Multi-core optical fiber and multi-core optical cable

Abstract

The MCF of the present invention can ensure sufficient manufacturing tolerance, has excellent mass producibility, and can also suppress the deterioration of connection loss. The MCF has four cores extending along the central axis and a common cladding. Each core has an adjacent relationship with two of the remaining cores, and the central interval Λ between the cores in the adjacent relationship is Λ nominal -0.9 μm or more and Λ nominal +0.9 μm or less. The outer diameter of the common cladding is 124 μm or more and 126 μm or less. The MFD, λ cc and d coat satisfy a specified relationship. The MFD has a value of 8.6 μm or more and 9.2 μm or less as the MFD reference value, and is -0.4 μm or more and +0.4 μm or less of the MFD reference value. The zero-dispersion wavelength has a value of 1312 nm or more and 1340 nm or less as the wavelength reference value, and is -12 nm or more and +12 nm or less of the wavelength reference value. The dispersion slope of the zero-dispersion wavelength is 0.092 ps / (nm 2 ·km) or less, λ cc is 1260 nm or less, and satisfies the specified structural conditions and optical conditions.

Description

Technical Field

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

[0002] This application claims priority based on Japanese Patent Application No. 2020-174958, filed on October 16, 2020, the entirety of which is incorporated herein by reference in accordance with the contents thereof. Background Art

[0003] Non-patent document 1 discloses a trench-assisted four-core optical fiber having four cores and a cladding with an outer diameter of 125 μm. The depth of the trench 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 or more and 8.6 μm or less. The cable cutoff wavelength is 1171 nm or more and 1195 nm or less. The zero dispersion wavelength is 1317 nm or more and 1319 nm or less. The wavelength dispersion slope of the zero dispersion wavelength is 0.090 ps / (nm 2 km) and above 0.091ps / (nm 2 The transmission loss at a wavelength of 1310 nm is 0.33 dB / km to 0.35 dB / km, and at a wavelength of 1550 nm is 0.19 dB / km to 0.21 dB / km. The core-to-core crosstalk (hereinafter referred to as "XT") at a wavelength of 1625 nm is -43 dB / km.

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

[0005] Patent Document 1 discloses a trenchless four-core optical fiber having four cores and a cladding with an outer diameter of 125 μm. The core pitch (center-to-center distance) is defined as a pitch reference value of 40 μm to 41.5 μm, and falls within a range of a value greater than the pitch reference value -1 μm and less than the pitch reference value +1 μm. coat The so-called OCT is based on a wavelength of 33μm and falls within the range of 32μm to 34μm. The V value is 2.50 to 2.58. The MFD at a wavelength of 1310nm is 8.0μm to 8.3μm. The bending loss at a wavelength of 1625nm is 0.1dB / 100 turns or less with a bending radius of 30mm. The effective cutoff wavelength (undefined) is 1260nm or less. The zero dispersion wavelength is 1300nm to 1324nm. The XT between the four cores after 10km of transmission at a wavelength of 1550nm is -30dB or less.

[0006] Patent Document 2 discloses a trenchless four-core optical fiber having four cores and a cladding having an outer diameter of 125 μm, wherein the XT between the cores is suppressed to a certain value or less. In addition, Patent Document 2 does not disclose the method required to suppress the XT between the cores to a certain value or less. In other words, the upper limit of the core spacing Λ for suppressing the leakage loss to the cladding is disclosed, but the lower limit of Λ required for suppressing the XT between the cores is not disclosed. In addition, there is no disclosure of the method for achieving the method of Patent Document 2. Figure 6 Λ of XT. In addition, in paragraph "0026" of Patent Document 2, the upper limit value of Λ is defined by formula (3) (in paragraph "0028", the minimum value of Λ is defined by formula (3), which creates a contradiction).

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2013-088458

[0008] Patent Document 2: International Publication WO2020 / 149158 Gazette

[0009] Patent Document 3: US Patent 9,933,331 Specification

[0010] 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>.

[0011] 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

[0012] <URL:https: / / doi.org / 10.1049 / cp.2019.0751>.

[0013] Non-Patent Document 3: R.J. Black and C.Pask, J. Opt. Soc. Am. A, JOSAA 1(11), p.1129 - 1131, 1984.

[0014] Non-Patent Document 4: T. Matsui et al., in Eur. Conf. Opt. Commun. (ECOC2017), p.W.1.B.2. Summary of the Invention

[0015] The MCF of the present invention is provided with 4 cores extending along the central axis and a common cladding to solve the above problems. Each core has an adjacent relationship with 2 cores among the remaining cores, and the central interval Λ between the cores in the adjacent relationship converges to Λ based on the nominal value Λnominal nominal -0.9μm or more and Λ nominalThe outer diameter of the common cladding is within the range of 124μm to 126μm based on 125μm. In each core, the MFD and λ of the wavelength of 1310nm are cc and d coat The prescribed relationship is satisfied. In each fiber core, the MFD is defined as the MFD reference value of 8.6 μm or more and 9.2 μm or less, and is within the range of the MFD reference value -0.4 μm or more and the MFD reference value +0.4 μm or less. In each fiber core, the zero dispersion wavelength is defined as the wavelength reference value of 1312 nm or more and 1340 nm or less, and is within the range of the wavelength reference value -12 nm or more and the wavelength reference value +12 nm or less. In each fiber core, the dispersion slope of the zero dispersion wavelength is 0.092 ps / (nm 2 km), in each fiber core, λ cc It is below 1260nm and meets the prescribed structural and optical conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 These are diagrams showing various structures of the MCF cable of the present invention (including the MCF of the present invention).

[0017] Figure 2 It is a diagram showing various core arrangements of the MCF of the present invention.

[0018] Figure 3 This is a diagram for explaining the main terms used in this specification.

[0019] Figure 4 This is a diagram showing the refractive index distribution around each core of an MCF applicable to the present invention.

[0020] Figure 5 The MFD / λ represents the center spacing Λ between adjacent cores and the MFD / λ when the counter-transmission XT at a wavelength of 1360nm becomes -20dB (= -20dB / 10km) after 10km of transmission (equivalent to a fiber length of 10km) in an MCF configured with four cores forming a square lattice. cc Graphic of the relationship.

[0021] Figure 6 The values ​​for the center spacing Λ and MFD / λ between adjacent cores are shown when the opposite transmission XT reaches -20dB after 10km of transmission (equivalent to a fiber length of 10km) through a 4-core MCF at both wavelengths of 1550nm and 1360nm, and when the parallel transmission XT reaches -20dB after 10km of transmission (equivalent to a fiber length of 10km) through a 4-core MCF. cc Graphic of the relationship.

[0022] Figure 7 The values ​​for the center spacing Λ and MFD / λ between adjacent cores are shown when the opposite transmission XT reaches -40dB after 10km of transmission (equivalent to a fiber length of 10km) through a 4-core MCF at both wavelengths of 1550nm and 1360nm, and when the parallel transmission XT reaches -40dB after 10km of transmission (equivalent to a fiber length of 10km) through a 4-core MCF. cc Graphic of the relationship.

[0023] Figure 8 The d value is when the leakage loss to the cladding becomes 0.01dB / km in a 4-core MCF with a wavelength of 1360nm. coat and MFD / λ cc Graphic of the relationship.

[0024] Figure 9 The d value is the value when the leakage loss to the cladding becomes 0.01dB / km at a wavelength of 1360nm in a 4-core MCF. coat Adding a 1μm margin, and adding a 1μm margin, the opposite transmission XT at a wavelength of 1360nm after 10km of transmission (equivalent to a fiber length of 10km) becomes -20dB (= -20dB / 10km). When the CD and MFD / λ are cc Graphic of the relationship.

[0025] Figure 10 This represents the d when the leakage loss to the cladding is 0.01 dB / km under the conditions that the counter-propagation XT after 10 km of transmission (equivalent to a fiber length of 10 km) in a 4-core MCF is -20 dB (= -20 dB / 10 km), and the parallel propagation XT (normal co-propagation XT) after 10 km of transmission (equivalent to a fiber length of 10 km) is -20 dB (= -20 dB / 10 km). coat Add a margin of 1 μm, and in the case of Λ plus a margin of 1 μm, CD (minimum allowable cladding diameter) and MFD / λ cc Graphic of the relationship.

[0026] Figure 11 This represents the conditions under which the counter transmission XT becomes -40dB (= -40dB / 10km) after 10km transmission (equivalent to a fiber length of 10km) in a 4-core MCF at both wavelengths of 1550nm and 1360nm, and the parallel transmission XT becomes -40dB (= -40dB / 10km) after 10km transmission (equivalent to a fiber length of 10km), and the leakage loss to the coating becomes 0.01dB / km. coatAdd a margin of 1 μm, and in the case of Λ plus a margin of 1 μm, CD (minimum allowable cladding diameter) and MFD / λ cc Graphic of the relationship. DETAILED DESCRIPTION

[0027] [Problems to be Solved by the Invention]

[0028] The inventors studied the above-mentioned prior art and discovered the following problem. Specifically, the MCF described in Non-Patent Document 1 exhibits significantly lower mass production performance and higher manufacturing costs compared to general-purpose single-mode optical fibers (hereinafter referred to as "SMFs"). This is because, in order to simultaneously achieve a reduction in the XT between cores, an increase in the number of cores, a reduction in the outer diameter of the cladding, and an increase in the MFD of each core, a low-refractive-index trench layer with a large relative refractive index difference from the cladding is required around the cores.

[0029] The MCFs described in Non-Patent Document 2, Patent Document 1, and Patent Document 2 have narrow manufacturing tolerances and high manufacturing costs. It has been proposed that a 1260nm to 1625nm MCF could be used for shorter distances. However, this MCF requires extremely high-precision control of the refractive index profile to achieve optical properties within the designed range, making it impossible to achieve the same manufacturing tolerances as a general-purpose SMF.

[0030] Furthermore, Patent Document 1 does not explicitly describe the presence or absence of trenches, but based on the disclosure (the definition and range of V values), it can be inferred that trench-type transmission is not actually included. The attempt to achieve good transmission characteristics beyond the O-band even for short distances has resulted in narrowing manufacturing tolerances.

[0031] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an MCF for short-distance transmission that ensures a sufficient manufacturing tolerance, excels in mass productivity, and suppresses degradation of connection loss.

[0032] [Description of Embodiments of the Invention]

[0033] First, the contents of the embodiments of the present invention will be individually listed and described.

[0034] (1) The MCF (multi-core fiber) of the present invention has, as one embodiment thereof, four cores extending along a central axis and a common cladding covering each of the four cores. In particular, each of the four cores is adjacent to two of the remaining cores. The center spacing Λ between the adjacent cores among the four cores is a predetermined core spacing nominal value Λ. nominal As the benchmark, it converges to Λ nominal -0.9μm value or more and Λ nominalThe outer diameter of the common cladding is within the range of 124 μm to 126 μm, based on a value of +0.9 μm. The MFD of each of the four cores at a wavelength of 1310 nm and the cable cutoff wavelength λ measured with a 22 m optical fiber are shown. cc The shortest distance d between the center of each of the four cores and the outer periphery of the common cladding is coat The following formula (1) is satisfied for any of the four cores:

[0035] d coat ≥2.88MFD / λ cc +5.36…(1).

[0036] Furthermore, in each of the four cores, the MFD is defined as the MFD reference value of 8.6 μm to 9.2 μm, and falls within the range of MFD reference value - 0.4 μm and MFD reference value + 0.4 μm. In each of the four cores, the zero dispersion wavelength is defined as the wavelength reference value of 1312 nm to 1340 nm, and falls within the range of wavelength reference value - 12 nm and wavelength reference value + 12 nm. The dispersion slope of the zero dispersion wavelength in each of the four cores is 0.092 ps / (nm 2 ·km) or less. In each of the four cores, the cutoff wavelength λ cc Furthermore, the MCF satisfies any of the following first and second conditions, and any of the following third and fourth conditions.

[0037] The first condition is defined as each of the four cores being in direct contact with the common cladding. The second condition is defined as each of the four cores having an optical cladding disposed between the corresponding core and the common cladding, and the relative refractive index difference Δ2 of the optical cladding with respect to the common cladding satisfies the relationship -0.1% ≤ Δ2 ≤ 0.1%.

[0038] The third condition is that the crosstalk between adjacent cores at a wavelength of 1360nm, which is equivalent to an optical fiber length of 10km, is -10dB or less, and the MFD / λ ratio is 0. cc The center spacing of adjacent cores is Λ a Any of the following equations (2) to (6) is satisfied:

[0039] 7.2≤MFD / λ cc ≤8.2≤0.443Λ a -5.33…(2)

[0040] 7.2≤MFD / λ cc ≤8.7≤0.443Λ a -5.33…(3)

[0041] 7.2≤MFD / λ cc ≤9.2≤0.443Λ a -5.33…(4)

[0042] 7.2≤MFD / λ cc ≤9.7≤0.443Λ a -5.33…(5)

[0043] 7.2≤MFD / λ cc ≤10.2≤0.443Λ a -5.33…(6)

[0044] , which is defined by this.

[0045] The fourth condition is that the crosstalk between adjacent cores corresponding to an optical fiber length of 10 km at a wavelength of 1360 nm is -20 dB or less, and the MFD / λ ratio of each of the four cores is cc The center spacing of adjacent cores is Λ a Any of the following equations (7) to (11) is satisfied:

[0046] 7.2≤MFD / λ cc ≤8.2≤0.392Λ a -4.88…(7)

[0047] 7.2≤MFD / λ cc ≤8.7≤0.392Λ a -4.88…(8)

[0048] 7.2≤MFD / λ cc ≤9.2≤0.392Λ a -4.88…(9)

[0049] 7.2≤MFD / λ cc ≤9.7≤0.392Λ a -4.88…(10)

[0050] 7.2≤MFD / λ cc ≤10.2≤0.392Λ a -4.88…(11)

[0051] , which is defined by this.

[0052] This MCF, constructed as described above, is a square-arranged four-core fiber with a standard cladding diameter, ensuring sufficient mass production tolerances and exhibiting optimal optical properties in the O-band. Furthermore, the outer diameter of the common cladding is limited to a range of 124 to 126 μm, based on a 125 μm reference. This reduces leakage loss from the outermost core to the cladding to below 0.01 dB / km at a wavelength of 1360 nm. Furthermore, when this MCF meets the third condition described above, it maintains a guaranteed tolerance for MFD / λcc, achieving high yields during mass production of the optical fiber and suppressing the total counter-transmission XT to the specified core to below -20 dB for a fiber length of 10 km at wavelengths below 1360 nm. When this MCF satisfies the fourth condition, it can maintain the tolerance for MFD / λcc and achieve a high yield in mass production of optical fibers. It can also suppress the total counter-transmission XT to a specified core, equivalent to a fiber length of 10 km, to below -40 dB at wavelengths of 1360 nm or less. "Leakage loss" can be determined using the method described in Patent Document 3, the disclosure of which is incorporated herein by reference.

[0053] (2) The MCF of the present invention, as one embodiment thereof, comprises: four cores extending along a central axis; and a common cladding covering each of the four cores. Each of the four cores is adjacent to two of the remaining cores. The center spacing Λ between the adjacent cores among the four cores is a predetermined core spacing nominal value Λ. nominal As the benchmark, it converges to Λ nominal -0.9μm value or more and Λ nominal The common cladding diameter CD is a nominal cladding diameter CD of less than 125 μm. nominal [μm] is the standard and converges to CD nominal -1μm value or more and CD nominal The range below the value of +1μm. The MFD at a wavelength of 1310nm for each of the four cores and the cable cutoff wavelength λ measured through a 22m optical fiber cc Satisfies the following formula (12):

[0054] d coat ≥2.88MFD / λ cc +5.36…(12)

[0055] In each of the four cores, the MFD is defined as the MFD reference value of 8.6 μm or more and 9.2 μm or less, and falls within the range of the MFD reference value - 0.4 μm or more and the MFD reference value + 0.4 μm or less. In each of the four cores, the zero dispersion wavelength is defined as the wavelength reference value of 1312 nm or more and 1340 nm or less, and falls within the range of the wavelength reference value - 12 nm or less and the wavelength reference value + 12 nm or less. The dispersion slope of the zero dispersion wavelength in each of the four cores is 0.092 ps / (nm 2 ·km) or less. In each of the four cores, the cutoff wavelength λ cc Furthermore, the MCF satisfies any of the following first and second conditions, and any of the following fifth and sixth conditions.

[0056] The first condition is defined as each of the four cores being in direct contact with the common cladding. The second condition is defined as each of the four cores having an optical cladding disposed between the corresponding core and the common cladding, and the relative refractive index difference Δ2 of the optical cladding with respect to the common cladding satisfies the relationship -0.1% ≤ Δ2 ≤ 0.1%.

[0057] The fifth condition is that the XT between adjacent cores, which corresponds to a fiber length of 10 km at a wavelength of 1360 nm, is -10 dB or less, and that each of the four cores satisfies the following equation (13):

[0058] CD nominal ≥13.31MFD / λ cc +24.47…(13)

[0059] , and in each of the four cores, the ratio of MFD / λ cc The center spacing of adjacent cores is Λ a Satisfy any of the following equations (14) to (18):

[0060] 6.5≤MFD / λ cc ≤7.5≤0.443Λ a -5.33…(14)

[0061] 6.5≤MFD / λ cc ≤8.0≤0.443Λ a -5.33…(15)

[0062] 6.5≤MFD / λ cc ≤8.5≤0.443Λ a -5.33…(16)

[0063] 6.5≤MFD / λ cc ≤9.0≤0.443Λ a -5.33…(17)

[0064] 6.5≤MFD / λ cc ≤9.5≤0.443Λ a -5.33…(18)

[0065] , which is defined by this.

[0066] The sixth condition is that the XT between adjacent cores, which corresponds to an optical fiber length of 10 km at a wavelength of 1360 nm, is less than -20 dB, and that each of the four cores satisfies the following equation (19):

[0067] CD nominal ≥9.37MFD / λ cc +31.73…(19)

[0068] , and in each of the four cores, the ratio of MFD / λ cc The center spacing of adjacent cores is Λ a Satisfy any of the following equations (20) to (24):

[0069] 6.5≤MFD / λ cc ≤7.5≤0.392Λ a -4.88…(20)

[0070] 6.5≤MFD / λ cc ≤8.0≤0.392Λ a -4.88…(21)

[0071] 6.5≤MFD / λ cc ≤8.5≤0.392Λ a -4.88…(22)

[0072] 6.5≤MFD / λ cc ≤9.0≤0.392Λ a -4.88…(23)

[0073] 6.5≤MFD / λ cc ≤9.5≤0.392Λ a -4.88…(24)

[0074] , which is defined by this.

[0075] The MCF with the above structure is a square-shaped 4-core fiber with a standard cladding diameter, which has excellent optical characteristics in the O band while ensuring sufficient mass production tolerance.cc The relationship (MFD / λ cc ) satisfies the above equations (12) and (13), thereby suppressing the leakage loss of the outermost core to the cladding at a wavelength of 1360nm to less than 0.01dB / km. In addition, when the MCF satisfies the above fifth condition, it is also possible to ensure MFD / λ cc The tolerance of MFD / λcc is guaranteed, and the yield is high during mass production of optical fibers. It can suppress the total amount of XT of the opposite transmission to the specified core at a wavelength of 1360nm or less, which is equivalent to a fiber length of 10km, to below -20dB. Moreover, when the MCF satisfies the sixth condition mentioned above, the tolerance of MFD / λcc can also be guaranteed, and the yield is high during mass production of optical fibers. It can suppress the total amount of XT of the opposite transmission to the specified core at a wavelength of 1360nm or less, which is equivalent to a fiber length of 10km, to below -40dB. The degradation of the optical characteristics of the long-wavelength C-band (above 1530nm and below 1565nm) and the L-band (above 1565nm and below 1625nm) is allowed, and thus it is an MCF with preferred optical characteristics in the O-band, which can achieve a large tolerance. It can also guarantee MFD / λ cc Furthermore, it is possible to maintain a high yield during mass production of optical fibers and suppress the total counter-transmission XT to a specified core within a 10 km optical fiber length to below -40 dB at wavelengths below 1360 nm.

[0076] (3) As one embodiment of the present invention, the MCF further includes a cladding portion, for example, made of resin, surrounding the common cladding. In the above-described structure, preferably, the leakage loss from at least one of the four cores to the cladding portion at a wavelength of 1550 nm or a wavelength of 1625 nm is greater than 0.05 dB / km, or the transmission loss of at least one of the four cores at a wavelength of 1550 nm is greater than 0.25 dB / km, or the transmission loss at a wavelength of 1625 nm is greater than 0.25 dB / km. In this case, degradation of the optical characteristics of the long-wavelength C-band (1530 nm to 1565 nm) and the L-band (1565 nm to 1625 nm) is tolerated, thereby achieving an MCF having preferred optical characteristics in the O-band, enabling a large tolerance.

[0077] (4) As one embodiment of the present invention, the MCF preferably satisfies the third or fifth condition above, and the X-T between adjacent cores corresponding to a fiber length of 10 km at a wavelength of 1550 nm is -10 dB or greater. Alternatively, the MCF preferably satisfies the fourth or sixth condition above, and the X-T between adjacent cores corresponding to a fiber length of 10 km at a wavelength of 1550 nm is -20 dB or greater. In this case, degradation of optical characteristics in the long-wavelength C-band and L-band is tolerated, resulting in an MCF having optimal optical characteristics in the O-band, enabling a wide tolerance.

[0078] (5) The MCF cable of the present invention preferably has a plurality of MCFs including an MCF having the structure described above. In addition, as one embodiment of the present invention, an MCF tape formed by bonding a plurality of MCFs including an MCF having the structure described above at intervals can be built in. As one embodiment of the present invention, the MCF cable has the MCF tape built in in a state twisted into a spiral. The transmission capacity can be increased by any method. Furthermore, as one embodiment of the present invention, the MCF cable preferably includes a multi-core optical fiber having an average value of a bending radius along the length direction of the optical fiber of not less than 0.03 m and not more than 0.14 m or not less than 0.14 m and not more than 0.3 m. In this case, the degradation of optical characteristics associated with the increase in bending loss can be effectively suppressed.

[0079] Each aspect listed above in the column [Description of Embodiments of the Present Invention] can be applied to each of all the remaining aspects, or to all combinations of the remaining aspects.

[0080] [Details of the embodiments of the present invention]

[0081] The specific structure of the multi-core optical fiber (MCF) and multi-core optical cable (MCF cable) involved in the present invention is described in detail below with reference to the accompanying drawings. The present invention is not limited to these examples but is defined by the claims, which include all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, identical elements are denoted by the same reference numerals, and duplicate descriptions are omitted.

[0082] Figure 1 These are diagrams showing various structures of the MCF cable of the present invention (including the MCF of the present invention).

[0083] An MCF cable 1A having structure (A) comprises: an outer sheath 300 containing an MCF storage space extending along the length 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 storage space are embedded in the outer sheath 300. Each MCF 100 includes a glass fiber 200, the outer peripheral surface of which is covered with a resin coating. Furthermore, the MCF 100 can form an MCF tape bonded at intervals. In this case, the MCF tape is twisted into a spiral and embedded in the MCF 1A.

[0084] On the other hand, an MCF cable 1B having structure (B) comprises: an outer sheath 500 containing an MCF storage space extending along the length of the MCF cable 1B; a slotted core 600 that divides the MCF storage space into multiple sections; and a plurality of MCFs 100 (MCFs of the present invention). The outer sheath 500 contains the slotted core 600 that divides the MCF storage space into multiple sections. The slotted core 600 is embedded with a tensile strength wire 700 extending along the length of the MCF cable 1B. Multiple MCFs 100 are housed in any of the sections divided by the slotted core 600.

[0085] Figure 2 : is a diagram showing various core configurations of the MCF of the present invention. In particular, Figure 2 The upper part shows a 4-core MCF 100A (MCF100 of the present invention) configured in a manner that 4 cores form a 3-sides equal trapezoid. Figure 2 The middle part shows a 4-core MCF 100B (MCF 100 of the present invention) in which the 4 cores are arranged at square lattice points set at positions offset from the cladding center. Figure 2 The lower part shows a cross-sectional view of a 4-core MCF 100C (MCF 100 of the present invention) in which a mark is also provided in the core configuration in the middle. Figure 2 The figures show, as an example, a combination of a first core 100a that transmits light in a predetermined direction and a second core 100b that transmits light in the opposite direction.

[0086] The MCF 100 of the present invention preferably has the center of the cladding as the axis of symmetry, and the core configuration consisting of the four cores does not have rotational symmetry of more than two times. In this case, even without markings, the core comparison can be performed during connection and when the MCF is rotated and core-aligned. At this time, it is preferably configured so that the center of each of the four cores is linearly symmetrical with the straight line passing through the center of the cladding as the axis of symmetry. Thus, when another MCF is connected to this MCF, none of the end faces of the MCF has polarity, and the cores can be aligned with each other.

[0087] Figure 2 The 4-core MCF 100A shown in the upper portion of FIG. 1 includes a glass fiber 200A and a resin coating 130 covering the glass fiber 200A. The glass fiber 200A has four cores (in this example, a first core 100a and a second core 100b) and a common cladding 120 surrounding the four cores. In the cross section of the 4-core MCF 100A, the three sides have equal lengths Λ nominal 、The length of the remaining side is greater than Λ nominal Four cores are assigned to the four vertices of a sufficiently long triangular equilateral trapezoid (core arrangement pattern 1). In this case, four cores are arranged so as to surround the cladding center (fiber axis AX1). In addition, the center positions of the first core 100a and the second core 100b are arranged within 1.0 μm, preferably within 0.5 μm, and more preferably within 0.25 μm from the vertex of the assigned triangular equilateral trapezoid. The length of the remaining side of the triangular equilateral trapezoid is preferably Λ nominal As a result, the core-to-core XT can be suppressed to a value below the specified value, and the rotational symmetry of more than 2 times is fully lost when observing the end face. In addition, the d of any core coat All are based on the specified d coat,nominal As a benchmark, it is preferred to converge to d coat,nominal -1μm value or more and d coat,nominal In this case, the leakage loss to the cladding portion can be suppressed to a predetermined value or less, and the rotational symmetry of two or more times can be sufficiently lost when observed at the end surface.

[0088] In addition, Figure 2 The example shown in the upper part of the embodiment may also have no structure that serves as a mark except for the core. If there is a structure that serves as a mark except for the core, the manufacturability is degraded in order to realize the structure (for example, in the case of a method of making a hole in the cladding mother material and inserting the core mother material, it is necessary to make a hole as a mark and insert a marker mother material having a different refractive index from the cladding into the hole). By not having a structure that serves as a mark except for the core, Figure 2 The example shown in the upper part of can improve the manufacturability of MCF.

[0089] Figure 2 The 4-core MCF 100B shown in the middle of FIG has a glass fiber 200B and a resin coating 130 covering the glass fiber 200B. The glass fiber 200B has four cores (in this example, including the first core 100a and the second core 100b) and a common cladding 120 surrounding the four cores. In the cross section of the 4-core MCF 100B, the four cores are distributed at a predetermined lattice point spacing Λ nominal And there are 4 pairs of adjacent grid points on the square grid. Figure 2 In the middle of the figure, as an example of core arrangement, an example is shown in which the center of the square lattice is offset from the center of the cladding (which coincides with the fiber axis AX2) (core arrangement pattern 2). However, the center of the square lattice and the center of the cladding may also coincide with each other. In this case, the d of each core coat Become equal, so that the optical characteristics can be made uniform. The center positions of the four cores are each arranged within 1.0μm from the lattice points allocated by the above-mentioned square lattice, preferably within 0.5μm, and more preferably within 0.25μm. Thus, when the four square lattice points are set as the design positions of the core centers, the dimensional tolerance of the core configuration can be allowed 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 the residual stress and the like applied to the cross-section of the four cores is improved, and it can be expected that the optical characteristics of the four cores will also become uniform. In addition, 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 -2.0μm value or more and Λ nominal +2.0μm value range below, preferably in Λ nominal -1.0μm value or more and Λ nominal +1.0μm value or less, more preferably in the range of Λ nominal -0.5μm value or more and Λ nominal The range is below the value of +0.5μm.

[0090] Figure 2 The 4-core MCF 100C shown in the lower part of FIG has a glass fiber 200C and a resin coating 130 covering the glass fiber 200C. The glass fiber 200C includes four cores (in this example, the first core 100a and the second core 100b) and a common cladding 120 surrounding the four cores. Figure 2In the lower part, as an example of a core configuration, an example is shown in which the core configuration consisting of four cores on the cross section of the 4-core MCF 100C is similar to the core configuration of the 4-core MCF 100B shown in the middle, and the center of the square lattice is offset from the center of the cladding (fiber axis AX3). The 4-core MCF 100B and the 4-core MCF 100C differ in that a mark 610 is provided (core configuration pattern 3). In addition, in the example in which the mark 610 is provided as described above, the center of the square lattice and the center of the cladding may also be consistent. In this case, the d of each core coat The refractive index of the mark 610 is preferably different from the refractive index of the common cladding 120 .

[0091] Figure 3 This is a diagram for explaining main terms used in this specification (adjacent relationship, cross-sectional structure around the core, parallel transmission, parallel transmission XT (crosstalk), and counter-transmission XT (crosstalk)).

[0092] (Neighbor Relationship)

[0093] In this specification, the adjacent relationship between cores refers to the cores that have the smallest center-to-center spacing relative to a specific core among the four cores arranged on the cross section of the MCF, and the cores whose center-to-center spacing differs from the smallest center-to-center spacing by 2 μm or less are defined as the cores that are adjacent to the specific core. Figure 3 As shown in FIG, when the core 111 (110a) is set to a specific core, the cores adjacent to the core 111 are the core 112 (110b) and the core 113 (110b). Figure 2 The core configurations of pattern 2 and pattern 3 both configure the cores in a manner that forms a square lattice. Figure 3 As shown, the adjacent relationship does not hold between the core 112 (110b) and the core 113 (110b). However, the core 114 (110a) is adjacent to both the core 112 (110b) and the core 113 (110b).

[0094] (Cross-sectional structure around the core)

[0095] In having Figure 2In the four cores of the present invention, each core (the first core 110a or the second core 110b) has a cross-sectional structure in which a common cladding 120 surrounds the outer periphery of the first core 110a or the second core 110b. The common cladding 120 can be provided in direct contact with the first core 110a or the second core 110b, but an optical cladding 121 can 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 can be provided between the optical cladding 121 and the common cladding 120. Furthermore, the optical cladding 121 is preferably provided for each core and has a relative refractive index difference Δ2 of not less than -0.1% and not more than 0.1% with respect 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 of −0.4% or more and less than 0% with respect to the refractive index of the common cladding layer.

[0096] (Parallel Transmission and Parallel Transmission XT)

[0097] exist Figure 3 The example shown shows three cores in an adjacent relationship (all first cores 110a transmitting light in the same direction). Specifically, an adjacent relationship exists between the left core and the center core, and an adjacent relationship exists between the center core and the right core. In other words, the state in which the cores in an adjacent relationship transmit light in the same direction is described as "parallel transmission." In this case, a normal inter-core XT (parallel transmission XT) occurs between adjacent cores transmitting light in the same direction (cores in an adjacent relationship).

[0098] (Counter-transmission and Counter-transmission XT)

[0099] On the other hand, counterpropagation is when two adjacent cores transmit light in different directions. Figure 3In the example, the left core and the center core are adjacent, but the left core functions as the first core 110a, while the center core functions as the second core 110b, which propagates light in a direction different from that of the first core 110a. The normal XT generated between these left and center cores is unlikely to affect communication quality. Similarly, the right core, which is adjacent to the center core, functions as the first core 110a, and the normal XT generated between these right cores and the center core is unlikely to affect communication quality. As described above, the state in which adjacent cores propagate light in different directions is described as "counter-propagation." However, between the left and right cores (both functioning as the first core 110a), XT affects communication quality through the center core (functioning as the second core 110b). As described above, the XT between cores that propagate light in the same direction, separated by cores that are adjacent and propagate light in opposite directions, is described as "counter-propagation XT."

[0100] In addition, in the following description, reference is made to Figure 3 The examples of "parallel transmission" and "counter-propagation" are described below. However, if the XT between the cores (hereinafter referred to as "adjacent cores") that are adjacent to each other within the optical fiber length L1 is calculated (parallel transmission XT: XT co ) is set to XT co (L1), when XT is expressed in decibels, it can be expressed as the following formula (25):

[0101]

[0102] At a distance 10 times greater, XT increases by 10dB.

[0103] When XT is expressed in decibels, for example, Figure 3 The example of counterpropagation shown is the XT from the right core to the left core across the center core (counterpropagation XT: XT counter ), using parallel transmission between the left core and the center core and between the center core and the right core XT: XT co , which can be expressed as the following formula (26):

[0104] XT counter =2XT co -10log 10 2…(26).

[0105] If the opposite transmission XT within the fiber length L1 is set to XT counter(L1), then when XT is expressed in decibels, the opposite transmission XT within the fiber length L2 is expressed as the following formula (27):

[0106]

[0107] At 10 times the distance, the noise level increases by 20dB.

[0108] XT from the adjacent core to the specified core co Total XT co，tot , if the number of adjacent cores of a given core is set to N, then the following equation (28) is obtained:

[0109] XT co,tot =XT co +10log 10 N…(28).

[0110] From a specific core that is adjacent to the adjacent core of a predetermined core (not adjacent to the predetermined core) to the XT of the predetermined core counter Total XT counter,tot , if the number of specific cores relative to the specified core is set to M, it seems to become the following formula (29):

[0111] XT counter,tot =XT counter +10log 10 M=2XT co -10log 10 2+10log 10 M…(29),

[0112] However, the inventors found that this is not the case. If the number of adjacent cores of the adjacent core n of the specified core (including the specified core) is set to K n , then the total XT counter,tot It becomes the following formula (30):

[0113]

[0114] Therefore, in a 4-core MCF with a core configuration in which 4 cores are arranged on a square lattice (hereinafter referred to as a “square core configuration”), the XT of any core is counter,tot It becomes the following formula (31):

[0115] XT counter,tot =XT counter +10log 10 2=2XT co …(31)

[0116] Therefore, in a 4-core optical fiber with only 3 pairs of adjacent cores (such as a 1×4 core configuration where 4 cores are arranged in a row), the XT counter,tot It becomes the following formula (32):

[0117] XT counter,tot =XT counter +10log 10 1=2XT co -10log 10 2…(32)

[0118] According to the above, in the 4-core MCF with square core configuration, in order to transmit XT after 10km counter,tot [dB] is set to -20dB or less, and the parallel transmission XT (XT) between adjacent cores is converted to the fiber length L [km] co ) is preferably the following formula (33):

[0119]

[0120] The sum of the parallel transmission XT from two adjacent cores to any core becomes the following formula (34):

[0121]

[0122] In the 4-core MCF with square core configuration, in order to transmit XT after 10km counter,tot [dB] is set to -40dB or less, and the parallel transmission XT (XT) between adjacent cores is converted to the fiber length L [km] co ) is preferably the following formula (35):

[0123]

[0124] The sum of the parallel transmission XT from two adjacent cores to any core is preferably the following formula (36):

[0125]

[0126] In a 4-core MCF (such as a 1×4 core configuration where 4 cores are arranged in a row) with only 3 pairs of adjacent cores, in order to increase the XT counter,tot [dB] is set to -20dB or less, and the parallel transmission XT (XT) between adjacent cores is converted to the fiber length L [km] co ) is preferably the following formula (37):

[0127]

[0128] The sum of the parallel transmission XT from two adjacent cores to any core is given by the following formula (38):

[0129]

[0130] In the 4-core MCF with square core configuration, in order to transmit XT after 10km counter,tot [dB] is set to -40dB or less, and the parallel transmission XT (XT) between adjacent cores is converted to the fiber length L [km] co ) is preferably the following formula (39):

[0131]

[0132] The sum of the parallel transmission XT from adjacent cores to any core having two adjacent cores is preferably the following formula (40):

[0133] .

[0134] Next, the distribution structure of MCF applicable to the present invention will be described. Figure 4 This is a diagram showing the refractive index distribution around each core of an MCF applicable to the present 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 silica glass).

[0135] Regarding the core structure of the MCF of the present invention, the refractive index distribution of the core and the optical properties thereof can be appropriately selected according to the application. For example, Figure 4 The refractive index distributions of the patterns (A) to (K) are shown. Figure 4 Here, 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 core. The system is represented by a local coordinate system with the center of each core·Δ=0% as the origin O. The structure may be the same or different between cores.

[0136] Figure 4The illustrated pattern (A) is a step-type refractive index profile, pattern (B) is a ring-type refractive index profile, pattern (C) is a double-step-type refractive index profile, pattern (D) is a gradient-type refractive index profile, and pattern (E) is a droop-type refractive index profile. These can be applied to the core structure of the MCF of the present invention. Furthermore, patterns (F) and (H) with a depressed refractive index profile around the core, patterns (G), (I), and (J) with a raised refractive index profile around the core, and pattern (E) with a matched refractive index profile around the core can also be applied to the core structure.

[0137] Refractive index distributions other than the step-type refractive index distribution of pattern (A) can be approximated using ESI (Equivalent-step-index) to determine the core radius a and the core Δ(Δ1) after the step-type approximation (non-patent document 3 mentioned above).

[0138] The above-mentioned non-patent document 3 can be easily applied when the boundary between the core and the cladding is clear, but it is difficult to apply when the boundary between the core and the cladding (common cladding 120 or optical cladding 121) is not clear, such as the droop-type refractive index distribution of pattern (E). For example, if b of pattern (E) is regarded as the radius of the core and the method of the above-mentioned non-patent document 3 is directly applied, the ESI approximation is poor. In such a case, it is preferable to regard r corresponding to Δ of 2 / 5 of Δ at a specific r as the core radius a and apply the above-mentioned non-patent document 3, 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 the simple average of Δ in the range from a to b using the following formula (41):

[0139]

[0140] Alternatively, the weighted average of r is given by the following equation (42):

[0141]

[0142] The values ​​obtained can be calculated based on the calculations in Non-Patent Document 3 to determine a and Δ1 (the maximum relative refractive index difference between the first and second cores 110a and 110b). Δ2 (the relative refractive index difference of the optical cladding 121) is preferably not less than -0.10% and not more than 0.10%. This significantly improves manufacturability.

[0143] A trench layer 122 having a lower refractive index than the optical cladding 121 and the common cladding 120 may be provided around the optical cladding 121. Figure 4 Pattern (K). However, if the relative refractive index difference Δ3 of the trench layer 122 relative to the refractive index of the common cladding layer 120 is -0.5% or less, manufacturability is significantly degraded. Therefore, Δ3 is preferably ≥ -0.4%, more preferably ≥ -0.3%, and most preferably ≥ -0.2%. Furthermore, from the perspective of manufacturability, the absence of a trench layer is more preferable.

[0144] Regarding the materials for the core and cladding (optical cladding 121 or common cladding 120), glass made of silica glass as the main component can achieve low transmission loss and high mechanical reliability, so it is preferred. Ge is preferably added to the core, thereby creating a refractive index difference between the core and cladding. Alternatively, F is preferably added to the cladding, thereby creating a refractive index difference between the core and cladding. Adding a small amount of F to the core and optical cladding improves manufacturability and enables a depressed type distribution, so it is preferred. Cl can be added to the core and cladding. This can suppress OH groups and the absorption loss caused by OH groups. The core and cladding can contain a small amount of P. This can improve manufacturability in some glass synthesis processes.

[0145] have Figure 2 The MCF of the present invention, shown in the cross-sectional structure, includes a resin coating 130. The diameter of this resin coating 130 is preferably within a range of 235 μm to 265 μm, based on a diameter of 250 μm. This allows the MCF of the present invention to be cabled without requiring major modifications to existing cable-making equipment.

[0146] In a typical general-purpose SMF, the nominal value of the cladding diameter CD nominal The nominal value of the coating diameter is 245 μm to 250 μm, but in order to increase the number of optical fibers accommodated per unit cross-section in the cable, the coating diameter is preferably 160 μm to 230 μm.

[0147] As described above, the MCF of the present invention is a 4-core MCF. The number of cores is an even number and is a power of 2, which is suitable for communication as a spatial channel number.

[0148] In addition, in the MCF of the present invention, the configuration of the centers of the four cores (in essence, the core configuration) is linearly symmetrical with a straight line passing through the center of the cladding as an axis of symmetry, and preferably does not have more than one rotational symmetry. Thus, even without a mark, the cores can be aligned when the optical fibers are connected or the MCF is rotated and core-aligned. At this time, it is preferably configured so that the centers of the above-mentioned four cores are linearly symmetrical with a straight line passing through the center of the cladding as an axis of symmetry. Thus, when the MCF is connected to another MCF, any end face of the MCF can mutually adjust the cores without polarity.

[0149] For example, Figure 2 In the example shown in the upper part, the MCF also has equal lengths Λ on three sides in the cross section. nominal 、The length of the remaining side is greater than Λ nominal The four vertices of the sufficiently long trilateral trapezoid are each provided with a core. In this case, the center position of each core is arranged within 1.0 μm, preferably within 0.5 μm, and more preferably within 0.25 μm from the corresponding vertex of the trilateral trapezoid. The length of the remaining side of the trilateral trapezoid is preferably Λ nominal As a result, the XT between the cores can be suppressed to a predetermined value or less, and the rotational symmetry of two or more times can be sufficiently lost when observing the end face.

[0150] In addition, it is preferred that the d of any core coat The specified nominal value d coat,nominal As the benchmark, it converges to d coat,nominal -1μm value or more and d coat,nominal Thereby, the leakage loss to the cladding portion can be suppressed to a predetermined value or less, and the rotational symmetry of two or more times can be sufficiently lost when observed at the end surface.

[0151] The MCF of the present invention preferably has no structure other than the core that serves as a marker. This is because having a structure other than the core that serves as a marker degrades manufacturability in order to implement this structure. For example, in a method in which a hole is opened in the cladding matrix and the core matrix is ​​inserted, it is necessary to insert a marker matrix (matrix that serves as a marker) having a different refractive index from the cladding into the hole. Conversely, having no structure other than the core that serves as a marker improves the manufacturability of the MCF of the present invention.

[0152] Alternatively, as shown in the middle of the MCF of the present invention, there may be a core configuration in which there are four pairs of adjacent cores among the four cores, i.e., a square lattice configuration. In this case, the rotational symmetry of the cladding center is fully lost, so the center position of the square lattice is offset from the cladding center. The centers of the four cores are spaced from the lattice point with a predetermined spacing Λ. nominal The four lattice points of the square lattice are arranged within 1.0 μm, preferably within 0.5 μm, and more preferably within 0.25 μm. Therefore, when the four lattice points of the square lattice are set as the design position of the core center, the dimensional tolerance of the core configuration can be allowed and the deviation of the core configuration can be suppressed. In addition, the four-core configuration of the square lattice is similar to Figure 2Compared with the three equilateral trapezoidal core arrangement shown in the upper part of FIG, the residual stress etc. applied to the cross section of the four cores becomes uniform, and the optical characteristics of the four cores also become uniform, which is preferable. In addition, the above structure can also be said to be approximately arranged in a square lattice shape with the center spacing Λ between adjacent cores converging to Λ nominal -2.0μm value or more and Λ nominal The range below the value of +2.0μm is preferably converged to Λ nominal -1.0μm value or more and Λ nominal The range below the value of +1.0μm is more preferably converged to Λ nominal -0.5μm value or more and Λ nominal The range is below the value of +0.5μm.

[0153] Furthermore, in the MCF of the present invention, as Figure 2 If the above conditions are summarized, it is preferred that there are three to four pairs of cores in an adjacent core relationship among the four cores, and the centers of the four cores are arranged so as to be linearly symmetrical about a line passing through the center of the cladding and not passing through the center of any core as the axis of symmetry. Moreover, when there are only three pairs of cores in an adjacent core relationship, the arrangement of the centers of the cores about the cladding center as the axis of symmetry does not have rotational symmetry of two or more times.

[0154] In addition, the MCF cable of the present invention preferably has a plurality of MCFs including an MCF having the structure described above. As an example, the MCF cable may be built with an MCF tape formed by bonding a plurality of MCFs including an MCF having the structure described above at intervals. The MCF cable has the MCF tape built in in a spirally twisted state. Depending on which structure, an increase in transmission capacity can be achieved. Furthermore, the MCF cable preferably includes a multi-core optical fiber having an average value of a bending radius along the length direction of the optical fiber of not less than 0.03 m and not more than 0.14 m or not less than 0.14 m and not more than 0.3 m. In this case, the degradation of optical characteristics associated with the increase in bending loss can be effectively suppressed.

[0155] Each core of the MCF of the present invention preferably has an MFD that is within the range of MFD reference value - 0.4 μm and MFD reference value + 0.4 μm at a wavelength of 1310 nm, with the MFD reference value being 8.6 μm or more and 9.2 μm or less. In this case, the MFD is equivalent to the nominal value MFD of the general SMF specified in ITU-TG.652. nominal Small (MFD nominalCompared with the connection loss between general SMFs of the type with reduced bending loss (≒8.6μm), the connection loss caused by axis offset between MCFs of the present invention (when a predetermined axis offset is given) can be suppressed to a level equal to or lower than that.

[0156] Each core of the MCF of the present invention preferably has an MFD within the range of 8.2 μm to 9.0 μm at a wavelength of 1310 nm, with 8.6 μm as the reference. Consequently, when connecting a general-purpose SMF specified in ITU-TG.652, which has a low nominal MFD and suppresses bending losses, to the MCF of the present invention, the connection loss caused by core center axis deviation (axial offset) can be made equivalent (when the specified axial offset is applied).

[0157] The MCF of the present invention preferably has a zero-dispersion wavelength of 1300 nm to 1324 nm, thereby suppressing distortion of the signal waveform after transmission in the O-band to the same degree as a general-purpose SMF.

[0158] The MCF of the present invention preferably has a zero-dispersion wavelength that falls within a range of -12 nm to +12 nm, using a wavelength reference value of 1312 nm to 1340 nm as a predetermined wavelength reference value. This reduces distortion in the signal waveform after transmission in the O-band compared to conventional SMFs (see Non-Patent Document 4).

[0159] In the operating band of the MCF of the present invention, it is preferred that the sum of XT from adjacent cores to any core is still below -20 dB after 10 km transmission. The XT from other than adjacent cores is sufficiently low and can be ignored, so a sufficient signal-to-noise ratio can be achieved even when coherent detection is performed. In addition, in the operating band of the MCF of the present invention, it is preferred that the sum of XT from adjacent cores to any core is still below -40 dB after 10 km transmission. The XT from other than adjacent cores is sufficiently low and can be ignored, so a sufficient signal-to-noise ratio can be achieved even when intensity modulation direct detection is performed. In the operating band of the MCF of the present invention, it is preferred that the parallel transmission XT is still below -10.0 dB after 10 km transmission. As a result, the opposite transmission XT can be set to below -20 dB after 10 km transmission. In addition, in the operating band of the MCF of the present invention, it is preferred that the parallel transmission XT is still below -20.0 dB after 10 km transmission. As a result, the counter-transmission XT can be kept below -40 dB even after 10 km of transmission.

[0160] In the following description, the Figure 4The research results on MCF are as follows: the core of the refractive index distribution of pattern (E), pattern (H) and pattern (J), and a is greater than 3μm and less than 5μm, Δ1-Δ2 is greater than 0.3% and less than 0.6%, Δ2 is greater than -0.1% and less than 0.1%, and b / a is greater than 2 and less than 5.

[0161] By calculating the wavelength dependence of the electric field distribution and effective refractive index of the fundamental mode using the finite element method or other methods, those skilled in the art can design a core structure with a predetermined zero-dispersion wavelength and MFD. For example, the relationship between a and (Δ1-Δ2) that achieves the zero-dispersion wavelength λ0 [μm] is in the range of 3μm ≤ a ≤ 5μm and 0.3% ≤ (Δ1-Δ2) ≤ 0.6%, as shown in the following equation (43):

[0162] a≈0.0667(λ0-1343.1)(Δ1-Δ2) 2 +0.0900(λ0-1354.6)(Δ1-Δ2)-0.0517(λ0-1411.2)…(43)

[0163] Therefore, in order for the zero dispersion wavelength λ0 [μm] to converge to λ 0nominal -12nm or more and λ 0nominal In the range below the value of +12 nm, the relationship between a and (Δ1-Δ2) preferably satisfies the following equations (44) and (45):

[0164] 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)…(44)

[0165] 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)…(45)

[0166] These two styles.

[0167] Furthermore, the relationship between a and (Δ1-Δ2) corresponding to the MFD [μm] at a wavelength of 1310 nm is expressed as the following equation (46) in the range of 3 μm ≤ a ≤ 5 μm and 0.3% ≤ (Δ1-Δ2) ≤ 0.6%:

[0168] (Δ1-Δ2)=(-0.0148MFD+0.213)[a-0.619MFD+2.01] 2 -0.0771MFD+1.033…(46)

[0169] Therefore, in order for MFD [μm] to converge to MFD nominal -0.4μm value or more and MFD nominal In the range below the value of +0.4 μm, the relationship between a and (Δ1-Δ2) preferably satisfies the following equations (47) and (48):

[0170] (Δ1-Δ2)≤[-0.0148(MFD nominal +0.4)+0.213][a-0.619(MFD nominal +0.4)+2.01] 2 ...-0.0771(MFD nominal +0.4)+1.033 (47)

[0172] (Δ1-Δ2)≥[-0.0148(MFD nominal -0.4)+0.213][a-0.619(MFD nominal -0.4)+2.01] 2 ...-0.0771(MFD nominal -0.4)+1.033 (48)

[0174] These two styles.

[0175] b / a and Δ2 can be set to λ cc The wavelength is 1260nm or less or 1360nm or less, and the zero dispersion slope is 0.092ps / (nm 2 Therefore, it is preferable that Δ2 is in the range of -0.1% to 0.0%, and b / a is in the range of 2 to 4.

[0176] Next, a preferred center interval Λ between adjacent cores will be described. Figure 4 In a 4-core MCF with a square core configuration, the center spacing Λ and MFD / λ between adjacent cores are expressed when the opposite transmission XT at a wavelength of 1360nm becomes -20dB (= -20dB / 10km) after 10km of transmission (equivalent to a fiber length of 10km). cc Here, the average value R of the optical fiber bending radius is 0.14m. If R is less than 0.14m, a lower XT can be achieved. In addition, λ ccThis is the cable cutoff wavelength measured using the structure of Figure 12 in ITU-T G.650.1 (03 / 2018) (non-cabled optical fiber).

[0177] In order to set the mutual transmission XT below -20dB (= -20dB / 10km) after 10km transmission at a wavelength of 1360nm, the center spacing Λ between adjacent cores and MFD / λ cc At least the following formula (49) or formula (50) is satisfied:

[0178] Λ≥2.26MFD / λ cc +12.0…(49)

[0179] MFD / λ cc ≤0.443Λ-5.33…(50)

[0180] ( Figure 5 ), and preferably satisfies the following formula (51) or formula (52):

[0181] Λ≥2.26MFD / λ cc +14.5…(51)

[0182] MFD / λ cc ≤0.443Λ-6.42…(52)

[0183] ( Figure 5 (the area above the upper dotted line shown in ).

[0184] Figure 6 The values ​​for the center spacing Λ and MFD / λ between adjacent cores are shown when the opposite transmission XT reaches -20dB after 10km of transmission (equivalent to a fiber length of 10km) through a 4-core MCF at both wavelengths of 1550nm and 1360nm, and when the parallel transmission XT reaches -20dB after 10km of transmission (equivalent to a fiber length of 10km) through a 4-core MCF. cc In addition, Figure 6 In the example, the symbol “○” represents the above relationship related to the parallel transmission XT at a wavelength of 1550 nm, and the symbol “● (in Figure 6 The symbol "□" represents the above relationship related to the parallel transmission XT at a wavelength of 1360 nm, the symbol "□" represents the above relationship related to the counter-propagation XT at a wavelength of 1550 nm, and the symbol "■ (in Figure 6 (shown by diagonal lines in the middle)" represents the above relationship related to the counter-transmission XT in the case of a wavelength of 1360 nm.

[0185] In this Figure 6The dashed line is not shown, but the Figure 5 Similarly, in order to set the parallel transmission XT below -20dB after 10km transmission at a wavelength of 1360nm, the center spacing Λ between adjacent cores and MFD / λ cc At least the following formula (53) or formula (54) is satisfied:

[0186] Λ≥2.64MFD / λ cc +12.6…(53)

[0187] MFD / λ cc ≤0.379Λ-4.76…(54)

[0188] , and preferably satisfies the following formula (55) or formula (56):

[0189] Λ≥2.64MFD / λ cc +15.0…(55)

[0190] MFD / λ cc ≤0.379Λ-5.68…(56)

[0191] At a wavelength of 1550nm, in order to set the opposite transmission XT to less than -20dB after 10km transmission, the center spacing Λ between adjacent cores and MFD / λ cc At least the following formula (57) or formula (58) is satisfied:

[0192] Λ≥3.13MFD / λ cc +10.7…(57)

[0193] MFD / λ cc ≤0.320Λ-3.42…(58)

[0194] , and preferably satisfies the following formula (59) or formula (60):

[0195] Λ≥3.13MFD / λ cc +13.4…(59)

[0196] MFD / λ cc ≤0.320Λ-4.29…(60)

[0197] At a wavelength of 1550nm, in order to set the parallel transmission XT after 10km transmission to less than -20dB, the center spacing Λ between adjacent cores and MFD / λ cc At least the following formula (61) or formula (62) is satisfied:

[0198] Λ≥3.66MFD / λ cc +10.8…(61)

[0199] MFD / λ cc ≤0.273Λ-2.95…(62)

[0200] , and preferably satisfies the following formula (63) or formula (64):

[0201] Λ≥3.66MFD / λ cc +13.7…(63)

[0202] MFD / λ cc ≤0.273Λ-3.74…(64)

[0203] Figure 7 The values ​​for the center spacing Λ and MFD / λ between adjacent cores are shown when the opposite transmission XT reaches -40dB after 10km of transmission (equivalent to a fiber length of 10km) through a 4-core MCF at both wavelengths of 1550nm and 1360nm, and when the parallel transmission XT reaches -40dB after 10km of transmission (equivalent to a fiber length of 10km) through a 4-core MCF. cc In addition, Figure 7 In the example, the symbol “○” represents the above relationship related to the parallel transmission XT at a wavelength of 1550 nm, and the symbol “● (in Figure 7 The symbol "□" represents the above relationship related to the parallel transmission XT at a wavelength of 1360 nm, the symbol "□" represents the above relationship related to the counter-propagation XT at a wavelength of 1550 nm, and the symbol "■ (in Figure 7 (shown by diagonal lines in the middle)" represents the above relationship related to the counter-transmission XT in the case of a wavelength of 1360 nm.

[0204] In this Figure 7 The dashed line is not shown, but the Figure 5 Similarly, in order to set the mutual transmission XT after 10 km transmission to less than -40dB at a wavelength of 1360nm, the center spacing Λ between adjacent cores and MFD / λ cc At least the following formula (65) or formula (66) is satisfied:

[0205] Λ≥2.55MFD / λ cc +12.4…(65)

[0206] MFD / λ cc ≤0.392Λ-4.88…(66)

[0207] , and preferably satisfies the following formula (67) or formula (68):

[0208] Λ≥2.55MFD / λ cc+14.9…(67)

[0209] MFD / λ cc ≤0.392Λ-5.83…(68)

[0210] In order to set the parallel transmission XT below -40dB after 10km transmission at a wavelength of 1360nm, the center spacing Λ between adjacent cores and MFD / λ cc At least the following formula (69) or formula (70) is satisfied:

[0211] Λ≥3.22MFD / λ cc +13.4…(69)

[0212] MFD / λ cc ≤0.310Λ-4.16…(70)

[0213] , and preferably satisfies the following formula (71) or formula (72):

[0214] Λ≥3.22MFD / λ cc +15.7…(71)

[0215] MFD / λ cc ≤0.310Λ-4.88…(72)

[0216] In order to set the mutual transmission XT below -40dB after 10km transmission at a wavelength of 1550nm, the center spacing Λ between adjacent cores and MFD / λ cc At least the following formula (73) or formula (74) is satisfied:

[0217] Λ≥3.54MFD / λ cc +10.8…(73)

[0218] MFD / λ cc ≤0.283Λ-3.05…(74)

[0219] , and preferably satisfies the following formula (75) or formula (76):

[0220] Λ≥3.54MFD / λ cc +13.6…(75)

[0221] MFD / λ cc ≤0.283Λ-3.85…(76)

[0222] In order to set the parallel transmission XT below -40dB after 10km transmission at a wavelength of 1550nm, the center spacing Λ between adjacent cores and MFD / λ ccAt least the following formula (77) or formula (78) is satisfied:

[0223] Λ≥4.47MFD / λ cc +11.0…(77)

[0224] MFD / λ cc ≤0.223Λ-2.46…(78)

[0225] , and preferably satisfies the following formula (79) or formula (80):

[0226] Λ≥4.47MFD / λ cc +14.1…(79)

[0227] MFD / λ cc ≤0.223Λ-3.16…(80)

[0228] In order to allow the position of each core to fluctuate from the design center, it is preferable to take 1 μm from the range of the above formulas as the margin of Λ. Therefore, in order to set the mutual transmission XT after 10 km transmission at a wavelength of 1360 nm to less than -20 dB, the nominal value of Λ is Λ nominal It is preferred to satisfy at least the following formula (81):

[0229] Λ nominal ≥2.26MFD / λ cc +12.0+1.0…(81)

[0230] , more preferably satisfies the following formula (82):

[0231] Λ nominal ≥2.26MFD / λ cc +14.5+1.0…(82)

[0232] In order to set the parallel transmission XT below -20dB after 10km transmission at a wavelength of 1360nm, the nominal value Λ nominal It is preferred to satisfy at least the following formula (83):

[0233] Λ nominal ≥2.64MFD / λ cc +12.6+1.0…(83)

[0234] , more preferably satisfies the following formula (84):

[0235] Λ nominal ≥2.64MFD / λ cc +15.0+1.0…(84)

[0236] In order to set the mutual transmission XT after 10 transmissions at a wavelength of 1550nm to less than -20dB, the nominal value Λ nominal It is preferred to satisfy at least the following formula (85):

[0237] Λ nominal ≥3.13MFD / λ cc +10.7+1.0…(85)

[0238] , more preferably satisfies the following formula (86):

[0239] Λ nominal ≥3.13MFD / λ cc +13.4+1.0…(86)

[0240] In order to set the parallel transmission XT below -20dB after 10km transmission at a wavelength of 1550nm, the nominal value Λ nominal It is preferred to satisfy at least the following formula (87):

[0241] Λ nominal ≥3.66MFD / λ cc +10.8+1.0…(87)

[0242] , more preferably satisfies the following formula (88):

[0243] Λ nominal ≥3.66MFD / λ cc +13.7+1.0…(88)

[0244] In order to set the opposite transmission XT at a wavelength of 1360nm to below -40dB, the nominal value Λ nominal It is preferred to satisfy at least the following formula (89):

[0245] Λ nominal ≥2.55MFD / λ cc +12.4+1.0…(89)

[0246] , more preferably satisfying the following formula (90):

[0247] Λ nominal ≥2.55MFD / λ cc +14.9+1.0…(90)

[0248] In order to set the parallel transmission XT below -40dB after 10km transmission at a wavelength of 1360nm, the nominal value Λ nominal It is preferred to satisfy at least the following formula (91):

[0249] Λ nominal ≥3.22MFD / λ cc+13.4+1.0…(91)

[0250] , more preferably satisfies the following formula (92):

[0251] Λ nominal ≥3.22MFD / λ cc +15.7+1.0…(92)

[0252] In order to set the mutual transmission XT below -40dB after 10km transmission at a wavelength of 1550nm, the nominal value Λ nominal It is preferred to satisfy at least the following formula (93):

[0253] Λ nominal ≥3.54MFD / λ cc +10.8+1.0…(93)

[0254] , more preferably satisfies the following formula (94):

[0255] Λ nominal ≥3.54MFD / λ cc +13.6+1.0…(94)

[0256] In order to set the parallel transmission XT below -40dB after 10km transmission at a wavelength of 1550nm, the nominal value Λ nominal It is preferred to satisfy at least the following formula (95):

[0257] Λ nominal ≥4.47MFD / λ cc +11.0+1.0…(95)

[0258] , more preferably satisfies the following formula (96):

[0259] Λ nominal ≥4.47MFD / λ cc +14.1+1.0…(96)

[0260] Relative to the above Λ nominal , Λ is preferably the following formula (97):

[0261] Λ nominal -0.9≤Λ≤Λ nominal +0.9…(97)

[0262] This is an approximation of the case where the position of each core can be independently considered to fluctuate as a Gaussian distribution with a 3σ=0.9μm value from the design center as a probability distribution. In this case, Λ does not satisfy the requirement for Λ nominal The probability of any of the above-defined equations (81) to (96) being true is suppressed to 1% or less.nominal , Λ preferably satisfies the following formula (98):

[0263] Λ nominal -0.7≤Λ≤Λ nominal +0.7…(98)

[0264] This is an approximation of the case where the position of each core can be independently considered to fluctuate as a Gaussian distribution with a 3σ=0.7μm value from the design center as a probability distribution. In this case, Λ does not satisfy the requirement for Λ nominal The probability of any of the above-defined equations (81) to (96) being true is suppressed to 0.1% or less. nominal , Λ preferably satisfies the following formula (99):

[0265] Λ nominal -0.5≤Λ≤Λ nominal +0.5…(99)

[0266] This is an approximation of the case where the position of each core can be independently considered to fluctuate as a Gaussian distribution with a 3σ=0.5μm value from the design center as a probability distribution. In this case, Λ does not satisfy the requirement for Λ nominal The probability of any of the defined equations (81) to (96) is suppressed to 0.001% or less.

[0267] Next, regarding the preferred d coat (The shortest distance from the interface between the resin coating and the cladding to the core center) will be described. Figure 8 The d value is when the leakage loss to the cladding becomes 0.01dB / km in a 4-core MCF with a wavelength of 1360nm. coat and MFD / λ cc Graphic of the relationship.

[0268] In order to set the leakage loss to the resin coating at 0.01 dB / km at a wavelength of 1360 nm, d coat and MFD / λ cc Satisfy the following formula (100) or formula (101):

[0269] d coat ≥2.88MFD / λ cc +5.36…(100)

[0270] MFD / λ cc ≤0.347d coat -1.86…(101)

[0271] (from Figure 8 The area above the dotted line on the lower side shown). And, dcoat and MFD / λ cc It is preferred to satisfy the following formula (102) or formula (103):

[0272] d coat ≥2.88MFD / λ cc +6.95…(102)

[0273] MFD / λ cc ≤0.347d coat -2.41…(103)

[0274] (from Figure 8 the area above the dotted line on the upper side shown).

[0275] d of the outermost core coat (ie d coat The minimum value of d is usually called the outer cladding thickness (OCT), but the d coat It is defined as a value that can be specified for each core.

[0276] In order to allow the position of each fiber core to fluctuate from the design center and the cladding diameter to fluctuate from the design center, d coat It is preferable that the range from the above formula (100) to the formula (103) has a margin of at least 1 μm. coat If set to d coat The nominal value d coat,nominal , then at least the following formula (104) is satisfied:

[0277] d coat,nominal ≥2.88MFD / λ cc +5.36+1.0…(104)

[0278] And, the nominal value of the cladding diameter CD nominal It is preferably set to satisfy the following formula (105):

[0279] d coat,nominal ≥2.88MFD / λ cc +6.95+1.0…(105)

[0280] At this time, it is preferred to satisfy the following equations (106) and (107):

[0281] Λ nominal -0.9≤Λ≤Λ nominal +0.9…(106)

[0282] CD nominal -0.9≤CD≤CD nominal +0.9…(107)

[0283] These two formulas, d coat The probability of not satisfying equation (100) or equation (102) is suppressed to less than 1%. In addition, it is preferable to satisfy the following equations (108) and (109):

[0284] Λ nominal -0.7≤Λ≤Λ nominal +0.7…(108)

[0285] CD nominal -0.7≤CD≤CD nominal +0.7…(109)

[0286] These two formulas. At this time, d coat The probability of not satisfying equation (100) or equation (102) is suppressed to less than 0.1%. In addition, it is preferable to satisfy the following equations (110) and (111):

[0287] Λ nominal -0.5≤Λ≤Λ nominal +0.5…(110)

[0288] CD nominal -0.5≤CD≤CD nominal +0.5…(111)

[0289] These two formulas. At this time, d coat The probability of not satisfying the formula (100) or the formula (102) is suppressed to 0.001% or less.

[0290] Next, the minimum CD that can be allowed nominal Provide explanation. Figure 9 The d value is the value when the leakage loss to the cladding becomes 0.01dB / km at a wavelength of 1360nm in a 4-core MCF. coat Adding a 1μm margin, and adding a 1μm margin, the opposite transmission XT at a wavelength of 1360nm after 10km of transmission (equivalent to a fiber length of 10km) becomes -20dB (= -20dB / 10km). When the CD and MFD / λ are cc In addition, Figure 9 , if we express MFD / λ cc Assuming that the axis representing CD is the x-axis and the y-axis, the upper dotted line is represented by y=8.95x+37.47 (x=0.1117y-4.186), and the lower dotted line is represented by y=8.95x+31.13 (x=0.1117y-3.478).

[0291] Taking into account the dimensional tolerances of the core position and cladding diameter, in order to reduce the leakage loss to the cladding to less than 0.01dB / km at a wavelength of 1360nm and to reduce the mutual transmission XT after 10km transmission to less than -20dB, CD nominal and MFD / λ cc The relationship satisfies the following formula (112) or formula (113):

[0292] CD nominal ≥8.95MFD / λ cc +31.13…(112)

[0293] MFD / λ cc ≤0.1117CD nominal -3.478…(113)

[0294] ( Figure 9 ), and preferably satisfies the following formula (114) or formula (115):

[0295] CD nominal ≥8.95MFD / λ cc +37.47…(114)

[0296] MFD / λ cc ≤0.1117CD nominal -4.186…(115)

[0297] ( Figure 9 (the area above the upper dotted line in the figure).

[0298] Figure 10 This represents the d when the leakage loss to the cladding is 0.01 dB / km under the conditions that the counter-propagation XT after 10 km transmission (equivalent to a fiber length of 10 km) in a 4-core MCF is -20 dB (= -20 dB / 10 km), and the parallel propagation XT (normal co-propagation XT) after 10 km transmission (equivalent to a fiber length of 10 km) is -20 dB (= -20 dB / 10 km). coat Add a margin of 1 μm, and in the case of Λ plus a margin of 1 μm, CD (minimum allowable cladding diameter) and MFD / λ cc In addition, Figure 10 In the example, the symbol “○” indicates the above relationship with the parallel transmission XT at a wavelength of 1550 nm, and the symbol “● (in Figure 10The symbol "□" indicates the above relationship with the parallel transmission XT at a wavelength of 1360 nm, the symbol "□" indicates the above relationship with the counter-propagation XT at a wavelength of 1550 nm, and the symbol "■ (in Figure 10 (shown by slashes in the middle)" represents the above relationship with the counter transmission XT at a wavelength of 1360 nm.

[0299] In this Figure 10 There is no record of dotted lines, but Figure 5 Similarly, in a 4-core MCF with a square core configuration, taking into account the dimensional tolerances of the core position and cladding diameter, in order to reduce the leakage loss to the cladding at a wavelength of 1360nm to less than 0.01dB / km and to reduce the parallel transmission XT after 10km transmission to less than -20dB, CD nominal and MFD / λ cc The relationship satisfies the following formula (116) or formula (117):

[0300] CD nominal ≥9.49MFD / λ cc +31.91…(116)

[0301] MFD / λ cc ≤0.1054CD nominal -3.363…(117)

[0302] , and preferably satisfies the following formula (118) or formula (119):

[0303] CD nominal ≥9.49MFD / λ cc +38.16…(118)

[0304] MFD / λ cc ≤0.1054CD nominal -4.021…(119)

[0305] In order to reduce the leakage loss to the cladding at a wavelength of 1550nm to less than 0.01dB / km and reduce the counter transmission XT after 10km transmission to less than -20dB, CD nominal and MFD / λ cc The relationship satisfies the following formula (120) or formula (121):

[0306] CD nominal ≥12.56MFD / λ cc +24.30…(120)

[0307] MFD / λ cc ≤0.07960CD nominal-1.934…(121)

[0308] , and preferably satisfies the following formula (122) or formula (123):

[0309] CD nominal ≥12.56MFD / λ cc +33.78…(122)

[0310] MFD / λ cc ≤0.07960CD nominal -2.688…(123)

[0311] In order to reduce the leakage loss to the cladding at a wavelength of 1550nm to less than 0.01dB / km and to reduce the parallel transmission XT after 10km transmission to less than -20dB, CD nominal and MFD / λ cc The relationship satisfies the following formula (124) or formula (125):

[0312] CD nominal ≥13.31MFD / λ cc +24.47…(124)

[0313] MFD / λ cc ≤0.07511CD nominal -1.838…(125)

[0314] , and preferably satisfies the following formula (126) or formula (127):

[0315] CD nominal ≥13.31MFD / λ cc +34.18…(126)

[0316] MFD / λ cc ≤0.07511CD nominal -2.567…(127)

[0317] In a 4-core fiber with a square core configuration, taking into account the dimensional tolerances of the core position and cladding diameter, in order to reduce the leakage loss to the cladding to less than 0.01dB / km at a wavelength of 1360nm and to reduce the relative transmission XT to less than -20dB after 10km of transmission, the CD nominal When 125μm, 120μm, 115μm, 110μm, 105μm, 100μm, 95μm, 90μm, 85μm, 80μm, MFD / λ cc Follow the CD nominalThe order of listing the values ​​of is preferably 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, 5.46 or less, and MFD / λ cc Follow the CD nominal The order of listing the numerical values ​​is preferably 9.78 or less, 9.22 or less, 8.66 or less, 8.10 or less, 7.54 or less, 6.99 or less, 6.43 or less, 5.87 or less, 5.31 or less, and 4.75 or less.

[0318] In order to reduce the leakage loss to the cladding at a wavelength of 1360 nm to less than 0.01 dB / km and to reduce the parallel transmission XT after 10 km transmission to less than -20 dB, MFD / λ cc Follow the CD nominal The numerical values ​​of the above enumeration order 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, 5.07 or less, and MFD / λ cc Follow the CD nominal The above-listed order of the numerical values ​​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.

[0319] In order to reduce the leakage loss to the cladding at a wavelength of 1550nm to less than 0.01dB / km and the counter transmission XT after 10km transmission to less than -20dB, MFD / λ cc Follow the CD nominal The order of the values ​​listed above is preferably 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, 4.43 or less, and MFD / λ cc Follow the CD nominal The above-listed order of the numerical values ​​is preferably 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.

[0320] In order to reduce the leakage loss to the cladding at a wavelength of 1550nm to less than 0.01dB / km and to reduce the parallel transmission XT after 10km transmission to less than -20dB, MFD / λ cc Follow the CD nominalThe order of the values ​​listed above is 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, 4.17 or less, and MFD / λ cc Follow the CD nominal The above-listed order of the numerical values ​​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.

[0321] Figure 11 This represents the conditions under which the counter transmission XT becomes -40dB (= -40dB / 10km) after 10km transmission (equivalent to a fiber length of 10km) in a 4-core MCF at both wavelengths of 1550nm and 1360nm, and the parallel transmission XT becomes -40dB (= -40dB / 10km) after 10km transmission (equivalent to a fiber length of 10km), and the leakage loss to the cladding becomes 0.01dB / km. coat Add a margin of 1 μm, and in the case of Λ plus a margin of 1 μm, CD (minimum allowable cladding diameter) and MFD / λ cc In addition, Figure 11 In the example, the symbol “○” indicates the above relationship with the parallel transmission XT at a wavelength of 1550 nm, and the symbol “● (in Figure 11 The symbol "□" indicates the above relationship with the parallel transmission XT at a wavelength of 1360 nm, the symbol "□" indicates the above relationship with the counter-propagation XT at a wavelength of 1550 nm, and the symbol "■ (in Figure 11 (shown by slashes in the middle)" represents the above relationship with the counter transmission XT at a wavelength of 1360 nm.

[0322] In this Figure 11 There is no record of dotted lines, but Figure 5 Similarly, taking into account the tolerance of the core position and cladding diameter, in order to reduce the leakage loss to the cladding at a wavelength of 1360nm to less than 0.01dB / km and the mutual transmission XT after 10km transmission to less than -40dB, CD nominal and MFD / λ cc The relationship satisfies the following formula (128) or formula (129):

[0323] CD nominal ≥9.37MFD / λ cc +31.73…(128)

[0324] MFD / λ cc≤0.1068CD nominal -3.388…(129)

[0325] , and preferably satisfies the following formula (130) or formula (131):

[0326] CD nominal ≥9.37MFD / λ cc +38.00…(130)

[0327] MFD / λ cc ≤0.1068CD nominal -4.058…(131)

[0328] In order to reduce the leakage loss to the cladding at a wavelength of 1360 nm to less than 0.01 dB / km and to reduce the parallel transmission XT after 10 km transmission to less than -40 dB, CD nominal and MFD / λ cc The relationship satisfies the following formula (132) or formula (133):

[0329] CD nominal ≥10.32MFD / λ cc +33.11…(132)

[0330] MFD / λ cc ≤0.09690CD nominal -3.208…(133)

[0331] , and preferably satisfies the following formula (134) or formula (135):

[0332] CD nominal ≥10.32MFD / λ cc +39.23…(134)

[0333] MFD / λ cc ≤0.09690CD nominal -3.802…(135)

[0334] In order to reduce the leakage loss to the cladding at a wavelength of 1550nm to less than 0.01dB / km and reduce the counter transmission XT after 10km transmission to less than -40dB, CD nominal and MFD / λ cc The relationship satisfies the following formula (136) or formula (137):

[0335] CD nominal ≥13.14MFD / λ cc +24.43…(136)

[0336] MFD / λ cc ≤0.07610CD nominal -1.859…(137)

[0337] , and preferably satisfies the following formula (138) or formula (139):

[0338] CD nominal ≥13.14MFD / λ cc +34.09…(138)

[0339] MFD / λ cc ≤0.07610CD nominal -2.594…(139)

[0340] In order to reduce the leakage loss to the cladding at a wavelength of 1550nm to less than 0.01dB / km and to reduce the parallel transmission XT after 10km transmission to less than -40dB, CD nominal and MFD / λ cc The relationship satisfies the following formula (140) or formula (141):

[0341] CD nominal ≥14.47MFD / λ cc +24.73…(140)

[0342] MFD / λ cc ≤0.06911CD nominal -1.709…(141)

[0343] , and preferably satisfies the following formula (142) or formula (143):

[0344] CD nominal ≥14.47MFD / λ cc +34.08…(142)

[0345] MFD / λ cc ≤0.06911CD nominal -2.406…(143)

[0346] In a 4-core MCF with a square core configuration, in order to reduce the leakage loss to the cladding to less than 0.01dB / km at a wavelength of 1360nm and to reduce the mutual transmission XT to less than -40dB after 10km transmission, the CD nominal When 125μm, 120μm, 115μm, 110μm, 105μm, 100μm, 95μm, 90μm, 85μm, 80μm, MFD / λ cc Follow the CDnominal The order of listing the numerical values ​​is preferably 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, 5.15 or less, and MFD / λ cc Follow the CD nominal The above-listed order of the numerical values ​​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.

[0347] In order to reduce the leakage loss to the cladding at a wavelength of 1360 nm to less than 0.01 dB / km and to reduce the parallel transmission XT after 10 km transmission to less than -40 dB, MFD / λ cc Follow the CD nominal The numerical values ​​of the above enumeration order are preferably 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, 4.54 or less, and MFD / λ cc Follow the CD nominal The above-listed order of the numerical values ​​is preferably 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.

[0348] In order to reduce the leakage loss to the cladding at a wavelength of 1550nm to less than 0.01dB / km and the counter transmission XT after 10km transmission to less than -40dB, MFD / λ cc Follow the CD nominal The order of the values ​​listed above is 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, 4.23 or less, and MFD / λ cc Follow the CD nominal The above-listed order of the numerical values ​​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.

[0349] In order to reduce the leakage loss to the cladding at a wavelength of 1550nm to less than 0.01dB / km and to reduce the parallel transmission XT after 10km transmission to less than -40dB, MFD / λ cc Follow the CD nominalThe numerical values ​​of the above enumeration order 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, 3.82 or less, and MFD / λ cc Follow the CD nominal The above-listed order of the numerical values ​​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.

[0350] λ cc In order to ensure single-mode operation in the O band, it is preferably below 1260nm. cc When the MFD is set to 6.5 or more, it is possible to adjust λ to 6.5 or more even when the MFD is within the range of 8.2 μm or more and 9.0 μm or less based on 8.6 μm. cc Set to 1260nm or less. cc By setting it to 7.2 or more, a larger MFD can be achieved, the connection loss between MCFs can be reduced, and λ cc Smaller than 1260nm (λ cc In this case, for example, MFD is set to be within the range of 8.8 μm to 9.6 μm based on 9.2 μm, which is λ cc ≤1.23μm, MFD / λ cc ≥7.2. In these cases, MFD / λ cc Preferably, the CD nominal The upper limit and the upper limit are defined according to MFD and λ cc The range is defined by the lower limit of the value.

[0351] When considering the mass production of MCF, the structure 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 2.5 or more, and most preferably MFD / λ cc The tolerance can be set to 3.0 or more MCF.

[0352] The MCF is preferably constructed with respect to MFD / λ cc A structure with a value between 6.5 and 7.5 is allowed. In addition, the structure of MCF can be about MFD / λ. cc The structure that allows 6.5 or more and 8.0 or less is more preferred, and the structure that allows 6.5 or more and 8.5 or less is even more preferred, and the structure that allows 6.5 or more and 9.0 or less is even more preferred. The most preferred MCF structure is about MFD / λcc Structures above 6.5 and below 9.5 are allowed.

[0353] The MCF can be constructed with respect to MFD / λ cc A structure with a value between 7.2 and 8.2 is allowed. cc The structure that allows 7.2 or more and 8.7 or less is more preferred, the structure that allows 7.2 or more and 9.2 or less is more preferred, and the structure that allows 7.2 or more and 9.7 or less is still more preferred. The most preferred MCF structure is about MFD / λ cc Structures above 7.2 and below 10.2 are allowed.

[0354] In λ cc When the wavelength is greater than 1260nm and less than 1360nm, a bend with a radius of 140mm or greater is added to 20m of the 22m sample fiber using the structure of Figure 12 of ITU-TG.650.1 (03 / 2018) (optical fiber without cable), and a bend with a radius of 40mm is added before and after the 20m section. When all modes are uniformly excited, the intensity of the high mode is set to P. h , set the strength of the fundamental mode to P f When h / (P f +P h )]=0.1dB as the wavelength λ cc However, regarding the MCF of the present invention, it is preferable to replace the radius of the bend added in the 20m section of the sample optical fiber 22m with a radius of 60mm to 100mm and measure the cutoff wavelength (λ ccR ) is below 1260nm. Thus, the single-mode operation of the O band after the cable installation can be guaranteed. In addition, the length L of the sample optical fiber sample [m] For a range exceeding 22m and below 1000m, sample -2[m] plus the bending radius of 140mm or more, in the L sample -2[m] section, add one turn of 40mm radius bend before and after, and preferably measure the cutoff wavelength (λ ccL ) is less than 1260nm. Therefore, when the cable length is L sample [m] cable, which can ensure single-mode operation in the O band.

[0355] Each fiber core of the MCF of the present invention preferably has a bending loss of 0.15 dB / turn or less, and more preferably 0.02 dB / turn or less, at a bending radius of 10 mm, at a wavelength of 1310 nm to 1360 nm. This can suppress the increase in loss after cable assembly, even when the MCF of the present invention is incorporated into an ultra-high-density cable with spacer adhesive tape.

[0356] When an MCF cable containing the MCF of the present invention is extended in a straight line (with a bending radius of at least 1 meter), the average bending radius of the MCF installed in the cable is preferably 0.14 meters or less, and more preferably 0.10 meters or less. Furthermore, when an MCF cable containing the MCF of the present invention is extended in a straight line, the average bending radius of the MCF installed in the cable is preferably 0.14 meters or more and 0.3 meters or less. This can reduce XT.

[0357] In addition, the MCF cable having the MCF of the present invention built therein preferably has an average bending radius of the MCF installed in the cable of 0.03 m or more, more preferably 0.06 m or more, thereby reducing loss due to bending.

[0358] Furthermore, the MCF cable with the MCF of the present invention built in is preferably a spacer tape cable. This allows the flexible spacer tape to be twisted into a spiral while being installed in the cable, allowing the MCF to be cabled while having a small bending radius, thereby reducing XT.

[0359] The MCF cable with the MCF of the present invention is a slotted cable, preferably with a tension member located in the center of the slot. This facilitates control of the MCF's bending radius and reduces XT. Furthermore, the presence of the tension member in the center of the slot makes the cable easily bendable in any direction, simplifying cable installation.

[0360] The MCF cable having the MCF of the present invention built in preferably has no grooves in the space inside the sheath and has a tension member 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.

[0361] As described above, according to the MCF of the present invention, a sufficient manufacturing tolerance can be secured, mass productivity is excellent, and degradation of connection loss is suppressed.

Claims

1. A multi-core optical fiber having: Four cores extending along a central axis; and A common cladding covering each of the four cores, In this multi-core optical fiber, Each of the four cores has an adjacent relationship with two of the remaining cores, The center spacing Λ between adjacent cores among the four cores converges to Λ with a specified core spacing nominal value Λ nominal as a reference, and converges to Λ nominal -0.9 μm or more and Λ nominal +0.9 μm or less, The outer diameter of the common cladding converges within a range of 124 μm or more and 126 μm or less based on 125 μm, For each of the four cores, the mode field diameter MFD at a wavelength of 1310 nm, the cable cut-off wavelength λ measured through 22 m of optical fiber cc and the shortest distance d among the distances from the centers of the four cores to the outer periphery of the common cladding coat If any of the four cores satisfies the following formula (1): d coat ≥2.88 MFD / λ cc +5.36…(1) Of the relationship, In each of the four cores, the mode field diameter MFD converges within a range of a value of MFD reference value - 0.4 μm or more and a value of MFD reference value + 0.4 μm or less with a value of 8.6 μm or more and 9.2 μm or less as the MFD reference value, In each of the four cores, the zero-dispersion wavelength converges within a range of a value of wavelength reference value - 12 nm or more and a value of wavelength reference value + 12 nm or less with a value of 1312 nm or more and 1340 nm or less as the wavelength reference value, In each of the four optical fibers, the dispersion slope at the zero-dispersion wavelength is 0.092 ps / (nm 2 ·km) or less. At each of the four cores, the cut-off wavelength λ cc is 1260 nm or less. Satisfies any condition of the first condition and the second condition, and satisfies any condition of the third condition and the fourth condition, The first condition is defined by each of the four cores being in direct contact with the common cladding, The second condition is defined such that corresponding to each of the four cores, there is also an optical cladding provided between the corresponding core among the four cores and the common cladding, and the relative refractive index difference Δ2 of the optical cladding with respect to the common cladding satisfies the relationship of - 0.1% ≤ Δ2 ≤ 0.1%, The 3rd condition is that the crosstalk between adjacent cores equivalent to a fiber length of 10 km at a wavelength of 1360 nm is -10 dB or less. For each of the 4 cores, compared to MFD / λ cc and the center spacing Λ of adjacent cores a satisfy the following formula (2): 7.2 ≤ MFD / λ cc ≤ 10.2 ≤ 0.443Λ a -5.33…(2) Thus defined, The 4th condition is that the crosstalk between adjacent cores equivalent to a fiber length of 10 km at a wavelength of 1360 nm is -20 dB or less, and for each of the 4 cores, the ratio MFD / λ cc and the center spacing Λ of adjacent cores a satisfy the following formula (3): 7.2 ≤ MFD / λ cc ≤ 10.2 ≤ 0.392Λ a -4.88…(3) Thus defined.

2. A multi-core optical fiber having: Four cores extending along a central axis; and A common cladding covering each of the four cores, In this multi-core optical fiber, Each of the four cores has an adjacent relationship with two of the remaining cores, The center interval Λ between the cores in adjacent relationship among the four cores converges to Λ nominal with a specified core interval nominal value Λ nominal as a reference, and is in the range of Λ nominal −0.9 μm or more and Λ +0.9 μm or less. The diameter CD of the common cladding is nominalized with a specified cladding diameter nominal value CD nominal [μm] as a reference and converges to CD nominal -1μm or more and CD nominal +1μm or less, For each of the four cores, the mode field diameter MFD at a wavelength of 1310 nm, the cable cut-off wavelength λ measured through a 22 m optical fiber cc and the shortest distance d among the distances from the centers of the four cores to the outer periphery of the common cladding coat In any of the four cores, the following formula (4) is satisfied: d coat ≥2.88 MFD / λ cc +5.36…(4) In each of the four cores, the mode field diameter MFD converges within a range of a value of MFD reference value - 0.4 μm or more and a value of MFD reference value + 0.4 μm or less with a value of 8.6 μm or more and 9.2 μm or less as the MFD reference value, In each of the four cores, the zero-dispersion wavelength converges within a range of a value of wavelength reference value - 12 nm or more and a value of wavelength reference value + 12 nm or less with a value of 1312 nm or more and 1340 nm or less as the wavelength reference value, In each of the four optical fibers, the dispersion slope at the zero-dispersion wavelength is 0.092 ps / (nm 2 ·km) or less. At each of the four cores, the cut-off wavelength λ cc is below 1260 nm, Satisfies any condition of the first condition and the second condition, and satisfies any condition of the fifth condition and the sixth condition, The first condition is defined by each of the four cores being in direct contact with the common cladding, The second condition is defined such that corresponding to each of the four cores, there is also an optical cladding provided between the corresponding core among the four cores and the common cladding, and the relative refractive index difference Δ2 of the optical cladding with respect to the common cladding satisfies the relationship of - 0.1% ≤ Δ2 ≤ 0.1%, The fifth condition is that the crosstalk between adjacent cores corresponding to a fiber length of 10 km at a wavelength of 1360 nm is - 10 dB or less, and in each of the four cores, the following formula (5) is satisfied: CD nominal ≥13.31 MFD / λ cc +24.47…(5) and for each of the four cores, the ratio of MFD / λ cc and the center-to-center spacing Λ of adjacent cores a satisfy the following equation (6): 6.5 ≤ MFD / λ cc ≤ 9.5 ≤ 0.443Λ a -5.33…(6) Defined hereby, The 6th condition is that the crosstalk between adjacent cores equivalent to a fiber length of 10 km at a wavelength of 1360 nm is -20 dB or less, and for each of the 4 cores, the following formula (7) is satisfied: CD nominal ≥9.37 MFD / λ cc +31.73…(7) and for each of the four cores, the ratio MFD / λ cc and the center separation Λ between cores in an adjacent relationship a satisfy the following formula (8): 6.5 ≤ MFD / λ cc ≤ 9.5 ≤ 0.392Λ a -4.88…(8) Defined hereby.

3. The multi-core optical fiber according to claim 1 or 2, wherein, It further has a coating portion that surrounds the common cladding, At a wavelength of 1550 nm or 1625 nm, the leakage loss from at least any one of the 4 cores to the coating portion is 0.05 dB / km or more, or the transmission loss of at least any one of the 4 cores at a wavelength of 1550 nm is 0.25 dB / km or more, or the transmission loss at a wavelength of 1625 nm is 0.25 dB / km or more.

4. The multi-core optical fiber according to claim 1, wherein, The 3rd condition is satisfied, The crosstalk between adjacent cores equivalent to a fiber length of 10 km at a wavelength of 1550 nm is -10 dB or more.

5. The multi-core optical fiber according to claim 1, wherein, The 4th condition is satisfied, The crosstalk between adjacent cores equivalent to a fiber length of 10 km at a wavelength of 1550 nm is -20 dB or more.

6. The multi-core optical fiber according to claim 2, wherein, The 5th condition is satisfied, The crosstalk between adjacent cores equivalent to a fiber length of 10 km at a wavelength of 1550 nm is -10 dB or more.

7. The multi-core optical fiber according to claim 2, wherein, The 6th condition is satisfied, The crosstalk between adjacent cores equivalent to a fiber length of 10 km at a wavelength of 1550 nm is -20 dB or more.

8. A multi-core optical cable having a plurality of multi-core optical fibers including the multi-core optical fiber according to any one of claims 1 to 7.

9. A multi-core optical cable having a multi-core optical fiber ribbon therein, which is formed by adhesively bonding a plurality of multi-core optical fibers including the multi-core optical fiber according to any one of claims 1 to 7 at intervals.

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

11. The multi-core optical cable according to any one of claims 8 to 10, wherein, The average value of the bending radius of the multi-core optical fiber in the fiber length direction is 0.03 m or more and 0.14 m or less, or 0.14 m or more and 0.3 m or less.

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