JP2023036400A - Multi-core optical fiber and optical transmission system - Google Patents

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

The present disclosure relates to multicore optical fibers that propagate light in multiple lightwave modes in each core.

Spatial division multiplexing technology has been extensively researched toward the realization of future large-capacity optical networks. Space division multiplexing technology is expected to dramatically improve the transmission capacity per optical fiber compared to conventional optical fibers by accommodating multiple spatial channels (core, light wave mode) in one optical fiber. there is For multi-core optical fibers with multiple cores, from the viewpoint of manufacturability and appraisal of compatibility with existing facilities, a homogenous core capable of propagating a single mode is placed in a cladding with a diameter of 125 μm (standard cladding), which is the same as a standard optical fiber. A structure that provides A few-mode multi-core optical fiber has been realized in which homogenous cores capable of propagating three light wave modes are arranged in a cladding with a diameter of 125 μm to further increase the transmission capacity. (For example, see Non-Patent Document 2.)

In order to obtain good transmission characteristics in a multi-core optical fiber transmission system, it is necessary to suppress core-to-core crosstalk (hereinafter sometimes referred to as XT), which causes signal interference between cores. XT increases with . In long-distance transmission using a multi-core optical fiber, an XT of −39 dB/km or less is required, 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 in order to suppress the leakage of higher-order modes, so it is difficult to sufficiently suppress XT. In Non-Patent Document 2, holes are arranged between the cores in order to alleviate the trade-off between the leakage of the higher-order mode and the XT, but this increases the manufacturing cost.

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. S. Nozoe et al. ， “１２５ μｍ－ｃｌａｄｄｉｎｇ ２ＬＰ－ｍｏｄｅ ａｎｄ ４－ｃｏｒｅ Ｍｕｌｔｉ－ｃｏｒｅ Ｆｉｂｒｅ 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.

An object of the present disclosure is to enable the design of a multi-core optical fiber with a standard clad diameter in consideration of manufacturability.

The multi-core optical fiber of the present disclosure is
having two or more cores through which two or more light wave modes propagate,
a cladding having a lower refractive index than the core and arranged to encompass all the cores;
An outermost layer having a refractive index greater than or equal to the clad and less than the core, and arranged on a concentric axis to surround the clad;
A crosstalk between the cores of the highest-order propagation mode propagated in the cores is -39 dB/km or less.

The optical transmission system of the present disclosure is
four or more transmitters;
two or more mode multiplexers for converting signal light generated by the transmitter into two or more lightwave modes;
an optical coupling unit that couples the light emitted from the multiplexer to each core of the multi-core optical fiber of the present disclosure;
a light extraction unit for extracting signal light from each core of the multi-core optical fiber;
two or more mode separators for separating the signal light emitted from the light extractor into two or more light wave modes;
four or more receivers for receiving signal light emitted from the mode separator;
characterized by having

According to the present disclosure, there is an effect of realizing a multi-core optical fiber with a standard clad diameter with high manufacturability.

1 shows an example of a cross-section of a multi-core optical fiber of the present disclosure; An example of a and Λ at Δ ₂ =0% is shown. An example of the outermost layer refractive index for realizing the 3-mode multi-core optical fiber of the present disclosure is shown. 1 shows an example of the cladding thickness of a 3-mode multi-core optical fiber of the present disclosure; 1 shows an example of the core-to-core distance of a 3-mode multi-core optical fiber of the present disclosure; 1 illustrates an example of a standard outer diameter 3-mode 4-core fiber of the present disclosure; An example of an optical transmission system using the optical fiber of the present disclosure is shown.

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 modified and improved forms based on the knowledge of those skilled in the art. In addition, in this specification and the drawings, constituent elements having the same reference numerals are the same as each other.

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 clad 12 surrounding all cores 11 in the circumferential direction, and an outermost layer 13 surrounding the clad 12 . The cladding 12 and the outermost layer 13 have circular cross-sectional shapes, and are arranged on a concentric axis. Although the number of cores 11 arranged in the clad 12 is arbitrary, a configuration in which four cores are arranged is shown here as an example.

The present disclosure realizes a few-mode multi-core optical fiber with a standard clad diameter having characteristics equivalent to those of Non-Patent Document 2 without using air holes, and has the following three points of difference from Non-Patent Document 2.
(i) A core 11 with a high optical confinement effect is used to reduce the trade-off between the leakage of higher-order modes and the increase in XT.
(ii) Unnecessary high-order modes propagating due to excessive confinement effects are cut off by providing the outermost layer 13 with a higher refractive index than the clad 12 .
(iii) Alleviate geometric restrictions by sharing the outermost layer 13 instead of providing it to each core.
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 will be described in detail below.

In the multi-core optical fiber of this embodiment, as shown in FIG. 1, four cores 11 propagating two or more light wave modes are arranged in a clad 12, and an outermost layer 13 is concentrically arranged to surround the clad 12. placed. At this time, the clad 12 has a lower refractive index than the core 11 , and the outermost layer 13 has a refractive index higher than the clad 12 and lower than the core 11 .

Here, the core radius is a, the relative refractive index difference of the core 11 with respect to the clad 12 is Δ ₁ , the relative refractive index difference of the outermost layer 13 with respect to the clad 12 is Δ ₂ , the distance between the cores is Λ, and the distance from the center of the core 11 to the outermost layer is Let s be the distance to the inner diameter of 13, and t be the distance from the center of core 11 to the outer diameter of outermost layer 13 (cladding thickness). Although FIG. 1 shows the case where the number of cores is 4 and they are arranged on a square lattice, the number of cores may be any number of 2 or more, and they may be arranged on a circular ring or arranged on a hexagonal close-packed arrangement.

As an example of the multi-core optical fiber according to the present disclosure in FIG _. The a and Λ dependences of the optical properties of the configuration with four cores are shown. Here, due to _Δ1 , the mode field diameter (hereinafter sometimes abbreviated as MFD) of the LP01 mode at a wavelength of 1.55 μm is set to 9.5 μm, which is equivalent to that of a standard multi-core optical fiber propagating a single mode. .

The straight line L21 in the figure is the boundary line of the structure that blocks the LP21 mode at a wavelength of 1.53 μm, the dashed line L22 is the boundary line of the structure where the XT between the LP11 modes is −39 dB/km or less, and the dashed line L23 is the LP11 mode. The boundary lines of the structure with a leakage loss of 0.01 dB/km are shown respectively. By using the a and Λ dependences that satisfy the gray regions enclosed by the respective boundaries, 3 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. It is possible to realize a multi-core optical fiber that propagates two light wave modes.

Each boundary line can be represented by the following formula.
L21: Λ=−1767+324.2a
L22: Λ = 956.5 - 307.9a + 25.7a ²
L23: Λ = -1336.2 + 467.9a - 39.5a ²

From the above, the gray area is a≦(Λ+1767)/324.2 and 956.5-307.9a+25.7a ² ≦Λ≦-1336.2+467.9a-39.5a ² when Δ ₁ =0%
can be expressed as

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

FIG. 3 shows the core radius dependence of _Δ2 of the multi-core optical fiber according to this embodiment. Here, as an example, a multi-core optical fiber that guides the LP01 mode and the LP11 mode, which are light wave modes, is shown, and the MFD at the wavelength of 1.55 μm in the LP01 mode is 9.5 μm due to _Δ1 at each core radius. Curve L3 in the figure shows _Δ2 that allows the LP21 mode to be blocked at a wavelength of 1.53 μm. In the gray region where _Δ2 is larger than 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 formula.
Δ ₂ =0.44+(a−5.6) ^0.63

FIG. 4 shows the clad thickness t of the multi-core optical fiber according to this embodiment. Here, as an example, a multi-core optical fiber capable of propagating three modes is shown, and with Δ ₁ and Δ ₂ , the MFD of the LP01 mode at a wavelength of 1.55 μm is 9.5 μm, and the cutoff wavelength of the LP21 mode is 1.53 μm. there is The solid line in the figure indicates the boundary line 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 the low loss property is realized in the region where t is larger than the boundary line L4. . This boundary line L4 can be expressed by the following equation.
t = 809.7 - 268.5a + 23.1a ²

FIG. 5 shows the core-to-core distance of the multi-core optical fiber according to this embodiment. Here _, as an example, a multi- _core optical fiber capable of propagating three light wave modes is shown. 0.53 μm. The solid line in the figure indicates the boundary line L5 of the structure where the XT is -39 dB/km or less at a wavelength of 1.625 μm, and the low XT property is realized in the gray region where Λ is larger than the boundary line L5. This boundary line L5 can be expressed by the following equation.
Λ=35.6×10 ⁻⁴ (a−5.4) ^−4.2 +35.3

From the above, when a≧5.6 μm, the outermost layer 13 has Δ ₂ ≧0.44+(a−5.6) ^0.63 and t≧809.7−268.5a+23.1a ²
and the inter-core distance is Λ≧35.6×10 ⁻⁴ (a−5.4) ^−4.2 +35.3
, it is possible to realize a few-mode multi-core optical fiber that achieves both sufficiently low XT properties and low loss properties.

As the multi-core optical fiber according to the present disclosure in FIG. 6, Λ and a of a multi-core optical fiber that has a standard clad with a diameter of 125 μm and includes four cores that can propagate the LP01 mode and the LP11 mode, which are light wave modes. show. Since the diameter of the cladding 12 is the same as that of the standard optical fiber, deterioration in manufacturability is small and compatibility with existing facilities is good, which is preferable. Here, Δ ₁ and Δ ₂ set the MFD at a wavelength of 1.55 μm for the LP01 mode to 9.5 μm and the cutoff wavelength for the LP21 mode to 1.53 μm.

A solid line L61 indicates a structure in which the leakage loss of the LP11 mode is 0.01 dB/km at a wavelength of 1.625 μm, and sufficiently low loss can be achieved in a region where Λ is smaller than that of the solid line L61. A dashed line L62 indicates a structure in which the core-to-core XT of LP11 is −39 dB/km at a wavelength of 1.625 μm, and a sufficiently low XT property can be realized in a region where Λ is larger than that of the dashed line L62. A solid line L61 and a 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

From the above, 35.6×10 ⁻⁴ (a−5.4) ^−4.2 +35.3≦Λ at a≧5.6 μm
≤ -1248.5 + 446.8a - 38.6a ²
A 3-mode 4-core fiber with sufficiently low loss and low XT properties can be realized with a standard cladding diameter in the gray area in FIG.

FIG. 7 shows a configuration diagram of an optical transmission system using a multi-core optical fiber according to the present disclosure. Signal lights generated by N transmitters 91 are input to M mode multiplexers 92 to multiplex the signal lights into two or more optical wave modes. where N≧4 and M≧2. Each light wave mode in which the signal light is multiplexed is coupled to each core of the multi-core optical fiber 94 of the present disclosure at the optical coupler 93 . A light extractor 95 extracts signal light propagating through each core.

The present disclosure can be applied to the information and communications industry.

11: Core 12: Cladding 13: Outermost layer 91: Transmitter 92: Mode multiplexer 93: Optical coupler 94: Multi-core optical fiber 95: Light extractor 96: Mode separator 97: Receiver

Claims (7)

having two or more cores through which two or more light wave modes propagate,
a cladding having a lower refractive index than the core and arranged to encompass all the cores;
An outermost layer having a refractive index greater than or equal to the clad and less than the core, and arranged on a concentric axis to surround the clad;
A multi-core optical fiber, wherein crosstalk between the cores of the highest-order propagation mode propagated in the cores is -39 dB/km or less. 2. The multi-core optical fiber according to claim 1, wherein the core propagates the LP01 mode and the LP11 mode and blocks the LP21 mode at wavelengths of 1.53 [mu]m or longer. The outermost layer has a diameter of 125 μm,
Assuming that the core radius and inter-core distance of the core are a and Λ,
a≦(Λ+1767)/324.2 and 956.5−307.9a+25.7a ² ≦Λ
≤ -1336.2 + 467.9a - 39.5a ²
3. The multi-core optical fiber according to claim 2, wherein: The core radius a of the core is 5.6 μm or more,
Assuming that the distance from the center of the core to the outer diameter of the outermost layer is t, and the refractive index of the outermost layer is _Δ2 ,
Δ ₂ ≧0.44+(a−5.6) ^0.63 and t≧809.7−268.5a+23.1a ²
and
The inter-core distance Λ of the cores is Λ≧35.6×10 ⁻⁴ (a−5.4) ^−4.2 +35.3
3. The multi-core optical fiber according to claim 2, wherein: The core radius a of the core is 5.6 μm or more,
The outermost layer has a diameter of 125 μm,
Four cores are arranged in the clad,
The mode field diameter of the LP01 mode of the core is 9.5 μm or more when the wavelength of light is 1.55 μm,
3. The multi-core optical fiber according to claim 2, wherein the leakage loss of the LP11 mode of the core is 0.01 dB/km or less when the wavelength of light is 1.625 [mu]m. The core radius a is 5.6 μm or more,
The outermost layer has a diameter of 125 μm,
Four cores are arranged in the clad,
The inter-core distance Λ and the core radius a are
35.6×10 ⁻⁴ (a−5.4) ^−4.2 +35.3≦Λ
≤ -1248.5 + 446.8a - 38.6a ²
3. The multi-core optical fiber according to claim 2, satisfying: four or more transmitters;
two or more mode multiplexers for converting signal light generated by the transmitter into two or more lightwave modes;
an optical coupling unit that couples the light emitted from the multiplexer to each core of the multi-core optical fiber according to any one of claims 1 to 6;
a light extraction unit for extracting signal light from each core of the multi-core optical fiber;
two or more mode separators for separating the signal light emitted from the light extractor into two or more light wave modes;
four or more receivers for receiving signal light emitted from the mode separator;
characterized by having
Optical transmission system.

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