Multi-core fiber

Abstract

To provide a highly practical multi-core fiber in which inter-core crosstalk is suppressed.SOLUTION: A multicore fiber includes a plurality of core portions, and a cladding portion that surrounds the plurality of core portions and has a refractive index lower than a maximum refractive index of the core portions, an outer diameter of the cladding portion is in a range of 125 μm ± 10 μ m, the number of the plurality of core portions is 2 to 4, the plurality of core portions is constituted by two or more core portion groups, and core portions belonging to different core portion groups among the core portions have effective core areas at a predetermined wavelength or cutoff wavelengths different from each other by 10% or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a multicore fiber. [Background technology]

[0002] In a multi-core fiber having multiple core portions, crosstalk between cores can become a problem. One method for solving this problem is to use heterogeneous core portions for the multiple core portions. Heterogeneous core portions are core portions whose effective refractive indices are different from one another (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5168702 Summary of the Invention [Problem to be solved by the invention]

[0004] However, known multi-core fibers have room for improvement in terms of realizing high practicality while suppressing inter-core crosstalk.

[0005] The present invention has been made in view of the above, and an object of the present invention is to provide a multi-core fiber in which inter-core crosstalk is suppressed and which has high practicality. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is a multicore fiber comprising a plurality of core portions and a cladding portion that surrounds the plurality of core portions and has a refractive index lower than the maximum refractive index of the core portions, wherein the outer diameter of the cladding portion is within a range of 125 μm±10 μm, the number of the plurality of core portions is 2 to 4, the plurality of core portions are configured into two or more core portion groups, and the core portions belonging to different core portion groups have effective core areas or cutoff wavelengths at a predetermined wavelength that differ by 10% or more.

[0007] At least one of the core portion groups may include a plurality of the core portions, and the effective core area and cutoff wavelength at a predetermined wavelength of the plurality of core portions included in the group may be within a range of ±10%.

[0008] The core parts closest to each other among the plurality of core parts may belong to different groups among the core part groups.

[0009] The number of the plurality of core portions may be four.

[0010] The plurality of cores propagate light in a single mode in a wavelength band including the predetermined wavelength, and have an effective core area of ​​60 to 180 μm 2 and the inter-core crosstalk may be -30 dB or less over a length of 100 km.

[0011] The core portion may include a center core made of silica glass containing germanium, and the cladding portion may be made of pure silica glass.

[0012] The core portion may include a center core made of silica glass containing at least one of fluorine, chlorine, potassium, and sodium, and the cladding portion may be made of silica glass containing fluorine.

[0013] With regard to the parameters defining the refractive index profiles of core portions belonging to different groups of the core portion groups, the parameter relating to the relative refractive index difference may be the same, and the parameter relating to the diameter may have a different magnification.

[0014] The core portion includes a center core and a structure located on the outer periphery of the center core, and the parameters defining the refractive index profile of core portions belonging to different core portion groups may be the same for the center core, but different for the structure located on the outer periphery of the center core.

[0015] The plurality of core portions include first core portions belonging to a first group of the core portion groups and second core portions belonging to a second group of the core portion groups, and at a wavelength of 1550 nm, an average effective core area of ​​the first core portions is 5 μm smaller than an average effective core area of ​​the second core portions. 2 or smaller, and the combination of the effective core area of ​​the first core portion and the effective core area of ​​the second core portion is 60 to 90 μm at a wavelength of 1550 nm. 2 and 90-120 μm 2 , 70-100μm 2 and 90 to 130 μm 2 , 90~120μm 2 and 110-140 μm 2 110~140μm 2 and 120-160 μm 2 , and 120-160 μm 2 and 140-180 μm 2 , or any one of them may be used.

[0016] the plurality of core units include a first core unit belonging to a first group of the core unit groups and a second core unit belonging to a second group of the core unit groups, The average cutoff wavelength of the first core region may be shorter than the average cutoff wavelength of the second core region by 100 nm or more, and the cutoff wavelength of the first core region and the cutoff wavelength of the second core region may be 1200 to 1500 nm and 1300 to 1520 nm.

[0017] The multi-core fiber may have a transmission loss of 0.25 dB / km or less at a wavelength of 1550 nm.

[0018] The core portion may include a center core, and the relative refractive index difference of the maximum refractive index of the center core with respect to the refractive index of the cladding portion may be 0.12 to 0.46%. [Effects of the Invention]

[0019] According to the present invention, it is possible to achieve an advantageous effect that a multi-core fiber in which inter-core crosstalk is suppressed and which has high practicality can be realized. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic cross-sectional view of a multi-core fiber according to a first embodiment, taken along a plane perpendicular to the longitudinal direction thereof. [Figure 2] FIG. 2 is an explanatory diagram of an example of a refractive index profile of the multi-core fiber shown in FIG. [Figure 3] FIG. 3 is a diagram illustrating an example of the relationship between the number of cores and inter-core crosstalk. [Figure 4] FIG. 4 is a diagram illustrating an example of the relationship between the number of cores and the confinement loss. [Figure 5] FIG. 5 is a diagram showing an example of the relationship between the difference in effective core cross-sectional area between cores and inter-core crosstalk. [Figure 6] FIG. 6 is a schematic cross-sectional view of a multi-core fiber according to the second embodiment in a plane perpendicular to the longitudinal direction. [Figure 7] FIG. 7 is a diagram illustrating an example of a refractive index profile of the multi-core fiber shown in FIG. [Figure 8] FIG. 8 is an explanatory diagram of a refractive index profile according to a modified example of the multi-core fiber according to the second embodiment. [Figure 9] FIG. 9 is a schematic cross-sectional view of a multi-core fiber according to the third embodiment in a plane perpendicular to the longitudinal direction. [Figure 10] FIG. 10 is a schematic cross-sectional view of a multi-core fiber according to the fourth embodiment in a plane perpendicular to the longitudinal direction. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the embodiments described below. Furthermore, in each drawing, the same or corresponding components are appropriately designated by the same reference numerals, and duplicate explanations are appropriately omitted. Furthermore, in this specification, cutoff wavelength or effective cutoff wavelength refers to the cable cutoff wavelength defined in ITU-T G.650.1 of the International Telecommunications Union (ITU). Furthermore, other terms not specifically defined in this specification shall follow the definitions and measurement methods in G.650.1 and G.650.2.

[0022] (Embodiment 1) 1 is a schematic cross-sectional view of a plane perpendicular to the longitudinal direction of a multi-core fiber according to embodiment 1. The multi-core fiber 10 includes four core portions 11, 12, 13, and 14 as multiple core portions, and a cladding portion 15.

[0023] The core portions 11 to 14 are arranged in a square shape in cross section. The core portions 11 to 14 have a step-type refractive index profile. The cladding portion 15 surrounds the core portions 11 to 14 and has a refractive index lower than the maximum refractive index of the core portions 11 to 14. In this specification, when the core portions have a step-type refractive index profile, the core portions themselves may be referred to as a center core.

[0024] The cores 11-14 are made of silica glass containing, for example, germanium, and the cladding 15 is made of pure silica glass. Here, pure silica glass refers to extremely high-purity silica glass that is substantially free of dopants that change the refractive index and has a refractive index of approximately 1.444 at a wavelength of 1550 nm. However, in this specification, pure silica glass also includes silica glass that contains a certain amount of chlorine, which is used in the manufacturing process. Note that the cores 11-14 may be made of silica glass containing at least one of fluorine, chlorine, potassium, and sodium, or pure silica glass, and the cladding 15 may be made of silica glass containing fluorine.

[0025] The outer diameter of the cladding portion 15 is set within the range of 125 μm±10 μm.

[0026] In this embodiment, the core portions 11 to 14 are configured into two core portion groups, a first group and a second group. The first group includes core portions 11 and 13 as the plurality of core portions, and the second group includes core portions 12 and 14 as the plurality of core portions.

[0027] Fig. 2 is an explanatory diagram of an example of the refractive index profile of the multi-core fiber 10. Fig. 2(a) shows the refractive index profiles of the core portions 11 and 13 belonging to the first group. Specifically, profile P11 is the refractive index profile of the core portions 11 and 13, and profile P12 is the refractive index profile of the cladding portion 15. Δ11 is a parameter that defines the refractive index profile of the core portions, and is the relative refractive index difference between the maximum refractive index of the core portion and the refractive index of the cladding portion. Furthermore, core diameter 2a1 is a parameter that defines the refractive index profile of the core portion, and is a parameter related to the diameter. Note that a1, which is half the value of core diameter 2a1, is sometimes referred to as the core radius.

[0028] 2(b) shows the refractive index profiles of the core portions 12 and 14 belonging to the second group. Specifically, profile P21 is the refractive index profile of the core portions 12 and 14, and profile P22 is the refractive index profile of the cladding portion 15. Core diameter 2k1a1 is a parameter that defines the refractive index profile of the core portion, and is a parameter related to the diameter, and is k1 times the core diameter 2a1 in FIG. 2(a). Here, k1 is a positive real number.

[0029] As shown in Fig. 2, the core portions 11 and 13 have the same refractive index profile, and therefore also have the same optical characteristics. Therefore, the core portions 11 and 13 have the same effective core area and cutoff wavelength at a predetermined wavelength. Similarly, the core portions 12 and 14 have the same refractive index profile, and therefore also have the same optical characteristics. Therefore, the core portions 12 and 14 have the same effective core area and cutoff wavelength at a predetermined wavelength. Note that the predetermined wavelength is a wavelength included in a wavelength band (used wavelength band) used as the wavelength of signal light when optical fiber communication is performed using the multicore fiber 10 as a transmission path. The used wavelength band is, for example, 1520 nm to 1620 nm. The predetermined wavelength is, for example, 1550 nm. Note that the multicore fiber 10 preferably propagates light in a single mode in the used wavelength band.

[0030] In the multicore fiber 10, cores belonging to different groups have effective core areas or cutoff wavelengths at a predetermined wavelength that differ by 10% or more. For example, the effective core areas (Aeff) of the cores 11 and 13 and the cores 12 and 14 at a wavelength of 1550 nm are 80 μm 2 , 110 μm 2 Alternatively, the cutoff wavelengths of the cores 11 and 13 and the cores 12 and 14 are 1300 nm and 1440 nm, respectively, which differ by 10% or more.

[0031] Furthermore, in the multi-core fiber 10, the most adjacent core portions among the core portions 11 to 14 belong to different core portion groups. For example, the core portions 12 and 14 most adjacent to the core portion 11, which belongs to the first group, belong to the second group.

[0032] In the multi-core fiber 10 configured as described above, the outer diameter of the cladding portion 15 is within a range of 125 μm±10 μm, which is approximately the same as the outer diameter of a standard optical fiber in optical fiber communications, and therefore, it is highly practical in that conventionally widely used tools and the like can be used. Furthermore, in the multi-core fiber 10, core portions belonging to different groups have effective core areas or cutoff wavelengths at a predetermined wavelength that differ by 10% or more, and therefore function as heterogeneous core portions. Therefore, inter-core crosstalk is suppressed. For example, in the multi-core fiber 10, the inter-core crosstalk at a wavelength of 1550 nm is −30 dB or less over a length of 100 km. Here, inter-core crosstalk refers to the value of crosstalk between a certain core portion and adjacent core portions.

[0033] Furthermore, in the multi-core fiber 10, the core portions 11 to 14 that are closest to each other belong to different core portion groups, which is more preferable in terms of suppressing inter-core crosstalk.

[0034] Furthermore, in the multi-core fiber 10, with regard to parameters defining the refractive index profiles of core portions belonging to different core portion groups, the parameter related to the relative refractive index difference is the same, but the parameter related to the diameter is a different magnification. Specifically, as shown in Fig. 2, core portions 11 and 13 and core portions 12 and 14 belonging to different groups have the same relative refractive index difference of Δ11, but the core diameter is different by a magnification of k1. In such a multi-core fiber 10, a common core preform is produced as a base for the core preforms for fabricating the core portions 11 and 13 and the core preforms for fabricating the core portions 12 and 14, and each core preform can be prepared by changing the thickness of the common core preform, which is preferable in terms of facilitating manufacturing.

[0035] (Suitable configuration for suppressing inter-core crosstalk) Next, the results of the inventor's intensive study to realize a multi-core fiber in which the inter-core crosstalk is suppressed and which is more practical will be described. Specifically, in a multi-core fiber in which the outer diameter of the cladding is 125 μm, the Aeff of the core is 80 μm. 2 and 110 μm 2 The structure was assumed to contain two types of heterogeneous cores and have a cutoff wavelength in the range of 1400±100nm. By varying the number of cores, the inter-core crosstalk (XT) at a wavelength of 1550nm and a length of 100km, and the confinement loss at a wavelength of 1625nm were investigated using simulation calculations and experiments.

[0036] Figure 3 is a diagram showing an example of the relationship between the number of cores and inter-core crosstalk. Figure 4 is a diagram showing an example of the relationship between the number of cores and confinement loss. Figures 3 and 4 plot data selected from patterns that provided good characteristics based on the results of numerous calculations and experiments.

[0037] As can be seen from Figures 3 and 4, to achieve an XT of -30 dB or less and a confinement loss of 0.001 dB / km or less, the number of cores is preferably between 2 and 4. If the number of cores is large, the distance between the cores becomes shorter, resulting in a larger XT, or the distance between the core and the outer periphery of the cladding becomes shorter, resulting in a larger confinement loss. The trends shown in Figures 3 and 4 vary slightly depending on the settings of Aeff and cutoff wavelength, but when Aeff is 60 to 180 μm 2 In addition, a core count of 4 is particularly preferable because it provides a large number of SDMs (Space Division Multiplex).

[0038] Next, the inventor investigated how much difference in Aeff is necessary to suppress inter-core crosstalk. Specifically, in a multi-core fiber with four cores as shown in Figure 1, the outer diameter of the cladding is set to 125 μm, and the confinement loss at a wavelength of 1625 nm in each core is set to 0.001 dB / km, and Aeff is set to 60 to 180 μm.2 We investigated the relationship between the difference in Aeff between cores and the inter-core crosstalk (XT) at a wavelength of 1550 nm and a length of 100 km in various multi-core fibers set to be

[0039] Figure 5 shows an example of the relationship between the difference in Aeff between cores and inter-core crosstalk (XT). XT is the average value of the four cores. As shown in Figure 5, it was confirmed that a difference in Aeff between cores of 10% or more is preferable in terms of keeping XT below -30 dB.

[0040] The inventors also investigated how much difference in cutoff wavelength is necessary to suppress inter-core crosstalk, and found that a difference of 10% or more in the cutoff wavelength between cores is also preferable in terms of keeping XT below -30 dB.

[0041] Even if there is a difference in cutoff wavelength between cores, as long as the difference is within the range in which light propagates in single mode in the wavelength band being used, there will be no significant difference in the optical characteristics of the cores. However, if there is a difference in Aeff, and the difference is too large, a difference in signal-to-noise ratio (SNR) will occur between the signal lights propagating through different cores. In this regard, it is better not to make the difference in Aeff between cores too large, for example, 70% or less is preferable.

[0042] According to the inventor's intensive study using calculations and experiments, it has been found that the four examples of Aeff combinations in Table 1 are preferable. Note that the Aeff, average confinement loss, and average XT in Table 1 are values ​​at wavelengths of 1550 nm, 1625 nm, and 1550 nm, respectively. That is, at a wavelength of 1550 nm, the combination of Aeff of the first core portion belonging to the first group and Aeff of the second core portion belonging to the second group is 60 to 90 μm. 2 and 90-120 μm 2 , 70-100μm 2 and 90 to 130 μm 2 , 90~120μm 2 and 110-140 μm 2 , 110~140μm2 and 120-160 μm 2 , and 120-160 μm 2 and 140-180 μm 2 Furthermore, it is preferable that the average Aeff of the first core portions belonging to the first group is 5 μm smaller than the average Aeff of the second core portions belonging to the second group. 2 It is preferable that the value is smaller than this.

[0043] [Table 1]

[0044] Note that Example 2 is particularly preferable from the viewpoints of low confinement loss and low XT. Furthermore, if the cutoff wavelength is too small, there is a risk of increased bending loss and deterioration of XT characteristics. Through extensive research by the present inventors, it has been found that it is preferable that the cutoff wavelength of the first group is 1200 to 1500 nm and that of the second group is 1300 to 1520 nm. It has also been found that it is preferable that the average cutoff wavelength of the first core portions belonging to the first group is 100 nm or more shorter than the average cutoff wavelength of the second core portions belonging to the second group. Furthermore, from the viewpoint of application to a system, it is preferable that the transmission loss at a predetermined wavelength is 0.25 dB / km.

[0045] Here, the difference between the Aeff of the first core portion belonging to the first group and the Aeff of the second core portion belonging to the second group by 10% or more means: (Aeff of the first core part) < (Aeff of the second core part) and ((Aeff of the second core part) - (Aeff of the first core part)) / (Aeff of the first core part) x 100 is 10% or more It can be defined as: Furthermore, the cutoff wavelength of the first core portion belonging to the first group differs from the cutoff wavelength of the second core portion belonging to the second group by 10% or more. (Cutoff wavelength of the first core) < (Cutoff wavelength of the second core) and ((Cutoff wavelength of the second core part) - (Cutoff wavelength of the first core part)) / (Cutoff wavelength of the first core part) x 100 is 10% or more It can be defined as:

[0046] Furthermore, it was found that in order to achieve the above-mentioned desirable Aeff and cutoff wavelength, it is preferable that the relative refractive index difference between the maximum refractive index of the center core in the core region and the refractive index of the cladding region is 0.12 to 0.46% in terms of Aeff, transmission loss, microbending loss, and XT characteristics. Note that if the relative refractive index difference is too low, there may be cases where the microbending loss and transmission loss increase and the XT characteristics deteriorate, while if the relative refractive index difference is too high, there may be cases where the transmission loss increases due to scattering loss and there may be cases where Aeff decreases.

[0047] (Embodiment 2) Fig. 6 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the multi-core fiber according to embodiment 2. The multi-core fiber 10A has a configuration in which the core portions 11 to 14 of the multi-core fiber 10 shown in Fig. 1 are replaced with core portions 11A, 12A, 13A, and 14A.

[0048] The core portions 11A to 14A are arranged in a square cross section. The core portions 11A to 14A have a trench-type refractive index profile. Specifically, the core portion 11A includes a center core 11A1, an intermediate layer 11A2, and a trench layer 11A3. The core portion 12A includes a center core 12A1, an intermediate layer 12A2, and a trench layer 12A3. The core portion 13A includes a center core 13A1, an intermediate layer 13A2, and a trench layer 13A3. The core portion 14A includes a center core 14A1, an intermediate layer 14A2, and a trench layer 14A3. In each of the core portions 11A to 14A, the intermediate layer surrounds the center core. The refractive index of the intermediate layer is the same as that of the cladding portion 15 or has a relative refractive index difference with respect to the cladding portion 15 within a range of ±0.05%. The trench layer surrounds the intermediate layer and has a refractive index lower than the refractive indexes of the intermediate layer and the cladding portion.

[0049] The center cores 11A1-14A1 are made of, for example, silica glass containing germanium, silica glass containing at least one of fluorine, chlorine, potassium, and sodium, or pure silica glass. The intermediate layers 11A2-14A2 are made of, for example, the same material as the cladding portion 15. The trench layers 11A3-14A3 are made of, for example, silica glass containing fluorine.

[0050] In this embodiment, the core portions 11A to 14A are configured into two core portion groups, a first group and a second group. The first group includes core portions 11A and 13A as the plurality of core portions, and the second group includes core portions 12A and 14A as the plurality of core portions.

[0051] 7 is an explanatory diagram of an example of the refractive index profile of the multi-core fiber 10. Fig. 7(a) shows the refractive index profiles of the core portions 11A and 13A belonging to the first group. Specifically, profile P31 is the refractive index profile of the center cores 11A1 and 13A1, profile P32 is the refractive index profile of the intermediate layers 11A2 and 13A2, profile P33 is the refractive index profile of the trench layers 11A3 and 13A3, and profile P34 is the refractive index profile of the cladding portion 15.

[0052] Δ12 is a parameter that defines the refractive index profile of the core portion and is the relative refractive index difference of the maximum refractive index of the center core of the core portion with respect to the refractive index of the cladding portion. Furthermore, the center core diameter 2a2 is a parameter that defines the refractive index profile of the core portion and is a parameter related to the diameter. Note that a2, which is half the center core diameter 2a2, is sometimes referred to as the center core radius. Δ22 is a parameter that defines the refractive index profile of the core portion and is the relative refractive index difference of the refractive index of the intermediate layer with respect to the refractive index of the cladding portion. Furthermore, the outer diameter 2b2, which is the outer diameter of the intermediate layer, is a parameter that defines the refractive index profile of the core portion and is a parameter related to the diameter. Δ32 is a parameter that defines the refractive index profile of the core portion and is the relative refractive index difference of the refractive index of the trench layer with respect to the refractive index of the cladding portion. Furthermore, the outer diameter 2c2, which is the outer diameter of the trench layer, is a parameter that defines the refractive index profile of the core portion and is a parameter related to the diameter.

[0053] 7(b) shows the refractive index profiles of the core portions 12A and 14A belonging to the second group. Specifically, profile P41 is the refractive index profile of the center cores 12A1 and 14A1, profile P42 is the refractive index profile of the intermediate layers 12A2 and 14A2, profile P43 is the refractive index profile of the trench layers 12A3 and 14A3, and profile P44 is the refractive index profile of the cladding portion 15.

[0054] Furthermore, the center core diameter 2k2a2 is a parameter that defines the refractive index profile of the core portion, and is a parameter related to the diameter, and is k2 times the center core diameter 2a2 in FIG. 7(a). Here, k2 is a positive real number. Similarly, the outer diameter 2k2b2, which is the outer diameter of the intermediate layer, is a parameter that defines the refractive index profile of the core portion, and is a parameter related to the diameter. Furthermore, the outer diameter 2k2c2, which is the outer diameter of the trench layer, is a parameter that defines the refractive index profile of the core portion, and is a parameter related to the diameter.

[0055] As shown in Figure 7, the core portions 11A and 13A have the same refractive index profile, and therefore also have the same optical characteristics. Therefore, the core portions 11A and 13A have the same effective core area and cutoff wavelength at a predetermined wavelength. Similarly, the core portions 12A and 14A have the same refractive index profile, and therefore also have the same optical characteristics. Therefore, the core portions 12A and 14A have the same effective core area and cutoff wavelength at a predetermined wavelength.

[0056] Here, also in the multi-core fiber 10A, core portions belonging to different groups have effective core areas or cutoff wavelengths at a predetermined wavelength that differ by 10% or more. Also in the multi-core fiber 10A, the core portions 11A to 14A that are closest to each other belong to different core portion groups. For example, the core portions 12A and 14A that are closest to the core portion 11A that belongs to the first group belong to the second group.

[0057] The multi-core fiber 10A configured as above also has a high practicality since the outer diameter of the cladding portion 15 is within the range of 125 μm±10 μm. Moreover, in the multi-core fiber 10A, the core portions belonging to different groups have effective core areas or cutoff wavelengths at a predetermined wavelength that differ by 10% or more, so inter-core crosstalk is suppressed. For example, in the multi-core fiber 10A, the inter-core crosstalk at a wavelength of 1550 nm is −30 dB or less at a length of 100 km.

[0058] Also in the multi-core fiber 10A, the core portions 11A to 14A that are closest to each other belong to different core portion groups, which is more preferable in terms of suppressing inter-core crosstalk.

[0059] Moreover, in the multi-core fiber 10A, with regard to parameters defining the refractive index profiles of core portions belonging to different core portion groups, the parameters related to the relative refractive index difference are the same, but the parameters related to the diameter are different in magnification. Specifically, as shown in Fig. 7 , the core portions 11A, 13A and the core portions 12A, 14A belonging to different groups have the same relative refractive index differences of Δ12, Δ22, and Δ32, respectively, but the parameters related to the diameter are different in magnification by a factor of k2. In such a multi-core fiber 10A, a common core preform is produced as a base for the core preforms for fabricating the core portions 11A and 13A and the core preforms for fabricating the core portions 12A and 14A, and each core preform can be prepared by changing the thickness of the common core preform, which is preferable in terms of facilitating manufacturing.

[0060] (Modification of the second embodiment) In the multi-core fiber 10A according to the embodiment 2, the refractive index profiles of the core portions 12A and 14A may be the following refractive index profiles. Fig. 8 is an explanatory diagram of a refractive index profile according to a modified example of the multi-core fiber 10 according to the embodiment 2. In this modified example, a profile P51 is the refractive index profile of the center cores 12A1 and 14A1, a profile P52 is the refractive index profile of the intermediate layers 12A2 and 14A2, a profile P53 is the refractive index profile of the trench layers 12A3 and 14A3, and a profile P54 is the refractive index profile of the cladding portion 15.

[0061] The outer diameter 2c3 of the trench layer is a parameter that defines the refractive index profile of the core portion, and is a parameter related to the structure that exists on the outer periphery of the center core. Note that c2 and c3 are different values.

[0062] In this modification, the core portions 12A and 14A have the same refractive index profile and therefore the same optical characteristics, and therefore the core portions 12A and 14A have the same effective core area and cutoff wavelength at a predetermined wavelength.

[0063] Also in the multicore fiber according to this modification, core portions belonging to different groups have effective core areas or cutoff wavelengths at a predetermined wavelength that differ by 10% or more. Also in the multicore fiber according to the modification, closest core portions belong to different core portion groups.

[0064] The multicore fiber according to this modified example configured as described above is also highly practical and suppresses inter-core crosstalk. Moreover, since the closest adjacent core portions belong to different core portion groups, it is more preferable in terms of suppressing inter-core crosstalk.

[0065] Furthermore, in the multicore fiber according to this modification, with regard to the parameters defining the refractive index profiles of core portions belonging to different core portion groups, the parameters related to the center core are the same, but the parameters related to the structure existing on the outer periphery of the center core are different. In such a multicore fiber, the Aeff characteristic is predominantly determined mainly by the center core structure, and therefore it is possible to configure core portions that have similar Aeff values ​​but have heterogeneous core structures.

[0066] (Embodiment 3) Fig. 9 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the multi-core fiber according to embodiment 3. The multi-core fiber 10B has a configuration in which the core portions 11 to 14 of the multi-core fiber 10 shown in Fig. 1 are replaced with core portions 11B, 12B, and 13B.

[0067] The core portions 11B to 13B are arranged in the shape of an equilateral triangle in cross section, and have a step-type or trench-type refractive index profile.

[0068] In this embodiment, the core portions 11B to 13B form three core portion groups, that is, each of the core portions 11B to 13B forms a group by itself.

[0069] In the multi-core fiber 10B, core portions belonging to different groups have effective core areas or cutoff wavelengths at a predetermined wavelength that differ by 10% or more. Also in the multi-core fiber 10B, the core portions closest to each other among the core portions 11B to 13B belong to different core portion groups.

[0070] The multi-core fiber 10B configured as above is also highly practical and suppresses inter-core crosstalk.

[0071] In this embodiment, each of the core units 11B to 13B constitutes a group by itself, but two core units may constitute one group and one other core unit may constitute another group.

[0072] (Embodiment 4) Fig. 10 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the multi-core fiber according to embodiment 4. The multi-core fiber 10C has a configuration in which the core portions 11 to 14 of the multi-core fiber 10 shown in Fig. 1 are replaced with core portions 11C and 12C.

[0073] The core portions 11C and 12C have a step-type or trench-type refractive index profile.

[0074] In this embodiment, the core units 11C and 12C constitute two core unit groups, that is, each of the core units 11C and 12C constitutes a group by itself.

[0075] In the multi-core fiber 10C, cores belonging to different groups also differ in effective core area or cutoff wavelength at a predetermined wavelength by 10% or more.

[0076] The multi-core fiber 10C configured as above is also highly practical and suppresses inter-core crosstalk.

[0077] (Manufacturing method) The multi-core fiber according to the embodiment can be manufactured using, for example, a method called a hole-punching method, which will be described below.

[0078] For example, the core preform is manufactured using a known VAD (Vapor-phase Axial Deposition) method or OVD (Outside Vapor Deposition) method. The core preform is a preform that has a portion that will become a part of the cladding so as to surround the portion that will become the core portion of the multi-core fiber. The number of types of core preforms prepared corresponds to the number of core portion groups. Note that the diameter of the core preform may differ for each core portion group.

[0079] In addition, a clad base material for forming the majority of the clad portion is prepared, and holes for inserting the core base material into the clad base material are formed in the clad base material in the number equal to the number of core portions. After the holes are formed, the insides of the holes may be cleaned.

[0080] Next, core preforms are inserted into the holes of the cladding preform and integrated by heat treatment, etc. to form an optical fiber preform. Furthermore, a multi-core fiber is drawn from the optical fiber preform using a known drawing furnace.

[0081] (Example) According to the above-mentioned manufacturing method, a multi-core fiber having the structure of the multi-core fiber 10A according to the second embodiment was manufactured. The center core was made of pure silica glass, and the intermediate layer, trench layer, and cladding portion were made of silica glass containing fluorine. The Aeff of the core portion was 80 μm at a wavelength of 1550 nm. 2 or 110 μm 2 The outer diameter of the cladding was designed to be 125 μm. The center-to-center distance between the most adjacent cores (core pitch) was 39 μm, and the distance between the center of the core closest to the outer periphery of the cladding and the outer periphery of the cladding (outer thickness) was 35 μm.

[0082] The structural parameters and optical properties of each core of the multicore fiber manufactured as described above are shown in Table 2. In Table 2, center core Δ is the relative refractive index difference of the center core, and trench Δ is the relative refractive index difference of the trench layer. The values ​​outside the parentheses are based on the refractive index of the cladding, and the values ​​inside the parentheses are based on the refractive index of pure silica glass. Cladding Δ is the relative refractive index difference of the cladding based on the refractive index of pure silica glass. Furthermore, b / a is the ratio of the outer diameter of the intermediate layer to the center core diameter, and c / a is the outer diameter of the trench layer to the center core diameter. λcc is the cutoff wavelength. λ0 is the zero-dispersion wavelength. Bending loss @ 1550 nm / 30 mm Φ is the bending loss at a wavelength of 1550 nm when the multicore fiber is bent at a diameter of 30 mm.

[0083] As shown in Table 2, in all of the multi-core fibers of the examples, the XT was suppressed to -30 dB or less even in the core, and the outer diameter of the cladding was 125 μm, making them highly practical. Furthermore, although not shown in Table 2, the confinement loss at a wavelength of 1550 nm in the multi-core fibers of the examples was 0.01 dB / km or less, which was a sufficiently low loss.

[0084] [Table 2]

[0085] In the above embodiments, the refractive index profile of the core portion is a step type or a trench type, but the present invention is not limited to this and may be, for example, a W-type. Also, in the above embodiments, the number of core portion groups is two or three, but it may be two or more.

[0086] In the above modification, the relative refractive index difference (trench Δ) of the trench layer of the core portion of the first group is the same as the relative refractive index difference (trench Δ) of the trench layer of the core portion of the second group, but they may be different from each other. In the above modification, b2 and b3 are different values ​​and c2 and c3 are different values, but only one of them may be different.

[0087] Furthermore, the effective core area and cutoff wavelength at a predetermined wavelength of core portions included in the same core portion group do not necessarily have to be the same, but it is preferable that they are within a range of ±10%.

[0088] Furthermore, the present invention is not limited to the above-described embodiments. The present invention also includes configurations in which the above-described components are appropriately combined. Furthermore, further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspects of the present invention are not limited to the above-described embodiments, and various modifications are possible. [Explanation of symbols]

[0089] 10, 10A, 10B, 10C: Multi-core fiber 11, 11A, 11B, 11C, 12, 12A, 12B, 12C, 13, 13A, 13B, 14, 14A: Core part 11A1, 12A1, 13A1, 14A1: Center core 11A2, 12A2, 13A2, 14A2: Middle layer 11A3, 12A3, 13A3, 14A3: trench layer 15: Cladding section P11, P12, P21, P22, P31, P32, P33, P34, P41, P42, P43, P44, P51, P52, P53, P54: Profiles

Claims

1. A plurality of cores; a cladding portion that surrounds the plurality of core portions and has a refractive index lower than the maximum refractive index of the core portions; Equipped with The outer diameter of the cladding portion is within the range of 125 μm ± 10 μm, the number of the plurality of core portions is 2 to 4, the plurality of core units are configured by two or more core unit groups, Among the core portions, core portions belonging to different core portion groups have effective core cross-sectional areas or cutoff wavelengths at a predetermined wavelength that differ by 10% or more. Multicore fiber.

2. At least one of the core portion groups includes a plurality of the core portions, and the effective core area and cutoff wavelength at a predetermined wavelength of the plurality of core portions included in the group are within a range of ±10%. The multicore fiber according to claim 1 .

3. The core parts closest to each other among the plurality of core parts belong to different groups among the core part groups. The multicore fiber according to claim 1 .

4. The number of the cores is four. The multicore fiber according to claim 1 .

5. The plurality of core portions, in a wavelength band including the predetermined wavelength, It propagates light in a single mode, Effective core area is 60 to 180 μm ２ and Core-to-core crosstalk is less than -30 dB at a length of 100 km. The multicore fiber according to claim 1 .

6. the core portion includes a center core made of silica glass containing germanium, The cladding portion is made of pure silica glass. The multicore fiber according to claim 1 .

7. the core portion includes a center core made of silica glass containing at least one of fluorine, chlorine, potassium, and sodium; The cladding portion is made of silica glass containing fluorine. The multicore fiber according to claim 1 .

8. Regarding the parameters defining the refractive index profiles of core portions belonging to different groups among the core portion groups, the parameters relating to the relative refractive index difference are the same, and the parameters relating to the diameter are different in magnification. The multicore fiber according to claim 1 .

9. the core portion includes a center core and a structure located on an outer periphery of the center core, With regard to the parameters defining the refractive index profiles of core portions belonging to different groups among the core portion groups, the parameters relating to the center core are the same, but the parameters relating to the structure existing on the outer periphery of the center core are different. The multicore fiber according to claim 1 .

10. the plurality of core units include a first core unit belonging to a first group of the core unit groups and a second core unit belonging to a second group of the core unit groups, At a wavelength of 1550 nm, the average effective core area of ​​the first core portion is 5 μm smaller than the average effective core area of ​​the second core portion. ２ Smaller than At a wavelength of 1550 nm, the combination of the effective core area of ​​the first core portion and the effective core area of ​​the second core portion is 60 to 90 μm ２ and 90 to 120 μm ２ , 70 to 100 μm ２ and 90 to 130 μm ２ , 90 to 120 μm ２ and 110 to 140 μm ２ , 110 to 140 μm ２ and 120 to 160 μm ２ , and 120 to 160 μm ２ and 140 to 180 μm ２ , It is one of The multicore fiber according to claim 1 .

11. the plurality of core units include a first core unit belonging to a first group of the core unit groups and a second core unit belonging to a second group of the core unit groups, an average cutoff wavelength of the first core portion is shorter than an average cutoff wavelength of the second core portion by 100 nm or more; The cutoff wavelength of the first core portion is 1200 to 1500 nm, and the cutoff wavelength of the second core portion is 1300 to 1520 nm. The multicore fiber according to claim 1 .

12. The transmission loss at a wavelength of 1550 nm is 0.25 dB / km or less. The multicore fiber according to claim 1 .

13. the core portion includes a center core, The relative refractive index difference of the maximum refractive index of the center core to the refractive index of the cladding portion is 0.12 to 0.46%. The multicore fiber according to claim 1 .

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