Multicore fiber and method of manufacturing the same

Abstract

To provide a multi-core fiber having many cores and having high manufacturability and connectivity, and a method for manufacturing the same.SOLUTION: A multi-core fiber includes 40 or more core parts and a clad part surrounding the peripheries of the core parts and having a refractive index lower than the maximum refractive index of the core parts. In a cross-section orthogonal to the longitudinal direction, 15 or less core parts are included in a row formed to be arranged in an approximately straight line; the relative refractive index difference Δ1 of the maximum refractive index of the core parts to the refractive index of the clad part is 0.35% or more and 1% or less; and a hole structure is not included in the cross-section.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a multicore fiber and a method for manufacturing a multicore fiber. [Background technology]

[0002] Space division multiplexing (SDM) is a new technology that can increase transmission capacity at a relatively low cost. One of the SDM technologies is multi-core fiber (MCF) (Patent Documents 1 to 4, Non-Patent Document 1). Patent Documents 2 and 4 and Non-Patent Document 1 disclose multi-core fibers with more than 100 cores. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6722271 [Patent Document 2] Patent Publication No. 2021-39340 [Patent Document 3] International Publication No. 2018 / 168170 [Patent Document 4] International Publication No. 2019 / 146750 [Non-patent literature]

[0004] [Non-Patent Document 1] Ming-Jun Li et al. “High Bandwidth Coupled Multicore Fiber for Data Center Applications”, ECOC2019, W.3.C.6 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the technologies disclosed in Patent Documents 2 and 4 and Non-Patent Document 1 use special structures, such as an air hole structure for light confinement and a core relative refractive index difference Δ1 of 1.2% or more. As a result, problems such as increased manufacturing costs due to reduced manufacturability and reduced connectivity have been raised. On the other hand, when no special structures are used, as disclosed in Patent Documents 1 and 3, only those with up to 30 cores have been proposed.

[0006] 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 having a large number of cores and high manufacturability and connectability, and a method for manufacturing the same. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is a multi-core fiber comprising 40 or more core portions and a cladding portion surrounding the outer periphery of the core portions and having a refractive index lower than the maximum refractive index of the core portions, wherein in a cross section perpendicular to the longitudinal direction, the core portions are arranged in a substantially linear array and the number of the core portions included in the array is 15 or less, the relative refractive index difference Δ1 of the maximum refractive index of the core portions with respect to the refractive index of the cladding portion is 0.35% or more and 1% or less, and the cross section does not include a hole structure.

[0008] The optical fiber may have 61 or more core portions, and the cladding portion may have an outer diameter of 270 μm or less.

[0009] The core pitch, which is the distance between the centers of the most adjacent core portions in the cross section, may be 15 μm or more.

[0010] The core portions may be arranged in a hexagonal close-packed lattice pattern in the cross section.

[0011] The Δ1 may be 0.8% or less.

[0012] The core may have a single-peak refractive index profile.

[0013] When the core portion exists alone, it transmits light in a single mode at a wavelength of 1550 nm and has an effective core area of ​​35 μm 2 More than 110μm 2 It may be designed to be:

[0014] After light of a wavelength of 1550 nm has propagated 1 m, the inter-core crosstalk of the two most adjacent core sections may be -20 dB or more, and after light of a wavelength of 1550 nm has propagated 1 km, the inter-core crosstalk of the two next-adjacent core sections may be 0 dB or less.

[0015] The effective core cross-sectional area of ​​the core portion may be larger than the effective core cross-sectional area when the core portion exists alone.

[0016] The core may have a transmission loss of 0.25 dB / km or less at a wavelength of 1550 nm.

[0017] The core pitch, which is the distance between the centers of the most adjacent core portions in the cross section, may be 25 μm or less.

[0018] One aspect of the present invention is a method for manufacturing a multi-core fiber, which includes stacking a plurality of core preforms, each of which includes a portion that becomes the core portion, in a glass tube to form an optical fiber preform, and drawing the multi-core fiber from the formed optical fiber preform.

[0019] The hole of the glass tube may be hexagonal.

[0020] When forming the optical fiber preform, a glass rod or glass powder may be filled into gaps between the plurality of core preforms in the glass tube. [Effects of the Invention]

[0021] According to the present invention, a multi-core fiber having a large number of cores and high manufacturability and connectability can be realized. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a schematic diagram showing a cross section of a multi-core fiber according to an embodiment. [Figure 2] FIG. 2 is a diagram showing an example of the relationship between the core pitch and the crosstalk (XT). [Figure 3] FIG. 3 is a diagram showing an example of the relationship between Δ1 and the mode field diameter (MFD). [Figure 4] FIG. 4 is a diagram showing an example of the relationship between the core pitch, Δ1, and power fluctuation. [Figure 5] FIG. 5 is a diagram showing an example of the relationship between the core pitch, Δ1, and the average transmission loss. [Figure 6] FIG. 6 is a diagram showing an example of the relationship between the core pitch and the power fluctuation when Δ1 is 0.45%. [Figure 7] FIG. 7 is a diagram showing an example of the relationship between the core pitch and the average transmission loss when Δ1 is 0.45%. [Figure 8] FIG. 8 is a diagram showing an example of the relationship between Δ1 and power fluctuation when the core pitch is 16 μm. [Figure 9] FIG. 9 is a diagram showing an example of the relationship between Δ1 and the average transmission loss when the core pitch is 16 μm. [Figure 10] FIG. 10 is a diagram showing an example of the relationship between the core pitch and the fiber diameter. [Figure 11] FIG. 11 is a diagram showing an example of the relationship between Δ1 and the effective core area (Aeff). [Figure 12] FIG. 12 is a diagram illustrating a method for manufacturing a multi-core fiber according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to these embodiments. In addition, in the drawings, identical or corresponding elements are appropriately designated by the same reference numerals. It should be noted that the drawings are schematic, and the dimensional relationships and ratios of elements may differ from the actual ones. The dimensional relationships and ratios may differ between drawings. In this specification, the cutoff wavelength refers to the effective cutoff wavelength, which means the cable cutoff wavelength defined in ITU-T (International Telecommunication Union) G.650.1. Other terms not specifically defined in this specification shall follow the definitions and measurement methods in G.650.1 and G.650.2.

[0024] 1 is a schematic diagram showing a cross section perpendicular to the longitudinal direction of a multicore fiber according to an embodiment. The multicore fiber 10 includes a plurality of cores 11 and a cladding 12 that surrounds the outer peripheries of the plurality of cores 11 and has a refractive index lower than the maximum refractive index of the plurality of cores 11, and extends in the longitudinal direction. This multicore fiber 10 has a structure in which 91 cores 11 are arranged in the cladding 12 in the form of a hexagonal close-packed lattice in the cross section perpendicular to the longitudinal direction. The 91 cores 11 are an example of 40 or more cores.

[0025] The core 11 is made of silica-based glass doped with a dopant, such as germanium, that increases the refractive index. The cladding 12 is made of pure silica glass. Here, pure silica glass refers to extremely high-purity silica glass that is substantially free of dopants that change the refractive index and has a refractive index of approximately 1.444 at a wavelength of 1550 nm. However, the core 11 may be made of pure silica glass and the cladding 12 may be made of silica-based glass doped with a dopant, such as fluorine, that decreases the refractive index. Alternatively, both the core 11 and the cladding 12 may be doped with a dopant for adjusting the refractive index.

[0026] The core 11 of the multi-core fiber 10 has, for example, a single-peak refractive index profile. The relative refractive index difference of the maximum refractive index of the core 11 with respect to the cladding 12 is Δ1. Δ1 is, for example, not less than 0.35% and not more than 1%. Δ1 may be not more than 0.8%.

[0027] Moreover, row C is a row formed by core portions 11 arranged in a substantially straight line in a cross section perpendicular to the longitudinal direction of the multi-core fiber 10. The number of core portions 11 included in row C is 15 or less, specifically 11.

[0028] In the multi-core fiber 10, the core pitch P is the distance between the centers of the two most adjacent core portions 11 in a cross section perpendicular to the longitudinal direction. In the multi-core fiber 10, the core pitch P is, for example, 15 μm or less.

[0029] Here, in a cross section perpendicular to the longitudinal direction, the core portion 11 next closest to the most adjacent core portion 11 from a certain core portion 11 is defined as the second adjacent core portion 11. In the multicore fiber 10, the core portions 11 are arranged in a hexagonal close-packed lattice pattern, and therefore the distance between the centers of two second adjacent core portions 11 is √3 times the core pitch P.

[0030] 1, the multi-core fiber 10 does not include a hole structure for light confinement in its cross section, that is, the multi-core fiber 10 has a solid structure.

[0031] The multi-core fiber 10 configured as described above has 91 core portions 11, but the number of core portions 11 included in row C is 11 (15 or less), Δ1 is 0.35% or more and 1% or less, and it does not include a hole structure, so it has a large number of cores and is easy to manufacture and connect.

[0032] Moreover, in the multi-core fiber 10, the cores 11 are arranged in a hexagonal close-packed lattice pattern, so the number of cores is large compared to the outer diameter (fiber diameter) of the cladding 12. Therefore, the number of cores can be increased without making the fiber diameter too large. Furthermore, since the cores 11 are arranged in a hexagonal close-packed lattice pattern, the multi-core fiber 10 can be easily manufactured by known methods such as a stack method.

[0033] The structure and characteristics of the multi-core fiber 10 will be described in more detail below using simulation calculation results.

[0034] In the multicore fiber 10, the crosstalk (XT) characteristics can be improved by increasing Δ1 of the core portion 11. For example, Fig. 2 is a diagram showing an example of the relationship between the core pitch and the crosstalk (XT). Fig. 1 shows the inter-core crosstalk between the two most adjacent core portions as XT after light with a wavelength of 1550 nm has propagated for 1 km in a case where the core diameter (2a) of the core portion is adjusted so that the effective cutoff wavelength is approximately 1520 nm. As shown in Fig. 1, even with the same core pitch, the larger Δ1 is, the smaller XT is.

[0035] On the other hand, Figure 3 shows an example of the relationship between Δ1 and mode field diameter (MFD). Note that MFD is a value for a single core 11 at a wavelength of 1550 nm. As shown in Figure 3, increasing Δ1 reduces MFD. An increase in MFD leads to an increase in nonlinearity and an increase in splice loss with other multicore fibers. Furthermore, as Δ1 increases, more dopant must be doped into the core 11, which tends to increase Rayleigh scattering loss. Note that the core 11 may be made of pure silica glass and the cladding may be made of silica-based glass with a low refractive index, but in this case, it is difficult to increase Δ1 above 1.0%. From the above perspectives, Δ1 is preferably 1% or less. Δ1 may also be 0.8 or less.

[0036] Furthermore, when Δ1 is made smaller, the interference of light between a certain core portion 11 and the core portions 11 arranged around it becomes stronger, and therefore, depending on the connection state and installation state of the multi-core fiber 10, the power fluctuation of the propagated light may become large.

[0037] Therefore, the inventors fabricated a 1 km long multicore fiber in which the core diameter of the core portion was adjusted and Δ1 and the core pitch were changed so that the effective cutoff wavelength would be approximately 1520 nm. The multicore fiber was a so-called 19-core type multicore fiber having 19 core portions arranged in a hexagonal close-packed lattice pattern. Light was then incident from one end of these multicore fibers onto a core portion located near the central axis, and the power of the light emitted from the other end was measured to investigate the power fluctuations.

[0038] FIG. 4 is a diagram showing an example of the relationship between the core pitch, Δ1, and power fluctuation in a fabricated 19-core multicore fiber. The power fluctuation is the fluctuation range of the measured power. As shown in FIG. 4, it was found that the power fluctuation increases rapidly in a region where the core pitch is less than 15 μm and in a region where Δ1 is less than 0.35%. From the above viewpoints, Δ1 is preferably 0.35% or more, and the core pitch is preferably 15 μm or more. If Δ1 is 0.35% or more and the core pitch is 15 μm or more, it is possible to achieve a state in which, for example, after light with a wavelength of 1550 nm has propagated 1 m, the inter-core crosstalk between the two most adjacent core portions 11 is −20 dB or more, and after light with a wavelength of 1550 nm has propagated 1 km, the inter-core crosstalk between the two second-adjacent core portions 11 is 0 dB or less.

[0039] Moreover, Fig. 5 is a diagram showing an example of the relationship between the core pitch, Δ1, and the average transmission loss in the fabricated 19-core multicore fiber. Here, the average transmission loss is the average value of the transmission loss that varies depending on the power fluctuation. As can be seen from Fig. 5, if the core pitch is 15 µm or more, an average transmission loss of, for example, 0.25 dB / km or less can be stably achieved. Furthermore, if Δ1 is 0.80% or less, an average transmission loss of, for example, 0.25 dB / km or less can be suitably achieved, and is preferable from the viewpoint of a balance between stability and reduction of the transmission loss.

[0040] FIG. 6 is a graph showing an example of the relationship between the core pitch and the power fluctuation when Δ1 is 0.45% in FIG. 4 . FIG. 7 is a graph showing an example of the relationship between the core pitch and the average transmission loss when Δ1 is 0.45% in FIG. 5 . FIG. 8 is a graph showing an example of the relationship between Δ1 and the power fluctuation when the core pitch is 16 μm in FIG. 4 . FIG. 9 is a graph showing an example of the relationship between Δ1 and the average transmission loss when the core pitch is 16 μm in FIG. 5 . As shown in FIGS. 6 and 7 , when the core pitch is less than 15 μm, the power fluctuation and the average transmission loss increase rapidly as the core pitch decreases. Furthermore, considering FIGS. 4 , 5 , 8 , and 9 together, when Δ1 is less than 0.35%, the power fluctuation and the average transmission loss increase rapidly as Δ1 decreases. Furthermore, when Δ1 is greater than 0.8%, the Rayleigh scattering loss itself increases, and therefore the average transmission loss also increases as Δ1 increases. From this perspective, Δ1 is preferably 0.80% or less.

[0041] Next, the core pitch and fiber diameter will be explained. Assuming a hexagonal close-packed lattice arrangement, the cladding 12 needs an outer thickness in addition to the portion where the cores 11 are arranged. The outer thickness can be defined, for example, by the cladding thickness T shown in Figure 1. The cladding thickness T is the distance from the center of the core 11, which is the portion of the core 11 closest to the outer periphery of the cladding 12, to the outer periphery of the cladding 12.

[0042] In the following, the fiber diameter was optimized by setting the cladding thickness T so that the fiber diameter is 1.2 times the diagonal length of the portion where the cores 11 are arranged in a hexagonal close-packed lattice pattern. Since the diagonal length of the portion where the cores 11 are arranged in a hexagonal close-packed lattice pattern is 11 times the core pitch in the multi-core fiber 10, the fiber diameter is 13.2 times the core pitch.

[0043] If the cladding thickness T is too thin, not only will the leakage loss in the core portion 11 close to the outer periphery of the cladding portion 12 increase, but structural disturbances and the like will also be more likely to occur during drawing of the multi-core fiber 10. The value of 1.2 times is an example of a preferable value from the viewpoints of suppressing leakage loss and suppressing structural disturbances during drawing.

[0044] Fig. 10 is a diagram showing an example of the relationship between the core pitch and the fiber diameter when optimized as described above. Note that 61 cores, 91 cores, 127 cores, and 169 cores are cases where the number of cores is 61, 91, 127, and 169, respectively. As shown in Fig. 10, when the core pitch is 15 µm or more, in order to make the fiber diameter 270 µm or less, the number of cores needs to be 169 or less, that is, the number of cores 11 included in a row formed by cores 11 lined up in a substantially straight line in a cross section perpendicular to the longitudinal direction when the cores are arranged in a hexagonal close-packed lattice pattern needs to be 15 or less.

[0045] Although not shown in Fig. 10, the number of cores may be, for example, 40 to 60. In this case, the arrangement of the core parts is, for example, such that 37 core parts are arranged in a hexagonal close-packed lattice pattern so that the overall shape is hexagonal, and then core parts are arranged on the outside of the sides of the hexagonal shape in a hexagonal close-packed lattice pattern.

[0046] In addition, while it is possible to increase the number of cores by increasing the fiber diameter, a larger fiber diameter requires higher screening strength to ensure the reliability of the multicore fiber, which leads to a decrease in yield. From this perspective, a fiber diameter of 270 μm or less is preferable.

[0047] For example, a suitable example is a multi-core fiber 10 having 61 or more cores 11 and an outer diameter of the cladding 12 of 270 μm or less. When having 61 or more cores 11, the core pitch needs to be 25 μm or less in order to make the outer diameter of the cladding 12 270 μm or less.

[0048] Furthermore, when the core portion 11 exists alone, it is preferable that the core portion 11 transmits light in a single mode at a wavelength of 1550 nm.

[0049] 11 is a diagram showing an example of the relationship between Δ1 and the effective core area (Aeff) when the cable cutoff wavelength λcc is fixed to 1520 nm in the multi-core fiber 10. In this case, if Δ1 is set to 0.35% or more and 1.0% or less, the core 11 has an effective core area (Aeff) of 35 μm 2 More than 110μm 2 In FIG. 11, the value of Aeff when Δ is 10.35% is 102 μm. 2 is.

[0050] Furthermore, assuming transmission in the C-Band (e.g., 1530 nm to 1565 nm), λcc is preferably 1520 nm or less, but may be set to a shorter wavelength depending on the wavelength band used for transmission. For example, when using the O-Band (e.g., 1260 nm to 1360 nm) for transmission, λcc may be set to 1260 nm or less.

[0051] It should be noted that the above design is an inherent characteristic of each core portion 11. In the multi-core fiber 10 according to the embodiment, each core portion 11 is strongly coupled with an adjacent core portion 11, and the field characteristics (e.g., Aeff characteristics) of the multi-core fiber 10 are different from the original characteristics of each core portion 11 (characteristics when there is no interference with the adjacent core portion 11). For example, the Aeff for a core portion 11 is larger than the Aeff when the core portion 11 exists alone. However, by setting the characteristics of each core portion 11 within the above-mentioned range, it is possible to realize a multi-core fiber 10 with excellent stability and low loss. For example, it has been confirmed that when each core portion 11 is designed to be multimode in the wavelength band used for transmission, inter-core interference of higher-order modes also occurs, resulting in a significant deterioration in power stability. Furthermore, in the structure of the multicore fiber 10 of the embodiment, a certain core portion 11 is strongly optically coupled to the nearest adjacent core portion 11 over a short distance, but this state is stable to a certain extent, and the XT with the second nearest adjacent core portion 11 is suppressed, so the structure is excellent in terms of stable characteristics and transmission loss.

[0052] An example of a manufacturing method for the multi-core fiber 10 will be described. First, a core preform having a cladding formed to surround the outer periphery of the core is prepared using the VAD method or the CVD method, and is drawn to an appropriate outer diameter using a fiber drawing machine to obtain 91 drawn core preforms. In addition, a quartz glass tube with a hexagonal hole is prepared using a tube manufacturing method.

[0053] 12, 91 core preforms 110 each having a core portion 111 and a cladding portion 112 are stacked in a quartz glass tube 120 to form an optical fiber preform 100. The core portion 111 is a portion that will become the core portion 11 of the multi-core fiber 10, and the cladding portion 112 and the quartz glass tube 120 are a portion that will become the cladding portion 12 of the multi-core fiber. When forming the optical fiber preform 100, it is preferable to fill the gaps between the multiple core preforms 110 in the quartz glass tube 120 with glass rods or glass powder. A glass rod is also called a cane.

[0054] Next, the optical fiber preform 100 is drawn under appropriate conditions to manufacture the multi-core fiber 10.

[0055] (Example) As an example of the present invention, a multi-core fiber having a structure similar to that of the multi-core fiber 10 according to the embodiment was produced by the above-described manufacturing method.

[0056] Although the fiber diameter tends to be large in such a multicore fiber with many cores, it is desirable to keep the fiber diameter at 270 μm or less to ensure reliability. On the other hand, if the pitch is too small, the transmission loss characteristics and its stability may deteriorate significantly. Therefore, based on the results described using Figure 10, the core pitch was set to 19 μm and the fiber diameter to approximately 250 μm. As assumed in the study of Figure 10, the size of the jacket tube (quartz glass tube) was set so that the fiber diameter was 1.2 times the diagonal length of the part where the cores are arranged in a hexagonal close-packed lattice pattern. Note that when the core pitch is 19 μm, the distance between the two next-neighboring cores is √3 times that distance, or approximately 33 μm. In addition, the multicore fiber of the example was coated with a primary coating with a thickness of 320 ± 20 μm and a secondary coating with a thickness of 370 ± 20 μm, on the outer periphery of the cladding.

[0057] As an example, the design parameters (Δ1 and core diameter (2a)) and optical characteristics of the multi-core fibers (MCF) of Samples No. 1 to 6 are shown in Table 1. Note that λcc, MFD, and Aeff are characteristics for one core, that is, characteristics when there is no interference with other cores.

[0058] As shown in Table 1, all samples have a λcc value that realizes single-mode transmission in the wavelength range of 1530 nm or more in the case of one core. 2 Good characteristics have been obtained. [Table 1]

[0059] Furthermore, after transmitting a few meters, adjacent cores are completely coupled and the light propagates through a wider field than a single core, but interference with the second-adjacent core is suppressed to a sufficiently low value of -10 dB or less even after 1 km transmission, and thanks to this effect, the variation in output power was suppressed to a small value of 1 dB or less even after 1 km transmission. Furthermore, the transmission loss shows the average value of the measured values ​​for each core, but because the structure of the multi-core fiber is optimized, measurements can be made with a certain degree of stability. The transmission loss value itself is also low, comparable to that of conventional multi-core fibers.

[0060] In the above embodiment, the cores 11 of the multi-core fiber 10 are arranged in a hexagonal close-packed lattice pattern, but may be arranged in other shapes, such as a square lattice pattern or a circular ring pattern.

[0061] In the above embodiment, the refractive index profile of the core portion 11 is a single-peak type that is easier to manufacture, but other refractive index profiles such as a W-type or trench type may also be used.

[0062] Furthermore, the present invention is not limited to the above-described embodiments. For example, 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]

[0063] 10: Multi-core fiber 11, 111: Core part 12, 112: Cladding section 100: Optical fiber base material 110: Core base material 120: Quartz glass tube C: Column P: Core pitch T: Cladding thickness

Claims

1. More than 40 core parts, a cladding portion surrounding the outer periphery of the core portion and having a refractive index lower than the maximum refractive index of the core portion; Equipped with In a cross section perpendicular to the longitudinal direction, the number of the core portions included in the row formed by the core portions arranged in a substantially straight line is 15 or less, a relative refractive index difference Δ1 of the maximum refractive index of the core portion with respect to the refractive index of the cladding portion is 0.35% or more and 1% or less; The cross section does not include a pore structure, The core portion is designed so that when it exists alone, it transmits light in a single mode at a wavelength of 1550 nm and has an effective core area of ​​35 μm 2 or more and 110 μm 2 or less. Multicore fiber.

2. 40 or more core parts; a cladding portion surrounding the outer periphery of the core portion and having a refractive index lower than the maximum refractive index of the core portion; Equipped with In a cross section perpendicular to the longitudinal direction, the number of the core portions included in the row formed by the core portions arranged in a substantially straight line is 15 or less, a relative refractive index difference Δ1 of the maximum refractive index of the core portion with respect to the refractive index of the cladding portion is 0.4% or more and 1% or less; The cross section does not include a pore structure, After light with a wavelength of 1550 nm has propagated for 1 m, the inter-core crosstalk between the two most adjacent core portions is −20 dB or more, and after light with a wavelength of 1550 nm has propagated for 1 km, the inter-core crosstalk between a certain core portion and the core portion next closest to the certain core portion is 0 dB or less. Multicore fiber.

3. 61 or more of the core portions, The outer diameter of the cladding portion is 270 μm or less. The multicore fiber according to claim 1 or 2.

4. The core pitch, which is the distance between the centers of the most adjacent core portions in the cross section, is 15 μm or more. The multicore fiber according to any one of claims 1 to 3.

5. The core portion is arranged in a hexagonal close-packed lattice pattern in the cross section. The multicore fiber according to any one of claims 1 to 4.

6. The Δ1 is 0.8% or less The multicore fiber according to any one of claims 1 to 5.

7. The core portion has a single-peak refractive index profile. The multicore fiber according to any one of claims 1 to 6.

8. The effective core cross-sectional area of ​​the core portion is larger than the effective core cross-sectional area when the core portion exists alone. The multicore fiber according to any one of claims 1 to 7.

9. The core has a transmission loss of 0.25 dB / km or less at a wavelength of 1550 nm. The multicore fiber according to any one of claims 1 to 8.

10. The core pitch, which is the distance between the centers of the most adjacent core portions in the cross section, is 25 μm or less. The multicore fiber according to any one of claims 1 to 9.

11. A method for manufacturing a multi-core fiber according to any one of claims 1 to 10, a plurality of core preforms each including a portion to be the core portion are stacked in a glass tube to form an optical fiber preform; The multi-core fiber is drawn from the formed optical fiber preform. Method for manufacturing multicore fiber.

12. The hole of the glass tube is hexagonal. The method for manufacturing a multicore fiber according to claim 11 .

13. When forming the optical fiber preform, a glass rod or glass powder is filled into the gaps between the plurality of core preforms in the glass tube. The method for manufacturing a multicore fiber according to claim 11 or 12.

Images