JPH0637385A - Rare-earth element doped multicore fiber and optical amplifier employing the same - Google Patents

Abstract

PURPOSE:To facilitate power amplification, realize a power transmission system and a distribution transmission system and improve the efficiency of an exciting light by a method wherein doping rates of rare-earth elements among cores are different and the refractive indices of the cores and a cladding are different. CONSTITUTION:Rare-earth elements are contained in the cladding 3 of a rare- earth element doped multicore fiber 1. Nine cores 2-1 to 2-9 having higher refractive indices than the cladding 3 are provided. The cores 2 are made of glass whose main component is SiO2 system material and which is doped with at least one among refractive index control dopants such as P, Ge and Al in addition to rare-earth elements. The cladding 3 is made of SiO2 or SiO2 doped with at least one among refractive index control dopants such as F, B and P. The shapes of the cores are circular, elliptical, polygonal, etc. The diameters of the respective cores 2 are so selected as to satisfy the conditions of single mode transmission.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rare earth element-doped multi-core fiber having a plurality of cores containing a rare earth element such as Er and Na in a clad having a low refractive index, and an optical amplifier using the same.

[0002]

2. Description of the Related Art In recent years, rare earth elements such as Er (erbium), Nd (neodymium), Pr (praseodymium) have been added to the core of an optical fiber, and the optical fiber has an absorption characteristic peculiar to the added rare earth element. Research and development of an optical fiber amplifier that amplifies signal light by exciting the pumping light has been activated.

FIG. 12 shows an example of the configuration of a conventional optical fiber amplifier.

This is because the signal light in the wavelength band of 1.5 μm is propagated in the core of the optical fiber 22 to which Er is added as shown by arrows 20 and 21, and the wavelength is passed through the optical directional coupler 23 from the middle thereof. 1.47μm (or 0.98μ
By driving the pumping semiconductor laser 24 of (m) by the drive circuit 25 and propagating the pumping light to the optical fiber 22 as well, a population inversion state is realized, whereby the signal light is multiplied by several hundred to 10,000 times. It has the effect of amplifying to a certain degree. The optical directional coupler 23 on the output side has a function of removing the light of the pumping semiconductor laser contained in the amplified signal light.

[0005]

However, the conventional optical fiber amplifier has the following problems to be solved.

(1) The power of the signal light incident on the core is +1
When it becomes 0 dBm or more, the amplification degree sharply decreases. That is, it is difficult to obtain large optical power on the output side.

(2) Therefore, it is difficult to realize a so-called distribution system that distributes signal light into several tens or more.

(3) The pumping light that has entered the core does not contribute efficiently to the amplifier, and a considerable amount of pumping light is discharged from the optical directional coupler on the output side, which is uneconomical.

Therefore, an object of the present invention is to solve the above-mentioned drawbacks of the prior art, to perform power amplification, and consequently to realize a power transmission and distribution transmission system and a rare earth-doped multi-core fiber which can efficiently use pumping light. It is to provide an optical amplifier using the same.

[0010]

In order to achieve the above object, the present invention uses a rare earth element-doped multicore fiber in which a plurality of high refractive index cores containing rare earth elements are provided in a low refractive index clad. , The amount of the rare earth element in each of the cores and the value of the refractive index, further different kinds of rare earth elements, also provided a low refractive index layer lower than the refractive index of the clad on the outer periphery of the core group, Another object of the present invention is to provide an optical fiber amplifier which transmits pumping light in the fiber.

That is, the first aspect of the present invention is to provide a plurality of high-refractive-index cores containing a rare-earth element in a low-refractive-index clad, and at least one of the cores has a rare-earth element addition amount. It is a rare earth element-doped multi-core fiber characterized by being different.

A second aspect of the invention is to provide a low refractive index clad in
A rare earth element-doped multi-core fiber characterized in that a plurality of high refractive index cores containing rare earth elements are provided, and at least one of the cores has a different refractive index value.

A third aspect of the invention is to provide a low refractive index clad in
A plurality of high-refractive-index cores containing rare earth elements are concentrated and distributed near the center of the clad, and a ring-shaped low-index core with a value lower than the clad's refractive index is placed around the center of the clad. A rare earth element-added multi-core fiber having a refractive index layer.

A fourth aspect of the present invention is the rare earth element-added multicore fiber according to the third aspect, wherein at least one of the cores has a different addition amount of the rare earth element.

A fifth aspect of the present invention is the rare earth element-doped multicore fiber according to the third aspect, characterized in that at least one of the cores has a different refractive index value.

A sixth aspect of the invention is to provide a cladding having a low refractive index,
A rare-earth element-doped multi-core fiber characterized in that a plurality of high-refractive-index cores containing rare-earth elements are provided, and at least one of the cores has a different shape.

A seventh invention is the rare-earth element-doped multicore fiber according to the second aspect, characterized in that the refractive index of the central core of the rare-earth element-doped multicore fiber has the highest value.

An eighth invention is a rare earth element-doped multicore fiber according to any one of the first to seventh aspects, wherein the cores are in contact with each other.

A ninth invention is the rare earth element-added multicore fiber according to any one of the first to seventh aspects, wherein the outer periphery of each core is coated with a coating cladding having a low refractive index.

A tenth aspect of the present invention is the rare earth element-added multicore fiber according to any one of the first to ninth aspects, characterized in that the cladding and the core are tapered in diameter from the input side toward the output side. .

An eleventh aspect of the present invention is to provide a narrow-diameter portion, in which the outer diameter is reduced from a large diameter to a tapered shape along the longitudinal direction of the rare earth element-added multicore fiber described in any one of the first to ninth aspects, and in which a propagation mode is coupled. After that, it is a rare earth element-added multi-core fiber configured to be thick again in a taper shape.

In a twelfth aspect of the invention, the input side is the rare earth element-doped multicore fiber described in any one of items 1 to 9, and each core is provided in each circular clad as proceeding from the input side to the output side. One input N output for optical distribution (N output (N
> 2) the coupler.

A thirteenth aspect of the present invention is to connect an optical demultiplexer to the input side of the rare earth element-doped multicore fiber described in any one of the first to eleventh aspects to propagate signal light and pumping light into the fiber,
The optical fiber amplifier for optical distribution is characterized in that an optical star coupler is connected to the output side to distribute the signal light.

[0024]

When the number of cores provided is N, as in the first aspect of the invention, the number of cores in the clad is increased N times as compared with the prior art. Therefore, the power of the signal light is + 10d
Even if it exceeds Bm, the amplification degree does not decrease, and the greatly amplified optical power can be obtained at the output side. Moreover, the amount of rare earth element added to the core at the center of the multi-core assembly composed of N pieces may be maximized, and the amount of rare earth element added may be reduced as the distance from the center to the outer periphery of the assembly increases. For example, the amount of rare earth element addition is the largest in the region where the optical power of the pumping light propagating in the optical fiber is maximum, and the amount of the rare earth element is small in the region where the optical power of the pumping light is weak. It contributes to an amplifier, and as a result, high power amplification can be realized.

As in the second invention, at least one of the cores is characterized in that its refractive index is different. Therefore, for example, as in the seventh invention, it is composed of N cores. In the multi-core of pumping light and signal light, the refractive index of the core in the central part of the multi-core assembly is set to be the highest, and the refractive index of the core is slightly lowered as the distance from the center to the outer circumference of the assembly is increased. Can be tightly confined to. As a result, the pumping light efficiently contributes to the optical amplifier and high power amplification can be realized.

As in the third aspect of the present invention, by covering the outer periphery of the multi-core assembly composed of N pieces with a ring-shaped low refractive index layer having a value lower than the refractive index of the cladding, pumping light and signal light It can be more strongly confined in the multi-core, and higher power amplification can be realized.

According to the fourth and fifth aspects, the confinement of the pumping light and the signal light in the multi-core is further increased, and the doping amount distribution of the rare earth element is provided according to the optical power distribution of the pumping light. As a result, the excitation light efficiency can be significantly improved.

According to the sixth aspect of the invention, at least one of the cores in the multi-core aggregate composed of N pieces is used.
One is that the shape is different, for example, if the core diameter of the center of the multi-core assembly is made the largest and a core with a small core diameter is arranged on the outer periphery of the multi-core assembly, the multi-core assembly can be densely packed. A circular multi-core aggregate can be constructed. That is, by making the gap between the cores extremely small, the pumping light and the signal light can be efficiently confined and propagated in the respective cores.

According to the eighth aspect of the present invention, by arranging the cores of the N multicore assembly in contact with each other, the signal light and the pumping light can be efficiently confined and propagated in the cores. It is possible to reduce the propagation loss of the signal light, to efficiently convert the energy of the pump light, and to contribute to the high power amplification of the signal light.

According to the ninth invention, by coating the outer periphery of each core with a coating cladding having a low refractive index,
The light is strongly confined in each core, and the scattering loss due to the interface irregularity between the cladding and the cladding can be reduced.

According to the tenth aspect of the invention, the taper type rare earth element-doped multi-core fiber having a taper-shaped crater from the input side to the output side is provided, whereby the light reflected from the output side is directed to the input side. You can suppress the return. That is, since it has an optical isolator function,
This is a very advantageous fiber structure for constructing an optical fiber amplifier.

According to the eleventh invention, by tapering from a large diameter to a tapered shape along the longitudinal direction of the optical fiber, and then again tapering to a large diameter through a small diameter portion, the N input and N output are themselves. Optical star coupler can be constructed. Moreover, by propagating the pumping light, it is possible to have an optical amplification function.

According to the twelfth aspect, an optical star coupler having a one-input N-output optical amplification function can be realized.

According to the thirteenth invention, it is possible to realize a system which distributes the power of the signal light amplified with a high gain to a large number of terminals with a more economical and simple structure.

[0035]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a first embodiment of the present invention. This is because the core 3 having a refractive index n _w (n _w > n _C1 ) containing a rare earth element in the clad 3 having a low refractive index n _C1 is 2-1.
9 shows the rare earth element-added multi-core fibers 1 arranged in a number of 2-9. 1A is a front view of the fiber 1, and FIG. 1B is a side view of the fiber 1, that is, an end view.

As the rare earth element, in addition to Er (erbium), Nd (neodymium), Pr (praseodymium), Sm
(Samarium), Tm (thulium), Yb (ytterbium), Ce (cerium), Ho (holmium), or the like may be used. Further, these cores 2 are composed of glass containing SiO ₂ as a main component, and those containing at least one kind of P, Ge, Al, Cr, etc. as a refractive index control additive in addition to the rare earth element are used. As the clad 3, SiO ₂ or SiO ₂ containing at least one kind of refractive index controlling additive such as F, B and P is used. Here, the relative refractive index difference Δ (= (n _w −
n _c1 ) ÷ n _w × 100%) is 0. It is selected from the range of several percent to several percent.

The shape of the core 2 may be circular, elliptical, polygonal, or any other structure. Diameter is 1μ
Although the range of m to several μm is preferable, Δ and the diameter of each core 2 are selected so as to satisfy the single mode transmission condition. That is,

[0038]

[Equation 1]

However, the values of a, n _w and Δ are selected so that λ: wavelength of signal light to be transmitted a: diameter of core 2 is satisfied.

The rare earth element-doped multicore fiber 1 shown in FIG. 1 can be manufactured as follows. That is, it can be obtained by bundling a large number of conventional rare earth element-doped fibers into a quartz glass tube, melting the above-mentioned quartz glass tube or a fused quartz glass tube in advance with a heating source, and drawing. This step is at least 1
The rare earth element in the core 2 is contained in a state closer to an atomic state as the number of times is increased, and this is due to concentration quenching when a large amount of rare earth element has been conventionally contained. This leads to the feature that it is possible to avoid the phenomenon of the decrease in amplification degree. The clads of the plurality of rare earth element-doped fibers are fused with each other to form an integrated clad 3. Here, each core 2-1 to 2-9
The amount of rare earth element in the core may be different from each other, and only one, for example, the amount of addition in the core 2-1 is the largest, and the amount of addition in the cores 2-2 to 2-9 is the same. You may make it less than. As a specific example, SiO ₂ is used for the clad 3 and SiO is used for the cores 2-1 to 2-9.
_{In the 2-} P ₂ O ₅ -Al ₂ O ₅ glass, 21 Er ions were used as an additive quantity to maximize the amount of the rare earth element added to the central core 2-1 and increase the amount of the rare earth element as the distance from the peripheral portion 14 increased. If the doping amount is gradually reduced, when the pumping light is propagated in the optical fiber 1, the distribution becomes close to the rare earth element doping distribution according to the optical power distribution of the pumping light. The energy can be well converted and a high gain optical amplifier can be realized. Moreover, even if the signal light is increased, the added amount of the rare earth element in each core is such that concentration quenching does not occur, so that the amplified signal light is less likely to be saturated. That is, high power amplification can be realized. Further, in FIGS. 1 and 2, if the refractive indexes in the respective cores are made different and the pumping light and the signal light are selected so as to be concentrated in the central portion of the optical fiber 1, the confinement of light becomes better, and a large power consumption is achieved. Amplification can be realized.

FIG. 3 shows a third embodiment of the rare earth element-doped multicore fiber of the present invention. This is M
A ring-shaped low refractive index layer 4 having a refractive index n _C2 (n _C2 <n _C1 ) lower than the refractive index n _C1 of the cladding 3 is provided on the peripheral portion 14 of each multi-core assembly. By providing the low-refractive index layer 4, the pumping light and the signal light are better confined in the multi-core aggregate, and higher power amplification can be realized. Also in this case, if the addition amount of the rare earth element and the value of the refractive index in each core are selected so as to correspond to the optical power distributions of the pumping light and the signal light,
Further, high power amplification can be realized.

FIG. 4 shows a fourth embodiment of the present invention. This is an example in which the shapes of the cores in a multi-core assembly composed of 13 pieces are different. In this embodiment, pm is used, and the amount of Er ions added in the cores 2-2 to 2-9 is 400 ppm. As shown in FIG. 2, the shape of the core 2-1 at the center of the core to which the rare earth element is added has the largest shape as shown in FIG. The shape of the cores 2-2 to 2-13 in the peripheral portion is selected to be small. With such a configuration, the outer peripheral portion 14 of the multi-core assembly can be made to have a substantially circular shape, the polarization dependency is small, and the propagation loss can be reduced. Also in this structure, the amount of the rare earth element added and the value of the refractive index in each core may be different. Further, the low refractive index layer 4 may be provided on the outer peripheral portion 14 as shown in FIG.

FIG. 5 shows a fifth embodiment of the present invention. This is a configuration in which 13 cores 2-1 to 2-13 are integrated together. With this united structure, almost all the signal light and the pumping light can be propagated in the united core. Therefore, since the clad that has been interposed between the cores until now is eliminated, unnecessary scattering in the clad that has been interposed between the cores is eliminated, and the signal light can be propagated with low loss. Further, the pumping light is also efficiently energy-converted, which contributes to the high power amplification of the signal light. In addition,
In FIG. 5, the shape of each core varies depending on the number of cores. That is, as the number of cores increases, the shape of the individual cores when combined together becomes closer to a circle. Further, the larger the number of cores, the more the amount of the rare earth element added into each core can be suppressed within a range where concentration quenching does not occur, so that high power amplification becomes easier. FIG. 6 shows another embodiment of the rare earth element-added multi-core fiber of the present invention. This is a structure in which the rare earth element-added cores 2 are dispersed and arranged almost all over the clad 3. In addition to high power amplification, this is used by inserting it between N input and output fibers to realize an N-input N-output high-power amplification optical star coupler. Then, it is a multi-core fiber having a structure in which the amount of Er ions added in the core 2-1 is 1000 p.

FIG. 7 also shows another embodiment of the rare earth element-added multi-core fiber of the present invention. This enhances the confinement of light in the core 2 and reduces the scattering loss due to the interface irregularity between the cladding 4 and the cladding 3.

FIG. 8 shows a fifth embodiment of the present invention.

In this example, the core 2 and the clad 3 are shown in FIG.
As shown in FIG. 8A, a tapered rare earth element-doped multi-core fiber 5 having a tapered diameter from the input side 6 shown in FIG. 8B to the output side 7 shown in FIG. 8C is formed. It is a thing. As a result, the light reflected from the output side 7 is suppressed from returning to the input side. That is, since it has an optical isolator function, it is an extremely advantageous fiber structure for constructing an optical fiber amplifier.

FIG. 9 shows a system application example of the rare earth element-doped multi-core fiber 1.

This is to power-amplify the signal light indicated by the arrow 8 which is incident on the rare earth element-added multi-core fiber 1 through the optical demultiplexer 10, and to output the amplified signal light to 1 input M output 1
The power is distributed by the xM type optical star coupler 11, and the distributed light is output from each output side as indicated by arrows 12-1, 12-2, ... 12-M.

In this case, for example, a multicore fiber in which Er is added to the rare earth element-doped multicore fiber 1 is used, and the signal light incident from the arrow 8 has a wavelength of 1.5 μm band. A wavelength of 1.48 μm (or 0.98 μm) is used for the excitation light indicated by the arrow 9. The optical demultiplexer 10 allows the signal light to pass through as it is, demultiplexes the pumping light, and both lights are incident on and propagated in the rare earth element-doped multicore fiber 1. Then, the signal light is power-amplified and enters the 1 × M type optical star coupler 11. The 1 × M type optical star coupler 11 divides the power-amplified signal light into M equal parts, and outputs arrows, 12-1, 12-2, from the respective output sides.
12-M is distributed and output. This is effective as a system for distributing the power of the amplified signal light to many terminals as described above.

FIG. 10 also shows a system application example, which is an effective configuration as a power distribution system having an optical isolator function using the tapered rare earth element-doped multi-core fiber 5. With this configuration, for example, the reflected light from the distributed light extraction end side propagates in the tapered rare earth element-doped multi-core fiber 5 and is emitted to other than the core 2 and does not return to the input end side. It has become a structure.

In FIGS. 9 and 10, the optical demultiplexer 1
The 0 and 1 × M type optical star couplers 11 may have an optical waveguide structure as well as an optical fiber structure.

11 and 12 between the rare earth element-doped multi-core fiber 1 (or tapered rare earth element multi-core fiber 5) and the 1 × M type optical star coupler 11 in the configurations of FIGS. 9 and 10. By thus inserting the optical isolator 13, more stable power amplification can be realized.

In FIGS. 9 and 10, the optical demultiplexer 10 is inserted between the rare earth element-doped multi-core fiber 1 (or the tapered rare earth element-doped multi-core fiber 5) and the 1 × M optical star coupler and left. By removing the pumping light that is generated, an optical amplifier with lower noise can be realized. Further, in the above-mentioned configuration, the position where the pumping light is incident is the case of so-called forward pumping (FIG. 9) in the same direction as the signal light propagation direction, and the so-called backward pumping (not shown) in the direction opposite to the signal light propagation direction. either will do.

FIG. 11 shows an embodiment of a 1-input N-output (N: 9) optical amplification type coupler for optical distribution of the present invention. This is a rare earth element-doped multi-core fiber in which the input side of the rare earth element-doped fiber bundle inserted into the glass tube 14 is tapered, and the output side is not stretched. The same figure (a) is the front view,
(B) is a side view of the input side, and (c) is a side view of the output side. When a normal optical fiber (for example, a single-mode optical fiber having a core diameter of 10 μm and a cladding diameter of 125 μm) is connected to the input side and the signal light and the excitation light are propagated from the input side optical fiber, the rare earth element-doped fiber 15 on the output side 15 The high power optical signals amplified from -1 to 15-9 are taken out. Here, the diameter of the core 2 and the diameter of the clad 3 of the rare earth element-doped fibers 15-1 to 15-9 on the output side are selected so that they can be easily connected to an ordinary optical fiber. Further, the diameter of the aggregate of the cores 2 of the rare earth element-doped multi-core fiber on the input side is also selected to be a value corresponding to the core diameter of an ordinary optical fiber.

The present invention is not limited to the above embodiment. First, the size and material of the rare earth element-added multicore fiber are not particularly limited, and can be selected from a wide range. For example, the fiber may be used for multimode transmission with a V value of 2.405 or more, in addition to single mode transmission. The material may be borosilicate-based, fluoride-based, phosphate-based, alkali-based glass, etc. in addition to the SiO ₂ system.

[0056]

The present invention has the following effects.

As in the first invention, if the number of cores provided is N, the number of cores in the clad is increased N times as compared with the conventional one. Therefore, the power of the signal light is +1
Even if it becomes 0 dBm or more, the amplification degree does not decrease, and the greatly amplified optical power can be obtained at the output side. Moreover, N
If the amount of rare earth element added to the core of the central part of the multi-core assembly composed of individual pieces is the largest, and the amount of rare earth element added is reduced as the distance from the center to the outer circumference of the assembly is reduced, Since the amount of rare earth elements added is the largest in the region where the optical power of the pumping light propagating in the fiber is maximum, and the amount of rare earth elements is small in the region where the optical power of the pumping light is weak, the pumping light is efficiently used for optical amplifiers. This contributes to the realization of high power amplification.

As in the second invention, it is characterized in that at least one of the cores has a different refractive index. Therefore, for example, as in the seventh invention, N By making the refractive index of the core in the central portion of the configured multi-core assembly the highest, and slightly lowering the refractive index of the core as it goes away from the center to the outer periphery of the assembly, a multi-core of pumping light and signal light You can strengthen the containment. As a result, the pumping light efficiently contributes to the optical amplifier, and high power amplification can be realized.

As in the third invention, by covering the outer periphery of the multi-core assembly composed of N pieces with a ring-shaped low refractive index layer having a value lower than the refractive index of the cladding, the pumping light and the signal light are It can be more strongly confined in the multi-core, and higher power amplification can be realized.

According to the fourth and fifth inventions, the confinement of the pumping light and the signal light in the multi-core is further increased,
Moreover, the excitation efficiency can be significantly improved by providing the distribution of the amount of the rare earth element added according to the optical power distribution of the excitation light.

According to the sixth aspect of the invention, at least one of the cores in the multi-core assembly composed of N pieces is used.
One is that the shape is different, for example, if the core diameter of the center of the multi-core assembly is made the largest and a core with a small core diameter is arranged on the outer periphery of the multi-core assembly, the multi-core assembly can be densely packed. A circular multi-core aggregate can be constructed. That is, by making the gap between the cores extremely small, it becomes possible to efficiently confine and propagate the pumping light and the signal light in the respective cores.

According to the eighth aspect of the present invention, by arranging the cores of the N multicore assembly in contact with each other, the signal light and the pumping light can be efficiently confined and propagated in the cores. It is possible to reduce the propagation loss of the signal light, to efficiently convert the energy of the pump light, and to contribute to the high power amplification of the signal light.

According to the ninth invention, by coating the outer periphery of each core with a coating cladding having a low refractive index,
The light is strongly confined in each core, and the scattering loss due to the interface irregularity between the cladding and the cladding can be reduced.

According to the tenth aspect of the invention, the taper type rare earth element-doped multicore fiber having a tapered crater diameter from the input side to the output side is provided, whereby the light reflected from the output side is directed to the input side. You can suppress the return. That is, since it has an optical isolator function,
This is a very advantageous fiber structure for constructing an optical fiber amplifier.

According to the eleventh aspect of the invention, along the longitudinal direction of the optical fiber, the taper is tapered from a large diameter to a taper large diameter through a small diameter portion. Optical star coupler can be constructed. Moreover, by propagating the pumping light, it is possible to have an optical amplification function.

According to the twelfth invention, it is possible to realize an optical star coupler having a one-input N-output optical amplification function.

According to the thirteenth invention, it is possible to realize a system that distributes the power of the signal light amplified with high gain to a large number of terminals with a more economical and simple structure.

[Brief description of drawings]

FIG. 1 is an explanatory view of an embodiment of a rare earth element-doped multicore fiber of the present invention.

FIG. 2 is an explanatory view of an example of a rare earth element-doped multicore fiber of the present invention.

FIG. 3 is an explanatory view of an embodiment of a rare earth element-doped multicore fiber of the present invention.

FIG. 4 is an explanatory view of an embodiment of the rare earth element-doped multicore fiber of the present invention.

FIG. 5 is an explanatory view of an embodiment of the rare earth element-doped multicore fiber of the present invention.

FIG. 6 is an explanatory view of an example of a rare earth element-doped multicore fiber of the present invention.

FIG. 7 is an explanatory view of an example of a rare earth element-doped multicore fiber of the present invention.

FIG. 8 is an explanatory diagram of an embodiment of a taper type rare earth element-doped multicore fiber of the present invention.

FIG. 9 is an explanatory diagram of an example of application of the system of the present invention.

FIG. 10 is an explanatory diagram of an example of application of the system of the present invention.

FIG. 11 is an explanatory diagram of an example of application of the system of the present invention.

FIG. 12 is an explanatory diagram showing a conventional example.

[Explanation of symbols]

 1 Rare earth element-doped multi-core fiber 2 Core 3 Clad 4 Low refractive index layer

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H01S 3/094 3/10 Z 8934-4M // C03B 37/012 A

Claims (13)

[Claims] 1. A plurality of high-refractive-index cores containing a rare-earth element are provided in a low-refractive-index clad, and at least one of the cores has a different addition amount of the rare-earth element. Characteristic rare earth element-added multi-core fiber. 2. A plurality of high-refractive-index cores containing a rare earth element are provided in a low-refractive-index clad, and at least one of the cores has a different refractive index value. A multi-core fiber doped with rare earth elements. 3. A plurality of high-refractive-index cores containing a rare earth element are concentrated and distributed near the central part of the clad in the low-refractive-index clad, and the clad is formed on the outer periphery of the concentrated area of the cores. A multi-core fiber doped with a rare earth element, which is provided with a ring-shaped low refractive index layer having a value lower than the refractive index of 4. The rare earth element-doped multi-core fiber according to claim 3, wherein at least one of the cores has a different amount of rare earth element added. 5. The rare earth element-doped multicore fiber according to claim 3, wherein at least one of the cores has a different refractive index value. 6. A plurality of high refractive index cores containing a rare earth element are provided in a low refractive index clad, and at least one of the cores has a different shape. Rare earth element-doped multi-core fiber. 7. The rare earth element-added multicore fiber according to claim 2, wherein the refractive index of the core at the center of the rare earth element-added multicore fiber has the highest value. 8. The rare earth element-doped multicore fiber according to claim 1, wherein the respective cores are in contact with each other. 9. The rare earth element-doped multicore fiber according to claim 1, wherein the outer circumference of each core is covered with a coating clad having a low refractive index. 10. The rare earth element-added multicore fiber according to claim 1, wherein the cladding and the core are tapered to have a large diameter from the input side toward the output side. 11. A structure in which an outer diameter of a rare earth element-added multicore fiber is tapered from a thick diameter to a tapered shape along a longitudinal direction, and is tapered again through a small diameter portion in which a propagation mode is coupled. The rare earth element-doped multicore fiber according to any one of claims 1 to 9. 12. The input side is the rare earth element-doped multicore fiber according to claim 1, wherein each core is provided in a circular clad as proceeding from the input side to the output side. A 1-input / N-output (N> 2) coupler for light distribution, which has one input side of the structure converted to the structure and which is composed of a rare earth element-doped fiber having only the number of multi-cores on the output side. 13. An optical demultiplexer is connected to the input side of the rare earth element-doped multicore fiber according to any one of claims 1 to 11 to propagate signal light and pump light into the fiber, and to the output side. An optical fiber amplifier for optical distribution, wherein an optical star coupler is connected to distribute the signal light.