Multi-core optical fiber, multi-core optical fiber design method and optical transmission method

Abstract

The purpose of the present invention is to provide a multi-core optical fiber having four cores with a standard 125±1μm cladding diameter of an existing single-mode optical fiber and capable of handling thousands of kilometers of transmission, a design method for the multi-core optical fiber, and an optical transmission method using the multi-core optical fiber. The multi-core optical fiber of the present invention configures two-stage claddings with different refractive indices around each core, so that the core radius a1, the radius a2 of the first cladding region surrounding each core, the relative refractive index Δ1 of the first cladding region relative to the core, and the relative refractive index Δ2 of the second cladding region including the four cores and the first cladding region relative to the core are within a specified range.

Description

[0001] This application is a divisional application of the invention patent application with the application date of November 8, 2019, the invention name of which is "Multi-core optical fiber, multi-core optical fiber design method and optical transmission method" and the national application number is "201980073966.9". Technical Field

[0002] The present invention relates to a multi-core optical fiber having a plurality of cores, a design method of the multi-core optical fiber, and a light transmission method using the multi-core optical fiber. Background Art

[0003] Multi-core optical fiber (MCF) with multiple core regions is being actively studied to significantly expand the transmission capacity by using space division multiplexing technology. In particular, it is known that power consumption can be reduced in optical transmission channels using space division multiplexing, and it is expected that MCF will be effectively used in long-distance optical communication systems represented by submarine communication systems. In non-patent document 1, MCF was successfully used in a 10,000 km level transmission experiment. Here, in MCF, it is necessary to prevent the degradation of transmission characteristics caused by crosstalk (XT) between cores. In order to reduce XT, in addition to the design of the core parameters in MCF, it is also necessary to appropriately set the distance between cores. In non-patent document 2, an MCF with a very low XT of less than -30 dB between cores at 10,000 km is reported.

[0004] Here, the MCF shown in multiple reports including non-patent literature 1 and 2 sets a sufficiently wide core spacing in order to reduce XT, so the cladding diameter is 150 to 230 μm, which is larger than that of conventional optical fibers. However, the length of an optical fiber manufactured from a fiber mother material becomes shorter in inverse proportion to the square of the cladding diameter, so the increase in the cladding diameter significantly deteriorates the productivity of the optical fiber. In addition, since existing optical fiber components are designed based on the previous cladding diameter of 125 μm, in order to effectively utilize the MCF with an increased cladding diameter, it is necessary to redesign peripheral components, and a lot of research and development is required for practical application.

[0005] Therefore, in recent years, MCF with a cladding diameter of 125μm, which is equivalent to the previous one, has been developed. The cladding diameter is the standard 125μm, which can maintain the mass production of optical fiber at or above the same level as before, and can effectively utilize existing peripheral items such as standard connecting parts and optical cables. In addition, each core of MCF has the same optical properties as existing optical fibers, which can ensure interchangeability with existing optical interfaces, so it can be easily upgraded from existing equipment to MCF.

[0006] Non-patent documents 3 and 4 report MCFs with four cores that have an XT of less than -30 dB at 100 km and have optical properties equivalent to those of existing single-mode optical fibers (SMFs). According to non-patent document 3, four cores can be configured using the same core structure, and according to non-patent document 5, five cores can be configured by using a plurality of core structures. In addition, non-patent document 6 studies MCFs with properties equivalent to those of low-loss optical fibers for the seafloor, and shows that two cores can be configured with a cladding diameter of 125 μm.

[0007] Prior art literature

[0008] Non-patent literature 1: H. Takahashi et al., “First Demonstration of MC-EDFA-Repeatered SDM Transmission of 40x 128-Gbit / s PDM-QPSK Signals per Core over 6,160-km 7-core MCF,” ECOC2012, Th3C3, Sep. 2012.

[0009] Non-patent document 2: T. Hayashi et al., “Design and fabrication of ultra-low crosstalk and low-loss multi-core fiber,” Opt. Express, vol. 19, pp. 16576-16592, Aug. 2011.

[0010] Non-patent document 3: T. Matsui et al., "Design of multi-core fiber in 125μm cladding diameter with full compliance to conventional SMF," ECOC2015, We.1.4.5, Sep.2015.

[0011] Non-patent document 4: T. Matsui et al., “118.5Tbit / s Transmission over 316km-Long Multi-Core Fiber with Standard Cladding Diameter” OECC2017, PDP2, Aug. 2017.

[0012] Non-patent document 5: T.Gonda et al., "125μm 5-core fiber with heterogeneousdesign suitable for migration from single-core system to multi-core system" ECOC2016, W2B1, Sep.2016.

[0013] Non-Patent Document 6: Tamura et al., “Aeff-amplified uncoupled dual-core optical fiber with 125 μm cladding diameter,” IEEE Conference on Electronics, Information and Communications Technology, B-10-2, September 2016

[0014] Non-patent document 7: PJ Winzer et al., “Penalties from In-Band Crosstalk for Advanced Optical Modulation Formats,” ECOC 2011, Tu5B7, Sep. 2011.

[0015] However, when considering maintaining the existing 125 μm cladding diameter and having four cores in MCF, there is a problem that the limit is about several hundred kilometers due to the limitation of XT, as shown in non-patent documents 3 to 5. In addition, as shown in non-patent document 6, in order to achieve XT that can cope with transmission of thousands of kilometers, for a cladding diameter of 125 μm, the limit of the number of cores is two, and there is a problem that it is difficult to configure more than three cores. Summary of the invention

[0016] Therefore, in order to solve the above-mentioned problem, the purpose of the present invention is to provide a multi-core optical fiber having four cores with the standard cladding diameter of 125±1μm of existing single-mode optical fiber and capable of handling transmission of thousands of kilometers, a design method of the multi-core optical fiber, and an optical transmission method using the multi-core optical fiber.

[0017] In order to achieve the above-mentioned purpose, the multi-core optical fiber of the present invention configures two-stage claddings with different refractive indices around each core, so that the core radius a1, the radius a2 of the first cladding region surrounding each core, the relative refractive index Δ1 of the first cladding region relative to the core, and the relative refractive index Δ2 of the second cladding region including the four cores and the first cladding region relative to the core are within specified ranges.

[0018] Specifically, the multi-core optical fiber of the present invention is characterized by having:

[0019] Four cores of radius a1 are arranged in a square lattice along the long side direction;

[0020] a first cladding region of radius a2 having a lower refractive index than the core, and having a refractive index around the core having a relative refractive index difference of Δ1 with the core; and

[0021] A second cladding region having a lower refractive index than the core, at the periphery of the first cladding region, having a greater refractive index than the first cladding region and having a relative refractive index difference with the core of Δ2,

[0022] The outer diameter of the second cladding region is 125±1 μm,

[0023] The effective cross-sectional area at a wavelength of 1550nm is 80μm 2 above,

[0024] The bending loss at a wavelength of 1625nm and a bending radius of 30mm is less than 0.1dB / 100turn.

[0025] The cut-off wavelength is below 1530nm.

[0026] The presence of the first cladding region makes it easier to confine the electric field distribution in the core, so the core spacing can be narrowed while reducing XT. Even with a cladding diameter of 125 μm, it can be a multi-core optical fiber with four cores configured for long-distance transmission.

[0027] The multi-core optical fiber of the present invention may have a region with a width w between the core and the first cladding region and having the same refractive index as the second cladding region. In this case, the width w is preferably 0 μm. <w≤1.3μm。

[0028] The parameters of the multi-core optical fiber of the present invention are as follows.

[0029] The characteristic lies in that the relationship between the Δ1 and the total value XT (dB / km) of the inter-core crosstalk per 1 km satisfies the number C1.

[0030] [Number C1]

[0031] Δ1≤4.93×10 -5 XT-0.00127 (C1)

[0032] The characteristic is that the relationship between a1 and Δ1 satisfies number C2.

[0033] [Number C2]

[0034]

[0035] The characteristic lies in that the relationship between the Δ2 and the total value XT (dB / km) of the inter-core crosstalk per 1 km satisfies the number C3.

[0036] [Number C3]

[0037] Δ2≥-1.14×10 -5 XT-0.00509 (C3)

[0038] The invention is characterized in that a ratio of a1 to a2, a2 / a1, is greater than or equal to 1.0 and less than or equal to 3.0,

[0039] The Δ1, the a2 / a1 and the effective cross-sectional area A at a wavelength of 1550nm eff The relationship satisfies number C4,

[0040] The Δ2, the a2 / a1 and the A eff The relationship satisfies number C5.

[0041] [Number C4]

[0042] Δ1≤(1.9×10 -6 A eff 2 -0.00033A eff +0.0163)a2 / a1+(-8.0×10 -6 A eff 2 +0.00139A eff -0.0705) (C4)

[0043] [Number C5]

[0044] Δ2≥(5.0×10 -8 A eff 2 +6.5×10 -6 A eff -0.00108)a2 / a1+(-2.9×10 -7 A eff 2 +5.58×10 - 5 A eff -0.00594)

[0045] and

[0046] Δ2≤(5.0×10 -7 A eff 2 -9.5×10 -5 A eff +0.0056)a2 / a1+(-1.6×10 -6 A eff 2 +0.00031A eff -0.0208) (C5)

[0047] The characteristic is that the Δ1 and the Δ2 satisfy the number C6.

[0048] [Number C6]

[0049] -0.7% ≤ Δ1 ≤ -0.49%

[0050] -0.4%≤Δ2≤-0.27% (C6)

[0051] Furthermore, various parameters of the multi-core optical fiber of the present invention can be expressed as follows.

[0052] The multi-core optical fiber is characterized by:

[0053] Four cores of radius a1 are arranged in a square lattice along the long side direction;

[0054] a first cladding region of radius a2 having a lower refractive index than the core, and having a refractive index around the core having a relative refractive index difference of Δ1 with the core; and

[0055] A second cladding region having a lower refractive index than the core, at the periphery of the first cladding region, having a greater refractive index than the first cladding region and having a relative refractive index difference with the core of Δ2,

[0056] a2 / a1=2.0 and the effective cross-sectional area at a wavelength of 1550nm is 80μm 2 ,

[0057] When the relative refractive index difference of the first cladding region is taken as the horizontal axis and the relative refractive index difference of the second cladding region is taken as the vertical axis, the Δ1 and the Δ2 are located at

[0058] A0(-0.800,-0.396)

[0059] A1a(-0.800,-0.340)

[0060] A1b(-0.750,-0.351)

[0061] A1(-0.682,-0.378)

[0062] The scope of encirclement.

[0063] The multi-core optical fiber is characterized by:

[0064] Four cores of radius a1 are arranged in a square lattice along the long side direction;

[0065] a first cladding region of radius a2 having a lower refractive index than the core, and having a refractive index around the core having a relative refractive index difference of Δ1 with the core; and

[0066] A second cladding region having a lower refractive index than the core, at the periphery of the first cladding region, having a greater refractive index than the first cladding region and having a relative refractive index difference with the core of Δ2,

[0067] a2 / a1=3.0 and the effective cross-sectional area at a wavelength of 1550nm is 80μm 2 ,

[0068] When the relative refractive index difference of the first cladding region is taken as the horizontal axis and the relative refractive index difference of the second cladding region is taken as the vertical axis, the Δ1 and the Δ2 are located at

[0069] B0a(-0.700,-0.376)

[0070] B1a(-0.700,-0.266)

[0071] B1b(-0.671,-0.286)

[0072] B1c (-0.618, -0.313)

[0073] B1d (-0.586, -0.338)

[0074] B1(-0.490,-0.396)

[0075] B0d(-0.586,-0.400)

[0076] B0c(-0.618,-0.391)

[0077] B0b(-0.671,-0.371)

[0078] The scope of encirclement.

[0079] The multi-core optical fiber is characterized by:

[0080] Four cores of radius a1 are arranged in a square lattice along the long side direction;

[0081] a first cladding region of radius a2 having a lower refractive index than the core, and having a refractive index around the core having a relative refractive index difference of Δ1 with the core; and

[0082] A second cladding region having a lower refractive index than the core, at the periphery of the first cladding region, having a greater refractive index than the first cladding region and having a relative refractive index difference with the core of Δ2,

[0083] a2 / a1=3.0 and the effective cross-sectional area at a wavelength of 1550nm is 100μm 2 ,

[0084] When the relative refractive index difference of the first cladding region is taken as the horizontal axis and the relative refractive index difference of the second cladding region is taken as the vertical axis, the Δ1 and the Δ2 are located at

[0085] C0a(-0.600,-0.274)

[0086] C1a(-0.600,-0.255)

[0087] C1b(-0.590,-0.262)

[0088] C1c(-0.570,-0.277)

[0089] C1(-0.528,-0.305)

[0090] C0c(-0.570,-0.297)

[0091] C0b(-0.590,-0.289)

[0092] The scope of encirclement.

[0093] The parameters of the above multi-core optical fiber are designed as follows.

[0094] The multi-core optical fiber has:

[0095] Four cores of radius a1 are arranged in a square lattice along the long side direction;

[0096] a first cladding region of radius a2 having a lower refractive index than the core, and having a refractive index around the core having a relative refractive index difference of Δ1 with the core; and

[0097] A second cladding region having a lower refractive index than the core, at the periphery of the first cladding region, having a greater refractive index than the first cladding region and having a relative refractive index difference with the core of Δ2,

[0098] The design method of the multi-core optical fiber is performed by:

[0099] The first step is to calculate the effective cross-sectional area A. eff (μm 2 ) and the required condition of the total value XT (dB / km) of the core crosstalk per 1 km, select the combination of the a1 and the Δ1;

[0100] In the second step, the a2 and the Δ2 that satisfy the cutoff wavelength and bending loss requirements in the combination selected in the first step are selected, and the core spacing relationship of the Δ1 and the core spacing Λ relative to the XT and the additional loss α are obtained. c an OCT relationship between the Δ1 and the shortest distance OCT from the outer periphery of the second cladding region to the center of the core under the requirement of

[0101] In the third step, the core spacing relationship and the OCT relationship obtained in the second step are used to obtain Calculate the outer diameter of the second cladding region at the Δ1 obtained in the first step And determine the outer diameter Is it less than 125μm?

[0102] Here, it is characterized in that when Aeff is 80 μm2 or more, in the first step, the combination satisfying the numbers C1 and C2 is selected. Also, it is characterized in that when a2 / a1 is 3.0 or less, in the second step, Δ2 satisfying the number C3 is selected.

[0103] Furthermore, the optical transmission method of the present invention is characterized in that

[0104] Using the multi-core optical fiber of more than 1000 km as an optical transmission channel,

[0105] Sending signal light with a wavelength of 1530 nm or more to each core of the multi-core optical fiber from four transmitters,

[0106] The signal light is received by four receivers for each of the cores of the multi-core optical fiber.

[0107] Furthermore, the optical transmission method of the present invention is characterized in that

[0108] The single-mode optical fiber of the submarine communication system with a communication distance of more than 1000 km is replaced by the multi-core optical fiber.

[0109] Sending signal light with a wavelength of 1530 nm or more to each core of the multi-core optical fiber from four transmitters,

[0110] The signal light is received by four receivers for each of the cores of the multi-core optical fiber.

[0111] The multi-core optical fiber of the present invention has 4 cores, and realizes a second cladding diameter of 125 μm and an XT of -54 dB / km or less. This characteristic is a characteristic that cannot be achieved by any of the multi-core optical fibers in non-patent documents 3 to 6. Therefore, the multi-core optical fiber can replace the single-mode optical fiber currently used in long-distance optical communication systems such as submarine communication systems. In addition, since it has 4 cores, it can achieve a dramatic increase in transmission capacity and a reduction in power consumption.

[0112] The present invention can provide a multi-core optical fiber having four cores with a standard cladding diameter of 125±1 μm of existing single-mode optical fibers and capable of handling transmission of thousands of kilometers, a design method of the multi-core optical fiber, and an optical transmission method using the multi-core optical fiber.

[0113] In this multi-core optical fiber, four cores are configured with a standard cladding diameter, which can achieve optical properties that are interchangeable with existing optical fibers and an XT that can transmit thousands of kilometers, thereby providing a high-density, large-capacity MCF suitable for ultra-long-distance communications such as submarine systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0114] Figure 1 This is a diagram for explaining an example of the structure of the multi-core optical fiber of the present invention.

[0115] Figure 2 It is a diagram for explaining the influence of the width w of the multi-core optical fiber of the present invention.

[0116] Figure 3 This is a characteristic diagram showing the relationship between the effective cross-sectional area and the structural parameters of the multi-core optical fiber of the present invention.

[0117] Figure 4 This is a characteristic diagram showing the relationship between the effective cross-sectional area and the structural parameters of the multi-core optical fiber of the present invention.

[0118] Figure 5 This is a characteristic diagram showing the relationship between the effective cross-sectional area and the structural parameters of the multi-core optical fiber of the present invention.

[0119] Figure 6 This is a characteristic diagram showing an example of the relationship between the core arrangement, XT, and excess loss of the multi-core optical fiber of the present invention.

[0120] Figure 7 This is a characteristic diagram showing the relationship between structural parameters for making the cladding diameter 125±1 μm or less under predetermined cutoff wavelength, bending loss, and effective cross-sectional area in the multi-core optical fiber of the present invention.

[0121] Figure 8 This is a characteristic diagram showing the relationship between structural parameters for making the cladding diameter 125±1 μm or less under predetermined cutoff wavelength, bending loss, and effective cross-sectional area in the multi-core optical fiber of the present invention.

[0122] Fig. 9 This is a characteristic diagram showing the relationship between structural parameters for making the cladding diameter 125±1 μm or less under predetermined cutoff wavelength, bending loss, and effective cross-sectional area in the multi-core optical fiber of the present invention.

[0123] Fig.10 This is a characteristic diagram showing the relationship between XT and the range of necessary structural parameters in the multi-core optical fiber of the present invention.

[0124] Fig.11 This is a characteristic diagram showing the relationship between XT and the range of necessary structural parameters in the multi-core optical fiber of the present invention.

[0125] Fig.12 This is a characteristic diagram showing the relationship between structural parameters for making the cladding diameter 125 μm or less at a predetermined cutoff wavelength, bending loss, and effective cross-sectional area in the multi-core optical fiber of the present invention.

[0126] Fig.13 This is a characteristic diagram showing the relationship between structural parameters for making the cladding diameter 125 μm or less at a predetermined cutoff wavelength, bending loss, and effective cross-sectional area in the multi-core optical fiber of the present invention.

[0127] Fig.14 This is a characteristic diagram showing the relationship between structural parameters for making the cladding diameter 125 μm or less at a predetermined cutoff wavelength, bending loss, and effective cross-sectional area in the multi-core optical fiber of the present invention.

[0128] Fig.15 This is a table illustrating a design example of the multi-core optical fiber of the present invention.

[0129] Fig.16 It is a characteristic diagram illustrating the relationship between the core spacing and XT in the design example of the multi-core optical fiber of the present invention.

[0130] Fig.17 It is a characteristic diagram explaining the relationship between OCT and excess loss in a design example of the multi-core optical fiber of the present invention.

[0131] Fig.18 is a cross-sectional view of a multi-core optical fiber of the present invention.

[0132] Fig.19 This is a table showing the evaluation results of the multi-core optical fiber of the present invention.

[0133] Fig. 20 is a flow chart illustrating the design method of the present invention.

[0134] Fig.21 It is a diagram for explaining the light transmission method of the present invention. DETAILED DESCRIPTION

[0135] Embodiments of the present invention are described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In addition, in this specification and the accompanying drawings, components with the same reference numerals represent components that are identical to each other.

[0136] Figure 1 It is a diagram for explaining an example of the structure of the optical fiber 15 according to the present embodiment. Figure 1 (a) is the cross-sectional structure, Figure 1 (b) and Figure 1 (c) is the refractive index distribution of each core. Optical fiber 15 is a multi-core optical fiber having:

[0137] Four cores 10 of radius a1 are arranged in a square lattice along the long side direction;

[0138] A first cladding region 11 of radius a2 having a lower refractive index than the core, surrounding the core 10, and having a relative refractive index difference of Δ1 with the core; and

[0139] The second cladding region 12, which has a lower refractive index than the core, is located at the outer periphery of the first cladding region 11, has a greater refractive index than the first cladding region and has a relative refractive index difference of Δ2 with the core.

[0140] Among them, the optical fiber 15 is characterized in that:

[0141] The outer diameter of the second cladding region 12 is 125±1 μm,

[0142] The effective cross-sectional area at a wavelength of 1550nm is 80μm 2 above,

[0143] The bending loss at a wavelength of 1625nm and a bending radius of 30mm is less than 0.1dB / 100turn.

[0144] The cut-off wavelength is below 1530nm.

[0145] The optical fiber 15 has four cores in a single-core optical fiber, and the cladding diameter (outer diameter of the second cladding region 12 ) is a standard 125±1 μm.

[0146] The refractive index distribution of each core is as follows Figure 1 As shown in (b), the first cladding region 11 having a lower refractive index than the core is provided around the core 10, and the second cladding region 12 having a lower refractive index than the core and higher than the first cladding region is provided around the core 10. Figure 1 As shown in (c), a region 13 may be provided between the core 10 and the first cladding region 11 , and the region 13 may have a refractive index equivalent to that of the second cladding region 12 .

[0147] Figure 2 Indicates that Figure 1 The refractive index distribution of (c) is the change in the cutoff wavelength relative to the width w of region 13. Figure 2 As shown in FIG. 1 , even if the core 10 and the first cladding region 11 have a region 13 with a refractive index equivalent to that of the second cladding region 12, the change in the cutoff wavelength is small. It can be confirmed from the figure that when the width w is less than 1.3 μm, the change in the cutoff wavelength is ±10 nm, which is equivalent to the measurement error, so it can be considered that Figure 1 (b) and Figure 1 The refractive index distribution of (c) is the same. Figure 1The structure (c) is preferred because it can reduce the variation of the core structure during manufacturing. For example, the refractive index of the second cladding region is easily stabilized in the case of pure quartz, which can improve manufacturing deviations and yields.

[0148] In addition, the relative refractive index difference is defined here based on the refractive index of the core, but the materials of the core and the cladding can be respectively made of pure silica and fluorine-doped glass, or a combination of glass doped with GeO2, pure silica glass and fluorine-doped glass, etc. Figure 1 The refractive index distribution of the glass material shown is selected by the combination of materials.

[0149] The “cutoff wavelength” of an optical fiber refers to a condition for single-mode propagation. If the wavelength is longer than the cutoff wavelength, light propagates in single mode, and if the wavelength is shorter than the cutoff wavelength, light propagates in multimode.

[0150] Figure 3 to Figure 5 This is to illustrate the method for obtaining a predetermined effective cross-sectional area A at a wavelength of 1550 nm of the optical fiber 15. eff The horizontal axis is the relative refractive index difference Δ1 of the first cladding region 11 with respect to the core, and the vertical axis is the radius a1 of the core 10. Figure 3 to Figure 5 The ratio a2 / a1 of the radius a2 of the first cladding region 11 to the core radius a1 is changed to 2.0, 2.5, and 3.0, respectively. The relative refractive index difference Δ of the cladding with respect to the core is set so that the cutoff wavelength is 1530 nm.

[0151] right Figure 3 to Figure 5 Compare to get a fixed A eff The relationship between a1 and Δ1 is roughly the same. This is considered to be because the electric field distribution is confined to the area surrounded by the first cladding region 11, and therefore has little dependence on the parameters a2 and Δ2 related to the second cladding region 12 (the relative refractive index difference of the second cladding region 12 with respect to the core).

[0152] Here, the conventional SMF has a wavelength of 1550nm. eff About 80μm 2 In order to ensure connectivity and low nonlinearity equal to or better than before, A eff Preferably 80 μm 2 According to Figure 3 to Figure 5 , in order to make A eff 80μm 2 , according to the approximate curve of the solid line, number 1 needs to be satisfied.

[0153] [Number 1]

[0154]

[0155] It is known that in order to reduce XT in MCF, the core spacing needs to be increased by a certain amount. The degradation of transmission characteristics caused by XT depends on the transmission method. Non-patent document 7 shows that for QPSK, 16QAM, and 64QAM, the crosstalk needs to be -18dB, -24dB, and -32dB or less, respectively. In the case of multi-core optical fiber, the crosstalk between cores becomes the sum of the crosstalk components from other cores relative to any core. For the crosstalk per unit distance (XT, unit dB / km) and the distance L (km), the crosstalk at any distance is given by

[0156] XT+log(L)

[0157] Given.

[0158] Therefore, for example, in order to use the 16QAM signal format for 1000 to 10000 km transmission, the crosstalk of the multi-core optical fiber needs to be below -54 to -64 dB / km. In order to reduce the crosstalk, the core spacing needs to be increased by a certain amount, but it is known that when the cladding diameter is fixed, as the core spacing increases, the thickness (OCT) between the core and the cladding end decreases, resulting in additional loss α caused by the leakage of light waves. c .

[0159] Figure 6 It is used to reduce the crosstalk between cores (XT) and the additional loss α c The figure is a design example of the optical fiber 15. The horizontal axis is the relative refractive index difference Δ1 of the first cladding region 11, the first vertical axis is the core spacing Λ or OCT, and the second vertical axis is the outer diameter of the second cladding region 12. XT and additional loss increase as the wavelength increases, so considering the effective use of the entire C+L band area, the wavelength is 1625nm. The solid line is the core spacing Λ (μm) when XT is -64dB / km, and the dotted line is the additional loss α c That is, in the area above the solid line, XT is less than -64 dB / km, and in the area above the dotted line, α c Less than 0.01dB / km.

[0160] The dotted line is for the case where XT is -64dB / km and α c The "necessary cladding diameter" is 0.01dB / km. In the case of a 4-core configuration,

[0161] Necessary cladding diameter = 2×(Λ / √2+OCT).

[0162] Here, a1 and Δ2 are set so that a2 / a1=3.0, A eff 110μm 2 , cut-off wavelength λ c It is 1530nm.

[0163] from Figure 6 It can be seen that as Δ1 increases, the confinement of the light wave becomes weaker. In order to obtain the desired XT and α c , the necessary core spacing Λ and OCT become larger. Figure 6 In order to make the cladding diameter 125±1μm, Δ1 needs to be -0.52% (see the dot-dash line). The required core spacing Λ and OCT are 42.5μm and 30μm respectively. Figure 6 The core spacing and OCT shown are the minimum values ​​that meet the specified XT and additional loss. If the value is set above this, XT and additional loss will become smaller values. In other words, if Δ1 is less than -0.52% in order to make the cladding diameter 125±1μm, the necessary cladding diameter is less than 125μm, and the design of Λ and OCT has a margin.

[0164] Figure 7 to Figure 9 Yes Description

[0165] The cut-off wavelength is below 1530nm.

[0166] An example of the structural parameters of an optical fiber 15 in which the bending loss is less than 0.1 dB / 100 turn at a wavelength of 1625 nm and a bending radius of 30 mm and the required cladding diameter is 125±1 μm. In any of the figures, in the shaded area, the conditions (XT is less than -64 dB / km and α c is below 0.01dB / km).

[0167] Figure 7 Yes A eff 80μm at a wavelength of 1550nm 2 , a2 / a1=2.0,

[0168] Figure 8 Yes A eff 80μm at a wavelength of 1550nm 2 , a2 / a1=3.0,

[0169] Fig. 9 Yes A eff 110 μm at a wavelength of 1550 nm 2 , the case where a2 / a1=3.0.

[0170] exist Figure 7 to Figure 9 In the figure, the dashed line represents the condition where the cutoff wavelength is 1530 nm (the cutoff wavelength is below 1530 nm in the region above the dashed line), and the dotted line represents the bending loss α b The bending loss α in the area below the dotted line is 0.1 dB / 100 turn. b is less than 0.1dB / 100turn).

[0171] exist Figure 7 to Figure 9 In the figure, the solid line represents the XT condition at a wavelength of 1625 nm, and the areas below the solid line are respectively below -54 dB / km, below -58 dB / km, below -61 dB / km, and below -64 dB / km.

[0172] The area that satisfies all conditions is the area surrounded by the solid line, the dashed line, and the dotted line, and for example, in the case of XT being -64 dB / km or less, it is the shaded area in the figure. Figure 7 to Figure 9 , the upper limit of Δ1 and the lower limit of Δ2 are determined by the requirements of XT and cutoff wavelength, and the lower limit of Δ2 changes less than Δ1. Therefore, the upper limit of Δ1 and the lower limit of Δ2 can be given by the intersection of the solid line and the dotted line. In addition, from Figure 7 to Figure 9 It can be seen that the larger a2 / a1 is, the more necessary A eff The smaller it is, the more relaxed the area can be designed.

[0173] Therefore, in A eff 80μm 2 When the cut-off wavelength is below 1530nm and the XT is below -64dB / km, Figure 8 It becomes the largest area, which is now 2.

[0174] [Number 2]

[0175] -0.7% ≤ Δ1 ≤ -0.49%

[0176] -0.4%≤Δ2≤-0.27% (2)

[0177] exist Figure 7 In the example, if the X coordinate is taken as Δ1 and the Y coordinate is taken as Δ2, then the shaded area satisfies

[0178] a2 / a1=2.0,

[0179] A at a wavelength of 1550nm eff =80μm 2 ,

[0180] Cladding diameter D≤125μm,

[0181] Additional loss α c ≤0.01dB / km,

[0182] Cut-off wavelength λc≤1530nm,

[0183] Bending loss α b ≤0.1dB / 100turn, and

[0184] The area where XT≤-64dB / km is

[0185] A0(-0.800,-0.396)

[0186] A1a(-0.800,-0.340)

[0187] A1b(-0.750,-0.351)

[0188] A1(-0.682,-0.378)

[0189] The scope of encirclement.

[0190] In addition, if the area meets XT≤-61dB / km, then

[0191] A0(-0.800,-0.396)

[0192] A2a(-0.800,-0.335)

[0193] A2b(-0.750,-0.346)

[0194] A2c (-0.700, -0.363)

[0195] A2(-0.668,-0.380)

[0196] The range of encirclement,

[0197] If the area meets XT≤-58dB / km, then

[0198] A0(-0.800,-0.396)

[0199] A3a(-0.800,-0.331)

[0200] A3b(-0.750,-0.339)

[0201] A3c (-0.700, -0.355)

[0202] A3d(-0.650,-0.380)

[0203] A3 (-0.639, -0.387)

[0204] The range of encirclement,

[0205] If the area meets XT≤-54dB / km, then it is

[0206] A0(-0.800,-0.396)

[0207] A4a (-0.800, -0.328)

[0208] A4b(-0.750,-0.323)

[0209] A4c (-0.700, -0.349)

[0210] A4d(-0.650,-0.370)

[0211] A4(-0.600,-0.400)

[0212] The scope of encirclement.

[0213] exist Figure 8 In the example, if the X coordinate is Δ1 and the Y coordinate is Δ2, then the shaded area satisfies

[0214] a2 / a1=3.0,

[0215] A at a wavelength of 1550nm eff =80μm 2 ,

[0216] Cladding diameter D≤125μm,

[0217] Additional loss α c ≤0.01dB / km,

[0218] Cut-off wavelength λc≤1530nm,

[0219] Bending loss α b ≤0.1dB / 100turn, and

[0220] The area where XT≤-64dB / km is

[0221] B0a(-0.700,-0.376)

[0222] B1a(-0.700,-0.266)

[0223] B1b(-0.671,-0.286)

[0224] B1c (-0.618, -0.313)

[0225] B1d (-0.586, -0.338)

[0226] B1(-0.490,-0.396)

[0227] B0d(-0.586,-0.400)

[0228] B0c(-0.618,-0.391)

[0229] B0b(-0.671,-0.371)

[0230] The scope of encirclement.

[0231] In addition, if the area meets XT≤-61dB / km, it is B0a(-0.700, -0.376)

[0232] B2a(-0.700,-0.256)

[0233] B2b (-0.671, -0.278)

[0234] B2c (-0.618, -0.306)

[0235] B2d (-0.586, -0.329)

[0236] B2(-0.484,-0.396)

[0237] B0d(-0.586,-0.400)

[0238] B0c(-0.618,-0.391)

[0239] B0b(-0.671,-0.371)

[0240] The range of encirclement,

[0241] If the area meets XT≤-58dB / km, then

[0242] B0a(-0.700,-0.376)

[0243] B3a(-0.700,-0.242)

[0244] B3b(-0.671,-0.267)

[0245] B3c (-0.618, -0.299)

[0246] B3d(-0.586,-0.319)

[0247] B3(-0.470,-0.396)

[0248] B0d(-0.586,-0.400)

[0249] B0c(-0.618,-0.391)

[0250] B0b(-0.671,-0.371)

[0251] The range of encirclement,

[0252] If the area meets XT≤-54dB / km, then

[0253] B0a(-0.700,-0.376)

[0254] B4a(-0.700,-0.234)

[0255] B4b(-0.671,-0.256)

[0256] B4c (-0.618, -0.278)

[0257] B4d(-0.586,-0.312)

[0258] B4(-0.458,-0.396)

[0259] B0d(-0.586,-0.400)

[0260] B0c(-0.618,-0.391)

[0261] B0b(-0.671,-0.371)

[0262] The scope of encirclement.

[0263] exist Fig. 9 In the example, if the X coordinate is taken as Δ1 and the Y coordinate is taken as Δ2, then the shaded area satisfies

[0264] a2 / a1=3.0,

[0265] A at a wavelength of 1550nm eff =110μm 2 ,

[0266] Cladding diameter D≤125μm,

[0267] Additional loss α c ≤0.01dB / km,

[0268] Cut-off wavelength λc≤1530nm,

[0269] Bending loss α b ≤0.1dB / 100turn, and

[0270] The area where XT≤-64dB / km is

[0271] C0a(-0.600,-0.274)

[0272] C1a(-0.600,-0.255)

[0273] C1b(-0.590,-0.262)

[0274] C1c(-0.570,-0.277)

[0275] C1(-0.528,-0.305)

[0276] C0c(-0.570,-0.297)

[0277] C0b(-0.590,-0.289)

[0278] The scope of encirclement.

[0279] In addition, if the area meets XT≤-61dB / km, it is C0a(-0.600, -0.274)

[0280] C2a (-0.600, -0.247)

[0281] C2b(-0.590,-0.254)

[0282] C2c (-0.570, -0.269)

[0283] C2(-0.513,-0.308)

[0284] C0c(-0.570,-0.297)

[0285] C0b(-0.590,-0.289)

[0286] The range of encirclement,

[0287] If the area meets XT≤-58dB / km, then

[0288] C0a(-0.600,-0.274)

[0289] C3a (-0.600, -0.234)

[0290] C3b (-0.590, -0.242)

[0291] C3c (-0.570, -0.257)

[0292] C3 (-0.495, -0.311)

[0293] C0c(-0.570,-0.297)

[0294] C0b(-0.590,-0.289)

[0295] The range of encirclement,

[0296] If the area meets XT≤-54dB / km, then

[0297] C0a(-0.600,-0.274)

[0298] C4a (-0.600, -0.220)

[0299] C4b (-0.590, -0.233)

[0300] C4c (-0.570, -0.249)

[0301] C4 (-0.479, -0.313)

[0302] C0c(-0.570,-0.297)

[0303] C0b(-0.590,-0.289)

[0304] The scope of encirclement.

[0305] Fig.10 and Fig.11 Is to explain by and Figure 7 to Figure 9 The upper limit of Δ1 is given by the intersection of the cutoff wavelength λc (dashed line) and the condition XT (solid line) of the graph produced as shown. Fig.10 ) and the lower limit of Δ2 ( Fig.11 ) with respect to XT. Here a2 / a1 is 3.0. From Fig.10 and Fig.11 It can be seen that the upper limit of Δ1 and the lower limit of Δ2 change linearly with respect to the required conditions of XT. eff 80μm 2 The above, according to Fig.10 and Fig.11 The number is 3.

[0306] [Number 3]

[0307] Δ1≤4.93×10 -5 XT-0.00127

[0308] Δ2≥-1.14×10 -5 XT-0.00509 (3)

[0309] Here, Figure 7 and Figure 8 By comparison, if a2 / a1 becomes smaller, the upper limit of Δ1 becomes smaller and the lower limit of Δ2 becomes larger. Therefore, when a2 / a1 is smaller than 3.0, it is included in the range of the upper limit of Δ1 and the lower limit of Δ2 represented by number 3.

[0310] Figure 12 to Figure 14 Is to explain by and Figure 7 to Figure 9The upper limit of Δ1 is given by the intersection of the cutoff wavelength λc (dashed line) and the condition XT (solid line) of the graph produced as shown. Fig.12 ), the lower limit of Δ2 ( Fig.13 ) and the upper limit of Δ2 ( Fig.14 ) is a graph showing the dependence of a2 / a1. The straight lines in each graph are the results of linear approximation.

[0311] Here, the requirement condition of XT is -64dB / km. Figure 12 to Figure 14 It can be seen that the upper limit of Δ1 and the lower limit of Δ2 (Δ 2_min ) and the upper limit of Δ2 (Δ 2_max ) can be approximated relatively well by linear approximation relative to a2 / a1. Figure 12 to Figure 14 ,

[0312] Relative to A eff =80μm 2 , which is 4,

[0313] [Number 4]

[0314]

[0315] Relative to A eff =90μm 2 , is number 5,

[0316] [Number 5]

[0317]

[0318] Relative to A eff =100μm 2 , which is 6.

[0319] [Number 6]

[0320]

[0321] If we take the approximate curve of the coefficients of numbers 4 to 6, we get number 7.

[0322] [Number 7]

[0323]

[0324] Within the parameter range satisfying number 7, an optical fiber with a standard 125 μm cladding diameter, four cores and characteristics equivalent to those of existing optical fibers can be achieved.

[0325] Fig.15 The structural parameters and optical characteristics are designed within the design range of number 7. The 4-core optical fibers of design examples (1) and (2) are A eff 85μm 2 and 102 μm2 , single-mode operation is achieved in the C-band and L-band, and the bending loss is also equal to or less than that of conventional SMF.

[0326] Fig.16 and Fig.17 is relative to Fig.15 The XT characteristics and additional loss of the design example are shown. The wavelength is 1625nm.

[0327] Fig.16 The relationship between the core spacing and XT is shown. In order to obtain an XT of -64 dB / km or less, the core spacings of 41.6 μm or less and 44 μm or less are required for design examples (1) and (2), respectively.

[0328] Fig.17 represents OCT and additional loss α c In order to achieve an additional loss of 0.01 dB / km or less, OCTs of 28.7 μm or less and 30.9 μm or less are required for design examples (1) and (2), respectively.

[0329] From these results, it can be seen that the minimum required cladding diameters are 116.2 μm and 124.0 μm in the four-core optical fibers of Design Example (1) and Design Example (2), respectively. In any design example, four cores can be arranged with a standard cladding diameter of 125 μm.

[0330] (Example)

[0331] Fig.18 This is a photograph of a cross section of the multi-core optical fiber of this example. Fig.19 Indicates the measurement result.

[0332] from Fig.18 From the cross-sectional photograph, it can be seen that the cladding diameter is 125 μm and there are four cores.

[0333] In addition, according to Fig.19 , the refractive index distribution of each core is

[0334] The core radius is 6 μm,

[0335] a2 / a1 is 3,

[0336] Δ1 and Δ2 are -0.6% and -0.4% respectively,

[0337] It can be confirmed that the mathematical formula representing the structural conditions is satisfied.

[0338] As an optical property, it is A eff 80μm 2 The cut-off wavelength is below 1480 nm.

[0339] The transmission loss was 0.18 dB / km or less and 0.25 dB / km or less at wavelengths of 1550 nm and 1625 nm, respectively, and no significant increase in loss was observed on the long wavelength side, so it is considered that sufficient OCT was ensured.

[0340] XT is below -66dB / km at a wavelength of 1625nm, and XT characteristics that can cope with transmission distances of more than 10,000km are obtained in the entire communication wavelength band.

[0341] (Design Method)

[0342] Fig. 20 This is a flowchart showing a method for designing a multi-core optical fiber.

[0343] The multi-core optical fiber has:

[0344] Four cores of radius a1 are arranged in a square lattice along the long side direction;

[0345] a first cladding region of radius a2 having a lower refractive index than the core, and having a refractive index around the core having a relative refractive index difference of Δ1 with the core; and

[0346] A second cladding region having a lower refractive index than the core, at the periphery of the first cladding region, having a greater refractive index than the first cladding region and having a relative refractive index difference with the core of Δ2,

[0347] The design method performs:

[0348] The first step S01, according to the effective cross-sectional area A eff (μm 2 ) and the required condition of the total value XT (dB / km) of the core crosstalk per 1 km, select the combination of the a1 and the Δ1;

[0349] In the second step S02, the a2 and Δ2 that satisfy the cutoff wavelength and bending loss requirements are selected in the combination selected in the first step S01, and the core spacing relationship of the Δ1 and the core spacing Λ relative to the XT and the additional loss α are obtained. c an OCT relationship between the Δ1 and the shortest distance OCT from the outer periphery of the second cladding region to the center of the core that meets the requirement; and

[0350] In the third step S03, the core spacing relationship and the OCT relationship obtained in the second step S02 are used to obtain Calculate the outer diameter of the second cladding region at the Δ1 obtained in the first step And determine the outer diameter Is it less than 125μm?

[0351] When designing a 4-core fiber with a standard 125 μm cladding diameter, first set the A eff , XT, bending loss, and cut-off wavelength requirements (step S00).

[0352] Next, in step S01, Figure 3 to Figure 5 As shown, select eff The combination of a1 and Δ1 of the required conditions. And, Fig.10 and Fig.11 As shown in FIG. 1 , set the conditions of Δ1 corresponding to the desired XT. For example, in A eff 80μm 2 In the above case, in the first step S01, a combination of a1 and Δ1 that satisfies equation 1 and equation 3 is selected.

[0353] Next, in step S02, for example, Figure 6 , Figure 7 to Figure 9 As shown in FIG. 1 , for the previously selected a1 and Δ1, a2 and Δ2 that satisfy the required conditions of bending loss and XT are selected. Specifically, according to Figure 7 to Figure 9 What you get Fig.12 Select a2 from the curve graph, according to Fig.13 and Fig.14 Δ2 is selected from the curve graph. At this point in time, the parameters of the core 10 and the first cladding region 11 are obtained. For example, when a2 / a1 is less than 3.0, in the second step S02, Δ2 that satisfies the number 3 is selected. Since a1, a2, Δ1, and Δ2 are all available at this point in time, Figure 6 Determine the required conditions for XT, the core spacing and OCT that can sufficiently reduce the excess loss, and determine the necessary cladding diameter.

[0354] Finally, in step S03, if the required cladding diameter is 125±1 μm or less, the design is complete. If the cladding diameter is larger than this, the parameters of the core and first cladding regions are reselected to reach 125±1 μm or less (repeatedly starting from step S01).

[0355] (Optical transmission method)

[0356] Fig.21 This is a diagram illustrating an optical transmission method using the above-mentioned multi-core optical fiber. The optical transmission method is characterized in that a multi-core optical fiber 15 with a length of more than 1000 km is used as an optical transmission channel, signal light with a wavelength of more than 1530 nm is transmitted from four transmitters 21 to each core 10 of the multi-core optical fiber 15, and the signal light is received by four receivers 22 for each core 10 of the multi-core optical fiber 15.

[0357] The optical fiber cable 100 has four optical fibers 15, and each of the cores 10 is connected to a transmitter 21 and a receiver 22 on the transmitting side and the receiving side, respectively. As described above, even if the crosstalk of the optical fiber cable 100 is large, it is below -54dB / km, and is suitable for long-distance transmission of thousands of kilometers. Here, it is assumed that there are connection points or optical amplifiers between optical fibers in the laying of the cable, but it can be considered that the crosstalk in the optical fiber is dominant in the transmission channel of thousands of kilometers, and the influence of crosstalk at other connection points or optical components is very small.

[0358] This optical transmission method can replace the single-mode optical fiber of the submarine communication system with a communication distance of more than 1000km with a multi-core optical fiber 15, and send signal light with a wavelength of more than 1530nm from four transmitters 21 to each core 10 of the multi-core optical fiber 15, and each core 10 of the multi-core optical fiber 15 is received by four receivers 22.

[0359] The optical fiber cable uses a multi-core optical fiber 15 with a standard cladding diameter, so the existing optical cable structure can be used, which is preferred.

[0360] Industrial Applicability

[0361] The present invention can be applied to optical fibers in optical communication systems.

[0362] Description of Reference Numerals

[0363] 10: Core

[0364] 11: First cladding region

[0365] 12: Second cladding region

[0366] 21: Transmitter

[0367] 22: Receiver

[0368] 100: Optical cable

Claims

1. A method for designing a multi-core optical fiber, characterized in that: The multi-core optical fiber has: Four cores of radius a1 are arranged in a square lattice along the long side direction; a first cladding region of radius a2 having a lower refractive index than the core, around the core, and having a relative refractive index difference of Δ1 with the core; as well as A second cladding region having a lower refractive index than the core, at the periphery of the first cladding region, having a greater refractive index than the first cladding region and having a relative refractive index difference with the core of Δ2, The design method of the multi-core optical fiber is performed by: The first step is to calculate the effective cross-sectional area A. eff μm 2 The combination of a1 and Δ1 is selected based on the requirement of the total value of the inter-core crosstalk per 1 km, XT dB / km; In the second step, the a2 and the Δ2 that satisfy the cutoff wavelength and bending loss requirements in the combination selected in the first step are selected, and the core spacing relationship of the Δ1 and the core spacing Λ relative to the XT and the additional loss α are obtained. c an OCT relationship between the Δ1 required by the condition and the shortest distance OCT from the outer periphery of the second cladding region to the center of the core; as well as In the third step, the core spacing relationship and the OCT relationship obtained in the second step are used to obtain Calculate the outer diameter of the second cladding region at the Δ1 obtained in the first step And determine the outer diameter Is it less than 125μm? In A eff 80μm 2 In the above cases, In the first step, the combination satisfying formula C1 and formula C2 is selected. [Formula C1] Δ1≤4.93×10 -5 XT-0.00127 (C1) [Formula C2] When a2 / a1 is 3.0 or less, In the second step, the Δ2 is selected to satisfy the formula C3, [Formula C3] Δ2≥-1.14×10 -5 XT-0.00509 (C3)。

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