Loopback optical system, optical repeater, optical transmission system, and optical loopback method

Abstract

To provide a turn-back optical system, an optical repeater, an optical transmission system and an optical turn-back method capable of suppressing deterioration in transmission quality.SOLUTION: The turn-back optical system includes a wavelength selection mirror that transmits first light of a first wavelength and reflects second light of a second wavelength, the first light and the second light being propagated through a first core and a second core included in the multicore fiber, and a reflector that turns back the second light reflected by the wavelength selection mirror to the wavelength selection mirror and couples the second light to the second core.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a folding optical system, an optical repeater, an optical transmission system, and an optical folding method. [Background technology]

[0002] In recent years, in order to increase the transmission capacity in optical communications, multi-core fibers in which multiple cores are formed in a single optical fiber have been used. For example, Patent Document 1 describes a multi-core fiber bidirectional optical amplifier that includes an FI / FO (Fan-in / Fan-out) and a single-core fiber bidirectional optical amplifier. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2023 / 105658 Summary of the Invention [Problem to be solved by the invention]

[0004] By relaying light by arranging optical amplifiers at predetermined intervals as in Patent Document 1, it is possible to suppress the degradation of light in multicore fiber transmission and enable long-distance transmission.

[0005] Meanwhile, in optical transmission systems, the transmitted light is reflected back to the transmitting side and the reflected light is measured in order to monitor the status of optical amplifiers, optical fibers, etc. However, related technologies such as Patent Document 1 do not take into consideration a configuration for monitoring in a multicore fiber optical transmission system, and therefore transmission quality may deteriorate depending on the configuration for reflecting the light.

[0006] In view of the above problems, one object of the present disclosure is to provide a folding optical system, an optical repeater, an optical transmission system, and an optical folding method that can suppress deterioration of transmission quality. [Means for solving the problem]

[0007] A folding optical system according to one embodiment of the present disclosure includes a wavelength-selective mirror that transmits first light of a first wavelength propagated through a first core among a first core and a second core included in a multicore fiber and reflects second light of a second wavelength, and a reflector that folds the second light reflected by the wavelength-selective mirror back to the wavelength-selective mirror and couples it to the second core.

[0008] An optical repeater according to one embodiment of the present disclosure comprises a multi-core optical amplifying fiber including a first core and a second core, and a folding optical system, wherein the folding optical system comprises a wavelength-selective mirror that transmits a first light of a first wavelength propagated through the first core and reflects a second light of a second wavelength, and a reflector that folds the second light reflected by the wavelength-selective mirror back to the wavelength-selective mirror and couples it to the second core.

[0009] An optical transmission system according to one aspect of the present disclosure includes an optical repeater connected between multicore fibers, the optical repeater including a multicore optical amplifying fiber including a first core and a second core, and a folding optical system, the folding optical system including a wavelength-selective mirror that transmits first light of a first wavelength propagated through the first core and reflects second light of a second wavelength, and a reflector that folds the second light reflected by the wavelength-selective mirror back to the wavelength-selective mirror and couples it to the second core.

[0010] An optical folding method according to one aspect of the present disclosure uses a wavelength-selective mirror to transmit first light of a first wavelength propagated through a first core of a first core and a second core included in a multicore fiber, and to reflect second light of a second wavelength, and to fold the second light reflected by the wavelength-selective mirror back to the wavelength-selective mirror and couple it to the second core. [Effects of the Invention]

[0011] According to the present disclosure, it is possible to suppress deterioration of transmission quality. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a configuration diagram illustrating an example of a configuration of a related optical transmission system. [Figure 2] FIG. 1 is a diagram illustrating a configuration example of a related optical repeater. [Figure 3] 1A and 1B are diagrams illustrating exemplary configurations of folding optical systems according to some embodiments. [Figure 4] 1A and 1B are diagrams illustrating exemplary configurations of folding optical systems according to some embodiments. [Figure 5] 1 illustrates an example configuration of an optical transmission system according to some embodiments. [Figure 6] FIG. 2 illustrates wavelength bands of light transmitted in an optical transmission system according to some embodiments. [Figure 7] 1 illustrates an example configuration of an optical repeater according to some embodiments. [Figure 8] 1A and 1B are diagrams for explaining an example of operation of an optical repeater according to some embodiments. [Figure 9] 1A and 1B are diagrams for explaining an example of operation of an optical repeater according to some embodiments. [Figure 10] 10A and 10B are diagrams illustrating other configurations of folded optical systems according to some embodiments. [Figure 11] 10A and 10B are diagrams illustrating other configurations of folded optical systems according to some embodiments. [Figure 12] 10A and 10B are diagrams illustrating other configurations of folded optical systems according to some embodiments. [Figure 13] 1 illustrates an example of a combination of core pairs in a multicore fiber according to some embodiments. [Figure 14] 1 illustrates an example of a combination of core pairs in a multicore fiber according to some embodiments. [Figure 15] 1 illustrates an example of a combination of core pairs in a multicore fiber according to some embodiments. [Figure 16] 1 illustrates an example configuration of an optical repeater according to some embodiments. [Figure 17] 1A and 1B are diagrams for explaining an example of operation of an optical repeater according to some embodiments. [Figure 18] 1A and 1B are diagrams for explaining an example of operation of an optical repeater according to some embodiments. [Figure 19] 1 illustrates an example configuration of an optical repeater according to some embodiments. [Figure 20] 1A and 1B are diagrams for explaining an example of operation of an optical repeater according to some embodiments. [Figure 21] 1A and 1B are diagrams for explaining an example of operation of an optical repeater according to some embodiments. [Figure 22] FIG. 2 illustrates wavelength bands of light transmitted in an optical transmission system according to some embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments will be described with reference to the drawings. In the drawings, the same elements are denoted by the same reference numerals, and redundant description will be omitted as necessary.

[0014] (Review of related technologies) 1 shows an example of the configuration of a related optical transmission system 9. For example, the optical transmission system 9 is an optical submarine transmission system, and performs bidirectional transmission between a terminal station 930-1 on the west side and a terminal station 930-2 on the east side via a single-core fiber 901 (901-1 to 901-4).

[0015] The optical transmission system 9 includes an optical repeater 900 that repeats signal light of a single-core fiber 901. For example, a plurality of optical repeaters 900 are arranged at predetermined intervals in the optical transmission system 9. In the example of Fig. 1, the optical repeaters 900 are arranged between single-core fibers 901-1 and 901-3 and single-core fibers 901-2 and 901-4.

[0016] The optical repeater 900 includes an SC-EDFA 920, which is an optical amplifier that amplifies signal light in a single-core fiber 901. The SC-EDFA (Single Core - Erbium Doped Fiber Amplifier) ​​920 amplifies light using a single-core EDF. In the example of FIG. 1, the signal light transmitted from the WEST side via a single-core fiber 901-1 is amplified by the SC-EDFA 920-1, and the amplified signal light is transmitted to the EAST side via a single-core fiber 901-2. The signal light transmitted from the EAST side via a single-core fiber 901-4 is amplified by the SC-EDFA 920-2, and the amplified signal light is transmitted to the WEST side via a single-core fiber 901-3.

[0017] Optical submarine transmission systems such as optical transmission system 9 are equipped with a configuration in which light is folded back inside an optical repeater in order to monitor the system and estimate loss locations. By folding back light, loopback measurements and OTDR (Optical Time Domain Reflectometer) measurements can be performed. For example, light (monitoring light) with a wavelength different from the signal light is folded back. In loopback measurements, the operating status of the optical amplifier is monitored by measuring the output light from the folded back optical amplifier. In OTDR measurements, optical fiber faults are monitored by measuring the scattered light from the optical fiber.

[0018] 1, an optical repeater 900 of an optical transmission system 9 includes a return unit 910 for loopback measurement and OTDR measurement. Since the SC-EDFA 920-1 and SC-EDFA 920-2 of the optical repeater 900 have isolators that transmit light in only one direction, return units 910-1 and 910-2 are arranged before and after the SC-EDFA 920-1 and SC-EDFA 920-2, and light is returned via an optical fiber in the return direction.

[0019] The folding unit 910 includes couplers 911 and 912, and wavelength-selective reflectors 913 and 914. The couplers 911 and 912 branch the light of the single-core fiber 901. The wavelength-selective reflectors 913 and 914 reflect light of a selected wavelength (light to be folded) from the light branched by the couplers 911 and 912.

[0020] For example, when loopback measurement is performed by returning light transmitted from the WEST side to the EAST side, light is output from the WEST side transmitter (TX) 931-1 to the WEST side receiver (RX) 932-2, and a portion of the light output from the SC-EDFA 920-1 is returned to the WEST side receiver 932-1 by the return unit 910-1. Specifically, a portion of the light output from the SC-EDFA 920-1 is branched by the coupler 911-2, and a wavelength for loopback measurement is selected from the branched light by the wavelength-selective reflector 913-2, and the light of the selected wavelength is reflected. The reflected light is output from the coupler 911-2 to the coupler 912-2 on the side of the single-core fiber transmitting in the opposite direction, and then transmitted from the coupler 912-2 to the receiver 932-1 via the SC-EDFA 920-2 and single-core fiber 901-3. The receiver 932-1 measures the power of the received light and monitors the output state of the SC-EDFA 920-1. In a bidirectional optical system in which multiple optical amplifiers are connected, the light from the output stage of each optical amplifier is looped back and the looped back light is measured, making it possible to identify optical amplifiers in an abnormal state, such as one with a reduced output.

[0021] For example, when light output from the EAST side to the WEST side is folded back for OTDR measurement, light is output from the EAST side transmitter 931-2 to the WEST side receiver 932-1, and a portion of the scattered light scattered in the opposite direction to the transmission direction in the single-core fiber 901-3 is folded back by the folding unit 910-1 to the EAST side receiver 932-2. Specifically, a portion of the scattered light in the single-core fiber 901-3 is branched by the coupler 911-1, and the branched light is output to the coupler 912-1 on the single-core fiber side that transmits in the reverse direction, and is transmitted from the coupler 912-1 to the receiver 932-2 via the SC-EDFA 920-1 and the single-core fiber 901-2. The receiver 932-2 measures the power of the received light and monitors the transmission status of the single-core fiber 901-3. In a bidirectional optical system in which multiple optical amplifiers are connected, the scattered light from each fiber is returned (to the transmitting side) and the returned light is measured, making it possible to identify any fiber locations that are in an abnormal state, such as a fiber break.

[0022] The inventors have considered applying the configuration of the return section of the single-core fiber transmission system of Fig. 1 to a multi-core fiber transmission system. Fig. 2 shows an example configuration of a related optical repeater 800 considered by the inventors.

[0023] 2, the optical repeater 800 is disposed between a multicore fiber 801-1 and a multicore fiber 801-2. For example, the multicore fiber 801-1 and the multicore fiber 801-2 are four-core fibers including a first core to a fourth core. A pair of the first and second cores performs bidirectional transmission, and a pair of the third and fourth cores performs bidirectional transmission. The first and third cores transmit signal light from the west side to the east side, and the second and fourth cores transmit signal light from the east side to the west side.

[0024] The optical repeater 800 includes FI / FOs 802-1 and 802-2 to separate / combine the signal light of the multicore fiber into the signal light of the single-core fiber. When performing bidirectional transmission using a pair of cores, an SC-EDFA 920 is provided for each pair of cores, as in Fig. 1, and folding units 910 are arranged before and after the SC-EDFA 920.

[0025] For example, the FI / FO 802-1 converts the signal light of the multicore fiber 801-1 into signal light for single-core fibers 901-1, 901-3, 901-5, and 901-7. The FI / FO 802-2 converts the signal light of the multicore fiber 801-2 into signal light for single-core fibers 901-2, 901-4, 901-6, and 901-8. For the signal light of the first core, an SC-EDFA 920-1 is arranged between the single-core fibers 901-1 and 901-2, and for the signal light of the second core, an SC-EDFA 920-2 is arranged between the single-core fibers 901-3 and 901-4. Folding units 910-1 and 910-2 are arranged before and after the SC-EDFA 920-1 and SC-EDFA 920-2. For the signal light of the third core, an SC-EDFA 920-3 is arranged between single-core fibers 901-5 and 901-6, and for the signal light of the fourth core, an SC-EDFA 920-4 is arranged between single-core fibers 901-7 and 901-8. Folding units 910-3 and 910-4 are arranged before and after the SC-EDFA 920-3 and SC-EDFA 920-4.

[0026] The configuration shown in Figure 2 enables loopback measurements and OTDR measurements for each core pair in a multicore fiber transmission system, just as in Figure 1. However, when the configuration of the single-core fiber transmission system in Figure 1 is applied to a multicore fiber transmission system, an FI / FO is required to branch the signal light for each core, as shown in Figure 2. This requires an increase in the number of components and a complex configuration. In particular, the use of an FI / FO causes issues such as deterioration of signal transmission quality due to increased optical signal loss and crosstalk noise between cores.

[0027] Therefore, in the embodiment, when the supervisory light is returned in a multi-core fiber transmission system, it is possible to suppress the deterioration of the transmission quality without using an FI / FO.

[0028] (Embodiment 1) Next, a first embodiment will be described. In this embodiment, an outline of several embodiments will be described.

[0029] 3 shows an example of the configuration of a folding optical system 10 according to some embodiments. For example, the folding optical system 10 folds back a monitor light in an optical transmission system that performs bidirectional transmission using a multicore fiber. The folding optical system 10 may be arranged before or after a multicore fiber optical amplifier in an optical repeater in the optical transmission system.

[0030] 3, the folding optical system 10 folds back light from a first core 21 or a second core 22 between multicore fibers 20 (20-1 and 20-2) including a first core 21 and a second core 22. The first core 21 and the second core 22 have different, opposite transmission directions of signal light. Either the multicore fiber 20-1 or the multicore fiber 20-2 may be a multicore fiber optical amplifier.

[0031] In the example of FIG. 3, the folding optical system 10 includes a wavelength selection mirror 11 and a reflector 12.

[0032] The wavelength-selective mirror 11 transmits a first light having a first wavelength out of the light propagated through the first core 21, and reflects a second light having a second wavelength. For example, the first light is signal light for communication in an optical transmission system. The second light is monitor light for monitoring the status of the optical transmission system (optical amplifier, optical fiber, etc.). The wavelength-selective mirror 11 may reflect a portion of the second light having a second wavelength and transmit the remaining light. For example, the wavelength-selective mirror 11 may be disposed so as to intersect with the optical axis direction of the first core 21 and the second core 22, and may reflect the second light in a direction perpendicular to the optical axis direction of the first core 21 and the second core 22. For example, the wavelength-selective mirror 11 may be a dichroic mirror.

[0033] The reflector 12 reflects the second light reflected by the wavelength selective mirror 11 back to the wavelength selective mirror 11 and couples it into the second core 22. This allows the second light (monitor light) to be reflected via the second core 22 for loopback measurement. The reflector 12 moves the optical path of the second light reflected by the wavelength selective mirror 11 in parallel and reflects the second light toward a position where it intersects with the optical axis direction of the second core 22 of the wavelength selective mirror 11. The reflector 12 may include a lens capable of focusing parallel light and a reflecting mirror capable of reflecting light from the lens at the focusing position of the lens. The reflector 12 may be a corner cube reflector.

[0034] 4, the folding optical system 10 may further include a reflecting mirror 13. The reflecting mirror 13 is disposed on the opposite side of the wavelength-selective mirror 11 from the reflector 12. In this case, the wavelength-selective mirror 11 reflects a portion of the light of the second wavelength (scattered light) propagating in the opposite direction to the first light in the first core 21, toward the opposite side of the reflector 12. The reflecting mirror 13 reflects the light reflected by the wavelength-selective mirror 11 toward the opposite side of the reflector 12 back to the wavelength-selective mirror 11. The reflector 12 reflects the light that has passed through the wavelength-selective mirror 11 back to the wavelength-selective mirror 11 and couples it into the second core 22. This allows light (scattered light of the monitor light) to be folded back via the second core 22 for OTDR measurement.

[0035] As described above, in this embodiment, two cores with different transmission directions are paired in the same multicore fiber, and light of a predetermined wavelength (monitor light) from the first core is folded back using a spatial optical system and coupled to the second core. This allows a folding section to be configured with a simple configuration without using an FI / FO for loopback measurement or OTDR measurement, and can suppress degradation of transmission quality.

[0036] (Embodiment 2) Next, a description will be given of a second embodiment of the present invention, which will explain a specific example of a folding optical system that folds back the monitor light for loopback measurement.

[0037] Fig. 5 shows an example of the configuration of an optical transmission system 1 according to some embodiments. For example, the optical transmission system 1 is an optical submarine transmission system, and similar to Fig. 1, performs bidirectional transmission between a terminal station on the west side and a terminal station on the east side via a multi-core fiber 300 (300-1 to 300-2).

[0038] The optical transmission system 1 includes an optical repeater 2 that repeats signal light of a multicore fiber 300. For example, a plurality of optical repeaters 2 are arranged at predetermined intervals in the optical transmission system 1. In the example of Fig. 5, the optical repeater 2 is arranged between the multicore fiber 300-1 and the multicore fiber 300-2.

[0039] The optical repeater 2 includes an MC-EDFA 200 and folding optical systems 100 (100-1 and 100-2). The MC-EDFA (Multi-Core-Erbium-Doped Fiber Amplifier) ​​200 amplifies light using a multi-core EDF (MC-EDF). For example, the multi-core fibers 300-1 and 300-2 and the MC-EDFA 200 are two-core multi-core fibers including a core C1 and a core C2. The core C1 and the core C2 have different transmission directions, and perform bidirectional transmission. The multi-core fibers 300-1 and 300-2 and the MC-EDFA 200 may include more than two cores, or may include multiple pairs of cores with different transmission directions.

[0040] The core C1 transmits the signal light from the east side to the west side, and the core C2 transmits the signal light from the west side to the east side. The signal light transmitted from the east side via the core C1 of the multi-core fiber 300-2 is amplified by the core C1 of the MC-EDFA200, and the amplified signal light is transmitted to the west side via the core C1 of the multi-core fiber 300-1. The signal light transmitted from the west side via the core C2 of the multi-core fiber 300-1 is amplified by the core C2 of the MC-EDFA200, and the amplified signal light is transmitted to the east side via the core C2 of the multi-core fiber 300-2.

[0041] The folding optical system 100-1 is disposed between the multi-core fiber 300-1 and the MC-EDFA 200 (on the west side of the MC-EDFA 200). The folding optical system 100-1 transmits, to the core C1 of the multi-core fiber 300-1, the signal light that is transmitted from the east side via the core C1 of the multi-core fiber 300-2 and amplified by the core C1 of the MC-EDFA 200. The folding optical system 100-1 folds back a portion of the monitor light (wavelength λ1) that is transmitted from the east side via the core C1 of the multi-core fiber 300-2 and amplified by the core C1 of the MC-EDFA 200, and transmits it to the east side via the core C2 of the MC-EDFA 200 and the core C2 of the multi-core fiber 300-2. The folding optical system 100-1 transmits the remaining monitor light that is not folded back to the core C1 of the multi-core fiber 300-1. This allows the supervisory light to be returned by the optical repeater 2 at the next stage (west side).

[0042] The folding optical system 100-2 is disposed between the multi-core fiber 300-2 and the MC-EDFA 200 (on the east side of the MC-EDFA 200). The folding optical system 100-2 transmits, to the core C2 of the multi-core fiber 300-2, the signal light that is transmitted from the west side via the core C2 of the multi-core fiber 300-1 and amplified by the core C2 of the MC-EDFA 200. The folding optical system 100-2 folds back a portion of the monitor light (wavelength λ2) that is transmitted from the west side via the core C2 of the multi-core fiber 300-1 and amplified by the core C2 of the MC-EDFA 200, and transmits it to the west side via the core C1 of the MC-EDFA 200 and the core C1 of the multi-core fiber 300-1. The folding optical system 100-2 transmits the remaining monitor light that is not folded back to the core C2 of the multi-core fiber 300-2. This allows the supervisory light to be returned by the optical repeater 2 at the next stage (EAST side).

[0043] 6 shows the wavelength bands of the signal light and supervisory light transmitted in the optical transmission system 1 according to some embodiments. The signal light is light for performing bidirectional communication between the WEST side and the EAST side (between terminal stations) in the optical transmission system 1. For example, the wavelength of the signal light may be the C band or the L band, or may be another wavelength. The wavelength of the signal light (core C2) transmitted from the WEST side to the EAST side and the wavelength of the signal light (core C1) transmitted from the EAST side to the WEST side may be the same or different.

[0044] The supervisory light is light used to monitor the optical transmission system 1 (optical amplifiers and optical fibers), and is, for example, light for loopback measurement. In the example of FIG. 6, the wavelength of the signal light and the wavelength of the supervisory light are different. For example, the wavelength of the supervisory light is longer than the wavelength of the signal light, but it may also be shorter than the wavelength of the signal light. Since the loopback measurement monitors the operating state of the optical amplifier, the wavelength of the supervisory light may be a wavelength included in the wavelength band of the signal light, or a wavelength close to the wavelength band of the signal light.

[0045] In the example of Figure 6, the supervisory light includes light of wavelength λ1 and light of wavelength λ2. The supervisory light of wavelength λ1 and the supervisory light of wavelength λ2 have different transmission directions. For example, the supervisory light of wavelength λ1 is transmitted from the EAST side to the WEST side (core C1), and a portion of the supervisory light of wavelength λ1 amplified by the MC-EDFA200 is returned to the EAST side (core C2). The supervisory light of wavelength λ2 is transmitted from the WEST side to the EAST side (core C2), and a portion of the supervisory light of wavelength λ2 amplified by the MC-EDFA200 is returned to the WEST side (core C1).

[0046] FIG. 7 shows an example of the configuration of an optical repeater 2 according to some embodiments. In the example of FIG. 7, an MC-EDFA 200 includes an MC-EDF 201 and isolators 202 (202-1 to 202-4). The MC-EDF 201 is a multi-core EDF (multi-core optical amplifier fiber). The MC-EDF 201 is an optical amplifier that amplifies input signal light. By inputting pump light together with the signal light into the MC-EDF 201, the signal light is pumped and amplified. As described above, for example, the MC-EDF 201 includes two cores C1 and C2, and amplifies the signal light (monitor light) input from different directions for each core, and outputs the amplified signal light (monitor light).

[0047] The isolators 202-1 to 202-4 transmit light in only one direction. The isolators 202-1 to 202-4 are arranged on the input side and output side of each core of the MC-EDF 201, respectively. The isolator 202-1 transmits the signal light (monitor light) transmitted from the EAST side via the core C1 of the multi-core fiber 300-2 to the WEST side, and outputs it toward the core C1 of the MC-EDF 201. The isolator 202-2 transmits the signal light (monitor light) amplified and output by C1 of the MC-EDF 201 to the WEST side, and outputs it toward the core C1 of the multi-core fiber 300-1 on the WEST side. The isolator 202-3 transmits the signal light (monitor light) transmitted from the WEST side via the core C2 of the multi-core fiber 300-1 to the EAST side, and outputs it toward the core C2 of the MC-EDF 201. The isolator 202-4 transmits the signal light (monitor light) amplified and output by the core C2 of the MC-EDF 201 to the EAST side, and outputs it toward the core C2 of the multi-core fiber 300-2 on the EAST side.

[0048] The folding optical system 100 (100-1 and 100-2) includes dichroic mirrors 110 (110-1 and 110-2) and reflectors 120 (120-1 and 120-2).

[0049] The dichroic mirror 110 transmits the signal light and reflects a portion of the monitor light. That is, the dichroic mirror 110 transmits the light of the wavelength of the signal light, reflects a portion of the light of the wavelength of the monitor light, and transmits the remaining light of the wavelength of the monitor light. For example, the dichroic mirror 110-1 of the folding optical system 100-1 transmits the signal light amplified by the core C1 of the MC-EDF 201, reflects a portion of the monitor light of wavelength λ1, and transmits the remaining monitor light of wavelength λ1. The dichroic mirror 110-2 of the folding optical system 100-2 transmits the signal light amplified by the core C2 of the MC-EDF 201, reflects a portion of the monitor light of wavelength λ2, and transmits the remaining monitor light of wavelength λ2.

[0050] The dichroic mirror 110 is disposed so as to intersect (overlap) with the optical axis direction of the cores C1 and C2 of the MC-EDF 201. The dichroic mirror 110 is tilted at an angle of 45° with respect to the optical axis direction of the cores C1 and C2, and reflects a portion of the monitor light in a direction (toward the reflector) perpendicular to the optical axis direction of the cores C1 and C2. The point at which the dichroic mirror 110 reflects (branches) the monitor light from the core C1 or C2 of the MC-EDF 201 toward the reflector 120 is called the branch point 111 (111-1 and 111-2). For example, the branch point 111-1 of the dichroic mirror 110-1 is the point at which the optical axis direction of the dichroic mirror 110-1 and the optical axis direction of the core C1 of the MC-EDF 201 intersect. The branch point 111-2 of the dichroic mirror 110-2 is a point where the optical axis directions of the dichroic mirror 110-2 and the core C2 of the MC-EDF 201 intersect.

[0051] Furthermore, the dichroic mirror 110 reflects a portion of the monitor light reflected by the reflector 120 in a direction (toward the core C1 or C2 of the MC-EDF 201) perpendicular to the direction of reflection. The points at which the dichroic mirror 110 reflects (couples) the monitor light from the reflector 120 toward the core C1 or C2 of the MC-EDF 201 are called coupling points 112 (112-1 and 112-2). For example, the coupling point 112-1 of the dichroic mirror 110-1 is the point where the optical axis directions of the dichroic mirror 110-1 and the core C2 of the MC-EDF 201 intersect. The coupling point 112-2 of the dichroic mirror 110-2 is the point where the optical axis directions of the dichroic mirror 110-2 and the core C1 of the MC-EDF 201 intersect.

[0052] The reflector 120 couples the monitor light reflected from the dichroic mirror 110 into another core by shifting (shifting) the optical path (optical axis) of the monitor light in parallel and returning it to the dichroic mirror 110. For example, the reflector 120-1 of the folding optical system 100-1 shifts the optical path of the monitor light reflected from the core C1 of the MC-EDF 201 by the distance between the cores C1 and C2 and returns it, thereby coupling the monitor light into the core C2 at the dichroic mirror 110-1. The reflector 120-2 of the folding optical system 100-2 shifts the optical path of the monitor light reflected from the core C2 of the MC-EDF 201 by the distance between the cores C1 and C2 and returns it, thereby coupling the monitor light into the core C1 at the dichroic mirror 110-2.

[0053] The reflector 120 includes a lens 121 and a mirror 122. For example, the lens 121 is a condensing lens such as a convex lens that can condense parallel light. The lens 121 tilts the optical path (optical axis) of the monitor light reflected at the branch point 111 of the dichroic mirror 110 toward the center of the lens 121 (the side where the optical path is shifted). For example, the side where the optical path is shifted is the side that approaches the MC-EDF 201. The point where the monitor light reflected from the dichroic mirror 110 is incident (input) on the lens 121 is called an input point 123. The input point 123 is an end of the lens 121 that is closer to the outer periphery than the center, and is located opposite the branch point 111 of the dichroic mirror 110. For example, the input point 123-1 of the reflector 120-1 is a point located in a direction perpendicular to the optical axis direction of the core C1 of the MC-EDF 201 from the branch point 111-1 of the dichroic mirror 110-1. The input point 123-2 of the reflector 120-2 is a point located in a direction perpendicular to the optical axis direction of the core C2 of the MC-EDF 201 from the branch point 111-2 of the dichroic mirror 110-2.

[0054] Furthermore, the lens 121 tilts the optical path (optical axis) of the monitor light reflected by the mirror 122 so that it is directed toward the coupling point 112 of the dichroic mirror 110. In other words, the lens 121 emits the monitor light reflected by the mirror 122 in a direction parallel to the monitor light reflected by the branching point 111 of the dichroic mirror 110. The point at which the lens 121 emits (outputs) the monitor light reflected from the mirror 122 is called the output point 124. The output point 124 is the other end of the lens 121 that is closer to the outer periphery than the center (the opposite side to the input point 123), and is a point located opposite the coupling point 112 of the dichroic mirror 110. The distance between the input point 123 and the output point 124 corresponds to the distance between the cores C1 and C2 of the MC-EDF 201. For example, output point 124-1 of reflector 120-1 is a point located in a direction perpendicular to the optical axis direction of core C2 of MC-EDF 201 from coupling point 112-1 of dichroic mirror 110-1. Output point 124-2 of reflector 120-2 is a point located in a direction perpendicular to the optical axis direction of core C1 of MC-EDF 201 from coupling point 112-2 of dichroic mirror 110-2.

[0055] The mirror 122 is a reflective mirror that reflects light from the lens 121. For example, the lens 121 is a condensing lens, and the mirror 122 reflects the light from the lens 121 at the condensing position of the condensing lens. The mirror 122 reflects the monitor light from the input point 123 of the lens 121 toward the output point 124 of the lens 121. The reflective surface of the mirror 122 is perpendicular to the optical path of the monitor light reflected from the dichroic mirror 110. The inclination of the light incident on the mirror 122 from the input point 123 of the lens 121 is equal to the inclination of the light exiting from the mirror 122 to the output point 124 of the lens 121. The point at which the mirror 122 reflects (reflects) the monitor light from the lens 121 is called the turning point 125. For example, the turning point 125 is a position opposite to the center of the lens 121 (halfway between the input point 123 and the output point 124). For example, the turning point 125-1 of the reflector 120-1 is a point located midway between the direction from the branching point 111-1 of the dichroic mirror 110-1 to a direction perpendicular to the optical axis direction of the core C1 of the MC-EDF 201 and the direction from the coupling point 112-1 of the dichroic mirror 110-1 to a direction perpendicular to the optical axis direction of the core C2 of the MC-EDF 201. For example, the turning point 125-2 of the reflector 120-2 is a point located midway between the direction from the branching point 111-2 of the dichroic mirror 110-2 to a direction perpendicular to the optical axis direction of the core C2 of the MC-EDF 201 and the direction from the coupling point 112-2 of the dichroic mirror 110-2 to a direction perpendicular to the optical axis direction of the core C1 of the MC-EDF 201.

[0056] Figure 8 shows an example of operation when loopback measurement is performed by folding back the supervisory light using the folding optical system 100-1 in Figure 7. In the example of Figure 8, supervisory light with wavelength λ1 from the EAST side is folded back to the EAST side using the folding optical system 100-1. Note that signal light is constantly transmitted from the EAST side to the WEST side via the optical repeater 2. Supervisory light with wavelength λ1 may be constantly transmitted from the EAST side to the WEST side to perform constant loopback measurement. Supervisory light may also be transmitted at required intervals and timing to perform loopback measurement.

[0057] First, the monitor light of wavelength λ1 propagates from the EAST side (core C1 of the multicore fiber 300-2) and is incident on the dichroic mirror 110-2 on the EAST side (S101). The dichroic mirror 110-2 reflects a part of the light of wavelength λ2 and transmits the remainder of the light of wavelength λ2 and light of other wavelengths, so that the incident monitor light of wavelength λ1 is transmitted to core C1 of the MC-EDF 201.

[0058] Next, the monitor light of wavelength λ1 that has passed through the dichroic mirror 110-2 is incident on the core C1 (east side) of the MC-EDF 201 via the isolator 202-1 (S102). The MC-EDF 201 amplifies the incident monitor light of wavelength λ1 and emits the amplified monitor light from the core C1 (west side).

[0059] Next, the monitor light amplified by the core C1 of the MC-EDF 201 is output to the branching point 111-1 of the dichroic mirror 110-1 via the isolator 202-2 (S103). The dichroic mirror 110-1 reflects a part of the light of wavelength λ1 and transmits the rest of the light of wavelength λ1 and light of other wavelengths, so when the monitor light of wavelength λ1 is incident from the isolator 202-2 to the branching point 111-1, the dichroic mirror 110-1 reflects a part of the monitor light of wavelength λ1 to the input point 123-1 of the lens 121-1 and transmits the rest of the monitor light to the WEST side (core C1 of the multi-core fiber 300-1).

[0060] Next, when the monitor light of wavelength λ1 is incident from branch point 111-1 of dichroic mirror 110-1 to input point 123-1, lens 121-1 outputs the monitor light to turning point 125-1 of mirror 122-1 (S104). Next, when the monitor light of wavelength λ1 is incident from lens 121-1 to turning point 125-1, mirror 122-1 reflects the monitor light to output point 124-1 of lens 121-1 (S105). Next, when the monitor light of wavelength λ1 is incident from mirror 122-1 to output point 124-1, lens 121-1 outputs the monitor light to coupling point 112-1 of dichroic mirror 110-1 (S106).

[0061] Next, the dichroic mirror 110-1 reflects a portion of the light of wavelength λ1 and transmits the remainder of the light of wavelength λ1 and light of other wavelengths, so when the monitoring light of wavelength λ1 is incident from the output point 124-1 of the lens 121-1 to the coupling point 112-1, the dichroic mirror 110-1 reflects a portion of the monitoring light of wavelength λ1 to the core C2 of the MC-EDF 201 (S107).

[0062] Next, the monitor light of wavelength λ1 reflected at coupling point 112-1 of dichroic mirror 110-1 is incident on core C2 (west side) of MC-EDF 201 via isolator 202-3 (S108). The MC-EDF 201 amplifies the incident monitor light of wavelength λ1 and emits the amplified monitor light from core C2 (east side).

[0063] Next, the monitor light amplified by the core C2 of the MC-EDF 201 is output to the dichroic mirror 110-2 via the isolator 202-4 (S109). The dichroic mirror 110-2 reflects a part of the light of wavelength λ2 and transmits the remainder of the light of wavelength λ2 and light of other wavelengths, so that the monitor light of wavelength λ1 is transmitted to the EAST side (core C2 of the multi-core fiber 300-2).

[0064] The terminal station on the EAST side then performs loopback measurement using the returned supervisory light. The terminal station on the EAST side measures the power of the received supervisory light and monitors the output state of the core C1 of the MC-EDF201.

[0065] Fig. 9 shows an example of operation when loopback measurement is performed by folding back the supervisory light using the folding optical system 100-2 in Fig. 7. In the example of Fig. 9, supervisory light with wavelength λ2 from the WEST side is folded back using the folding optical system 100-2. As in Fig. 8, signal light is constantly transmitted from the WEST side to the EAST side via the optical repeater 2. Supervisory light with wavelength λ2 may be constantly transmitted from the WEST side to the EAST side, and loopback measurement may be constantly performed. Supervisory light may also be transmitted at required intervals and timing, and loopback measurement may be performed.

[0066] First, the monitor light of wavelength λ2 is propagated from the WEST side (core C2 of the multi-core fiber 300-1) and is incident on the WEST side dichroic mirror 110-1 (S201). The dichroic mirror 110-1 reflects a part of the light of wavelength λ1 and transmits the remainder of the light of wavelength λ1 and light of other wavelengths, so that the incident monitor light of wavelength λ2 is transmitted to core C2 of the MC-EDF 201.

[0067] Next, the monitor light of wavelength λ2 that has passed through the dichroic mirror 110-1 is incident on the core C2 (west side) of the MC-EDF 201 via the isolator 202-3 (S202). The MC-EDF 201 amplifies the incident monitor light of wavelength λ2 and emits the amplified monitor light from the core C2 (east side).

[0068] Next, the monitor light amplified by core C2 of the MC-EDF 201 is output to branch point 111-2 of dichroic mirror 110-2 via isolator 202-4 (S203). Since the dichroic mirror 110-2 reflects a part of the light of wavelength λ2 and transmits the rest of the light of wavelength λ2 and light of other wavelengths, when the monitor light of wavelength λ2 is incident from the isolator 202-4 to branch point 111-2, the dichroic mirror 110-2 reflects a part of the monitor light of wavelength λ2 to input point 123-2 of lens 121-2 and transmits the rest of the monitor light to the EAST side (core C2 of multicore fiber 300-2).

[0069] Next, when the monitor light of wavelength λ2 is incident from branch point 111-2 of dichroic mirror 110-2 to input point 123-2, lens 121-2 outputs the monitor light to turning point 125-2 of mirror 122-2 (S204). Next, when the monitor light of wavelength λ2 is incident from lens 121-2 to turning point 125-2, mirror 122-2 reflects the monitor light to output point 124-2 of lens 121-2 (S205). Next, when the monitor light of wavelength λ2 is incident from mirror 122-2 to output point 124-2, lens 121-2 outputs the monitor light to coupling point 112-2 of dichroic mirror 110-2 (S206).

[0070] Next, the dichroic mirror 110-2 reflects a portion of the light of wavelength λ2 and transmits the remainder of the light of wavelength λ2 and light of other wavelengths, so when the monitoring light of wavelength λ2 is incident from the output point 124-2 of the lens 121-2 to the coupling point 112-2, it reflects a portion of the monitoring light of wavelength λ2 to the core C1 of the MC-EDF 201 (S207).

[0071] Next, the monitor light of wavelength λ2 reflected at coupling point 112-2 of dichroic mirror 110-2 is incident on core C1 (east side) of MC-EDF 201 via isolator 202-1 (S208). The MC-EDF 201 amplifies the incident monitor light of wavelength λ2 and emits the amplified monitor light from core C1 (west side).

[0072] Next, the monitor light amplified by the core C1 of the MC-EDF 201 is output to the dichroic mirror 110-1 via the isolator 202-2 (S209). The dichroic mirror 110-1 reflects a part of the light of wavelength λ1 and transmits the remainder of the light of wavelength λ1 and light of other wavelengths, so that the monitor light of wavelength λ2 is transmitted to the WEST side (core C1 of the multi-core fiber 300-1).

[0073] The terminal station on the west side then performs loopback measurement using the returned supervisory light. The terminal station on the west side measures the power of the received supervisory light and monitors the output state of the core C2 of the MC-EDF 201.

[0074] As described above, in this embodiment, in multicore fiber bidirectional transmission, each core uses supervisory light with a wavelength different from that of the signal light, and cores with different transmission directions within the same multicore fiber are paired with a folding optical system that couples part of the supervisory light propagating through one core to the other core. For example, the folding optical system is configured with a reflector that uses dichroic mirrors and lenses with different reflection wavelengths. This makes it possible to perform loopback measurements without branching for each core using FI / FO, eliminating the need for FI / FO and suppressing degradation of transmission quality.

[0075] (Modification 1 of Embodiment 2) In the above example, the reflector 120 in the folding optical system 100 is configured by the lens 121 and the mirror 122, but the present invention is not limited to this and other configurations may also be used.

[0076] 10 shows another example of the configuration of the reflector 120 in the folding optical system 100. In the example of Fig. 10, the reflector 120 is a corner cube type reflector. The other configurations are the same as those in Fig. 7.

[0077] For example, the reflector 120 includes reflectors 126a and 126b connected at a right angle. The reflector 126a (input point 123) reflects the monitor light from the branch point 111 of the dichroic mirror 110 to the reflector 126b. The reflector 126b (output point 124) reflects the monitor light from the reflector 126a to the connection point 112 of the dichroic mirror 110 and returns it.

[0078] (Modification 2 of Embodiment 2) For example, since the distance between two cores in a multicore fiber is several tens of μm, it may be difficult to implement the folding optical system 100. For this reason, the distance between the light beams from the two cores (the distance between the optical axes) may be increased in the folding optical system. This can improve the folding accuracy of the dichroic mirror 110 and the reflector 120.

[0079] Fig. 11 shows another example of the configuration of the folding optical system 100. Although Fig. 11 shows only the configuration of the folding optical system 100-1, the folding optical system 100-2 also has a similar configuration.

[0080] 11, in order to increase the distance between core C1 and core C2, folding optical system 100-1 includes magnifying lenses 131-1 to 134-1 and parallel lenses 141-1 to 143-1. Magnifying lenses 131-1 to 134-1 tilt the optical path (optical axis) of the monitor light (signal light) in a direction that increases the distance between core C1 and core C2. Parallel lenses 141-1 to 143-1 return the optical path of the monitor light (signal light), which is tilted in the direction that increases the distance between core C1 and core C2, to the same direction (parallel) as the optical axis of the cores.

[0081] The magnifying lenses 131-1 and 132-1 are arranged between the dichroic mirror 110-1 and the MC-EDF 201 (isolators 202-2 and 202-3). The magnifying lenses 131-1 and 132-1 may be combined into one lens. The parallel lenses 141-1 and 142-1 are arranged between the dichroic mirror 110-1 and the magnifying lenses 131-1 and 132-1. The parallel lenses 141-1 and 142-1 may be combined into one lens. The magnifying lenses 133-1 and 134-1 are arranged between the dichroic mirror 110-1 and the WEST side (multi-core fiber 300-1). The magnifying lenses 133-1 and 134-1 may be combined into one lens. The parallel lens 143-1 is arranged between the dichroic mirror 110-1 and the magnifying lenses 133-1 and 134-1. The parallel lens 143-1 may be replaced by two lenses.

[0082] For example, the magnifying lens 131-1 outputs the monitor light (signal light) from the core C1 of the MC-EDF 201 to the collimating lens 141-1 in a direction that widens the distance between the optical axes of the cores C1 and C2. The collimating lens 141-1 outputs the monitor light (signal light) from the magnifying lens 131-1 to the branch point 111-1 of the dichroic mirror 110-1 in a direction parallel to the optical axes of the cores. The dichroic mirror 110-1 reflects a portion of the monitor light that has passed through the magnifying lens 131-1 and the collimating lens 141-1, and outputs the monitor light that has been folded back by the reflector 120-1 to the collimating lens 142-1.

[0083] The collimating lens 142-1 outputs the monitor light (signal light from the WEST side) from the coupling point 112-1 of the dichroic mirror 110-1 to the magnifying lens 132-1 in a direction that reduces the distance between the optical axes of the cores C1 and C2. The magnifying lens 132-1 outputs the monitor light (signal light) from the collimating lens 142-1 to the core C2 of the MC-EDF 201 in the same direction as the optical axes of the cores.

[0084] The collimating lens 143-1 outputs the signal light (part of the monitor light) that has passed through the branching point 111-1 of the dichroic mirror 110-1 to the magnifying lens 133-1 in a direction that reduces the distance between the optical axes of the cores C1 and C2. The magnifying lens 133-1 outputs the signal light (part of the monitor light) from the collimating lens 143-1 to the WEST side (core C1 of the multi-core fiber 300-2) in the same direction as the optical axes of the cores.

[0085] The magnifying lens 134-1 outputs the signal light from the WEST side (core C2 of the multicore fiber 300-1) to the parallel lens 143-1 in a direction that expands the distance between the optical axes of the cores C1 and C2. The parallel lens 143-1 outputs the signal light from the magnifying lens 134-1 to the dichroic mirror 110-1 in a direction parallel to the optical axes of the cores.

[0086] (Third Modification of Second Embodiment) The multicore fiber 300 and the MC-EDF 201 may be a four-core multicore fiber. Fig. 12 shows another configuration example of the folded optical system 100, which is a configuration example of a four-core multicore fiber. In the example of Fig. 12, the multicore fiber 300 and the MC-EDF 201 include cores C1 to C4. The cores C1 and C2 are adjacent to each other and form a core pair with different transmission directions, similar to the above example. The cores C3 and C4 are also adjacent to each other and form a core pair with different transmission directions, similar to the cores C1 and C2.

[0087] Note that cores C1 and C3 are also adjacent to each other but have different transmission directions. Cores C2 and C4 are also adjacent to each other but have different transmission directions. For this reason, cores C1 and C3 may be paired together, and cores C2 and C4 may be paired together.

[0088] For example, a folding optical system 100 similar to the above example may be arranged for each pair of cores C1 and C2, and for each pair of cores C3 and C4. That is, a folding optical system 100 (100-1 and 100-2) that folds back the monitor light between cores C1 and C2, and a folding optical system 100 (100-1 and 100-2) that folds back the monitor light between cores C3 and C4 may be provided.

[0089] 12, a single folding optical system 100 (one on the west side and one on the east side) may be used to fold back the monitor light within the pair of cores C1 and C2, and also within the pair of cores C3 and C4. That is, a single dichroic mirror 110, a single lens 121, and a single mirror 122 may be used to fold back the monitor light for two pairs of cores. The dichroic mirror 110 reflects the monitor light from one of cores C1 and C2 and one of cores C3 and C4, and the reflector 120 (lens 121 and mirror 122) folds back the monitor light and couples it into the other of cores C1 and C2 and the other of cores C3 and C4, respectively. Note that the light flow in the folding optical system 100-2 is not shown in FIG. 12, but it is the same as that for the folding optical system 100-1.

[0090] 12, the dichroic mirror 110-1 transmits the signal light and reflects a portion of the monitor light with wavelength λ1, similar to Fig. 7. The dichroic mirror 110-1 reflects a portion of the monitor light with wavelength λ1 from the cores C1 and C4 of the MC-EDF 201 to the reflector 120-1. The dichroic mirror 110-1 reflects a portion of the monitor light with wavelength λ1 (the monitor light from the cores C1 and C4) reflected from the reflector 120-1 to the cores C2 and C3 of the MC-EDF 201, respectively.

[0091] 7, the dichroic mirror 110-2 transmits the signal light and reflects a portion of the monitor light with wavelength λ2. The dichroic mirror 110-2 reflects a portion of the monitor light with wavelength λ2 from cores C2 and C3 of the MC-EDF 201 to the reflector 120-2. The dichroic mirror 110-2 reflects a portion of the monitor light with wavelength λ2 (monitor light from cores C2 and C3) reflected from the reflector 120-2 to cores C1 and C4 of the MC-EDF 201, respectively.

[0092] The reflector 120 includes a lens 121 and a mirror 122, similar to that shown in FIG. 7. In this case, by using a cylindrical lens for the lens 121, each pair of monitor light can be reflected by a single lens. The reflector 120-1 reflects the monitor light of wavelength λ1 reflected from the cores C1 and C4 by the dichroic mirror 110-1 back to the dichroic mirror 110-1 using the lens 121-1 and the mirror 122-1, and couples it to the cores C2 and C3, respectively. The reflector 120-2 reflects the monitor light of wavelength λ2 reflected from the cores C2 and C3 by the dichroic mirror 110-2 using the lens 121-2 and the mirror 122-2, and couples it to the cores C1 and C4, respectively.

[0093] (Fourth Modification of the Second Embodiment) The multicore fiber 300 and the MC-EDF 201 may be multicore fibers with six or more cores. Even in the case of a multicore fiber with six or more cores (an even number of cores), cores with different transmission directions may be paired, and each pair of cores may be provided with a folding optical system 100 as in Fig. 7, and the monitor light may be folded between the cores in the pair.

[0094] 13 to 15 show examples of combinations of core pairs in a six-core multicore fiber. In the examples of Fig. 13 to 15, the multicore fiber 300 and the MC-EDF 201 include cores C1 to C6. The cores C1 to C6 are arranged side by side along the outer periphery of the fiber. Adjacent cores among the cores C1 to C6 have different transmission directions.

[0095] 13, adjacent cores form pairs of cores with different transmission directions. Cores C1 and C2 form a core pair, cores C3 and C4 form a core pair, and cores C5 and C6 form a core pair. In this case, a folding optical system 100 that folds back the monitor light between cores C1 and C2, a folding optical system 100 that folds back the monitor light between cores C3 and C4, and a folding optical system 100 that folds back the monitor light between cores C5 and C6 may be provided.

[0096] 14, cores facing each other across the center of the fiber are paired in different transmission directions. Cores C1 and C4 are paired, cores C2 and C5 are paired, and cores C3 and C6 are paired. In this case, a folding optical system 100 that folds back the monitor light between cores C1 and C4, a folding optical system 100 that folds back the monitor light between cores C2 and C5, and a folding optical system 100 that folds back the monitor light between cores C3 and C6 may be provided.

[0097] In the example of FIG. 15, cores facing each other in one direction (for example, the vertical direction in the drawing) in the fiber form pairs of cores with different transmission directions. Cores C1 and C2 form a core pair, cores C3 and C6 form a core pair, and cores C4 and C5 form a core pair. In this case, a folding optical system 100 that folds back the monitor light between cores C1 and C2, a folding optical system 100 that folds back the monitor light between cores C3 and C6, and a folding optical system 100 that folds back the monitor light between cores C4 and C5 may be provided. Also, as in FIG. 12, a single folding optical system 100 (dichroic mirror 110, lens 121, and mirror 122) may be used to fold back the monitor light between cores C1 and C2, between cores C3 and C6, and between cores C4 and C5, respectively.

[0098] (Embodiment 3) Next, a third embodiment will be described. In this embodiment, an example in which the dichroic mirror of the folding optical system shown in the second embodiment reflects monitor light of two wavelengths will be described.

[0099] Fig. 16 shows an example of the configuration of an optical repeater 2 according to some embodiments. In the example of Fig. 16, a dichroic mirror 110-1 transmits the signal light and reflects a portion of the supervisory light with wavelength λ1 and a portion of the supervisory light with wavelength λ2. Similarly, a dichroic mirror 110-2 transmits the signal light and reflects a portion of the supervisory light with wavelength λ1 and a portion of the supervisory light with wavelength λ2.

[0100] 7, filters 127 (127-1 and 127-2) are provided that transmit only the wavelengths reflected by the folding optical system 100. For example, the filter 127-1 transmits only the monitoring light of wavelength λ1. The filter 127-2 transmits only the monitoring light of wavelength λ2. For example, the filter 127 is disposed between the lens 121 and the mirror 122. The filter 127 may also be disposed between the lens 121 and the dichroic mirror 110. The rest is the same as in FIG. 7.

[0101] Fig. 17 shows an example of operation when loopback measurement is performed by folding back the monitoring light using the folding optical system 100-1 in Fig. 16. In the example of Fig. 17, similar to Fig. 8, monitoring light with wavelength λ1 from the EAST side is folded back to the EAST side by the folding optical system 100-1.

[0102] First, the monitor light of wavelength λ1 propagates from the EAST side (core C1 of the multicore fiber 300-2) and is incident on the dichroic mirror 110-2 on the EAST side (S101). The dichroic mirror 110-2 reflects a part of the light of wavelengths λ1 and λ2 and transmits the rest of the wavelengths λ1 and λ2 and the light of other wavelengths, so it reflects a part of the incident monitor light of wavelength λ1 and transmits the rest of the monitor light to core C1 of the MC-EDF 201.

[0103] 8, the monitor light of wavelength λ1 that has passed through the dichroic mirror 110-2 passes through the isolator 202-1 and is amplified by the core C1 of the MC-EDF 201 (S102). The amplified monitor light passes through the isolator 202-2 and is output to the branching point 111-1 of the dichroic mirror 110-1 (S103). The dichroic mirror 110-1 reflects a portion of the light of wavelengths λ1 and λ2 and transmits the remaining wavelengths λ1 and λ2 and light of other wavelengths. Therefore, when the monitor light of wavelength λ1 is input from the isolator 202-2 to the branching point 111-1, the dichroic mirror 110-1 reflects a portion of the monitor light of wavelength λ1 to the input point 123-1 of the lens 121-1 and transmits the remaining monitor light to the WEST side (core C1 of the multi-core fiber 300-1).

[0104] Next, when monitor light of wavelength λ1 is incident on input point 123-1 from dichroic mirror 110-1, lens 121-1 outputs the monitor light to turning point 125-1 of mirror 122-1 (S104). Next, filter 127-1 transmits only light of wavelength λ1, so it transmits the monitor light of wavelength λ1 output from lens 121-1 (S110).

[0105] Next, when the monitor light of wavelength λ1 is incident on turning point 125-1 from lens 121-1 via filter 127-1, mirror 122-1 reflects the monitor light to output point 124-1 of lens 121-1 (S105). Next, filter 127-1 transmits the monitor light of wavelength λ1 emitted from mirror 122-1 (S111). Next, when the monitor light of wavelength λ1 is incident on output point 124-1 from mirror 122-1 via filter 127-1, lens 121-1 emits the monitor light to coupling point 112-1 of dichroic mirror 110-1 (S106).

[0106] Next, the dichroic mirror 110-1 reflects a portion of the light of wavelengths λ1 and λ2 and transmits the remaining wavelengths λ1 and λ2 and the light of other wavelengths, so that when the monitoring light of wavelength λ1 is incident from the lens 121-1 to the coupling point 112-1, a portion of the monitoring light of wavelength λ1 is reflected to the core C2 of the MC-EDF 201 (S107).

[0107] 8, the monitor light of wavelength λ1 reflected by the dichroic mirror 110-1 passes through the isolator 202-3 and is amplified by the core C2 of the MC-EDF 201 (S108). The amplified monitor light passes through the isolator 202-4 and is output to the dichroic mirror 110-2 (S109). The dichroic mirror 110-2 reflects a portion of the light of wavelengths λ1 and λ2 and transmits the remaining wavelengths λ1 and λ2 and the light of other wavelengths, so that the dichroic mirror 110-2 reflects a portion of the monitor light of wavelength λ1 to the input point 123-2 of the lens 121-2 and transmits the remaining monitor light to the EAST side (core C2 of the multi-core fiber 300-2).

[0108] Next, when monitor light of wavelength λ1 is incident from dichroic mirror 110-2 to input point 123-2, lens 121-2 outputs the monitor light to turning point 125-2 of mirror 122-2 (S112). Filter 127-2 transmits only light of wavelength λ2, so it blocks the monitor light of wavelength λ1 output from lens 121-2 (S113). Thereafter, loopback measurement is performed at the terminal station on the EAST side, as in Figure 8.

[0109] Fig. 18 shows an example of operation when loopback measurement is performed by folding back the monitoring light by the folding optical system 100-2 in Fig. 16. In the example of Fig. 18, similar to Fig. 9, the monitoring light with wavelength λ2 from the WEST side is folded back by the folding optical system 100-2.

[0110] First, the monitor light of wavelength λ2 propagates from the WEST side (core C2 of multicore fiber 300-1) and is incident on the WEST-side dichroic mirror 110-1 (S201). The dichroic mirror 110-1 reflects a portion of the light of wavelengths λ1 and λ2 and transmits the remaining wavelengths λ1 and λ2 and light of other wavelengths, so it reflects a portion of the incident monitor light of wavelength λ2 and transmits the remaining monitor light to core C2 of MC-EDF 201.

[0111] 9, the monitor light of wavelength λ2 that has passed through the dichroic mirror 110-1 passes through the isolator 202-3 and is amplified by the core C2 of the MC-EDF 201 (S202). The amplified monitor light passes through the isolator 202-4 and is output to the branching point 111-2 of the dichroic mirror 110-2 (S203). The dichroic mirror 110-2 reflects a portion of the light of wavelengths λ1 and λ2 and transmits the remaining wavelengths λ1 and λ2 and light of other wavelengths. Therefore, when the monitor light of wavelength λ2 is input from the isolator 202-4 to the branching point 111-2, the dichroic mirror 110-2 reflects a portion of the monitor light of wavelength λ2 to the input point 123-2 of the lens 121-2 and transmits the remaining monitor light to the EAST side (core C2 of the multi-core fiber 300-2).

[0112] Next, when the monitor light of wavelength λ2 is incident on input point 123-2 from dichroic mirror 110-2, lens 121-2 outputs the monitor light to turning point 125-2 of mirror 122-2 (S204). Next, filter 127-2 transmits only light of wavelength λ2, so it transmits the monitor light of wavelength λ2 output from lens 121-2 (S210).

[0113] Next, when the monitor light of wavelength λ2 is incident on turning point 125-2 from lens 121-2 via filter 127-2, mirror 122-2 reflects the monitor light to output point 124-2 of lens 121-2 (S205). Next, filter 127-2 transmits the monitor light of wavelength λ2 emitted from mirror 122-2 (S211). Next, when the monitor light of wavelength λ2 is incident on output point 124-2 from mirror 122-2 via filter 127-2, lens 121-2 emits the monitor light to coupling point 112-2 of dichroic mirror 110-2 (S206).

[0114] Next, the dichroic mirror 110-2 reflects the light of wavelengths λ1 and λ2 and transmits the remaining wavelengths λ1 and λ2 and the light of other wavelengths, so that when the monitoring light of wavelength λ2 is incident from the lens 121-2 to the coupling point 112-2, a portion of the monitoring light of wavelength λ2 is reflected to the core C1 of the MC-EDF 201 (S207).

[0115] 9, the monitor light of wavelength λ2 reflected by the dichroic mirror 110-2 passes through the isolator 202-1 and is amplified by the core C1 of the MC-EDF 201 (S208). The amplified monitor light passes through the isolator 202-2 and is output to the dichroic mirror 110-1 (S209). The dichroic mirror 110-2 reflects a portion of the light of wavelengths λ1 and λ2 and transmits the remaining wavelengths λ1 and λ2 and the light of other wavelengths, so that the dichroic mirror 110-2 reflects a portion of the monitor light of wavelength λ2 to the input point 123-1 of the lens 121-1 and transmits the remaining monitor light to the WEST side (core C1 of the multi-core fiber 300-1).

[0116] Next, when monitor light of wavelength λ2 is incident on input point 123-1 from dichroic mirror 110-1, lens 121-1 outputs the monitor light to turning point 125-1 of mirror 122-1 (S212). Filter 127-1 transmits only light of wavelength λ1, and therefore blocks the monitor light of wavelength λ2 output from lens 121-1 (S213). Thereafter, loopback measurement is performed at the terminal station on the WEST side, as in FIG. 9.

[0117] As described above, the dichroic mirror of the folding optical system may reflect light of two wavelengths, and the folding optical system may be provided with a filter that transmits only the required wavelength. For example, it may be difficult to form a dichroic mirror that reflects only wavelength λ1 or only wavelength λ2. For this reason, the folding optical system may be configured using a dichroic mirror that reflects wavelengths λ1 and λ2, which is easy to form.

[0118] (Fourth embodiment) Next, a fourth embodiment will be described. In this embodiment, an example will be described in which folding for OTDR is further performed in the folding optical system shown in the second embodiment. Note that this embodiment may also be applied to the folding optical system shown in the third embodiment.

[0119] Fig. 19 shows a configuration example of an optical repeater 2 according to some embodiments. In the example of Fig. 19, the folding optical system 100 further includes mirrors 128 (128-1 and 128-2) in addition to the configuration of Fig. 7. The mirror 128 reflects the monitor light scattered by the multi-core fiber and reflected from the dichroic mirror 110 back to the dichroic mirror 110 for OTDR measurement.

[0120] Mirror 128-1 is disposed on the opposite side of branch point 111-1 of dichroic mirror 110-1 from reflector 120-1. Mirror 128-1 reflects the monitor light reflected from branch point 111-1 of dichroic mirror 110-1 (toward the opposite side of reflector 120-1) back to branch point 111-1 of dichroic mirror 110-1. Mirror 128-2 is disposed on the opposite side of branch point 111-2 of dichroic mirror 110-2 from reflector 120-2. Mirror 128-2 reflects the monitor light reflected from branch point 111-2 of dichroic mirror 110-2 (toward the opposite side of reflector 120-2) back to branch point 111-2 of dichroic mirror 110-2. The rest is the same as in FIG. 7.

[0121] 19 is similar to that of the second embodiment. That is, the folding optical system 100-1 returns the monitor light with wavelength λ1 from the east side to the east side, as in FIG. 8. The folding optical system 100-2 returns the monitor light with wavelength λ2 from the west side to the west side, as in FIG. 9.

[0122] Fig. 20 shows an example of operation when OTDR measurement is performed by folding back the monitoring light using the folding optical system 100-1 in Fig. 19. In the example of Fig. 20, the monitoring light with wavelength λ1 transmitted from the east side to the west side and scattered by the multi-core fiber on the west side is transmitted to the east side using the folding optical system 100-1.

[0123] First, the monitor light of wavelength λ1 is scattered on the WEST side (core C1 of multicore fiber 300-1), propagates to the EAST side, and is incident on branching point 111-1 (west-side surface) of dichroic mirror 110-1 on the WEST side (S301). Since dichroic mirror 110-1 reflects a portion of the light of wavelength λ1 and transmits the remainder of the light of wavelength λ1 and light of other wavelengths, it reflects a portion of the monitor light of wavelength λ1 to mirror 128-1 and transmits the remaining monitor light to isolator 202-2. The monitor light of wavelength λ1 that has transmitted through dichroic mirror 110-1 is blocked by isolator 202-2 and is not input to core C1 of MC-EDF 201.

[0124] Next, when the monitor light with wavelength λ1 is incident from dichroic mirror 110-1, mirror 128-1 reflects the monitor light again toward branch point 111-1 of dichroic mirror 110-1 (S302).

[0125] Next, dichroic mirror 110-1 reflects a portion of the light of wavelength λ1 and transmits the remainder of the light of wavelength λ1 and light of other wavelengths, so when monitoring light of wavelength λ1 is incident on branch point 111-1 (the surface on the west side) from mirror 128-1, it reflects a portion of the monitoring light of wavelength λ1 to the west side and transmits the remaining monitoring light to input point 123-1 of lens 121-1 (S303).

[0126] 8, the monitor light of wavelength λ1 that has passed through the dichroic mirror 110-1 is reflected by the lens 121-1 and the mirror 122-1 toward the coupling point 112-1 of the dichroic mirror 110-1 (S104 to S106), and a portion of the monitor light of wavelength λ1 is reflected from the dichroic mirror 110-1 toward the core C2 of the MC-EDF 201 (S107). The monitor light of wavelength λ1 that has been reflected by the dichroic mirror 110-1 is amplified by the core C2 of the MC-EDF 201 via the isolator 202-3 (S108). The monitor light amplified by the core C2 of the MC-EDF 201 is propagated toward the EAST side (core C2 of the multi-core fiber 300-2) via the isolator 202-4 and the dichroic mirror 110-2 (S109).

[0127] Thereafter, the terminal station on the EAST side performs OTDR measurement using the returned monitor light. The terminal station on the EAST side measures the power of the received monitor light and monitors the transmission state of the core C1 of the multi-core fiber 300-1.

[0128] Fig. 21 shows an example of operation when OTDR measurement is performed by folding back the monitoring light using the folding optical system 100-2 in Fig. 19. In the example of Fig. 21, the monitoring light with wavelength λ2 transmitted from the west side to the east side and scattered by the multi-core fiber on the east side is transmitted to the west side by the folding optical system 100-2.

[0129] First, the monitor light of wavelength λ2 is scattered on the EAST side (core C2 of multicore fiber 300-2), propagates to the WEST side, and is incident on branch point 111-2 (EAST side surface) of dichroic mirror 110-2 on the EAST side (S401). Since dichroic mirror 110-2 reflects a part of the light of wavelength λ2 and transmits the rest of the light of wavelength λ2 and light of other wavelengths, it reflects a part of the monitor light of wavelength λ2 to mirror 128-2 and transmits the rest of the monitor light to isolator 202-4. The monitor light of wavelength λ2 that has transmitted through dichroic mirror 110-2 is blocked by isolator 202-4 and is not input to core C2 of MC-EDF 201.

[0130] Next, when the monitor light with wavelength λ2 is incident from dichroic mirror 110-2, mirror 128-2 reflects the monitor light again toward branch point 111-2 of dichroic mirror 110-2 (S402).

[0131] Next, dichroic mirror 110-2 reflects a portion of the light of wavelength λ2 and transmits the remainder of the light of wavelength λ2 and light of other wavelengths, so when supervisory light of wavelength λ2 is incident on branch point 111-2 (the surface on the EAST side) from mirror 128-2, it reflects a portion of the supervisory light of wavelength λ2 to the EAST side and transmits the remaining supervisory light to input point 123-2 of lens 121-2 (S403).

[0132] 9, the monitor light of wavelength λ2 that has passed through the dichroic mirror 110-2 is reflected by the lens 121-2 and the mirror 122-2 toward the coupling point 112-2 of the dichroic mirror 110-2 (S204 to S206), and a portion of the monitor light of wavelength λ2 is reflected from the dichroic mirror 110-2 toward the core C1 of the MC-EDF 201 (S207). The monitor light of wavelength λ2 that has been reflected by the dichroic mirror 110-2 is amplified by the core C1 of the MC-EDF 201 via the isolator 202-1 (S208). The monitor light amplified by the core C1 of the MC-EDF 201 is propagated toward the WEST side (core C1 of the multi-core fiber 300-1) via the isolator 202-2 and the dichroic mirror 110-1 (S209).

[0133] Thereafter, the terminal station on the west side performs OTDR measurement using the returned monitor light. The terminal station on the west side measures the power of the received monitor light and monitors the transmission state of the core C2 of the multicore fiber 300-2.

[0134] The monitor light for loopback measurement and the monitor light for OTDR measurement may have the same wavelength or different wavelengths. For example, monitor light with wavelength λ1 may be transmitted from the east side to the west side, and loopback measurement and OTDR measurement may be performed using the monitor light reflected back to the east side. Monitor light with wavelength λ2 may be transmitted from the west side to the east side, and loopback measurement and OTDR measurement may be performed using the monitor light reflected back to the west side. In this case, the same folding optical system 100 may be used to return the monitor light for loopback measurement and the monitor light for OTDR measurement. For example, as shown in FIG. 19 , a folding optical system 100-1 that reflects the monitor light with wavelength λ1 for loopback measurement and OTDR measurement, and a folding optical system 100-2 that reflects the monitor light with wavelength λ2 for loopback measurement and OTDR measurement may be provided.

[0135] Fig. 21 shows an example of wavelength bands when the monitor light for loopback measurement and the monitor light for OTDR measurement have different wavelengths. In the example of Fig. 21, the monitor light for loopback measurement has wavelengths λ1 and λ2, and the monitor light for OTDR measurement has wavelengths λ3 and λ4. Since the loopback measurement monitors the operating state of the optical amplifier, the wavelength of the monitor light for loopback measurement is closer to the wavelength band of the signal light than the wavelength of the monitor light for OTDR measurement.

[0136] For example, monitor light of wavelength λ1 may be transmitted from the east side to the west side and loopback measurement may be performed using the monitor light reflected back to the east side, while monitor light of wavelength λ2 may be transmitted from the west side to the east side and loopback measurement may be performed using the monitor light reflected back to the west side. Monitor light of wavelength λ3 may be transmitted from the east side to the west side and OTDR measurement may be performed using the monitor light scattered on the west side and reflected back to the east side, while monitor light of wavelength λ4 may be transmitted from the west side to the east side and scattered on the east side and OTDR measurement may be performed using the monitor light reflected back to the west side. In this case, the reflection of the monitor light for the loopback measurement and the reflection of the monitor light for the OTDR measurement may be performed using separate reflection optical systems 100. For example, as shown in FIG. 7, a folding optical system 100-1 that folds back the monitoring light of wavelength λ1 for loopback measurement, a folding optical system 100-2 that folds back the monitoring light of wavelength λ2 for loopback measurement, and, as shown in FIG. 19, a folding optical system 100-1 that folds back the monitoring light of wavelength λ3 for OTDR measurement (the folding optical system 100-1 reflects the wavelength λ3 with a dichroic mirror 110-1), and a folding optical system 100-2 that folds back the monitoring light of wavelength λ4 for OTDR measurement (the folding optical system 100-2 reflects the wavelength λ4 with a dichroic mirror 110-2) may be provided.

[0137] As described above, the monitoring light (scattered light) scattered in the multi-core fiber may be reflected by a dichroic mirror, and a mirror that reflects the reflected light back to the dichroic mirror may be added to the folding optical system. This allows the folding optical system to fold back the monitoring light for loop-back measurement and the monitoring light (scattered light) for OTDR measurement.

[0138] Although the present disclosure has been described above with reference to the embodiments, the present disclosure is not limited to the above-described 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. Furthermore, each embodiment can be combined with other embodiments as appropriate.

[0139] Each drawing is merely an example for describing one or more embodiments. Each drawing may relate not only to one particular embodiment, but also to one or more other embodiments. As will be understood by those skilled in the art, various features or steps described with reference to any one drawing can be combined with features or steps shown in one or more other drawings to create, for example, an embodiment not explicitly shown or described. Not all features or steps shown in any one drawing are necessary to describe an exemplary embodiment, and some features or steps may be omitted. The order of steps described in any drawing may be changed as appropriate.

[0140] A part or all of the above-described embodiments can be described as, but not limited to, the following supplementary notes. (Appendix 1) a wavelength selection mirror that transmits a first light having a first wavelength propagated through the first core of a first core and a second core included in the multicore fiber and reflects a second light having a second wavelength; a reflector that reflects the second light reflected by the wavelength selection mirror back to the wavelength selection mirror and couples the second light to the second core; A folded optical system comprising: (Appendix 2) the first light is signal light in an optical transmission system including the folding optical system, the second light is a monitor light for monitoring a state of the optical transmission system; 10. The folded optical system of claim 1. (Appendix 3) the first core and the second core have different transmission directions of the signal light; 2. The folded optical system described in Appendix 2. (Appendix 4) the wavelength-selective mirror reflects a portion of the second light at the second wavelength; 4. The folded optical system according to any one of claims 1 to 3. (Appendix 5) the wavelength selection mirror is disposed so as to intersect with the optical axis direction of the first core and the second core, and reflects the second light in a direction perpendicular to the optical axis direction. 4. The folded optical system according to any one of claims 1 to 3. (Appendix 6) The wavelength-selective mirror is a dichroic mirror. 4. The folded optical system according to any one of claims 1 to 3. (Appendix 7) the reflector moves the optical path of the second light reflected by the wavelength selection mirror in parallel, and reflects the second light toward a position intersecting the optical axis direction of the second core of the wavelength selection mirror. 4. The folded optical system according to any one of claims 1 to 3. (Appendix 8) The reflector includes a lens capable of condensing parallel light, and a reflecting mirror capable of reflecting light from the lens at a condensing position of the lens. 4. The folded optical system according to any one of claims 1 to 3. (Appendix 9) The reflector is a corner cube type reflector. 4. The folded optical system according to any one of claims 1 to 3. (Appendix 10) a magnifying lens that enlarges the distance between the optical axis of the first core and the optical axis of the second core; the wavelength selection mirror reflects the second light passing through the magnifying lens, and couples the second light returned from the reflector to the second core via the magnifying lens. 4. The folded optical system according to any one of claims 1 to 3. (Appendix 11) the multicore fiber includes a third core and a fourth core; the wavelength selection mirror transmits the third light having the first wavelength propagated through the third core and reflects the fourth light having the second wavelength; the reflector reflects the fourth light reflected by the wavelength selection mirror back to the wavelength selection mirror and couples it into the fourth core. 4. The folded optical system according to any one of claims 1 to 3. (Appendix 12) the wavelength-selective mirror further reflects light of other wavelengths; a filter that transmits second light having the second wavelength out of the light reflected by the wavelength selection mirror; the reflector reflects the second light that has passed through the filter back to the wavelength-selective mirror; 4. The folded optical system according to any one of claims 1 to 3. (Appendix 13) the wavelength-selective mirror reflects the second light having the second wavelength propagated in a direction opposite to the first light toward a side opposite to the reflector; a reflecting mirror that reflects the second light reflected by the wavelength selecting mirror back to the wavelength selecting mirror; 4. The folded optical system according to any one of claims 1 to 3. (Appendix 14) a multi-core optical amplifying fiber including a first core and a second core; a folded optical system; The folding optical system is a wavelength selection mirror that transmits a first light having a first wavelength propagated through the first core and reflects a second light having a second wavelength; a reflector that reflects the second light reflected by the wavelength selection mirror back to the wavelength selection mirror and couples the second light to the second core; An optical repeater comprising: (Appendix 15) an optical repeater connected between the multi-core fibers; The optical repeater comprises: a multi-core optical amplifying fiber including a first core and a second core; a folded optical system; The folding optical system is a wavelength selection mirror that transmits a first light having a first wavelength propagated through the first core and reflects a second light having a second wavelength; a reflector that reflects the second light reflected by the wavelength selection mirror back to the wavelength selection mirror and couples the second light to the second core; An optical transmission system comprising: (Appendix 16) a wavelength-selective mirror that transmits a first light having a first wavelength propagated through the first core of a first core and a second core included in the multicore fiber and reflects a second light having a second wavelength; the second light reflected by the wavelength selection mirror is returned to the wavelength selection mirror and coupled to the second core; Optical folding method.

[0141] Some or all of the elements (e.g., configurations and functions) described in Supplementary Notes 2 to 13 that are dependent on Supplementary Note 1 (folded optical system) may also be dependent on Supplementary Note 14 (optical repeater), Supplementary Note 15 (optical transmission system), and Supplementary Note 16 (optical folding method) in the same dependent relationship as Supplementary Notes 2 to 13. Some or all of the elements described in any Supplementary Note may be applied to various hardware, software, recording means for recording software, systems, and methods. [Explanation of symbols]

[0142] 1 Optical transmission system 2 Optical repeater 10. Folded optical system 11 Wavelength-selective mirror 12 Reflector 13 Reflective mirror 20 Multicore fiber 21 First Core 22 Second Core 100 folded optical system 110 Dichroic Mirror 111 Junction 112 Connection point 120 Reflector 121 Lens 122 Mirror 123 input points 124 output points 125 Turning Point 126a, 126b Reflector 127 filters 128 Mirror 131~134 Magnifying Lens 141~143 Parallel Lens 201 MC-EDF 202 Isolator 300 Multicore Fiber

Claims

1. a wavelength selection mirror that transmits a first light having a first wavelength propagated through the first core of a first core and a second core included in the multicore fiber and reflects a second light having a second wavelength; a reflector that reflects the second light reflected by the wavelength selection mirror back to the wavelength selection mirror and couples the second light to the second core; A folded optical system comprising:

2. the first light is signal light in an optical transmission system including the folding optical system, the second light is a monitor light for monitoring a state of the optical transmission system; The folded optical system of claim 1 .

3. the first core and the second core have different transmission directions of the signal light; The folded optical system of claim 2 .

4. the wavelength-selective mirror reflects a portion of the second light at the second wavelength. The folded optical system according to any one of claims 1 to 3.

5. the wavelength selection mirror is disposed so as to intersect with the optical axis direction of the first core and the second core, and reflects the second light in a direction perpendicular to the optical axis direction; The folded optical system according to any one of claims 1 to 3.

6. The wavelength-selective mirror is a dichroic mirror. The folded optical system according to any one of claims 1 to 3.

7. the reflector moves parallel to the optical path of the second light reflected by the wavelength selection mirror, and reflects the second light toward a position intersecting the optical axis direction of the second core of the wavelength selection mirror. The folded optical system according to any one of claims 1 to 3.

8. a multi-core optical amplifying fiber including a first core and a second core; a folded optical system; The folding optical system is a wavelength selection mirror that transmits a first light having a first wavelength propagated through the first core and reflects a second light having a second wavelength; a reflector that reflects the second light reflected by the wavelength selection mirror back to the wavelength selection mirror and couples the second light to the second core; An optical repeater comprising:

9. an optical repeater connected between the multi-core fibers; The optical repeater comprises: a multi-core optical amplifying fiber including a first core and a second core; a folded optical system; The folding optical system is a wavelength selection mirror that transmits a first light having a first wavelength propagated through the first core and reflects a second light having a second wavelength; a reflector that reflects the second light reflected by the wavelength selection mirror back to the wavelength selection mirror and couples the second light to the second core; An optical transmission system comprising:

10. a wavelength-selective mirror that transmits a first light having a first wavelength propagated through the first core of a first core and a second core included in the multicore fiber and reflects a second light having a second wavelength; the second light reflected by the wavelength selection mirror is returned to the wavelength selection mirror and coupled to the second core; Optical folding method.

Images