symmetrical locations with respect to the fiber axis C, the 2-core optical fiber 1F exhibits non-inversion symmetry with respect to the fiber axis C in each of the lateral observation images 2 and 3. In other words, in the 2-core optical fiber 1F, the lateral observation images 2 and 3 can be made distinct from each other, and the two cores 10 can be distinguished, even without positioning the two cores 10 in non-inversion-symmetrical locations.

[0063] As explained above, the 2-core optical fiber 1F exhibits asymmetry that breaks the inversion symmetry with respect to the fiber axis C in the lateral observation images 2 and 3 taken from two directions perpendicular to the fiber axis C. Furthermore, the core identification method acquires lateral observation images 2 and 3 of the 2-core optical fiber 1F from two directions perpendicular to the fiber axis C, and identifies the two cores 10 based on the non-inversion symmetry of the 2-core optical fiber 1F with respect to the lateral observation images 2 and 3. Therefore, it can be said that the 2-core optical fiber 1F and the core identification method can also distinguish the two cores 10 due to non-inversion symmetry.

[0064] While embodiments have been described above, this disclosure is not necessarily limited to the embodiments described above, and various modifications are possible without departing from its essence. For example, there may be two or more cores.

[0065] In the cross-section, the midpoint M only needs to be spaced away from the fiber axis C in at least one of the directions parallel to line segment L and the direction perpendicular to line segment L, and may be spaced away from the fiber axis C in both the direction parallel to line segment L and the direction perpendicular to line segment L.

[0066] In a 2-core optical fiber 1C, the two cores 10 are similar to each other, but they do not have to be. For example, the cross-sectional shape of the other core 10 may not be circular. [Explanation of symbols]

[0067] 1A, 1B, 1C, 1D, 1E, 1F, 100… 2-core optical fiber 2,3…Lateral observation images 10... Cores 10c…center 20... Clad 20c…center 21…Optical cladding 22... Physical cladding 30... Covering 40... Low refractive index area C... Fiber axis L... line segment M…Midway point R... yen

Claims

1. Multiple cores extending along the fiber axis, A cladding that encloses the plurality of cores and, excluding the portion where the plurality of cores are provided, has rotational symmetry with respect to the fiber axis, A low refractive index portion having a refractive index lower than that of the cladding, A multicore optical fiber comprising a covering that surrounds the cladding and has rotational symmetry with respect to the fiber axis, In the lateral observation image from at least one of the two directions perpendicular to the fiber axis, the image has non-inversion symmetry with respect to the fiber axis, In a lateral observation image from a second direction perpendicular to both the fiber axis and at least one of the directions, the arrangement of the plurality of cores has inversion symmetry with respect to the fiber axis, allowing the plurality of cores to be identified. In a cross-section perpendicular to the fiber axis, the centroids of the plurality of core groups are spaced apart from the fiber axis. In a cross-section perpendicular to the fiber axis, the low refractive index portion is positioned at a location that overlaps with the fiber axis. The cladding, except for the portion where the plurality of cores and the low refractive index portion are provided, has rotational symmetry with respect to the fiber axis. Multicore optical fiber.

2. The plurality of cores have the same shape as each other in the lateral observation image. A multicore optical fiber according to claim 1.

3. At least one of the plurality of cores has a different shape from the other cores in the lateral observation image. A multicore optical fiber according to claim 1.

4. Multiple cores extending along the fiber axis, In a cross-section perpendicular to the fiber axis, a low refractive index portion is provided within a circle having a circumference that is centered on the fiber axis and passes through the center of the core closest to the fiber axis among the plurality of cores, A cladding that encloses the plurality of cores and the low refractive index portion, and which is symmetrical with respect to the fiber axis except for the portion in which the plurality of cores and the low refractive index portion are provided, A multicore optical fiber comprising a covering that surrounds the cladding and has rotational symmetry with respect to the fiber axis, In a lateral observation image from at least one of two directions perpendicular to the fiber axis, the arrangement of the plurality of cores has inversion symmetry with respect to the fiber axis. In a cross-section perpendicular to the fiber axis, the centroids of the plurality of core groups are spaced apart from the fiber axis. Multicore optical fiber.

5. In a cross-section perpendicular to the fiber axis, the low refractive index portion has an elliptical shape with a noncircularity higher than 0% and 10% or less. A multicore optical fiber according to any one of claims 1 to 4.

6. The aforementioned non-yen rate is between 6% and 10%. The multicore optical fiber according to claim 5.

7. The plurality of cores consist of two cores, and the line segment connecting the centers of the two cores coincides with a second direction perpendicular to both the fiber axis and at least one of the directions. A multicore optical fiber according to any one of claims 1 to 6.

8. A core identification method comprising acquiring lateral observation images from two directions perpendicular to the fiber axis of a multicore optical fiber according to any one of claims 1 to 7, and identifying the plurality of cores based on the non-inversion symmetry with respect to the fiber axis in the lateral observation images.

9. The core identification method according to claim 8, wherein the two directions are orthogonal to each other.

Images