Multicore optical fiber

Abstract

A multicore optical fiber (FB) comprises at least two core regions (10), a cladding region (20) that has a lower refractive index than the refractive index of the core regions (10) and surrounds all the core regions, and one or more coating regions (31, 32) that surround the cladding region (20), the refractive index of at least one coating region (31) being lower than the refractive index of the core regions (10) and higher than the refractive index of the cladding region (20).

Description

Multi-core optical fiber

[0001] The present disclosure relates to multi-core optical fibers.

[0002] Optical fibers, which support optical communication systems, are composed of a core through which light is guided, a cladding with a lower refractive index than the core, and a polymer resin with a higher refractive index than the cladding, arranged from the center outward. Each lightwave mode has its own propagation constant depending on the refractive index distribution, and the propagation constant divided by the wavenumber is called the effective refractive index. Lightwave modes with an effective refractive index smaller than the cladding refractive index leak out of the core.

[0003] As a refractive index profile form of an optical fiber that can strengthen the light confinement of a lightwave mode in the core, Non-Patent Document 1 discloses a trench-assisted (TA) refractive index profile having a low-refractive index region with a lower refractive index than the cladding. Non-Patent Document 2 discloses a W-type refractive index profile. With the TA-type and W-type refractive index profiles, the refractive index of the cladding is made larger than the effective refractive index of the unwanted lightwave mode, thereby removing the unwanted lightwave mode from the core. Furthermore, it is possible to suppress bending loss and expand the mode field diameter.

[0004] Non-Patent Document 3 discloses a configuration in which a low refractive index resin having a lower refractive index than the cladding is used to confine light in the cladding region of an optical fiber used in erbium-doped optical fiber amplification. Non-Patent Document 4 discloses, for example, a polymer resin having a refractive index (1.457 or 1.458) that is approximately 0.3% or 0.37% lower than the refractive index of pure silica glass at a wavelength of 852 nm.

[0005] Non-Patent Document 5 discloses a multi-core optical fiber having a three-mode four-core fiber structure in which four three-mode waveguide cores are arranged in a cladding region with a standard cladding diameter, from the viewpoint of the applicability of existing communication facility technology toward future increases in capacity of optical communication systems.

[0006] K. Himeno, S. Matsuo, N. Guan and A. Wada, “Low-Bending-Loss Single-Mode Fibers for Fiber-to-the-Home,” Journal of Lightwave technology, vol. 23, no. 11, pp. 3494-3499, Nov. 2005.T. Kato, M. Hirano, M. Onishi and M. Nishimura, "Ultra-low nonlinearity low-loss pure silica core fiber for long-haul WDM transmission," Electron. Lett., vol. 35, no. 19, pp. 1615-1617 (1999).S. Takasaka et al., "Absorption Ratio Dependence of Gain Characteristics on Cladding Pumped Multicore EDFAs," ECOC2019, P77, (2019).https: / / www.fospia.com / spc-seriesSagae, Yuto, et al. "Weakly Coupled Homogeneous 3-Mode 4-Core Fiber With Standard Cladding Diameter." Journal of Lightwave Technology, vol.41, no. 12, pp. 3950-3956 (2023).

[0007] However, according to the multi-core optical fiber disclosed in Non-Patent Document 5, the high-order light wave mode LP １１ The inter-core crosstalk is relatively high, limiting the transmission distance. This is because the inter-core distance cannot be sufficiently widened in the cladding region, which is limited in diameter to 125 μm. Therefore, there is a problem that it is difficult to construct an efficient long-distance transmission line.

[0008] The present disclosure has been made in view of the above-mentioned problems, and an object of the present disclosure is to provide a multi-core optical fiber that is less likely to be limited in transmission distance and that can realize the construction of an efficient long-distance transmission line.

[0009] In order to solve the above-mentioned problems, the multi-core optical fiber according to the present disclosure comprises at least two or more core regions, a cladding region having a refractive index lower than that of the core regions and surrounding all of the core regions, and one or more coating regions surrounding the cladding regions, wherein the refractive index of at least one coating region is lower than that of the core regions and higher than that of the cladding region.

[0010] According to the present disclosure, it is possible to provide a multi-core optical fiber that is less likely to limit the transmission distance and that can realize the construction of an efficient long-distance transmission line.

[0011] Fig. 1 is a cross-sectional view showing an example of the structure of a multi-core optical fiber according to the present disclosure; Fig. 2 is a diagram showing an example of a refractive index profile in a multi-core optical fiber according to the present disclosure; Fig. 3 is a characteristic diagram relating to leakage loss and cutoff wavelength; Fig. 4 is a diagram showing an example of the relationship between the radius of a core region and the inter-core distance; Fig. 5 is a cross-sectional view showing a modified example of the structure of a multi-core optical fiber according to the present disclosure; Fig. 6 is a diagram showing an example of a refractive index profile having a common trench structure;

[0012] Next, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description, the same components are designated by the same reference numerals and redundant description will be omitted.

[0013] [1. Configuration of the multi-core optical fiber] Fig. 1 is a cross-sectional view showing an example of the structure of a multi-core optical fiber according to the present disclosure. Fig. 2 is a diagram showing an example of a refractive index profile in the multi-core optical fiber according to the present disclosure. Hereinafter, the multi-core optical fiber will be referred to as an optical fiber FB.

[0014] Fig. 1 shows a cross-sectional view of an optical fiber FB at a cross section perpendicular to the extending direction (direction of the central axis) thereof. The optical fiber FB includes a core region 10, a cladding region 20, and coating regions 31 and 32. Fig. 2 shows the refractive index profile in a cross section passing through the center of the optical fiber FB and the center of the core region 10.

[0015] For the following description, the radius a of the core region 10, the distance Λ between the core regions 10, the distance t between the end of the core region 10 and the cladding region 20, the thickness w of the coating regions 31 and 32, and the diameter D of the cladding region 20 are used.

[0016] The optical fiber FB includes at least two or more core regions 10. The optical fiber FB also includes one or more coating regions. In Fig. 1, the optical fiber FB is shown to include four core regions 10 and two coating regions 31 and 32, but is not limited to this example.

[0017] The core region 10 propagates multiple lightwave modes (LP modes). In the example shown in Figures 1 and 2, the core region 10 has a step-index (SI) structure. That is, the core region 10 is composed of at least two or more different regions having different refractive indices and arranged concentrically around the center of the core region 10.

[0018] The cladding region 20 has a refractive index lower than that of the core regions 10, and is arranged to surround all of the core regions 10 included in the optical fiber FB. The diameter D of the cladding region 20 may be 124 μm or more and 126 μm or less.

[0019] Light rays propagate through the optical fiber FB by repeated total reflection in the core region 10. Here, the inclination of the light rays is not allowed to be any arbitrary value; only light rays with a specific angle can propagate. Such a form of light rays is called a light wave mode. The light wave modes that can propagate through the core region 10 can be determined, for example, by electromagnetic field analysis.

[0020] For example, a linearly polarized mode exists in the light wave mode. ｍｌ Here, "m" is the mode order indicating how the intensity distribution of the transverse electric field of a light ray propagating through the core region 10 changes in the angular direction, and "l" is the mode order indicating how the intensity distribution of the transverse electric field of a light ray propagating through the core region 10 changes in the radial direction.

[0021] The coating regions 31 and 32 are arranged to surround the cladding region 20. For example, as shown in FIG. 1 , the coating region 31 is arranged concentrically to surround the cladding region 20, and the coating region 32 is arranged concentrically to surround the coating region 31.

[0022] Here, the refractive index of at least one of the coating regions included in the optical fiber FB is set to be lower than the refractive index of the core region 10 and higher than the refractive index of the cladding region 20. Fig. 2 shows that the refractive index of the coating region 31 is set to be lower than the refractive index of the core region 10 and higher than the refractive index of the cladding region 20.

[0023] The refractive index n of the core region 10 and the refractive index n of the cladding region 20 ０ , the refractive index n of the coating region 31 １ , the refractive index n of the coating region 32 ２ The relative refractive index difference of the core region 10 and the coating regions 31 and 32 with respect to the cladding region 20 is defined using the following formula:

[0024] The relative refractive index difference Δ of the core region 10 with respect to the cladding region 20 is "Δ=(n ２ -n ０ ２ ) / 2n ２ ". The relative refractive index difference Δ of the coating region 31 with respect to the cladding region 20 is defined as follows: １ is "Δ １ = (n １ ２ -n ０ ２ ) / 2n １ ２ The relative refractive index difference Δ of the coating region 32 with respect to the cladding region 20 is defined as ２ is "Δ ２ = (n ２ ２ -n ０ ２ ) / 2n ２ ２ " is defined as:

[0025] The refractive index of at least one of the coating regions is set lower than the refractive index of the core region 10 and higher than the refractive index of the cladding region 20, so that unwanted light wave modes leaking from the core region 10 are removed by the coating region 31.

[0026] In addition, "Δ １ <Δ<Δ ２ " is satisfied, a common depressed structure can be realized by utilizing the coating regions 31 and 32. Although FIG. 2 shows a case where the refractive index profile in the core region is of a step-index (SI) type, the present invention is not limited to this example. For example, the refractive index profile in the core region may be of a graded-index (GI) type or a multi-step type.

[0027] 3 is a characteristic diagram of leakage loss and cutoff wavelength. The hatched area in FIG. 3 represents the optical wave mode LP. ０１ When the mode field diameter (MFD) of ０１ = 10 μm), the light wave mode LP ２１ and the lightwave mode LP ０２ The cutoff wavelength is 1530 nm or less, and the light wave mode is LP. １１ 1 shows a design region where the leakage loss is 0.01 dB / km or less at a wavelength of 1625 nm.

[0028] Here, "Δ １ = 0.3%." When the radius a of the core region 10 and the distance t between the end of the core region 10 and the end of the cladding region 20 are within the range included in the shaded area shown in Fig. 3, it is possible to design a multi-core optical fiber that is less likely to be limited in transmission distance and that can realize the construction of an efficient long-distance transmission line.

[0029] The solid line portion of the boundary of the design domain shown in FIG. 3 is expressed as follows: "t = 152.36 × 10 ３ -726.86 x 10 ２ × a + 115.64 × 10 ２ ×a ２ -613.49 x a ３ "

[0030] The dotted line portion of the boundary of the design domain shown in FIG. 3 is expressed as follows: "t = 240.42 × 10 ３ -116.41 x 10 ３ × a + 187.92 × 10 ２ ×a ２ -1011.30 x a ３ "

[0031] Therefore, a multi-core optical fiber can be designed by satisfying the following relationship, where the unit of the radius a of the core region 10 is "μm" and the unit of the distance t between the end of the core region 10 and the end of the cladding region 20 is "μm": ３ -726.86 x 10 ２ × a + 115.64 × 10 ２ ×a ２ -613.49 x a ３ ≦t≦240.42×10 ３ -116.41 x 10 ３ × a + 187.92 × 10 ２ ×a ２ -1011.30 x a ３ "

[0032] 4 is a diagram showing an example of the relationship between the radius of the core region and the inter-core distance, which shows a characteristic diagram of the radius a of the core region 10 and the inter-core distance Λ in a three-mode four-core fiber having a standard cladding diameter (125 μm).

[0033] In particular, the lightwave mode LP １１ The solid line shows the relationship between the distance t at which the leakage loss of the fiber becomes 0.01 dB / km at a wavelength of 1625 nm and the distance Λ in the structure of four core regions 10 arranged in a square. On the other hand, the dashed line shown in Fig. 4 shows the inter-core distance of a three-mode four-core fiber (commonly depressed 3M-4CF) that has been reported so far.

[0034] 4, it can be seen that the solid line is located on the side where the distance Λ is greater than that of the dashed line. Therefore, when arranging the core regions 10, the distance Λ between the cores can be increased. As a result, it is possible to reduce crosstalk between the core regions 10. Furthermore, it becomes possible to apply the optical fiber FB to long-distance transmission lines.

[0035] 5 is a cross-sectional view showing a modified example of the structure of the multi-core optical fiber according to the present disclosure. In relation to the reduction in crosstalk obtained by providing the coating region, the number of core regions 10 included in the optical fiber FB can be increased.

[0036] 5, for example, the radius of the core region 10 is set to 6.3 μm, and the distance between the end of the core region 10 and the end of the cladding region 20 is set to 13 μm. In this case, even if seven core regions 10 are arranged, the distance between the core regions 10 can be secured to be about 50 μm.

[0037] In comparison with the previously reported three-mode four-core fiber with an inter-core distance of about 41 μm, the optical fiber FB of the present disclosure makes it possible to increase the inter-core distance, thereby achieving a sufficient reduction in crosstalk, and furthermore, an improvement in spatial multiplexing can be expected due to the increased core region 10.

[0038] Fig. 6 is a diagram showing an example of a refractive index profile having a common trench structure. Unlike the refractive index profile shown in Fig. 2, in the refractive index profile shown in Fig. 6, the refractive index of the coating region 31 is set to be smaller than the refractive index of the cladding region 20. In this way, a common trench structure may be realized by utilizing the coating region.

[0039] 6 shows a case where the refractive index profile in the core region is of a step-index (SI) type, but is not limited to this example. For example, the refractive index profile in the core region may be of a graded-index (GI) type or a multi-step type.

[0040] [Effects of the embodiment] As described above in detail, the multi-core optical fiber according to the present disclosure includes at least two or more core regions, a cladding region having a refractive index lower than that of the core regions and surrounding all of the core regions, and one or more coating regions surrounding the cladding regions, and the refractive index of at least one coating region is lower than that of the core regions and higher than that of the cladding region.

[0041] This makes it possible to provide a multi-core optical fiber that is less likely to limit the transmission distance and enables the construction of an efficient long-distance transmission line. In particular, it is possible to reduce crosstalk between core regions and further remove unwanted lightwave modes leaking from the core regions from the cores. Furthermore, it is possible to suppress bending loss and increase the mode field diameter.

[0042] In the multi-core optical fiber according to the present disclosure, among the light wave modes propagating through the core region, the light wave mode LP ０２ and lightwave mode LP ２１ The cutoff wavelength of the core region may be 1530 nm, which makes it possible to reliably remove from the core any unwanted optical wave modes that have leaked from the core region.

[0043] Furthermore, in the multi-core optical fiber according to the present disclosure, among the light wave modes propagating through the core region, the light wave mode LP １１ The leakage loss at a wavelength of 1625 nm may be 0.01 dB / km or less. This makes it possible to provide a multi-core optical fiber that is less likely to be limited in transmission distance and that can realize the construction of an efficient long-distance transmission line.

[0044] In the multi-core optical fiber according to the present disclosure, among the light wave modes propagating through the core region, the light wave mode LP ０２ and lightwave mode LP ２１ The cutoff wavelength of the core region is 1530 nm, the radius of the core region is a, and the distance between the end of the core region and the cladding region is t. The units of the radius a and the distance t are μm, and the formula is "152.36×10 ３ -726.86 x 10 ２ × a + 115.64 × 10 ２ ×a ２-613.49 x a ３ ≦t≦240.42×10 ３ -116.41 x 10 ３ × a + 187.92 × 10 ２ ×a ２ -1011.30 x a ３ " may also satisfy the above.

[0045] This makes it possible to design a multi-core optical fiber that is less likely to be limited in transmission distance and that can realize the construction of an efficient long-distance transmission line.

[0046] Furthermore, in the multi-core optical fiber according to the present disclosure, the refractive index profile in the core region may be at least one of a step-index (SI) type, a graded-index (GI) type, and a multi-step type, thereby realizing the effect of reducing crosstalk obtained by providing a coating region in an optical fiber having core regions with various refractive index profiles.

[0047] Although the contents of the present disclosure have been described above based on the embodiments, the present disclosure is not limited to these descriptions, and various modifications and improvements are possible, which will be apparent to those skilled in the art. The descriptions and drawings that form part of this disclosure should not be understood as limiting the present disclosure. Various alternative embodiments, examples, and operating techniques will be apparent to those skilled in the art from this disclosure.

[0048] Of course, the present disclosure includes various embodiments not described herein. Therefore, the technical scope of the present disclosure is defined only by the invention-specifying matters according to the scope of the claims that are appropriate from the above description.

[0049] 10 Core region 20 Cladding region 31 Coating region 32 Coating region FB Optical fiber

Claims

1. A multi-core optical fiber comprising: at least two or more core regions; a cladding region having a refractive index lower than that of the core regions and surrounding all of the core regions; and one or more coating regions surrounding the cladding regions, wherein the refractive index of at least one of the coating regions is lower than that of the core regions and higher than that of the cladding region.

2. Among the light wave modes propagating through the core region, the light wave mode LP ０２ and lightwave mode LP ２１ The multi-core optical fiber according to claim 1, wherein the cutoff wavelength is 1530 nm.

3. Among the light wave modes propagating through the core region, the light wave mode LP １１ 2. The multi-core optical fiber according to claim 1, wherein the leakage loss at a wavelength of 1625 nm is 0.01 dB / km or less.

4. Among the light wave modes propagating through the core region, the light wave mode LP ０２ and lightwave mode LP ２１ the cutoff wavelength of the core region is 1530 nm, the radius of the core region is a, and the distance between the end of the core region and the cladding region is t, and the units of the radius a and the distance t are μm, and ３ -726.86 x 10 ２ × a + 115.64 × 10 ２ ×a ２ -613.49 x a ３ ≦t≦240.42×10 ３ -116.41 x 10 ３ × a + 187.92 × 10 ２ ×a ２ -1011.30 x a ３ The multi-core optical fiber according to claim 1 , which satisfies the following:

5. A multi-core optical fiber according to any one of claims 1 to 4, wherein the refractive index profile in the core region is at least one of a step-index (SI) type, a graded-index (GI) type, and a multi-step type.

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