Multicore fiber

Abstract

To provide a multi-core fiber that suppresses inter-core crosstalk and exhibits excellent microbend loss characteristics. [Solution] The multicore fiber comprises a plurality of core portions and a cladding portion surrounding the plurality of core portions and having a refractive index lower than the maximum refractive index of the core portions, wherein the plurality of core portions include a first core portion and a second core portion having an effective core cross-sectional area 10% or more larger than that of the first core portion, and the distance from the center of the cladding portion is smaller than the distance from the center of the cladding portion to the first core portion.

Description

[Technical Field]

[0001] This invention relates to multicore fibers. [Background technology]

[0002] In multicore fibers with multiple core sections, inter-core crosstalk can be a problem. One way to solve this problem is to use different types of core sections. Different types of core sections are core sections with different effective refractive indices (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Patent No. 5168702 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] However, depending on the arrangement of different core sections in the cladding, microbend losses may become excessively large in certain core sections compared to others, so there is room for improvement from a practical standpoint.

[0005] The present invention has been made in view of the above, and its object is to provide a multicore fiber that suppresses intercore crosstalk and has excellent microbend loss characteristics. [Means for solving the problem]

[0006] To solve the above-mentioned problems and achieve the objective, one aspect of the present invention is a multicore fiber comprising a plurality of core portions and a cladding portion surrounding the plurality of core portions and having a refractive index lower than the maximum refractive index of the core portions, wherein the plurality of core portions include a first core portion and a second core portion whose effective core cross-sectional area at a predetermined wavelength is 10% or more larger than that of the first core portion, and whose distance from the center of the cladding portion is smaller than the distance from the center of the cladding portion to the first core portion.

[0007] The aforementioned multiple core portions propagate light in single mode within the operating wavelength band including the predetermined wavelength, and have an effective core cross-sectional area of ​​60 to 180 μm². 2 But that's fine.

[0008] The core portion includes a center core made of silica glass containing germanium, and the cladding portion may be made of pure silica glass.

[0009] The core portion includes 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.

[0010] The outer diameter of the cladding portion is in the range of 125 ± 25 μm, and the microbend loss of light at a wavelength of 1550 nm may be 10 times or less than that of a standard optical fiber having a resin coating portion with an outer diameter of 250 μm on the outer circumference of a cladding portion with an outer diameter of 125 μm.

[0011] The relative refractive index difference between the refractive index of the cladding portion and the maximum refractive index of the center core may be 0.12% or more and 0.46% or less.

[0012] The microbend loss of light with a wavelength of 1550 nm in the second core portion may be 200% or less of the microbend loss of light with a wavelength of 1550 nm in the first core portion.

[0013] The microbend loss of light with a wavelength of 1550 nm in the second core part may be 150% or less of the microbend loss of light with a wavelength of 1550 nm in the first core part.

Advantages of the Invention

[0014] According to the present invention, there is an effect that a multi-core fiber with suppressed inter-core crosstalk and excellent microbend loss characteristics can be realized.

Brief Description of the Drawings

[0015] [Figure 1] FIG. 1 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the multi-core fiber according to Embodiment 1. [Figure 2] FIG. 2 is an explanatory diagram of an example of a trench-type refractive index profile. [Figure 3] FIG. 3 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 4] FIG. 4 is a diagram showing an example of the relationship between the effective core cross-sectional area and the microbend loss. [Figure 5] FIG. 5 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the multi-core fiber according to a modification of Embodiment 1. [Figure 6] 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. [Figure 7] FIG. 7 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the multi-core fiber according to a modification of Embodiment 2. [Figure 8] FIG. 8 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the multi-core fiber according to a modification of Embodiment 2.

Modes for Carrying Out the Invention

[0016] Embodiments of the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited to the embodiments described below. In each drawing, the same or corresponding components are denoted by the same reference numerals as appropriate, and redundant explanations are omitted as appropriate. Furthermore, in this specification, the cutoff wavelength or effective cutoff wavelength refers to the cable cutoff wavelength as defined in ITU-T G.650.1 of the International Telecommunication Union (ITU). In addition, terms not specifically defined in this specification shall follow the definitions and measurement methods in G.650.1 and G.650.2.

[0017] (Embodiment 1) Figure 1 is a schematic cross-sectional view of a multicore fiber according to Embodiment 1 in a plane perpendicular to the longitudinal direction. The multicore fiber 10 comprises two core portions 11 and 12, which are multiple core portions, a cladding portion 13, and a resin coating portion 14. The multicore fiber 10 is a so-called two-core type multicore fiber having two core portions.

[0018] The core portions 11 and 12 are arranged in a substantially straight line in cross-section, with the center O of the cladding portion 13 in between. The core portions 11 and 12 have a so-called trench-type refractive index profile. The cladding portion 13 surrounds the core portions 11 and 12 and has a refractive index lower than the maximum refractive index of the core portions 11 and 12.

[0019] The core parts 11 and 12 are made of silica glass containing germanium, for example, and the cladding part 13 is made of pure silica glass. Here, pure silica glass is extremely high-purity silica glass that substantially does not contain 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 includes those that contain a certain amount of chlorine used in the manufacturing process. The core parts 11 and 12 may be made of silica glass or pure silica glass containing at least one of fluorine, chlorine, potassium, and sodium, and the cladding part 15 may be made of silica glass containing fluorine.

[0020] Furthermore, the clad portion 13 is set to a practically desirable value, with an outer diameter within the range of 125 μm ± 25 μm.

[0021] The resin coating portion 14 consists of known structures and materials, such as those used to protect glass optical fibers in optical fibers. The resin coating portion 14 comprises, for example, a two-layer coating structure. The outer diameter of the resin coating portion 14 is, for example, 250 μm.

[0022] Figure 2 is an explanatory diagram of an example of a trench-type refractive index profile. In Figure 2, profile P1 is the refractive index profile of the center core of the core, profile P2 is the refractive index profile of the intermediate layer of the core, profile P3 is the refractive index profile of the trench layer of the core, and profile P4 is the refractive index profile of the cladding. The intermediate layer is the layer surrounding the center core. The refractive index of the intermediate layer is the same as that of the cladding or the refractive index difference with respect to the cladding is within ±0.05%. The trench layer is the layer surrounding the intermediate layer. The refractive index of the trench layer is lower than that of the intermediate layer and the cladding.

[0023] Δ1 is a parameter that defines the refractive index profile of the core, and is the relative refractive index difference between the maximum refractive index of the center core and the refractive index of the cladding. The center core diameter 2a is a parameter that defines the refractive index profile of the core, and is a parameter related to diameter. Note that 'a', which is half the value of the center core diameter 2a, is sometimes called the center core radius. Δ2 is a parameter that defines the refractive index profile of the core, and is the relative refractive index difference between the refractive index of the intermediate layer and the refractive index of the cladding. The outer diameter 2b, which is the outer diameter of the intermediate layer, is a parameter that defines the refractive index profile of the core, and is a parameter related to diameter. Δ3 is a parameter that defines the refractive index profile of the core, and is the relative refractive index difference between the refractive index of the trench layer and the refractive index of the cladding. The outer diameter 2c, which is the outer diameter of the trench layer, is a parameter that defines the refractive index profile of the core, and is a parameter related to diameter.

[0024] The effective core cross-sectional area (Aeff) of the core sections 11 and 12 is determined by the combination of parameters that define the refractive index profile of the core section as described above.

[0025] In this embodiment, the distance from the center O of the cladding portion 13 to the core portion 11 is d11. Also, the distance from the center O to the core portion 12 is d12.

[0026] In this embodiment, the effective core cross-sectional area of ​​the core portion 12 is 10% or more larger than the effective core cross-sectional area of ​​the core portion 11. Also, the distance d12 of the core portion 12 from the center O of the cladding portion 13 is smaller than the distance d11 of the core portion 11 from the center O of the cladding portion 13. The core portion 11 is an example of a first core portion included in a plurality of core portions, and the core portion 12 is an example of a second core portion included in a plurality of core portions.

[0027] The following explains the effect of the above-mentioned relationship between the effective core cross-sectional area and the distance from the center O. First, the inventors investigated how much the Aeff of the two core sections should differ in order to suppress inter-core crosstalk. Specifically, as shown in Figure 1, in a two-core type multicore phi, the outer diameter of the cladding section is 125 μm, and the confinement loss of each core section at a wavelength of 1625 nm is 0.001 dB / km, and the Aeff is 60-180 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 fiber length of 100 km in various multi-core fibers configured to achieve the desired result. The core pitch (distance between the centers of two core sections) was set to 40 μm.

[0028] Figure 3 shows an example of the relationship between the difference in effective core cross-sectional area between cores and intercore crosstalk, as investigated in this way. From Figure 3, it can be seen that if the difference in Aeff between cores is 10% or more, intercore crosstalk can be stably reduced to -30 dB or less.

[0029] On the other hand, the inventors found that the larger the effective core cross-sectional area of ​​the core, the greater the microbend loss tends to be. Specifically, the inventors first investigated the microbend loss of a multicore fiber with two cores, as shown in Figure 1, where the outer diameter of the cladding is 125 μm and the outer diameter of the resin coating is 250 μm, by calculation and experiment. The multicore fiber was optimized so that the cutoff wavelength at each core was 1400 ± 100 nm.

[0030] Microbend loss can also be measured using the following measurement method called the sandpaper method, which is similar to the fixed-diameter drum method specified in JIS C6823:2010. The sandpaper method defines the microbend loss as the difference between the transmission loss in state A, where a predetermined length (400m or more) of optical fiber is wound in a single layer without overlapping at a tension of 100gf on a fixed drum wound with #1000 grit sandpaper, and the transmission loss of the optical fiber in state B, where the optical fiber is wound on the same fixed drum as in state A, but without sandpaper, at the same tension and length as in state A. Here, the transmission loss of the optical fiber in state B does not include microbend loss and is considered to be the transmission loss inherent to the optical fiber itself. Also, in this measurement method, the transmission loss is measured at a wavelength of 1550nm, for example, so the microbend loss is also the value at a wavelength of 1550nm. Hereafter, unless otherwise specified, the microbend loss is the value at a wavelength of 1550nm.

[0031] Figure 4 shows an example of the relationship between effective core cross-sectional area and microbend loss. As can be seen from Figure 4, the microbend loss tended to increase as the effective core cross-sectional area increased. However, it was found that for the same effective core cross-sectional area, the microbend loss was smaller in the core portion closer to the center of the cladding. In this invention, when various distances from the center of the cladding were examined, it was found that, regardless of the distance set in Figure 4 (20 μm and 40 μm), for the same effective core cross-sectional area, the microbend loss was smaller in the core portion closer to the center of the cladding.

[0032] Therefore, in this embodiment, the distance from the center O of the cladding portion 15 of the core portion 12, which has an effective core cross-sectional area 10% or more larger than that of the core portion 11, is made smaller than the distance from the center O of the cladding portion 15 of the core portion 11. This suppresses microbend loss of the core portion 12 while suppressing intercore crosstalk by treating the core portions 11 and 12 as different cores.

[0033] In this case, it is preferable that the microbend loss of light with a wavelength of 1550 nm in the core portion 12 (an example of the second core portion) is 200% or less, and more preferably 150% or less, of the microbend loss of light with a wavelength of 1550 nm in the core portion 11 (an example of the first core portion). This prevents the difference between the two microbend losses from becoming excessive.

[0034] In the multicore fiber 10 configured as described above, intercore crosstalk is suppressed and excellent microbend loss characteristics are achieved.

[0035] Furthermore, because the multicore fiber 10 has excellent microbend loss characteristics, it can achieve relatively low transmission loss. From the viewpoint of application to a transmission system, it is preferable that the transmission loss of the multicore fiber 10 is 0.25 dB / km or less at a wavelength of 1550 nm.

[0036] Furthermore, in the multicore fiber 10, the arrangement of the core portion is asymmetrical with respect to the center of the cladding portion, so there is no need to provide a marker to identify the core portion.

[0037] Furthermore, in the multicore fiber 10, the cores 11 and 12 propagate light in single mode within the usable wavelength band including a predetermined wavelength, and the effective core cross-sectional area is 60 to 180 μm². 2 It is preferable that this is the case. The predetermined wavelength is the wavelength used for optical fiber transmission, for example, 1550 nm. The wavelength band used is the wavelength band used for optical fiber transmission, which includes the predetermined wavelength, for example, the C band (1530 nm to 1565 nm).

[0038] Furthermore, the microbend loss of the core sections 11 and 12 is preferably 10 times or less that of a standard SMF. A standard SMF is a single-mode optical fiber having characteristics conforming to the standard defined in ITU-T G.652, and having a resin coating layer with an outer diameter of 250 μm on the outer circumference of a cladding section with an outer diameter of 125 μm. In the case of a two-layer structure, for example, the coating layer consists of a primary layer with a thickness of approximately 37.5 μm and a secondary layer with a thickness of approximately 25 μm that surrounds the outer circumference of the primary layer.

[0039] According to the inventors' research, it has been confirmed that a multicore fiber with a cladding outer diameter in the range of 125 ± 25 μm can achieve a microbend loss of 10 times or less that of a standard SMF.

[0040] In particular, the relationship between the effective core cross-sectional area in the core and the distance from the center of that core is preferably as shown in Table 1. This makes it easy to achieve a characteristic in which the microbend loss is 10 times or less than that of a standard SMF. For example, if the effective core cross-sectional area in a certain core is 80 μm 2 More than 100μm 2 If the effective core cross-sectional area is less than 100 μm, it is preferable that the distance from the center of the core is less than 35 μm. Similarly, if the effective core cross-sectional area is 100 μm 2 More than 120μm 2When it is less than, the distance from the center of the core part is preferably less than 31 μm, and the effective core cross-sectional area is 120 μm 2 or more and 140 μm 2 When it is less than, the distance from the center of the core part is preferably less than 27 μm, and the effective core cross-sectional area is 140 μm 2 or more and 160 μm 2 When it is less than, the distance from the center of the core part is preferably less than 23 μm, and the effective core cross-sectional area is 160 μm 2 or more and 180 μm 2 When it is less than, the distance from the center of the core part is preferably less than 19 μm, and the effective core cross-sectional area is 180 μm 2 When it is or more, the distance from the center of the core part is preferably less than 14 μm.

[0041]

Table 1

[0042] Also, in the multi-core fiber 10, the distance between the centers of the core parts 11 and 12 is preferably 20 μm or more. Thereby, it becomes easy to make the crosstalk between cores at a stripe length of 100 km at a wavelength of 1550 nm a preferable value of -15 dB or less.

[0043] Also, the distance between the centers of the core parts 11 and 12 is preferably a value such that the distance between the centers of the core parts 11 and 12 and the outer peripheral surface of the cladding part 13 (also called the cladding thickness) is 15 μm or more. Thereby, it becomes easy to make the leakage loss at a wavelength of 1550 nm a preferable value of 0.01 dB / km or less.

[0044] Also, in the multi-core fiber 10, the relative refractive index difference (Δ1 in FIG. 2) of the maximum refractive index of the center core with respect to the refractive index of the cladding part 13 is preferably 0.12% or more and 0.46% or less. Thereby, while suppressing an increase in microbend loss and an increase in crosstalk between cores, the effective core cross-sectional area is 60 to 180 μm 2It is easy to set it to a suitable range like this. Furthermore, if Δ1 is between 0.12% and 0.46%, the increase in transmission loss due to scattering loss by dopants (e.g., germanium) contained in the center core can also be suppressed.

[0045] (Modified version of Embodiment 1) Figure 5 is a schematic cross-sectional view of a multicore fiber in a plane perpendicular to the longitudinal direction, relating to a modified example of Embodiment 1. These multicore fibers 10A, 10B, 10C, and 10D differ from the multicore fiber 10 in that the core portions 11 and 12 are not arranged in a straight line with respect to the center O in the cross-section.

[0046] The same effects as those obtained with multicore fiber 10 can be obtained with multicore fiber 10A to 10D.

[0047] (Manufacturing method) The multicore fiber according to this embodiment can be manufactured, for example, using a method called a drilling method, as described below.

[0048] For example, core materials are manufactured using known methods such as VAD (Vapor-phase Axial Deposition) or OVD (Outside Vapor Deposition). A core material is a material that has a portion that will become part of the cladding portion, surrounding the portion that will become the core portion of the multicore fiber. Core materials are prepared according to the number of core portions.

[0049] Furthermore, a cladding base material is prepared to form the majority of the cladding portion, and voids are made in it equal to the number of core portions for inserting the core base material. After making the voids, the inside of the voids may be cleaned.

[0050] Next, core materials are inserted into each void of the clad material and integrated through heat treatment or other means to form an optical fiber material. Furthermore, multicore fibers are drawn from the optical fiber material using a known drawing furnace.

[0051] (Examples, Comparative Examples) According to the manufacturing method described above, a multicore fiber of the embodiment corresponding to Embodiment 1 and a multicore fiber of the comparative example were manufactured. In the multicore fiber of the embodiment, the effective core cross-sectional area of ​​the core portion corresponding to the first core portion, core number #1 (hereinafter sometimes referred to as core #1), was 80 μm². 2 The design ensured that the distance from the center of the cladding portion was 28 μm. Furthermore, the effective core cross-sectional area of ​​the core portion corresponding to the second core portion, core number #2 (hereinafter sometimes referred to as core #2), was 120 μm. 2 The design ensures that the distance from the center of the cladding is 22 μm. In this case, the effective core cross-sectional area of ​​core #2 is more than 10% larger than the effective core cross-sectional area of ​​core #1, specifically by 50%.

[0052] On the other hand, the comparative example's multicore fiber had an effective core cross-sectional area of ​​120 μm² in core #1 compared to the example. 2 The design ensured that the distance from the center of the cladding portion was 28 μm. Furthermore, the effective core cross-sectional area of ​​core #2 was 80 μm. 2 The design was such that the distance from the center of the cladding portion is 22 μm.

[0053] In both the examples and comparative examples, the multicore fibers were designed with trench-type refractive index profiles for cores #1 and #2. The center core was made of pure silica, and the cladding was made of fluorine-containing silica glass. The cladding diameter was designed to be 125 μm.

[0054] Table 2 shows the structural parameters and optical properties of each core section of the multicore fiber manufactured as described above. In Table 2, "Center Core Δ" corresponds to Δ1, and "Trench Δ" corresponds to Δ3. "Cladding Ratio" is a value based on the refractive index of the cladding section, and "Pure Silica Ratio" is a value based on the refractive index of pure silica glass. "Cladding Δ" is the difference in relative refractive index of the cladding section based on the refractive index of pure silica glass. "b / a" indicates the ratio of the outer diameter of the intermediate layer to the center core diameter, and "c / a" indicates the outer diameter of the trench layer to the center core diameter. "λcc" indicates the cutoff wavelength. "Dispersion Slope" indicates the value at the zero-dispersion wavelength. Bending loss @1550nm / 30mmΦ is the bending loss at a wavelength of 1550nm when the multicore fiber is bent to a diameter of 30mm.

[0055] As shown in Table 2, in both the examples and comparative examples, the multicore fibers had different core sections for cores #1 and #2, resulting in low intercore crosstalk (XT) of -44 dB or less.

[0056] On the other hand, the multicore fibers in the examples had microbend loss ratios of 2.2 and 3.3 compared to standard SMF. However, in the comparative examples, the ratios were 12.5 and 1.8, with one being greater than 10 and one being more than 690% greater than the other.

[0057] [Table 2]

[0058] (Embodiment 2) Figure 6 is a schematic cross-sectional view of a multicore fiber according to Embodiment 2 in a plane perpendicular to the longitudinal direction. The multicore fiber 20 has a configuration in which core portions 21 and 22 are added to the multicore fiber 10 according to Embodiment 1. The multicore fiber 10 is a so-called 4-core type multicore fiber.

[0059] The core sections 21 and 22 are arranged in a nearly straight line with respect to the center O. Furthermore, the line connecting the core sections 21 and 22 to the center O is nearly perpendicular to the line connecting the core sections 11 and 12 to the center O. The core sections 21 and 22 have a trench-type refractive index profile. The cladding section 13 surrounds the core sections 21 and 22 and has a refractive index lower than the maximum refractive index of the core sections 11 and 12. The materials constituting the core sections 21 and 22 may be the same as those constituting the core sections 11 and 12.

[0060] Furthermore, the distance of the core portion 21 from the center O is d21. Also, the distance of the core portion 22 from the center O is d22.

[0061] In this embodiment, the effective core cross-sectional area of ​​the core portion 22 is 10% or more larger than the effective core cross-sectional area of ​​the core portion 12. The effective core cross-sectional area of ​​the core portion 12 is 10% or more larger than the effective core cross-sectional area of ​​the core portion 21. Furthermore, the effective core cross-sectional area of ​​the core portion 21 is 10% or more larger than the effective core cross-sectional area of ​​the core portion 11.

[0062] Furthermore, regarding the distances d11, d12, d21, and d22 from the center O of the core parts 11, 12, 21, and 22, d22 is smaller than d12. Also, d12 is smaller than d21. Also, d21 is smaller than d11.

[0063] In other words, in this embodiment, if core portion 11 is considered the first core portion, then core portions 12, 21, and 22 all correspond to the second core portion. Also, if core portion 21 is considered the first core portion, then core portions 12 and 22 all correspond to the second core portion. Also, if core portion 12 is considered the first core portion, then core portion 22 corresponds to the first core portion.

[0064] The same effects as those obtained with the multicore fiber 10 can be obtained with the multicore fiber 20 configured as described above.

[0065] (Modified version of Embodiment 2) Figures 7 and 8 are schematic cross-sectional views of a multicore fiber in a plane perpendicular to the longitudinal direction, relating to a modified example of Embodiment 2. These multicore fibers 20A, 20B, 20C, 20D, 20E, 20F, 20G, and 20H differ from multicore fiber 20 in the positional relationship of the core portions 11, 12, 21, and 22. Specifically, in multicore fibers 20A to 20H, at least one of the following conditions is met: the core portions 11 and 12 are not arranged in a straight line with respect to the center O in the cross-section, and the core portions 21 and 22 are not arranged in a straight line with respect to the center O in the cross-section.

[0066] The same effects as with multicore fiber 20 can be obtained with multicore fiber 20A~20H.

[0067] In the above embodiment, the refractive index profile of the core is trench-type, but it may also be step-type or W-type. Furthermore, if the core has a step-type refractive index profile, the core itself can be considered a center core.

[0068] Furthermore, the present invention is not limited by the embodiments described above. Configurations that appropriately combine the above-described components are also included in the present invention. Moreover, further effects and modifications can be easily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the embodiments described above, and various modifications are possible. [Explanation of symbols]

[0069] 10, 10A, 10B, 10C, 10D, 20, 20A, 20B, 20C, 20D, 20E, 20F, 20G, 20H Multicore Fiber 11, 12, 21, 22 Core section 13,15 Clad section 14 Resin coating O center P1, P2, P3, P4 Profiles

Claims

1. Multiple core sections, A cladding portion surrounding the plurality of core portions and having a refractive index lower than the maximum refractive index of the core portions, Equipped with, The plurality of core portions include a first core portion and a second core portion whose effective core cross-sectional area at a predetermined wavelength is 10% or more larger than that of the first core portion, and whose distance from the center of the cladding portion is smaller than the distance from the center of the cladding portion to the first core portion. Multicore fiber.

2. The plurality of core portions are, in the operating wavelength band including the predetermined wavelength, It propagates light in single mode, Effective core cross-sectional area: 60–180 μm ２ That is The multicore fiber according to claim 1.

3. 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.

4. 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.

5. The outer diameter of the cladding portion is in the range of 125 ± 25 μm. The microbend loss of light at a wavelength of 1550 nm is 10 times or less than that of a standard optical fiber having characteristics conforming to the standard defined in ITU-T G. 652 and having a resin coating with an outer diameter of 250 μm on the outer circumference of a cladding portion with an outer diameter of 125 μm. The multicore fiber according to feature 1.

6. The relative refractive index difference between the refractive index of the cladding portion and the maximum refractive index of the center core is 0.12% or more and 0.46% or less. The multicore fiber according to claim 3 or 4.

7. The microbend loss of light with a wavelength of 1550 nm in the second core is 200% or less of the microbend loss of light with a wavelength of 1550 nm in the first core. The multicore fiber according to claim 1.

8. The microbend loss of light with a wavelength of 1550 nm in the second core is 150% or less of the microbend loss of light with a wavelength of 1550 nm in the first core. The multicore fiber according to claim 1.

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