Multicore optical fiber and optical cable

Abstract

This multicore optical fiber (1) is provided with a glass fiber (2). The glass fiber (2) comprises a plurality of cores (10) that extends along the central axis of the glass fiber (2), and a first cladding (20) that surrounds the plurality of cores (10). The plurality of cores (10) includes a first core (11) and a second core (12) that are closest to each other. When the effective refractive index of the first core (11) and the effective refractive index of the second core (12) at at least one wavelength in the range of 1260-1625 nm are denoted by n1 and n2, respectively, 250 × 10- 6 ≤ n1 - n2 ≤ 950 × 10- 6 holds, and when the mode field diameter of the first core (11) and the mode field diameter of the second core (12) at the wavelength are denoted by d1 [μm] and d2 [μm], respectively, d1 - d2 ≤ - 0.1 [μm] holds.

Description

Multicore optical fibers and optical cables

[0001] This application claims priority to Japanese Patent Application No. 2024-034104, filed on March 6, 2024, and incorporates by reference all the contents of said Japanese application.

[0002] In uncoupled multi-core optical fibers (hereinafter also referred to as "MCFs"), reducing inter-core crosstalk (hereinafter also referred to as "XT") is an important issue. Patent Document 1 describes providing heterogeneity between cores to reduce XT. Patent Document 2 also describes providing heterogeneity between cores. Non-Patent Document 1 describes that in an MCF with heterogeneity between cores, XT has bending radius dependence. Non-Patent Document 2 describes a formula for calculating the peak position of the bending radius dependence of XT. Non-Patent Document 3 describes that an MCF with heterogeneity between cores was fabricated and the bending radius dependence of XT was actually measured in the MCF.

[0003] International Publication No. 2023 / 189621 U.S. Patent Application Publication No. 2024 / 0053530

[0004] Koshiba et al., “Analytical Expression of Average Power-Coupling Coefficients for Estimating Intercore Crosstalk in Multicore Fibers” October 2012, IEEE Photonics Journal Vol. 4, No. 5, pp. 1987-1995Hayashi et al., “Physical interpretation of intercore crosstalk in multicore fiber: effects of macrobend, structure fluctuation, and microbend” 11 March 2013, OPTICS EXPRESS Vol. 21, No. 5, pp.5401-5412Kobayashi et al., “Characterization of Inter-core Crosstalk of Multi-core Fiber as a Function of Bending Radius with Multi-channel OTDR” OECC / PSC 2022 TuC2-2

[0005] An MCF according to one aspect of the present disclosure is an MCF including a glass fiber, the glass fiber having a plurality of cores extending along a central axis of the glass fiber and a first cladding surrounding the plurality of cores, the plurality of cores including a first core and a second core that are closest to each other, and an effective refractive index of the first core at at least one wavelength in the range of 1260 nm to 1625 nm, where n1 is the effective refractive index of the first core and n2 is the effective refractive index of the second core, －６ ≦n1−n2≦950×10 －６ When the mode field diameter of the first core at the wavelength is d1 and the mode field diameter of the second core is d2, d1-d2≦-0.1 μm.

[0006] FIG. 1 is a graph showing the bending radius dependence of XT of an MCF. FIG. 2 is a graph showing the effective refractive index difference dependence of XT at a bending radius of 300 mm. FIG. 3 is a graph showing the effective refractive index difference dependence of XT at a bending radius of 150 mm. FIG. 4 is a graph showing the MFD dependence of cutoff wavelength. FIG. 5 is a graph showing the relationship between the effective refractive index and the cutoff wavelength. FIG. 6 is a graph showing the relationship between the MFD difference and the cutoff wavelength difference. FIG. 7 is a graph showing the relationship between the effective refractive index difference and the MFD difference when the cutoff wavelength difference is 100 nm. FIG. 8 is a diagram showing a cross section perpendicular to the central axis of an MCF according to an embodiment, and a refractive index profile. FIG. 9 is a diagram showing an optical cable according to an embodiment. FIG. 10 is a diagram showing a cross section perpendicular to the central axis of an MCF according to a first modification, and a refractive index profile. FIG. 11 is a diagram showing a cross section perpendicular to the central axis of an MCF according to a second modification, and a refractive index profile. FIG. 12 is a diagram showing a cross section perpendicular to the central axis of an MCF according to a third modification, and a refractive index profile. Fig. 13 is a diagram showing a cross section perpendicular to the central axis of an MCF according to a fourth modification and its refractive index distribution. Fig. 14 is a diagram showing a cross section perpendicular to the central axis of an MCF according to a fifth modification and its refractive index distribution. Fig. 15 is a diagram showing a cross section perpendicular to the central axis of an MCF according to a sixth modification. Fig. 16 is a diagram showing a cross section perpendicular to the central axis of an MCF according to a seventh modification. Fig. 17 is a diagram showing a cross section perpendicular to the central axis of an MCF according to an eighth modification. Fig. 18 is a diagram showing a cross section perpendicular to the central axis of an MCF according to a ninth modification. Fig. 19 is a diagram showing a cross section perpendicular to the central axis of an MCF according to a tenth modification.

[0007] [Problem to be Solved by the Present Disclosure] In an MCF that simply provides heterogeneity between cores, the XT reduction effect may not be achieved depending on the bending radius at which the MCF is used. If the heterogeneity between cores is increased to achieve the XT reduction effect while taking into account the winding diameter of the bobbin, the difference in the effective refractive index of the fundamental mode between the cores increases. This makes it difficult to simultaneously meet the cutoff and bending loss specifications, reducing mass productivity. Hereinafter, unless otherwise specified, "effective refractive index" means "effective refractive index of the fundamental mode."

[0008] [Advantages of the Present Disclosure] The present disclosure provides an MCF and an optical cable that can improve mass productivity while reducing XT.

[0009] [Description of Embodiments of the Present Disclosure] The contents of embodiments of the present disclosure will be described. (1) An MCF according to one aspect of the present disclosure is an MCF including a glass fiber, the glass fiber having a plurality of cores extending along a central axis of the glass fiber and a first cladding surrounding the plurality of cores, the plurality of cores including a first core and a second core that are closest to each other, and where n1 is the effective refractive index of the first core and n2 is the effective refractive index of the second core at at least one wavelength in the range of 1260 nm to 1625 nm, the MCF has a refractive index of 250×10 －６ ≦n1−n2≦950×10 －６ In this MCF, the difference in the effective refractive index between the first core and the second core is 250×10 －６ Furthermore, the difference in mode field diameter between the first core and the second core is 0.1 μm or more, and the difference in effective refractive index between the first core and the second core is 950×10 －６ Since the difference in cutoff wavelength between the first core and the second core is 100 nm or less, the cutoff and bending loss specifications can be satisfied while reducing XT.

[0010] (2) In the above (1), n1-n2≧400×10 －６ and d1-d2≦−0.15 μm, in which case the cutoff and bending loss specifications can be met simultaneously while further reducing XT.

[0011] (3) In the above (2), d1-d2 may be ≦-0.45 μm, in which case the cutoff and bending loss specifications can be satisfied while further reducing XT.

[0012] (4) In any of the above (1) to (3), d1-d2 may be equal to or greater than -1.4 μm, which can reduce connection loss when connecting MCFs together.

[0013] (5) In any of the above (1) to (4), the cores may be arranged in a square lattice pattern in a cross section perpendicular to the central axis. From the viewpoint of using cores of different types between adjacent cores, arranging the cores in a square lattice pattern can increase the core density.

[0014] (6) In any of the above (1) to (4), the number of the multiple cores may be 2. In this case, it is possible to simultaneously satisfy the cutoff and bending loss specifications while reducing the XT between the two cores.

[0015] (7) In the above (5) or (6), the cores may be arranged such that the center of gravity of the cores is offset from the central axis in a cross section perpendicular to the central axis. In this case, the cores can be identified.

[0016] (8) In the above (5) or (6), the glass fiber may further include a marker surrounded by the first cladding, and the refractive index of the marker may be different from the refractive index of the first cladding. In this case, it is possible to identify multiple cores.

[0017] (9) In any one of the above (1) to (8), the glass fiber may further have a second cladding surrounding the first cladding, and the refractive index of the second cladding may be higher than the refractive index of the first cladding and lower than the refractive index of any of the plurality of cores. In this case, even if the effective cross-sectional area of ​​the core is increased, the bending loss is unlikely to increase.

[0018] (10) In the above (9), the first cladding may be a common cladding that surrounds multiple cores. In this case, since the first cladding is a common cladding, the cores can be positioned close to the central axis. Therefore, when connecting MCFs, positional deviation due to rotational deviation is reduced. As a result, connection loss can be reduced.

[0019] (11) In the above (9), the first cladding may include a plurality of individual claddings surrounding the plurality of cores, respectively. In this case, the first cladding can be formed by the same manufacturing method as that for a single-core optical fiber. Therefore, the manufacturing cost can be reduced.

[0020] (12) In any one of the above (1) to (11), the glass fiber may further have a low-refractive-index portion having a refractive index lower than that of the first cladding, and the low-refractive-index portion may be provided on a line connecting the central axis of the first core and the central axis of the second core in a cross section perpendicular to the central axis. In this case, XT can be further reduced.

[0021] (13) An optical cable according to an aspect of the present disclosure includes a plurality of MCFs according to any one of (1) to (12) above and a jacket that houses the plurality of MCFs. In this case, since the optical cable includes the MCFs, it is possible to simultaneously satisfy cutoff and bending loss specifications while reducing XT.

[0022] [Details of the Embodiments of the Present Disclosure] Specific examples of the MCF and optical cable according to the present embodiment will be described with reference to the drawings as necessary. The present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the following description, the same elements in the drawings will be given the same reference numerals, and duplicate explanations will be omitted.

[0023] Fig. 1 is a graph showing the bending radius dependency of XT of an MCF. The vertical axis of Fig. 1 shows XT [dB] between the closest cores (hereinafter also referred to as "neighboring cores") among multiple cores of the MCF. The horizontal axis of Fig. 1 shows the bending radius (bending diameter) [mm] of the MCF. Fig. 1 shows the case where the effective refractive index difference (absolute value) between neighboring cores at a desired wavelength where XT is to be reduced is 300 × 10 －６ , 250 x 10 －６ , 200 x 10 －６ , 150 x 10 －６ , 100 x 10 －６ and 50 x 10 －６The figure shows the bending radius dependence of XT between adjacent cores when the distance between the core centers of the adjacent cores is 35 μm and the wavelength is 1550 nm.

[0024] The desired wavelength is, for example, at least one wavelength in the range of 1260 nm to 1625 nm. The desired wavelength may be at least one wavelength in the range of 1260 nm to 1360 nm, particularly 1310 nm. The desired wavelength may be at least one wavelength in the range of 1530 nm to 1565 nm, particularly 1550 nm. When considering single-mode properties, the cutoff wavelengths of the adjacent cores may both be shorter than the desired wavelength. While XT generally has wavelength dependence, the peak bending radius of XT is almost independent of wavelength. Therefore, the same effect can be obtained at any wavelength within the above wavelength range.

[0025] Optical fibers are usually housed in cables. It is known that the bending radius of optical fibers inside a cable is approximately 300 mm. As shown in Figure 1, XT is dependent on the bending radius. XT increases as the bending radius increases from 0, and after reaching a peak (maximum value) at a certain bending radius R_pk, decreases as the bending radius increases. The value of R_pk increases as the effective refractive index difference decreases. When the heterogeneity between cores is low (i.e., when the effective refractive index difference is small), the value of R_pk becomes equal to or greater than the bending radius of the cable, resulting in a small XT reduction effect. The inventors have determined that in order to obtain a sufficient XT reduction effect, the effective refractive index difference should be set to 250 x 10 －６ I found that more was needed.

[0026] Fig. 2 is a graph showing the dependency of XT on the effective refractive index difference when the bending radius is 300 mm. That is, Fig. 2 shows the relationship between XT between adjacent cores and the effective refractive index difference between the adjacent cores when the bending radius of the MCF is 300 mm. The vertical axis of Fig. 2 shows XT [dB] between adjacent cores. The horizontal axis of Fig. 2 shows the effective refractive index difference between adjacent cores [×10 －６ As shown in FIG. 2, the effective refractive index difference is 150×10 －６The effective refractive index difference is 250×10. －６ Then, the effective refractive index difference is 150 × 10 －６ In comparison, XT improves by 20 dB.

[0027] 3 is a graph showing the dependency of XT on the effective refractive index difference at a bending radius of 150 mm. That is, FIG. 3 shows the relationship between XT between adjacent cores and the effective refractive index difference between the adjacent cores when the bending radius of the MCF is 150 mm. The vertical axis of FIG. 3 represents XT [dB]. The horizontal axis of FIG. 3 represents the effective refractive index difference [×10 －６ Depending on the internal structure of the cable, the bending radius of the optical fiber inside the cable may be about 150 mm. In this case, as shown in FIG. 3, the effective refractive index difference is set to 400×10 －６ The effective refractive index difference must be 400×10 or more. －６ By doing so, it is possible to accommodate any cable structure.

[0028] Increasing the difference in effective refractive index between cores increases the difference in confinement force between the cores. This increases the difference (absolute value) in cutoff wavelength between the cores. Here, of the adjacent cores, the one with a higher effective refractive index and a longer cutoff wavelength is referred to as the first core, and the one with a lower effective refractive index and a shorter cutoff wavelength is referred to as the second core. When the difference in cutoff wavelength between cores is large, if an attempt is made to keep the cutoff wavelength of the first core below the wavelength band used, the cutoff wavelength of the second core becomes too short, increasing the bending loss of the second core and making the second core unsuitable for practical use. Therefore, it is necessary to reduce the difference in cutoff wavelength.

[0029] The inventors discovered that by creating a difference in mode field diameter (hereinafter also referred to as "MFD") between adjacent cores, it is possible to reduce the difference in cutoff wavelength while maintaining the difference in effective refractive index. Specifically, the MFD of the first core is reduced and the MFD of the second core is increased. By increasing the MFD while keeping the effective refractive index of the fundamental mode (LP01 mode) of the core fixed, it is possible to increase the effective refractive index of the lowest higher-order mode (LP11 mode). This allows the cutoff wavelength to be longer.

[0030] FIG. 4 is a graph showing the MFD dependency of the cutoff wavelength. The vertical axis of FIG. 4 represents the cutoff wavelength [nm] when the effective refractive index of the fundamental mode is 1.4416. The horizontal axis of FIG. 4 represents the MFD [μm]. It can be seen from FIG. 4 that as the MFD increases, the effective refractive index of the lowest higher-order mode increases, thereby lengthening the cutoff wavelength. Therefore, in order to reduce the difference in cutoff wavelength between the first core and the second core, it is sufficient to shorten the cutoff wavelength of the first core by reducing the MFD of the first core, and to lengthen the cutoff wavelength of the second core by increasing the MFD of the second core.

[0031] Fig. 5 is a graph showing the relationship between the effective refractive index and the cutoff wavelength. The vertical axis of Fig. 5 represents the cutoff wavelength [nm], and the horizontal axis of Fig. 5 represents the effective refractive index. Fig. 5 shows the relationship when the MFD is 11.2 μm, 11.3 μm, 11.4 μm, 11.5 μm, 11.6 μm, 11.7 μm, and 11.8 μm. For example, when the effective refractive index difference is 250 × 10 －６ Consider a case where the effective refractive index of the first core is 1.4418 and the effective refractive index of the second core is 1.44155 to achieve the above. For example, if the MFDs of the first and second cores are the same, 11.5 μm, the difference in cutoff wavelength is approximately 80 nm. In contrast, if the MFD of the first core is 11.2 μm and the MFD of the second core is 11.5 μm, the difference in cutoff wavelength is improved to approximately 30 nm.

[0032] Fig. 6 is a graph showing the relationship between the difference in MFD and the difference in cutoff wavelength. The vertical axis of Fig. 6 shows the difference in cutoff wavelength between adjacent cores (=cutoff wavelength of the first core - cutoff wavelength of the second core) [nm]. The horizontal axis of Fig. 6 shows the difference in MFD between adjacent cores (=MFD of the first core - MFD of the second core) [μm]. Fig. 6 shows the relationship between the difference in effective refractive index between adjacent cores when the difference in effective refractive index between adjacent cores is 200 × 10 －６ , 250 x 10 －６ , 300 x 10 －６ , 400 x 10 －６ , 500 x 10 －６ and 600 x 10 －６ The relationship is shown for the case where

[0033] Considering mass productivity, the difference in cutoff wavelength needs to be 100 nm or less. If the difference in cutoff wavelength exceeds 100 nm, it becomes difficult for the cutoff wavelength of each core to fall within the specified range, resulting in a decrease in yield. The difference in cutoff wavelength may be 50 nm or less. This further improves mass productivity.

[0034] From FIG. 6, it can be seen that the effective refractive index difference between adjacent cores is 250×10 －６ When the effective refractive index difference between adjacent cores is 400×10, it can be seen that the difference in cutoff wavelength can be made 50 nm or less by making the difference in MFD between adjacent cores (=MFD of the first core - MFD of the second core) -0.17 μm or less. －６ It can be seen that, when the difference in MFD between adjacent cores is set to −0.15 μm or less, the difference in cutoff wavelength can be made 100 nm or less, and when the difference in MFD between adjacent cores is set to −0.45 μm or less, the difference in cutoff wavelength can be made 50 nm or less.

[0035] According to ITU-T G.654, an international standard for single-core optical fibers, the MFD tolerance of a single-core optical fiber is ±0.7 μm. Although there is no international standard such as ITU-T for MCF, assuming that the same standard as for single-core optical fibers is applied, the difference in MFD (=MFD of the first core−MFD of the second core) may be −1.4 μm or more.

[0036] Fig. 7 is a graph showing the relationship between the effective refractive index difference and the MFD difference when the difference in cutoff wavelength is 100 nm. The vertical axis of Fig. 7 shows the difference in MFD between adjacent cores (=MFD of the first core - MFD of the second core) [µm]. The horizontal axis of Fig. 7 shows the difference in effective refractive index between adjacent cores (=effective refractive index of the first core - effective refractive index of the second core) [×10 －６ From FIG. 7, the effective refractive index difference is 1100×10 －６ From FIG. 7, it can be seen that the effective refractive index difference can be set to 1080×10 －６ The following may also be used.

[0037] 8 is a diagram showing a cross section perpendicular to the central axis and a refractive index profile of an MCF according to an embodiment. The MCF 1 according to an embodiment includes a glass fiber 2 and a resin coating 3 (see FIG. 9 ) covering the outer peripheral surface of the glass fiber 2. The glass fiber 2 has a central axis AX. The glass fiber 2 is made of silica-based glass. The glass fiber 2 has multiple cores 10 and a first cladding 20. In this embodiment, the number of the multiple cores 10 (the number of cores) is two, and the MCF 1 is a two-core optical fiber, but this is not limited to this.

[0038] The multiple cores 10 extend along the central axis AX of the glass fiber 2. Each of the multiple cores 10 has, for example, a circular shape in a cross section perpendicular to the central axis AX (hereinafter also referred to as a "cross section"). The multiple cores 10 have, for example, the same circular shape in the cross section. The diameter (core diameter) of each of the multiple cores 10 is, for example, 4 μm or more and 15 μm or less, and may be 6 μm or more and 13 μm or less.

[0039] The multiple cores 10 include a first core 11 and a second core 12 that are closest to each other. In the cross section, the distance between the center position (central axis) of the first core 11 and the center position (central axis) of the second core 12 (core center distance) is, for example, 20 μm or more and 60 μm or less, and may be 30 μm or more and 50 μm or less. The first core 11 and the second core 12 have different effective refractive indices. The refractive index of the first core 11 and the refractive index of the second core 12 do not necessarily have to be equal. The multiple cores 10 are made of silica-based glass. The multiple cores 10 may contain a dopant for refractive index adjustment or may be pure silica. The effective refractive index can be obtained, for example, by measuring the refractive index distribution of the MCF 1 and calculating the effective refractive index from the measured refractive index distribution using a finite element method.

[0040] When the effective refractive index of the first core 11 is n_eff1 and the effective refractive index of the second core 12 is n_eff2, 250×10 －６ ≦n_eff1−n_eff2≦950×10 －６ n_eff1-n_eff2 is 250 x 10 －６By setting n_eff1-n_eff2 to 950×10 or more, the XT reduction effect can be fully achieved. －６ By setting n_eff1-n_eff2 to 400×10 or less, the difference in cutoff wavelength can be set to 100 nm or less. This makes it possible to improve mass productivity. －６ This further enhances the XT reduction effect.

[0041] When the MFD of the first core 11 is d1 and the MFD of the second core 12 is d2, d1-d2 is equal to or less than -0.1 μm. This makes it possible to reduce the difference in cutoff wavelength between the first core 11 and the second core 12.

[0042] The difference d1-d2 may be equal to or less than −0.15 μm, or may be equal to or less than −0.45 μm, which can further reduce the difference in cutoff wavelength between the first core 11 and the second core 12.

[0043] The difference d1-d2 may be -1.4 μm or more, which can reduce connection loss when connecting MCFs 1. The MFD is measured in accordance with, for example, 6.1 of ITU-T G650.1.

[0044] The first core 11 and the second core 12 are arranged to face each other across the central axis AX in the cross section. The first core 11 and the second core 12 are arranged, for example, equidistant from the central axis AX in the cross section.

[0045] The first cladding 20 surrounds the cores 10. The first cladding 20 is a common cladding that collectively surrounds the cores 10. The central axis of the first cladding 20 coincides with the central axis AX of the glass fiber 2. In this embodiment, the outer peripheral surface of the first cladding 20 forms the outer peripheral surface of the glass fiber 2.

[0046] When the refractive index of the first core 11 is n11, the refractive index of the second core 12 is n12, and the refractive index of the first cladding 20 is n20, n11>n20 and n12>n20 are satisfied. In this embodiment, n11>n12. The first cladding 20 is made of silica-based glass. The first cladding 20 may contain a dopant for adjusting the refractive index, or may be pure silica.

[0047] 9 is a diagram illustrating an optical cable according to an embodiment. As shown in the figure, the optical cable 100 according to the embodiment includes a plurality of MCFs 1 and an outer jacket 200. The outer jacket 200 forms a cylindrical storage space that stores the plurality of MCFs 1. The outer jacket 200 stores the plurality of MCFs 1 within the storage space. Two tension members 300 extending along the storage space are embedded in the outer jacket 200. Because the optical cable 100 includes the MCFs 1, it is possible to simultaneously satisfy cutoff and bending loss specifications while reducing XT.

[0048] Although the embodiments have been described above, the present disclosure is not necessarily limited to the above-described embodiments and modifications, and various modifications are possible without departing from the spirit of the present disclosure. Below, modifications of MCF1 will be described while omitting explanations of points of commonality with MCF1 as appropriate.

[0049] FIG. 10 shows the cross section perpendicular to the central axis and the refractive index profile of an MCF according to a first modification. In an MCF 1A according to this modification, the glass fiber 2 further includes a second cladding 30. The second cladding 30 surrounds the first cladding 20. The outer peripheral surface of the first cladding 20 is covered by the second cladding 30. The outer peripheral surface of the second cladding 30 constitutes the outer peripheral surface of the glass fiber 2. The refractive index of the second cladding 30 is higher than that of the first cladding 20 and lower than the effective refractive index of any of the multiple cores 10. If the refractive index of the second cladding 30 is n30, then n11 > n30 > n20 and n12 > n30 > n20. The second cladding 30 is made of silica-based glass and contains a dopant for adjusting the refractive index. The second cladding 30 prevents the bending loss from increasing even when the effective cross-sectional area of ​​the core 10 is increased.

[0050] In the MCF1A, the first cladding 20 is common, so that the core 10 can be arranged closer to the central axis AX of the glass fiber 2 than in the MCF1B (see FIG. 11 ) described below. Therefore, when splicing MCFs 1A together, positional deviation due to rotational deviation is reduced, resulting in reduced splice loss.

[0051] 11 is a diagram showing a cross section perpendicular to the central axis and a refractive index profile of an MCF according to a second modification. In an MCF 1B according to this modification, the glass fiber 2 further has a second cladding 30. The second cladding 30 surrounds the first cladding 20. The first cladding 20 includes a plurality of individual claddings 21, 22 that respectively surround the plurality of cores 10. The number of the individual claddings 21 matches the number of the cores 10. In this modification, the number of the individual claddings 21 is two. The individual cladding 21 surrounds the first core 11. The individual cladding 22 surrounds the second core 12.

[0052] The outer peripheral surface of the first cladding 20 (i.e., the outer peripheral surfaces of the individual claddings 21 and 22) is covered with the second cladding 30. The outer peripheral surface of the second cladding 30 forms the outer peripheral surface of the glass fiber 2. The second cladding 30 is a common cladding that surrounds the multiple cores 10 together with the multiple individual claddings 21 and 22.

[0053] The refractive index of the second cladding 30 is higher than that of the first cladding 20 and lower than the effective refractive index of any of the multiple cores 10. If the refractive index of the second cladding 30 is n30, then n11 > n30 > n20 and n12 > n30 > n20 are satisfied. The second cladding 30 is made of silica-based glass. The second cladding 30 may contain a dopant for adjusting the refractive index, or may be pure silica. The second cladding 30 has the effect of preventing deterioration of bending loss even when the effective cross-sectional area of ​​the core 10 is increased. In the MCF1B, individual claddings 21 and 22 are provided for each core 10, so that the first cladding 20 can be formed using the same manufacturing method as a single-core optical fiber. This reduces manufacturing costs.

[0054] 12 is a diagram showing the cross section perpendicular to the central axis and the refractive index profile of an MCF according to a third modified example. In an MCF 1C according to this modified example, the glass fiber 2 further includes a low refractive index portion 40. In this modified example, the number of low refractive index portions 40 is one. In the cross section, the low refractive index portion 40 is provided on a line LS connecting the core center 11a of the first core 11 and the core center 12a of the second core 12. In the cross section, the low refractive index portion 40 has, for example, a circular shape, and the diameter of the low refractive index portion 40 is larger than the diameter of the core 10.

[0055] The low-refractive-index portion 40 has a refractive index lower than the refractive index of the first cladding 20. When the refractive index of the low-refractive-index portion 40 is n40, the following relationships hold: n11>n20>n40, and n12>n20>n40. In the MCF1C, the low-refractive-index portion 40 can further reduce XT.

[0056] 13 is a diagram showing a cross section perpendicular to the central axis and a refractive index profile of an MCF according to a fourth modification. In an MCF 1D according to this modification, the glass fiber 2 further has a plurality of low refractive index portions 40. In this modification, the number of low refractive index portions 40 is two, which is the same as the number of cores 10. The plurality of low refractive index portions 40 are provided on a line LS connecting the core center 11 a of the first core 11 and the core center 12 a of the second core in the cross section. The low refractive index portion 40 includes a portion disposed between the first core 11 and the second core 12 in the cross section.

[0057] In cross section, the low refractive index portion 40 has, for example, an annular shape and surrounds one corresponding core 10. In cross section, the inner diameter of the low refractive index portion 40 is larger than the diameter of the core 10, and the low refractive index portion 40 is spaced apart from the core 10 at equal intervals over the entire circumference. The central axis of the low refractive index portion 40 coincides with the central axis of the corresponding core 10. The low refractive index portion 40 is arranged so as to be coaxial with the corresponding core 10.

[0058] The low-refractive-index portion 40 has a refractive index lower than the refractive index of the first cladding 20. When the refractive index of the low-refractive-index portion 40 is n40, the following relationships hold: n11>n20>n40, and n12>n20>n40. In the MCF1D, the low-refractive-index portion 40 can further reduce XT.

[0059] FIG. 14 shows the cross section perpendicular to the central axis and the refractive index profile of an MCF according to the fifth modification. This modification corresponds to a combination of the first and third modifications. In MCF1E according to this modification, the glass fiber 2 further includes a second cladding 30 and a low-refractive-index portion 40. The second cladding 30 has the same configuration as the second cladding 30 of MCF1A. The low-refractive-index portion 40 has the same configuration as the low-refractive-index portion 40 of MCF1C. In MCF1E, the second cladding 30 prevents the bending loss from increasing even when the effective cross-sectional area of ​​the core 10 is increased. Furthermore, the low-refractive-index portion 40 further reduces XT.

[0060] 15 is a diagram showing a cross section perpendicular to the central axis of an MCF according to a sixth modified example. In an MCF 1F according to this modified example, the glass fiber 2 further has a marker 50. The marker 50 is surrounded by the first cladding 20. In the cross section, the marker 50 is arranged at a position that breaks the symmetry (linear symmetry, rotational symmetry, etc.) of the central positions of the multiple cores 10. The refractive index of the marker 50 is different from the refractive index of the first cladding 20. The marker 50 makes it possible to identify the multiple cores 10 even when the core arrangement has rotational symmetry.

[0061] 16 is a diagram showing a cross section perpendicular to the central axis of an MCF according to a seventh modification. In an MCF 1G according to this modification, the multiple cores 10 are arranged in the cross section so that the center of gravity GC of the multiple cores 10 is offset from the central axis AX. This eliminates rotational symmetry in the core arrangement, making it possible to distinguish the multiple cores 10 from one another.

[0062] FIG. 17 is a diagram showing a cross section perpendicular to the central axis of an MCF according to an eighth modification. In the MCF 1H according to this modification, the multiple cores 10 are arranged in a square lattice pattern in the cross section. In this modification, the number of cores is four, and the MCF 1H is a four-core optical fiber. In this modification, the multiple cores 10 include two first cores 11 and two second cores 12. In FIG. 17 , the first cores 11 are shown without hatching, and the second cores 12 are shown with hatching. The multiple cores 10 have four adjacent core combinations. The multiple cores 10 are arranged so that in all adjacent core combinations, one adjacent core is the first core 11 and the other is the second core 12. From the perspective of using different types of cores between adjacent cores, arranging the multiple cores 10 in a square lattice pattern can increase core density while maintaining a low XT. In a square lattice arrangement, all adjacent cores can be configured with different types of cores.

[0063] FIG. 18 is a diagram showing a cross section perpendicular to the central axis of an MCF according to a ninth modification. In MCF1I according to this modification, the multiple cores 10 are arranged in a square lattice pattern in the cross section. In this modification, the number of cores is 12, and MCF1I is a 12-core optical fiber. In this modification, the multiple cores 10 include six first cores 11 and six second cores 12. In FIG. 16, the first cores 11 are shown without hatching and the second cores 12 are shown with hatching. The multiple cores 10 have 16 adjacent core combinations. The multiple cores 10 are arranged so that in all adjacent core combinations, one adjacent core is a first core 11 and the other is a second core 12. As with MCF1H, the core density can be increased while maintaining a low XT.

[0064] FIG. 19 is a diagram showing a cross section perpendicular to the central axis of an MCF according to a tenth modification. In the cross section of MCF1J according to this modification, the multiple cores 10 are arranged in a square lattice pattern. In this modification, the number of cores is 16, and MCF1J is a 16-core optical fiber. In this modification, the multiple cores 10 include eight first cores 11 and eight second cores 12. In FIG. 19, the first cores 11 are shown without hatching, and the second cores 12 are shown with hatching. The multiple cores 10 have 24 adjacent core combinations. The multiple cores 10 are arranged so that in all adjacent core combinations, one adjacent core is a first core 11 and the other is a second core 12. As with MCF1H, the core density can be increased while maintaining a low XT.

[0065] The above-described embodiments and modifications may be combined as appropriate. For example, the optical cable 100 may include an MCF1A instead of the MCF1. The MCF1 may be a five-core optical fiber, and the five cores 10 may be arranged in a cross shape. The MCF1 may be a nine-core optical fiber, and the nine cores 10 may be arranged in a 3 × 3 square lattice shape. The second modification and the fourth modification may be combined.

[0066] DESCRIPTION OF SYMBOLS 1, 1A, 1B, 1C, 1D, 1E, 1F, 1G, 1H, 1I, 1J...MCF 2...Glass fiber 3...Resin coating 10...Core 11...First core 11a...Core center 12...Second core 12a...Core center 20...First clad 21, 22...Individual clads 30...Second clad 40...Low refractive index portion 50...Marker 100...Optical cable 200...Outer jacket 300...Tensile strength wire AX...Central axis GC...Center of gravity LS...Line n11...Effective refractive index of first core n12...Effective refractive index of second core n20...Refractive index of first clad n30...Refractive index of second clad n40...Refractive index of low refractive index portion

Claims

1. A multi-core optical fiber comprising a glass fiber, wherein the glass fiber has a plurality of cores extending along the central axis of the glass fiber and a first cladding surrounding the plurality of cores, wherein the plurality of cores include a first core and a second core that are closest to each other, and wherein the effective refractive index of the first core at at least one wavelength in the range of 1260 nm to 1625 nm is n1 and the effective refractive index of the second core is n2, the effective refractive index is 250×10 －６ ≦n1−n2≦950×10 －６ wherein, when a mode field diameter of the first core at the wavelength is d1 [μm] and a mode field diameter of the second core is d2 [μm], d1-d2≦-0.17 [μm].

2. n1-n2≧400×10 －６ The multi-core optical fiber according to claim 1, wherein d1-d2≦−0.15 μm.

3. The multi-core optical fiber according to claim 2, wherein d1-d2≦-0.45 μm.

4. A multi-core optical fiber according to any one of claims 1 to 3, wherein d1-d2≧-1.4 [μm].

5. The multi-core optical fiber according to any one of claims 1 to 4, wherein the plurality of cores are arranged in a square lattice pattern in a cross section perpendicular to the central axis.

6. The multi-core optical fiber according to any one of claims 1 to 4, wherein the number of the plurality of cores is two.

7. The multi-core optical fiber according to claim 5 or 6, wherein, in a cross section perpendicular to said central axis, said plurality of cores are arranged so that the center of gravity of all of said plurality of cores is shifted from said central axis.

8. The multi-core optical fiber according to claim 5 or 6, wherein the glass fiber further comprises a marker surrounded by the first cladding, and the refractive index of the marker is different from the refractive index of the first cladding.

9. The multi-core optical fiber according to any one of claims 1 to 8, wherein the glass fiber further has a second cladding surrounding the first cladding, and the refractive index of the second cladding is higher than the refractive index of the first cladding and lower than the refractive index of any of the plurality of cores.

10. The multi-core optical fiber according to claim 9, wherein the first cladding is a common cladding that surrounds the plurality of cores together.

11. The multi-core optical fiber according to claim 9, wherein the first cladding includes a plurality of individual claddings surrounding the plurality of cores, respectively.

12. A multi-core optical fiber according to any one of claims 1 to 11, wherein the glass fiber further has a low-refractive-index portion having a refractive index lower than the refractive index of the first cladding, and the low-refractive-index portion is provided on a line connecting the central axis of the first core and the central axis of the second core in a cross section perpendicular to the central axis.

13. An optical cable comprising: a plurality of multi-core optical fibers according to any one of claims 1 to 12; and an outer jacket that houses the plurality of multi-core optical fibers.

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