Multi-core optical amplifying fiber and optical amplifier

Abstract

To reduce the difference in gain among a plurality of core sections.SOLUTION: A multi-core optical amplifying fiber includes a plurality of core portions doped with a rare earth element, an inner cladding portion surrounding the plurality of core portions and having a refractive index lower than the maximum refractive index of each core portion, and an outer cladding portion surrounding the inner cladding portion and having a refractive index lower than the refractive index of the inner cladding portion. The plurality of core portions include a first core portion having a first distance from a central axis of the inner cladding portion extending along a longitudinal direction, and a second core portion having a second distance from the central axis that is greater than the first distance. When exciting light of predetermined power for optically exciting the rare earth element is caused to propagate through the inner cladding and signal light optically amplified by the rare earth element is caused to propagate through each of the first core portion and the second core portion, the difference between a first gain obtained by the signal light in the first core portion and a second gain obtained by the signal light in the second core portion is less than 0.9 dB.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a multi-core optical amplifying fiber and an optical amplifier. [Background technology]

[0002] For example, in applications such as undersea optical communications, it is expected that the power consumption of optical amplifiers can be reduced by using multi-core EDFAs (Erbium-Doped Optical Fiber Amplifiers) as optical amplifiers.

[0003] As a multi-core EDFA, a configuration is known in which a double-clad multi-core EDF is used to optically pump erbium (Er), which is an optical amplification medium contained in the core, by a cladding pumping method (see Non-Patent Documents 1 and 2). The multi-core EDF is a type of multi-core optical amplifying fiber. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Kazi S Abedin et al, “Multimode Erbium Doped Fiber Amplifiers for Space Division Multiplexing Systems”, JOURNAL OF LIGHTWAVE TECHNOLOGY,VOL.32,NO.16,AUGUST 15,2014 pp.2800-2808. [Non-Patent Document 2] Kazi S Abedin et al, “Clading-pumped erbium-doped multicore fiber amplifier”, OPTICS EXPRESS Vol.20,No.18 27 August 2012 pp.20191-20200. Summary of the Invention [Problem to be solved by the invention]

[0005] In a multi-core EDFA, signal light is input to each of the multiple cores separately and amplified. In this case, it is preferable that the difference in gain given to the signal light between the cores is small. If the difference in gain between the cores is large, power variation occurs between the signal lights, and the signal transmission distance and signal transmission quality also vary.

[0006] The present invention has been made in view of the above, and has an object to provide a multi-core optical amplifying fiber capable of reducing the gain difference among a plurality of core portions, and an optical amplifier using the same. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is a multi-core optically amplifying fiber comprising: a plurality of core portions doped with a rare earth element; an inner cladding portion surrounding the plurality of core portions and having a refractive index lower than the maximum refractive index of each of the core portions; and an outer cladding portion surrounding the inner cladding portion and having a refractive index lower than the refractive index of the inner cladding portion, wherein the plurality of core portions include a first core portion having a first distance from a central axis of the inner cladding portion extending along a longitudinal direction, and a second core portion having a second distance from the central axis that is larger than the first distance, wherein when pumping light of a predetermined power that optically pumps the rare earth element is propagated through the inner cladding portion and signal light optically amplified by the rare earth element is propagated through each of the first core portion and the second core portion, a difference between a first gain obtained by the signal light in the first core portion and a second gain obtained by the signal light in the second core portion is less than 0.9 dB.

[0008] A first refractive index, which is the refractive index of the inner cladding portion in the vicinity of the first core portion, may be higher than a second refractive index, which is the refractive index of the inner cladding portion in the vicinity of the second core portion.

[0009] The refractive index of a central region of the inner cladding portion, the central region including the central axis, may be higher than the refractive index of a peripheral region of the inner cladding portion surrounding the central region.

[0010] The inner cladding may have a GI refractive index profile.

[0011] The inner cladding may have a stepped refractive index profile.

[0012] The cross-sectional shape of the inner cladding portion in a plane perpendicular to the longitudinal direction may be circular, polygonal, or a shape consisting of arcs and straight lines.

[0013] A first mode field diameter, which is a mode field diameter of the first core portion at the wavelength of the signal light, may be larger than a second mode field diameter, which is a mode field diameter of the second core portion at the wavelength of the signal light.

[0014] The multiple cores may be arranged to have n-fold rotational symmetry within the inner cladding, where n is an integer of 2 or more.

[0015] The inner cladding portion may have a plurality of layered cladding portions formed concentrically, and one or more of the plurality of core portions may be disposed in each of the plurality of layered cladding portions.

[0016] The mode field diameter at the wavelength of the signal light of the core portion arranged in a first layered cladding portion among the plurality of layered cladding portions may be greater than the mode field diameter at the wavelength of the signal light of the core portion arranged in a second layered cladding portion adjacent to the outer circumferential side of the first layered cladding portion by 10% to 20%.

[0017] One aspect of the present invention is an optical amplifier including the multi-core optical amplifying fiber, an excitation light source that outputs excitation light that optically excites the rare earth element in the multi-core optical amplifying fiber, and an optical coupler that optically couples the excitation light to the inner cladding portion. Effect of the Invention

[0018] According to the present invention, it is possible to realize a multi-core optical amplifying fiber capable of reducing the gain difference among a plurality of cores, and an optical amplifier using the same. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to a first embodiment. [Diagram 2] FIG. 2 is a schematic diagram showing a refractive index profile of the multi-core optical amplifying fiber shown in FIG. [Diagram 3] FIG. 3 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to the second embodiment. [Figure 4] FIG. 4 is a schematic diagram showing a refractive index profile of the multi-core optical amplifying fiber shown in FIG. [Diagram 5] FIG. 5 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to the third embodiment. [Figure 6] FIG. 6 is a schematic diagram showing a refractive index profile of the multi-core optical amplifying fiber shown in FIG. [Figure 7] FIG. 7 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to the fourth embodiment. [Figure 8] FIG. 8 is a schematic cross-sectional view of an optical amplifier according to the fifth embodiment. [Figure 9] FIG. 9 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to a sixth embodiment. [Figure 10] FIG. 10 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to the seventh embodiment. [Figure 11] FIG. 11 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to the eighth embodiment. [Figure 12] FIG. 12 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to the ninth embodiment. [Figure 13]FIG. 13 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to a tenth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, an embodiment will be described with reference to the drawings. Note that the present invention is not limited to this embodiment. In addition, in the description of the drawings, the same or corresponding elements are appropriately given the same reference numerals, and duplicated descriptions are appropriately omitted. It should be noted that the drawings are schematic, and the dimensional relationship of each element, the ratio of each element, etc. may differ from reality. There may be parts in which the dimensional relationship and ratio differ between the drawings. In addition, in this specification, the cutoff wavelength means the cable cutoff wavelength defined in ITU-T (International Telecommunication Union) G.650.1. In addition, other terms not specifically defined in this specification shall follow the definitions and measurement methods in G.650.1 and G.650.2.

[0021] (Embodiment 1) 1 is a schematic cross-sectional view of a multi-core light amplifying fiber according to embodiment 1, showing a cross section perpendicular to the longitudinal direction of the multi-core light amplifying fiber. The multi-core light amplifying fiber 10 is a double-clad, seven-core multi-core optical fiber including seven cores 11, an inner cladding 12 surrounding the cores 11, and an outer cladding 13 surrounding the inner cladding 12. The cores 11 are an example of a plurality of cores.

[0022] The core portions 11 are made of glass such as silica glass. The core portions 11 are arranged in a triangular lattice pattern that realizes a closest-packed state. One of the core portions 11, a core portion 11a, is arranged so that its central axis coincides with a central axis X extending along the longitudinal direction in the inner cladding portion 12. The central axis X of the inner cladding portion 12 can be defined as the center of gravity of the inner cladding portion 12. Six of the core portions 11b are arranged so as to be located at the corner positions of a regular hexagon around the central axis X. In this case, the core portions 11 are arranged so as to have n-fold rotational symmetry within the inner cladding portion, where n is 2, 3, and 6.

[0023] The core section 11 contains, for example, germanium (Ge) or aluminum (Al) as a refractive index adjusting dopant that increases the refractive index. The core section 11 also contains Er as a rare earth element that is an amplification medium. Er is doped at a concentration such that the absorption coefficient peak is 2.5 dB / m to 11 dB / m around a wavelength of 1530 nm. The doping concentration is, for example, 250 ppm to 2000 ppm. However, the absorption coefficient and the doping concentration are not particularly limited. Note that Al also has a function of suppressing concentration quenching of Er.

[0024] The inner cladding portion 12 is made of glass such as silica glass. The inner cladding portion 12 has layered cladding portions 12a and 12b as a plurality of layered cladding portions formed concentrically. The layered cladding portion 12a is a portion having a circular cross section whose central axis is substantially the same as the central axis X. The layered cladding portion 12b is a portion having an annular cross section that surrounds the outer periphery of the layered cladding portion 12a. The layered cladding portion 12a surrounds the core portion 11a, and the layered cladding portion 12b surrounds the core portion 11b. That is, one or more core portions of the core portion 11 are disposed in each of the layered cladding portions 12a and 12b.

[0025] The outer cladding portion 13 is made of, for example, a resin. The resin in question is, for example, a known resin used as a resin cladding portion of a double-cladding type optical fiber.

[0026] Fig. 2 is a schematic diagram showing a refractive index profile in the AA cross section of the multi-core optical amplifying fiber 10. In Fig. 2, profiles P11a and P11b show the refractive index profiles of the cores 11a and 11b, respectively. Profiles P12a and P12b show the refractive index profiles of the layered cladding portions 12a and 12b of the inner cladding portion 12, respectively. Profile P13 shows the refractive index profile of the outer cladding portion 13.

[0027] As shown in FIG. 2, the maximum refractive index of each of the cores 11 is n1. The refractive index profile of the cores 11 is a step index (SI) type or a graded index (GI) type. The inner cladding 12 has a refractive index lower than the maximum refractive index of each of the cores 11. The inner cladding 12 has a GI type refractive index profile in which the refractive index is high on the central axis X and decreases toward the periphery in the radial direction. A dopant such as fluorine is distributed in the glass constituting the inner cladding 12 so as to obtain a GI type refractive index profile. As a result, the refractive index of the layered cladding 12a (central region) shown by the profile P12a is higher than the refractive index of the layered cladding 12b (peripheral region) shown by the profile P12b. Furthermore, the refractive index (an example of a first refractive index) n21 of the inner cladding portion 12 (layered cladding portion 12a) in the vicinity of the core portion 11a is higher than the refractive index (an example of a second refractive index) n22 of the inner cladding portion 12 (layered cladding portion 12b) in the vicinity of the core portion 11b. Note that the vicinity of the core portion refers to a range away from the outer edge of the core portion by about several μm, for example. Moreover, the outer cladding portion 13 has a refractive index n3 lower than the refractive index of the inner cladding portion 12.

[0028] If the relative refractive index difference of the maximum refractive index of the core portion 11a or 11b with respect to the inner cladding portion 12 is core Δ, in this embodiment, the core Δ of the core portions 11a and 11b is, for example, 0.35% to 2% at a wavelength of 1550 nm. However, as shown in FIG. 2, in this embodiment, the core Δ of the core portion 11b is larger than the core Δ of the core portion 11a. It is preferable that the core diameter of each of the core portions 11a and 11b is set in relation to the core Δ so as to realize a cutoff wavelength shorter than the optical amplification wavelength band in which the rare earth element can amplify light. This allows each of the core portions 11a and 11b to propagate signal light in the optical amplification wavelength band in single mode. The optical amplification wavelength band in which the rare earth element can amplify light is, for example, 1530 nm to 1565 nm, which is called the C band, or 1565 nm to 1625 nm, which is called the L band, in the case of Er. At a wavelength at which Er amplifies light, for example 1550 nm, the mode field diameter of each of the cores 11a and 11b is, for example, 5 μm or more and 11 μm or less.

[0029] Of the core portions 11, one core portion 11a which is a first core portion has a first distance d11 from the central axis X of the inner cladding portion 12 (i.e., the distance from the central axis X to the central axis of the core portion 11a). As described above, the first distance d11 is zero. On the other hand, six core portions 11b which are second core portions have a second distance d12 from the central axis X of the inner cladding portion 12. The distances of the core portions 11b from the central axis X are approximately equal. The second distance d12 is equal to the inter-core distance in the core portion 11, and is, for example, 38.5 μm.

[0030] Here, in order to investigate the gain characteristics of each core of a multi-core optical amplifying fiber, the present inventor fabricated a test multi-core optical amplifying fiber which is a double-clad type and seven-core type multi-core optical amplifying fiber like the multi-core optical amplifying fiber 10, and in which the refractive index of the inner cladding is uniform from the central axis in the radial direction. Then, pumping light of a predetermined power (e.g., a wavelength of 975 nm and a power of 17 W) for optically exciting a rare earth element was propagated through the inner cladding, and signal light (e.g., a wavelength of 1550 nm and a power of −5.0 dBm) optically amplified by a rare earth element was propagated through each of the cores. Then, it was discovered that the gain obtained by the signal light in the core close to the central axis of the inner cladding was lower than the gain obtained by the signal in the core far from the central axis of the inner cladding (i.e., the gain difference was large).

[0031] In contrast, in the multi-core optical amplifying fiber 10, the inner cladding 12 has a GI type refractive index profile such that the refractive index is high at and near the central axis X and decreases toward the periphery in the radial direction. As a result, compared to the case of a test multi-core optical amplifying fiber, the pumping light propagating through the inner cladding 12 is distributed such that the power density increases closer to the central axis X. As a result, the difference between the gain in the core 11a closer to the central axis X (an example of a first gain) and the gain in the core 11b farther from the central axis X (an example of a second gain) becomes smaller.

[0032] Furthermore, in the multi-core optical amplifier fiber 10, the core 11a closer to the central axis X has a smaller relative refractive index difference with respect to the inner cladding 12 than the core 11b. As a result, the core 11a has a larger mode field diameter than the core 11b. As a result, the gain difference between the gain in the core 11a and the gain in the core 11b is also reduced by the effect of the difference in mode field diameter.

[0033] As described above, according to the multi-core optical amplifying fiber 10 according to the first embodiment, the difference between the first gain and the second gain is small. For example, the difference is less than 0.9 dB when the pump light has a wavelength of 975 nm and a power of 17 W, the signal light has a wavelength of 1550 nm and a power of -5.0 dBm, and a gain of 18 dB is obtained in each core. However, the difference in gain is more preferably 0.5 dB or less, and further preferably 0.3 dB or less.

[0034] (Embodiment 2) Fig. 3 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to embodiment 2, showing a cross section perpendicular to the longitudinal direction of the multi-core optical amplifying fiber. The multi-core optical amplifying fiber 20 is a double-clad type and seven-core type multi-core optical fiber having a configuration in which the inner cladding portion 12 in the multi-core optical amplifying fiber 10 shown in Fig. 1 is replaced with an inner cladding portion 22.

[0035] The inner cladding portion 22 is made of glass such as silica glass. The inner cladding portion 22 has layered cladding portions 22a and 22b as a plurality of layered cladding portions formed concentrically. The layered cladding portion 22a is a portion having a circular cross section whose central axis is substantially the same as the central axis X. The layered cladding portion 22b is a portion having an annular cross section that surrounds the outer periphery of the layered cladding portion 22a. The layered cladding portion 22a surrounds the core portion 11a, and the layered cladding portion 22b surrounds the core portion 11b. That is, one or more core portions of the core portion 11 are disposed in each of the layered cladding portions 22a and 22b.

[0036] Fig. 4 is a schematic diagram showing a refractive index profile in a BB line cross section of the multi-core optical amplifying fiber 20. In Fig. 4, profiles P22a and P22b show the refractive index profiles of the layered cladding portions 22a and 22b of the inner cladding portion 22, respectively.

[0037] As shown in FIG. 4, the inner cladding portion 22 has a refractive index lower than the maximum refractive index of each core portion 11. The inner cladding portion 22 has a stepped refractive index profile in which the refractive index of the layered cladding portion 22a including the central axis X is approximately uniform and relatively high at n23, and the refractive index of the layered cladding portion 22b is approximately uniform and relatively low at n24. In this case, the layered cladding portion 22a is made of, for example, pure silica glass. The layered cladding portion 22b is made of, for example, silica glass to which a dopant such as fluorine is added so as to have a stepped refractive index profile. The relative refractive index difference of the layered cladding portion 22b with respect to the layered cladding portion 22a is, for example, -1.2% or more and -0.7% or less. As a result, the refractive index of the layered cladding portion 22a (central region) is higher than the refractive index of the layered cladding portion 22b (peripheral region). Furthermore, the refractive index (an example of a first refractive index) n23 of the inner cladding portion 22 (layered cladding portion 22a) in the vicinity of the core portion 11a is higher than the refractive index (an example of a second refractive index) n24 of the inner cladding portion 22 (layered cladding portion 22b) in the vicinity of the core portion 11b.

[0038] The core Δ of the core portion 11a or 11b relative to the inner cladding portion 22 is also, for example, 0.35% to 2% at a wavelength of 1550 nm.

[0039] Also in the multi-core optical amplifying fiber 20, the distance from the central axis X of the core 11a is the first distance d11, which is zero as described above. On the other hand, the distance from the central axis X of the core 11b is the second distance d12, which is, for example, 38.5 μm.

[0040] As described above, in the multi-core optical amplifying fiber 20, the inner cladding 22 has a stepped refractive index profile such that the refractive index is high in the central region including the central axis X and low in the radially peripheral region. In this case as well, compared with the case of the test multi-core optical amplifying fiber, the pumping light propagating through the inner cladding 22 is distributed such that the power density increases closer to the central axis X. As a result, the difference between the gain in the core 11a closer to the central axis X and the gain in the core 11b farther from the central axis X becomes smaller.

[0041] Furthermore, in the multi-core optical amplifying fiber 20, the core 11a closer to the central axis X has a larger mode field diameter than the core 11b. As a result, the gain difference between the core 11a and the core 11b is also reduced by the effect of the difference in mode field diameter.

[0042] As described above, according to the multi-core optical amplifying fiber 20 according to the second embodiment, the difference between the first gain and the second gain is small. For example, the difference is less than 0.9 dB when the pump light has a wavelength of 975 nm and a power of 17 W, the signal light has a wavelength of 1550 nm and a power of -5.0 dBm, and a gain of 18 dB is obtained in each core. However, the difference in gain is more preferably 0.5 dB or less, and further preferably 0.3 dB or less.

[0043] (Embodiment 3) Fig. 5 is a schematic cross-sectional view of a multi-core light amplifying fiber according to embodiment 3, showing a cross section perpendicular to the longitudinal direction of the multi-core light amplifying fiber. The multi-core light amplifying fiber 30 is a double-clad 19-core multi-core optical fiber having a configuration in which the number of cores 11 in the multi-core light amplifying fiber 20 shown in Fig. 3 is increased to 19, and the inner cladding 22 is replaced with an inner cladding 32.

[0044] The core portions 11 are arranged in a triangular lattice pattern that realizes a closest-packed state. One core portion 11a among the core portions 11 is arranged so that its central axis coincides with a central axis X that extends along the longitudinal direction of the inner cladding portion 32. Six core portions 11c among the core portions 11 are arranged so as to be positioned at the corners of a regular hexagon around the central axis X. Six core portions 11d among the core portions 11 are arranged so as to be positioned at the corners of a regular hexagon around the central axis X. Six core portions 11e among the core portions 11 are arranged so as to be positioned at the corners of a regular hexagon around the central axis X.

[0045] The inner cladding portion 32 is made of glass such as silica glass. The inner cladding portion 32 has layered cladding portions 32a, 32b, and 32c as a plurality of layered cladding portions formed concentrically. The layered cladding portion 32a is a portion having a circular cross section whose central axis is substantially the same as the central axis X. The layered cladding portion 32b is a portion having a circular cross section surrounding the outer periphery of the layered cladding portion 32a. The layered cladding portion 32c is a portion having a circular cross section surrounding the outer periphery of the layered cladding portion 32b. The layered cladding portion 32a surrounds the core portion 11a, the layered cladding portion 32b surrounds the core portion 11c, and the layered cladding portion 32c surrounds the core portions 11d and 11e. That is, one or more core portions of the core portion 11 are arranged in each of the layered cladding portions 32a, 32b, and 32c.

[0046] Fig. 6 is a schematic diagram showing a refractive index profile in a CC line cross section of the multi-core optical amplifying fiber 30. In Fig. 6, profiles P11a, P11c, and P11e show the refractive index profiles of the core portions 11a, 11c, and 11e, respectively. Profiles P32a, P32b, and P32c show the refractive index profiles of the layered cladding portions 32a, 32b, and 32c of the inner cladding portion 32, respectively.

[0047] As shown in FIG. 6, the maximum refractive index of each of the cores 11 is n1. The inner cladding 32 has a refractive index lower than the maximum refractive index of each of the cores 11. The inner cladding 32 has a stepped refractive index profile in which the layered cladding 32a including the central axis X has a relatively high refractive index of approximately uniform n25, the layered cladding 32b has a relatively high refractive index of approximately uniform n26 lower than n25, and the layered cladding 32c has a relatively high refractive index of approximately uniform n27 lower than n26. In this case, the layered cladding 32a is made of, for example, pure silica glass. The layered cladding 32b and 32c are made of, for example, silica glass to which a dopant such as fluorine is added so as to have a stepped refractive index profile. The relative refractive index difference of the layered cladding 32c with respect to the layered cladding 32a is, for example, -1.2% or more and -0.7% or less. As a result, the refractive index of the layered cladding portion 32a (central region) is higher than the refractive indexes of the layered cladding portions 32b and 32c (peripheral regions). Furthermore, the refractive index (an example of a first refractive index) n25 of the inner cladding portion 32 (layered cladding portion 32a) in the vicinity of the core portion 11a is higher than the refractive index (an example of a second refractive index) n26 of the inner cladding portion 32 (layered cladding portion 32b) in the vicinity of the core portion 11b. Moreover, the refractive index (an example of a first refractive index) n26 of the inner cladding portion 32 (layered cladding portion 32b) in the vicinity of the core portion 11b is higher than the refractive index (an example of a second refractive index) n27 of the inner cladding portion 32 (layered cladding portion 32c) in the vicinity of the core portion 11c. Furthermore, the refractive index (an example of a first refractive index) n25 of the inner cladding portion 32 (layered cladding portion 32a) in the vicinity of the core portion 11a is higher than the refractive index (an example of a second refractive index) n27 of the inner cladding portion 32 (layered cladding portion 32c) in the vicinity of the core portion 11c.

[0048] Also in the multi-core optical amplifying fiber 30, among the cores 11, one core 11a has a distance d11 from the central axis X of the inner cladding 32. As described above, d11 is zero. Moreover, the six cores 11c have a distance d32 from the central axis X of the inner cladding 12. The distances from the central axis X of the cores 11c are approximately equal. Moreover, the six cores 11d have a distance d33 from the central axis X of the inner cladding 32. The distances from the central axis X of the cores 11d are approximately equal. Moreover, the six cores 11e have a distance d34 from the central axis X of the inner cladding 32. The distances from the central axis X of the cores 11e are approximately equal. d32 is equal to the inter-core distance in the cores 11, for example, 38.5 μm. Here, if any one of d11, d32, d33, and d34 is taken as an example of a first distance, any distance that is larger (farther) than the first distance is an example of a second distance relative to the first distance.

[0049] As described above, in the multi-core optical amplifying fiber 30, the inner cladding portion 32 has a step-type refractive index profile such that the refractive index is high in the central portion including the central axis X and is low in the peripheral portion in the radial direction. In this case as well, compared with the case of the test multi-core optical amplifying fiber, the pumping light propagating through the inner cladding portion 32 is distributed such that the power density is higher closer to the central axis X. As a result, the difference between the gain (an example of the first gain) in the core portion 11 (for example, the core portion 11a) closer to the central axis X among the core portions 11 and the gain (an example of the second gain) in the core portion 11 (for example, the core portion 11c) farther from the central axis X becomes small. Furthermore, the difference between the gain (an example of the first gain) in the core portion 11 (for example, the core portion 11a) closer to the central axis X among the core portions 11 and the gain (an example of the second gain) in the core portion 11 (for example, the core portion 11e) farther from the central axis X becomes small.

[0050] Furthermore, also in the multi-core optical amplifying fiber 30, a core portion closer to the central axis X (for example, core portion 11a) has a larger mode field diameter than a core portion farther from the central axis X (for example, core portion 11e). As a result, the gain difference between the gain in core portion 11a and the gain in core portion 11e is also reduced by the effect of the difference in mode field diameter.

[0051] As described above, according to the multi-core optical amplifying fiber 30 according to the third embodiment, the difference between the first gain and the second gain is small. For example, the difference is less than 0.9 dB when the pump light has a wavelength of 975 nm and a power of 17 W, the signal light has a wavelength of 1550 nm and a power of -5.0 dBm, and a gain of 18 dB is obtained in each core. However, the difference in gain is more preferably 0.5 dB or less, and further preferably 0.3 dB or less.

[0052] In this embodiment, the cores 11c and 11e at different distances from the central axis X are arranged in the same layered clad portion 32c, but the cores 11c and 11e may be arranged in different layered clad portions. In this case, the refractive index of the layered clad portion of the core 11e at a greater distance from the central axis X may be lower than the refractive index of the layered clad portion of the core 11d at a closer distance from the central axis X.

[0053] (Embodiment 4) Fig. 7 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to embodiment 4, showing a cross section perpendicular to the longitudinal direction of the multi-core optical amplifying fiber. The multi-core optical amplifying fiber 40 is a double-clad, 19-core multi-core optical fiber having a configuration in which the number of cores 11 in the multi-core optical amplifying fiber 10 shown in Fig. 1 is increased to 19, and the inner cladding 12 is replaced with an inner cladding 42.

[0054] The core portions 11 are arranged in a triangular lattice pattern that realizes a closest-packed state. One core portion 11f among the core portions 11 is arranged so that its central axis coincides with a central axis X that extends along the longitudinal direction of the inner cladding portion 42. Six core portions 11g among the core portions 11 are arranged so as to be positioned at the corners of a regular hexagon around the central axis X. Six core portions 11h among the core portions 11 are arranged so as to be positioned at the corners of a regular hexagon around the central axis X. Six core portions 11i among the core portions 11 are arranged so as to be positioned at the corners of a regular hexagon around the central axis X.

[0055] Also in the multi-core optical amplifying fiber 40, among the cores 11, one core 11f has a distance d41 from the central axis X of the inner cladding 42. As described above, d41 is zero. On the other hand, the six cores 11g have a distance d42 from the central axis X of the inner cladding 42. The distances from the central axis X of the cores 11g are approximately equal. Moreover, the six cores 11h have a distance d43 from the central axis X of the inner cladding 42. The distances from the central axis X of the cores 11h are approximately equal. Moreover, the six cores 11i have a distance d44 from the central axis X of the inner cladding 42. The distances from the central axis X of the cores 11i are approximately equal. d42 is equal to the inter-core distance in the cores 11, and is, for example, 38.5 μm. Here, if any one of d11, d42, d43, and d44 is taken as an example of a first distance, any distance that is larger (farther) than the first distance is an example of a second distance relative to the first distance.

[0056] Moreover, the core portion 11f has the largest core diameter among the core portions 11, and also has the largest mode field diameter at the wavelength of the signal light. The six core portions 11g, which are the second core portions, have the second largest core diameter among the core portions 11, and also have the second largest mode field diameter at the wavelength of the signal light. The twelve core portions 11h and 11i have the third largest core diameter among the core portions 11, and also have the third largest mode field diameter at the wavelength of the signal light. Here, if any of the core portions 11 is an example of a first core portion, the mode field diameter of the first core portion is the first mode field diameter. Moreover, a core portion that is farther away from the central axis X than the first core portion is an example of a second core portion, and the mode field diameter of the second core portion is an example of a second mode field diameter.

[0057] The inner cladding portion 42 is made of glass such as silica glass. The inner cladding portion 42 has layered cladding portions 42a, 42b, and 42c as a plurality of layered cladding portions formed concentrically. The layered cladding portion 42a is a portion having a circular cross section whose central axis is substantially the same as the central axis X. The layered cladding portion 42b is a portion having a circular cross section that surrounds the outer periphery of the layered cladding portion 42a. The layered cladding portion 42c is a portion having a circular cross section that surrounds the outer periphery of the layered cladding portion 42b. The layered cladding portion 42a surrounds the core portion 11f, the layered cladding portion 42b surrounds the core portion 11g, and the layered cladding portion 42c surrounds the core portions 11h and 11i. That is, one or more core portions of the core portion 11 are arranged in each of the layered cladding portions 42a, 42b, and 42c.

[0058] In the inner cladding 42, the layered cladding portions 42a, 42b, 42c all have the same and uniform refractive index.

[0059] As described above, in a test multi-core optical amplifying fiber in which the refractive index of the inner cladding portion is uniform in the radial direction from the central axis, the gain obtained by the signal light in the core portion close to the central axis of the inner cladding is lower than the gain obtained by the signal light in the core portion farther from the central axis of the inner cladding (i.e., the gain difference is larger).

[0060] However, in the multi-core optical amplifying fiber 40, the mode field diameter is largest in the core portion 11f closer to the central axis X, and the mode field diameter decreases as the distance from the central axis increases. As a result, the gain difference between the gain in the core portion 11f and the gain in, for example, the core portion 11i becomes smaller due to the effect of the difference in the mode field diameter. As a result, the difference between the gain in the core portion 11a closer to the central axis X (first gain) and the gain in, for example, the core portion 11f farther from the central axis X (second gain) becomes smaller.

[0061] Here, the mode field diameter of the core portion arranged in the first layered cladding portion among the plurality of layered cladding portions 42a, 42b, and 42c at the wavelength of the signal light may be 10% to 20% larger than the mode field diameter of the core portion arranged in the second layered cladding portion adjacent to the outer periphery side of the first layered cladding portion at the wavelength of the signal light. That is, for example, if the layered cladding portion 42a is the first layered cladding portion, the second layered cladding portion is the layered cladding portion 42b. In this case, the mode field diameter of the core portion 11f arranged in the layered cladding portion 42a at the wavelength of the signal light is 10% to 20% larger than the mode field diameter of the core portion 11g arranged in the layered cladding portion 42b at the wavelength of the signal light. This makes it possible to preferably exert the effect of reducing the difference in gain due to the difference in mode field diameter.

[0062] As described above, according to the multi-core optical amplifying fiber 40 according to the fourth embodiment, the difference between the first gain and the second gain is small. For example, the difference is less than 0.9 dB when the pump light has a wavelength of 975 nm and a power of 17 W, the signal light has a wavelength of 1550 nm and a power of -5.0 dBm, and a gain of 18 dB is obtained in each core. However, the difference in gain is more preferably 0.5 dB or less, and further preferably 0.3 dB or less.

[0063] In this embodiment, the core diameter and the mode field diameter of the core portions 11h and 11i are equal to each other; however, the core diameter and the mode field diameter of the core portion 11h that is closer to the central axis X may be larger than those of the core portion 11i that is farther away from the central axis X, and may be smaller than those of the core portion 11i that is farther away from the central axis X.

[0064] (Embodiment 5) 8 is a schematic configuration diagram of an optical amplifier according to embodiment 5. The optical amplifier 100 includes seven optical isolators 1, an optical fiber fan-in (FAN IN) 2, a pumping light source 3, an optical coupler 4, the multi-core optical amplifying fiber 10 according to embodiment 1, a pump stripper 5, an optical fiber fan-out (FAN OUT) 6, and seven optical isolators 7. In the figure, the symbol "x" indicates a fusion splicing point of the optical fibers.

[0065] The optical fiber fan-in 2 includes seven single-mode optical fibers bundled at one end and one multi-core fiber having seven cores. Each core of the bundled side of the seven single-mode optical fibers is optically coupled to each core of the multi-core fiber. In this multi-core fiber, the cores are arranged in a triangular lattice pattern similar to the core 11 of the multi-core optical amplifier fiber 10 shown in FIG. 1. The seven single-mode optical fibers are standard single-mode optical fibers defined in, for example, ITU-T (International Telecommunication Union) G.652, and each is provided with an optical isolator 1. The optical isolators 1 and 7 allow light to pass in the direction indicated by the arrow and block the passage of light in the opposite direction. Each core of the multi-core fiber of the optical fiber fan-in 2 is connected to each core of the multi-core fiber 4a of the optical coupler 4. In this multi-core fiber 4a, the cores are arranged in a triangular lattice pattern similar to the core 11 of the multi-core optical amplifier fiber 10 shown in FIG. 1. In the optical fiber fan-in, the end faces of the bundled seven single-mode optical fibers and the end face of the multi-core fiber that optically couples are processed at an angle to the optical axis to suppress reflection, but they may also be perpendicular to the optical axis.

[0066] When signal light is input to each single mode optical fiber of the optical fiber fan-in 2, each optical isolator 1 passes each signal light. Each core of the multi-core fiber propagates each signal light and outputs the signal light to the connected core of the multi-core fiber 4a. The core of the multi-core fiber 4a propagates the input signal light.

[0067] The pumping light source 3 outputs pumping light. The wavelength of the pumping light is 975 nm, which is approximately the same as the wavelength of the absorption peak of erbium ions in the 900 nm wavelength band. Therefore, the pumping light can optically pump erbium ions. The optical coupling optical fibers 3a and 4b propagate the pumping light output from the pumping light source. As described above, the pumping light propagates through the optical coupling optical fiber 4b, and then couples with the inner cladding part of the multi-core fiber 4a in the optical coupler 4, and propagates through the inner cladding part.

[0068] Each of the cores 11 of the multi-core optical amplifying fiber 10 is optically connected to each of the cores of the multi-core fibers 4a of the optical coupler 4. Furthermore, the inner clad 12 of the multi-core optical amplifying fiber 10 is optically connected to the inner clad of the multi-core fiber 4a. Therefore, when each of the signal lights and pumping lights propagating through the multi-core fiber 4a is input to the multi-core optical amplifying fiber 10, they propagate in the same direction through each of the cores 11 and the inner clad 12. The pumping light optically pumps erbium in each of the cores 11 while propagating through the inner clad 12. Each of the signal lights propagating through each of the cores 11 is optically amplified by the action of stimulated emission of erbium. The multi-core optical amplifying fiber 10 outputs each of the optically amplified signal lights and the pumping light that did not contribute to the optical amplification.

[0069] The pump stripper 5 is a known device that removes pump light while passing each signal light output from the multi-core optical amplifier fiber 10. The pump stripper 5 has, for example, a double-clad multi-core fiber having seven cores arranged in a triangular lattice pattern, with a part of the outer cladding removed, and has a configuration in which pump light is extracted from the surface of the inner cladding of the removed part, irradiated onto a heat sink or the like, and absorbed to convert the energy of the pump light into thermal energy and dissipate heat. The pump stripper 5 passes each signal light through the multi-core fiber, and reduces the power of the pump light to a level that does not cause any problems when it is output from the optical amplifier 100.

[0070] The optical fiber fan-out 6, like the optical fiber fan-in 2, includes seven single-mode optical fibers bundled at one end and one multi-core fiber having seven core parts arranged in a triangular lattice, and is configured so that each core part on the bundled side of the seven single-mode optical fibers is optically coupled to each core part of the multi-core fiber. Each single-mode optical fiber is provided with an optical isolator 7. The multi-core fiber is connected to a pump stripper 5. Note that the end face on the bundled side of the seven single-mode optical fibers and the optically coupled end face of the multi-core fiber are processed at an angle with respect to the optical axis to suppress reflection, but may be perpendicular to the optical axis.

[0071] When signal light is input from each core of the multi-core fiber of the pump stripper 5 to each core of the optical fiber fan-out 6, each signal light propagates through each core of each single mode optical fiber and is output through the optical isolator 7.

[0072] The optical amplifier 100 configured in this manner is provided with a multi-core optical amplifying fiber 10 in which the gain difference between the multiple cores 11 is small, and therefore the gain difference imparted to the multiple input signal light beams can be made small.

[0073] (Embodiment 6) Fig. 9 is a schematic cross-sectional view of a multi-core light amplifying fiber according to embodiment 6, showing a cross section perpendicular to the longitudinal direction of the multi-core light amplifying fiber. The multi-core light amplifying fiber 50 is a double-clad type and seven-core type multi-core optical fiber having a configuration in which the inner cladding 12 and the outer cladding 13 in the multi-core light amplifying fiber 10 shown in Fig. 1 are replaced with an inner cladding 52 and an outer cladding 53, respectively.

[0074] The inner cladding portion 52 is made of glass such as silica glass. The inner cladding portion 52 has a hexagonal cross section. The inner cladding portion 52 has layered cladding portions 52a and 52b as a plurality of layered cladding portions formed concentrically. The layered cladding portion 52a is a portion having a circular cross section whose central axis is substantially the same as the central axis X. The layered cladding portion 52b is a ring-shaped portion having a hexagonal cross section that surrounds the outer periphery of the layered cladding portion 52a. The layered cladding portion 52a surrounds the core portion 11a, and the layered cladding portion 52b surrounds the core portion 11b. That is, one or more core portions of the core portion 11 are disposed in each of the layered cladding portions 52a and 52b. The inner cladding portion 52 is an example of an inner cladding portion having a polygonal cross section in a plane perpendicular to the longitudinal direction.

[0075] The inner cladding 52 has a refractive index lower than the maximum refractive index of each core 11. The inner cladding 52 has a GI type refractive index profile similar to the inner cladding 12 shown in FIG. 1. As a result, the refractive index of the layered cladding 52a is higher than the refractive index of the layered cladding 52b. Furthermore, the refractive index of the inner cladding 52 in the vicinity of the core 11a is higher than the refractive index of the inner cladding 52 in the vicinity of the core 11b.

[0076] The outer cladding portion 53 is made of, for example, a resin. The resin in question is, for example, a known resin used as a resin cladding portion of a double-cladding type optical fiber.

[0077] In the multi-core optical amplifying fiber 50 configured as described above, the difference in gain between the core portion 11a and the core portion 11b is also reduced due to the effect of the GI refractive index profile of the inner cladding portion 52 and the effect of the difference in mode field diameter between the core portion 11a and the core portion 11b.

[0078] Furthermore, in the multi-core optical amplifying fiber 50, the inner cladding 52 has a hexagonal cross section, so that the pumping light propagating through the inner cladding 52 is distributed so that the power density becomes higher toward the central axis X. As a result, the difference between the gain in the core 11a and the gain in the core 11b becomes smaller. For example, the difference is less than 0.9 dB when the pumping light has a wavelength of 975 nm and a power of 17 W, the signal light has a wavelength of 1550 nm and a power of -5.0 dBm, and each core is set to have a gain of 18 dB. However, the difference in gain is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

[0079] (Embodiment 7) Fig. 10 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to embodiment 7, showing a cross section perpendicular to the longitudinal direction of the multi-core optical amplifying fiber. The multi-core optical amplifying fiber 60 is a double-clad type and seven-core type multi-core optical fiber having a configuration in which the inner cladding 12 and the outer cladding 13 in the multi-core optical amplifying fiber 10 shown in Fig. 1 are replaced with an inner cladding 62 and an outer cladding 63, respectively.

[0080] The inner cladding portion 62 is made of glass such as silica glass. The inner cladding portion 62 has a D-shaped cross section consisting of arcs and straight lines. Specifically, the inner cladding portion 62 has layered cladding portions 62a and 62b as a plurality of layered cladding portions formed concentrically. The layered cladding portion 62a is a portion having a circular cross section whose central axis is substantially the same as the central axis X. The layered cladding portion 62b is an annular portion surrounding the outer periphery of the layered cladding portion 62a, but since a part of the outer periphery has a flat surface 62b1, the layered cladding portion 62b has a D-shaped cross section consisting of arcs and straight lines. The layered cladding portion 62a surrounds the core portion 11a, and the layered cladding portion 62b surrounds the core portion 11b. That is, one or more core portions of the core portion 11 are arranged in each of the layered cladding portions 62a and 62b. The inner cladding portion 62 is an example of an inner cladding portion whose cross-sectional shape in a plane perpendicular to the longitudinal direction is made up of arcs and straight lines.

[0081] The inner cladding 62 has a refractive index lower than the maximum refractive index of each core 11. The inner cladding 62 has a GI type refractive index profile similar to the inner cladding 12 shown in FIG. 1. As a result, the refractive index of the layered cladding 62a is higher than the refractive index of the layered cladding 62b. Furthermore, the refractive index of the inner cladding 62 in the vicinity of the core 11a is higher than the refractive index of the inner cladding 62 in the vicinity of the core 11b.

[0082] The outer cladding portion 63 is made of, for example, a resin. The resin in question is, for example, a known resin used as a resin cladding portion of a double-cladding type optical fiber.

[0083] In the multi-core optical amplifying fiber 60 configured as described above, the difference in gain between the core portion 11a and the core portion 11b is also reduced due to the effect of the GI refractive index profile of the inner cladding portion 62 and the effect of the difference in mode field diameter between the core portion 11a and the core portion 11b.

[0084] Furthermore, in the multi-core optical amplifying fiber 60, the inner cladding 62 has a D-shaped cross section, so that the pumping light propagating through the inner cladding 62 is distributed so that the power density becomes higher toward the central axis X. As a result, the difference between the gain in the core 11a and the gain in the core 11b becomes smaller. For example, the difference is less than 0.9 dB when the pumping light has a wavelength of 975 nm and a power of 17 W, the signal light has a wavelength of 1550 nm and a power of -5.0 dBm, and each core is set to have a gain of 18 dB. However, the difference in gain is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

[0085] (Embodiment 8) Fig. 11 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to embodiment 8, showing a cross section perpendicular to the longitudinal direction of the multi-core optical amplifying fiber. The multi-core optical amplifying fiber 90 is a double-clad 12-core multi-core optical fiber having a configuration in which the number of cores 11 in the multi-core optical amplifying fiber 10 shown in Fig. 1 is increased to 12, and the inner cladding 12 is replaced with an inner cladding 72.

[0086] The core portions 11 are arranged in a triangular lattice pattern that realizes a closest-packed state. Three core portions 11j of the core portions 11 are arranged at the corners of an equilateral triangle around a central axis X that extends along the longitudinal direction of the inner cladding portion 72. Three core portions 11k of the core portions 11 are arranged at the corners of an equilateral triangle around the central axis X. Six core portions 11l of the core portions 11 are arranged at the corners of a hexagon around the central axis X.

[0087] The inner cladding portion 72 is made of glass such as silica glass. The inner cladding portion 72 has layered cladding portions 72a, 72b, and 72c as a plurality of layered cladding portions formed concentrically. The layered cladding portion 72a is a portion having a circular cross section whose central axis is substantially the same as the central axis X. The layered cladding portion 72b is a portion having a circular cross section surrounding the outer periphery of the layered cladding portion 72a. The layered cladding portion 72c is a portion having a circular cross section surrounding the outer periphery of the layered cladding portion 72b. The layered cladding portion 72a surrounds the core portion 11j, the layered cladding portion 72b surrounds the core portion 11k, and the layered cladding portion 72c surrounds the core portion 11l. That is, one or more core portions of the core portion 11 are arranged in each of the layered cladding portions 72a, 72b, and 72c.

[0088] The inner cladding 72 has a refractive index lower than the maximum refractive index of each core 11. The inner cladding 72 has a GI type refractive index profile similar to the inner cladding 12 shown in FIG. 1. As a result, the refractive index of the layered cladding 72a is higher than that of the layered cladding 72b. The refractive index of the layered cladding 72b is higher than that of the layered cladding 72c. Furthermore, the refractive index of the inner cladding 52 in the vicinity of the core 11j is higher than that of the inner cladding 52 in the vicinity of the core 11k, and the refractive index of the inner cladding 52 in the vicinity of the core 11k is higher than that of the inner cladding 52 in the vicinity of the core 11l.

[0089] In the multi-core optical amplifying fiber 70, the three cores 11j out of the cores 11 are spaced at a distance d71 from the central axis X of the inner cladding 72. The distances from the central axis X of the cores 11j are approximately equal. The three cores 11k are spaced at a distance d72 from the central axis X of the inner cladding 72. The distances from the central axis X of the cores 11k are approximately equal. The six cores 11l are spaced at a distance d73 from the central axis X of the inner cladding 72. The distances from the central axis X of the cores 11l are approximately equal. If any one of d71, d72, and d73 is an example of a first distance, any distance that is greater (farther) than the first distance is an example of a second distance relative to the first distance.

[0090] In the multi-core optical amplifying fiber 70 configured as above, the difference between the gain in the core 11j and the gain in the core 11l is reduced due to the effect of the GI type refractive index profile of the inner cladding 72 and the effect of the difference in mode field diameter between the core 11j, the core 11k, and the core 11l. For example, the difference is less than 0.9 dB when the pump light has a wavelength of 975 nm and a power of 17 W, the signal light has a wavelength of 1550 nm and a power of -5.0 dBm, and each core is set to have a gain of 18 dB. However, the difference in gain is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

[0091] (Embodiment 9) Fig. 12 is a schematic cross-sectional view of a multi-core light amplifying fiber according to embodiment 9, showing a cross section perpendicular to the longitudinal direction of the multi-core light amplifying fiber. The multi-core light amplifying fiber 80 is a double-clad 12-core multi-core optical fiber having a configuration in which the arrangement of the cores 11 in the multi-core light amplifying fiber 70 shown in Fig. 11 is changed and the inner cladding 72 is replaced with an inner cladding 82.

[0092] The core portions 11 are arranged in a square shape. Four of the core portions 11m are arranged at the corners of a square around a central axis X extending along the longitudinal direction in the inner cladding portion 82. Eight of the core portions 11k are arranged at the corners of an octagon around the central axis X. In this case, the core portions 11 are arranged to have n-fold rotational symmetry within the inner cladding portion, where n is 2 or 4.

[0093] The inner cladding portion 82 is made of glass such as silica glass. The inner cladding portion 82 has layered cladding portions 82a and 82b as a plurality of layered cladding portions formed concentrically. The layered cladding portion 82a is a portion having a circular cross section whose central axis is substantially the same as the central axis X. The layered cladding portion 82b is a portion having an annular cross section that surrounds the outer periphery of the layered cladding portion 82a. The layered cladding portion 82a surrounds the core portion 11m, and the layered cladding portion 82b surrounds the core portion 11n. That is, one or more of the core portions 11 are disposed in each of the layered cladding portions 82a and 82b.

[0094] The inner cladding 82 has a refractive index lower than the maximum refractive index of each core 11. The inner cladding 82 has a GI type refractive index profile similar to the inner cladding 12 shown in FIG. 1. As a result, the refractive index of the layered cladding 82a is higher than the refractive index of the layered cladding 82b. Furthermore, the refractive index of the inner cladding 82 in the vicinity of the core 11m is higher than the refractive index of the inner cladding 82 in the vicinity of the core 11n.

[0095] In the multi-core optical amplifying fiber 80 configured as above, the difference between the gain in the core portion 11m and the gain in the core portion 11n is reduced, for example, due to the effect of the GI type refractive index profile of the inner cladding portion 82 and the effect of the difference in mode field diameter between the core portion 11m and the core portion 11n. For example, the difference is less than 0.9 dB when the pump light has a wavelength of 975 nm and a power of 17 W, the signal light has a wavelength of 1550 nm and a power of -5.0 dBm, and a gain of 18 dB is obtained in each core portion. However, the gain difference is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

[0096] (Embodiment 10) Fig. 13 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to embodiment 10, showing a cross section perpendicular to the longitudinal direction of the multi-core optical amplifying fiber. The multi-core optical amplifying fiber 90 is a double-clad 16-core multi-core optical fiber having a configuration in which the number of cores 11 in the multi-core optical amplifying fiber 80 shown in Fig. 12 is increased to 16, and the inner cladding portion 82 is replaced with an inner cladding portion 92.

[0097] The core portions 11 are arranged in a square shape. Four of the core portions 11o are arranged at the corners of a square around a central axis X extending along the longitudinal direction of the inner cladding portion 92. Eight of the core portions 11p are arranged at the corners of an octagon around the central axis X. Four of the core portions 11k are arranged at the corners of a square around the central axis X.

[0098] The inner cladding portion 92 is made of glass such as silica glass. The inner cladding portion 92 has layered cladding portions 92a, 92b, and 92c as a plurality of layered cladding portions formed concentrically. The layered cladding portion 92a is a portion having a circular cross section whose central axis is substantially the same as the central axis X. The layered cladding portion 92b is a portion having a circular cross section surrounding the outer periphery of the layered cladding portion 92a. The layered cladding portion 92c is a portion having a circular cross section surrounding the outer periphery of the layered cladding portion 92b. The layered cladding portion 92a surrounds the core portion 11o, the layered cladding portion 92b surrounds the core portion 11p, and the layered cladding portion 92c surrounds the core portion 11q. That is, one or more core portions of the core portion 11 are disposed in each of the layered cladding portions 92a, 92b, and 92c.

[0099] The inner cladding portion 92 has a refractive index lower than the maximum refractive index of each core portion 11. The inner cladding portion 92 has a GI type refractive index profile similar to the inner cladding portion 12 shown in FIG. 1. As a result, the refractive index of the layered cladding portion 92a is higher than the refractive index of the layered cladding portion 92b. The refractive index of the layered cladding portion 92b is higher than the refractive index of the layered cladding portion 92c. Furthermore, the refractive index of the inner cladding portion 92 in the vicinity of the core portion 11o is higher than the refractive index of the inner cladding portion 92 in the vicinity of the core portion 11p, and the refractive index of the inner cladding portion 92 in the vicinity of the core portion 11p is higher than the refractive index of the inner cladding portion 92 in the vicinity of the core portion 11q.

[0100] In the multi-core optical amplifying fiber 90 configured as above, the difference between the gain in the core 11o and the gain in the core 11q is reduced due to the effect of the GI type refractive index profile of the inner cladding 92 and the effect of the difference in mode field diameter between the core 11o, the core 11p, and the core 11q. For example, the difference is less than 0.9 dB when the pump light has a wavelength of 975 nm and a power of 17 W, the signal light has a wavelength of 1550 nm and a power of -5.0 dBm, and each core is set to have a gain of 18 dB. However, the difference in gain is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

[0101] In the above-mentioned embodiments, the number of core parts in the multi-core optical amplifying fiber is 7, 12, 16, or 19, but the number of core parts is not particularly limited. The arrangement of the core parts is not limited to a triangular lattice or a square lattice. In the above-mentioned embodiments, the optical amplifying medium in the multi-core optical amplifying fiber is erbium, but other optical amplifying media such as ytterbium (Yb), thulium (Tm), or praseodymium (Pr) may be used. The wavelength of the pumping light may be appropriately set to a wavelength that can optically pump the optical amplifying medium.

[0102] Furthermore, in the above embodiment, each core of the multi-core optical amplifying fiber is configured to propagate the signal light in a single mode, but it may be configured to propagate the signal light in multiple modes.

[0103] In the above embodiment, the cores are arranged to have n-fold rotational symmetry in the inner cladding, where n is 2, 3, 4, or 6, but n may be another integer greater than or equal to 2. Examples of other integers include 5, 8, 9, 10, 12, and 16.

[0104] Furthermore, in the above embodiment, the core portion is arranged so as not to straddle the two layered cladding portions, but the core portion may be arranged so as to straddle the boundary between the two layered cladding portions.

[0105] In the above embodiment, there is one pumping light source and one optical fiber for optical coupling in the optical coupler, but there may be multiple pumping light sources and multiple optical fibers for optical coupling in the optical coupler. In the above embodiment, the optical amplifier is a forward pumping type, but it may be a backward pumping type or a bidirectional pumping type. To configure a backward pumping type optical amplifier, for example, in the optical amplifier 100, the optical coupler 4 may be connected to the optical fiber fan-out 6 side and the pump stripper 5 may be connected to the optical fiber fan-in 2 side of the multi-core optical amplifying fiber 10. In addition, when the power of the pumping light output from the multi-core optical amplifying fiber 10 that has not contributed to optical amplification is relatively low, there are cases where the pump stripper 5 does not need to be provided.

[0106] Moreover, the present invention is not limited to the above-mentioned embodiment. The present invention also includes a configuration in which the above-mentioned components are appropriately combined. For example, in the above-mentioned embodiment in which the inner cladding portion has a GI type refractive index profile, the refractive index profile may be replaced with a step type. Further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspects of the present invention are not limited to the above-mentioned embodiment, and various modifications are possible. [Explanation of symbols]

[0107] 1, 7: Optical isolator 2: Fiber optic fan-in 3: Excitation light source 3a, 4b: Optical fiber for optical coupling 4: Optical coupler 4a: Multicore fiber 5: Pump stripper 6: Optical fiber fan-out 10, 20, 30, 40, 50, 60, 70, 80, 90: Multi-core optical amplifier fiber 11, 11a, 11b, 11c, 11d, 11e, 11f, 11g, 11h, 11i, 11j, 11k, 11l, 11m, 11n, 11o, 11p, 11q: Core part 12, 22, 32, 42, 52, 62, 72, 82, 92: Inner cladding 12a, 12b, 22a, 22b, 32a, 32b, 32c, 42a, 42b, 42c, 52a, 52b, 62a, 62b, 72a, 72b, 72c, 82a, 82b, 92a, 92b, 92c: layered clad portion 13, 53, 63: Outer cladding 62b1: Plane 100: Optical amplifier X: Central axis

Claims

1. A plurality of core portions doped with rare earth elements; an inner cladding portion surrounding the plurality of core portions and having a refractive index lower than the maximum refractive index of each of the core portions; an outer cladding portion surrounding the inner cladding portion and having a refractive index lower than that of the inner cladding portion; Equipped with the plurality of core portions include a first core portion that is a first distance from a central axis of the inner cladding portion extending along a longitudinal direction, and a second core portion that is a second distance from the central axis that is greater than the first distance, When pumping light of a predetermined power for optically pumping the rare earth element is propagated through the inner cladding portion, and signal light optically amplified by the rare earth element is propagated through each of the first core portion and the second core portion, a difference between a first gain obtained by the signal light in the first core portion and a second gain obtained by the signal light in the second core portion is less than 0.9 dB. Multicore optical amplifying fiber.

2. A first refractive index, which is a refractive index of the inner cladding portion in the vicinity of the first core portion, is higher than a second refractive index, which is a refractive index of the inner cladding portion in the vicinity of the second core portion. The multi-core optical amplifying fiber according to claim 1 .

3. The refractive index of a central region of the inner cladding portion, the central region including the central axis of the inner cladding portion, is higher than the refractive index of a peripheral region surrounding the central region of the inner cladding portion. The multi-core optical amplifying fiber according to claim 1 .

4. The inner cladding has a GI type refractive index profile. The multi-core optical amplifying fiber according to claim 3 .

5. The inner cladding has a stepped refractive index profile. The multi-core optical amplifying fiber according to claim 3 .

6. The cross-sectional shape of the inner cladding portion in a plane perpendicular to the longitudinal direction is a circle, a polygon, or a shape consisting of arcs and straight lines. The multi-core optical amplifying fiber according to claim 3 .

7. A first mode field diameter, which is a mode field diameter of the first core portion at the wavelength of the signal light, is larger than a second mode field diameter, which is a mode field diameter of the second core portion at the wavelength of the signal light. The multi-core optical amplifying fiber according to claim 1 .

8. The plurality of cores are arranged in the inner cladding to have n-fold rotational symmetry, where n is an integer of 2 or more. The multi-core optical amplifying fiber according to any one of claims 1 to 7.

9. The inner cladding portion has a plurality of layered cladding portions formed concentrically, One or more of the core portions are disposed in each of the layered cladding portions. The multi-core optical amplifying fiber according to any one of claims 1 to 7.

10. A mode field diameter at the wavelength of the signal light of the core portion disposed in a first layered cladding portion among the plurality of layered cladding portions is greater than a mode field diameter at the wavelength of the signal light of the core portion disposed in a second layered cladding portion adjacent to the outer periphery of the first layered cladding portion by 10% to 20%. The multi-core optical amplifying fiber according to claim 9.

11. A multi-core optical amplifying fiber according to claim 1; a pumping light source that outputs pumping light for optically pumping the rare earth element in the multi-core optical amplifier fiber; an optical coupler that optically couples the pumping light to the inner cladding portion; Equipped Optical amplifier.

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