Optical fiber transmission line and optical transmission method

Abstract

To suppress a decrease in monitoring accuracy of an optical fiber transmission line caused by crosstalk.SOLUTION: An optical fiber transmission line includes a first optical fiber consisting of multiple cores that share a cladding, a second optical fiber that does not share a cladding with the first optical fiber, and a first coupling circuit that couples the core of the first optical fiber with the core of the second optical fiber, and the coupling circuit couples the return light of the first light propagating through the core of the first optical fiber to the core of the second optical fiber as second light.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to optical fiber transmission lines and the like. [Background technology]

[0002] In optical transmission systems, a multicore fiber (MCF) having multiple cores housed in a single optical cable is sometimes used to efficiently accommodate rapidly increasing traffic demand. Meanwhile, an OTDR (optical time-domain reflectometer) device is known as a device for monitoring the status of an optical transmission system. An OTDR device is usually installed in a terminal station of the optical transmission system. The OTDR device can monitor an optical fiber transmission line by transmitting monitoring light and measuring Rayleigh scattered light generated by the monitoring light. By switching the core to which the monitoring light output from the OTDR device is input, each core of the MCF can be monitored one by one. In relation to the present disclosure, Patent Document 1 describes a technology related to reducing XT (crosstalk) in a multi-core optical fiber. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2023 / 026486 Summary of the Invention [Problem to be solved by the invention]

[0004] In optical transmission systems using MCF (MCF transmission systems), crosstalk occurs, where light propagating through one adjacent core leaks into the other core. There are two types of crosstalk between cores: codirectional crosstalk and counter-directional crosstalk. Codirectional crosstalk is crosstalk between light propagating in the same direction between two cores. Counter-directional crosstalk is crosstalk between light propagating in opposite directions between two cores. When comparing the magnitude of codirectional crosstalk and counter-directional crosstalk, counter-directional crosstalk is generally smaller. For this reason, in the FP and CP of MCF transmission systems, counter-directional transmission, a transmission method in which the light propagates in different directions in the two cores, is often used to suppress degradation of transmission performance due to crosstalk.

[0005] However, when monitoring an MCF using an OTDR, crosstalk light may be superimposed on the Rayleigh scattered light due to back-channel crosstalk caused by the monitoring light. Because the crosstalk light becomes noise in the Rayleigh scattered light in the OTDR, the influence of back-channel crosstalk may reduce the accuracy of the monitoring results.

[0006] (Purpose of disclosure) An object of the present disclosure is to provide a technique for suppressing a decrease in monitoring accuracy of an optical fiber transmission line caused by crosstalk. [Means for solving the problem]

[0007] The optical fiber transmission line of the present disclosure comprises a first optical fiber consisting of multiple cores that share a cladding, a second optical fiber that does not share a cladding with the first optical fiber, and a first coupling circuit that couples the core of the first optical fiber with the core of the second optical fiber, and the coupling circuit couples return light of a first light propagating through the core of the first optical fiber to the core of the second optical fiber as a second light.

[0008] An optical transmission method according to the present disclosure is an optical transmission method used in an optical fiber transmission system including an optical fiber transmission line and an OTDR (optical time-domain reflectometer) device, wherein the optical fiber transmission line includes: a first optical fiber consisting of multiple cores that share a cladding; a second optical fiber that does not share a cladding with the first optical fiber; and a first coupling circuit that couples a core of the first optical fiber with a core of the second optical fiber; The method includes the steps of: coupling, by the coupling circuit, return light of the first light propagating through the core of the first optical fiber into the core of the second optical fiber as second light; and transmitting, by the OTDR device, the first light to the first optical fiber and receiving the second light from the second optical fiber. [Effects of the Invention]

[0009] The technology according to the present disclosure can suppress a decrease in the monitoring accuracy of an optical fiber transmission line caused by crosstalk. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram illustrating an example of the configuration of an optical fiber transmission line. [Figure 2] 1A and 1B are diagrams showing examples of the results of monitoring an optical fiber using common techniques. [Figure 3] 10A and 10B are diagrams illustrating an example of the results of monitoring an optical fiber when the influence of inter-core crosstalk is small. [Figure 4] FIG. 1 is a diagram illustrating an example of the configuration of an optical fiber transmission line. [Figure 5] FIG. 1 is a diagram illustrating an example of the configuration of an optical fiber transmission line. [Figure 6] FIG. 1 is a diagram illustrating an example of the configuration of an optical fiber transmission line. [Figure 7] FIG. 1 is a diagram illustrating an example of the configuration of an optical fiber transmission system. [Figure 8] 1 is a diagram illustrating an example of the propagation of monitoring light and Rayleigh scattered light in an optical fiber transmission system. [Figure 9]1A and 1B are diagrams illustrating an example of propagation of crosstalk light in an optical fiber transmission system. [Figure 10] FIG. 1 is a diagram illustrating an example of the configuration of an optical fiber transmission system. [Figure 11] FIG. 1 is a diagram illustrating an example of the configuration of an optical fiber transmission system. [Figure 12] FIG. 1 is a diagram showing a general configuration for monitoring an optical fiber transmission line. DETAILED DESCRIPTION OF THE INVENTION

[0011] Embodiments of the present disclosure will be described below with reference to the drawings. In the embodiments and drawings, elements already mentioned are given the same reference symbols, and duplicated descriptions may be omitted. Furthermore, the direction and width of arrows in the drawings are not intended to limit the direction or amplitude of signals, etc. Note that the multicore fiber used in each embodiment is an uncoupled multicore fiber unless otherwise specified.

[0012] (First embodiment) FIG. 1 is a diagram illustrating an example configuration of an optical fiber transmission line 100 according to the present disclosure. The optical fiber transmission line 100 includes a first optical fiber 110, a second optical fiber 120, and a first coupling circuit 130. The first optical fiber 110 and the second optical fiber 120 are different optical fibers. The first optical fiber 110 is an uncoupled multicore fiber consisting of multiple cores that share a cladding, and includes a first core 111. The second optical fiber 120 is an optical fiber that does not share a cladding with the first optical fiber 110, and includes a second core 121. In FIG. 1 , the first light propagates through the first core 111 from left to right on the page. The first optical fiber 110 may include a third core 112. Furthermore, when the first optical fiber 110 is used as a transmission line for signal light, a communication method may be used in which crosstalk between signal light is suppressed between the multiple cores in the first optical fiber 110. For example, the first optical fiber 110 may be configured to perform counter-directional transmission using the first core 111 and the third core 112. That is, the direction of the signal light propagating through the first core 111 and the direction of the signal light propagating through the third core 112 may be opposite to each other.

[0013] The first coupling circuit 130 is inserted midway through each of the first optical fiber 110 and the second optical fiber 120. The first coupling circuit 130 couples the return light 103 to the second core 121 as second light 102. Here, the return light 103 is light generated by backscattering or reflecting the first light 101, and the second light 102 and the return light 103 are light propagating in the opposite direction to the first light 101 in the optical fiber transmission line 100.

[0014] The optical fiber transmission line 100 having such a configuration can suppress a decrease in monitoring accuracy caused by inter-core crosstalk when monitoring the first optical fiber 110 by propagating the first light through the first core 111. The reason for this is that the second core 121 is included in the second optical fiber 120 that does not share a clad with the first optical fiber 110, and therefore the counter-crosstalk between the light propagating through the first optical fiber 110 and the light propagating through the second core 121 can be ignored.

[0015] FIG. 12 is a diagram showing a general configuration for monitoring an optical fiber transmission line 900 using an OTDR device 10. A core 911 and a core 912 are provided in the same optical fiber 910. A monitor light 901 is input to the core 911 from the OTDR device 10. An optical repeater 930 is provided midway through the optical fiber 910. The optical repeater 930 includes an optical circuit that guides Rayleigh scattered light 902 generated by the monitor light 901 to the core 912. The OTDR device 10 receives the Rayleigh scattered light 902 from the core 912 (indicated by the dashed arrow). In this case, because the cores 911 and 912 are in the same optical fiber 910, crosstalk light caused by the monitor light 901 is superimposed on the Rayleigh scattered light 902 due to opposing crosstalk within the optical fiber 910. This crosstalk light acts as noise to the Rayleigh scattered light, which may reduce measurement accuracy when measuring the state of the optical fiber transmission line 900 using the Rayleigh scattered light.

[0016] On the other hand, the optical fiber transmission line 100 in FIG. 1 can guide the return light 103 to the second core 121 via the first coupling circuit 130 and propagate the return light 103 as second light 102 through the second core 121. Here, backscattered light generated by the first light 101 is an example of the return light 103. The backscattered light may be Rayleigh scattered light. That is, the OTDR device 10 can receive the Rayleigh scattered light (return light 103) from the second core 121 as the second light 102. Because the second core 121 is included in a different optical fiber from the first core 111, crosstalk between the first core 111 and the second core 121 can be negligible. Therefore, compared to general technologies, the optical fiber transmission line 100 can reduce noise superimposed on the Rayleigh scattered light due to back-to-back crosstalk, thereby improving the monitoring accuracy of the optical fiber transmission line 100 using the OTDR device 10.

[0017] The second core 121 shown in FIG. 1 may be included in one single-core fiber. Alternatively, the second core 121 may be included in another multi-core fiber (second multi-core fiber). The second multi-core fiber may be included in the optical fiber transmission line 100. Furthermore, the optical fiber transmission line 100 may include a second coupling circuit. The second coupling circuit couples light propagating through the second core 121 in the opposite direction to the second light 102 into the direction of the first light 101 at the first core 111. Furthermore, the first core 111, the second core 121, and the third core 112 may be housed in the same optical cable.

[0018] Figure 2 shows an example of the results of monitoring an optical fiber using a common technique. The vertical axis represents the intensity of Rayleigh scattered light, and the horizontal axis represents the distance from the OTDR device. Long-distance optical fiber may have fusion splices due to repairs, and monitoring using an OTDR device reveals these splices as changes in the intensity of Rayleigh scattered light. Figure 2 shows that the intensity of Rayleigh scattered light varies depending on the position within the optical fiber due to the effects of crosstalk. Figure 2 shows an example in which, even if a section repaired by fusion exists at the position indicated by the downward arrow, the change in the intensity of Rayleigh scattered light is masked by the effects of crosstalk light.

[0019] Such irregular intensity variations in Rayleigh scattered light can occur when the magnitude of the back-to-back crosstalk is non-uniform along the length of the optical fiber. Non-uniform back-to-back crosstalk can also be caused by bending or twisting during optical fiber manufacturing or after optical fiber installation. Furthermore, long-distance optical fibers are sometimes constructed by connecting multiple optical fibers with different properties and structures in order to adjust optical properties such as dispersion. The mixture of different types of optical fibers can also cause non-uniform crosstalk.

[0020] Figure 3 shows an example of the results of monitoring an optical fiber when the influence of back-to-back crosstalk can be ignored. In Figure 3, the influence of back-to-back crosstalk on the Rayleigh scattered light is small, so the intensity of the Rayleigh scattered light decreases almost linearly with distance. The fusion point is observed as a point where the intensity of the Rayleigh scattered light changes more sharply than in other sections.

[0021] In this way, when monitoring an optical fiber using an OTDR device, the accuracy of monitoring the optical fiber by the OTDR device can be improved by reducing the effect of crosstalk between the core through which the monitoring light propagates and the core through which the Rayleigh scattered light propagates.

[0022] (Second embodiment) FIG. 4 is a diagram illustrating an example configuration of an optical fiber transmission line 200 according to the present disclosure. The optical fiber transmission line 200 includes optical fibers 210, 220, 230, and 240, and an optical repeater 250. The optical fiber 210 includes at least cores 211 and 212, and the cores 211 and 212 form a core pair. A core pair (CP) is formed by selecting and assigning two cores from multiple cores included in an MCF. The core pair of a two-core MCF is also called a fiber pair (FP). Like the optical fiber 210, the optical fibers 220, 230, and 240 also include at least two cores, and these cores form a core pair. The reference symbols for each component of the optical fibers 220, 230, and 240 are assigned in accordance with those for the optical fiber 210. In this diagram, the direction from left to right on the page is referred to as the "downstream," and the opposite direction from the "downstream" is referred to as the "upstream." Furthermore, for example, light propagating in the downstream direction will be referred to as “downstream light.” The optical fiber transmission line 200 is connected to the OTDR device 20 via FIFOs 265 and 266. A FIFO (Fan-in / Fan-out) is a known optical component that connects a multi-core fiber and a single-core fiber.

[0023] The optical path inside the optical repeater 250 is wired by a single-core fiber. Therefore, FIFOs 261-264 are provided between the inside of the optical repeater 250 and each of the optical fibers 210, 220, 230, and 240. The FIFOs 261-264 may be provided in the optical repeater 250. In FIG. 4 and the subsequent figures, the optical paths connected to the FIFOs are not shown.

[0024] The optical repeater 250 includes optical amplifiers 271-274. The optical amplifier 271 amplifies downstream light input from the core 211 and outputs the amplified light to the core 221. The optical amplifier 272 amplifies upstream light input from the core 222 and outputs the amplified light to the core 212. Similarly, the optical amplifier 273 amplifies downstream light input from the core 231 and outputs the amplified light to the core 241. The optical amplifier 274 amplifies upstream light input from the core 242 and outputs the amplified light to the core 232. The optical amplifiers 271-274 are one form of repeater circuits that repeat light propagating through the cores.

[0025] The optical repeater 250 includes a coupling circuit 281 that connects the core 221 and the core 242. The coupling circuit 281 guides the upstream light propagating through the core 221 from the output side of the optical amplifier 271 to the input side of the optical amplifier 274. The light propagating through the coupling circuit 281 is amplified by the optical amplifier 274 and propagates through the core 232 as upstream light. The coupling circuit 281 can be configured by optical couplers provided on the output side of the optical amplifier 271 and the input side of the optical amplifier 274, and optical paths connecting these optical couplers. The optical couplers are, for example, 2x1 optical couplers or 2x2 optical couplers. The optical couplers branch and couple the input light. These optical paths may be configured using, for example, single-core fibers or optical waveguides.

[0026] In the optical fiber transmission line 200 having such a configuration, when a monitor light is input into the core 211 in the downstream direction using the OTDR device 20, Rayleigh scattered light generated in the core 221 propagates in the upstream direction through the core 221. The Rayleigh scattered light is then input into the optical amplifier 274 via the coupling circuit 281 and propagates in the upstream direction through the core 232. The Rayleigh scattered light that has propagated through the core 232 is received by the OTDR device 20. The OTDR device 20 acquires the state of the core 221 using the received Rayleigh scattered light. The Rayleigh scattered light generated in the core 221 propagates through the optical fiber 230, which is different from the core 211, and is received by the OTDR device 20. Therefore, the Rayleigh scattered light is transmitted to the OTDR device 20 without being significantly affected by crosstalk caused by the monitor light. Therefore, the configuration of the optical fiber transmission line 200 suppresses a decrease in the accuracy of monitoring the core 221 by the OTDR device 20.

[0027] (First modified example of the second embodiment) 5 is a diagram showing an example configuration of an optical fiber transmission line 200A according to the present disclosure. The optical fiber transmission line 200A includes an optical repeater 250A instead of the optical repeater 250 of the optical fiber transmission line 200. The optical repeater 250A is a modified example of the optical repeater 250. The following mainly describes the changes in the optical repeater 250A from the optical repeater 250.

[0028] The optical repeater 250A includes coupling circuits 281 to 284. The coupling circuit 281 couples the upstream light of the core 221 with the upstream optical path of the core 242 at the input of the optical amplifier 274.

[0029] The combining circuit 282 combines the upstream light in the core 241 with the upstream light path in the core 212 at the input of the optical amplifier 272 .

[0030] The combiner circuit 283 combines the downstream light in the core 212 with the downstream light path in the core 231 at the input of the optical amplifier 273 .

[0031] The combining circuit 284 combines the downstream light in the core 232 with the downstream optical path in the core 221 at the input of the optical amplifier 271 .

[0032] By including the coupling circuits 281-284, the optical repeater 250A can guide light traveling in the opposite direction to the amplified light to the cores of other fibers in each of the cores to which the light amplified in the optical amplifiers 271-274 is input. For example, when monitor light is input in the upstream direction to the core 242, Rayleigh scattered light propagating in the downstream direction is generated in the core 232. This Rayleigh scattered light is guided to the input of the optical amplifier 271 via the coupling circuit 284. The Rayleigh scattered light is then amplified in the optical amplifier 271 and can be propagated as downstream light in the core 221. In this case, the Rayleigh scattered light is transmitted without being affected by crosstalk caused by the monitor light propagating in the core 242. Therefore, the core 232 can be monitored by inputting monitor light in the upstream direction to the core 242 and analyzing the Rayleigh scattered light propagating in the downstream direction through the core 221.

[0033] When the monitoring light is input in the upstream direction of the core 222 or in the downstream direction of the core 231, the core 212 and the core 241 can be monitored by the same action. Therefore, the optical repeater 250A and the optical fiber transmission line 200A incorporating the optical repeater 250A instead of the optical repeater 250 can suppress a decrease in monitoring accuracy using an OTDR device.

[0034] (Second Modification of the Second Embodiment) 6 is a diagram showing an example configuration of an optical fiber transmission line 200B according to the present disclosure. The optical fiber transmission line 200B includes an optical repeater 250B. The optical repeater 250B is a modified example of the optical repeater 250A. The following description will mainly focus on the changes made to the optical repeater 250B from the optical repeater 250A.

[0035] The optical repeater 250B includes coupling circuits 281B-284B, which reflect the input light and cause it to propagate in the opposite direction. The coupling circuits 281B-284B include reflectors 291B-294B, respectively.

[0036] The reflector 291B reflects part or all of the input light. In this embodiment, the reflector 291B is configured to reflect part of the light output from the optical amplifier 271 and guide it to the combining circuit 281. The same applies to the reflectors 292B-294B. The reflectors 291B-294B may be arranged at one end of the 2×2 optical couplers that make up the combining circuits 281B-284B. In this case, part of the light output from the optical amplifiers 271-274 is incident on the reflectors 291B-294B via the 2×2 optical couplers and reflected there. The reflected light is input to the combining circuits 281B-284B via the 2×2 optical couplers, respectively.

[0037] The reflectors 291B-294B may selectively reflect light of some wavelengths of the transmitted light. For example, the reflectors 291B-294B may reflect light (control light) including a control signal used for monitoring and controlling the optical repeater 250 in the optical fiber transmission line 200B. The control light is looped back to the cores connected to the coupling circuits 281-284 via the coupling circuits. The looped back control light is received by a monitor and control device in the terminal station. The monitor and control device monitors the state of the optical fiber transmission line 200 using the reflected light input from any of the reflectors 291B-294B. The OTDR device connected to the optical fiber transmission line 200 may have the function of the monitor and control device.

[0038] (Third embodiment) FIG. 7 is a diagram illustrating an example configuration of an optical fiber transmission system 1000 according to the present disclosure. The optical fiber transmission system 1000 includes an optical fiber transmission line 300 and a terminal station 330. The optical fiber transmission line 300 includes two-core MCFs 310 and 320, and optical repeaters 351-353. FIFOs 341 and 342 are disposed between the terminal station 330 and the MCFs 310 and 320, respectively. The optical repeaters 351-353 have the same configuration. Therefore, the optical repeaters 351-353 may be collectively referred to as the optical repeater 350. The optical fiber transmission line 300 has a configuration in which the optical fiber transmission lines 200B described in FIG. 6 are cascaded. That is, the MCF 310 includes the optical fibers 210 and 220 of FIG. 6, and the MCF 320 includes the optical fibers 230 and 240 of FIG. 4. As the optical repeater 350, any one of the optical repeaters 250, 250A, and 250B in FIG. 4 can be used.

[0039] The terminal station 330 includes an OTDR device 331 and a switch 332. The OTDR device 331 outputs a monitor light to the switch 332. Rayleigh scattered light generated in the optical fiber transmission line 300 by the monitor light is input from the switch 332 to the OTDR device 331. The OTDR device 331 analyzes the received Rayleigh scattered light to monitor the state of the core through which the monitor light has propagated.

[0040] The switch 332 connects the OTDR device 331 to the cores of the MCF 310 and the cores of the MCF 320. The switch 332 independently selects one core from which the OTDR device 331 transmits monitoring light and one core from which it receives Rayleigh scattered light. The OTDR device 331 and the switch 332 may be operated by a maintenance technician working at a terminal station, or may be controlled by a monitoring system located at a remote location.

[0041] FIG. 8 is a diagram illustrating an example of the propagation of monitor light and Rayleigh scattered light in the optical fiber transmission system 1000 according to the present disclosure. In FIG. 8, the switch 332 is configured so that the monitor light output from the OTDR device 331 is input to the core 311 of the MCF 310. The switch 332 is also configured so that the Rayleigh scattered light is received from the core 322 of the MCF 320. In this configuration, the upstream Rayleigh scattered light generated by the monitor light propagating downstream through the MCF 310 is guided to the core 322 of the MCF 320 via the coupling circuits 281 provided in each of the optical repeaters 251-253. The Rayleigh scattered light propagating upstream through the core 322 is received by the OTDR device 331 via the switch 332. Here, the coupling circuits 281 provided in each optical repeater do not connect the core 311 to the core 312, but connect the core 311 to the core 322. Therefore, the Rayleigh scattered light generated by the monitoring light propagating through the core 311 does not propagate through the core 312, but is guided to the core 322. As a result, the OTDR device 331 can monitor the core 311 by analyzing the Rayleigh scattered light that has propagated through the core 322.

[0042] FIG. 9 is a diagram illustrating an example of propagation of crosstalk light in the optical fiber transmission system 1000 according to the present disclosure. The connection between the OTDR device 331 and the optical fiber transmission line 300 in FIG. 9 is the same as that in FIG. 8. In FIG. 9, counter crosstalk light generated in response to monitoring light propagating downstream through the core 311 of the MCF 310 propagates through the core 312 adjacent to the core 311. However, since the core 322 is in a different optical fiber from the cores 311 and 312, no crosstalk occurs between the cores 312 and 322. Furthermore, the coupling circuit 281 does not connect these cores. Therefore, the upstream crosstalk light propagating through the core 312 does not affect the Rayleigh scattered light propagating through the core 322. Therefore, the optical fiber transmission system 1000 can suppress degradation of monitoring accuracy caused by crosstalk light when analyzing the Rayleigh scattered light propagating through the core 322.

[0043] (Fourth embodiment) 10 is a diagram showing an example configuration of an optical fiber transmission system 2000 according to the present disclosure. The optical fiber transmission system 2000 includes SCFs 410 and 420 instead of the MCF 320 included in the optical fiber transmission system 1000. An SCF (single-core fiber) is an optical fiber having only one core. The core 411 of the SCF 410 and the core 421 of the SCF 420 correspond to the core 321 and the core 322 included in the optical fiber transmission system 1000, respectively.

[0044] 8 , in each optical repeater 250, the coupling circuit 281 connects the core 311 of the MCF 310 and the core 421 of the SCF 420. Furthermore, the core 312 is not connected to the core 421. Therefore, in the optical fiber transmission system 2000, the OTDR device 331 can monitor the core 311 by analyzing the Rayleigh scattered light that has propagated through the core 421. Furthermore, the optical fiber transmission system 2000 can suppress a decrease in monitoring accuracy caused by crosstalk light when analyzing the Rayleigh scattered light that propagates through the core 421.

[0045] (Fifth embodiment) 11 is a diagram illustrating an example configuration of an optical fiber transmission system 3000 according to the present disclosure. The optical fiber transmission system 3000 includes MCFs 510 and 520 instead of the MCFs 310 and 320 included in the optical fiber transmission system 1000. Both the MCFs 510 and 520 are four-core MCFs. The MCF 510 includes cores 511-514, and the MCF 520 includes cores 521-524. The cores 511 and 512 form one core pair, and the cores 513 and 514 form another core pair. Similarly, the cores 521 and 522 form one core pair, and the cores 523 and 524 form another core pair. Furthermore, optical repeaters 551-553 amplify light propagating through the two cores that form one core pair. The optical repeaters 551-553 amplify light propagating in opposite directions through the two cores of one core pair.

[0046] In each of the optical repeaters 551-553, a coupling circuit 561 connects the core 511 of the MCF 510 and the core 524 of the MCF 520.

[0047] Because MCF 510 is a four-core MCF, Rayleigh scattered light generated in core 511 also propagates through cores 512-514 due to crosstalk within MCF 510. However, because cores 512-514 are not connected to core 524, the crosstalk light propagating through cores 512-514 does not affect the light propagating through core 524. In particular, Rayleigh scattered light leaking from core 511 to core 513 due to crosstalk is blocked by optical isolators provided at the outputs of the optical amplifiers in optical repeaters 551-553. That is, Rayleigh scattered light leaking to core 513 is blocked by the nearest optical repeater.

[0048] In this way, also in the optical fiber transmission system 3000, the OTDR device 331 can monitor the core 511 by analyzing the Rayleigh scattered light that has propagated through the core 524. Then, the optical fiber transmission system 3000 can suppress a decrease in monitoring accuracy caused by crosstalk light when analyzing the Rayleigh scattered light that has propagated through the core 524.

[0049] The embodiments of the present disclosure can also be described as follows, but are not limited to these.

[0050] (Appendix 1) a first optical fiber consisting of multiple cores sharing a cladding; a second optical fiber that does not share a cladding with the first optical fiber; a first coupling circuit that couples a core of the first optical fiber with a core of the second optical fiber; Equipped with the coupling circuit couples return light of the first light propagating through the core of the first optical fiber to the core of the second optical fiber as second light; Optical fiber transmission line.

[0051] (Appendix 2) 2. The optical fiber transmission line according to claim 1, wherein the return light is light generated by backscattering or reflecting the first light.

[0052] (Appendix 3) 3. The optical fiber transmission line according to claim 1, wherein the coupling circuit connects the first optical fiber and the second optical fiber so as to suppress crosstalk between the first light and the return light between a plurality of cores in the first optical fiber.

[0053] (Appendix 4) 3. The optical fiber transmission line according to claim 1, wherein a communication method is used in which crosstalk between signal lights is suppressed between a plurality of cores in the first optical fiber.

[0054] (Appendix 5) 5. The optical fiber transmission line according to claim 4, wherein the communication method includes a method of transmitting signal light in opposite directions in the first optical fiber.

[0055] (Appendix 6) 6. The optical fiber transmission line according to any one of claims 1 to 5, wherein the first light is monitor light for monitoring a state of the optical fiber transmission line.

[0056] (Appendix 7) 7. The optical fiber transmission line according to any one of claims 1 to 6, wherein the second optical fiber is a multi-core fiber.

[0057] (Appendix 8) 7. The optical fiber transmission line according to any one of claims 1 to 6, wherein the second optical fiber is a single-core fiber.

[0058] (Appendix 9) the coupling circuit comprises an optical coupler and a reflector; 9. The optical fiber transmission line according to any one of appendixes 1 to 8, wherein the optical coupler branches the first light, guides the branched first light to the reflector, and couples the return light, which is the first light reflected by the reflector, with the second optical fiber as the second light.

[0059] (Appendix 10) 9. The optical fiber transmission line according to any one of appendixes 1 to 8, further comprising a second coupling circuit that couples light propagating through a core of the second optical fiber in a direction opposite to that of the second light to the direction of the first light in the first optical fiber.

[0060] (Appendix 11) 11. The optical fiber transmission line according to any one of claims 1 to 10, wherein the first optical fiber and the second optical fiber are provided in a single optical cable.

[0061] (Appendix 12) 12. The optical fiber transmission line according to any one of claims 1 to 11, further comprising an optical repeater including a relay circuit that relays the first light and the second light and the first coupling circuit.

[0062] (Appendix 13) An optical fiber transmission system including an optical fiber transmission line and an OTDR (optical time-domain reflectometer) device, The optical fiber transmission line is a first optical fiber consisting of multiple cores sharing a cladding; a second optical fiber that does not share a cladding with the first optical fiber; a first coupling circuit that couples a core of the first optical fiber with a core of the second optical fiber; Equipped with the coupling circuit couples return light of the first light propagating through the core of the first optical fiber to the core of the second optical fiber as second light; the OTDR device transmits the first light to the first optical fiber and receives the second light from the second optical fiber; Fiber optic transmission system.

[0063] (Appendix 14) 14. The optical fiber transmission system of claim 13, wherein the return light is light generated by backscattering or reflecting the first light.

[0064] (Appendix 15) An optical transmission method used in an optical fiber transmission line including a first optical fiber consisting of multiple cores sharing a cladding, and a second optical fiber that does not share a cladding with the first optical fiber, Return light of the first light propagating through the core of the first optical fiber is coupled to the core of the second optical fiber as second light. Optical transmission method.

[0065] (Appendix 16) 16. The optical transmission method according to claim 15, wherein the return light is light generated by backscattering or reflecting the first light.

[0066] (Appendix 17) 17. The optical transmission method according to claim 15, wherein the first optical fiber and the second optical fiber are connected so as to suppress crosstalk between the first light and the return light between a plurality of cores in the first optical fiber.

[0067] (Appendix 18) 18. The optical transmission method according to any one of claims 15 to 17, using a communication system in which crosstalk between signal lights is suppressed between multiple cores in the first optical fiber.

[0068] (Appendix 19) 19. The optical transmission method according to claim 18, wherein the communication method includes a method of transmitting signal light in opposite directions in the first optical fiber.

[0069] (Appendix 20) 20. The optical transmission method according to any one of claims 15 to 19, wherein the first light is a monitor light for monitoring a state of the optical fiber transmission line.

[0070] (Appendix 21) 21. The optical transmission method according to any one of claims 15 to 20, wherein the second optical fiber is a multi-core fiber.

[0071] (Appendix 22) 21. The optical transmission method according to any one of claims 15 to 20, wherein the second optical fiber is a single-core fiber.

[0072] (Appendix 23) 23. The optical transmission method according to any one of claims 15 to 22, wherein light propagating through a core of the second optical fiber in a direction opposite to that of the second light is coupled to the direction of the first light in the first optical fiber.

[0073] 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. For example, the optical fiber transmission system described in each embodiment also discloses an optical transmission method that can be applied to an MCF transmission system.

[0074] 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]

[0075] 10, 20, 331 OTDR equipment 100, 200, 200A, 200B, 300 optical fiber transmission line 101 First Light 102 Second Light 103 Return light 110 First Optical Fiber 120 Second Optical Fiber 111 First Core 112 Third Core 121 Second Core 130 first coupling circuit 210, 220, 230, 240 optical fiber 211, 212, 221, 222, 231, 232, 241, 242 cores 250, 250A, 250B Optical Repeater 251-253 Optical repeater 261-266 FIFO 271-274 Optical amplifier 281-284 Combined circuit 281B-284B combination circuit 291B-294B Reflector 310, 320 MCF 311, 312, 321, 322 cores 330 Terminal 332 Switch 341, 342 FIFO 350-353 Optical repeater 411, 421 core 510, 520 MCF 511-514 Core 521-524 Core 531 OTDR equipment 551-553 Optical repeater 561 Combined circuit 900 Optical fiber transmission line 901 Surveillance light 902 Rayleigh scattered light 910 Optical Fiber 911, 912 Core 930 Optical Repeater 1000, 2000, 3000 optical fiber transmission systems

Claims

1. a first optical fiber consisting of multiple cores sharing a cladding; a second optical fiber that does not share a cladding with the first optical fiber; a first coupling circuit that couples a core of the first optical fiber with a core of the second optical fiber; Equipped with the coupling circuit couples return light of the first light propagating through the core of the first optical fiber to the core of the second optical fiber as second light; Optical fiber transmission line.

2. 2. The optical fiber transmission line according to claim 1, wherein the return light is light generated by backscattering or reflecting the first light.

3. 3. The optical fiber transmission line according to claim 1, wherein the coupling circuit connects the first optical fiber and the second optical fiber so as to suppress crosstalk between the first light and the return light between a plurality of cores in the first optical fiber.

4. 3. The optical fiber transmission line according to claim 1, wherein a communication method is used in which crosstalk between signal lights is suppressed between a plurality of cores in the first optical fiber.

5. 5. The optical fiber transmission line according to claim 4, wherein the communication method includes a method of transmitting signal light in opposite directions in the first optical fiber.

6. 3. The optical fiber transmission line according to claim 1, wherein the first light is a monitor light for monitoring a state of the optical fiber transmission line.

7. 3. The optical fiber transmission line according to claim 1, wherein the second optical fiber is a multi-core fiber.

8. 3. The optical fiber transmission line according to claim 1, wherein the second optical fiber is a single-core fiber.

9. the coupling circuit comprises an optical coupler and a reflector; 3. The optical fiber transmission line according to claim 1, wherein the optical coupler branches the first light, guides the branched first light to the reflector, and couples the return light, which is the first light reflected by the reflector, with the second optical fiber as the second light.

10. An optical transmission method used in an optical fiber transmission line including: a first optical fiber consisting of a plurality of cores that share a cladding; a second optical fiber that does not share a cladding with the first optical fiber; and a first coupling circuit that couples a core of the first optical fiber with a core of the second optical fiber, Return light of the first light propagating through the core of the first optical fiber is coupled as second light to the core of the second optical fiber by the first coupling circuit. Optical transmission method.

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