Multicore optical amplification fiber and optical amplifier

Abstract

The purpose of the present invention is to reduce a difference in gain between a plurality of core parts. This multicore optical amplification fiber is provided with: a plurality of core parts to which a rare earth element is added; an inner cladding part that surrounds the plurality of core parts and has a refractive index lower than the maximum refractive index of each core part; and an outer cladding part that surrounds the inner cladding part and has a refractive index lower than the refractive index of the inner cladding part. The plurality of core parts include a first core part of which the distance from the central axis of the inner cladding part, extending along the longitudinal direction, is a first distance, and a second core part of which the distance from the central axis is a second distance greater than the first distance. When excitation light of a prescribed power that optically excites the rare earth element is propagated through the inner cladding and signal light that is optically amplified by the rare earth element is propagated through each of the first core part and the second core part, the difference between a first gain the signal light obtains in the first core part and the second gain the signal light obtains in the second core part is less than 0.9 dB.

Description

Multi-core optical amplifying fiber and optical amplifier

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

[0002] For example, in applications such as undersea optical communications, it is expected that the power consumption of optical amplifiers will be reduced by using multi-core erbium-doped optical fiber amplifiers (EDFAs) as optical amplifiers.

[0003] A known multi-core EDFA is configured to use a double-clad multi-core EDF and optically pump erbium (Er), an optical amplifying medium included 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.

[0004] 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.Kazi S Abedin et al, “Clading-pumped erbium-doped multicore fiber amplifier”, OPTICS EXPRESS Vol.20,No.18 27 August 2012 pp.20191-20200.

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

[0006] The present invention has been made in view of the above, and has an object to provide a multi-core optical amplifying fiber that can reduce the difference in gain among a plurality of cores, and an optical amplifier using the same.

[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 first core portions that are distanced a first distance from a central axis of the inner cladding portion extending along a longitudinal direction, and second core portions that are distanced 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 when signal light optically amplified by the rare-earth element is propagated through each of the first core portions and the second core portions, a difference between a first gain obtained by the signal light in the first core portions and a second gain obtained by the signal light in the second core portions 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 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-type 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 plurality of cores may be arranged to have n-fold rotational symmetry within the inner cladding, where n is an integer of 2 or greater.

[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 periphery 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; a pumping light source that outputs pumping light that optically pumps the rare-earth element in the multi-core optical amplifying fiber; and an optical coupler that optically couples the pumping light to the inner cladding portion.

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

[0019] Fig. 1 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 1. Fig. 2 is a schematic diagram showing a refractive index profile of the multi-core optical amplifying fiber shown in Fig. 1 . Fig. 3 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 2. Fig. 4 is a schematic diagram showing a refractive index profile of the multi-core optical amplifying fiber shown in Fig. 3 . Fig. 5 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 3. Fig. 6 is a schematic diagram showing a refractive index profile of the multi-core optical amplifying fiber shown in Fig. 5 . Fig. 7 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 4. Fig. 8 is a schematic cross-sectional view of an optical amplifier according to Embodiment 5. Fig. 9 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 6. Fig. 10 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 7. Fig. 11 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 8. Fig. 12 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 9. Fig. 13 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 10.

[0020] The following describes embodiments with reference to the drawings. Note that the present invention is not limited to these embodiments. In the drawings, identical or corresponding elements are appropriately designated by the same reference numerals, and redundant explanations are omitted. It should be noted that the drawings are schematic, and the dimensional relationships and ratios of elements may differ from the actual figures. The dimensional relationships and ratios may differ between drawings. In this specification, the term "cutoff wavelength" refers to the cable cutoff wavelength defined in ITU-T (International Telecommunication Union) G.650.1. Other terms not specifically defined in this specification are defined and measured in accordance with the definitions and measurement methods in G.650.1 and G.650.2.

[0021] 1 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to embodiment 1, showing a cross section perpendicular to the longitudinal direction of the multi-core optical amplifying fiber. The multi-core optical 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 cores 11 are made of glass such as silica-based glass. The cores 11 are arranged in a triangular lattice pattern that achieves a closest-packed state. One core 11a of the cores 11 is arranged so that its central axis coincides with a central axis X extending along the longitudinal direction of the inner cladding 12. The central axis X of the inner cladding 12 can be defined as the center of gravity of the inner cladding 12. Six cores 11b of the cores 11 are arranged so as to be positioned at the corners of a regular hexagon around the central axis X. In this case, the cores 11 are arranged so as to have n-fold rotational symmetry within the inner cladding, where n is 2, 3, or 6.

[0023] The core region 11 contains, for example, germanium (Ge) or aluminum (Al) as a refractive index adjusting dopant that increases the refractive index. The core region 11 also contains Er as a rare earth element that serves as an amplification medium. Er is doped at a concentration such that the peak absorption coefficient at a wavelength of, for example, 1530 nm is 2.5 dB / m to 11 dB / m. 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 the function of suppressing concentration quenching of Er.

[0024] The inner cladding portion 12 is made of glass such as silica-based glass. The inner cladding portion 12 has layered cladding portions 12a and 12b as multiple concentrically formed layered cladding portions. The layered cladding portion 12a is a portion with a circular cross section whose central axis is approximately the same as the central axis X. The layered cladding portion 12b is a portion with 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. In other words, one or more of the core portions 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, which is a known resin used as a resin cladding portion of a double-cladding optical fiber.

[0026] Fig. 2 is a schematic diagram showing the refractive index profile at the A-A 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, respectively, of the inner cladding portion 12. 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 core 11 is n1. The refractive index profile of the core 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 core 11. The inner cladding 12 has a GI type refractive index profile in which the refractive index is high along the central axis X and decreases toward the periphery in the radial direction. The glass constituting the inner cladding 12 is doped with a dopant such as fluorine in a distributed manner to achieve a GI type refractive index profile. As a result, the refractive index of the layered cladding 12a (central region) indicated by profile P12a is higher than the refractive index of the layered cladding 12b (peripheral region) indicated by profile P12b. Furthermore, the refractive index (an example of a first refractive index) n21 of the inner cladding 12 (layered cladding 12a) near the core 11a is higher than the refractive index (an example of a second refractive index) n22 of the inner cladding 12 (layered cladding 12b) near the core 11b. Note that the vicinity of the core refers to, for example, a range several μm away from the outer edge of the core. The outer cladding 13 has a refractive index n3 lower than the refractive index of the inner cladding 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 defined as 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. The core diameter of each of the core portions 11a and 11b is preferably set, in relation to core Δ, to achieve 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. In the case of Er, the optical amplification wavelength band in which the rare earth element can amplify light is called the C band, for example, 1530 nm to 1565 nm, or called the L band, for example, 1565 nm to 1625 nm. The mode field diameter of each of the cores 11a and 11b at a wavelength at which Er amplifies light, for example, 1550 nm, is, for example, 5 μm or more and 11 μm or less.

[0029] Of the core portions 11, one core portion 11a, which is the first core portion, is spaced a distance 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) that is a first distance d11. As described above, the first distance d11 is zero. On the other hand, six core portions 11b, which are the second core portions, are spaced a distance from the central axis X of the inner cladding portion 12 that is a second distance d12. 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 inventors fabricated a test multi-core optical amplifying fiber that was a double-clad, seven-core multi-core optical amplifying fiber such as the multi-core optical amplifying fiber 10, in which the refractive index of the inner cladding was uniform in the radial direction from the central axis. Pumping light of a predetermined power (e.g., 17 W power at a wavelength of 975 nm) that optically pumps a rare earth element was propagated through the inner cladding, and signal light (e.g., −5.0 dBm at a wavelength of 1550 nm) optically amplified by the rare earth element was propagated through each of the cores. As a result, 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 in which the refractive index is high at and near the central axis X and decreases radially outward. 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 so that the power density increases toward the central axis X. As a result, the difference between the gain in the core 11 a closer to the central axis X (an example of a first gain) and the gain in the core 11 b farther from the central axis X (an example of a second gain) becomes smaller.

[0032] Furthermore, in the multi-core optical amplifying 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 core 11a and 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 gain difference is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

[0034] 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, seven-core multi-core optical fiber having a configuration in which the inner cladding 12 in the multi-core optical amplifying fiber 10 shown in FIG. 1 is replaced with an inner cladding 22.

[0035] The inner cladding portion 22 is made of glass such as silica-based glass. The inner cladding portion 22 has layered cladding portions 22a and 22b as multiple concentrically formed layered cladding portions. The layered cladding portion 22a is a portion with a circular cross section whose central axis is substantially the same as the central axis X. The layered cladding portion 22b is a portion with 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. In other words, one or more of the core portions 11 are disposed in each of the layered cladding portions 22a and 22b.

[0036] Fig. 4 is a schematic diagram showing the refractive index profile in the cross section taken along line BB 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 22 has a refractive index lower than the maximum refractive index of each core 11. The inner cladding 22 has a stepped refractive index profile in which the layered cladding 22a, including the central axis X, has a relatively high refractive index of approximately uniform n23, and the layered cladding 22b has a relatively low refractive index of approximately uniform n24. In this case, the layered cladding 22a is made of, for example, pure silica glass. The layered cladding 22b is made of, for example, silica glass doped with a dopant such as fluorine to form a stepped refractive index profile. The relative refractive index difference of the layered cladding 22b with respect to the layered cladding 22a is, for example, −1.2% or more and −0.7% or less. As a result, the refractive index of the layered cladding 22a (central region) is higher than the refractive index of the layered cladding 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, while 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 step-type refractive index profile such that the refractive index is high in the central region including the central axis X and low in the peripheral region in the radial direction. In this case as well, compared to the case of the test multi-core optical amplifying fiber, the pumping light propagating through the inner cladding 22 is distributed so that the power density increases toward the central axis X. As a result, the difference between the gain in the core 11 a closer to the central axis X and the gain in the core 11 b 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 gain difference is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

[0043] 5 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to embodiment 3, showing a cross section perpendicular to the longitudinal direction of the multi-core optical amplifying fiber. The multi-core optical 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 optical 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 to achieve a closest-packed state. One core portion 11a of the core portions 11 is arranged so that its central axis coincides with the central axis X extending along the longitudinal direction of the inner cladding portion 32. Six core portions 11c of 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 of 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 of 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-based glass. The inner cladding portion 32 has multiple concentrically formed layered cladding portions, layered cladding portions 32a, 32b, and 32c. The layered cladding portion 32a is a portion with a circular cross section whose central axis is approximately the same as the central axis X. The layered cladding portion 32b is a portion with a circular cross section that surrounds the outer periphery of the layered cladding portion 32a. The layered cladding portion 32c is a portion with a circular cross section that surrounds 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. In other words, one or more of the core portions 11 are disposed in each of the layered cladding portions 32a, 32b, and 32c.

[0046] Fig. 6 is a schematic diagram showing the refractive index profile at the CC 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 cores 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 core 11 is n1. The inner cladding 32 has a refractive index lower than the maximum refractive index of each core 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, which is lower than n25, and the layered cladding 32c has a relatively high refractive index of approximately uniform n27, which is even lower than n26. In this case, the layered cladding 32a is made of, for example, pure silica glass. The layered claddings 32b and 32c are made of, for example, silica glass doped with a dopant such as fluorine so as to form 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) near the core 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) near the core 11b. Furthermore, the refractive index (an example of a first refractive index) n26 of the inner cladding portion 32 (layered cladding portion 32b) near the core 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) near the core 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, of the cores 11, one core 11a is at a distance d11 from the central axis X of the inner cladding 32. As described above, d11 is zero. Furthermore, the six cores 11c are at 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. Furthermore, the six cores 11d are at 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. Furthermore, the six cores 11e are at 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, and is, for example, 38.5 μm. Here, if any of d11, d32, d33, and d34 is 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 32 has a stepped refractive index profile such that the refractive index is high in the central portion including the central axis X and low in the peripheral portion in the radial direction. In this case, compared to the test multi-core optical amplifying fiber, the pumping light propagating through the inner cladding 32 is distributed such that the power density increases toward the central axis X. As a result, the difference between the gain (an example of a first gain) in the core portion 11 closer to the central axis X (for example, the core portion 11a) and the gain (an example of a second gain) in the core portion 11 farther from the central axis X (for example, the core portion 11c) becomes smaller. Furthermore, the difference between the gain (an example of a first gain) in the core portion 11 closer to the central axis X (for example, the core portion 11a) and the gain (an example of a second gain) in the core portion 11 farther from the central axis X (for example, the core portion 11e) becomes smaller.

[0050] Furthermore, in the multi-core optical amplifying fiber 30, the core portion closer to the central axis X (for example, the core portion 11a) has a larger mode field diameter than the core portion farther from the central axis X (for example, the core portion 11e). As a result, the gain difference between the gain in the core portion 11a and the gain in the 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 gain difference is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

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

[0053] 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 to achieve a closest-packed state. One core portion 11f of the core portions 11 is arranged so that its central axis coincides with the central axis X extending along the longitudinal direction of the inner cladding portion 42. Six core portions 11g of 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 of 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 of 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, of the cores 11, one core 11f is at 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 are at 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. The six cores 11h are at 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. The six cores 11i are at 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 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] Furthermore, core portion 11f has the largest core diameter among 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 second core portions, have the second largest core diameter among 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 core portions 11 and also have the third largest mode field diameter at the wavelength of the signal light. Here, if any of core portions 11 is an example of a first core portion, the mode field diameter of that first core portion is the first mode field diameter. Furthermore, 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 that 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-based glass. The inner cladding portion 42 includes multiple concentrically formed layered cladding portions, layered cladding portions 42a, 42b, and 42c. The layered cladding portion 42a is a circular portion having a central axis substantially coincident with the central axis X. The layered cladding portion 42b is an annular portion surrounding the outer periphery of the layered cladding portion 42a. The layered cladding portion 42c is an annular portion surrounding 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 of the core portions 11 are disposed in each of the layered cladding portions 42a, 42b, and 42c.

[0058] In the inner cladding portion 42, the layered cladding portions 42a, 42b, and 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 from the central axis to the radial direction, 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 far from the central axis of the inner cladding (i.e., the gain difference is large).

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

[0061] Here, the 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 42 a, 42 b, and 42 c may be 10% to 20% larger than the 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. That is, for example, if the layered cladding portion 42 a is the first layered cladding portion, the second layered cladding portion is the layered cladding portion 42 b. In this case, the mode field diameter at the wavelength of the signal light of the core portion 11 f disposed in the layered cladding portion 42 a is 10% to 20% larger than the mode field diameter at the wavelength of the signal light of the core portion 11 g disposed in the layered cladding portion 42 b. This effectively reduces 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 gain difference is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

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

[0064] 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 splice 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 on the bundled side of the seven single-mode optical fibers is optically coupled to a 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 amplifying fiber 10 shown in FIG. 1 . The seven single-mode optical fibers are standard single-mode optical fibers defined, for example, in 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 arrows and block light from passing in the opposite direction. Each core of the multi-core fiber in the optical fiber fan-in 2 is connected to a core of a multi-core fiber 4a in 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 amplifying 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 optically coupled end face of the multi-core fiber 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 the signal light. Each core of the multi-core fiber propagates the signal light and outputs the signal light to the connected core in the multi-core fiber 4 a. The core of the multi-core fiber 4 a 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 3 a and 4 b propagate the pumping light output from the pumping light source. As described above, the pumping light propagates through the optical coupling optical fiber 4 b, and then couples with the inner cladding portion of the multi-core fiber 4 a in the optical coupler 4, and propagates through the inner cladding portion.

[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 4 a of the optical coupler 4. Furthermore, the inner cladding 12 of the multi-core optical amplifying fiber 10 is optically connected to the inner cladding of the multi-core fiber 4 a. Therefore, when each of the signal light and pumping light propagating through the multi-core fiber 4 a 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 cladding 12. The pumping light optically pumps erbium in each of the cores 11 while propagating through the inner cladding 12. Each of the signal light 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 light and pumping light that did not contribute to the optical amplification.

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

[0070] Similar to the optical fiber fan-in 2, the optical fiber fan-out 6 includes seven single-mode optical fibers bundled at one end and one multi-core fiber having seven cores arranged in a triangular lattice pattern, and is configured so that each core on the bundled side of the seven single-mode optical fibers is optically coupled to each core 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 faces of 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 also be perpendicular to the optical axis.

[0071] When signal light is input from each core of the multicore 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 includes a multi-core optical amplifying fiber 10 in which the difference in gain between the multiple core portions 11 is small, and therefore the difference in gain given to the multiple input signal lights can be made small.

[0073] 9 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to embodiment 6, showing a cross section perpendicular to the longitudinal direction of the multi-core optical amplifying fiber. The multi-core optical amplifying fiber 50 is a double-clad, seven-core 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 52 and an outer cladding 53, respectively.

[0074] The inner cladding portion 52 is made of glass such as silica-based glass. The inner cladding portion 52 has a hexagonal cross section. The inner cladding portion 52 includes layered cladding portions 52a and 52b as multiple concentrically formed layered cladding portions. The layered cladding portion 52a is a circular portion having a central axis substantially coincident with the central axis X. The layered cladding portion 52b is an annular 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. In other words, one or more of the core portions 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 near the core 11a is higher than the refractive index of the inner cladding 52 near the core 11b.

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

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

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

[0079] 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, seven-core 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-based 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 multiple concentrically formed layered cladding portions. The layered cladding portion 62a has a circular cross section whose central axis is approximately 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. However, since a portion of the outer periphery has a flat surface 62b1, the layered cladding portion 62a 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. In other words, one or more of the core portions 11 are disposed 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 section 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 near the core 11a is higher than the refractive index of the inner cladding 62 near the core 11b.

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

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

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

[0085] 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 to achieve a closest-packed state. Three of the core portions 11j are arranged at the corners of an equilateral triangle around a central axis X extending along the longitudinal direction of the inner cladding portion 72. Three of the core portions 11k are arranged at the corners of an equilateral triangle around the central axis X. Six of the core portions 11l 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-based glass. The inner cladding portion 72 has multiple concentrically formed layered cladding portions, layered cladding portions 72a, 72b, and 72c. The layered cladding portion 72a is a portion with a circular cross section whose central axis is approximately the same as the central axis X. The layered cladding portion 72b is a portion with a circular cross section that surrounds the outer periphery of the layered cladding portion 72a. The layered cladding portion 72c is a portion with a circular cross section that surrounds 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. In other words, one or more of the core portions 11 are disposed 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 the refractive index of the layered cladding 72b. The refractive index of the layered cladding 72b is also higher than the refractive index of the layered cladding 72c. Furthermore, the refractive index of the inner cladding 52 near the core 11j is higher than the refractive index of the inner cladding 52 near the core 11k, and the refractive index of the inner cladding 52 near the core 11k is higher than the refractive index of the inner cladding 52 near the core 11l.

[0089] In the multi-core optical amplifying fiber 70, of the cores 11, the three cores 11j are spaced 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 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 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. Here, if any 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 described above, the difference in gain between the core 11j and 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 a gain of 18 dB is obtained in each core. However, the gain difference is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

[0091] 12 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to embodiment 9, showing a cross section perpendicular to the longitudinal direction of the multi-core optical amplifying fiber. The multi-core optical 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 optical 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-based glass. The inner cladding portion 82 has layered cladding portions 82a and 82b as multiple concentrically formed layered cladding portions. The layered cladding portion 82a is a portion with a circular cross section whose central axis is approximately the same as the central axis X. The layered cladding portion 82b is a portion with 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. In other words, 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 near the core 11m is higher than the refractive index of the inner cladding 82 near the core 11n.

[0095] In the multi-core optical amplifying fiber 80 configured as described above, the difference in gain between the core 11m and the core 11n is also reduced 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 11m and the core 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. However, the gain difference is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

[0096] 13 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to a tenth embodiment, 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 82 is replaced with an inner cladding 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-based glass. The inner cladding portion 92 has multiple concentrically formed layered cladding portions, layered cladding portions 92a, 92b, and 92c. The layered cladding portion 92a is a portion with a circular cross section whose central axis is approximately the same as the central axis X. The layered cladding portion 92b is a portion with a circular cross section that surrounds the outer periphery of the layered cladding portion 92a. The layered cladding portion 92c is a portion with a circular cross section that surrounds 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. In other words, one or more of the core portions 11 are disposed in each of the layered cladding portions 92a, 92b, and 92c.

[0099] The inner cladding 92 has a refractive index lower than the maximum refractive index of each core 11. The inner cladding 92 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 92a is higher than the refractive index of the layered cladding 92b. The refractive index of the layered cladding 92b is also higher than the refractive index of the layered cladding 92c. Furthermore, the refractive index of the inner cladding 92 near the core 11o is higher than the refractive index of the inner cladding 92 near the core 11p, and the refractive index of the inner cladding 92 near the core 11p is higher than the refractive index of the inner cladding 92 near the core 11q.

[0100] In the multi-core optical amplifying fiber 90 configured as described above, the difference in gain between the core 11o and 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 a gain of 18 dB is obtained in each core. However, the gain difference is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

[0101] In the above embodiments, the number of cores in the multi-core optical amplifying fiber is 7, 12, 16, or 19, but the number of cores is not particularly limited. Furthermore, the arrangement of the cores is not limited to a triangular lattice or a square lattice. In the above 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 also 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 signal light in a single mode, but it may be configured to propagate signal light in multiple modes.

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

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

[0105] Furthermore, in the above-described embodiments, there is one pumping light source and one optical coupling optical fiber in the optical coupler, but there may be multiple pumping light sources and multiple optical coupling optical fibers in the optical coupler. Furthermore, in the above-described embodiments, 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 of the multi-core optical amplifying fiber 10, and the pump stripper 5 may be connected to the optical fiber fan-in 2 side. Furthermore, 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, the pump stripper 5 may not be provided.

[0106] Furthermore, the present invention is not limited to the above-described embodiments. Configurations in which the above-described components are appropriately combined are also included in the present invention. For example, in the above-described embodiments in which the inner cladding portion has a GI-type refractive index profile, the refractive index profile may be replaced with a stepped 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-described embodiments, and various modifications are possible.

[0107] The present invention can be used in optical amplifying fibers, optical fiber amplifiers, and optical communication systems.

[0108] 1, 7: Optical isolator 2: Optical fiber fan-in 3: Pumping light source 3a, 4b: Optical fiber for optical coupling 4: Optical coupler 4a: Multi-core fiber 5: Pump stripper 6: Optical fiber fan-out 10, 20, 30, 40, 50, 60, 70, 80, 90: Multi-core optical amplifying fiber 11, 11a, 11b, 11c, 11d, 11e, 11f, 11g, 11h, 11i, 11j, 11k, 11l, 11m, 11n, 11o, 11p, 11q: Core portion 12, 22, 32, 42, 52, 62, 72, 82, 92: Inner cladding portion 12a, 12b, 22a, 22b, 32a, 32b, 32c, 42a, 42b, 42c, 52a, 52b, 62a, 62b, 72a, 72b, 72c, 82a, 82b, 92a, 92b, 92c: layered cladding portion 13, 53, 63: outer cladding portion 62b1: plane 100: optical amplifier X: central axis

Claims

1. A multi-core optically amplifying fiber comprising: a plurality of core sections doped with a rare earth element; an inner cladding section surrounding the plurality of core sections and having a refractive index lower than the maximum refractive index of each core section; and an outer cladding section surrounding the inner cladding section and having a refractive index lower than the refractive index of the inner cladding section, wherein the plurality of core sections include a first core section that is a first distance away from a central axis of the inner cladding section extending along a longitudinal direction, and a second core section that is a second distance away 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 section, and when signal light optically amplified by the rare earth element is propagated through each of the first core section and the second core section, a difference between a first gain obtained by the signal light in the first core section and a second gain obtained by the signal light in the second core section is less than 0.9 dB.

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

3. The multi-core optical amplifying fiber according to claim 1, wherein the refractive index of a 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.

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

5. The multi-core optical amplifying fiber according to claim 3, wherein the inner cladding portion has a step-type refractive index profile.

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

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

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

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

10. A multi-core optical amplifying fiber according to claim 9, wherein a mode field diameter at the wavelength of the signal light of said core portion arranged in a first layered cladding portion among said plurality of layered cladding portions is greater than a mode field diameter at the wavelength of the signal light of said core portion arranged in a second layered cladding portion adjacent to the outer periphery of said first layered cladding portion by not less than 10% and not more than 20%.

11. An optical amplifier comprising: a multi-core optical amplifying fiber according to claim 1; an excitation light source that outputs excitation light for optically exciting 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.

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