Multicore optical fiber and multicore optical fiber preform

Abstract

A multicore optical fiber (1) according to the present invention includes silica-based glass. The multicore optical fiber (1) comprises: a plurality of cores (2); cladding (3, 4) that surrounds the plurality of cores (2) and has a refractive index lower than the refractive index of the plurality of cores; and a low-refractive-index part (5) that is disposed on a line segment (m) that connects the central axes (2a) of two cores disposed closest to each other among the plurality of cores (2) in a cross section orthogonal to the central axis (1a) of the multicore optical fiber, and that has a refractive index lower than the refractive index of the cladding. In the cross section, the low-refractive-index part (5) has a non-circular shape.

Description

Multi-core optical fiber and multi-core optical fiber preform

[0001] This application claims priority to Japanese Patent Application No. 2024-087489, filed on May 29, 2024, and incorporates by reference all the contents of said Japanese application.

[0002] In recent years, communication traffic has continued to increase. This has raised concerns about the capacity limits of optical communication systems that use single-mode fibers, which are used in conventional optical communications. One solution proposed is a multi-core optical fiber having multiple cores. Hereinafter, a multi-core optical fiber will also be referred to as an MCF. One form of MCF is an uncoupled MCF, in which each core transmits information independently. Generally, when multiple cores are provided in a single optical fiber, light leakage from one core to another occurs unless the inter-core distance is set to a certain length or longer. This results in degradation of signal quality. Inter-core crosstalk is used as an indicator of such degradation of signal quality. Hereinafter, inter-core crosstalk will also be referred to as inter-core XT.

[0003] One known means for controlling inter-core XT is to provide a low refractive index portion between adjacent cores, which has a refractive index lower than that of the cladding. This is disclosed in, for example, Patent Document 1 and Non-Patent Document 1.

[0004] International Publication No. 2017 / 033584

[0005] S. Nozoe et al., “Low Crosstalk 125 μm-Clading Multi-Core Fiber with Limited Air-Holes Fabricated with Over-Clading Bundled Rods Technique”, OFC 2017, Th1H. 6

[0006] The MCF according to the present disclosure is an MCF containing silica-based glass, and comprises a plurality of cores, a cladding surrounding the plurality of cores and having a refractive index lower than that of the plurality of cores, and a low-refractive-index portion, which is located on a line segment connecting the central axes of two of the plurality of cores that are arranged closest to each other in a cross section perpendicular to the central axis of the MCF, and has a refractive index lower than that of the cladding, and in the cross section, the low-refractive-index portion has a non-circular shape.

[0007] Fig. 1 is a diagram showing the cross section and refractive index profile of an MCF according to an embodiment. Fig. 2 is a cross section showing the relationship between a line segment connecting the central axes of the cores and the major axis of a low refractive index portion. Fig. 3 is a flowchart showing a method for manufacturing an MCF. Fig. 4 is a graph showing the relationship between the ellipticity of the low refractive index portion and inter-core XT.

[0008] To reduce the inter-core XT, it is desirable for the low-refractive-index portion to have a large cross-sectional area and a low refractive index. To form such a low-refractive-index portion, it is necessary to add a high concentration of fluorine to the glass, which increases the manufacturing cost of the MCF.

[0009] The present disclosure provides MCFs and MCF preforms that can reduce inter-core XT without increasing manufacturing costs.

[0010] According to the present disclosure, an MCF and an MCF preform capable of reducing inter-core XT without increasing manufacturing costs is provided.

[0011] The content of an embodiment of the present disclosure will be described. (1) An MCF according to one embodiment of the present disclosure is an MCF containing silica-based glass, and includes: a plurality of cores; a cladding surrounding the plurality of cores and having a refractive index lower than that of the plurality of cores; and a low-refractive-index portion, which is located on a line segment connecting the central axes of two of the plurality of cores that are closest to each other in a cross section perpendicular to the central axis of the MCF and has a refractive index lower than that of the cladding, and which has a non-circular shape in the cross section. In this MCF, the low-refractive-index portion provided between two adjacent cores has a non-circular cross section. The power density of light is high on the line segment connecting the central axes of the two cores. By arranging the longitudinal direction of the low-refractive-index portion along this line segment in the cross section, inter-core XT can be efficiently reduced. Therefore, inter-core XT can be reduced without increasing manufacturing costs.

[0012] (2) In the above (1), the low refractive index portion may have an elliptical cross section. In this case, by arranging the major axis of the low refractive index portion along the line segment, it is possible to reliably reduce the inter-core XT without increasing the manufacturing cost.

[0013] (3) In the above (2), the angle between the major axis of the low refractive index portion and the line segment in the cross section may be within ±25 degrees. In this case, it is possible to reliably reduce the inter-core XT without increasing the manufacturing cost.

[0014] (4) In the above (2) or (3), the ellipticity of the low refractive index portion in the cross section may be 0.5 or more and 0.97 or less. In this case, it is possible to further reduce the inter-core XT without increasing the manufacturing cost.

[0015] (5) In the above (2) or (3), the ellipticity of the low refractive index portion in the cross section may be 0.5 or more and 0.8 or less. In this case, it is possible to further reduce the inter-core XT without increasing the manufacturing cost.

[0016] (6) In the above (2) or (3), the ellipticity of the low refractive index portion in the cross section may be 0.5 or more and 0.6 or less. In this case, it is possible to further reduce the inter-core XT without increasing the manufacturing cost.

[0017] (7) In any of the above (1) to (6), the low refractive index portion may contain fluorine, and the fluorine concentration of the low refractive index portion may be higher than the fluorine concentration of the clad. In this case, the refractive index of the low refractive index portion may be lower than the refractive index portion of the clad.

[0018] (8) In any of the above (1) to (7), the plurality of cores may contain an alkali metal element or an alkaline earth metal element, in which case transmission loss due to Rayleigh scattering is reduced.

[0019] (9) In any of the above (1) to (8), the plurality of cores may contain germanium, which can increase the refractive index of the cores.

[0020] (10) In any of the above (1) to (9), the plurality of cores may contain chlorine. In this case, the occurrence of glass defects can be reduced, and the transmission loss component due to concentration fluctuations can be reduced compared to when other elements are added.

[0021] (11) An MCF preform according to one embodiment of the present disclosure is an MCF preform containing silica-based glass, comprising: a plurality of cores; a cladding surrounding the plurality of cores and having a refractive index lower than that of the plurality of cores; and a low-refractive-index portion located on a line connecting the central axes of two of the plurality of cores that are closest to each other in a cross section perpendicular to the central axis of the MCF preform, the low-refractive-index portion having a refractive index lower than that of the cladding portion, the low-refractive-index portion having a non-circular shape in the cross section. In this MCF preform, the low-refractive-index portion provided between two adjacent cores has a non-circular cross section. When an optical fiber is manufactured, the power density of the light is high on the line connecting the central axes of the two cores. By arranging the longitudinal direction of the low-refractive-index portion along this line in the cross section, inter-core XT can be efficiently reduced. Therefore, inter-core XT can be reduced without increasing manufacturing costs.

[0022] [Details of the embodiment of the present disclosure] Specific examples of MCFs and MCF preforms according to the present embodiment will be described with reference to the drawings as necessary. The present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims. In the following description, the same elements in the drawings will be designated by the same reference numerals, and duplicate explanations will be omitted.

[0023] (MCF) Fig. 1 is a diagram showing the cross section and refractive index profile of an MCF according to an embodiment. As shown in Fig. 1, the MCF 1 according to the first embodiment includes multiple cores 2, a first cladding 3, a second cladding 4, and one or more low refractive index sections 5. Fig. 1 shows a cross section perpendicular to the central axis 1a of the MCF 1. Hereinafter, the cross section perpendicular to the central axis 1a of the MCF 1 will also be simply referred to as the "cross section."

[0024] The MCF 1 is an uncoupled MCF. The MCF 1 includes silica-based glass. The MCF 1 is formed from silica-based glass. The silica-based glass contains silica as a main component, with the silica content being 90% or more by mass fraction. The silica-based glass may contain silica at a mass fraction of 95% or more. The multiple cores 2, the first cladding 3, the second cladding 4, and the low refractive index portion 5 each include silica-based glass. The multiple cores 2, the first cladding 3, the second cladding 4, and the low refractive index portion 5 each are formed from silica-based glass.

[0025] In the cross section, the multiple cores 2 are arranged around the central axis 1a at a distance from each other. The multiple cores 2 form a core group. In this embodiment, the number of cores 2 is two. Two cores 2 are arranged side by side in a direction perpendicular to the central axis 1a. The two cores 2 are arranged so as to sandwich the central axis 1a. In the cross section, the central axis 1a is located on a line segment m that connects the central axes 2a of the two cores 2. The central axis 1a is located at a position that bisects the line segment m.

[0026] The multiple cores 2 may have, for example, the same shape as one another. The multiple cores 2 may have, for example, a circular shape, i.e., a perfect circular shape, in cross section. The diameter of the core 2 may be, for example, 10 μm or more and 12 μm or less, or 11 μm or more and 11.5 μm or less. The multiple cores 2 may have shapes different from one another. The multiple cores 2 may have a non-circular shape in cross section. The shortest distance between adjacent cores 2 may be, for example, 20 μm or more and 50 μm or less, or 30 μm or more and 40 μm or less. Hereinafter, the diameter of the core 2 may also be referred to as the core diameter. Hereinafter, the shortest distance between adjacent cores 2 may also be referred to as the core pitch.

[0027] The multiple cores 2 contain, for example, an alkali metal element or an alkaline earth metal element. This reduces the viscosity of the cores when the MCF preform is drawn to manufacture the MCF 1, promoting rearrangement of the glass and reducing transmission loss due to Rayleigh scattering. The multiple cores 2 may contain, for example, multiple types of elements selected from alkali metal elements or alkaline earth metal elements. The multiple cores 2 contain, for example, germanium. This makes it easy to make the refractive index of the cores 2 higher than the refractive index of the first cladding. The multiple cores 2 contain, for example, chlorine. This reduces the occurrence of glass defects and reduces transmission loss components due to concentration fluctuations compared to when other elements are added.

[0028] The first cladding 3 is a common cladding that surrounds the cores 2 and the low refractive index portion 5. The first cladding 3 is an optical cladding. The first cladding 3 has a refractive index that is 0.3% or more and 0.5% or less lower than the refractive index of the cores 2. The first cladding 3 contains, for example, fluorine.

[0029] The second cladding 4 surrounds the first cladding 3. The second cladding 4 is a common cladding that, together with the first cladding 3, surrounds the cores 2 and the low refractive index portion 5. The second cladding 4 is a physical cladding. The outer diameter of the second cladding 4 is, for example, 124 μm or more and 126 μm or less. The second cladding 4 has a refractive index lower than that of the cores 2 and higher than that of the first cladding 3. The second cladding 4 contains, for example, fluorine. The fluorine concentration of the second cladding 4 is lower by 1000 ppm or more and 5000 ppm by mass fraction than that of the first cladding 3.

[0030] In the cross section, the low refractive index portion 5 is disposed on a line segment m connecting the central axes 2a of two of the multiple cores 2 that are disposed closest to each other. In the cross section, the central axis 5a of the low refractive index portion 5 is disposed on the line segment m and overlaps with the central axis 1a. In this embodiment, the number of cores 2 is two, and therefore the number of low refractive index portions 5 is one. If the number of cores 2 is three or more, the number of low refractive index portions 5 may be two or more. The low refractive index portion 5 has a refractive index that is 0.1% or more and 1.0% or less lower than the refractive index of the first cladding 3. The low refractive index portion 5 contains, for example, fluorine. The fluorine concentration of the low refractive index portion 5 is higher than the fluorine concentration of the first cladding 3.

[0031] The low refractive index portion 5 has a non-circular shape in cross section. In this embodiment, the low refractive index portion 5 has an elliptical shape in cross section. The ellipticity of the low refractive index portion 5 in cross section may be, for example, 0.97 or less, 0.8 or less, or 0.6 or less. The ellipticity of the low refractive index portion 5 is, for example, 0.5 or more. The ellipticity of the low refractive index portion 5 is expressed as the ratio rb / ra of the major axis ra, which is the length of the major axis D1 of the ellipse, to the minor axis rb, which is the length of the minor axis D2 of the ellipse. In FIG. 1 , the low refractive index portion 5 has a major axis D1 along the line segment m. That is, the non-circular shape of the low refractive index portion 5 is long in the direction along the line segment m.

[0032] The ellipticity of the low refractive index portion 5 can be obtained, for example, as follows: The MCF 1 is cut to obtain a cross section perpendicular to the central axis 1a. An electron microscope photograph of the cross section is taken. In the cross section photograph, the boundary surface between the low refractive index portion 5 and the first cladding 3 is approximated using the least squares method to obtain an ellipse. Instead of taking a cross section photograph of the MCF 1, a two-dimensional refractive index distribution of the MCF 1 may be obtained, and in the obtained refractive index distribution, the boundary surface between the low refractive index portion 5 and the first cladding 3 may be approximated using the least squares method to obtain an ellipse. Next, the major axis ra and minor axis rb of the obtained ellipse are obtained, and the ellipticity is calculated.

[0033] Fig. 2 is a cross-sectional view showing the relationship between the line segment connecting the central axes of the cores 2 and the major axis of the low refractive index portion. In the cross section, the major axis D1 of the low refractive index portion 5 may be parallel to the line segment m as shown in Fig. 1, or may intersect with the line segment m at an angle θ as shown in Fig. 2. In the cross section, the angle θ formed between the major axis D1 and the line segment m is, for example, within ±25 degrees.

[0034] (MCF Preform) The MCF preform according to the embodiment has a configuration corresponding to the MCF1. The MCF preform includes multiple core portions, a first cladding portion, a second cladding portion, and one or more low refractive index portions. The cross-sectional structure of the MCF preform is similar to that of the MCF1, and is therefore not shown. The composition of each portion of the MCF preform is substantially the same as that of the MCF1. Therefore, the MCF preform has a refractive index profile corresponding to the refractive index profile of the MCF1.

[0035] The multiple core portions are portions that become the multiple cores 2. The first cladding portion is a portion that becomes the first cladding 3. The first cladding portion surrounds the multiple core portions and has a refractive index that is 0.3% to 0.5% lower than the refractive index of the multiple core portions. The second cladding portion is a portion that becomes the second cladding 4. The second cladding portion surrounds the first cladding portion and has a refractive index that is 0.2% to 0.4% lower than the refractive index of the multiple core portions.

[0036] The low refractive index portion is a portion that becomes the low refractive index portion 5. In a cross section perpendicular to the central axis of the MCF base material, the low refractive index portion is arranged on a line segment connecting the central axes of two core portions that are arranged closest to each other among the multiple core portions. The low refractive index portion has a refractive index that is 0.1% to 1.0% lower than the refractive index of the first cladding 3. In a cross section perpendicular to the central axis of the MCF base material, the low refractive index portion 5 has a non-circular shape.

[0037] The ellipticity of the low refractive index portion of the MCF base material can be determined, for example, as follows: The MCF base material is cut to obtain a cross section perpendicular to the central axis. An electron microscope photograph of the cross section is taken. In the cross section photograph, the boundary surface between the low refractive index portion and the first cladding portion is approximated using the least squares method to obtain an ellipse. Instead of using a cross section photograph of the MCF base material, a two-dimensional refractive index distribution of the MCF base material may be obtained, and in the obtained refractive index distribution, the boundary surface between the low refractive index portion and the first cladding portion may be approximated using the least squares method to obtain an ellipse. Next, the major axis RA and minor axis RB of the obtained ellipse are determined, and the ellipticity RB / RA is calculated.

[0038] (Manufacturing Method of MCF) Fig. 3 is a flowchart showing a manufacturing method of MCF. The manufacturing method of MCF 1 includes steps S1 to S6. Steps S1 to S3 may be performed sequentially or in parallel. If performed sequentially, they may be performed in any order. Steps S4 to S6 are performed in this order after steps S1 to S3 have been performed.

[0039] Step S1 is a step of preparing a cladding tube that will become the first cladding portion of the optical fiber preform. Step S1 includes a step of manufacturing a glass rod to be used as the cladding tube and a step of drilling holes in the glass rod. Step S2 is a step of preparing multiple core rods that will become multiple core portions. Step S2 includes a step of manufacturing a glass rod to be used as the core portions and a step of drawing and grinding the glass rod. When the designs of the multiple cores 2 are different from each other, core rods are prepared according to the designs of the corresponding cores 2.

[0040] Step S3 is a step of preparing one or more low-refractive-index rods. Step S3 includes a step of manufacturing a glass rod to be used as the low-refractive-index rod and a step of elongating the glass rod. In the elongating step, the glass rod is elongated so that its cross section has a non-circular shape. This results in a low-refractive-index rod having a non-circular cross section. Here, the non-circular cross section of the low-refractive-index rod is an ellipse.

[0041] In step S4, the cladding tube is integrated with the core rods and one or more low-refractive-index rods inserted into the holes of the cladding tube by heating using a rod-in-tube method. The low-refractive-index rods are arranged in the cladding tube so that the angle between the major axis of the cross section and the line segment connecting the central axes of the two core rods is within ±25 degrees.

[0042] Step S5 is a step of forming a glass layer that will become a second cladding portion on the outside of a glass rod formed by integrating a cladding tube, multiple core rods, and one or multiple low refractive index rods, by an outside vapor deposition (OVD) method. Through the above steps, an MCF preform is manufactured. Step S6 is a step of drawing the MCF preform. Through the above steps, an MCF 1 is manufactured.

[0043] As described above, the MCF 1 has a low-refractive index portion 5 between two adjacent cores 2, thereby reducing inter-core XT. The low-refractive index portion 5 is formed, for example, from glass doped with a high concentration of fluorine. Such glass is expensive and may increase the manufacturing cost of the MCF 1. The optical power density is high on the line segment m connecting the central axes 2a of the two cores 2. The low-refractive index portion 5 has a non-circular cross section. By arranging the longitudinal direction of the low-refractive index portion 5 along the line segment m in the cross section, the amount of expensive glass used can be reduced, and the inter-core XT can be efficiently reduced. Therefore, the inter-core XT can be reduced without increasing the manufacturing cost. Since the low-refractive index portion 5 of this embodiment has an elliptical cross section, the longitudinal direction of the low-refractive index portion 5 is the direction of the major axis of the low-refractive index portion 5. If the low-refractive index portion 5 has a non-circular cross section other than an elliptical shape, the longitudinal direction of the low-refractive index portion 5 is the direction in which the low-refractive index portion 5 is longest. In this case, the length of the low refractive index portion 5 in the longitudinal direction is longer than the diameter of a circle having the same area as the low refractive index portion 5 .

[0044] Although the embodiments have been described above, the present disclosure is not necessarily limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present disclosure.

[0045] The following describes the present disclosure in more detail by showing the results of evaluation tests according to the present disclosure, but the present disclosure is not limited to these examples.

[0046] In order to evaluate the relationship between the ellipticity of the low refractive index portion and the inter-core XT, MCFs of Samples 1 to 6 were manufactured. In the MCFs of Samples 1 to 6, the cross section of the low refractive index portion was elliptical, and the major axis of the low refractive index portion was parallel to the line segment connecting the central axes of the cores. The core pitch was a constant value. The core pitch was 35 μm. The number of cores was 2, and the number of low refractive index portions was 1. The reason for reducing the number of cores and low refractive index portions in this way is to perform evaluation in a state where external disturbance is reduced. However, even if the number of cores is 3 or more and the number of low refractive index portions is 2 or more, the relationship between the ellipticity of the low refractive index portion and the inter-core XT is thought to be similar.

[0047] Table 1 summarizes the specifications of the MCFs of Samples 1 to 6. Table 1 lists the core diameter (μm) and the effective cross-sectional area Aeff (μm) at a wavelength of 1550 nm for each of the first and second cores. ２ ), cutoff wavelength λcc (nm), and transmission loss (dB / km) at a wavelength of 1550 nm are shown in Table 1. Inter-core XT (dB / 100 km) is also shown in Table 1. Table 1 also shows the major axis (μm), minor axis (μm), ellipticity, and cross-sectional area (μm ２ The cross-sectional area is a value calculated by π × major axis × minor axis. The ellipticity of the low refractive index portion of the optical fiber preform was also measured, and the difference from the ellipticity of the low refractive index portion of the optical fiber was about 0.01.

[0048]

[0049] As shown in Table 1, in Samples 1 to 6, the specifications of the two cores were the same, but the shapes of the low refractive index portions were different. Specifically, the minor axis of the low refractive index portion was gradually shortened from 13.4 μm in Sample 1 to 12.9 μm in Sample 2, 12.5 μm in Sample 3, 12.1 μm in Sample 4, 10.0 μm in Sample 5, and 7.2 μm in Sample 6. The major axis of the low refractive index portion was maintained at 13.5 μm in the MCFs of Samples 1 to 6. As the minor axis of the low refractive index portion became shorter, the ellipticity and cross-sectional area of ​​the low refractive index portion gradually decreased from Sample 1 to Sample 6.

[0050] FIG. 4 is a graph showing the relationship between the ellipticity of the low refractive index section and the inter-core XT. The horizontal axis of the graph shows the ellipticity of the low refractive index section, and the vertical axis shows the inter-core XT (dB / 100 km). It can be seen from the graph that the inter-core XT tends to worsen as the ellipticity of the low refractive index section decreases. The amount of change in the inter-core XT is small, and the low refractive index section has the effect of reducing the inter-core XT in all of the MCFs from Samples 1 to 6. The inter-core XT when no low refractive index section is provided is approximately -30 dB / km. From these results, it was confirmed that, according to the above embodiment, it is possible to manufacture an MCF capable of reducing the inter-core XT without increasing manufacturing costs.

[0051] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above meaning, and is intended to include all modifications within the meaning and scope of the claims.

[0052] 1...MCF 1a...Central axis 2...Core 2a...Central axis 3...First cladding 4...Second cladding 5...Low refractive index portion 5a...Central axis D1...Long axis D2...Short axis m...Line segment θ...Angle S1...Clad tube preparation process S2...Core rod preparation process S3...Low refractive index rod preparation process S4...Integration process S5...OVD process S6...Drawing process

Claims

1. A multi-core optical fiber containing silica-based glass, comprising: a plurality of cores; a cladding surrounding the plurality of cores and having a refractive index lower than that of the plurality of cores; and a low-refractive-index portion having a refractive index lower than that of the cladding, the low-refractive-index portion being arranged on a line segment connecting the central axes of two of the plurality of cores that are arranged closest to each other in a cross section perpendicular to the central axis of the multi-core optical fiber, wherein the low-refractive-index portion has a non-circular shape in the cross section.

2. The multi-core optical fiber according to claim 1, wherein the low refractive index portion has an elliptical shape in the cross section.

3. The multi-core optical fiber according to claim 2, wherein in the cross section, an angle formed between the major axis of the low refractive index portion and the line segment is within ±25 degrees.

4. The multi-core optical fiber according to claim 2 or 3, wherein the ellipticity of the low refractive index portion in the cross section is 0.5 or more and 0.97 or less.

5. The multi-core optical fiber according to claim 2 or 3, wherein the ellipticity of the low refractive index portion in the cross section is 0.5 or more and 0.8 or less.

6. The multi-core optical fiber according to claim 2 or 3, wherein the ellipticity of the low refractive index portion in the cross section is 0.5 or more and 0.6 or less.

7. The multi-core optical fiber according to any one of claims 1 to 6, wherein the low refractive index portion contains fluorine, and the fluorine concentration in the low refractive index portion is higher than the fluorine concentration in the cladding.

8. The multi-core optical fiber according to any one of claims 1 to 7, wherein the plurality of cores contain an alkali metal element or an alkaline earth metal element.

9. The multi-core optical fiber according to any one of claims 1 to 8, wherein the plurality of cores contain germanium.

10. The multi-core optical fiber according to any one of claims 1 to 9, wherein the plurality of cores contain chlorine.

11. A multi-core optical fiber preform containing silica-based glass, comprising: a plurality of core portions; a cladding portion surrounding the plurality of core portions and having a refractive index lower than that of the plurality of core portions; and a low-refractive-index portion, which is located on a line segment connecting the central axes of two of the plurality of core portions that are located closest to each other in a cross section perpendicular to the central axis of the multi-core optical fiber preform, and has a refractive index lower than that of the cladding portion, wherein the low-refractive-index portion has a non-circular shape in the cross section.

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