Multi-core optical fiber and core identification method

Abstract

A multicore optical fiber (1A) has a plurality of cores (10) extending along an optical fiber axis (C), a cladding (20) in which the plurality of cores (10) are built-in and which has rotational symmetry with respect to the optical fiber axis (C) except for a portion in which the plurality of cores (10) are provided, and a coating portion (30) that surrounds the cladding (20) and which has rotational symmetry with respect to the optical fiber axis (C). The above-described multicore optical fiber (1A) has non-inversion symmetry with respect to the optical fiber axis (C) when viewed as (2, 3) from the side in at least one of two directions orthogonal to the optical fiber axis (C).

Description

Technical Field

[0001] This invention relates to multi-core optical fibers and a method for identifying fiber cores. This application claims priority to Japanese Application No. 2021-063042, filed on April 1, 2021, the entire contents of which are incorporated herein by reference. Background Technology

[0002] When the interference (crosstalk) between the signals transmitted in each fiber core is sufficiently small, each fiber core can be considered an independent transmission path. Multi-core optical fibers with such cores are classified as uncoupled optical fibers. In uncoupled optical fibers, multiple fiber cores within the same fiber operate as independent transmission paths, therefore, structures are needed at both ends of the fiber for identifying the fiber cores. Patent Document 1 describes a multi-core optical fiber with notches or dummy cores as markers. Patent Document 2 describes a multi-core optical fiber where the core group is separated from the cladding center by a predetermined distance, thereby compromising cladding symmetry.

[0003] Patent Document 1: International Publication No. 2000 / 016131

[0004] Patent Document 2: International Publication No. 2016 / 027896 Summary of the Invention

[0005] One embodiment of the present invention relates to a multi-core optical fiber comprising: a plurality of cores extending along an optical fiber axis; a cladding having the plurality of cores internally and having rotational symmetry with respect to the optical fiber axis except for the portion containing the plurality of cores; and a covering portion surrounding the cladding and having rotational symmetry with respect to the optical fiber axis. The aforementioned multi-core optical fiber, when viewed laterally from at least one of two directions orthogonal to the optical fiber axis, exhibits non-reversed symmetry with respect to the optical fiber axis.

[0006] Another embodiment of the present invention relates to a multi-core optical fiber comprising: a plurality of cores extending along an optical fiber axis; a low-refractive-index portion disposed in a cross-section orthogonal to the optical fiber axis within a circle having a circumference passing through the center of the core among the plurality of cores closest to the optical fiber axis, centered on the optical fiber axis; a cladding having the plurality of cores and the low-refractive-index portion internally, and having rotational symmetry with respect to the optical fiber axis except for the portion having the plurality of cores and the low-refractive-index portion; and a covering portion surrounding the cladding, and having rotational symmetry with respect to the optical fiber axis. The above-described multi-core optical fiber, when viewed laterally from at least one of two directions orthogonal to the optical fiber axis, exhibits non-reversed symmetry about the optical fiber axis. Attached Figure Description

[0007] Figure 1 This is a cross-sectional view of the dual-core optical fiber involved in the scale.

[0008] Figure 2 This is a cross-sectional view of the dual-core optical fiber according to the first embodiment.

[0009] Figure 3 This is a diagram showing the side view image and brightness distribution of the dual-core optical fiber involved in the first embodiment.

[0010] Figure 4 This is a cross-sectional view of the dual-core optical fiber according to the second embodiment.

[0011] Figure 5 This is a cross-sectional view of the dual-core optical fiber according to the third embodiment.

[0012] Figure 6 This is a cross-sectional view of the dual-core optical fiber according to the fourth embodiment.

[0013] Figure 7 This is a cross-sectional view of the dual-core optical fiber according to the fifth embodiment.

[0014] Figure 8 This is a cross-sectional view of the dual-core optical fiber according to the sixth embodiment. Detailed Implementation

[0015] [The problem to be solved by this invention]

[0016] In the invention described in Patent Document 1, the marking not only complicates the manufacturing of the optical fiber mother material but may also affect the transmission characteristics of the signal transmitted in the fiber core. In the invention described in Patent Document 2, although the fiber core can be identified by observing the cross-section of the optical fiber, cross-sectional observation of the optical fiber is sometimes difficult, such as during optical fiber splicing.

[0017] Therefore, the object of the present invention is to provide a multi-core optical fiber and a fiber core identification method that can identify the fiber core without being subject to marking and cross-sectional observation.

[0018] [Effects of the Invention]

[0019] According to the present invention, a multi-core optical fiber and a fiber core identification method are provided that enable core identification regardless of marking and cross-sectional observation.

[0020] [Description of Embodiments of the Invention]

[0021] First, embodiments of the present invention will be described. One embodiment of the multi-core optical fiber includes: a plurality of cores extending along the fiber axis; a cladding having the plurality of cores internally and having rotational symmetry with respect to the fiber axis except for the portion containing the plurality of cores; and a covering portion surrounding the cladding and having rotational symmetry with respect to the fiber axis. The aforementioned multi-core optical fiber, when viewed laterally from at least one of two directions orthogonal to the fiber axis, exhibits non-reversed symmetry with respect to the fiber axis.

[0022] In one embodiment of the multi-core optical fiber, the fiber core can be identified by the non-reversed symmetry of the lateral observation image.

[0023] In a cross-section orthogonal to the fiber axis, the centroids of multiple fiber core assemblies are separated from the fiber axis. Here, the "centroid of the fiber core assembly" is also the point with the average coordinates of the position coordinates of the centers of each fiber core in the cross-section.

[0024] Multiple fiber cores can appear to have the same shape when viewed from the side. In this case, the fiber cores can also be identified by the non-reversed symmetry based on their configuration.

[0025] At least one of the multiple fiber cores can be viewed from the side and has a shape that differs from the other fiber cores. In this case, the fiber cores can be identified by the non-reversed symmetry based on their shape.

[0026] The aforementioned multi-core optical fiber can have the following structure: It further includes a low-refractive-index section with a refractive index lower than that of the cladding. In a cross-section orthogonal to the fiber axis, the low-refractive-index section is positioned coinciding with the fiber axis. The cladding, except for the portion containing multiple cores and the low-refractive-index section, exhibits rotational symmetry with respect to the fiber axis. In this case, crosstalk between signals transmitted between two cores separated by the low-refractive-index section can be suppressed. As a result, the inter-core distance required to ensure certain crosstalk characteristics can be reduced, thus reducing connection loss caused by rotational angular deviations around the fiber axis during fiber connection.

[0027] Other embodiments of the multi-core optical fiber include: a plurality of cores extending along the fiber axis; a low-refractive-index portion disposed in a cross-section orthogonal to the fiber axis within a circle having a circumference passing through the center of the core closest to the fiber axis among the plurality of cores; a cladding containing the plurality of cores and the low-refractive-index portion, and having rotational symmetry with respect to the fiber axis except for the portion containing the plurality of cores and the low-refractive-index portion; and a covering portion surrounding the cladding, and having rotational symmetry with respect to the fiber axis. The aforementioned multi-core optical fiber, when viewed laterally from at least one of two directions orthogonal to the fiber axis, exhibits non-reversed symmetry about the fiber axis.

[0028] In other implementations of multi-core optical fibers, the fiber cores can also be identified by the non-reversal symmetry of the lateral observation image.

[0029] Alternatively, in a cross-section orthogonal to the fiber axis, the low-refractive-index portion can have an elliptical shape with a non-circularity greater than 0% and less than 10%. The non-circularity is defined as the difference between the diameter of the circumscribed circle and the diameter of the inscribed circle relative to the average diameter. In this case, by making the shape of the low-refractive-index portion flat, the area of ​​the low-refractive-index portion can be ensured to be constant, and the spacing between the fiber core and the low-refractive-index portion is narrowed compared to the case of a perfect circle. This improves crosstalk suppression.

[0030] The non-circularity can be between 6% and 10%. In this case, the gap between the fiber core and the low-refractive-index section can be further narrowed. This further improves the crosstalk suppression effect.

[0031] Multiple fiber cores can be two fiber cores.

[0032] One embodiment of the fiber core identification method obtains lateral observation images from two directions orthogonal to the fiber axis of the multi-core fiber, and identifies multiple fiber cores based on the non-reversal symmetry of the lateral observation images related to the fiber axis.

[0033] The fiber core identification method described in the above embodiments can identify the fiber core by the non-reversed symmetry of the side-view image.

[0034] The two directions mentioned above can be orthogonal to each other. In this case, the non-reversed symmetry of the side-view image can be detected most sensitively.

[0035] [Detailed Description of Embodiments of the Invention]

[0036] Hereinafter, specific examples of the multi-core optical fiber of the present invention will be described with reference to the accompanying drawings. Furthermore, the present invention is not limited to these examples, but is shown in the claims, which are intended to include all modifications equivalent to and within the scope of the claims. In the description of the drawings, the same reference numerals are used for the same elements, and repeated descriptions are omitted.

[0037] Figure 1 This is a cross-sectional view of the dual-core optical fiber involved in the scale. For example... Figure 1 As shown, the dual-core optical fiber 100 involved in the comparative example has two cores 10, a cladding 20, and a covering portion 30.

[0038] Two fiber cores 10 extend along the fiber axis C. The two fiber cores 10 have the same shape. In a cross-section orthogonal to the fiber axis C (hereinafter referred to as the "section"), the fiber core 10 has a circular shape. In the section, a line segment L connecting the centers 10c of the fiber cores 10 passes through the fiber axis C, and the line segment L is divided into two parts by the fiber axis C. That is, in the section, the fiber axis C constitutes the midpoint of the line segment L, and the distances from the centers 10c of the two fiber cores 10 to the fiber axis C are equal. The fiber cores 10 are, for example, made of silica glass containing halogens such as chlorine. The glass constituting the two fiber cores 10 may, for example, have the same composition. The glass constituting the two fiber cores 10 may also have different compositions.

[0039] Cladding 20 is a common cladding that houses (encloses) two fiber cores 10. The center 20c of cladding 20 coincides with the fiber axis C. Except for the portion containing the two fiber cores 10, cladding 20 is rotationally symmetric with respect to the fiber axis C. Cladding 20 has an optical cladding 21 and a physical cladding 22. The optical cladding 21 houses the two fiber cores 10 and is disposed in contact with the outer peripheral surfaces of the two fiber cores 10. The physical cladding 22 houses the optical cladding 21 and is disposed in contact with the outer peripheral surface of the optical cladding 21.

[0040] 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 fiber 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, but lower than that of the fiber core 10.

[0041] The covering portion 30 is disposed on the outer peripheral surface of the cladding 20. The covering portion 30 is embedded (enclosed) within the cladding 20 and is disposed in contact with the outer peripheral surface of the cladding 20. The covering portion 30 has rotational symmetry with respect to the fiber axis C. The covering portion 30 is made of resin. Examples of resins constituting the covering portion 30 include, for example, UV-curable resins of polyurethane acrylate.

[0042] The dual-core fiber 100 does not have a notch or dummy core for marking purposes. Furthermore, the two cores 10 have the same shape and are arranged in a rotationally symmetrical position about the fiber axis C in cross-section. Therefore, the two cores 10 cannot be identified.

[0043] Figure 2 This is a cross-sectional view of the dual-core optical fiber according to the first embodiment. Figure 2As shown, the dual-core optical fiber 1A according to the first embodiment differs from the dual-core optical fiber 100 in the configuration of the two cores 10, but is otherwise identical. The dual-core optical fiber 1A has: two cores 10 having the same shape; a cladding 20 comprising an optical cladding 21 and a physical cladding 22; and a covering portion 30. The cladding 20 has no markings and, except for the portion containing the two cores 10, is rotationally symmetric with respect to the fiber axis C. The covering portion 30 is rotationally symmetric with respect to the fiber axis C. The outer peripheral surface of the covering portion 30 has no notches and is rotationally symmetric with respect to the fiber axis C.

[0044] In a dual-core fiber 1A, the two fiber cores 10 are positioned after being moved parallel to the line segment L, with the distance between the midpoint M of the line segment L and the fiber axis C being Δx in the cross-section. That is, the midpoint M of the line segment L is separated from the fiber axis C (the center 20c of the cladding 20) by a distance Δx in a direction parallel to the line segment L.

[0045] Next, refer to Figure 3 The fiber core identification method for the dual-core optical fiber 1A according to this embodiment will be described. In the fiber core identification method, side view images are obtained from two directions orthogonal to the fiber axis C, and the two fiber cores 10 are identified based on the non-reversal symmetry of the side view images relative to the fiber axis C. Specifically, the fiber core identification method includes a step of obtaining side view images, a step of evaluating the reversal symmetry of the obtained side view images, and a step of identifying the fiber core 10 using side view images evaluated as lacking symmetry.

[0046] In the process of acquiring a side-view image, the two directions that constitute the observation direction are different from each other. The two directions are, for example, orthogonal to each other. The side-view image is, for example, a transmission image of a dual-core fiber 1A, which is acquired by illuminating the dual-core fiber 1A with parallel light parallel to the observation direction.

[0047] Figure 3 This is a diagram showing the side view image and brightness distribution of the dual-core optical fiber according to the first embodiment. Figure 3 The covering part 30 is omitted (see reference). Figure 2 The illustration is shown in the image. Figure 3 Images 2 and 3 are shown as side views from two directions orthogonal to the fiber axis C. Here, side views 2 and 3 are obtained by observing from the side from directions orthogonal to each other. The observation direction of side view image 2 is parallel to line segment L. The observation direction of side view image 3 is orthogonal to line segment L.

[0048] exist Figure 3The brightness distribution corresponding to each side view image 2 and 3 is also shown. The horizontal axis (orthogonal to the observation direction) of each brightness distribution indicates the position in the radial direction, and the vertical axis (parallel to the observation direction) indicates the brightness. In the brightness distribution corresponding to side view image 2, the horizontal axis is orthogonal to line segment L. In the brightness distribution corresponding to side view image 3, the horizontal axis is parallel to line segment L. The brightness of the portion corresponding to the fiber core 10 is higher than the brightness of the portion corresponding to the cladding 20.

[0049] When viewing image 2 from the side, the viewing direction is parallel to line segment L, so the two fiber cores 10 are observed in a state of overlap. Line segment L passes through the fiber axis C, so in viewing image 2 from the side, both fiber cores 10 coincide with the fiber axis C. The dual-core fiber 1A exhibits inversion symmetry related to the fiber axis C when viewed from the side in image 2.

[0050] In the side view of image 3, the observation direction is perpendicular to line segment L, so the two fiber cores 10 are observed in a separated state. The two fiber cores 10 have the same shape, therefore the side view image 3 also has the same shape (same width), and cannot be distinguished based on their individual shapes. As described above, in the dual-core fiber 1A, the two fiber cores 10 are positioned after being moved parallel to line segment L. Therefore, the side view image 3 of the dual-core fiber 1A does not have reverse symmetry about the fiber axis C.

[0051] In the process of evaluating symmetry, for example, rotating the dual-core fiber 1A 180 degrees relative to the fiber axis C to invert the side-view image, the orthogonality (degree of coincidence) of the brightness distributions before and after the inversion is evaluated. The evaluation of orthogonality is, for example, based on the inner product of the brightness distributions before and after the inversion. The higher the consistency, the higher the inner product. For example, if the inner product is above a predetermined threshold, it is judged as inverted symmetry; if it is below the threshold, it is judged as asymmetry. The threshold is, for example, set to 60% of the maximum value of the inner product. The inner product becomes the maximum value under the same brightness distribution.

[0052] The dual-core fiber 1A, viewed from the side in at least one of two directions orthogonal to the fiber axis C, exhibits non-reversal symmetry about the fiber axis C. This non-reversal symmetry of the dual-core fiber 1A in the side-view image 3 is an asymmetry that breaks the reversal symmetry associated with the fiber axis C. In the identification process, the fiber core 10 is identified using the side-view image 3, which is evaluated as lacking reversal symmetry. Based on the side-view image 3, two fiber cores 10 that cannot be distinguished by their individual shapes can be identified. Furthermore, as... Figure 2 As shown in the dual-core fiber 1A, a more preferable cross-sectional structure has symmetry related to mirror reversal. This allows the end faces of the two ends of the fiber to be made identical, ensuring that both ends have the same connectivity.

[0053] As explained above, the dual-core fiber 1A, when viewed from the side in at least one of two directions orthogonal to the fiber axis C, exhibits an asymmetry that disrupts the inversion symmetry associated with the fiber axis C. Furthermore, the fiber core identification method obtains side views 2 and 3 from two directions orthogonal to the fiber axis C of the dual-core fiber 1A, and identifies the two fiber cores 10 based on the non-inversion symmetry of the dual-core fiber 1A associated with the side view 3.

[0054] By adding the fiber core 10 identification function as described above to the multi-core fiber fusion splicer, the fiber core 10 can be identified without obtaining a cross-sectional image of the dual-core fiber 1A. Furthermore, alignment around the fiber axis C of the dual-core fiber 1A can be performed, thus suppressing splice loss between the multi-core fibers.

[0055] Figure 4 This is a cross-sectional view of the dual-core optical fiber according to the second embodiment. Figure 4 As shown, the dual-core optical fiber 1B according to the second embodiment differs from the dual-core optical fiber 100 in the configuration of the two cores 10, but is otherwise identical to the dual-core optical fiber 100. Specifically, the dual-core optical fiber 1B has: two cores 10 having the same shape; a cladding 20 comprising an optical cladding 21 and a physical cladding 22; and a covering portion 30. The cladding 20, except for the portion containing the two cores 10, is rotationally symmetric with respect to the fiber axis C. The covering portion 30 is rotationally symmetric with respect to the fiber axis C.

[0056] In the dual-core fiber 1B, the two fiber cores 10 are positioned after being moved perpendicular to the line segment L, with the distance between the midpoint M of the line segment L and the fiber axis C being Δy in the cross-section. That is, the midpoint M of the line segment L is separated from the fiber axis C (the center 20c of the cladding 20) by a distance Δy in a direction orthogonal to the line segment L.

[0057] Although the illustration is omitted, the image 3 observed from the side of the dual-core fiber 1B with the observation direction orthogonal to line segment L (see reference) Figure 3 The fiber does not possess non-reversed symmetry relative to the fiber axis C. Therefore, it is impossible to identify the two fiber cores 10 solely based on the side view image 3. The side view image 2 of the dual-core fiber 1B, with the observation direction parallel to line segment L (refer to...) Figure 3 The fiber cores 10 exhibit non-reversed symmetry relative to the fiber axis C. However, in the side view image 2, the observation direction is parallel to the line segment L, so the two fiber cores 10 are observed in a state of overlap. Therefore, it is impossible to identify the two fiber cores 10 based solely on the side view image 2. In the dual-core fiber 1B, both side view images 2 and 3 are used to identify the two fiber cores 10.

[0058] As explained above, the dual-core fiber 1B, when viewed laterally from at least one of two directions orthogonal to the fiber axis C, exhibits an asymmetry that disrupts the inversion symmetry associated with the fiber axis C. Furthermore, the fiber core identification method obtains lateral views 2 and 3 from two directions orthogonal to the fiber axis C of the dual-core fiber 1B, and identifies the two fiber cores 10 based on the non-inversion symmetry of the dual-core fiber 1B associated with lateral view 2 and lateral view 3. Therefore, in both the dual-core fiber 1B and the fiber core identification method, it can be said that the two fiber cores 10 can be identified through asymmetry.

[0059] Figure 5 This is a cross-sectional view of the dual-core optical fiber according to the third embodiment. For example... Figure 5 As shown, the dual-core optical fiber 1C according to the third embodiment differs from the dual-core optical fiber 100 in that the core diameters of the two cores 10 are different from each other, but is otherwise identical to the dual-core optical fiber 100. Specifically, the dual-core optical fiber 1C has: a cladding 20, which includes an optical cladding 21 and a physical cladding 22; and a covering portion 30. The cladding 20, except for the portion containing the two cores 10, is rotationally symmetric with respect to the fiber axis C. The covering portion 30 is rotationally symmetric with respect to the fiber axis C.

[0060] The core diameter of one fiber core 10 is the same as the core diameter of the two fiber cores 10 of the dual-core fiber 100. The core diameter of the other fiber core 10 is larger than the core diameter of the first fiber core 10. The positions of the centers 10c of the two fiber cores 10 are the same as the positions of the centers 10c of the two fiber cores 10 of the dual-core fiber 100. Therefore, in cross-section, the shortest distance between one fiber core 10 and the fiber axis C is longer than the shortest distance between the other fiber core 10 and the fiber axis C. The two fiber cores 10 are similar to each other. Furthermore, the shortest distance between a point and a fiber core is defined as the minimum value of the set of distances between all points contained within the fiber core and that point.

[0061] Although the illustration is omitted, image 3 is observed from the side with the observation direction orthogonal to line segment L (see reference). Figure 3 The two fiber cores 10 have different shapes (different widths). The side-view image 3 of the dual-core fiber 1C does not exhibit reverse symmetry with respect to the fiber axis C. Therefore, in the dual-core fiber 1C, the fiber cores 10 can be identified based on the side-view image 3. The side-view image 2, with the viewing direction parallel to line segment L (refer to...) Figure 3 The two fiber cores 10 are observed with their surfaces overlapping. Therefore, the two fiber cores 10 cannot be identified solely from the side view image 2.

[0062] As explained above, the dual-core fiber 1C, when viewed laterally from at least one of two directions orthogonal to the fiber axis C, exhibits an asymmetry that disrupts the inversion symmetry associated with the fiber axis C. Furthermore, the fiber core identification method obtains laterally viewed images 2 and 3 from two directions orthogonal to the fiber axis C of the dual-core fiber 1C, and identifies the two fiber cores 10 based on the non-inversion symmetry of the dual-core fiber 1C associated with the laterally viewed image 3. Therefore, in both the dual-core fiber 1C and the fiber core identification method, it can be said that the two fiber cores 10 can be identified through non-inversion symmetry.

[0063] Figure 6 This is a cross-sectional view of the dual-core optical fiber according to the fourth embodiment. For example... Figure 6 As shown, the dual-core optical fiber 1D according to the fourth embodiment differs from the dual-core optical fiber 1B in that it has a low-refractive-index portion 40, but is otherwise identical to the dual-core optical fiber 1B. The low-refractive-index portion 40 is disposed 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, for example, made of fluorine-containing silica glass. The diameter of the low-refractive-index portion 40 is, for example, smaller than the core diameter of the fiber core 10.

[0064] The low-refractive-index portion 40 is disposed between the two fiber cores 10 in cross-section. Specifically, the low-refractive-index portion 40 is configured in cross-section to coincide with the fiber axis C. The low-refractive-index portion 40 has a circular shape in cross-section, and its center is configured to coincide with the fiber axis C. Except for the portion of the cladding 20 that contains the two fiber cores 10 and the low-refractive-index portion 40, it has rotational symmetry with respect to the fiber axis C.

[0065] Although the illustration is omitted, similar to the dual-core fiber 1B, the dual-core fiber 1D is viewed from the side as image 3 (see reference). Figure 3 ), which does not have non-reversed symmetry relative to the fiber axis C, and when viewed from the side, image 2 (refer to Figure 3 The fiber cores 10 exhibit non-reversed symmetry relative to the fiber axis C. In side-view image 2, the observation direction is parallel to line segment L, thus the two fiber cores 10 are observed in a state of overlap. Therefore, in the dual-core fiber 1D, side-view images 2 and 3 are used to identify the two fiber cores 10.

[0066] As explained above, the dual-core fiber 1D, when viewed laterally from at least one of two directions orthogonal to the fiber axis C, exhibits an asymmetry that disrupts the inversion symmetry associated with the fiber axis C. Furthermore, the fiber core identification method obtains lateral views 2 and 3 from two directions orthogonal to the fiber axis C of the dual-core fiber 1D. Based on the non-inversion symmetry of the dual-core fiber 1D associated with lateral view 2 and lateral view 3, the two fiber cores 10 are identified. Therefore, in both the dual-core fiber 1D and the fiber core identification method, it can be said that the two fiber cores 10 can be identified through non-inversion symmetry.

[0067] The dual-core optical fiber 1D has a low-refractive-index section 40. This suppresses crosstalk between signals transmitted in the two cores 10 sandwiching the low-refractive-index section. Consequently, the inter-core distance required to ensure certain crosstalk characteristics can be reduced, thus decreasing connection loss caused by rotational angular deviation 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 deviation of the center 10c relative to a certain rotational angle around the fiber axis C. Therefore, as described above, connection loss caused by angular deviation can be reduced. The low-refractive-index section 40 is positioned to coincide with the fiber axis C and therefore does not function as a marker for identifying the cores 10.

[0068] Figure 7 This is a cross-sectional view of the dual-core optical fiber according to the fifth embodiment. For example... Figure 7 As shown, the dual-core optical fiber 1E according to the fifth embodiment differs from the dual-core optical fiber 1D in the shape of the low-refractive-index portion 40, but is otherwise identical to the dual-core optical fiber 1D. The low-refractive-index portion 40 has an elliptical shape in cross-section with a non-circularity greater than 0% and less than 10%, and is configured such that its major axis is parallel to line segment L. The length of the major axis of the low-refractive-index portion 40 is, for example, equal to the core diameter of the fiber core 10.

[0069] In the dual-core fiber 1E, the lateral view image 2, viewed from at least one of the two directions orthogonal to the fiber axis C, also exhibits an asymmetry that disrupts the inversion symmetry associated with the fiber axis C. Furthermore, the fiber core identification method obtains lateral view images 2 and 3 from two directions orthogonal to the fiber axis C of the dual-core fiber 1E. Based on the non-inversion symmetry of the dual-core fiber 1E associated with lateral view image 2 and lateral view image 3, the two fiber cores 10 are identified. Therefore, in both the dual-core fiber 1E and the fiber core identification method, it can be said that the two fiber cores 10 can be identified through non-inversion symmetry.

[0070] In the dual-core fiber 1E, the cross-sectional shape of the low-refractive-index portion 40 is made flat, thereby keeping the cross-sectional area of ​​the low-refractive-index portion 40 constant, and compared with the case of a perfect circle, the spacing between the fiber core 10 and the low-refractive-index portion 40 can be narrowed. This improves the crosstalk suppression effect between signals transmitted between the two fiber cores 10 sandwiching the low-refractive-index portion. The non-circularity can be 6% or more and 10% or less, in which case the crosstalk suppression effect can be further improved. Compared with the case where the cross-sectional shape of the low-refractive-index portion 40 is perfect, the cross-sectional area of ​​the low-refractive-index portion 40 is reduced, thus also reducing manufacturing costs. Furthermore, the influence of the low-refractive-index portion 40 on the optical characteristics of the dual-core fiber 1E is also reduced.

[0071] Figure 8 This is a cross-sectional view of the dual-core optical fiber according to the sixth embodiment. For example... Figure 8 As shown, the dual-core optical fiber 1F according to the sixth embodiment differs from the dual-core optical fiber 100 in that it has a low-refractive-index portion 40, but is otherwise identical to the dual-core optical fiber 100. The low-refractive-index portion 40 is disposed 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, for example, made of fluorine-containing silica glass. The diameter of the low-refractive-index portion 40 is, for example, smaller than the core diameter of the fiber core 10. The low-refractive-index portion 40 is circular in cross-section, but may also be elliptical. In this case, the low-refractive-index portion 40 has an elliptical shape with a non-circularity greater than 0% and less than 10%, and is configured such that its major axis is parallel to the line segment L. The non-circularity may also be 6% or more and less than 10%.

[0072] A low-refractive-index section 40 is disposed in cross-section between the two fiber cores 10. Specifically, the low-refractive-index section 40 is disposed in cross-section within a circle R centered on the fiber axis C, having a circumference passing through the center of the fiber core 10 closest to the fiber axis C. In the bi-core fiber 1F, the distances from the two fiber cores 10 to the fiber axis C are equal. Thus, the circle R, centered on the fiber axis C, passes through the center 10c of each of the two fiber cores 10.

[0073] The low-refractive-index portion 40 is separated from the fiber axis C in cross-section. The low-refractive-index portion 40 is separated from the fiber axis C in cross-section in both directions parallel to and orthogonal to line segment L. The cladding 20, except for the portion containing the two fiber cores 10 and the low-refractive-index portion 40, exhibits rotational symmetry with respect to the fiber axis C.

[0074] Although the illustration is omitted, images 2 and 3 are observed from the side in dual-core fiber 1F (refer to...). Figure 3In each of the two fiber cores 10, the low-refractive-index portion 40 is positioned differently from the fiber axis C. Therefore, although the two fiber cores 10 are positioned in a reverse-symmetric manner relative to the fiber axis C, the two-core fiber 1F exhibits non-reverse-symmetry relative to the fiber axis C in each of the side-view images 2 and 3. That is, in the two-core fiber 1F, even if the two fiber cores 10 are not positioned in a non-reverse-symmetric manner, the side-view images 2 and 3 can be made distinct from each other, and the two fiber cores 10 can be identified.

[0075] As explained above, the dual-core fiber 1F, when viewed laterally from two directions orthogonal to the fiber axis C, exhibits an asymmetry that disrupts the inversion symmetry associated with the fiber axis C. Furthermore, the fiber core identification method obtains the laterally viewed images 2 and 3 from two directions orthogonal to the fiber axis C of the dual-core fiber 1F, and identifies the two fiber cores 10 based on the non-inversion symmetry of the dual-core fiber 1F associated with the laterally viewed images 2 and 3. Therefore, in both the dual-core fiber 1F and the fiber core identification method, it can be said that the two fiber cores 10 can be identified through non-inversion symmetry.

[0076] The embodiments have been described above, but the present invention is not necessarily limited to the embodiments described above, and various modifications can be made without departing from its spirit. For example, there can be two or more fiber cores.

[0077] In the cross section, the midpoint M can be separated from the fiber axis C in at least one of the directions parallel to line segment L and orthogonal to line segment L. Separation can occur in both the direction parallel to line segment L and the direction orthogonal to line segment L.

[0078] In a dual-core fiber 1C, the two cores 10 are similar to each other, but they can also be dissimilar. For example, the cross-sectional shape of the other core 10 can be anything other than a circle.

[0079] Explanation of the label

[0080] 1A, 1B, 1C, 1D, 1E, 1F, 100… Dual-core optical fiber

[0081] 2, 3... Lateral view images

[0082] 10… fiber core

[0083] 10c…center

[0084] 20…cladding

[0085] 20c…center

[0086] 21…Optical Cladding

[0087] 22…Physical cladding

[0088] 30… Covering section

[0089] 40…low refractive index section

[0090] C…fiber axis

[0091] L…line segment

[0092] M…midpoint

[0093] R…circle

Claims

1. A multi-core optical fiber, having the following characteristics: Multiple fiber cores, which extend along the fiber axis; The cladding has the plurality of fiber cores embedded therein, and except for the portion where the plurality of fiber cores are disposed, it has rotational symmetry with respect to the fiber axis; The low-refractive-index portion has a refractive index lower than that of the cladding; as well as The cladding portion surrounds the cladding layer and has rotational symmetry with respect to the fiber axis. Its features are, The image viewed from the side from at least one of two directions orthogonal to the fiber axis exhibits non-reversed symmetry about the fiber axis. When viewed from the side in a second direction orthogonal to both the fiber axis and the at least one direction, the arrangement of the plurality of fiber cores exhibits reverse symmetry about the fiber axis, thereby enabling the identification of the plurality of fiber cores. In a cross-section orthogonal to the fiber axis, the centroids of the plurality of fiber cores separate from the fiber axis. In a cross-section orthogonal to the fiber axis, the low-refractive-index portion is positioned coinciding with the fiber axis. The cladding, except for the portion containing the plurality of fiber cores and the low-refractive-index portion, has rotational symmetry with respect to the fiber axis.

2. The multi-core optical fiber according to claim 1, wherein, The plurality of fiber cores appear to have the same shape when viewed from the side.

3. The multi-core optical fiber according to claim 1, wherein, At least one of the plurality of fiber cores has a shape that is different from the other fiber cores when viewed from the side.

4. A multi-core optical fiber, which has the following characteristics: Multiple fiber cores, which extend along the fiber axis; The low refractive index section is disposed in a cross section orthogonal to the fiber axis, within a circle having a circumference passing through the center of the fiber core closest to the fiber axis among the plurality of fiber cores, with the fiber axis as the center. The cladding has the plurality of fiber cores and the low-refractive-index portion built inside, and except for the portion where the plurality of fiber cores and the low-refractive-index portion are provided, it has rotational symmetry with respect to the fiber axis; as well as The cladding portion surrounds the cladding layer and has rotational symmetry with respect to the fiber axis. In this multi-core optical fiber, When viewed from the side from at least one of two directions orthogonal to the fiber axis, the arrangement of the plurality of fiber cores exhibits reverse symmetry about the fiber axis. In a cross-section orthogonal to the fiber axis, the center of gravity of the plurality of fiber cores is separated from the fiber axis.

5. The multi-core optical fiber according to any one of claims 1 to 4, wherein, In a cross-section orthogonal to the fiber axis, the low-refractive-index portion has an elliptical shape with a non-circularity greater than 0% and less than 10%.

6. The multi-core optical fiber according to claim 5, wherein, The non-circularity rate is 6% or more and 10% or less.

7. The multi-core optical fiber according to any one of claims 1 to 4, wherein, The plurality of fiber cores are two fiber cores, and the line segment connecting the centers of the two fiber cores is consistent with the second direction, which is orthogonal to both the fiber axis and the at least one direction.

8. A fiber core identification method comprising obtaining lateral observation images from two directions orthogonal to the fiber axis of a multi-core optical fiber as described in any one of claims 1 to 7, and identifying the plurality of fiber cores based on the non-reversal symmetry of the lateral observation images in relation to the fiber axis.

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

Images