Optical amplifier

Abstract

This optical amplifier comprises: a multicore optical fiber (1) provided with a plurality of cores; an amplification optical fiber (2) that is provided with a plurality of cores to which rare earth elements are added, and that amplifies light transmitted through the multicore optical fiber (1); an excitation light source (5) that outputs excitation light for exciting the rare earth elements; an excitation optical coupler (6) that causes the excitation light to be incident on cores the number of which is smaller than the total number of cores of the multicore optical fiber (1); and a scrambler (7) that diffuses the excitation light incident on the multicore optical fiber (1) to all the cores included in the multicore optical fiber (1).

Description

Optical amplifier

[0001] The present disclosure relates to an optical amplifier that amplifies light transmitted through a multi-core optical fiber.

[0002] A spatial multiplexing transmission system has been proposed that can improve spatial efficiency by using multiple cores in a multi-core optical fiber as channels. In the spatial multiplexing transmission system, an optical amplifier using a coupled multi-core optical fiber is used to amplify attenuated light.

[0003] In such optical amplifiers, it is necessary to inject pump light as evenly as possible into each core in order to reduce the gain variation between spatial channels. Non-Patent Document 1 discloses that the gain variation between spatial channels can be reduced by injecting pump light with controlled intensity into all cores.

[0004] NKFontaine et al.,”Coupled-Core Optical Amplifier”,Proc. OFC,Th5D.3,2017.

[0005] However, the technology disclosed in Non-Patent Document 1 requires that pump light be injected into all cores, resulting in a complex device configuration. Branching pump light emitted from a single pump light source results in increased loss, resulting in a deterioration in amplification efficiency. Furthermore, when the transmission path is a coupled multi-core optical fiber, random mode coupling occurs in signal light in the C band (a wavelength band of 1.530 to 1.565 μm). On the other hand, since the pump light has a short wavelength (e.g., 0.98 μm), a sufficient amount of optical coupling cannot be obtained. This results in a large gain deviation between spatial channels.

[0006] The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide an optical amplifier that can input pump light to all cores included in a multi-core optical fiber with a simple configuration and reduce gain deviation between spatial channels.

[0007] An optical amplifier according to one aspect of the present disclosure includes: a multi-core optical fiber having a plurality of cores; an amplification optical fiber having a plurality of cores doped with a rare earth and amplifying light transmitted through the multi-core optical fiber; an excitation light source that outputs excitation light that excites the rare earth; an excitation light coupler that inputs the excitation light to a number of cores that is less than the total number of cores of the multi-core optical fiber; and a scrambler that diffuses the excitation light input to the multi-core optical fiber to all cores included in the multi-core optical fiber.

[0008] According to the present disclosure, it is possible to input pump light to all cores included in a multi-core optical fiber with a simple configuration and reduce the gain deviation between spatial channels.

[0009] FIG. 1 is an explanatory diagram schematically showing the configuration of an optical amplifier according to an embodiment. FIG. 2 is a cross-sectional view of an amplification optical fiber. FIG. 3 is an explanatory diagram showing the inter-core distance and refractive index of a two-core amplification optical fiber. FIG. 4 is a graph showing the relationship between the core radius a, the relative refractive index difference Δ, and the coupling length Lc (m) when the inter-core distance D is 25 μm. FIG. 5 is a graph showing the relationship between the core radius a, the relative refractive index difference Δ, and the coupling length Lc (m) when the inter-core distance D is 35 μm. FIG. 6A(a) is a cross-sectional view of a multi-core optical fiber before TEC treatment, and FIG. 6B(b) is a cross-sectional view of a multi-core optical fiber after TEC treatment, and FIG. 6B(b) is a graph showing the relative refractive index difference Δ. FIG. 7 is a graph showing the relationship between the core radius, the relative refractive index difference Δ, and the optical coupling amount in a scrambler according to a first embodiment. FIG. 8 is an explanatory diagram showing the configuration of a scrambler according to a second embodiment. Fig. 9 is a graph showing the relationship between the fiber diameter reduction ratio (D2 / D1) and the amount of optical coupling in the scrambler according to Example 2. Fig. 10 is an explanatory diagram showing a schematic configuration of an optical amplifier using the scrambler according to Example 3. Fig. 11 is a graph showing the relationship between the fiber length and the amount of optical coupling in the scrambler according to Example 4.

[0010] Hereinafter, an embodiment will be described with reference to the drawings. Fig. 1 is a configuration diagram of an optical amplifier 100 according to an embodiment. As shown in Fig. 1, the optical amplifier 100 includes a multi-core optical fiber 1 (hereinafter abbreviated as "MCF 1"), a multi-core optical fiber 2 for coupled amplification (hereinafter abbreviated as "amplification optical fiber 2"), isolators 3 and 4, a pumping light source 5, a pumping light coupler 6, and a scrambler 7.

[0011] The MCF1 is an optical fiber in which multiple cores are arranged in one cladding.

[0012] The amplification optical fiber 2 is an optical fiber having multiple cores arranged in one cladding, similar to the MCF 1 described above, and each core is further doped with a rare earth ion. The rare earth ion is, for example, an erbium ion (Er). The number of cores in the amplification optical fiber 2 may be the same as the number of cores in the MCF 1. The amplification optical fiber 2 and the MCF 1 are connected by fusion splicing or the like at connection portions 8A and 8B. That is, the amplification optical fiber 2 has multiple cores doped with rare earth elements and amplifies light transmitted through the MCF 1.

[0013] The isolators 3 and 4 restrict the light transmission direction to one direction and block the light in the opposite direction. Specifically, the isolator 3 restricts the light transmitted through the MCF 1 to the direction of the scrambler 7, and the isolator 4 restricts the light amplified by the amplification optical fiber 2 to the downstream direction.

[0014] The pumping light source 5 emits pumping light. The pumping light is light that pumps the rare earth ions doped in the core of the amplification optical fiber 2. For example, when the rare earth ions are erbium ions, the pumping light source 5 emits light with a wavelength in the 980 nm band. In other words, the pumping light source 5 outputs pumping light that excites the rare earth elements.

[0015] The pumping light coupler 6 inputs pumping light into one of the multiple cores arranged in the MCF 1. A side-coupled optical fiber coupler, a spatial optical element, a PLC (Planar Lightwave Circuit), or the like can be used as the pumping light coupler 6. The pumping light coupler 6 is not limited to one core, and for example, if the MCF 1 has N cores, the pumping light may be input into fewer than N cores (cores equal to or less than "N-1"). In other words, the pumping light coupler 6 inputs pumping light into fewer cores than the total number of cores in the MCF 1.

[0016] The scrambler 7 diffuses the pumping light emitted from the pumping light source 5 and incident on one core of the MCF 1 to the other cores of the MCF 1. For example, if the MCF 1 has two cores (first and second cores), the scrambler 7 diffuses the pumping light incident on the first core to the second core. Also, if the MCF 1 has four cores (first to fourth cores), the scrambler 7 diffuses the pumping light incident on the first core to the second to fourth cores. In other words, by using the scrambler 7, the pumping light emitted from the pumping light source 5 can be uniformly incident on the multiple cores (e.g., four cores) of the MCF 1. In other words, the scrambler 7 diffuses the pumping light incident on the MCF 1 to all of the cores included in the MCF 1.

[0017] The principle of amplification by the amplification optical fiber 2 will be described below. First, the relationship between the coupling of light propagating through the core C of the amplification optical fiber 2 and the parameters of the core C will be described. Fig. 2 is a cross-sectional view of the amplification optical fiber 2 when cut in the longitudinal direction. Fig. 2 shows an example in which the amplification optical fiber 2 has two cores C1 and C2. As shown in Fig. 2, the amplification optical fiber 2 has two cores C1 and C2, a first cladding 12 surrounding each of the cores C1 and C2, and a second cladding 13 surrounding the first cladding 12. Each of the cores C1 and C2 is doped with rare earth ions such as erbium ions.

[0018] 2 shows an example in which the number of cores in the amplification optical fiber 2 is two, but the number of cores is not limited to two. Furthermore, each of the cores C1 and C2 of the amplification optical fiber 2 may propagate propagation light of a plurality of propagation modes.

[0019] The amplification optical fiber 2 may further include a coating (not shown) that contacts the second cladding 13 and surrounds the second cladding 13. In this embodiment, a case will be described in which the amplification optical fiber 2 is a step index (SI) type in which the core radius of the core C is a and the relative refractive index difference is Δ.

[0020] In the amplification optical fiber 2, when adjacent cores C are sufficiently far apart and do not interact with each other, the signal light, which is the propagating light in each core C, propagates in its own eigenmode. On the other hand, as the distance between adjacent cores C becomes closer, the electric field distribution leaking out from each core C has an effect, causing a transition of power between the cores C.

[0021] 3, the refractive index of the cores C1 and C2 is n1, the refractive index of the first cladding 12 is n0, and the distance between the centers of the cores C1 and C2 is the inter-core distance D. In this case, when the two cores C1 and C2 have the same structure and the propagation constants of the fundamental modes propagating through each core C1 and C2 are the same, the electric field components coupled from the core C1 to the core C2 are in phase. This phase relationship is maintained over a long distance, and power transition occurs between the cores C1 and C2.

[0022] The above describes the case of the same core structure and the same propagation constant, but in a core structure with different propagation constants, a mismatch in the electric field components occurs between adjacent cores C as the light propagates, resulting in a partial power transfer. In the two core structures, we will consider light propagating in the z direction through core C1 and core C2.

[0023] The optical power of the forward wave after propagating through the core C1 and the core C2 by the position z can be expressed by the following equations (1) and (2), respectively.

[0024]

[0025]

[0026] where A(z) and B(z) are the electric field amplitude coefficients in the core C1 and the core C2 at the position z, respectively, and κ is the mode coupling coefficient between the two cores C1 and C2.

[0027] When light is incident on only one of the two cores, the optical power in the core C1 and the core C2 changes periodically according to the change in the position z. The relative intensity |B(z) / A(0)| shown in equation (2) ２ The maximum value of is (2κ / δβ) ２ and the optical power is transferred between the two cores C1 and C2 for each coupling length Lc.

[0028] The coupling length Lc between the core 11 and the adjacent core 11 is given by the following equation (3).

[0029] Lc=π / (δβ) (3) In particular, when the adjacent cores C1 and C2 have the same core structure and the propagation constants of the propagation modes are the same, the coupling length Lc can be expressed by the following equation (4).

[0030] Lc=π / (2κ) (4) In order to generate sufficient mode coupling between adjacent cores C in a coupled-type amplification optical fiber 2, the length of the amplification optical fiber 2 used must be equal to or greater than the coupling length Lc.

[0031] An example of calculation of the relationship between the core parameters, core radius a and relative refractive index difference Δ, and the coupling coefficient κ and coupling length Lc is shown below. The coupling coefficient κ of the fundamental mode propagating through the two parallel cores 11 is expressed by the following equation (5).

[0032]

[0033] The u and w shown in equation (5) are expressed by the following equations (6) and (7).

[0034]

[0035]

[0036] In equations (5) to (7), v is the V value, K is the Bessel function, k is the wave number, and β is the propagation constant of the propagation mode. The coupling length Lc was calculated when the core radius a, relative refractive index difference Δ, and inter-core distance D were set to 4.0 to 8.0 μm, 0.4 to 0.8%, 25, and 35 μm, respectively.

[0037] 4 is a graph showing the value of the coupling length Lc (m) as a function of the core radius a and the relative refractive index difference Δ when D=25 μm. In FIG. 4, the curve L405 shows the value for Lc=5 m. Similarly, L410 has Lc=10m, L415 has Lc=15m, L420 has Lc=20m, L425 has Lc=25m, L430 has Lc=30m, L435 has Lc=35m, L440 has Lc=40m, L445 has Lc=45m, L450 has Lc=50m, L455 has Lc=55m, L460 has Lc=60m, L465 has Lc=65m, L470 has Lc=70m, L475 has Lc=75m, L480 has Lc=80m, L485 has Lc=85m, L490 has Lc=90m, and L495 has Lc=95m.

[0038] 4, the coupling length Lc tends to increase in regions with a higher relative refractive index difference Δ and a smaller core radius a. In an amplification optical fiber 2 doped with erbium ions (Er), which is generally used for C-band optical amplification, the length used is approximately several tens of meters. Therefore, it can be seen that sufficient mode coupling occurs in the amplification optical fiber 2 by using optimal fiber parameters.

[0039] Figure 5 is a graph showing the coupling length Lc as a function of the core radius a and the relative refractive index difference Δ when the inter-core distance D is 35 μm. In Figure 5, L510 indicates the value where Lc = 100 m. Similarly, L520 indicates the value where Lc = 200 m, L530 indicates the value where Lc = 300 m, L540 indicates the value where Lc = 400 m, L550 indicates the value where Lc = 500 m, L560 indicates the value where Lc = 600 m, L570 indicates the value where Lc = 700 m, L580 indicates the value where Lc = 800 m, and L590 indicates the value where Lc = 900 m.

[0040] 5 with FIG. 4 , it can be seen that the required coupling length Lc increases over the entire region due to the increased inter-core distance D, and that an inter-core distance D of approximately 35 μm is not suitable as a coupled-type amplifying multi-core optical fiber. Therefore, it is preferable that the inter-core distance D is less than 35 μm. As shown above, it is possible to design the amplifying optical fiber 2 by optimally adjusting the parameters of the multi-core optical fiber, such as the inter-core distance D, the core radius a, and the relative refractive index difference Δ.

[0041] Hereinafter, embodiments (first to third embodiments) showing specific configurations of the scrambler 7 will be described.

[0042] First Embodiment In the first embodiment, a TEC fiber obtained by subjecting a dopant-doped multi-core optical fiber to a TEC (Thermally-Diffused Expanded Core) treatment is used as the scrambler 7. A TEC fiber is an optical fiber in which the diameter of a dopant-doped multi-core fiber is locally thermally diffused to approximately two to three times its original size. The dopant is, for example, germanium (Ge). Note that the dopant is not limited to germanium, and any dopant may be used as long as it increases the refractive index when added to the core and has the property of being thermally diffused. In other words, the scrambler 7 according to the first embodiment includes a TEC fiber in which the core diameter of an MCF 1 is locally expanded.

[0043] The refractive index profile can be locally changed by thermally diffusing the dopant added to the core of an optical fiber. Thermal diffusion expands the core diameter and diffuses the dopant, reducing the relative refractive index difference Δ.

[0044] 6A(a) is a cross-sectional view of a two-core multi-core optical fiber (before TEC treatment), (b) is a graph of the relative refractive index difference Δ(%), and Fig. 6B(a) is a cross-sectional view of a multi-core optical fiber (TEC fiber) after its diameter has been enlarged by TEC treatment, (b) is a graph of the relative refractive index difference Δ(%).

[0045] 6A and 6B, the core diameter is increased by the TEC treatment. Specifically, the core diameter of cores 22A and 22B after the TEC treatment is increased and the relative refractive index difference Δ is reduced compared to cores 21A and 21B before the TEC treatment.

[0046] The TEC fiber cannot be applied to a multi-core optical fiber with a pure silica core structure in which no dopant is added to the core. For this reason, a multi-core optical fiber doped with a dopant such as germanium is used as the scrambler 7. In addition, by connecting multiple TEC fibers in series, the coupling of pump light can be promoted.

[0047] 7 is a graph showing the calculation results of the core radius, relative refractive index difference Δ, and inter-core optical coupling (dB) for a two-core TEC fiber. The core pitch was 16 μm, the coupling length was 0.1 m, and the wavelength of the propagating light was 0.98 μm. The optical coupling amount increases as the core radius increases and the relative refractive index difference Δ decreases. By performing TEC processing until the region R1 in the figure is reached, it becomes possible to divide the pump light equally (-3 dB).

[0048] That is, the optical coupling amount is "-3 dB = 10 －０．３ = 0.5", and the pumping light incident on one core is coupled to the other core at a ratio of 0.5. Note that the pumping light does not necessarily have to be equally divided depending on the amount of coupled signal light.

[0049] The pumping light uniformly diffused into each core is incident on each core of the amplifying optical fiber 2 shown in FIG.

[0050] As described above, the optical amplifier 100 according to this embodiment includes a multi-core optical fiber 1 (MCF1) having a plurality of cores, an amplification optical fiber 2 having a plurality of cores doped with rare earth elements and amplifying light transmitted through the MCF1, an excitation light source 5 that outputs excitation light for exciting the rare earth elements, an excitation light coupler 6 that inputs the excitation light to a number of cores that is less than the total number of cores in the MCF1, and a scrambler 7 that diffuses the excitation light input to the MCF1 to all of the cores included in the MCF1.

[0051] Moreover, in the first embodiment, a TEC fiber obtained by applying TEC treatment to a dopant-added multi-core optical fiber is used as the scrambler 7. By applying the TEC treatment so that the optical coupling amount between the cores becomes 3 dB, it is possible to uniformly diffuse the pumping light in a two-core multi-core fiber.

[0052] Therefore, there is no need to input pumping light to all cores of the multicore fiber, and the configuration can be simplified. Since the pumping light is uniformly input to each core of the amplification optical fiber 2, it is possible to reduce the gain deviation between the spatial channels of each core. In addition, it is possible to improve the light transmission efficiency and reduce power consumption. Since there is no need to split the pumping light emitted from the pumping light source 5 using a power coupler, loss can be suppressed.

[0053] In the above-described first embodiment, an example has been described in which the number of cores in the MCF 1 is two, but even when the number of cores is three or more, a TEC fiber can be adopted as the scrambler 7. That is, TEC processing is applied to a multi-core optical fiber having three or more cores, and pump light is incident on one of these cores from the pumping light source 5. Then, by adjusting the thermal diffusion by the TEC processing so that the pump light is diffused evenly to all the cores, the pump light can be diffused evenly to each core.

[0054] Second Example Next, a second example of the scrambler 7 will be described. In the second example, a portion of the MCF 1 is melt-stretched, and the stretched portion of the MCF 1 is used as the scrambler 7. Melt-stretching refers to heating a portion of the MCF 1 and stretching it by applying tension to both ends. By melt-stretching a portion of the MCF 1, it is possible to reduce the diameter of the entire MCF 1 while maintaining the cross-sectional dimension ratio of the MCF 1. In other words, the scrambler 7 according to the second example includes a stretched portion formed by stretching a portion of the MCF 1.

[0055] 8(a) shows a cross-sectional view of a two-core MCF 1 before stretching, i.e., before its diameter is reduced, and FIG. 8(b) shows a cross-sectional view (cross-sectional view of the stretched portion) of the two-core MCF 1 after stretching, i.e., after its diameter is reduced. The diameter of the MCF 1 before stretching is D1, and the diameter of the MCF 1 after stretching is D2. The cores 23A and 23B of the MCF 1 before stretching and the cores 24A and 24B of the MCF 1 after stretching have the same dimensional ratio. Reducing the diameter of the MCF 1 promotes coupling of pump light between the two cores 24A and 24B.

[0056] 9 is a graph showing the calculated results of the fiber diameter reduction ratio (D2 / D1) and the amount of optical coupling to adjacent cores when the pitch of each core 23A, 23B of the MCF1 is 16 μm, the extension length is 0.05 m, and the wavelength of the propagating light is 0.98 μm. It can be seen that with the MCF1 configured as described above, when the reduction ratio (D2 / D1) is 0.86, the amount of optical coupling is 3 dB, i.e., the intensity of the pump light can be equally divided. That is, by melt-drawing the MCF1 and appropriately setting the fiber diameter reduction ratio (D2 / D1) (0.86 in the example of FIG. 9), it is possible to equally divide the pump light incident on one core and diffuse it to the other core.

[0057] The pumping light uniformly diffused into each core is incident on each core of the amplifying optical fiber 2 shown in FIG.

[0058] As described above, in the second embodiment, the melt-stretched MCF 1 is used as the scrambler 7. That is, the scrambler 7 for uniformly diffusing pump light to each core can be formed by the extremely simple operation of melt-stretching a portion of the MCF 1. Therefore, as in the first embodiment described above, it is not necessary to input pump light to all of the cores of the MCF 1, and the configuration can be simplified.

[0059] Since the pumping light is uniformly incident on each core of the amplification optical fiber 2, it is possible to reduce the gain deviation between the spatial channels of each core. In addition, it is possible to improve the light transmission efficiency and reduce power consumption. Since it is not necessary to split the pumping light emitted from the pumping light source 5 using a power coupler, it is possible to suppress loss.

[0060] Furthermore, when the diameter of MCF1 is reduced, on the other hand, as the diameter is reduced further, losses begin to occur not only in the coupling of the pump light but also in the signal light wavelength. Therefore, it is desirable to extend the MCF1 within a range where no loss of the signal light occurs (for example, within 1 cm).

[0061] Third Example Next, a third example of the scrambler 7 will be described. Fig. 10 is an explanatory diagram showing the configuration of an optical amplifier 100 using the scrambler 7 according to the third example. In the third example, a transmission multi-core optical fiber 31 not doped with rare earth ions is used as the scrambler 7. In addition, in the third example, the length of the multi-core optical fiber 31 is adjusted to an appropriate length, so that pumping light incident on one core is diffused to the other core. That is, in the third example, the multi-core optical fiber 31 not doped with rare earth ions is installed between the isolator 3 and the amplification optical fiber 2 and used as the scrambler 7.

[0062] The multi-core optical fiber 31 may be the same optical fiber as the MCF 1 or may be a different optical fiber.

[0063] That is, the scrambler 7 according to the third embodiment is disposed between the isolator 3 that regulates the light transmission direction and the amplification optical fiber 2, and includes a multi-core optical fiber 31 whose length is adjusted.

[0064] 7 is a graph showing the relationship between the fiber length and the optical coupling amount when pumping light having a wavelength of 0.98 μm is incident on one core of the above-mentioned multi-core optical fiber 31 (two cores). The pitch of each core of the multi-core optical fiber 31 is 16 μm, the core radius of each core is 5.0 μm, and the relative refractive index difference Δ is 0.4%.

[0065] 7, the optical coupling amount increases monotonically as the fiber length of the multi-core optical fiber 31 increases. It can be seen that the optical coupling amount is "-3 dB" when the fiber length is about 700 m, i.e., the intensity of the pumping light can be equally divided. Therefore, by installing the multi-core optical fiber 31 with a length of 700 m between the isolator 3 and the amplification optical fiber 2, it becomes possible to equally divide the pumping light incident on one core and diffuse it to the other core.

[0066] The pumping light uniformly diffused into each core is incident on each core of the amplifying optical fiber 2 shown in FIG.

[0067] As described above, in the third embodiment, the scrambler 7 can be formed by an extremely simple method of adjusting the length of the multi-core optical fiber 31 for transmission that is not doped with rare earth ions. Therefore, as in the first and second embodiments described above, it is not necessary to input pump light to all the cores of the MCF 1, and the configuration can be simplified.

[0068] Since the pumping light is uniformly incident on each core of the amplification optical fiber 2, it is possible to reduce the gain deviation between the spatial channels of each core. In addition, it is possible to improve the light transmission efficiency and reduce power consumption. Since it is not necessary to split the pumping light emitted from the pumping light source 5 using a power coupler, it is possible to suppress loss.

[0069] Furthermore, a multi-core fiber with high coupling may be used as the scrambler 7 shown in Fig. 7. In addition, the amount of mode coupling can be promoted by providing a long-period grating, bending, or twisting, and these can be used alone or in combination with the conventional methods.

[0070] In the above-described embodiments, an example has been described in which the scrambler 7 is installed between the pumping light coupler 6 and the amplification optical fiber 2, but the scrambler 7 may also be installed between the pumping light source 5 and the pumping light coupler 6. Furthermore, the scrambler 7 may also be incorporated into the amplification optical fiber 2.

[0071] In the above-described optical amplifier 100, an example has been described in which the pumping light output from the pumping light source 5 is incident on one of the multiple cores included in the MCF 1, but a configuration in which the pumping light is incident on a number of cores that is fewer than the number of cores in the MCF 1 may also be used. That is, if the number of cores is N, the pumping light may be incident on (N-1) cores. By incidenting the pumping light on (N-1) cores, it is possible to reduce loss compared to when the pumping light is incident on all N cores.

[0072] The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the present disclosure.

[0073] REFERENCE SIGNS LIST 1 Multi-core optical fiber (MCF) 2 Amplifying optical fiber (multi-core optical fiber for coupled amplification) 3, 4 Isolator 5 Pumping light source 6 Pumping light coupler 7 Scrambler 8A, 8B Connection section 31 Multi-core optical fiber 100 Optical amplifier

Claims

1. An optical amplifier comprising: a multi-core optical fiber having a plurality of cores; an amplifying optical fiber having a plurality of cores doped with a rare earth and amplifying light transmitted through the multi-core optical fiber; a pumping light source outputting pumping light for exciting the rare earth; a pumping light coupler for inputting the pumping light into cores that are fewer than the total number of cores of the multi-core optical fiber; and a scrambler for diffusing the pumping light input to the multi-core optical fiber to all cores included in the multi-core optical fiber.

2. The optical amplifier according to claim 1, wherein the scrambler includes a TEC (Thermally-diffused Expanded Core) fiber in which the core diameter of a multi-core optical fiber is locally expanded.

3. The optical amplifier according to claim 1, wherein the scrambler includes an extension portion formed by extending a portion of a multi-core optical fiber.

4. The optical amplifier according to claim 1, further comprising an isolator between the multi-core optical fiber and the amplification optical fiber for regulating the direction of light transmission, and the scrambler is disposed between the isolator and the amplification optical fiber and includes a multi-core optical fiber with an adjusted length.

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