Multi-core optical fiber and multi-core optical fiber cable

Abstract

An MCF (100) according to the present disclosure is provided with: 12 or 16 core units each including a core (110) and a recessed layer (122); a common cladding (120); and a resin coating (130). In a cross section, the core units are arranged such that the adjacent relationship between the cores (110) in the adjacent relationship with respect to the specific core (110) is not established, and the centers of the core units are each arranged so as to be linearly symmetric about an axis that intersects the central axis (AX) and does not pass through the center of any one of the core units as an axis of symmetry (LA). The outer diameter of the resin coating (130) is 250 + / -15 [mu] m, the effective cross-sectional area (Aeff1550) at a wavelength of 1.550 [mu] m is 70 [mu] m2 or more, and the cable cutoff wavelength ([lambda] cc) at a length of 22 m is 1.530 [mu] m or less. The distance ([lambda]) between the centers of adjacent cores, the shortest distance (dcooat) from the center of the core to the interface between the common cladding and the resin coating (130), and the minimum outer diameter (CD) of the common cladding (120) satisfy a specific relationship.

Description

Technical Field

[0001] The present disclosure relates to a multi-core optical fiber (hereinafter referred to as "MCF") and an MCF cable.

[0002] This application claims priority based on Japanese Patent Application No. 2023-017438, filed on February 8, 2023, is based on the contents thereof, and is incorporated herein by reference in its entirety. Background Art

[0003] Non-Patent Document 1 discloses two MCFs with different core configurations on the optical fiber cross section. The first MCF is a 12-core MCF with a common cladding having an outer diameter of 147 μm and 12 cores arranged in a square lattice on the optical fiber cross section. The second MCF is a 12-core MCF with a common cladding having an outer diameter of 145 μm and 12 cores arranged in a hexagonal lattice on the optical fiber cross section. Both 12-core MCFs lack a trench layer. In addition, these 12-core MCFs (hereinafter referred to as "MCFs") have: mode field diameters of 5.4 μm at a wavelength of 1.310 μm and 6.1 μm at a wavelength of 1.550 μm, a cutoff wavelength of 1.26 μm, a zero dispersion wavelength of 1.41 μm, a leakage loss from the cladding to the cladding of 0.01 dB / km at a wavelength of 1.565 μm, and an inter-core crosstalk of -30 dB / km at a wavelength of 1.565 μm (hereinafter referred to as "XT").

[0004] Patent Document 1 discloses a 12-core MCF and a 16-core MCF having a cladding outer diameter of approximately 250 μm as MCFs suitable for short-distance communications of approximately 10 km using a wavelength band of 1.260 μm to 1.360 μm.

[0005] In addition, in the seventh embodiment of patent document 2, as an MCF capable of reducing XT and leakage loss at a wavelength of 1.550 μm, a 12-core MCF in which 12 cores are arranged in a square lattice on the cross section of the optical fiber is disclosed. In this seventh embodiment, it is qualitatively disclosed that from the viewpoint of reducing XT, the center-to-center distance between adjacent cores is preferably 30 μm or more, 35 μm or more, or 40 μm or more. However, in patent document 2, except for the numerical values ​​shown in the fourth embodiment, there is no description as to what specific XT values ​​are preferred, and what XT values ​​can be achieved. In addition, as specific numerical values ​​of the MCF shown in the fourth embodiment, the relative refractive index difference of the core is 0.23%, the relative refractive index difference of the groove layer is -0.65%, the thickness of the inner cladding is 6 μm, the thickness of the groove layer is 4.3 μm, and the effective cross-sectional area is 118.2 μm 2 Above and 125.2μm 2The cutoff wavelength is 1.28 μm to 1.39 μm, and the center-to-center distance between adjacent cores is 40.5 μm. In addition, in the fourth embodiment, it is disclosed that under the conditions of a wavelength of 1.550 μm and an optical fiber length of 3.96 km, the inter-core XT can be achieved to be -38.6 dB to -41.6 dB, that is, 1.75×10 -5 / km and above and 3.49×10 -5 / km or less.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 2022-41625

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 2015-163972

[0010] Patent Document 3: Japanese Patent Application Laid-Open No. 2017-509171

[0011] Non-patent literature

[0012] Non-patent document 1:

[0013] Yusuke Sasaki, et al., "High Density Multicore Fibers Employing SmallMFD Cores for Datacenters", OECC2018, Technical Digest, P2-07, July 02-06, 2018, Jeju, Korea.

[0014] Non-patent document 2:

[0015] RJ Black and C. Pask, J. Opt. Soc. Am. A, JOSAA 1(11), 1129-1131(1984).

[0016] Non-patent document 3:

[0017] Y.Kobayashi and T.Hayashi, “Behavior and measurement method of inter-core crosstalk in multicore fibers with core-dependent loss,”

[0018] Opt.Express 31(1),pp.502-508(2023). Summary of the Invention

[0019] The MCF disclosed in the present invention comprises: 12 or 16 core units, each comprising a core and a recessed layer; a common cladding; and a resin coating. In cross section, the core units are arranged so that the adjacent relationship between cores adjacent to a specific core does not hold, and the centers of the core units are arranged so as to be linearly symmetrical with an axis intersecting the central axis and not passing through the center of any of the core units as the symmetry axis. The outer diameter of the resin coating is 250±15μm, and the effective cross-sectional area at a wavelength of 1.550μm is 70μm. 2 The cutoff wavelength of a 22 m cable is 1.530 μm or less or 1.460 μm or less. The center-to-center distance between adjacent cores, the shortest distance from the core center to the cladding interface, and the cladding diameter satisfy a specific relationship. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 1 and 2 are diagrams showing various structures of an MCF cable of the present disclosure including the MCF of the present disclosure.

[0021] Figure 2 It is a diagram for explaining various conditions for determining the core arrangement in the MCF of the present disclosure.

[0022] Figure 3 1 and 2 are diagrams showing core configurations in the first and second embodiments of the MCF of the present disclosure.

[0023] Figure 4 1 and 2 are diagrams illustrating core configurations in third and fourth embodiments of the MCF of the present disclosure.

[0024] Figure 5 It is a diagram showing a core configuration in a fifth embodiment of the MCF of the present disclosure.

[0025] Figure 6 This is a diagram for explaining the main terms used in this specification.

[0026] Figure 7 Graph showing the refractive index distribution around each core of an MCF applicable to the present disclosure.

[0027] Figure 8 This is a table showing the specifications of a plurality of samples of MCF according to the present disclosure and a plurality of samples according to comparative examples. DETAILED DESCRIPTION

[0028] [Problems to be Solved by the Present Disclosure]

[0029] The inventors studied the above-mentioned prior art and found the following problems.

[0030] That is, when using the MCF of non-patent document 1, there is a problem that the connection loss is significantly deteriorated. In order to embed multiple cores in a common cladding that is not too thick, it is necessary to significantly reduce the MFD compared to a general single-mode optical fiber (hereinafter referred to as "SMF"). For example, the connection loss caused by the axis offset in the MCF of non-patent document 1 having an MFD of 5.4 μm at a wavelength of 1.310 μm is 2.54 times worse than the connection loss caused by the axis offset in a general SMF with a nominal MFD value of 8.6 μm. In other words, as an example, the connection loss between general SMFs is 0.5 dB or less, while in the connection between the 12-core MCFs, the connection loss is degraded to 1.27 dB or less. As another example, the connection loss between general SMFs is 0.35 dB or less, while the connection loss between the 12-core MCFs is degraded to 0.89 dB or less.

[0031] Furthermore, the MCF of Patent Document 1 is unsuitable for transmission in the C-band (1.530 μm to 1.565 μm) and L-band (1.565 μm to 1.625 μm), which are suitable for high-density wavelength multiplexing transmission. This is because the high-density packing of 12 or 16 cores within the common cladding sacrifices inter-core XT in long wavelength bands such as the C-band.

[0032] Furthermore, the MCF of Patent Document 2 has a problem of poor manufacturability. This is because a trench layer with a large absolute value of relative refractive index difference is required to reduce XT or leakage loss, and the formation of this trench layer makes it difficult to manufacture the MCF base material.

[0033] The present disclosure is proposed to solve the above-mentioned problems. Its purpose is to provide an MCF and MCF cable that effectively reduces XT or leakage loss at a wavelength of 1.565μm or 1.625μm in a structure with 12 or 16 built-in cores and a standard sheath outer diameter of approximately 250μm, as a transmission medium suitable for bidirectional transmission.

[0034] [Effects of the Present Disclosure]

[0035] The MCF and MCF cable disclosed herein are transmission media suitable for bidirectional transmission. They have 12 or 16 cores built in and a standard outer diameter of about 250 μm. They effectively reduce XT or leakage loss at a wavelength of 1.565 μm or 1.625 μm.

[0036] [Description of Embodiments of the Present Disclosure]

[0037] First, the contents of the embodiments of the present disclosure will be individually listed and described.

[0038] MCF of the present disclosure

[0039] (1) It has 12 or 16 units, a common cladding, and a resin coating. The 12 or 16 units respectively include: a core extending along the central axis; and a depressed layer covering the periphery of the core and having a refractive index lower than the maximum refractive index of the core. The common cladding has a refractive index higher than that of the depressed layer and covers the periphery of each of the 12 or 16 core units. The resin coating covers the periphery of the common cladding. By embedding the 12 or 16 core units in the common cladding, in the fusion connection between such MCFs, a connection of the same number of cores as in the fusion connection of optical fiber ribbons containing 12 or 16 optical fibers can be performed in a single connection operation.

[0040] Furthermore, in a cross section of the MCF perpendicular to the central axis, the 12 or 16 core units are arranged so that adjacent cores are not adjacent to each other relative to a specific core selected from the 12 or 16 core units. Furthermore, in the cross section of the MCF, the centers of the 12 or 16 core units are arranged so as to be linearly symmetrical about an axis that intersects the central axis and does not pass through the centers of any of the 12 or 16 core units. With this core arrangement, MCFs can be fusion-bonded regardless of the polarity of the MCF end faces.

[0041] In addition, in the MCF disclosed herein, the outer diameter of the resin coating is 250±15μm, that is, not less than 235μm and not more than 265μm. As a result, the MCF disclosed herein achieves a coating outer diameter equivalent to that of a conventional MCF. In addition, it is possible to suppress the possibility of damage to the common cladding layer composed of glass material and achieve a practical coating thickness. The effective cross-sectional area Aeff_1550[μm] at a wavelength of 1.550μm is 2 ] is 70μm 2 In this case, the degradation of connection loss and nonlinear interference noise can be reduced in the connection between MCFs. The cutoff wavelength λcc [μm] of the 22m cable is 1.530μm or less or 1.460μm or less. As a result, single-mode operation suitable for long-distance transmission can be guaranteed in the frequency band from the C-band to the long wavelength side, or from the S-band (1.460μm to 1.530μm) to the long wavelength side.

[0042] In the MCF of the present disclosure, when the core radius is ra [μm], the inner radius of the depressed layer is rb [μm], the outer radius of the depressed layer is rc [μm], and the absolute value of the relative refractive index difference of the depressed layer with respect to the refractive index of the common cladding is Δdep [%], the center-to-center distance Λ [μm] between adjacent cores satisfies the following formula (1). In this case, XT at a wavelength of 1.565 μm can be reduced to a level sufficient for counterpropagation, for example, to 10 in parallel propagation. -3 / km or less.

[0043]

[0044] In the MCF of the present disclosure, the shortest distance d_coat [μm] from the core center to the interface between the common cladding and the resin coating satisfies the following formula (2): In this case, the leakage loss at a wavelength of 1.565 μm can be reduced to 0.01 dB / km or less.

[0045]

[0046] In the configuration of the MCF of the present disclosure in which the common cladding surrounds the outer periphery of each of the twelve core units, the minimum outer diameter CD of the common cladding is 185 μm or less, 190 μm or less, or 195 μm or less, and satisfies the following formula (3).

[0047]

[0048] Furthermore, in the MCF configuration of the present disclosure, in which a common cladding surrounds the outer periphery of each of the 16 core units, the minimum outer diameter CD of the common cladding is 195 μm or less, and furthermore, the following equation (4) is satisfied. This configuration effectively reduces XT or leakage loss at a wavelength of 1.565 μm or 1.625 μm, even with a standard cladding outer diameter of approximately 250 μm.

[0049]

[0050] (2) In the above (1), the center-to-center distance Λ may satisfy the following equation (5). In this case, XT at a wavelength of 1.565 μm can be reduced to a level sufficient for counterpropagation, for example, to 10 in parallel propagation. -4 / km or less.

[0051]

[0052] The shortest distance d_coat may also satisfy the following formula (6): In this case, the leakage loss at a wavelength of 1.565 μm can be reduced to 0.001 dB / km or less.

[0053]

[0054] In the configuration of the MCF of the present disclosure in which the common cladding surrounds the outer circumferences of the twelve core units, the minimum outer diameter CD of the common cladding may also satisfy the following formula (7).

[0055]

[0056] (3) In the above (1), the center-to-center distance Λ [μm] may satisfy the following formula (8). In this case, XT at a wavelength of 1.625 μm can be reduced to a level sufficient for counter-propagation, for example, to 10 in parallel propagation. -3 / km or less.

[0057]

[0058] The shortest distance d_coat [μm] may also satisfy the following formula (9): In this case, the leakage loss at a wavelength of 1.625 μm can be reduced to 0.01 dB / km or less.

[0059]

[0060] In the configuration of the MCF of the present disclosure in which the common cladding surrounds the outer circumferences of the twelve core units, the minimum outer diameter CD of the common cladding may also satisfy the following formula (10).

[0061]

[0062] (4) In the above (1), the center-to-center distance Λ [μm] may satisfy the following formula (11). In this case, XT at a wavelength of 1.625 μm can be reduced to a level sufficient for counterpropagation, for example, to 10 in parallel propagation. -4 / km or less.

[0063]

[0064] The shortest distance d_coat [μm] may also satisfy the following formula (12): In this case, the leakage loss at a wavelength of 1.625 μm can be reduced to 0.001 dB / km or less.

[0065]

[0066] In the configuration of the MCF of the present disclosure in which the common cladding surrounds the outer circumferences of the twelve core units, the minimum outer diameter CD of the common cladding may also satisfy the following formula (13).

[0067]

[0068] (5) In any of (1) to (4), Δdep is 0.5% or less or 0.35% or less. By suppressing the refractive index difference between the depressed layer and the common cladding layer in this way, the manufacturability of the MCF base material can be improved.

[0069] The MCF of the present disclosure is,

[0070] (6) It comprises: 12 or 16 cores extending along the central axis; a cladding covering the outer periphery of each of the 12 or 16 cores; and a resin coating covering the outer periphery of the common cladding. On a cross section of the MCF perpendicular to the central axis, the 12 or 16 cores are arranged so that the adjacent relationship between the cores that are adjacent to each other relative to a specific core selected from the 12 or 16 cores does not hold. Moreover, on the cross section of the MCF, the centers of the 12 or 16 cores are arranged so as to be linearly symmetrical with an axis that intersects the central axis and does not pass through the center of any of the 12 or 16 cores as the axis of symmetry. The outer diameter of the resin coating is 250±15μm. The effective cross-sectional area Aeff_1550[μm at a wavelength of 1.550μm 2 ] is 70μm 2 The cutoff wavelength λcc [μm] of a 22m cable is 1.530μm or less or 1.460μm or less. The outer diameter of the common cladding is 143μm or more and 195μm or less. The center-to-center distance between adjacent cores is 28.5μm or more and 40μm or less. The shortest distance from the center of each of the 12 or 16 cores to the interface between the common cladding and the resin coating is 26μm or more and 35μm or less. The parallel propagation XT between adjacent cores at a wavelength of 1.565μm is 10 -3 / km or less. The leakage loss from the common cladding to the resin coating at a wavelength of 1.565μm is less than 0.01dB / km. According to this configuration, similar to the MCF described in (1), in a configuration with a standard cladding outer diameter of approximately 250μm, it is possible to effectively reduce XT or leakage loss at a wavelength of 1.565μm or 1.625μm.

[0071] The MCF cable disclosed herein is,

[0072] (7) The MCF described in any one of (1) to (6) above is installed in a state where the average bending radius is 0.06 m or more and 0.6 m or less. Thus, an MCF cable that effectively reduces XT or leakage loss at a wavelength of 1.565 μm or 1.625 μm can be obtained.

[0073] Each aspect listed in the above section [Description of Embodiments of the Present Disclosure] is applicable to each of all the remaining aspects, or to all combinations of the remaining aspects.

[0074] [Details of the embodiment of the present disclosure]

[0075] The specific structures of the MCF and MCF cable disclosed herein are described in detail below with reference to the accompanying drawings. The present disclosure is not limited to these examples and is intended to encompass all modifications within the meaning and scope of the claims and their equivalents. In the description of the drawings, identical elements are denoted by the same reference numerals, and duplicate descriptions are omitted.

[0076] Figure 1 is a diagram showing various structures of the MCF cable of the present disclosure (in Figure 1 The MCF of the present disclosure is built into such an MCF cable.

[0077] exist Figure 1 The upper section ( Figure 1 The MCF cable 1A shown in FIG. 1 (denoted as "Configuration (A)") comprises an outer sheath 300 containing an MCF housing space extending along the length of the MCF cable 1A, and a plurality of MCFs 100, serving as the MCFs of the present disclosure. Two tension members 400A and 400B extending along the MCF housing space are embedded in the outer sheath 300. Each MCF 100 includes a glass fiber 200, the outer circumference of which is coated with resin.

[0078] on the other hand, Figure 1 The lower part ( Figure 1 The MCF cable 1B shown in FIG1 (denoted as "Structure (B)") comprises: an outer sheath 500 including an MCF storage space extending along the longitudinal direction of the MCF cable 1B; a slotted core 600 dividing the MCF storage space into multiple spaces; and a plurality of MCFs 100 serving as the MCF of the present disclosure. The outer sheath 500 contains a slotted core 600 (slotted core) dividing the MCF storage space into multiple spaces. The slotted core 600 is embedded with a tension wire 700 extending along the longitudinal direction of the MCF cable 1B. The plurality of MCFs 100 are stored in any space divided by the slotted core 600.

[0079] Figure 2 Graph for explaining various conditions for determining the core arrangement in the MCF of the present disclosure ( Figure 2 In the Figure 2 The upper section ( Figure 2 The square grid defined on the cross section of the MCF is shown in FIG. Figure 2 The lower part ( Figure 2 2 (referred to as “adjacent relationship between inner and outer cores” in the figure) shows a diagram for explaining the arrangement state of two outer cores that maintain an adjacent relationship with respect to one inner core.

[0080] The MCF disclosed herein has 12 or 16 cores. Thus, even when the MCFs to be connected are individually rotated and aligned before being fused, the number of connected cores per fusion can be made the same as with ribbon fusion, which is used in cables with multiple 12 or 16 optical fibers integrated with resin when connecting super-multi-core cables. Furthermore, ribbon fusion refers to a connection method in which the optical fibers contained in the optical fiber ribbon are individually fused to the optical fibers to be connected.

[0081] In addition, the MCF of the present disclosure has a core configuration in which a plurality of cores that are adjacent to a specific core selected from 12 or 16 cores do not have an adjacent relationship with each other. As a result, in bidirectional communication in which signals are transmitted in different propagation directions between cores that are adjacent to each other (hereinafter referred to as "adjacent cores"), the XT from another core that is also adjacent to the adjacent core of the specific core to the specified core, i.e., the counter-propagation XT, can be reduced. Here, the adjacent cores of the specific core will be described later (refer to Figure 6 ) refers to cores that have a significant influence on the parallel propagation XT toward a specific core. Specifically, adjacent cores are those whose center-to-center distance is closest to that of the specific core, as well as cores with the same center-to-center distance (a difference of 2 μm or less). Parallel propagation XT refers to the normal XT when light propagates in the same direction.

[0082] As an example, Figure 2 As shown, 12 or 16 cores are arranged in a manner similar to a square lattice core arrangement in which a part of the 12 or 16 cores are deviated from the lattice points, that is, a core arrangement in which the adjacent relationship does not hold true among adjacent cores that are adjacent to each other with respect to a specific core. At this time, on the cross section of the MCF, the center of each core is arranged at a position that is line-symmetrical with respect to a prescribed axis that passes through the central axis of the MCF that coincides with the center of the common cladding and does not pass through the center of each core. Thus, when connecting the MCFs, it is possible to connect a prescribed end of a certain MCF to any end of another MCF without worrying about "polarity". In addition, in the case where the core arrangement of the MCF disclosed in the present invention has a single rotational symmetry, that is, in the case where the core arrangement of the MCF disclosed in the present invention does not have a rotational symmetry of more than two times, each core can be distinguished even without adding a "mark" other than the core.

[0083] In addition, Figure 2In the example, although the deviation of the core position is not taken into account, the center position of the core can also deviate within 0.5μm from the specified grid point. This can increase the manufacturing tolerance. In an actual MCF, the positions of the grid points to which the inner core and the outer core are respectively assigned (inner grid points and outer grid points) can be obtained by optimizing the grid point spacing, orientation, and position of the square grid in such a way that the sum of the square errors of the positional offsets of the grid points corresponding to the centers of the cores is minimized.

[0084] Specifically, Figure 2 The illustrated MCF 100 includes a glass fiber 200 extending along a central axis AX and a resin coating 130 covering the glass fiber 200. Glass fiber 200 includes 12 or 16 cores, each extending along the central axis AX, and a common cladding 120 covering each of the 12 cores. In this specification, whenever "12 or 16 cores" are mentioned, they are referred to as cores 110.

[0085] Figure 2 The square lattice 800, which serves as a reference for the core configuration and is shown in the upper section of FIG100 , is defined on a cross section of the MCF 100 that is orthogonal to the central axis AX. In addition, on the cross section of the MCF 100, the shape of the outer periphery 210 of the common cladding 120 is circular. That is, the square lattice 800 is a square lattice having a lattice point spacing that is consistent with the center-to-center distance Λ between adjacent cores, and is composed of a plurality of lattice points that are arranged in point symmetry with respect to the central axis AX in such a manner that the four inner lattice points 810 surround the central axis AX at the shortest distance. More specifically, the square lattice 800 includes: four inner lattice points 810; and eight outer lattice points 820 that surround the four inner lattice points 810 and are adjacent to any one of the four inner lattice points 810. Furthermore, when 12 cores are arranged, no core is arranged at the non-peripheral lattice points 830 that are not adjacent to any of the four inner lattice points 810 .

[0086] For example, the 12 cores belong to either the inner core 110A (core 110) assigned to the four inner lattice points 810 or the outer core (core 110) assigned to the eight outer lattice points 820. The distance between the center position of each of the four inner cores 110A and the corresponding inner lattice point among the four inner lattice points 810 is less than 0.5 μm. On the other hand, the eight outer cores belong to either the lattice point configuration core 110Ba or the lattice point non-configuration core 110Bb. The lattice point configuration core 110Ba is a core whose center is configured at a position that is less than 0.5 μm away from the corresponding outer lattice point among the eight outer lattice points 820. In addition, the lattice point non-configuration core 110Bb is a core whose center is configured at a position that is a distance D1 greater than 2 μm away from the corresponding outer lattice point 820. In addition, for the angle θ formed by two line segments extending from one inner peripheral core 110A to two outer peripheral cores in an adjacent relationship, it is 90 [deg] when both of the two outer peripheral cores are lattice point configuration cores 110Ba, and is less than 90 [deg] when one of the two outer peripheral cores is a lattice point non-configuration core 110Bb. The lattice point non-configuration cores 110Bb are respectively arranged such that their centers are located at positions away from a specific inner peripheral lattice point that is in an adjacent relationship with the corresponding outer peripheral lattice point among the four inner peripheral lattice points 810. That is, the lattice point non-configuration cores 110Bb are respectively arranged on a circle centered on a specific inner peripheral lattice point. In addition, D3 is greater than (Λ-0.5μm) and less than (Λ+0.5μm). The centers of the respective lattice point non-configuration cores 110Bb are arranged so as to be at a distance D3 from a specific outer peripheral lattice point that is in an adjacent relationship with the corresponding outer peripheral lattice point (refer to Figure 2 The upper part shows that the distance between the adjacent peripheral lattice points is longer than the distance D1 from the corresponding peripheral lattice point, and is away from the center of any core of the remaining lattice point non-configuration core 110Bb by a distance D2. In addition, D2 is greater than Λ+3μm. In this specification, when the above conditions are met, the lattice point non-configuration core 110Bb is also defined as the adjacent relationship with the lattice point. Similarly, the adjacent relationship with other cores, that is, the inner core 110A or other peripheral cores is maintained.

[0087] Figure 3 is a diagram showing the core configuration in the first embodiment and the second embodiment of the MCF of the present disclosure ( Figure 3 In the Figure 3 The upper section ( Figure 3 The cross section of MCF100A having 12 cores is disclosed. Figure 3 The lower part ( Figure 3 hereinafter referred to as "Embodiment 2" discloses a cross section of an MCF100B having 16 cores. Figure 4is a diagram showing the core configuration in the third embodiment and the fourth embodiment of the MCF of the present disclosure ( Figure 4 In the Figure 4 The upper section ( Figure 4 The cross section of MCF100C having 12 cores is disclosed. Figure 4 The lower part ( Figure 4 The cross section of MCF100D having 12 cores is disclosed. Figure 5 100E is a diagram showing a core arrangement in a fifth embodiment of the MCF of the present disclosure, and discloses a cross section of an MCF 100E having 12 cores. Figures 3 to 5 The core positions and core sizes are not based on actual scale for easy visual recognition. Figures 3 to 5 In each of the figures, the symmetry axis LA is shown so that it can be recognized that 12 or 16 cores are arranged in line symmetry.

[0088] Figure 3 The MCF100A shown in the upper section includes a glass fiber 200A surrounded by a resin coating 130. The glass fiber 200A has: 12 cores extending along the central axis AX; and a common cladding 120 covering the 12 cores respectively. The 12 cores are classified into 4 inner cores 110A and 8 outer cores according to the type of lattice points assigned. In addition, the 8 outer cores are classified into lattice point configuration cores 110Ba and lattice point non-configuration cores 110Bb. In the case of MCF100A, the 8 outer cores are all classified as lattice point configuration cores 110Ba. That is, on the cross section of the glass fiber 200A that is orthogonal to the central axis AX of the MCF100A, Figure 2 Among the lattice points of the square lattice 800 shown in the upper section, four inner cores 110A are arranged on the inner lattice points 810, and eight outer cores serving as lattice point arrangement cores 110Ba are respectively arranged on the outer lattice points 820 surrounding the inner lattice points 810.

[0089] on the other hand, Figure 3The MCF100B shown in the lower section includes a glass fiber 200B surrounded by a resin coating 130. The glass fiber 200B has: 16 cores extending along the central axis AX; and a common cladding 120 covering these 16 cores respectively. The 16 cores are classified into 4 inner cores 110A and 12 outer cores according to the type of lattice points assigned. In addition, the 12 outer cores are classified into lattice point configuration cores 110Ba and lattice point non-configuration cores 110Bb. In the case of MCF100B, all 12 outer cores are classified as lattice point configuration cores 110Ba. That is, in a cross section of glass fiber 200B perpendicular to the central axis AX of MCF 100B, four inner cores 110A are arranged at inner lattice points 810 among the lattice points of square lattice 800, and twelve outer cores, serving as lattice point arrangement cores 110Ba, are arranged at outer lattice points 820 surrounding inner lattice points 810. Thus, both MCF 100A and MCF 100B have rotational symmetry of two or more degrees.

[0090] also, Figure 4 The MCF100C shown in the upper paragraph, Figure 4 The MCF100D shown in the lower part of Figure 5 The MCF100E shown has the same features as the core configuration. Figure 3 The structure of MCF100A shown in the upper section is the same as that of MCF100C. That is, MCF100C to 100E include glass fibers 200C to 200E corresponding to the above-mentioned glass fiber 200A, and a resin coating 130. In addition, each of the glass fibers 200C to 200E includes: 12 cores extending along the central axis AX; and a common cladding 120 covering each of the 12 cores. On each cross section of the glass fibers 200C to 200E that is orthogonal to the central axis AX, the common cladding 120 has a circular outer periphery. The 12 cores are classified into 4 inner cores 110A and 8 outer cores according to the type of lattice points to which they are assigned. In addition, the 8 outer cores are classified into lattice point configuration cores 110Ba and lattice point non-configuration cores 110Bb. In any of MCF100C to MCF100E, lattice point non-arrangement cores 110Bb are arranged at positions line-symmetrical with respect to the symmetry axis LA. The core arrangements of these MCF100C to MCF100E do not have rotational symmetry of two or more orders of magnitude around the central axis AX.

[0091] exist Figure 4In the MCF 100C shown in the upper section, glass fiber 200C, surrounded by resin coating 130, comprises 12 cores extending along central axis AX and a common cladding 120 covering each of the 12 cores. The 12 cores are divided into four inner cores 110A and eight outer cores, depending on the type of lattice points assigned to them. The eight outer cores are divided into lattice point-arranged cores 110Ba and lattice point-unarranged cores 110Bb.

[0092] In MCF100C, four inner cores 110A are arranged on the inner lattice points 810 of the square lattice 800 set on the cross section perpendicular to the central axis AX. At this time, the distance between the center of the inner core 110A corresponding to the inner lattice point 810 is less than 0.5 μm. Eight outer cores are arranged around these four inner cores 110A. Six of the eight outer cores are arranged as lattice point arrangement cores 110Ba so that the distance between the assigned outer lattice point 820 and the core center is less than 0.5 μm. The remaining two outer cores are arranged as lattice point non-arrangement cores 110Bb to maintain Figure 2 The core centers are arranged offset from the assigned peripheral lattice points 820 in the adjacent relationship shown in the lower section of FIG. Therefore, in this MCF 100C, the ratio of the number of lattice point arrangement cores to the number of lattice point non-arrangement cores is 6 to 2.

[0093] In MCF 100C, as shown in the figure, one lattice point arrangement core 110Ba and one lattice point non-arrangement core 110Bb adjacent to one inner circumferential core 110A are arranged to form an angle θ around the inner circumferential core 110A.

[0094] exist Figure 4 In the MCF100D shown in the lower section, four inner cores 110A are respectively arranged on the inner lattice points 810 of the square lattice 800 set on the cross section perpendicular to the central axis AX. At this time, the distance between the center of the inner core 110A corresponding to the inner lattice point 810 is less than 0.5 μm. Eight outer cores are arranged around these four inner cores 110A. Among the eight outer cores, six outer cores are allocated as lattice point allocation cores 110Ba so that the distance between the allocated outer lattice point 820 and the core center is less than 0.5 μm. The remaining two outer cores are used as lattice point non-allocation cores 110Bb to maintain Figure 2 The adjacent relationship shown in the lower section of FIG is arranged so that the core centers are offset from the respectively assigned peripheral lattice points 820. Therefore, in this MCF 100D, as in the MCF 100C, the ratio of the number of lattice point arranged cores to the number of lattice point non-arranged cores is 6 to 2.

[0095] Figure 5The MCF100E involved in the fifth embodiment shown is also similar to the MCF100C and MCF100D. Four inner cores 110A are arranged on the inner lattice points 810 of the square lattice set on the cross section perpendicular to the central axis AX, and eight outer cores are arranged around them. Among the eight outer cores, four outer cores are arranged as lattice point arrangement cores 110Ba on the respectively assigned outer lattice points 820. The remaining four outer cores are arranged as lattice point non-arrangement cores 110Bb to maintain Figure 2 The state of the adjacent relationship shown in the lower section is configured in a state in which the core center is offset relative to the respectively assigned peripheral lattice points 820. Therefore, in this MCF100E, the ratio of the number of lattice point configured cores to the number of lattice point non-configured cores is 4 to 4. In addition, the two paired lattice point non-configured cores 110Bb are configured in positions that are line-symmetrical with respect to the symmetry axis LA, but the core configuration in this MCF100E does not have more than two rotational symmetries centered on the central axis AX. In addition, with respect to the pairs of lattice point non-configured cores 110Bb configured in line-symmetrical manner, the offset angle of the first pair is θ1, and the offset angle of the second pair is θ2. However, θ2 and θ1 do not need to be consistent.

[0096] Moreover, compared with MCF100C and MCF100D in which two of the eight peripheral cores are set as lattice point non-configured cores 110Bb, the asymmetry of MCF100E in which four of the eight peripheral cores are set as lattice point non-configured cores 110Bb is increased compared with these MCF100C and MCF100D.

[0097] Figure 6 This is a diagram for explaining the adjacent relationship of the inner cores, the cross-sectional structure of the core periphery, parallel propagation and parallel propagation XT, and counterpropagation XT, which are main terms used in this specification.

[0098] (Adjacent relationship of inner core)

[0099] In this specification, when focusing on a specific core among the 12 or 16 cores arranged on the cross section of the MCF, the adjacent relationship between the cores is defined as the core that has the minimum center distance relative to the specific core and the difference from the minimum center distance is 2 μm or less as the core that is adjacent to the specific core. In particular, Figure 6 , the following describes four inner cores 110A (cores 110) arranged at four inner lattice points 810 surrounding the central axis AX at a lattice point spacing corresponding to the center-to-center distance Λ between adjacent cores. Among these four inner cores 110A, pairs of inner cores 110A that form the sides of the square lattice are adjacent. On the other hand, pairs of inner cores 110A located on the diagonals of the square lattice do not have a centrally adjacent relationship.

[0100] (Cross-sectional structure around the core)

[0101] In each of MCFs 100A to 100E, the cross-sectional structure around each core 110 is that a common cladding 120 surrounds the outer periphery of the core 110. Furthermore, the core 110 includes an inner core 110A, a lattice point arrangement core 110Ba, and a lattice point non-arrangement core 110Bb. The common cladding 120 can be provided in direct contact with the core 110, but an optical cladding 121 can also be provided between the common cladding 120 and the core 110. Furthermore, a depressed layer 122 having a small absolute value of Δdep can be provided between the optical cladding 121 and the common cladding 120. Furthermore, the optical cladding 121 can be prepared for each core 110, and have a relative refractive index difference Δ2 of not less than -0.1% and not more than 0.1% relative to the refractive index of the common cladding 120. When Δ2 is a negative value, the optical cladding 121 functions as a depressed layer, and the absolute value of the relative refractive index difference is given by Δdep. Furthermore, when the depressed layer 122 is provided, the depressed layer 122 may have a relative refractive index difference Δ3 of -2.0% or more and less than -1.0%, -1.0% or more and less than -0.7%, -0.7% or more and less than -0.4%, or -0.4% or more and less than 0% with respect to the refractive index of the common cladding layer 120. The absolute value Δdep of the relative refractive index difference of the depressed layer 122 may be 0.5% or less or 0.35% or less.

[0102] (Parallel Propagation and Parallel Propagation XT)

[0103] exist Figure 6 The example shown shows three cores in an adjacent relationship (all first cores 110a propagating light in the same direction). Specifically, an adjacent relationship exists between the left core and the center core, and an adjacent relationship exists between the center core and the right core. In other words, the state in which each core propagates light in the same direction in an adjacent relationship is referred to as "parallel propagation." In this case, parallel propagation XT occurs between adjacent cores propagating light in the same direction, as in normal XT.

[0104] (Counterpropagation and Counterpropagation XT)

[0105] On the other hand, counterpropagation propagates light in mutually different directions between two adjacent cores. Figure 6In the example, while the left and center cores have an adjacent relationship, the left core functions as the first core 110a, while the center core functions as the second core 110b, which propagates light in a direction different from the first core 110a. The normal XT generated between these left and center cores is unlikely to affect communication quality. Similarly, the right cores, which have an adjacent relationship with the center core, function as the first core 110a, and the normal XT generated between these right cores and the center core is unlikely to affect communication quality. In this way, the state in which the cores with an adjacent relationship propagate light in different directions is denoted as "counterpropagation." However, between the left and right cores, both functioning as the first core 110a, XT affects communication quality via the center core, which functions as the second core 110b. Thus, XT between cores that propagate light in the same direction via cores with an adjacent relationship and propagating light in opposite directions is denoted as "counterpropagation XT."

[0106] In addition, in the following description, reference is made to Figure 6 The examples of "parallel propagation" and "counterpropagation" shown in FIG2 are used to illustrate this. However, when the XT (parallel propagation XT: XT_co) between cores (hereinafter referred to as "adjacent cores") in an optical fiber with a length of L1 and an adjacent relationship is set to XT_co(L1), when XT is expressed in decibels, the parallel propagation XT in an optical fiber with a length of L2 is expressed as the following formula (14), and XT increases by 10 dB with 10 times the distance.

[0107] XT_co(L2)=XT_co(L1)+10log10(L2 / L1)…(14)

[0108] In the case of expressing XT in decibel values, for example, Figure 7 In the counter-propagation example shown, the XT from the right core to the left core via the center core (counter-propagation XT: XT_counter) can be expressed as the following equation (15) using the parallel propagation XT between the left core and the center core, and between the center core and the right core, i.e., XT_co.

[0109] XT_counter = 2XT_co - 10log 10 2…(15)

[0110] When the counterpropagation XT in an optical fiber with a length of L1 is set to XT_counter(L1), when XT is expressed in decibels, the counterpropagation XT in an optical fiber with a length of L2 is expressed as the following equation (16), and XT_counter increases by 20 dB with 10 times the distance.

[0111] XT_counter(L2)=XT_counter(L1)+20log 10 (L2 / L1)…(16)

[0112] When the number of adjacent cores to a given core is N, the total XT_co from the adjacent cores to the given core, XT_co_tot, is expressed as follows: Equation (17). Equation (17) assumes that the XT_co between adjacent cores is uniform. If the difference in XT_co between adjacent cores cannot be ignored, XT_co from core n among the N adjacent cores to the given core is expressed as XT_co(n), and the total XT_co_tot is expressed as follows: Equation (18).

[0113] XT_co_tot=XT_co+10log 10 N…(17)

[0114]

[0115] The inventors have found that the total XT_counter_tot of the counter-propagation XT to a given core is obtained by adding the neighboring cores of the neighboring cores to the given core. Figure 6 In the example of counterpropagation shown in FIG. 1 , when the specified core is set as the core on the left, and the number of "corresponding to the core on the right" is set as M, it may become the following equation (19). However, it is not the case. When the number of adjacent cores (including the specified core) of core n among the N adjacent cores (central core) relative to the specified core (the core on the left) is set as K(n), XT_counter_tot becomes the following equation (20). Therefore, in an MCF having 12 cores, XT_counter_tot to any of the four cores belonging to the inner core group can be expressed as the following equation (21).

[0116] XT_counter_tot=XT_counter+10log 10 M=2XT_co-10log 10 2+10log 10 M…(19)

[0117]

[0118] Therefore, in order to set the counter-propagation XT after 10 km of propagation using a 12-core MCF (equivalent to a fiber length of 10 km) to below -20 dB (= -20 dB / 10 km), the parallel propagation XT (XT_co) between adjacent cores converted to the fiber length L (km) only needs to satisfy the following formula (22). In addition, the sum of the parallel propagation XT from the four adjacent cores to any one of the four cores belonging to the inner core group only needs to satisfy the following formula (23).

[0119]

[0120]

[0121] In order to set the counter-propagation XT after 10 km of optical fiber length equivalent to 10 km using a 12-core MCF to below -40 dB (= -40 dB / 10 km), the parallel propagation XT (XT_co) between adjacent cores converted to the optical fiber length L (km) only needs to satisfy the following formula (24). In addition, the sum of the parallel propagation XT from the four adjacent cores to any one of the four cores belonging to the inner core group only needs to satisfy the following formula (25).

[0122]

[0123] Next, the distribution structure of MCF applicable to the present disclosure will be described. Figure 7 This diagram shows the refractive index distribution around each core of an MCF applicable to the present disclosure. Unless otherwise specified, "relative refractive index difference Δ" refers to the relative refractive index difference relative to the refractive index of the common cladding. That is, "relative refractive index difference Δ" does not refer to the relative refractive index difference relative to the refractive index of pure silica glass.

[0124] Regarding the core structure of the MCF of the present disclosure, the core refractive index distribution and the optical properties thereof can be appropriately selected according to the application, for example, Figure 7 The refractive index distribution from mode (A) to mode (K) is shown. Figure 7 Here, Δ represents the relative refractive index difference relative to the refractive index of the common cladding, and r represents the radius from the center of each core. The system is represented using a local coordinate system with the origin O being the center of each core and Δ = 0%. The structure can be consistent or different between cores. Furthermore, Δcore represents the absolute value of the relative refractive index difference between each core relative to the refractive index of the common cladding, and Δdep represents the absolute value of the relative refractive index difference of the depressed layer or the portion functioning as the depressed layer.

[0125] Figure 7The illustrated mode (A) is a step-type refractive index distribution, mode (B) is a ring-type refractive index distribution, mode (C) is a double-step-type refractive index distribution, mode (D) is a gradient-type refractive index distribution, and mode (E) is a droop-type refractive index distribution. These can be applied to the core structure of the MCF disclosed herein. Furthermore, modes (F) and (H) having a depressed refractive index distribution around the core, modes (G), (I), and (J) having a raised refractive index distribution around the core, and mode (K) having a matched refractive index distribution around the core can also be applied to the core structure.

[0126] In refractive index distributions other than the step-type refractive index distribution of mode (A), the core radius ra or the core Δ(Δ1) can be obtained using ESI (Equivalent-step-index) approximation when approximated by the step type (Non-Patent Document 2 mentioned above).

[0127] The above-mentioned non-patent document 2 can be easily applied when the boundary between the core and the cladding is clear, but it is difficult to apply when the boundary between the core and the cladding is unclear, such as in the case of the droop-type refractive index distribution of mode (E). For example, when rd in mode (E) is regarded as the radius of the core and the method of the above-mentioned non-patent document 2 is directly applied, the ESI approximation is not smooth. In addition, the cladding means the common cladding 120 or the optical cladding 121. In this case, it is preferable to make the slope of the refractive index distribution The core radius ra is taken as the value of Δ r which is 2 / 5 of Δ in r with the largest absolute value of negative value, and the above-mentioned non-patent document 2 is applied. In this case, the refractive index r of the cladding can be obtained by using the simple average of Δ in the range from ra to rd as shown in the following formula (26), or the weighted average of r as shown in the following formula (27), and ra or Δ1 can be obtained by calculation based on the above-mentioned non-patent document 2. Δ2 is preferably greater than or equal to -0.10% and less than or equal to 0.10%. This is because the manufacturability is greatly improved. In addition, the optical cladding in mode (F) and mode (H) has a negative relative refractive index difference and essentially functions as a depressed layer with an absolute value of Δdep.

[0128]

[0129] A depressed layer 122 ( ) having a lower refractive index than the optical cladding 121 and the common cladding 120 may be provided around the optical cladding 121. Figure 7Mode (K). However, if the relative refractive index difference Δ3 of the depressed layer 122 relative to the refractive index of the common cladding 120 is -0.5% or less, manufacturability is significantly degraded. Therefore, Δ3 ≥ -0.4% is sufficient, Δ3 ≥ -0.3% is more preferred, and Δ3 ≥ -0.2% is even more preferred. Furthermore, from the perspective of manufacturability, the absolute value Δdep of the relative refractive index difference of the depressed layer 122 or the optical cladding 121 functioning as a depressed layer may be 0.5% or less, or 0.35% or less.

[0130] Regarding the materials of the core and cladding, glass with silica glass as the main component is preferred because it can achieve low transmission loss and high mechanical reliability. It is preferred to create a refractive index difference between the core and the cladding by adding Ge to the core. Alternatively, it is preferred to create a refractive index difference between the core and the cladding by adding F to the cladding. By adding a small amount of F to the core and the optical cladding, a depressed type distribution can be achieved with good manufacturability, which is preferred. Cl can also be added to the core or the cladding. This can reduce OH groups and reduce absorption losses caused by OH groups. The core or the cladding can also contain a small amount of P. This can improve manufacturability in some glass synthesis processes.

[0131] In addition, Figures 3 to 5 The MCF of the present disclosure with the cross-sectional structure shown has a resin coating 130 having a diameter of 250±15 μm, that is, 235 μm to 265 μm. This allows the MCF of the present disclosure to be cabled without major changes to existing cable equipment.

[0132] In a typical general-purpose SMF, the nominal minimum outer diameter CD of the coating, equivalent to the diameter of the glass fiber 200, is 125 μm. The nominal diameter of the resin coating 130 is 245 μm or greater and approximately 250 μm. However, in SMFs with thinner coatings, nominal diameters of the resin coating are also found to be 180 μm, 190 μm, and 200 μm. In these cases, the nominal thicknesses of the resin coating 130 are 27.5 μm, 32.5 μm, and 37.5 μm, respectively. If the resin coating 130 is thinner, damage to the coating surface caused by sand, dust, or the like may occur, and the damage may reach the glass coating, weakening the optical fiber's strength. Therefore, a sufficient nominal coating thickness is desirable.

[0133] In the MCF of the present disclosure, in order to achieve a nominal value of 250 μm for the diameter of the resin coating 130 and a nominal value of 27.5 μm or more for the coating thickness, the nominal value of CD only needs to be 195 μm or less.

[0134] Furthermore, to achieve a nominal diameter of 245 μm and a nominal coating thickness of 27.5 μm or greater for the resin coating 130, the nominal CD value need only be 190 μm or less. To achieve a nominal diameter of 50 μm and a nominal coating thickness of 32.5 μm or greater for the resin coating 130, the nominal CD value need only be 185 μm or less. To achieve a nominal diameter of 245 μm and a nominal coating thickness of 32.5 μm or greater for the resin coating 130, the nominal CD value need only be 180 μm or less. To achieve a nominal diameter of 250 μm and a nominal coating thickness of 37.5 μm or greater for the resin coating 130, the nominal CD value need only be 175 μm or less. Furthermore, to achieve a nominal diameter of 245 μm and a nominal coating thickness of 37.5 μm or greater for the resin coating 130 , the nominal CD value only needs to be 170 μm or less. In each case, the coating thickness tolerance only needs to be ±15 μm or less, and more preferably ±10 μm or less.

[0135] Next, as another example of the MCF disclosed herein, an MCF suitable for bidirectional transmission that reduces XT and leakage loss at a wavelength of 1.565 μm or 1.625 μm will be described. Furthermore, a wavelength of 1.565 μm is the upper limit of the C-band (1.530 μm to 1.565 μm), and a wavelength of 1.625 μm is the upper limit of the L-band (1.565 μm to 1.625 μm).

[0136] Non-patent document 1 discloses an MCF with a small MFD in order to reduce inter-core XT and leakage loss. However, the application of the MCF of patent document 1 to bidirectional transmission will lead to a significant deterioration of connection loss. Patent document 1 discloses an MCF for short-distance transmission in the O band (above 1.260 μm and below 1.360 μm). However, the use of the MCF of patent document 1 cannot avoid the deterioration of inter-core XT in long wavelength bands such as the C band, and such an MCF is not suitable for high-density wavelength multiplexing transmission in the C band or L band. Moreover, in patent document 2, in order to reduce inter-core XT or leakage loss, a refractive index distribution around each core is adopted. Figure 7 In this case, the absolute value of the relative refractive index difference of the depressed layer around each core needs to be increased, which degrades the manufacturability of the MCF base material.

[0137] The MCF disclosed herein has a resin coating 130 with a standard outer diameter of 250 μm ± 15 μm, i.e., not less than 235 μm and not more than 265 μm, and has 12 or 16 cores 110 built in. Specifically, the MCF disclosed herein comprises: 12 cores 110, each extending along a central axis AX; a common cladding 120, each covering the 12 or 16 cores 110; and a resin coating 130, covering the outer periphery of the common cladding 120. In a cross section of the MCF perpendicular to the central axis AX, the 12 or 16 cores 110 are arranged so that the adjacent relationship does not hold between cores that are adjacent to a specific core selected from the 12 or 16 cores 110. Furthermore, the 12 or 16 cores 110 are arranged so that the centers of the 12 or 16 cores 110 are linearly symmetrical about an axis of symmetry LA that intersects the central axis and does not pass through the center of any of the 12 or 16 cores 110. That is, the MCF of the present disclosure has Figures 3 to 5 In addition, the refractive index distribution around each core can be applied Figure 8 Any of the modes (A) to (K) shown in FIG. Figure 8 In a structure where only the depressed layer 122, or the optical cladding 121 functioning as a depressed layer, or both the optical cladding 121 and the depressed layer 122 are provided around the core 110, as in Mode (F), Mode (H), and Mode (K), a single core unit is composed of a combination of the core 110 and the depressed layer 122 corresponding to the core 110, or a combination of the core 110, the optical cladding 121, and the depressed layer 122. A single core unit includes a single core 110, and the center of the single core unit coincides with the center of the single core 110 it includes. In the following description, when only the "depressed layer 122" is mentioned, not only the depressed layer but also the optical cladding 121, which substantially functions as a depressed layer, is included.

[0138] In the MCF of the present disclosure, the effective cross-sectional area Aeff_1550[μm] at a wavelength of 1.550 μm is 2 ] is 70μm 2 The cutoff wavelength λcc [μm] of the cable with a length of 22 m is 1.530 μm or less or 1.460 μm or less. The outer diameter of the common cladding 120 is 143 μm or more and 195 μm or less. The center-to-center distance between adjacent cores is 28.5 μm or more and 40 μm or less. The shortest distance from the center of each of the 12 or 16 cores 110 to the interface between the common cladding 120 and the resin coating 130 is 26 μm or more and 35 μm or less. The parallel propagation XT between adjacent cores at a wavelength of 1.565 μm is 10 -3The leakage loss from the common cladding 120 to the resin coating 130 at a wavelength of 1.565 μm is 0.01 dB / km or less.

[0139] In the following description, Figure 7 The following describes the refractive index distributions of MCF samples of modes (F), (H), and (K) in which a depressed layer 122 or an optical cladding 121 functioning as a depressed layer is provided around each core, among the modes (A) to (K) shown. The various conditions shown below also apply to the group without a depressed layer, i.e., modes (A) to (E), (G), (I), and (J), as the refractive index distribution around each core. Specifically, in MCF samples in which any mode of the group without a depressed layer is applied as the refractive index distribution around each core, the parameter of the depressed layer 122, i.e., the absolute value of the relative refractive index difference Δdep, described later, is set to 0. Furthermore, the inner radius rb and the outer radius rc of the depressed layer 122 satisfy the relationship rb = rc.

[0140] exist Figure 8 The specifications of samples 1 to 12 of the embodiment of the MCF disclosed herein and samples 1 to 5 of the comparative examples are shown. Figure 8 In the table, Sample 2, Sample 4, Sample 6, and Sample 8 of the embodiment are MCFs of 12 or 16 cores 110 respectively, each of which includes a specific inner core 110A, a lattice point arrangement core 110Ba, and a lattice point non-arrangement core 110Bb, and the refractive index distribution around each core is Figure 7 Any of the modes (F), (H) and (K) of the refractive index distribution shown. Figure 8 In the table, each of Samples 1, 3, 5, 7, 9 to 12 of the embodiment and Samples 1 to 5 of the comparative example is an MCF in which 12 or 16 cores 110 are arranged at each lattice point of the square lattice. In Sample 1 of the comparative example, the refractive index distribution around each core is a pattern without a depressed layer. In Samples 2 to 5 of the comparative example, the refractive index distribution around each core is Figure 7 The refractive index distribution is any of the modes (F), (H), and (K) shown. In Samples 7, 8, and 12 of the embodiment and Sample 5 of the comparative example, the refractive index distribution around each core is mode (K), and Δ2 is 0%.

[0141] Figure 8 Among the parameters shown in the table, ra [μm] is the radius of the core 110. rb [μm] is the inner radius of the recessed layer 122. rc [μm] is the outer radius of the recessed layer 122. Aeff_1550 [μm] 2] is the effective cross-sectional area at a wavelength of 1.550 μm. λcc[μm] is the cut-off wavelength of a cable with a length of 22 m. Δcore[%] is the maximum refractive index difference of the core 110 relative to the refractive index of the common cladding 120. Δdep[%] is the absolute value of the relative refractive index difference of the recessed layer 122 relative to the refractive index of the common cladding 120. Λ[μm] is the center-to-center distance between adjacent cores. d_coat[μm] is the shortest distance from the core 110 to the interface between the common cladding 120 and the resin coating 130. θ[deg] is an indicator showing the positional relationship between the inner core 110A whose center is located on the inner circumference lattice point 810 and the lattice point configured core 110Ba and the lattice point non-configured core 110Bb that are adjacent to the inner core 110A and have the same center-to-center distance Λ relative to the inner core 110A. θ is the angle between a line segment extending from the center of inner core 110A to the center of lattice-point-configured core 110Ba and a line segment extending from the center of inner core 110A to the center of non-lattice-point-configured core 110Bb. CD [μm] is the minimum outer diameter of common cladding 120. XT@1.565μm [1 / km] is the parallel propagation XT between adjacent cores at a wavelength of 1.565μm. XT@1.625μm [1 / km] is the parallel propagation XT between adjacent cores at a wavelength of 1.625μm. Leakage loss@1.565μm [dB / km] is the leakage loss at a wavelength of 1.565μm. Leakage loss@1.625μm [dB / km] is the leakage loss at a wavelength of 1.625μm.

[0142] In addition, Figure 3 and Figure 5 In the example, the center-to-center distance Λ between adjacent cores is set to be uniform across all cores. However, it can vary within a specified range from the nominal value Λ_nominal. In this case, manufacturing tolerances can be increased. Similarly, the minimum outer diameter CD of the common cladding 120 can also vary within a specified range from the nominal value CD_nominal.

[0143] The following will be Figure 8 The "lower limit of Aeff", "desired range of XT", "desired center-to-center distance between adjacent cores", "desired d_coat", and "allowable minimum outer diameter CD" consistent with the specifications of samples 1 to 4 of the illustrated embodiment are shown below.

[0144] (Lower limit of Aeff)

[0145] Each core of the MCF of the present disclosure may have a 70 μm wavelength at a wavelength of 1.550 μm. 2The effective cross-sectional area Aeff_1550 is greater than or equal to 1550. This can reduce noise caused by nonlinear interference. In addition, it can also reduce connection losses caused by axis offset between the MCFs disclosed in this disclosure.

[0146] (Expected XT range)

[0147] In the MCF disclosed herein, the sum of the counter-propagating XT from adjacent cores to any core remains below -20 dB even after 10 km of propagation at a wavelength of 1.565 μm. Because the counter-propagating XT from sources other than adjacent cores is sufficiently low and negligible, a sufficient signal-to-noise ratio (SNR) can be achieved even when performing coherent detection.

[0148] Furthermore, in the MCF disclosed herein, the sum of the counterpropagation XT from adjacent cores to any core remains below -40 dB even after 10 km of propagation within the operating band. Since the counterpropagation XT from sources other than adjacent cores is sufficiently low and negligible, sufficient SNR can be achieved even when performing intensity modulation direct detection (IM-DD).

[0149] In the MCF of the present disclosure, the parallel propagation XT between adjacent cores can be 10 at a wavelength of 1.565 μm. -3 Therefore, when 12 or 16 cores are arranged in a standard square lattice core configuration, the sum of the counter-propagation XT for all pairs of adjacent cores can be kept below 10 even with a repeater spacing of 10 km or after 10 km of propagation in the link. -3 / km (=-30dB). Specifically, when 12 cores are arranged, the value is less than 7×10 -4 / km, when 16 cores are configured, it is less than 8×10 -4 / km.

[0150] Furthermore, in the MCF of the present disclosure, the parallel propagation XT between adjacent cores is preferably 10 at a wavelength of 1.565 μm. -4 Therefore, when 12 or 16 cores are arranged in a standard square lattice core configuration, the sum of the counter-propagation XT between adjacent cores can be set to 10 even after 100 km of propagation in the repeater spacing or link. -3 / km or less (-30dB or less). Specifically, when 12 cores are arranged, the density is less than 8×10-4 / km, when 16 cores are arranged, it is less than 9×10 -4 / km.

[0151] (Desired center-to-center distance between adjacent cores)

[0152] Reduce the parallel propagation XT between adjacent cores to 10 at a wavelength of 1.565 μm -3 When the center-to-center distance Λ between adjacent cores is less than 1 / km, the center-to-center distance Λ between adjacent cores can satisfy the following formula (28). Here, ra [μm] is the radius of the core 110, rb [μm] is the inner radius of the recessed layer 122, rc [μm] is the outer radius of the recessed layer 122, and Aeff_1550 [μm] is the outer radius of the recessed layer 122. 2 ] is the effective cross-sectional area at a wavelength of 1.550 μm, λcc [μm] is the cut-off wavelength of a cable with a length of 22 m, and Δdep [%] is the absolute value of the relative refractive index difference of the depressed layer 122 based on the refractive index of the common cladding 120 and is defined as a value greater than 0.

[0153]

[0154] In addition, the parallel propagation XT between adjacent cores is reduced to 10 at a wavelength of 1.565 μm. -4 When Λ is less than / km, it can satisfy the following formula (29).

[0155]

[0156] Reduce the parallel propagation XT between adjacent cores to 10 at a wavelength of 1.625 μm -3 When Λ is less than / km, it can satisfy the following formula (30).

[0157]

[0158] Furthermore, the parallel propagation XT between adjacent cores is reduced to 10 at a wavelength of 1.625 μm. -4 When Λ is less than / km, it is preferable that Λ satisfies the following formula (31).

[0159]

[0160] (desired d_coat)

[0161] When the leakage loss from the common cladding 120 to the resin coating 130 is reduced to 0.01 dB / km or less at a wavelength of 1.565 μm, the shortest distance d_coat from the core center to the outer peripheral surface of the common cladding 120 can satisfy the following equation (32). The outer peripheral surface of the common cladding 120 corresponds to the interface between the common cladding 120 and the resin coating 130. While d_coat of the outermost core, i.e., the minimum value of d_coat, is generally referred to as the outer cladding thickness (OCT), in this specification, d_coat means a value that can be specified for each core.

[0162]

[0163] When the leakage loss from the common cladding 120 to the resin coating 130 is reduced to 0.001 dB / km or less at a wavelength of 1.565 μm, d_coat can satisfy the following formula (33).

[0164]

[0165] When the leakage loss from the common cladding 120 to the resin coating 130 is reduced to 0.0005 dB / km or less at a wavelength of 1.565 μm, d_coat can satisfy the following equation (34).

[0166]

[0167] On the other hand, when the leakage loss from the common cladding 120 to the resin coating 130 is reduced to 0.01 dB / km or less at a wavelength of 1.625 μm, d_coat preferably satisfies the following formula (35).

[0168]

[0169] When the leakage loss from the common cladding 120 to the resin coating 130 is reduced to 0.001 dB / km or less at a wavelength of 1.625 μm, d_coat preferably satisfies the following formula (36).

[0170]

[0171] When the leakage loss from the common cladding 120 to the resin coating 130 is reduced to 0.0005 dB / km or less at a wavelength of 1.625 μm, d_coat preferably satisfies the following formula (37).

[0172]

[0173] (Minimum allowable outer diameter CD)

[0174] Reduce the parallel propagation XT between adjacent cores to 10 at a wavelength of 1.565 μm -3 / km or less, and the leakage loss from the common cladding 120 to the resin coating 130 is reduced to less than 0.01dB / km, in the square lattice core configuration as a reference, the minimum outer diameter CD of the MCF configured with 16 cores can satisfy the following formula (38), and the minimum outer diameter CD of the MCF configured with 12 cores can satisfy the following formula (39).

[0175]

[0176] Reduce the parallel propagation XT between adjacent cores to 10 at a wavelength of 1.565 μm -4 / km or less, and the leakage loss from the common cladding 120 to the resin coating 130 is reduced to less than 0.001dB / km, in the square lattice core configuration as a reference, the minimum outer diameter CD of the MCF configured with 16 cores can satisfy the following formula (40), and the CD of the MCF configured with 12 cores can satisfy the following formula (41).

[0177]

[0178] On the other hand, the parallel propagation XT between adjacent cores is reduced to 10 at a wavelength of 1.625 μm. -3 / km or less, and the leakage loss from the common cladding 120 to the resin coating 130 is reduced to less than 0.01dB / km, in the square lattice core configuration as a reference, the minimum outer diameter CD of the MCF configured with 16 cores can satisfy the following formula (42), and the minimum outer diameter CD of the MCF configured with 12 cores can satisfy the following formula (43).

[0179]

[0180] Reduce the parallel propagation XT between adjacent cores to 10 at a wavelength of 1.625 μm -4 / km or less, and the leakage loss from the common cladding 120 to the resin coating 130 is reduced to less than 0.001dB / km, in the square lattice core configuration as a reference, the CD of the MCF configured with 16 cores can satisfy the following formula (44), and the CD of the MCF configured with 12 cores can satisfy the following formula (45).

[0181]

[0182] Furthermore, when λcc is 1.530μm or less, single-mode operation in the C-band is guaranteed. When λcc is 1.460μm or less, single-mode operation in the S-band is guaranteed. Furthermore, since λcc is longer than 1.260μm, light confinement within the core is enhanced, reducing counterpropagation XT and leakage losses. Since λcc is longer than 1.360μm, light confinement within the core is further enhanced, further reducing counterpropagation XT and leakage losses.

[0183] Δdep only needs to be 0.5% or less. By reducing the absolute value of the refractive index difference between the depressed layer 122 and the common cladding layer 120, the manufacturability of the MCF matrix can be improved. Δdep can also be 0.35% or less, which can further improve the manufacturability of the MCF matrix. Furthermore, the most preferred range for Δdep is 0.20% or less.

[0184] In MCF cables that can incorporate the MCF disclosed herein, the upper limit of the average bend radius of the installed MCFs need only be 0.60m or less, more preferably 0.30m or less. This condition is effective in reducing counterpropagation XT. Furthermore, the lower limit of the average bend radius of the installed MCFs need only be 0.06m or greater, more preferably 0.10m or greater. This condition is effective in reducing the probability of bending fracture of the MCFs in the cable.

[0185] The MCF cable that can be built into the MCF of the present disclosure can also be a grooved cable. In this case, the bending radius of the MCF can be easily controlled and the XT can be reduced.

[0186] The MCF cable that can incorporate the MCF disclosed herein can also be an intermittently bonded tape cable. Intermittently bonded tape refers to a tape in which adjacent MCFs within the tape are bonded together at regular intervals along the length. In this case, the flexible intermittently bonded tape can be twisted into a spiral shape while being installed within the cable. In other words, by providing the MCFs with a small bend radius and cable-like construction, XT can be effectively reduced.

[0187] In addition, the characteristic quantities or characteristics of the MCF disclosed in the present invention can be measured by the following method. The effective cross-sectional area Aeff_1550 can be measured, for example, by the method described in Appendix III of ITU-T G.650.2 (08 / 2015). The cable cutoff wavelength λcc can be measured, for example, by the method described in Section 6.3 of ITU-T G.650.1 (10 / 2020). The center-to-center distance Λ of adjacent cores can be measured, for example, by the refractive near-field method or the lateral interference method, or the microscopic observation image of the MCF cross section (transmission near-field method). The shortest distance d_coat from the center of the core to the interface between the common cladding and the resin coating can be measured, for example, by the refractive near-field method or the lateral interference method, or the microscopic observation image of the MCF cross section (transmission near-field method). The minimum outer diameter CD of the common cladding can be measured, for example, by the refractive near-field method or the lateral interference method, or the microscopic observation image of the MCF cross section (transmission near-field method). XT during parallel propagation can be measured by the method described in non-patent document 3. The leakage loss can be measured by the method described in Patent Document 3.

[0188] Although units are omitted in the above formulas, in formulas (1) to (13), the numerator Aeff_1550 1 / 2 The unit of the coefficient of the constant of the / λcc term and the constant term is [μm]. That is, Formulas (1) to (13) are mathematical formulas when Λ, d_coat, and CD are expressed in units of [μm].

[0189] Description of Reference Numerals

[0190] 1A, 1B...MCF cables

[0191] 100, 100A, 100B, 100C, 100D, 100E…MCF

[0192] 110…core

[0193] 110A...Inner core

[0194] 110Ba… lattice point configuration core

[0195] 110Bb… lattice point non-configured core

[0196] 110a…first core

[0197] 110b…second core

[0198] 120…Public cladding

[0199] 121…Optical cladding

[0200] 122…Depression layer

[0201] 130…resin coating

[0202] 200, 200A, 200B, 200C, 200E…glass fiber

[0203] 210…Periphery

[0204] 300, 500... skin

[0205] 400A, 400B, 700...Tension-resistant wire

[0206] 600...slotted core

[0207] 800…square grid

[0208] 810…Inner circumference grid points

[0209] 820…Peripheral grid points

[0210] 830…non-peripheral grid points

[0211] AX…Center axis

[0212] LA…axis of symmetry.

Claims

1. A multi-core optical fiber having: 12 or 16 core units, each including a core extending along a central axis and a depressed layer covering an outer circumference of the core and having a refractive index lower than a maximum refractive index of the core; a common cladding layer having a refractive index higher than that of the depressed layer, covering the outer circumference of each of the 12 or 16 core units; and a resin coating covering the outer periphery of the common cladding, On a cross section of the multi-core optical fiber orthogonal to the central axis, the 12 or 16 core units are arranged so that adjacent relationships do not hold between cores that are adjacent to each other relative to specific cores selected from the 12 or 16 core units, and the centers of the 12 or 16 core units are arranged so as to be linearly symmetrical with an axis that intersects the central axis and does not pass through the center of any of the 12 or 16 core units as an axis of symmetry. The outer diameter of the resin coating is 250±15μm, The effective cross-sectional area at a wavelength of 1.550 μm is Aeff_1550[μm 2 ] is 70μm 2 Above, the cutoff wavelength λcc[μm] of the 22m cable is 1.530μm or less or 1.460μm or less, When the radius of the core is ra [μm], the inner radius of the depressed layer is rb [μm], the outer radius of the depressed layer is rc [μm], and the absolute value of the relative refractive index difference of the depressed layer with respect to the refractive index of the common cladding is Δdep [%], the center-to-center distance Λ [μm] between the adjacent cores satisfies the following formula (1): The shortest distance d_coat [μm] from the center of the core to the interface between the common cladding and the resin coating satisfies the following formula (2): In the configuration in which the common cladding surrounds the outer periphery of each of the twelve core units, the minimum outer diameter CD [μm] of the common cladding is 185 μm or less, 190 μm or less, or 195 μm or less, and further satisfies the following formula (3): In the configuration in which the common cladding surrounds the outer periphery of each of the 16 core units, the minimum outer diameter CD [μm] of the common cladding is 195 μm or less, and further satisfies the following formula (4):

2. The multi-core optical fiber according to claim 1, wherein: The center-to-center distance Λ satisfies the following formula (5): The shortest distance d_coat [μm] satisfies the following formula (6), In the configuration in which the common cladding surrounds the outer peripheries of the twelve core units, the minimum outer diameter CD [μm] of the common cladding satisfies the following formula (7):

3. The multi-core optical fiber according to claim 1, wherein: The center-to-center distance Λ [μm] satisfies the following formula (8), The shortest distance d_coat [μm] satisfies the following formula (9), In the configuration in which the common cladding surrounds the outer peripheries of the twelve core units, the minimum outer diameter CD [μm] of the common cladding satisfies the following formula (10):

4. The multi-core optical fiber according to claim 1, wherein: The center-to-center distance Λ [μm] satisfies the following formula (11), The shortest distance d_coat [μm] satisfies the following formula (12), In the configuration in which the common cladding surrounds the outer peripheries of the twelve core units, the minimum outer diameter CD [μm] of the common cladding satisfies the following formula (13):

5. The multi-core optical fiber according to claim 1, wherein: The Δdep is 0.5% or less or 0.35% or less. 6 . A multi-core optical fiber cable, comprising the multi-core optical fiber according to claim 1 installed in a state where the average bending radius is 0.06 m or more and 0.6 m or less.

7. A multi-core optical fiber comprising: 12 or 16 cores, extending along the central axis; a common cladding covering the outer periphery of each of the 12 or 16 cores; as well as a resin coating covering the outer periphery of the common cladding, On a cross section of the multi-core optical fiber orthogonal to the central axis, the 12 or 16 cores are arranged so that adjacent relationships do not hold between cores that are adjacent to each other with respect to specific cores selected from the 12 or 16 cores, and the centers of the 12 or 16 cores are arranged so as to be linearly symmetrical about an axis that intersects the central axis and does not pass through the center of any of the 12 or 16 cores as an axis of symmetry. The outer diameter of the resin coating is 250±15μm, The effective cross-sectional area at a wavelength of 1.550 μm is Aeff_1550[μm 2 ] is 70μm 2 Above, the cutoff wavelength λcc[μm] of the 22m cable is 1.530μm or less or 1.460μm or less, The outer diameter of the common cladding is not less than 143 μm and not more than 195 μm, The center-to-center distance between the adjacent cores is 28.5 μm or more and 40 μm or less. The shortest distance from the center of each of the 12 or 16 cores to the interface between the common cladding and the resin coating is 26 μm or more and 35 μm or less, At a wavelength of 1.565 μm, the parallel propagation crosstalk between the adjacent cores is 10 -3 / km or less, The leakage loss from the common cladding to the resin coating at a wavelength of 1.565 μm is 0.01 dB / km or less. 8 . A multi-core optical fiber cable, comprising the multi-core optical fiber according to claim 7 installed in a state where the average bending radius is 0.06 m or more and 0.6 m or less.

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