Multi-core optical fiber

Abstract

A multicore optical fiber includes: a group of M (M is a positive integer of 1 or more) core regions arranged in a straight line in a cross section; a cladding region surrounding the plurality of core regions, having a lower refractive index than the plurality of core regions; and a coating region surrounding the cladding region, the multicore optical fiber being characterized in that the plurality of core regions are respectively arranged in line symmetry with respect to two imaginary lines orthogonal at the center of the cladding region, a diameter of the cladding region is 180 μm or less, and a diameter of the coating region is 235 μm or more and 265 μm or less.

Description

Technical Field

[0001] This invention relates to a multi-core optical fiber with multiple cores. Background Technology

[0002] The rapid development of high-speed and miniaturized optical transceivers has led to the utilization of parallel transmission using multiple optical channels. Generally, multi-core optical fibers with ribbon cores are used, but for higher density, narrow-pitch ribbon cores using thin-diameter optical fibers have been studied.

[0003] Furthermore, for further high-density applications, the application of multi-core fiber (MCF), which has multiple cores within a single optical fiber, has been investigated. While laser arrays and photodetector arrays in optical transceivers have achieved high densities at the tens of μm level through advanced technologies such as silicon photonics, there are limits to the reduction of optical fiber diameter, requiring optical converters for connections. On the other hand, MCF allows for core configuration with spacing of tens of μm, enabling direct connection to high-density laser arrays and photodetector arrays, thus facilitating high-density, low-loss optical cabling.

[0004] Existing technical documents

[0005] Patent Document 1: Japanese Patent Publication No. 6560806

[0006] Patent Document 2: Japanese Patent Publication No. 2020-115191

[0007] Non-patent document 1: T. Matsui, et al., "Design of multi-core fiber in 125μm cladding diameter with full compliance to conventional SMF", in Proc.ECOC, Valencia, Spain, Sep. 2015, We.4.3.

[0008] Non-patent literature 2: M.-J., Li, et al., “Multicore Fiber for Optical Interconnect Applications”, in Proc. OECC, Busan, Korea, July 2012, 5E4-2.

[0009] Non-patent document 3: T.Hayashi et al., "End-to-End Multi-Core FibreTransmission Link Enabled by Silicon Photonics Transceiver with GratingCoupler Array", in Proc.ECOC, Gothenburg, Sweden, Sep. 2017.

[0010] Non-patent document 4: https: / / www.fujikura.co.jp / rd / gihou / backnumber / pages / _icsFiles / afieldfile / 2017 / 06 / 06 / 130_R2.pdf

[0011] However, as disclosed in Patent Documents 1 and 2 and Non-Patent Document 1, typical multi-core optical fibers have hexagonally close-packed cores, which differs from the configuration of lasers and photodetector arrays within optical transceivers, necessitating the use of optical converters. Non-Patent Document 2's multi-core optical fiber, due to its sufficient core spacing, results in a very large cladding diameter, making it incompatible with existing cable and other related technologies. Non-Patent Document 3's multi-core optical fiber is optimized for a wavelength of 1.31 μm, but suffers from increased loss, particularly at longer wavelengths, and lacks compatibility with existing optical fibers, thus limiting its usable wavelength range.

[0012] Furthermore, when considering direct coupling with a laser array, as shown in Non-Patent Document 4, a sufficiently small beam diameter is preferred. However, the multi-core optical fibers described in Patent Documents 1 and 2, and Non-Patent Documents 1, 2, and 3 have a relatively large mode field diameter relative to the laser beam diameter. To obtain good coupling characteristics, a spot size converter is required, posing challenges in reducing loss and achieving high density. Summary of the Invention

[0013] Therefore, the purpose of this invention is to provide a multi-core optical fiber with excellent connectivity and high density to laser arrays and photodetector arrays.

[0014] To achieve the above objectives, the multi-core optical fiber of the present invention comprises:

[0015] A group of M (M is a positive integer greater than or equal to 1) core regions arranged in a straight line within the cross section, comprising N (N is a positive integer greater than or equal to 2) core regions;

[0016] A cladding region, surrounding the plurality of core regions, has a lower refractive index than the plurality of core regions; and

[0017] The cladding area surrounds the cladding region.

[0018] The multi-core optical fiber is characterized in that...

[0019] The plurality of core regions are arranged linearly symmetrically relative to two imaginary lines orthogonal to the center of the cladding region.

[0020] The diameter of the cladding region is less than 180 μm.

[0021] The diameter of the covered area is greater than 235 μm and less than 265 μm.

[0022] The present invention provides a multi-core optical fiber with excellent connectivity and high density to laser arrays and photodetector arrays. Attached Figure Description

[0023] Figure 1A This is a schematic diagram showing the cross-sectional structure of a multi-core optical fiber.

[0024] Figure 1B This is a schematic diagram showing the cross-sectional structure of a multi-core optical fiber.

[0025] Figure 2A This is a diagram showing the refractive index distribution in the core region of a multi-core optical fiber.

[0026] Figure 2B This is a diagram showing the refractive index distribution in the core region of a multi-core optical fiber.

[0027] Figure 3 This is a diagram showing the relationship between the MFD and the core region of a multi-core optical fiber.

[0028] Figure 4 This is a graph showing the relationship between the core spacing and XT of a multi-core optical fiber.

[0029] Figure 5 This is a diagram showing the relationship between the MFD and the core region of a multi-core optical fiber.

[0030] Figure 6 This is a graph showing the relationship between the core spacing and XT of a multi-core optical fiber.

[0031] Figure 7 This is a diagram illustrating the core structure of a multi-core optical fiber.

[0032] Figure 8 This is a graph showing the relationship between the cladding thickness and confinement loss of a multi-core optical fiber.

[0033] Figure 9 This is a graph showing the relationship between the core spacing and XT of a multi-core optical fiber.

[0034] Figure 10 This is a graph representing the minimum cladding diameter of the group M of a multi-core optical fiber relative to the core region.

[0035] Figure 11 This is a diagram representing the minimum cladding diameter of a multi-core optical fiber relative to the number N of cores arranged in a straight line. Detailed Implementation

[0036] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments shown below. These embodiments are merely illustrative, and the present invention can be implemented in various ways with modifications and alterations based on the knowledge of those skilled in the art. Furthermore, in this specification and the accompanying drawings, the same reference numerals denote identical constituent elements.

[0037] (Implementation Method 1)

[0038] Figure 1A and Figure 1B A schematic diagram illustrating the cross-sectional structure of the multi-core optical fiber of the present invention is shown. Figure 1A , Figure 1B In the diagram, 11 is the core region, 12 is the cladding region, and 13 is the covering region.

[0039] exist Figure 1A , Figure 1B In this multi-core optical fiber, there is a core region 11, a cladding region 12, and a covering region 13. The cladding region 12 surrounds the core region 11 and has a lower refractive index than the core region 11. The covering region 13 surrounds the cladding region 12.

[0040] exist Figure 1A In a multi-core optical fiber, there is a group of N (N is a positive integer greater than 2) core regions on a straight line passing through the center of the cladding region within the cross section. Figure 1B This includes a group of N core regions arranged in a straight line within the cross-section of a multi-core optical fiber, comprising M (where M is a positive integer greater than or equal to 1). Figure 1A , Figure 1B In the diagram, the two dashed lines are two imaginary lines orthogonal to the center of the cladding region. Any number of core regions are arranged symmetrically with respect to these two imaginary lines.

[0041] Laser arrays and photodetector arrays are configured in a linear fashion, or linear arrays are configured in layers. Through... Figure 1A , Figure 1B The core configuration enables the multi-core optical fiber of this invention to be directly connected to laser arrays and photodetector arrays.

[0042] The diameter of the cladding region is 235 μm or more and 265 μm or less, and the diameter of the cladding layer is 180 μm or less. By making the diameter of the cladding region 235 μm or more and 265 μm or less, which is the same standard as existing optical fibers, the multi-core optical fiber of the present invention can be applied to existing optical cables. Furthermore, if we consider an optical fiber with a cladding region diameter of 180 μm or more and 220 μm or less relative to a cladding region diameter of 125 μm, then the cladding thickness only needs to be at least 27.5 μm or more. On the other hand, if we consider the lower limit of the diameter of the cladding region of the optical fiber of the present invention as 235 μm, then in order to make the cladding thickness 27.5 μm or more, it is only necessary to make the diameter of the cladding region 180 μm or less.

[0043] Furthermore, if the diameter of the cladding region is 125±1μm, it is equal to the diameter of the cladding region of existing optical fibers, which is more preferable. In this case, the diameter of the cladding region can be either 235μm or more and 265μm or less, as is generally the case, or 180μm or more and 220μm or less, as described above.

[0044] As described above, the multi-core optical fiber of the present invention can provide a multi-core optical fiber with excellent connectivity and high density to laser arrays and photodetector arrays.

[0045] (Implementation Method 2)

[0046] Figure 2A , Figure 2B This indicates the refractive index distribution in the core region of the multi-core optical fiber of the present invention. Figure 2A It is a step-type refractive index distribution with a central core of radius a and relative refractive index difference Δ. Figure 2A The multi-core optical fiber structure shown exhibits excellent manufacturability and stability. Figure 2B It is a groove-type refractive index distribution, with grooves of width d and a lower relative refractive index difference Δt than the cladding region around the central core of radius a, at a position a1 away from the center. Figure 2B The structure of the multi-core fiber shown exhibits excellent light confinement, enabling reduced XT (Crosstalk) and high-density core configuration in the multi-core fiber.

[0047] if Figure 2A , Figure 2B The optical characteristics of each core shown are compatible with existing optical fibers, so the multi-core optical fiber of the present invention can be wired in the same way as existing optical fibers.

[0048] Figure 3 This indicates the relationship between the mode field diameter (MFD) and the core region of the multi-core optical fiber of the present invention. Figure 3In the diagram, the horizontal axis represents the MFD at a wavelength of 1.31 μm, and the vertical axis represents the outer cladding thickness (OCT) or the inter-center distance between core regions (core spacing Λ). OCT refers to the shortest distance from the center of the core region closest to the cladding region to the cladding region itself. Here, the refractive index distribution of each core is step-type, with the core structure configured such that the cutoff wavelength is below 1.26 μm. The solid line represents the OCT for each MFD, limiting the loss to below 0.01 dB / km at a wavelength of 1.625 μm.

[0049] If the OCT is a value greater than the solid line, additional losses can be adequately suppressed throughout the communication band. The dashed and dotted lines represent the core spacing required to achieve M=1 and M=2 core configurations respectively for N=4 when the OCT condition is met, with a cladding diameter below 180μm. The MFD of general-purpose single-mode fiber is 8.6μm to 9.2μm at a wavelength of 1.31μm; therefore, considering connectivity with single-mode fiber, if the MFD is greater than 8.6μm, then from... Figure 3 It can be seen that for M=1, the distance between the centers of the core regions needs to be less than 36.2μm, and for M=2, the distance between the centers of the core regions needs to be less than 34.5μm.

[0050] Figure 4 This indicates the relationship between the core spacing and XT of the multi-core optical fiber of the present invention. Figure 4 In the figure, the horizontal axis represents the distance between the centers of the core regions (core spacing Λ), and the vertical axis represents XT. Here, the core structure is step-type, with an MFD of 8.6 μm at a wavelength of 1.31 μm and a cutoff wavelength below 1.26 μm. The solid line, dashed line, and dotted line in the figure represent wavelengths of 1.625, 1.55, and 1.31 μm, respectively.

[0051] according to Figure 3 When the center-to-center distance of the core region is less than 36.2 μm and 34.5 μm, the XT values ​​at wavelength 1.625 μm are above -11 dB / km and above -6 dB / km, respectively. In the IM-DD (Intensity Modulation-Direct Detection) method, if an allowable XT value of around -15 dB is considered, it can be seen that for M=1 and 2, the transmission distance can be applied to a maximum of approximately 300 m and 100 m, respectively, across the entire frequency band. Here, as... Figure 4 As shown, a lower XT is obtained at shorter wavelengths than 1.625 μm. For example, at wavelengths of 1.31 and 1.55 μm, the effect of XT can be ignored even at transmission distances of several kilometers or more.

[0052] Figure 5 This illustrates the relationship between the MFD and the core region of the multi-core optical fiber of the present invention. Figure 5 In the diagram, the horizontal axis represents the MFD at a wavelength of 1.31 μm, and the vertical axis represents the cladding thickness (OCT) or the inter-center distance of the core regions (core spacing Λ). Here, the refractive index distribution of each core is grooved, and the core structure is configured such that the cutoff wavelength is below 1.26 μm. Figure 2B In this context, a1 / a is set to 2.5, d / a to 1, and Δt to -0.7%. The solid line represents the cladding thickness (OCT) relative to the limiting loss of each MFD at a wavelength of 1.625 μm, which is below 0.01 dB / km.

[0053] If the OCT is a value greater than the solid line, additional losses can be sufficiently suppressed throughout the communication band. The dashed and dotted lines represent the center-to-center distance (core spacing Λ) of the core regions required to achieve core configurations of M=1 and M=2, respectively, with N=4 and a cladding diameter of less than 180 μm, when the OCT condition is met. Figure 5 It can be seen that for M=1, the center-to-center distance (core spacing Λ) of the core regions needs to be less than 38.5μm, and for M=2, the center-to-center distance (core spacing Λ) of the core regions needs to be less than 36.5μm.

[0054] Figure 6 This indicates the relationship between the core spacing and XT of the multi-core optical fiber of the present invention. Figure 6 In the diagram, the horizontal axis represents the center-to-center distance of the core regions (core spacing Λ), and the vertical axis represents XT. Here, the core structure is trench type, with an MFD of 8.6 μm at a wavelength of 1.31 μm and a cutoff wavelength below 1.26 μm.

[0055] according to Figure 5 When the center-to-center distance of the core region is below 38.5 μm and below 36.5 μm, the XT at a wavelength of 1.625 μm is above -45 dB / km and above -39 dB / km, respectively. If we consider obtaining an even lower XT at a shorter wavelength than 1.625 μm, then even with a transmission distance of over 10 km when using a trench type, the effect of XT can be ignored.

[0056] As explained above, the multi-core optical fiber of the present invention provides a high-density multi-core optical fiber with excellent connectivity to laser arrays and photodetector arrays. Furthermore, low-loss optical interconnects can be achieved using the multi-core optical fiber of the present invention.

[0057] (Implementation Method 3)

[0058] use Figure 7 The core structure of the multi-core optical fiber of the present invention is explained. In Figure 7In the diagram, the horizontal axis represents the core radius *a*, and the vertical axis represents the relative refractive index difference *Δ* between the core and cladding regions. The refractive index distribution in the core region of the multi-core optical fiber of the present invention, as shown in Embodiment 3, is step-type.

[0059] In short optical interconnects of several tens of centimeters within the board, it is envisioned that the laser array and the optical fiber used for wiring can be directly connected. According to Non-Patent Document 4, if the MFD of the optical fiber is about 4 μm or less, it can be coupled to the laser array with high efficiency.

[0060] exist Figure 7 In the diagram, the solid line represents a core structure with an MFD of 4μm at a wavelength of 1.31μm. In the region further to the left of the solid line, the MFD is below 4μm. The dashed line represents a core structure with a cutoff wavelength of 1.26μm. In the region further to the left of the dashed line, single-mode operation is achieved in the communication band (wavelengths above 1.26μm and below 1.625μm). Therefore, in the upper left region enclosed by the solid and dashed lines, an MFD below 4μm can be achieved, and single-mode operation in the aforementioned communication band can be obtained. More specifically, this becomes... Figure 7 The area inside the polygon enclosed by the ● mark (black circle mark).

[0061] That is, if the core structure is set to be composed of Figure 7 The upper left region of the figure enclosed by the solid and dashed lines, especially where a is below 1.9 μm and Δ is above 1.8%, can improve the coupling efficiency with the laser array.

[0062] Figure 8 This diagram illustrates the relationship between the cladding thickness and confinement loss of the multi-core optical fiber of this invention. The horizontal axis represents the cladding thickness (OCT), and the vertical axis represents the confinement loss. The core structure has an MFD of 4 μm, a = 1.9 μm, Δ = 1.8%, and a wavelength of 1.625 μm. From... Figure 8 It is known that as the OCT increases, the confinement loss decreases. Here, if the confinement loss is below 0.01 dB / km, it can be considered sufficiently small compared to the inherent loss of the optical fiber. Therefore, according to... Figure 8 The OCT needs to be 18μm or higher. Here, since shorter wavelengths result in lower confinement loss, a low confinement loss is obtained across the entire communication band based on the above conditions. Furthermore, since a smaller MFD results in lower confinement loss, by setting the OCT to 18μm or higher, a similar result is obtained when the MFD is less than 4μm. Figure 8 The same or lower level of limitation loss.

[0063] Figure 9 This indicates the relationship between the core spacing and XT of the multi-core optical fiber of the present invention. Figure 9 In the diagram, the horizontal axis represents the distance between the centers of the core regions (core spacing Λ), and the vertical axis represents XT. The core structure and wavelength are related... Figure 8 same.

[0064] XT decreases linearly with increasing core spacing. Here, if we assume the transmission distance of the optical interconnect using the fiber of the present invention is approximately tens of centimeters within the board, then XT is preferably below -30 dB / km. Figure 9 The core spacing needs to be at least 16 μm. Here, since the XT decreases with shorter wavelengths, a smaller XT is required in the communication band. Furthermore, since a smaller MFD results in less inter-core interference and a smaller XT, if the core spacing is at least 16 μm, then when the MFD is below 4 μm, the desired effect is achieved. Figure 9 The XT characteristics are equivalent to those below. Furthermore, if the core spacing is 20 μm or more, the effect of XT can be ignored even at depths below 1 km.

[0065] Figure 10 The diameter of the smallest cladding region represents the number M of groups of core regions in the multi-core optical fiber of the present invention. According to... Figure 8 and Figure 9 The OCT and core spacing Λ are set to 18 μm and 20 μm, respectively. Here, the diameter D of the core spacing Λ, OCT, and cladding region is relative to... Figure 1A , Figure 1B The number of core regions N and the number of core regions M are related as follows.

[0066] [Mathematical Expression 1]

[0067]

[0068] When N and M are set to arbitrary numbers, it is necessary to ensure that OCT and Λ are in the range of 18 μm and 20 μm or more, respectively, and D is below 180 μm. Furthermore, if D is 125 ± 1 μm, it becomes the same diameter of the cladding region as existing optical fibers, which is more preferable. Here, in Figure 10 In this case, N=4. It is known that when M is 7 or less, the diameter of the cladding region becomes 180 μm or less. Furthermore, it is known that if M is 4 or less, a core region with N=4 can be configured with a cladding region diameter of 125 μm.

[0069] Figure 11 This represents the diameter of the smallest cladding region of the multi-core optical fiber of the present invention relative to the number N of cores arranged in a straight line. Figure 11 In the diagram, the horizontal axis represents the number N of cores arranged in a straight line, and the vertical axis represents the diameter of the minimum necessary cladding region.

[0070] exist Figure 11 In the middle, OCT and core spacing Λ and Figure 10 Same, M=1. From Figure 11It can be seen that when M=1, and D is below 180μm, a maximum of eight cores can be arranged in a straight line. Furthermore, it can be seen that when D is 125μm, a maximum of five cores can be arranged in a straight line.

[0071] As explained above, the multi-core optical fiber of the present invention provides a high-density multi-core optical fiber with excellent connectivity to laser arrays and photodetector arrays. Furthermore, low-loss optical interconnects can be achieved using the multi-core optical fiber of the present invention.

[0072] Industrial applicability

[0073] This invention can be applied to the information and communication industry.

[0074] Explanation of reference numerals in the attached figures

[0075] 11: Core region, 12: Cladding region, 13: Covering region.

Claims

1. A multi-core optical fiber, comprising: A group of M core regions arranged in a straight line within a cross section, where M is a positive integer greater than 1 and N is a positive integer greater than 2; A cladding region, surrounding the plurality of core regions, has a lower refractive index than the plurality of core regions; and The cladding area surrounds the cladding region. The multi-core optical fiber is characterized in that... The plurality of core regions are arranged linearly symmetrically relative to two imaginary lines orthogonal to the center of the cladding region. The diameter of the cladding region is less than 180 μm. The diameter of the covered region is greater than 235 μm and less than 265 μm. The OCT is above 18μm, and the limiting loss is below 0.01dB / km. The OCT is the shortest distance from the center of the core region closest to the end of the cladding region to the end of the cladding region among the plurality of core regions.

2. A multi-core optical fiber, comprising: A group of M core regions arranged in a straight line within a cross section, where M is a positive integer greater than 1 and N is a positive integer greater than 2; A cladding region, surrounding the plurality of core regions, has a lower refractive index than the plurality of core regions; and The cladding area surrounds the cladding region. The multi-core optical fiber is characterized in that... The plurality of core regions are arranged linearly symmetrically relative to two imaginary lines orthogonal to the center of the cladding region. The diameter of the cladding region is less than 180 μm. The diameter of the covered region is greater than 235 μm and less than 265 μm. The refractive index distribution of the multiple core regions is step-type. N is 4 and M is 1. The center-to-center distance between the multiple core regions is less than 36.2 μm. The mode field diameter at a wavelength of 1.31 μm is greater than 8.6 μm and less than 9.2 μm. The cutoff wavelength is below 1.26 μm.

3. A multi-core optical fiber, comprising: A group of M core regions arranged in a straight line within a cross section, where M is a positive integer greater than 1 and N is a positive integer greater than 2; A cladding region, surrounding the plurality of core regions, has a lower refractive index than the plurality of core regions; and The cladding area surrounds the cladding region. The multi-core optical fiber is characterized in that... The plurality of core regions are arranged linearly symmetrically relative to two imaginary lines orthogonal to the center of the cladding region. The diameter of the cladding region is less than 180 μm. The diameter of the covered region is greater than 235 μm and less than 265 μm. The refractive index distribution of the multiple core regions is step-type. The N is 4 and the M is 2. The center-to-center distance between the multiple core regions is less than 34.5 μm. The mode field diameter at a wavelength of 1.31 μm is greater than 8.6 μm and less than 9.2 μm. The cutoff wavelength is below 1.26 μm.

4. A multi-core optical fiber, comprising: A group of M core regions arranged in a straight line within a cross section, where M is a positive integer greater than 1 and N is a positive integer greater than 2; A cladding region, surrounding the plurality of core regions, has a lower refractive index than the plurality of core regions; and The cladding area surrounds the cladding region. The multi-core optical fiber is characterized in that... The plurality of core regions are arranged linearly symmetrically relative to two imaginary lines orthogonal to the center of the cladding region. The diameter of the cladding region is less than 180 μm. The diameter of the covered region is greater than 235 μm and less than 265 μm. The refractive index distribution of the multiple core regions is grooved. N is 4 and M is 1. The center-to-center distance between the multiple core regions is less than 38.5 μm. The mode field diameter at a wavelength of 1.31 μm is greater than 8.6 μm and less than 9.2 μm. The cutoff wavelength is below 1.26 μm.

5. A multi-core optical fiber, comprising: A group of M core regions arranged in a straight line within a cross section, where M is a positive integer greater than 1 and N is a positive integer greater than 2; A cladding region, surrounding the plurality of core regions, has a lower refractive index than the plurality of core regions; and The cladding area surrounds the cladding region. The multi-core optical fiber is characterized in that... The plurality of core regions are arranged linearly symmetrically relative to two imaginary lines orthogonal to the center of the cladding region. The diameter of the cladding region is less than 180 μm. The diameter of the covered region is greater than 235 μm and less than 265 μm. The refractive index distribution of the multiple core regions is grooved. The N is 4 and the M is 2. The center-to-center distance between the multiple core regions is less than 36.5 μm. The mode field diameter at a wavelength of 1.31 μm is greater than 8.6 μm and less than 9.2 μm. The cutoff wavelength is below 1.26 μm.

6. A multi-core optical fiber, comprising: A group of M core regions arranged in a straight line within a cross section, where M is a positive integer greater than 1 and N is a positive integer greater than 2; A cladding region, surrounding the plurality of core regions, has a lower refractive index than the plurality of core regions; and The cladding area surrounds the cladding region. The multi-core optical fiber is characterized in that... The plurality of core regions are arranged linearly symmetrically relative to two imaginary lines orthogonal to the center of the cladding region. The diameter of the cladding region is less than 180 μm. The diameter of the covered region is greater than 235 μm and less than 265 μm. The refractive index distribution of the multiple core regions is step-type. The core radii of the plurality of core regions are less than 1.9 μm, and the relative refractive index difference between the plurality of core regions and the cladding region is greater than 1.8%. Among the plurality of core regions, the shortest distance from the center of the core region closest to the end of the cladding region to the end of the cladding region is greater than 18 μm. The center-to-center distance between the multiple core regions is greater than 16 μm, and [Mathematical Expression 2] in, OCT represents the shortest distance from the center of the core region closest to the end of the cladding region to the end of the cladding region. Λ represents the distance between the centers of the multiple core regions.

7. The multi-core optical fiber according to claim 6, characterized in that, The N is 4 and the M is 7 or less.

8. The multi-core optical fiber according to claim 6, characterized in that, M is 1 and N is 8 or less.

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