WO2024166598A1 - マルチコア光ファイバおよびマルチコア光ファイバケーブル - Google Patents

マルチコア光ファイバおよびマルチコア光ファイバケーブル Download PDF

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WO2024166598A1
WO2024166598A1 PCT/JP2024/000504 JP2024000504W WO2024166598A1 WO 2024166598 A1 WO2024166598 A1 WO 2024166598A1 JP 2024000504 W JP2024000504 W JP 2024000504W WO 2024166598 A1 WO2024166598 A1 WO 2024166598A1
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core
cores
less
mcf
center
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French (fr)
Japanese (ja)
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哲也 林
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Sumitomo Electric Industries Ltd
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Sumitomo Electric Industries Ltd
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Priority to CN202480009463.6A priority Critical patent/CN120604150A/zh
Priority to EP24753045.4A priority patent/EP4664166A1/en
Priority to JP2024576178A priority patent/JPWO2024166598A1/ja
Publication of WO2024166598A1 publication Critical patent/WO2024166598A1/ja
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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02042—Multicore optical fibres
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
    • G02B6/03616—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference
    • G02B6/03622—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 2 layers only
    • G02B6/03627—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 2 layers only arranged - +
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
    • G02B6/03616—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference
    • G02B6/03638—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 3 layers only
    • G02B6/0365—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 3 layers only arranged - - +

Definitions

  • MCF multi-core optical fiber
  • MCF cable a multi-core optical fiber
  • This application claims priority from Japanese Patent Application No. 2023-017438, filed on February 8, 2023, the contents of which are incorporated herein by reference in their entirety.
  • Non-Patent Document 1 discloses two types of MCFs with different core arrangements on the fiber cross section.
  • the first MCF is a 12-core MCF including a common cladding with an outer diameter of 147 ⁇ m and 12 cores arranged in a square lattice pattern on the fiber cross section.
  • the second MCF is a 12-core MCF including a common cladding with an outer diameter of 145 ⁇ m and 12 cores arranged in a hexagonal lattice pattern on the fiber cross section.
  • Neither 12-core MCF has a trench layer.
  • these 12-core MCFs have a mode field diameter (hereinafter referred to as "MCF") of 5.4 ⁇ m at a wavelength of 1.310 ⁇ m and 6.1 ⁇ m at a wavelength of 1.550 ⁇ m, a cutoff wavelength of 1.26 ⁇ m, a zero dispersion wavelength of 1.41 ⁇ m, a cladding-to-coating leakage loss of 0.01 dB/km at a wavelength of 1.565 ⁇ m, and inter-core crosstalk (hereinafter referred to as "XT”) of -30 dB/km at a wavelength of 1.565 ⁇ m.
  • MCF mode field diameter
  • Patent Document 1 discloses a 12-core MCF and a 16-core MCF with a coating outer diameter of approximately 250 ⁇ m, which are MCFs suitable for short-distance communications of approximately 10 km using a wavelength band of 1.260 ⁇ m to 1.360 ⁇ m.
  • Patent Document 2 discloses a 12-core MCF in which 12 cores are arranged in a square lattice pattern on the fiber cross section as an MCF that can reduce XT and leakage loss at a wavelength of 1.550 ⁇ m.
  • the center-to-center distance between adjacent cores is preferably 30 ⁇ m or more, 35 ⁇ m or more, or 40 ⁇ m or more from the viewpoint of reducing XT.
  • Patent Document 2 does not disclose anything about what specific XT value is desirable or what XT value can be realized.
  • Specific numerical values of the MCF shown in the fourth embodiment are a core relative refractive index difference of 0.23%, a trench layer relative refractive index difference of -0.65%, an inner cladding thickness of 6 ⁇ m, a trench layer thickness of 4.3 ⁇ m, an effective area of 118.2 ⁇ m2 to 125.2 ⁇ m2 , a cutoff wavelength of 1.28 ⁇ m to 1.39 ⁇ m, and a center-to-center distance between adjacent cores of 40.5 ⁇ m.
  • the fourth embodiment also discloses that, under conditions of a wavelength of 1.550 ⁇ m and a fiber length of 3.96 km, a core-to-core XT of -38.6 dB to -41.6 dB can be achieved, that is, a core-to-core XT of 1.75 ⁇ 10 -5 /km to 3.49 ⁇ 10 -5 /km at a wavelength of 1.55 ⁇ m.
  • the MCF of the present disclosure includes 12 or 16 core units each including a core and a depressed layer, a common clad, and a resin coating.
  • the core units are arranged so that there is no adjacent relationship between cores adjacent to a specific core, and are arranged so that the centers of the core units are linearly symmetrical with respect to an axis that intersects the central axis and does not pass through any center of the core units.
  • the outer diameter of the resin coating is 250 ⁇ 15 ⁇ m
  • the effective cross-sectional area at a wavelength of 1.550 ⁇ m is 70 ⁇ m2 or more
  • the cutoff wavelength of a 22 m long cable is 1.530 ⁇ m or less or 1.460 ⁇ m or less.
  • the center-to-center distance between adjacent cores, the shortest distance from the core center to the clad interface, and the clad diameter satisfy a specific relationship.
  • FIG. 1 illustrates various configurations of MCF cables of the present disclosure that include an MCF of the present disclosure.
  • FIG. 2 is a diagram for explaining various conditions for determining the core arrangement in the MCF of the present disclosure.
  • FIG. 3 is a diagram showing core arrangements in the first and second embodiments of the MCF of the present disclosure.
  • FIG. 4 is a diagram showing core arrangements in the third and fourth embodiments of the MCF of the present disclosure.
  • FIG. 5 is a diagram showing a core arrangement in a fifth embodiment of the MCF of the present disclosure.
  • FIG. 6 is a diagram for explaining main terms used in this specification.
  • FIG. 7 is a diagram showing a refractive index profile around each core that can be applied to the MCF of the present disclosure.
  • FIG. 8 is a table showing the specifications of multiple samples of the MCF of the present disclosure and multiple samples according to a comparative example.
  • connection loss when using the MCF of Non-Patent Document 1, there is a problem that the connection loss is significantly worse.
  • a significant reduction in MFD is required compared to a general-purpose single-mode optical fiber (hereinafter referred to as "SMF").
  • SMF single-mode optical fiber
  • the connection loss caused by the axial misalignment in the MCF of Non-Patent Document 1, which has an MFD of 5.4 ⁇ m at a wavelength of 1.310 ⁇ m is 2.54 times worse than the connection loss caused by the axial misalignment in a general-purpose SMF, which has a nominal MFD value of 8.6 ⁇ m.
  • connection loss between general-purpose SMFs is 0.5 dB or less, while the connection loss between the 12-core MCFs is 1.27 dB or less, and as another example, the connection loss between general-purpose SMFs is 0.35 dB or less, while the connection loss between the 12-core MCFs is 0.89 dB or less.
  • the MCF of Patent Document 1 also has the problem that it is not suitable for transmission in the C-band (1.530 ⁇ m to 1.565 ⁇ m) or L-band (1.565 ⁇ m to 1.625 ⁇ m), which are suitable for dense wavelength division multiplexing transmission. This is because inter-core XT and other characteristics in long wavelength bands such as the C-band are sacrificed in order to densely pack 12 or 16 cores into the common cladding.
  • the MCF of Patent Document 2 has the problem of poor manufacturability. This is because it is necessary to prepare a trench layer with a large absolute value of the relative refractive index difference in order to reduce XT and leakage loss, and the formation of this trench layer makes it difficult to manufacture the MCF base material.
  • the present disclosure has been made to solve the problems described above, and aims to provide an MCF and MCF cable that, as a transmission medium suitable for bidirectional transmission, effectively reduces XT and leakage loss at a wavelength of 1.565 ⁇ m or 1.625 ⁇ m in a configuration that incorporates 12 or 16 cores and has a standard coating outer diameter of approximately 250 ⁇ m.
  • the MCF and MCF cable disclosed herein are transmission media suitable for bidirectional transmission, and in a configuration having 12 or 16 built-in cores and a standard coating outer diameter of approximately 250 ⁇ m, they effectively reduce XT and leakage loss at a wavelength of 1.565 ⁇ m or 1.625 ⁇ m.
  • the MCF of the present disclosure is (1)
  • the MCF includes 12 or 16 units, a common clad, and a resin coating.
  • Each of the 12 or 16 units includes a core extending along a central axis and a depressed layer covering the outer periphery of the core and having a refractive index lower than the maximum refractive index of the core.
  • the common clad has a refractive index higher than the refractive index of the depressed layer and covers the outer periphery of each of the 12 or 16 core units.
  • the resin coating covers the outer periphery of the common clad.
  • the fusion splicing of such MCFs allows the same number of cores to be connected in one splicing operation as in the fusion splicing of fiber ribbons including 12 or 16 optical fibers.
  • the 12 or 16 core units are arranged such that adjacent cores are not adjacent to a specific core selected from the 12 or 16 core units.
  • the 12 or 16 core units are arranged such that the centers of the 12 or 16 core units are line-symmetrical with respect to an axis that intersects the central axis and does not pass through any of the centers of the 12 or 16 core units.
  • the outer diameter of the resin coating is 250 ⁇ 15 ⁇ m, that is, 235 ⁇ m or more and 265 ⁇ m or less.
  • the MCF of the present disclosure realizes a coating outer diameter equivalent to that of a general-purpose MCF.
  • a proven coating thickness can be realized while suppressing the possibility of scratching the common cladding made of a glass material.
  • the effective cross-sectional area Aeff_1550 [ ⁇ m 2 ] at a wavelength of 1.550 ⁇ m is 70 ⁇ m 2 or more.
  • the cable cutoff wavelength ⁇ cc [ ⁇ m] at a length of 22 m is 1.530 ⁇ m or less or 1.460 ⁇ m or less.
  • ⁇ cc [ ⁇ m] at a length of 22 m is 1.530 ⁇ m or less or 1.460 ⁇ m or less.
  • the radius of the core is ra [ ⁇ m]
  • the inner radius of the depressed layer is rb [ ⁇ m]
  • the outer radius of the depressed layer is rc [ ⁇ m]
  • the absolute value of the relative refractive index difference of the depressed layer based on the refractive index of the common cladding is ⁇ dep [%]
  • the center-to-center distance ⁇ [ ⁇ m] between adjacent cores satisfies the following formula (1).
  • the XT at a wavelength of 1.565 ⁇ m can be reduced to a level sufficient for counter propagation, for example, 10 ⁇ 3 /km or less in parallel propagation.
  • the shortest distance d_coat [ ⁇ m] from the center of the core to the interface between the common cladding and the resin coating satisfies the following formula (2):
  • the leakage loss at a wavelength of 1.565 ⁇ m can be reduced to 0.01 dB/km or less.
  • the minimum outer diameter CD of the common cladding is 185 ⁇ m or less, 190 ⁇ m or less, or 195 ⁇ m or less, and further satisfies the following formula (3).
  • the minimum outer diameter CD of the common cladding is 195 ⁇ m or less and further satisfies the following formula (4):
  • the center distance ⁇ may satisfy the following formula (5):
  • XT at a wavelength of 1.565 ⁇ m can be reduced to a level sufficient for counter propagation, for example, 10 ⁇ 4 /km or less in parallel propagation.
  • the shortest distance d_coat may satisfy the following formula (6):
  • the leakage loss at a wavelength of 1.565 ⁇ m can be reduced to 0.001 dB/km or less.
  • the minimum outer diameter CD of the common cladding may satisfy the following formula (7):
  • the center-to-center distance ⁇ [ ⁇ m] may satisfy the following formula (8).
  • XT at a wavelength of 1.625 ⁇ m can be reduced to a level sufficient for counter propagation, for example, 10 ⁇ 3 /km or less in parallel propagation.
  • the shortest distance d_coat [ ⁇ m] may satisfy the following formula (9):
  • the leakage loss at a wavelength of 1.625 ⁇ m can be reduced to 0.01 dB/km or less.
  • the minimum outer diameter CD of the common cladding may satisfy the following formula (10):
  • the center-to-center distance ⁇ [ ⁇ m] may satisfy the following formula (11).
  • XT at a wavelength of 1.625 ⁇ m can be reduced to a level sufficient for counter propagation, for example, 10 ⁇ 4 /km or less in parallel propagation.
  • the shortest distance d_coat [ ⁇ m] may satisfy the following formula (12):
  • the leakage loss at a wavelength of 1.625 ⁇ m can be reduced to 0.001 dB/km or less.
  • the minimum outer diameter CD of the common cladding may satisfy the following formula (13):
  • ⁇ dep is 0.5% or less or 0.35% or less.
  • the MCF of the present disclosure is (6)
  • the MCF comprises 12 or 16 cores each extending along a central axis, a clad covering the outer periphery of each of the 12 or 16 cores, and a resin coating covering the outer periphery of the common clad.
  • the 12 or 16 cores are arranged such that there is no adjacent relationship between cores adjacent to a specific core selected from the 12 or 16 cores.
  • the 12 or 16 cores are arranged such that the centers of the 12 or 16 cores are line-symmetrical with respect to an axis that intersects the central axis and does not pass through any of the centers of the 12 or 16 cores.
  • the outer diameter of the resin coating is 250 ⁇ 15 ⁇ m.
  • the effective cross-sectional area Aeff_1550 [ ⁇ m 2 ] at a wavelength of 1.550 ⁇ m is 70 ⁇ m 2 or more.
  • the cable cutoff wavelength ⁇ cc [ ⁇ m] at 22 m length is 1.530 ⁇ m or less or 1.460 ⁇ m or less.
  • the outer diameter of the common cladding is 143 ⁇ m or more and 195 ⁇ m or less.
  • the center-to-center distance between adjacent cores is 28.5 ⁇ m or more and 40 ⁇ m 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 ⁇ m or more and 35 ⁇ m or less.
  • the parallel propagation XT between adjacent cores at a wavelength of 1.565 ⁇ m is 10 ⁇ 3 /km or less.
  • the leakage loss from the common cladding to the resin coating at a wavelength of 1.565 ⁇ m is 0.01 dB/km or less.
  • the MCF cable of the present disclosure comprises: (7)
  • the MCF according to any one of (1) to (6) above is mounted with an average bending radius of 0.06 m or more and 0.6 m or less, thereby obtaining an MCF cable that effectively reduces XT and leakage loss at a wavelength of 1.565 ⁇ m or 1.625 ⁇ m.
  • FIG. 1 shows various structures of the MCF cable of the present disclosure (marked “cable structure” in FIG. 1).
  • the MCF of the present disclosure is embedded in such an MCF cable.
  • the MCF cable 1A shown in the upper part of Figure 1 (labeled "Structure (A)" in Figure 1) comprises an outer sheath 300 including an MCF storage space extending along the longitudinal direction of the MCF cable 1A, and a plurality of MCFs 100, which are MCFs of the present disclosure.
  • the outer sheath 300 has two tension members 400A, 400B embedded therein that extend along the MCF storage space.
  • Each of the MCFs 100 includes a glass fiber 200 whose outer circumferential surface is covered with a resin coating.
  • the MCF cable 1B shown in the lower part of FIG. 1 (marked “structure (B)" in FIG. 1) comprises an outer jacket 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 multiple MCFs 100, which are the MCFs of the present disclosure.
  • the slotted core 600 dividing the MCF storage space into multiple spaces is housed within the outer jacket 500.
  • a tensile wire 700 extending along the longitudinal direction of the MCF cable 1B is embedded in the slotted core 600.
  • the multiple MCFs 100 are housed within any of the spaces divided by the slotted core 600.
  • Figure 2 is a diagram for explaining the conditions for determining the core arrangement in the MCF of the present disclosure (labeled "Core Arrangement” in Figure 2).
  • the upper part of Figure 2 (labeled “Square Lattice” in Figure 2) shows a square lattice defined on the cross section of the MCF, and the lower part of Figure 2 (labeled "Adjacency relationship between inner core and outer core” in Figure 2) shows a diagram for explaining the arrangement of two outer cores that maintain an adjacency relationship with one inner core.
  • the MCF disclosed herein has 12 or 16 cores. As a result, even when the MCFs to be connected are fused after being rotated one by one, it is possible to connect the same number of cores per fusion as with ribbon fusion of a cable that contains many fiber ribbons in which 12 or 16 optical fibers are integrated with resin when connecting an ultra-multi-core cable.
  • Ribbon fusion refers to a connection method in which each optical fiber included in a fiber ribbon is fused to the optical fiber to be connected at the same time.
  • the MCF of the present disclosure also has a core arrangement in which multiple cores adjacent to a specific core selected from 12 or 16 cores are not adjacent to each other.
  • This allows for reduced XT, i.e., counter-propagation XT, from another core adjacent to the specific core in bidirectional communication in which signals are transmitted in different propagation directions between adjacent cores (hereinafter referred to as "adjacent cores").
  • the adjacent core of a specific core means a core that is greatly affected by parallel propagation XT to the specific core, as will be described later (see FIG. 6).
  • the adjacent cores are the core closest to the specific core and the core with the same center distance (difference of 2 ⁇ m or less) between the adjacent cores.
  • parallel propagation XT means normal XT when light is propagated in the same direction.
  • 12 or 16 cores are arranged in a square lattice-like core arrangement in which some of the 12 or 16 cores are shifted from the lattice points, that is, in a core arrangement in which adjacent cores that are adjacent to a specific core are not adjacent to each other.
  • the centers of each core are arranged in a position that is linearly symmetrical with respect to a predetermined 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.
  • the center position of the core may vary within 0.5 ⁇ m from a given lattice point. This allows for a larger manufacturing tolerance.
  • the positions of the lattice points (inner lattice point and outer lattice point) to which the inner and outer cores are respectively assigned can be found by optimizing the spacing, orientation, and position of the lattice points of the square lattice so that the sum of the square errors of the positional deviations between the centers of the cores and the corresponding lattice points is minimized.
  • the MCF 100 shown in FIG. 2 comprises a glass fiber 200 extending along a central axis AX and a resin coating 130 covering the glass fiber 200.
  • the glass fiber 200 comprises 12 or 16 cores each extending along the central axis AX and a common cladding 120 covering each of the 12 cores.
  • cores 110 when simply referring to “12 or 16 cores", this will be referred to as cores 110.
  • the square lattice 800 shown in the upper part of FIG. 2, which is the basis for core placement, is defined on a cross section of the MCF 100 perpendicular to the central axis AX. Note that 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 with a lattice point interval that matches the center-to-center distance ⁇ between adjacent cores, and is composed of a plurality of lattice points that are arranged point-symmetrically with respect to the central axis AX so that four inner lattice points 810 surround the central axis AX at the shortest distance.
  • the square lattice 800 includes four inner lattice points 810 and eight outer periphery lattice points 820 that surround the four inner lattice points 810 and are adjacent to any of the four inner lattice points 810. Note that when 12 cores are placed, no cores are placed on the non-outer periphery lattice points 830 that are not adjacent to any of the four inner lattice points 810.
  • each of the 12 cores belongs 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 0.5 ⁇ m or less.
  • each of the eight outer periphery cores belongs to either the lattice point placement core 110Ba or the lattice point non-placement core 110Bb.
  • the lattice point placement core 110Ba is a core whose center is located at a position where the distance from the corresponding outer periphery lattice point among the eight outer periphery lattice points 820 is 0.5 ⁇ m or less.
  • the lattice point non-placement core 110Bb is a core whose center is located at a distance D1 of 2 ⁇ m or more from the corresponding outer periphery lattice point 820.
  • the angle ⁇ formed by two line segments extending from one inner core 110A toward two adjacent outer cores is 90 [deg] when both of the two outer cores are lattice point arrangement cores 110Ba, and is less than 90 [deg] when one of the two outer cores is a lattice point non-arranged core 110Bb.
  • Each of the lattice point non-arranged cores 110Bb has its center arranged at a position away from a specific inner lattice point that is adjacent to the corresponding outer lattice point among the four inner lattice points 810 by a distance D3. That is, each of the lattice point non-arranged cores 110Bb is arranged on a circumference centered on the specific inner lattice point.
  • D3 is equal to or greater than ( ⁇ -0.5 ⁇ m) and equal to or less than ( ⁇ +0.5 ⁇ m).
  • the centers of the non-lattice-point-placed cores 110Bb are placed so that they are at a longer distance from a specific outer periphery lattice point (see the pair of outer periphery lattice points whose adjacency is shown in the upper part of FIG. 2) that is adjacent to the corresponding outer periphery lattice point than the distance D1 from the corresponding outer periphery lattice point, and are at a distance D2 from the centers of the remaining non-lattice-point-placed cores 110Bb.
  • D2 is ⁇ +3 ⁇ m or more.
  • the non-lattice-point-placed cores 110Bb are defined as maintaining their adjacency with other cores, i.e., the inner core 110A and other outer periphery cores, in the same way as the adjacency between the lattice points.
  • FIG. 3 is a diagram showing the core arrangement in the first and second embodiments of the MCF of the present disclosure (indicated as "Core Arrangement" in FIG. 3).
  • the upper part of FIG. 3 (indicated as “Embodiment 1" in FIG. 3) discloses a cross section of MCF 100A having 12 cores, and the lower part of FIG. 3 (indicated as “Embodiment 2" in FIG. 3) discloses a cross section of MCF 100B having 16 cores.
  • FIG. 4 is a diagram showing the core arrangement in the third and fourth embodiments of the MCF of the present disclosure (indicated as "Core Arrangement" in FIG. 4).
  • the upper part of FIG. 4 (indicated as "Embodiment 3" in FIG.
  • FIG. 5 is a diagram showing the core arrangement in the fifth embodiment of the MCF of the present disclosure, and discloses a cross section of MCF 100E having 12 cores.
  • Figures 3 to 5 are schematic diagrams in which the core positions and dimensions are not based on actual scale. Also, in each of Figures 3 to 5, an axis of symmetry LA is shown so that it can be seen that 12 or 16 cores are arranged in line symmetry.
  • the MCF 100A shown in the upper part of Figure 3 includes a glass fiber 200A surrounded by a resin coating 130.
  • the glass fiber 200A has 12 cores extending along a central axis AX and a common cladding 120 covering each of these 12 cores.
  • the 12 cores are classified into four inner cores 110A and eight outer cores depending on the type of lattice point to which they are assigned.
  • the eight outer cores are also classified into lattice point placement cores 110Ba and non-lattice point placement cores 110Bb. In the case of the MCF 100A, all eight outer cores are classified as lattice point placement cores 110Ba.
  • MCF 100B shown in the lower part of Figure 3 includes a glass fiber 200B surrounded by a resin coating 130.
  • Glass fiber 200B has 16 cores extending along central axis AX and a common cladding 120 covering each of these 16 cores.
  • the 16 cores are classified into four inner cores 110A and 12 outer cores depending on the type of lattice point to which they are assigned.
  • the 12 outer cores are also classified into lattice point placement cores 110Ba and non-lattice point placement cores 110Bb. In the case of MCF 100B, all of the 12 outer cores are classified as lattice point placement cores 110Ba.
  • both the MCF 100A and the MCF 100B have two-fold or more rotational symmetry.
  • MCF 100C shown in the upper part of FIG. 4, MCF 100D shown in the lower part of FIG. 4, and MCF 100E shown in FIG. 5 have the same structure as MCF 100A shown in the upper part of FIG. 3, except for the core arrangement. That is, MCF 100C to 100E have glass fibers 200C to 200E corresponding to glass fiber 200A, and resin coating 130. Also, each of glass fibers 200C to 200E has 12 cores extending along the central axis AX and a common clad 120 covering each of these 12 cores. On the cross section of each of glass fibers 200C to 200E perpendicular to the central axis AX, common clad 120 has a circular outer periphery.
  • the 12 cores are classified into four inner cores 110A and eight outer cores depending on the type of lattice point assigned.
  • the eight outer cores are classified into lattice-point-placed cores 110Ba and non-lattice-point-placed cores 110Bb.
  • the non-lattice-point-placed cores 110Bb are arranged in a position that is linearly symmetrical with respect to the axis of symmetry LA, and the core arrangements in each of these MCF100C to MCF100E do not have two-fold or more rotational symmetry about the central axis AX.
  • the glass fiber 200C surrounded by the resin coating 130 has 12 cores extending along the central axis AX and a common cladding 120 that covers each of these 12 cores.
  • the 12 cores are classified into four inner cores 110A and eight outer cores depending on the type of lattice points assigned to them.
  • the eight outer cores are classified into lattice point placement cores 110Ba and lattice point non-placement cores 110Bb.
  • four inner cores 110A are arranged on the inner lattice points 810 of a square lattice 800 set on a cross section perpendicular to the central axis AX. At this time, the distance between the inner lattice point 810 and the center of the corresponding inner core 110A is 0.5 ⁇ m or less.
  • Eight outer cores are arranged around these four inner cores 110A. Of the eight outer cores, six are arranged as lattice point arrangement cores 110Ba so that the distance between the outer lattice point 820 to which they are assigned and the core center is 0.5 ⁇ m or less.
  • the remaining two outer cores are arranged as lattice point non-arrangement cores 110Bb so that the core centers are shifted from the outer lattice point 820 to which they are assigned while maintaining the adjacent relationship shown in the lower part of FIG. 2. 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 6:2.
  • one lattice point placement core 110Ba and one non-lattice point placement core 110Bb, each of which is adjacent to one inner core 110A, are arranged at an angle ⁇ with the inner core 110A as the center.
  • four inner cores 110A are arranged on the inner lattice points 810 of a square lattice 800 set on a cross section perpendicular to the central axis AX.
  • the distance between the inner lattice point 810 and the center of the corresponding inner core 110A is 0.5 ⁇ m or less.
  • Eight outer cores are arranged around these four inner cores 110A. Of the eight outer cores, six are arranged as lattice point arrangement cores 110Ba such that the distance between the outer lattice point 820 assigned to each of them and the core center is 0.5 ⁇ m or less.
  • the remaining two outer cores are arranged as lattice point non-arrangement cores 110Bb with the core centers shifted from the outer lattice points 820 assigned to each of them while maintaining the adjacent relationship shown in the lower part of FIG. 2. Therefore, in this MCF100D, as in MCF100C, the ratio of the number of lattice point placement cores to the number of non-lattice point placement cores is 6:2.
  • MCF100E In the MCF100E according to the fifth embodiment shown in FIG. 5, like the MCF100C and MCF100D, four inner cores 110A are arranged on inner lattice points 810 of a square lattice set on a cross section perpendicular to the central axis AX, and eight outer cores are arranged around them. Of the eight outer cores, four are arranged as lattice point arrangement cores 110Ba on the outer lattice points 820 to which they are assigned. The remaining four outer cores are arranged as non-lattice point arrangement cores 110Bb with their core centers shifted from the outer lattice points 820 to which they are assigned, while maintaining the adjacency relationship shown in the lower part of FIG. 2.
  • the ratio of the number of lattice point arrangement cores to the number of non-lattice point arrangement cores is four to four.
  • the two non-lattice-point-placed cores 110Bb that make up a pair are arranged in positions that are linearly symmetrical with respect to the axis of symmetry LA, but the core arrangement in this MCF 100E does not have two-fold or more rotational symmetry centered on the central axis AX.
  • the deviation angle of the first pair is ⁇ 1
  • the deviation angle of the second pair is ⁇ 2.
  • ⁇ 2 and ⁇ 1 do not need to be the same.
  • MCF100E in which four of the eight outer cores are set to non-lattice point placement cores 110Bb, has increased asymmetry compared to MCF100C and MCF100D.
  • Figure 6 is a diagram to explain the main terms used in this specification, namely the adjacency relationship of the inner cores, the cross-sectional structure around the cores, parallel propagation and parallel propagation XT, and counter propagation XT.
  • FIG. 6 illustrates four inner cores 110A (cores 110) arranged on four inner lattice points 810 surrounding the central axis AX with a lattice point interval that matches the center-to-center distance ⁇ between adjacent cores.
  • the inner cores 110A pairs that form each side of the square lattice are adjacent to each other.
  • the inner cores 110A pairs located on the diagonal lines of the square lattice do not have an adjacent relationship.
  • the cross-sectional structure around each core 110 is such that the outer periphery of the core 110 is surrounded by a common clad 120.
  • the core 110 includes an inner core 110A, a lattice-point-placed core 110Ba, and a lattice-point-unplaced core 110Bb.
  • the common clad 120 may be provided so as to be in direct contact with the core 110, or an optical clad 121 may be provided between the common clad 120 and the core 110.
  • a depressed layer 122 having a small absolute value of ⁇ dep may be provided between the optical clad 121 and the common clad 120.
  • the optical clad 121 may be provided for each core 110, and may have a relative refractive index difference ⁇ 2 of ⁇ 0.1% or more and 0.1% or less with respect to the refractive index of the common clad 120. However, when ⁇ 2 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 ⁇ dep.
  • the depressed layer 122 When the depressed layer 122 is provided, the depressed layer 122 has a relative refractive index difference ⁇ 3 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 120.
  • the absolute value ⁇ dep of the relative refractive index difference of the depressed layer 122 may be 0.5% or less, or 0.35% or less.
  • Parallel Propagation and Parallel Propagation XT In the example shown in Fig. 6, three cores (first core 110a all propagating light in the same direction) that are adjacent to each other are shown. That is, an adjacent relationship is established between the left core and the center core, and an adjacent relationship is established between the center core and the right core. That is, a state in which each of the adjacent cores propagates light in the same direction is referred to as "parallel propagation". In this case, parallel propagation XT occurs as normal XT between adjacent cores that propagate light in the same direction.
  • Counterpropagation and Counterpropagation XT On the other hand, in counter propagation, light is propagated in different directions between two cores that are adjacent to each other. That is, in the example of FIG. 6, the left core and the center core are adjacent to each other, but the left core functions as the first core 110a, and the center core functions as the second core 110b that propagates light in a direction different from the first core 110a. The normal XT that occurs between these left core and center core is unlikely to affect communication quality. Similarly, the right core that is adjacent to the center core functions as the first core 110a, and the normal XT that occurs between these right core and center core is unlikely to affect communication quality.
  • counter propagation the state in which the cores that are adjacent to each other propagate light in different directions.
  • XT affects communication quality via the center core that functions as the second core 110b.
  • counter-propagating XT XT between cores that are adjacent to each other and propagate light in the same direction via cores that propagate light in opposite directions.
  • XT counter propagation XT: XT_counter
  • XT_co parallel propagation XT between the left core and the central core, and between the central core and the right core.
  • the counter propagation XT in fiber length L1 is XT_counter(L1), and XT is expressed in decibels
  • the counter propagation XT in fiber length L2 can be expressed by the following equation (16), and XT_counter increases by 20 dB when the distance is 10 times larger.
  • the total XT_co_tot of XT_co from adjacent cores to a given core is expressed by the following formula (17), where N is the number of adjacent cores to the given core.
  • the above formula (17) is based on the premise that XT_co between adjacent cores is uniform. If the difference in XT_co between adjacent cores cannot be ignored, the total XT_co_tot is expressed by the following formula (18), where XT_co from core n among the N adjacent cores to the given core is XT_co(n).
  • the total XT_counter_tot of counter propagation XT to a given core seems to be expressed by the following formula (19) when the number of "adjacent cores of adjacent cores (corresponding to the right core when the given core is the left core in the example of counter propagation shown in FIG. 6)" of the given core is M.
  • XT_counter_tot is expressed by the following formula (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 by the following formula (21).
  • the parallel propagation XT (XT_co) between adjacent cores in terms of fiber length L (km) needs to satisfy the following formula (24), and the sum of the parallel propagation XT from four cores adjacent to any of the four cores belonging to the inner core group needs to satisfy the following formula (25).
  • FIG. 7 is a diagram showing the refractive index profile around each core applicable to the MCF of the present disclosure.
  • the "relative refractive index difference ⁇ " refers to the relative refractive index difference with respect to the refractive index of the common cladding.
  • the "relative refractive index difference ⁇ " is not the relative refractive index difference with respect to the refractive index of pure silica glass.
  • the refractive index profile of the core and the associated optical properties can be selected from appropriate structures depending on the application.
  • the refractive index profiles of patterns (A) to (K) shown in FIG. 7 can be applied.
  • ⁇ is the relative refractive index difference based on the refractive index of the common cladding
  • the structures may be the same between cores, or may be different.
  • ⁇ core means the absolute value of the relative refractive index difference of each core with respect to the refractive index of the common cladding
  • ⁇ dep means the absolute value of the relative refractive index difference of the depressed layer or the portion that functions as the depressed layer.
  • Pattern (A) shown in FIG. 7 is a step-type refractive index profile
  • pattern (B) is a ring-type refractive index profile
  • pattern (C) is a double-step-type refractive index profile
  • pattern (D) is a graded-type refractive index profile
  • pattern (E) is a tapered-type refractive index profile, which are applicable to the core structure in the MCF of the present disclosure.
  • patterns (F) and (H) in which a depressed-type refractive index profile is provided around the core patterns (G), (I) and (J) in which a raised-type refractive index profile is provided around the core, and pattern (K) in which a matched-type refractive index profile is provided around the core are also applicable to the core structure.
  • the ESI (Equivalent-step-index) approximation can be used to determine the core radius ra and core ⁇ ( ⁇ 1) when approximated by a step type (Non-Patent Document 2 above).
  • 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 it to the case where the boundary between the core and the cladding is unclear, such as the skirt-shaped refractive index profile of pattern (E).
  • the skirt-shaped refractive index profile of pattern (E) For example, if rd in pattern (E) is regarded as the radius of the core and the method of Non-Patent Document 2 is applied as it is, the ESI approximation does not work well.
  • the cladding means the common cladding 120 or the optical cladding 121.
  • the refractive index of the cladding r can be calculated based on the above-mentioned Non-Patent Document 2 by using the value obtained by the simple average of ⁇ in the range from ra to rd shown in the following formula (26) or the weighted average by r shown in the following formula (27). It is preferable that ⁇ 2 is -0.10% or more and 0.10% or less, because this significantly improves manufacturability.
  • the optical cladding in the pattern (F) and the pattern (H) has a negative relative refractive index difference, and substantially functions as a depressed layer with an absolute value of ⁇ dep.
  • a depressed layer 122 having a refractive index lower than that of the optical cladding 121 and the common cladding 120 may be provided around the optical cladding 121 (pattern (K) in FIG. 7).
  • the relative refractive index difference ⁇ 3 of the depressed layer 122 based on the refractive index of the common cladding 120 is -0.5% or less, manufacturability is significantly deteriorated, so ⁇ 3 ⁇ -0.4%, preferably ⁇ 3 ⁇ -0.3%, and more preferably ⁇ 3 ⁇ -0.2%.
  • the absolute value ⁇ dep 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 from the viewpoint of manufacturability.
  • the core and cladding materials are preferably made of glass mainly composed of silica glass, which can achieve low transmission loss and high mechanical reliability.
  • the core is preferably doped with Ge, which creates a refractive index difference between the core and the cladding.
  • the cladding is preferably doped with F, which creates a refractive index difference between the core and the cladding. Doping the core and optical cladding with a trace amount of F is preferable, which can achieve a depressed profile with good manufacturability.
  • the core and cladding may be doped with Cl, which can reduce OH groups and reduce absorption losses caused by OH groups.
  • the core and cladding may contain a trace amount of P, which can improve manufacturability in some glass synthesis processes.
  • the MCF of the present disclosure having the cross-sectional structure shown in Figures 3 to 5 has a resin coating 130, and the diameter of the resin coating 130 is 250 ⁇ 15 ⁇ m, i.e., 235 ⁇ m or more and 265 ⁇ m or less. This makes it possible to cable the MCF of the present disclosure without making major changes to existing cabling equipment, etc.
  • the nominal value of the minimum outer diameter CD of the cladding which corresponds to the diameter of the glass fiber 200, is 125 ⁇ m, and the nominal value of the diameter of the resin coating 130 is about 245 ⁇ m to 250 ⁇ m, but in small-diameter coated SMF, the nominal value of the diameter of the resin coating is also seen to be 180 ⁇ m, 190 ⁇ m, and 200 ⁇ m. In these cases, the nominal value of the thickness of the resin coating 130 is 27.5 ⁇ m, 32.5 ⁇ m, and 37.5 ⁇ m, respectively. If the thickness of the resin coating 130 is thin, if sand or dust scratches the coating surface, the scratches may reach the glass cladding, weakening the strength of the optical fiber, so a sufficient nominal coating thickness is desired.
  • the nominal CD value needs to be 195 ⁇ m or less.
  • the nominal value of the CD may be 190 ⁇ m or less.
  • the nominal value of the CD may be 185 ⁇ m or less.
  • the nominal value of the CD may be 180 ⁇ m or less.
  • the nominal value of the CD may be 175 ⁇ m or less. Furthermore, in order to realize a nominal value of the diameter of the resin coating 130 of 245 ⁇ m and a nominal value of the coating thickness of 37.5 ⁇ m or more, the nominal value of the CD may be 170 ⁇ m or less. In each case, the coating thickness tolerance should be ⁇ 15 ⁇ m or less, and more preferably ⁇ 10 ⁇ m or less.
  • the wavelength of 1.565 ⁇ m is the upper limit of the C-band (1.530 ⁇ m or more and 1.565 ⁇ m or less)
  • the wavelength of 1.625 ⁇ m is the upper limit of the L-band (1.565 ⁇ m or more and 1.625 ⁇ m or less).
  • Non-Patent Document 1 discloses an MCF with a small MFD to reduce inter-core XT and leakage loss.
  • application of the MCF of Patent Document 1 to bidirectional transmission results in significant deterioration of connection loss.
  • Patent Document 1 discloses an MCF for short-distance transmission in the O-band (1.260 ⁇ m or more and 1.360 ⁇ m or less).
  • use of the MCF of Patent Document 1 inevitably leads to 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.
  • Patent Document 2 adopts a depressed type shown in FIG.
  • the MCF of the present disclosure has a resin coating 130 with a standard outer diameter of 250 ⁇ m ⁇ 15 ⁇ m, i.e., 235 ⁇ m to 265 ⁇ m, and incorporates 12 or 16 cores 110.
  • the MCF of the present disclosure comprises 12 cores 110 each extending along the central axis AX, a common clad 120 covering each of the 12 or 16 cores 110, and a resin coating 130 covering the outer periphery of the common clad 120.
  • the 12 or 16 cores 110 are arranged such that there is no adjacent relationship between the cores adjacent to a specific core selected from the 12 or 16 cores 110.
  • the 12 or 16 cores 110 are arranged so that the centers of the 12 or 16 cores 110 are symmetrical with respect to an axis of symmetry LA that intersects with the central axis and does not pass through any of the centers of the 12 or 16 cores 110. That is, the MCF of the present disclosure has any of the core arrangements shown in Figs. 3 to 5.
  • any of the patterns (A) to (K) shown in Fig. 8 can be applied to the refractive index profile around each core.
  • one core unit is composed of a group of the core 110 and the depressed layer 122 corresponding to the core 110, or a group of the core 110, the optical cladding 121, and the depressed layer 122.
  • One core unit includes one core 110, and the center of the core unit coincides with the center of the core 110 contained therein.
  • the effective area Aeff_1550 [ ⁇ m 2 ] at a wavelength of 1.550 ⁇ m is 70 ⁇ m 2 or more.
  • the cable cutoff wavelength ⁇ cc [ ⁇ m] at a length of 22 m is 1.530 ⁇ m or less or 1.460 ⁇ m or less.
  • the outer diameter of the common cladding 120 is 143 ⁇ m or more and 195 ⁇ m or less.
  • the center-to-center distance between adjacent cores is 28.5 ⁇ m or more and 40 ⁇ m 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 ⁇ m or more and 35 ⁇ m or less.
  • the parallel propagation XT between adjacent cores at a wavelength of 1.565 ⁇ m is 10 ⁇ 3 /km or less.
  • the leakage loss from the common cladding 120 to the resin coating 130 is 0.01 dB/km or less.
  • MCF samples having refractive index profiles of patterns (F), (H), and (K) in which the depressed layer 122 or the optical cladding 121 functioning as the depressed layer is provided around each core among patterns (A) to (K) shown in FIG. 7 will be described.
  • the various conditions shown below also apply to the group in which the depressed layer is not provided as the refractive index profile around each core, that is, patterns (A) to (E), (G), (I), and (J).
  • Samples 1 to 12 of the embodiment of the MCF of the present disclosure shows the specifications of Samples 1 to 12 of the embodiment of the MCF of the present disclosure and Samples 1 to 5 of the comparative example.
  • Samples 2, 4, 6, and 8 of the embodiment are MCFs having 12 or 16 cores 110 including a specific set of inner core 110A, lattice point arranged core 110Ba, and lattice point non-arranged core 110Bb, and the refractive index profile around each core is any of the patterns (F), (H), and (K) of the refractive index profile shown in FIG. 7.
  • FIG. 7 shows the specifications of Samples 1 to 12 of the embodiment of the MCF of the present disclosure and Samples 1 to 5 of the comparative example.
  • Samples 2, 4, 6, and 8 of the embodiment are MCFs having 12 or 16 cores 110 including a specific set of inner core 110A, lattice point arranged core 110Ba, and lattice point non-arranged core 110Bb, and the refractive index profile around each core is any
  • Samples 1, 3, 5, 7, and 9 to 12 of the embodiment and Samples 1 to 5 of the comparative example are MCFs in which 12 or 16 cores 110 are arranged on each lattice point of a square lattice.
  • the refractive index profile around each core is a pattern without a depressed layer.
  • the refractive index profile around each core is any one of the patterns (F), (H), and (K) of the refractive index profile shown in Figure 7.
  • the refractive index profile around each core is pattern (K), and ⁇ 2 is 0%.
  • ra [ ⁇ m] is the radius of the core 110.
  • rb [ ⁇ m] is the inner radius of the depressed layer 122.
  • rc [ ⁇ m] is the outer radius of the depressed layer 122.
  • Aeff_1550 [ ⁇ m 2 ] is the effective cross-sectional area at a wavelength of 1.550 ⁇ m.
  • ⁇ cc [ ⁇ m] is the cable cutoff wavelength at a length of 22 m.
  • ⁇ core [%] is the maximum refractive index difference of the core 110 with respect to the refractive index of the common cladding 120.
  • ⁇ dep [%] is the absolute value of the relative refractive index difference of the depressed layer 122 with respect to the refractive index of the common cladding 120.
  • ⁇ [ ⁇ m] is the center-to-center distance between adjacent cores.
  • d_coat [ ⁇ m] is the shortest distance from the core 110 to the interface between the common cladding 120 and the resin coating 130.
  • ⁇ [deg] is an index showing the positional relationship between the inner core 110A whose center is located on the inner lattice point 810 and the lattice-point-placed core 110Ba and the lattice-point-free core 110Bb that are adjacent to the inner core 110A and have the same center-to-center distance ⁇ with respect to the inner core 110A.
  • ⁇ is the angle between a line segment extending from the center of the inner core 110A toward the center of the lattice-point-placed core 110Ba and a line segment extending from the center of the inner core 110A toward the center of the lattice-point-free core 110Bb.
  • CD [ ⁇ m] is the minimum outer diameter of the common cladding 120.
  • XT@1.565 ⁇ m [1/km] is the parallel propagation XT between adjacent cores at a wavelength of 1.565 ⁇ m.
  • XT@1.625 ⁇ m [1/km] is the parallel propagation XT between adjacent cores at a wavelength of 1.625 ⁇ m.
  • the leakage loss @ 1.565 ⁇ m [dB/km] is the leakage loss at a wavelength of 1.565 ⁇ m.
  • the leakage loss @ 1.625 ⁇ m [dB/km] is the leakage loss at a wavelength of 1.625 ⁇ m.
  • the center-to-center distances ⁇ between adjacent cores are all set to be equal, but they may vary within a specified range from the nominal value ⁇ _nominal. In this case, it is possible to increase the manufacturing tolerance.
  • the minimum outer diameter CD of the common cladding 120 may also vary within a specified range from the nominal value CD_nominal.
  • Each core of the MCF of the present disclosure may have an effective cross-sectional area Aeff_1550 of 70 ⁇ m2 or more at a wavelength of 1.550 ⁇ m. This allows for a reduction in noise caused by nonlinear interference. It also allows for a reduction in connection loss caused by axial misalignment between the MCFs of the present disclosure.
  • the sum of counter-propagating XT from adjacent cores to any core should be -20 dB or less even after 10 km propagation at a wavelength of 1.565 ⁇ m.
  • Counter-propagating XT from cores other than adjacent cores is sufficiently low and negligible, thereby achieving a sufficient signal-to-noise rate (SNR) even when performing coherent detection.
  • SNR signal-to-noise rate
  • the sum of counter-propagating XT from adjacent cores to any core should be -40 dB or less even after 10 km of propagation in the wavelength band used.
  • Counter-propagating XT from cores other than adjacent cores is sufficiently low and negligible, which makes it possible to achieve a sufficient SNR even when performing Intensity Modulation-Direct Detection (IM-DD).
  • IM-DD Intensity Modulation-Direct Detection
  • the parallel propagation XT between adjacent cores is preferably 10 ⁇ 4 /km or less ( ⁇ 40 dB or less) at a wavelength of 1.565 ⁇ m.
  • 12 cores when 12 cores are arranged, it can be less than 8 ⁇ 10 ⁇ 4 /km, and when 16 cores are arranged, it can be less than 9 ⁇ 10 ⁇ 4 /km.
  • the center-to-center distance ⁇ between adjacent cores should satisfy the following formula (28): where ra [ ⁇ m] is the radius of the core 110, rb [ ⁇ m] is the inner radius of the depressed layer 122, rc [ ⁇ m] is the outer radius of the depressed layer 122, Aeff_1550 [ ⁇ m 2 ] is the effective area at a wavelength of 1.550 ⁇ m, ⁇ cc [ ⁇ m] is the cable cutoff wavelength at a length of 22 m, and ⁇ dep [%] 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, which is a value of 0 or more by definition.
  • ⁇ satisfies the following formula (29).
  • ⁇ should satisfy the following formula (30).
  • ⁇ satisfies the following formula (31).
  • d_coat When the leakage loss from the common cladding 120 to the resin coating 130 at a wavelength of 1.565 ⁇ m is reduced to 0.01 dB/km or less, the shortest distance d_coat from the core center to the outer peripheral surface of the common cladding 120 should satisfy the following formula (32). Note that the outer peripheral surface of the common cladding 120 corresponds to the interface between the common cladding 120 and the resin coating 130.
  • d_coat of the outermost core i.e., the minimum value of d_coat, is generally called the outer cladding thickness (OCT), but in this specification, d_coat means a value that can be defined for each core.
  • d_coat satisfies the following formula (33).
  • d_coat satisfies the following formula (34).
  • d_coat satisfies the following formula (35).
  • d_coat satisfies the following formula (36).
  • d_coat satisfies the following equation (37).
  • Minimum allowable outer diameter CD When the parallel propagation XT between adjacent cores at a wavelength of 1.565 ⁇ m is reduced to 10 ⁇ 3 /km or less and the leakage loss from the common cladding 120 to the resin coating 130 is reduced to 0.01 dB/km or less, in a standard square lattice core arrangement, the minimum outer diameter CD of an MCF having 16 cores should satisfy the following formula (38), and the minimum outer diameter CD of an MCF having 12 cores should satisfy the following formula (39).
  • the minimum outer diameter CD of an MCF having 16 cores should satisfy the following formula (40), and the CD of an MCF having 12 cores should satisfy the following formula (41).
  • the minimum outer diameter CD of an MCF having 16 cores should satisfy the following formula (42), and the minimum outer diameter CD of an MCF having 12 cores should satisfy the following formula (43).
  • the CD in an MCF having 16 cores should satisfy the following equation (44), and the CD in an MCF having 12 cores should satisfy the following equation (45).
  • ⁇ cc When ⁇ cc is 1.530 ⁇ m or less, single mode operation in C band can be ensured. When ⁇ cc is 1.460 ⁇ m or less, single mode operation in S band can be ensured.
  • ⁇ cc when ⁇ cc is longer than 1.260 ⁇ m, light confinement in the core is strengthened, making it possible to reduce counter-propagation XT and leakage loss. When ⁇ cc is longer than 1.360 ⁇ m, light confinement in the core is further strengthened, making it possible to further reduce counter-propagation XT and leakage loss.
  • ⁇ dep may be 0.5% or less.
  • ⁇ dep may be 0.35% or less. This can further improve the manufacturability of the MCF base material.
  • the most preferable range for ⁇ dep is 0.20% or less.
  • the upper limit of the average bending radius of the mounted MCF may be 0.60 m or less, and more preferably 0.30 m or less. This condition is effective in reducing counter-propagation XT.
  • the lower limit of the average bending radius of the mounted MCF may be 0.06 m or more, and more preferably 0.10 m or more. This condition is effective in reducing the probability of bending breakage of the MCF in the cable.
  • the MCF cable capable of incorporating the MCF disclosed herein may be a ribbon slot type cable.
  • the bending radius of the MCF can be easily controlled, making it possible to reduce XT.
  • the MCF cable capable of incorporating the MCF of the present disclosure may be an intermittently bonded ribbon type cable.
  • An intermittently bonded ribbon is a ribbon in which adjacent MCFs among the multiple MCFs constituting the ribbon are bonded to each other at regular intervals along the longitudinal direction.
  • the flexible intermittently bonded ribbon can be mounted in the cable while being twisted in a spiral shape.
  • the features and characteristics of the MCF disclosed herein can be measured by the following methods.
  • the effective 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 ⁇ cc 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 ⁇ between adjacent cores can be measured, for example, by the refraction near-field method, the lateral interference method, or a microscopic 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 refraction near-field method, the lateral interference method, or a microscopic 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 refraction near-field method, the lateral interference method, or a microscope 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.
  • Leakage loss can be measured by the method described in Patent Document 3.
  • formulas (1) to (13) are formulas when ⁇ , d_coat, and CD are expressed in units of ⁇ m.

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