Multi-core optical fiber and multi-core optical cable

Abstract

The MCF of the present application can ensure sufficient manufacturing tolerance, is excellent in mass productivity, and can also suppress deterioration of connection loss. The MCF has four cores and a common cladding extending along a central axis. In a cross section, the common cladding has a circular outer periphery, and the four cores are arranged at positions that become line-symmetric with respect to a straight line that intersects the central axis and does not intersect any of the four cores. The core arrangement defined by the four cores has one-fold rotational symmetry with the central axis as a center of rotation.

Description

Technical Field

[0001] This invention relates to multi-core optical fiber (hereinafter referred to as "MCF") and multi-core optical cable (hereinafter referred to as "MCF cable").

[0002] This application claims priority based on Japanese Patent Application No. 2020-174975, filed on October 16, 2020, the contents of which are incorporated herein by reference in their entirety. Background Technology

[0003] Non-Patent Document 1 discloses a trench-assisted four-core optical fiber with four cores arranged in a square configuration and a cladding having an outer diameter of 125 μm. The trench depth is approximately -0.7% or less. The mode field diameter (hereinafter referred to as "MFD") at a wavelength of 1310 nm is 8.4 μm to 8.6 μm. The cable cutoff wavelength is 1171 nm to 1195 nm. The zero-dispersion wavelength is 1317 nm to 1319 nm, and the wavelength dispersion slope of this zero-dispersion wavelength is 0.090 ps / (nm). 2 ·km) or more 0.091ps / (nm) 2 The transmission loss at a wavelength of 1310 nm is 0.33 dB / km or less and 0.35 dB / km or less. The transmission loss at a wavelength of 1550 nm is 0.19 dB / km or more and 0.21 dB / km or less. The crosstalk between fiber cores (hereinafter referred to as "XT") at a wavelength of 1625 nm is -43 dB / km.

[0004] Non-Patent Document 2 discloses a trenchless four-core optical fiber with four cores arranged in a square configuration and a cladding having an outer diameter of 125 μm. The MFD (Mean Dispersion) at a wavelength of 1310 nm is 8.6 μm to 8.8 μm, and at a wavelength of 1550 nm, the MFD is 9.6 μm to 9.8 μm. The cable cutoff wavelength is 1234 nm to 1244 nm. The zero-dispersion wavelength is 1318 nm to 1322 nm, and the wavelength dispersion slope of this zero-dispersion wavelength is 0.088 ps / (nm). 2 ·km) or above 0.089ps / (nm) 2·km) or less. The transmission loss at a wavelength of 1310 nm is 0.328 dB / km or more and 0.330 dB / km or less, the transmission loss at a wavelength of 1550 nm is 0.188 dB / km or more and 0.193 dB / km or less, and the transmission loss at a wavelength of 1625 nm is 0.233 dB / km or more and 0.245 dB / km or less. The inter-core XT in the O-band (1260 nm or more and 1360 nm or less) is -56 dB / km or less, and the inter-core XT in the C-band (1530 nm or more and 1565 nm or less) is -30 dB / km or less. In addition, MFD / λ calculated according to the values in Table 1 of Non-Patent Document 2 cc is 6.97 or more and 7.08 or less, with extremely small fluctuations.

[0005] Non-Patent Document 3 discloses a four-core optical fiber in a 1×4 configuration (a core arrangement in which four cores are arranged in a column). The relative refractive index difference Δ of the core is 0.34%, the outer diameter of the core is 8.4 μm, the core pitch (center-to-center distance) is 50 μm or more, and the outer diameter of the cladding is 200 μm or more as estimated from Fig. 3.

[0006] Patent Document 1: US Patent No. 9,933,331 Specification

[0007] Non-Patent Document 1: Takashi Matsui, et al., “Design of 125μm cladding multi-core fiber with full-band compatibility to conventional single-mode fiber,” Eur. Conf. Opt. Commun. (ECOC) 2015, Internet <URL:https: / / doi.org / 10.1109 / ECOC.2015.<7341966>.

[0008] Non-Patent Document 2: T. Matsui et al., “Step-index profile multi-core fibre with standard 125-μm cladding to full-band application,” in Eur. Conf. Opt. Commun. (ECOC) (2019), Internet

[0009] <URL:https: / / doi.org / 10.1049 / cp.2019.0751>.

[0010] Non-patent literature 3: Ming-Jun Li, et al., “Multicore Fiber for Optical Interconnect Applications,” OECC 2012 Technical Design, 5E4-2 (July 2012).

[0011] Non-patent literature 4: RJ Black and C. Pask, J. Opt. Soc. Am. A, JOSAA 1(11), p. 1129-1131, 1984.

[0012] Non-patent literature 5: T. Matsui et al., in Eur. Conf. Opt. Commun. (ECOC 2017), pW1. B. 2. Summary of the Invention

[0013] The MCF (Multi-core Optical Fiber) of the present invention comprises: four cores extending along a central axis; and a common cladding covering each of the four cores. Specifically, in a cross-section of the multi-core optical fiber orthogonal to the central axis, the common cladding has a circular outer periphery. In the cross-section, the four cores are arranged in a linearly symmetrical position relative to a straight line intersecting the central axis but not intersecting any of the four cores. Furthermore, in the cross-section, the core configuration defined by the four cores does not possess more than one order of rotational symmetry, even if any point is taken as the center of rotation. Attached Figure Description

[0014] Figure 1 This is a diagram illustrating various constructions of the MCF cable (containing the MCF of the present invention).

[0015] Figure 2 This is a diagram illustrating various core configurations of the MCF of the present invention.

[0016] Figure 3 This is a diagram used to illustrate the main terms used in this specification.

[0017] Figure 4 This is a diagram showing the refractive index distribution around each fiber core of the MCF that can be applied to this invention.

[0018] Figure 5 This refers to the center-to-center spacing Λ and MFD / λ between adjacent fiber cores when the phase-to-phase transmission XT at a wavelength of 1360nm becomes -20dB (=-20dB / 10km) after 10km transmission in an MCF (4-core MCF) configured in a square lattice of 4 fiber cores at a wavelength of 10km (equivalent to a fiber length of 10km). cc A graph showing the relationship between the two.

[0019] Figure 6 This refers to the center spacing Λ and MFD / λ between adjacent fiber cores when the phase transmission XT becomes -20dB after 10km transmission of 4-core MCF at wavelengths of 1550nm and 1360nm (equivalent to a fiber length of 10km), and when the parallel transmission XT becomes -20dB after 10km transmission of 4-core MCF at wavelengths of 1360nm. cc A graph showing the relationship between the two.

[0020] Figure 7 This refers to the center spacing Λ and MFD / λ between adjacent fiber cores when the phase transmission XT becomes -40dB after 10km transmission of 4-core MCF at wavelengths of 1550nm and 1360nm (equivalent to a fiber length of 10km), and when the parallel transmission XT becomes -40dB after 10km transmission of 4-core MCF at wavelengths of 1360nm. cc A graph showing the relationship between the two.

[0021] Figure 8 This indicates that when the leakage loss to the cladding is 0.01 dB / km in a 4-core MCF with a wavelength of 1360 nm, d coat and MFD / λ cc A graph showing the relationship between the two.

[0022] Figure 9 This indicates that in a 4-core MCF at a wavelength of 1360nm, the leakage loss to the cladding is 0.01dB / km. coat Adding a 1μm margin, and furthermore, after 10km transmission (equivalent to a 10km fiber length), the phase-to-phase transmission XT at a wavelength of 1360nm becomes -20dB (=-20dB / 10km) Λ with a 1μm margin, the CD and MFD / λ are calculated. cc A graph showing the relationship between the two.

[0023] Figure 10 This means that, under the conditions that the phase-wise transmission XT is -20dB (= -20dB / 10km) after 10km transmission in a 4-core MCF at both wavelengths of 1550nm and 1360nm (equivalent to a fiber length of 10km), and the parallel transmission XT (XT in the case of normal co-directional transmission) is also -20dB (= -20dB / 10km) after 10km transmission (equivalent to a fiber length of 10km), and the leakage loss to the cladding is 0.01dB / km, d coat Adding a 1μm margin, and with an additional 1μm margin in Λ, the CD (minimum permissible cladding diameter) and MFD / λ cc A graph showing the relationship between the two.

[0024] Figure 11 This indicates that, at wavelengths of 1550nm and 1360nm, after 10km of transmission over a 4-core MCF fiber (equivalent to a 10km fiber length), the phase-wise transmission XT becomes -40dB (=-40dB / 10km), and after 10km of transmission (equivalent to a 10km fiber length), the parallel transmission XT also becomes -40dB (=-40dB / 10km), under the condition that the leakage loss to the cladding is 0.01dB / km. coat Adding a 1μm margin, and with an additional 1μm margin in Λ, the CD (minimum permissible cladding diameter) and MFD / λ cc A graph showing the relationship between the two. Detailed Implementation

[0025] [The problem to be solved by this invention]

[0026] The inventors studied the aforementioned prior art and, as a result, discovered the following problem: In the MCFs described in Non-Patent Documents 1 to 3, the core arrangement exhibits rotational symmetry more than twice around the cladding center. Therefore, without markings, the core cannot be identified in these MCFs.

[0027] More specifically, the MCF in Non-Patent Document 1 described above has significantly worse mass production performance and higher manufacturing costs compared to general single-mode fiber (hereinafter referred to as "SMF"). This is because, in order to simultaneously reduce XT, increase the number of cores, reduce the outer diameter of the cladding, and increase the MFD of each core, a low-refractive-index trench layer with a large relative refractive index difference from the cladding needs to be set around the core.

[0028] Furthermore, the MCFs in Non-Patent Documents 2 and 3 have narrow manufacturing tolerances, resulting in higher manufacturing costs. While it is proposed that MCFs of 1260nm to 1625nm can be used for shorter distances, these MCFs require very high precision control of the refractive index distribution to achieve the desired optical characteristics within their design range, thus making it impossible to achieve the same manufacturing tolerances as general-purpose SMFs.

[0029] Furthermore, while non-patent document 3 does not explicitly describe the presence or absence of grooves, it can be factually determined, based on the disclosed content (definition and range of V-values), that it does not contain grooves. Even for short distances, attempts are made to achieve good transmission characteristics beyond the O-band, resulting in narrower manufacturing tolerances.

[0030] The present invention was proposed to solve the above-mentioned problems, and its purpose is to provide an MCF for short-distance transmission that can ensure sufficient manufacturing tolerance, excellent mass production performance, and suppress the degradation of connection loss.

[0031] [Description of embodiments of the present invention]

[0032] First, the contents of each embodiment of the present invention will be described separately.

[0033] (1) The MCF (Multi-core Optical Fiber) of the present invention, as one embodiment, comprises: four fiber cores extending along a central axis; and a common cladding covering each of the four fiber cores. Specifically, in a cross-section of the MCF orthogonal to the central axis, the common cladding has a circular outer periphery. In the cross-section, the four fiber cores are arranged in a linearly symmetrical position relative to a straight line intersecting the central axis but not intersecting any of the four fiber cores. Furthermore, in the cross-section, the fiber core configuration defined by the four fiber cores does not possess more than one rotational symmetry, even when any point is taken as the center of rotation. In other words, in the cross-section, the configuration of the centers of the four fiber cores is such that even when rotated around any point, it only becomes the same configuration as itself after a 360-degree rotation.

[0034] In addition, each of the four fiber cores can have a grooved structure.

[0035] An MCF with the above-described structure, consisting of a common cladding with a standard outer diameter and four-core fiber arranged in a quadrilateral configuration, achieves desired optical characteristics in the O-band while ensuring sufficient production tolerances. Furthermore, in the cross-section, the four cores are arranged in a linearly symmetrical position relative to a straight line intersecting the central axis but not intersecting any of the four cores. The core configuration has no polarity (the core configuration is the same at both ends of the MCF), therefore, connections to other MCFs of the same type can be made at either end of the MCF. Additionally, since the straight line representing the target axis does not intersect any of the cores, polarity does not need to be considered in the transmission link when connecting MCFs to each other. For example, consider a multi-core connector with an even number of fiber ribbons. If half of the fibers are used for transmitting from the left and the other half for receiving from the right, no structural changes are needed at either end, and polarity issues are avoided. However, in the case of an MCF with a core at the center of the cladding, if the core at the center of the cladding is used for transmitting at one end, it needs to be used for receiving at the other end. This requires a connection / link structure that considers polarity (using different fan-in / fan-out structures at both ends, or different transceiver structures). In cross-section, a core configuration defined by four cores does not exhibit more than one rotational symmetry, even if any point is set as the center of rotation. In this case, core identification and matching during connection can be performed without markings.

[0036] (2) As one aspect of the present invention, it is preferred that the four fiber cores are configured such that the center of each of the four fiber cores is located on the cross-section, and each of the three sides has a length Λ. nominal And the length of one side is Λ nominal The region has a radius of 1.0 μm, with each of the four vertices of the 3-sides equal trapezoid as its center. In this case, the XT between adjacent fiber cores can be suppressed to below a specified value, and the rotational symmetry of more than two orders can be sufficiently lost on the end face of the MCF.

[0037] (3) As one aspect of the invention, the preferred distance d from the center of each of the four fiber cores to the outer periphery of the common cladding is... coat , with the specified nominal value d coat,nominal All converge to d based on the criterion coat,nominal Values ​​above -1μm and d coat,nominal The value is within the range of +1 μm or less. In this case, the leakage loss from the common coating to the coating portion (the coating portion composed of resin or the like that surrounding the outer periphery of the common coating) can be suppressed to a predetermined value or less, and the rotational symmetry of more than two times can be sufficiently lost on the end face of the MCF. The "leakage loss" can be obtained by the method described in Patent Document 1 above, the disclosure of which is incorporated herein by reference to Patent Document 1.

[0038] (4) As one aspect of the invention, it is preferable that the diameter CD of the common cladding is 126 μm or less, and that the MFD (mode field diameter) of each of the four fiber cores at a wavelength of 1310 nm and the cable cutoff wavelength λ measured through a 22 m long optical fiber are also considered. cc It satisfies the following equation (1):

[0039] d coat ≥2.88MFD / λ cc +5.36…(1)

[0040] Furthermore, preferably, in each of the four fiber cores, the MFD is 8.2 μm to 9.6 μm and the difference between the maximum and minimum values ​​is 0.8 μm or less; in each of the four fiber cores, the zero-dispersion wavelength is 1300 nm to 1352 nm and the difference between the maximum and minimum values ​​is 24 nm or less; and in each of the four fiber cores, the dispersion slope of the zero-dispersion wavelength is 0.092 ps / (nm). 2 Below ·km, in each of the 4 fiber cores, the cable cutoff wavelength λ cc The wavelength is below 1260 nm. Furthermore, it is preferable that the MCF satisfies either condition 1 or condition 2.

[0041] Furthermore, the first condition mentioned above is that, at a wavelength of 1360nm, the XT-10dB between adjacent fiber cores with a fiber length of 10km is less than 10dB, and in each of the four fiber cores, the following equation (2) is satisfied:

[0042] CD nominal ≥13.31MFD / λ cc +24.47…(2)

[0043] The relationship, and in each of the four fiber cores, compared to MFD / λ cc The center spacing Λ between the fiber cores and the adjacent fiber cores satisfies any one of the following equations (3) to (7):

[0044] 6.5≤MFD / λ cc ≤7.5≤0.443Λ-5.33…(3)

[0045] 6.5≤MFD / λ cc ≤8.0≤0.443Λ-5.33…(4)

[0046] 6.5≤MFD / λ cc ≤8.5≤0.443Λ-5.33…(5)

[0047] 6.5≤MFD / λ cc ≤9.0≤0.443Λ-5.33…(6)

[0048] 6.5≤MFD / λ cc ≤9.5≤0.443Λ-5.33…(7)

[0049] This is how it is defined.

[0050] The second condition mentioned above is that, at a wavelength of 1360nm, the XT between adjacent fiber cores with a fiber length of 10km is below -20dB, and in each of the four fiber cores, the following equation (8) is satisfied:

[0051] CD nominal ≥9.37MFD / λ cc +31.73…(8)

[0052] The relationship, and in each of the four fiber cores, compared to MFD / λ cc The center spacing Λ between the fiber cores and the adjacent fiber cores satisfies any one of the following equations (9) to (13):

[0053] 6.5≤MFD / λ cc ≤7.5≤0.392Λ-4.88…(9)

[0054] 6.5≤MFD / λ cc ≤8.0≤0.392Λ-4.88…(10)

[0055] 6.5≤MFD / λ cc ≤8.5≤0.392Λ-4.88…(11)

[0056] 6.5≤MFD / λ cc ≤9.0≤0.392Λ-4.88…(12)

[0057] 6.5≤MFD / λ cc ≤9.5≤0.392Λ-4.88…(13)

[0058] This is how it is defined.

[0059] By satisfying the above structure and conditions, an MCF with desired optical characteristics in the O-band is obtained using a 4-core fiber with a common cladding and a standard outer diameter, while ensuring sufficient mass production tolerance. The leakage loss from the outermost core to the cladding at a wavelength of 1360 nm is suppressed to below 0.01 dB / km. While satisfying the first condition, the MFD / λ can also be guaranteed. cc The tolerance is high. Furthermore, it can maintain a high yield rate during mass production, and the total amount of back-to-back transmission XT to a specified fiber core at wavelengths below 1360nm (equivalent to a fiber length of 10km) is suppressed to below -20dB. Additionally, while satisfying the second condition above, it allows for degradation of optical characteristics in the long-wavelength C-band (1530nm to 1565nm) and L-band (1565nm to 1625nm), thus enabling the MCF to achieve large tolerances while maintaining superior optical characteristics in the O-band. MFD / λ can also be guaranteed. cc The tolerance is high. In addition, it can maintain a high yield during the mass production of optical fibers and suppress the total amount of back-to-back transmission XT to the specified fiber core of a 10km optical fiber to below -40dB at wavelengths below 1360nm.

[0060] (5) As one aspect of the invention, it also includes a covering portion that surrounds the common cladding. Furthermore, it is preferable that the leakage loss from at least any of the four fiber cores to the covering portion is 0.05 dB / km or more at a wavelength of 1550 nm or 1625 nm, or the transmission loss is 0.25 dB / km or more at a wavelength of 1550 nm, or the transmission loss of at least any of the four fiber cores is 0.25 dB / km or more at a wavelength of 1625 nm. In this case, the degradation of optical characteristics in the long-wavelength C-band and L-band is permissible, thereby creating an MCF with preferred optical characteristics in the O-band, enabling large tolerances.

[0061] (6) As one aspect of the invention, it is preferable to satisfy the first condition described above, and the XT between adjacent fiber cores with a fiber length of 10 km at a wavelength of 1550 nm is -10 dB or more; or, it is preferable to satisfy the second condition described above, and the XT between adjacent fiber cores with a fiber length of 10 km at a wavelength of 1550 nm is -20 dB or more. In this case, the degradation of optical characteristics in the C-band and L-band of the long wavelength band is permissible, thereby becoming an MCF with preferred optical characteristics in the O-band, and achieving a large tolerance.

[0062] (7) The MCF cable of the present invention preferably comprises multiple MCFs having the structure described above. Furthermore, as an embodiment of the present invention, an MCF strip formed by bonding multiple MCFs having the structure described above at intervals can be incorporated. As an embodiment of the present invention, the MCF cable incorporates the MCF strip in a twisted spiral shape. Transmission capacity can be increased in any manner. Moreover, as an embodiment of the present invention, it is preferable to include a multi-core optical fiber with an average bending radius along the fiber length direction of 0.03m to 0.14m or 0.14m to 0.3m. In this case, the degradation of optical properties associated with increased bending loss can be effectively suppressed.

[0063] The methods listed above in the "[Description of Embodiments of the Invention]" section can be applied to each of the remaining methods, or to all combinations of the remaining methods.

[0064] [Detailed Description of Embodiments of the Invention]

[0065] The specific construction of the multi-core optical fiber (MCF) and multi-core optical cable (MCF cable) according to the present invention will be described in detail below with reference to the accompanying drawings. Furthermore, the present invention is not limited to these illustrations, but is shown in the claims and includes all modifications within the same meaning and scope as the claims. Additionally, in the description of the drawings, the same reference numerals are used to denote the same elements, and repeated descriptions are omitted.

[0066] Figure 1 This is a diagram illustrating various constructions of the MCF cable (containing the MCF of the present invention).

[0067] The MCF cable 1A having structure (A) includes: an outer sheath 300 containing an MCF housing space extending along the length direction of the MCF cable 1A; and a plurality of MCFs 100 (MCFs of the present invention). Two tension members 400A and 400B extending along the MCF housing space are embedded in the outer sheath 300. Each MCF 100 contains glass fiber 200, the outer peripheral surface of which is covered by a resin coating. Furthermore, the MCFs 100 can form a spaced-apart bonded MCF strip, in which case the MCF strip is incorporated into the MCF 1A in a twisted helical state.

[0068] On the other hand, the MCF cable 1B having structure (B) includes: an outer sheath 500 containing an MCF housing space extending along the length direction of the MCF cable 1B; a slotted core 600 dividing the MCF housing space into multiple portions; and a plurality of MCFs 100 (MCFs of the present invention). The slotted core 600, which divides the MCF housing space into multiple portions, is housed within the outer sheath 500. Tensile wires 700 extending along the length direction of the MCF cable 1B are embedded in the slotted core 600. The plurality of MCFs 100 are housed within arbitrary spaces divided by the slotted core 600.

[0069] Figure 2 This is a diagram illustrating various core configurations of the MCF of the present invention. Specifically, in... Figure 2 The upper part shows a 4-core MCF100A (MCF 100 of the present invention) arranged in a trigonal trapezoidal configuration of 4 fiber cores. Figure 2 The middle section shows a 4-core MCF 100B (MCF100 of the present invention) with 4 cores arranged in a square grid pattern at positions offset from the cladding center. Figure 2 The lower part shows a cross-sectional view of a 4-core MCF 100C (MCF100 of the present invention) with markings in the middle core configuration. Furthermore, in Figure 2 The figures, as an example, show a combination of a first fiber core 100a that transmits light in a specified direction and a second fiber core 100b that transmits light in the opposite direction.

[0070] The MCF 100 of the present invention preferably has a core configuration consisting of four cores with the cladding center as the axis of symmetry, and does not exhibit rotational symmetry more than twice. In this case, core alignment during connection and MCF rotational core alignment can be performed even without markings. Preferably, the configuration of the centers of the four cores is linearly symmetrical with a straight line passing through the cladding center as the axis of symmetry. Therefore, when connecting other MCFs to this MCF, none of the end faces of that MCF exhibits polarity, allowing for core alignment between the cores.

[0071] Figure 2 The upper part of the 4-core MCF 100A shown has glass fiber 200A and a resin-coated portion 130 covering the glass fiber 200A. The glass fiber 200A has four cores (in this example, including a first core 100a and a second core 100b) and a common cladding 120 surrounding these four cores. In the cross-section of the 4-core MCF 100A, it has equal lengths Λ on three sides. nominal The length of the remaining side is greater than Λ nominal Four fiber cores are assigned to the four vertices of a sufficiently long triequilateral trapezoid (fiber core configuration pattern 1). In this case, the four fiber cores are arranged such that they surround the cladding center (fiber axis AX1). Furthermore, the center positions of the first fiber core 100a and the second fiber core 100b are located within 1.0 μm from the vertices of the assigned triequilateral trapezoid, preferably within 0.5 μm, and more preferably within 0.25 μm. The length of the remaining side of the aforementioned triequilateral trapezoid is preferably Λ. nominal More than 1.2 times. Therefore, it is possible to suppress the inter-core XT to below a specified value, and to sufficiently lose more than two rotational symmetries when viewed from the end face. Furthermore, the d of any core... coat All are based on the prescribed d coat,nominal Based on this, it is preferred to converge to d. coat,nominal Values ​​above -1μm and d coat,nominal The range is below +1 μm. In this case, leakage loss to the covering part can be suppressed to below the specified value, and rotational symmetry of more than two orders is sufficiently lost when viewed from the end face.

[0072] In addition, Figure 2 The example shown above can also have a structure that does not serve as a marker except for the fiber core. If a structure that serves as a marker except for the fiber core is present, then the manufacturing process deteriorates in order to achieve that structure (for example, in the case of a method where the cladding material is perforated and the fiber core material is inserted, a marker material that serves as a marker and will be inserted into the perforation to serve as a marker, thus becoming a marker with a different refractive index than the cladding). By having a structure that does not serve as a marker except for the fiber core, thereby... Figure 2 The example shown at the top can improve the manufacturability of MCF.

[0073] Figure 2 The 4-core MCF 100B shown in the middle section has glass fibers 200B and a resin-coated portion 130 covering the glass fibers 200B. The glass fibers 200B have four cores (in this example, including a first core 100a and a second core 100b) and a common cladding 120 surrounding these four cores. In the cross-section of the 4-core MCF 100B, the four cores are distributed at a predetermined grid point spacing Λ. nominalFurthermore, there are four pairs of square lattice points in adjacent lattice pairs. However, the center of the aforementioned square lattice is offset from the cladding center (aligned with the fiber axis AX2) (core configuration pattern 2). The center positions of the four cores are each arranged within 1.0 μm from the lattice points assigned to the aforementioned square lattice, preferably within 0.5 μm, and more preferably within 0.25 μm. Thus, by setting the four square lattice points as the core center design positions, the dimensional tolerance of the core configuration can be tolerated and the offset of the core configuration can be suppressed. In addition, compared with the core configuration of the 4-core MCF 100A shown above, the uniformity of residual stress, etc., applied in the cross-section of the four cores is improved, and it is expected that the optical characteristics of the four cores will also become more uniform. Furthermore, in the example in the middle, the four cores are approximately arranged in a square lattice shape, and the center spacing Λ between adjacent cores is Λ nominal Values ​​above -2.0 μm and Λ nominal The range below +2.0 μm is preferably within the Λ range. nominal Values ​​above -1.0 μm and Λ nominal The range below +1.0 μm, more preferably in the Λ range. nominal Values ​​above -0.5μm and Λ nominal The range below +0.5μm.

[0074] Figure 2 The lower part of the 4-core MCF 100C shows a glass fiber 200C and a resin-coated portion 130 covering the glass fiber 200C. The glass fiber 200C comprises four cores (in this example, a first core 100a and a second core 100b) and a common cladding 120 surrounding these four cores. In the cross-section of the 4-core MCF 100C, the core configuration consisting of the four cores is similar to that of the 4-core MCF 100B shown in the middle, with the center of the square lattice offset from the cladding center (fiber axis AX3). The 4-core MCF 100B and the 4-core MCF 100C differ in that they are marked with a mark 610 (core configuration pattern 3). Furthermore, the refractive index of the mark 610 is preferably different from the refractive index of the common cladding 120.

[0075] Figure 3 This is a diagram used to illustrate the main terms used in this specification (adjacent relationship, cross-sectional structure around the fiber core, parallel transmission and parallel transmission XT (crosstalk) and opposite transmission XT (crosstalk)).

[0076] (Adjacent relationship)

[0077] In this specification, the adjacency relationship between fiber cores is defined as follows when considering one specific fiber core among the four fiber cores arranged in the cross-section of the MCF: the fiber core having the smallest center-to-center spacing relative to that specific fiber core, and the fiber core with a center-to-center spacing difference of 2 μm or less from that minimum center-to-center spacing, are defined as fiber cores that are adjacent to that specific fiber core. That is, as follows: Figure 3 As shown, when fiber core 111 (110a) is positioned at a specific fiber core, the fiber cores adjacent to fiber core 111 are fiber core 112 (110b) and fiber core 113 (110b). Furthermore, Figure 2 Both Pattern 2 and Pattern 3 have their fiber cores configured in a way that forms a square lattice, therefore, as Figure 3 As shown, the adjacency relationship does not hold between fiber core 112 (110b) and fiber core 113 (110b). However, fiber core 114 (110a) is adjacent to both fiber core 112 (110b) and fiber core 113 (110b).

[0078] (Cross-sectional structure around the fiber core)

[0079] In having Figure 2 The four-core configuration of the present invention shown in Models 1 to 3 has a cross-sectional structure around each core (first core 110a or second core 110b) in which a common cladding 120 surrounds the outer periphery of the first core 110a or the second core 110b. The common cladding 120 may be configured to directly contact the first core 110a or the second core 110b, but an optical cladding 121 may also be provided between the common cladding 120 and the first core 110a or the second core 110b. Furthermore, a trench layer 122 having a small relative refractive index difference Δ3 may be provided between the optical cladding 121 and the common cladding 120. Moreover, the optical cladding 121 is preferably prepared for each core and has a relative refractive index difference Δ2 of -0.1% to 0.1% relative to the refractive index of the common cladding 120. Furthermore, when the trench layer 122 is provided, the trench layer 122 preferably has a relative refractive index difference Δ3 relative to the refractive index of the common cladding layer 120 that is -2.0% or more but less than -1.0%, -1.0% or more but less than -0.7%, -0.7% or more but less than -0.4%, or -0.4% or more but less than 0%.

[0080] (Parallel transmission and parallel transmission XT)

[0081] exist Figure 3The example shown illustrates three fiber cores with an adjacency relationship (all being the first fiber core 110a transmitting light in the same direction). That is, an adjacency relationship exists between the left and central fiber cores, and between the central and right fiber cores. In other words, the state where each fiber core with an adjacency relationship transmits light in the same direction is described as "parallel transmission." In this case, a typical inter-fiber XT (parallel transmission XT) occurs between adjacent fiber cores transmitting light in the same direction (between adjacent fiber cores).

[0082] (Reciprocal transmission and reciprocal transmission XT)

[0083] On the other hand, opposite-direction transmission refers to two adjacent fiber cores transmitting light in different directions. That is, in Figure 3 In the example, the left fiber core and the central fiber core are adjacent, but the left fiber core functions as the first fiber core 110a, while the central fiber core functions as the second fiber core 110b that transmits light in a direction different from the first fiber core 110a. The normal XT generated between these left and central fiber cores does not significantly affect communication quality. Similarly, the right fiber core, which is adjacent to the central fiber core, functions as the first fiber core 110a, and the normal XT generated between these right and central fiber cores does not significantly affect communication quality. As described above, the state where adjacent fiber cores transmit light in different directions is described as "opposite transmission". However, between the left and right fiber cores (both functioning as the first fiber core 110a), the XT, separated by the central fiber core (functioning as the second fiber core 110b), affects communication quality. As described above, the XT between fiber cores that are adjacent and transmit light in opposite directions but transmit light in the same direction is described as "opposite transmission XT".

[0084] In addition, please refer to the following instructions. Figure 3 The examples of "parallel transmission" and "opposite transmission" shown will be explained one by one, but if we consider the XT (parallel transmission XT: XT) between fiber cores (hereinafter referred to as "adjacent fiber cores") within the fiber length L1 range that have an adjacent relationship, co Set as XT co (L1), then when XT is expressed in decibels, it is expressed as the following equation (14):

[0085]

[0086] At a distance of 10 times, XT increases by 10dB.

[0087] When XT is expressed in decibels, for example, in Figure 3The example of counter-current transmission shown refers to the XT (counter-current transmission XT: XT) from the right fiber core to the left fiber core across the central fiber core. counter Parallel transmission XT is used between the left and central fiber cores and between the central and right fiber cores. co It can be expressed as the following formula (15):

[0088] XT counter =2XT co -10log 10 2…(15).

[0089] If we define the phased transmission XT within the fiber length L1 as XT counter (L1), then when XT is expressed in decibels, the phase-to-phase transmission XT within the fiber length L2 is expressed as the following equation (16):

[0090]

[0091] Increase by 20 dB at a distance of 10 times.

[0092] XT from adjacent fiber cores to the specified fiber core co Total XT co，tot If the number of adjacent fiber cores of a given fiber core is set to N, then it becomes the following equation (17):

[0093] XT co,tot =XT co +10log 10 N…(17)

[0094] From a specific fiber core that is adjacent to the specified fiber core (but not adjacent to the specified fiber core) to the XT of the specified fiber core. counter Total XT counter,tot If the number of specific cores relative to the specified core is set to M, then it seems to become the following equation (18):

[0095] XT counter,tot =XT counter +10log 10 M = 2XT co -10log 10 2+10log 10 M…(18)

[0096] However, the inventors discovered that this was not the case. If the number of adjacent cores (including the specified core) of the adjacent core n of the specified core is set to K... n The total is XT counter,tot It becomes the following equation (19):

[0097]

[0098] Therefore, in a 4-core MCF with 4 cores arranged on a square grid (hereinafter referred to as "square core configuration"), the XT of any core... counter,tot It becomes the following formula (20):

[0099] XT counter,tot =XT counter +10log 10 2 = 2XT co …(20)

[0100] Therefore, in a 4-core fiber (such as a 1×4 core configuration with 4 cores arranged in a row) where only 3 pairs of cores are adjacent, an XT fiber with any core having 2 adjacent cores can be used. counter,tot It becomes the following formula (21):

[0101] XT counter,tot =XT counter +10log 10 1 = 2XT co -10log 10 2…(21).

[0102] Based on the above, the 4-core MCF configured in the square fiber core is designed to transmit the XT signal after 10km. counter,tot [dB] is set to below -20dB, representing the parallel transmission XT (XT) between adjacent fiber cores, converted to fiber length L [km]. co The preferred form is (22):

[0103]

[0104] The sum of parallel transmissions XT from two adjacent fiber cores to any fiber core is given by the following equation (23):

[0105]

[0106] The 4-core MCF in the positive fiber core is designed to transmit XT signals over a distance of 10km. counter,tot [dB] is set to below -40dB, representing the parallel transmission XT (XT) between adjacent fiber cores, converted to fiber length L [km]. co The preferred form is (24):

[0107]

[0108] The sum of parallel transmissions XT from two adjacent fiber cores to any fiber core is preferably given by the following equation (25):

[0109]

[0110] In a 4-core MCF configuration with only 3 pairs of adjacent fiber cores (such as a 1×4 core configuration with 4 fiber cores arranged in a row), in order to transmit 10km of XT... counter,tot [dB] is set to below -20dB, representing the parallel transmission XT (XT) between adjacent fiber cores, converted to fiber length L [km]. co The preferred form is (26):

[0111]

[0112] The sum of parallel transmissions XT from any adjacent fiber core to any fiber core with two adjacent fiber cores is given by the following equation (27):

[0113]

[0114] The 4-core MCF in the positive fiber core is designed to transmit XT signals over a distance of 10km. counter,tot [dB] is set to below -40dB, representing the parallel transmission XT (XT) between adjacent fiber cores, converted to fiber length L [km]. co The preferred form is (28):

[0115]

[0116] The sum of parallel transmissions XT from adjacent fiber cores to any fiber core having two adjacent fiber cores is preferably expressed by the following equation (29):

[0117]

[0118] Next, the distribution structure of the MCF that can be applied to the present invention will be described. Figure 4 This is a diagram showing the refractive index distribution around each fiber core of the MCF applicable to this invention. Furthermore, unless otherwise stated, "relative refractive index difference Δ" refers to the relative refractive index difference relative to the refractive index of the common cladding (and therefore not the relative refractive index difference relative to the refractive index of pure silicon glass).

[0119] Regarding the core structure of the MCF of the present invention, the refractive index distribution of the core and its associated optical properties can be selected according to the application, for example, the following can be applied: Figure 4 The refractive index distributions of samples (A) to (K) are shown. Furthermore, in Figure 4In this context, Δ is the relative refractive index difference based on the refractive index of the common cladding, and r is the radius from the center of each fiber core. This is represented by a local coordinate system with the center of each fiber core (Δ = 0%) as the origin O. The construction can be consistent or different between fiber cores.

[0120] Figure 4 The patterns shown (A) are stepped refractive index distributions, (B) are toroidal refractive index distributions, (C) are double-step refractive index distributions, (D) are gradient refractive index distributions, and (E) are drooping refractive index distributions. These patterns can be applied to the core structure of the MCF of the present invention. Furthermore, patterns (F) and (H) with a depressed refractive index distribution around the core, patterns (G), (I), and (J) with a raised refractive index distribution around the core, and pattern (E) with a matched refractive index distribution around the core can also be applied to the core structure.

[0121] The refractive index distribution other than that of the stepped refractive index distribution in pattern (A) can be approximated using ESI (Equivalent-step-index) to obtain the core radius a and the core Δ(Δ1) after the step approximation (see Non-Patent Document 4 above).

[0122] Non-Patent Document 4 can be easily applied when the boundaries between the core and cladding are clear, but it is difficult to apply to cases where the boundaries between the core and cladding (common cladding 120 or optical cladding 121) are unclear, such as the drooping refractive index distribution of Pattern (E). For example, if the method of Non-Patent Document 4 is directly applied by taking b of Pattern (E) as the radius of the core, the ESI approximation is poor. In the case described above, it is preferable to take the r corresponding to 2 / 5 of Δ at a specific r as the core radius a and apply Non-Patent Document 4, where the specific r is the slope of the refractive index distribution. The refractive index of the cladding (common cladding 120 or optical cladding 121) is then the largest negative value of r. This is achieved by simply averaging Δ over the range of a to b, as shown by the following equation (30):

[0123]

[0124] Alternatively, the weighted average of r can be expressed by the following equation (31):

[0125]

[0126] The obtained values ​​can be used to determine a and Δ1 (the maximum relative refractive index difference between the first and second fiber cores 110a and 110b) based on calculations in Non-Patent Document 4. Δ2 (the relative refractive index difference of the optical cladding 121) is preferably between -0.10% and 0.10%. Therefore, manufacturability is greatly improved.

[0127] A trench layer 122 with a lower refractive index than the optical cladding 121 and the common cladding 120 can be provided around the optical cladding 121. Figure 4 The pattern (K) is used. However, when the relative refractive index difference Δ3 of the trench layer 122, based on the refractive index of the common cladding 120, is less than -0.5%, manufacturability deteriorates significantly. Therefore, Δ3 ≥ -0.4%, more preferably Δ3 ≥ -0.3%, and most preferably Δ3 ≥ -0.2%. Furthermore, from a manufacturability point of view, it is more preferable to have no trench layer. On the other hand, the depth of the trench layer 122 in the refractive index distribution can enhance the blocking of light toward the fiber core, and it is desirable to suppress leakage loss between fiber cores XT and from the fiber core to the cladding. Preferably, Δ3 is -0.7% or more and less than -0.4%, more preferably -1.0% or more and less than -0.7%, and even more preferably -2.0% or more and less than -1.0%.

[0128] Regarding the materials of the core and cladding (optical cladding 121 or common cladding 120), glass with silicon glass as the main component is preferred as it can achieve low transmission loss and high mechanical reliability. It is preferable to add Ge to the core, thereby creating a refractive index difference between the core and the cladding. Alternatively, it is preferable to add F to the cladding, thereby creating a refractive index difference between the core and the cladding. Adding trace amounts of F to the core and optical cladding results in good manufacturability and enables a depressed distribution, which is therefore preferred. Cl can be added to the core and cladding. This suppresses OH groups and absorption losses caused by OH groups. Trace amounts of P can be contained in the core and cladding. This improves manufacturability in some glass synthesis processes.

[0129] have Figure 2 The cross-sectional structure of the MCF of the present invention shown has a resin-coated portion 130, the diameter of which preferably converges to a range of 235 μm to 265 μm based on 250 μm. Therefore, the cable-compatible MCF of the present invention can be realized without making significant changes to existing cable-connecting devices.

[0130] In a typical general-purpose SMF, the nominal value of the cladding diameter CD nominal The nominal value of the cladding diameter is 245μm or more to about 250μm, but in order to increase the number of optical fibers per unit section in the cable, the cladding diameter is preferably 160μm or more and 230μm or less.

[0131] The MCF of the present invention, as described above, is a 4-core MCF. The number of cores is an even number and a power of 2, thus making it suitable as a spatial channel for communication.

[0132] Furthermore, in the MCF of the present invention, the center configuration of the four fiber cores (essentially the fiber core configuration) is linearly symmetrical with a straight line passing through the cladding center as the axis of symmetry, and preferably does not have more than one rotational symmetry. Thus, fiber core alignment during fiber connection and MCF rotational alignment can be performed even without markings. In this case, it is preferable that the centers of the four fiber cores are linearly symmetrical with a straight line passing through the cladding center as the axis of symmetry. Therefore, when connecting this MCF to other MCFs, the fiber cores can be aligned non-polarly between any end face of the MCF.

[0133] For example in Figure 2 The example shown above also shows that the MCF has equal lengths Λ on three sides in the cross-section. nominal The length of the remaining side is greater than Λ nominal A fiber core is disposed at each of the four vertices of a sufficiently long triangular trapezoid. The center position of each fiber core is positioned within 1.0 μm from the corresponding vertex of the triangular trapezoid, preferably within 0.5 μm, and more preferably within 0.25 μm. The length of the remaining side of the triangular trapezoid is preferably Λ. nominal More than 1.2 times. Thus, the XT between fiber cores can be suppressed to below a specified value, and the rotational symmetry of more than two orders is sufficiently lost when viewed from the end face.

[0134] In addition, the preferred d of any fiber core coat All are specified by the nominal value d coat,nominal Based on d, it converges to d coat,nominal Values ​​above -1μm and d coat,nominal The range of values ​​below +1μm is thus achieved. This allows leakage loss to the covering portion to be suppressed to below a specified value, and also sufficiently eliminates rotational symmetry of more than two orders when observed from the end face.

[0135] The MCF of the present invention preferably does not have a marking structure other than the fiber core. This is because having a marking structure other than the fiber core degrades manufacturability to achieve this structure. For example, in a manufacturing method where the fiber core material is inserted through an opening in the cladding material, a marking material (the marking material) with a refractive index different from the cladding material needs to be inserted into the opening for marking. Conversely, by not having a marking structure other than the fiber core, the manufacturability of the MCF of the present invention can be improved.

[0136] Furthermore, as shown in the middle of the MCF of the present invention, there can be a core configuration where four pairs of cores are adjacent to each other, i.e., a square lattice configuration. In this case, the rotational symmetry at least twice around the cladding center is sufficiently lost, thus the center position of the square lattice is offset from the cladding center. The centers of the four cores are spaced apart by a predetermined lattice point interval Λ. nominal The four grid points of the square lattice are arranged within 1.0 μm, preferably within 0.5 μm, and more preferably within 0.25 μm. Therefore, by setting the four grid points of the square lattice as the design position of the core center, dimensional tolerances in the core arrangement can be tolerated while suppressing core arrangement offset. Furthermore, the square lattice four-core arrangement... Figure 2 Compared to the triangular trapezoidal core configuration shown in the upper part, the residual stress applied to the cross-section of the four cores becomes more uniform, and the optical properties of the four cores also become more uniform, therefore it is preferred. Furthermore, the above structure can also be described as approximately arranging the four cores in a square lattice shape, with the center spacing Λ between adjacent cores converging at Λ. nominal Values ​​above -2.0 μm and Λ nominal The range below +2.0 μm preferably converges to Λ. nominal Values ​​above -1.0 μm and Λ nominal The range below +1.0 μm is more preferably converged to Λ. nominal Values ​​above -0.5μm and Λ nominal The range below +0.5μm.

[0137] Furthermore, in the MCF of the present invention, such as Figure 2 As shown in the example below, a structure with markings is also possible. If the above conditions are summarized, it is preferable that there are 3 to 4 pairs of adjacent fiber cores among the 4 fiber cores, and the center positions of the 4 fiber cores are arranged to be linearly symmetric with a straight line passing through the cladding center but not through the center of any fiber core as the axis of symmetry. Furthermore, when there are only 3 pairs of adjacent fiber cores, the arrangement of the centers of the fiber cores with the cladding center as the axis of symmetry does not have more than 2 rotational symmetries.

[0138] Furthermore, the MCF cable of the present invention preferably comprises multiple MCFs including the MCFs with the structure described above. As an example, the MCF cable may incorporate an MCF strip formed by bonding multiple MCFs including the MCFs with the structure described above at intervals. The MCF cable is incorporated with the MCF strip spirally twisted. Regardless of the structure, an increase in transmission capacity can be achieved. Moreover, it is preferable to include multi-core optical fibers with an average bending radius along the fiber length direction of 0.03m to 0.14m or 0.14m to 0.3m. In this case, the degradation of optical properties associated with increased bending loss can be effectively suppressed.

[0139] Each core of the MCF of the present invention preferably has an MFD that converges to a value between 8.6 μm and 9.2 μm (MFD reference value) and 0.4 μm (MFD reference value + 0.4 μm) at a wavelength of 1310 nm. In this case, the MFD value is consistent with the nominal MFD value in the general SMFs specified in ITU-T G.652. nominal Small (MFD) nominal Compared to the connection loss of general SMFs of the type with ≥8.6μm and suppressed bending loss, the connection loss caused by the axial offset of the MCFs of the present invention (in the case of given axial offset) can be suppressed to the same level or below.

[0140] Each core of the MCF of the present invention preferably has an MFD ranging from 8.2 μm to 9.0 μm based on 8.6 μm at a wavelength of 1310 nm. Therefore, when connecting a general SMF (such as the type of general SMF specified in ITU-T G.652, which has a small nominal MFD value and suppresses bending loss) with the MCF of the present invention, the connection loss caused by core center axis offset (axis offset) can be set to be equal (in the case of a specified axis offset).

[0141] The MCF of the present invention preferably has a zero-dispersion wavelength of 1300 nm to 1324 nm. Therefore, the distortion of the signal waveform transmitted in the O-band can be suppressed to the same level as that of a general-purpose SMF.

[0142] The MCF of the present invention preferably has a zero-dispersion wavelength that converges to a value between -12 nm and +12 nm, with a wavelength reference value of 1312 nm to 1340 nm. Therefore, compared to a general SMF, it can suppress distortion of the signal waveform transmitted in the O-band (see Non-Patent Document 5 above).

[0143] In the operating band, the MCF of the present invention preferably maintains a sum of XT values ​​from adjacent fiber cores to any fiber core below -20 dB after 10 km of transmission. The XT values ​​from sources other than adjacent fiber cores are sufficiently low to be negligible, thus achieving a sufficient signal-to-noise ratio even during coherent detection. Furthermore, in the operating band, the MCF of the present invention preferably maintains a sum of XT values ​​from adjacent fiber cores to any fiber core below -40 dB after 10 km of transmission. The XT values ​​from sources other than adjacent fiber cores are sufficiently low to be negligible, thus achieving a sufficient signal-to-noise ratio even during intensity-modulated direct detection. In the operating band, the MCF of the present invention preferably maintains a parallel transmission XT value below -10.0 dB after 10 km of transmission. This allows the phased transmission XT value to remain below -20 dB after 10 km of transmission. Furthermore, in the operating band, the MCF of the present invention preferably maintains a parallel transmission XT value below -20.0 dB after 10 km of transmission. Therefore, the relative transmission XT can still be set to below -40dB after 10km of transmission.

[0144] The following description shows the relationship with... Figure 4 The results of the study are related to the refractive index distribution of the core of the samples (E), (H) and (J), where a is 3 μm or more and 5 μm or less, Δ1-Δ2 is 0.3% or more and 0.6% or less, Δ2 is -0.1% or more and 0.1% or less, and b / a is 2 or more and 5.

[0145] By calculating the electric field distribution of the fundamental mode and the wavelength dependence of the effective refractive index using methods such as the finite element method, those skilled in the art can design the structure of a fiber core with a specified zero-dispersion wavelength and MFD. For example, the relationship between a and (Δ1-Δ2) for the zero-dispersion wavelength λ0 [μm] is in the range of 3μm≤a≤5μm and 0.3%≤(Δ1-Δ2)≤0.6%, which is as follows (32):

[0146] a≈0.0667(λ0-1343.1)(Δ1-Δ2) 2 +0.0900(λ0-1354.6)(Δ1-Δ2)-0.0517(λ0-1411.2)…(32)

[0147] Therefore, in order for the zero-dispersion wavelength λ0[μm] to converge to λ 0nominal Values ​​above -12nm and λ 0nominal For values ​​below +12nm, the relationship between a and (Δ1-Δ2) preferably satisfies the following equations (33) and (34):

[0148] a≤0.0667(λ 0nominal -12-1343.1)(Δ1-Δ2) 2+0.0900(λ 0nominal -12-1354.6)(Δ1-Δ2)-0.0517(λ 0nominal -12-1411.2)…(33)

[0149] a≥0.0667(λ 0nominal +12-1343.1)(Δ1-Δ2) 2 +0.0900(λ 0nominal +12-1354.6)(Δ1-Δ2)-0.0517(λ 0nominal +12-1411.2)…(34)

[0150] These two formulas.

[0151] Furthermore, the relationship between a and (Δ1-Δ2) corresponding to the MFD[μm] at a wavelength of 1310nm is in the range of 3μm≤a≤5μm and 0.3%≤(Δ1-Δ2)≤0.6%, which becomes the following equation (35):

[0152] (Δ1-Δ2)=(-0.0148MFD+0.213)[a-0.619MFD+2.01] 2 -0.0771MFD+1.033…(35)

[0153] Therefore, in order for MFD[μm] to be MFD nominal Converging to MFD as a benchmark nominal Values ​​above -0.4 μm and MFD nominal For values ​​below +0.4 μm, the relationship between a and (Δ1-Δ2) preferably satisfies the following equations (36) and (37):

[0154]

[0155]

[0156] These two formulas.

[0157] b / a and Δ2 only need to be set to λ cc It becomes below 1260nm or 1360nm, and the zero-dispersion slope becomes 0.092ps / (nm). 2 ·km) is sufficient. Therefore, it is preferable that Δ2 is between -0.1% and 0.0%, and b / a is between 2 and 4.

[0158] Next, the preferred center spacing Λ between adjacent fiber cores will be explained. Figure 4This refers to the center-to-center spacing Λ and MFD / λ between adjacent fiber cores when the phase-to-phase transmission XT becomes -20dB (=-20dB / 10km) at a wavelength of 1360nm after 10km transmission with a 4-core MCF configuration in a square fiber core. cc The graph shows the relationship between the two. Here, the average bending radius R of the optical fiber is 0.14 m. If R is below 0.14 m, a lower XT can be achieved. Furthermore, λ... cc The cable cutoff wavelength was determined using the structure (without cabled optical fiber) in Figure 12 of ITU-T G.650.1 (03 / 2018).

[0159] To ensure that the phase-to-phase transmission XT after 10km transmission at a wavelength of 1360nm is below -20dB (=-20dB / 10km), the center spacing Λ between adjacent fiber cores and MFD / λ are... cc At least the following equation (38) or equation (39) must be satisfied:

[0160] Λ≥2.26MFD / λ cc +12.0…(38)

[0161] MFD / λ cc ≤0.443Λ-5.33…(39)

[0162] ( Figure 5 The region shown above the dotted line on the lower side), and preferably satisfies the following equation (40) or equation (41):

[0163] Λ≥2.26MFD / λ cc +14.5…(40)

[0164] MFD / λ cc ≤0.443Λ-6.42…(41)

[0165] ( Figure 5 The area shown is above the dashed line on the top side.

[0166] Figure 6 This refers to the center spacing Λ and MFD / λ between adjacent fiber cores when the phase transmission XT becomes -20dB after 10km transmission of 4-core MCF at wavelengths of 1550nm and 1360nm (equivalent to a fiber length of 10km), and when the parallel transmission XT becomes -20dB after 10km transmission of 4-core MCF at wavelengths of 1360nm. cc A diagram illustrating the relationship. Furthermore, in Figure 6 In the diagram, the symbol “○” indicates the above relationship related to parallel transmission XT at a wavelength of 1550 nm, and the symbol “●” (in the diagram) indicates the relationship related to parallel transmission XT at a wavelength of 1550 nm. Figure 6The symbol “□” indicates the above relationship related to parallel transmission XT at a wavelength of 1360nm, and the symbol “■” indicates the above relationship related to opposing transmission XT at a wavelength of 1550nm. Figure 6 (The text in the middle is displayed as a slash) indicates the above relationship related to the opposite transmission XT at a wavelength of 1360nm.

[0167] In Figure 6 The dashed line is not shown, but it is consistent with the above. Figure 5 Similarly, in order to set the parallel transmission XT after 10km transmission to below -20dB at a wavelength of 1360nm, the center spacing Λ between adjacent fiber cores and MFD / λ cc At least the following equation (42) or equation (43) must be satisfied:

[0168] Λ≥2.64MFD / λ cc +12.6…(42)

[0169] MFD / λ cc ≤0.379Λ-4.76…(43)

[0170] Furthermore, it is preferable to satisfy the following equation (44) or equation (45):

[0171] Λ≥2.64MFD / λ cc +15.0…(44)

[0172] MFD / λ cc ≤0.379Λ-5.68…(45).

[0173] In order to set the phase transmission XT below -20dB after 10km transmission at a wavelength of 1550nm, the center spacing Λ between adjacent fiber cores and the MFD / λ are... cc At least the following equation (46) or equation (47) must be satisfied:

[0174] Λ≥3.13MFD / λ cc +10.7…(46)

[0175] MFD / λ cc ≤0.320Λ-3.42…(47)

[0176] Furthermore, it is preferable to satisfy the following equation (48) or equation (49):

[0177] Λ≥3.13MFD / λ cc +13.4…(48)

[0178] MFD / λ cc ≤0.320Λ-4.29…(49).

[0179] In order to set the parallel transmission XT below -20dB after 10km transmission at a wavelength of 1550nm, the center spacing Λ between adjacent fiber cores and the MFD / λ are... cc At least the following equation (50) or equation (51) must be satisfied:

[0180] Λ≥3.66MFD / λ cc +10.8…(50)

[0181] MFD / λ cc ≤0.273Λ-2.95…(51)

[0182] Furthermore, it is preferable to satisfy either equation (52) or equation (53):

[0183] Λ≥3.66MFD / λ cc +13.7…(52)

[0184] MFD / λ cc ≤0.273Λ-3.74…(53).

[0185] Figure 7 This refers to the center spacing Λ and MFD / λ between adjacent fiber cores when the phase transmission XT becomes -40dB after 10km transmission of 4-core MCF at wavelengths of 1550nm and 1360nm (equivalent to a fiber length of 10km), and when the parallel transmission XT becomes -40dB after 10km transmission of 4-core MCF at wavelengths of 1360nm. cc A diagram illustrating the relationship. Furthermore, in Figure 7 In the diagram, the symbol “○” indicates the above relationship related to parallel transmission XT at a wavelength of 1550 nm, and the symbol “●” (in the diagram) indicates the relationship related to parallel transmission XT at a wavelength of 1550 nm. Figure 7 The symbol “□” indicates the above relationship related to parallel transmission XT at a wavelength of 1360nm, and the symbol “■” indicates the above relationship related to opposing transmission XT at a wavelength of 1550nm. Figure 7 (The text in the middle is displayed as a slash) indicates the above relationship related to the opposite transmission XT at a wavelength of 1360nm.

[0186] In Figure 7 The dashed line is not shown, but it is consistent with the above. Figure 5 Similarly, in order to set the phase-to-phase transmission XT below -40dB after 10km transmission at a wavelength of 1360nm, the center-to-center spacing Λ between adjacent fiber cores and the MFD / λ cc At least the following equation (54) or equation (55) must be satisfied:

[0187] Λ≥2.55MFD / λ cc+12.4…(54)

[0188] MFD / λ cc ≤0.392Λ-4.88…(55)

[0189] Furthermore, it is preferable to satisfy the following equation (56) or equation (57):

[0190] Λ≥2.55MFD / λ cc +14.9…(56)

[0191] MFD / λ cc ≤0.392Λ-5.83…(57).

[0192] To ensure that the parallel transmission XT after 10km transmission at a wavelength of 1360nm is below -40dB, the center spacing Λ between adjacent fiber cores and MFD / λ are... cc At least the following equation (58) or equation (59) must be satisfied:

[0193] Λ≥3.22MFD / λ cc +13.4…(58)

[0194] MFD / λ cc ≤0.310Λ-4.16…(59)

[0195] Furthermore, it is preferable to satisfy either equation (60) or equation (61):

[0196] Λ≥3.22MFD / λ cc +15.7…(60)

[0197] MFD / λ cc ≤0.310Λ-4.88…(61).

[0198] To ensure that the phase-to-phase transmission XT after 10km transmission at a wavelength of 1550nm is below -40dB, the center-to-center spacing Λ between adjacent fiber cores and the MFD / λ are... cc At least the following equation (62) or equation (63) must be satisfied:

[0199] Λ≥3.54MFD / λ cc +10.8…(62)

[0200] MFD / λ cc ≤0.283Λ-3.05…(63)

[0201] Furthermore, it is preferable to satisfy the following equation (64) or equation (65):

[0202] Λ≥3.54MFD / λ cc +13.6…(64)

[0203] MFD / λ cc ≤0.283Λ-3.85…(65).

[0204] To ensure that the parallel transmission XT after 10km transmission at a wavelength of 1550nm is below -40dB, the center-to-center spacing Λ between adjacent fiber cores and the MFD / λ are... cc At least the following equation (66) or equation (67) must be satisfied:

[0205] Λ≥4.47MFD / λ cc +11.0…(66)

[0206] MFD / λ cc ≤0.223Λ-2.46…(67)

[0207] Furthermore, it is preferable to satisfy the following equation (68) or equation (69):

[0208] Λ≥4.47MFD / λ cc +14.1…(68)

[0209] MFD / λ cc ≤0.223Λ-3.16…(69).

[0210] To allow for fluctuations in the position of each fiber core from the design center, it is preferable to take a margin of 1 μm from the ranges described above as Λ. Therefore, to ensure that the phase-to-phase transmission XT after 10 km transmission at a wavelength of 1360 nm is below -20 dB, the nominal value of Λ is... nominal Preferably, at least the following equation (70) is satisfied:

[0211] Λ nominal ≥2.26MFD / λ cc +12.0+1.0…(70)

[0212] More preferably, it satisfies the following equation (71):

[0213] Λ nominal ≥2.26MFD / λ cc +14.5+1.0…(71).

[0214] To ensure that the parallel transmission XT after 10km transmission at a wavelength of 1360nm is below -20dB, the nominal value Λ nominal Preferably, at least the following (72) is satisfied:

[0215] Λ nominal ≥2.64MFD / λ cc +12.6+1.0…(72)

[0216] More preferably, it satisfies the following equation (73):

[0217] Λ nominal ≥2.64MFD / λ cc +15.0+1.0…(73).

[0218] To ensure that the phase transmission XT after 10-wavelength transmission at 1550nm is below -20dB, the nominal value Λ nominal Preferably, at least the following equation (74) is satisfied:

[0219] Λ nominal ≥3.13MFD / λ cc +10.7+1.0…(74)

[0220] More preferably, it satisfies the following equation (75):

[0221] Λ nominal ≥3.13MFD / λ cc +13.4+1.0…(75).

[0222] To ensure that the parallel transmission XT after 10km transmission at a wavelength of 1550nm is below -20dB, the nominal value Λ nominal Preferably, at least the following equation (76) is satisfied:

[0223] Λ nominal ≥3.66MFD / λ cc +10.8+1.0…(76)

[0224] More preferably, it satisfies the following equation (77):

[0225] Λ nominal ≥3.66MFD / λ cc +13.7+1.0…(77).

[0226] To ensure that the phase transmission XT at a wavelength of 1360nm is below -40dB, the nominal value Λ nominal Preferably, at least the following equation (78) is satisfied:

[0227] Λ nominal ≥2.55MFD / λ cc +12.4+1.0…(78)

[0228] More preferably, it satisfies the following equation (79):

[0229] Λ nominal ≥2.55MFD / λ cc +14.9+1.0…(79).

[0230] To ensure that the parallel transmission XT after 10km transmission at a wavelength of 1360nm is below -40dB, the nominal value Λ nominal Preferably, at least the following equation (80) is satisfied:

[0231] Λ nominal ≥3.22MFD / λ cc +13.4+1.0…(80)

[0232] More preferably, it satisfies the following equation (81):

[0233] Λ nominal ≥3.22MFD / λ cc +15.7+1.0…(81).

[0234] To ensure that the phase-to-phase transmission XT after 10km transmission at a wavelength of 1550nm is below -40dB, the nominal value Λ nominal Preferably, at least the following equation (82) is satisfied:

[0235] Λ nominal ≥3.54MFD / λ cc +10.8+1.0…(82)

[0236] More preferably, it satisfies the following equation (83):

[0237] Λ nominal ≥3.54MFD / λ cc +13.6+1.0…(83).

[0238] To ensure that the parallel transmission XT after 10km transmission at a wavelength of 1550nm is below -40dB, the nominal value Λ nominal Preferably, at least the following equation (84) is satisfied:

[0239] Λ nominal ≥4.47MFD / λ cc +11.0+1.0…(84)

[0240] More preferably, it satisfies the following equation (85):

[0241] Λ nominal ≥4.47MFD / λ cc +14.1+1.0…(85).

[0242] Compared to the above Λ nominal Λ is preferably the following formula (86):

[0243] Λ nominal -0.9≤Λ≤Λ nominal +0.9…(86)

[0244] This is an approximation of the situation where the position of each fiber core can be independently considered as fluctuating within a Gaussian distribution with a probability distribution of 3σ = 0.9 μm starting from the design center. In this case, Λ does not satisfy the condition for Λ. nominal The probability of any of the defined expressions (70) to (85) above is suppressed to less than 1%. Furthermore, relative to Λ nominal Λ preferably satisfies the following equation (87):

[0245] Λ nominal -0.7≤Λ≤Λ nominal +0.7…(87)

[0246] This is an approximation of the situation where the position of each fiber core can be independently considered as fluctuating within a Gaussian distribution with a probability distribution of 3σ = 0.7 μm from the design center. In this case, Λ does not satisfy the condition for Λ. nominal The probability of any of the defined expressions (70) to (85) above is suppressed to less than 0.1%. Furthermore, relative to Λ nominal Λ preferably satisfies the following equation (88):

[0247] Λ nominal -0.5≤Λ≤Λ nominal +0.5…(88)

[0248] This is an approximation of the situation where the position of each fiber core can be independently considered as fluctuating within a Gaussian distribution with a probability distribution of 3σ = 0.5μm starting from the design center. In this case, Λ does not satisfy the condition for Λ. nominal The probability of any of the above-defined equations (70) to (85) is suppressed to less than 0.001%.

[0249] Next, regarding the preferred d coat (The shortest distance from the resin-coated portion and the cladding interface to the center of the fiber core) will be explained. Figure 8 This indicates that when the leakage loss to the cladding is 0.01 dB / km in a 4-core MCF with a wavelength of 1360 nm, d coat and MFD / λ cc A graph showing the relationship between the two.

[0250] To set the leakage loss to the resin-coated portion at a wavelength of 1360 nm to 0.01 dB / km, d coat and MFD / λ cc Satisfying the following equation (89) or equation (90):

[0251] d coat ≥2.88MFD / λ cc +5.36…(89)

[0252] MFD / λ cc ≤0.347d coat -1.86…(90)

[0253] (from Figure 8 (The area above the dashed line on the lower side is shown). And, d coat and MFD / λ cc Preferably, the following equation (91) or equation (92) is satisfied:

[0254] d coat ≥2.88MFD / λ cc +6.95…(91)

[0255] MFD / λ cc ≤0.347d coat -2.41…(92)

[0256] (from Figure 8 The area above the dotted line on the upper side is shown.

[0257] The outermost core d coat (i.e. d) coat The minimum value of the outer cladding thickness (OCT) is usually referred to as the outer cladding thickness (OCT), but the d of this invention coat It is defined as a value that can be specified for each fiber core.

[0258] To allow for fluctuations in the position of each fiber core from the design center, and for the cladding diameter to fluctuate from the design center, d coat Preferably, the range from equation (100) to equation (103) has a margin of at least 1 μm. Therefore, d coat If set to d coat The nominal value d coat,nominal Then at least the following equation (93) must be satisfied:

[0259] d coat,nominal ≥2.88MFD / λ cc +5.36+1.0…(93)

[0260] Furthermore, the nominal value of the cladding diameter, CD... nominal The preferred setting is to satisfy the following equation (94):

[0261] d coat,nominal ≥2.88MFD / λ cc +6.95+1.0…(94)

[0262] At this point, it is preferable to satisfy the following equations (95) and (96):

[0263] Λ nominal -0.9≤Λ≤Λ nominal+0.9…(95)

[0264] CD nominal -0.9≤CD≤CD nominal +0.9…(96)

[0265] These two equations, d coat The probability of not satisfying equation (89) or equation (91) is suppressed to less than 1%. Furthermore, it is preferable to satisfy the following equations (97) and (98):

[0266] Λ nominal -0.7≤Λ≤Λ nominal +0.7…(97)

[0267] CD nominal -0.7≤CD≤CD nominal +0.7…(98)

[0268] These two equations. At this time, d coat The probability of not satisfying equation (89) or equation (91) is suppressed to less than 0.1%. Furthermore, it is preferable to satisfy the following equations (99) and (100):

[0269] Λ nominal -0.5≤Λ≤Λ nominal +0.5…(99)

[0270] CD nominal -0.5≤CD≤CD nominal +0.5…(100)

[0271] These two equations. At this time, d coat The probability of not satisfying equation (89) or equation (91) is suppressed to below 0.001%.

[0272] Next, for the smallest allowable CD nominal Please provide an explanation. Figure 9 This indicates that in a 4-core MCF at a wavelength of 1360nm, the leakage loss to the cladding is 0.01dB / km. coat Adding a 1μm margin, and furthermore, after 10km transmission (equivalent to a 10km fiber length) at a wavelength of 1360nm, the phase-to-phase transmission XT becomes -20dB (=-20dB / 10km). With a 1μm margin added, CD and MFD / λ... cc A diagram illustrating the relationship. Furthermore, in Figure 9 If we represent MFD / λ ccLet the x-axis be the axis and the y-axis be the axis representing CD. Then the upper dashed line is represented by y = 8.95x + 37.47 (x = 0.1117y - 4.186), and the lower dashed line is represented by y = 8.95x + 31.13 (x = 0.1117y - 3.478).

[0273] Taking into account the tolerances of the core position and cladding diameter, in order to set the leakage loss to the cladding at a wavelength of 1360nm to below 0.01dB / km, and the back-to-back transmission XT after 10km transmission to below -20dB, CD nominal and MFD / λ cc The relationship satisfies either equation (101) or equation (102):

[0274] CD nominal ≥8.95MFD / λ cc +31.13…(101)

[0275] MFD / λ cc ≤0.1117CD nominal -3.478…(102)

[0276] ( Figure 9 The region above the dotted line on the lower side), and preferably satisfies the following equation (103) or equation (104):

[0277] CD nominal ≥8.95MFD / λ cc +37.47…(103)

[0278] MFD / λ cc ≤0.1117CD nominal -4.186…(104)

[0279] ( Figure 9 (The area above the dotted line on the top side).

[0280] Figure 10 This means that, at wavelengths of 1550nm and 1360nm, after 10km of transmission over a 4-core MCF fiber (equivalent to a 10km fiber length), the phase-wise transmission XT becomes -20dB (=-20dB / 10km), and after 10km of transmission over a parallel transmission XT (the XT for typical co-directional transmission) also becomes -20dB (=-20dB / 10km), with a leakage loss to the cladding of 0.01dB / km. coat With a 1μm margin added, and with an additional 1μm margin added to Λ, CD (minimum permissible cladding diameter) and MFD / λ ccA diagram illustrating the relationship. Furthermore, in Figure 10 In the diagram, the symbol “○” indicates the above relationship with parallel transmission XT at a wavelength of 1550nm, and the symbol “●” (in the case of…) Figure 10 The symbol “□” indicates the above relationship with parallel transmission XT at wavelength 1360nm, and the symbol “■” indicates the above relationship with opposing transmission XT at wavelength 1550nm. Figure 10 (The text in the middle is displayed as a slash) indicates the above relationship with the opposite transmission XT at a wavelength of 1360nm.

[0281] In Figure 10 No dotted line is recorded, but it is consistent with... Figure 5 Similarly, in a 4-core MCF with a square core configuration, considering the tolerances of core position and cladding diameter, in order to set the leakage loss to the cladding at a wavelength of 1360nm to below 0.01dB / km, and the parallel transmission XT after 10km transmission to below -20dB, CD nominal and MFD / λ cc The relationship satisfies either equation (105) or equation (106):

[0282] CD nominal ≥9.49MFD / λ cc +31.91…(105)

[0283] MFD / λ cc ≤0.1054CD nominal -3.363…(106)

[0284] Furthermore, it is preferable to satisfy the following equation (107) or equation (108):

[0285] CD nominal ≥9.49MFD / λ cc +38.16…(107)

[0286] MFD / λ cc ≤0.1054CD nominal -4.021…(108).

[0287] To reduce the leakage loss to the cladding at 1550nm wavelength to below 0.01dB / km, and to reduce the back-to-back transmission XT after 10km transmission to below -20dB, CD nominal and MFD / λ cc The relationship satisfies either equation (109) or equation (110):

[0288] CD nominal ≥12.56MFD / λ cc+24.30…(109)

[0289] MFD / λ cc ≤0.07960CD nominal -1.934…(110)

[0290] Furthermore, it is preferable to satisfy either equation (111) or equation (112):

[0291] CD nominal ≥12.56MFD / λ cc +33.78…(111)

[0292] MFD / λ cc ≤0.07960CD nominal -2.688…(112).

[0293] To reduce the leakage loss to the cladding at 1550nm wavelength to below 0.01dB / km, and to reduce the parallel transmission XT after 10km transmission to below -20dB, CD nominal and MFD / λ cc The relationship satisfies the following equation (113) or equation (114):

[0294] CD nominal ≥13.31MFD / λ cc +24.47…(113)

[0295] MFD / λ cc ≤0.07511CD nominal -1.838…(114)

[0296] Furthermore, it is preferable to satisfy the following equation (115) or equation (116):

[0297] CD nominal ≥13.31MFD / λ cc +34.18…(115)

[0298] MFD / λ cc ≤0.07511CD nominal -2.567…(116).

[0299] In a 4-core optical fiber with a square core configuration, considering the tolerances of core position and cladding diameter, in order to set the leakage loss to the cladding at a wavelength of 1360nm to below 0.01dB / km, and the back-to-back transmission XT after 10km transmission to below -20dB, in CD... nominalWhen 125μm, 120μm, 115μm, 110μm, 105μm, 100μm, 95μm, 90μm, 85μm, 80μm, MFD / λ cc According to CD nominal The preferred order of the numerical values ​​is 10.49 or less, 9.93 or less, 9.37 or less, 8.81 or less, 8.25 or less, 7.69 or less, 7.14 or less, 6.58 or less, 6.02 or less, and 5.46 or less, and MFD / λ cc According to CD nominal The preferred order of the numerical values ​​is below 9.78, below 9.22, below 8.66, below 8.10, below 7.54, below 6.99, below 6.43, below 5.87, below 5.31, and below 4.75.

[0300] To reduce the leakage loss to the cladding at 1360nm wavelength to below 0.01dB / km, and to reduce the parallel transmission XT after 10km transmission to below -20dB, MFD / λ cc According to CD nominal The numerical values ​​listed above are preferably 9.81 or less, 9.28 or less, 8.76 or less, 8.23 ​​or less, 7.70 or less, 7.17 or less, 6.65 or less, 6.12 or less, 5.59 or less, and 5.07 or less, and MFD / λ cc According to CD nominal The order of the values ​​listed above is preferably 9.15 or less, 8.62 or less, 8.10 or less, 7.57 or less, 7.04 or less, 6.52 or less, 5.99 or less, 5.46 or less, 4.94 or less, and 4.41 or less.

[0301] To reduce the leakage loss to the cladding at 1550 nm wavelength to below 0.01 dB / km, and to reduce the back-to-back transmission XT after 10 km transmission to below -20 dB, MFD / λ cc According to CD nominal The numerical values ​​listed above are preferably in the following order: 8.02 or less, 7.62 or less, 7.22 or less, 6.82 or less, 6.42 or less, 6.03 or less, 5.63 or less, 5.23 or less, 4.83 or less, and 4.43 or less, and MFD / λ cc According to CD nominal The preferred order of the values ​​listed above is 7.26 or less, 6.86 or less, 6.47 or less, 6.07 or less, 5.67 or less, 5.27 or less, 4.87 or less, 4.48 or less, 4.08 or less, and 3.68 or less.

[0302] To reduce the leakage loss to the cladding at 1550nm wavelength to below 0.01dB / km, and to reduce the parallel transmission XT after 10km transmission to below -20dB, MFD / λ cc According to CD nominal The numerical values ​​listed above are preferably 7.55 or less, 7.18 or less, 6.80 or less, 6.42 or less, 6.05 or less, 5.67 or less, 5.30 or less, 4.92 or less, 4.55 or less, and 4.17 or less, and MFD / λ cc According to CD nominal The order of the values ​​listed above is preferably 6.82 or less, 6.45 or less, 6.07 or less, 5.69 or less, 5.32 or less, 4.94 or less, 4.57 or less, 4.19 or less, 3.82 or less, and 3.44 or less.

[0303] Figure 11 This refers to the condition that, under the following conditions, after 10km of transmission in a 4-core MCF fiber at wavelengths of 1550nm and 1360nm (equivalent to a fiber length of 10km), the phase-to-phase transmission XT becomes -40dB (=-40dB / 10km), and the parallel transmission XT becomes -40dB (=-40dB / 10km) after 10km of transmission (equivalent to a fiber length of 10km), the leakage loss to the cladding is 0.01dB / km. coat With a 1μm margin added, and with an additional 1μm margin added to Λ, CD (minimum permissible cladding diameter) and MFD / λ cc A graph showing the relationship between the two.

[0304] In addition, Figure 11 In the diagram, the symbol “○” indicates the above relationship with parallel transmission XT at a wavelength of 1550nm, and the symbol “●” (in the case of…) Figure 11 The symbol “□” indicates the above relationship with parallel transmission XT at wavelength 1360nm, and the symbol “■” indicates the above relationship with opposing transmission XT at wavelength 1550nm. Figure 11 (The text in the middle is displayed as a slash) indicates the above relationship with the opposite transmission XT at a wavelength of 1360nm.

[0305] In Figure 11 No dotted line is recorded, but it is consistent with... Figure 5 Similarly, considering the tolerances of the core position and cladding diameter, in order to set the leakage loss to the cladding at a wavelength of 1360nm to below 0.01dB / km, and the back-to-back transmission XT after 10km transmission to below -40dB, CD nominal and MFD / λ ccThe relationship satisfies either equation (117) or equation (118):

[0306] CD nominal ≥9.37MFD / λ cc +31.73…(117)

[0307] MFD / λ cc ≤0.1068CD nominal -3.388…(118)

[0308] Furthermore, it is preferable to satisfy either equation (119) or equation (120):

[0309] CD nominal ≥9.37MFD / λ cc +38.00…(119)

[0310] MFD / λ cc ≤0.1068CD nominal -4.058…(120).

[0311] To reduce the leakage loss to the cladding at 1360nm wavelength to below 0.01dB / km, and to reduce the parallel transmission XT after 10km transmission to below -40dB, CD nominal and MFD / λ cc The relationship satisfies either equation (121) or equation (122):

[0312] CD nominal ≥10.32MFD / λ cc +33.11…(121)

[0313] MFD / λ cc ≤0.09690CD nominal -3.208…(122)

[0314] Furthermore, it is preferable to satisfy the following equation (123) or equation (124):

[0315] CD nominal ≥10.32MFD / λ cc +39.23…(123)

[0316] MFD / λ cc ≤0.09690CD nominal -3.802…(124).

[0317] To reduce the leakage loss to the cladding at 1550nm wavelength to below 0.01dB / km, and to reduce the back-to-back transmission XT after 10km transmission to below -40dB, CD nominaland MFD / λ cc The relationship satisfies either equation (125) or equation (126):

[0318] CD nominal ≥13.14MFD / λ cc +24.43…(125)

[0319] MFD / λ cc ≤0.07610CD nominal -1.859…(126)

[0320] Furthermore, it is preferable to satisfy either equation (127) or equation (128):

[0321] CD nominal ≥13.14MFD / λ cc +34.09…(127)

[0322] MFD / λ cc ≤0.07610CD nominal -2.594…(128).

[0323] To reduce the leakage loss to the cladding at 1550nm wavelength to below 0.01dB / km, and to reduce the parallel transmission XT after 10km transmission to below -40dB, CD nominal and MFD / λ cc The relationship satisfies either equation (129) or equation (130):

[0324] CD nominal ≥14.47MFD / λ cc +24.73…(129)

[0325] MFD / λ cc ≤0.06911CD nominal -1.709…(130)

[0326] Furthermore, it is preferable to satisfy either equation (131) or equation (132):

[0327] CD nominal ≥14.47MFD / λ cc +34.08…(131)

[0328] MFD / λ cc ≤0.06911CD nominal -2.406…(132).

[0329] In a 4-core MCF with a square core configuration, considering the tolerances of core position and cladding diameter, to ensure that the leakage loss to the cladding at a wavelength of 1360nm is below 0.01dB / km and the back-to-back transmission XT after 10km transmission is below -40dB, in CD... nominal When 125μm, 120μm, 115μm, 110μm, 105μm, 100μm, 95μm, 90μm, 85μm, 80μm, MFD / λ cc According to CD nominal The preferred order of the numerical values ​​is 9.96 or less, 9.42 or less, 8.89 or less, 8.36 or less, 7.82 or less, 7.29 or less, 6.76 or less, 6.22 or less, 5.69 or less, and 5.15 or less, and MFD / λ cc According to CD nominal The order of the values ​​listed above is preferably 9.29 or less, 8.76 or less, 8.22 or less, 7.69 or less, 7.15 or less, 6.62 or less, 6.09 or less, 5.55 or less, 5.02 or less, and 4.48 or less.

[0330] To reduce the leakage loss to the cladding at 1360nm wavelength to below 0.01dB / km, and to reduce the parallel transmission XT after 10km transmission to below -40dB, MFD / λ cc According to CD nominal The numerical values ​​listed above are preferably in the order of 8.90 or less, 8.42 or less, 7.94 or less, 7.45 or less, 6.97 or less, 6.48 or less, 6.00 or less, 5.51 or less, 5.03 or less, and 4.54 or less, and MFD / λ cc According to CD nominal The preferred order of the values ​​listed above is 8.31 or less, 7.83 or less, 7.34 or less, 6.86 or less, 6.37 or less, 5.89 or less, 5.40 or less, 4.92 or less, 4.43 or less, and 3.95 or less.

[0331] To reduce the leakage loss to the cladding at 1550 nm wavelength to below 0.01 dB / km, and to reduce the back-to-back transmission XT after 10 km transmission to below -40 dB, MFD / λ cc According to CD nominal The numerical values ​​listed above are preferably 7.65 or less, 7.27 or less, 6.89 or less, 6.51 or less, 6.13 or less, 5.75 or less, 5.37 or less, 4.99 or less, 4.61 or less, and 4.23 or less, and MFD / λ cc According to CD nominalThe order of the values ​​listed above is preferably 6.92 or less, 6.54 or less, 6.16 or less, 5.78 or less, 5.40 or less, 5.02 or less, 4.64 or less, 4.25 or less, 3.87 or less, and 3.49 or less.

[0332] To reduce the leakage loss to the cladding at 1550nm wavelength to below 0.01dB / km, and to reduce the parallel transmission XT after 10km transmission to below -40dB, MFD / λ cc According to CD nominal The numerical values ​​listed above are preferably 6.93 or less, 6.58 or less, 6.24 or less, 5.89 or less, 5.55 or less, 5.20 or less, 4.86 or less, 4.51 or less, 4.17 or less, and 3.82 or less, and MFD / λ cc According to CD nominal The order of the values ​​listed above is preferably 6.23 or less, 5.89 or less, 5.54 or less, 5.20 or less, 4.85 or less, 4.51 or less, 4.16 or less, 3.81 or less, 3.47 or less, and 3.12 or less.

[0333] λ cc To ensure single-mode operation in the O-band, a wavelength below 1260 nm is preferred. In this case, by adjusting the MFD / λ... cc Setting it to 6.5 or higher allows for λ to be optimized even when the MFD converges to a range between 8.2μm and 9.0μm with a baseline of 8.6μm. cc Set below 1260nm. By using MFD / λ cc Setting it to 7.2 or higher allows for a larger MFD, reduces connection losses between MCFs, and increases λ. cc Significantly reduce λ compared to 1260nm cc (Set to below 1.2μm) (= Remainder). In this case, for example, MFD converges to the range of 8.8μm to 9.6μm with a reference of 9.2μm, becoming λ. cc ≤1.23μm, MFD / λ cc ≥7.2. In these cases, MFD / λ cc Preferred choice is based on the CD described above. nominal The defined upper limit and according to MFD and λ cc The values ​​between the lower limit defined by the range.

[0334] When considering the mass production of MCF, the construction of MCF is MFD / λ. cc The tolerance is 1.0 or more, preferably 1.5 or more, more preferably 2.0 or more, and preferably set to 2.5 or more. The most preferred value is MFD / λ.cc The tolerance can be 3.0 or higher for the construction of MCF.

[0335] The preferred construction of MCF is related to MFD / λ cc Constructions with a resolution between 6.5 and 7.5 are permitted. Furthermore, the construction of the MCF can be related to MFD / λ. cc The structure allows for a value between 6.5 and 8.0, more preferably a value between 6.5 and 8.5, and even more preferably a value between 6.5 and 9.0. The most preferred MCF structure is related to MFD / λ. cc Constructions with a value between 6.5 and 9.5 are permitted.

[0336] The construction of the MCF can be related to MFD / λ cc Constructions with a resolution between 7.2 and 8.2 are permitted. Furthermore, the construction of the MCF can be related to MFD / λ. cc A configuration that allows for a value between 7.2 and 8.7 is preferred, a configuration that allows for a value between 7.2 and 9.2 is even more preferred, and a configuration that allows for a value between 7.2 and 9.7 is even more preferred. The most preferred MCF configuration is related to MFD / λ. cc Constructions with a tolerance of 7.2 to 10.2 are permitted.

[0337] In λ cc For wavelengths exceeding 1260 nm but below 1360 nm, using the structure in Figure 12 of ITU-T G.650.1 (03 / 2018) (non-cable-connected optical fiber), a bend with a radius of 140 mm or greater is added to 20 m of a 22 m sample optical fiber. One bend with a radius of 40 mm is then added before and after this 20 m interval. With uniform excitation of all modes, the intensity of the higher mode is set to P. h Let the strength of the fundamental model be P. f At that time, it will become 10log10[P] h / (P f +P h The wavelength with a value of 0.1 dB is used as λ. cc Measurements were performed, but regarding the MCF of the present invention, it is preferable to replace the radius of the bend added in the 20m interval of the 22m sample fiber with a radius of 60mm to 100mm and the bend added during the measurement of the cutoff wavelength (λ). ccR The wavelength is below 1260nm. This ensures single-mode operation in the O-band after cable installation. Additionally, the sample fiber length L... sample [m] The range exceeding 22m and below 1000m, in L sample -2 [m] plus a bend with a bending radius of 140 mm or more, in the L sampleAdd one bend with a radius of 40 mm before and after the -2 [m] interval, and the optimal cutoff wavelength (λ) is determined. ccL The wavelength is below 1260nm. Therefore, in a cable length of L... sample The [m] cable can guarantee single-mode operation in the O-band.

[0338] In the MCF of the present invention, the bending loss of each fiber core at wavelengths between 1310 nm and 1360 nm is preferably 0.15 dB / turn or less, more preferably 0.02 dB / turn or less, when the bending radius is 10 mm. Therefore, even when the MCF of the present invention is installed in an ultra-high density cable of the spacer adhesive tape type, the increase in loss after cabling can be suppressed.

[0339] When the MCF cable incorporating the MCF of the present invention is stretched in a straight line (with a bending radius of at least 1m), the average bending radius of the MCF installed within the cable is preferably 0.14m or less, more preferably 0.10m or less. Furthermore, the average bending radius of the MCF installed within the cable is preferably 0.14m or more and 0.3m or less. This reduces the XT (thickness factor).

[0340] Furthermore, in the MCF cable in which the MCF of the present invention is incorporated, the average bending radius of the MCF installed within the cable is preferably 0.03 m or more, more preferably 0.06 m or more. This reduces losses caused by bending.

[0341] Furthermore, the MCF cable incorporating the MCF of the present invention is preferably a spacer tape type cable. This allows the flexible spacer tape to be twisted into a spiral shape while being installed inside the cable, enabling the cable to be made with a small bending radius of the MCF, thus reducing the XT (thickness factor).

[0342] The MCF cable incorporating the present invention is a grooved cable, preferably with a tensile strength member at the center of the groove component. This facilitates control of the MCF's bending radius and reduces the bending radius (XT). Furthermore, the presence of the tensile strength member at the center of the groove component allows for easy bending of the cable in any orientation, facilitating cable laying operations.

[0343] The MCF cable incorporating the MCF of the present invention preferably does not have slotted components inside the sheath, but has a tensile-resistant body inside the sheath. This allows for efficient use of the space inside the sheath and increases the number of fiber cores per unit cross-sectional area of ​​the MCF cable.

[0344] As described above, the MCF according to the present invention can guarantee sufficient manufacturing tolerance, excellent mass production performance, and also suppress the degradation of connection loss.

Claims

1. A multi-core optical fiber, having the following characteristics: Four fiber cores, each extending along the central axis; and A common cladding layer covers each of the four fiber cores. In this multi-core optical fiber, In the cross-section of this multi-core optical fiber orthogonal to the central axis, the common cladding has a circular outer perimeter. On the cross-section, relative to a straight line intersecting the central axis but not intersecting any of the four fiber cores, the four fiber cores are arranged in a position that is linearly symmetrical. In the cross-section, the core configuration defined by the four cores does not possess more than one order of rotational symmetry, even if any point is taken as the center of rotation. The diameter CD of the common cladding is less than 126 μm. The mode field diameter (MFD) of each of the four fiber cores at a wavelength of 1310 nm and the cable cutoff wavelength (λ) measured through a 22 m long optical fiber. cc It satisfies the following equation (1): …（1）， In each of the four fiber cores, the mode field diameter (MFD) is between 8.2 μm and 9.6 μm, and the difference between the maximum and minimum values ​​among them is less than 0.8 μm. In each of the four fiber cores, the zero-dispersion wavelength is between 1300 nm and 1352 nm, and the difference between the maximum and minimum values ​​among them is less than 24 nm. In each of the four fiber cores, the dispersion slope of the zero-dispersion wavelength is 0.092 ps / (nm). 2 •km) and below, In each of the four fiber cores, the cable cutoff wavelength λ cc Below 1260nm Any condition that satisfies both condition 1 and condition 2 The first condition is that, at a wavelength of 1360nm, the crosstalk between adjacent fiber cores with a fiber length of 10km is below -10dB, and in each of the four fiber cores, the following equation (2) is satisfied: …（2） The relationship, and in each of the four fiber cores, compared to MFD / λ cc The center spacing Λ between the fiber cores and the adjacent fiber cores satisfies the following equation (3): …（3）， This is the definition. The second condition is that, at a wavelength of 1360nm, the crosstalk between adjacent fiber cores with a fiber length of 10km is below -20dB, and in each of the four fiber cores, the following equation (4) is satisfied: …（4） The relationship, and in each of the four fiber cores, the ratio of MFD / λ cc The center spacing Λ between the fiber cores and the adjacent fiber cores satisfies the following equation (5): …（5）， This is the definition. in, d coat CD is the distance from the center of each of the four fiber cores to the outer periphery of the common cladding. nominal The nominal value of the diameter of the common cladding.

2. The multi-core optical fiber according to claim 1, wherein, The four fiber cores are configured such that the center of each of the four fiber cores is located on the cross-section, where each of the three sides has a length Λ. nominal And the length of one side is the Λ nominal Within a radius of 1.0 μm, with each of the four vertices of a triequilateral trapezoid being more than 1.2 times larger than the radius of the trapezoid.

3. The multi-core optical fiber according to claim 1 or 2, wherein, The distance d from the center of each of the four fiber cores to the outer periphery of the common cladding coat , with the specified nominal value d coat,nominal All converge to d based on the criterion coat,nominal Values ​​above -1μm and d coat,nominal The range below +1μm.

4. The multi-core optical fiber according to claim 1, wherein, It also has a covering portion that surrounds the common cladding. The leakage loss from at least any one of the four fiber cores to the covering portion is 0.05 dB / km or more at a wavelength of 1550 nm or 1625 nm, or the transmission loss of at least any one of the four fiber cores is 0.25 dB / km or more at a wavelength of 1550 nm or 1625 nm.

5. The multi-core optical fiber according to claim 4, wherein, The first condition is met, and at a wavelength of 1550nm, the crosstalk between adjacent fiber cores with a fiber length of 10km is greater than -10dB.

6. The multi-core optical fiber according to claim 4, wherein, The second condition is met, and at a wavelength of 1550nm, the crosstalk between adjacent fiber cores with a fiber length of 10km is greater than -20dB.

7. A multi-core optical cable having a plurality of multi-core optical fibers comprising any one of claims 1 to 6.

8. The multi-core optical cable according to claim 7, wherein, The multi-core optical cable comprises multi-core optical fibers with an average bending radius along the fiber length of 0.03m to 0.14m or 0.14m to 0.3m.

9. A multi-core optical cable having a built-in multi-core optical fiber ribbon, the multi-core optical fiber ribbon being formed by bonding together at intervals a plurality of multi-core optical fibers comprising the multi-core optical fiber of any one of claims 1 to 6.

10. The multi-core optical cable according to claim 9, wherein, The multi-core optical cable incorporates the multi-core optical fiber ribbon in a twisted spiral shape.

11. The multi-core optical cable according to claim 9, wherein, The multi-core optical cable comprises multi-core optical fibers with an average bending radius along the fiber length of 0.03m to 0.14m or 0.14m to 0.3m.

12. The multi-core optical cable according to claim 10, wherein, The multi-core optical cable comprises multi-core optical fibers with an average bending radius along the fiber length of 0.03m to 0.14m or 0.14m to 0.3m.

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