Multi-core optical fiber

Abstract

A multi-core optical fiber is provided with: four cores each extending along the central axis of the multi-core optical fiber; the common cladding wraps the four fiber cores and has a refractive index lower than the refractive index of each of the four fiber cores; and a coating resin wrapping the common cladding. The common cladding has a diameter of 124.5 [mu] m or more and 125.5 [mu] m or less. In each of the four cores, Aeff satisfies formula (1) and formula (2), and [lambda] cc satisfies formula (1) and formula (3). The four cores are disposed such that Aeff, [lambda] cc, and Dc for any one of the first core and the second core satisfy formula (4), and the OCT, Aeff, and [lambda] cc for the first core satisfy formula (5). Aeff < =-11.919 [lambda] cc2 + 153.67 [lambda] cc-105.98 (1) 70 < = Aeff < = 101.2 (2) 1.270 < = [lambda] cc < = 1.530 (3) Dc > = 62.67-44.75 [lambda] cc + 0.2217 Aeff + 9.911 [lambda] cc2-8.461 * 10-4 Aeff 2 + 3.981 * 10-2 [lambda] ccAeff (4) OCT > = 76.53-70.55 [lambda] cc + 0.3821 Aeff + 19.56 [lambda] cc2-6.480 * 10-4 Aeff 2 + 7.279 * 10-2 [lambda] ccAeff (5).

Description

Technical Field

[0001] The present disclosure relates to a multi-core optical fiber. This application claims priority based on Japanese Application No. 2022-192004 filed on November 30, 2022, and all the contents described in the Japanese Application are incorporated herein by reference. Background Art

[0002] Patent Document 1 and Non-Patent Document 1 describe a step-index multi-core optical fiber (hereinafter referred to as MCF) having four cores and a common cladding. Patent Document 2 describes an MCF that suppresses crosstalk (hereinafter referred to as XT) between cores by providing a first cladding region with a low refractive index between the cores and the common cladding.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Publication No. 2013-88458

[0006] Patent Document 2: Japanese Patent Application Publication No. 2020-86054

[0007] Patent Document 3: Japanese Patent Application Publication No. 2017-509171

[0008] Non-patent literature

[0009] Non-patent literature 1: T. Matsui et al., “STEP-INDEX PROFILE MULTI-CORE FIBRE WITH STANDARD 125μM CLADDING TO FULL-BAND APPLICATION”, ECOC 2019, M.1.D.3

[0010] Non-patent document 2: Y. Sagae et al., "Ultra-Low-XT Multi-Core Fiber with Standard 125-μm Cladding for Long-Haul Transmission", OECC 2019, TuC3-4

[0011] Non-patent document 3: T.Hayashi et al., “Uncoupled Multi-core Fiber Design for Practical Bidirectional Optical Communications,” OFC, M1E.1

[0012] Non-patent document 4: RJ Black and C. Pask, J. Opt. Soc. Am. A, JOSAA 1(11), p.1129-1131, 1984

[0013] Non-patent document 5: Y.Kobayashi and T.Hayashi, "Behavior and measurementmethod of inter-core crosstalk in multicore fibers with core-dependent loss," Opt.Express 31(1), pp.502-508(2023). Summary of the invention

[0014] The MCF disclosed in the present invention comprises: four cores extending along the central axis of the MCF; a common cladding covering the four cores and having a refractive index lower than that of each of the four cores; and a coating resin covering the common cladding. The diameter of the common cladding is greater than 124.5 μm and less than 125.5 μm. In each of the four cores, if the effective cross-sectional area at a wavelength of 1550 nm is set to Aeff [μm 2 ], and set the optical cable cut-off wavelength to λcc [μm], then Aeff satisfies equations (1) and (2), and λcc satisfies equations (1) and (3).

[0015] Aeff≤-11.919λcc 2 +153.67λcc-105.98 (1)

[0016] 70≤Aeff≤101.2 (2)

[0017] 1.270≤λcc≤1.530 (3)

[0018] The four fiber cores are arranged as follows: in a cross section orthogonal to the central axis, if the center interval between a first fiber core, which is one of the four fiber cores, and a second fiber core located closest to the first fiber core is set to Dc [μm], and the shortest distance between the interface between the common cladding and the coating resin and the center of the first fiber core is set to OCT [μm], then for Aeff and λcc of any one of the first fiber core and the second fiber core, Dc satisfies equation (4), and OCT, Aeff and λcc of the first fiber core satisfy equation (5).

[0019] Dc≥62.67-44.75λcc+0.2217Aeff+9.911λcc 2 -8.461×10 -4 Aeff 2 +3.981×10-2 λccAeff (4)

[0020] OCT≥76.53-70.55λcc+0.3821Aeff+19.56λcc 2 -6.480×10 -4 Aeff 2 +7.279×10 -2 ×ccAeff (5) BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a cross-sectional view perpendicular to the central axis of the MCF according to the embodiment.

[0022] Figure 2 The XT is 10 when propagating in opposite directions at a wavelength of 1625nm. -4 / (100km) 2 The following is a graph showing the relationship between the minimum value of the adjacent core spacing and the effective cross-sectional area.

[0023] Figure 3 This is a graph showing the relationship between the lower limit value of the outer cladding thickness at which the leakage loss is 0.001 dB / km or less at a wavelength of 1625 nm and the effective cross-sectional area for a plurality of cutoff frequencies.

[0024] Figure 4 This is a graph showing the relationship between the lower limit value of the outer cladding thickness at which the leakage loss is 0.0005 dB / km or less at a wavelength of 1625 nm and the effective cross-sectional area for a plurality of λcc.

[0025] Figure 5 This is a graph showing the relationship between the cutoff wavelength and the upper limit value of the effective cross-sectional area of ​​the design center value of the core spacing, in which the XT at a wavelength of 1625nm satisfies the XT below -40dB / km and the leakage loss below 0.001dB / km even when the core spacing deviates by ±1μm from the design center value among multiple cladding diameters.

[0026] Figure 6 This is a graph showing the relationship between the upper limit value of the effective cross-sectional area and the cutoff wavelength for the design center value of the core spacing, in which the XT at a wavelength of 1625 nm satisfies the design center value of the core spacing of less than -40 dB / km and the leakage loss of less than 0.0005 dB / km, even when the core spacing deviates by ±1 μm from the design center value, among multiple cladding diameters.

[0027] Figure 7 This is a cross-sectional view perpendicular to the central axis of the MCF according to the first modification.

[0028] Figure 8It is a cross-sectional view perpendicular to the central axis of the MCF according to the second modification.

[0029] Fig. 9 This is a diagram showing the refractive index distribution around the core of an MCF that can be applied to the present disclosure.

[0030] Fig.10 This is a diagram showing the refractive index distribution around the core of an MCF that can be applied to the present disclosure. DETAILED DESCRIPTION

[0031] [Technical Problems to be Solved by the Present Disclosure]

[0032] Non-patent document 2 states that in the MCF described in patent document 2, the loss increases in the region above 1580nm. Thus, in the prior art, the leakage loss at a wavelength of 1625nm is not completely suppressed. Therefore, in long-distance transmission, if the L band (wavelength 1565nm to 1625nm) is used for transmission, the transmission loss increases and the signal transmission quality deteriorates.

[0033] The present disclosure provides an MCF capable of suppressing degradation of signal transmission quality within a wavelength range of 1530 nm to 1625 nm.

[0034] [Effects of the present disclosure]

[0035] According to the MCF of the present disclosure, it is possible to suppress degradation of signal transmission quality within a wavelength range of 1530 nm to 1625 nm.

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

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

[0038] (1) An MCF according to one aspect of the present disclosure comprises: four cores extending along the central axis of the MCF; a common cladding covering the four cores and having a refractive index lower than that of each of the four cores; and a coating resin covering the common cladding. The diameter of the common cladding is greater than or equal to 124.5 μm and less than or equal to 125.5 μm. In each of the four cores, if the effective cross-sectional area at a wavelength of 1550 nm is set to Aeff [μm 2 ], and set the optical cable cut-off wavelength to λcc [μm], then Aeff satisfies equations (1) and (2), and λcc satisfies equations (1) and (3).

[0039] The four fiber cores are arranged as follows: in a cross section orthogonal to the central axis, if the center interval between a first fiber core, which is one of the four fiber cores, and a second fiber core located closest to the first fiber core is set to Dc [μm], and the shortest distance between the interface between the common cladding and the coating resin and the center of the first fiber core is set to OCT [μm], then for Aeff and λcc of any one of the first fiber core and the second fiber core, Dc satisfies equation (4), and OCT, Aeff and λcc of the first fiber core satisfy equation (5).

[0040] Aeff≤-11.919λcc 2 +153.67λcc-105.98 (1)

[0041] 70≤Aeff≤101.2 (2)

[0042] 1.270≤λcc≤1.530 (3)

[0043] Dc≥62.67-44.75λcc+0.2217Aeff+9.911λcc 2 -8.461×10 -4 Aeff 2 +3.981×10 -2 λccAeff (4)

[0044] OCT≥76.53-70.55λcc+0.3821Aeff+19.56λcc 2 -6.480×10 -4 Aeff 2 +7.279×10 -2 λccAeff (5)

[0045] In this MCF, the XT for parallel propagation between the first core and the second core at a wavelength of 1625 nm is 10 -4 / km or less. In addition, the leakage loss at a wavelength of 1625nm is less than 0.001dB / km. Therefore, the degradation of signal transmission quality can be suppressed at a wavelength of 1625nm. According to the MCF, the degradation of signal transmission quality can be suppressed in long-distance transmission based on counter-propagation in a wavelength range of at least 1530nm and below 1625nm.

[0046] (2) In the above (1), in each of the four cores, Aeff may satisfy the equation (6) and λcc may satisfy the equation (7).

[0047] 70≤Aeff≤93.0 (6)

[0048] 1.270≤λcc≤1.460 (7)

[0049] In this case, at least in the long-distance transmission based on counter-propagation within the wavelength range of 1460 nm to 1625 nm, the degradation of signal transmission quality can be suppressed. That is, the degradation of signal transmission quality can be suppressed within the wavelength range of 1460 nm to 1625 nm or within the wavelength range of 1530 nm to 1625 nm.

[0050] (3) In the above (1) or (2), in each of the four cores, λcc may satisfy equation (8).

[0051] 1.360≤λcc (8)

[0052] In this case, light confinement to the core can be enhanced, and thus XT and leakage loss can be further suppressed.

[0053] (4) In any of the above (1) to (3), the four cores may be arranged so that OCT, Aeff, and λcc of the first core satisfy equation (9).

[0054] OCT≥78.90-72.75λcc+0.3936Aeff+20.14λcc 2 -6.704×10 -4 Aeff 2 +7.480×10 -2 λccAeff (9)

[0055] In this case, the leakage loss at a wavelength of 1625 nm is 0.0005 dB / km or less.

[0056] (5) In any of the above (1) to (4), in the cross section, the centers of the four cores may be arranged one at each of the four vertices of a square having a side length of Dc. In this case, since the four cores are arranged symmetrically, the optical characteristics among the four cores can be made uniform.

[0057] (6) In any of the above (1) to (4), the centers of the four cores may be arranged one at each of the four vertices of an isosceles trapezoid having three sides of length Dc and one side longer than Dc in the cross section. In this case, the cores can be identified even without providing a mark.

[0058] (7) In any of the above (1) to (6), the common cladding may be provided in contact with the outer peripheral surfaces of the four cores. In this case, since a complicated refractive index structure is not used, the manufacturability can be improved.

[0059] (8) In the above (7), the relative refractive index difference of each of the four cores based on the refractive index of the common cladding may be 0.50% or less. In this case, since no deep depressed cladding or refractive index groove is used, the manufacturability can be improved. In addition, by not making the refractive index difference between the core and the common cladding too large, the transmission loss can be suppressed.

[0060] (9) It may also be that the MCF of any one of the above (1) to (6) further comprises four independent claddings wrapping each of the four cores on the inner side of the common cladding, and if the relative refractive index difference of each of the four independent claddings based on the refractive index of the common cladding is set to Δic [%], then Δic satisfies formula (10).

[0061] -0.20≤Δic<0 (10)

[0062] In this case, since a deep depressed cladding or a refractive index groove is not used, manufacturability can be improved.

[0063] (10) The MCF of any one of (1) to (6) above may further include four independent claddings covering each of the four cores on the inner side of the common cladding, and the relative refractive index difference of the cores on the inner side of the independent claddings based on the refractive index of each independent cladding is 0.50% or less. In this case, since a deep depressed cladding or refractive index groove is not used, the manufacturability can be improved. In addition, by not making the refractive index difference between the core and the independent cladding too large, the transmission loss can be suppressed.

[0064] [Details of the embodiments of the present disclosure]

[0065] The specific examples of the MCF involved in this embodiment are described with reference to the accompanying drawings as needed. It should be noted that the present disclosure is not limited to these examples, but is shown by the claims, and is intended to include all changes within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are marked with the same figure numbers, and repeated descriptions are omitted.

[0066] Figure 1 : is a cross-sectional view orthogonal to the central axis of the MCF involved in the embodiment. Figure 1 As shown, the MCF1 involved in the embodiment is a quad-core optical fiber having four cores 2, a common cladding 3 and a coating resin 4. The core 2 is composed of glass with silica as the main component. In a cross section orthogonal to the central axis AX, the four cores 2 have the same circular shape as each other. The four cores 2 extend along the central axis AX of the MCF1 respectively.

[0067] In a cross section orthogonal to the central axis AX, if the center interval between the first core, which is one of the four cores 2, and the second core located closest to the first core is set to Dc [μm], Dc has the same value regardless of which of the four cores 2 the first core is. That is, the center of the four cores 2 is arranged at each of the four vertices of a square whose side length is Dc [μm]. The first core and the second core are adjacent cores, and Dc is the center interval between adjacent cores. In MCF1, since the four cores 2 are arranged symmetrically, the optical characteristics between the four cores 2 can be made uniform.

[0068] In each of the four cores 2, if the effective cross-sectional area at a wavelength of 1550 nm is set to Aeff [μm 2 ], the optical cable cutoff wavelength is set to λcc [μm], then Aeff satisfies equations (1) and (2), and λcc satisfies equations (1) and (3). It should be noted that the Aeff of the four cores 2 can be the same or different from each other. In addition, the λcc of the four cores 2 can be the same or different from each other.

[0069] Aeff≤-11.919λcc 2 +153.67λcc-105.98 (1)

[0070] 70≤Aeff≤101.2 (2)

[0071] 1.270≤λcc≤1.530 (3)

[0072] By making Aeff 70 μm 2 In the above, it is possible to suppress the degradation of signal transmission quality caused by nonlinear interference. In formula (1), in order to make Aeff 70 μm 2 Above, λcc needs to be above 1270nm. By making λcc below 1530nm, single-mode operation can be achieved in the C band (wavelength 1530nm to wavelength 1565nm) and L band (wavelength 1565nm to wavelength 1625nm). Therefore, it is possible to realize an optical fiber suitable for optical signal transmission in the C band and L band. In formula (1), in order to make λcc below 1530nm, Aeff needs to be 101.2μm 2 the following.

[0073] In each of the four cores 2, Aeff may also satisfy equation (6), and λcc may also satisfy equation (7). In each of the four cores 2, λcc may also satisfy equation (8).

[0074] 70≤Aeff≤93.0 (6)

[0075] 1.270≤λcc≤1.460 (7)

[0076] 1.360≤λcc (8)

[0077] In formula (1), in order to make λcc less than 1460nm, Aeff needs to be 93.0μm 2 the following.

[0078] The four cores 2 are arranged so that Aeff, λcc, and Dc of any of the first core and the second core all satisfy the equation (4).

[0079] Dc≥62.67-44.75λcc+0.2217Aeff+9.911λcc 2 -8.461×10 -4 Aeff 2 +3.981×10 -2 λccAeff (4)

[0080] Therefore, the XT for parallel propagation between adjacent cores at a wavelength of 1625 nm is 10 -4 Therefore, the XT (opposite XT) during counter-propagation between adjacent cores at a wavelength of 1625 nm can be set to 10 -4 / (100km) 2 As follows. Thus, it is possible to fully suppress the degradation of signal quality caused by XT during counter-propagation. It should be noted that in parallel propagation, the transmission directions of optical signals between adjacent fiber cores are the same. In counter-propagation, the transmission directions of optical signals between adjacent fiber cores are different.

[0081] In particular, in long-distance transmission using counter-propagation, the transmission loss (span loss) of the optical fiber between adjacent amplifier repeaters (span) is small. Therefore, based on non-patent document 3, from the perspective of suppressing signal quality degradation, as an allowable XT, indirect XT, in which the optical signal that propagates in parallel becomes XT via the adjacent fiber core, is dominant. Indirect XT is proportional to the square of the span length in each span. Therefore, 1 / (100km) is used here. 2 As a unit. 10 -4 / (100km) 2 Equal to 10 -8 / km 2 The accumulation of indirect XT across multiple spans is linear, that is, the simple addition of the indirect XT of each span. Therefore, if the XT during counter-propagation is 10 -4 / (100km) 2 In a counter-propagating multi-core optical fiber transmission system with an average span length of about 100 km or less, the counter-propagating XT in each span can be suppressed to about 10-4 / km or less. As a whole, the transmission system can suppress the signal quality degradation caused by XT regardless of the fiber length (or the number of spans). In other words, the noise caused by XT can be suppressed compared with the noise caused by the optical amplifier and the noise caused by nonlinear interference.

[0082] For parallel propagation, XT is allowed to increase to a level that degrades the optical signal transmission quality, i.e., 10 -4 / km or less, but XT during counter-propagation is set to a level that can suppress degradation of optical signal transmission quality. As a result, a value that suppresses leakage loss can be achieved for OCT described below.

[0083] Figure 2 The XT is 10 when the two sides propagate in opposite directions at a wavelength of 1625 nm. -4 / (100km) 2 The following is a graph showing the relationship between the lower limit (Dmin) of Dc and Aeff. The plurality of λcc are 1.26 μm, 1.36 μm, 1.46 μm, and 1.53 μm. Figure 2 The horizontal axis represents Aeff [μm 2 ], the vertical axis represents Dmin [μm]. For example, when λcc = 1.53 μm, the relationship between Dmin and Aeff must be Figure 2 The upper range of the bottom curve in .

[0084] Formula (4) Figure 2 The relationships shown are organized. Figure 2 It is obtained by creating multiple combinations of Aeff, λcc and Dmin. Specifically, multiple combinations of Aeff and λcc are created by variously changing the radius ra of the core 2 and the relative refractive index difference Δ of the core, and Dmin in each combination is calculated.

[0085] The common cladding 3 surrounds the four cores 2. The common cladding 3 is provided in contact with the outer peripheral surfaces of the four cores 2. No depressed cladding is provided between the cores 2 and the common cladding 3. Thus, since the MCF 1 does not use a complicated refractive index structure, the manufacturability can be improved.

[0086] The common cladding 3 is made of glass with silicon dioxide as the main component. The common cladding 3 has a refractive index lower than that of each of the four cores 2. In order to generate a refractive index difference between the core 2 and the common cladding 3, germanium (Ge) may be added to the core 2. Alternatively, fluorine (F) may be added to the common cladding 3. By adding a small amount of F to the core 2 and the common cladding 3, a depressed type distribution can be achieved with good manufacturability.

[0087] The relative refractive index difference Δc of each core 2 is 0.50% or less based on the refractive index of the cladding in contact with the core 2. In the present embodiment, since the common cladding 3 is in contact with each core 2, the cladding that serves as the reference for the refractive index is the common cladding 3. The relative refractive index difference Δc of each core 2 based on the refractive index of the common cladding 3 is represented by Δ1. That is, the relative refractive index difference Δ1 of each of the four cores 2 based on the refractive index of the common cladding 3 is 0.50% or less. Since a deep depressed cladding or refractive index groove is not used, the manufacturability of MCF1 can be improved. By not making the refractive index difference between the core 2 and the common cladding 3 too large, the transmission loss can be suppressed.

[0088] The diameter of the common cladding 3 (cladding diameter) is 124.5 μm or more and 125.5 μm or less. Since the common cladding 3 has the same diameter (2rb) as the cladding diameter of a widely used general-purpose single-mode optical fiber, equivalent handling properties and mechanical reliability can be achieved.

[0089] The coating resin 4 covers the common cladding 3. The coating resin 4 is provided in contact with the outer peripheral surface of the common cladding 3. The coating resin 4 is made of, for example, an ultraviolet curing resin.

[0090] The four cores 2 are arranged such that, in a cross section orthogonal to the central axis AX, if the shortest distance between the interface between the common cladding 3 and the coating resin 4 and the center of the first core is set to OCT (outer cladding thickness) [μm], then the OCT, Aeff and λcc of the first core satisfy equation (5).

[0091] OCT≥76.53-70.55λcc+0.3821Aeff+19.56λcc 2 -6.480×10 -4 Aeff 2 +7.279×10 -2 λccAeff (5)

[0092] This makes it possible to reduce the leakage loss at a wavelength of 1625 nm to 0.001 dB / km or less.

[0093] The four cores 2 may be arranged so that the OCT, Aeff and λcc of the first core satisfy the equation (9).

[0094] OCT≥78.90-72.75λcc+0.3936Aeff+20.14λcc 2 -6.704×10 -4 Aeff 2 +7.480×10 -2 λccAeff (9)

[0095] This makes it possible to reduce the leakage loss at a wavelength of 1625 nm to 0.0005 dB / km or less.

[0096] Figure 3 The graph shows the relationship between the lower limit value of OCT (OCTmin) at which the leakage loss is 0.001 dB / km or less at a wavelength of 1625 nm and Aeff for a plurality of λcc. The plurality of λcc are 1.26 μm, 1.36 μm, 1.46 μm, and 1.53 μm. Figure 3 The horizontal axis represents Aeff [μm 2 ], the vertical axis represents OCTmin [μm]. For example, when λcc = 1.53 μm, the relationship between OCTmin and Aeff must be Figure 3 The upper range of the bottom curve in .

[0097] Formula (5) is Figure 3 The relationships shown are organized. Figure 3 It is obtained by creating multiple combinations of Aeff, λcc and OCTmin. Specifically, multiple combinations of Aeff and λcc are created by making various changes to the radius ra of the core 2 and the relative refractive index difference Δ1 of the core 2 based on the refractive index of the common cladding 3, and OCTmin in each combination is calculated.

[0098] Figure 4 The graph shows the relationship between the lower limit value of OCT (OCTmin) at which the leakage loss is 0.0005 dB / km or less at a wavelength of 1625 nm and Aeff for a plurality of λcc. The plurality of λcc are 1.26 μm, 1.36 μm, 1.46 μm, and 1.53 μm. Figure 4 The horizontal axis represents Aeff [μm 2 ], the vertical axis represents OCTmin [μm]. For example, when λcc = 1.53 μm, the relationship between OCTmin and Aeff must be Figure 4 The upper range of the bottom curve in .

[0099] Formula (9) Figure 4 The relationships shown are organized. Figure 4 It is obtained by creating multiple combinations of Aeff, λcc and OCTmin. Specifically, multiple combinations of Aeff and λcc are created by variously changing the radius ra of the core 2 and the relative refractive index difference Δ of the core, and OCTmin in each combination is calculated.

[0100] Since the diameter of the common cladding 3 is greater than 124.5 μm and 125.5 μm, the leakage loss is maximum when the diameter of the common cladding 3 is 124.5 μm. The relationship between Aeff and λcc, which has the following design center value of Dc, is expressed by equation (1): Even if the diameter of the common cladding 3 is 124.5 μm and the core spacing (Dc) deviates by ±1 μm from the design center value, the opposite XT at a wavelength of 1625 nm satisfies 10 -4 / (100km) 2 , the leakage loss is less than 0.001 dB / km. That is, if the relationship between Aeff and λcc satisfies equation (1), there is a design center value of Dc as follows: when the diameter of the common cladding 3 is greater than 124.5 μm and less than 125.5 μm, even if the core spacing (Dc) deviates by ±1 μm from the design center value, the opposite XT at a wavelength of 1625 nm satisfies 10 -4 / (100km) 2 , leakage loss is less than 0.001dB / km.

[0101] Figure 5 It is shown that in multiple cladding diameters, even if the core spacing deviates by ±1 μm from the designed center value, the relative XT at a wavelength of 1625 nm satisfies 10 -4 / (100km) 2 The following is a graph showing the relationship between the upper limit value of Aeff and λcc when the leakage loss is 0.001 dB / km or less. The multiple cladding diameters are set within a range of 124.5 μm to 125.5 μm. The multiple cladding diameters are 124.5 μm, 124.75 μm, 125 μm, 125.25 μm, and 125.5 μm. Figure 5 The horizontal axis represents λcc[μm], and the vertical axis represents the upper limit value of Aeff. Figure 5 In , the innermost side of the roughly triangular region represented by the curve and the straight line corresponds to the range satisfying the equations (1), (2), and (3), respectively.

[0102] Figure 6 It is shown that in a plurality of cladding diameters in the range of 124.5 μm to 125.5 μm, even if the core pitch deviates by ±1 μm from the designed center value, the opposite XT at a wavelength of 1625 nm satisfies 10 -4 / (100km) 2 , a graph showing the relationship between the upper limit value of Aeff and λcc when the leakage loss is 0.0005 dB / km or less. Figure 6 The horizontal axis represents λcc[μm], and the vertical axis represents the upper limit value of Aeff. Figure 6In , the innermost side of the roughly triangular region represented by the curve and the straight line corresponds to the range satisfying equations (1), (6) and (7), respectively.

[0103] When equations (1), (2), (3) and (5) are satisfied, the bending loss when a bend with a radius of 25 mm or more is much lower than 0.1 dB per 100 turns. Therefore, in repeaters and station buildings, even if the excess length of MCF1 is wound and stored with a radius of 25 mm or more, the increase in loss can be suppressed.

[0104] As described above, in the MCF1 according to the present embodiment, the XT at the time of parallel propagation between adjacent cores at a wavelength of 1625 nm is 10 -4 / km or less. In addition, the leakage loss at a wavelength of 1625nm is less than 0.001dB / km. Therefore, the degradation of signal transmission quality can be suppressed at a wavelength of 1625nm. According to MCF1, the degradation of signal transmission quality can be suppressed in long-distance transmission based on counter-propagation in a wavelength range of at least 1530nm and less than 1625nm.

[0105] As mentioned above, although embodiment was described, this disclosure is not necessarily limited to the said embodiment, Various changes can be made within the range which does not deviate from the summary.

[0106] Figure 7 : is a cross-sectional view orthogonal to the central axis of the MCF involved in the first modification. Figure 8 As shown, in the MCF1A involved in the first variant, in the cross section orthogonal to the central axis AX, the centers of the four cores 2 are arranged one at each of the four vertices of an isosceles trapezoid having three sides with a length of Dc and one side longer than Dc. In the MCF1A, the cores 2 can be identified even without providing a mark.

[0107] Figure 8 : is a cross-sectional view orthogonal to the central axis of the MCF involved in the second modification. Fig. 9 As shown, the MCF1B according to the second modification further includes four independent claddings 5. The four independent claddings 5 ​​respectively wrap the four cores 2 inside the common cladding 3. If the relative refractive index difference of each of the four independent claddings 5 ​​based on the refractive index of the common cladding 3 is Δic [%], Δic satisfies the formula (10).

[0108] -0.20≤Δic<0 (10)

[0109] As described above, the relative refractive index difference Δc of each core 2 is 0.50% or less, based on the refractive index of the cladding in contact with the core 2. In this modification, the cladding that serves as the reference for the refractive index is the corresponding independent cladding 5, that is, the independent cladding 5 that wraps the core 2 inside. Since no deep depressed cladding or refractive index groove is used in MCF1B, manufacturability can be improved. By not making the refractive index difference between the core 2 and the corresponding independent cladding 5 too large, transmission loss can be suppressed.

[0110] Fig. 9 The figure shows the refractive index distribution around the core of the MCF that can be applied to the present invention. Regarding the core structure of the MCF of the present invention, the refractive index distribution of the core and the optical properties associated therewith can be selected according to the application, for example, it can be applied Fig. 9 The refractive index distribution of the patterns (A) to (J) shown in FIG. Fig. 9 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, which is represented by a local coordinate system with the center of each core and Δ=0% as the origin O. The structure may be the same or different between cores.

[0111] Fig. 9 The pattern (A) shown is a step-type refractive index distribution, the pattern (B) is a ring-type refractive index distribution, the pattern (C) is a double-step-type refractive index distribution, the pattern (D) is a gradient-type refractive index distribution, and the pattern (E) is a droop-type refractive index distribution, which can be applied to the core structure in the MCF disclosed in the present invention. Furthermore, the patterns (F) and (H) having a depressed refractive index distribution around the core, the pattern (G), (I) and (J) having a raised refractive index distribution around the core, and the pattern (E) having a matched refractive index distribution around the core can also be applied to the core structure.

[0112] Patterns (A), (B), (C), and (D) correspond to MCF1 according to the embodiment and MCF1A according to the first modification. Patterns (F) and (H) correspond to MCF1B according to the second modification when equation (10) is satisfied.

[0113] For refractive index distributions other than the step-type refractive index distribution of pattern (A), the radius ra of the core and the relative refractive index difference Δ1 can be obtained by using ESI (Equivalent-step-index: equivalent step refractive index) approximation when the step-type approximation is performed (Non-patent document 4). Non-patent document 4 can be easily applied when the boundary between the core and the cladding is clear. In the case of the refractive index distributions of pattern (E), pattern (H), pattern (I) and pattern (J) where the boundary between the core and the common cladding is not clear, the r when the dΔ / dr of the refractive index distribution before ESI approximation is the smallest (the slope to the lower right is the steepest) can be regarded as the core radius of the core before approximation to perform ESI approximation. The relative refractive index difference (Δic) of the independent cladding can use the average value of the relative refractive index difference of the core before approximation at the independent cladding part. That is, the average value (rb-ra) / 3+ra≤r≤2(rb-ra) / 3+ra at the central part from ra to rb on the horizontal axis (r axis) can be used.

[0114] Fig.10 : is a diagram showing the refractive index distribution around the core of the MCF that can be applied to the present disclosure. Fig. 9 In the equation, Δ is the relative refractive index difference based on the refractive index of the common cladding. Fig.10 In the equation, Δ is the relative refractive index difference based on the refractive index of the cladding layer in contact with the core. Fig.10 In the figure, the relative refractive index difference of the core is represented by Δc. Fig.10 In the patterns (A) to (E) shown, since the cladding layer that serves as the reference of the refractive index is the common cladding layer, Fig. 9 The patterns (A) to (E) shown have substantially the same refractive index distribution. Fig.10 In the patterns (F) to (J) shown, since the claddings other than the common cladding are arranged in contact with the core, they are similar to Fig. 9 Patterns (F) to (J) shown have different refractive index distributions.

[0115] The refractive index distribution around the core is not limited to Fig. 9 and Fig.10 The refractive index distributions of patterns (A) to (J) are shown in FIG.

[0116] It should be noted that the characteristic quantities and characteristics of the MCF disclosed in the present invention can be measured by the following methods. The refractive indices of the core, common cladding and independent cladding can be measured, for example, by the refraction near-field method and the lateral interference method. The diameter of the common cladding can be measured, for example, according to the refraction near-field method, the lateral interference method, and the microscopic observation image of the MCF cross section (transmission near-field method). The effective cross-sectional area Aeff can be measured, for example, by the method described in Appendix III of ITU-T G.650.2 (08 / 2015). The optical cable cutoff wavelength λcc can be measured, for example, by the method described in Section 6.3 of ITU-T G.650.1 (10 / 2020). The center spacing Dc between the first core and the second core located closest to the first core can be measured, for example, according to the refraction near-field method, the lateral interference method, and the microscopic observation image of the MCF cross section (transmission near-field method). The shortest distance OCT between the interface of the common cladding and the coating resin and the center of the first core can be measured by, for example, a refraction near-field method, a lateral interference method, or a microscopic observation image of an MCF cross section (a transmission near-field method). The XT during parallel propagation can be measured by the method described in non-patent document 5. The XT during counter-propagation can be predicted based on the formula described in non-patent document 3 according to the XT during parallel propagation. The leakage loss can be measured by the method described in patent document 3. The components constituting the multi-core optical fiber can be measured by fluorescent X-ray analysis.

[0117] Although units are omitted in the above formulae, the units of constants and coefficients in the formulae (1) to (10) are as follows: [-] represents a dimensionless quantity.

[0118] Formula (1): -11.919[-], 153.67[μm], -105.98[μm 2 ]

[0119] Formula (2): 70[μm 2 ],101.2[μm 2 ]

[0120] Formula (3): 1.270[μm], 1.530[μm]

[0121] Formula (4): 62.67[μm], -44.75[-], 0.2217[μm -1 ],9.911[μm -1 ], -8.461×10 -4 [μm -3 ], 3.981×10 -2 [μm -2 ]

[0122] Formula (5): 76.53[μm], -70.55[-], 0.3821[μm -1 ],19.56[μm -1 ], -6.480×10 -4 [μm -3 ], 7.279×10 -2 [μm -2 ]

[0123] Formula (6): 70[μm 2 ],93.0[μm 2 ]

[0124] Formula (7): 1.270[μm], 1.460[μm]

[0125] Formula (8): 1.360 [μm]

[0126] Formula (9): 78.90[μm], -72.75[-], 0.3936[μm -1 ],20.14[μm -1 ], -6.704×10 -4 [μm -3 ], 7.480×10 -2 [μm -2 ]

[0127] Formula (10): -0.20[%], 0[%]

[0128] In addition, it is considered that the symbols of the above-mentioned physical quantities do not include units, and the symbols are regarded as representing the numerical values ​​of the units recorded together with the symbols. Even if the equations (1) to (10) are considered to be dimensionless, the equations are valid. In this case, the numerical value of λcc used in the equation is a numerical value in [μm]. In the above content, in order to compare with the wavelength of the optical signal, the numerical value of λcc is sometimes recorded in the unit of [nm].

[0129] Description of Reference Numerals

[0130] 1, 1A, 1B: MCF; 2: fiber core; 3: common cladding; 4: coating resin; 5: independent cladding; AX: central axis; Dc: center interval between adjacent fiber cores; OCT: outer cladding thickness; Δ: relative refractive index difference; Δc: relative refractive index difference of the core based on the cladding connected to the core; Δ1: relative refractive index difference of the core based on the refractive index of the common cladding; Δic: relative refractive index difference of the independent cladding; ra: radius of the core; rb: radius of the cladding.

Claims

1. A multi-core optical fiber having: The four fiber cores extend respectively along the central axis of the multi-core optical fiber; a common cladding, surrounding the four cores, having a refractive index lower than that of each of the four cores; and A coating resin encapsulates the common coating, The diameter of the common cladding is greater than or equal to 124.5 μm and less than or equal to 125.5 μm, In each of the four fiber cores, if the effective cross-sectional area at a wavelength of 1550 nm is set to Aeff and the optical cable cutoff wavelength is set to λcc, then Aeff satisfies equations (1) and (2), and λcc satisfies equations (1) and (3), where the unit of Aeff is μm 2 , the unit of λcc is μm, The four cores are arranged such that, in a cross section orthogonal to the central axis, if the center interval between a first core, which is one of the four cores, and a second core located closest to the first core is set to Dc, and the shortest distance between the interface between the common cladding and the coating resin and the center of the first core is set to OCT, then for Aeff and λcc, Dc of any one of the first core and the second core, they all satisfy equation (4), and OCT, Aeff and λcc of the first core satisfy equation (5), wherein the units of Dc and OCT are μm, Aeff≤-11.919λcc 2 +153.67λcc-105.98 (1) 70≤Aeff≤101.2 (2) 1.270≤λcc≤1.530 (3) Dc≥62.67-44.75λcc+0.2217Aeff+9.911λcc 2 -8.461×10 -4 Aeff 2 +3.981×10 -2 λccAeff(4) OCT≥76.53-70.55λcc+0.3821Aeff+19.56λcc 2 -6.480×10 -4 Aeff 2 +7.279×10 -2 λccAeff (5).

2. The multi-core optical fiber according to claim 1, wherein: In each of the four cores, Aeff satisfies equation (6), λcc satisfies equation (7), 70≤Aeff≤93.0 (6) 1.270≤λcc≤1.460 (7).

3. The multi-core optical fiber according to claim 1 or 2, wherein: In each of the four cores, λcc satisfies equation (8), 1.360≤λcc (8).

4. The multi-core optical fiber according to any one of claims 1 to 3, wherein: The four cores are arranged so that the OCT, Aeff and λcc of the first core satisfy equation (9), OCT≥78.90-72.75λcc+0.3936Aeff+20.14λcc 2 -6.704×10 -4 Aeff 2 +7.480×10 -2 λccAeff (9).

5. The multi-core optical fiber according to any one of claims 1 to 4, wherein: In the cross section, the centers of the four cores are arranged one at each of the four vertices of a square with a side length of Dc.

6. The multi-core optical fiber according to any one of claims 1 to 4, wherein: In the cross section, the centers of the four cores are arranged one at each of the four vertices of an isosceles trapezoid having three sides with lengths Dc and one side longer than Dc.

7. The multi-core optical fiber according to any one of claims 1 to 6, wherein: The common cladding is arranged in contact with the outer circumferences of the four cores.

8. The multi-core optical fiber according to claim 7, wherein: The relative refractive index difference between the four cores based on the refractive index of the common cladding is 0.50% or less.

9. The multi-core optical fiber according to any one of claims 1 to 6, wherein: The multi-core optical fiber further comprises four independent claddings inside the common cladding, which wrap each of the four cores. If the relative refractive index difference of each of the four independent claddings based on the refractive index of the common cladding is set to Δic%, then Δic satisfies equation (10), -0.20≤Δic<0 (10).

10. The multi-core optical fiber according to any one of claims 1 to 6, wherein: The multi-core optical fiber further comprises four independent claddings inside the common cladding, which wrap each of the four cores. The relative refractive index difference of the core inside each independent cladding with respect to the refractive index of each independent cladding is 0.50% or less.

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