Multi-core fiber optical amplifier and optical amplification method

Abstract

To provide a multi-core fiber optical amplifier and an optical amplification method that enable effective utilization of a core of an MCF without using an excitation fiber using a special MCF.SOLUTION: A multi-core fiber optical amplifier includes: a multi-core excitation fiber which includes a first core and a second core; and a clad excitation circuit which injects excitation light into a clad of the multi-core excitation fiber. In the multi-core fiber optical amplifier, signal light input to one end of the first core is output from the other end of the first core, the signal light output from the other end of the first core is input to one end of the second core, and the signal light input to the one end of the second core is output from the other end of the second core.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a multi-core fiber optical amplifier and an optical amplification method. [Background technology]

[0002] The practical application of spatial optical multiplexing transmission systems using multi-core fibers (MCFs) is progressing. MCFs have multiple cores in one optical fiber. By using MCFs instead of single-core fibers (SCFs), it is possible to increase the transmission capacity of optical transmission systems. MCF optical amplifiers that use MCFs as pumping fibers (multi-core pumping fibers) are also known. In such pumping fibers, rare earth elements such as erbium (Er) are doped into the cores of MCFs. MCF optical amplifiers allow two types of pumping: "core pumping," in which pumping light is directly injected into the core, and "cladding pumping," in which pumping light is injected into the cladding to pump the erbium in multiple cores all at once.

[0003] Wavelength division multiplexing signal light is usually used for the signal light transmitted by MCF. Two bands of wavelength division multiplexing signal light are known: C-band and L-band. The C-band is approximately 1520-1560 nm, and the L-band is approximately 1570-1610 nm. In the following, wavelength division multiplexing signal light is referred to as WDM (wavelength division multiplexing) light.

[0004] In relation to the present disclosure, an optical amplifier described in Patent Document 1 has a pumping core in the center, and is configured to fold back signal light propagating through a signal light core located around the pumping core using a lens array and mirrors. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2013-058651 A Summary of the Invention [Problem to be solved by the invention]

[0006] C-band signal light is mainly used for the transmission of WDM signals. To further expand the transmission capacity, the transmission of WDM signals using L-band signal light is also being considered. However, the efficiency of a typical optical fiber amplifier in the L-band is lower than its efficiency in the C-band. For this reason, in order to amplify L-band signal light using an optical fiber amplifier, it is necessary to use a pumping fiber that is several times longer than that for the C-band.

[0007] The technology described in Patent Document 1 aims to obtain the effect of extending the pumping fiber by propagating one signal light to multiple cores in an optical amplifier using an MCF as the pumping fiber. However, the technology described in Patent Document 1 requires the use of a special MCF with a pumping core at the center.

[0008] The present disclosure describes a technique for providing a multi-core fiber optical amplifier and an optical amplification method that can effectively utilize the cores of an MCF without using a pumping fiber that uses a special MCF. [Means for solving the problem]

[0009] A multi-core optical fiber amplifier according to the present disclosure is a multi-core fiber optical amplifier including a multi-core pumping fiber having a first core and a second core, and a cladding pumping means for injecting pumping light into a cladding of the multi-core pumping fiber, A signal light input to one end of the first core is output from the other end of the first core, The signal light output from the other end of the first core is input to one end of the second core; The signal light input to one end of the second core is output from the other end of the second core.

[0010] An optical amplification method according to the present disclosure is an optical amplification method used in a multi-core fiber optical amplifier including a multi-core pumping fiber including a first core and a second core, comprising: injecting pump light into the cladding of the multi-core pump fiber; A signal light input to one end of the first core is output from the other end of the first core, The signal light output from the other end of the first core is input to one end of the second core; The signal light input to one end of the second core is output from the other end of the second core. Effect of the Invention

[0011] The present disclosure provides a multi-core fiber optical amplifier and an optical amplification method that can effectively utilize the cores of an MCF without using a pumping fiber that uses a special MCF. [Brief description of the drawings]

[0012] [Figure 1] FIG. 2 is a diagram illustrating a configuration example of an MCF optical amplifier. [Diagram 2] FIG. 2 is a diagram showing an example of a cross section of an excitation fiber. [Diagram 3] 1A and 1B are diagrams illustrating examples of optical paths between end faces of excitation fibers. [Figure 4] 1A and 1B are diagrams illustrating examples of optical paths between end faces of excitation fibers. [Diagram 5] 1A and 1B are diagrams illustrating examples of optical paths between end faces of excitation fibers. [Figure 6] 1A and 1B are diagrams illustrating examples of optical paths between end faces of excitation fibers. [Figure 7] 1A and 1B are diagrams illustrating examples of how end faces of excitation fibers face each other. [Figure 8] 1A and 1B are diagrams illustrating examples of how end faces of excitation fibers face each other. [Figure 9] 1A and 1B are diagrams illustrating examples of how end faces of excitation fibers face each other. [Figure 10] 1A and 1B are diagrams illustrating examples of the arrangement of cores of excitation fibers. [Figure 11] 1A and 1B are diagrams illustrating examples of the arrangement of cores of excitation fibers. [Figure 12] 1A and 1B are diagrams illustrating examples of how end faces of excitation fibers face each other. [Figure 13] 1A and 1B are diagrams illustrating examples of how end faces of excitation fibers face each other. [Figure 14] 1A and 1B are diagrams illustrating examples of how end faces of excitation fibers face each other. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] (First embodiment) FIG. 1 is a diagram showing a configuration example of an MCF optical amplifier 1000 according to the present disclosure. The MCF optical amplifier 1000 is a multi-core fiber optical amplifier, and includes an MCF having two or more cores as a pumping fiber 100. The pumping fiber is a multi-core pumping fiber in which a rare earth element such as erbium is doped in the core. In FIG. 1, a case in which the pumping fiber 100 includes four cores (core 1 to core 4) will be described. However, the number of cores in the pumping fiber 100 is not limited to four as long as it is a plurality of cores. In FIG. 1, the end faces of core 1 to core 4 appearing at end face 101, which is one end of the pumping fiber 100, are indicated as end face 1A to end face 4A, respectively. Moreover, the end faces of core 1 to core 4 appearing at end face 102, which is the other end of the pumping fiber 100, are indicated as end face 1B to end face 4B, respectively. The end face 101 and the end face 102 face each other in parallel. In addition, the central axis of the excitation fiber 100 at the end face 101 coincides with the central axis of the excitation fiber 100 at the end face 102. Furthermore, as will be described later, in FIG.

[0014] FIG. 2 is a diagram showing an example of a cross section of the excitation fiber 100. The excitation fiber 100 is an MCF with a circular cross section. Core 1 to core 4 are arranged at equal intervals on a circumference centered on the central axis of the excitation fiber 100. Therefore, when the excitation fiber 100 is rotated about the central axis, the positions of core 1 to core 4 in the cross section of the excitation fiber 100 overlap at every 90 degrees. In the following, the position of the central axis of the excitation fiber in the longitudinal direction at the end face of the excitation fiber is simply referred to as the "center of the excitation fiber." In FIG. 2, the center of the excitation fiber 100 is indicated by a black dot.

[0015] The pumping fiber 100 shown in FIG. 1 has a pumping light coupling section 110 for injecting pumping light into the clad of the pumping fiber 100. The pumping light coupling section 110 serves as a clad pumping means for injecting pumping light into the clad of the multi-core pumping fiber. A known configuration may be applied to the pumping light coupling section 110. For example, the pumping light can be injected into the clad of the pumping fiber 100 by winding an optical fiber that propagates the pumping light around the pumping fiber 100 from which the coating has been removed. The rare earth ions doped in the cores 1 to 4 are excited by the pumping light (clad pumping light) injected into the clad, and the pumping fiber 100 amplifies the signal light propagating through the cores 1 to 4.

[0016] The pumping fiber 100 is configured such that one end face (end face 101) faces the other end face (end face 102) and the cores appearing on the end faces 101 and 102 are optically coupled one-to-one. Then, by applying a rotation (twist) to the end face 101 or the end face 102, the signal light propagated through a certain core (for example, core 1) can be propagated through the space between the end face 101 and the end face 102 and input to another core (for example, core 2). The arrangement of the cores shown in FIG. 1 can be obtained by twisting the pumping fiber 100 by 90 degrees (i.e., rotating the end face 101 or 102 by 90 degrees in a direction perpendicular to the central axis of the pumping fiber 100). In order to maintain the positional relationship of the cores between the end face 101 and the end face 102, the pumping fiber 100 may be provided with a holding section. The holding portion is, for example, a member that simultaneously holds and fixes the vicinity of the end face 101 and the vicinity of the end face 102 of the excitation fiber 100.

[0017] When signal light is input to end face 1A of core 1 at end face 101, the signal light propagates through core 1 and is output from end face 1B of core 1 at end face 102. As shown in FIG. 1, the signal light output from end face 1B is input to end face 2A of core 2. Then, the signal light propagates through core 2 and is output from end face 2B of core 2 at end face 102. The signal light output from end face 2B is input to end face 3A of core 3. The signal light propagates through core 3 and is output from end face 3B of core 3 at end face 102. The signal light output from end face 3B is input to end face 4A of core 4. The signal light input to end face 4A of core 4 propagates through core 4 and is output from end face 4B of core 4 at end face 102. The signal light output from end face 4B is output to the outside of pumping fiber 100.

[0018] Incidentally, JP2014-021225A describes an optical amplifier having a configuration in which a coupler is used to input signal light and pump light from the outer periphery of an MCF. However, the optical amplifier described in JP2014-021225A uses a coupler to input signal light and pump light to the pump fiber, so only a core close to the outer periphery of the pump fiber can be used for optical amplification. In contrast, the MCF optical amplifier 1000 can use a core at any position of the pump fiber for optical amplification as long as the cores are optically connected between end face 101 and end face 102.

[0019] With such a configuration, the MCF optical amplifier 1000 described in this embodiment can effectively utilize the core of the MCF without using a pumping fiber that uses a special MCF.

[0020] The signal light input to core 1 propagates through core 1, core 2, core 3, and core 4 in this order, and is output from core 4. Then, by injecting cladding pumping light into the pumping fiber 100, the signal light is amplified in each core while propagating from core 1 to core 4. That is, since the MCF optical amplifier 100 amplifies the signal light using the four cores of one pumping fiber 100, the length of the pumping fiber through which the signal light propagates is four times that of a pumping fiber using an SCF of the same length as the pumping fiber 100. As a result, the MCF optical amplifier 1000 can reduce the length of the pumping fiber required for the pumping fiber, compared to an optical amplifier using an SCF as the pumping fiber.

[0021] (Another expression of the first embodiment) The effects of the MCF optical amplifier 1000 described above can also be obtained by a multi-core fiber optical amplifier having the following configuration: The numbers in parentheses are reference numerals in FIG.

[0022] The multicore fiber optical amplifier (1000) includes a multicore pumping fiber (100) having a first core and a second core, and a clad pumping means (110) for injecting pumping light into the clad of the multicore pumping fiber (100). The multicore fiber optical amplifier (1000) is configured to output signal light input to one end (1A) of the first core from the other end (1B) of the first core. The multicore fiber optical amplifier (1000) is also configured to input the signal light output from the other end (1B) of the first core to one end (2A) of the second core, and to output the signal light input to the one end (2A) of the second core from the other end (2B) of the second core.

[0023] The first core (1) and the second core (2) may be arranged at equal intervals on a circumference centered on the center of the end face of the multicore pumping fiber (100). The other end (102) of the multicore pumping fiber (100) may be held in a state rotated by a predetermined angle with respect to one end (101) of the multicore pumping fiber, thereby optically coupling the other end (1B) of the first core and one end (2A) of the second core.

[0024] The multi-core fiber optical amplifier (1000) may further include a third core. The multi-core fiber optical amplifier (1000) may be configured to input the signal light output from the other end (2B) of the second core to one end of the third core (3A) and to output the signal light input to the one end (3A) of the third core from the other end (3B) of the third core.

[0025] Second embodiment Hereinafter, as the second embodiment, an example of the configuration of an optical path between the end face 101 and the end face 102 of the pumping fiber 100 will be described. Figures 3 to 6 each show a part of the configuration of the MCF optical amplifier 1000 in Figure 1, and the configurations in Figures 3 to 6 can be applied to the MCF optical amplifier 1000.

[0026] Fig. 3 is a diagram for explaining an example of the configuration of an optical path between end face 101 and end face 102 of pumping fiber 100 in MCF optical amplifier 1000. In Fig. 3, end face 1A of core 1 and end face 4B of core 4 are arranged to face each other between end face 101 and end face 102. Similarly, end face 1B faces end face 2A, end face 2B faces end face 3A, and end face 3B faces end face 4A.

[0027] The end face 1B and the end face 2A are optically coupled by two optical collimators 1021 and 1012. The optical collimator 1021 is disposed on the end face 102 side, and the optical collimator 1012 is disposed on the end face 101 side. The optical collimator 1021 converts the signal light output from the end face 1B into collimated light. The optical collimator 1012 collects the collimated light output from the optical collimator 1021 and inputs it to the end face 2A. The optical axes of the optical collimators 1021 and 1012 are adjusted so that the core 1 and the core 2 are optically coupled between the end face 1B and the end face 2A. The signal light propagates between the optical collimators 1021 and 1012 as collimated light.

[0028] Similarly, two optical collimators are used for each of the coupling between the core 2 and the core 3 and the coupling between the core 3 and the core 4 between the end face 101 and the end face 102. That is, the optical collimator 1022 and the optical collimator 1013 couple the core 2 and the core 3, and the optical collimator 1023 and the optical collimator 1014 couple the core 3 and the core 4. With this configuration, the signal light input to the end face 1A of the core 1 propagates through the core 1-core 4 and is output from the end face 4B of the core 4. Then, by injecting the pumping light from the pumping light coupling unit 110 into the pumping fiber 100, the signal light is pumped over a length four times that of the pumping fiber 100. The optical collimators 1012-1014 and the optical collimators 1021-1023 may couple collimated light with each core using a lens array.

[0029] 3, end face 1A of core 1 and end face 4B of core 4 face each other between end face 101 and end face 102. However, end face 1A is used for inputting signal light, and end face 4B is used for output. Therefore, end face 1A and end face 4B are not optically connected in the space between end face 101 and end face 102. The configuration for inputting signal light to end face 1A and the configuration for outputting the signal light output from end face 4B to the outside are arbitrary.

[0030] FIG. 4 is a diagram showing another example of the configuration of the optical path between end face 101 and end face 102. FIGS. 4 to 6 show only parts necessary for explanation. In FIG. 4, similar to FIG. 3, end face 1A of core 1 and end face 4B of core 4 face each other between end face 101 and end face 102. As described above, end face 1A is used for inputting the signal light, and end face 4B is used for outputting the signal light. FIG. 4 shows a configuration including a mirror 801 for separating the signal light input to core 1 from the signal light output from core 4.

[0031] The mirror 801 is provided between the end face 1A and the end face 4B. Both surfaces of the mirror 801 reflect the signal light. The mirror 801 is, for example, a reflective film using a metal thin film. The end face 1A and the end face 4B face each other via the optical collimators 1011 and 1024. The mirror 801 is disposed so as to form an angle of 45 degrees with respect to the optical axis connecting the end face 1A and the end face 4B. With this configuration, the signal light introduced as collimated light from outside the MCF optical amplifier 1000 can be reflected by the mirror 801 and guided to the end face 1A. Furthermore, the signal light output from the end face 4B can be reflected by the mirror 801 and extracted to the outside of the MCF optical amplifier 1000.

[0032] The reflective film of the mirror 801 may reflect the signal light and the pumping light. When the signal light is guided to the end face 1A, the signal light multiplexed with the pumping light may be input to the end face 1A via the mirror 801 and the optical collimator 1011. In general, the wavelength bands of the pumping light and the signal light do not overlap. For example, the wavelength of the pumping light is in the 980 nm band, and the wavelength of the signal light is in the C band or the L band (i.e., approximately 1520 nm to 1610 nm). Therefore, by wavelength-multiplexing the pumping light and the signal light using a wavelength filter and converting the wavelength-multiplexed pumping light and the signal light into collimated light, the pumping light and the signal light can be input to the core 1 through the same optical path. The pumping light directly input to the core is hereinafter referred to as core pumping light.

[0033] By the core pumping light input from the mirror 801, the core 1 can amplify the signal light by core pumping in addition to cladding pumping by the pumping light coupling unit 110. Furthermore, the core pumping light remaining in the core 1 without being used for amplification may be input to the core 2 via the end face 1B, the optical collimators 1021 and 1012, and the end face 2A. The core pumping light input to the core 2 may further propagate to the core 3 and the core 4 together with the signal light. Note that the mirror 801 is not essential in the configuration in which the pumping light and the signal light are multiplexed and input to the core 1. That is, this configuration can be applied to the configuration not including the mirror 801 described in FIG. 1 and FIG. 3. Furthermore, the configuration in which the signal light multiplexed with the pumping light is input to the core from the end face 101 can be applied to the configurations of FIG. 5 and FIG. 6 described later.

[0034] Fig. 5 is a diagram showing another example of the configuration of the optical path between the end face 101 and the end face 102. The configuration of Fig. 5 differs from the configuration of Fig. 4 in that it further includes a mirror 802. The mirror 802 is provided between the end face 3A of the core 3 and the end face 2B of the core 2. Both surfaces of the mirror 802 reflect the signal light. The mirror 802 is a reflective film using, for example, a metal thin film. The end face 3A and the end face 2B face each other via the optical collimator 1013 and the optical collimator 1022. The mirror 802 is also disposed so as to form an angle of 45 degrees with respect to the optical axis connecting the end face 3A and the end face 2B.

[0035] With this configuration, the second signal light introduced as collimated light from outside the MCF optical amplifier 1000 can be reflected by the mirror 802 and input to the core 3. The second signal light input to the core 3 propagates through the cores 3 and 4 and is output to the outside by the mirror 801. On the other hand, the first signal light input to the core 1 by the mirror 801 propagates through the cores 1 and 2, is output from the end face 2B of the core 2, and is output to the outside by the mirror 802.

[0036] FIG. 5 shows a configuration example in which mirrors are arranged according to the number of signal lights to be amplified. In the example of FIG. 5, two of the four cores 1-4 are used, so that two signal lights can be amplified using one pumping fiber 100. In addition, by changing the position of the mirror, the length of the cores through which the two signal lights propagate can be changed. In FIG. 5, the first signal light propagates through cores 1 and 2, and the second signal light propagates through cores 3 and 4. However, for example, if the mirror 802 is arranged between the optical collimator 1012 and the optical collimator 1021, the first signal light propagates only through core 1, while the second signal light propagates through cores 2-4. Similarly, when the position of the mirror 801 is changed, the length of the cores through which the multiple signal lights propagate can be changed for each of the multiple signal lights. Therefore, the configuration of FIG. 5 allows the length of the core to be selected according to the amplification factor required for the signal light, so that the multiple signal lights can be amplified under more preferable conditions.

[0037] FIG. 6 is a diagram showing another example of the configuration of the optical path between end face 101 and end face 102. FIG. 6 shows a configuration in which mirror 803 is provided instead of mirror 801 in FIG. 4. Mirror 803 is composed of a reflective film and an optical filter. The reflective film is formed only in a portion necessary for reflecting the signal light input to end face 1A and the signal light output from end face 4B. The other portion of mirror 803 is an optical filter. The optical filter transmits light of the wavelength of the signal light and reflects light of the wavelength of the pump light. The optical filter is composed of, for example, a dielectric multilayer film. The reflective film may also reflect both the signal light and the pump light.

[0038] The mirror 803 is disposed so as to form an angle of 45 degrees with respect to the collimated light that connects the end face 1A and the end face 4B. In this case, the mirror 803 also forms an angle of 45 degrees with respect to the collimated light that connects other cores between the end face 101 and the end face 102. The optical axis of the optical collimator 1012 and the optical axis of the optical collimator 1021 are adjusted so that the signal light is optically coupled by passing through the optical filter of the mirror 803. The same is true for the optical axis of the optical collimator 1013 and the optical axis of the optical collimator 1022, and the optical axis of the optical collimator 1014 and the optical axis of the optical collimator 1023. With this configuration, the mirror 803 reflects the excitation light that is incident from a direction perpendicular to the optical axis that connects the cores between the end face 101 and the end face 102. The excitation light incident on the mirror 803 can be generated by converting light output from a general excitation light source into collimated light having approximately the same diameter as the excitation fiber 100 using an optical collimator.

[0039] The pumping light reflected by the mirror 803 passes between the optical collimators 1011-1014 and enters the clad of the pumping fiber 100 at the end face 101. On the other hand, the optical filter portion of the mirror 803 transmits the signal light, so the signal light propagating in order from core 1 to core 4 is not affected by the mirror 802. As a result, the pumping fiber 100 amplifies the signal light propagating from core 1 to core 4 by clad pumping.

[0040] A part of the pumping light reflected by the mirror 803 is input to the end faces 1A-4A via the optical collimators 1011-1014. The pumping light input to the end faces 1A-4A pumps the cores 1-4 as core pumping light. That is, the configuration of FIG. 6 can simultaneously realize cladding pumping and core pumping of the pumping fiber 100 by one pumping light. When core pumping is not required, an optical filter that blocks the pumping light and transmits the signal light may be formed on the lens of the optical collimator. This allows the pumping light reflected by the mirror 803 and traveling toward the end face 101 to be used only for cladding pumping of the pumping fiber 100.

[0041] Thus, the mirror 803 has a function of inputting pumping light into the clad of the pumping fiber 100, in addition to the function of inputting and outputting the signal light. Therefore, the mirror 803 has the function of the pumping light coupling unit 110 shown in Fig. 1. The mirror 803 may reflect the residual pumping light output from the clad of the end face 102, and extract it to the outside of the MCF optical amplifier 1000. The residual pumping light is pumping light that has not been used for pumping in the pumping fiber 100. The residual pumping light may be used to pump another pumping fiber.

[0042] 5. That is, mirror 803 has a reflective film at a position where the optical path along which the signal light to be reflected propagates as collimated light intersects with mirror 803. The portion of mirror 803 other than the reflective film may be composed of an optical filter that transmits the signal light and reflects the excitation light.

[0043] 6, both the signal light and the pumping light travel in the direction of the end face 101. Therefore, the pumping fiber 100 operates by forward pumping. However, the direction of the pumping light input to the pumping fiber 100 may be opposite to that in FIG. 6. By inputting the pumping light to the end face 102, the pumping fiber 100 can operate by backward pumping.

[0044] 1 to 6, the MCF optical amplifier 1000 using a pumping fiber with four cores has been described. However, the above-mentioned configuration in which the end face 101 and the end face 102 are optically connected can also be applied to a pumping fiber with two or more cores by increasing or decreasing the number of parallel optical collimators.

[0045] 3 to 6 are applicable to the MCF optical amplifier 1000. Therefore, the MCF optical amplifier 1000 having these configurations makes it possible to effectively utilize the core of the MCF without using a pumping fiber using a special MCF.

[0046] (Third embodiment) Hereinafter, as a third embodiment, pumping fibers having various core arrangements that can be applied to the MCF optical amplifier 1000 will be described with reference to Figs. 7 to 14. Any of the pumping fibers shown in Figs. 7 to 14 can be applied to the MCF optical amplifier 1000 in Fig. 1. Therefore, the MCF optical amplifier 1000 to which the following configuration is applied also makes it possible to effectively utilize the MCF cores without using a pumping fiber that uses a special MCF.

[0047] Figures 7 to 14 are diagrams for explaining examples of optical paths of signal light between end faces of pumping fibers having various core arrangements. The configurations explained in Figures 3 to 6 for optically connecting cores can also be applied to the pumping fibers shown in Figures 7 to 14. Note that in the following figures, optical components such as optical collimators and mirrors are omitted.

[0048] Fig. 7 shows the state where the end face 101 and the end face 102 of the excitation fiber 100 having four cores face each other. Fig. 7 again describes the case where the end face 101 and the end face 102 of the excitation fiber 100 described in Fig. 1 face each other. The end faces 101 and 102 in Fig. 7 are views of the end face of the excitation fiber 100 as seen from the outside of the excitation fiber 100, similar to Fig. 1. The four cores are arranged at equal intervals on a circumference centered on the center of the excitation fiber in the cross section of the excitation fiber 100. That is, the positions of adjacent cores of the excitation fiber 100 are all at the positions of cores when the cross section of the excitation fiber 100 is rotated 90 degrees.

[0049] The arrows in FIG. 7 show schematic propagation paths of signal light between cores. Signal light input to end face 1A of core 1 is output from end face 1B of core 1. When end face 101 and end face 102 are opposed to each other, pumping fiber 100 is rotated 90 degrees around the central axis of pumping fiber 100. As a result, end face 1B faces end face 2A. Signal light output from end face 1B is input to core 2 from end face 2A. Similarly, signal light output from end face 2B is input to end face 3A, and signal light output from end face 3B is input to end face 4A. Signal light propagating from core 1 to core 4 is output from end face 4B.

[0050] FIG. 8 is a diagram showing an example of the core arrangement of the pumping fiber 200. The pumping fiber 200 has a core (core 5) along its central axis. Core 5 is not included in the optical path of the signal light propagating through core 1 to core 4. Core 5 may be used alone as an optical amplifier having a length equivalent to that of one pumping fiber 100. For example, core 1 to core 4 may be used to amplify the signal light in the L band, and core 5 may be used to amplify the signal light in the C band. Between the end face 101 and the end face 102, the end face 5A and the end face 5B of the core 5 face each other. Therefore, by providing a mirror that reflects the signal light propagating through the core 5 on the optical path connecting the end face 5A and the end face 5B, the signal light can be input and output to the core 5. This mirror realizes a function similar to that of the mirror 801 in FIG. 4. In addition, when the configuration of FIG. 5 is applied to the pumping fiber 200, the mirror on the optical path connecting the end face 5A and the end face 5B may be provided as a part of the mirror 802.

[0051] In an optical fiber amplifier using an erbium-doped pumping fiber, the amplification efficiency is generally lower in the L band than in the C band. Therefore, in order to make the gain of the L band signal light closer to that of the C band signal light, it is necessary to prepare a separate pumping fiber several times longer than the pumping fiber for the C band and use it as the pumping fiber for the L band. The pumping fiber 200 amplifies the signal light using core 1 to core 4, thereby extending the length of the pumping fiber and improving the amplification efficiency. Therefore, the pumping fiber 200 can amplify the signal light in the C band and the signal light in the L band using a single pumping fiber, and can reduce the difference in pumping efficiency between the C band and the L band. That is, the MCF optical amplifier 1000 using the pumping fiber 200 can miniaturize the optical amplifier. In the following FIG. 9 and subsequent figures, the core arranged at the center of the pumping fiber can be used to amplify the signal light in the C band. Moreover, the core used to amplify the C band does not have to be the central core.

[0052] 9 is a diagram showing an example of the core arrangement of the pumping fiber 300. The pumping fiber 300 has six cores (core 1 to core 6) on the circumference of a circle centered on the center of the pumping fiber 300, and has a core 7 along the central axis of the pumping fiber 300. Cores 1 to 6 are arranged at equal intervals on the circumference centered on the center of the pumping fiber 300. The pumping fiber 300 is rotated 120 degrees to the left when looking at the end face 302. Therefore, for example, the end face 3A of the core 3 and the end face 1B of the core 1 face each other. Therefore, the first signal light input to the end face 1A of the core 1 at the end face 301 and output from the end face 1B is input to the end face 3A of the core 3. Then, the first signal light propagates through the cores 3 and 5 and is output from the end face 5B of the core 5.

[0053] On the other hand, the second signal light inputted to the end face 2A of the core 2 at the end face 301 is outputted from the end face 2B at the end face 302 and inputted to the end face 4A of the core 4. This light propagates through the cores 4 and 6 and is outputted from the end face 6B of the core 6. In this way, the pumping fiber 300 divides the cores into two groups (cores 1, 3, 5 and cores 2, 4, 6) and can amplify different signal lights for each group. Moreover, the core 7 is not included in the optical path of the light propagating through the cores 1-6. As with the pumping fiber 300 in FIG. 8, the core 7 may be used alone as an optical amplifier having the length of one pumping fiber 300. For example, the cores 1, 3, 5 and the cores 2, 4, 6 may be used to amplify different L-band signal lights, and the core 7 may be used to amplify C-band signal light. In the pumping fiber 300 as well, the signal light in the C band and the signal light in the L band can be amplified by a single pumping fiber, and the difference in pumping efficiency between the C band and the L band can be reduced.

[0054] FIG. 10 is a diagram showing an example of a core arrangement on an end face 401 of the pumping fiber 400. In FIG. 10 and FIG. 11, the description of the core arrangement on the other end face of the pumping fiber is omitted. The pumping fiber 400 has 12 cores (core 1 to core 12). Core 1 to core 4 are on a circumference centered on the center of the pumping fiber 400 and are arranged at equal intervals on the circumference. Core 5 to core 8 are arranged at equal intervals on the same circumference as core 1 to core 4. The positions of core 1 to core 8 do not overlap. Core 9 to core 12 are arranged at equal intervals on a circumference of a circle with a smaller radius than core 1 to core 8. Therefore, by applying a 90-degree rotation to the pumping fiber 400, the first signal light input to one end face of core 1 can be propagated through core 1 to core 4 and output from the other end face of core 4, similar to the pumping fibers 100 to 300. Similarly, the second signal light input to the end face of core 5 can be propagated from core 5 to core 8 and output from the end face of core 8. Furthermore, the third signal light input to the end face of core 9 can be propagated from core 9 to core 12 and output from the end face of core 12. In this way, by dividing the 12 cores into three groups of four cores, the pumping fiber 400 can independently amplify the first to third signal lights.

[0055] FIG. 11 is a diagram showing an example of a core arrangement on an end face 501 of a pumping fiber 500. The pumping fiber 500 has 19 cores (core 1 to core 19). Core 1 to core 6 are on a circumference centered on the center of the pumping fiber 500 and are arranged at equal intervals on the circumference. Core 6 to core 12 are arranged at equal intervals on the same circumference as core 1 to core 6. The positions of core 1 to core 12 do not overlap. Core 13 to core 18 are arranged at equal intervals on a circumference of a circle whose radius is smaller than that of core 1 to core 8. By applying a 60-degree rotation to the pumping fiber 500, the first signal light inputted to core 1 from one end face of the pumping fiber 500 can be propagated through core 1 to core 6 and outputted from core 6 at the other end face of the pumping fiber 500. Similarly, the second signal light inputted to core 7 can be propagated through core 7 to core 12 and outputted from core 12. Furthermore, the third signal light input to the core 13 can be propagated through the core 13 and the core 18 and output from the core 16 .

[0056] In this way, the pumping fiber 500 can amplify the first to third signal lights independently by dividing the 18 cores into three groups of six cores each. Also, in the pumping fiber 500, the first to third signal lights each propagate through six cores (i.e., core 1 to core 6, core 7 to core 12, and core 13 to core 18). Therefore, compared to the pumping fiber 400, the pumping fiber 500 has a greater effect of extending the length of the cores for amplification.

[0057] The pumping fiber 500 has a core 19 at its center. The core 19 is not included in the optical path of the light propagating through the cores 1-18. The core 19 may be used alone as an optical amplifier having a length equivalent to that of the pumping fiber 500. For example, the cores 1-18 may be used to amplify the signal light in the L band, and the core 19 may be used to amplify the signal light in the C band.

[0058] FIG. 12 is a diagram for explaining an example of the core arrangement of the pumping fiber 600. The pumping fiber 600 includes a core 1 to a core 4. The cores 1 to 4 of the pumping fiber 600 are arranged at equal intervals on a straight line passing through the cross section of the pumping fiber 600. In the example of FIG. 13, the cores 1 to 4 are arranged at equal intervals on a straight line passing through the center of the pumping fiber 600, at positions point-symmetric with respect to the center of the pumping fiber 600. By rotating the end face 602 of such a pumping fiber 600 by 180 degrees to face the end face 601, the first signal light input to the end face 1A of the core 1 can be propagated through the cores 1 and 4 and output from the end face 4B of the core 4. Furthermore, the second signal light input to the end face 2A of the core 2 can be propagated through the cores 2 and 3 and output from the end face 3B of the core 3.

[0059] Fig. 13 is a diagram for explaining another opposing example of the excitation fiber 600. For ease of understanding, in this figure, an end face 601 shows the core arrangement as seen from the outside of the excitation fiber 600, and an end face 602 shows the core arrangement as seen from a virtual inside of the excitation fiber 600. Also, in Fig. 13, in order to easily show the positions of the end faces of the cores 1-4 of the end faces 601 and 602, the end face 602 is drawn shifted downward.

[0060] 13, when end face 601 and end face 602 of excitation fiber 600 are opposed to each other, the lateral position of end face 602 is shifted by one core spacing in the lateral direction with respect to end face 101. That is, in a state in which end face 601 and end face 602 are opposed to each other, end face 1B and end face 2A, end face 2B and end face 3A, and end face 3B and end face 4A are opposed to each other, respectively.

[0061] Signal light is input to end face 1A of core 1. The input signal light is input from end face 1B to end face 2A and propagates through core 2. The signal light propagates from core 2 to core 4 and is output from end face 4B of core 4. According to the configuration in Fig. 13, end face 1A of the core that inputs the signal light and end face 4B of the core that outputs the signal light do not face each other, so there is no need to provide mirror 801 in Fig. 4 for inputting and outputting the signal light, and the configuration of MCF optical amplifier 1000 is simplified.

[0062] FIG. 14 is a diagram for explaining an example of opposing excitation fibers 700. The excitation fiber 700 includes a core 1-core 16. The cores 1-16 of the excitation fiber 700 are arranged in 4 rows and 4 columns on the cross section of the excitation fiber 700. The row intervals are all equal, and the column intervals are also all equal. In this figure, the end face 701 of the excitation fiber 700 shows the core arrangement as seen from the outside of the excitation fiber 700, and the end face 702 shows the core arrangement as seen virtually from the inside of the excitation fiber 700. In addition, in FIG. 14, the end face 702 is shifted downward in order to clearly show the positions of the end faces of the cores 1-16 of the end faces 701 and 702. Note that the reference numerals of the end faces of the cores 5-core 16 are omitted in the drawing.

[0063] End surface 701 and end surface 702 are disposed such that end surface 1B-end surface 3B and end surface 2A-end surface 4A face each other in a one-to-one relationship. Similarly, end surface 5B-end surface 7B and end surface 6A-end surface 8A also face each other in a one-to-one relationship, end surface 9B-end surface 11B and end surface 10A-end surface 12A also face each other in a one-to-one relationship, and end surface 13B-end surface 15B and end surface 14A-end surface 16A also face each other in a one-to-one relationship.

[0064] A first signal light is input to the end face 1A of the core 1. The first signal light is input from the end face 1B to the end face 2A and propagates through the core 2. The first signal light then propagates from the core 2 to the core 4 and is output from the end face 4B of the core 4. The second signal light input to the core 5 is input from the end face 5B to the end face 6A and propagates through the core 6. The second signal light propagates from the core 6 to the core 8 and is output from the end face 8B of the core 8. Similarly, the third signal light input to the core 9 propagates through the cores 9-12 and is output from the end face 12B of the core 12. The fourth signal light input to the core 13 propagates through the cores 13-16 and is output from the end face 16B of the core 16. In the configuration of FIG. 14 using the pumping fiber 700, the end faces of the cores that input and output the signal light are not opposed to each other, as in FIG. 13. This makes it easier to configure an optical path with the outside for inputting and outputting the signal light. Moreover, the pumping fiber 700 can simultaneously amplify four signal lights independently.

[0065] In addition, the embodiment of the present disclosure can be described as follows, but is not limited to the following.

[0066] (Appendix 1) a multi-core pumping fiber having a first core and a second core; A cladding pumping means for injecting pumping light into the cladding of the multi-core pumping fiber; A multi-core fiber optical amplifier comprising: A signal light input to one end of the first core is output from the other end of the first core, The signal light output from the other end of the first core is input to one end of the second core; a multi-core fiber optical amplifier configured to output the signal light input to one end of the second core from the other end of the second core.

[0067] (Appendix 2) the first core and the second core are arranged at equal intervals on a circumference centered on the center of the end face of the multi-core pumping fiber, 2. The multi-core fiber optical amplifier according to claim 1, wherein the other end of the multi-core pumping fiber is held in a state rotated by a predetermined angle with respect to one end of the multi-core pumping fiber, thereby optically coupling the other end of the first core and the other end of the second core.

[0068] (Appendix 3) the multi-core pumping fiber comprises a third core; The signal light output from the other end of the second core is input to one end of the third core; The signal light input to one end of the third core is output from the other end of the third core. 1. A multicore fiber optical amplifier according to claim 1 or 2.

[0069] (Appendix 4) 3. The multi-core fiber optical amplifier according to claim 1, wherein the other end of the first core and the other end of the second core are optically coupled by misaligning a center of one end of the multi-core pumping fiber and a center of the other end of the multi-core pumping fiber.

[0070] (Appendix 5) 5. The multi-core fiber optical amplifier according to claim 1, wherein the other end of the first core and the one end of the second core are optically coupled by opposing optical collimators.

[0071] (Appendix 6) 6. The multi-core fiber optical amplifier according to claim 5, further comprising a mirror between the opposing optical collimators, the mirror reflecting the signal light to couple the signal light with the first core.

[0072] (Appendix 7) 7. The multi-core fiber optical amplifier of claim 6, wherein the mirror couples the pumping light with the first core by reflecting the pumping light.

[0073] (Appendix 8) The cladding pumping means comprises: 8. The multi-core fiber optical amplifier according to claim 6 or 7, further comprising a wavelength filter disposed between the opposing optical collimators, and guiding the pumping light input from outside the multi-core pumping fiber to a clad of the multi-core pumping fiber.

[0074] (Appendix 9) The wavelength filter is transmitting light having a wavelength of the signal light from the other end of the first core to the other end of the second core; 10. A multicore fiber optical amplifier according to claim 8.

[0075] (Appendix 10) 10. The multi-core fiber optical amplifier according to claim 8 or 9, wherein the wavelength filter is formed in a part of the mirror.

[0076] (Appendix 11) Multi-core pumping fiber having a first core and a second core An optical amplification method for use in a multi-core fiber optical amplifier comprising: injecting pump light into the cladding of the multi-core pump fiber; A signal light input to one end of the first core is output from the other end of the first core, The signal light output from the other end of the first core is input to one end of the second core; The signal light input to one end of the second core is output from the other end of the second core.

[0077] (Appendix 12) The first core and the second core are arranged at equal intervals on a circumference centered on the center of the end face of the multi-core pumping fiber, Rotating the other end of the multi-core pumping fiber by a predefined angle with respect to the one end of the multi-core pumping fiber; the other end of the first core and the other end of the second core are optically coupled to each other. 12. An optical amplification method according to claim 11.

[0078] (Appendix 13) The signal light output from the other end of the second core is input to one end of a third core of the multi-core pumping fiber; The signal light input to one end of the third core is output from the other end of the third core. 13. An optical amplification method according to claim 11 or 12.

[0079] (Appendix 14) A center of one end of the multi-core pumping fiber is offset from a center of the other end of the multi-core pumping fiber; the other end of the first core and the other end of the second core are optically coupled to each other. 13. An optical amplification method according to claim 11 or 12.

[0080] (Appendix 15) 15. The optical amplification method according to any one of claims 11 to 14, wherein the other end of the first core and the one end of the second core are optically coupled by opposing optical collimators.

[0081] (Appendix 16) 16. The optical amplification method according to claim 15, wherein the signal light is reflected by a mirror provided between the opposing optical collimators to couple the signal light with the first core.

[0082] (Appendix 17) 17. The optical amplification method according to claim 16, wherein the pumping light is reflected by the mirror to couple the first core with the pumping light.

[0083] (Appendix 18) 18. The optical amplification method according to claim 16, further comprising guiding the pumping light input from outside the multi-core pumping fiber to a cladding of the multi-core pumping fiber by a wavelength filter disposed between the opposing optical collimators.

[0084] (Appendix 19) The wavelength filter Transmitting light of the wavelength of the signal light; The signal light is propagated from the other end of the first core to the other end of the second core. 19. An optical amplification method according to claim 18.

[0085] (Appendix 20) 20. The optical amplification method according to claim 18 or 19, wherein the wavelength filter is formed as part of the mirror.

[0086] Although the present disclosure has been described above with reference to the embodiments, the present disclosure is not limited to the above embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present disclosure within the scope of the present disclosure. For example, the multi-core fiber optical amplifier described in each embodiment also discloses an optical amplification method applicable to the multi-core fiber optical amplifier.

[0087] In addition, the configurations described in the respective embodiments are not necessarily mutually exclusive. The actions and effects of the present disclosure may be achieved by a configuration that combines all or part of the above-described embodiments. [Explanation of symbols]

[0088] 1-18 Core 1A-12A end face 1B-16B End face 100-700 Excitation Fiber 101, 102, 201, 202, 301, 302 End face 401, 501, 601, 602, 701, 702 End face 110 Excitation light coupling section 801-803 Mirror 1000 MCF Optical Amplifier 1011-1014, 1021-1024 Optical collimators

Claims

1. a multi-core pumping fiber including a first core and a second core; A cladding pumping means for injecting pumping light into the cladding of the multi-core pumping fiber; A multi-core fiber optical amplifier comprising: A signal light input to one end of the first core is output from the other end of the first core, The signal light output from the other end of the first core is input to one end of the second core; a multi-core fiber optical amplifier configured to output the signal light input to one end of the second core from the other end of the second core.

2. the first core and the second core are arranged at equal intervals on a circumference centered on the center of the end face of the multi-core pumping fiber, 2. The multi-core fiber optical amplifier according to claim 1, wherein the other end of the multi-core pumping fiber is held in a state rotated by a predetermined angle with respect to one end of the multi-core pumping fiber, thereby optically coupling the other end of the first core and one end of the second core.

3. the multi-core pump fiber comprises a third core; The signal light output from the other end of the second core is input to one end of the third core; The signal light input to one end of the third core is output from the other end of the third core.

3. A multi-core fiber optical amplifier according to claim 1 or 2.

4. 3. The multi-core fiber optical amplifier according to claim 1, wherein the other end of the first core and the other end of the second core are optically coupled by misaligning a center of one end of the multi-core pumping fiber and a center of the other end of the multi-core pumping fiber.

5. 3. The multi-core fiber optical amplifier according to claim 1, wherein the other end of the first core and the one end of the second core are optically coupled by opposing optical collimators.

6. The multi-core fiber optical amplifier according to claim 5 , further comprising a mirror disposed between the opposing optical collimators, the mirror reflecting the signal light to couple the signal light with the first core.

7. The multi-core fiber optical amplifier according to claim 6 , wherein the mirror couples the pumping light with the first core by reflecting the pumping light.

8. The cladding pumping means comprises:

7. The multi-core fiber optical amplifier according to claim 6, further comprising a wavelength filter disposed between the opposing optical collimators, the wavelength filter guiding the pumping light input from outside the multi-core pumping fiber to a clad of the multi-core pumping fiber.

9. The wavelength filter is transmitting light having a wavelength of the signal light from the other end of the first core to the other end of the second core; 9. A multi-core fiber optical amplifier according to claim 8.

10. An optical amplification method for use in a multi-core fiber optical amplifier including a multi-core pumping fiber including a first core and a second core, comprising: injecting pump light into the cladding of the multi-core pump fiber; A signal light input to one end of the first core is output from the other end of the first core, The signal light output from the other end of the first core is input to one end of the second core; The signal light input to one end of the second core is output from the other end of the second core.

Images