JP7809580B2 - Multicore fiber and method of manufacturing the same - Google Patents

Description

The present invention relates to a multicore fiber and a manufacturing method thereof.

Various core arrangements have been investigated for multicore fibers with multiple cores, including hexagonal close-packed arrangements, annular arrangements, and square arrangements. Using a multicore fiber has the advantage of improving the spatial density of the cores, but if the spacing between cores is made too small, inter-core crosstalk (inter-core XT) increases, causing signal penalties when used as an optical transmission line. Therefore, technology has been disclosed that suppresses inter-core crosstalk by placing air holes with circular cross sections between cores (see Non-Patent Documents 1 and 2). However, achieving circular air holes requires the use of complex fabrication methods such as drilling and capillary stacking.

Meanwhile, Patent Document 1 proposes a method of providing gaps between cores without a separate process by providing a planar portion (parallel portion) when fabricating the core preform of a multicore fiber and inserting it into a hole in the cladding preform. However, even with this method, it is necessary to fabricate the parallel portion when fabricating the core preform, and at least the hole in the cladding preform into which the core preform is inserted must be prepared by drilling, posing an issue with the complexity of the manufacturing process.

JP 2015-121642 A

K. Saitoh et al, “Multi-core hole-assisted fibers for high core density space division multiplexing”, OECC 2010, 7C2-1 (2010). B. Yao et al., “Reduction of Crosstalk by Hole-Walled Multi-Core Fiber” OFC 2012, paper OM2D.5. (2012).

As mentioned above, known multicore fibers have room for improvement in terms of ease of manufacturing structures that suppress inter-core crosstalk.

The present invention has been made in light of the above, and its purpose is to provide a multicore fiber that is easy to manufacture and in which inter-core crosstalk is suppressed, and a method for manufacturing the same.

In order to solve the above-mentioned problems and achieve the objectives, one aspect of the present invention is a multi-core fiber comprising a plurality of core portions arranged in a substantially rectangular shape in a cross section perpendicular to the longitudinal direction, and a cladding portion that has a refractive index lower than the maximum refractive index of the core portions and surrounds the outer periphery of the core portions, the cladding portion having a substantially cross-shaped or diamond-shaped first hole between the most adjacent core portions.

The cladding portion may have a low refractive index region between the second-neighboring core portions, the refractive index of which is lower than that of the surrounding glass.

The cladding portion may have a second air hole between the second-neighboring core portions.

The core may be made of pure silica glass or a silica-based glass containing at least one of germanium, fluorine, chlorine, potassium, and sodium.

The cladding portion may be made of pure silica glass or silica-based glass containing at least one of fluorine and chlorine.

The number of core parts may be five or more.

The number of core parts may be 13 or more.

One aspect of the present invention is a method for manufacturing a multicore fiber, comprising: a first step of forming a glass assembly and forming a multicore fiber preform including the glass assembly by alternately arranging a first glass body, which has a core portion and a cladding portion surrounding the outer periphery of the core portion and has a refractive index lower than the maximum refractive index of the core portion, and a second glass body, which is made of a homogeneous silica-based glass and has a substantially circular cross section perpendicular to the longitudinal direction, in a substantially rectangular shape in the cross section perpendicular to the longitudinal direction; and a second step of heating and melting the multicore fiber preform and drawing a multicore fiber.

The refractive index of the second glass body may be lower than the refractive index of the cladding portion of the first glass body.

The second glass body may have holes.

In the first step, a glass tube may be inserted into a gap formed in the glass assembly.

The first step may include a third step of using the glass aggregate as the multicore fiber preform and forming a coating layer around the outer periphery of the multicore fiber drawn in the second step.

In the first step, a glass layer may be formed to surround the outer periphery of the glass assembly.

The present invention has the advantage of realizing a multi-core fiber that is easy to manufacture and has reduced inter-core crosstalk.

FIG. 1 is a schematic cross-sectional view of a multi-core fiber according to a first embodiment, taken along a plane perpendicular to the longitudinal direction thereof. FIG. 2 is a diagram illustrating an example of a method for manufacturing a multi-core fiber according to the first embodiment. FIG. 3 is a diagram illustrating an example of a method for manufacturing a multi-core fiber according to the first embodiment. FIG. 4 is a diagram illustrating an example of a method for manufacturing a multi-core fiber according to the first embodiment. FIG. 5 is a diagram illustrating an example of a method for manufacturing a multi-core fiber according to the first embodiment. FIG. 6 is a diagram illustrating another example of the method for manufacturing the multi-core fiber according to the first embodiment. FIG. 7 is a schematic cross-sectional view of a multi-core fiber according to the second embodiment taken along a plane perpendicular to the longitudinal direction thereof. FIG. 8 is a diagram illustrating an example of a method for manufacturing a multi-core fiber according to the second embodiment. FIG. 9 is a schematic cross-sectional view of a multi-core fiber according to the third embodiment, taken along a plane perpendicular to the longitudinal direction thereof. FIG. 10 is a diagram illustrating an example of a method for manufacturing a multi-core fiber according to the third embodiment. FIG. 11 is a diagram illustrating another example of the method for manufacturing a multi-core fiber according to the third embodiment. FIG. 12 is a schematic cross-sectional view of a multi-core fiber according to the fourth embodiment taken along a plane perpendicular to the longitudinal direction. FIG. 13 is a diagram showing another example of a glass assembly. FIG. 14 is a schematic cross-sectional view of a multi-core fiber according to a comparative example taken along a plane perpendicular to the longitudinal direction.

Embodiments of the present invention will be described in detail below 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 will be omitted where appropriate. Furthermore, in this specification, 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.

(Embodiment 1)
1 is a schematic cross-sectional view of a multi-core fiber in a plane perpendicular to the longitudinal direction of the multi-core fiber according to embodiment 1. The multi-core fiber 10 includes 13 core portions 11, which are a plurality of core portions, and a cladding portion 12.

The core portions 11 are arranged in a substantially rectangular shape in a cross section perpendicular to the longitudinal direction. In this embodiment, the core portions 11 are arranged on the lattice points of a square lattice L1, represented by a dashed-dotted line. The center-to-center distance (also called the core pitch) between the most adjacent core portions 11 is equal to the lattice constant of the square lattice L1.

The cladding portion 12 has a refractive index lower than the maximum refractive index of the core portion 11 and surrounds the outer periphery of the core portion 11.

The cladding portion 12 has a first air hole 13, which is approximately cross-shaped or diamond-shaped, between the most adjacent core portions 11. For example, a first air hole 13a is provided between the most adjacent core portions 11a and 11b in FIG. 1. Core portion 11c is the core portion second most adjacent to core portion 11a. A core portion second most adjacent to a core portion, such as core portion 11c, may be referred to as the second adjacent core portion hereinafter. The center-to-center distance between core portion 11a and core portion 11c is √2 times the center-to-center distance between core portion 11a and core portion 11b.

Examples of materials for the core 11 and cladding 12 are given below. The materials for the core 11 and cladding 12 are not particularly limited as long as they satisfy the refractive index relationship described above. For example, the core 11 may be made of pure silica glass or silica-based glass containing at least one of germanium, fluorine, chlorine, potassium, and sodium. Chlorine is a dopant added during the manufacturing process of the multicore fiber 10 (such as the dehydration process) and may not be added intentionally. Germanium is a dopant that increases the refractive index of silica-based glass. Potassium and sodium are dopants that increase the refractive index of silica-based glass. Fluorine is a dopant that decreases the refractive index of silica-based glass. Pure silica glass is extremely high-purity silica glass with a refractive index of approximately 1.444 at a wavelength of 1550 nm.

Also, for example, the cladding portion 12 is made of pure silica glass or silica-based glass containing at least one of fluorine and chlorine.

For example, the core portion 11 may be made of silica-based glass containing germanium, and the cladding portion 12 may be made of silica-based glass containing no dopants other than chlorine. Alternatively, the core portion 11 may be made of pure silica-based glass or silica-based glass containing at least one of fluorine, chlorine, potassium, and sodium, and the cladding portion 12 may be made of silica-based glass containing fluorine.

In the multicore fiber 10 configured as described above, the first air holes 13 are provided between the most adjacent core portions 11, thereby suppressing inter-core crosstalk (crosstalk between adjacent cores) between the most adjacent core portions 11. For example, according to the inventor's extensive research, the multicore fiber 10 can achieve a cutoff wavelength of 1530 nm or less and crosstalk between adjacent cores of -20 dB or less over a length of 100 km, even when the core pitch is made extremely small, at 35 μm or less.

Furthermore, in the multicore fiber 10, inter-core crosstalk between second adjacent core portions is effectively suppressed because the center-to-center distance is √2 times larger than the center-to-center distance between adjacent cores.

Furthermore, the multi-core fiber 10 can be easily manufactured using the manufacturing method described below.

The manufacturing method according to this example comprises a first step and a second step. In the first step, first glass bodies and second glass bodies are alternately arranged in a substantially rectangular shape in a cross section perpendicular to the longitudinal direction to form a glass assembly, and a multicore fiber preform containing the glass assembly is formed. In the second step, the multicore fiber preform is heated and melted, and a multicore fiber is drawn.

The first step will be described with reference to Figures 2 to 5. First, as shown in Figure 2, 13 first glass bodies 110 and 12 second glass bodies 120 are arranged alternately in a generally rectangular shape in a cross section perpendicular to the longitudinal direction to form a glass assembly 100.

The first glass body 110 has a core portion 111 and a cladding portion 112 that has a refractive index lower than the maximum refractive index of the core portion 111 and surrounds the outer periphery of the core portion 111, and is approximately circular in cross section perpendicular to the longitudinal direction. The core portion 111 is the portion that will become the core portion 11 of the multicore fiber 10, and the cladding portion 112 is the portion that will become part of the cladding portion 12. The first glass body 110 can be produced, for example, by the VAD method (Vapor-phase Axial Deposition).

The second glass body 120 is made of a uniform silica-based glass and has a substantially circular cross section perpendicular to the longitudinal direction. The second glass body 120 is a portion that will become part of the cladding portion 12 of the multicore fiber 10. The constituent material of the second glass body 120 may be the same as the constituent material of the cladding portion 112 of the first glass body 110.

In addition, in the glass assembly 100, a roughly cross-shaped or diamond-shaped gap G is formed surrounded by two first glass bodies 110 and two second glass bodies 120, and this gap G is the part that becomes the first hole 13 of the multi-core fiber 10.

Next, as shown in Figure 3, the glass assembly 100 is bound with a jig 1 to fix the positional relationship between the first glass body 110 and the second glass body 120, while the outer periphery of the glass assembly 100 is heated and melted with a burner 2 to integrate the first glass body 110 and the second glass body 120.

Next, as shown in Figure 4, while the glass assembly 100 is rotated around its axis, glass soot is deposited around the outer periphery of the glass assembly 100 using a flame deposition method (e.g., OVD (Outside Vapor Deposition) method), and the glass soot is then sintered to form a glass layer 130. This glass layer 130 becomes part of the cladding portion 12 of the multicore fiber 10. Note that the cross-sectional shape of the glass assembly 100 perpendicular to the longitudinal direction is approximately rectangular, but by adjusting the rotation speed of the glass assembly 100, the cross-sectional shapes of the glass soot and glass layer 130 can be made approximately circular.

Next, as shown in Figure 5, a tapered glass member 140 for starting drawing is heat-fused to one end of the glass assembly 100 and the glass layer 130 to produce a multi-core fiber preform 1000. The jig 1 may then be removed.

In the second step, the multicore fiber preform 1000 is heated and melted using a known drawing device, and then drawn. This allows the multicore fiber 10 to be manufactured. During drawing, the pressure is controlled so that the voids G are not completely crushed, thereby forming the first air holes 13. It is also preferable to control the pressure so that the first glass body 110 and the second glass body 120 are not excessively deformed.

In the above manufacturing example, the glass layer 130 was formed on the outer periphery of the glass assembly 100 using flame deposition. However, as shown in FIG. 6, the glass assembly 100 may be inserted into a glass tube 150 having an approximately circular outer periphery in a cross section perpendicular to the longitudinal direction and an approximately rectangular hole on the inside, thereby forming a multicore fiber preform 1000A for manufacturing the multicore fiber 10. Although the inner hole of the glass tube 150 is approximately rectangular, if it has an irregularity shaped to match the outer periphery of the glass assembly 100, the gap between the glass assembly 100 and the glass tube 150 can be reduced.

(Embodiment 2)
7 is a schematic cross-sectional view of a multi-core fiber 20 in a plane perpendicular to the longitudinal direction of the multi-core fiber according to embodiment 2. The multi-core fiber 20 includes 13 core portions 21, which are a plurality of core portions, and a cladding portion 22.

The core portions 21 are arranged in a substantially rectangular shape in a cross section perpendicular to the longitudinal direction. In this embodiment, the core portions 21 are arranged on the lattice points of a square lattice L2, represented by the dashed-dotted line. The center-to-center distance between the most adjacent core portions 21 is equal to the lattice constant of the square lattice L2.

The cladding portion 22 has a refractive index lower than the maximum refractive index of the core portion 21 and surrounds the outer periphery of the core portion 21.

The cladding portion 22 has a first air hole 23, which is generally cross-shaped or diamond-shaped, between the most adjacent core portions 21. For example, a first air hole 23a is provided between the most adjacent core portions 21a and 21b of the core portions 21 in Figure 7.

The cladding portion 22 also has a low-refractive index region 24, which has a lower refractive index than the surrounding glass, between the second adjacent core portions 21. For example, a low-refractive index region 24a is provided between the second adjacent core portions 21a and 21c of the core portions 21 in Figure 7. In this embodiment, the low-refractive index region 24 has a substantially circular shape in a cross section perpendicular to the longitudinal direction.

The constituent materials of the core region 21 and cladding region 22 are not particularly limited as long as they satisfy the above-mentioned refractive index relationship, but may be the same as the constituent materials of the core region 11 and cladding region 12. Furthermore, the low refractive index region 24 may be, for example, silica-based glass containing an appropriate amount of fluorine, a dopant that lowers the refractive index.

In the multicore fiber 20 configured as described above, similar to the multicore fiber 10, a first air hole 23 is provided between the most adjacent core portions 21, thereby suppressing crosstalk between adjacent cores. Furthermore, a low refractive index region 24 is provided between the second adjacent core portions 21, thereby suppressing crosstalk between second adjacent cores.

For example, according to the inventors' intensive research, in the multicore fiber 20, even when the core pitch is made extremely small, at 30 μm or less, by setting the relative refractive index difference of the low refractive index region 24 with respect to the surrounding area to -0.1% to -1%, it is possible to achieve second-adjacent core crosstalk of -20 dB or less over a length of 100 km.

The multicore fiber 20 can be easily manufactured by a manufacturing method similar to the manufacturing method of the multicore fiber 10 illustrated with reference to Figures 2 to 5, or by a manufacturing method similar to the manufacturing method illustrated with reference to Figure 6. When manufacturing the multicore fiber 20, a glass assembly 200 such as that shown in Figure 8 is used as the glass assembly. The glass assembly 200 is formed by arranging 13 first glass bodies 210 and 12 second glass bodies 220 alternately in a substantially rectangular shape in a cross section perpendicular to the longitudinal direction.

The first glass body 210 has a core portion 211 and a cladding portion 212 that has a refractive index lower than the maximum refractive index of the core portion 211 and surrounds the outer periphery of the core portion 211, and is approximately circular in cross section perpendicular to the longitudinal direction. The core portion 211 is the portion that will become the core portion 21 of the multicore fiber 20, and the cladding portion 212 is the portion that will become part of the cladding portion 22. This first glass body 210 may be the same as the first glass body 110 shown in Figure 2.

The second glass body 220 is made of a uniform silica-based glass and has a substantially circular shape in a cross section perpendicular to the longitudinal direction. The second glass body 220 is the portion that will become the low refractive index region 24 of the multicore fiber 20. The material that makes up the second glass body 220 may have a lower refractive index than the material that makes up the cladding portion 112 of the first glass body 110, for example, a relative refractive index difference that is lower by approximately -0.1% to -1%.

(Embodiment 3)
9 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of a multi-core fiber according to embodiment 3. The multi-core fiber 30 includes 13 core portions 31 as a plurality of core portions, and a cladding portion 32.

The core portions 31 are arranged in a substantially rectangular shape in a cross section perpendicular to the longitudinal direction. In this embodiment, the core portions 31 are arranged on the lattice points of a square lattice L3 represented by a dashed line. The center-to-center distance between the most adjacent core portions 21 among the core portions 31 is equal to the lattice constant of the square lattice L3.

The cladding portion 32 has a refractive index lower than the maximum refractive index of the core portion 31 and surrounds the outer periphery of the core portion 31. The cladding portion 32 has a substantially rectangular shape in a cross section perpendicular to the longitudinal direction.

The cladding portion 32 has a first air hole 33, which is generally cross-shaped or diamond-shaped, between the most adjacent core portions 31. For example, a first air hole 33a is provided between the most adjacent core portions 31a and 31b of the core portions 31 in Figure 9.

The cladding portion 32 also has second air holes 34 between the second adjacent core portions 31. For example, a second air hole 34a is provided between the second adjacent core portions 31a and 31c of the core portions 31 in FIG. 9. In this embodiment, the second air holes 34 have a substantially circular shape in a cross section perpendicular to the longitudinal direction, but this is not particularly limited.

The constituent materials of the core portion 31 and the cladding portion 32 are not particularly limited as long as they satisfy the refractive index relationship described above, but may be the same as the constituent materials of the core portion 11 and the cladding portion 12.

In the multicore fiber 30 configured as described above, similar to the multicore fiber 10, a first air hole 33 is provided between the most adjacent core portions 31, thereby suppressing crosstalk between adjacent cores. Furthermore, a second air hole 34 is provided between the second adjacent core portions 31, thereby suppressing crosstalk between second adjacent cores.

For example, according to the inventors' intensive research, in the multicore fiber 30, even when the core pitch is made extremely small, at 25 μm or less, by making the diameter of the second air holes 34 1.0 to 1.5 times the core diameter of the core portion 31, it is possible to achieve second adjacent core crosstalk of -20 dB or less over a length of 100 km.

The multicore fiber 30 is also an example of a multicore fiber in which the outer periphery of the cladding is approximately polygonal. By making it approximately polygonal in this way, for example, when the multicore fiber 30 is arranged on a flat surface of a device or the like, the flat surface and the core 11 are automatically aligned approximately parallel or perpendicular to each other. This may eliminate or reduce the need for complicated angle adjustments when connecting the multicore fiber 30 to other multicore fibers of the same type, optical waveguide arrays, optical fiber ribbons, etc.

The multicore fiber 30 can be easily manufactured by a manufacturing method similar to the manufacturing method of the multicore fiber 10 illustrated with reference to Figures 2 to 5, or by a manufacturing method similar to the manufacturing method illustrated with reference to Figure 6. When manufacturing the multicore fiber 30, a glass assembly 300 such as that shown in Figure 10 is used as the glass assembly. The glass assembly 300 is formed by arranging 13 first glass bodies 310 and 12 second glass bodies 320 alternately in a substantially rectangular shape in a cross section perpendicular to the longitudinal direction.

The first glass body 310 has a core portion 311 and a cladding portion 312 that has a refractive index lower than the maximum refractive index of the core portion 311 and surrounds the outer periphery of the core portion 311, and is approximately circular in cross section perpendicular to the longitudinal direction. The core portion 311 is the portion that will become the core portion 31 of the multicore fiber 30, and the cladding portion 312 is the portion that will become part of the cladding portion 32. This first glass body 310 may be the same as the first glass body 110 shown in Figure 2.

The second glass body 320 is made of uniform silica-based glass, has a substantially circular shape in a cross section perpendicular to the longitudinal direction, and has a hole 321 formed in its center. The second glass body 320 becomes part of the cladding portion 32 of the multicore fiber 30, and the hole 321 becomes the second hole 34 of the multicore fiber 30. The diameter of the hole 321 is, for example, 1.0 to 1.5 times the core diameter of the core portion 311.

When the multicore fiber 30 is manufactured using a manufacturing method similar to the manufacturing method exemplified with reference to Figure 6, as shown in Figure 11, the glass aggregate 300 may be inserted into a glass tube 350 having a substantially rectangular outer periphery in a cross section perpendicular to the longitudinal direction and a substantially rectangular hole on the inside, thereby forming a multicore fiber preform 3000 for manufacturing the multicore fiber 30.

(Embodiment 4)
12 is a schematic cross-sectional view of a multi-core fiber in a plane perpendicular to the longitudinal direction of the multi-core fiber according to embodiment 4. The multi-core fiber 40 includes 13 core portions 41, which are a plurality of core portions, a cladding portion 42, and a coating layer 45 surrounding the outer periphery of the cladding portion 42.

The cores 41 are arranged in a substantially rectangular shape in a cross section perpendicular to the longitudinal direction, similar to the cores 11 of the multicore fiber 10 in Figure 1.

The cladding portion 42 has a refractive index lower than the maximum refractive index of the core portion 31 and surrounds the outer periphery of the core portion 41. The cladding portion 42 has a substantially rectangular shape in a cross section perpendicular to the longitudinal direction. The cladding portion 42 also has a substantially cross-shaped or diamond-shaped first air hole 43 between the most adjacent core portions 41.

The coating layer 45 surrounds the outer periphery of the cladding portion 42 and has a substantially circular shape in a cross section perpendicular to the longitudinal direction.

The constituent materials of the core 41 and cladding 42 are not particularly limited as long as they satisfy the refractive index relationship described above, but may be the same as the constituent materials of the core 11 and cladding 12. The coating layer 45 is made of resin. This resin is, for example, an ultraviolet-curing resin, but is not particularly limited as long as it is a resin used as a coating for optical fiber.

The multicore fiber 40 can be manufactured, for example, by bundling or integrating the glass aggregate 100 shown in FIG. 2 into a multicore fiber preform, heating and melting it, and drawing it, and then forming a coating layer 45 to surround the outer periphery of the drawn multicore fiber. The step of forming the coating layer 45 is an example of the third step.

In the multicore fiber 40 configured as described above, similar to the multicore fiber 10, a first air hole 43 is provided between the most adjacent core portions 41, thereby suppressing crosstalk between adjacent cores.

(Another example of glass assembly)
Fig. 13 is a diagram showing another example of a glass assembly. The glass assembly 100A shown in Fig. 13 has a configuration in which a glass tube 160 is inserted into a gap formed between the first glass body 110 and the second glass body 120 of the glass assembly 100 shown in Fig. 2.

In this way, with a glass assembly 100A having a glass tube 160 inserted, deformation of the first glass body 110 and the second glass body 120 can be suppressed by optimizing the process so that the glass tube 160 does not deform during a heating process such as drawing. Alternatively, by constructing the glass tube 160 from a material that is easily deformed and causing the glass tube 160 to deform instead of the first glass body 110 or the second glass body 120, deformation of the first glass body 110 or the second glass body 120 can be suppressed.

(Comparative Example, Examples 1 to 3)
As a comparative example, a multi-core fiber having the structure shown in Fig. 14 was manufactured using the VAD method and the hole-punching method. This multi-core fiber 50 includes four core portions 51 and a cladding portion 52 surrounding the four core portions 51. Specifically, a core rod serving as a preform for the core portions 51 and a portion of the cladding portion 52 was manufactured by the VAD method. Then, a glass rod serving as a preform for the remaining portion of the cladding portion 52 was prepared, and four holes were drilled in it, and a core rod was inserted into each hole, thereby manufacturing a multi-core fiber preform. Then, this multi-core fiber preform was heated and melted using a drawing device, and drawn, thereby manufacturing the multi-core fiber of the comparative example.

The core pitch P1, which is the distance between the centers of adjacent core portions 51, was set to 40 μm. The minimum cladding thickness T1, which is the shortest distance from the center of each core portion 51 to the outer edge of the cladding portion 52, was set to 34 μm, and the cladding diameter of the cladding portion 52 was set to 125 μm.

The relative refractive index difference Δ1 of the core portion 51 with respect to the refractive index of the cladding portion 52 was set to 0.37%, and the core diameter was set to 9 μm. As a result, the multicore fiber of the comparative example was designed to have a zero-dispersion wavelength λ0 of 1305 nm, a dispersion slope at λ0 of 0.0900 ps/nm ² /km, a mode field diameter (MFD) at a wavelength of 1310 nm of 9.16 μm, a cutoff wavelength (λcc) of 1223 nm, and a transmission loss at a wavelength of 1550 nm of 0.187 dB/km.

As Example 1, a multicore fiber with the structure shown in Figure 1 was fabricated. As Example 2, a multicore fiber with the structure shown in Figure 7 was fabricated. As Example 3, a multicore fiber with the structure shown in Figure 9 was fabricated.

In Example 1, the core was made of silica glass doped with chlorine and potassium, with a relative refractive index of 0.1% compared to pure silica glass. The cladding was made of silica glass doped with fluorine, with a relative refractive index difference of -0.27% compared to pure silica glass. Therefore, the relative refractive index difference Δ1 of the core with respect to the refractive index of the cladding was 0.37%, the same as in the comparative example. The core diameter was 9 μm, the same as in the comparative example.

In Example 2, the core portion was made of germanium-doped silica glass, with a relative refractive index of 0.39% compared to pure silica glass. The cladding portion other than the low refractive index region was made of silica glass containing no dopants other than chlorine, with a relative refractive index difference of 0.02% compared to pure silica glass. Therefore, the relative refractive index difference Δ1 of the core portion relative to the refractive index of the cladding portion was 0.37%, the same as in the comparative example. The low refractive index region was made of fluorine-doped silica glass, with a relative refractive index difference of -0.5% compared to pure silica glass. The core diameter was 9 μm, the same as in the comparative example.

In Example 3, the core was made of germanium-doped silica glass, with a relative refractive index of 0.39% compared to pure silica glass. The cladding was made of silica glass containing no dopants other than chlorine, with a relative refractive index difference of 0.02% compared to pure silica glass. Therefore, the relative refractive index difference Δ1 of the core with respect to the refractive index of the cladding was 0.37%, the same as in the comparative example. The core diameter was 9 μm, the same as in the comparative example. The diameter of the second air hole was 1.1 times the core diameter.

Table 1 shows the core pitch, cladding diameter, and optical characteristics of the multicore fibers of the comparative example and examples 1 to 3. In Table 1, "adjacent core XT" refers to the inter-core crosstalk between the two most adjacent core portions, and "second adjacent core XT" refers to the total inter-core crosstalk between the second adjacent core portions.

As shown in Table 1, the multi-core fibers of Examples 1 to 3 had optical properties such as λ0, dispersion slope, MFD, λcc, and transmission loss that were almost the same as those of the multi-core fiber of the comparative example, despite the core pitch being even smaller than that of the multi-core fiber of the comparative example, and the XT between adjacent cores was −20 dB or less. Furthermore, in Example 2, despite the core pitch being as small as 25.0 μm, the XT between second adjacent cores was as very small as −27.9 dB due to the effect of the low refractive index region. Furthermore, in Example 3, despite the core pitch being even smaller at 21.0 μm, the XT between second adjacent cores was as very small as −27.5 dB due to the effect of the low refractive index region.

From the above results, the multicore fibers of Examples 1 to 3 are suitable from the perspective of increasing transmission distances and increasing core density.

In the above embodiments and examples, the multicore fiber has 13 cores, but the number of cores is not particularly limited. For example, the multicore fiber may have any number of cores, such as 5 or more, or 13 or more.

For example, if a multi-core fiber has five cores, the multi-core can be manufactured by forming a glass assembly using, for example, five first glass bodies and four second glass bodies.

In addition, a coating portion may be provided on the outer periphery of the cladding portion of the multi-core fiber according to embodiments 1 to 3.

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, second air holes may be provided in the multicore fiber 10 of embodiment 1, or low refractive index regions may be provided in place of the second air holes 34 in the multicore fiber 30 of embodiment 3.

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.

1: Jig 2: Burner 10, 20, 30, 40: Multicore fiber 11, 11a, 11b, 11c, 21, 21a, 21b, 21c, 31a, 31b, 31c, 41, 111, 211, 311: Core portion 12, 22, 32, 42, 112, 212, 312: Cladding portion 13, 13a, 23, 23a, 33, 33a, 43: First hole 24, 24a: Low refractive index region 34, 34a: Second hole 45: Coating layer 100, 100A, 200, 300: Glass assembly 110, 210, 310: First glass body 120, 220, 320: Second glass body 130: Glass layer 140 : Glass members 150, 160, 350: Glass tube 321: Holes 1000, 1000A, 3000: Multicore fiber preform G: Air gaps L1, L2, L3: Square lattice P1: Core pitch T1: Minimum cladding thickness

Claims (12)

a plurality of core portions arranged in a substantially rectangular shape in a cross section perpendicular to the longitudinal direction;
a clad portion having a refractive index lower than the maximum refractive index of the core portion and surrounding the outer periphery of the core portion;
Equipped with
a first hole having a substantially cross shape or a substantially diamond shape between the core portions that are closest to each other among the core portions; The multicore fiber according to claim 1 , wherein the cladding portion has a low refractive index region between the core portions that are second to each other among the core portions, the low refractive index region having a refractive index lower than that of the surrounding glass. The multicore fiber according to claim 1 , wherein the cladding portion has a second hole between the core portions that are second adjacent to each other among the core portions. The multi-core fiber according to claim 1 , wherein the core portion is made of pure silica glass or silica-based glass containing at least one of germanium, fluorine, chlorine, potassium, and sodium. The multi-core fiber according to claim 1 , wherein the cladding portion is made of pure silica glass or silica-based glass containing at least one of fluorine and chlorine. The multicore fiber according to claim 1 , wherein the number of the core portions is five or more. The multicore fiber according to claim 6 , wherein the number of the core portions is 13 or more. a first step of forming a glass assembly by arranging a first glass body, which has a core portion and a cladding portion whose refractive index is lower than the maximum refractive index of the core portion and which surrounds the outer periphery of the core portion, and which is substantially circular in a cross section perpendicular to the longitudinal direction, and a second glass body, which is made of a uniform silica-based glass, which is substantially circular in a cross section perpendicular to the longitudinal direction, and which has substantially the same outer diameter as the first glass body, alternately and in a square lattice pattern in the cross section perpendicular to the longitudinal direction to form a glass assembly, and forming a multicore fiber preform including the glass assembly;
a second step of heating and melting the multi-core fiber preform and drawing a multi-core fiber;
Equipped with
In the second step, the pressure applied to the multi-core fiber preform is controlled to maintain the gap surrounded by the two first glass bodies and the two second glass bodies so as not to collapse.
Method for manufacturing multicore fiber. The method for manufacturing a multicore fiber according to claim 8 , wherein the refractive index of the second glass body is lower than the refractive index of the cladding portion of the first glass body. The method for manufacturing a multi-core fiber according to claim 8 , wherein the second glass body has holes. In the first step, the glass aggregate is used as the multi-core fiber preform;
The method for manufacturing a multi-core fiber according to claim 8 , further comprising a third step of forming a coating layer so as to surround an outer periphery of the multi-core fiber drawn in the second step. The method for manufacturing a multi-core fiber according to claim 8 , wherein in the first step, a glass layer is formed so as to surround an outer periphery of the glass aggregate.