Optical amplification device, optical transmission system and optical amplification method

Abstract

To provide an optical amplification device capable of suppressing quality deterioration of signal light, an optical transmission system and an optical amplification method.SOLUTION: An optical amplification device is connected between a first multicore fiber and a second multicore fiber. The optical amplification device comprises: a switching section which switches in a bidirectional manner transmission directions of signal light transmitted in the same direction by the first multicore fiber and the second multicore fiber; and an amplification section which amplifies the signal light transmitted in the bidirectional manner.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present disclosure relates to an optical amplifier, an optical transmission system, and an optical amplification 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 an optical amplifier using an MC-EDF (Multi-Core-Erbium-Doped Fiber). Also, Non-Patent Document 1 describes a multi-core fiber capable of reducing inter-core crosstalk. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-145255 [Non-patent literature]

[0004] [Non-Patent Document 1] Masanori Takahashi et al., "Uncoupled 4-core Fiber with Ultra-low Loss and Low Inter Core Crosstalk", 2020 European Conference on Optical Communications (ECOC), IEEE, December 2020 Summary of the Invention [Problem to be solved by the invention]

[0005] For example, in related technologies such as Patent Document 1 and Non-Patent Document 1, the influence of inter-core crosstalk that occurs in optical amplifiers using MC-EDFs or the like is not taken into consideration, which may result in degradation of the quality of signal light.

[0006] An object of the present disclosure is to provide an optical amplifier, an optical transmission system, and an optical amplification method that are capable of suppressing deterioration in the quality of signal light. [Means for solving the problem]

[0007] An optical amplification device according to one aspect of the present disclosure is an optical amplification device connected between a first multicore fiber and a second multicore fiber, and includes a switching unit that switches the transmission direction of signal light transmitted in the same direction through the first multicore fiber and the second multicore fiber to bidirectional, and an amplification unit that amplifies the signal light transmitted in the two directions.

[0008] An optical transmission system according to one embodiment of the present disclosure includes a first multicore fiber, a second multicore fiber, and an optical amplifier connected between the first multicore fiber and the second multicore fiber, wherein the optical amplifier includes a switching unit that switches the transmission direction of signal light transmitted in the same direction through the first multicore fiber and the second multicore fiber to bidirectional, and an amplifier that amplifies the signal light transmitted in the two directions.

[0009] An optical amplification method according to one aspect of the present disclosure is an optical amplification method in an optical amplification device connected between a first multicore fiber and a second multicore fiber, in which a transmission direction of signal light transmitted in the same direction through the first multicore fiber and the second multicore fiber is switched to bidirectional, and the signal light transmitted in the two directions is amplified. [Effects of the Invention]

[0010] According to the present disclosure, deterioration in the quality of signal light can be suppressed. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram for explaining codirectional transmission in a multicore fiber. [Figure 2]FIG. 1 is a diagram for explaining bidirectional transmission in a multicore fiber. [Figure 3] FIG. 1 is a configuration diagram illustrating an example of a configuration of a related optical transmission system. [Figure 4] FIG. 1 is a configuration diagram illustrating an example of a configuration of a related optical transmission system. [Figure 5] 1 is a graph illustrating signal quality measurements according to some embodiments. [Figure 6] 1 is a diagram illustrating an example of a configuration of an optical transmission system according to some embodiments. [Figure 7] 1 is a diagram illustrating an example of a configuration of an optical transmission system according to some embodiments. [Figure 8] 1A and 1B are diagrams for explaining the principle of transmission direction switching of an optical amplifier according to some embodiments. [Figure 9] 1 is a configuration diagram illustrating an example of a configuration of an optical amplifier according to some embodiments; [Figure 10] 1A and 1B are diagrams for explaining the principle of transmission direction switching of an optical amplifier according to some embodiments. [Figure 11] 1 is a configuration diagram illustrating an example of a configuration of an optical amplifier according to some embodiments; [Figure 12] 1A and 1B are diagrams for explaining the principle of transmission direction switching of an optical amplifier according to some embodiments. [Figure 13] 1 is a configuration diagram illustrating an example of a configuration of an optical amplifier according to some embodiments; [Figure 14] 1A and 1B are diagrams for explaining the principle of transmission direction switching of an optical amplifier according to some embodiments. [Figure 15] 1 is a configuration diagram illustrating an example of a configuration of an optical amplifier according to some embodiments; [Figure 16] 1A and 1B are diagrams for explaining the principle of transmission direction switching of an optical amplifier according to some embodiments. [Figure 17] 1 is a configuration diagram illustrating an example of a configuration of an optical amplifier according to some embodiments; [Figure 18] 1 is a configuration diagram illustrating an example of a configuration of an optical amplifier according to some embodiments; [Figure 19] 1 is a configuration diagram illustrating an example of a configuration of an optical amplifier according to some embodiments; [Figure 20] 1 is a configuration diagram illustrating an example of a configuration of an optical amplifier according to some embodiments; DETAILED DESCRIPTION OF THE INVENTION

[0012] 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.

[0013] (Review of related technologies) Figure 1 is a diagram explaining co-directional transmission in a multi-core fiber. Co-directional transmission is a transmission method in which signal light is transmitted in the same direction through multiple cores in a multi-core fiber. In the example of Figure 1, signal light is transmitted in the same direction through two cores. In the case of co-directional transmission, for example, the signal light transmitted through the two cores interferes with each other, causing crosstalk (inter-core crosstalk) and degrading transmission quality, which is an issue.

[0014] Figure 2 is a diagram explaining bidirectional transmission in a multicore fiber. Bidirectional transmission is a transmission method in which signal light is transmitted in different directions for each core in a multicore fiber. In the example of Figure 2, signal light is transmitted in opposite directions in two cores. In the case of bidirectional transmission, for example, interference between signal light transmitted in two cores can be suppressed, thereby reducing inter-core crosstalk.

[0015] First, the inventors studied the issues involved in performing bidirectional transmission and unidirectional transmission using a multicore fiber. Fig. 3 shows a configuration example of a related optical transmission system performing bidirectional transmission. In the example of Fig. 3, the optical transmission system 8 is a system that amplifies and transmits signals between multicore transmission line fibers 801 and 802 using an MC-EDF 810, which is an optical amplifier. As shown in Fig. 3, the optical transmission system 8 includes an isolator (ISO) 820, a gain flattening filter (GFF) 830, and a fan-in / fan-out (FIFO) 840.

[0016] An ISO is required for each signal path to input and output optical signals in one direction. Furthermore, because the power of the light amplified by the MC-EDF 810 varies depending on the wavelength due to wavelength dependency, a GFF must be placed after the MC-EDF 810. Therefore, in the example of Figure 3, on the path where the signal light propagates from the first core of the transmission line fiber 801 to the first core of the transmission line fiber 802, an ISO 820-1 is placed before the MC-EDF 810, and GFFs 830-1 and 820-1 are placed after the MC-EDF 810. Furthermore, on the path where the signal light propagates from the second core of the transmission line fiber 802 to the second core of the transmission line fiber 801, an ISO 820-3 is placed before the MC-EDF 810, and GFFs 830-2 and 820-4 are placed after the MC-EDF 810.

[0017] Furthermore, in order to separate / combine multi-core signal light into single-core signal light, a FIFO 840-1 is arranged between the transmission line fiber 801 and ISO 820-1 and ISO 820-4, a FIFO 840-2 is arranged between ISO 820-1 and GFF 830-2 and the MC-EDF 810, a FIFO 840-3 is arranged between the MC-EDF 810 and GFF 830-1 and ISO 820-3, and a FIFO 840-4 is arranged between ISO 820-2 and ISO 820-3 and the transmission line fiber 802.

[0018] As shown in Figure 3, inter-core crosstalk between the transmission fiber and MC-EDF can be reduced by performing bidirectional transmission using the transmission fiber and MC-EDF. However, to perform bidirectional transmission, a FIFO is required to effectively allocate GFF and ISO for each core, as shown in Figure 3. This increases the loss due to the FIFO and poses the problem of an increased number of components.

[0019] Fig. 4 shows an example of the configuration of a related optical transmission system that performs co-directional transmission. When performing co-directional transmission, as shown in Fig. 4, the optical transmission system 9 is configured only with an MC-EDF 810, ISOs 820-1 and 820-2, and a GFF 830. That is, to propagate signal light from two cores of a transmission line fiber 801 to two cores of a transmission line fiber 802, the ISO 820-1 is placed before the MC-EDF 810, and the GFF 830 and ISO 820-1 are placed after the MC-EDF 810.

[0020] As shown in Figure 4, in codirectional transmission, the signal light in each core travels in the same direction, allowing the ISO and GFF for each core to be integrated. Therefore, codirectional transmission does not require a FIFO, as in bidirectional transmission, which reduces loss due to the FIFO and the number of components. However, when performing codirectional transmission, there is the issue of inter-core crosstalk occurring between the transmission fiber and the MC-EDF, as shown in Figure 1.

[0021] Therefore, in the embodiment, it is possible to suppress the occurrence of loss due to optical components while suppressing inter-core crosstalk.

[0022] (Embodiment 1) Next, a first embodiment will be described. In this embodiment, a basic example of an optical transmission system including an optical amplifier device will be described.

[0023] With regard to transmission line fibers in optical transmission systems, recent advances in fiber technology have made it possible to suppress inter-core crosstalk even in co-directional transmission. For example, as shown in Non-Patent Document 1, a multi-core fiber has already been developed that suppresses crosstalk to less than -60 dB / km in co-directional transmission.

[0024] On the other hand, with regard to MC-EDF, progress has not been made in reducing crosstalk in codirectional transmission. For this reason, although inter-core crosstalk can be reduced in transmission line fibers by using related technologies, it is not possible to reduce inter-core crosstalk in MC-EDF.

[0025] In order to study methods for suppressing inter-core crosstalk in MC-EDF, the inventors measured signal quality when unidirectional transmission and bidirectional transmission were performed in MC-EDF. Figure 5 shows the results of experimental measurements by the inventors on signal quality (Q factor) as a function of transmission distance in an optical transmission system. That is, Figure 5 shows the difference in transmission distance depending on the transmission direction of MC-EDF. In Figure 5, 501 shows the signal quality when the transmission line fiber is used for unidirectional transmission and the MC-EDF is used for unidirectional transmission, and 502 shows the signal quality when the transmission line fiber is used for unidirectional transmission and the MC-EDF is used for bidirectional transmission.

[0026] The experimental results in Fig. 5 show that inter-core crosstalk is relatively large when unidirectional transmission is performed using MC-EDF, and that the transmission distance increases by 1.4 times when bidirectional transmission is performed using MC-EDF (for example, 700 km for unidirectional transmission and 1000 km for bidirectional transmission). Therefore, bidirectional transmission is an effective method for reducing inter-core crosstalk in MC-EDF. Therefore, in the embodiment, inter-core crosstalk can be reduced by performing bidirectional transmission in MC-EDF.

[0027] 6 shows an example of the configuration of an optical transmission system 1 according to some embodiments. In the example of Fig. 6, the optical transmission system 1 includes an optical amplifier 100 between a transmission line fiber 200 and a transmission line fiber 300.

[0028] The transmission line fiber 200 (for example, a first multi-core fiber) and the transmission line fiber 300 (for example, a second multi-core fiber) are multi-core fibers. Each of the transmission line fiber 200 and the transmission line fiber 300 may include two cores, or may include more cores.

[0029] The transmission line fiber 200 is an optical fiber that transmits signal light input to the optical amplifier 100 on the input side of the optical amplifier 100. The transmission line fiber 200 includes a core 201a (e.g., one of the first and second input side cores) and a core 201b (e.g., the other of the first and second input side cores).

[0030] The transmission line fiber 300 is an optical fiber that transmits signal light output from the optical amplifier 100 on the output side of the optical amplifier 100. The transmission line fiber 300 includes a core 301a (e.g., one of the first and second output side cores) and a core 301b (e.g., the other of the first and second output side cores).

[0031] 6, co-directional transmission is performed through the transmission line fiber 200 and the transmission line fiber 300. For example, by using a multi-core fiber capable of low crosstalk as described in Non-Patent Document 1 as the transmission line fiber 200 and the transmission line fiber 300, it is possible to reduce inter-core crosstalk in the transmission line fiber 200 and the transmission line fiber 300. Note that other multi-core fibers may also be used as the transmission line fiber 200 and the transmission line fiber 300.

[0032] The optical amplifier 100 is an amplifier that amplifies light using an MC-EDF. The optical amplifier 100 is connected between a transmission line fiber 200 and a transmission line fiber 300. The optical amplifier 100 amplifies two signal lights output from cores 201a and 201b of the transmission line fiber 200, and outputs the amplified two signal lights to cores 301a and 301b of the transmission line fiber 300.

[0033] The optical amplifier 100 includes a transmission direction switching unit 110 and an MC-EDF 120. In the optical amplifier 100, bidirectional transmission is performed using the MC-EDF 120. This reduces inter-core crosstalk in the MC-EDF 120.

[0034] The transmission direction switching unit 110 is a switching unit that switches the transmission direction between the transmission line fibers 200 and 300 and the MC-EDF 120. To perform bidirectional transmission using the MC-EDF 120, the transmission direction switching unit 110 switches the transmission direction of signal light transmitted in the same direction through the transmission line fibers 200 and 300 between two directions. For example, the transmission direction switching unit 110 inputs two signal lights output in the same direction from cores 201a and 201b of the transmission line fiber 200 to the MC-EDF 120 in two directions (reverse directions). The transmission direction switching unit 110 outputs the two signal lights amplified by the MC-EDF 120 and output in both directions to cores 301a and 301b of the transmission line fiber 300 in the same direction.

[0035] The MC-EDF 120 is an amplifier that amplifies input signal light. When pumping light is input to the MC-EDF 120 together with the signal light, the signal light is pumped and amplified. The pumping method may be core pumping, in which pumping light is input to the core, or cladding pumping, in which pumping light is input to the cladding. The MC-EDF 120 is a multi-core optical amplifying fiber, and may include two cores like the transmission line fibers 200 and 300, or may include more cores.

[0036] For example, the MC-EDF 120 includes two cores (a first amplifying core and a second amplifying core), amplifies the signal light input in both directions from the transmission direction switching unit 110, and outputs the amplified bidirectional signal light to the transmission direction switching unit 110.

[0037] For example, the transmission direction switching unit 110 may input the signal light output from a first input core (one of the cores 201a and 201b) of the transmission line fiber 200 to a first amplification core at a first end facet of the MC-EDF 120, and input the signal light output from the first amplification core at a second end facet of the MC-EDF 120 to a first output core (one of the cores 301a and 301b) of the transmission line fiber 300. The transmission direction switching unit 110 may input the signal light output from a second input core (the other of the cores 201a and 201b) of the transmission line fiber 200 to a second amplification core at the second end facet of the MC-EDF 120, and input the signal light output from the second amplification core at the first end facet of the MC-EDF 120 to a second output core (the other of the cores 301a and 301b) of the transmission line fiber 300. As long as the transmission direction switching unit 110 can realize such an input / output relationship, there are no limitations on the configuration. For example, as in the example described below, the transmission direction switching unit 110 may be configured to switch the transmission direction using a mirror.

[0038] Although the ISO, GFF, etc. are omitted in the example of Fig. 6, the optical transmission system 1 may include the ISO, GFF, etc. as necessary. Fig. 7 shows an example of the configuration of the optical transmission system 1 according to some embodiments. In the example of Fig. 7, the optical transmission system 1 includes an ISO 410-1, a GFF 420, and an ISO 410-2, similar to Fig. 4.

[0039] The ISO 410-1 is disposed between the transmission line fiber 200 and the optical amplifier 100. The ISO 410-1 passes the multi-core signal light output from the transmission line fiber 200 to the optical amplifier 100. For example, the ISO 410-1 may be configured by integrating a plurality of isolators that pass the signal light for each core, respectively.

[0040] The GFF 420 is disposed between the optical amplifier 100 and the ISO 410-2. The GFF 420 flattens the wavelength dependency of the multi-core signal light amplified by the MC-EDF 120. For example, the GFF 420 may be configured by integrating multiple filters for each of the signal lights for each core.

[0041] The ISO 410-2 is disposed between the GFF 420 and the transmission line fiber 300. The ISO 410-2 passes the multi-core signal light flattened by the GFF 420 to the transmission line fiber 300. For example, the ISO 410-2 may be configured by integrating a plurality of isolators that pass the signal light for each core, respectively.

[0042] As described above, in this embodiment, an optical transmission system is provided with an optical amplifier having a transmission direction switching function. Specifically, unidirectional transmission is performed in the transmission line fiber, and switching is made to bidirectional transmission in the optical amplifier (MC-EDF) with large inter-core crosstalk. By performing bidirectional transmission in the optical amplifier, it is possible to reduce the occurrence of inter-core crosstalk in the optical amplifier. Furthermore, by performing unidirectional transmission in the transmission line fiber, it is possible to reduce the number of optical components such as FIFOs, GFFs, and isolators, and thereby reduce loss due to the optical components.

[0043] (Embodiment 2) Next, a description will be given of a second embodiment. In this embodiment, an example in which a two-core MC-EDF is used will be described as a specific example of the optical amplifying device shown in the first embodiment.

[0044] The principle of switching the transmission direction in the optical amplifier 100 according to some embodiments will be described with reference to Fig. 8. Fig. 8 is a front view of the end faces 122-1 and 122-2 of the MC-EDF 120.

[0045] 8, the MC-EDF 120 is a two-core MC-EDF. That is, the MC-EDF 120 includes cores 121a and 121b. Signal light is input and output in both directions to and from the cores 121a and 121b at end faces 122-1 and 122-2 on both ends of the MC-EDF 120.

[0046] As shown in FIG. 8, the transmission direction switching unit 110 includes a double-sided mirror 111. The double-sided mirror 111 (reflector) switches the direction of the signal light by reflecting the signal light input / output to either the core 121a or the core 121b. For example, the double-sided mirror 111 switches the direction of the signal light input / output to the core 121b. One side 111a (e.g., a first reflector) of the double-sided mirror 111 switches the direction of the signal light input to the core 121b. One side 111b (e.g., a second reflector) of the double-sided mirror 111 switches the direction of the signal light output from the core 121b. The one side 111a and the one side 111b of the double-sided mirror 111 are mirrors that totally reflect light. Note that the double-sided mirror 111 is not limited to the double-sided mirror 111, and a first single-sided mirror having the same configuration as the one side 111b and a second single-sided mirror having the same configuration as the one side 111b may also be provided.

[0047] The transmission direction switching function of the transmission direction switching unit 110 is realized by the positional relationship (including the angle) between the double-sided mirror 111 and the cores (e.g., core 121b) that change the transmission direction at the end faces 122-1 and 122-2 of the MC-EDF 120. That is, the end faces 122-1 and 122-2 of the MC-EDF 120 and the double-sided mirror 111 are fixed so that the cores that change the transmission direction at the end faces 122-1 and 122-2 of the MC-EDF 120 and the double-sided mirror 111 have a predetermined positional relationship. The transmission direction switching unit 110 may include a fixing unit (not shown) that fixes the end faces 122-1 and 122-2 of the MC-EDF 120 and the double-sided mirror 111 so that the cores that change the transmission direction at the end faces 122-1 and 122-2 of the MC-EDF 120 and the double-sided mirror 111 have a predetermined positional relationship.

[0048] In the example of Fig. 8, the end faces 122-1 and 122-2 of the MC-EDF 120 face each other at a predetermined angle when the MC-EDF 120 is folded back. That is, the MC-EDF 120 is folded back without twisting. The end faces 122-1 and 122-2 face each other without rotating. For example, the core 121a of the end face 122-1 and the core 121a of the end face 122-2 are located inside the area where the end faces 122-1 and 122-2 face each other. When the end faces 122-1 and 122-2 are placed opposite each other, the core 121a of the end face 122-1 and the core 121a of the end face 122-2 face each other. Core 121b of end face 122-1 and core 121b of end face 122-2 are located on the outside where end face 122-1 and end face 122-2 face each other. When end face 122-1 and end face 122-2 face each other, core 121b of end face 122-1 and core 121b of end face 122-2 face each other.

[0049] 8, the core 121a is a core that does not switch the transmission direction, and the core 121b is a core that switches the transmission direction. Note that the core 121b may be a core that does not switch the transmission direction, and the core 121a may be a core that switches the transmission direction. That is, one of the two cores is a core that does not switch the transmission direction, and the other core is a core that switches the transmission direction. This can reduce inter-core crosstalk in the MC-EDF 120.

[0050] 8, when end faces 122-1 and 122-2 of MC-EDF 120 are arranged opposite each other at a certain angle, double-sided mirror 111 is placed at the intersection of cores 121b that change the transmission direction within MC-EDF 120. Specifically, double-sided mirror 111 is placed at the intersection of the central axes (straight lines extending in the optical input / output direction) of cores 121b that change the transmission direction within MC-EDF 120. This makes it possible to switch the transmission direction of signal light in core 121b to the opposite direction to the transmission direction of signal light in core 121a.

[0051] In the path through core 121a, signal light is input to core 121a at end face 122-2, and the signal light amplified by MC-EDF 120 is output from core 121a at end face 122-1. In addition, in the path through core 121b, the input signal light is reflected by one surface 111a of double-sided mirror 111, input to core 121b at end face 122-1, and the signal light amplified by MC-EDF 120 is output from core 121b at end face 122-1. Double-sided mirror 111 is fixed so that one surface 111a reflects the signal light input from transmission line fiber 200 to core 121b at end face 122-1. The signal light output from core 121b is reflected by the other surface 111b of double-sided mirror 111 and output. The double-sided mirror 111 is fixed at an angle such that one surface 111b reflects the signal light output from the core 121b into the transmission line fiber 300. In this way, by fixing the positional relationship between the core 121b, which changes the transmission direction, and the double-sided mirror 111 as shown in Fig. 8, the transmission direction within the MC-EDF 120 can be switched between two directions.

[0052] Fig. 9 shows a specific example of the configuration of the optical amplifying device 100 described in Fig. 8. Fig. 9 is a view of the end faces 122-1 and 122-2 of the MC-EDF 120 as seen from above.

[0053] 8, the optical amplifier 100 includes the MC-EDF 120 and the double-sided mirror 111. The optical amplifier 100 may include a fixing portion that fixes the MC-EDF 120 and the double-sided mirror 111. The end faces 122-1 and 122-2 of the MC-EDF 120 and the double-sided mirror 111 are fixed so as to have the positional relationship shown in FIG.

[0054] 9, the end faces 122-1 and 122-2 of the MC-EDF 120 are fixed so as to face each other at a right angle. For example, the MC-EDF 120 is folded back without being twisted, and fixed so as to face each end face 122-1 and 122-2 at a right angle. If necessary, the MC-EDF 120 may be wound around to pull out the end faces 122-1 and 122-2. Note that the MC-EDF 120 may be twisted by 360 degrees × n (n is an integer).

[0055] 8, core 121a of end face 122-1 and core 121a of end face 122-2 are located on the inside where end face 122-1 and end face 122-2 face each other. Core 121b of end face 122-1 and core 121b of end face 122-1 are located on the outside where end face 122-1 and end face 122-2 face each other.

[0056] 9, the input-side transmission line fiber 200 is disposed so that the end face 202 of the transmission line fiber 200 faces the end face 122-2 of the MC-EDF 120. Specifically, the core 201a at the end face 202 of the transmission line fiber 200 faces the core 121a at the end face 122-2 of the MC-EDF 120. It can also be said that the central axis of the core 201a at the end face 202 of the transmission line fiber 200 overlaps with the central axis of the core 121a at the end face 122-2 of the MC-EDF 120. Furthermore, the core 201b at the end face 202 of the transmission line fiber 200 faces the core 121b at the end face 122-2 of the MC-EDF 120 (through the double-sided mirror 111). It can also be said that the central axis of the core 201b at the end face 202 of the transmission fiber 200 overlaps with the central axis of the core 121b at the end face 122-2 of the MC-EDF 120.

[0057] The output-side transmission line fiber 300 is fixed so that the end face 302 of the transmission line fiber 300 faces the end face 122-1 of the MC-EDF 120. Specifically, the core 301a at the end face 302 of the transmission line fiber 300 faces the core 121a at the end face 122-1 of the MC-EDF 120. It can also be said that the central axis of the core 301a at the end face 302 of the transmission line fiber 300 overlaps with the central axis of the core 121a at the end face 122-1 of the MC-EDF 120. Furthermore, the core 301b at the end face 302 of the transmission line fiber 300 faces the core 121b at the end face 122-1 of the MC-EDF 120 (through the double-sided mirror 111). It can also be said that the central axis of the core 301b at the end face 302 of the transmission fiber 300 overlaps with the central axis of the core 121b at the end face 122-1 of the MC-EDF 120.

[0058] 8, for example, core 121b of MC-EDF 120 is the core that changes the transmission direction. For this reason, double-sided mirror 111 is disposed at the intersection between the central axis of core 121b at end face 122-1 of MC-EDF 120 and the central axis of core 121b at end face 122-1 of MC-EDF 120. In other words, double-sided mirror 111 is disposed at the intersection between the central axis connecting core 201b at end face 202 of transmission line fiber 200 and core 121b at end face 122-2 of MC-EDF 120 and the central axis connecting core 301b at end face 302 of transmission line fiber 300 and core 121b at end face 122-1 of MC-EDF 120.

[0059] One surface 111a of the double-sided mirror 111 is disposed to face the core 201b at the end surface 202 of the transmission line fiber 200 and the core 121b at the end surface 122-1 of the MC-EDF 120. The one surface 111a of the double-sided mirror 111 is fixed at an angle of 45 degrees with respect to the central axis of the core 201b at the end surface 202 of the transmission line fiber 200 and the central axis of the core 121b at the end surface 122-1 of the MC-EDF 120. The one surface 111a of the double-sided mirror 111 reflects light from the core 201b at the end surface 202 of the transmission line fiber 200 at a right angle toward the core 121b at the end surface 122-1 of the MC-EDF 120.

[0060] One surface 111b of the double-sided mirror 111 is disposed to face the core 301b at the end surface 302 of the transmission line fiber 300 and the core 121b at the end surface 122-2 of the MC-EDF 120. The one surface 111b of the double-sided mirror 111 is fixed at an angle of 45 degrees with respect to the central axis of the core 301b at the end surface 302 of the transmission line fiber 300 and the central axis of the core 121b at the end surface 122-2 of the MC-EDF 120. The one surface 111b of the double-sided mirror 111 reflects light from the core 121b at the end surface 122-2 of the MC-EDF 120 at a right angle toward the core 301b at the end surface 302 of the transmission line fiber 300.

[0061] The signal light output from the core 201a at the end face 202 of the transmission line fiber 200 is input to the core 121a at the end face 122-2 of the MC-EDF 120. The signal light input to the core 121a at the end face 122-2 of the MC-EDF 120 is propagated through the MC-EDF 120 and amplified, and the amplified signal light is output from the core 121a at the end face 122-1 to the core 301a at the end face 302 of the transmission line fiber 300.

[0062] The signal light output from core 201b at end face 202 of transmission line fiber 200 is reflected by one surface 111a of double-sided mirror 111 and is input from one surface 111a to core 121b at end face 122-1. The signal light input to core 121b at end face 122-1 propagates through MC-EDF 120 and is amplified, and the amplified signal light is output from core 121b at end face 122-1 toward double-sided mirror 111. The signal light output from core 121b is reflected by one surface 111b of double-sided mirror 111 and is output from one surface 111b to core 301b at end face 302 of transmission line fiber 300.

[0063] As described above, the function of the transmission direction switching unit in the optical amplifier may be realized by a mirror that switches the transmission direction. For example, by arranging the core that switches the transmission direction in a two-core MC-EDF and the mirror in a predetermined positional relationship, the transmission direction of the MC-EDF can be switched between two directions.

[0064] (Modification of the second embodiment) In the modification of the second embodiment, an example in which a 180-degree twist is applied to the MC-EDF in the optical amplifying device shown in the second embodiment will be described.

[0065] The principle of switching the transmission direction in the optical amplifier 100 according to some embodiments will be described with reference to Fig. 10. Fig. 10 is a front view of the end faces 122-1 and 122-2 of the MC-EDF 120.

[0066] In the example of Fig. 10, the positional relationship between the cores 121a and 121b at the end faces 122-1 and 122-2 of the MC-EDF 120 is different from that of Fig. 8. That is, the MC-EDF 120 is folded back with a 180-degree twist around the central axis of the fiber. The end faces 122-1 and 122-2 of the MC-EDF 120 face each other at a predetermined angle after the MC-EDF 120 is folded back while being twisted 180 degrees. It can also be said that either the end face 122-1 or the end face 122-2 faces each other while rotated 180 degrees. The 180-degree twist is the angle by which the core 121 rotates from the state in which the end face 122 is not rotated (Fig. 8) to the position of the adjacent core 121. For example, core 121a of end face 122-1 and core 121b of end face 122-2 are located on the inside where end face 122-1 and end face 122-2 face each other. It can also be said that when end face 122-1 and end face 122-2 are faced to each other, core 121a of end face 122-1 and core 121b of end face 122-2 face each other. Core 121b of end face 122-1 and core 121a of end face 122-2 are located on the outside where end face 122-1 and end face 122-2 face each other. It can also be said that when end face 122-1 and end face 122-2 are faced to each other, core 121b of end face 122-1 and core 121a of end face 122-2 face each other.

[0067] As in Fig. 8, when end faces 122-1 and 122-2 of MC-EDF 120 are arranged opposite each other at a certain angle, double-sided mirror 111 is placed at the intersection of cores 121b that change the transmission direction within MC-EDF 120. In the example of Fig. 10, the position of core 121b of end face 122-2 is different from that of Fig. 8, and therefore the position of double-sided mirror 111 is shifted from that of Fig. 8. In other words, double-sided mirror 111 is placed at the intersection of the central axis of core 121b of end face 122-2, which is located on the inside where end faces 122-1 and 122-2 face each other, and the central axis of core 121b of end face 122-1, which is located on the outside where end faces 122-1 and 122-2 face each other.

[0068] 10, in the path of core 121a, the signal light is input to core 121a of end face 122-2, which is located on the outside where end face 122-1 and end face 122-2 face each other, and the signal light amplified by MC-EDF 120 is output from core 121a of end face 122-1, which is located on the inside where end face 122-1 and end face 122-2 face each other. In addition, in the path of core 121b, the input signal light is reflected by one surface 111a of double-sided mirror 111 and input to core 121b of end face 122-1, which is located on the outside where end face 122-1 and end face 122-2 face each other, and the signal light amplified by MC-EDF 120 is output from core 121b of end face 122-1, which is located on the inside where end face 122-1 and end face 122-2 face each other. The signal light output from core 121b is reflected by the other surface 111b of double-sided mirror 111 and is output.

[0069] Fig. 11 shows a specific example of the configuration of the optical amplifying device 100 described in Fig. 10. Fig. 11 is a view of the end faces 122-1 and 122-2 of the MC-EDF 120 as seen from above.

[0070] In the example of FIG. 11, the positions of the cores 121a and 121b and the double-sided mirror 111 at the end faces 122-1 and 122-2 of the MC-EDF 120 are different from those in FIG.

[0071] 11, the end faces 122-1 and 122-2 of the MC-EDF 120 are fixed so as to face each other at a right angle. For example, the MC-EDF 120 is twisted 180 degrees and folded back, and fixed so as to face each end face 122-1 and 122-2 at a right angle. If necessary, the MC-EDF 120 may be wound around to pull out the end faces 122-1 and 122-2. Alternatively, the MC-EDF 120 may be twisted by 180 degrees + 360 degrees × n (n is an integer).

[0072] 10, core 121a of end face 122-1 and core 121b of end face 122-2 are located on the inside where end face 122-1 and end face 122-2 face each other. Core 121b of end face 122-1 and core 121a of end face 122-2 are located on the outside where end face 122-1 and end face 122-2 face each other.

[0073] 9, the input-side transmission line fiber 200 is fixed so that the end face 202 of the transmission line fiber 200 faces the end face 122-2 of the MC-EDF 120. The output-side transmission line fiber 300 is fixed so that the end face 302 of the transmission line fiber 300 faces the end face 122-1 of the MC-EDF 120.

[0074] 10 , for example, core 121b of MC-EDF 120 is assumed to be the core that changes the transmission direction. Therefore, double-sided mirror 111 is disposed at the intersection of the central axis of core 121b at end face 122-2 of MC-EDF 120, which is located inside where end face 122-1 and end face 122-2 face each other, and the central axis of core 121b at end face 122-1 of MC-EDF 120, which is located outside where end face 122-1 and end face 122-2 face each other. In other words, double-sided mirror 111 is disposed at the intersection of the central axis connecting core 201a at end face 202 of transmission line fiber 200 and core 121b at end face 122-2 of MC-EDF 120, and the central axis connecting core 301b at end face 302 of transmission line fiber 300 and core 121b at end face 122-1 of MC-EDF 120.

[0075] One surface 111a of the double-sided mirror 111 is disposed to face the core 201a at the end surface 202 of the transmission fiber 200 and the core 121b at the end surface 122-1 of the MC-EDF 120. The one surface 111a of the double-sided mirror 111 is fixed at an angle of 45 degrees with respect to the central axis of the core 201a at the end surface 202 of the transmission fiber 200 and the central axis of the core 121b at the end surface 122-1 of the MC-EDF 120. The one surface 111a of the double-sided mirror 111 reflects light from the core 201a at the end surface 202 of the transmission fiber 200 at a right angle toward the core 121b at the end surface 122-1 of the MC-EDF 120.

[0076] One surface 111b of the double-sided mirror 111 is disposed to face the core 301b at the end surface 302 of the transmission line fiber 300 and the core 121b at the end surface 122-2 of the MC-EDF 120. The one surface 111b of the double-sided mirror 111 is fixed at an angle of 45 degrees with respect to the central axis of the core 301b at the end surface 302 of the transmission line fiber 300 and the central axis of the core 121b at the end surface 122-2 of the MC-EDF 120. The one surface 111b of the double-sided mirror 111 reflects light from the core 121b at the end surface 122-2 of the MC-EDF 120 at a right angle toward the core 301b at the end surface 302 of the transmission line fiber 300.

[0077] The signal light output from the core 201b at the end face 202 of the transmission line fiber 200 is input to the core 121a at the end face 122-2 of the MC-EDF 120. The signal light input to the core 121a at the end face 122-2 of the MC-EDF 120 is propagated through the MC-EDF 120 and amplified, and the amplified signal light is output from the core 121a at the end face 122-1 to the core 301a at the end face 302 of the transmission line fiber 300.

[0078] The signal light output from core 201a at end face 202 of transmission line fiber 200 is reflected by one surface 111a of double-sided mirror 111 and is input from one surface 111a to core 121b at end face 122-1. The signal light input to core 121b at end face 122-1 propagates through MC-EDF 120 and is amplified, and the amplified signal light is output from core 121b at end face 122-1 toward double-sided mirror 111. The signal light output from core 121b is reflected by one surface 111b of double-sided mirror 111 and is output from one surface 111b to core 301b at end face 302 of transmission line fiber 300.

[0079] As described above, when switching the transmission direction of a two-core MC-EDF in an optical amplifier, the MC-EDF may be twisted by 180 degrees. Even in this case, the transmission direction of the MC-EDF can be switched in both directions by arranging the core that switches the transmission direction in the two-core MC-EDF and the mirror in a predetermined positional relationship.

[0080] (Embodiment 3) Next, a description will be given of a third embodiment. In this embodiment, an example in which a four-core MC-EDF is used will be described as a specific example of the optical amplifying device shown in the first embodiment.

[0081] The principle of switching the transmission direction in the optical amplifier 100 according to some embodiments will be described with reference to Fig. 12. Fig. 12 is a front view of the end faces 122-1 and 122-2 of the MC-EDF 120.

[0082] 12, the MC-EDF 120 is a four-core MC-EDF. That is, the MC-EDF 120 includes cores 121a to 121d. Signal light is input and output in both directions to and from the cores 121a to 121d at end faces 122-1 and 122-2 on both ends of the MC-EDF 120. In bidirectional transmission, the transmission directions of adjacent cores are opposite to each other.

[0083] 12, the end faces 122-1 and 122-2 of the MC-EDF 120 face each other at a predetermined angle when the MC-EDF 120 is folded back. That is, the MC-EDF 120 is folded back without twisting. The end faces 122-1 and 122-2 face each other without rotating. For example, the cores 121a and 121c of the end face 122-1 and the cores 121a and 121c of the end face 122-2 are located on the inner side of the end faces 122-1 and 122-2 that face each other. For example, the core 121a is located above the end faces 122-1 and 122-2 (inner upper side), and the core 121c is located below the end faces 122-1 and 122-2 (inner lower side). When end face 122-1 and end face 122-2 are placed opposite each other, core 121a of end face 122-1 faces core 121a of end face 122-2, and core 121c of end face 122-1 faces core 121c of end face 122-2.

[0084] Cores 121b and 121d of end face 122-1 and cores 121b and 121d of end face 122-2 are located on the outside where end face 122-1 and end face 122-2 face each other. For example, core 121b is located on the upper side (outer upper side) of end faces 122-1 and 122-2, and core 121d is located on the lower side (outer lower side) of end faces 122-1 and 122-2. It can also be said that when end face 122-1 and end face 122-2 face each other, core 121b of end face 122-1 faces core 121b of end face 122-2, and core 121d of end face 122-1 faces core 121d of end face 122-2.

[0085] In the example of Figure 12, cores 121a and 121d are cores that do not switch the transmission direction, and cores 121b and 121c are cores that switch the transmission direction. Note that the combination of cores that do not switch the transmission direction and cores that switch the transmission direction is not limited to this. Any other combination of cores may be used as long as the traveling direction of light propagating between adjacent cores can be reversed. This makes it possible to effectively reduce inter-core crosstalk.

[0086] 12, when end faces 122-1 and 122-2 of MC-EDF 120 are arranged opposite each other at a certain angle, double-sided mirrors 111-1 and 111-2 are placed at the intersection of cores 121b and 121c, respectively, where the transmission direction within MC-EDF 120 is changed. Specifically, double-sided mirror 111-1 is placed at the intersection of the central axes of core 121b, where the transmission direction within MC-EDF 120 is changed, and double-sided mirror 111-2 is placed at the intersection of the central axes of core 121c, where the transmission direction within MC-EDF 120 is changed. This allows the transmission direction of signal light from cores 121b and 121c to be switched to the opposite direction to the transmission direction of cores 121a and 121d.

[0087] In the path through core 121a, the signal light is input to core 121a at end face 122-2, and the signal light amplified by MC-EDF 120 is output from core 121a at end face 122-1. In addition, in the path through core 121b, the input signal light is reflected by one surface 111-1a of double-sided mirror 111-1, input to core 121b at end face 122-1, and the signal light amplified by MC-EDF 120 is output from core 121b at end face 122-1. The signal light output from core 121b is reflected by the other surface 111-1b of double-sided mirror 111-1 and output.

[0088] In the path through core 121c, the input signal light is reflected by one surface 111-2a of double-sided mirror 111-2 and input to core 121c at end face 122-1, and the signal light amplified by MC-EDF 120 is output from core 121c at end face 122-1. The signal light output from core 121c is reflected by the other surface 111-2b of double-sided mirror 111-2 and output. In addition, in the path through core 121d, the signal light is input to core 121d at end face 122-2 and the signal light amplified by MC-EDF 120 is output from core 121d at end face 122-1.

[0089] Fig. 13 shows a specific example of the configuration of the optical amplifying device 100 described in Fig. 12. Fig. 13 is a top view of the end faces 122-1 and 122-2 of the MC-EDF 120.

[0090] 13, the end faces 122-1 and 122-2 of the MC-EDF 120 are fixed so as to face each other at a right angle. For example, the MC-EDF 120 is folded back without twisting, and fixed so as to face each end face 122-1 and 122-2 at a right angle. If necessary, the MC-EDF 120 may be wound around to pull out the end faces 122-1 and 122-2. Note that the MC-EDF 120 may be twisted by 360 degrees × n (n is an integer).

[0091] 12, cores 121a and 121c of end face 122-1 and cores 121a and 121c of end face 122-2 are located on the inside where end face 122-1 and end face 122-2 face each other. Cores 121b and 121d of end face 122-1 and cores 121b and 121d of end face 122-2 are located on the outside where end face 122-1 and end face 122-2 face each other.

[0092] 13, the input-side transmission line fiber 200 is fixed so that the end face 202 of the transmission line fiber 200 faces the end face 122-2 of the MC-EDF 120. Specifically, the core 201a at the end face 202 of the transmission line fiber 200 faces the core 121a at the end face 122-2 of the MC-EDF 120. It can also be said that the central axis of the core 201a at the end face 202 of the transmission line fiber 200 overlaps with the central axis of the core 121a at the end face 122-2 of the MC-EDF 120. The core 201b at the end face 202 of the transmission line fiber 200 faces the core 121b at the end face 122-2 of the MC-EDF 120. It can also be said that the central axis of the core 201b at the end face 202 of the transmission line fiber 200 overlaps with the central axis of the core 121b at the end face 122-2 of the MC-EDF 120.

[0093] The core 201c at the end face 202 of the transmission line fiber 200 faces the core 121c at the end face 122-2 of the MC-EDF 120. It can also be said that the central axis of the core 201c at the end face 202 of the transmission line fiber 200 overlaps with the central axis of the core 121c at the end face 122-2 of the MC-EDF 120. The core 201d at the end face 202 of the transmission line fiber 200 faces the core 121d at the end face 122-2 of the MC-EDF 120. It can also be said that the central axis of the core 201d at the end face 202 of the transmission line fiber 200 overlaps with the central axis of the core 121d at the end face 122-2 of the MC-EDF 120.

[0094] The output-side transmission line fiber 300 is fixed so that the end face 302 of the transmission line fiber 300 faces the end face 122-1 of the MC-EDF 120. Specifically, the core 301a at the end face 302 of the transmission line fiber 300 faces the core 121a at the end face 122-1 of the MC-EDF 120. It can also be said that the central axis of the core 301a at the end face 302 of the transmission line fiber 300 overlaps with the central axis of the core 121a at the end face 122-1 of the MC-EDF 120. The core 301b at the end face 302 of the transmission line fiber 300 faces the core 121b at the end face 122-1 of the MC-EDF 120. It can also be said that the central axis of the core 301b at the end face 302 of the transmission line fiber 300 overlaps with the central axis of the core 121b at the end face 122-1 of the MC-EDF 120.

[0095] The core 301c at the end face 302 of the transmission line fiber 300 faces the core 121c at the end face 122-1 of the MC-EDF 120. It can also be said that the central axis of the core 301c at the end face 302 of the transmission line fiber 300 overlaps with the central axis of the core 121c at the end face 122-1 of the MC-EDF 120. The core 301d at the end face 302 of the transmission line fiber 300 faces the core 121d at the end face 122-1 of the MC-EDF 120. It can also be said that the central axis of the core 301d at the end face 302 of the transmission line fiber 300 overlaps with the central axis of the core 121d at the end face 122-1 of the MC-EDF 120.

[0096] 12, for example, cores 121b and 121c of MC-EDF 120 are assumed to be cores that change the transmission direction. For this reason, double-sided mirror 111-1 is disposed at the intersection between the central axis of core 121b at end face 122-2 of MC-EDF 120 and the central axis of core 121b at end face 122-1 of MC-EDF 120. In other words, double-sided mirror 111-1 is disposed at the intersection between the central axis connecting core 201b at end face 202 of transmission line fiber 200 and core 121b at end face 122-2 of MC-EDF 120 and the central axis connecting core 301b at end face 302 of transmission line fiber 300 and core 121b at end face 122-1 of MC-EDF 120.

[0097] The double-sided mirror 111-2 is disposed at the intersection of the central axis of the core 121c at the end face 122-2 of the MC-EDF 120 and the central axis of the core 121c at the end face 122-1 of the MC-EDF 120. It can also be said that the double-sided mirror 111-2 is disposed at the intersection of the central axis connecting the core 201c at the end face 202 of the transmission line fiber 200 and the core 121c at the end face 122-2 of the MC-EDF 120 and the central axis connecting the core 301c at the end face 302 of the transmission line fiber 300 and the core 121c at the end face 122-1 of the MC-EDF 120.

[0098] One surface 111-1a of the double-sided mirror 111-1 is disposed to face the core 201b at the end surface 202 of the transmission line fiber 200 and the core 121b at the end surface 122-1 of the MC-EDF 120. The one surface 111-1a of the double-sided mirror 111-1 is fixed at an angle of 45 degrees with respect to the central axis of the core 201b at the end surface 202 of the transmission line fiber 200 and the central axis of the core 121b at the end surface 122-1 of the MC-EDF 120. The one surface 111-1a of the double-sided mirror 111-1 reflects light from the core 201b at the end surface 202 of the transmission line fiber 200 at a right angle toward the core 121b at the end surface 122-1 of the MC-EDF 120.

[0099] One surface 111-1b of the double-sided mirror 111-1 is disposed to face the core 301b at the end surface 302 of the transmission line fiber 300 and the core 121b at the end surface 122-2 of the MC-EDF 120. The one surface 111-1b of the double-sided mirror 111-1 is fixed at an angle of 45 degrees with respect to the central axis of the core 301b at the end surface 302 of the transmission line fiber 300 and the central axis of the core 121b at the end surface 122-2 of the MC-EDF 120. The one surface 111-1b of the double-sided mirror 111-1 reflects light from the core 121b at the end surface 122-2 of the MC-EDF 120 at a right angle toward the core 301b at the end surface 302 of the transmission line fiber 300.

[0100] One surface 111-2a of the double-sided mirror 111-2 is disposed to face the core 201c at the end surface 202 of the transmission line fiber 200 and the core 121c at the end surface 122-1 of the MC-EDF 120. The one surface 111-2a of the double-sided mirror 111-2 is fixed at an angle of 45 degrees with respect to the central axis of the core 201c at the end surface 202 of the transmission line fiber 200 and the central axis of the core 121c at the end surface 122-1 of the MC-EDF 120. The one surface 111-2a of the double-sided mirror 111-2 reflects light from the core 201c at the end surface 202 of the transmission line fiber 200 at a right angle toward the core 121c at the end surface 122-1 of the MC-EDF 120.

[0101] One surface 111-2b of the double-sided mirror 111-2 is disposed to face the core 301c at the end surface 302 of the transmission line fiber 300 and the core 121c at the end surface 122-2 of the MC-EDF 120. The one surface 111-2b of the double-sided mirror 111-2 is fixed at an angle of 45 degrees with respect to the central axis of the core 301c at the end surface 302 of the transmission line fiber 300 and the central axis of the core 121c at the end surface 122-2 of the MC-EDF 120. The one surface 111-2b of the double-sided mirror 111-2 reflects light from the core 121c at the end surface 122-2 of the MC-EDF 120 at a right angle toward the core 301c at the end surface 302 of the transmission line fiber 300.

[0102] The signal light output from the core 201a at the end face 202 of the transmission line fiber 200 is input to the core 121a at the end face 122-2 of the MC-EDF 120. The signal light input to the core 121a at the end face 122-2 of the MC-EDF 120 is propagated through the MC-EDF 120 and amplified, and the amplified signal light is output from the core 121a at the end face 122-1 to the core 301a at the end face 302 of the transmission line fiber 300.

[0103] The signal light output from core 201b at end face 202 of transmission line fiber 200 is reflected by one surface 111-1a of double-sided mirror 111-1 and input from one surface 111-1a to core 121b at end face 122-1. The signal light input to core 121b at end face 122-1 propagates through MC-EDF 120 and is amplified, and the amplified signal light is output from core 121b at end face 122-1 toward double-sided mirror 111-1. The signal light output from core 121b is reflected by one surface 111-1b of double-sided mirror 111-1 and output from one surface 111-1b to core 301b at end face 302 of transmission line fiber 300.

[0104] The signal light output from core 201c at end face 202 of transmission line fiber 200 is reflected by one surface 111-2a of double-sided mirror 111-2 and input from one surface 111-2a to core 121c at end face 122-1. The signal light input to core 121c at end face 122-1 propagates through MC-EDF 120 and is amplified, and the amplified signal light is output from core 121c at end face 122-1 toward double-sided mirror 111-2. The signal light output from core 121c is reflected by one surface 111-2b of double-sided mirror 111-2 and output from one surface 111-2b to core 301c at end face 302 of transmission line fiber 300.

[0105] The signal light output from the core 201d at the end face 202 of the transmission line fiber 200 is input to the core 121d at the end face 122-2 of the MC-EDF 120. The signal light input to the core 121d at the end face 122-2 of the MC-EDF 120 is propagated through the MC-EDF 120 and amplified, and the amplified signal light is output from the core 121d at the end face 122-1 to the core 301d at the end face 302 of the transmission line fiber 300.

[0106] As described above, a four-core MC-EDF may be used when switching the transmission direction of an MC-EDF in an optical amplifier. Even in this case, the transmission direction of the MC-EDF can be switched between two directions by arranging the core that switches the transmission direction in the four-core MC-EDF and the mirror in a predetermined positional relationship.

[0107] (Modification 1 of Embodiment 3) In the first modification of the third embodiment, an example will be described in which a 90-degree twist is added to the MC-EDF 120 in the optical amplifying device 100 shown in the third embodiment.

[0108] The principle of switching the transmission direction in the optical amplifier 100 according to some embodiments will be described with reference to Fig. 14. Fig. 14 is a front view of the end faces 122-1 and 122-2 of the MC-EDF 120.

[0109] In the example of Fig. 14, the positional relationship of the cores 121a to 121d at the end faces 122-1 and 122-2 of the MC-EDF 120 is different from that of Fig. 12. That is, the MC-EDF 120 is folded back with a 90-degree twist around the central axis of the fiber. The end faces 122-1 and 122-2 of the MC-EDF 120 face each other at a predetermined angle when the MC-EDF 120 is folded back while being twisted 90 degrees. It can also be said that either the end face 122-1 or the end face 122-2 faces each other with a 90-degree rotation. The 90-degree twist is the angle by which the core 121 rotates from the state in which the end face 122 is not rotated (Fig. 12) to the position of the adjacent core 121.

[0110] For example, cores 121a and 121c of end face 122-1 and cores 121b and 121a of end face 122-2 are located on the inner side where end face 122-1 and end face 122-2 face each other. For example, core 121a is located on the upper side (inner upper side) of end face 122-1, core 121c is located on the lower side (inner lower side) of end face 122-1, core 121b is located on the upper side (inner upper side) of end face 122-2, and core 121a is located on the lower side (inner lower side) of end face 122-2. It can also be said that when end face 122-1 and end face 122-2 face each other, core 121a of end face 122-1 faces core 121b of end face 122-2 faces each other, and core 121c of end face 122-1 faces core 121a of end face 122-2 faces each other. Cores 121b and 121d of end face 122-1 and cores 121d and 121c of end face 122-2 are located on the outside where end face 122-1 and end face 122-2 face each other. For example, core 121b is located above (outside upper side) end face 122-1, core 121d is located below (outside lower side) end face 122-1, core 121d is located above (outside upper side) end face 122-2, and core 121c is located below (outside lower side) end face 122-2. It can also be said that when end face 122-1 and end face 122-2 face each other, core 121b of end face 122-1 faces core 121d of end face 122-2, and core 121d of end face 122-1 faces core 121c of end face 122-2.

[0111] 12, when end faces 122-1 and 122-2 of MC-EDF 120 are arranged opposite each other at a certain angle, double-sided mirror 111-1 is placed at the intersection of cores 121b that change the transmission direction within MC-EDF 120, and double-sided mirror 111-2 is placed at the intersection of cores 121c that change the transmission direction within MC-EDF 120. In the example of Fig. 14, the positions of cores 121b and 121c on end face 122-2 are different from those in Fig. 12, and therefore the positions of double-sided mirrors 111-1 and 111-2 are shifted from those in Fig. 12. That is, double-sided mirror 111-1 is placed at the intersection of the central axis of core 121b of end face 122-2, which is on the upper inner side where end face 122-1 and end face 122-2 face each other, and the central axis of core 121b of end face 122-1, which is on the upper outer side where end face 122-1 and end face 122-2 face each other. Double-sided mirror 111-2 is placed at the intersection of the central axis of core 121c of end face 122-2, which is on the lower outer side where end face 122-1 and end face 122-2 face each other, and the central axis of core 121c of end face 122-1, which is on the lower inner side where end face 122-1 and end face 122-2 face each other.

[0112] As shown in Figure 14, in the path of core 121a, signal light is input to core 121a of end face 122-2, which is located on the inner lower side where end face 122-1 and end face 122-2 face each other, and the signal light amplified by MC-EDF 120 is output from core 121a of end face 122-1, which is located on the inner upper side where end face 122-1 and end face 122-2 face each other.

[0113] In the path of core 121b, the input signal light is reflected by one surface 111-1a of double-sided mirror 111-1 and input to core 121b of end surface 122-1, which is located on the upper outer side where end surfaces 122-1 and 122-2 face each other, and the signal light amplified by MC-EDF 120 is output from core 121b of end surface 122-1, which is located on the upper inner side where end surfaces 122-1 and 122-2 face each other. The signal light output from core 121b is reflected by the other surface 111-1b of double-sided mirror 111-1 and output.

[0114] In the path of core 121c, the input signal light is reflected by one surface 111-2a of double-sided mirror 111-2 and input to core 121c of end surface 122-1, which is located on the inner lower side where end surfaces 122-1 and 122-2 face each other, and the signal light amplified by MC-EDF 120 is output from core 121c of end surface 122-1, which is located on the outer lower side where end surfaces 122-1 and 122-2 face each other. The signal light output from core 121c is reflected by the other surface 111-2b of double-sided mirror 111-2 and output.

[0115] In the path of core 121d, the signal light is input to core 121d of end face 122-2, which is located on the upper outer side where end face 122-1 and end face 122-2 face each other, and the signal light amplified by MC-EDF 120 is output from core 121d of end face 122-1, which is located on the lower outer side where end face 122-1 and end face 122-2 face each other.

[0116] Fig. 15 shows a specific example of the configuration of the optical amplifying device 100 described in Fig. 14. Fig. 15 is a view of the end faces 122-1 and 122-2 of the MC-EDF 120 as seen from above.

[0117] In the example of FIG. 15, the positions of the cores 121a to 121d and the double-sided mirrors 111-1 and 111-2 at the end faces 122-1 and 122-2 of the MC-EDF 120 are different from those in FIG.

[0118] 15, the end faces 122-1 and 122-2 of the MC-EDF 120 are fixed so as to face each other at a right angle. For example, the MC-EDF 120 is twisted 90 degrees and folded back, and fixed so as to face each end face 122-1 and 122-2 at a right angle. If necessary, the MC-EDF 120 may be wound around to pull out the end faces 122-1 and 122-2. Alternatively, the MC-EDF 120 may be twisted by 90 degrees + 360 degrees × n (n is an integer).

[0119] 14, cores 121a and 121c of end face 122-1 and cores 121b and 121a of end face 122-2 are located on the inside where end face 122-1 and end face 122-2 face each other. Cores 121b and 121d of end face 122-1 and cores 121d and 121c of end face 122-2 are located on the outside where end face 122-1 and end face 122-2 face each other.

[0120] 13, the input-side transmission line fiber 200 is fixed so that the end face 202 of the transmission line fiber 200 faces the end face 122-2 of the MC-EDF 120. The output-side transmission line fiber 300 is fixed so that the end face 302 of the transmission line fiber 300 faces the end face 122-1 of the MC-EDF 120.

[0121] 14, for example, cores 121b and 121c of MC-EDF 120 are used as cores that change the transmission direction. To this end, double-sided mirror 111-1 is placed at the intersection of the central axis of core 121b at end face 122-2 of MC-EDF 120, which is located on the inner upper side where end face 122-1 and end face 122-2 face each other, and the central axis of core 121b at end face 122-1 of MC-EDF 120, which is located on the outer upper side where end face 122-1 and end face 122-2 face each other. It can also be said that double-sided mirror 111-1 is arranged at the intersection of a central axis connecting core 201a at end face 202 of transmission line fiber 200 and core 121b at end face 122-2 of MC-EDF 120, and a central axis connecting core 301b at end face 302 of transmission line fiber 300 and core 121b at end face 122-1 of MC-EDF 120.

[0122] Double-sided mirror 111-2 is disposed at the intersection of the central axis of core 121c at end face 122-2 of MC-EDF 120, which is located on the outer lower side where end face 122-1 and end face 122-2 face each other, and the central axis of core 121c at end face 122-1 of MC-EDF 120, which is located on the inner lower side where end face 122-1 and end face 122-2 face each other. In other words, double-sided mirror 111-2 is disposed at the intersection of the central axis connecting core 201d at end face 202 of transmission line fiber 200 and core 121c at end face 122-2 of MC-EDF 120, and the central axis connecting core 301c at end face 302 of transmission line fiber 300 and core 121c at end face 122-1 of MC-EDF 120.

[0123] One surface 111-1a of the double-sided mirror 111-1 is disposed to face the core 201a at the end surface 202 of the transmission line fiber 200 and the core 121b at the end surface 122-1 of the MC-EDF 120. The one surface 111-1a of the double-sided mirror 111-1 is fixed at an angle of 45 degrees with respect to the central axis of the core 201a at the end surface 202 of the transmission line fiber 200 and the central axis of the core 121b at the end surface 122-1 of the MC-EDF 120. The one surface 111-1a of the double-sided mirror 111-1 reflects light from the core 201a at the end surface 202 of the transmission line fiber 200 at a right angle toward the core 121b at the end surface 122-1 of the MC-EDF 120.

[0124] One surface 111-1b of the double-sided mirror 111-1 is disposed to face the core 301b at the end surface 302 of the transmission line fiber 300 and the core 121b at the end surface 122-2 of the MC-EDF 120. The one surface 111-1b of the double-sided mirror 111-1 is fixed at an angle of 45 degrees with respect to the central axis of the core 301b at the end surface 302 of the transmission line fiber 300 and the central axis of the core 121b at the end surface 122-2 of the MC-EDF 120. The one surface 111-1b of the double-sided mirror 111-1 reflects light from the core 121b at the end surface 122-2 of the MC-EDF 120 at a right angle toward the core 301b at the end surface 302 of the transmission line fiber 300.

[0125] One surface 111-2a of the double-sided mirror 111-2 is disposed to face the core 201d at the end surface 202 of the transmission line fiber 200 and the core 121c at the end surface 122-1 of the MC-EDF 120. The one surface 111-2a of the double-sided mirror 111-2 is fixed at an angle of 45 degrees with respect to the central axis of the core 201d at the end surface 202 of the transmission line fiber 200 and the central axis of the core 121c at the end surface 122-1 of the MC-EDF 120. The one surface 111-2a of the double-sided mirror 111-2 reflects light from the core 201d at the end surface 202 of the transmission line fiber 200 at a right angle toward the core 121c at the end surface 122-1 of the MC-EDF 120.

[0126] One surface 111-2b of the double-sided mirror 111-2 is disposed to face the core 301c at the end surface 302 of the transmission line fiber 300 and the core 121c at the end surface 122-2 of the MC-EDF 120. The one surface 111-2b of the double-sided mirror 111-2 is fixed at an angle of 45 degrees with respect to the central axis of the core 301c at the end surface 302 of the transmission line fiber 300 and the central axis of the core 121c at the end surface 122-2 of the MC-EDF 120. The one surface 111-2b of the double-sided mirror 111-2 reflects light from the core 121c at the end surface 122-2 of the MC-EDF 120 at a right angle toward the core 301c at the end surface 302 of the transmission line fiber 300.

[0127] The signal light output from the core 201c at the end face 202 of the transmission line fiber 200 is input to the core 121a at the end face 122-2 of the MC-EDF 120. The signal light input to the core 121a at the end face 122-2 of the MC-EDF 120 is propagated through the MC-EDF 120 and amplified, and the amplified signal light is output from the core 121a at the end face 122-1 to the core 301a at the end face 302 of the transmission line fiber 300.

[0128] The signal light output from core 201a at end face 202 of transmission line fiber 200 is reflected by one surface 111-1a of double-sided mirror 111-1 and input from one surface 111-1a to core 121b at end face 122-1. The signal light input to core 121b at end face 122-1 is propagated through MC-EDF 120 and amplified, and the amplified signal light is output from core 121b at end face 122-1 toward double-sided mirror 111-1. The signal light output from core 121b is reflected by one surface 111-1b of double-sided mirror 111-1 and output from one surface 111-1b to core 301b at end face 302 of transmission line fiber 300.

[0129] The signal light output from core 201d at end face 202 of transmission line fiber 200 is reflected by one surface 111-2a of double-sided mirror 111-2 and input from one surface 111-2a to core 121c at end face 122-1. The signal light input to core 121c at end face 122-1 propagates through MC-EDF 120 and is amplified, and the amplified signal light is output from core 121c at end face 122-1 toward double-sided mirror 111-2. The signal light output from core 121c is reflected by one surface 111-2b of double-sided mirror 111-2 and output from one surface 111-2b to core 301c at end face 302 of transmission line fiber 300.

[0130] The signal light output from the core 201b at the end face 202 of the transmission line fiber 200 is input to the core 121d at the end face 122-2 of the MC-EDF 120. The signal light input to the core 121d at the end face 122-2 of the MC-EDF 120 is propagated through the MC-EDF 120 and amplified, and the amplified signal light is output from the core 121d at the end face 122-1 to the core 301d at the end face 302 of the transmission line fiber 300.

[0131] As described above, when switching the transmission direction of a four-core MC-EDF in an optical amplifier, the MC-EDF may be twisted by 90 degrees. Even in this case, the transmission direction of the MC-EDF can be switched in both directions by arranging the core that switches the transmission direction in the four-core MC-EDF and the mirror in a predetermined positional relationship.

[0132] (Modification 2 of Embodiment 3) In the second modification of the third embodiment, an example will be described in which a 180-degree twist is added to the MC-EDF 120 in the optical amplifying device 100 shown in the third embodiment.

[0133] The principle of switching the transmission direction in the optical amplifier 100 according to some embodiments will be described with reference to Fig. 16. Fig. 16 is a front view of the end faces 122-1 and 122-2 of the MC-EDF 120.

[0134] In the example of Fig. 16, the positional relationship of the cores 121a to 121d at the end faces 122-1 and 122-2 of the MC-EDF 120 is different from that of Fig. 12. That is, the MC-EDF 120 is folded back with a 180-degree twist around the central axis of the fiber. The end faces 122-1 and 122-2 of the MC-EDF 120 face each other in a state where the MC-EDF 120 is folded back while being twisted 180 degrees. It can also be said that either the end face 122-1 or the end face 122-2 faces each other in a state where it is rotated 180 degrees. The 180-degree twist is the angle by which the core 121 rotates from the state where the end face 122 is not rotated (Fig. 12) to the position of the core 121 next to it.

[0135] For example, cores 121a and 121c of end face 122-1 and cores 121d and 121b of end face 122-2 are located on the inner side where end face 122-1 and end face 122-2 face each other. For example, core 121a is located on the upper side (inner upper side) of end face 122-1, core 121c is located on the lower side (inner lower side) of end face 122-1, core 121d is located on the upper side (inner upper side) of end face 122-2, and core 121b is located on the lower side (inner lower side) of end face 122-2. It can also be said that when end face 122-1 and end face 122-2 face each other, core 121a of end face 122-1 faces core 121d of end face 122-2 faces each other, and core 121c of end face 122-1 faces core 121b of end face 122-2 faces each other. Cores 121b and 121d of end face 122-1 and cores 121c and 121a of end face 122-2 are located on the outside where end face 122-1 and end face 122-2 face each other. For example, core 121b is located above end face 122-1 (upper outer side), core 121d is located below end face 122-1 (lower outer side), core 121c is located above end face 122-2 (upper outer side), and core 121a is located below end face 122-2 (lower outer side). When end face 122-1 and end face 122-2 face each other, core 121b of end face 122-1 faces core 121c of end face 122-2, and core 121d of end face 122-1 faces core 121a of end face 122-2.

[0136] In the example of Fig. 16, cores 121b and 121c of MC-EDF 120 are the cores (group) that change the transmission direction. As shown in Fig. 16, when four-core MC-EDF 120 is twisted 180 degrees and end face 122-1 and end face 122-2 are arranged opposite each other, the central axes of the cores that change the transmission direction do not intersect. For this reason, when end face 122-1 and end face 122-2 of MC-EDF 120 are arranged opposite each other at a certain angle, double-sided mirrors 111-1 and 111-2 are placed at the intersection of one core and the other core of the group of cores 121b and 121c that change the transmission direction in MC-EDF 120. That is, double-sided mirror 111-1 is placed at the intersection of the central axis of core 121b of end face 122-2, which is located on the inner lower side where end face 122-1 and end face 122-2 face each other, and the central axis of core 121c of end face 122-1, which is located on the inner lower side where end face 122-1 and end face 122-2 face each other. Double-sided mirror 111-2 is placed at the intersection of the central axis of core 121c of end face 122-2, which is located on the outer upper side where end face 122-1 and end face 122-2 face each other, and the central axis of core 121b of end face 122-1, which is located on the outer upper side where end face 122-1 and end face 122-2 face each other.

[0137] As shown in Figure 16, in the path of core 121a, signal light is input to core 121a of end face 122-2, which is located on the outer lower side where end face 122-1 and end face 122-2 face each other, and the signal light amplified by MC-EDF 120 is output from core 121a of end face 122-2, which is located on the inner upper side where end face 122-1 and end face 122-2 face each other.

[0138] In the path through core 121b, the input signal light is reflected by one surface 111-1a of double-sided mirror 111-1 and input to core 121b of end surface 122-1, which is located on the upper outer side where end surfaces 122-1 and 122-2 face each other, and the signal light amplified by MC-EDF 120 is output from core 121b of end surface 122-1, which is located on the lower inner side where end surfaces 122-1 and 122-2 face each other. The signal light output from core 121b is reflected by the other surface 111-2b of double-sided mirror 111-2 and output.

[0139] In the path of core 121c, the input signal light is reflected by one surface 111-2a of double-sided mirror 111-2 and input to core 121c of end surface 122-1, which is located on the inner lower side where end surfaces 122-1 and 122-2 face each other, and the signal light amplified by MC-EDF 120 is output from core 121c of end surface 122-1, which is located on the outer upper side where end surfaces 122-1 and 122-2 face each other. The signal light output from core 121c is reflected by the other surface 111-2b of double-sided mirror 111-1 and output.

[0140] In the path of core 121d, the signal light is input to core 121d of end face 122-2, which is located on the inner upper side where end face 122-1 and end face 122-2 face each other, and the signal light amplified by MC-EDF 120 is output from core 121d of end face 122-1, which is located on the outer lower side where end face 122-1 and end face 122-2 face each other.

[0141] Fig. 17 shows a specific example of the configuration of the optical amplifying device 100 described in Fig. 16. Fig. 17 is a view of the end faces 122-1 and 122-2 of the MC-EDF 120 as seen from above.

[0142] In the example of FIG. 17, the positions of the cores 121a to 121d and the double-sided mirrors 111-1 and 111-2 at the end faces 122-1 and 122-2 of the MC-EDF 120 are different from those in FIG.

[0143] 17, the end faces 122-1 and 122-2 of the MC-EDF 120 are fixed so as to face each other at a right angle. For example, the MC-EDF 120 is twisted 180 degrees and folded back, and fixed so as to face each end face 122-1 and 122-2 at a right angle. If necessary, the MC-EDF 120 may be wound around to pull out the end faces 122-1 and 122-2. Alternatively, the MC-EDF 120 may be twisted by 180 degrees + 360 degrees × n (n is an integer).

[0144] 16, cores 121a and 121c of end face 122-1 and cores 121d and 121b of end face 122-2 are located on the inside where end face 122-1 and end face 122-2 face each other. Cores 121b and 121d of end face 122-1 and cores 121c and 121a of end face 122-2 are located on the outside where end face 122-1 and end face 122-2 face each other.

[0145] 13, the input-side transmission line fiber 200 is fixed so that the end face 202 of the transmission line fiber 200 faces the end face 122-2 of the MC-EDF 120. The output-side transmission line fiber 300 is fixed so that the end face 302 of the transmission line fiber 300 faces the end face 122-1 of the MC-EDF 120.

[0146] 16, for example, cores 121b and 121c of MC-EDF 120 are set as a core (group) that changes the transmission direction. To this end, double-sided mirror 111-1 is placed at the intersection of the central axis of core 121c at end face 122-2 of MC-EDF 120, which is located on the outer upper side where end face 122-1 and end face 122-2 face each other, and the central axis of core 121b at end face 122-1 of MC-EDF 120, which is located on the outer upper side where end face 122-1 and end face 122-2 face each other. It can also be said that double-sided mirror 111-1 is arranged at the intersection of a central axis connecting core 201b at end face 202 of transmission line fiber 200 and core 121c at end face 122-2 of MC-EDF 120, and a central axis connecting core 301b at end face 302 of transmission line fiber 300 and core 121b at end face 122-1 of MC-EDF 120.

[0147] Double-sided mirror 111-2 is disposed at the intersection of the central axis of core 121b at end face 122-2 of MC-EDF 120, which is located on the inner lower side of where end face 122-1 and end face 122-2 face, and the central axis of core 121c at end face 122-1 of MC-EDF 120, which is located on the inner lower side of where end face 122-1 and end face 122-2 face. In other words, double-sided mirror 111-2 is disposed at the intersection of the central axis connecting core 201c at end face 202 of transmission line fiber 200 and core 121b at end face 122-2 of MC-EDF 120, and the central axis connecting core 301c at end face 302 of transmission line fiber 300 and core 121c at end face 122-1 of MC-EDF 120.

[0148] One surface 111-1a of the double-sided mirror 111-1 is disposed to face the core 201b at the end surface 202 of the transmission line fiber 200 and the core 121b at the end surface 122-1 of the MC-EDF 120. The one surface 111-1a of the double-sided mirror 111-1 is fixed at an angle of 45 degrees with respect to the central axis of the core 201b at the end surface 202 of the transmission line fiber 200 and the central axis of the core 121b at the end surface 122-1 of the MC-EDF 120. The one surface 111-1a of the double-sided mirror 111-1 reflects light from the core 201b at the end surface 202 of the transmission line fiber 200 at a right angle toward the core 121b at the end surface 122-1 of the MC-EDF 120.

[0149] One surface 111-1b of the double-sided mirror 111-1 is disposed to face the core 301b at the end surface 302 of the transmission line fiber 300 and the core 121c at the end surface 122-2 of the MC-EDF 120. The one surface 111-1b of the double-sided mirror 111-1 is fixed at an angle of 45 degrees with respect to the central axis of the core 301b at the end surface 302 of the transmission line fiber 300 and the central axis of the core 121c at the end surface 122-2 of the MC-EDF 120. The one surface 111-1b of the double-sided mirror 111-1 reflects light from the core 121c at the end surface 122-2 of the MC-EDF 120 at a right angle toward the core 301b at the end surface 302 of the transmission line fiber 300.

[0150] One surface 111-2a of the double-sided mirror 111-2 is disposed to face the core 201c at the end surface 202 of the transmission line fiber 200 and the core 121c at the end surface 122-1 of the MC-EDF 120. The one surface 111-2a of the double-sided mirror 111-2 is fixed at an angle of 45 degrees with respect to the central axis of the core 201c at the end surface 202 of the transmission line fiber 200 and the central axis of the core 121c at the end surface 122-1 of the MC-EDF 120. The one surface 111-2a of the double-sided mirror 111-2 reflects light from the core 201c at the end surface 202 of the transmission line fiber 200 at a right angle toward the core 121c at the end surface 122-1 of the MC-EDF 120.

[0151] One surface 111-2b of the double-sided mirror 111-2 is disposed to face the core 301c at the end surface 302 of the transmission line fiber 300 and the core 121b at the end surface 122-2 of the MC-EDF 120. The one surface 111-2b of the double-sided mirror 111-2 is fixed at an angle of 45 degrees with respect to the central axis of the core 301c at the end surface 302 of the transmission line fiber 300 and the central axis of the core 121b at the end surface 122-2 of the MC-EDF 120. The one surface 111-2b of the double-sided mirror 111-2 reflects light from the core 121b at the end surface 122-2 of the MC-EDF 120 at a right angle toward the core 301c at the end surface 302 of the transmission line fiber 300.

[0152] The signal light output from the core 201d at the end face 202 of the transmission line fiber 200 is input to the core 121a at the end face 122-2 of the MC-EDF 120. The signal light input to the core 121a at the end face 122-2 of the MC-EDF 120 is propagated through the MC-EDF 120 and amplified, and the amplified signal light is output from the core 121a at the end face 122-1 to the core 301a at the end face 302 of the transmission line fiber 300.

[0153] The signal light output from core 201b at end face 202 of transmission line fiber 200 is reflected by one surface 111-1a of double-sided mirror 111-1 and input from one surface 111-1a to core 121b at end face 122-1. The signal light input to core 121b at end face 122-1 propagates through MC-EDF 120 and is amplified, and the amplified signal light is output from core 121b at end face 122-1 toward double-sided mirror 111-2. The signal light output from core 121b is reflected by one surface 111-2b of double-sided mirror 111-2 and output from one surface 111-2b to core 301c at end face 302 of transmission line fiber 300.

[0154] The signal light output from core 201c at end face 202 of transmission line fiber 200 is reflected by one surface 111-2a of double-sided mirror 111-2 and input from one surface 111-2a to core 121c at end face 122-1. The signal light input to core 121c at end face 122-1 propagates through MC-EDF 120 and is amplified, and the amplified signal light is output from core 121c at end face 122-1 toward double-sided mirror 111-1. The signal light output from core 121c is reflected by one surface 111-1b of double-sided mirror 111-1 and output from one surface 111-1b to core 301b at end face 302 of transmission line fiber 300.

[0155] The signal light output from the core 201a at the end face 202 of the transmission line fiber 200 is input to the core 121d at the end face 122-2 of the MC-EDF 120. The signal light input to the core 121d at the end face 122-2 of the MC-EDF 120 is propagated through the MC-EDF 120 and amplified, and the amplified signal light is output from the core 121d at the end face 122-1 to the core 301d at the end face 302 of the transmission line fiber 300.

[0156] As described above, when switching the transmission direction of a four-core MC-EDF in an optical amplifier, the MC-EDF may be twisted by 180 degrees. Even in this case, the transmission direction of the MC-EDF can be switched in both directions by arranging the core that switches the transmission direction in the four-core MC-EDF and the mirror in a predetermined positional relationship.

[0157] Although a 90-degree twist is added to the MC-EDF in Variation 1 of Embodiment 3 and a 180-degree twist is added to the MC-EDF in Variation 2 of Embodiment 3, the twist angle is not limited to these. As long as a mirror can be placed at the intersection of the cores that changes the transmission direction, the twist angle of the MC-EDF can be freely chosen. However, the core arrangement at the time of output will be shifted by the twist angle from the core arrangement at the time of input. Furthermore, this is not limited to multi-cores with two or four cores, but also applies to multi-cores with an even number of cores arranged concentrically, such as six or eight cores.

[0158] (Fourth embodiment) Next, a fourth embodiment will be described. In this embodiment, a specific example of the pumping method in the optical amplifier shown in the second embodiment will be described. Note that the pumping method of this embodiment may be applied to the first modification of the second embodiment, the third embodiment, and the first and second modifications of the third embodiment.

[0159] 18 shows an example of the configuration when core pumping is performed in an optical amplifier 100 according to some embodiments. The example of FIG. 18 includes a multiplexing section 130 and a demultiplexing section 131 in addition to the configuration of FIG.

[0160] The multiplexing unit 130 (coupler) multiplexes the signal light and pumping light for each core for core pumping. The multiplexing unit 130 multiplexes the signal light and pumping light input from the transmission line fiber 200 for each core, and inputs the multiplexed light to the MC-EDF 120.

[0161] 18, the multiplexing unit 130 is disposed between the end face 122-2 and double-sided mirror 111 of the MC-EDF 120 and the end face 202 of the transmission line fiber 200 (in front of the end face 202 of the transmission line fiber 200). The multiplexing unit 130 multiplexes the signal light and pumping light from the core 201a of the transmission line fiber 200, and inputs the multiplexed light to the core 121a of the end face 122-2 of the MC-EDF 120. As a result, the signal light input to the core 121a is pumped and amplified by core pumping.

[0162] The multiplexer 130 multiplexes the signal light and pumping light from the core 201b of the transmission line fiber 200, and inputs the multiplexed light to the core 121b at the end face 122-1 of the MC-EDF 120 via one surface 111a of the double-sided mirror 111. As a result, the signal light input to the core 121b is excited and amplified by the core pumping.

[0163] The demultiplexing unit 131 (separator) demultiplexes the optical signal amplified by core pumping from the pump light. The demultiplexing unit 131 demultiplexes the pump light from the light output from the MC-EDF 120 for each core, and outputs the amplified signal light to the transmission line fiber 300.

[0164] 18, the demultiplexing unit 131 is disposed between the end face 122-1 and double-sided mirror 111 of the MC-EDF 120 and the end face 302 of the transmission line fiber 300 (in front of the end face 302 of the transmission line fiber 300). The demultiplexing unit 131 demultiplexes the light output from the core 121a of the end face 122-1 of the MC-EDF 120 into pump light and signal light, and outputs the demultiplexed signal light to the core 301a of the transmission line fiber 300. As a result, only the signal light amplified in the core 121a of the MC-EDF 120 propagates.

[0165] The demultiplexing unit 131 demultiplexes the light output from the core 121b of the end face 122-1 of the MC-EDF 120 via one surface 111b of the double-sided mirror 111 into pump light and signal light, and outputs the demultiplexed signal light to the core 301b of the transmission line fiber 300. As a result, only the signal light amplified in the core 121b of the MC-EDF 120 propagates.

[0166] 19 shows a configuration example in which cladding pumping is performed in an optical amplifier 100 according to some embodiments. In this case, the MC-EDF 120 is a fiber with a double-clad structure including an inner cladding 122a (a cladding on the central axis side) and an outer cladding 122b (a cladding on the outer periphery side).

[0167] The example in Fig. 19 includes multiplexer / demultiplexer units 132-1 and 132-2 in addition to the configuration in Fig. 9. The multiplexer / demultiplexer units 132-1 and 132-2 (combiners / separators) multiplex signal light for each core with pumping light for cladding pumping for cladding pumping, and also demultiplex the optical signal amplified by cladding pumping from the pumping light. The multiplexer / demultiplexer units 132-1 and 132-2 pump the MC-EDF 120 in both directions.

[0168] 19, the multiplexing / demultiplexing unit 132-1 is disposed between the end face 122-1 of the MC-EDF 120 and the double-sided mirror 111 (in front of the end face 122-1 of the MC-EDF 120). The multiplexing / demultiplexing unit 132-1 multiplexes the signal light and pumping light input from the core 201b of the transmission line fiber 200 via one face 111a of the double-sided mirror 111, inputs the signal light to the core 121b of the end face 122-1, and inputs the pumping light to the inner cladding 122a of the end face 122-1. As a result, the signal light input to the core 121b is pumped and amplified by cladding pumping.

[0169] The multiplexing / demultiplexing unit 132-2 is disposed between the end face 122-2 of the MC-EDF 120 and the double-sided mirror 111 (in front of the end face 122-2 of the MC-EDF 120). The multiplexing / demultiplexing unit 132-2 multiplexes the signal light and pumping light input from the core 201a of the transmission line fiber 200, inputs the signal light to the core 121a of the end face 122-2, and inputs the pumping light to the inner cladding 122a of the end face 122-2. As a result, the signal light input to the core 121a is pumped and amplified by cladding pumping.

[0170] Furthermore, the multiplexer / demultiplexer 132-1 demultiplexes the light output from the core 121a and inner cladding 122a of the end face 122-1 of the MC-EDF 120 into pump light and signal light, and outputs the demultiplexed signal light to the core 301a of the transmission line fiber 300. As a result, only the signal light amplified in the core 121a of the MC-EDF 120 propagates.

[0171] The multiplexer / demultiplexer 132-2 demultiplexes the light output from the core 121b and inner cladding 122a of the end face 122-2 of the MC-EDF 120 into pump light and signal light, and outputs the demultiplexed signal light to the core 301b of the transmission line fiber 300 via one surface 111b of the double-sided mirror 111. As a result, only the signal light amplified in the core 121b of the MC-EDF 120 propagates.

[0172] As described above, the MC-EDF of an optical amplifier may use either core pumping or cladding pumping. Core pumping allows the signal light of each core to be pumped individually. Cladding pumping allows the signal light of multiple cores to be pumped efficiently with a single pump light. For example, in cladding pumping, by placing a multiplexer / demultiplexer immediately before both end facets of the MC-EDF and performing bidirectional pumping, it is possible to prevent the gain and NF (Noise Figure) from changing depending on the transmission direction of the signal light.

[0173] (Embodiment 5) Next, a fourth embodiment will be described. In this embodiment, an example will be described in which the propagation area of ​​signal light is expanded between the MC-EDF and the transmission line fiber in the optical amplifier shown in the second embodiment. Note that the method of expanding the propagation area of ​​signal light of this embodiment may be applied to the first modification of the second embodiment, the third embodiment, and the first and second modifications of the third embodiment.

[0174] Fig. 20 shows an example of the configuration of an optical amplifier 100 according to some embodiments. In the example of Fig. 20, in addition to the configuration of Fig. 9, magnifying lenses 140-1 to 140-4 and collimating lenses 141-1 to 141-4 are provided. As shown in Fig. 20, the magnifying lenses 140-1 to 140-4 and the collimating lenses 141-1 to 141-4 are arranged in this order in front of the end face of each fiber.

[0175] The magnifying lenses 140-1 to 140-4 expand (in the width direction) the propagation region of the signal light output from the fiber. Expanding the propagation region of the signal light means shifting the optical axis of the signal light (parallel light) so that the intervals between the signal light input and output at multiple cores are increased. The parallel lenses 141-1 to 141-4 convert the signal light whose propagation region has been expanded by the magnifying lenses 140-1 to 140-4 into parallel light (collimated light). The magnifying lenses 140-1 to 140-4 and the parallel lenses 141-1 to 141-4 may shift only the optical axis of the signal light input and output to the double-sided mirror 111. Note that by inputting the signal light into the fiber via the parallel lenses 141-1 to 141-4 and the magnifying lenses 140-1 to 140-4, the propagation region of the signal light can be reduced to the size between the cores in the fiber. That is, the magnifying lenses 140-1 to 140-4 and the parallel lenses 141-1 to 141-4 also function as a magnifying / contracting unit that enlarges and reduces the propagation area of ​​the signal light.

[0176] In the example of FIG. 20, a magnifying lens 140-1 and a collimating lens 141-1 are arranged in front of the end face 122-1 of the MC-EDF 120. The magnifying lens 140-1 outputs the signal light output from the core 121a of the end face 122-1 in a direction in which the propagation region expands (toward the side where the distance from the core 121b widens). The collimating lens 141-1 outputs the signal light output from the core 121a of the end face 122-1 and directed by the magnifying lens 140-1 in a direction in which the propagation region expands, in a direction in which the signal light becomes a parallel light. Specifically, the signal light is output to a collimating lens 141-4 on the transmission line fiber 300 side. This allows the signal light output from the core 121a of the end face 122-1 to propagate as a parallel light with an expanded propagation region.

[0177] Furthermore, the collimating lens 141-1 outputs the signal light input from the core 201b of the transmission fiber 200 via the magnifying lens 140-3, the collimating lens 141-3, and one surface 111a of the double-sided mirror 111 in a direction in which the propagation area narrows (toward the core 121b of the end surface 122-1). The magnifying lens 140-1 outputs the signal light input via the one surface 111a of the double-sided mirror 111 and directed by the collimating lens 141-1 in a direction in which the propagation area narrows, toward the core 121b of the end surface 122-1. This allows the signal light from the core 201b of the transmission fiber 200, which is collimated light with an expanded propagation area, to be input to the core 121b of the end surface 122-1.

[0178] A magnifying lens 140-2 and a collimating lens 141-2 are arranged in front of the end face 122-2 of the MC-EDF 120. The magnifying lens 140-2 outputs the signal light output from the core 121b of the end face 122-2 in a direction in which the propagation area expands (toward the side where the distance from the core 121a becomes wider). The collimating lens 141-2 outputs the signal light output from the core 121b of the end face 122-2 and directed by the magnifying lens 140-2 in a direction in which the propagation area expands, in a direction in which the signal light becomes parallel. Specifically, the signal light is output to one surface 111b of the double-sided mirror 111. This allows the signal light output from the core 121b of the end face 122-2 to propagate as parallel light with an expanded propagation area.

[0179] Furthermore, the collimating lens 141-2 outputs the signal light input from the core 201a of the transmission fiber 200 via the expanding lens 140-3 and the collimating lens 141-3 in a direction in which the propagation region narrows (towards the core 121a of the end face 122-2). The expanding lens 140-2 outputs the signal light input via the collimating lens 141-3 and directed by the collimating lens 141-2 in a direction in which the propagation region narrows, towards the core 121a of the end face 122-2. This allows the signal light from the core 201a of the transmission fiber 200, which is collimated light with an expanded propagation region, to be input to the core 121a of the end face 122-2.

[0180] An expanding lens 140-3 and a collimating lens 141-3 are disposed in front of the end face 202 of the transmission line fiber 200. The expanding lens 140-3 outputs the signal light output from the core 201a of the end face 202 in a direction in which the propagation region expands (toward the side in which the distance from the core 201b increases). The collimating lens 141-3 outputs the signal light output from the core 201a of the end face 202 and directed by the expanding lens 140-3 in a direction in which the signal light becomes parallel light. Specifically, the signal light is output to the collimating lens 141-2 on the end face 122-2 side of the MC-EDF 120. This allows the signal light output from the core 201a of the end face 202 of the transmission line fiber 200 to propagate as parallel light with an expanded propagation region.

[0181] Furthermore, the magnifying lens 140-3 outputs the signal light output from the core 201b of the end face 202 in a direction in which the propagation area expands (toward the side in which the distance from the core 201a becomes wider). The collimating lens 141-3 outputs the signal light output from the core 201b of the end face 202 and directed by the magnifying lens 140-3 in a direction in which the signal light becomes parallel light. Specifically, the collimating lens 141-3 outputs the signal light to one surface 111a of the double-sided mirror 111. This allows the signal light output from the core 201b of the end face 202 of the transmission line fiber 200 to propagate as parallel light with an expanded propagation area.

[0182] An expanding lens 140-4 and a collimating lens 141-4 are disposed in front of the end face 302 of the transmission line fiber 300. The collimating lens 141-4 outputs the signal light output from the core 121a of the end face 122-1 of the MC-EDF 120 via the expanding lens 140-1 and the collimating lens 141-1 in a direction in which the propagation region narrows (towards the core 301b of the end face 302). The expanding lens 140-4 outputs the signal light output via the collimating lens 141-1 and directed by the collimating lens 141-4 in a direction in which the propagation region narrows, towards the core 301a of the end face 302. This allows the signal light from the core 121a of the end face 122-1 of the MC-EDF 120, which is collimated light with an expanded propagation region, to be input to the core 301a of the end face 302 of the transmission line fiber 300.

[0183] Furthermore, the collimating lens 141-4 outputs the signal light output from the core 121b of the end face 122-2 of the MC-EDF 120 via the magnifying lens 140-2, the collimating lens 141-2, and one surface 111b of the double-sided mirror 111 in a direction in which the propagation region narrows (toward the core 301a of the end face 302). The magnifying lens 140-4 outputs the signal light output from the one surface 111b of the double-sided mirror 111 and directed by the collimating lens 141-4 in a direction in which the propagation region narrows, toward the core 301b of the end face 302. This allows the signal light, which is output from the core 121b of the end face 122-2 of the MC-EDF 120 via the one surface 111b of the double-sided mirror 111 and is collimated light with an expanded propagation region, to be input to the core 301b of the end face 302 of the transmission line fiber 300.

[0184] As described above, in an optical amplifier, optical components such as lenses may be added in front of each fiber to expand the propagation area of ​​the signal light. This makes it easier to arrange a double-sided mirror. For example, simply collimating the light output from the core requires micrometer-level processing to arrange the double-sided mirror. However, by using optical components to shift the optical axis of the light output from the core to parallelism, the double-sided mirror can be arranged with millimeter- to centimeter-level precision.

[0185] 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.

[0186] 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.

[0187] A part or all of the above-described embodiments can be described as, but not limited to, the following supplementary notes.

[0188] (Appendix 1) An optical amplifier connected between a first multi-core fiber and a second multi-core fiber, a switching unit that switches the transmission direction of signal light transmitted in the same direction through the first multi-core fiber and the second multi-core fiber between two directions; an amplifier that amplifies the bidirectionally transmitted signal light; An optical amplifier comprising: (Appendix 2) the amplifying section includes a multi-core optical amplifying fiber, the multi-core optical amplifying fiber is fixed so that a first end face of the multi-core optical amplifying fiber and a second end face of the multi-core optical amplifying fiber face each other at a predetermined angle; The switching unit is a first reflecting section that reflects signal light output from a first input-side core of the first multi-core fiber so that the signal light is input to a first amplifying core at the first end face of the multi-core optical amplifying fiber; a second reflecting section that reflects the signal light output from the first amplifying core at the second end face of the multi-core optical amplifying fiber so that the signal light is input to a first output-side core of the second multi-core fiber, 10. The optical amplifier device of claim 1. (Appendix 3) the first reflecting portion and the second reflecting portion are disposed at an intersection between a central axis of the first amplifying core at the first end face of the multi-core optical amplifying fiber and a central axis of the first amplifying core at the second end face. 3. An optical amplifier according to claim 2. (Appendix 4) The first reflecting portion is a first single-sided mirror, and the second reflecting portion is a second single-sided mirror. 4. The optical amplifier according to claim 2 or 3. (Appendix 5) The first reflecting portion and the second reflecting portion are double-sided mirrors. 4. The optical amplifier according to claim 2 or 3. (Appendix 6) The first end surface and the second end surface are fixed so as to face each other in a state where either the first end surface or the second end surface is rotated by a predetermined angle. 4. The optical amplifier according to claim 2 or 3. (Appendix 7) the switching unit is configured to input the signal light output from the second input core of the first multi-core fiber to a second amplifying core at the second end face of the multi-core optical amplifying fiber, and to input the signal light output from the second amplifying core at the first end face of the multi-core optical amplifying fiber to a second output core of the second multi-core fiber. 4. The optical amplifier according to claim 2 or 3. (Appendix 8) the multi-core optical amplifying fiber is a multi-core fiber with a double clad structure, a first multiplexing unit that multiplexes signal light and pumping light input to the first end face of the multi-core optical amplifying fiber and inputs the multiplexed pumping light into a clad on a central axis side of the first end face; a second multiplexing section that multiplexes signal light and pumping light input to the second end face of the multi-core optical amplifying fiber and inputs the multiplexed pumping light into a clad on a central axis side of the second end face; 4. The optical amplifier according to claim 2, further comprising: (Appendix 9) an expansion section for expanding a propagation region of input / output signal light between the first end face of the multi-core optical amplifying fiber and the first multi-core fiber, and between the second end face of the multi-core optical amplifying fiber and the second multi-core fiber, 4. The optical amplifier according to claim 2 or 3. (Appendix 10) a first multicore fiber; and a second multicore fiber; and an optical amplifier connected between the first multi-core fiber and the second multi-core fiber, The optical amplifier device comprises: a switching unit that switches the transmission direction of signal light transmitted in the same direction through the first multi-core fiber and the second multi-core fiber between two directions; an amplifier that amplifies the bidirectionally transmitted signal light; An optical transmission system comprising: (Appendix 11) An optical amplification method in an optical amplifier connected between a first multi-core fiber and a second multi-core fiber, comprising: switching the transmission direction of signal light transmitted in the same direction through the first multi-core fiber and the second multi-core fiber to bidirectional; amplifying the bidirectionally transmitted signal light; Optical amplification method.

[0189] Some or all of the elements (e.g., configurations and functions) described in Supplementary Notes 2 to 9 that are dependent on Supplementary Note 1 (optical amplifier) ​​may also be dependent on Supplementary Note 10 (optical transmission system) and Supplementary Note 11 (optical amplification method) in the same dependent relationship as Supplementary Notes 2 to 9. 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]

[0190] 1 Optical transmission system 100 Optical amplifier 110 Transmission direction switching unit 120 MC-EDF 111 Double-sided mirror 111a, 111b single side 121a~121d Core 122-1, 122-2 end face 122a Inner cladding 122b outer cladding 130 Multiplexing section 131 Demultiplexer 132-1, 132-2 Multiplexing / demultiplexing section 140-1~140-4 Magnifying Lens 141-1~141-4 Parallel Lenses 200 Transmission fiber 201a~201d Core 202 End face 300 Transmission Line Fiber 301a~301d Core 302 End face 410-1, 410-2 ISO 420 GFF

Claims

1. An optical amplifier connected between a first multi-core fiber and a second multi-core fiber, a switching unit that switches a transmission direction of signal light transmitted in the same direction through the first multi-core fiber and the second multi-core fiber between two directions; an amplifier that amplifies the bidirectionally transmitted signal light; An optical amplifier comprising:

2. the amplifying section includes a multi-core optical amplifying fiber, the multi-core optical amplifying fiber is fixed such that a first end face of the multi-core optical amplifying fiber and a second end face of the multi-core optical amplifying fiber face each other at a predetermined angle; The switching unit is a first reflecting section that reflects signal light output from a first input-side core of the first multi-core fiber so that the signal light is input to a first amplifying core at the first end face of the multi-core optical amplifying fiber; a second reflecting section that reflects the signal light output from the first amplifying core at the second end face of the multi-core optical amplifying fiber so that the signal light is input to a first output-side core of the second multi-core fiber, 2. The optical amplifier according to claim 1.

3. the first reflecting portion and the second reflecting portion are disposed at an intersection between a central axis of the first amplifying core at the first end face of the multi-core optical amplifying fiber and a central axis of the first amplifying core at the second end face.

3. The optical amplifier according to claim 2.

4. the first reflecting portion is a first single-sided mirror, and the second reflecting portion is a second single-sided mirror; 4. An optical amplifier according to claim 2 or 3.

5. the first reflecting portion and the second reflecting portion are double-sided mirrors; 4. An optical amplifier according to claim 2 or 3.

6. the first end surface and the second end surface are fixed so as to face each other in a state in which either the first end surface or the second end surface is rotated by a predetermined angle; 4. An optical amplifier according to claim 2 or 3.

7. the switching unit is configured to input the signal light output from the second input core of the first multi-core fiber to a second amplifying core at the second end face of the multi-core optical amplifying fiber, and to input the signal light output from the second amplifying core at the first end face of the multi-core optical amplifying fiber to a second output core of the second multi-core fiber.

4. An optical amplifier according to claim 2 or 3.

8. the multi-core optical amplifying fiber is a multi-core fiber with a double clad structure, a first multiplexing unit that multiplexes signal light and pumping light input to the first end face of the multi-core optical amplifying fiber and inputs the multiplexed pumping light into a clad on a central axis side of the first end face; a second multiplexing section that multiplexes signal light and pumping light input to the second end face of the multi-core optical amplifying fiber and inputs the multiplexed pumping light into a clad on a central axis side of the second end face; 4. The optical amplifier according to claim 2, further comprising:

9. a first multicore fiber; a second multicore fiber; and an optical amplifier connected between the first multi-core fiber and the second multi-core fiber, The optical amplifier device comprises: a switching unit that switches a transmission direction of signal light transmitted in the same direction through the first multi-core fiber and the second multi-core fiber between two directions; an amplifier that amplifies the bidirectionally transmitted signal light; An optical transmission system comprising:

10. An optical amplification method in an optical amplifier device connected between a first multi-core fiber and a second multi-core fiber, comprising: switching the transmission direction of signal light transmitted in the same direction through the first multi-core fiber and the second multi-core fiber to bidirectional; amplifying the bidirectionally transmitted signal light; Optical amplification method.

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