Multi-core fiber

Abstract

To provide a multi-core fiber which can suppress propagation of a higher mode even with a short length.SOLUTION: A multi-core fiber is composed of a plurality of core parts composed of glass, a clad part which is composed of glass and surrounds the outer periphery of the plurality of core parts, a primary layer which is composed of a resin and surrounds the outer periphery of the clad part, and a secondary layer which is composed of a resin and surrounds the outer periphery of the primary layer. When a maximum specific refractive index difference of the core part to an average refractive index of the clad part is represented by Δ, Δ is 0.2% or more and 0.5% or less, a core diameter of the core part is 4.0 μm or more and 15 μm or less, a clad diameter of the clad part is 125 μm or more, a ratio of thickness of the primary layer to thickness of the secondary layer is 0.35 or less, a cutoff wavelength when being measured with a length of less than 2 m is less than 1,530 nm, and a cutoff wavelength when being measured with the length is 1,260 nm or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Optical fibers are often used in applications requiring long lengths, such as communication lines. In contrast, when optical fibers are used as optical components or as optical fibers installed in optical components, the length of the optical fibers may be shortened. Furthermore, with the recent development of the Internet of Things (IoT) leading to an increase in the capacity of information communication, multicore fibers may be used for wiring within communication devices. When wiring within communication devices, the length of multicore fibers is also relatively short.

[0003] According to ITU-T G.650.1 of the International Telecommunications Union (ITU), the cutoff wavelength of an optical fiber is determined by the characteristics measured when the length is set to 2 m. However, as mentioned above, optical fibers are sometimes used at lengths shorter than 2 m. In such short optical fibers, the cutoff wavelength shifts to the longer wavelength side compared to when the length is 2 m. As a result, the optical fiber may unintentionally become a multimode fiber in the desired wavelength band.

[0004] In order to prevent an optical fiber from unintentionally becoming a multimode fiber, it is useful to suppress the propagation of higher-order modes. As a technique for suppressing the propagation of higher-order modes in an optical fiber, for example, a technique employing a trench structure as disclosed in Patent Document 1 is known.

[0005] Furthermore, in multi-core fibers, crosstalk between multiple cores can become a problem, and a trench structure is also used to solve the problem of crosstalk (for example, Patent Document 2). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-310328 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-54252 Summary of the Invention [Problem to be solved by the invention]

[0007] However, Patent Document 1 employs a complex trench structure, which makes its design and manufacturing difficult. Furthermore, in the case of a multicore fiber such as that described in Patent Document 2, employing a trench structure may cause a problem in that the cutoff wavelength shifts to the long wavelength side, particularly in the core portion close to the central axis of the cladding portion. Furthermore, Patent Document 2 increases the refractive index of the core portion to suppress propagation of higher-order modes, but increasing the refractive index may result in a deterioration in crosstalk. Therefore, it is necessary to design the refractive index taking crosstalk into consideration, which may narrow the design range.

[0008] 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 that can suppress propagation of higher-order modes even when the fiber length is short. [Means for solving the problem]

[0009] 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 made of glass; a cladding portion made of glass and surrounding the outer periphery of the plurality of core portions; a primary layer made of resin and surrounding the outer periphery of the cladding portion; and a secondary layer made of resin and surrounding the outer periphery of the primary layer; wherein, when a maximum relative refractive index difference of the core portions with respect to an average refractive index of the cladding portion is Δ, Δ is 0.2% or more and 0.5% or less; the core portions have a core diameter of 4.0 μm or more and 15 μm or less; the cladding diameter of the cladding portion is 125 μm or more; a ratio of a thickness of the primary layer to a thickness of the secondary layer is 0.35 or less; and a cutoff wavelength when measured at a length of less than 2 m is less than 1530 nm, and a cutoff wavelength when measured at the above length is 1260 nm or more.

[0010] The primary layer may have an elastic modulus of 0.75 MPa or more.

[0011] One aspect of the present invention is a multicore fiber comprising: a plurality of core portions made of glass; a cladding portion made of glass and surrounding the outer periphery of the plurality of core portions; a primary layer made of resin and surrounding the outer periphery of the cladding portion; and a secondary layer made of resin and surrounding the outer periphery of the primary layer; wherein, when a maximum relative refractive index difference of the core portions with respect to an average refractive index of the cladding portions is Δ, Δ is 0.2% or more and 0.5% or less; the core portions have a core diameter of 4.0 μm or more and 15 μm or less; the cladding diameter of the cladding portion is 125 μm or more; a ratio of a thickness of the primary layer to a thickness of the secondary layer is 0.35 or less; and a cutoff wavelength when measured at a length of less than 2 m is less than 1260 nm, and a cutoff wavelength when measured at the above length is 990 nm or more.

[0012] The primary layer may have an elastic modulus of 0.75 MPa or more.

[0013] One aspect of the present invention is a multicore fiber comprising: a plurality of core portions made of glass; a cladding portion made of glass and surrounding the outer periphery of the plurality of core portions; a primary layer made of resin and surrounding the outer periphery of the cladding portion; and a secondary layer made of resin and surrounding the outer periphery of the primary layer; wherein, when a maximum relative refractive index difference of the core portions with respect to an average refractive index of the cladding portions is Δ, Δ is 0.2% or more and 0.5% or less; the core portions have a core diameter of 4.0 μm or more and 15 μm or less; the cladding diameter of the cladding portion is 125 μm or more; a ratio of a thickness of the primary layer to a thickness of the secondary layer is 0.35 or less; and a cutoff wavelength when measured at a length of less than 2 m is less than 1565 nm, and a cutoff wavelength when measured at the above length is 1295 nm or more.

[0014] The primary layer may have an elastic modulus of 0.75 MPa or more.

[0015] The multi-core fiber may have a length of less than 2 m.

[0016] The multicore fiber may have the ratio less than 0.3.

[0017] The multicore fiber may have the ratio less than 0.25.

[0018] The multicore fiber may comprise a reference core portion made of glass, a reference cladding portion made of glass and surrounding the outer periphery of the reference core portion, a reference primary layer made of resin and surrounding the outer periphery of the reference cladding portion, and a reference secondary layer made of resin and surrounding the outer periphery of the reference primary layer, and the ratio of the microbending loss sensitivity to the microbending loss sensitivity of a reference optical fiber in which the reference cladding portion has a cladding diameter of 125 μm, the reference cladding portion has a modulus of elasticity of 70,000 MPa, the reference primary layer has a modulus of elasticity of 0.5 MPa, and the reference secondary layer has a modulus of elasticity of 1,000 MPa may be 2 or more. [Effects of the Invention]

[0019] The present invention has an effect of realizing a multi-core fiber that can suppress propagation of higher-order modes even if it has a short length. [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. [Figure 2] FIG. 2 is a diagram showing an example of the relationship between the core diameter and the relative refractive index difference Δ. [Figure 3] FIG. 3 is a diagram showing an example of the relationship between the P / S thickness ratio and the relative MB sensitivity. [Figure 4] FIG. 4 is a diagram showing an example of the relationship between the P / S thickness ratio and the relative MB sensitivity. [Figure 5] FIG. 5 is a diagram showing an example of the relationship between the P / S thickness ratio and the relative MB sensitivity. [Figure 6] FIG. 6 is a diagram showing an example of the relationship between the primary elastic modulus and the primary diameter. [Figure 7] FIG. 7 is a diagram showing an example of the relationship between the primary elastic modulus and the primary diameter. [Figure 8] FIG. 8 is a diagram showing an example of the relationship between the primary elastic modulus and the primary diameter. [Figure 9] FIG. 9 is a diagram showing an example of the relationship between the primary elastic modulus and the primary diameter. [Figure 10] FIG. 10 is a diagram showing an example of the relationship between the primary elastic modulus and the primary diameter. [Figure 11] FIG. 11 is a diagram showing an example of the relationship between the primary elastic modulus and the P / S thickness ratio. [Figure 12] FIG. 12 is a diagram showing an example of the relationship between the core diameter and the relative refractive index difference Δ. [Figure 13] FIG. 13 is a diagram showing an example of the relationship between the core diameter and the relative refractive index difference Δ. [Figure 14] FIG. 14 is a schematic cross-sectional view of a multi-core fiber according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that 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. 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 multicore fiber according to embodiment 1. A multicore fiber 10 includes seven cores 11 made of glass such as silica-based glass, a cladding 12 made of glass such as silica-based glass and surrounding the outer periphery of the cores 11, a primary layer 13 made of resin and surrounding the outer periphery of the cladding 12, and a secondary layer 14 made of resin and surrounding the outer periphery of the primary layer 13. In this embodiment, the cores 11 are arranged to form a triangular lattice.

[0023] The seven core portions 11 are located approximately at the center of the cladding portion 12 and are composed of two types of core portions: a first core portion close to the center, and six second core portions surrounding the first core portion and far from the center.

[0024] The portion of the multi-core fiber 10 that includes the core 11 and the cladding 12 is made of glass, and is sometimes called a glass optical fiber. The primary layer 13 and the secondary layer 14 are sometimes called coating layers. A multi-core fiber that includes a coating layer is sometimes called a bare multi-core fiber.

[0025] The glass optical fiber in the multicore fiber 10 has, for example, a step-index type refractive index profile. However, the refractive index profile of the glass optical fiber may be other refractive index profiles, for example, W-type or trench type. In this specification, Δ is defined as the relative refractive index difference (maximum relative refractive index difference) of the maximum refractive index of the core portions 11 with respect to the average refractive index of the cladding portion 12. In this embodiment, Δ is 0.2% or more and 0.5% or less. The core diameter of the core portions 11 is 4.0 μm or more and 15 μm or less. The outer diameter (cladding diameter) of the cladding portion 12 is 125 μm or more. The center-to-center distance (also called core pitch) between adjacent core portions 11 is, for example, 5.80 μm, but is not limited to this.

[0026] The resin constituting the primary layer 13 and the secondary layer 14 is, for example, a UV-curable resin. The UV-curable resin is a blend of various resin materials and additives, such as an oligomer, a diluent monomer, a photopolymerization initiator, a silane coupling agent, a sensitizer, and a lubricant. As the oligomer, conventionally known materials such as polyether-based urethane acrylate, epoxy acrylate, polyester acrylate, and silicone acrylate can be used. As the diluent monomer, conventionally known materials such as monofunctional monomers and polyfunctional monomers can be used. Furthermore, the additives are not limited to those described above, and a wide variety of conventionally known additives used for UV-curable resins and the like can be used.

[0027] The primary modulus of elasticity of the primary layer 13 is smaller than the secondary modulus of elasticity of the secondary layer 14. The secondary modulus of elasticity is, for example, 500 MPa or more and 1000 MPa or less. The primary modulus of elasticity and the secondary modulus of elasticity are also called Young's moduli. These moduli can be achieved by adjusting the resin components, manufacturing conditions, etc. Specifically, the primary modulus of elasticity and the secondary modulus of elasticity can be adjusted by adjusting the type, molecular weight, and content of oligomers in the materials constituting the primary layer 13 and the secondary layer 14, the type and amount of diluent monomer, the type and content of other components, and curing conditions such as UV irradiation intensity.

[0028] In the multicore fiber 10 configured as above, it is preferable that the ratio of the thickness of the primary layer 13 to the thickness of the secondary layer 14, i.e., (thickness of primary layer) / (thickness of secondary layer), is 0.35 or less, the cutoff wavelength when measured at a length of less than 2 m is less than 1530 nm, and the cutoff wavelength when measured at the above length is 1260 nm or more. With such a multicore fiber 10, propagation of higher-order modes is suppressed even when the length is less than 2 m, and light with wavelengths of 1530 nm to 1565 nm, known as the C-band, can be transmitted in single mode.

[0029] The cutoff wavelength measured at a length of less than 2 m refers to the cutoff wavelength obtained by performing the cutoff wavelength measurement method specified in ITU-T G.650.1, with the only change being that the length of the optical fiber being measured is less than 2 m.

[0030] A more detailed explanation follows: First, in order to suppress propagation of higher-order modes, the inventors investigated the characteristics of the cutoff wavelength (hereinafter sometimes referred to as the 2m cutoff wavelength) based on the characteristics measured with a length set to 2m, as specified in ITU-T G.650.1.

[0031] Fig. 2 is a diagram showing an example of the relationship between the core diameter and the relative refractive index difference (Δ) in a glass optical fiber having a step-index refractive index profile, such that the 2m cutoff wavelength (λc) is 1530nm or 1550nm. Note that Fig. 2 shows the results of a simulation calculation using the weak waveguide approximation. In Fig. 2, the dashed line is an approximation curve of a quartic function of the data points when λc is 1530nm. The approximation curve is expressed as y=0.0001x where x is the core diameter and y is the relative refractive index difference. 4 -0.0063x 3 +0.1235x 2 -1.1514x+4.531 (r 2 =0.9999). Therefore, y<0.0001x4 -0.0063x 3 +0.1235x 2 It was confirmed that the 2m cutoff wavelength was less than 1530nm when -1.1514x+4.531 was used.

[0032] However, as mentioned above, when the length of an optical fiber is less than 2 m, the cutoff wavelength shifts to longer wavelengths than the 2 m cutoff wavelength. Therefore, even if the 2 m cutoff wavelength is less than 1530 nm, the cutoff wavelength may be 1530 nm or greater. Furthermore, when the refractive index profile is the W-type or trench-type mentioned above, the low-refractive-index region included in the core may cause higher-order modes to propagate more easily in the core near the center of the cladding. This may result in the cutoff wavelength shifting to longer wavelengths, resulting in a multimode propagation state. Therefore, the present inventors have come up with a technical idea that, when the length is short, such as less than 2 m, the properties of the coating layer are adjusted to increase the microbending loss caused by the coating layer, thereby increasing the propagation loss of higher-order modes and suppressing their propagation. This technical idea is significantly different from the usual technical idea that the increase in microbending loss due to the coating layer should be minimized.

[0033] The present inventors have investigated the microbending loss sensitivity φbend as an index of an increase in microbending loss. Hereinafter, the microbending loss sensitivity may be referred to as MB loss sensitivity.

[0034] For example, according to J. Baldauf, N. Okada, and M. Miyamoto, “Relationship of mechanical characteristics of dual coated single mode optical fibers and microbending loss,” IEICE Trans. Commun., vol. E76-B, no. 4, pp. 352-357, 1993, φbend can be expressed by the following equation:

[0035] φbend=3f0xs 2 / (H g 2 D0 0.375 H s 0.625 ) f0=τ w / r D H g =πE g r g 4 / 4 D0=E p +E s (r s -r p ) 3 / r s 3 H s =πE s (r s 4 -r p 4 ) / 4 x s =E p r g / (r p -r g )

[0036] In addition, E g is the elastic modulus of the cladding, and E p is the primary elastic modulus, E s is the secondary modulus, and r g is the outer radius of the cladding (1 / 2 of the cladding diameter), and r p is the outer radius of the primary layer (1 / 2 of the primary diameter), and r s is the outer radius of the secondary layer (1 / 2 of the secondary diameter), and r D is the drum radius (=0.3m), and τ w is the winding tension (=10N).

[0037] Here, an optical fiber that serves as a reference for MB loss sensitivity is defined as a reference optical fiber. The reference optical fiber includes a reference core made of glass, a reference cladding made of glass and surrounding the outer periphery of the reference core, a reference primary layer made of resin and surrounding the outer periphery of the reference cladding, and a reference secondary layer made of resin and surrounding the outer periphery of the reference primary layer. The reference optical fiber has a cladding diameter of 125 μm, a primary diameter of 195 μm, a secondary diameter of 250 μm, and an elastic modulus of the reference cladding of 70,000 MPa, a primary elastic modulus of 0.5 MPa, and a secondary elastic modulus of 1,000 MPa. The MB loss sensitivity of this reference optical fiber is defined as a reference MB sensitivity φbend_ref. The inventor then defined the ratio of the MB loss sensitivity φbend_s of a target optical fiber to φbend_ref as R (=φbend_s / φbend_ref, i.e., relative MB loss sensitivity), and when R is 1, the microbending loss is negligibly small. However, it was found that if R is 2 or more, the microbending loss is large and this is effective in suppressing the propagation of higher-order modes, that is, in suppressing the long-wavelength shift of the cut wavelength.

[0038] Next, the inventors investigated the optimal (primary layer thickness) / (secondary layer thickness) (=P / S thickness ratio) for various combinations of cladding diameter (GD) and secondary diameter (SD) in the multi-core fiber 10 shown in Fig. 1 as shown in Table 1. Specifically, the investigated cladding diameters (GD) were 125 µm, 150 µm, 180 µm, 210 µm, and 250 µm. [Table 1]

[0039] FIG. 3 shows an example of the relationship between the P / S thickness ratio and the relative MB sensitivity, where the primary elastic modulus is 0.5 MPa and the secondary elastic modulus is 1000 MPa. FIG. 4 shows an example of the relationship between the P / S thickness ratio and the relative MB sensitivity, where the primary elastic modulus is 1.0 MPa and the secondary elastic modulus is 1000 MPa. FIG. 5 shows an example of the relationship between the P / S thickness ratio and the relative MB sensitivity, where the primary elastic modulus is 2.0 MPa and the secondary elastic modulus is 1000 MPa. The numbers in the legends in FIGS. 3 to 5 correspond to the data numbers in Table 1. As shown in FIGS. 3 to 5, the inventors' investigations have revealed that, for various combinations of GD and SD and primary elastic moduli, a P / S thickness ratio of 0.35 or less is preferable to achieve an R of 2 or greater. Furthermore, it has been found that a P / S thickness ratio of less than 0.3 is more preferable, and a P / S thickness ratio of less than 0.25 is even more preferable. Furthermore, the inventors investigated the relationship between the primary elastic modulus and R by setting various P / S thickness ratios, and found that a primary elastic modulus of 0.75 MPa or more is more suitable for making R 2 or more.

[0040] Next, the inventors investigated the relationship between the primary elastic modulus and the primary diameter. Fig. 6 is a diagram showing an example of the relationship between the primary elastic modulus and the primary diameter in the multi-core fiber 10 shown in Fig. 1 when the cladding diameter is 125 µm, the secondary diameter is 250 µm, and the secondary elastic modulus is 1000 MPa. In Fig. 6, P-ISM on the horizontal axis means the primary elastic modulus, and P-diameter on the vertical axis means the primary diameter. Furthermore, the dashed line indicates the case where R is 1, and the solid line indicates the case where R is 2. In Fig. 6, the area to the lower right of the solid line indicated by the arrow is the condition that R is 2 or more. Note that when the solid line is represented as an approximate curve using a quartic function, where x is the primary elastic modulus and y is the primary diameter, y=587.69x 4 -682x 3 +295.88x 2 +13.713x+126.81 (r 2 =0.9996). Therefore, y≦587.69x 4 -682x 3+295.88x 2 It was confirmed that R can be set to 2 or more when +13.713x+126.81 is used. For example, when R=2, the primary diameter is in the range of 128μm to 190μm. In this case, the thickness of the primary layer (primary thickness) is in the range of 1.5μm to 32.5μm, and the thickness of the secondary layer (secondary thickness) is in the range of 30μm to 61μm.

[0041] Next, Fig. 7 is a diagram showing an example of the relationship between the primary elastic modulus and the primary diameter in the multi-core fiber 10 shown in Fig. 1 when the cladding diameter is 180 µm, the secondary diameter is 250 µm, and the secondary elastic modulus is 1000 MPa. In Fig. 7, P-ISM on the horizontal axis means the primary elastic modulus, and P-diameter on the vertical axis means the primary diameter. Furthermore, the dashed line indicates the case where R is 1, and the solid line indicates the case where R is 2. In Fig. 7, the area to the lower right of the solid line indicated by the arrow is the condition where R is 2 or more. When the solid line is expressed as an approximate curve using a quartic function, where x is the primary elastic modulus and y is the primary diameter, y=135.14x 4 -158.37x 3 +78.077x 2 +16.624x+180.55 (r 2 =0.9999). Therefore, y≦135.14x 4 -158.37x 3 +78.077x 2 It was confirmed that R can be set to 2 or more when +16.624x+180.55 is used. For example, when R=2, the primary diameter is in the range of 183μm to 213μm. In this case, the primary thickness is in the range of 1.5μm to 16.5μm, and the secondary layer thickness is in the range of 18.5μm to 33.5μm.

[0042] Next, Fig. 8 is a diagram showing an example of the relationship between the primary elastic modulus and the primary diameter in the multi-core fiber 10 shown in Fig. 1 when the cladding diameter is 180 µm, the secondary diameter is 300 µm, and the secondary elastic modulus is 1000 MPa. In Fig. 8, P-ISM on the horizontal axis means the primary elastic modulus, and P-diameter on the vertical axis means the primary diameter. Furthermore, the dashed line indicates the case where R is 1, and the solid line indicates the case where R is 2. In Fig. 8, the area to the lower right of the solid line indicated by the arrow is the condition where R is 2 or more. Note that when the solid line is expressed as an approximate curve using a quartic function, where x is the primary elastic modulus and y is the primary diameter, y = 1.3726x 4 -3.1534x 3 +5.0183x 2 +15.993x+180.17 (r 2 = 1). Therefore, y ≦ 1.3726x 4 -3.1534x 3 +5.0183x 2 It was confirmed that R can be set to 2 or more by setting it to +15.993x+180.17. For example, when R=2, the primary diameter is in the range of 181μm to 229μm. In this case, the primary thickness is in the range of 0.5μm to 24.5μm, and the secondary layer thickness is in the range of 35.5μm to 59.5μm.

[0043] Next, Fig. 9 is a diagram showing an example of the relationship between the primary elastic modulus and the primary diameter in the multi-core fiber 10 shown in Fig. 1 when the cladding diameter is 240 µm, the secondary diameter is 300 µm, and the secondary elastic modulus is 1000 MPa. In Fig. 9, P-ISM on the horizontal axis means the primary elastic modulus, and P-diameter on the vertical axis means the primary diameter. Furthermore, the dashed line indicates the case where R is 1, and the solid line indicates the case where R is 2. In Fig. 9, the area to the lower right of the solid line indicated by the arrow is the condition where R is 2 or more. When the solid line is expressed as an approximate curve using a quartic function, where x is the primary elastic modulus and y is the primary diameter, y = 0.0229x 4 -0.1312x 3 +0.7097x 2 +6.6821x+240.14 (r 2= 1). Therefore, y ≦ 0.0229x 4 -0.1312x 3 +0.7097x 2 It was confirmed that R can be set to 2 or more when +6.6821x+240.14 is used. For example, when R=2, the primary diameter is in the range of 241μm to 299μm. In this case, the primary thickness is in the range of 0.5μm to 29.5μm, and the secondary layer thickness is in the range of 0.5μm to 29.5μm.

[0044] Next, Fig. 10 is a diagram showing an example of the relationship between the primary elastic modulus and the primary diameter in the multi-core fiber 10 shown in Fig. 1 when the cladding diameter is 240 µm, the secondary diameter is 350 µm, and the secondary elastic modulus is 1000 MPa. In Fig. 10, P-ISM on the horizontal axis means the primary elastic modulus, and P-diameter on the vertical axis means the primary diameter. Furthermore, the dashed line indicates the case where R is 1, and the solid line indicates the case where R is 2. In Fig. 10, the area to the lower right of the solid line indicated by the arrow is the condition where R is 2 or more. When the solid line is expressed as an approximate curve using a quartic function, where x is the primary elastic modulus and y is the primary diameter, y = 0.0669x 4 -0.6164x 3 +2.4088x 2 +4.6485x+240.69 (r 2 =0.9996). Therefore, y≦0.0669x 4 -0.6164x 3 +2.4088x 2 It was confirmed that R can be set to 2 or more when +4.6485x+240.69 is used. For example, when R=2, the primary diameter is in the range of 240μm to 322μm. In this case, the primary thickness is in the range of 0μm to 41μm, and the secondary layer thickness is in the range of 14μm to 55μm.

[0045] FIG. 11 is a diagram showing an example of the relationship between the primary elastic modulus (P-ISM) (MPa) and the P / S thickness ratio. FIG. 11 shows an example where R is 2 for a combination of GD (cladding diameter) and SD (secondary diameter) values ​​as shown in Table 1. For example, GD125SD250 means that the cladding diameter is 125 μm and the secondary diameter is 250 μm. However, the secondary elastic modulus is set to 1000 MPa. R can be set to 2 for combinations of primary elastic modulus and P / S thickness ratio as shown by the various lines in FIG. 11.

[0046] Table 2, in relation to Figure 6, shows examples of R obtained when the cladding diameter, primary diameter, secondary diameter, primary elastic modulus, and secondary elastic modulus are appropriately set. In all of the examples shown in Table 2, it can be seen that an R of 2 or more can be obtained. On the other hand, Table 3 shows examples where R is 1, and in these examples, the effect of suppressing the propagation of higher-order modes cannot be obtained. [Table 2] [Table 3]

[0047] As described above, in the multi-core fiber 10 according to the first embodiment, a preferable lower limit of the cutoff wavelength when measured at a length of less than 2 m is 1260 nm. The reason for this is that the cutoff wavelength of an optical fiber may shift by 90 nm to the longer wavelength side when the length is shortened by 0.5 m (for example, Louis-Anne de Montmorillon et al., “Recent Developments of Bend-Insensitive and Ultra-Bend-Insensitive Fibers Fully Compliant with Both G.657.B and G.652.D ITU-T Recommendations,” Proceedings of the 58th IWCS / IICIT International Wire & Cable Symposium, pp. 270-276.). Therefore, even when the multicore fiber 10 according to the first embodiment is used at a length of less than 0.5 m, in order to transmit light having a wavelength of 1530 nm in single mode, the cutoff wavelength measured at that length must be 1260 nm or more, since (1530 nm - 90 nm x 3) = 1260 nm.

[0048] (Modification 1 of Embodiment 1) The multicore fiber 10 according to the first embodiment suppresses propagation of higher-order modes even when the length is less than 2 m, and is therefore capable of transmitting light with wavelengths of 1530 nm to 1565 nm, known as the C-band, in single mode. In contrast, the multicore fiber according to the first modification of the first embodiment suppresses propagation of higher-order modes even when the length is less than 2 m, and is configured to be able to transmit light with wavelengths of 1260 nm to 1360 nm, known as the O-band, in single mode.

[0049] For example, a multicore fiber according to a first modification of the first embodiment includes a plurality of core portions, a cladding portion, a primary layer, and a secondary layer similar to those of the multicore fiber 10. Δ is 0.2% or more and 0.5% or less, the core diameter is 4.0 μm or more and 15 μm or less, the cladding diameter is 125 μm or more, (thickness of the primary layer) / (thickness of the secondary layer) is 0.35 or less, the cutoff wavelength when measured at a length of less than 2 m is less than 1260 nm, and the cutoff wavelength when measured at the above length is 990 nm or more. Note that the lower limit of 990 nm is a value derived as follows: (1260 nm - 270 nm) = 990 nm.

[0050] Fig. 12 is a diagram showing an example of the relationship between the core diameter and the relative refractive index difference (Δ) in a glass optical fiber having a step-index refractive index profile, such that the 2m cutoff wavelength (λc) is 1260nm or 1310nm. Note that Fig. 12 shows the results of a simulation calculation using the weak waveguide approximation. In Fig. 12, the dashed line is an approximation curve of a quartic function of the data points when λc is 1260nm. The approximation curve is expressed as y=0.0002x where x is the core diameter and y is the relative refractive index difference. 4 -0.0101x 3 +0.1701x 2 -1.3472x+4.466 (r 2 =0.9998). Therefore, y<0.0002x 4 -0.0101x 3 +0.1701x 2 It was confirmed that the 2m cutoff wavelength was less than 1260nm when -1.3472x+4.466 was used.

[0051] (Modification 2 of Embodiment 1) The multicore fiber according to the second modification of the first embodiment is configured so that propagation of higher-order modes is suppressed even when the length is less than 2 m, and light having wavelengths of 1565 nm to 1625 nm, known as the L band, can be transmitted in single mode.

[0052] For example, a multicore fiber according to a second modification of the first embodiment includes a plurality of core portions, a cladding portion, a primary layer, and a secondary layer similar to those of the multicore fiber 10. Δ is 0.2% or more and 0.5% or less, the core diameter is 4.0 μm or more and 15 μm or less, the cladding diameter is 125 μm or more, (thickness of the primary layer) / (thickness of the secondary layer) is 0.35 or less, the cutoff wavelength when measured at a length of less than 2 m is less than 1565 nm, and the cutoff wavelength when measured at the above length is 1295 nm or more. Note that the lower limit of 1295 nm is a value derived as follows: (1565 nm - 270 nm) = 1295 nm.

[0053] Fig. 13 is a diagram showing an example of the relationship between the core diameter and the relative refractive index difference (Δ) in a glass optical fiber having a step-index refractive index profile, such that the 2m cutoff wavelength (λc) is 1565nm or 1625nm. Note that Fig. 13 shows the results of a simulation calculation using the weak waveguide approximation. In Fig. 13, the dashed line is an approximation curve of a quartic function of the data points when λc is 1565nm. The approximation curve is expressed as y=0.00008x where x is the core diameter and y is the relative refractive index difference. 4 -0.0045x 3 +0.0991x 2 -1.0101x+4.276 (r 2 =0.9999). Therefore, y≧0.00008x 4 -0.0045x 3 +0.0991x 2 It was confirmed that the 2m cutoff wavelength was less than 1565nm when -1.0101x+4.276 was used.

[0054] (Embodiment 2) 14 is a schematic cross-sectional view of a multi-core fiber according to a second embodiment. The multi-core fiber 20 includes 19 cores 21 made of glass such as silica-based glass, a clad 22 made of glass such as silica-based glass and surrounding the outer periphery of the cores 21, a primary layer 23 made of resin and surrounding the outer periphery of the clad 22, and a secondary layer 24 made of resin and surrounding the outer periphery of the primary layer 23. In this embodiment, the cores 21 are arranged to form a triangular lattice. The 19 cores 21 are located approximately at the center of the clad 22 and include three types of cores: a first core close to the center, six second cores surrounding the first cores and far from the center, and twelve third cores further farther from the center than the second cores. The multi-core fiber 20 is an example of a multi-core fiber including two or more types of cores far from the center of the clad 22.

[0055] The cores 21, the cladding 22, the primary layer 23, and the secondary layer 24 are each made of the same materials and have the same characteristics as the corresponding elements of the multicore fiber 10 according to the first embodiment shown in Fig. 1, and therefore detailed description thereof will be omitted. In this way, even in the multicore fiber 20 having 19 cores 21, if (thickness of primary layer) / (thickness of secondary layer) is 0.35 or less and the cutoff wavelength measured at a length of less than 2 m is 1260 nm or more and less than 1530 nm, light of a wavelength in the C band can be transmitted in single mode even at a length of less than 2 m.

[0056] Furthermore, as a first variant of the second embodiment, if the (thickness of the primary layer) / (thickness of the secondary layer) is 0.35 or less and the cutoff wavelength measured over a length of less than 2 m is 990 nm or more and less than 1260 nm, then light of the O-band wavelength can be transmitted in single mode even over a length of less than 2 m.

[0057] Furthermore, as a second variant of the second embodiment, if the (thickness of the primary layer) / (thickness of the secondary layer) is 0.35 or less and the cutoff wavelength when measured over a length of less than 2 m is 1295 nm or more and less than 1565 nm, light of an L-band wavelength can be transmitted in single mode even over a length of less than 2 m.

[0058] In the above-described embodiments, the number of core parts is 7 or 19, but the number of core parts is not limited to these numbers, and the arrangement of the core parts is not limited to a triangular lattice pattern.

[0059] 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]

[0060] 10, 20: Multi-core fiber 11, 21: Core section 12, 22: Cladding section 13, 23: Primary layer 14, 24: Secondary layer

Claims

1. a plurality of core portions made of glass; a clad portion made of glass and surrounding the outer periphery of the plurality of core portions; a primary layer made of resin and surrounding the outer periphery of the clad portion; a secondary layer made of resin and surrounding the outer periphery of the primary layer; Equipped with a maximum relative refractive index difference Δ of the core portion with respect to the average refractive index of the cladding portion is 0.2% or more and 0.5% or less; The core diameter of the core portion is 4.0 μm or more and 15 μm or less, The cladding diameter of the cladding portion is 125 μm or more, the ratio of the thickness of the primary layer to the thickness of the secondary layer is 0.35 or less; The cutoff wavelength when measured at a strip length of less than 2 m is less than 1530 nm, The cutoff wavelength measured at the above length is 1260 nm or more. Multicore fiber.

2. The multi-core fiber according to claim 1 , wherein the primary layer has an elastic modulus of 0.75 MPa or more.

3. a plurality of core portions made of glass; a clad portion made of glass and surrounding the outer periphery of the plurality of core portions; a primary layer made of resin and surrounding the outer periphery of the clad portion; a secondary layer made of resin and surrounding the outer periphery of the primary layer; Equipped with a maximum relative refractive index difference Δ of the core portion with respect to the average refractive index of the cladding portion is 0.2% or more and 0.5% or less; The core diameter of the core portion is 4.0 μm or more and 15 μm or less, The cladding diameter of the cladding portion is 125 μm or more, the ratio of the thickness of the primary layer to the thickness of the secondary layer is 0.35 or less; The cutoff wavelength when measured at a strip length of less than 2 m is less than 1260 nm, The cutoff wavelength measured at the above length is 990 nm or more. Multicore fiber.

4. The multi-core fiber according to claim 3 , wherein the primary layer has an elastic modulus of 0.75 MPa or more.

5. a plurality of core portions made of glass; a clad portion made of glass and surrounding the outer periphery of the plurality of core portions; a primary layer made of resin and surrounding the outer periphery of the clad portion; a secondary layer made of resin and surrounding the outer periphery of the primary layer; Equipped with a maximum relative refractive index difference Δ of the core portion with respect to the average refractive index of the cladding portion is 0.2% or more and 0.5% or less; The core diameter of the core portion is 4.0 μm or more and 15 μm or less, The cladding diameter of the cladding portion is 125 μm or more, the ratio of the thickness of the primary layer to the thickness of the secondary layer is 0.35 or less; The cutoff wavelength when measured at a strip length of less than 2 m is less than 1565 nm, The cutoff wavelength measured at the above length is 1295 nm or more. Multicore fiber.

6. The multi-core fiber according to claim 5 , wherein the primary layer has an elastic modulus of 0.75 MPa or more.

7. The length is less than 2m The multicore fiber according to any one of claims 1 to 6.

8. The ratio is less than 0.3 The multicore fiber according to any one of claims 1 to 6.

9. The ratio is less than 0.25 The multicore fiber according to any one of claims 1 to 6.

10. a reference cladding portion made of glass, a reference cladding portion made of glass and surrounding the outer periphery of the reference core portion, a reference primary layer made of resin and surrounding the outer periphery of the reference cladding portion, and a reference secondary layer made of resin and surrounding the outer periphery of the reference primary layer, wherein the cladding diameter of the reference cladding portion is 125 μm, the elastic modulus of the reference cladding portion is 70,000 MPa, the elastic modulus of the reference primary layer is 0.5 MPa, and the elastic modulus of the reference secondary layer is 1,000 MPa, and the ratio of the microbending loss sensitivity to the microbending loss sensitivity of the reference optical fiber is 2 or more. The multicore fiber according to any one of claims 1 to 6.

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