Multi-core optical fiber and optical transmission system

Abstract

To make it possible to design a multi-core optical fiber having a standard cladding diameter taken in consideration of manufacturability.SOLUTION: The present disclosure provides a multi-core optical fiber comprising: two or more cores through which two or more lightwave modes propagate; a cladding a refractive index of which is lower than that of the cores and which is disposed so as to include all the cores; and an outermost layer a refractive index of which is equal to or more than that of the cladding and less than that of the cores and which is disposed on a concentric axis so as to surround the cladding. A crosstalk between the cores in the highest order propagation mode propagating through the cores is -39 dB / km or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a multi-core optical fiber that propagates light of multiple lightwave modes in each core. [Background technology]

[0002] Space division multiplexing technology is being actively researched to realize future high-capacity optical networks. Space division multiplexing technology is expected to dramatically improve the transmission capacity per optical fiber compared to conventional optical fibers by incorporating multiple spatial channels (cores, lightwave modes) into a single optical fiber. For multi-core optical fibers with multiple cores, a structure in which homogeneous cores capable of single-mode propagation are arranged in a cladding (standard cladding) with a diameter of 125 μm, equivalent to that of standard optical fibers, has attracted attention from the perspectives of manufacturability and compatibility with existing facilities (see, for example, Non-Patent Document 1). To further increase transmission capacity, a few-mode multi-core optical fiber has also been realized, in which homogeneous cores capable of propagating three lightwave modes are arranged in a cladding with a diameter of 125 μm (see, for example, Non-Patent Document 2).

[0003] In a multi-core optical fiber transmission system, to obtain good transmission characteristics, it is necessary to suppress inter-core crosstalk (hereinafter sometimes referred to as XT), which is signal interference between cores, and XT increases as the core spacing decreases. Long-distance transmission using a multi-core optical fiber requires an XT of -39 dB / km or less, and Non-Patent Document 2 achieves an XT of -50 dB / km or less. In a multi-core optical fiber that allows propagation of several modes, it is necessary to reduce the core spacing to suppress leakage of higher-order modes, making it difficult to sufficiently suppress XT. Non-Patent Document 2 employs air holes between cores to mitigate the trade-off between the leakage of higher-order modes and XT, but this increases manufacturing costs. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] T. Matsui et al., “Design of 125 μm cladding multi-core fiber with full-band compatibility to conventional single-mode fiber,” 2015 European Conference on Optical Communication (ECOC), Valencia, Spain, 2015, pp. 1-3, doi: 10.1109 / ECOC.2015.7341966. [Non-patent document 2] S. Nozoe et al., “125 μm-cladding 2LP-mode and 4-core Multi-core Fiber with Air-hole Structure for Low Crosstalk in C+L Band,” 2017 European Conference on Optical Communication (ECOC), Gothenburg, 2017, pp. 1-3, doi: 10.1109 / ECOC.2017.8346167. Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present disclosure is to enable the design of a multi-core optical fiber with a standard cladding diameter that takes manufacturability into consideration. [Means for solving the problem]

[0006] The multi-core optical fiber of the present disclosure comprises: having two or more cores through which two or more lightwave modes propagate; a cladding having a refractive index lower than that of the cores and arranged to encompass all of the cores; an outermost layer having a refractive index equal to or greater than that of the cladding and less than that of the core, the outermost layer being arranged concentrically around the cladding; The crosstalk between the cores in the highest order propagation mode propagated in the cores is −39 dB / km or less.

[0007] The optical transmission system of the present disclosure includes: Four or more transmitters; two or more mode combiners that convert the signal light generated by the transmitter into two or more lightwave modes; an optical coupling unit that couples the light output from the multiplexer to each core of the multi-core optical fiber of the present disclosure; a light extraction unit that extracts signal light from each core of the multi-core optical fiber; two or more mode separators that separate the signal light output from the light extraction unit into two or more lightwave modes; four or more receivers that receive signal light emitted from the mode separator; The present invention is characterized by having the following. [Effects of the Invention]

[0008] According to the present disclosure, it is possible to achieve an effect of realizing a multi-core optical fiber with a standard cladding diameter with high manufacturability. [Brief explanation of the drawings]

[0009] [Figure 1] 1 shows an example of a cross section of a multi-core optical fiber according to the present disclosure. [Figure 2] An example of a and Λ when Δ2=0% is shown below. [Figure 3] 1 shows an example of the refractive index of the outermost layer that realizes a three-mode multi-core optical fiber according to the present disclosure. [Figure 4] 2 shows an example of the cladding thickness of a three-mode multi-core optical fiber according to the present disclosure. [Figure 5] 2 shows an example of the inter-core distance of a three-mode multi-core optical fiber according to the present disclosure. [Figure 6] 1 shows an example of a standard outer diameter three-mode four-core fiber according to the present disclosure. [Figure 7] 1 shows an example of an optical transmission system using the optical fiber of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings indicate the same components.

[0011] Fig. 1 shows a cross-sectional view of a multi-core optical fiber according to an embodiment of the present disclosure. The multi-core optical fiber of this embodiment includes two or more cores 11, a cladding 12 that surrounds all of the cores 11 in the circumferential direction, and an outermost layer 13 that surrounds the cladding 12. The cross-sectional shapes of the cladding 12 and the outermost layer 13 are circular, and they are arranged on a concentric axis. The number of cores 11 arranged in the cladding 12 is arbitrary, but here, a configuration in which four cores are arranged is shown as an example.

[0012] This disclosure realizes a few-mode multi-core optical fiber with a standard cladding diameter that has the same characteristics as Non-Patent Document 2 without using holes, and differs from Non-Patent Document 2 in the following three points. (i) In order to alleviate the trade-off between the leakage of higher-order modes and the increase in XT, a core 11 with a high light confinement effect is used. (ii) Unwanted higher-order modes propagating due to excessive confinement effects are cut off by providing the outermost layer 13 with a refractive index higher than that of the cladding 12 . (iii) Geometrical constraints are alleviated by making the outermost layer 13 common rather than providing it for each core. A detailed description will be given below of a case where the core 11 guides the LP01 mode and the LP11 mode, which are light wave modes, and the unnecessary higher-order mode is the LP21 mode.

[0013] 1, in the multi-core optical fiber of this embodiment, four cores 11 that propagate two or more light wave modes are arranged in a cladding 12, and an outermost layer 13 is arranged concentrically around the cladding 12. In this case, the refractive index of the cladding 12 is lower than that of the cores 11, and the refractive index of the outermost layer 13 is set to be equal to or higher than that of the cladding 12 and lower than that of the cores 11.

[0014] Here, the core radius is a, the relative refractive index difference of the core 11 with respect to the cladding 12 is Δ1, the relative refractive index difference of the outermost layer 13 with respect to the cladding 12 is Δ2, the inter-core distance is Λ, the distance from the center of the core 11 to the inner diameter of the outermost layer 13 is s, and the distance from the center of the core 11 to the outer diameter of the outermost layer 13 (cladding thickness) is t. Note that while Figure 1 shows a case where there are four cores arranged in a square lattice, any number of cores greater than two and an annular arrangement or a hexagonal close-packed arrangement may also be used.

[0015] 2 shows the a and Λ dependence of the optical characteristics of an example of a multi-core optical fiber according to the present disclosure, in which four cores are arranged to guide the LP01 mode and the LP11 mode, which are optical wave modes, when Δ2=0% and the diameter of the outermost layer 13 is 125 μm, which is equivalent to that of a standard cladding. Here, Δ1 is used to set the mode field diameter (hereinafter sometimes abbreviated as MFD) of the LP01 mode at a wavelength of 1.55 μm to 9.5 μm, which is equivalent to that of a multi-core optical fiber that propagates a standard single mode.

[0016] In the figure, the straight line L21 indicates the boundary of the structure that cuts off the LP21 mode at a wavelength of 1.53 μm, the dashed line L22 indicates the boundary of the structure where the XT between the LP11 modes is -39 dB / km or less, and the dash-dot line L23 indicates the boundary of the structure where the leakage loss of the LP11 mode is 0.01 dB / km. By using the a and Λ dependencies that satisfy the gray regions enclosed by each boundary, it is possible to realize a multi-core optical fiber that propagates three lightwave modes with sufficiently low XT and low loss at a wavelength of 1.55 μm when the diameter of the outermost layer 13 is 125 μm.

[0017] Each boundary line can be expressed by the following equation: L21:Λ=-1767+324.2a L22:Λ=956.5-307.9a+25.7a 2 L23:Λ=-1336.2+467.9a-39.5a 2

[0018] From the above, the gray area is Δ When 2=0% a≦(Λ+1767) / 324.2 and 956.5-307.9a+25.7a 2 ≦Λ≦-1336.2+467.9a-39.5a 2 It can be expressed as:

[0019] 2 shows an example where Δ2=0%, but the present disclosure is not limited to this. For example, by setting Δ2>0%, the cutoff wavelength condition in FIG. 2 is relaxed, and the design region is expanded to the region of a≧5.6 μm.

[0020] Figure 3 shows the core radius dependence of Δ2 of the multi-core optical fiber according to this embodiment. Here, a multi-core optical fiber guiding the lightwave modes LP01 mode and LP11 mode is shown as an example, and the MFD at a wavelength of 1.55 μm of the LP01 mode is set to 9.5 μm by Δ1 at each core radius. Curve L3 in the figure shows Δ2 that enables blocking of the LP21 mode at a wavelength of 1.53 μm. In the gray region where Δ2 is larger than that of curve L3, a three-mode multi-core optical fiber is realized in the region of a≧5.6 μm. This boundary line can be expressed by the following equation. Δ2=0.4 4( a-5.6) 0.63

[0021] Figure 4 shows the cladding thickness t of the multi-core optical fiber according to this embodiment. Here, a multi-core optical fiber enabling three-mode propagation is shown as an example, and the MFD of the LP01 mode at a wavelength of 1.55 μm is set to 9.5 μm and the cutoff wavelength of the LP21 mode is set to 1.53 μm by Δ1 and Δ2. The solid line in the figure indicates the boundary L4 of the structure where the leakage loss of the LP11 mode is 0.01 dB / km or less at a wavelength of 1.625 μm, and low loss is achieved in the region where t is larger than the boundary L4. This boundary L4 can be expressed by the following equation: t=809.7-268.5a+23.1a 2

[0022] Figure 5 shows the inter-core distance of the multi-core optical fiber according to this embodiment. Here, as an example, a multi-core optical fiber that allows propagation of three lightwave modes is shown, and the MFD at a wavelength of 1.55 μm of the LP01 mode is set to 9.5 μm and the cutoff wavelength of the LP21 mode is set to 1.53 μm by Δ1 and Δ2. The solid line in the figure indicates the boundary L5 of the structure where XT is -39 dB / km or less at a wavelength of 1.625 μm, and low XT is achieved in the gray region where Λ is larger than the boundary L5. This boundary L5 can be expressed by the following equation: Λ=35.6×10 -4 (a-5.4) -4.2 +35.3

[0023] From the above, when a≧5.6 μm, the outermost layer 13 Δ2≧0.44+(a-5.6) 0.63 and t≧809.7-268.5a+23.1a 2 and the core distance is Λ≧35.6×10 -4 (a-5.4) -4.2 +35.3 In this case, a few-mode multi-core optical fiber that has both sufficiently low XT and low loss can be realized.

[0024] Figure 6 shows Λ and a of a multi-core optical fiber according to the present disclosure, which has a standard cladding with a diameter of 125 μm and includes four cores capable of propagating the optical wave modes LP01 mode and LP11 mode. Having the same diameter of the cladding 12 as that of a standard optical fiber is preferable because it minimizes degradation in manufacturability and provides good compatibility with existing equipment. Here, the MFD at a wavelength of 1.55 μm for the LP01 mode is set to 9.5 μm and the cutoff wavelength of the LP21 mode is set to 1.53 μm using Δ1 and Δ2.

[0025] The solid line L61 shows a structure where the leakage loss of the LP11 mode is 0.01 dB / km at a wavelength of 1.625 μm, and sufficient low loss can be achieved in the region where Λ is smaller than the solid line L61. The dashed line L62 shows a structure where the LP11 inter-core XT is -39 dB / km at a wavelength of 1.625 μm, and sufficient low XT can be achieved in the region where Λ is larger than the dashed line L62. The solid line L61 and dashed line L62 can be expressed by the following equations. L61:Λ=-1248.5+446.8a-38.6a 2 L62:Λ=35.6×10 -4 (a-5.4) -4.2 +35.3

[0026] From the above, when a≧5.6μm 35.6×10 -4 (a-5.4) -4.2 +35.3≦Λ ≦-1248.5+446.8a-38.6a 2 In the gray area in Figure 6, shown by , a 3-mode 4-core fiber with sufficiently low loss and low XT can be realized with a standard cladding diameter.

[0027] 7 shows a configuration diagram of an optical transmission system using a multi-core optical fiber according to the present disclosure. Signal light generated by N transmitters 91 is input to M mode multiplexers 92, where the signal light is multiplexed into two or more lightwave modes. Here, N≧4 and M≧2. Each lightwave mode into which the signal light is multiplexed is coupled to each core of a multi-core optical fiber 94 according to the present disclosure in an optical coupling unit 93. The signal light propagating through each core is extracted in an optical extraction unit 95, and each mode is demultiplexed into each signal light by M mode separators 96, and the signal light is received by N receivers 97. [Industrial Applicability]

[0028] The present disclosure can be applied to the information and communications industry. [Explanation of symbols]

[0029] 11: Core 12: Clad 13: Outermost layer 91:Transmitter 92: Mode multiplexer 93: Optical coupling part 94: Multi-core optical fiber 95:Light extraction part 96: Mode separator 97: Receiver

Claims

1. having two or more cores through which two or more lightwave modes propagate; a clad having a refractive index lower than that of the core and disposed so as to encompass all of the core; an outermost layer having a refractive index greater than that of the cladding and less than that of the core, the outermost layer being concentrically arranged to surround the cladding; The diameter of the outermost layer is 125 μm, a core radius a of the core, a distance s from the center of the core to the inner diameter of the outermost layer, and a relative refractive index difference Δ 2 of the outermost layer with respect to the cladding satisfy the following formula: Δ 2 ≧0.44 (a-5.6) 0.63 s / a=3.0 a>5.6

2. 2. The multi-core optical fiber according to claim 1, wherein a distance t from the center of the core to the outer diameter of the outermost layer satisfies the following formula: t≧809.7-268.5a+23.1a 2

3. The inter-core distance Λ of the cores is Λ≧35.6×10 －４ (a-5.4) －４．２ +35.3 The multi-core optical fiber according to claim 1,

4. Four of the cores are arranged in the clad, The mode field diameter of the LP01 mode of the core when the wavelength of light is 1.55 μm is 9.5 μm or more, The leakage loss of the LP11 mode of the core when the wavelength of light is 1.625 μm is 0.01 dB / km or less. The multi-core optical fiber according to claim 1 .

5. Four of the cores are arranged in the clad, The inter-core distance Λ and the core radius a of the cores are 35.6×10 －４ (a-5.4) －４．２ +35.3≦Λ ≦-1248.5+446.8a-38.6a ２ The multi-core optical fiber according to claim 1 , which satisfies the following:

6. four or more transmitters; two or more mode combiners that convert the signal light generated by the transmitter into two or more lightwave modes; an optical coupling unit that couples the light emitted from the mode multiplexer to each core of the multi-core optical fiber according to any one of claims 1 to 5; a light extraction unit that extracts signal light from each core of the multi-core optical fiber; two or more mode separators that separate the signal light output from the light extraction unit into two or more lightwave modes; four or more receivers that receive the signal light emitted from the mode separator; characterized in that it has Optical transmission system.

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