Multicore fiber

Abstract

A multicore fiber (100) comprises a plurality of core parts (111, 112) made of glass, a crosstalk suppression part (130) made of glass and disposed between any two core parts of the plurality of core parts, and a cladding part (120) made of glass and surrounding the plurality of core parts and the outer periphery of the crosstalk suppression part, the maximum refractive index of the core parts being higher than the average refractive index of the cladding part, the minimum refractive index of the crosstalk suppression part being lower than the average refractive index of the cladding part, and the distances (A, B) between each of the two core parts and the crosstalk suppression part being different from each other.

Description

Multicore Fiber

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

[0002] In a multicore fiber having multiple cores, the most common method for identifying the cores in a cross section is to provide a portion called a marker in the cladding. The marker has a refractive index different from that of the cladding and does not contribute to light propagation. When the cross section of the multicore fiber is observed under a microscope, such a marker is observed as a portion with a different contrast from the cladding. However, this method has a problem in that a glass preform for forming the marker must be prepared.

[0003] In response to this, a method has been proposed in which, in a multicore fiber, core portions are arranged asymmetrically with respect to the axial center of the cladding portion, making it possible to identify the core portions (Patent Document 1, Non-Patent Document 1). Also, a method has been proposed in which, in order to identify the core portions, multiple core portions are configured with step-type core portions and trench-type core portions (Non-Patent Document 2). In the method described in Non-Patent Document 2, the multiple trench-type core portions are further configured with core portions having center core portions with different diameters and refractive indices, thereby enabling the core portions to be identified. The trench layer in the trench-type core portion also has the function of suppressing inter-core crosstalk in the multicore fiber.

[0004] International Publication No. 2022 / 210786

[0005] Y. Sasaki et al., IEICE EXAT Nov. 2020-13 (2020)Y. Amma et al., OFC2015, Th4C.4, (2015)

[0006] However, in the method using an asymmetric arrangement such as those in Patent Document 1 and Non-Patent Document 1, it is difficult to fabricate optical components that are optically coupled to core portions, such as an optical component called a FIFO. On the other hand, in the method in Non-Patent Document 2, if the difference between the diameter and refractive index of the center core portions in multiple trench-type core portions is large, the difference in optical properties of the center core portions will be large, and if the difference between the diameter and refractive index of the center core portions is small, the core portions will be difficult to distinguish. Furthermore, in Non-Patent Document 2, the number of trench-type core portions is large, and the number of trench layers is also large, resulting in a complex configuration.

[0007] 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 with a simple configuration and the core portions are easily identified.

[0008] In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is a multi-core fiber comprising: a plurality of core portions made of glass; a crosstalk suppression portion made of glass and arranged between any two of the plurality of core portions; and a cladding portion made of glass and surrounding the outer peripheries of the plurality of core portions and the crosstalk suppression portion, wherein the maximum refractive index of the core portions is higher than the average refractive index of the cladding portions, the minimum refractive index of the crosstalk suppression portion is lower than the average refractive index of the cladding portions, and the distances between each of the two core portions and the crosstalk suppression portion are different from each other.

[0009] One aspect of the present invention is a multi-core fiber comprising three or more core portions made of glass, and two or more crosstalk suppression portions made of glass, each of which is arranged between any two of the three or more core portions, and a cladding portion made of glass and surrounding the outer peripheries of the three or more core portions and the crosstalk suppression portions, wherein the maximum refractive index of the core portions is higher than the average refractive index of the cladding portions, the minimum refractive index of the crosstalk suppression portions is lower than the average refractive index of the cladding portions, and at least two of the two or more crosstalk suppression portions have different diameters.

[0010] The distances between the two core units and the crosstalk suppression unit may be different from each other.

[0011] The cladding portion may be free of any markers that have a refractive index different from that of the cladding portion and that do not contribute to light propagation.

[0012] At least a portion of the crosstalk suppressor may be made of fluorine-containing silica glass.

[0013] The core may be made of silica glass containing at least one of germanium, fluorine, chlorine, potassium, and sodium.

[0014] The core propagates the input light in a single mode, and has an effective core area of ​​60 μm at the wavelength of the light. ２ 180 μm or more ２ It may be the following:

[0015] The number of the core portions may be 2 or more and 19 or less.

[0016] The core portion may be arranged to have rotational symmetry with respect to the central axis of the cladding portion.

[0017] According to the present invention, it is possible to realize a multi-core fiber in which inter-core crosstalk is suppressed with a simple configuration and the core portions are easily identified.

[0018] Fig. 1 is a schematic cross-sectional view of a multi-core fiber according to embodiment 1. Fig. 2A is a diagram showing changes in XT. Fig. 2B is a diagram showing changes in cut-off wavelength. Fig. 2C is a diagram showing changes in cut-off wavelength. Fig. 3A is a diagram showing changes in XT. Fig. 3B is a diagram showing changes in cut-off wavelength. Fig. 3C is a diagram showing changes in cut-off wavelength. Fig. 4 is an explanatory diagram of a method for manufacturing the multi-core fiber shown in Fig. 1. Fig. 5 is a schematic cross-sectional view of a multi-core fiber according to embodiment 2. Fig. 6 is a schematic cross-sectional view of a multi-core fiber according to embodiment 3. Fig. 7 is a schematic cross-sectional view of a multi-core fiber according to embodiment 4. Fig. 8 is a schematic cross-sectional view of a multi-core fiber according to embodiment 5.

[0019] 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, identical or corresponding components are appropriately designated by the same reference numerals, and duplicate explanations are appropriately omitted. Furthermore, in this specification, the cutoff wavelength or effective cutoff wavelength refers to the cable cutoff wavelength (λcc) 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.

[0020] 1 is a schematic cross-sectional view of a multi-core fiber according to embodiment 1. The multi-core fiber 100 is made of glass such as silica glass, and includes core portions 111 and 112, a cladding portion 120, and a crosstalk suppression portion 130. The core portions 111 and 112 are an example of a plurality of core portions.

[0021] The cores 111 and 112 have a substantially circular cross section and are arranged to have two-fold rotational symmetry with respect to the central axis of the cladding 120, which also has a substantially circular cross section. The maximum refractive index of the core 111 is higher than the average refractive index of the cladding 120. The average refractive index of the cladding 120 is the average refractive index of the cladding 120 in the radial direction. The maximum refractive index of the core 112 is higher than the average refractive index of the cladding 120. The cores 111 and 112 may contain at least one of germanium, fluorine, chlorine, potassium, and sodium. The refractive index profile of the cores 111 and 112 is, for example, a step type.

[0022] The crosstalk suppression section 130 has a substantially circular cross section and is disposed between the core sections 111 and 112. Disposing the crosstalk suppression section 130 between the core sections 111 and 112 means that an imaginary line segment connecting the central axes of the core sections 111 and 112 intersects with the crosstalk suppression section 130 in a cross section perpendicular to the longitudinal direction. The minimum refractive index of the crosstalk suppression section 130 is lower than the average refractive index of the cladding section 120 in the radial direction. The crosstalk suppression section 130 may be made of silica glass, at least a portion of which contains fluorine. The crosstalk suppression section 130 is an example of a crosstalk suppression section disposed between any two of a plurality of core sections.

[0023] The cladding section 120 surrounds the outer peripheries of the core sections 111 and 112 and the crosstalk suppression section 130. The cladding section 120 may be made of silica glass containing at least a portion of fluorine, or may be made of pure silica glass. Pure silica glass is extremely high-purity silica glass that does not substantially contain dopants that change the refractive index and has a refractive index of approximately 1.444 at a wavelength of 1550 nm.

[0024] The core portion 111 and the core portion 112 have the same diameter (core diameter) of 2a. The relative refractive index difference of the maximum refractive index of the core portion 111 with respect to the average refractive index of the cladding portion 120 and the relative refractive index difference of the maximum refractive index of the core portion 112 with respect to the average refractive index of the cladding portion 120 are also equal to Δ1, where Δ1 is a positive value. The relative refractive index difference of the minimum refractive index of the crosstalk suppression portion 130 with respect to the average refractive index of the cladding portion 120 is Δ-, where Δ- is a negative value.

[0025] Here, the distances between the two core units 111 and 112 and the crosstalk suppression unit 130 are different from each other. That is, the distance A between the central axis of the core unit 111 and the central axis of the crosstalk suppression unit 130 and the distance B between the central axis of the core unit 112 and the central axis of the crosstalk suppression unit 130 shown in FIG. 1 are different from each other.

[0026] In the multi-core fiber 100 configured as described above, the relative refractive index difference and core diameter between the core portions 111 and 112 are equal to within a range of ±10%. As a result, in the multi-core fiber 100, the optical characteristics of the core portions 111 and 112 are roughly determined by their respective relative refractive index differences and core diameters, and therefore there is little difference in the optical characteristics between the two. Furthermore, since the crosstalk suppression portion 130 has a negative Δ- value, it has a function of suppressing inter-core crosstalk between the core portions 111 and 112, similar to a trench layer, despite its simple structure. Furthermore, since the distances between the two core portions 111 and 112 and the crosstalk suppression portion 130 are different from each other, it is easy to distinguish between the core portions 111 and 112 when the cross-section of the multi-core fiber 100 is observed with a microscope or the like. In other words, the crosstalk suppression portion 130 has both the function of suppressing inter-core crosstalk and the function of a marker for identifying the core portions.

[0027] Therefore, in the multicore fiber 100, inter-core crosstalk is suppressed with a simple configuration, and it is easy to identify the core portions 111 and 112. Furthermore, it is not necessary to provide a marker in the cladding portion 120 that has a refractive index different from that of the cladding portion 120 and does not contribute to light propagation, and it is not necessary to arrange the core portions 111 and 112 asymmetrically so as not to have rotational symmetry with respect to the central axis of the cladding portion 120.

[0028] Here, the present inventors performed the following simulation calculations to investigate the influence of the crosstalk suppression section 130 on the optical characteristics of each of the core sections 111 and 112 in the multicore fiber 100. That is, the values ​​of A, B, Δ-, or the diameter C of the crosstalk suppression section 130 in the multicore fiber 100 of Fig. 1 were changed, and changes in the cutoff wavelength of the core sections 111 and 112 and the inter-core crosstalk were calculated.

[0029] In FIG. 1 , d1 is the shortest distance from the central axis of the core 111 to the outer circumferential surface of the cladding 120. d2 is the shortest distance from the central axis of the core 112 to the outer circumferential surface of the cladding 120. These shortest distances are also called cladding thicknesses. In this calculation, d1 and d2 were both set to 40 μm. The outer diameter (cladding diameter) of the cladding 120 was set to 125 μm. Therefore, the distance between the central axes of the cores 111 and 112 (core pitch) was 45 μm. Furthermore, the structural parameters and optical characteristics of the cores 111 and 112 in the multicore fiber 100 in the absence of the crosstalk suppression section 130 are as shown in Table 1 below. Note that "MFD" means mode field diameter, Aeff means effective core area, and "XT" means inter-core crosstalk at a length of 100 km. As shown in Table 1, XT in the absence of the crosstalk suppression section is -28.5 dB.

[0030]

[0031] Next, the crosstalk suppression unit 130 is provided, and the changes in XT and cutoff wavelength when A [μm], B [μm] (= 45 - A), C, and D (= Δ-) are changed are shown in Figures 2A, 2B, 2C, 3A, 3B, and 3C. Figures 2A, 2B, and 2C show the case where A is 15 μm and B is 30 μm, with Figure 2A showing the XT characteristics, Figure 2B showing the cutoff wavelength of the core unit 111, and Figure 2C showing the cutoff wavelength of the core unit 112. Figures 3A, 3B, and 3C show the case where A is 20 μm and B is 25 μm, with Figure 3A showing the XT characteristics, Figure 3B showing the cutoff wavelength of the core unit 111, and Figure 3C showing the cutoff wavelength of the core unit 112.

[0032] From Table 1 and Figures 2A, 2B, 2C, 3A, 3B, and 3C, it was confirmed that providing the crosstalk suppression section 130 reduces XT and suppresses inter-core crosstalk. It was also confirmed that the larger C or the larger the absolute value of D, the more XT decreases, and inter-core crosstalk is further suppressed. Furthermore, by comparing Figures 2A, 2B, and 2C with Figures 3A, 3B, and 3C, it was confirmed that changing the value of A does not significantly change the inter-core crosstalk suppression effect. In other words, arranging the crosstalk suppression section 130 closer to either the core section 111 or 112 does not significantly change the inter-core crosstalk suppression effect. It was also confirmed that changing the values ​​of A, B, C, and D does not significantly change the cutoff wavelength. For example, while the cutoff wavelength is 1254 nm in Table 1, the cutoff wavelength is 1300 nm or less in any of Figures 2A, 2B, 2C, 3A, 3B, and 3C, and light with a wavelength of, for example, 1310 nm can be transmitted in single mode.

[0033] The inventors also investigated the changes in dispersion, mode field diameter, and effective core area when the values ​​of A, B, C, and D were changed, and confirmed that these values ​​also did not change significantly when the values ​​of A, B, C, and D were changed.

[0034] (Manufacturing Method) The multicore fiber 100 according to the first embodiment can be manufactured, for example, as follows. First, a cylindrical cladding preform 1000 that will become a part of the cladding portion 120 of the multicore fiber 100 is prepared, as shown in FIG. 4 . The cladding preform 1000 can be manufactured using a well-known method for manufacturing a glass preform, such as a VAD (Vapor Phase Axial Deposition) method or an MCVD (Modified Chemical Vapor Deposition) method. Next, holes 1001, 1002, and 1003 are formed in the cladding preform 1000 using a well-known drilling method, and the surface is cleaned as necessary. At this time, if the glass walls between the holes 1001, 1002, and 1003 are thin, manufacturing becomes difficult. Therefore, attention is paid to the inner diameters and relative positions of the holes 1001, 1002, and 1003 so that the walls have an appropriate thickness, i.e., so that the hole 1003 is not too close to the other holes 1001 and 1002.

[0035] Meanwhile, core preforms 1010 and 1020 and a glass rod 1030 are prepared. The core preforms 1010 and 1020 and the glass rod 1030 can also be manufactured using a well-known glass preform manufacturing method, such as a VAD method or an MCVD method. The core preform 1010 includes a core portion 1011 that will become the core portion 111 of the multicore fiber 100, and a clad portion 1012 that surrounds the outer periphery of the core portion 1011 and will become part of the clad portion 120 of the multicore fiber 100. The core preform 1020 includes a core portion 1021 that will become the core portion 112 of the multicore fiber 100, and a clad portion 1022 that surrounds the outer periphery of the core portion 1021 and will become part of the clad portion 120 of the multicore fiber 100. The glass rod 1030 is a member that will become the crosstalk suppression portion 130 of the multicore fiber 100, and is made of silica glass doped with fluorine, for example, approximately uniformly.

[0036] Next, the core preforms 1010, 1020 and the glass rod 1030 are inserted into the holes 1001, 1002, 1003 of the cladding preform 1000, respectively, and heated to be integrated into an optical fiber preform. Next, the multi-core fiber 100 is drawn using a well-known drawing method.

[0037] 5 is a schematic cross-sectional view of a multi-core fiber according to embodiment 2. The multi-core fiber 200 is made of glass such as silica glass, and includes core portions 111, 112, 113, and 114, a cladding portion 120, and crosstalk suppression portions 131, 132, 133, and 134.

[0038] The cores 111 to 114 have a substantially circular cross section and are arranged in a square lattice pattern, and are arranged so as to have four-fold rotational symmetry with respect to the central axis of the cladding 120, which has a substantially circular cross section.

[0039] The crosstalk suppression unit 131 has a substantially circular cross section and is disposed between the core units 111 and 114 closer to the core unit 111 than the core unit 114. The crosstalk suppression unit 132 has a substantially circular cross section and is disposed between the core units 111 and 112 closer to the core unit 111 than the core unit 112. The crosstalk suppression unit 133 has a substantially circular cross section and is disposed between the core units 112 and 113 closer to the core unit 113 than the core unit 112. The crosstalk suppression unit 134 has a substantially circular cross section and is disposed between the core units 113 and 114 at an equal distance from the core units 113 and 114. In other words, the crosstalk suppression units 131, 132, and 133 are examples of crosstalk suppression units that are disposed between two core units and have different distances from each of the two core units.

[0040] The core portions 111 to 114 and the crosstalk suppression portions 131 to 134 have the same structural parameters (i.e., diameter and relative refractive index difference) as the core portion 111 or 112 and the crosstalk suppression portion 130 of the multicore fiber 100 according to the first embodiment.

[0041] In the multicore fiber 200 configured as described above, there is little difference in optical characteristics among the core portions 111 to 114. Furthermore, the crosstalk suppression portions 131 to 134 suppress inter-core crosstalk despite having a simple structure. Furthermore, in the multicore fiber 200, for the core portion 111, the distance between the crosstalk suppression portions 131 and 132 arranged between adjacent core portions is closer than the distance between the adjacent core portions. For the core portion 112, the distance between the crosstalk suppression portions 132 and 133 arranged between adjacent core portions is farther than the distance between the adjacent core portions. For the core portion 113, the distance between the crosstalk suppression portions 132 and 133 arranged between adjacent core portions is closer or equal than the distance between the adjacent core portions. For the core portion 114, the distance between the crosstalk suppression portions 134 and 131 arranged between adjacent core portions is equal or farther than the distance between the adjacent core portions. Therefore, the core units 111 to 114 are easily distinguishable from one another because the positional relationships between the two crosstalk suppression units disposed between adjacent core units are different.

[0042] Therefore, in the multicore fiber 200, inter-core crosstalk is suppressed with a simple configuration, and it is easy to identify the core portions 111 to 114. Furthermore, it is not necessary to provide a marker in the cladding portion 120, and it is not necessary to arrange the core portions 111 to 114 asymmetrically with respect to the central axis of the cladding portion 120.

[0043] 6 is a schematic cross-sectional view of a multi-core fiber according to embodiment 3. The multi-core fiber 300 is made of glass such as silica glass, and includes core portions 111, 112, 113, 114, 115, 116, and 117, a cladding portion 120, crosstalk suppression portions 131, 132, 133, 134, 135, and 136, and a trench layer 140.

[0044] The cores 111 to 117 have a substantially circular cross section and are arranged in a triangular lattice pattern. The cores 111 to 114 are arranged to have six-fold rotational symmetry with respect to the central axis of the cladding 120, which has a substantially circular cross section. The core 117 is arranged at a position that substantially coincides with the central axis of the cladding 120.

[0045] The crosstalk suppressor 131 has a substantially circular cross section and is disposed between the core portions 111 and 116, closer to the core portion 111 than the core portion 116. The crosstalk suppressor 132 has a substantially circular cross section and is disposed between the core portions 111 and 112, closer to the core portion 111 than the core portion 112. The crosstalk suppressor 133 has a substantially circular cross section and is disposed between the core portions 112 and 113, closer to the core portion 113 than the core portion 112. The crosstalk suppressor 134 has a substantially circular cross section and is disposed between the core portions 113 and 114, equidistant from the core portions 113 and 114. The crosstalk suppressor 135 has a substantially circular cross section and is disposed between the core portions 114 and 115, equidistant from the core portions 114 and 115. The crosstalk suppression unit 136 has a substantially circular cross section and is disposed between the core units 115 and 116 at an equal distance from the core units 115 and 116. In other words, the crosstalk suppression units 131 to 136 are examples of crosstalk suppression units that are disposed between two core units and are at mutually different distances from the two core units.

[0046] The trench layer 140 has a substantially circular cross section and surrounds the outer periphery of the core portion 117 .

[0047] The core portions 111 to 117 and the crosstalk suppression portions 131 to 136 have the same structural parameters as the core portion 111 or 112 and the crosstalk suppression portion 130 of the multicore fiber 100 according to embodiment 1. Similarly to the crosstalk suppression portion 131, the minimum refractive index of the trench layer 140 is lower than the average refractive index in the radial direction of the cladding portion 120. At least a portion of the trench layer 140 may be made of silica glass containing fluorine.

[0048] In the multicore fiber 300 configured as described above, there is little difference in the optical characteristics of the cores 111 to 117. Furthermore, the crosstalk suppression sections 131 to 136 suppress inter-core crosstalk despite their simple structure. Furthermore, the trench layer 140 suppresses inter-core crosstalk between the core 117 and the cores 111 to 116 arranged around it. Furthermore, in the multicore fiber 300, the cores 111 to 114 and 116 have mutually different positional relationships with the two crosstalk suppression sections arranged between adjacent cores, making it easy to distinguish between the cores 111 to 114. Therefore, it is also easy to distinguish between the cores 111 to 116.

[0049] Therefore, in the multicore fiber 300, inter-core crosstalk is suppressed with a simple configuration, and it is easy to identify the core portions 111 to 116. Furthermore, it is not necessary to provide a marker in the cladding portion 120, and it is not necessary to arrange the core portions 111 to 116 asymmetrically with respect to the central axis of the cladding portion 120.

[0050] (Embodiment 4) Fig. 7 is a schematic cross-sectional view of a multi-core fiber according to embodiment 4. The multi-core fiber 200A has a configuration in which the crosstalk suppression units 131 to 134 in the multi-core fiber 200 according to embodiment 2 shown in Fig. 5 are replaced with crosstalk suppression units 131A, 132A, 133A, and 134A.

[0051] The crosstalk suppression unit 131A has a substantially circular cross section and is disposed between the core units 111 and 114 at an equal distance from the core units 111 and 114. The crosstalk suppression unit 132A has a substantially circular cross section and is disposed between the core units 111 and 112 at an equal distance from the core units 111 and 112. The crosstalk suppression unit 133A has a substantially circular cross section and is disposed between the core units 112 and 113 at an equal distance from the core units 112 and 113. The crosstalk suppression unit 134A has a substantially circular cross section and is disposed between the core units 113 and 114 at an equal distance from the core units 113 and 114. The crosstalk suppression units 131A to 134A have the same relative refractive index difference but different diameters. Specifically, the crosstalk suppression units 131A, 132A, 133A, and 134A have the largest diameters, in ascending order.

[0052] In the multicore fiber 200A, the core portions 111 to 114 are an example of three or more core portions. Also, the crosstalk suppression portions 131A to 134A are an example of crosstalk suppression portions that are arranged between any two of the three or more core portions, and at least two of the two or more crosstalk suppression portions have different diameters.

[0053] In the multi-core fiber 200A configured as described above, there is little difference in the optical characteristics of the cores 111 to 114. Furthermore, the crosstalk suppression units 131A to 134A suppress inter-core crosstalk despite having a simple structure. Furthermore, in the multi-core fiber 200A, the crosstalk suppression units 131A to 134A have different diameters, making it easy to distinguish between the cores 111 to 114.

[0054] Therefore, in the multicore fiber 200A, inter-core crosstalk is suppressed with a simple configuration, and it is easy to identify the core portions 111 to 114. Furthermore, it is not necessary to provide a marker in the cladding portion 120, and it is not necessary to arrange the core portions 111 to 114 asymmetrically with respect to the central axis of the cladding portion 120.

[0055] (Embodiment 5) Fig. 8 is a schematic cross-sectional view of a multi-core fiber according to embodiment 5. The multi-core fiber 300A has a configuration in which the crosstalk suppression units 131 to 136 in the multi-core fiber 300 according to embodiment 2 shown in Fig. 6 are replaced with crosstalk suppression units 131B, 132B, 133B, 134B, 135B, and 136B.

[0056] The crosstalk suppressor 131B has a substantially circular cross section and is disposed between the core portions 111 and 116 at an equal distance from the core portions 111 and 116. The crosstalk suppressor 132B has a substantially circular cross section and is disposed between the core portions 111 and 112 at an equal distance from the core portions 111 and 112. The crosstalk suppressor 133B has a substantially circular cross section and is disposed between the core portions 112 and 113 at an equal distance from the core portions 112 and 113. The crosstalk suppressor 134B has a substantially circular cross section and is disposed between the core portions 113 and 114 at an equal distance from the core portions 113 and 114. The crosstalk suppressor 135B has a substantially circular cross section and is disposed between the core portions 114 and 115 at an equal distance from the core portions 114 and 115. The crosstalk suppression unit 136B has a substantially circular cross section and is disposed between the core units 115 and 116 at an equal distance from the core units 115 and 116. The crosstalk suppression units 131B to 133B have the same relative refractive index difference but different diameters. Specifically, the crosstalk suppression units 132B, 133B, and 131B have the largest diameters, respectively. The crosstalk suppression units 133B to 136B have the same relative refractive index difference and diameter.

[0057] In the multicore fiber 300B, the core portions 111 to 114 are an example of three or more core portions. The crosstalk suppression portions 131B to 133B are an example of crosstalk suppression portions that are arranged between any two of the three or more core portions, and at least two of the two or more crosstalk suppression portions have different diameters.

[0058] In the multi-core fiber 300B configured as described above, there is little difference in the optical characteristics of the cores 111 to 114. Furthermore, the crosstalk suppression units 131B to 136B suppress inter-core crosstalk despite having a simple structure. Furthermore, in the multi-core fiber 300B, the crosstalk suppression units 131B to 133B have different diameters, making it easy to distinguish between the cores 111 to 116. Specifically, by knowing the information that crosstalk suppression section 131B, which is on the counterclockwise side of core section 111, has a relatively small diameter, crosstalk suppression section 132B, which is on the counterclockwise side of core section 112 and on the clockwise side of core section 111, has a relatively large diameter, and crosstalk suppression section 132B, which is on the counterclockwise side of core section 113 and on the clockwise side of core section 112, has a diameter between those of crosstalk suppression sections 131B and 132B, core sections 111 to 113 can be identified, and further, other core sections 114 to 116 can also be identified.

[0059] Therefore, in the multicore fiber 300B, inter-core crosstalk is suppressed with a simple configuration, and it is easy to identify the core portions 111 to 116. Furthermore, it is not necessary to provide a marker in the cladding portion 120, and it is not necessary to arrange the core portions 111 to 116 asymmetrically with respect to the central axis of the cladding portion 120.

[0060] In any of the multicore fibers of the above embodiments, the core preferably propagates input light in a single mode, where the input wavelength is a wavelength used as a signal light in optical fiber communications, for example, light in the 1.55 μm band.

[0061] From the viewpoint of compatibility with many conventional optical fibers, the core portion propagates the input light in a single mode and has an effective core area (Aeff) of 60 μm or less at the wavelength of the input light. ２ 180 μm or more ２ It is preferable that:

[0062] Example 1 As Example 1, a multicore fiber was manufactured having the configuration of the multicore fiber 100 according to the first embodiment shown in Fig. 1. The core preform, the cladding preform, and the glass rod were manufactured by a method based on the VAD method. The core preform was configured to include a core portion and a cladding portion, and the diameter of the cladding portion was twice that of the core portion.

[0063] The structural parameters and optical characteristics of the multicore fiber of Example 1 are shown in Table 2. In the table, "core 1" and "core 2" refer to core portions corresponding to the core portions 111 and 112 of the multicore fiber 100, respectively. The distance between the central axis of core 1 and the crosstalk suppression portion (XT suppression portion) was set to 21 μm, and the distance between the central axis of core 2 and the XT suppression portion was set to 24 μm. Furthermore, the central axis of the XT suppression portion was positioned on an imaginary line connecting the central axes of core 1 and core 2 in the cross section.

[0064] In the table, the cladding ratio Δ1 is the relative refractive index difference between the average refractive index of the cladding and the maximum refractive index of the core, and the pure silica ratio is the relative refractive index difference between the maximum refractive index of the core and the refractive index of pure silica glass. Similarly, the cladding ratio Δ- is the relative refractive index difference between the average refractive index of the cladding and the minimum refractive index of the crosstalk suppression section, and the pure silica ratio is the relative refractive index difference between the minimum refractive index of the crosstalk suppression section and the refractive index of pure silica glass. Furthermore, Δclad is the relative refractive index difference between the average refractive index of the cladding and the refractive index of pure silica glass.

[0065] As shown in Table 2, the optical properties such as the effective core area, cutoff wavelength, and transmission loss were approximately the same for Core 1 and Core 2. ２ The design was such that the fiber was able to achieve values ​​roughly in line with the design. It was also confirmed that the cutoff wavelength was 1530 nm or less, and that light in the 1.55 μm band could be propagated in single mode. Furthermore, the inter-core crosstalk was -37.5 dB, which was below -30 dB and below -35 dB.

[0066]

[0067] Furthermore, when an operator observed the end face of the multi-core fiber of Example 1 under a microscope and performed a number of tests to identify the core portion, the accuracy rate was 99.3%, confirming that the identification was possible with almost no problems.

[0068] Example 2 As Example 2, a multicore fiber was manufactured having the configuration of the multicore fiber 200 according to the second embodiment shown in Fig. 5. The core preform, the cladding preform, and the glass rod were manufactured by a method based on the VAD method. The core preform was configured to include a core portion and a cladding portion, and the diameter of the cladding portion was twice that of the core portion.

[0069] Table 3 shows the structural parameters and optical characteristics of the multicore fiber of Example 2. In the table, "Core 1," "Core 2," "Core 3," and "Core 4" refer to core portions corresponding to core portions 111, 112, 113, and 114 of the multicore fiber 200, respectively. The distance between the central axis of Core 1 and the crosstalk suppression portion (XT suppression portion) was set to 19 μm, and the distance between the central axis of Core 2 and the XT suppression portion was set to 22 μm. The central axis of the XT suppression portion was positioned on an imaginary line connecting the central axes of adjacent cores in the cross section. The diameter of the crosstalk suppression portion was set to 14 μm. The relative refractive index difference of the minimum refractive index of the crosstalk suppression portion with respect to the average refractive index of the cladding portion was set to -0.20%, and the relative refractive index difference of the minimum refractive index of the crosstalk suppression portion with respect to the refractive index of pure silica glass was set to -0.45%.

[0070] As shown in Table 3, the optical properties such as the effective core area, cutoff wavelength, and transmission loss were almost the same for cores 1 to 4. ２ The design was such that the fiber was able to achieve values ​​roughly in line with the design. It was also confirmed that the cutoff wavelength was 1530 nm or less, and that light in the 1.55 μm band could be propagated in single mode. Furthermore, the inter-core crosstalk was -36.7 dB between core 1 and core 2, -35.9 dB between core 2 and core 3, -37.1 dB between core 3 and core 4, and -36.4 dB between core 4 and core 1, all of which were -30 dB or less and -35 dB or less.

[0071]

[0072] Furthermore, when an operator observed the end face of the multi-core fiber of Example 2 under a microscope and conducted a number of tests to identify the core portion, the accuracy rate was 99.5%, confirming that the identification was possible with almost no problems.

[0073] In the above embodiment, the relative refractive index difference and core diameter between the multiple core portions are equal, but if the absolute values ​​of the differences in the relative refractive index difference and core diameter are equal within a range of ±10%, the optical characteristics such as the effective core area, dispersion, dispersion slope, and transmission loss will be approximately equal, and for example, the absolute value of the difference will be 10% or less, which is preferable.

[0074] In addition, in the above-described embodiments, the number of core parts is 2, 4, or 7, but is not particularly limited thereto. For example, the number of core parts is 2 or more and 19 or less. Furthermore, in the above-described embodiments, the core parts are arranged so as to have 2-fold, 4-fold, or 6-fold rotational symmetry with respect to the central axis of the cladding part, but is not particularly limited thereto, and the core parts may be arranged so as to have n-fold rotational symmetry, where n is any integer equal to or greater than 2.

[0075] Furthermore, the present invention is not limited to the above-described embodiments. Configurations in which the above-described components are appropriately combined are also included in the present invention. For example, in a configuration in which the diameters of the crosstalk suppression sections are different from each other, as in embodiment 4 shown in FIG. 7, the distances between the two core sections and the crosstalk suppression section disposed between them may be different from each other, as in embodiment 2 shown in FIG. 5. The crosstalk suppression section may also be an air hole. 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.

[0076] The present invention can be used in a multi-core fiber.

[0077] 100, 200, 200A, 300, 300A, 300B: Multi-core fiber 111, 112, 113, 114, 115, 116, 117, 1011, 1021: Core portion 120, 1012, 1022: Cladding portion 130, 131, 131A, 131B, 132, 132A, 132B, 133, 133A, 133B, 134, 134A, 134B, 135, 135B, 136, 136B: Crosstalk suppression portion 140: Trench layer 1000: Cladding base material 1001, 1002, 1003: Hole 1010, 1020: Core base material 1030: Glass rod

Claims

1. A multi-core fiber comprising: a plurality of core portions made of glass; a crosstalk suppression portion made of glass and arranged between any two of the plurality of core portions; and a cladding portion made of glass and surrounding the outer periphery of the plurality of core portions and the crosstalk suppression portion, wherein the maximum refractive index of the core portions is higher than the average refractive index of the cladding portion, the minimum refractive index of the crosstalk suppression portion is lower than the average refractive index of the cladding portion, and the distances between each of the two core portions and the crosstalk suppression portion are different from each other.

2. A multi-core fiber comprising: three or more core portions made of glass; two or more crosstalk suppression portions made of glass, each of which is arranged between any two of the three or more core portions; and a cladding portion made of glass and surrounding the outer periphery of the three or more core portions and the crosstalk suppression portions, wherein the maximum refractive index of the core portions is higher than the average refractive index of the cladding portion, the minimum refractive index of the crosstalk suppression portions is lower than the average refractive index of the cladding portion, and at least two of the two or more crosstalk suppression portions have different diameters.

3. The multi-core fiber according to claim 2, wherein the distances between the two core portions and the crosstalk suppression portion are different from each other.

4. A multicore fiber according to any one of claims 1 to 3, wherein the cladding portion does not contain any marker that has a refractive index different from that of the cladding portion and does not contribute to optical propagation.

5. A multi-core fiber according to any one of claims 1 to 3, wherein at least a part of the crosstalk suppression portion is made of fluorine-containing silica glass.

6. The multi-core fiber according to any one of claims 1 to 3, wherein the core portion is made of silica glass containing at least one of germanium, fluorine, chlorine, potassium, and sodium.

7. The core portion propagates the input light in a single mode, and the effective core area at the wavelength of the light is 60 μm ２ 180 μm or more ２ The multicore fiber according to any one of claims 1 to 3, wherein:

8. A multicore fiber according to any one of claims 1 to 3, wherein the number of the core portions is 2 or more and 19 or less.

9. The multicore fiber according to any one of claims 1 to 3, wherein the core portions are arranged so as to have rotational symmetry with respect to the central axis of the cladding portion.

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