JP2023119383A - Optical amplification fiber, optical fiber amplifier, and optical communication system - Google Patents

Abstract

To provide an optical amplification fiber whose excitation efficiency is improved, and an optical fiber amplifier and an optical communication system using the optical amplification fiber.SOLUTION: The optical amplification fiber comprises: at least one core part to which a rare earth element is added; an inside clad part that encloses the at least one core part, and which has a lower refractive index than the maximum refractive index of each core part; and an outside clad part that encloses the inside clad part, and which has a lower refractive index than the refractive index of the inside clad part. The inside clad part includes a different refractive index region whose refractive index is different from those of the adjacent regions. The length of the inside clad part is such that, when multi-mode pump light of power equal to or smaller than the number of core parts multiplied by 500 mW is inputted to the inside clad part and output signal light of power of 20 dBm or greater is outputted in each core part, the absorption-strip length product of the peak value (dB/m) of absorption vector at a wavelength of 1530 nm multiplied by the length (m) of the optical amplification fiber is less than 100 dB.SELECTED DRAWING: Figure 1

Description

The present invention relates to an optical amplifying fiber, an optical fiber amplifier and an optical communication system.

For example, in applications such as submarine optical communication, it is expected that power consumption of optical amplifiers will be reduced by using multi-core EDFAs (Erbium-Doped optical fiber amplifiers) as optical amplifiers.

As for the multi-core EDFA, a configuration is known in which a double-clad multi-core EDF is used as a multi-core optical amplification fiber, and erbium (Er), which is a rare earth element contained in the core portion, is optically pumped by a clad pumping method (Non-Patent Document 1, 2).

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, “Cladding-pumped erbium-doped multicore fiber amplifier”, OPTICS EXPRESS Vol.20, No.18 27 August 2012 pp.20191-20200.

Since communication traffic is constantly increasing, even more suitable characteristics of multi-core optical amplifying fibers are required in order to increase the communication capacity.

In particular, if the pumping efficiency of the multi-core optical amplifier fiber 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 represented by, for example, the ratio of the energy of the pumping light used for optical amplification to the energy of the pumping light input to the multi-core optical amplification fiber. The improvement in pumping efficiency is beneficial not only for multi-core optical amplification fibers but also for single-core optical amplification fibers.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical amplifying fiber with improved pumping efficiency, an optical fiber amplifier and an optical communication system using the same.

In order to solve the above-described problems and achieve the object, one aspect of the present invention provides at least one core portion doped with a rare earth element, and surrounding the at least one core portion, and an outer cladding portion surrounding the inner cladding portion and having a lower refractive index than the inner cladding portion, the inner cladding portion being separated from an adjacent region includes modified refractive index regions with different refractive indices, multimode pump light with power equal to or less than the value obtained by multiplying the number of cores by 500 mW is input to the inner cladding portion, and each core portion has a power of 20 dBm or more. In the state of outputting the output signal light, the absorption striation product obtained by multiplying the peak value (dB/m) of the absorption spectrum at a wavelength of 1530 nm by the length (m) of the optical amplification fiber is less than 100 dB. is an optical amplifying fiber having a length of

The modified refractive index region is made of quartz 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), alkali. It may be metal, chlorine (Cl), or aluminum (Al).

In the inner cladding portion, two or more layers of the modified refractive index regions may be present in the radial direction in a cross section perpendicular to the axial direction of the optical amplifying fiber.

The modified refractive index region may be present at a position spaced apart from the core portion by a core diameter or more in a cross section perpendicular to the axial direction of the optical amplifying fiber.

The modified refractive index regions may be located at rotationally symmetrical positions about the center of the optical amplifying fiber.

When a hexagonal close-packed lattice is defined in a cross section perpendicular to the axial direction of the optical amplifying fiber, the modified refractive index region may be located at a lattice point of the lattice point.

When a hexagonal close-packed lattice is defined in a cross section orthogonal to the axial direction of the optical amplifying fiber, the modified refractive index region is a circle centered at a certain lattice point and having a radius of 1/2 or less of the distance between lattice points. It may exist in a ring shape.

A plurality of the modified refractive index regions may exist dispersedly only within the inner clad portion.

In a cross section orthogonal to the axial direction of the optical amplifying fiber, the total cross-sectional area of the plurality of modified refractive index regions with respect to the cross-sectional area of the inner cladding portion may be 0.1% or more and 50% or less.

The plurality of modified refractive index regions exist dispersedly only within the inner clad portion, and the ratio of the plurality of modified refractive index regions to the cross-sectional area of the inner clad portion in a cross section orthogonal to the axial direction of the optical amplification fiber. The total cross-sectional area may be greater than 30% and less than or equal to 50%.

The diameter of the modified refractive index region may be 1/2000 to 2 times the wavelength of light propagating through the inner clad.

The modified refractive index region may exist in an annular region separated from the core portion by a core diameter or more in a cross section perpendicular to the axial direction of the optical amplifying fiber.

The modified refractive index regions may be distributed substantially uniformly in the radial direction of each core portion of the optical amplifying fiber.

The modified refractive index regions may be distributed substantially uniformly in the axial direction of the optical amplifying fiber.

The modified refractive index regions may be distributed substantially uniformly in a direction around the axis of each core portion of the optical amplifying fiber.

A plurality of the core portions are provided, and the modified refractive index is defined between the inner side and the outer side of a circular tubular boundary passing through the core portion that is the most distant from the center of the optical amplification fiber with the center of the optical amplification fiber as an axis among the plurality of core portions. The existence density of the regions may be different.

The rare earth element may include erbium.

The wavelength of the pumping light is 976 nm ± 2 nm, and the cladding absorption rate = -10 x log ((pumping light power (W) transmitted through the optical amplification fiber having the plurality of cores) / (the plurality of the When defined as pumping light power (W)) incident on the inner cladding portion of the optical amplifying fiber having a core portion/length (m) of the optical amplifying fiber, the cladding absorption rate is 0.05 dB/ m or more may be used.

In one aspect of the present invention, the optical amplifying fiber, a pumping light source that outputs pumping light for optically pumping the rare earth element of the optical amplifying fiber, an optical coupler that optically couples the pumping light to the inner cladding, An optical fiber amplifier comprising:

A plurality of core portions may be provided, and a gain difference between the plurality of core portions may be 3 dB or less.

One aspect of the present invention is an optical communication system comprising the optical fiber amplifier.

According to the present invention, an optical amplifying fiber with improved pumping efficiency can be realized.

FIG. 1 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 1. FIG. FIG. 2 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 2. FIG. FIG. 3 is an explanatory diagram of an example of a method for manufacturing a multi-core optical amplifying fiber according to Embodiment 2. FIG. FIG. 4 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 3. FIG. FIG. 5 is an explanatory diagram of an example of a method for manufacturing a multi-core optical amplifying fiber according to Embodiment 3. FIG. FIG. 6 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 4. FIG. FIG. 7 is a schematic cross-sectional view of the multi-core optical amplifying fiber shown in FIG. 6 in a cross section different from that in FIG. FIG. 8 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 5. FIG. FIG. 9 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 6. FIG. 10 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 7. FIG. FIG. 11 is a schematic diagram showing the configuration of a multi-core optical fiber amplifier according to Embodiment 8. FIG. FIG. 12 is a diagram showing an example of an absorption spectrum of a multi-core optical amplification fiber. FIG. 13 is a schematic diagram showing the configuration of an optical communication system according to the twelfth embodiment.

Embodiments will be described below with reference to the drawings. In addition, this invention is not limited by this embodiment. In addition, in the description of the drawings, the same or corresponding elements are given the same reference numerals as appropriate. Also, it should be noted that the drawings are schematic, and the relationship of dimensions of each element, the ratio of each element, and the like may differ from reality. Even between the drawings, there are cases where portions with different dimensional relationships and ratios are included. Further, in this specification, the cut-off wavelength refers to ITU-T (International Telecommunications Union) G.35. 650.1 means the cable cutoff wavelength. For other terms not specifically defined in this specification, see G.I. 650.1 and G.I. 650.2 shall comply with the definition and measurement method.

(Embodiment 1)
FIG. 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. A multi-core optical amplifying fiber 1 is a double clad comprising seven core portions 1a as a plurality of core portions, an inner clad portion 1b surrounding the core portions 1a, and an outer clad portion 1c surrounding the inner clad portion 1b. It is a 7-core type multi-core optical fiber.

The core portions 1a are arranged in a triangular lattice shape that achieves the closest packed state. That is, one core portion 1a is arranged at or near the center of the inner clad portion 1b. With this core portion 1a as the center, the six core portions 1a are arranged at the corners of a regular hexagon. If a hexagonal close-packed lattice is defined in a cross section orthogonal to the axial direction of the multi-core optical amplifying fiber 1, the core portion 1a can be said to be located at the lattice point. Core portion 1a contains, for example, germanium (Ge) or aluminum (Al) as a refractive index adjusting dopant that increases the refractive index. Further, the core portion 1a contains erbium (Er) as a rare earth element which is an amplification medium. Er is added at a concentration such that the peak of the absorption coefficient near the wavelength of 1530 nm is 2.5 dB/m to 11 dB/m. Also, for example, the additive concentration is 250 ppm to 2000 ppm. However, the absorption coefficient and addition concentration are not particularly limited. Al also has a function of suppressing concentration quenching of Er.

The inner clad portion 1b has a lower refractive index than the maximum refractive index of each core portion 1a. A refractive index profile of each core portion 1a and inner clad portion 1b is, for example, a step index type. In addition, the inner clad portion 1b may have a trench portion positioned 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 that of the other portions of the inner clad portion 1b. It has a refractive index. In this case, the refractive index profile of each core portion 1a and the inner clad portion 1b becomes a trench type.

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, and an annular and layered modified refractive index region 1bb surrounding the modified refractive index region 1bb. It includes a modified refractive index region 1bc, 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 having different refractive indices from adjacent regions. Specifically, the modified refractive index region 1bb has a refractive index different from that of the inner region 1ba and the modified refractive index region 1bc, which are adjacent regions. The modified refractive index region 1bc differs in refractive index from the adjacent modified refractive index regions 1bb and 1bd. The modified refractive index region 1bd has a refractive index different from that of the adjacent modified refractive index regions 1bc and 1be.

The inner region 1ba is made of, for example, pure silica glass containing no refractive index adjusting dopant, and the modified refractive index regions 1bb, 1bc, 1bd, and 1be are made of, for example, silica glass containing a refractive index adjusting dopant. Dopants for adjusting the refractive index are, for example, F, Ge, phosphorus (P), boron (B), alkali metals such as sodium (Na) and potassium (K), chlorine (Cl), or Al. One or more dopants selected from these dopants are added to the modified refractive index regions 1bb, 1bc, 1bd, and 1be. By changing the type and amount of the dopant to be added, it is possible to configure the difference in refractive index among the modified refractive index regions 1bb, 1bc, 1ed, and 1ee. The difference in refractive index between the inner region 1ba and the modified refractive index regions 1bb, 1bc, 1ed, and 1ee should be as large as the absolute value of the relative refractive index difference of one to the other. Yes, more preferably 0.7% or more. Also, if the layer thickness of the modified refractive index regions 1bb, 1bc, 1ed, and 1ee is at least several times the wavelength of the propagating light, the difference in refractive index tends to occur as the average value of the wavelength order. The inner clad portion 1b preferably has a layer thickness of at least 1 μm or more because excitation light having a wavelength capable of optically exciting Er, for example, excitation light in a 900 nm wavelength band such as 976 nm, propagates through the inner clad portion 1b, as will be described later. Also, the layer thickness can be appropriately set according to the outer diameter of the inner clad portion 1b.

In the multi-core optical amplifying fiber 1, the inner cladding portion 1b has four layers of modified refractive index regions in the radial direction due to the modified refractive index regions 1bb, 1bc, 1bd, and 1be.

Assuming that the relative refractive index difference of each core portion 1a with respect to the glass of the inner region 1ba is core Δ, the core Δ of each core portion 1a in the present embodiment is substantially equal, for example, 0.35% to 2% at a wavelength of 1550 nm. . The core diameter of the core portion 1a is preferably set so as to realize a cutoff wavelength shorter than the optical amplification wavelength band in which the rare earth element can optically amplify, in relation to the core Δ. In the case of Er, the optical amplification wavelength band is, for example, 1530 nm to 1565 nm called C band and 1565 nm to 1625 nm called L band. The core diameter is, for example, about 5 μm to 10 μm.

The outer clad portion 1c has a refractive index lower than that of the inner clad portion 1b, and is made of resin, for example. When the inner cladding portion 1b has a trench portion for the core portion 1a, the refractive index of the outer cladding portion 1c may be higher than the refractive index of the trench portion. It is lower than the refractive index and the average refractive index of the inner clad portion 1b.

When pumping light having a wavelength capable of optically exciting Er, for example, pumping light in a 900 nm wavelength band such as 976 nm, is input to the inner clad portion 1b, the pumping light propagates inside the inner clad portion 1b and reaches each core portion 1a. The doped Er is photoexcited. As a result, each core portion 1a can optically amplify the signal light input to each core portion 1a. In this way, the multi-core optical amplifying fiber 1 is configured to be able to apply the clad pumping method.

In the multi-core optical amplifying fiber 1, there are interfaces (hereinafter sometimes referred to as modified refractive index interfaces) having different refractive indices across the inner region 1ba and the modified refractive index regions 1bb, 1bc, 1bd, and 1be. , scatter the excitation light propagating in the inner cladding portion 1b. As a result, more components of the excitation light propagating through the inner clad portion 1b reach the core portion 1a. For example, in the case of a clad pumping method such as the multi-core optical amplifying fiber 1, there are normally unused components that do not contribute to pumping, such as the skew component S that propagates so as not to reach the core portion 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 interface and part of them reaches the core portion 1a and can be used for optical excitation of Er. Since the pumping light propagates through the inner cladding portion 1b in multiple modes, the skew component S may also have modes propagating at various angles. When these various skew components S are scattered by the interface with the modified refractive index, some of them reach the core portion 1a and can be easily used for photoexcitation of Er.

In the multi-core optical amplifying fiber 1 configured as described above, the pumping light propagating through the inner clad portion 1b is scattered by the interface of the modified refractive index, so that more components of the pumping light reach the core portion 1a. Efficiency is improved. In addition, the effect of the modified refractive index interface can be adjusted by adjusting the number and thickness of the layers of the modified refractive index region and the refractive index difference in the radial direction. For example, the layers of the modified refractive index regions can be appropriately designed to have one or more layers, and a total of two or more layers.

In general, the longer the length of an optical amplifying fiber, the greater the amplified output light power. However, in order to uniformly amplify the C band in the wavelength range of 1530 nm to 1565 nm at an appropriate signal light input power (for example, the total input power of signal light is -10 dBm to 0 dBm), the multicore optical amplifying fiber 1 has an inner cladding Multimode pump light (wavelength: 976 nm, for example) with power equal to or less than the number of cores 1a multiplied by 500 mW is input to portion 1b, and output signal light with power of 20 dBm or more is output from each core portion 1a. , it is preferable that the absorption line length product has a length of less than 100 dB. Here, the absorption length product is obtained by multiplying the peak value (dB/m) of the absorption spectrum at a wavelength of 1530 nm by the length (m) of the multi-core optical amplifying fiber 1 . Further, in this specification, uniformly amplifying the C band means amplifying so that the gain difference between wavelengths within the C band is within 3 dB.

If the absorption product is 100 dB or more, the amplified light output power increases, but the wavelength to be amplified shifts to longer wavelengths, and the amplification gain in the C band decreases. However, if the absorption product is 80 dB or less, the amplified light output power may decrease.

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 punching method. For example, in the case of the perforation method, seven holes extending parallel to the axial direction are formed in the base material rod, and each hole includes a portion that will become the core portion 1a and a portion that will become a part of the inner region 1ba. A core rod is inserted to form the base material. Subsequently, the base material is drawn to form the outer clad portion 1c.

The base material rod used in the above method can be produced using, for example, a VAD (Vapor-phase Axial Deposition) method, an OVD (Outside Vapor Deposition) method, an MCVD (Modified Chemical Vapor Deposition) method, or a plasma CVD method. . At this time, the base material rod includes, as soot layers made of glass fine particles, a soot layer serving as the inner region 1ba, a soot layer serving as the modified refractive index region 1bb, a soot layer serving as the modified refractive index region 1bc, and a soot layer serving as the modified refractive index region 1bd. A soot layer that becomes the modified refractive index region 1be is deposited, dehydrated and vitrified by heat treatment.

The base material rod used in the above method can also be produced by the jacket method. In this case, the jacket pipes forming the modified refractive index regions 1bb, 1bc, 1bd, and 1be are sequentially inserted into the glass rod forming the inner region 1ba so as to cover the glass rod, and are integrated by heat treatment.

(Embodiment 2)
FIG. 2 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 2. FIG. This multi-core optical amplifying fiber 2 has a configuration in which the inner cladding portion 1b in the multi-core optical amplifying fiber 1 according to Embodiment 1 shown in FIG. 1 is replaced with an inner cladding portion 1d. In the inner clad portion 1d, the modified refractive index regions 1bb, 1bc, 1bd, and 1be of the inner clad portion 1b are replaced with regions of the same constituent material as the inner region 1ba, and a plurality of modified refractive index regions 1da having a circular cross section are provided. configuration. Although the number of the modified refractive index areas 1da is six in this embodiment, the number is not limited.

The modified refractive index region 1da has a refractive index different from that of adjacent regions in the inner cladding portion 1d. In addition, the modified refractive index region 1da is located on the outer peripheral side of the regular hexagon formed by the core portion 1a. In addition, the modified refractive index region 1da is located at a rotationally symmetrical position about the center of the multi-core optical amplifying fiber 2, and is located at a 6-fold rotationally symmetrical position in this embodiment. Further, each modified refractive index region 1da is located at a lattice point when a hexagonal close-packed lattice is defined in a cross section perpendicular to the axial direction of the multi-core optical amplifying fiber 2 .

In the multi-core optical amplifying fiber 2 configured as described above, similarly to the multi-core optical amplifying fiber 1, the pumping efficiency is improved by the effect of the modified refractive index interface of the modified refractive index region 1da. Further, 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 2-fold rotational symmetry or 3-fold rotational symmetry.

In addition, the modified refractive index region 1da may be located on the inner circumference side or the same circumference of the regular hexagon formed by the core portion 1a.

The multi-core optical amplifying fiber 2 can be manufactured using a known multi-core fiber manufacturing method. For example, the drilling method will be described with reference to FIG.

That is, as shown in FIG. 3, seven holes 21a and six holes 21b extending parallel to the axial direction are formed in the base material rod 21, which forms part of the inner clad portion 1d. Then, the core rod 22 is inserted into the hole 21a, and the glass rod 23 that becomes the modified refractive index region 1da is inserted into the hole 21b to form a base material. The core rod 22 is a glass rod including a core portion 22a that forms the core portion 1a and a clad portion 22b that surrounds the core portion 22a and forms a part of the inner clad portion 1d. Subsequently, the base material is drawn to form the outer clad portion 1c.

(Embodiment 3)
FIG. 4 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 3. FIG. This multi-core optical amplifying fiber 3 has a configuration in which the inner clad portion 1d in the multi-core optical amplifying fiber 2 according to Embodiment 2 shown in FIG. 2 is replaced with an inner clad portion 1e. The inner cladding portion 1e has a configuration in which the modified refractive index region 1da in the inner cladding portion 1d is removed and a plurality of modified refractive index regions 1ea having a circular cross section are provided. Although the number of the modified refractive index areas 1ea is 12 in this embodiment, the number is not limited.

The modified refractive index region 1ea is located on the outer peripheral side of the regular hexagon formed by the core portion 1a. Further, the modified refractive index region 1ea is located at a rotationally symmetrical position about the center of the multi-core optical amplifying fiber 3, and is located at a 6-fold rotationally symmetrical position in this embodiment. Further, each modified refractive index region 1ea is located at a lattice point when a hexagonal close-packed lattice is defined in a cross section orthogonal to the axial direction of the multi-core optical amplifying fiber 3 . Furthermore, in the present embodiment, each core portion 1a and each modified refractive index region 1ea are located at lattice points of the same hexagonal close-packed lattice.

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

In addition, the modified refractive index interface may be located on the inner circumference side or the same circumference of the regular hexagon formed by the core portion 1a. Also, the number of the modified refractive index regions 1ea may be appropriately increased or decreased from twelve.

The multi-core optical amplifying fiber 3 can be manufactured using a known multi-core fiber manufacturing method. For example, the stack method will be described with reference to FIG.

That is, as shown in FIG. 5, seven core rods 22 are stacked inside a glass tube 31 that forms part of the inner clad portion 1e. At the same time, 12 glass rods 32 that form the modified refractive index region 1ea are stacked in the gap 33 between the core rod 22 and the glass tube 31 to form a base material. Here, by equalizing the diameters of the core rod 22 and the glass rod 32, each core portion 1a and each modified refractive index region 1ea can be configured at the same lattice point position of the hexagonal close-packed lattice. Also, in the remaining portion of the gap 33, a glass rod which is made of the same material as that of the clad portion 22b and becomes a part of the inner clad portion 1e is stacked. Subsequently, the base material is drawn to form the outer clad portion 1c. In order to reduce the number of modified refractive index regions 1ea from 12, one or more of the twelve glass rods 32 should be replaced with glass rods made of the same material as the clad portion 22b.

(Embodiment 4)
FIG. 6 is a schematic cross-sectional view of a multi-core optical amplifying fiber according to Embodiment 4. FIG. In FIG. 6 , the direction perpendicular to the drawing is the axial direction Dz of the multi-core optical amplification fiber 4 . In addition, 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 drawings, the axial direction Dz, the radial direction Dr, and the direction around the axis Dt can be defined in the same manner as in FIG.

This multi-core optical amplifying fiber 4 has a configuration in which the inner clad portion 1d in the multi-core optical amplifying fiber 2 according to Embodiment 2 shown in FIG. 2 is replaced with an inner clad portion 1f. The inner cladding portion 1f has a configuration in which the modified refractive index region 1da in the inner cladding portion 1d is removed and a plurality of modified refractive index regions 1fa are provided.

The modified refractive index regions 1fa are distributed only inside the inner clad portion 1f. The modified refractive index region 1fa includes, for example, microcrystals and clusters dispersed in quartz glass. The modified refractive index region 1fa is made of, for example, Al, Ge, or an alkali metal. The larger the refractive index difference between the modified refractive index region 1fa and the surrounding glass material, the better.

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

Further, in the multi-core optical amplifying fiber 4, the modified refractive index region 1fa does not exist in the region near the boundary between the inner clad portion 1f and the outer clad portion 1c. As a result, the scattering of the skew component S is moderately adjusted, and the generation of scattered light traveling away from the core portion 1a in the vicinity of the boundary between the inner clad portion 1f and the outer clad portion 1c can be suppressed.

In the multi-core optical amplifying fiber 4, in a cross section perpendicular to the axial direction Dz as shown in FIG. , the cross-sectional area ratio is, for example, preferably 0.1% or more and 50% or less, and more preferably 1% or more. If the cross-sectional area ratio is 0.1% or more, the effect of improving the excitation efficiency by the plurality of modified refractive index regions 1fa is likely to be exhibited, and if it is greater than 30%, the improvement effect is more likely to be exhibited. Moreover, if the cross-sectional area ratio is 50% or less, it is easy to manufacture the multi-core optical amplifying fiber 4 with desired optical characteristics (amplification characteristics, etc.). On the other hand, if the cross-sectional area ratio is more than 50%, 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 due to scattering of the pumping light by the modified refractive index region 1fa may be overwhelmed by the effect of deteriorating the pumping efficiency due to an increase in propagation loss.

Moreover, the diameter of the modified refractive index region 1fa in the cross section as shown in FIG. 6 is preferably 1/2000 to 2 times the wavelength of the light (excitation light) propagating through the inner clad portion 1b. When the diameter of the modified refractive index region 1fa, which is a scatterer, is 1/20 to 2 times the wavelength of the excitation light, the scattering of light by the modified refractive index region 1fa is mainly Mie scattering. When the diameter of the modified refractive index region 1fa is 1/2000 to 1/20 times the wavelength of the excitation light, light scattering by the modified refractive index region 1fa is mainly Rayleigh scattering. As described above, when a light wave collides with a particulate matter or a change in refractive index, the type of scattering differs depending on the size of the particle. Here, Mie scattering is mainly forward scattering, and Rayleigh scattering is isotropic scattering. For example, using the scattering direction characteristics of these scattering, the modified refractive index regions 1fa with different diameters are spatially distributed according to the arrangement of the core portion 1a and the electric field distribution of the excitation light in the inner cladding portion 1f. may be used to increase the degree of improvement in excitation efficiency. For example, Rayleigh scattering contributes significantly to improving pump efficiency when it is more effective to change the direction of propagation significantly.

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

Further, it is preferable that the plurality of modified refractive index regions 1fa are randomly present in the inner cladding portion 1f because the excitation light scattering effect tends to uniformly act on each core portion 1a. A plurality of modified refractive index regions 1fa randomly existing in the inner cladding portion 1f means that the distribution of the modified refractive index regions 1fa is substantially uniformly distributed in the inner cladding portion 1f without bias. can be paraphrased. Therefore, for example, the modified refractive index regions 1fa are preferably distributed substantially uniformly in the axial direction Dz.

Here, the term “substantially uniform” means that, for example, in the inner cladding portion 1f, when a square lattice with a side of 30 μm or a square lattice with a side of 1/3 the diameter of the inner cladding portion 1f is set, all It can be determined that the variation in the total area of the modified refractive index regions 1fa is in the range of ±30%. Also, the modified refractive index region 1fa does not need to include a region in contact with the core portion 1a.

FIG. 7 is a schematic cross-sectional view of the multi-core optical amplifying fiber 4 shown in FIG. 6 in a cross section different from that in FIG. The cross section shown in FIG. 7 is shifted from the cross section shown in FIG. 6 in the axial direction Dz by a small distance of about 1% to 5% of the length of the multi-core optical amplifying fiber 4, for example. is a cross section of In the cross section shown in FIG. 7, the existing position of the modified refractive index region 1fa is different from that in the cross section shown in FIG. In this way, the positions of the modified refractive index regions 1fa may differ for each cross section.

Similarly, for example, the modified refractive index regions 1fa are preferably distributed substantially uniformly in the radial direction Dr. Moreover, the modified refractive index regions 1fa are preferably distributed substantially uniformly in the radial direction of each core portion 1a. Further, for example, the modified refractive index regions 1fa are preferably distributed substantially uniformly in the direction Dt around the axis. In this case, the existing position of the modified refractive index region 1fa in the range of 0 to 60 degrees from the reference angular position in the axial direction Dt and the existing position of the modified refractive index region 1fa in the range of 60 to 120 degrees. can be different. Moreover, the modified refractive index regions 1fa are preferably distributed substantially uniformly in the direction around the axis of each core portion 1a.

Here, the term “substantially uniform in the radial direction” means, for example, a ring-shaped ring (circular ring) obtained by dividing the inner cladding portion 1f in the radial direction by 30 μm or ⅓ of the diameter with the center position of the inner cladding portion 1f as a reference. However, in the case where the area is set such that only the central portion is circular, it can be determined that the variation in the total area of the modified refractive index areas 1fa is within ±30% in all the areas.

Further, the term “substantially uniform in the axial direction” means, for example, that the inner cladding portion 1f is divided by 30° in the circumferential direction with respect to the center position of the inner cladding portion 1f, and the circular arc is divided by 30 μm in the radial direction or 1/3 of the diameter. When a ring-toric area (only the center part is circular) divided by length is set, the variation in the total area of the modified refractive index area 1fa is within a range of ±30% in all the areas. and can be determined.

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

Moreover, in the method using fine particles, for example, fine particles including the modified refractive index region 1fa may be used.

(Embodiment 5)
FIG. 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 the fourth embodiment in that the inner cladding portion 1f has a region R1 surrounding each core portion 1a.

Each region R1 is a region that is concentric with each core portion 1a, has a diameter that is, for example, three times or more the core diameter of the surrounding core portion 1a, and extends in the axial direction Dz along each core portion 1a. It is a tubular area. This region R1 does not include the modified refractive index region 1fa.

In the multi-core optical amplifying fiber 5 configured as described above, the modified refractive index region 1fa is separated from each core portion 1a by a core diameter or more in a cross section perpendicular to the axial direction Dz of the multi-core optical amplifying fiber 5. It exists at a position spaced apart by three times or more. As a result, in the multi-core optical amplifying fiber 5, the pumping efficiency is improved as in the multi-core optical amplifying fibers 1-4. Furthermore, it is possible to suppress the excitation light that is once scattered by the modified refractive index region 1fa and directed toward the core portion 1a from being scattered again by the modified refractive index region in the vicinity of the core portion 1a. Furthermore, since the modified refractive index region 1fa is relatively distant from each core portion 1a, it is possible to suppress the influence on the light propagation characteristics of each core portion 1a.

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, core rods, which are seven glass rods including a portion that will become the core portion 1a and a portion that will become the region R1, are stacked in a glass tube that will become a part of the inner clad portion 1f. Subsequently, glass rods including the modified refractive index region 1fa and forming a part of the inner clad portion 1f are stacked in the gap between the core rod and the glass tube to form a base material. Subsequently, the base material is drawn to form the outer clad portion 1c. In the case of the perforation method, seven holes extending parallel to the axial direction are formed in a base material rod, which is a relatively large-diameter glass rod including the modified refractive index region 1fa, which is part of the inner cladding portion 1f. Then, a core rod is inserted into each hole to form a base material. Subsequently, the base material is drawn to form the outer clad portion 1c. Note that the modified refractive index region 1fa may not be formed in the glass rod from the beginning, and may be formed by, for example, aggregating a dopant by heat treatment in the manufacturing process.

(Embodiment 6)
FIG. 9 is a schematic cross-sectional view of a multicore optical amplifying fiber according to Embodiment 6, showing a cross section perpendicular to the axial direction of the multicore optical amplifying fiber. The multi-core optical amplifying fiber 6 differs from the multi-core optical amplifying fiber 4 according to the fourth embodiment in that regions R2 and R3 are present in the inner clad portion 1f. In addition, in FIG. 9, illustration of the modified refractive index region is omitted.

The region R2 is a cylindrical region extending in the axial direction Dz through the centers of the six core portions 1a arranged at the corners of the regular hexagon in the inner clad portion 1f. The region R3 is a tubular region located on the outer peripheral side of the region R2 in the inner clad portion 1b and extending in the axial direction Dz. A boundary between the region R2 and the region R3 is an example of a cylindrical boundary passing through the core portion 1a that is the farthest from the center of the multi-core optical amplifying fiber 6. FIG. Note that the boundary axis is the center of the multi-core optical amplifying fiber 6 .

In the multi-core optical amplifying fiber 6, the existence density of the modified refractive index regions differs between the region R2 inside the boundary and the region R3 outside the boundary. For example, the presence density in region R2 is higher than the presence density in region R3. Also, for example, the presence density in the region R2 is lower than the presence density in the region R3.

In the multi-core optical amplifying fiber 6 constructed as described above, the pumping efficiency is improved as in the multi-core optical amplifying fibers 1-5. Further, in the multi-core optical amplifying fiber 6, the degree of scattered light generation is differentiated between the region R2 where the core portion 1a mainly exists and the region R3 where the skew component is relatively large on the outer edge side of the inner cladding portion 1f. can be done. For example, the presence density of the modified refractive index regions in the region R2 may be increased to cause a large amount of scattering in the region R2 where the core portion 1a mainly exists, or the presence density of the modified refractive index regions in the region R3 may be increased. may be used to generate a large number of scattering skew components. The density of existence in each region can be appropriately set according to the design and required characteristics of the multi-core optical amplifying fiber 6 .

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 perforation method, a base material rod that forms part of the inner clad portion 1f is formed so that the existence density of the modified refractive index regions varies depending on the location. Such a base material rod can be produced, for example, by the jacket method. Subsequently, seven holes extending parallel to the axial direction are formed in the base material rod, and a core rod is inserted into each hole to form the base material. Subsequently, the base material is drawn to form the outer clad portion 1c. Moreover, in the case of the stacking method, the core rod is stacked inside the glass tube that forms part of the inner clad portion 1f. Subsequently, a base material is formed by stacking a glass rod including a modified refractive index region and forming a part of the inner clad portion 1f in the gap between the core rod and the glass tube. At this time, by using a glass rod in which the existence density of the modified refractive index regions differs depending on the stacking location, the existence density of the modified refractive index regions can be made different depending on the location.

(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 the fourth embodiment in that the inner cladding portion 1f has a region R4 and the modified refractive index region exists only in the region R4. In addition, in FIG. 10, illustration of the modified refractive index region is omitted.

Each region R4 is a circular region concentric with each core portion 1a and is a tubular region extending in the axial direction Dz along each core portion. Each region R4 may exist in an annular region separated from each core portion 1a by a core diameter or more in a cross section of the multi-core optical amplifying fiber 7 perpendicular to the axial direction Dz. Further, when a hexagonal close-packed lattice having lattice points corresponding to the core portions 1a is defined in a cross section perpendicular to the axial direction of the multi-core optical amplifying fiber 4, the modified refractive index region is centered at a certain lattice point and It can be said that they exist in an annular shape with a radius equal to or less than 1/2 of the inter-point distance. The inter-lattice point distance is the center-to-center distance between adjacent core portions 1a.

In the multi-core optical amplifying fiber 7 constructed as described above, the pumping efficiency is improved as in the multi-core optical amplifying fibers 1-6. Further, in the multi-core optical amplifying fiber 7, it is possible to suppress the second scattering of the pumping light, and it is possible to suppress the modified refractive index region from affecting the light propagation characteristics of each core portion 1a. Further, if the existence density of the modified refractive index regions in the region R4 for each core portion 1a is set to a different value, the effect of the modified refractive index regions for each core portion 1a can be adjusted to different extents.

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 perforation method, seven holes extending parallel to the axial direction are formed in the base material rod, a core rod inserted in a glass tube containing a modified refractive index region is inserted into each hole, and the base material is to form Subsequently, the base material is drawn to form the outer clad portion 1c. The glass tube including the modified refractive index regions may have a structure in which the modified refractive index regions form a plurality of concentric layers, such as the modified refractive index regions 1bb, 1bc, 1bd, and 1be in FIG. A glass tube including such a modified refractive index region is manufactured to have a larger diameter than the hole to be inserted, and is thinned into a similar shape by stretching to have an outer diameter that can be inserted into the hole. Therefore, for example, by first forming a glass tube having a large diameter and a multi-layered structure and then stretching the tube, it is possible to easily realize a multi-layered and fine refractive index distribution.

In addition, in a configuration in which a double-clad multi-core EDF is used as a multi-core optical amplification fiber and erbium (Er), which is a rare earth element contained in the core portion, is optically pumped by a clad pumping method, a normal normal The cladding absorptance of the multi-core optical amplifying fiber is about 0.02 dB/m.
On the other hand, the cladding absorptance is 0.05 dB/m or more regardless of which of the multi-core optical amplifying fibers 1 to 7 is used.
here,
Cladding absorption rate = -10 x log ((pumping light power (W) transmitted through the multi-core optical amplifying fiber)/(pumping light power (W) incident on the inner cladding of the multi-core optical amplifying fiber))/ Length of multi-core optical amplification fiber (m)
is calculated by The wavelength of excitation light is 976 nm±2 nm.

(Embodiment 8)
FIG. 11 is a schematic diagram showing the configuration of a multi-core optical fiber amplifier according to Embodiment 8. FIG. 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, a multi-core optical amplifying fiber 1 according to Embodiment 1, a pump stripper 150, an optical fiber fan-out. (FAN OUT) 160 and seven optical isolators 170 are provided. In the figure, the symbol "x" indicates the fusion splicing point of the optical fiber.

The fiber optic fan-in 120 comprises seven bundled single-mode optical fibers and one multi-core fiber with seven cores, each core of the seven single-mode fibers at the junction. A section is configured to optically couple to each core section of the multicore fiber. It should be noted that the seven single-mode optical fibers are, for example, ITU-TG. 652 standard single mode optical fiber, each provided with an optical isolator 110 . Optical isolators 110 and 170 pass light in the direction indicated by the arrows and block light from passing in the opposite direction. The multi-core fibers of optical fiber fan-in 120 are connected to optical coupler 140 . The end surfaces of the bundled seven single-mode optical fibers and the multi-core fibers to be optically coupled are processed obliquely with respect to the optical axis for reflection suppression, but may be perpendicular to the optical axis. . Instead of the seven optical isolators 110 and 170, an optical isolator in which a plurality (seven in this embodiment) of single-mode optical fibers are integrated may be used.

The multi-core fiber of the optical fiber fan-in 120, like the multi-core optical amplifying fiber 1, has seven cores arranged in a triangular lattice pattern, and located on the outer circumference of each core. and a cladding portion with a low refractive index. 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 portion of the multi-core fiber propagates each signal light.

A semiconductor laser 130, which is an excitation light source, is a lateral multimode semiconductor laser and outputs excitation light. The wavelength of the excitation light is 976 nm, which is substantially the same as the wavelength of the absorption peak of Er in the 900 nm wavelength band. This allows the excitation light to photoexcite the erbium ions. A semiconductor laser 130 outputs excitation light from a multimode optical fiber. This multimode optical fiber is a step index type with a core diameter/cladding diameter of 105 μm/125 μm, for example, and an NA of 0.16 or 0.22, for example.

The optical coupler 140 includes a main optical fiber and an excitation light supply optical fiber. The main optical fiber has seven cores arranged in a triangular lattice like the cores of the multi-core fiber of the optical fiber fan-in 120, and the outer circumference of each core. The optical fiber is a double clad type optical fiber comprising an inner clad portion having a lower refractive index than the inner clad portion and an outer clad portion positioned on the outer periphery of the inner clad portion and having a lower refractive index than the inner clad portion. The core portion and the inner clad portion are made of quartz-based glass, and the outer clad portion is made of resin.

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

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 portion of the multi-core fiber is connected to each core portion of the main optical fiber. Therefore, each signal light propagated through each core portion of the multi-core fiber is optically coupled to each core portion when input to the main optical fiber. Each core propagates each signal light. Pumping light and signal light are output from the main optical fiber to the multi-core optical amplifying fiber 1 .

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

Pump stripper 150 is a known device that removes pump light that has not contributed to light amplification. The pump stripper 150 has, for example, a portion of the outer clad of a double-clad multi-core fiber having seven cores removed, extracts pumping light from the surface of the inner clad of the removed portion, and supplies it to a radiator plate or the like. It has a configuration in which the energy of the excitation light is converted into heat energy by irradiation and absorption, and the heat is released. The pump stripper 150 propagates each signal light through the multi-core fiber, and reduces the power of the pump light to a level that causes no problem even if it is output from the optical amplifier 100 .

The fiber optic fan-out 160, similar to the fiber optic fan-in 120, comprises seven bundled single-mode optical fibers and one multi-core fiber having seven cores, with seven cores at the joint. Each core portion of the single-mode optical fiber is configured to be optically coupled to each core portion of the multi-core fiber. Each single-mode optical fiber is provided with an optical isolator 170 . The multicore fiber is connected to pump stripper 150 . The end surfaces of the bundled seven single-mode optical fibers and the multi-core fibers to be optically coupled are processed obliquely with respect to the optical axis for reflection suppression, but may be perpendicular to the optical axis. .

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

Since this optical amplifier 100 performs optical amplification using the multi-core optical amplifying fiber 1 with improved pumping efficiency, the power consumption of the semiconductor laser 130 for obtaining the same amplification characteristics can be reduced. The gain difference at a certain wavelength in the amplification band between the core portions 1a of the multi-core optical amplifying fiber 1 is preferably 3 dB or less. The gain difference can be adjusted by changing the properties of the multi-core optical amplifying fiber 1, such as its length.

In the optical amplifier 100, any one of the multi-core optical amplifying fibers 2 to 7 may be used in place of the multi-core optical amplifying fiber 1. FIG.

(Example of absorption spectrum of multi-core optical amplification fiber)
FIG. 12 is a diagram showing an example of the absorption spectrum of a multi-core optical amplification fiber, which is the absorption spectrum of one core. In the example shown in FIG. 12, the absorption peak value is approximately 3.1 dB/m.

(Embodiment 9)
FIG. 13 is a schematic diagram showing the configuration of an optical communication system according to the ninth embodiment. An optical communication system 1000 includes an optical transmitter 1010, an optical receiver 1020, an optical amplifier 100 according to the eleventh embodiment, and optical transmission fibers 1031 to 1037 and 1041 to 1047 which are 14 single-core optical fibers. I have.

The optical transmitter 1010 comprises seven transmitters 1011-1017. Transmitters 1011 to 1017 each transmit signal light. The seven optical transmission fibers 1031 to 1037 transmit the signal lights output from the transmitters 1011 to 1017 respectively and input them to the optical amplifier 100 . The optical amplifier 100 collectively optically amplifies seven signal lights input from the optical transmission fibers 1031 to 1037 and outputs them to the seven optical transmission fibers 1041 to 1047, respectively. The optical transmission fibers 1041 to 1047 transmit the amplified signal light and input it to the optical receiver 1020 . The optical receiver 1020 has seven receivers 1021-1027. The receivers 1021-1027 receive the amplified signal light transmitted by the optical transmission fibers 1041-1047 and convert it into an electrical signal.

Since the optical communication system 1000 uses the optical amplifier 100 with reduced power consumption to obtain the same amplification characteristics, optical communication with reduced power consumption can be realized. In this embodiment, the optical transmission fiber is seven single-core optical fibers, but an optical transmission fiber made up of one seven-core multi-core fiber may be used.

If the optical communication system 1000 is a long distance communication system, etc., the optical amplifier 100 can be used as a repeater amplifier, preamplifier, or 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.

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, such as ytterbium (Yb), or both Er and Yb. may contain.

Further, in the above embodiment, the core portions in the multi-core optical amplifying fiber are arranged in a triangular lattice pattern, but they may be arranged in a square lattice pattern. The number of core portions in the multi-core optical amplifying fiber is also not particularly limited as long as it is plural. Further, in the above embodiments, the optical amplifying fiber is a multi-core optical amplifying fiber, but as an embodiment, it may be a single-core optical amplifying fiber having only one core portion surrounded by an inner cladding portion.

Moreover, the present invention is not limited by the above embodiments. The present invention also includes those configured by appropriately combining the respective constituent elements described above. Further effects and modifications can be easily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the above-described embodiments, and various modifications are possible.

1, 2, 3, 4, 5, 6, 7 Multi-core optical amplification fiber 1a Core portions 1b, 1d, 1e, 1f Inner clad portion 1ba Inner region 1c Outer clad portions 1bb, 1bc, 1bd, 1be, 1da, 1ea, 1fa Modified refractive index region 21 Base material rods 21a, 21b Hole 22 Core rod 22a Core portion 22b Cladding portions 23, 32 Glass rod 31 Glass tube 33 Gap 110, 170 Optical isolator 120 Optical fiber fan-in 130 Semiconductor laser 140 Optical coupler 150 Pump stripper 160 optical fiber fanout 100 optical amplifier 1000 optical communication system 1010 optical transmitter 1020 optical receiver 1011 to 1017 transmitters 1021 to 1027 receivers 1031 to 1037, 1041 to 1047 optical transmission fiber Dr radial direction Dt axial direction Dz axial direction R1, R2, R3, R4 Region S Skew component

Claims (21)

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 lower refractive index than the maximum refractive index of each core portion;
an outer cladding portion surrounding the inner cladding portion and having a lower refractive index than the inner cladding portion;
wherein the inner cladding portion includes a modified refractive index region having a different refractive index from an adjacent region,
In a state in which multimode pump light with power equal to or less than the number obtained by multiplying the number of cores by 500 mW is input to the inner cladding, and output signal light with power equal to or greater than 20 dBm is output from each core,
An optical amplifying fiber having a length such that the absorption length product, which is obtained by multiplying the peak value (dB/m) of the absorption spectrum at a wavelength of 1530 nm by the length (m) of the optical amplifying fiber, is less than 100 dB. The modified refractive index region is made of quartz 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), alkali. The optical amplifying fiber according to claim 1, which is metal, chlorine (Cl), or aluminum (Al). 3. The light amplifying fiber according to claim 1, wherein the inner cladding portion has two or more layers of the modified refractive index regions in the radial direction in a cross section perpendicular to the axial direction of the light amplifying fiber. The optical amplifying fiber according to any one of claims 1 to 3, wherein the modified refractive index region exists at a position spaced apart from the core by a core diameter or more in a cross section orthogonal to the axial direction of the optical amplifying fiber. . The light amplifying fiber according to any one of claims 1 to 4, wherein the modified refractive index regions are positioned rotationally symmetric about the center of the light amplifying fiber. The light according to any one of claims 1 to 5, wherein the modified refractive index region is located at a lattice point when a hexagonal close-packed lattice is defined in a cross section perpendicular to the axial direction of the optical amplification fiber. amplification fiber. When a hexagonal close-packed lattice is defined in a cross section orthogonal to the axial direction of the optical amplifying fiber, the modified refractive index region is a circle centered at a certain lattice point and having a radius of 1/2 or less of the distance between lattice points. The optical amplifying fiber according to any one of claims 1 to 6, which exists in a ring. 2. The optical amplifying fiber according to claim 1, wherein the plurality of modified refractive index regions are distributed only within the inner clad portion. The total cross-sectional area of the plurality of modified refractive index regions with respect to the cross-sectional area of the inner cladding portion is 0.1% or more and 50% or less in a cross section perpendicular to the axial direction of the optical amplifying fiber. Optical amplification fiber. A plurality of the modified refractive index regions exist dispersedly only within the inner cladding,
2. The light according to claim 1, wherein in a cross section orthogonal to the axial direction of the optical amplifying fiber, the total cross-sectional area of the plurality of modified refractive index regions with respect to the cross-sectional area of the inner cladding portion is more than 30% and 50% or less. amplification fiber. The optical amplifying fiber according to any one of claims 8 to 10, wherein the diameter of the modified refractive index region is 1/2000 to 2 times the wavelength of light propagating through the inner clad portion. 12. The modified refractive index region according to any one of claims 8 to 11, wherein the modified refractive index region exists in an annular region separated from the core portion by a core diameter or more in a cross section perpendicular to the axial direction of the optical amplification fiber. Optical amplification fiber. The light amplifying fiber according to any one of claims 8 to 12, wherein the modified refractive index regions are distributed substantially uniformly in the radial direction of each core portion of the light amplifying fiber. The optical amplifying fiber according to any one of claims 8 to 13, wherein the modified refractive index regions are distributed substantially uniformly in the axial direction of the optical amplifying fiber. The light amplifying fiber according to any one of Claims 8 to 14, wherein the modified refractive index regions are distributed substantially uniformly in a direction around the axis of each core portion of the light amplifying fiber. A plurality of the core portions are provided, and the modified refractive index is defined between the inner side and the outer side of a circular tubular boundary passing through the core portion that is the most distant from the center of the optical amplification fiber with the center of the optical amplification fiber as an axis among the plurality of core portions. 16. The optical amplifying fiber according to any one of claims 8 to 15, wherein the regions have different density. The optical amplifying fiber according to any one of claims 1 to 16, wherein said rare earth element includes erbium. The wavelength of the excitation light is 976 nm ± 2 nm,
Cladding absorption rate = -10 × log ((pumping light power (W) transmitted through the optical amplifying fiber having a plurality of cores)/(the above-mentioned Pumping light power (W)) incident on the inner cladding portion/length of the optical amplification fiber (m),
If we define
The optical amplifying fiber according to any one of claims 1 to 17, wherein the cladding absorption rate is 0.05 dB/m or more. an optical amplifying fiber according to any one of claims 1 to 18;
a pumping light source for outputting pumping light for optically pumping the rare earth element of the optical amplifying fiber;
an optical coupler that optically couples the excitation light to the inner cladding;
An optical fiber amplifier. 20. The optical fiber amplifier according to claim 19, comprising a plurality of said core portions, wherein a gain difference between said plurality of core portions is 3 dB or less. An optical communication system comprising the optical fiber amplifier according to claim 19 or 20.

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