Multi-core optical fiber and method for designing multi-core optical fiber

Abstract

The present invention is provided with: a plurality of cores (10) arranged in a square lattice or a row along the longitudinal direction of a multi-core optical fiber (1); a plurality of first cladding regions (11) each surrounding the plurality of cores (10) and having a refractive index lower than that of the surrounding core (10); a plurality of second cladding regions (12) that surround each of the plurality of first cladding regions (11) and have a refractive index lower than that of the surrounded first cladding regions (11); and a third cladding region (13) surrounding the plurality of second cladding regions (12) and having a refractive index lower than that of the plurality of cores (10), the mode field diameter and core spacing of the plurality of cores (10) being corresponding to the mode field diameter and core spacing within a region surrounded by the intersection of the first line, the second line, and the third line in a graph representing the relationship between the mode field diameter and core spacing, the first line indicates an upper limit of a core spacing at which excess loss of the core is equal to or less than a predetermined value, the second line indicates a lower limit of the core spacing at which crosstalk between the cores is equal to or less than a predetermined value, and the third line indicates a lower limit of a mode field diameter at which an increase amount of Rayleigh scattering loss of the core is equal to or less than a predetermined value.

Description

Technical Field

[0001] The present disclosure relates to multi-core optical fibers and a method for designing multi-core optical fibers. Background Art

[0002] In recent years, multi-core optical fibers (MCFs) with a standard cladding diameter that have high mass productivity and interchangeability with existing standard technologies have attracted attention. For example, Patent Document 1 and Non-Patent Document 1 disclose MCFs having a trench-type refractive index profile with a strong optical confinement effect. Patent Document 2 discloses an MCF having a step-type refractive index profile suitable for mass productivity.

[0003] In MCFs with a standard cladding diameter, single-mode operation in the entire communication band is ensured in the same manner as conventional single-mode optical fibers, but crosstalk occurs in which optical signals interfere with each other between the cores. Therefore, Patent Document 3 discloses an MCF for long-distance transmission in which crosstalk is reduced by limiting the transmission band of the single-mode operation region of each core to 1.53 μm to 1.625 μm in the C and L bands or 1.46 μm to 1.625 μm in the S, C, and L bands.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: International Publication No. 2022 / 034662

[0007] Patent Document 2: Japanese Patent No. 7172634

[0008] Patent Document 3: Japanese Patent No. 6560806

[0009] Non-Patent Documents

[0010] Non-Patent Document 1: Takashi Matsui, et al., "Design of 125μm cladding multi-core fiber with full-band compatibility to conventional single-mode fiber", Date of Conference: 27 September 2015 - 01 October 2015, DOI: 10.1109 / ECOC.2015.7341966, <URL:https: / / ieeexplore.ieee.org / document / <7341966> Summary of the Invention

[0011] Problems to be Solved by the Invention

[0012] However, in the design of MCFs with standard cladding diameters, the size of the extended mode field diameter (MFD), which represents the intensity distribution of the optical signal across the cross section, is limited to suppress crosstalk and excessive loss in the design. Therefore, increasing the MFD is extremely difficult compared to existing single-core single-mode designs, and there is a tendency for loss to increase as well. For example, in the case of a 4-core MCF, the MFD is as small as 9 μm to 10 μm, while the loss coefficient is as large as 0.155 dB / km to 0.18 dB / km.

[0013] This disclosure was made in view of the above circumstances, and the purpose of this disclosure is to provide a multi-core optical fiber and a design method for multi-core optical fibers that can increase the mode field diameter while ensuring the suppression of crosstalk and excessive loss in the design.

[0014] Methods for solving problems

[0015] One aspect of this disclosure of a multi-core optical fiber comprises: a plurality of cores arranged in a square lattice or a row along the length of the multi-core optical fiber; a plurality of first cladding regions surrounding the plurality of cores, each having a lower refractive index than the surrounded cores; a plurality of second cladding regions surrounding the plurality of first cladding regions, each having a lower refractive index than the surrounded first cladding regions; and a third cladding region surrounding the plurality of second cladding regions, each having a lower refractive index than the plurality of cores, wherein the mode field diameter and core spacing of the plurality of cores are corresponding to the mode field diameter and core spacing within the region enclosed by the intersection of a first line, a second line, and a third line in a graph representing the relationship between the mode field diameter and the core spacing, wherein the first line represents an upper limit of the core spacing for which the excess loss of the cores is below a predetermined value, the second line represents a lower limit of the core spacing for which crosstalk between the cores is below a predetermined value, and the third line represents a lower limit of the mode field diameter for which the increase in Rayleigh scattering loss of the cores is below a predetermined value.

[0016] One embodiment of this disclosure of a multi-core optical fiber comprises: a plurality of cores arranged in a square lattice or a row along the length of the multi-core optical fiber; a plurality of first cladding regions surrounding the plurality of cores, each having a lower refractive index than the cores it surrounds; a plurality of second cladding regions surrounding the plurality of first cladding regions, each having a lower refractive index than the first cladding regions it surrounds; and a third cladding region surrounding the plurality of second cladding regions, each having a lower refractive index than the cores, wherein the diameter of the third cladding region is 125±1μm, the cutoff wavelength is below 1.53μm, the mode field diameter of the plurality of cores at a wavelength of 1.55μm is 9.5μm to 15.0μm, and the core spacing of the plurality of cores is 33μm to 50μm.

[0017] This disclosure discloses a method for designing a multi-core optical fiber, wherein the multi-core optical fiber comprises: a plurality of cores arranged in a square lattice or a row along the length direction of the multi-core optical fiber; a plurality of first cladding regions surrounding the plurality of cores, each having a lower refractive index than the cores they surround; a plurality of second cladding regions surrounding the plurality of first cladding regions, each having a lower refractive index than the first cladding regions they surround; and a third cladding region surrounding the plurality of second cladding regions, each having a lower refractive index than the cores. A computer calculates the mode field diameter and core spacing corresponding to the region enclosed by the intersection of the first line, the second line, and the third line in a graph representing the relationship between mode field diameter and core spacing, as the mode field diameter and core spacing of the plurality of cores, wherein the first line represents an upper limit of core spacing for which the excess loss of the cores is below a predetermined value, the second line represents a lower limit of core spacing for which crosstalk between the cores is below a predetermined value, and the third line represents a lower limit of mode field diameter for which the increase in Rayleigh scattering loss of the cores is below a predetermined value.

[0018] Invention Effects

[0019] According to this disclosure, a technique can be provided that can increase the mode field diameter while ensuring the suppression of crosstalk and excessive losses in the design. Attached Figure Description

[0020] Figure 1 This is a diagram showing the cross-sectional structure of the MCF in the first embodiment.

[0021] Figure 2 This is a diagram showing the refractive index distribution in the region near the fiber core.

[0022] Figure 3 This is a graph showing the relationship between MFD and the increase in Rayleigh scattering loss.

[0023] Figure 4 This is a diagram illustrating the relationship between in-direction crosstalk and bidirectional crosstalk.

[0024] Figure 5 This is a graph showing the relationship between co-directional crosstalk and wavelength.

[0025] Figure 6 This is a diagram showing the relationship between MFD and fiber core spacing.

[0026] Figure 7A This is a graph showing the relationship between MFD and core spacing relative to Δ1.

[0027] Figure 7B This is a graph showing the relationship between MFD and core spacing relative to Δ1.

[0028] Figure 8A This is a graph showing the relationship between MFD and core spacing relative to Δ1.

[0029] Figure 8B This is a graph showing the relationship between MFD and core spacing relative to Δ1.

[0030] Figure 9A This is a graph showing the relationship between MFD and core spacing relative to Δ1.

[0031] Figure 9B This is a graph showing the relationship between MFD and core spacing relative to Δ1.

[0032] Figure 10 This is a diagram showing the functional block structure of the MCF design device.

[0033] Figure 11 This is a diagram illustrating the design methodology of MCF.

[0034] Figure 12 This is a diagram showing the cross-sectional structure of the MCF in the second embodiment.

[0035] Figure 13 This is a diagram showing the hardware structure of the MCF design device. Detailed Implementation

[0036] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. In the drawings, the same symbols are used to refer to the same parts and descriptions are omitted.

[0037] [First Implementation Method]

[0038] (Structure of MCF1)

[0039] Figure 1 This is a diagram showing the cross-sectional structure of the MCF according to the first embodiment. MCF1 is an MCF with a standard cladding diameter of approximately 125 ± 1 μm. MCF1 includes: four fiber cores 10 arranged in a square lattice pattern along the length direction (extension direction) of the paper depth; four first cladding regions 11 surrounding the four fiber cores 10; four second cladding regions 12 surrounding the four first cladding regions 11; and one third cladding region 13 surrounding all of the second cladding regions 12. The diameter of the third cladding region 13 is approximately 125 ± 1 μm.

[0040] Figure 2 This is a graph showing the refractive index distribution in the region near the fiber core. The horizontal axis is the direction of the short side of MCF1 (the radial direction passing through the center of the fiber core 10), and the vertical axis is the relative refractive index difference. The refractive index of the first cladding region 11 is lower than that of the fiber core 10. The refractive index of the second cladding region 12 is lower than that of the first cladding region 11. The refractive index of the third cladding region 13 is lower than that of the four fiber cores 10. Each fiber core 10 has a groove-type refractive index distribution or an equivalent refractive index distribution. The refractive index distributions of each fiber core 10 are approximately the same.

[0041] Hereinafter, the radius of the fiber core 10 is defined as 'a'. The radius of the inner diameter of the second cladding region 12 is defined as 'a1'. The radius of the outer diameter of the second cladding region 12 is defined as 'a2'. The absolute value of the relative refractive index difference between the fiber core 10 and the first cladding region 11 is defined as 'Δ'. The absolute value of the relative refractive index difference between the fiber core 10 and the second cladding region 12 is defined as 'Δ1'. The relative refractive index difference between the fiber core 10 and the third cladding region 13 may be equal to or different from 'Δ'.

[0042] In addition, the transmission band is set to 1.53μm to 1.625μm in the C and L bands or 1.46μm to 1.625μm in the S, C, and L bands. The fiber core 10 is made of pure quartz glass.

[0043] Rayleigh scattering loss

[0044] Rayleigh scattering loss, which is the loss of a single core, is explained.

[0045] In MCF1 with the above structure and refractive index distribution, let the wavelength λ = 1.55 μm, a1 / a = 2.0, a2 / a = 3.0, and Δ1 = 0.6%, with a cutoff wavelength λ... c The values ​​of a and Δ are adjusted to be 1.53 μm, thereby changing the MFD.

[0046] Figure 3 This graph shows the relationship between the MFD (μm) and the increase in Rayleigh scattering loss (dB / km). It can be confirmed that the increase in Rayleigh scattering loss decreases as the MFD increases. This is believed to be because the electric field component contained in the fiber core 10, which is pure quartz glass, increases with the increase in MFD. Furthermore, it is also believed that the decrease in the electric field component leaking from the fiber core 10 to or outside the first cladding region 11 reduces scattering loss and structural misalignment loss at the fiber core-cladding interface.

[0047] according to Figure 3 It can be seen that, for example, if the goal is to reduce the increase in Rayleigh scattering loss to less than 0.003 dB per 1 km, the MFD should be set to approximately 10.8 μm or higher. That is, MFD = 10.8 μm can be used as the lower limit of the MFD for reducing the increase in Rayleigh scattering loss to less than 0.003 dB / km.

[0048] (crosstalk)

[0049] Crosstalk, which is a loss between fiber cores, is explained.

[0050] Figure 4This is a characteristic graph showing the crosstalk relationship between co-directional transmission (transmitting optical signals in the same direction) and bi-directional transmission (transmitting them in opposite directions) between adjacent fiber cores. The horizontal axis represents co-directional crosstalk (dB / 1km) after 1km of transmission, and the vertical axis represents bi-directional crosstalk (dB / 80km) after 80km of transmission.

[0051] Generally, bidirectional transmission has the characteristic of reducing crosstalk compared to unidirectional transmission. Therefore, as... Figure 4 As shown, the crosstalk per 80 km in bidirectional transmission is approximately the same as that per 1 km in co-directional transmission. Furthermore, the crosstalk relationship between co-directional and bidirectional transmission is wavelength-independent.

[0052] Here, we consider reducing crosstalk. For example, to minimize the impact of crosstalk on the received signal during bidirectional transmission, we consider reducing the crosstalk at the receiver to approximately -25 dB.

[0053] In this case, for example, for transmission distances of 1000km and 10000km, the bidirectional crosstalk needs to be set to approximately -35dB and -45dB per 80km, respectively. At this point, according to... Figure 4 The corresponding crosstalk in the same direction needs to be set to approximately -35dB and -45dB per 1km.

[0054] Figure 5 This is a diagram illustrating an example of co-directional crosstalk in MCF1. The horizontal axis represents wavelength (μm), and the vertical axis represents co-directional crosstalk (dB / 1km) after 1km of transmission. The values ​​of a1 / a, a2 / a, and Δ1 are the same as those mentioned above, with a = 5.5μm and Δ = 0.3%.

[0055] according to Figure 5 It can be confirmed that the co-directional crosstalk of MCF1 increases approximately linearly with increasing wavelength. The wavelength corresponding to the aforementioned co-directional crosstalk of approximately -35dB per 1km is approximately 1.625μm. It can be determined that the crosstalk at approximately 1.55μm below this wavelength is reduced by approximately 10dB.

[0056] Furthermore, in terrestrial relay systems with a range of approximately 1000 km, the high-capacity transmission of the L-band is utilized in addition to the C-band. In seabed relay systems ranging from several thousand to 10000 km, due to limitations in transmission power, only the C-band is generally used.

[0057] according to Figure 4 and Figure 5Based on the bands used, by setting the co-directional crosstalk to approximately -35dB or less per kilometer at λ = 1.625μm, sufficiently small crosstalk characteristics below "crosstalk at the receiving end of -25dB" can be obtained in bidirectional transmission at the 1000km level in the C and L bands, and at the thousands to 10000km level in the C band.

[0058] Furthermore, the same applies when reducing the impact of crosstalk on the received signal during co-directional transmission. This is achieved by obtaining bi-directional crosstalk corresponding to the wavelength, based on... Figure 4 The crosstalk relationship between unidirectional and bidirectional transmission shown can reduce the crosstalk at the receiver to below a predetermined level during unidirectional transmission.

[0059] (MCF design methodology (MFD and core spacing calculation methods))

[0060] Next, we will explain the calculation method for ensuring loss suppression in MCF1 and expanding MFD, as well as the core spacing.

[0061] First, calculate the lower limit of the MFD that reduces Rayleigh scattering loss to a predetermined level and the lower limit of the core spacing that reduces crosstalk to a predetermined level. These calculation methods are as described above. Additionally, calculate the upper limit of the core spacing that reduces excess loss in the design to a predetermined level using known methods.

[0062] Next, these calculation results are input into the database representing the relationship between MFD and core spacing. Figure 6 The graph shows the MFD (μm) at a wavelength of 1.55 μm and the core spacing (μm). Here, a1 / a = 2.0, a2 / a = 3.0, Δ1 = 0.6%, and λ = 1.625 μm.

[0063] For example, if λ c =1.53μm, then the upper limit of the fiber core spacing that makes the design excess loss αc below 0.01dB / km is the first line L1, if λ c =1.46μm, then the upper limit of the fiber core spacing that makes the design excess loss αc less than 0.01dB / km is the first line L1'. For setting the lower limit of the fiber core spacing with co-directional crosstalk XT as less than -35dB per 1km, if λ c =1.53μm then becomes the second line L2, if λ c =1.46μm then becomes the second line L2'. The lower limit of the increase in Rayleigh scattering loss ΔαR relative to the pure quartz glass core 10 for the MFD is less than 0.003dB / km, which is the third line L3. Furthermore, the increase in Rayleigh scattering loss ΔαR relative to the cutoff wavelength λ... c Its dependence is small enough.

[0064] according to Figure 6 The MCF1 structure, satisfying the design conditions for reduced excessive loss, co-directional crosstalk, and Rayleigh scattering loss, is defined as follows: for a cutoff wavelength below 1.53 μm, the region is enclosed by the intersection point A of the first line L1 and the second line L2, the intersection point B of the first line L1 and the third line L3, and the intersection point C of the second line L2 and the third line L3. For a cutoff wavelength below 1.46 μm, the region is enclosed by the intersection point A' of the first line L1' and the second line L2', the intersection point B' of the first line L1' and the third line L3, and the intersection point C' of the second line L2' and the third line L3. Therefore, intersection points A, B, C, A', B', and C' are used as the construction conditions for the MCF1.

[0065] Then, the MFD and core spacing corresponding to the regions surrounded by construction conditions A, B, and C are calculated (set) to be the MFD and core spacing of MCF1 with a cutoff wavelength of 1.53 μm or less. Additionally, the MFD and core spacing corresponding to the regions surrounded by construction conditions A', B', and C' are calculated (set) to be the MFD and core spacing of MCF1 with a cutoff wavelength of 1.46 μm or less.

[0066] Furthermore, when the cutoff wavelength is below 1.46 μm, the area is smaller compared to when the cutoff wavelength is below 1.53 μm. Therefore, the MFD and core spacing are smaller, but it can be said that single-mode operation is guaranteed in the S-band. From the viewpoint of expanding the bands used and the stability of Raman amplification for the C-band, it is preferred.

[0067] In this way, by setting the MFD and core spacing corresponding to the region surrounded by construction conditions A, B, C or construction conditions A', B', C', it is possible to ensure the suppression of crosstalk and excessive losses in the design, and to increase the mode field diameter.

[0068] A more detailed explanation of the calculation method is provided.

[0069] For example, change any one or more of a1 / a, a2 / a, and Δ1. Then, calculate the values ​​of the modified construction conditions A, B, C, A', B', and C'. For example, change them to any one of a1 / a = 1.5, 2.0, or 2.5, a2 / a = 2.5, 3.0, or 3.5, or Δ1 = 0.40% to 0.80%.

[0070] The dependencies of the modified construction conditions A, B, C, A', B', C' on Δ1 are shown in the figure. Figure 7A , Figure 7B , Figure 8A , Figure 8B , Figure 9A , Figure 9BThe horizontal axis represents Δ1, the vertical axis on the left represents the MFD (μm) at a wavelength of 1.55μm, and the vertical axis on the right represents the core spacing (μm). Figure 7A , Figure 7B This applies to the cases where a1 / a = 1.5 and a2 / a = 2.5. Figure 8A , Figure 8B This refers to the cases where a1 / a = 2.0 and a2 / a = 3.0. Figure 9A , Figure 9B This applies to the cases where a1 / a = 2.5 and a2 / a = 3.5. Figure 7A , Figure 8A , Figure 9A It refers to the MFD of construction conditions A and A', as well as the core spacing. Figure 7B , Figure 8B , Figure 9B It refers to the MFD and core spacing of construction conditions B, B' and construction conditions C, C'.

[0071] Within the range of Δ1 = 0.40% to 0.80%, the highest and lowest values ​​among the modified construction conditions A, B, C, A', B', and C' are used to calculate (set) the MFD and core spacing. Based on... Figure 7A , Figure 7B , Figure 8A , Figure 8B , Figure 9A , Figure 9B When the cutoff wavelength is below 1.53 μm, the MFD at a wavelength of 1.55 μm is about 9.5 μm to about 15.0 μm, and the core spacing is about 33 μm to about 50 μm.

[0072] By setting the MFD and core spacing to their respective values, when the optical signal is propagated in the same direction for 1 km in all cores, the co-directional crosstalk between cores at a wavelength of 1.625 μm can be reduced to below -35 dB. The MFD setting value is within the range of 9.5 μm to 15.0 μm specified by the international standard "ITU-T G.654 (Category D)" related to single-mode optical fiber.

[0073] By changing any one or more of a1 / a, a2 / a, and Δ1, and setting the corresponding MFD and core spacing within the area surrounded by the changed construction conditions A, B, C or construction conditions A', B', C', the crosstalk and excessive loss in the design can be appropriately suppressed, and the mode field diameter can be appropriately increased.

[0074] (MCF design unit and design flow (MFD and core spacing calculation flow))

[0075] Figure 10This is a diagram showing the functional block structure of the MCF design device 2. The MCF design device 2 is a computer used to design the MCF1, and includes an arithmetic unit 21 for calculating the MFD and core spacing, an output unit 22 for outputting the calculated MFD and core spacing, and a storage unit 23 for storing various data required for calculating the MFD and core spacing.

[0076] Figure 11 This is a diagram showing the calculation process for MFD and core spacing.

[0077] First, the arithmetic unit 21 calculates a lower limit for the increase in Rayleigh scattering loss to be below a predetermined MFD, a lower limit for crosstalk to be below a predetermined core spacing, and an upper limit for design excess loss to be below a predetermined core spacing (step S1).

[0078] Next, the calculation unit 21 inputs the upper limit of the core spacing, the lower limit of the core spacing, and the lower limit of the MFD calculated in step S1 into a graph showing the relationship between the MFD and the core spacing (step S2).

[0079] Next, the arithmetic unit 21 calculates, for each cutoff wavelength, the intersection point A (A') of the first line L1 representing the upper limit of the fiber core spacing and the second line L2 representing the lower limit of the fiber core spacing, the intersection point B (B') of the first line L1 and the third line L3 representing the lower limit of the MFD, and the intersection point C (C') of the second line L2 and the third line L3 (step S3).

[0080] Next, the calculation unit 21 calculates the MFD and core spacing corresponding to the area surrounded by intersections A, B, and C (A', B', and C') in the above diagram, as the MFD and core spacing of MCF1 (step S4). At this time, the calculation unit 21 changes any one or more of a1 / a, a2 / a, and Δ1, and uses the highest and lowest values ​​among the changed values ​​of intersections A, B, and C to calculate the MFD and core spacing of MCF1.

[0081] Finally, the output unit 22 outputs the MFD of MCF1 and the core spacing (step S5).

[0082] (Formulas for calculating MFD and core spacing)

[0083] Based on the above results, the MFD and core spacing of construction conditions A, B, and C can be approximated as shown in equation (1). A W B W C W These are the MFDs for constructing conditions A, B, and C, respectively. A Λ B Λ and C Λ These are the core spacings for construction conditions A, B, and C, respectively.

[0084] [Formula 1]

[0085]

[0086] like Figure 6 As shown, B W and C W They are equal. K Δ1 ~K Δ3 It is a proportionality constant. K is the condition for constructing conditions A, B, and C. Δ1 ~K Δ3 As shown in Table 1.

[0087] [Table 1]

[0088]

[0089] The relationship between MFD and core spacing and Δ1 is as follows: Figure 7A , Figure 7B , Figure 8A , Figure 8B , Figure 9A , Figure 9B As shown, it depends on a1 / a, therefore K is constructed based on conditions A, B, and C. Δ1 ~K Δ3 They can be approximated as in equation (2).

[0090] [Formula 2]

[0091]

[0092] K 1a ~K 1c K 2a ~K 2c K 3a ~K 3c K is a proportionality constant. Construct conditions A, B, and C with respect to K. 1a ~K 1c K 2a ~K 2c K 3a ~K 3c As shown in Table 2.

[0093] [Table 2]

[0094] <![CDATA[A W ]]> <![CDATA[A Λ ]]> <![CDATA[B W ,C W ]]> <![CDATA[B Λ ]]> <![CDATA[C Λ ]]> <![CDATA[K 1a ]]> -267786 699182 20715 -89461 93858 <![CDATA[K 1b ]]> 1424041 -3124798 -131365 518887 -473595 <![CDATA[K 1c ]]> -1554861 2937634 211061 -855785 730568 <![CDATA[K 2a ]]> 2440 -7950 -480 1856 -1499 <![CDATA[K 2b ]]> -13009 35480 2775 -9953 7296 <![CDATA[K 2c ]]> 14698 -34816 -4115 16287 -12524 <![CDATA[K 3a ]]> -6.0 21.3 1.4 -5.8 4.4 <![CDATA[K 3b ]]> 31.2 -94.7 -7.5 28.8 -19.7 <![CDATA[K 3c ]]> -25.2 136.6 22.1 -7.0 77.8

[0095] Furthermore, the MFD and core spacing of construction conditions A', B', and C' can be approximated as in equation (3). A' W B' W C' W These are the MFDs for constructing conditions A', B', and C', respectively. A' Λ B' Λ and C' ΛThese are the core spacings for construction conditions A', B', and C', respectively.

[0096] [Formula 3]

[0097]

[0098] like Figure 6 As shown, B' W and C' w They are all equal. Construct K for conditions A', B', and C'. Δ1 ~K Δ3 As shown in Table 3, K is used to construct conditions A', B', and C'. Δ1 ~K Δ3 They are the same as in equation (2). Construct K for conditions A', B', and C'. 1a ~K 1c K 2a ~K 2c K 3a ~K 3c As shown in Table 4.

[0099] [Table 3]

[0100]

[0101] [Table 4]

[0102] <![CDATA[A’ W ]]> <![CDATA[A’ Λ ]]> <![CDATA[B’ W ,C’ W ]]> <![CDATA[B’ Λ ]]> <![CDATA[C’ Λ ]]> <![CDATA[K 1a ]]> -34012 43309 13234 -30443 105834 <![CDATA[K 1b ]]> 467994 -338612 -103738 247577 -496611 <![CDATA[K 1c ]]> -608285 38133 180207 -536187 726884 <![CDATA[K 2a ]]> -194 -1144 -371 1093 -1583 <![CDATA[K 2b ]]> -2478 6763 2369 -6337 7150 <![CDATA[K 2c ]]> 4558 -4778 -3675 12001 -12010 <![CDATA[K 3a ]]> 1.5 4.4 0.9 -3.3 4.2 <![CDATA[K 3b ]]> 1.7 -24.4 -5.4 15.8 -16.8 <![CDATA[K 3c ]]> 2.1 62.9 19.9 6.7 74.8

[0103] (Effect)

[0104] According to the first embodiment, in a graph showing the relationship between MFD and fiber core spacing, the MFD and fiber core spacing corresponding to the region enclosed by the intersection of a first line representing an upper limit of fiber core spacing that reduces excess loss of the fiber core to a predetermined level, a second line representing a lower limit of fiber core spacing that reduces crosstalk between fiber cores to a predetermined level, and a third line representing a lower limit of MFD that reduces the increase in Rayleigh scattering loss of the fiber core to a predetermined level, are calculated as the MFD and fiber core spacing of the fiber core. Therefore, a technique can be provided that increases MFD while ensuring suppression of crosstalk and excess loss in the design. Since MFD can be increased, this is also preferred from the viewpoint of suppressing nonlinear effects in the optical fiber transmission path.

[0105] That is, in an MCF with a standard cladding diameter, it can ensure the crosstalk characteristics required for long-distance / bidirectional transmission of more than 1000km and reduce excessive loss including the L-band, while also reducing losses caused by Rayleigh scattering due to the extension of MFD and suppressing nonlinear effects.

[0106] Furthermore, according to the first embodiment, the mode field diameter and core spacing within the region after calculating and changing any one or more of the following: the ratio of the inner diameter radius of the second cladding region to the core radius, the ratio of the outer diameter radius of the second cladding region to the core radius, and the relative refractive index difference between the core and the second cladding region, are used as the MFD and core spacing of the core. Therefore, it is possible to provide a technique that appropriately ensures the suppression of crosstalk and excessive losses in the design, and can appropriately expand the MFD.

[0107] Furthermore, according to the first embodiment, since the MFD and core spacing are calculated using an approximation of Equation (1) or Equation (3), it is possible to provide a technique that more appropriately and easily ensures the suppression of crosstalk and excessive losses in the design, and can more appropriately and easily expand the MFD.

[0108] Furthermore, according to the first embodiment, when the cutoff wavelength is below 1.53 μm, the mode field diameter at a wavelength of 1.55 μm is 9.5 μm to 15.0 μm, and the core spacing is 33 μm to 50 μm. Therefore, it is possible to provide a technique that can more appropriately and easily ensure the suppression of crosstalk and excessive loss in the design, and can more appropriately and easily expand the MFD.

[0109] [Second Implementation]

[0110] Figure 12 This is a diagram showing the cross-sectional structure of the MCF1 according to the second embodiment. Similar to the first embodiment, the MCF1 is an MCF with a standard cladding diameter of approximately 125 ± 1 μm. Two fiber cores 10 are arranged in a row along the length direction. The two fiber cores 10 possess... Figure 2 The grooved refractive index distribution shown is or is equivalent to it. The refractive index distributions of each fiber core 10 are approximately the same.

[0111] In the 2-core MCF1, with Figure 6 Similarly, consider a construction that simultaneously suppresses excessive losses, crosstalk, and Rayleigh scattering losses in the design. In this case, the increase in crosstalk as the loss between adjacent fiber cores and the increase in Rayleigh scattering loss as the loss of each fiber core are constant and independent of the number of fiber cores. Therefore, similar to the 4-fiber core construction of MCF1, a region surrounded by three intersection points A, B, and C, or a region surrounded by three intersection points A', B', and C', is obtained.

[0112] On the other hand, regarding the excessive loss in the design, based on the cladding thickness defined by the shortest distance from the center of each fiber core to the cladding end, in the standard cladding diameter MCF1, if the cladding thickness is set to OCT, the fiber core spacing is set to Λ, and the cladding diameter is set to D, then the relationship of equation (4) exists.

[0113] [Formula 4]

[0114]

[0115] The 2 in Λ2 is the identifier for a 2-core structure. The 4 in Λ4 is the identifier for a 4-core structure.

[0116] OCT and D are the same in both the 2-core and 4-core structures. Therefore, according to equation (4), the upper limit of the core spacing in the 2-core structure for suppressing excessive loss is relative to that in the 4-core structure. Therefore, in the case of a 2-core structure, in Figure 6 In the middle, the line connecting intersection points A and B, and the line connecting intersection points A' and B', are moved upwards along the vertical axis. times.

[0117] Therefore, in a 2-core structure, the design excess loss αc is below 0.01 dB / km (λ = 1.625 μm), the co-directional crosstalk XT is below -35 dB per 1 km (λ = 1.625 μm), and the Rayleigh scattering loss increment ΔαR is below 0.003 dB / km relative to the pure quartz glass core 10. W-2 B W-2 C W-2 A Λ-2 B Λ-2 C Λ-2 Equation (1) can be approximated as Equation (5) using Equation (1).

[0118] [Formula 5]

[0119]

[0120] In addition, construct condition A' W-2 B' W-2 C' W-2 A' Λ-2 B' Λ-2 C' Λ-2 Equation (3) can be approximated as Equation (6).

[0121] [Formula 6]

[0122]

[0123] According to the second embodiment, since the MFD and core spacing are calculated using an approximation of Equation (5) or Equation (6), it is possible to provide a more appropriate and convenient way to ensure the suppression of crosstalk and excessive losses in the design, and to more appropriate and conveniently expand the MFD.

[0124] [Third Implementation Method]

[0125] (Covering layer)

[0126] To protect the optical fiber, the standard optical fiber is coated with a coating layer of UV resin or the like, with a diameter of approximately 250 ± 15 μm, relative to a cladding diameter of 125 μm. Therefore, in the MCF1 of the first and second embodiments, the cladding can also be applied with a coating layer of the same diameter. This makes it suitable in size for existing optical cables, connector interfaces, etc., with the same diameter, and is therefore preferred.

[0127] Alternatively, it can be clad in accordance with the values ​​specified in the international standard "IEC 60793-2-50" related to single-mode fiber. That is, it can be clad with a diameter of approximately 200±20μm. This allows for a significant expansion of the overall core count, density, and transmission capacity, making it a preferred option.

[0128] (Bending loss)

[0129] Considering the installation on existing optical communication systems, MCF1 preferably meets the bending loss conditions specified in the international standards "ITU-TG.652" and "ITU-TG.654 (Category E)" related to single-mode optical fiber at a wavelength of 1.625μm and a bending radius of 30mm, specifically below 0.1dB / 100turn.

[0130] According to the third embodiment, since the diameter of the MCF1 covered by the coating layer is set to about 250±15μm or about 200±20μm, it is possible to provide an MCF1 that is suitable for international standards, etc.

[0131] [other]

[0132] This disclosure is not limited to the embodiments described above. The first to third embodiments can also be combined. Various modifications can be made within the scope of this disclosure. For example, the fiber core 10 can be any combination of multiple fiber cores arranged in a square grid or a row along the length of the multi-core optical fiber. It can be a total of 9 fiber cores arranged in a 3×3 square grid, or 3 fiber cores arranged in a row.

[0133] For example, such as Figure 13 As shown, the aforementioned MCF design device 2 can be implemented using a general-purpose computer system including a CPU 901, a memory 902, a storage device 903, a communication device 904, an input device 905, and an output device 906. The memory 902 and storage device 903 are storage devices. In this computer system, the CPU 901 executes a predetermined program loaded on the memory 902, thereby implementing the various functions of the MCF design device 2.

[0134] MCF design device 2 can be implemented by a single computer. MCF design device 2 can be implemented by multiple computers. MCF design device 2 can be a virtual machine installed on a computer.

[0135] The program used by the MCF design device 2 can be stored on computer-readable recording media such as HDDs, SSDs, USB storage devices, CDs, and DVDs. Computer-readable recording media include, for example, non-transient recording media. The program used by the MCF design device 2 can also be distributed via communication networks.

[0136] Symbol Explanation

[0137] 1MCF

[0138] 10 fiber cores

[0139] 11 First cladding area

[0140] 12 Second cladding area

[0141] 13 Third cladding area

[0142] 2MCF design device,

[0143] 21. Computational Unit

[0144] 22 Output Section

[0145] 23 Storage Department

[0146] 901 CPU

[0147] 902 memory,

[0148] 903 storage device,

[0149] 904 communication device,

[0150] 905 input device

[0151] 906 output device.

Claims

1. A multi-core optical fiber, characterized in that, have: Multiple fiber cores are arranged in a square lattice or a row along the length of the multi-core optical fiber; Multiple first cladding regions, each surrounding a plurality of fiber cores, have a lower refractive index than the surrounding fiber cores; Multiple second cladding regions, each surrounding the multiple first cladding regions, have a lower refractive index than the surrounded first cladding regions; as well as The third cladding region, which surrounds the plurality of second cladding regions, has a lower refractive index than the plurality of fiber cores. The mode field diameter and core spacing of the plurality of fiber cores are the corresponding mode field diameter and core spacing in the region enclosed by the intersection of the first line, the second line and the third line in the graph showing the relationship between the mode field diameter and the core spacing, wherein the first line represents the upper limit of the core spacing that makes the excess loss of the fiber core below a predetermined value, the second line represents the lower limit of the core spacing that makes the crosstalk between the fiber cores below a predetermined value, and the third line represents the lower limit of the mode field diameter that makes the increase in Rayleigh scattering loss of the fiber core below a predetermined value.

2. The multi-core optical fiber according to claim 1, characterized in that, The plurality of fiber cores are four fiber cores arranged in a square lattice pattern along the length direction of the multi-core optical fiber. The diameter of the third cladding region is 125±1μm. The cutoff wavelength is below 1.53 μm. When the identifier for the mode field diameter at a wavelength of 1.55 μm is set to W, the identifier for the fiber core spacing is set to Λ, the radius of the fiber core is set to a, and the radius of the inner diameter of the second cladding region is set to a1, the mode field diameter A corresponding to the intersection point A of the first line and the second line is... W and fiber core spacing A Λ The mode field diameter B corresponding to the intersection point B of the first line and the third line. W and fiber core spacer B Λ The mode field diameter C corresponding to the intersection point C of the second line and the third line. W and fiber core spacing C Λ For the following formula 7 and Table 5: [Formula 7] [Table 5] 。 3. The multi-core optical fiber according to claim 1, characterized in that, The plurality of fiber cores are four fiber cores arranged in a square lattice pattern along the length direction of the multi-core optical fiber. The diameter of the third cladding region is 125±1μm. The cutoff wavelength is below 1.46 μm. When the identifier for the mode field diameter at a wavelength of 1.55 μm is set to W, the identifier for the core spacing is set to Λ, the radius of the core is set to a, and the radius of the inner diameter of the second cladding region is set to a1, the mode field diameter A' corresponding to the intersection point A' of the first line and the second line is... W and fiber core spacing A' Λ The mode field diameter B' corresponding to the intersection point B' of the first line and the third line W and fiber core spacing B' Λ The mode field diameter C' corresponding to the intersection point C' of the second line and the third line W and fiber core spacing C' Λ The following formulas and Table 6 are provided: [Formula 8] [Table 6] 。 4. The multi-core optical fiber according to claim 1, characterized in that, The plurality of fiber cores are two fiber cores arranged in a row along the length direction of the multi-core optical fiber. The diameter of the third cladding region is 125±1μm. The cutoff wavelength is below 1.53 μm. The identifier for the mode field diameter at a wavelength of 1.55 μm is set to W-2, the identifier for the core spacing is set to Λ-2, the radius of the core is set to a, the radius of the inner diameter of the second cladding region is set to a1, and the identifier for the mode field diameter at a wavelength of 1.55 μm when there are 4 cores is set to W, the identifier for the core spacing is set to Λ, and the mode field diameter corresponding to the intersection point A of the first line and the second line is set to A. W The fiber core spacing is set to A. Λ Let the diameter of the mode field corresponding to the intersection point B of the first line and the third line be B. W The fiber core spacing is set to B. Λ Let the diameter of the mode field corresponding to the intersection point C of the second line and the third line be C. W The fiber core spacing is set to C. Λ hour, The mode field diameter A corresponding to the intersection point A W-2 and fiber core spacing A Λ-2 The mode field diameter B corresponding to the intersection point B W-2 and fiber core spacing B Λ-2 The mode field diameter C corresponding to the intersection point C W-2 and fiber core spacing C Λ-2 For the following numerical formula 9 and Table 7: [Formula 9] B W-2 =C W-2 =B W =C W C Λ-2 =CA in, [Table 7] 。 5. The multi-core optical fiber according to claim 1, characterized in that, The plurality of fiber cores are two fiber cores arranged in a row along the length direction of the multi-core optical fiber. The diameter of the third cladding region is 125±1μm. The cutoff wavelength is below 1.46 μm. The identifier for the mode field diameter at a wavelength of 1.55 μm is set as W-2, the identifier for the fiber core spacing is set as Λ-2, the radius of the fiber core is set as a, and the radius of the inner diameter of the second cladding region is set as a1. The identifier for the mode field diameter at a wavelength of 1.55 μm when there are four fiber cores is set as W, the identifier for the fiber core spacing is set as Λ, and the mode field diameter corresponding to the intersection point A' of the first line and the second line is set as A'. W The fiber core spacing is set to A' Λ Let the diameter of the mode field corresponding to the intersection point B' of the first line and the third line be B'. W The fiber core spacing is set to B' Λ Let the diameter of the mode field corresponding to the intersection point C' of the second line and the third line be C'. W The fiber core spacing is set to C' Λ hour, The mode field diameter A' corresponding to the intersection point A' W-2 and fiber core spacing A' Λ-2 The mode field diameter B' corresponding to the intersection point B' W-2 and fiber core spacing B' Λ-2 The mode field diameter C' corresponding to the intersection point C' W-2 and fiber core spacing C' Λ-2 For the following formula 10 and Table 8: [Formula 10] B′ W-2 =C′ W-2 =B′ W =C′ W C′ Λ-2 =C′ Λ in, [Table 8] 。 6. The multi-core optical fiber according to claim 1, characterized in that, The diameter of the third cladding region is 125±1μm. The diameter of the multi-core optical fiber surrounded by the cladding layer is 200±20μm.

7. A multi-core optical fiber, characterized in that, have: Multiple fiber cores are arranged in a square lattice or a row along the length of the multi-core optical fiber; Multiple first cladding regions, each surrounding a plurality of fiber cores, have a lower refractive index than the surrounding fiber cores; Multiple second cladding regions, each surrounding the multiple first cladding regions, have a lower refractive index than the surrounded first cladding regions; as well as The third cladding region, which surrounds the plurality of second cladding regions, has a lower refractive index than the plurality of fiber cores. The diameter of the third cladding region is 125±1μm. The cutoff wavelength is below 1.53 μm. The mode field diameters of the multiple fiber cores at a wavelength of 1.55 μm range from 9.5 μm to 15.0 μm. The spacing between the multiple fiber cores is 33μm to 50μm.

8. A design method for multi-core optical fibers, characterized in that, The multi-core optical fiber has the following characteristics: Multiple fiber cores are arranged in a square lattice or a row along the length of the multi-core optical fiber; Multiple first cladding regions, each surrounding a plurality of fiber cores, have a lower refractive index than the surrounding fiber cores; Multiple second cladding regions, each surrounding the multiple first cladding regions, have a lower refractive index than the surrounded first cladding regions; as well as The third cladding region, which surrounds the plurality of second cladding regions, has a lower refractive index than the plurality of fiber cores. The computer calculates the mode field diameter and core spacing within the region enclosed by the intersection of a first line, a second line, and a third line in a graph representing the relationship between mode field diameter and core spacing. These values ​​are used as the mode field diameter and core spacing of the plurality of cores. The first line represents an upper limit of core spacing that minimizes excess loss of the cores below a predetermined value. The second line represents a lower limit of core spacing that minimizes crosstalk between cores below a predetermined value. The third line represents a lower limit of mode field diameter that minimizes the increase in Rayleigh scattering loss of the cores below a predetermined value.

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