JP2025046040A - Multi-core optical amplifying fiber and optical amplifier - Google Patents

Abstract

The present invention aims to reduce the difference in gain among a plurality of core sections.
[Solution] The multi-core optical amplifying fiber comprises 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, 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 the longitudinal direction, and a second core portion having a second distance from the central axis that is larger than the first distance, and when pumping light of a predetermined power that optically pumps a rare earth element is propagated through the inner cladding, and when signal light optically amplified by the rare earth element is propagated 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 Figure] Figure 1

Description

The present invention relates to a multi-core optical amplifier fiber and an optical amplifier.

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 erbium-doped optical fiber amplifiers (EDFAs) as optical amplifiers.

A known multi-core EDFA is configured to use a double-clad multi-core EDF to optically pump erbium (Er), an optical amplification medium contained in the core, using a cladding pumping method (see Non-Patent Documents 1 and 2). A multi-core EDF is a type of multi-core optical amplifier fiber.

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.

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 variations will occur between the signal lights, leading to variations in the signal transmission distance and signal transmission quality.

The present invention has been made in view of the above, and aims to provide a multi-core optical amplifier fiber that can reduce the gain difference between multiple cores, and an optical amplifier using the same.

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 parts doped with a rare earth element; an inner cladding part surrounding the plurality of core parts and having a refractive index lower than the maximum refractive index of each core part; and an outer cladding part surrounding the inner cladding part and having a refractive index lower than the refractive index of the inner cladding part, the plurality of core parts including a first core part having a first distance from a central axis of the inner cladding part extending along a longitudinal direction, and a second core part having a second distance from the central axis that is greater than the first distance; and when a pumping light of a predetermined power that optically pumps the rare earth element is propagated through the inner cladding part, and a signal light 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 obtained by the signal light in the first core part and a second gain obtained by the signal light in the second core part is less than 0.9 dB.

A first refractive index, which is the refractive index of the inner cladding portion near the first core portion, may be higher than a second refractive index, which is the refractive index of the inner cladding portion near the second core portion.

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

The inner cladding portion may have a GI type refractive index profile.

The inner cladding may have a stepped refractive index profile.

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.

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.

The multiple cores may be arranged to have n-fold rotational symmetry within the inner cladding, where n is an integer equal to or greater than 2.

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.

The mode field diameter of the core portion arranged in a first layered cladding portion of the plurality of layered cladding portions at the wavelength of the signal light may be greater than the mode field diameter 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%.

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

The present invention makes it possible to realize a multi-core optical amplifier fiber that can reduce the gain difference between multiple cores, and an optical amplifier using the same.

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

The following describes the embodiments with reference to the drawings. Note that the present invention is not limited to these embodiments. 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 relationships and ratios of each element may differ from reality. There may be parts in which the dimensional relationships and ratios 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.

(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.

The core portion 11 is made of glass such as silica glass. The core portions 11 are arranged in a triangular lattice pattern that realizes a close-packed state. One of the core portions 11, core portion 11a, is arranged so that its central axis coincides with the central axis X that extends 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 that they are positioned at the corners of a regular hexagon around the central axis X. In this case, the core portions 11 are arranged so that they have n-fold rotational symmetry within the inner cladding portion, where n is 2, 3, and 6.

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 that gives a peak absorption coefficient of 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, there are no particular limitations on the absorption coefficient or doping concentration. Note that Al also has the function of suppressing concentration quenching of Er.

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 multiple layered cladding portions formed concentrically. The layered cladding portion 12a is a portion having a circular cross section whose central axis is approximately 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. In other words, one or more of the core portions of the core portion 11 are arranged in each of the layered cladding portions 12a and 12b.

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-clad type optical fiber.

Figure 2 is a schematic diagram showing the refractive index profile at the A-A cross section of the multi-core optical amplifier fiber 10. In Figure 2, profiles P11a and P11b show the refractive index profiles of the core portions 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.

As shown in FIG. 2, the maximum refractive index of each core portion 11 is n1. The refractive index profile of the core portion 11 is SI (Step Index) type or GI (Graded Index) type. The inner cladding portion 12 has a refractive index lower than the maximum refractive index of each core portion 11. The inner cladding portion 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. The glass constituting the inner cladding portion 12 is doped with a dopant such as fluorine in a distributed manner so as to have a GI type refractive index profile. As a result, the refractive index of the layered cladding portion 12a (central region) shown by the profile P12a is higher than the refractive index of the layered cladding portion 12b (peripheral region) shown by the 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 is, 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.

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 be optically amplified. 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 be optically amplified is, for example, 1530 nm to 1565 nm, called the C band, or, for example, 1565 nm to 1625 nm, called the L band, in the case of Er. The mode field diameter of each of the cores 11a and 11b at a wavelength at which Er is optically amplified, for example 1550 nm, is, for example, 5 μm or more and 11 μm or less.

Of the core portions 11, the one core portion 11a, which is the 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, the six core portions 11b, which are the 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.

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

In contrast, in the multi-core optical amplifier 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 radial periphery. As a result, compared to the case of a test multi-core optical amplifier fiber, the pump light propagating through the inner cladding 12 is distributed so 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.

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

As described above, according to the multi-core optical amplifying fiber 10 of 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 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.

(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.

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 multiple layered cladding portions formed concentrically. The layered cladding portion 22a is a portion having a circular cross section whose central axis is approximately 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. In other words, one or more of the core portions 11 are arranged in each of the layered cladding portions 22a and 22b.

Figure 4 is a schematic diagram showing the refractive index profile at the B-B cross section of the multi-core optical amplifier fiber 20. In Figure 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.

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 at n23 and relatively high, and the refractive index of the layered cladding portion 22b is approximately uniform at n24 and relatively low. 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.

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.

Also, in the multi-core optical amplifier 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.

As described above, in the multi-core optical amplifier 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 radial peripheral region. In this case, compared to the test multi-core optical amplifier fiber, the pumping light propagating through the inner cladding 22 is distributed such that the power density is higher 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.

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

As described above, according to the multi-core optical amplifying fiber 20 of 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 each core is set to have a gain of 18 dB. However, the gain difference is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

(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 core portions 11 in the multi-core light amplifying fiber 20 shown in Fig. 3 is increased to 19, and the inner cladding portion 22 is replaced with an inner cladding portion 32.

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

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 almost the same as the central axis X. The layered cladding portion 32b is a portion having a circular cross section that surrounds the outer periphery of the layered cladding portion 32a. The layered cladding portion 32c is a portion having 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. 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.

Figure 6 is a schematic diagram showing the refractive index profile at the C-C line cross section of the multi-core optical amplifier fiber 30. In Figure 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.

As shown in FIG. 6, the maximum refractive index of each core portion 11 is n1. The inner cladding portion 32 has a refractive index lower than the maximum refractive index of each core portion 11. The inner cladding portion 32 has a stepped refractive index profile in which the refractive index of the layered cladding portion 32a including the central axis X is approximately uniform and relatively high at n25, the refractive index of the layered cladding portion 32b is approximately uniform and n26 is lower than n25, and the refractive index of the layered cladding portion 32c is approximately uniform and n27 is even lower than n26. In this case, the layered cladding portion 32a is made of, for example, pure silica glass. The layered cladding portions 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 portion 32c with respect to the layered cladding portion 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.

In the multi-core optical amplifier fiber 30, the distance of one core 11a from the central axis X of the inner cladding 32 is d11. As described above, d11 is zero. The distance of the six cores 11c from the central axis X of the inner cladding 12 is d32. The distances of the cores 11c from the central axis X are approximately equal. The distance of the six cores 11d from the central axis X of the inner cladding 32 is d33. The distances of the cores 11d from the central axis X are approximately equal. The distance of the six cores 11e from the central axis X of the inner cladding 32 is d34. The distances of the cores 11e from the central axis X are approximately equal. d32 is equal to the inter-core distance in the cores 11, 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 greater (farther) than the first distance is an example of a second distance relative to the first distance.

As described above, in the multi-core optical amplifier fiber 30, the inner cladding portion 32 has a step-type refractive index profile such that the refractive index is high in the center including the central axis X and low in the radial peripheral portion. In this case, compared with the case of the test multi-core optical amplifier fiber, the pumping light propagating through the inner cladding portion 32 is distributed so 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, core portion 11a) closer to the central axis X and the gain (an example of the second gain) in the core portion 11 (for example, core portion 11c) farther from the central axis X becomes smaller. Furthermore, the difference between the gain (an example of the first gain) in the core portion 11 (for example, core portion 11a) closer to the central axis X and the gain (an example of the second gain) in the core portion 11 (for example, core portion 11e) farther from the central axis X becomes smaller.

Furthermore, even in the multi-core optical amplifier fiber 30, the core portion closer to the central axis X (for example, core portion 11a) has a larger mode field diameter than the 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.

As described above, according to the multi-core optical amplifying fiber 30 of 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 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.

In this embodiment, the cores 11c and 11e, which are 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, which is farther away from the central axis X, may be lower than the refractive index of the layered clad portion of the core 11d, which is closer to the central axis X.

(Embodiment 4)
Fig. 7 is a schematic cross-sectional view of a multi-core light amplifying fiber according to embodiment 4, showing a cross section perpendicular to the longitudinal direction of the multi-core light amplifying fiber. The multi-core light 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 light amplifying fiber 10 shown in Fig. 1 is increased to 19, and the inner cladding 12 is replaced with an inner cladding 42.

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

In the multi-core optical amplifier fiber 40, the distance of one core 11f from the central axis X of the inner cladding 42 is d41. As described above, d41 is zero. On the other hand, the distance of the six cores 11g from the central axis X of the inner cladding 42 is d42. The distances from the central axis X of the cores 11g are approximately equal. The distance of the six cores 11h from the central axis X of the inner cladding 42 is d43. The distances from the central axis X of the cores 11h are approximately equal. The distance of the six cores 11i from the central axis X of the inner cladding 42 is d44. 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, for example, 38.5 μm. Here, if any of d11, d42, d43, and d44 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.

In addition, 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 the 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 the first core portion is the first mode field diameter. In addition, 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.

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 approximately 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 of the core portions 11 are arranged in each of the layered cladding portions 42a, 42b, and 42c.

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

As described above, in a test multi-core optical amplifier fiber in which the refractive index of the inner cladding is uniform from the central axis in 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).

However, in the multi-core optical amplifier 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.

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

As described above, according to the multi-core optical amplifying fiber 40 of 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 each core is set to have a gain of 18 dB. However, the gain difference is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

In this embodiment, the core diameter and mode field diameter of the core portions 11h and 11i are equal, but 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 portion 11i, which is farther away, may be smaller.

(Embodiment 5)
8 is a schematic 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.

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 shape 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 shape similar to the core 11 of the multi-core optical amplifier fiber 10 shown in FIG. 1. In addition, in an optical fiber fan-in, the end face 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.

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 part of the multi-core fiber propagates each signal light and outputs the signal light to the connected core part in the multi-core fiber 4a. The core part of the multi-core fiber 4a propagates the input signal light.

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 excite 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 portion of the multi-core fiber 4a in the optical coupler 4, and propagates through the inner cladding portion.

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 fiber 4a of the optical coupler 4. 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 signal light and pumping light propagated through the multi-core fiber 4a are 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 excites the 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 the erbium. The multi-core optical amplifying fiber 10 outputs each optically amplified signal light and pumping light that did not contribute to the optical amplification.

The pump stripper 5 is a known device that passes each signal light output from the multi-core optical amplifier fiber 10 while rejecting the pump light. The pump stripper 5 has, for example, a double-clad multi-core fiber having seven cores arranged in a triangular lattice pattern, with part of the outer cladding removed, and extracts pump light from the surface of the inner cladding of the removed part, irradiates it onto a heat sink or the like, and absorbs it to convert the energy of the pump light into thermal energy and dissipate it. The pump stripper 5 passes each signal light through the multi-core fiber, while reducing the power of the pump light to a level that will not cause any problems when it is output from the optical amplifier 100.

The optical fiber fan-out 6, like the optical fiber fan-in 2, comprises seven single-mode optical fibers bundled at one end and one multi-core fiber having seven cores arranged in a triangular lattice pattern, and 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 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 to the optical axis to suppress reflection, but may be perpendicular to the optical axis.

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.

The optical amplifier 100 configured in this manner is equipped with a multi-core optical amplifying fiber 10 in which the gain difference between the multiple cores 11 is small, so that the gain difference between the multiple input signal lights can be reduced.

(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.

The inner cladding portion 52 is made of glass such as quartz 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 almost 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. In other words, one or more core portions of the core portion 11 are arranged 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.

The inner cladding portion 52 has a refractive index lower than the maximum refractive index of each core portion 11. The inner cladding portion 52 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 52a is higher than the refractive index of the layered cladding portion 52b. Furthermore, the refractive index of the inner cladding portion 52 in the vicinity of the core portion 11a is higher than the refractive index of the inner cladding portion 52 in the vicinity of the core portion 11b.

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-clad type optical fiber.

Even in the multi-core optical amplifier fiber 50 configured as described above, the difference between the gain in the core portion 11a and the gain in the core portion 11b is 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 11a and the core portion 11b.

Furthermore, in the multi-core optical amplifier 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 is higher the closer it is 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 each core is set to have a gain of 18 dB, 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.

(Embodiment 7)
Fig. 10 is a schematic cross-sectional view of a multi-core light amplifying fiber according to embodiment 7, showing a cross section perpendicular to the longitudinal direction of the multi-core light amplifying fiber. The multi-core light 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 light amplifying fiber 10 shown in Fig. 1 are replaced with an inner cladding 62 and an outer cladding 63, respectively.

The inner cladding portion 62 is made of glass such as quartz 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 circular portion having a cross section with a central axis 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, it 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.

The inner cladding portion 62 has a refractive index lower than the maximum refractive index of each core portion 11. The inner cladding portion 62 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 62a is higher than the refractive index of the layered cladding portion 62b. Furthermore, the refractive index of the inner cladding portion 62 in the vicinity of the core portion 11a is higher than the refractive index of the inner cladding portion 62 in the vicinity of the core portion 11b.

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-clad type optical fiber.

Even in the multi-core optical amplifier fiber 60 configured as described above, the difference between the gain in core portion 11a and the gain in core portion 11b is 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 core portion 11a and core portion 11b.

Furthermore, in the multi-core optical amplifier 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 is higher the closer it is 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 each core is set to have a gain of 18 dB, 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.

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

The core portions 11 are arranged in a triangular lattice pattern that achieves a closest-packed state. Three of the core portions 11j are arranged at the corners of an equilateral triangle around the central axis X that extends 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.

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 almost the same as the central axis X. The layered cladding portion 72b is a portion having a circular cross section that surrounds the outer periphery of the layered cladding portion 72a. The layered cladding portion 72c is a portion having 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. 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.

The inner cladding portion 72 has a refractive index lower than the maximum refractive index of each core portion 11. The inner cladding portion 72 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 72a is higher than the refractive index of the layered cladding portion 72b. The refractive index of the layered cladding portion 72b is also higher than the refractive index of the layered cladding portion 72c. Furthermore, the refractive index of the inner cladding portion 52 in the vicinity of the core portion 11j is higher than the refractive index of the inner cladding portion 52 in the vicinity of the core portion 11k, and the refractive index of the inner cladding portion 52 in the vicinity of the core portion 11k is higher than the refractive index of the inner cladding portion 52 in the vicinity of the core portion 11l.

In the multi-core optical amplifier fiber 70, the distance from the central axis X of the inner cladding 72 to the three cores 11j is d71. The distances from the central axis X of the cores 11j are approximately equal. The distance from the central axis X of the three cores 11k is d72. The distance from the central axis X of the cores 11k is approximately equal. The distance from the central axis X of the six cores 11l is d73 from the central axis X of the inner cladding 72. The distance from the central axis X of the cores 11l is 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.

Even in the multi-core optical amplifier fiber 70 configured as described above, the difference between the gain in the core portion 11j and the gain in the core portion 11l is reduced due to the effect of the GI-type refractive index profile of the inner cladding portion 72 and the effect of the difference in mode field diameter between the core portion 11j, the core portion 11k, and the core portion 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 portion is set to have a gain of 18 dB. However, the gain difference is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

(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 type and 12-core type 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 portion 72 is replaced with an inner cladding portion 82.

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 the central axis X that extends 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.

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 multiple layered cladding portions formed concentrically. The layered cladding portion 82a is a portion having a circular cross section whose central axis is approximately 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. In other words, one or more of the core portions 11 are arranged in each of the layered cladding portions 82a and 82b.

The inner cladding portion 82 has a refractive index lower than the maximum refractive index of each core portion 11. The inner cladding portion 82 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 82a is higher than the refractive index of the layered cladding portion 82b. Furthermore, the refractive index of the inner cladding portion 82 in the vicinity of the core portion 11m is higher than the refractive index of the inner cladding portion 82 in the vicinity of the core portion 11n.

Even in the multi-core optical amplifier fiber 80 configured as described above, the difference between the gain in the core portion 11m and the gain in the core portion 11n is 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 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 each core portion is set to have a gain of 18 dB. However, the gain difference is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

(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 type and 16-core type 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.

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

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 almost the same as the central axis X. The layered cladding portion 92b is a portion having a circular cross section that surrounds the outer periphery of the layered cladding portion 92a. The layered cladding portion 92c is a portion having 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. That is, one or more core portions of the core portion 11 are arranged in each of the layered cladding portions 92a, 92b, and 92c.

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 also 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.

Even in the multi-core optical amplifier fiber 90 configured as described above, the difference between the gain in the core portion 11o and the gain in the core portion 11q is reduced due to the effect of the GI-type refractive index profile of the inner cladding portion 92 and the effect of the difference in mode field diameter between the core portion 11o, the core portion 11p, and the core portion 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 portion is set to have a gain of 18 dB. However, the gain difference is more preferably 0.5 dB or less, and even more preferably 0.3 dB or less.

In the above embodiment, the number of core parts in the multi-core optical amplifier 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 embodiment, the optical amplifier medium in the multi-core optical amplifier fiber is erbium, but it may be other optical amplifier medium such as ytterbium (Yb), thulium (Tm), or praseodymium (Pr). The wavelength of the excitation light may be appropriately set to a wavelength that can optically excite the optical amplifier medium.

In addition, in the above embodiment, each core of the multi-core optical amplifier 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.

In addition, in the above embodiment, the multiple cores are arranged to have n-fold rotational symmetry within the inner cladding, where n is 2, 3, 4, or 6, but 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.

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

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 of the multi-core optical amplification fiber 10, and the pump stripper 5 may be connected to the optical fiber fan-in 2 side. In addition, if the power of the pumping light output from the multi-core optical amplification fiber 10 that does not contribute to optical amplification is relatively low, there are cases where it is not necessary to provide the pump stripper 5.

Furthermore, the present invention is not limited to the above-mentioned embodiment. The present invention also includes configurations 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.

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 section 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 (11)

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 away from a central axis of the inner cladding portion extending along a longitudinal direction, and a second core portion that is a second distance away from the central axis that is greater than the first distance;
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 when pumping light of a predetermined power that optically pumps the rare earth element is propagated through the inner cladding section and signal light that is optically amplified by the rare earth element is propagated through each of the first core section and the second core section. 2. The multi-core optical amplifying fiber according to claim 1, wherein 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 , wherein a refractive index of a central region including the central axis of the inner cladding portion is higher than a refractive index of a peripheral region surrounding the central region of the inner cladding portion. The multi-core optical amplifying fiber according to claim 3 , wherein the inner cladding portion has a GI type refractive index profile. The multi-core optical amplifying fiber according to claim 3 , wherein the inner cladding portion has a step-type refractive index profile. The multi-core optical amplifying fiber according to claim 3 , wherein a 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. 2. The multi-core optical amplifying fiber according to claim 1, wherein 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. 8. The multi-core optical amplifying fiber according to claim 1, wherein the plurality of cores are arranged so as to have n-fold rotational symmetry in the inner cladding, n being an integer of 2 or more. The inner cladding portion has a plurality of layered cladding portions formed concentrically,
The multi-core optical amplifying fiber according to any one of claims 1 to 7, wherein one or more of the plurality of cores are arranged in each of the plurality of layered cladding portions. 10. The multi-core optical amplifying fiber according to claim 9, wherein a 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 is greater than a mode field diameter at the wavelength of the signal light of the core portion arranged in a second layered cladding portion adjacent to an outer periphery of the first layered cladding portion by 10% to 20%. 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 of the multi-core optical amplifier fiber;
an optical coupler that optically couples the pumping light to the inner cladding portion;
An optical amplifier comprising:

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