Multi-core fiber transmission line, multi-core fiber transmission system and multi-core fiber connection method

Abstract

To reduce variance in loss between cores of an MCF transmission line while suppressing an optical transmission line from increasing in loss.SOLUTION: An MCF transmission line has a plurality of MCFs, including a first MCF and a second MCF, connected, core by core, in series, and the core of the first MCF and the core of the second MCF which are connected are so selected that a plurality of cores connected between the first MCF and second MCF have variance in loss reduced.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a multicore fiber transmission line and a transmission system, a multicore fiber connection method, and a program related thereto. [Background technology]

[0002] The practical application of spatial optical multiplexing transmission systems using multicore fiber (MCF) is progressing. MCF has multiple cores in a single optical fiber. By using MCF instead of single-core fiber (SCF), it is possible to expand the transmission capacity of optical transmission systems.

[0003] The loss of an MCF varies from core to core. Therefore, in a long-distance MCF transmission line in which multiple MCFs are cascaded, the loss differences between cores accumulate, and the signal light power at the output end of the MCF transmission line can vary significantly from core to core. Such variations in signal light power between cores in an MCF transmission line can degrade the transmission quality of optical transmission systems using MCFs.

[0004] A gain flattening filter (GFF) is sometimes used to reduce the power variation of signal light between cores in an MCF transmission line. Using a GFF can reduce the power variation of signal light between MCF cores. Also, in a core-pumped MC-EDFA (erbium-doped fiber amplifier), where pump light is directly injected into the core, the gain of the EDFA can be adjusted for each core by adjusting the pump light power for each core. For example, in a core with high loss, the core loss can be compensated for by increasing the gain by increasing the pump light power.

[0005] In relation to the present disclosure, Patent Document 1 describes a configuration in which a spatial channel switching element is used to switch the connection relationship of cores between MCFs. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-082318 Summary of the Invention [Problem to be solved by the invention]

[0007] The GFF mentioned above can reduce the level difference of signal light between cores in an MCF transmission line by adding loss to a core with low loss. In this case, the power of the signal light passing through the GFF is attenuated according to the core with the lowest signal light power. Therefore, when a GFF is used, the power of light propagating through the MCF transmission line may be significantly reduced. Furthermore, core-pumped MC-EDFAs have the problem that increasing the pumping light power of a specific core with a low amplification factor worsens the noise figure of that core, degrading the signal-to-noise ratio of the light amplified by that core.

[0008] Patent Document 1 describes a configuration in which a spatial channel switching element (a planar waveguide or a few-mode fiber) is used to switch signal light between cores. However, the technology described in Patent Document 1 requires the insertion of a spatial channel switching element at each connection point of the MCF. For this reason, the technology described in Patent Document 1 has a problem in that loss in the switching element increases the loss of the entire optical transmission line.

[0009] The present disclosure makes it possible to reduce the variation in loss between cores in an MCF transmission line while suppressing an increase in loss in the optical transmission line. [Means for solving the problem]

[0010] A multicore fiber transmission line according to the present disclosure is a multicore fiber transmission line in which a plurality of multicore fibers including a first multicore fiber and a second multicore fiber are connected in series for each core, The cores of the first multicore fiber and the cores of the second multicore fiber to be connected are selected so that the variation in loss of the multiple cores connected between the first multicore fiber and the second multicore fiber is reduced.

[0011] A multi-core fiber connection method according to the present disclosure is a multi-core fiber connection method for connecting a plurality of multi-core fibers including a first multi-core fiber and a second multi-core fiber, core by core, comprising: selecting cores of the first multicore fiber and the second multicore fiber to be connected such that a variation in loss of multiple cores connected between the first multicore fiber and the second multicore fiber is reduced. [Effects of the Invention]

[0012] The present disclosure provides a technique for reducing the variation in loss between cores in an MCF transmission line while suppressing an increase in loss in the optical transmission line. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 10 is a diagram illustrating an example of the configuration of an MCF transmission line. [Figure 2] FIG. 1 is a diagram showing an example of a cross section of an MCF. [Figure 3] FIG. 10 is a diagram illustrating an example of a connection between cores at a connection point. [Figure 4] 10A and 10B are diagrams illustrating examples of losses in each core of an MCF and cores connected at connection points. [Figure 5] 10 is a flowchart illustrating an example of a procedure for an MCF connection method. [Figure 6] FIG. 1 is a diagram illustrating an example of the configuration of an MCF transmission system. [Figure 7] 10 is a flowchart illustrating an example of a procedure for an MCF connection method. [Figure 8] FIG. 1 illustrates an example of the configuration of an MCF transmission system. [Figure 9]10 is a flowchart illustrating an example of a control procedure of a control circuit. DETAILED DESCRIPTION OF THE INVENTION

[0014] (First embodiment) FIG. 1 is a diagram illustrating an example of the configuration of an MCF transmission line 100 according to the first embodiment. In the MCF transmission line 100, MCFs 10, 20, and 30 are connected in series, core by core, at connection points 51 and 52. FIG. 2 is a diagram illustrating an example of a cross section of the MCF 10. The core arrangements of MCFs 20 and 30 are the same as those of the MCF 10. The MCF 10 is a seven-core MCF including cores 101-107. The cores 102-107 are arranged at equal intervals on a circumference 108. In other words, the cores 102-107 are located at the vertices of a regular hexagon 109 inscribed in the circumference 108. The core 101 is located at the center of the circumference 108. More generally, when there are n cores on the circumference 108 (where n is an integer greater than or equal to 3), the cores on the circumference 108 may be arranged at the vertices of a regular polygon inscribed in the circumference 108 and consisting of n vertices. That is, Figure 2 shows the case where n = 6. Note that hereinafter, the center of the regular polygon (i.e., the center of circumference 108) will be referred to as the "center of MCF" or simply as the "center." For example, core 101 is located at the center of MCF 10. Note that when the cores of an MCF are arranged at multiple vertices that form a square lattice, the center of the lattice will be the center of the MCF.

[0015] In addition, unless otherwise specified, the explanations in each embodiment do not limit the number of cores or the arrangement of cores. In this embodiment, cores 101-107 in the cross-sectional view are written as cores 201-207 and cores 301-307 in MCFs 20 and 30, respectively.

[0016] Referring to FIG. 1, MCF 10 has a fan-out 12 on the connection point 51 side. MCF 20 has a fan-in 21 on the connection point 51 side and a fan-out 22 on the connection point 52 side. MCF 30 has a fan-in 31 on the connection point 52 side. The fan-outs 12 and 22 and the fan-ins 21 and 31 are all known optical components with an MCF on one end and an SCF on the other end, and are generally called fan-in / fan-out. In this embodiment, MCF 10 and MCF 20 are connected core by core using fan-in / fan-out, and MCF 20 and MCF 30 are connected core by core. In FIG. 1, for example, fusion splicing or an optical connector is used to connect core by core.

[0017] The only difference between the fan-outs 12 and 22 and the fan-ins 21 and 31 is the orientation in which they are arranged in FIG. 1, and fan-ins / fan-outs of the same specifications can be used for all four of these optical components.

[0018] SCFs 121-127 are connected one-to-one to cores 101-107 of MCF 10, respectively. SCFs 211-217 and SCFs 221-227 are connected one-to-one to cores 201-207 of MCF 20, respectively. SCFs 311-317 are connected one-to-one to cores 301-307 of MCF 30, respectively.

[0019] FIG. 3 shows an example of connections between cores at connection points 51 and 52. When connecting MCF10 and MCF20, any combination of SCFs 121-127 in fan-out 12 and SCFs 211-217 in fan-in 21 can be selected. In FIG. 3, SCF121 and SCF217, SCF122 and SCF211, SCF123 and SCF212, and SCF124 and SCF213 are connected. Furthermore, SCF125 and SCF214, SCF126 and SCF215, and SCF127 and SCF216 are connected. That is, MCF10 and MCF20 are connected at connection point 51 so that their core numbers are offset by one. Similarly, MCF20 and MCF30 are connected at connection point 52 so that their core numbers are offset by one. Note that core 101 of MCF 10 is connected to core 207 of MCF 20. Core 201 of MCF 20 is connected to core 307 of MCF 30. With such connections, for example, light propagating through core 101 of MCF 10 propagates through SCF 121, SCF 217, core 207, SCF 227, SCF 316, and core 306 in this order.

[0020] In a long MCF, the transmission loss may vary from core to core. Therefore, when the number of MCFs connected in series (the number of spans) increases, the variation in cumulative loss between cores may increase. Therefore, when MCFs 10, 20, and 30 are connected in series in the MCF transmission line 100, the cores to be connected at the connection points 51 and 52 can be selected so that the variation in cumulative loss between cores after connection is reduced.

[0021] For example, consider a case where the loss of the central core (core 101, core 201, core 301) of each MCF is smaller than that of the other cores in MCF10, MCF20, and MCF30. In such a case, core 101, core 201, and core 301 may be connected so that they are not connected to each other at connection points 51 and 52. This type of connection makes it possible to prevent the loss of the path that passes through the central core more frequently among the seven cores included in MCF transmission line 100 from becoming too small compared to the loss of the other paths.

[0022] 4 is a diagram showing an example of the loss of each core of MCFs 10, 20, and 30 in FIG. 3 and the cores connected at connection points 51 and 52. The loss of MCF 10 is the loss of each core between one end of MCF 10 and fan-out 12 connected to the other end of the MCF. The loss of MCF 20 is the loss of each core between fan-in 21 and fan-out 22 connected to MCF 20. The loss of MCF 30 is the loss of each core between fan-in 31 connected to one end of MCF 30 and the other end of MCF 30.

[0023] 4 shows an example in which the loss of the central cores 101, 201, and 301 of MCFs 10, 20, and 30 is 1 dB, and the loss of the other cores is 2 dB. Core 101 of MCF 10 is connected to core 207 of MCF 20, and core 207 is connected to core 306 of MCF 30. The other cores are also connected as shown in FIG.

[0024] For example, when cores 101, 201, and 301, each with a loss of 1 dB, are connected in series, the loss (cumulative loss) of the path to which these cores are connected after the connection is 3 dB. In contrast, the cumulative loss of the paths of other cores (e.g., the path in which cores 102, 202, and 302 are connected in series) is 2 dB × 3 = 6 dB. Therefore, the difference in cumulative loss between cores in the path after the connection is 3 dB. In contrast, as shown in FIG. 4, by swapping the cores connected at connection points 51 and 52, the cumulative loss of each of the seven paths after the connection is 5 dB or 6 dB. In other words, the variation in cumulative loss between cores is reduced to 1 dB. Note that the combination of SCFs connecting fan-out 12 and fan-in 21 at connection point 51 is not limited to the example shown in FIG. 4. The same applies to connection point 52.

[0025] The combination of cores to be connected at connection points 51 and 52 may be determined by the procedure illustrated in the flowchart of FIG. 5. That is, the loss of each core of each of the MCFs to be connected (MCFs 10, 20, and 30) is measured in advance (step S01 in FIG. 5). Then, a combination of cores to be connected that minimizes the variation in the total core loss of the MCFs to be connected is selected (S02). Here, the combination with the smallest variation may be selected, or a combination of cores with the variation equal to or less than a predetermined value may be selected. Finally, the cores are connected based on the selected combination (S03).

[0026] When three or more MCFs are to be connected in series, in step S02, the combination of cores to be connected may be determined so as to reduce the variation in loss between the cores of two adjacent MCFs at each connection point. Alternatively, the combination of cores at each connection point may be determined so as to reduce the variation in cumulative loss between cores when all MCFs are connected in series. As described above, the MCF transmission line and MCF connection method of this embodiment can reduce the variation in loss between cores in the MCF transmission line while suppressing an increase in loss in the optical transmission line.

[0027] The MCF transmission line 100 having the above features and achieving similar effects can also be described as follows: The reference numerals in FIG.

[0028] The multicore fiber transmission line (100) is a multicore fiber transmission line in which a plurality of multicore fibers including a first multicore fiber (10) and a second multicore fiber (20) are connected in series for each core. The cores of the first multicore fiber (10) and the second multicore fiber (20) to be connected are selected so as to reduce the variation in loss of the plurality of cores connected between the first multicore fiber (10) and the second multicore fiber (20).

[0029] The multicore fiber transmission line 100 may also include a fan-out 12 connected to the first multicore fiber 10 and a fan-in 21 connected to the second multicore fiber 20. The connection between the first multicore fiber 10 and the second multicore fiber 20 may be made between a single-core fiber included in the fan-out 12 and a single-core fiber included in the fan-in 21.

[0030] (Another example of the first embodiment) When splicing two long MCFs, each of the two MCFs may contain a fusion splice point between them. For example, as described below, when MCFs are directly fusion spliced together, the splice loss of the cores near the periphery of the MCF may increase compared to the cores near the center due to misalignment of the rotational angle during rotational alignment. As a result, the loss of the MCF with the fusion splice may be greater in the cores near the periphery.

[0031] When connecting two MCFs in which the cores near the center and the cores near the periphery of the MCF have different losses, the cores may be connected at the connection points so that the cores near the center and the cores near the periphery are swapped. For example, in the configuration example shown in Figure 3, the central cores and the outer cores are swapped for each MCF connection point at connection points 51 and 52. As a result, in the configuration of Figure 3, the difference in cumulative loss between the cores near the center and the outer cores of the MCF can be reduced in the MCF transmission line 100 in which the MCFs are connected in series.

[0032] (Second embodiment) In this embodiment, an example of a procedure for directly fusion-splicing MCFs that are closely arranged and face each other without using fan-out and fan-in will be described.

[0033] When MCFs with cores arranged around the center are directly spliced by fusion, a procedure may be used to adjust the core position at the splice point by rotating the MCF around the center axis and fusion splicing at an angle that minimizes the loss of light propagating through the cores. This core position adjustment procedure will be referred to as "rotational alignment" below. When adjusting the core position of an MCF by rotational alignment, the splice loss of each core is expressed by equation (1).

[0034] Loss=10×log[exp(d 2 / w 2 )](dB) (1) Here, d is the amount of axial misalignment of the cores when spliced, and w is MFD / 2. MFD is the mode field diameter of the core. Meanwhile, the amount of axial misalignment d between opposing cores on a concentric circle of radius r is expressed by equation (2), where θ is the angular misalignment from the center of the MCF when spliced.

[0035] d=r×θ (2) The angular misalignment θ is the angle between the opposing cores and the rotation axis (i.e., the center of the MCF) immediately before fusion splicing. Equation (1) shows that the splice loss (Loss) increases with the axial misalignment d.

[0036] Equation (2) indicates that the axial misalignment d relative to the angular misalignment θ during alignment increases with distance from the center of the MCF. Therefore, when MCFs are spliced with an angular misalignment of θ during rotational alignment, the connection loss of cores positioned closer to the outer periphery of the MCF is greater than the connection loss of cores positioned further inward. Therefore, when MCFs are directly spliced together using rotational alignment, the loss of the path connecting cores near the center of the MCF may be small, due to the difference in splice loss described above, while the loss of the path connecting cores positioned closer to the outer periphery of the MCF may be large. Therefore, even when MCFs are directly fusion-spliced together using rotational alignment, it is preferable to be able to suppress the variation in loss between cores in the fusion-spliced MCF.

[0037] FIG. 6 illustrates a configuration example of a multicore fiber transmission system (MCF transmission system) 1000 in which MCFs 10A, 20A, and 30A are connected in series. In this embodiment, the MCF 10A is a nine-core MCF, and in FIG. 6, the core numbers of each MCF are indicated as 1 to 9. The nine cores are arranged at multiple lattice points in a 3-row by 3-column arrangement. The spacing between the columns and rows constituting the multiple lattice points is the same, and these multiple lattice points form a single square lattice. The MCFs 20A and 30A also have the same core arrangement as the MCF 10A. Core 5 is located at the center of the MCFs 10A, 20A, and 30A. Therefore, at the connection point 51A between the MCF 10A and the MCF 20A, when one of the MCFs 10A and 20A is rotated around core 5, the positions of the other cores overlap at every 90 degrees. In other words, the core arrangements of MCFs 10A, 20A, and 30A are rotationally symmetric (four-fold symmetric) around the center of each MCF. At the rotation angle where the core positions overlap, the cores can be fusion-spliced together while remaining optically connected.

[0038] A fusion splicer 1011 is disposed near the splicing point 51A, and a fusion splicer 1012 is disposed near the splicing point 52A. The fusion splicers 1011 and 1012 have a rotational alignment function and splice the MCFs by fusion after the rotational alignment is completed. The fusion is performed using heat generated by, for example, electric discharge. The fusion splicer 1011 holds the end of the MCF 10A and the end of the MCF 20A at the splicing point 51A and has a mechanism for adjusting the core position so that the core of the MCF 10A and the core of the MCF 20A are optically coupled. The fusion splicer 1012 has the same function as the fusion splicer 1011. The mechanisms for adjusting the core position that the fusion splicers 1011 and 1012 have also include a mechanism for rotational alignment. The rotational alignment mechanism aligns the cross sections of one MCF to be spliced with the cross section of the other MCF on a straight line, and rotates the cross section of the other MCF on a straight line relative to the cross section of the other MCF. This straight line passes through the centers of both MCFs. Fusion splicers 1011 and 1012 equipped with this function serve as the fusion means for fusion-splicing the MCFs.

[0039] The light source 1021 outputs reference light of a predetermined transmission power to one of the SCFs 111-119 in the fan-in 11A. The light source 1021 may incorporate a light-emitting element and a 1×9 optical switch, and the optical switch may be used to select one of the SCFs 111-119 in the fan-in 11A to which the light-emitting element is connected. The optical power meter 1022 receives the reference light transmitted by the light source 1021 and measures the power of the received light. The optical power meter 1022 may store the value of the transmission power of the reference light as data in advance. The optical power meter 1022 then calculates the loss of the reference light propagating through a path including the desired connection point from the difference between the transmission power and the reception power of the reference light. Note that the reference light propagates through only one core in each of the MCFs 10A, 20A, and 30A. Therefore, the fan-out 32A is not required when the optical power meter 1022 measures the power of the reference light. For example, the end face of MCF 30A may be directly connected to the optical power meter 1022 to measure the power of the reference light output from MCF 30A. The optical power meter 1022 serves as loss measurement means for measuring the loss before the connection between MCF 10A and MCF 20A and before the connection between MCF 20A and MCF 30A.

[0040] When performing rotational alignment at connection points 51A and 52A, by changing the rotation angle from the initial position by 90 degrees, paths using different cores are configured for MCFs 10A, 20A, and 30A. The initial position is the position where cores of opposing MCFs with the same core number face each other. The core arrangement at connection point 51A in Figure 6 is shown as an example where the core position of MCF 20A, which faces MCF 10A, is rotated 90 degrees clockwise from the initial position. In this case, cores 1, 2, and 3 of MCF 10A face cores 3, 6, and 9 of MCF 20A, respectively. The same applies to the other cores. When MCF 10A and MCF 20A are fusion-spliced in this state, cores 1 to 9 of MCF 10A are connected to cores 3, 6, 9, 2, 5, 8, 1, 4, and 7 of MCF 20A, respectively. By further increasing the rotation angle of MCF 20A by 90 degrees, the connection relationships between the cores can be made even more different. Therefore, the connection relationship of the cores between MCF10A and MCF20A can be changed using the rotational alignment function of fusion splicer 1011. Similarly, the connection relationship of the cores between MCF20A and MCF30A can be changed using the rotational alignment function of fusion splicer 1012.

[0041] In this embodiment, the light source 1021 sequentially transmits reference light to nine cores at the splicing points 51A and 52A each time one of the MCFs is rotated 90 degrees clockwise or counterclockwise from its initial position. The optical power meter 1022 measures and outputs the loss of the reference light for each core in each of the spliced states of MCF10A, MCF20A, and MCF30A. The operator operating the fusion splicers 1011 and 1012 selects the rotation angles of the splicing points 51A and 52A to be set during splicing based on the measurement results of the loss for each rotation angle and each core. The selected rotation angles are, for example, the rotation angles of the splicing points 51A and 52A that minimize the variation in loss between the cores. The operator operates the fusion splicers 1011 and 1012 to splice MCF10 and MCF20, and MCF20 and MCF30 at each angle. The rotation angle may be selected by the optical power meter 1022 or a control circuit connected thereto. This procedure can reduce the variation in loss among the cores after connecting the MCFs 10A, 20A, and 30A. That is, the MCF transmission system 1000 of this embodiment can reduce the variation in loss among the cores of the MCF transmission line while suppressing an increase in loss in the optical transmission line.

[0042] 7 is a flowchart showing an example of the procedure for splicing MCFs according to this embodiment. Each time one of the opposing MCFs at a splice point is rotated a predetermined angle from its initial position, reference light is sequentially transmitted to multiple cores (step S11 in FIG. 7). Then, with the core positions of the MCFs adjusted, the loss of the reference light per core is measured (S12). From the results of the loss measurements for each rotation angle and per core, a rotation angle at the splice point that minimizes the variation in loss between cores after the core positions are adjusted is selected (S13), and the opposing MCFs are fused at the selected rotation angle (S14). The selected rotation angle is a combination of the rotation angle at splice point 51A and the rotation angle at splice point 52A.

[0043] The above-described MCF splicing procedure involves measuring the total loss of MCFs 10A, 20A, and 30A while changing the rotation angle, and then determining the rotation angle at splicing points 51A and 52A. Alternatively, an optical power meter 1022 may first be placed at splicing point 52A, and a preferable rotation angle may be found between MCF 10A and MCF 20A using the same procedure as described above, followed by rotation alignment and fusion splicing. Next, rotation alignment and fusion splicing may be performed similarly between MCF 20A and MCF 30A. In this case, the reference light transmitted by light source 1021 propagates through MCF 10A and MCF 20A, emerges at splicing point 52A, and then propagates through MCF 30A to be received by optical power meter 1022.

[0044] (Third embodiment) 8 is a diagram showing an example of the configuration of an MCF transmission system 2000. The MCF transmission system 2000 is an optical transmission system in which an optical repeater 510 and an optical repeater 520 are connected by MCFs 10A, 20A, and 30A. The optical repeaters 510 and 520 have an optical amplification function for amplifying signal light. The optical fibers at the input and output of the optical repeaters 510 and 520 are MCFs. The MCFs 10A, MCFs 20A, and MCFs 30A are the 9-core MCFs described in FIG. 6.

[0045] The optical repeater 510 has a fan-out 511 at its input and a fan-in 512 at its output. An optical amplifier 513 is arranged between the fan-out 511 and the fan-in 512. The optical amplifier 513 includes erbium-doped fiber amplifiers (EDFAs) arranged in parallel, the same number as the number of cores in the MCF 10A. The input / output interface of each EDFA is an SCF. The input side of the optical amplifier 513 is connected to the fan-out 511 by nine SCFs, and the output side of the optical amplifier 513 is connected to the fan-in 512 by nine SCFs. The optical transmitter circuit 514 couples the reference light to one SCF selected from the nine SCFs included in the fan-in 512. The reference light has a wavelength (e.g., 1510 nm) that does not overlap with the signal light, and is coupled with the signal light amplified by the EDFA and transmitted toward the MCF 10A.

[0046] The configuration of the optical repeater 520 differs from the optical repeater 510 in that it includes an optical receiving circuit 525 instead of the optical transmitting circuit 514. The functions of the fan-out 521, the fan-in 522, and the optical amplifier 523 are similar to those of the fan-out 511, the fan-in 512, and the optical amplifier 513 of the optical repeater 510. The optical receiving circuit 525 has a function of selecting and receiving the reference light input from the MCF 30A for each core and measuring its power.

[0047] The optical transmitting circuit 514 and the optical receiving circuit 525 may be included in a general optical supervisory circuit that uses an optical supervisory channel (OSC). The supervisory light transmitted by the OSC may be used as the reference light. Since the configuration for multiplexing the supervisory light and the signal light in an optical repeater and the configuration for demultiplexing the supervisory light and the signal light are well known, detailed description of the multiplexing and demultiplexing of the reference light and the signal light will be omitted.

[0048] The optical repeater 510 and the optical repeater 520 are connected via MCFs 10A, 20A, and 30A. Each core of the MCF 10A is connected to each SCF in a fan-in 512 of the optical repeater 510. Each core of the MCF 30A is connected to each SCF in a fan-out 521 of the optical repeater 520. Fusion splicers 1011 and 1012 with rotational alignment functions are provided at a connection point 51A between the MCF 10A and the MCF 20A and at a connection point 52A between the MCF 20A and the MCF 30A. A control circuit 600 is an electric circuit communicatively connected to the optical transmitting circuit 514, the optical receiving circuit 525, and the fusion splicers 1011 and 1012. The control circuit 600 controls some or all of these connected devices.

[0049] The optical transmitting circuit 514 of the optical repeater 510 selects one of the nine SCFs in the fan-in 512 and outputs reference light to the selected SCF. The reference light input to the selected SCF propagates through the core of the MCF 10A connected to that SCF, and then propagates through MCF 20A and MCF 30A via the gaps at connection points 51A and 52A. The optical receiving circuit 525 receives the reference light transmitted by the optical transmitting circuit 514 from the SCF in the fan-out 521 connected to the core through which the reference light propagates, and calculates the loss of the path along which the reference light propagates from the difference between the transmitted power and the received power of the reference light.

[0050] At this time, each time one of the MCFs at connection points 51A and 52A is rotated by a predetermined angle from its initial position, the optical transmitter circuit 514 sequentially transmits reference light to each of the nine SCF cores in the fan-in 512. The optical receiver circuit 525 then measures the loss of the path through which the reference light propagates in each case. Based on the loss measurement results, the control circuit 600 selects the rotation angle of the connection point that minimizes the variation in path loss.

[0051] When calculating the path loss, the path loss inside the optical repeaters 510 and 520 may be compensated for. The calculated loss is notified to the control circuit 600 via a communication line. The loss calculation may be performed in the control circuit 600.

[0052] The control circuit 600 may instruct the optical transmitting circuit 514 on the SCF from which to send the reference light and the transmission power of the reference light. In this case, the control circuit 600 may determine the core loss of the MCFs 10A, 20A, and 30A connected to the selected SCF from the difference between the transmission power and the power of the reference light measured by the optical receiving circuit 525 after sending the reference light. Alternatively, the transmission power of the reference light sent by the optical transmitting circuit 514 may be a fixed value defined in the MCF transmission system 2000.

[0053] The optical receiving circuit 525 may acquire, through communication using a communication line, information on the number of the SCF (i.e., the core number) selected by the optical transmitting circuit 514 and the core number of the MCF opposing the splicing points 51A and 52A. The optical receiving circuit 525 may correspond these pieces of information and transmit them to the control circuit 600. The optical receiving circuit 525 may acquire the number of the SCF selected by the optical transmitting circuit 514 from the optical transmitting circuit 514. The optical receiving circuit 525 may acquire information on the core numbers of the MCF opposing the splicing points 51A and 52A from the fusion splicers 1011 and 1012. The information on the core numbers of the opposing MCFs may be set in the fusion splicers 1011 and 1012 by an operator.

[0054] The optical receiving circuit 525 may use an optical switch to select an SCF that measures the received power of the reference light, thereby identifying the SCF in the fan-out 521 through which the reference light is transmitted. If the optical receiving circuit 525 cannot identify the core through which the reference light is received, the optical receiving circuit 525 may sequentially switch the SCFs that measure the received power of the reference light, searching for an SCF in which the reference light is detected.

[0055] The optical transmitting circuit 514 transmits the reference light to the selected SCF. The fusion splicer 1011 adjusts the core positions so that the core of MCF 10A and the core of MCF 20A are optically coupled. The fusion splicer 1012 adjusts the core positions so that the core of MCF 20A and the core of MCF 30A are optically coupled. The reference light propagates through one core of each of MCFs 10A, 20A, and 30A and is received by the optical receiving circuit 525. As a result, the loss of the propagation path of the reference light can be calculated from the difference between the transmission power of the reference light and the reception power of the reference light at the optical receiving circuit 525.

[0056] The above loss measurement procedure is performed for nine cores at connection point 51A, with rotation angles of 0, 90, 180, and 270 degrees. Furthermore, loss measurements are performed for nine cores at connection point 52A, with rotation angles of 0, 90, 180, and 270 degrees. The control circuit 600 controls fusion splicers 1011 and 1012 to control the rotation angles at connection points 51A and 52A. The control circuit 600 acquires information on the measured loss and the rotation angles of connection points 51A and 52A from the optical receiving circuit 525 and the fusion splicers 1011 and 1012. The control circuit 600 generates measured loss data for each core for each combination of the rotation angle at connection point 51A and the rotation angle at connection point 52A. From this loss data, the control circuit 600 extracts the rotation angle at connection point 51A and the rotation angle at connection point 52A that have the smallest core-to-core variation. Then, control circuit 600 outputs an instruction to fusion splicer 1011 to fusion splice MCF 10A and MCF 20A at the rotation angle at extracted connection point 51A. Control circuit 600 also outputs an instruction to fusion splicer 1012 to fusion splice MCF 20A and MCF 30A at the rotation angle at extracted connection point 52A.

[0057] In this way, the MCF transmission system 2000 selects a rotation angle that minimizes the variation in loss between cores, and at that angle, fusion-splices MCF 10 with MCF 20 and MCF 20 with MCF 30. As a result, the variation in loss between cores can be reduced after splicing MCFs 10A, 20A, and 30A.

[0058] The above-mentioned MCF connection procedure is the same as that of the second embodiment, in which the total loss of MCFs 10A, 20A, and 30A is measured while changing the rotation angle for each core, and the optical axes of the MCFs are adjusted so that the rotation angles at connection points 51A and 52A are respectively preferable values.

[0059] In this embodiment, the optical transmitting circuit 514, the optical receiving circuit 525, and the fusion splicers 1011 and 1012 are communicatively connected to the control circuit 600. The control circuit 600 controls these devices to automatically fusion-splice the MCFs at the splicing points 51A and 52A. The control circuit 600 may be a computer including a CPU and a recording device. The control circuit 600 may implement part or all of its functions by executing a program recorded in the recording device.

[0060] 9 is a flowchart showing an example of a control procedure for connecting MCF 10A, MCF 20A, and MCF 30A in control circuit 600. Control circuit 600 controls fusion splicers 1011 and 1012 so that cores with the same core number of opposing MCFs at connection points 51A and 52A are positioned (initial positions) to face each other (step S21 in FIG. 9). Through this control, cores with the same core number face each other between MCFs facing each other at connection points 51A and 52A.

[0061] The control circuit 600 controls the optical transmitter circuit 514 to transmit the reference light to one of the cores of the MCF 10A (i.e., one of the SCFs in the fan-in 512) (S22). The control circuit 600 controls the optical receiver circuit 525 to receive the reference light and measure the loss of the reference light (S23). The control circuit 600 repeats steps S22 and S23 for the other cores of the MCF 10A (S24).

[0062] The control circuit 600 controls the fusion splicers 1011 and 1012 to independently change the rotation angles at the connection points 51A and 52A. The control circuit 600 then measures the loss of the path formed at the changed rotation angles. Specifically, the control circuit 600 performs measurements in steps S22 to S24 for each combination of the rotation angles at the connection points 51A and 52A, excluding the initial state (S25). If any of the combinations of rotation angles is known in advance to be difficult to form into a path, that combination may be excluded from the loss measurement. For example, if there are multiple cores known to have high loss, combinations in which such cores are connected may be excluded from the loss measurement.

[0063] The optical receiving circuit 525 notifies the control circuit 600 of the loss measurement result of the reference light. The loss measurement result is associated with the conditions used during measurement and stored in the control circuit 600. The conditions used during loss measurement include at least the rotation angles of the connection points 51A and 52A used during measurement. The control circuit 600 also stores information indicating the arrangement of cores corresponding to the core numbers. In the initial state, MCF10A, MCF20A, and MCF30A configure paths such that cores with the same core number face each other. Therefore, the core number of the core through which the reference light propagates during loss measurement can be determined from the core arrangement information and the rotation angles of the connection points 51A and 52A. In this way, the control circuit 600 can store the loss of the reference light in association with the core numbers of MCF10A, MCF20A, and MCF30A that configure the corresponding path (S26).

[0064] Based on the loss measurement results for each rotation angle and each core, the control circuit 600 selects a rotation angle that constitutes a path with the smallest variation in loss among the paths formed by MCF10A, MCF20A, and MCF30A (S27). Alternatively, the control circuit 600 may select a rotation angle that constitutes a path in which the variation in loss among cores is equal to or less than a predetermined value. The selected rotation angle is a combination of the rotation angle at connection point 51A and the rotation angle at connection point 52A. The control circuit 600 then controls the fusion splicers 1011 and 1012 to fuse the opposing MCFs at the selected rotation angle (S28).

[0065] In the second and third embodiments, the MCFs 10A, 20A, and 30A are described as 9-core MCFs with a 3-row x 3-column arrangement. However, the core arrangement of the MCFs is not limited to this. The core position adjustment procedures in the second and third embodiments can be applied to the connection of MCFs in which the core arrangement is rotationally symmetrical about the center of the MCF. In other words, if the MCF has a core arrangement in which the positions of the cores around the center overlap by rotating the MCF around the center, the connection procedure in the second embodiment can be applied, and similar effects can be obtained.

[0066] That is, in the second and third embodiments, the cores of MCFs 10A, 20A, and 30A are arranged in the same manner, and the cores other than the central core of MCFs 10A, 20A, and 30A are arranged in positions that are rotationally symmetrical about the center. Fig. 6 shows that the cores of MCF 10A and MCF 20A are connected at connection point 51A in a rotationally symmetrical position facing each other. Fig. 6 also shows that the cores of MCF 20A and MCF 30A are connected at connection point 52A in a rotationally symmetrical position facing each other.

[0067] For example, in an MCF in which cores are arranged at the vertices of one or more regular hexagons centered at the center of the MCF, the core arrangement has six-fold rotational symmetry. Therefore, such a six-core MCF can be applied to the MCF transmission systems of the second and third embodiments. In this case, a different combination of cores is configured each time the six-core MCF is rotated 60 degrees from its initial position. Loss is measured for each of these paths, and cores with minimal loss variation are selected. The same effects as those of the second and third embodiments can be achieved with the six-core MCF. Furthermore, similar procedures can be used when cores are arranged at the vertices of a regular polygon other than a regular hexagon, and the center of the regular polygon is located at the center of the MCF.

[0068] The embodiment of the present disclosure can also be described as follows, but is not limited to the following.

[0069] (Appendix 1) A multi-core fiber transmission line in which a plurality of multi-core fibers including a first multi-core fiber and a second multi-core fiber are connected in series for each core, the cores of the first multicore fiber and the cores of the second multicore fiber to be connected are selected so that a variation in loss of a plurality of cores connected between the first multicore fiber and the second multicore fiber is reduced. Multicore fiber transmission line.

[0070] (Appendix 2) a fan-out connected to the first multi-core fiber and a fan-in connected to the second multi-core fiber, the connection is made between a single-core fiber included in the fan-out and a single-core fiber included in the fan-in; Multicore fiber transmission line described in Appendix 1.

[0071] (Appendix 3) 3. The multi-core fiber transmission line according to claim 2, wherein a core of the first multi-core fiber and a core of the second multi-core fiber to be connected are selected depending on a connection relationship between a single-core fiber provided in the fan-out and a single-core fiber provided in the fan-in.

[0072] (Appendix 4) the cores of the first multi-core fiber and the cores of the second multi-core fiber are arranged on a plurality of concentric circles centered on the center of each multi-core fiber, Some or all of the cores to be connected are selected from the cores arranged on the concentric circles having different radii. Multicore fiber transmission line described in Appendix 1.

[0073] (Appendix 5) 2. The multi-core fiber transmission line according to claim 1, wherein an arrangement of cores in the first multi-core fiber and an arrangement of cores in the second multi-core fiber are identical, and the cores are arranged at positions that are rotationally symmetric about a center.

[0074] (Appendix 6) 6. The multi-core fiber transmission line according to claim 5, wherein a core of the first multi-core fiber and a core of the second multi-core fiber are connected in a state where they face each other at the rotationally symmetric positions.

[0075] (Appendix 7) The multi-core fiber transmission line according to any one of Supplementary Note 1 to 6, wherein a core of the first multi-core fiber and a core of the second multi-core fiber are connected by fusion splicing.

[0076] (Appendix 8) 5. The multi-core fiber transmission line according to any one of appendixes 1 to 4, wherein a core of the first multi-core fiber and a core of the second multi-core fiber are connected by an optical connector.

[0077] (Appendix 9) A multicore fiber transmission line according to any one of Supplementary Notes 1 to 6; loss measurement means for measuring the loss before the connection; a fusion splicing means for directly fusion-splicing the first multi-core fiber and the second multi-core fiber; A multicore fiber transmission system comprising:

[0078] (Appendix 10) The multi-core fiber transmission system according to Supplementary Note 9, wherein the fusion means includes a mechanism that makes a cross section of the first multi-core fiber and a cross section of the second multi-core fiber face each other on a straight line, and rotates the cross section of the second multi-core fiber on the straight line relative to the cross section of the first multi-core fiber.

[0079] (Appendix 11) 11. A multi-core fiber transmission system according to claim 10, wherein the fusion splicing means performs the fusion splicing at an angle of rotation at which the loss variation in the path formed by the cores to be spliced is reduced.

[0080] (Appendix 12) 12. The multicore fiber transmission system according to claim 9, wherein the loss measurement means measures the loss for each core using a reference light propagating through the fusion spliced portion.

[0081] (Appendix 13) A multi-core fiber connection method for connecting a plurality of multi-core fibers including a first multi-core fiber and a second multi-core fiber for each core, comprising: selecting cores of the first multicore fiber and the second multicore fiber to be connected so that a variation in loss of a plurality of cores connected between the first multicore fiber and the second multicore fiber is reduced; Multicore fiber connection method.

[0082] (Appendix 14) the connection is made between a single-core fiber included in a fan-out connected to the first multicore fiber and a single-core fiber included in a fan-in connected to the second multicore fiber; A multicore fiber splicing method as described in Appendix 13.

[0083] (Appendix 15) 15. The multi-core fiber connection method according to claim 14, wherein a core of the first multi-core fiber and a core of the second multi-core fiber to be connected are selected based on a connection relationship between a single-core fiber provided in the fan-out and a single-core fiber provided in the fan-in.

[0084] (Appendix 16) the cores of the first multi-core fiber and the cores of the second multi-core fiber are arranged on a plurality of concentric circles centered on the center of each multi-core fiber; a part or all of the cores to be connected are selected from the cores arranged on the concentric circles and having different radii; A multicore fiber splicing method as described in Appendix 13.

[0085] (Appendix 17) 14. The multi-core fiber connection method according to claim 13, wherein the arrangement of cores of the first multi-core fiber and the arrangement of cores of the second multi-core fiber are the same, and the cores are arranged at positions that are rotationally symmetric about the center.

[0086] (Appendix 18) 18. The multi-core fiber connection method according to any one of appendices 13 to 17, wherein a core of the first multi-core fiber and a core of the second multi-core fiber are connected by fusion splicing.

[0087] (Appendix 19) measuring the loss before the connection; directly fusion-splicing the first multi-core fiber and the second multi-core fiber; A multi-core fiber connection method comprising any one of appendixes 13 to 18.

[0088] (Appendix 20) a cross section of the first multi-core fiber and a cross section of the second multi-core fiber are arranged to face each other on a straight line; rotating the cross section of the second multi-core fiber on the straight line relative to the cross section of the first multi-core fiber; A multicore fiber splicing method as described in Appendix 19.

[0089] (Appendix 21) 21. The multicore fiber splicing method according to claim 19, wherein the fusion splicing is performed at an angle of rotation at which the loss variation in the path formed by the cores to be spliced is reduced.

[0090] (Appendix 22) 22. A multicore fiber splicing method according to any one of appendixes 13 to 21, wherein the loss is measured for each core using a reference light propagating through the fusion spliced portion.

[0091] (Appendix 23) A computer of a multi-core fiber transmission system configured by connecting a plurality of multi-core fibers including a first multi-core fiber and a second multi-core fiber for each core, executing a procedure of selecting cores of the first multicore fiber and cores of the second multicore fiber to be connected such that a variation in loss of a plurality of cores connected between the first multicore fiber and the second multicore fiber is reduced; program.

[0092] (Appendix 24) 24. The program according to claim 23, further comprising a step of measuring the loss for each core using a reference light propagating through the connected portion.

[0093] Although the present disclosure has been described above with reference to the embodiments, the present disclosure is not limited to the above embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present disclosure within the scope of the present disclosure.

[0094] For example, the multi-core fiber transmission line and multi-core fiber transmission system described in each embodiment also disclose a multi-core fiber connection method and a program including part or all of the procedure thereof.

[0095] Furthermore, the configurations described in the respective embodiments are not necessarily mutually exclusive, and the functions and effects of the present disclosure may be achieved by a configuration that combines all or part of the above-described embodiments. [Explanation of symbols]

[0096] 1-9 Core 10, 10A, 20, 20A, 30, 30A MCF 11A, 21, 21A, 31 Fan-in 12, 22, 32A fanout 51, 51A, 52, 52B connection points 100 MCF transmission line 101-107, 201-207, 301-307 Core 108 circumference 109 Regular hexagon 510, 520 Optical repeater 511, 521 fanout 512, 522 Fan-in 513, 523 Optical amplifier 514 Optical transmitter circuit 525 Optical receiving circuit 600 control circuit 1000, 2000 MCF transmission system 1011, 1012 fusion splicer 1021 light source 1022 Optical Power Meter

Claims

1. A multi-core fiber transmission line in which a plurality of multi-core fibers including a first multi-core fiber and a second multi-core fiber are connected in series for each core, the cores of the first multicore fiber and the cores of the second multicore fiber to be connected are selected so that a variation in loss of a plurality of cores connected between the first multicore fiber and the second multicore fiber is reduced. Multicore fiber transmission line.

2. a fan-out connected to the first multi-core fiber and a fan-in connected to the second multi-core fiber, the connection is made between a single-core fiber included in the fan-out and a single-core fiber included in the fan-in; The multicore fiber transmission line according to claim 1 .

3. 3. The multicore fiber transmission line according to claim 2, wherein cores of the first multicore fiber and the second multicore fiber to be connected are selected depending on a connection relationship between a single-core fiber provided in the fan-out and a single-core fiber provided in the fan-in.

4. the cores of the first multicore fiber and the cores of the second multicore fiber are arranged on a plurality of concentric circles centered on the center of each multicore fiber, Some or all of the cores to be connected are selected from the cores arranged on the concentric circles having different radii. The multicore fiber transmission line according to claim 1 .

5. 2. The multicore fiber transmission line according to claim 1, wherein an arrangement of cores in the first multicore fiber and an arrangement of cores in the second multicore fiber are the same, and the cores are arranged at positions that are rotationally symmetric about a center.

6. The multicore fiber transmission line according to claim 5 , wherein the cores of the first multicore fiber and the cores of the second multicore fiber are connected in a state where they face each other at the rotationally symmetric positions.

7. The multi-core fiber transmission line according to claim 1 , wherein the cores of the first multi-core fiber and the cores of the second multi-core fiber are connected by fusion splicing.

8. The multicore fiber transmission line according to claim 1 , wherein the cores of the first multicore fiber and the cores of the second multicore fiber are connected by an optical connector.

9. a multicore fiber transmission line according to any one of claims 1 to 4; loss measurement means for measuring the loss before the connection; a fusion splicing means for directly fusion-splicing the first multi-core fiber and the second multi-core fiber; A multicore fiber transmission system comprising:

10. A multi-core fiber connection method for connecting a plurality of multi-core fibers including a first multi-core fiber and a second multi-core fiber for each core, comprising: selecting cores of the first multicore fiber and cores of the second multicore fiber to be connected such that a variation in loss of a plurality of cores connected between the first multicore fiber and the second multicore fiber is reduced; Multicore fiber connection method.

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