Multi-core optical fiber and design method

Abstract

The present invention aims to provide a multi-core optical fiber and a design method thereof, which configures four cores of a single-peak type with a standard cladding diameter, satisfies a desired specification, and is excellent in mass productivity, quality, and yield. The multi-core optical fiber of the present invention has four cores of a single-peak type refractive index profile with a radius a arranged in a square lattice shape in a long side direction, and a cladding region with a diameter of 125 ± 1 μm, which has a lower refractive index than the cores, and an absolute value of a relative refractive index difference from the cores is Δ, on an outer peripheral portion of the cores, and the four cores are arranged in a manner that a relationship of a minimum distance (OCT) from a center of the core to an outer periphery of the cladding region, a minimum value Λ of a pitch of the cores, and an MFD at a wavelength of 1310 nm satisfy a number C1, and the relative refractive index difference Δ of the cores and the cladding region and the radius a of the cores are set. [Number C1] OCT ≥ 3.73 MFD + 3.43 Λ ≤ -5.28 MFD + 83.54.

Description

[0001] This application is a divisional application of the invention patent application filed on January 6, 2020, entitled "Multi-core optical fiber and design method", with application number "202080008176.5". Technical Field

[0002] This invention relates to multi-core optical fibers (MCFs) having multiple core regions and their design methods. Background Technology

[0003] Research is actively underway to significantly increase transmission capacity using space division multiplexing (SDM) technology. For example, numerous reports, including Non-Patent Literature 1 and 2, document high-capacity SDM transmission using MCF. However, the MCFs described in these reports, including Non-Patent Literature 1 and 2, require a sufficiently wide core spacing Λ to reduce crosstalk (XT), resulting in a cladding diameter of 150–230 μm, which is larger than existing optical fibers. However, the length of an optical fiber manufactured from a single fiber material decreases inversely proportional to the square of the cladding diameter, thus significantly degrading fiber productivity due to the increased cladding diameter. Furthermore, existing fiber components are designed to correspond to the existing 125 μm cladding diameter; therefore, to effectively utilize the increased cladding diameter of the MCF, redesigning peripheral components is necessary, requiring extensive research and development for practical application.

[0004] Therefore, in recent years, MCFs with a cladding diameter of 125 μm, the same as previous optical fibers, have been developed. By using a standard cladding diameter of 125 μm, the mass production capability of optical fibers can be maintained at or above the previous level, and existing peripheral components such as standard connectors and optical cables can be effectively utilized. Furthermore, by ensuring that each core of the MCF has the same optical characteristics as existing optical fibers, interchangeability with existing optical interfaces can be ensured, thus allowing for easy upgrades from existing equipment to MCFs.

[0005] Non-patent documents 3 and 4 report four-core MCFs with optical properties equivalent to existing single-mode fibers (SMFs) at 100km with an XT of less than -30dB. According to non-patent document 3, four cores can be configured using the same core structure; furthermore, according to non-patent document 5, five cores can be configured by using multiple core structures.

[0006] However, because these MCFs use complex trench-type refractive index distribution shapes as their core structures, the problems lie in the complexity of the processes used to form the refractive index distribution and the yield rate; the issues are mass production and economy. General-purpose SMFs employ a simple single-peak refractive index distribution, offering high mass production capability and good quality. Non-patent document 6 reports a two-core optical fiber with a single-peak shape and a cladding diameter of 125 μm.

[0007] Existing technical documents

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

[0009] Non-patent literature 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 125 μm cladding diameter with full compliance to conventional SMF", ECOC2015, We.1.4.5, Sep.2015.

[0011] Non-patent literature 4: T. Matsui et al., “118.5 Tbit / s Transmission over 316 km-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: Y.Geng, et.al., "High speed, bidirectional dual-core fibertransmission system for high density, short-reach optical interconnects", Photonics West, 9380-8, Feb. 2015.

[0014] Non-patent literature 7: PJWinzer et al., “Penalties from In-Band Crosstalk for Advanced Optical Modulation Formats”, ECOC2011, Tu5B7, Sep.2011.

[0015] Non-patent literature 8: D. Marcuse, “Loss analysis of single-mode fiber splices”, Bell System Tech.J., vol.565, no.5, May-June, 1977. Summary of the Invention

[0016] However, non-patent document 6 does not describe configuring three or more single-peak cores with a standard cladding diameter of 125 μm. That is, in existing MCF designs, the challenge is that it is difficult to configure three or more single-peak cores with a standard cladding diameter of 125 μm. Therefore, to solve the above-mentioned problem, the object of the present invention is to provide a multi-core optical fiber and its design method, wherein the multi-core optical fiber has four single-peak cores configured with a standard cladding diameter, meeting the desired specifications, and exhibiting excellent mass production feasibility, quality, and yield.

[0017] To achieve the above objectives, the multi-core optical fiber of the present invention has four cores configured within a cladding with a diameter of 125±1μm based on a specified formula.

[0018] Specifically, the first multi-core optical fiber of the present invention has:

[0019] Four cores with a unimodal refractive index distribution of radius a, arranged in a square lattice along the long side; and

[0020] A cladding region with a diameter of 125±1μm, having a refractive index lower than that of the core, and an absolute value of Δ for the relative refractive index difference with respect to the core, located on the outer periphery of the core.

[0021] The multi-core optical fiber is characterized in that...

[0022] The mode field diameter (MFD) at a wavelength of 1310 nm is 8.2–9.6 μm.

[0023] The bending loss at a wavelength of 1625nm and a bending radius of 30mm is below 0.1dB / 100turns.

[0024] The cutoff wavelength is below 1260nm.

[0025] Four cores are configured such that the minimum distance (OCT) from the center of the core to the outer periphery of the cladding region, the value of the core spacing Λ, and the relationship between the MFD at a wavelength of 1310 nm satisfy a number C1, and the relative refractive index difference Δ between the core and the cladding region and the radius a of the core are set.

[0026] [Number C1]

[0027]

[0028] Furthermore, the second multi-core optical fiber of the present invention has:

[0029] Four cores with a unimodal refractive index distribution of radius a, arranged in a square lattice along the long side; and

[0030] A cladding region with a diameter of 125±1μm, having a refractive index lower than that of the core, and an absolute value of Δ for the relative refractive index difference with respect to the core, located on the outer periphery of the core.

[0031] The multi-core optical fiber is characterized in that...

[0032] The mode field diameter (MFD) at a wavelength of 1550 nm is 9–12 μm.

[0033] The bending loss at a wavelength of 1625nm and a bending radius of 30mm is below 0.1dB / 100turns.

[0034] The cutoff wavelength is below 1530nm.

[0035] Four cores are configured such that the minimum distance (OCT) from the center of the core to the outer periphery of the cladding region, the value of the core spacing Λ, and the relationship between the MFD at a wavelength of 1550 nm satisfy the number C4, and the relative refractive index difference Δ between the core and the cladding region and the radius a of the core are set.

[0036] [Number C4]

[0037]

[0038] Based on the MFD obtained according to the crosstalk per unit length required for multi-core optical fibers, OCT and Δ are obtained. Furthermore, this MFD can be achieved using the core radius a and the relative refractive index difference Δ. Since this multi-core optical fiber is single-peaked, it exhibits excellent mass production capability, quality, and yield. Therefore, this invention provides a multi-core optical fiber with four single-peaked cores configured with a standard cladding diameter, meeting desired specifications, and exhibiting excellent mass production capability, quality, and yield.

[0039] The relationship between crosstalk per unit length and MFD is as follows.

[0040] In the case of the first multi-core optical fiber, the characteristic is that,

[0041] The relationship between the total value XT of crosstalk per unit length at wavelength 1625nm received by any core of the core from other cores and the MFD at wavelength 1310nm satisfies number C2, or the relationship between the total value XT of crosstalk per unit length at wavelength 1360nm received by any core of the core from other cores and the MFD at wavelength 1310nm satisfies number C3.

[0042] [Number C2]

[0043]

[0044] [Number C3]

[0045]

[0046] In the case of a second multi-core optical fiber, the characteristic is that,

[0047] The relationship between the total value XT of crosstalk per unit length at wavelength 1625nm received by any core of the core from other cores and the MFD at wavelength 1550nm satisfies number C5, or the relationship between the total value XT of crosstalk per unit length at wavelength 1565nm received by any core of the core from other cores and the MFD at wavelength 1550nm satisfies number C6.

[0048] [Number C5]

[0049]

[0050] [Number C6]

[0051]

[0052] In addition, the first multi-core optical fiber and the second multi-core optical fiber are designed as follows.

[0053] The design method of the multi-core optical fiber of the present invention is as follows:

[0054] The specification determination step involves determining the required cutoff wavelength of the multi-core optical fiber and the total value XT of crosstalk per unit length received by any core from other cores.

[0055] First MFD calculation steps,

[0056] When the cutoff wavelength determined in the specification determination step is below 1260 nm and the total crosstalk value XT per unit length is a value at a wavelength of 1625 nm, the mode field diameter (MFD) at a wavelength of 1310 nm is calculated by the number C2.

[0057] When the cutoff wavelength determined in the specification determination step is below 1260 nm and the total crosstalk value XT per unit length is the value at wavelength 1360 nm, the mode field diameter (MFD) at wavelength 1310 nm is calculated by the number C3.

[0058] In the first structural calculation step, the MFD calculated in the first MFD calculation step is substituted into the number C1 to calculate the minimum distance (OCT) from the center of the core to the outer periphery of the cladding region and the value Λ of the core spacing.

[0059] The second MFD calculation step,

[0060] When the cutoff wavelength determined in the specification determination step is below 1530 nm and the total crosstalk value XT per unit length is a value at a wavelength of 1625 nm, the mode field diameter (MFD) at a wavelength of 1550 nm is calculated by C5.

[0061] When the cutoff wavelength determined in the specification determination step is below 1530 nm and the total crosstalk per unit length XT is a value at a wavelength of 1565 nm, the mode field diameter (MFD) at a wavelength of 1550 nm is calculated using the number C6; and

[0062] In the second structural calculation step, the MFD calculated in the second MFD calculation step is substituted into the number C4 to calculate the minimum distance (OCT) from the center of the core to the outer periphery of the cladding region and the value Λ of the core spacing.

[0063] This invention provides a multi-core optical fiber and its design method, wherein the multi-core optical fiber has four cores configured in a single-peak shape with a standard cladding diameter to meet the desired specifications, and has excellent mass production capability, quality and yield. Attached Figure Description

[0064] Figure 1 (a) is a diagram illustrating the cross-sectional structure of the multi-core optical fiber of the present invention. (b) is a diagram illustrating an example of the refractive index distribution of the multi-core optical fiber of the present invention.

[0065] Figure 2 This is a structural diagram illustrating an example of the relationship between the core radius and the relative refractive index difference of the multi-core optical fiber of the present invention.

[0066] Figure 3 This is a characteristic diagram showing the relationship between MFD, OCT, and Λ of the multi-core optical fiber at a wavelength of 1310nm according to the present invention.

[0067] Figure 4 This is a characteristic diagram illustrating an example of the relationship between MFD and XT at a wavelength of 1310nm for the multi-core optical fiber of the present invention.

[0068] Figure 5 This is a characteristic diagram showing the relationship between MFD, OCT, and Λ of the multi-core optical fiber at a wavelength of 1550nm.

[0069] Figure 6 This is a characteristic diagram illustrating an example of the relationship between MFD and XT at a wavelength of 1550nm for the multi-core optical fiber of the present invention.

[0070] Figure 7 This is a flowchart illustrating the design method of the multi-core optical fiber of the present invention. Detailed Implementation

[0071] Embodiments of the present invention will be 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 these embodiments. Furthermore, in this specification and the accompanying drawings, elements with the same reference numerals denote identical constituent elements.

[0072] Figure 1 This is an example of the structure of a multi-core optical fiber (MCF) according to this embodiment. Figure 1 (a) is the cross-sectional structure of the MCF. Figure 1 (b) is a diagram illustrating the refractive index distribution of each core. This MCF has four cores 11 in a single-core fiber, and the diameter of the cladding region 12 is a standard 125 ± 1 μm. The refractive index distribution of each core is as follows: Figure 1 As shown in (b), a cladding region 12 with a lower refractive index than the core 11 surrounds the core. Here, the relative refractive index difference is defined based on the refractive index of the cladding region; however, the materials of the core and cladding region can be pure quartz and fluorine-doped glass, or combinations of GeO2-doped glass, pure quartz glass, and fluorine-doped glass, respectively. Figure 1 The combination of glass materials with the shown refractive index distribution is used to select the material.

[0073] Figure 2This indicates the design range of core parameters used to obtain the specified optical properties of the multi-core optical fiber of the present invention. The horizontal axis is the core radius, and the vertical axis is the relative refractive index difference between the core and cladding. In the region above the solid line, bending loss at a wavelength of 1625 nm and a bending radius of 30 mm can be suppressed to below 0.1 dB / 100 turns.

[0074] The dashed and dotted lines in the figure represent the relationship between the core radius and the relative refractive index difference for cutoff wavelengths of 1260 nm and 1530 nm, respectively. To the left of the dashed and dotted lines, cutoff wavelengths below 1260 nm and 1530 nm can be achieved, respectively. Furthermore, the dotted line in the figure represents the relationship between the core radius and the relative refractive index difference for an MFD of 10 μm at a wavelength of 1550 nm. MFDs larger than 10 μm can be achieved in the region below the dotted line.

[0075] Therefore, by setting the core radius and relative refractive index difference within the region enclosed by the solid lines, dotted lines (or dashed lines), and dotted-dashed lines in the diagram, the desired bending loss, cutoff wavelength, and MFD characteristics can be simultaneously satisfied. Here, in Figure 2 The figure illustrates the case where the MFD is 10 μm at a wavelength of 1550 nm, but it is known that the relationship between the core radius, relative refractive index difference and MFD in a single-peak fiber can be described by the following empirical formula (for example, refer to Non-Patent Document 8).

[0076] (Equation 1)

[0077] MFD / 2a≒0.65+1.619V -1.5 +2.879V -6 (1)

[0078] Where V is the normalized frequency, and it can be defined using the core radius a, the core refractive index n1, the cladding refractive index n2, and the wavelength λ.

[0079] V≡2πa / λ(n1 2 -n2 2 ) 0.5 .

[0080] Therefore, it is possible to derive the desired MFD characteristics at any wavelength. Figure 2 .

[0081] Furthermore, while 1260nm and 1530nm are used here as cutoff wavelengths, they represent the lower limits of single-mode transmission bands as defined by multiple international standards (e.g., ITU-T recommendations G.652 and G.654, respectively). Additionally, bending loss and MFD also represent the loss characteristics and interconnectivity after optical cabling, as specified by international standards such as ITU-T.

[0082] (Implementation Method 1)

[0083] The first multi-core optical fiber will be described as an embodiment 1.

[0084] In MCF, it is necessary to appropriately set the minimum distance (OCT) from the core center to the outer periphery of the cladding to effectively suppress additional losses αc. αc tends to increase at longer wavelengths, and generally, it is preferable to be below 0.01 dB / km at the upper limit wavelength used. Figure 3 This illustrates the relationship between MFD and OCT at a wavelength of 1310 nm. Here, as an example, the wavelength range of 1260 nm to 1625 nm will be used. That is, the cutoff wavelength is set to 1260 nm, and the αc at a wavelength of 1625 nm is below 0.01 dB / km. Furthermore, the bending loss is equivalent to that of existing SMFs, with a value of below 0.1 dB / 100 turns at a bending radius of 30 mm at a wavelength of 1625 nm.

[0085] At this moment, as if in Figure 2 As explained, the relationship between the core radius and the relative refractive index difference can be derived to satisfy the requirements of cutoff wavelength, bending loss, and MFD. Figure 3 The solid line in the middle represents the minimum OCT required for αc to be below 0.01 dB / km, based on the conditions of core radius and relative refractive index difference satisfying the above relationship. Figure 3 It can be seen that OCT increases with MFD, and their relationship can be approximated by the following formula.

[0086] (Equation 2)

[0087] OCT≥3.73MFD+3.43 (2)

[0088] Here, Figure 3 The dashed line represents the upper limit of the center-to-center distance Λ between the four cores configured in a square lattice when the cladding diameter is 125 μm. From Figure 3 It can be seen that the relationship between MFD and Λ can be approximated by the following formula.

[0089] (Equation 3)

[0090] Λ≤-5.28MFD+83.54 (3)

[0091] Therefore, it can be seen that in the MCF of the present invention, when the lower limit of the band used is 1260nm, the specified wavelength of MFD is 1310nm, and αc at wavelength 1625nm is less than 0.01dB / km, the setting conditions of OCT and Λ can be determined according to the design center value of MFD and by equations (2) and (3).

[0092] That is, the first multi-core optical fiber has:

[0093] Four cores with a unimodal refractive index distribution of radius a, arranged in a square lattice along the long side; and

[0094] A cladding region with a diameter of 125±1μm, having a refractive index lower than that of the core, and an absolute value of Δ for the relative refractive index difference with respect to the core, located on the outer periphery of the core.

[0095] The multi-core optical fiber is characterized in that...

[0096] The mode field diameter (MFD) at a wavelength of 1310 nm is 8.2–9.6 μm.

[0097] The bending loss at a wavelength of 1625nm and a bending radius of 30mm is below 0.1dB / 100turns.

[0098] The cutoff wavelength is below 1260nm.

[0099] Four cores are configured such that the minimum distance (OCT) from the center of the core to the outer periphery of the cladding region, the value of the core spacing Λ, and the relationship between the MFD at a wavelength of 1310 nm satisfy equations (2) and (3), and the relative refractive index difference Δ between the core and the cladding region and the radius a of the core are set.

[0100] Figure 4 This indicates that the MCF of the present invention satisfies Figure 3 The relationship between the required MFD at 1310nm wavelength and the maximum XT is shown. Furthermore, the most commonly used SMF at 1310nm wavelength is defined by international standards as having an MFD in the range of 8.2–9.6 μm. Generally, mismatch in the MFD between connected fibers leads to increased connection loss; therefore, the MFD at 1310nm wavelength for this MCF is also preferably set within the range of 8.2–9.6 μm.

[0101] The solid line in the figure represents the calculation results when the upper limit of the band is used as the wavelength of 1625nm. From Figure 4 It can be seen that XT increases with MFD, and the relationship between the two can be approximated by equation (4). In addition, XT becomes saturated in the region above 0 dB / km.

[0102] (Equation 4)

[0103] XT≤27.0MFD-251.8 (4)

[0104] Here, the transmission characteristic degradation caused by XT in MCF depends on the transmission mode. Non-patent document 7 shows that crosstalk below -16dB, -24dB, and -32dB is required for QPSK, 16QAM, and 64QAM transmission modes, respectively. In the case of multi-core fiber, inter-core crosstalk becomes the sum of crosstalk components from other cores relative to any given core. For crosstalk per unit distance (dB / km) and distance L (km), crosstalk at any distance is given by the following formula.

[0105] XT+10log (L)

[0106] For example, to achieve transmission over distances of 1000–10000 km using the 16QAM signal format, the crosstalk of multi-core optical fibers needs to be below -54–-64 dB / km. That is, according to… Figure 4 Regardless of the MFD at a wavelength of 1310nm, the first multi-core fiber cannot transmit at a wavelength of 1625nm using the 16QAM signal format for 1000-10000km.

[0107] On the other hand, from Figure 4 It can be seen that, in the case of implementing MCF for L=1km transmission using QPSK signals, the first multi-core fiber can set the MFD at a wavelength of 1310nm to a range below 8.75μm.

[0108] Furthermore, if the MFD at a wavelength of 1310nm is 9μm and QPSK signal transmission is desired, the XT at a wavelength of 1625nm for the MFD is -9dB / km. Therefore, in order to achieve the -16dB / km XT characteristic required for QPSK signal transmission, it is sufficient to keep the maximum transmission distance below 0.2km.

[0109] also, Figure 4 The dashed line in the figure represents the calculation result when the upper limit wavelength of the band is used as 1360nm. It can be seen that the relationship between MFD at wavelength 1310nm and XT at wavelength 1360nm can be described by equation (5).

[0110] (Equation 5)

[0111] XT≤33.7MFD-342.3 (5)

[0112] As mentioned above, from Figure 4It can be seen that by utilizing the 8.2–9.6 μm MFD characteristics that match the existing SMF standard and appropriately setting the transmission distance, a transmission system compatible with QPSK, 16QAM, and 64QAM can be realized. Therefore, using the above relationship (4) or (5), when the upper limit of the band used is set as 1625 nm or 1360 nm respectively, the relationship between XT and MFD for realizing the desired transmission system can be derived.

[0113] As described above, the relationship between OCT, Λ and MFD in an MCF that has the same cutoff wavelength, bending loss and MFD characteristics as existing SMFs, with an additional loss of less than 0.01dB / km at a wavelength of 1625nm, and achieves the desired XT characteristics in the wavelength range of 1260nm to 1625nm can be specified using equations (2), (3) and (4).

[0114] Similarly, the relationship between OCT, Λ and MFD in the MCF of the present invention, which has the same cutoff wavelength, bending loss and MFD characteristics as the existing SMF, with an additional loss of less than 0.01dB / km at a wavelength of 1625nm and achieves the desired XT characteristics in the wavelength range of 1260nm to 1360nm, can be defined using the relations (2), (3) and (5).

[0115] (Implementation Method 2)

[0116] The second multi-core optical fiber will be described as an embodiment 2.

[0117] This MCF is based on a cutoff wavelength of 1530nm. Figure 5 This illustrates the relationship between MFD and OCT at a wavelength of 1550 nm. Here, as an example, the wavelength range of 1530 nm to 1625 nm will be used. That is, the cutoff wavelength is set to 1530 nm, and the αc at a wavelength of 1625 nm is 0.01 dB / km or less. Furthermore, the bending loss is the same as that of existing SMFs, with a value of 0.1 dB / 100 turns or less at a bending radius of 30 mm at a wavelength of 1625 nm.

[0118] At this moment, as if in Figure 2 As explained, the relationship between the core radius and the relative refractive index difference can be derived to satisfy the requirements of cutoff wavelength, bending loss, and MFD. Figure 5 The solid line in the middle represents the minimum OCT required for αc to be below 0.01 dB / km, based on the conditions of core radius and relative refractive index difference satisfying the above relationship. Figure 5 It can be seen that OCT increases with MFD, and their relationship can be approximated by the following formula.

[0119] (Equation 6)

[0120] OCT≥2.82MFD+3.7 (6)

[0121] Here, Figure 5 The dashed line represents the upper limit of the center-to-center distance Λ between the four cores configured in a square lattice when the cladding diameter is 125 μm. From Figure 5 It can be seen that the relationship between MFD and Λ can be approximated by the following formula.

[0122] (Equation 7)

[0123] Λ≤-3.99MFD+83.15 (7)

[0124] Therefore, it can be seen that in the MCF of the present invention, when the lower limit of the band used is 1530nm, the specified wavelength of MFD is 1550nm, and αc at wavelength 1625nm is less than 0.01dB / km, the setting conditions of OCT and Λ can be determined according to the design center value of MFD and by equations (6) and (7).

[0125] That is, the second multi-core optical fiber has:

[0126] Four cores with a single-peaked refractive index distribution of radius a, arranged in a square lattice along the long side;

[0127] A cladding region with a diameter of 125±1μm, having a refractive index lower than that of the core, and an absolute value of Δ for the relative refractive index difference with respect to the core, located on the outer periphery of the core.

[0128] The multi-core optical fiber is characterized in that...

[0129] The mode field diameter (MFD) at a wavelength of 1550 nm is 9–12 μm.

[0130] The bending loss at a wavelength of 1625nm and a bending radius of 30mm is below 0.1dB / 100turns.

[0131] The cutoff wavelength is below 1530nm.

[0132] Four cores are configured such that the minimum distance (OCT) from the center of the core to the outer periphery of the cladding region, the value of the core spacing Λ, and the relationship between the MFD at a wavelength of 1550 nm satisfy equations (6) and (7), and the relative refractive index difference Δ between the core and the cladding region and the radius a of the core are set.

[0133] Figure 6 This indicates that the MCF of the present invention satisfies Figure 5The relationship between the required MFD at 1550nm wavelength and the maximum XT is shown. Furthermore, the MFD at 1550nm wavelength for commonly used high-speed transmission SMFs is generally in the range of 9–12 μm. Generally, mismatch in the MFD between connected fibers leads to increased connection loss; therefore, the MFD at 1550nm wavelength for this MCF is also preferably set in the range of 9–12 μm.

[0134] The solid line in the figure represents the calculation results when the upper limit of the band is used as the wavelength of 1625nm. From Figure 6 It can be seen that XT increases with MFD, and the relationship between the two can be approximated by equation (8). In addition, XT becomes saturated in the region above 0 dB / km.

[0135] (Equation 8)

[0136] XT≤24.6MFD-294.4 (8)

[0137] also, Figure 6 The dashed line in the equation represents the relationship when the upper limit wavelength of the band is used as 1565nm. It can be seen that the relationship between MFD at wavelength 1550nm and XT at wavelength 1565nm can be described by equation (9).

[0138] (Equation 9)

[0139] XT≤26.0MFD-315.8 (9)

[0140] This MCF is also the same as the MCF described in Implementation Method 1, from... Figure 6 It can be seen that by utilizing the 9-12μm MFD characteristics, which are highly compatible with existing high-speed transmission SMFs, and by appropriately setting the transmission distance, a transmission system compatible with QPSK, 16QAM, and 64QAM can be realized. Therefore, using the above relationship (8) or (9), when the upper limit of the band used is set as 1625nm or 1565nm respectively, the relationship between XT and MFD for realizing the desired transmission system can be derived.

[0141] As described above, the relationship between OCT, Λ and MFD in an MCF that has the same cutoff wavelength, bending loss and MFD characteristics as existing high-speed transmission SMFs, with an additional loss of less than 0.01dB / km at a wavelength of 1625nm, and achieves the desired XT characteristics in the wavelength range of 1530nm to 1625nm can be specified using equations (6), (7) and (8).

[0142] Similarly, the relationship between OCT, Λ and MFD in an MCF that has the same cutoff wavelength, bending loss and MFD characteristics as existing SMFs, with an additional loss of less than 0.01dB / km at a wavelength of 1625nm, and achieves the desired XT characteristics in the wavelength range of 1530nm to 1565nm, can be specified using equations (6), (7) and (9).

[0143] (Optical fiber design methods)

[0144] Figure 7 This is a flowchart illustrating the MCF design method described in embodiments 1 and 2. The characteristic of this design method is that it performs:

[0145] Specification determination step S01: Determine the required cutoff wavelength of the multi-core optical fiber and the total value XT of crosstalk per unit length received by any core from other cores.

[0146] First MFD calculation step S03,

[0147] If the cutoff wavelength determined in specification determination step S01 is below 1260nm (below 1260nm in step S02) and the total crosstalk value XT per unit length is the value at wavelength 1625nm, the mode field diameter (MFD) at wavelength 1310nm is calculated by equation (4).

[0148] If the cutoff wavelength determined in specification determination step S01 is below 1260nm (below 1260nm in step S02) and the total crosstalk value XT per unit length is the value at wavelength 1360nm, the mode field diameter (MFD) at wavelength 1310nm is calculated by equation (5).

[0149] In the first structural calculation step S04, the MFD calculated in the first MFD calculation step S03 is substituted into equations (2) and (3) to calculate the minimum distance (OCT) from the center of the core to the outer periphery of the cladding region and the value Λ of the core spacing.

[0150] Second MFD calculation step S06,

[0151] If the cutoff wavelength determined in specification determination step S01 is below 1530nm (below 1530nm in step S02) and the total crosstalk value XT per unit length is the value at wavelength 1625nm, the mode field diameter (MFD) at wavelength 1550nm is calculated by equation (8).

[0152] If the cutoff wavelength determined in specification determination step S01 is below 1530 nm (below 1530 nm in step S02), and the total crosstalk value XT per unit length is a value at wavelength 1565 nm, the mode field diameter (MFD) at wavelength 1550 nm is calculated using equation (9); and

[0153] In the second structural calculation step S07, the MFD calculated in the second MFD calculation step S06 is substituted into equations (6) and (7) to calculate the minimum distance (OCT) from the center of the core to the outer periphery of the cladding region and the value Λ of the core spacing.

[0154] This design method further substitutes the MFD calculated in the first MFD calculation step S03 or the second MFD calculation step S06 into equation (1) to calculate the core radius a and the relative refractive index difference Δ (steps S05 and S08).

[0155] By configuring the cores using the core radius a, relative refractive index difference Δ, core spacing Λ, and OCT calculated in this design method, it is possible to configure four single-peaked cores with a standard cladding diameter, resulting in an MCF that meets the desired specifications and offers excellent mass production capabilities, quality, and yield.

[0156] Industrial applicability

[0157] The multi-core optical fiber of the present invention can be used as an optical fiber in optical communication systems.

[0158] Explanation of reference numerals in the attached figures

[0159] 11: Core

[0160] 12: Cladding region

Claims

1. A multi-core optical fiber, having: Four cores with a unimodal refractive index distribution of radius a; and A cladding region with a diameter of 125±1μm, having a refractive index lower than that of the core, and an absolute value of Δ for the relative refractive index difference with respect to the core, located on the outer periphery of the core. The multi-core optical fiber is characterized in that... The mode field diameter (MFD) at a wavelength of 1550 nm is 9–12 μm. The bending loss at a wavelength of 1625nm and a bending radius of 30mm is below 0.1dB / 100turns. The cutoff wavelength is below 1530nm. Four cores are configured such that the minimum distance OCT from the center of the core to the outer periphery of the cladding region, the core spacing value Λ, and the relationship between the MFD at a wavelength of 1550 nm satisfy number C4. The relative refractive index difference Δ between the core and the cladding region and the radius a of the core are also set. [Number C4] OCT ≥ 2.82 MFD + 3.7 Λ≤-3.99MFD+83.15 (C4).

2. The multi-core optical fiber according to claim 1, characterized in that, The total crosstalk per unit length at wavelength 1625nm received by any core from other cores, XT, is related to the MFD at wavelength 1550nm by the number C5. [Number C5] XT≥24.6MFD-294.8 (C5).

3. The multi-core optical fiber according to claim 1, characterized in that, The total value XT of crosstalk per unit length at wavelength 1565nm received by any core from other cores and the relationship between the crosstalk at wavelength 1550nm and the MFD satisfy number C6. [Number C6] XT≥26.0MFD-315.8 (C6).

4. A design method for a multi-core optical fiber, wherein the multi-core optical fiber has: Four cores with a unimodal refractive index distribution of radius a; and A cladding region with a diameter of 125±1μm, having a refractive index lower than that of the core, and an absolute value of Δ for the relative refractive index difference with respect to the core, located on the outer periphery of the core. The design method is characterized by performing: The specification determination step involves determining the required cutoff wavelength of the multi-core optical fiber and the total value XT of crosstalk per unit length received by any core from other cores. First MFD calculation steps, When the cutoff wavelength determined in the specification determination step is below 1260 nm and the total crosstalk per unit length XT is a value at a wavelength of 1625 nm, the mode field diameter MFD at a wavelength of 1310 nm is calculated using C2. [Number C2] XT≥27.0MFD-251.8 (C2) When the cutoff wavelength determined in the specification determination step is below 1260nm and the total crosstalk value XT per unit length is the value at wavelength 1360nm, the mode field diameter MFD at wavelength 1310nm is calculated by the number C3. [Number C3] XT≥33.7MFD-342.3 (C3) In the first structural calculation step, the MFD calculated in the first MFD calculation step is substituted into the number C1 to calculate the minimum distance OCT from the center of the core to the outer periphery of the cladding region and the value Λ of the core spacing. [Number C1] OCT ≥ 3.73 MFD + 3.43 A≤-5.28MFD+83.54 (C1) The second MFD calculation step, When the cutoff wavelength determined in the specification determination step is below 1530 nm, and the total crosstalk value XT per unit length is a value at a wavelength of 1625 nm, the mode field diameter MFD at a wavelength of 1550 nm is calculated using the number C5. [Number C5] XT≥24.6MFD-294.8 (C5) When the cutoff wavelength determined in the specification determination step is below 1530nm and the total crosstalk value XT per unit length is the value at wavelength 1565nm, the mode field diameter MFD at wavelength 1550nm is calculated by the number C6. as well as [Number C6] XT≥26.0MFD-315.8 (C6) In the second structural calculation step, the MFD calculated in the second MFD calculation step is substituted into the number C4 to calculate the minimum distance OCT from the center of the core to the outer periphery of the cladding region and the value Λ of the core spacing. [Number C4] OCT ≥ 2.82 MFD + 3.7 A≤-3.99MFD+83.15 (C4)

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