Optically amplifying fiber, optical fiber amplifier and optical communication system

Abstract

An optical amplification fibre is provided with at least one core part to which a rare earth element is added, an inside cladding part that surrounds the core part and that has a refractive index lower than the maximum refractive index of each of the core parts, and an outside cladding part that surrounds the inside cladding part and that has a refractive index lower than the refractive index of the inside cladding part, the inside cladding part having a refractive index difference region in which the refractive index is different to that of the adjacent region.

Description

[Technical Field]

[0001] The present invention relates to an optical amplifying fiber, an optical fiber amplifier, and an optical communication system. [Background technology]

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

[0003] Regarding multi-core EDFA, a configuration is known in which a double-clad multi-core EDF is used as a multi-core optical amplifying fiber, and erbium (Er), a rare earth element contained in the core, is optically pumped by a cladding pumping method (see Non-Patent Documents 1 and 2). [Prior art documents] [Non-patent literature]

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

[0005] Communication traffic is constantly increasing, and therefore, in order to increase communication capacity, more suitable characteristics are required for multi-core optical amplifying fibers.

[0006] In particular, if the pumping efficiency of a multi-core optical fiber amplifier can be improved, it is preferable from the viewpoint of reducing the power consumption of the multi-core optical fiber amplifier. Here, the pumping efficiency is expressed, for example, as the ratio of the energy of the pumping light used for optical amplification to the energy of the pumping light input into the multi-core optical fiber amplifier. Note that the improvement of the pumping efficiency is beneficial not only for multi-core optical fiber amplifiers but also for single-core optical fiber amplifiers.

[0007] The present invention has been made in view of the above, and has as its object to provide an optical amplifying fiber with improved pumping efficiency, as well as an optical fiber amplifier and an optical communication system using the same. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is an optical amplifying fiber comprising: at least one core portion doped with a rare earth element; an inner cladding portion surrounding the at least one core portion and having a refractive index lower than the maximum refractive index of any of the core portions; and an outer cladding portion surrounding the inner cladding portion and having a refractive index lower than that of the inner cladding portion, wherein the inner cladding portion includes a modified refractive index region having a refractive index different from that of an adjacent region.

[0009] The modified refractive index area is made of silica glass containing a dopant for adjusting the refractive index, and the dopant for adjusting the refractive index may be fluorine (F), germanium (Ge), phosphorus (P), boron (B), an alkali metal, chlorine (Cl), or aluminum (Al).

[0010] The inner cladding portion may have two or more layers of the modified refractive index areas in the radial direction in a cross section perpendicular to the axial direction of the light amplifying fiber.

[0011] The modified refractive index area may be present at a position spaced apart from the core portion by at least the core diameter in a cross section perpendicular to the axial direction of the light amplifying fiber.

[0012] The modified refractive index area may be located in a rotationally symmetrical position with the center of the light amplifying fiber as the axis.

[0013] The modified refractive index areas may be located at the positions of lattice points when a hexagonal close-packed lattice is defined in a cross section perpendicular to the axial direction of the light-amplifying fiber.

[0014] The modified refractive index area may be present in a circular ring shape having a radius equal to or less than half the distance between lattice points, with a hexagonal close-packed lattice centered at a certain lattice point, when a hexagonal close-packed lattice is defined in a cross section perpendicular to the axial direction of the optical amplifying fiber.

[0015] A plurality of the modified refractive index areas may be dispersed within the inner cladding portion.

[0016] In a cross section perpendicular to the axial direction of the optical amplifying fiber, the total cross-sectional area of ​​the plurality of modified refractive index areas may be 0.1% or more and 30% or less of the cross-sectional area of ​​the inner cladding portion.

[0017] The diameter of the modified refractive index area may be 1 / 2000 to 2 times the wavelength of light propagating through the inner cladding portion.

[0018] The modified refractive index area may be present in an annular area spaced apart from the core portion by at least the core diameter in a cross section perpendicular to the axial direction of the light amplifying fiber.

[0019] The modified refractive index areas may be distributed approximately uniformly in the radial direction of each core portion of the light amplifying fiber.

[0020] The modified refractive index areas may be distributed approximately uniformly in the axial direction of the light amplifying fiber.

[0021] The modified refractive index areas may be distributed approximately uniformly in a direction around the axis of each core portion of the light amplifying fiber.

[0022] The optical fiber may have a plurality of the core portions, and the density of the modified refractive index areas may differ between the inside and outside of a cylindrical boundary that passes through the core portion farthest from the center of the optical amplifying fiber, with the center of the optical amplifying fiber as its axis.

[0023] The rare earth element may include erbium.

[0024] When the wavelength of the pumping light is 976 nm±2 nm and the cladding absorption coefficient is defined as -10×log((pumping light power (W) that has passed through the optical amplifying fiber having a plurality of core portions and exited) / (pumping light power (W) that has entered the inner cladding portion of the optical amplifying fiber having a plurality of core portions)) / length (m) of the optical amplifying fiber, the cladding absorption coefficient may be 0.05 dB / m or more.

[0025] One aspect of the present invention is an optical fiber amplifier including: the optical amplifying fiber; a pumping light source that outputs pumping light that optically pumps the rare earth element in the optical amplifying fiber; and an optical coupler that optically couples the pumping light to the inner cladding portion.

[0026] The optical fiber may include a plurality of the core sections, and the gain difference between the plurality of core sections may be 3 dB or less.

[0027] One aspect of the present invention is an optical communication system including the optical fiber amplifier. [Effects of the Invention]

[0028] According to the present invention, an optical amplifying fiber with improved pumping efficiency can be realized. [Brief explanation of the drawings]

[0029] [Figure 1] FIG. 1 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to a first embodiment. [Figure 2] FIG. 2 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to a second embodiment. [Figure 3] FIG. 3 is an explanatory diagram of an example of a method for manufacturing a multi-core optical amplifying fiber according to the second embodiment. [Figure 4] FIG. 4 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to a third embodiment. [Figure 5] FIG. 5 is an explanatory diagram of an example of a method for manufacturing a multi-core optical amplifying fiber according to the third embodiment. [Figure 6] FIG. 6 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to a fourth embodiment. [Figure 7] FIG. 7 is a schematic cross-sectional view of the multi-core optical amplifying fiber shown in FIG. 6 at a cross section different from that shown in FIG. [Figure 8] FIG. 8 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to the fifth embodiment. [Figure 9] FIG. 9 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to a sixth embodiment. [Figure 10] FIG. 10 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to the seventh embodiment. [Figure 11] FIG. 11 is a schematic diagram showing the configuration of a multi-core optical fiber amplifier according to the eighth embodiment. [Figure 12] FIG. 12 is a diagram showing an example of the absorption spectrum of a multi-core optical amplifying fiber. [Figure 13] FIG. 13 is a schematic diagram showing the configuration of an optical communication system according to the twelfth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0030] Hereinafter, embodiments will be described with reference to the drawings. However, the present invention is not limited to these embodiments. In addition, in the drawings, identical or corresponding elements are appropriately designated by the same reference numerals. 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 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 follow the definitions and measurement methods in G.650.1 and G.650.2.

[0031] (Embodiment 1) 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 axial direction of the multi-core optical amplifying fiber. The multi-core optical amplifying fiber 1 is a double-clad seven-core multi-core optical fiber including seven core portions 1a as a plurality of core portions, an inner cladding portion 1b surrounding the core portions 1a, and an outer cladding portion 1c surrounding the inner cladding portion 1b.

[0032] The core portions 1a are arranged in a triangular lattice pattern to achieve a close-packed state. That is, one core portion 1a is arranged at or near the center of the inner cladding portion 1b. The six core portions 1a are arranged at the corner positions of a regular hexagon with this core portion 1a at the center. When a hexagonal close-packed lattice is defined in a cross section perpendicular to the axial direction of the multi-core optical amplifying fiber 1, the core portions 1a can also be said to be located at the lattice points. The core portions 1a contain, for example, germanium (Ge) or aluminum (Al) as a refractive index adjusting dopant that increases the refractive index. The core portions 1a also contain erbium (Er) as a rare earth element that serves as an amplification medium. Er is doped at a concentration such that the absorption coefficient peaks at a wavelength of 1530 nm, for example, between 2.5 dB / m and 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. Al also has the function of suppressing concentration quenching of Er.

[0033] The inner cladding portion 1b has a refractive index lower than the maximum refractive index of each core portion 1a. The refractive index profile of each core portion 1a and the inner cladding portion 1b is, for example, a step-index type. The inner cladding portion 1b may have a trench portion located on the outer periphery of each core portion 1a. In this case, the trench portion is made of silica glass doped with a refractive index adjusting dopant such as fluorine (F) that lowers the refractive index, and the refractive index of the trench portion is lower than the refractive index of the rest of the inner cladding portion 1b. In this case, the refractive index profile of each core portion 1a and the inner cladding portion 1b is a trench type.

[0034] The inner cladding portion 1b includes an inner region 1ba having a circular cross section and surrounding the core portion 1a, an annular and layered modified refractive index region 1bb surrounding the inner region 1ba, an annular and layered modified refractive index region 1bc surrounding the modified refractive index region 1bb, an annular and layered modified refractive index region 1bd surrounding the modified refractive index region 1bc, and an annular and layered modified refractive index region 1be surrounding the modified refractive index region 1bd. The modified refractive index regions 1bb, 1bc, 1bd, and 1be are regions with refractive indices different from those of their adjacent regions. Specifically, the modified refractive index region 1bb has a refractive index different from that of the adjacent regions, the inner region 1ba and the modified refractive index region 1bc. The modified refractive index region 1bc has a refractive index different from that of the adjacent regions, the modified refractive index region 1bb and the modified refractive index region 1bd. The modified refractive index region 1bd has a refractive index different from that of the adjacent regions, the modified refractive index region 1bc and the modified refractive index region 1be.

[0035] The inner region 1ba is made of pure silica glass containing no refractive index-adjusting dopants, while the modified refractive index regions 1bb, 1bc, 1bd, and 1be are made of silica glass containing refractive index-adjusting dopants. Examples of refractive index-adjusting dopants include F, Ge, phosphorus (P), boron (B), alkali metals such as sodium (Na) and potassium (K), chlorine (Cl), and Al. One or more dopants selected from these dopants are added to the modified refractive index regions 1bb, 1bc, 1bd, and 1be. The difference in refractive index between the modified refractive index regions 1bb, 1bc, 1ed, and 1ee can be achieved by changing the type and amount of the dopant added. The greater the absolute value of the relative refractive index difference between one region and the other, the better. The difference in refractive index between the inner region 1ba and the modified refractive index regions 1bb, 1bc, 1ed, and 1ee, the better. It is, for example, 0.3% or more, and more preferably 0.7% or more. Furthermore, if the layer thickness of the modified refractive index areas 1bb, 1bc, 1ed, and 1ee is at least several times the wavelength of the propagating light, differences in refractive index tend to occur as an average value on the order of wavelength. As will be described later, the inner cladding portion 1b propagates pumping light with a wavelength capable of optically pumping Er, for example, pumping light in the 900 nm wavelength band such as 976 nm. Therefore, a layer thickness of at least 1 μm or more is preferable. Furthermore, the layer thickness can be appropriately set depending on the outer diameter of the inner cladding portion 1b.

[0036] In the multi-core optical amplifying fiber 1, the modified refractive index areas 1bb, 1bc, 1bd, and 1be provide the inner cladding portion 1b with at least two layers of modified refractive index areas in the radial direction, that is, four layers.

[0037] If the relative refractive index difference of each core portion 1a with respect to the glass of the inner region 1ba is designated as core Δ, in this embodiment, the core Δ of each core portion 1a is approximately equal, for example, 0.35% to 2% at a wavelength of 1550 nm. The core diameter of the core portion 1a is preferably set in relation to core Δ so as to achieve a cutoff wavelength shorter than the optical amplification wavelength band in which the rare earth element can amplify light. In the case of Er, the optical amplification wavelength band is, for example, 1530 nm to 1565 nm, known as the C band, or 1565 nm to 1625 nm, known as the L band. The core diameter is, for example, approximately 5 μm to 10 μm.

[0038] The outer cladding 1c has a refractive index lower than that of the inner cladding 1b and is made of, for example, a resin. If the inner cladding 1b has a trench portion corresponding to the core 1a, the refractive index of the outer cladding 1c may be higher than that of the trench portion, but lower than the refractive index of the rest of the inner cladding 1b and the average refractive index of the inner cladding 1b.

[0039] When pumping light of a wavelength capable of optically pumping Er, for example, pumping light in the 900 nm wavelength band such as 976 nm, is input to the inner cladding 1b, the pumping light optically pumps Er doped in each core 1a while propagating inside the inner cladding 1b. This enables each core 1a to optically amplify the signal light input to it. In this way, the multi-core optical amplifying fiber 1 is configured to be applicable to the cladding pumping method.

[0040] In the multi-core optical amplifying fiber 1, interfaces (hereinafter sometimes referred to as modified refractive index interfaces) in the inner region 1ba and the modified refractive index regions 1bb, 1bc, 1bd, and 1be, where the refractive index differs across the interface, scatter the pumping light propagating through the inner cladding 1b. As a result, more components of the pumping light propagating through the inner cladding 1b reach the core 1a. For example, in the case of a cladding pumping method such as the multi-core optical amplifying fiber 1, unused components that do not contribute to pumping, such as a skew component S, usually propagate so as not to reach the core 1a. However, in the multi-core optical amplifying fiber 1, unused components such as the skew component S are scattered by the modified refractive index interfaces, and some of them reach the core 1a and can be used for optical pumping of Er. Note that since the pumping light propagates through the inner cladding 1b in multiple modes, the skew component S may also have modes that propagate at various angles. By scattering these various skew components S by the modified refractive index interfaces, some of them can reach the core 1a and be more likely to be used for optical pumping of Er.

[0041] In the multi-core optical amplifying fiber 1 configured as described above, the modified refractive index interfaces scatter the pumping light propagating through the inner cladding 1b, increasing the component of the pumping light that reaches the core 1a, thereby improving the pumping efficiency. In addition, the effect of the modified refractive index interfaces can be adjusted by adjusting the number, thickness, and refractive index difference of the layers of the modified refractive index regions in the radial direction. For example, the layers of the modified refractive index regions can be designed to have one or more layers each, for a total of two or more layers.

[0042] The multi-core optical amplifying fiber 1 can be manufactured using a known multi-core fiber manufacturing method, such as a stacking method or a drilling method. For example, in the case of the drilling method, seven holes extending parallel to the axial direction are formed in a preform rod, and a core rod, which is a glass rod including a portion that will become the core portion 1a and a portion that will become part of the inner region 1ba, is inserted into each hole to form a preform. Next, this preform is drawn to form the outer cladding portion 1c.

[0043] The base material rod used in the above method can be produced by, for example, a vapor-phase axial deposition (VAD) method, an outside vapor deposition (OVD) method, a modified chemical vapor deposition (MCVD) method, or a plasma CVD method. In this case, the base material rod is formed by depositing a soot layer made of glass particles, which will become the inner region 1ba, the modified refractive index region 1bb, the modified refractive index region 1bc, the modified refractive index region 1bd, and the modified refractive index region 1be, and then dehydrating and vitrifying them by heat treatment.

[0044] The base material rod used in the above method can also be produced by the jacket method, in which a glass rod that will become the inner region 1ba is inserted into a jacket tube that will become each of the modified refractive index regions 1bb, 1bc, 1bd, and 1be, and the jacket tubes are then inserted into the glass rod so that they successively cover the modified refractive index regions 1bb, 1bc, 1bd, and 1be, and the resulting tubes are integrated by heat treatment.

[0045] (Embodiment 2) Fig. 2 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to embodiment 2. This multi-core optical amplifying fiber 2 has a configuration in which the inner cladding 1b in the multi-core optical amplifying fiber 1 according to embodiment 1 shown in Fig. 1 is replaced with an inner cladding 1d. The inner cladding 1d has a configuration in which the modified refractive index areas 1bb, 1bc, 1bd, and 1be of the inner cladding 1b are replaced with areas made of the same material as the inner area 1ba, and is provided with a plurality of modified refractive index areas 1da each having a circular cross section. In this embodiment, the number of modified refractive index areas 1da is six, but the number is not limited thereto.

[0046] The modified refractive index areas 1da have a refractive index different from that of adjacent areas in the inner cladding portion 1d. The modified refractive index areas 1da are located on the outer periphery of the regular hexagon formed by the core portions 1a. The modified refractive index areas 1da are located at positions that are rotationally symmetric with respect to the center of the multi-core light amplifying fiber 2, and in this embodiment, are located at positions that are six-fold rotationally symmetric. When a hexagonal close-packed lattice is defined in a cross section orthogonal to the axial direction of the multi-core light amplifying fiber 2, each modified refractive index area 1da is located at the position of the lattice point.

[0047] In the multi-core optical amplifying fiber 2 configured as above, the pumping efficiency is improved by the effect of the modified refractive index interface of the modified refractive index region 1da, as in the multi-core optical amplifying fiber 1. In addition, the effect of the modified refractive index interface can be adjusted by adjusting the position and rotational symmetry of the modified refractive index region 1da. For example, the rotational symmetry may be two-fold or three-fold rotational symmetry.

[0048] The modified refractive index area 1da may be located on the inner periphery of the regular hexagon formed by the core portion 1a or on the same periphery.

[0049] The multi-core optical amplifying fiber 2 can be manufactured by utilizing a known method for manufacturing a multi-core fiber. For example, the case of the hole-punching method will be described with reference to FIG.

[0050] That is, as shown in Fig. 3, seven holes 21a and six holes 21b extending parallel to the axial direction are formed in a base material rod 21 that will become part of the inner cladding portion 1d. Then, a core rod 22 is inserted into the holes 21a, and a glass rod 23 that will become the modified refractive index area 1da is inserted into the holes 21b to form a base material. The core rod 22 is a glass rod that includes a core portion 22a that will become the core portion 1a, and a cladding portion 22b that surrounds the core portion 22a and will become part of the inner cladding portion 1d. Next, this base material is drawn to form the outer cladding portion 1c.

[0051] (Embodiment 3) Fig. 4 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to embodiment 3. This multi-core optical amplifying fiber 3 has a configuration in which the inner cladding 1d in the multi-core optical amplifying fiber 2 according to embodiment 2 shown in Fig. 2 is replaced with an inner cladding 1e. The inner cladding 1e has a configuration in which the modified refractive index area 1da in the inner cladding 1d is deleted and a plurality of modified refractive index areas 1ea each having a circular cross section are provided. In this embodiment, the number of modified refractive index areas 1ea is 12, but the number is not limited thereto.

[0052] The modified refractive index areas 1ea are located on the outer periphery of the regular hexagon formed by the core portions 1a. The modified refractive index areas 1ea are located at positions with rotational symmetry about the center of the multi-core light amplifying fiber 3, and in this embodiment, are located at positions with six-fold rotational symmetry. When a hexagonal close-packed lattice is defined in a cross section perpendicular to the axial direction of the multi-core light amplifying fiber 3, each modified refractive index area 1ea is located at the position of the lattice point. Furthermore, in this embodiment, each core portion 1a and each modified refractive index area 1ea are located at the position of the lattice point of the same hexagonal close-packed lattice.

[0053] In the multi-core optical amplifying fiber 3 configured as above, the pumping efficiency is improved by the effect of the modified refractive index interface of the modified refractive index region 1ea, similar to the multi-core optical amplifying fibers 1 and 2. In addition, the effect of the modified refractive index interface can be adjusted by adjusting the position, number, and rotational symmetry of the modified refractive index region 1ea.

[0054] The modified refractive index interface may be located on the inner periphery of the regular hexagon formed by the core portion 1a or on the same periphery.

[0055] The multi-core optical amplifying fiber 3 can be manufactured by utilizing a known method for manufacturing a multi-core fiber. For example, the stack method will be described with reference to FIG.

[0056] Specifically, as shown in FIG. 5, seven core rods 22 are stacked in a glass tube 31 that will form part of the inner cladding portion 1e. At the same time, 12 glass rods 32 that will form the modified refractive index areas 1ea are stacked in the gap 33 between the core rods 22 and the glass tube 31 to form a base material. By making the diameters of the core rods 22 and the glass rods 32 equal, a structure can be achieved in which each core portion 1a and each modified refractive index area 1ea is located at the same lattice point of a hexagonal close-packed lattice. Furthermore, glass rods that will form part of the inner cladding portion 1e and are made of the same material as the cladding portion 22b are stacked in the remaining portion of the gap 33. Next, this base material is drawn to form the outer cladding portion 1c. Note that the number of modified refractive index areas 1ea can be reduced from 12 by replacing one or more of the 12 glass rods 32 with glass rods made of the same material as the cladding portion 22b.

[0057] (Embodiment 4) Fig. 6 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to embodiment 4. In Fig. 6, the direction perpendicular to the drawing is the axial direction Dz of the multi-core optical amplifying fiber 4. Also, in Fig. 6, the radial direction Dr and the axial direction Dt of the multi-core optical amplifying fiber 4 are defined. Note that in other figures, the axial direction Dz, radial direction Dr and axial direction Dt can be defined in the same way as in Fig. 6.

[0058] This multi-core optical amplifying fiber 4 has a configuration in which the inner cladding 1d is replaced with an inner cladding 1f in the multi-core optical amplifying fiber 2 according to the second embodiment shown in Fig. 2. The inner cladding 1f has a configuration in which the modified refractive index area 1da in the inner cladding 1d is deleted and a plurality of modified refractive index areas 1fa are provided.

[0059] The modified refractive index areas 1fa are dispersed within the inner cladding portion 1f. The modified refractive index areas 1fa are composed of, for example, microcrystals or clusters dispersed in silica glass. The modified refractive index areas 1fa are composed of, for example, Al, Ge, or an alkali metal. The greater the difference in refractive index between the modified refractive index areas 1fa and the surrounding glass material, the better.

[0060] In the multi-core optical amplifying fiber 4 configured as above, the pumping efficiency is improved by the effect of the modified refractive index interface of the modified refractive index region 1fa, similar to the multi-core optical amplifying fibers 1 to 3. Furthermore, the effect of the modified refractive index interface can be adjusted by adjusting the size and density of the modified refractive index region 1fa.

[0061] Furthermore, in the multi-core optical amplifying fiber 4, no modified refractive index area 1fa exists in the area of ​​the inner cladding 1f near the boundary with the outer cladding 1c, which allows the scattering of the skew component S to be appropriately adjusted, and makes it possible to suppress the generation of scattered light that travels away from the core 1a near the boundary between the inner cladding 1f and the outer cladding 1c.

[0062] In the multi-core optical amplifier fiber 4, in a cross section perpendicular to the axial direction Dz as shown in FIG. 6 , if the total cross-sectional area of ​​the multiple modified refractive index regions 1fa to the cross-sectional area of ​​the inner cladding portion 1f is defined as the cross-sectional area ratio, the cross-sectional area ratio is preferably, for example, 0.1% to 30%, more preferably 1% or more. If the cross-sectional area ratio is 0.1% or more, the multiple modified refractive index regions 1fa are likely to exhibit an improvement in pumping efficiency, and if it is 1% or more, the improvement effect is more likely to be exhibited. If the cross-sectional area ratio is 30% or less, it is easy to manufacture the multi-core optical amplifier fiber 4 with desired optical characteristics (amplification characteristics, etc.). If the cross-sectional area ratio is greater than 30%, the effect of scattering the pumping light becomes too strong, which may increase the propagation loss of the pumping light in the inner cladding portion 1f. In this case, the effect of improving the pumping efficiency by scattering the pumping light by the modified refractive index regions 1fa may be outweighed by the effect of deteriorating the pumping efficiency due to the increased propagation loss.

[0063] Furthermore, the diameter of the modified refractive index region 1fa in the cross section shown in FIG. 6 is preferably between 1 / 2000 and 2 times the wavelength of the light (excitation light) propagating through the inner cladding portion 1b. When the diameter of the modified refractive index region 1fa, which serves as a scatterer, is between 1 / 20 and 2 times the wavelength of the excitation light, the scattering of light by the modified refractive index region 1fa is primarily Mie scattering. When the diameter of the modified refractive index region 1fa is between 1 / 2000 and 1 / 20 times the wavelength of the excitation light, the scattering of light by the modified refractive index region 1fa is primarily Rayleigh scattering. Thus, when a light wave collides with a particulate material or a refractive index change, the type of scattering varies depending on the size of the particle. Here, forward scattering is dominant in Mie scattering, while isotropic scattering is dominant in Rayleigh scattering. However, both types of scattering are considered to contribute to improving the excitation efficiency. For example, by utilizing the scattering direction characteristics of these types of scattering, the degree of improvement in the excitation efficiency may be enhanced by spatially distributing modified refractive index regions 1fa with different diameters depending on the arrangement of the core portion 1a and the electric field distribution of the excitation light in the inner cladding portion 1f. For example, Rayleigh scattering contributes greatly to improving excitation efficiency when it is more effective to significantly change the propagation direction.

[0064] If the cross section of the modified refractive index area 1fa is not circular, the diameter of the modified refractive index area 1fa may be defined as the diameter of a circle equal to the cross-sectional area of ​​the modified refractive index area 1fa.

[0065] Furthermore, it is preferable that the multiple modified refractive index areas 1fa are randomly present within the inner cladding portion 1f, since this facilitates the excitation light scattering effect to be uniform for each core portion 1a. The random presence of the multiple modified refractive index areas 1fa within the inner cladding portion 1f can be rephrased as meaning that the distribution of the modified refractive index areas 1fa within the inner cladding portion 1f is not biased and is distributed approximately uniformly. Therefore, for example, it is preferable that the modified refractive index areas 1fa are distributed approximately uniformly in the axial direction Dz.

[0066] Fig. 7 is a schematic cross-sectional view of the multi-core optical amplifying fiber 4 shown in Fig. 6 at a cross-section different from that shown in Fig. 6. The cross-section shown in Fig. 7 is taken at a position moved in the axial direction Dz from the cross-section shown in Fig. 6 by a distance small relative to the length, for example, about 1% to 5% of the length of the multi-core optical amplifying fiber 4. In the cross-section shown in Fig. 7, the position of the modified refractive index region 1fa is different from that in the cross-section shown in Fig. 6 just by moving slightly in the axial direction Dz. In this way, the position of the modified refractive index region 1fa may differ from one cross-section to another.

[0067] Similarly, for example, it is preferable that the modified refractive index areas 1fa are distributed approximately uniformly in the radial direction Dr. It is also preferable that the modified refractive index areas 1fa are distributed approximately uniformly in the radial direction of each core portion 1a. It is also preferable that the modified refractive index areas 1fa are distributed approximately uniformly in the axial direction Dt. In this case, the positions of the modified refractive index areas 1fa in the range of 0 to 60 degrees from the reference angle position in the axial direction Dt may be different from the positions of the modified refractive index areas 1fa in the range of 60 to 120 degrees from the reference angle position. It is also preferable that the modified refractive index areas 1fa are distributed approximately uniformly in the axial direction of each core portion 1a.

[0068] The multi-core optical amplifying fiber 4 can be manufactured by utilizing a known method for manufacturing a multi-core fiber. The modified refractive index region 1fa can be formed by adding a constituent material of the modified refractive index region 1fa to a base material when manufacturing a portion that will become the inner cladding portion 1f, and aggregating the material by heat treatment.

[0069] In the method using fine particles, for example, fine particles including modified refractive index areas 1fa may be used.

[0070] (Embodiment 5) 8 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to embodiment 5, showing a cross section perpendicular to the axial direction of the multi-core optical amplifying fiber. The multi-core optical amplifying fiber 5 differs from the multi-core optical amplifying fiber 4 according to embodiment 4 in that the inner cladding 1f has a region R1 surrounding each core 1a.

[0071] Each region R1 is a concentric region with respect to each core portion 1a, has a diameter, for example, three times or more the core diameter of the surrounding core portion 1a, and is a cylindrical region extending in the axial direction Dz along each core portion 1a. The modified refractive index region 1fa is not included in this region R1.

[0072] In the multi-core light amplifying fiber 5 configured as above, the modified refractive index region 1fa is present at a position at a distance of at least the core diameter, for example, at a distance of at least three times the core diameter, from each core 1a in a cross section orthogonal to the axial direction Dz of the multi-core light amplifying fiber 5. As a result, the multi-core light amplifying fiber 5 improves pumping efficiency, similar to the multi-core light amplifying fibers 1 to 4. Furthermore, it is possible to prevent pumping light that has been scattered by the modified refractive index region 1fa and headed toward the core 1a from being scattered again by a modified refractive index region in the vicinity of the core 1a. Furthermore, since the modified refractive index region 1fa is relatively far from each core 1a, it is also possible to prevent the modified refractive index region 1fa from affecting the light propagation characteristics of each core 1a.

[0073] The multi-core optical amplifying fiber 5 can be manufactured using a known multi-core fiber manufacturing method. For example, in the case of the stacking method, seven glass rods, each including a core portion 1a and a region R1, are stacked in a glass tube that will form part of the inner cladding portion 1f. Next, a glass rod that will form part of the inner cladding portion 1f and includes a modified refractive index region 1fa is stacked in the gap between the core rod and the glass tube to form a preform. Next, this preform is drawn to form the outer cladding portion 1c. In the case of the drilling method, seven holes extending parallel to the axial direction are formed in a preform rod, which is a relatively large-diameter glass rod including the modified refractive index region 1fa that will form part of the inner cladding portion 1f, and a core rod is inserted into each hole to form the preform. Next, this preform is drawn to form the outer cladding portion 1c. Note that the modified refractive index region 1fa does not have to be formed in the glass rod from the beginning; for example, it may be formed by agglomerating a dopant by heat treatment during the manufacturing process.

[0074] (Embodiment 6) Fig. 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 axial direction of the multi-core optical amplifying fiber. The multi-core optical amplifying fiber 6 differs from the multi-core optical amplifying fiber 4 according to embodiment 4 in that regions R2 and R3 exist in the inner cladding 1f. Note that the modified refractive index regions are not shown in Fig. 9.

[0075] Region R2 is a cylindrical region that passes through the centers of the six core portions 1a that are arranged at the corner positions of a regular hexagon in the inner cladding portion 1f and extends in the axial direction Dz. Region R3 is a cylindrical region that is located on the outer periphery of Region R2 in the inner cladding portion 1b and extends in the axial direction Dz. The boundary between Region R2 and Region R3 is an example of a cylindrical boundary that passes through the core portion 1a that is farthest from the center of the multi-core optical amplifier fiber 6. The axis of the boundary is the center of the multi-core optical amplifier fiber 6.

[0076] In the multi-core optical amplifying fiber 6, the density of the modified refractive index areas differs between an area R2 inside the boundary and an area R3 outside the boundary. For example, the density of the modified refractive index areas in the area R2 is higher than the density of the modified refractive index areas in the area R3. Also, for example, the density of the modified refractive index areas in the area R2 is lower than the density of the modified refractive index areas in the area R3.

[0077] The multi-core optical amplifying fiber 6 configured as above has an improved pumping efficiency, similar to the multi-core optical amplifying fibers 1 to 5. Furthermore, in the multi-core optical amplifying fiber 6, the degree of scattered light generation can be made different between the region R2 where the cores 1a are mainly present and the region R3 on the outer edge side of the inner cladding 1f where there are relatively many skew components. For example, the density of the modified refractive index regions in the region R2 may be increased to generate a lot of scattering in the region R2 where the cores 1a are mainly present, or the density of the modified refractive index regions in the region R3 may be increased to generate a lot of scattering of skew components. The density of the modified refractive index regions in each region can be designed as appropriate depending on the design and required characteristics of the multi-core optical amplifying fiber 6.

[0078] The multi-core optical amplifying fiber 6 can be manufactured using a known multi-core fiber manufacturing method. For example, in the case of the hole-punching method, a preform rod that will become part of the inner cladding portion 1f is formed so that the density of the modified refractive index regions varies depending on the location. Such a preform rod can be manufactured, for example, by the jacket method. Next, seven holes extending parallel to the axial direction are formed in this preform rod, and a core rod is inserted into each hole to form a preform. Next, this preform is drawn to form the outer cladding portion 1c. Also, in the case of the stacking method, the core rod is stacked in a glass tube that will become part of the inner cladding portion 1f. Next, a glass rod that includes the modified refractive index regions and will become part of the inner cladding portion 1f is stacked in the gap between the core rod and the glass tube to form the preform. In this case, by using glass rods with different densities of the modified refractive index regions depending on the stacking location, the density of the modified refractive index regions can be made to vary depending on the location.

[0079] (Embodiment 7) Fig. 10 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to embodiment 7, showing a cross section perpendicular to the axial direction of the multi-core optical amplifying fiber. The multi-core optical amplifying fiber 7 differs from the multi-core optical amplifying fiber 4 according to embodiment 4 in that a region R4 is present in the inner cladding 1f, and a modified refractive index region is present only in region R4. Note that the modified refractive index region is not shown in Fig. 10.

[0080] Each region R4 is a region concentric with each core 1a and is a cylindrical region extending in the axial direction Dz along each core 1a. Each region R4 may exist in an annular region spaced apart from each core 1a by at least the core diameter in a cross section perpendicular to the axial direction Dz of the multi-core optical amplifier fiber 7. Furthermore, when a hexagonal close-packed lattice is defined with each core 1a as a lattice point in a cross section perpendicular to the axial direction of the multi-core optical amplifier fiber 4, the modified refractive index region can be said to exist in an annular shape with a certain lattice point as its center and a radius equal to or less than half the distance between lattice points. The distance between lattice points is the center-to-center distance between adjacent cores 1a.

[0081] The multi-core light amplifying fiber 7 configured as above has improved pumping efficiency, similar to the multi-core light amplifying fibers 1 to 6. Furthermore, the multi-core light amplifying fiber 7 can suppress re-scattering of pumping light and can suppress the modified refractive index regions from affecting the light propagation characteristics of each core 1a. Furthermore, by setting the existence densities of the modified refractive index regions in region R4 for each core 1a to different values, the effects of the modified refractive index regions for each core 1a can be adjusted to different degrees.

[0082] The multi-core optical amplifying fiber 7 can be manufactured using a known multi-core fiber manufacturing method. For example, in the case of the hole-punching method, seven holes extending parallel to the axial direction are formed in a preform rod, and a core rod inserted in a glass tube including modified refractive index regions is inserted into each hole to form a preform. Next, this preform is drawn to form the outer cladding portion 1c. The glass tube including modified refractive index regions may have a structure in which the modified refractive index regions are arranged in multiple concentric layers, such as the modified refractive index regions 1bb, 1bc, 1bd, and 1be in FIG. 1. Such a glass tube including modified refractive index regions can be manufactured with a diameter larger than the hole to be inserted, and then drawn to a similar shape to achieve a thin outer diameter that can be inserted into the hole. Therefore, for example, a multilayered and fine refractive index distribution can be easily achieved by first forming a glass tube with a large diameter and a multilayer structure, and then drawing it.

[0083] In a configuration in which a double-clad multi-core EDF is used as the multi-core optical amplifier fiber and erbium (Er), a rare earth element contained in the core, is optically pumped by the cladding pumping method, the cladding absorption coefficient of a normal multi-core optical amplifier fiber that does not include a modified refractive index region is approximately 0.02 dB / m. In contrast, the cladding absorption is 0.05 dB / m or more when any of the multi-core optical amplifying fibers 1 to 7 is used. where: Cladding absorption coefficient = -10 × log ((pumping light power (W) transmitted through the multi-core optical amplifier fiber)) / (pumping light power (W) incident on the inner cladding of the multi-core optical amplifier fiber)) / length of the multi-core optical amplifier fiber (m) The wavelength of the excitation light is 976 nm ± 2 nm.

[0084] (Embodiment 8) Fig. 11 is a schematic diagram showing the configuration of a multi-core optical fiber amplifier according to an eighth embodiment. Hereinafter, the multi-core optical fiber amplifier may be simply referred to as an optical amplifier. The optical amplifier 100 includes seven optical isolators 110, an optical fiber fan-in (FAN IN) 120, a semiconductor laser 130, an optical coupler 140, the multi-core optical amplifying fiber 1 according to the first embodiment, a pump stripper 150, an optical fiber fan-out (FAN OUT) 160, and seven optical isolators 170. In the figure, the symbol "x" indicates a fusion splice point of the optical fibers.

[0085] The optical fiber fan-in 120 includes seven bundled single-mode optical fibers and one multi-core fiber having seven cores, and is configured such that at a coupling section, each core of the seven single-mode optical fibers is optically coupled to each core of the multi-core fiber. The seven single-mode optical fibers are, for example, standard single-mode optical fibers defined in ITU-TG.652, and each is provided with an optical isolator 110. The optical isolators 110 and 170 allow light to pass in the direction indicated by the arrow and block light from passing in the opposite direction. The multi-core fiber of the optical fiber fan-in 120 is connected to an optical coupler 140. The end faces of the bundled seven single-mode optical fibers and the multi-core fiber, where the fibers are optically coupled, are processed at an angle to the optical axis to suppress reflection, but may also be perpendicular to the optical axis. Instead of the seven optical isolators 110 and 170, an optical isolator configured to integrate multiple (seven in this embodiment) single-mode optical fibers may be used.

[0086] The multi-core fiber of the optical fiber fan-in 120 has seven cores arranged in a triangular lattice pattern, and claddings located on the outer periphery of each core and having a refractive index lower than the maximum refractive index of each core, similar to the multi-core optical amplifying fiber 1. When signal light is input to each single-mode optical fiber of the optical fiber fan-in 120, each optical isolator 110 passes each signal light, and each core of the multi-core fiber propagates each signal light.

[0087] The semiconductor laser 130, which serves as the pumping light source, is a transverse multimode semiconductor laser and outputs pumping light. The wavelength of the pumping light is 976 nm, which is approximately the same as the wavelength of the absorption peak of Er in the 900 nm wavelength band. This allows the pumping light to optically pump erbium ions. The semiconductor laser 130 outputs the pumping light from a multimode optical fiber. This multimode optical fiber is a step-index type with a core diameter / cladding diameter of, for example, 105 μm / 125 μm, and an NA of, for example, 0.16 or 0.22.

[0088] The optical coupler 140 includes a main optical fiber and an optical fiber for supplying pumping light. The main optical fiber is a double-clad optical fiber including seven cores arranged in a triangular lattice pattern similar to the cores of the multi-core fiber of the optical fiber fan-in 120, inner clads located on the outer periphery of each core and having a refractive index lower than the maximum refractive index of each core, and an outer clad located on the outer periphery of the inner clad and having a refractive index lower than that of the inner clad. The cores and inner clad are made of silica-based glass, and the outer clad is made of resin.

[0089] The pumping light supplying optical fiber is a multimode optical fiber of the same type, with the other end connected to the multimode optical fiber of the semiconductor laser 130, and is a step-index type with a core diameter / cladding diameter of, for example, 105 μm / 125 μm, and an NA of, for example, 0.16 or 0.22. The pumping light supplying optical fiber receives pumping light from the semiconductor laser 130 and supplies this pumping light to the main optical fiber. The inner cladding propagates the pumping light.

[0090] One end of the main optical fiber of the optical coupler 140 is connected to the multi-core fiber of the optical fiber fan-in 120. Each core of the multi-core fiber is connected to each core of the main optical fiber. Therefore, when each signal light propagating through each core of the multi-core fiber enters the main optical fiber, it is optically coupled to each core. Each core propagates each signal light. The pump light and the signal light are output from the main optical fiber to the multi-core optical amplifying fiber 1.

[0091] One end of the multi-core optical amplifying fiber 1 is connected to the main optical fiber of the optical coupler 140. Each core 1a of the multi-core optical amplifying fiber 1 is connected to each core of the main optical fiber. Furthermore, the inner cladding 1b of the multi-core optical amplifying fiber 1 is connected to the inner cladding of the main optical fiber. Therefore, when each signal light and pumping light propagating through the main optical fiber are input to the multi-core optical amplifying fiber 1, they propagate in the same direction through each core 1a and inner cladding 1b. The pumping light optically excites Er in each core 1a while propagating through the inner cladding 1b. Each signal light propagating through each core 1a is optically amplified by the action of stimulated emission of Er. The multi-core optical amplifying fiber 1 outputs each optically amplified signal light and pumping light that did not contribute to optical amplification.

[0092] The pump stripper 150 is a known device that removes pump light that did not contribute to optical amplification. The pump stripper 150 has a configuration in which, for example, a part of the outer cladding of a double-clad multicore fiber having seven cores is removed, and the pump light is extracted from the surface of the inner cladding of the removed part, irradiated onto a heat sink or the like, and absorbed, converting the energy of the pump light into thermal energy and dissipating it. The pump stripper 150 propagates each signal light through the multicore 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.

[0093] Similar to the optical fiber fan-in 120, the optical fiber fan-out 160 includes seven bundled single-mode optical fibers and one multi-core fiber having seven cores, and is configured so that the cores of the seven single-mode optical fibers are optically coupled to the cores of the multi-core fiber at a coupling section. Each single-mode optical fiber is provided with an optical isolator 170. The multi-core fiber is connected to a pump stripper 150. Note that the end faces of the bundled seven single-mode optical fibers and the multi-core fiber, where the fibers are optically coupled, are processed at an angle with respect to the optical axis to suppress reflection, but may also be perpendicular to the optical axis.

[0094] When signal light is input from each core of the multi-core fiber of the pump stripper 150 to each core of the optical fiber fan-out 160, each signal light propagates through each core of each single-mode optical fiber and is output through the optical isolator 170.

[0095] This optical amplifier 100 performs optical amplification using a multi-core optical amplifying fiber 1 with improved pumping efficiency, and therefore can reduce the power consumption of the semiconductor laser 130 to obtain the same amplification characteristics. Note that the gain difference between the cores 1a of the multi-core optical amplifying fiber 1 at a certain wavelength in the amplification band is preferably 3 dB or less. The gain difference can be adjusted by changing the characteristics of the multi-core optical amplifying fiber 1, for example, its length.

[0096] In the optical amplifier 100, the multi-core optical amplifying fiber 1 may be replaced with any of the multi-core optical amplifying fibers 2 to 7.

[0097] (Example of absorption spectrum of multi-core optical amplifier fiber) 12 is a diagram showing an example of the absorption spectrum of a multi-core optical amplifying fiber, which is the absorption spectrum of one core portion. In the example shown in FIG. 12, the absorption peak value is about 3.1 dB / m.

[0098] (Embodiment 9) 13 is a schematic diagram showing the configuration of an optical communication system according to the 9th embodiment. The optical communication system 1000 includes an optical transmitting device 1010, an optical receiving device 1020, an optical amplifier 100 according to the 11th embodiment, and 14 optical transmission fibers 1031 to 1037 and 1041 to 1047, which are single-core optical fibers.

[0099] The optical transmitting device 1010 includes seven transmitters 1011 to 1017. The transmitters 1011 to 1017 each transmit an optical signal. Seven optical transmission fibers 1031 to 1037 transmit the optical signals output from the transmitters 1011 to 1017, respectively, and input them to the optical amplifier 100. The optical amplifier 100 optically amplifies the seven optical signals input from the optical transmission fibers 1031 to 1037 collectively, and outputs the amplified optical signals to seven optical transmission fibers 1041 to 1047, respectively. The optical transmission fibers 1041 to 1047 transmit the amplified optical signals and input them to the optical receiving device 1020. The optical receiving device 1020 includes seven receivers 1021 to 1027. The receivers 1021 to 1027 receive the amplified signal light transmitted through the optical transmission fibers 1041 to 1047 and convert it into an electrical signal.

[0100] The optical communication system 1000 uses the optical amplifier 100, which consumes less power to obtain the same amplification characteristics, and therefore can realize optical communication with reduced power consumption. Note that, although the optical transmission fiber in this embodiment is seven single-core optical fibers, an optical transmission fiber consisting of one seven-core multi-core fiber may also be used.

[0101] If the optical communication system 1000 is a long-distance communication system, the optical amplifier 100 can be used as a repeater amplifier, a preamplifier, or a booster amplifier. If the optical communication system 1000 is a network system using a ROADM (Reconfigurable Optical Add / Drop Multiplexer), the optical amplifier 100 can be used for loss compensation.

[0102] In the above embodiment, the core portion of the multi-core optical amplifying fiber contains only Er as a rare earth element, but it may contain only a rare earth element other than Er, for example, ytterbium (Yb), or may contain both Er and Yb.

[0103] Furthermore, in the above-mentioned embodiments, the cores in the multi-core optical amplifying fiber are arranged in a triangular lattice pattern, but they may also be arranged in a square lattice pattern. The number of cores in the multi-core optical amplifying fiber is not particularly limited as long as there are multiple cores. Furthermore, in the above-mentioned embodiments, the optical amplifying fiber is a multi-core optical amplifying fiber, but in some embodiments, it may be a single-core optical amplifying fiber having only one core surrounded by an inner cladding.

[0104] Furthermore, the present invention is not limited to the above-described embodiments. The present invention also includes configurations in which the above-described components are appropriately combined. Furthermore, 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. [Explanation of symbols]

[0105] 1, 2, 3, 4, 5, 6, 7 Multi-core optical amplifier fiber 1a Core part 1b, 1d, 1e, 1f Inner cladding 1ba inner area 1c Outer cladding 1bb, 1bc, 1bd, 1be, 1da, 1ea, 1fa modified refractive index area 21 Base material rod 21a, 21b holes 22 Core Rod 22a Core 22b Cladding part 23, 32 Glass rod 31 Glass tube 33 Gap 110, 170 Optical isolator 120 Fiber Optic Fan-in 130 Semiconductor laser 140 Optical coupler 150 Pump Stripper 160 optical fiber fanout 100 Optical Amplifier 1000 Optical Communication Systems 1010 Optical transmitter 1020 Optical receiving device 1011~1017 Transmitters 1021~1027 Receiver 1031~1037, 1041~1047 Optical transmission fiber Dr radial direction Dt axis rotation direction Dz axis direction R1, R2, R3, R4 area S skew component

Claims

1. At least one core portion doped with a rare earth element; an inner cladding portion surrounding the at least one core portion and having a refractive index lower than the maximum refractive index of each core portion; an outer cladding portion surrounding the inner cladding portion and having a refractive index lower than that of the inner cladding portion; an optical amplifying fiber comprising: the inner cladding portion includes a modified refractive index region having a refractive index different from that of an adjacent region; In the inner cladding portion, the positions of the modified refractive index areas in cross sections at different positions in the axial direction are different, excitation light for optically exciting the rare earth element is input to the inner cladding portion; a plurality of the modified refractive index areas are dispersed within the inner cladding portion; The plurality of modified refractive index areas are distributed approximately uniformly in the radial direction of each core portion of the light amplifying fiber and are also distributed approximately uniformly in the axial direction of the light amplifying fiber. Optically amplifying fiber.

2. The modified refractive index area is made of silica glass containing a dopant for adjusting the refractive index, and the dopant for adjusting the refractive index is fluorine (F), germanium (Ge), phosphorus (P), boron (B), an alkali metal, chlorine (Cl), or aluminum (Al).

2. The optical amplifying fiber according to claim 1.

3. The modified refractive index area is present at a position spaced apart from the core portion by at least the core diameter in a cross section perpendicular to the axial direction of the light amplifying fiber.

3. The optical amplifying fiber according to claim 1.

4. In a cross section perpendicular to the axial direction of the optical amplifying fiber, the total cross-sectional area of ​​the plurality of modified refractive index areas is 0.1% or more and 30% or less of the cross-sectional area of ​​the inner cladding portion.

4. The optical amplifying fiber according to claim 1.

5. The diameter of the modified refractive index area is 1 / 2000 to 2 times the wavelength of the light propagating through the inner cladding portion.

5. An optical amplifying fiber according to claim 1.

6. The plurality of modified refractive index areas are distributed approximately uniformly in a direction around the axis of each core portion of the optical amplifying fiber.

6. An optical amplifying fiber according to claim 1.

7. The rare earth element includes erbium.

7. An optical amplifying fiber according to claim 1.

8. The wavelength of the excitation light is 976 nm ± 2 nm, cladding absorption coefficient=−10×log((pumping light power (W) transmitted through the light amplifying fiber having a plurality of core portions and exiting) / (pumping light power (W) incident on the inner cladding portion of the light amplifying fiber having a plurality of core portions)) / length (m) of the light amplifying fiber, If we define Cladding absorption is 0.05 dB / m or more An optical amplifying fiber according to any one of claims 1 to 7.

9. an optical amplifying fiber according to any one of claims 1 to 8; a pumping light source that outputs pumping light that optically pumps the rare earth element in the optical amplifying fiber; an optical coupler that optically couples the pumping light to the inner cladding portion; Equipped with Fiber optic amplifier.

10. The core section has a plurality of core sections, and the gain difference between the plurality of core sections is 3 dB or less.

10. The optical fiber amplifier according to claim 9.

11. The optical fiber amplifier according to claim 9 or 10 is provided. Optical communication system.

Images