JP6734100B2 - Optical fiber amplifier and multistage optical amplification fiber structure - Google Patents

Description

The present invention relates to an optical fiber amplifier and a multistage optical amplification fiber structure.

An optical amplification fiber having a structure called a so-called multi-core fiber, which includes a plurality of core portions doped with rare earth elements in one optical fiber, is disclosed, and an optical fiber amplifier and a fiber laser using the same are disclosed. (Patent Documents 1 and 2).

Patent Document 1 discloses a fiber laser using a multi-core rare earth-doped optical fiber having a clad pumping configuration. In this fiber laser, in order to efficiently and efficiently couple the pumping light to the core portion, the core in the multi-core rare earth-doped optical fiber is circularly connected to reduce the amount of the remaining pumping light. On the other hand, in Patent Document 2, by connecting the core portions of the multi-core rare earth-doped optical fiber in a circular manner, the rare-earth-doped optical fiber can be used for the amplification characteristics and the resonator characteristics that change depending on the length of the rare-earth-doped optical fiber in the fiber laser. A technique is disclosed in which the characteristics can be adjusted and the size can be reduced without replacement.

Japanese Patent No. 3889746 Japanese Patent No. 5811056

By the way, in many optical fiber amplifiers, optical amplification fibers are connected in multiple stages. In this case, as the number of stages of the optical amplification fiber increases, the size of the optical fiber amplifier increases, and the number of pumping light sources used for optically pumping the optical amplification fiber increases, resulting in an increase in power consumption of the optical fiber amplifier. There is.

The present invention has been made in view of the above, and provides an optical fiber amplifier that can be downsized and have low power consumption even when connected in multiple stages, and a multistage optical amplification fiber structure that can be suitably used for this. The purpose is to

In order to solve the problems described above and achieve the object, an optical fiber amplifier according to an aspect of the present invention is formed with a plurality of rare earth element-doped core portions and the outer periphery of the plurality of core portions. An optical amplification fiber having an inner cladding portion having a refractive index lower than that of a plurality of core portions, and an outer cladding portion formed on the outer periphery of the inner cladding portion and having a refractive index lower than the inner cladding portion, and the optical amplification fiber. An optical fiber amplifier of the cladding pumping type, wherein the inner cladding part of the fiber comprises at least one pumping light source for supplying pumping light for optically pumping the rare earth element, wherein the plurality of core parts are a plurality of core part groups. The optical amplification fiber is configured such that the gains of the core parts belonging to different core part groups are different from each other, the core parts belonging to the different core part groups are connected in series, and a multistage optical fiber is provided. It is characterized in that it constitutes an amplifying fiber structure.

In the optical fiber amplifier according to the aspect of the present invention, the inner clad section has a plurality of inner sub-cladding sections having different refractive indexes, and the core sections belonging to the different core section groups are different inner sub-cladding sections. It is characterized by being surrounded by.

An optical fiber amplifier according to an aspect of the present invention is characterized in that core portions belonging to the different core portion groups have different doping concentrations of the rare earth element.

An optical fiber amplifier according to an aspect of the present invention is characterized in that core parts belonging to the different core part groups have different kinds of the added rare earth elements.

An optical fiber amplifier according to an aspect of the present invention is characterized in that core portions belonging to the different core portion groups have different core diameters.

In the optical fiber amplifier according to one aspect of the present invention, n and m are integers of 2 or more, the number of stages in the multistage optical amplifying fiber structure is n, the number of core groups in the optical amplifying fiber is n or more, and When the number of core portions in the optical amplifying fiber is n×m, it is characterized by having m multistage optical amplifying fiber structures.

An optical fiber amplifier according to one aspect of the present invention is characterized in that the number of pumping light sources is smaller than m.

A multi-stage optical amplification fiber structure according to an aspect of the present invention has a plurality of core portions doped with a rare earth element, and an inner clad formed on the outer periphery of the plurality of core portions and having a lower refractive index than the plurality of core portions. And an optical amplification fiber having an outer clad formed on the outer periphery of the inner clad and having a refractive index lower than that of the inner clad, wherein the plurality of cores comprises a plurality of cores. The optical amplification fiber is configured such that the gains of the core parts belonging to different core part groups are different from each other, and the core parts belonging to the different core part groups are connected in series. ..

According to the present invention, there is an effect that it is possible to realize an optical fiber amplifier that can be downsized and consume less power even if connected in multiple stages.

FIG. 1 is a schematic configuration diagram of the optical fiber amplifier according to the first embodiment. FIG. 2 is a diagram showing a schematic cross section and a refractive index profile of the optical amplification fiber shown in FIG. FIG. 3 is a schematic cross-sectional view of Configuration Example 1 of the optical amplification fiber. FIG. 4 is a schematic cross-sectional view of Configuration Example 2 of the optical amplification fiber. FIG. 5 is a schematic cross-sectional view of Configuration Example 3 of the optical amplification fiber. FIG. 6 is a schematic cross-sectional view of Configuration Example 4 of the optical amplification fiber. FIG. 7 is a schematic configuration diagram of the optical fiber amplifier according to the second embodiment. FIG. 8 is a schematic configuration diagram of the optical fiber amplifier according to the third embodiment. FIG. 9 is a diagram showing a schematic cross section and a refractive index profile of the optical amplification fiber shown in FIG. FIG. 10 is a diagram showing a schematic cross section and a refractive index profile of Structural Example 5 of the optical amplification fiber. FIG. 11 is a schematic cross-sectional view of Configuration Example 6 of the optical amplification fiber. FIG. 12 is a schematic cross-sectional view of Structural Example 7 of the optical amplification fiber. FIG. 13 is a schematic cross-sectional view of Structural Example 8 of the optical amplification fiber. FIG. 14 is a schematic sectional view of Configuration Example 9 of the optical amplification fiber.

Hereinafter, embodiments of an optical fiber amplifier and a multistage optical amplification fiber structure according to the present invention will be described in detail with reference to the drawings. The present invention is not limited to this embodiment. Moreover, in each drawing, the same or corresponding elements are appropriately denoted by the same reference numerals.

(Embodiment 1)
FIG. 1 is a schematic configuration diagram of the optical fiber amplifier according to the first embodiment. As shown in FIG. 1, an optical fiber amplifier 100 includes an input optical fiber 1, an optical isolator 2, an optical multiplexer/demultiplexer 3, a WDM (Wavelength Division-Multiplexing) coupler 4, an excitation light source 5, and a multimode light. Fiber 6, optical amplification fiber 7, residual pumping light processing unit 8, optical multiplexer/demultiplexer 9, optical isolator 10, connection optical fiber 11, ASE (Amplified Spontaneous Emission) cut filter 12, and optical isolator 13 And an output optical fiber 14.

First, the configuration of the optical amplification fiber 7 will be described. FIG. 2 is a diagram showing a refractive index profile in a schematic cross section of the optical amplification fiber 7 and a cross section taken along the line AA. The optical amplification fiber 7 includes a plurality of (12 in the optical amplification fiber 7) core portions 7aa to 7al to which erbium (Er), which is a rare earth element, is added, and an inner clad portion formed on the outer periphery of the core portions 7aa to 7al. This is an optical amplification fiber having a so-called double-clad structure having 7b and an outer clad portion 7c formed on the outer periphery of the inner clad portion 7b.

The core portions 7aa to 7al include a core portion group 7d including six core portions 7aa to 7af and a core portion group 7e including six core portions 7ag to 7al. The core portions 7aa to 7af of the core portion group 7d and the core portions 7ag to 7al of the core portion group 7e are arranged in a hexagonal shape.

The inner clad portion 7b has a plurality of (two in the optical amplification fiber 7) inner sub-clad portions 7ba and 7bb, and the core portions 7aa to 7af and 7ag to 7al belonging to different core portion groups 7d and 7e are It is surrounded by different inner sub-cladding portions 7ba and 7bb. Specifically, the core portions 7aa to 7af belonging to the core portion group 7d are surrounded by the inner sub-cladding portion 7ba having a circular cross section, and the core portions 7ag to 7al belonging to the core portion group 7e are inner sub-cladding portions 7ba. It is surrounded by a ring-shaped inner sub-cladding portion 7bb formed on the outer periphery of the.

Moreover, as the refractive index profile P1 shows, the outer cladding portion 7c, the inner sub-cladding portion 7ba, the inner sub-cladding portion 7bb, the core portions 7aa to 7af, and the core portions 7ag to 7al have refractive indices of n1, n2, and n3, n4, and n5. Therefore, the inner cladding portion 7b has a lower refractive index than the core portions 7aa to 7al. The outer cladding portion 7c has a lower refractive index than the inner cladding portion 7b. The inner sub-cladding portions 7ba and 7bb have different refractive indexes. Specifically, the inner sub-cladding portion 7bb has a higher refractive index than the inner sub-cladding portion 7ba.

In order to realize the above relationship of the refractive index, the core portions 7aa to 7al are made of quartz glass to which germanium (Ge) that is an additive for increasing the refractive index is added, and the inner sub-cladding portion 7ba increases the refractive index. Fluorine (F), which is an additive, is made of quartz glass, the inner sub-cladding portion 7bb is made of pure silica glass to which an additive for adjusting the refractive index is not added, and the outer cladding portion 7c has a low refractive index. Made of resin material. However, these materials are mere examples, and are not particularly limited as long as they are materials that can realize the above relationship of the refractive index.

In addition, in the optical amplification fiber 7, the core diameter, the concentration of Er added, and the relative refractive index difference with respect to the enclosed cladding portion are substantially the same in all the core portions 7aa to 7al. In addition, for example, the core diameter is 2 to 6 μm, the concentration of Er added is 300 to 3000 ppm, and the relative refractive index difference with respect to the enclosed clad portion is 0.3 to 1.5%.

Next, returning to FIG. 1, the optical fiber amplifier 100 will be described. Hereinafter, each component of the optical fiber amplifier 100 and its function will be described in order of connection from the input optical fiber 1 side. The input optical fiber 1 receives an input of signal light to be optically amplified by the optical fiber amplifier 100, and propagates in a single mode. The signal light is, for example, signal light included in the 1.55 μm wavelength band and the 1.57 μm wavelength band used in optical communication. The signal light may be signal light of a single wavelength or WDM signal light in which signal lights of different wavelengths are wavelength-multiplexed.

The optical isolator 2 has a function of passing signal light propagating toward the optical multiplexer/demultiplexer 3 through the input optical fiber 1 and blocking light propagating from the optical multiplexer/demultiplexer 3 toward the input optical fiber 1.

The optical multiplexer/demultiplexer 3 is a fiber bundle type optical multiplexer/demultiplexer, and includes 12 input side optical fibers 3a to 3l and one output side multi-core optical fiber 3m in the first embodiment. .. The input side optical fibers 3a to 3l all propagate the signal light in a single mode. The input optical fiber 1 is connected to the input side optical fiber 3a via the optical isolator 2.

The output side multi-core optical fiber 3m is a multi-core optical fiber having the same core arrangement as the optical amplification fiber 7. The cores of the input side optical fibers 3a to 3l are connected to any of the cores of the output side multi-core optical fiber 3m. The output side multi-core optical fiber 3m is connected to the WDM coupler 4. As a result, the optical multiplexer/demultiplexer 3 outputs the signal light input from the input optical fiber 1 to the WDM coupler 4. In the first embodiment, the optical multiplexer/demultiplexer 3 is a fiber bundle type, but the present invention is not limited to this, and a spatially coupled optical multiplexer/demultiplexer may be used.

The pumping light source 5 is composed of one high-power semiconductor LD (Laser Diode) that outputs multimode pumping light having a wavelength of 980 nm band. The central wavelength of the excitation light is, for example, 975 nm or 976 nm. The pumping light source 5 is for supplying pumping light that optically pumps Er to the inner cladding portion 7b of the optical amplification fiber 7.

The multimode optical fiber 6 is, for example, a multimode optical fiber having a core diameter of 105 μm, a cladding diameter of 125 μm, and an NA of 0.22 or 0.15. The multimode optical fiber 6 propagates the pumping light output from the pumping light source 5 in multimode and outputs it to the WDM coupler 4.

The WDM coupler 4 multiplexes the signal light and the pumping light and outputs the multiplexed light to the output side multi-core optical fiber 4a fusion-spliced to the optical amplification fiber 7 at the connection point CP1. The output-side multi-core optical fiber 4a is a multi-core optical fiber having the same core arrangement as that of the optical amplification fiber 7, and has a double clad structure. The core portion of the output-side multi-core optical fiber 4a is connected to any one of the core portions of the optical amplification fiber 7. The output-side multi-core optical fiber 4 a propagates the signal light in the single mode by the core portion, propagates the pump light in the multi mode by the inner cladding portion, and outputs the signal light to the optical amplification fiber 7. In the first embodiment, the WDM coupler 4 is a fiber type, but the present invention is not limited to this, and a spatially coupled WDM coupler may be used.

In the optical fiber amplifier 100, the optical multiplexer/demultiplexer 3, the WDM coupler 4, and the optical amplification fiber 7 are arranged so that the signal light input from the input optical fiber 1 is input to the core portion 7aa of the optical amplification fiber 7. The connection is made.

The optical amplification fiber 7 propagates the pumping light in the multimode by the inner cladding portion 7b. Then, Er added to each of the core portions 7aa to 7al of the optical amplification fiber 7 absorbs the excitation light and is photoexcited. That is, the optical fiber amplifier 100 is a cladding pump type optical fiber amplifier. The signal light is propagated in the single mode by the core portion 7aa, so that the stimulated emission phenomenon occurs and the signal light is optically amplified. The optical amplification fiber 7 outputs the amplified signal light to the optical multiplexer/demultiplexer 9.

As described above, since the inner sub-cladding portion 7bb has a higher refractive index than the inner sub-cladding portion 7ba in the inner cladding portion 7b, the pumping light is generated in the inner sub-cladding portion 7bb more than in the inner sub-cladding portion 7ba. , Uneven distribution with stronger power. As a result, the core portions 7ag to 7al of the core portion group 7e surrounded by the inner sub-cladding portion 7bb are more than the core portions 7aa to 7af of the core portion group 7d surrounded by the inner sub-cladding portion 7ba. It is in a state of being more strongly excited by the excitation light.

The optical multiplexer/demultiplexer 9 is a fiber bundle type optical multiplexer/demultiplexer, and in the first embodiment, includes one input-side multi-core optical fiber 9m and twelve output-side optical fibers 9a to 9l. .. The input side multi-core optical fiber 9m is a multi-core optical fiber having the same core arrangement as the optical amplification fiber 7. The cores of the input side multi-core optical fiber 9m are connected to any of the cores of the optical amplification fiber 7.

The output side optical fibers 9a to 9l all propagate the signal light in a single mode. The cores of the output side optical fibers 9a to 9l are connected to any of the cores of the input side multi-core optical fiber 9m. In the optical fiber amplifier 100, the optical amplification fiber 7 and the optical multiplexer/demultiplexer 9 are connected so that the signal light output from the core portion 7aa of the optical amplification fiber 7 is output from the output side optical fiber 9a. .. Accordingly, the optical multiplexer/demultiplexer 9 outputs the signal light output from the core portion 7aa of the optical amplification fiber 7 to the output side optical fiber 9a. In the first embodiment, the optical multiplexer/demultiplexer 9 is a fiber bundle type, but the present invention is not limited to this, and a spatially coupled optical multiplexer/demultiplexer may be used.

The input side multi-core optical fiber 9m and the optical amplification fiber 7 are fusion-spliced at the connection point CP2. The residual excitation light processing unit 8 is provided at the connection point CP2. The residual excitation light processing unit 8 has a structure in which the outer cladding portion 7c is removed and exposed in the vicinity of the connection point CP2 for fusion splicing, and the exposed inner cladding portion 7b is covered with a material having a high refractive index. The residual pumping light processing unit 8 has a function of processing the residual pumping light that has been propagated through the inner cladding portion 7b and output without being used for optical amplification.

The optical isolator 10 is connected between the output side optical fiber 9 a of the optical multiplexer/demultiplexer 9 and the connection optical fiber 11. The optical isolator 10 has a function of passing the signal light output from the output side optical fiber 9a toward the connection optical fiber 11 and blocking the light propagating from the connection optical fiber 11 side toward the output side optical fiber 9a. Have. The optical isolator 10 preferably has a wavelength-dependent optical loss characteristic that prevents residual pumping light that could not be removed by the residual pumping light processing unit 8 from passing toward the connecting optical fiber 11.

The connection optical fiber 11 is a single mode optical fiber that connects the output side optical fiber 9a of the optical multiplexer/demultiplexer 9 and the input side optical fiber 3k of the optical multiplexer/demultiplexer 3 and propagates the signal light in a single mode. The connection optical fiber 11 inputs the signal light output from the output side optical fiber 9a into the input side optical fiber 3k of the optical multiplexer/demultiplexer 3.

The ASE cut filter 12 is inserted in the middle of the connecting optical fiber 11. The ASE cut filter 12 has a function of passing the signal light and blocking the ASE light generated in the core portion 7aa of the optical amplification fiber 7, and is configured by, for example, a bandpass filter. However, the insertion position of the ASE cut filter 12 is not limited to this, and may be, for example, between the output side optical fiber 9 a and the optical isolator 10.

In the optical fiber amplifier 100, the optical multiplexer/demultiplexer 3, the WDM coupler 4, and the WDM coupler 4 are provided so that the signal light input to the input side optical fiber 3k of the optical multiplexer/demultiplexer 3 is input to the core portion 7ak of the optical amplification fiber 7. The optical amplification fiber 7 is connected. As a result, the core portions 7aa and the core portions 7ak respectively belonging to the different core portion groups 7d and 7e are connected in series to form the multistage optical amplification fiber structure 15 having two stages. As a result, the signal light is propagated in the single mode by the core portion 7ak in which the added Er is photoexcited, so that the stimulated emission phenomenon occurs and the signal light is optically amplified again.

Here, as described above, the core portion 7ak is more strongly excited by the excitation light than the core portion 7aa, so that the core portions 7aa and the core portions that belong to the different core portion groups 7d and 7e, respectively. The gain given to the signal light is different from that of 7 ak. Specifically, the core portion 7ak gives a higher gain to the signal light than the core portion 7aa. Here, the gain means a so-called small signal gain, which is a gain for signal light having a power that does not cause gain saturation. The gain difference between the core portion 7aa and the core portion 7ak is, for example, about 2 to 5 dB, preferably 5 dB or more, but the value of the gain difference is not limited to this. Further, the gain difference can be adjusted by the refractive index difference between the inner sub-cladding portion 7ba and the inner sub-cladding portion 7bb, the excitation light intensity, and the like.

Further, in the optical fiber amplifier 100, the optical amplification fiber 7 and the optical multiplexer/demultiplexer are arranged so that the signal light output from the core portion 7ak of the optical amplification fiber 7 is output from the output side optical fiber 9k of the optical multiplexer/demultiplexer 9. The container 9 is connected. As a result, the signal light optically amplified by the core unit 7ak is output from the output side optical fiber 9k of the optical multiplexer/demultiplexer 9, passes through the optical isolator 13, and propagates the signal light in a single mode. Is output from.

As described above, in the cladding-pumped optical fiber amplifier 100 according to the first embodiment, the optical amplification fiber 7 has the signal between the core portions 7aa and the core portions 7ak belonging to the different core portion groups 7d and 7e, respectively. The multi-stage light is configured such that the gains given to light are different from each other, the core portions 7aa and 7ak are connected in series, and the core portion 7aa is the first stage and the core portion 7ak is the second stage. The amplification fiber structure 15 is formed. As described above, since the core portion 7aa and the core portion 7ak included in the optical amplification fiber 7 which is a multi-core fiber are connected in series, the optical fiber amplifier 100 can be downsized while having a two-stage optical amplification fiber structure. Is. Further, since the core portion 7aa and the core portion 7ak are collectively pumped by the cladding pumping, the number of pumping light sources 5 is one while having the two-stage optical amplification fiber structure, that is, the number of stages of the two-stage optical amplification fiber structure. The number may be less than a certain number 2, and the optical fiber amplifier 100 can reduce power consumption.

The distance between the adjacent core portions of the core portions 7aa to 7al is preferably set to a distance such that the crosstalk of the signal light between the adjacent core portions is −40 dB or less. The same applies to other configuration examples of the optical amplification fiber shown below.

(Example of optical amplification fiber configuration)
Next, a configuration example of an optical amplification fiber that can be used instead of the optical amplification fiber 7 in the optical fiber amplifier 100 will be described.

(Structure example 1)
FIG. 3 is a schematic cross-sectional view of Configuration Example 1 of the optical amplification fiber. The optical amplification fiber 7A includes six core portions 7Aaa to 7Aaf containing Er, six core portions 7Aag to 7Aal containing Er and ytterbium (Yb) co-doped, and outer peripheries of the core portions 7Aaa to 7Aal. A double-clad optical amplification fiber having an inner clad portion 7Ab formed on the inner side and an outer clad portion 7c formed on the outer periphery of the inner clad portion 7Ab.

The core portions 7Aaa to 7Aal include a core portion group 7Ad composed of six core portions 7Aaa to 7Aaf and a core portion group 7Ae composed of six core portions 7Aag to 7Aal. The core portions 7Aaa to 7Aaf of the core portion group 7Ad and the core portions 7Aag to 7Aal of the core portion group 7Ae are arranged in a hexagonal shape.

Moreover, the inner cladding portion 7Ab has a lower refractive index than the core portions 7Aaa to 7Aal. The outer cladding portion 7c has a lower refractive index than the inner cladding portion 7Ab.

In order to realize the above relationship of the refractive index, the core portions 7Aaa to 7Aal are made of silica glass to which Ge is added, the inner cladding portion 7Ab is made of pure silica glass, and the outer cladding portion 7c is made of a resin having a low refractive index. Made of material. However, these materials are mere examples, and are not particularly limited as long as they are materials that can realize the above relationship of the refractive index.

Here, in the optical amplifying fiber 7A, the core diameter and the relative refractive index difference with respect to the inner cladding portion 7Ab are substantially equal in all the core portions 7Aaa to 7Aal. Further, the concentration of Er added is substantially equal in all of the core portions 7Aaa to 7Aaf. Further, the addition concentrations of Er and Yb are substantially equal in all the core portions 7Aag to 7Aal.

A case where the optical amplification fiber 7 is used in place of the optical amplification fiber 7 in the optical fiber amplifier 100 will be described. The optical multiplexer/demultiplexer 3, the WDM coupler 4, and the optical amplification fiber 7A are connected so that the signal light input from the input optical fiber 1 is input to the core portion 7Aaa of the optical amplification fiber 7A. The optical amplification fiber 7A and the optical multiplexer/demultiplexer 9 are connected so that the signal light output from the core unit 7Aaa is output from the output side optical fiber 9a of the optical multiplexer/demultiplexer 9. Further, the optical multiplexer/demultiplexer 3, so that the signal light propagated through the connection optical fiber 11 and input to the input side optical fiber 3k of the optical multiplexer/demultiplexer 3 is input to the core portion 7Aak of the optical amplification fiber 7A. The WDM coupler 4 and the optical amplification fiber 7A are connected. Further, the optical amplification fiber 7A and the optical multiplexer/demultiplexer 9 are connected so that the signal light output from the core unit 7Aak is output from the output side optical fiber 9k of the optical multiplexer/demultiplexer 9. As a result, the core portions 7Aaa and 7Aak belonging to the different core portion groups 7Ad and 7Ae are connected in series to form a multistage optical amplification fiber structure.

The optical amplification fiber 7A propagates the excitation light in multimode by the inner clad portion 7Ab. Since the refractive index of the inner cladding portion 7Ab is substantially uniform in the cross section perpendicular to the longitudinal direction of the optical amplification fiber 7A, the intensity distribution of the excitation light in that cross section is also substantially uniform. Then, Er added to each of the core portions 7Aaa to 7Aaf of the optical amplification fiber 7A absorbs the excitation light and is photoexcited. On the other hand, in the core portions 7Aag to 7Aal, Yb added to the core portions mainly absorbs excitation light and photoexcites, and energy is transferred from Yb to Er, so that Er is excited. The signal light is propagated there in a single mode by the core portion 7Aaa, so that the stimulated emission phenomenon occurs and the signal light is optically amplified. The optical amplification fiber 7A outputs the amplified signal light to the optical multiplexer/demultiplexer 9.

Further, the signal light propagates through the connection optical fiber 11 and is input to the core unit 7Aak. Then, the signal light is propagated in the single mode by the core portion 7Aak, the stimulated emission phenomenon occurs, the signal light is optically amplified again, and finally output from the output optical fiber 14.

Here, the absorption coefficient of Er that absorbs the excitation light in the core portions 7Aaa to 7Aaf to the excitation light is, for example, about 5 to 10 dB/m, but the excitation light of Yb that mainly absorbs the excitation light in the core portions 7Aag to 7Aal. The absorption coefficient with respect to can be, for example, about 50 to 100 dB/m. As a result, the core portion 7Aak absorbs more excitation light than the core portion 7Aaa, and is in a more strongly excited state. As a result, the gains given to the signal light are different between the core portions 7Aaa and 7Aak belonging to the different core portion groups 7Ad and 7Ae, respectively. Specifically, the core section 7Aak gives a higher gain to the signal light than the core section 7Aaa. The gain difference between the core portion 7Aaa and the core portion 7Aak is, for example, about 3 to 4 dB, preferably 5 dB or more, but the value of the gain difference is not limited to this. The gain difference can be adjusted by the Er addition concentration in the core group 7Ad, the Er and Yb addition concentrations in the core group 7Ae, the excitation light intensity, or the like.

As described above, in the optical fiber amplifier which uses the optical amplification fiber 7A in place of the optical amplification fiber 7 in the optical fiber amplifier 100, the optical amplification fiber 7A has core portions 7Aaa belonging to different core portion groups 7Ad and 7Ae, respectively. The gains given to the signal light are different from each other with the core portion 7Aak, and the core portion 7Aaa and the core portions 7Aak are connected in series. The core portion 7Aaa is the first stage, and the core portion 7Aak. Is a second-stage optical amplification fiber structure. As a result, this fiber amplifier can be downsized while having a two-stage optical amplification fiber structure. Furthermore, since the core portion 7Aaa and the core portion 7Aak are collectively pumped by the cladding pumping, this optical fiber amplifier can have low power consumption.

In the optical amplification fiber 7A of the configuration example 1, Er, Er and Yb are added to the core portions 7Aaa to 7Aaf and 7Aag to 7Aal belonging to different core portion groups 7Ad and 7Ae, respectively, and the rare earth element added is added. By making the types different, the gains given to the signal lights are made different. However, the present invention is not limited to this, and Er, or Er and Yb are added to each of the core portions belonging to different core portion groups, but the addition concentration of Er or Er and Yb is set between the core portion groups. , The gains given to the signal lights may be different from each other.

(Structure example 2)
FIG. 4 is a schematic cross-sectional view of Configuration Example 2 of the optical amplification fiber. The optical amplification fiber 7B includes twelve Er-doped core portions 7Baa to 7Bal, an inner clad portion 7Bb formed on the outer circumferences of the core portions 7Baa to 7Bal, and an outer clad formed on the outer circumference of the inner clad portion 7Bb. An optical amplification fiber having a double clad structure having a portion 7c.

The core portions 7Baa to 7Bal include a core portion group 7Bd composed of six core portions 7Baa to 7Baf and a core portion group 7Be composed of six core portions 7Bag to 7Bal. The core portions 7Baa to 7Baf of the core portion group 7Bd and the core portions 7Bag to 7Bal of the core portion group 7Be are arranged in a hexagonal shape.

The inner cladding portion 7Bb has a lower refractive index than the core portions 7Baa to 7Bal. The outer clad 7c has a lower refractive index than the inner clad 7Bb.

An example of the constituent material of the optical amplification fiber 7B for realizing the above relationship of the refractive index is the same as that of the optical amplification fiber 7A of the configuration example 1.

Here, in the optical amplification fiber 7B, the Er addition concentration and the relative refractive index difference with respect to the inner cladding portion 7Bb are substantially the same in all the core portions 7Baa to 7Bal. However, the core diameters of all the core portions 7Baa to 7Baf are substantially equal and all of the core portions 7Bag to 7Bal are substantially equal, but the core portions 7Baa to 7Baf and the core portions 7Bag to 7Bal are different from each other. Specifically, the core portions 7Bag to 7Bal have a larger core diameter than the core portions 7Baa to 7Baf.

A case where the optical amplification fiber 7B is used instead of the optical amplification fiber 7 in the optical fiber amplifier 100 will be described. The optical multiplexer/demultiplexer 3, the WDM coupler 4, and the optical amplification fiber 7B are connected so that the signal light input from the input optical fiber 1 is input to the core portion 7Baa of the optical amplification fiber 7B. The optical amplification fiber 7B and the optical multiplexer/demultiplexer 9 are connected so that the signal light output from the core portion 7Baa is output from the output side optical fiber 9a of the optical multiplexer/demultiplexer 9. In addition, the optical multiplexer/demultiplexer 3, so that the signal light propagated through the connection optical fiber 11 and input to the input side optical fiber 3k of the optical multiplexer/demultiplexer 3 is input to the core portion 7Bak of the optical amplification fiber 7B. The WDM coupler 4 and the optical amplification fiber 7B are connected. Further, the optical amplification fiber 7B and the optical multiplexer/demultiplexer 9 are connected so that the signal light output from the core portion 7Bak is output from the output side optical fiber 9k of the optical multiplexer/demultiplexer 9. As a result, the core portions 7Baa and 7Bak belonging to the different core portion groups 7Bd and 7Be are connected in series to form a multistage optical amplification fiber structure.

The optical amplification fiber 7B propagates the excitation light in multimode by the inner clad 7Bb. Since the refractive index of the inner cladding portion 7Bb is substantially uniform in the cross section perpendicular to the longitudinal direction of the optical amplification fiber 7B, the intensity distribution of the excitation light in that cross section is also substantially uniform. Then, Er added to each of the core portions 7Baa to 7Bal of the optical amplification fiber 7B absorbs the excitation light and is optically excited. The signal light is propagated there in the single mode by the core portion 7Baa, so that the stimulated emission phenomenon occurs and the signal light is optically amplified. The optical amplification fiber 7B outputs the amplified signal light to the optical multiplexer/demultiplexer 9.

Further, the signal light propagates through the connection optical fiber 11 and is input to the core unit 7Bak. Then, the signal light is propagated in the single mode by the core portion 7Bak, an induced emission phenomenon occurs, the signal light is optically amplified again, and finally output from the output optical fiber 14.

Here, as described above, the core portions 7Bag to 7Bal have a larger core diameter than the core portions 7Baa to 7Baf. As a result, the core portion 7Bak absorbs more excitation light than the core portion 7Baa, and is more strongly excited. As a result, the gain given to the signal light is different between the core portions 7Baa and 7Bak belonging to the different core portion groups 7Bd and 7Be. Specifically, the core portion 7Bak gives a higher gain to the signal light than the core portion 7Baa. The gain difference between the core portion 7Baa and the core portion 7Bak is, for example, about 3 to 4 dB, preferably 5 dB or more, but the value of the gain difference is not limited to this. The gain difference can be adjusted by the Er addition concentration, the core diameter, the excitation light intensity, or the like.

As described above, in the optical fiber amplifier using the optical amplification fiber 7B in place of the optical amplification fiber 7 in the optical fiber amplifier 100, the optical amplification fiber 7B has core portions 7Baa belonging to different core portion groups 7Bd and 7Be, respectively. The gains given to the signal light are different from each other with the core portion 7Bak, and the core portion 7Baa and the core portions 7Bak are connected in series, and the core portion 7Baa is the first stage and the core portion 7Bak. Is a second-stage optical amplification fiber structure. As a result, this fiber amplifier can be downsized while having a two-stage optical amplification fiber structure. Further, since the core portion 7Baa and the core portion 7Bak are collectively pumped by the cladding pumping, the power consumption of this optical fiber amplifier can be reduced.

(Configuration example 3)
FIG. 5 is a schematic cross-sectional view of Configuration Example 3 of the optical amplification fiber. The optical amplification fiber 7C includes 12 core parts 7Caa to 7Cal containing Er, an inner clad part 7Cb formed on the outer circumference of the core parts 7Caa to 7Cal, and an outer clad formed on the outer circumference of the inner clad part 7Cb. An optical amplification fiber having a double clad structure having a portion 7c.

The core portions 7Caa to 7Cal are composed of a core portion group 7Cd composed of six core portions 7Caa to 7Caf and a core portion group 7Ce composed of six core portions 7Cag to 7Cal. The core portions 7Caa to 7Caf of the core portion group 7Cd and the core portions 7Cag to 7Cal of the core portion group 7Ce are arranged in a hexagonal shape.

The inner clad portion 7Cb has two inner sub-clad portions 7Cba and 7Cbb, and the core portions 7Caa to 7Caf and 7Cag to 7Cal belonging to different core portion groups 7Cd and 7Ce are different from each other in the inner sub-clad portion 7Cba and 7Cba. Surrounded by 7 Cbb. Specifically, the core parts 7Caa to 7Caf belonging to the core part group 7Cd are surrounded by the inner sub-cladding part 7Cba having a circular cross section, and the core parts 7Cag to 7Cal belonging to the core part group 7Ce are the inner sub-cladding part 7Cba. It is surrounded by a ring-shaped inner sub-cladding portion 7Cbb formed on the outer periphery of.

Further, like the optical amplification fiber 7, the inner cladding portion 7Cb has a lower refractive index than the core portions 7Caa to 7Cal. The outer clad 7c has a lower refractive index than the inner clad 7Cb. The inner sub-cladding portions 7Cba and 7Cbb have different refractive indexes. Specifically, the inner sub-cladding portion 7Cbb has a higher refractive index than the inner sub-cladding portion 7Cba. An example of the constituent material of the optical amplification fiber 7C for realizing such a relationship of the refractive index is the same as that of the case of the optical amplification fiber 7.

Further, in the optical amplification fiber 7C, the core diameter, the Er addition concentration, and the relative refractive index difference with respect to the enclosed cladding portion are substantially the same in all the core portions 7Caa to 7Cal. In addition, for example, the core diameter is 2 to 6 μm, the concentration of Er added is 300 to 3000 ppm, and the relative refractive index difference with respect to the enclosed clad portion is 0.3 to 1.5%.

Comparing the optical amplification fiber 7 and the optical amplification fiber 7C, the outer diameter of the inner sub-cladding portion 7Cba of the optical amplification fiber 7C is larger than the outer diameter of the inner sub-cladding portion 7ba of the optical amplification fiber 7. The radial width D of the inner sub-cladding portion 7Cbb of the optical amplification fiber 7C is smaller than the radial width of the inner sub-cladding portion 7bb of the optical amplification fiber 7.

The inner sub-cladding portion 7Cbb is made of, for example, pure silica glass, and its width D is freely set to be 10 to 90% of the diameter of the optical amplification fiber 7C (outer diameter of the outer cladding portion 7c). You can choose.

By adjusting the outer diameter of the inner sub-cladding portion 7Cba and the radial width D of the inner sub-cladding portion 7Cbb, the distance is set such that the crosstalk of the signal light between the adjacent core portions is -40 dB or less. It will be easier.

An optical fiber amplifier using an optical amplification fiber 7C instead of the optical amplification fiber 7 in the optical fiber amplifier 100 can be downsized while having a two-stage optical amplification fiber structure, and can be reduced in power consumption. is there.

(Structure example 4)
FIG. 6 is a schematic cross-sectional view of Configuration Example 4 of the optical amplification fiber. The optical amplification fiber 7D is formed on the outer periphery of the six core portions 7Daa to 7Daf containing Er, the six core portions 7Dag to 7Dal containing Er and Yb together, and the core portions 7Daa to 7Dal. A double-clad optical amplification fiber having an inner clad portion 7Db and an outer clad portion 7c formed on the outer periphery of the inner clad portion 7Db.

The core parts 7Daa to 7Dal are composed of a core part group 7Dd composed of six core parts 7Daa to 7Daf and a core part group 7De composed of six core parts 7Dag to 7Dal. The core portions 7Daa to 7Daf of the core portion group 7Dd and the core portions 7Dag to 7Dal of the core portion group 7De are arranged in a hexagonal shape.

The inner cladding portion 7Db has a lower refractive index than the core portions 7Daa to 7Dal. The outer cladding portion 7c has a lower refractive index than the inner cladding portion 7Db. An example of the constituent material of the optical amplification fiber 7D for realizing such a relationship of the refractive index is the same as that of the case of the optical amplification fiber 7A.

Here, in the optical amplifying fiber 7D, the core diameter and the relative refractive index difference with respect to the inner cladding portion 7Db are substantially the same in all the core portions 7Daa to 7Dal. Further, the concentration of Er added is substantially the same in all the core portions 7Daa to 7Daf. Further, the addition concentrations of Er and Yb are substantially equal in all of the core portions 7Dag to 7Dal.

Comparing the optical amplifying fiber 7A and the optical amplifying fiber 7D, in the optical amplifying fiber 7A, the core portions arranged in the radial direction, for example, the core portion 7Aaa and the core portion 7Aag, are radial from the central axis of the optical amplifying fiber 7A. Are evenly spaced. On the other hand, in the optical amplification fiber 7D, for example, the core portion 7Daa and the core portion 7Dag are arranged at unequal intervals in the radial direction from the central axis of the optical amplification fiber 7D.

As described above, by adjusting the interval between the core parts arranged in the radial direction, it becomes easy to set the distance such that the crosstalk of the signal light between the adjacent core parts is −40 dB or less.

An optical fiber amplifier using an optical amplification fiber 7D instead of the optical amplification fiber 7 in the optical fiber amplifier 100 can be downsized while having a two-stage optical amplification fiber structure and can be reduced in power consumption. is there.

(Embodiment 2)
FIG. 7 is a schematic configuration diagram of the optical fiber amplifier according to the second embodiment. As shown in FIG. 7, the optical fiber amplifier 100A is different from the optical fiber amplifier 100 shown in FIG. 1 in that an input optical fiber 1A, an optical isolator 2A, an optical isolator 10A, a connecting optical fiber 11A, and an ASE cut filter 12A are provided. The optical isolator 13A and the output optical fiber 14A are added.

The input optical fiber 1A receives an input of a signal light (hereinafter, referred to as a second signal light) different from a signal light (hereinafter, referred to as a first signal light) input to the input optical fiber 1, and is in a single mode. Propagate. The second signal light is, for example, signal light included in the 1.55 μm wavelength band and the 1.57 μm wavelength band used in optical communication. The second signal light may be signal light of a single wavelength or WDM signal light in which signal lights of different wavelengths are wavelength-multiplexed.

The optical isolator 2A has a function of passing the second signal light propagating toward the optical multiplexer/demultiplexer 3 through the input optical fiber 1A and blocking the light propagating from the optical multiplexer/demultiplexer 3 toward the input optical fiber 1A. Have. The input optical fiber 1A is connected to the input side optical fiber 3b of the optical multiplexer/demultiplexer 3 via the optical isolator 2A.

In the optical fiber amplifier 100A, the optical multiplexer/demultiplexer 3, the WDM coupler 4, and the optical amplification fiber 7 are arranged so that the second signal light input from the input optical fiber 1A is input to the core portion 7ab of the optical amplification fiber 7. The connection is made.

The optical amplification fiber 7 propagates the second signal light in the single mode by the core portion 7ab. As a result, stimulated emission phenomenon occurs due to Er that is added to the core portion 7ab and optically excited, and the second signal light is optically amplified. The optical amplification fiber 7 outputs the amplified second signal light to the optical multiplexer/demultiplexer 9.

In the optical fiber amplifier 100A, the optical amplifier fiber 7 and the optical multiplexer/demultiplexer are arranged so that the second signal light output from the core portion 7ab of the optical amplifier fiber 7 is output from the output side optical fiber 9b of the optical multiplexer/demultiplexer 9. The container 9 is connected. Accordingly, the optical multiplexer/demultiplexer 9 outputs the second signal light output from the core portion 7ab of the optical amplification fiber 7 to the output side optical fiber 9b.

The optical isolator 10A is connected between the output side optical fiber 9b of the optical multiplexer/demultiplexer 9 and the connection optical fiber 11A. The optical isolator 10A allows the second signal light output from the output side optical fiber 9b to pass toward the connection optical fiber 11A, and blocks the light propagating from the connection optical fiber 11A side toward the output side optical fiber 9b. Have a function. The optical isolator 10A preferably has a wavelength-dependent optical loss characteristic that prevents residual pumping light that could not be removed by the residual pumping light processing unit 8 from passing toward the connecting optical fiber 11A.

The connection optical fiber 11A is a single mode optical fiber that connects the output side optical fiber 9b of the optical multiplexer/demultiplexer 9 and the input side optical fiber 3l of the optical multiplexer/demultiplexer 3 and propagates the second signal light in a single mode. .. The connection optical fiber 11A inputs the signal light output from the output side optical fiber 9b into the input side optical fiber 3l of the optical multiplexer/demultiplexer 3.

The ASE cut filter 12A is inserted in the middle of the connecting optical fiber 11A. The ASE cut filter 12A has a function of passing the second signal light and blocking the ASE light generated in the core portion 7ab of the optical amplification fiber 7, and is configured by, for example, a bandpass filter. However, the insertion position of the ASE cut filter 12A is not limited to this, and may be, for example, between the output side optical fiber 9b and the optical isolator 10A.

In the optical fiber amplifier 100A, the optical multiplexer/demultiplexer 3, the WDM coupler 4, and the WDM coupler 4 are provided so that the signal light input to the input side optical fiber 3l of the optical multiplexer/demultiplexer 3 is input to the core portion 7al of the optical amplification fiber 7. The optical amplification fiber 7 is connected. As a result, the core portions 7ab and 7al, which belong to different core portion groups 7d and 7e and have different gains, are connected in series to form the multistage optical amplification fiber structure 15A. As a result, the second signal light is propagated in the single mode by the core portion 7al in which the added Er is photoexcited, so that the stimulated emission phenomenon occurs and the second signal light is optically amplified again.

Further, in the optical fiber amplifier 100A, the optical amplification fiber 7 and the optical multiplexer/demultiplexer are arranged so that the signal light output from the core portion 7al of the optical amplification fiber 7 is output from the output side optical fiber 9l of the optical multiplexer/demultiplexer 9. The container 9 is connected. As a result, the second signal light optically amplified by the core portion 7al is output from the output side optical fiber 9l of the optical multiplexer/demultiplexer 9, passes through the optical isolator 13A, and propagates the second signal light in a single mode. It is output from the output optical fiber 14A.

As with the optical fiber amplifier 100, the first signal light is optically amplified by the multistage optical amplification fiber structure 15 formed by the core portion 7aa and the core portion 7ak.

In the cladding-pumped optical fiber amplifier 100A according to the second embodiment, the optical amplification fiber 7 is provided between the core portion 7aa and the core portion 7ak belonging to different core portion groups 7d and 7e, and between the core portion 7ab and the core portion 7ab. 7al are configured so that the gains given to the signal light are different from each other, and the core portions 7aa and 7ak are connected in series, and the core portion 7aa is the first stage and the core portion 7ak is 2 The two-stage optical amplification fiber structure 15, which is the stage, is configured. Further, the core portion 7ab and the core portion 7al are connected in series, and the two-stage optical amplification fiber structure 15A in which the core portion 7ab is the first stage and the core portion 7al is the second stage is configured. In this way, since the core portion 7aa and the core portion 7ak included in the optical amplification fiber 7 which is a multi-core fiber are connected in series, and the core portion 7ab and the core portion 7al are connected in series, the optical fiber amplifier 100 is It is possible to reduce the size while having two two-stage optical amplification fiber structures. Furthermore, since the core part 7aa, the core part 7ak, the core part 7ab, and the core part 7al are collectively pumped by cladding pumping, the number of pumping light sources 5 is one while having two two-stage optical amplification fiber structures. Often, the optical fiber amplifier 100A can reduce power consumption.

Furthermore, an optical fiber amplifier having six two-stage optical amplification fiber structures is configured by appropriately adding an input optical fiber, an optical isolator, a connecting optical fiber, an ASE cut filter, and an output optical fiber to the optical fiber amplifier 100A. be able to. As a result, it is possible to realize an optical fiber amplifier with further reduced size and lower power consumption. In such an optical fiber amplifier, the number of stages in the multistage optical amplification fiber structure is 2, the multistage optical amplification fiber structure is 6, and the number of core portions of the optical amplification fiber is 2×6=12. ..

(Embodiment 3)
FIG. 8 is a schematic configuration diagram of the optical fiber amplifier according to the third embodiment. As shown in FIG. 3, the optical fiber amplifier 200 includes an input optical fiber 1, an optical isolator 2, an optical multiplexer/demultiplexer 23, a WDM coupler 24, a pumping light source 5, a multimode optical fiber 6, and an optical amplifier. The fiber 27, the residual pumping light processing unit 8, the optical multiplexer/demultiplexer 29, the optical isolator 10, the connection optical fibers 11 and 31, the ASE cut filters 12 and 32, the optical isolator 13, and the output optical fiber 14. , Are provided.

First, the configuration of the optical amplification fiber 27 will be described. FIG. 9 is a diagram showing a refractive index profile in a schematic cross section of the optical amplification fiber 27 and a cross section taken along line BB. The optical amplifying fiber 27 includes 18 core portions 27aa to 27ar to which Er is added, an inner clad portion 27b formed on the outer periphery of the core portions 27aa to 27ar, and an outer clad formed on the outer periphery of the inner clad portion 27b. And an optical amplification fiber having a double-clad structure having a portion 27c.

The core portions 27aa to 27ar include a core portion group 27d including six core portions 27aa to 27af, a core portion group 27e including six core portions 27ag to 27al, and six core portions 27am to 27am. It is composed of a core part group 27f composed of 27ar. The core portions 27aa to 27af of the core portion group 27d, the core portions 27ag to 27al of the core portion group 27e, and the core portions 27am to 27ar of the core portion group 27f are arranged in a hexagonal shape.

The inner clad part 27b has three inner sub-clad parts 27ba, 27bb, 27bc, and the core parts 27aa to 27af, 27ag to 27al, 27am to 27ar belonging to different core part groups 27d, 27e and 27f are It is surrounded by different inner sub-cladding portions 27ba, 27bb, 27bc. Specifically, the core portions 27aa to 27af belonging to the core portion group 27d are surrounded by the inner sub-cladding portion 27ba having a circular cross section, and the core portions 27ag to 27al belonging to the core portion group 27e are inside sub-cladding portions 27ba. The core portions 27am to 27ar, which are surrounded by the ring-shaped inner sub-cladding portion 27bb formed on the outer periphery of the core portion group 27f, are ring-shaped inner sub-cladding portions formed on the outer periphery of the inner sub-cladding portion 27bb. It is surrounded by the portion 27bc.

Further, as shown by the refractive index profile P2, the outer clad portion 27c, the inner sub-clad portion 27ba, the inner sub-clad portion 27bb, the inner sub-clad portion 27bc, the core portions 27aa to 27af, the core portions 27ag to 27al, the core portion 27am to. The refractive indices of 27ar are n11, n12, n13, n14, n15, n16, and n17, respectively. Therefore, the inner cladding portion 27b has a lower refractive index than the core portions 27aa to 27ar. The outer cladding portion 27c has a lower refractive index than the inner cladding portion 27b. The inner sub-cladding portions 27ba, 27bb, and 27bc have different refractive indexes. Specifically, the inner sub-clad portion 27bb has a higher refractive index than the inner sub-clad portion 27ba, and the inner sub-clad portion 27bc has a higher refractive index than the inner sub-clad portion 27bb.

In order to realize the above refractive index relationship, the core parts 27aa to 27am are made of Ge-doped silica glass, the inner sub-cladding part 27ba is made of F-doped silica glass, and the inner sub-cladding part 27bb is formed. Is made of quartz glass to which F is added at a concentration lower than that of the inner sub-cladding portion 27ba, the inner sub-cladding portion 27bc is made of pure silica glass, and the outer cladding portion 27c is made of a resin material having a low refractive index. However, these materials are mere examples, and are not particularly limited as long as they are materials that can realize the above relationship of the refractive index.

In addition, in the optical amplification fiber 27, the core diameter, the concentration of Er added, and the relative refractive index difference with respect to the enclosed cladding portion are substantially the same in all the core portions 27aa to 27ar. In addition, for example, the core diameter is 2 to 6 μm, the concentration of Er added is 300 to 3000 ppm, and the relative refractive index difference with respect to the enclosed clad portion is 0.3 to 1.5%.

Next, returning to FIG. 8, the optical fiber amplifier 200 will be described. Hereinafter, each component of the optical fiber amplifier 100 and its function will be described in order of connection from the input optical fiber 1 side. However, the input optical fiber 1, the optical isolator 2, the pumping light source 5, the multimode optical fiber 6, the residual pumping light processing unit 8, the optical isolator 10, the connection optical fiber 11, the ASE cut filter 12, the optical isolator 13, and the output light Since the fiber 14 is the same as the corresponding constituent element of the optical fiber amplifier 100 shown in FIG. 1, the description thereof will be appropriately omitted.

The optical multiplexer/demultiplexer 23 is a fiber bundle type optical multiplexer/demultiplexer, and includes 18 input side optical fibers 23a to 23r and one output side multi-core optical fiber 23s in the first embodiment. .. The input side optical fibers 23a to 23r all propagate the signal light in a single mode. The input optical fiber 1 is connected to the input side optical fiber 23a via the optical isolator 2.

The output side multi-core optical fiber 23s is a multi-core optical fiber having the same core arrangement as the optical amplification fiber 27. The cores of the input side optical fibers 23a to 23r are connected to any of the cores of the output side multi-core optical fiber 23s. The output side multi-core optical fiber 23s is connected to the WDM coupler 24. As a result, the optical multiplexer/demultiplexer 23 outputs the signal light input from the input optical fiber 1 to the WDM coupler 24. In the third embodiment, the optical multiplexer/demultiplexer 23 is a fiber bundle type, but the present invention is not limited to this, and a spatially coupled optical multiplexer/demultiplexer may be used.

The pumping light source 5 is composed of one high-power semiconductor LD that outputs multimode pumping light having a wavelength of 980 nm band. The pumping light source 5 is for supplying pumping light that optically pumps Er to the inner cladding portion 27b of the optical amplification fiber 27.

The multimode optical fiber 6 propagates the pumping light output from the pumping light source 5 in multimode and outputs it to the WDM coupler 24.

The WDM coupler 24 multiplexes the signal light and the pumping light output from the pumping light source 5, and outputs the multiplexed light to the output side multi-core optical fiber 24a fusion-spliced to the optical amplification fiber 27 at the connection point CP1. The output side multi-core optical fiber 24a is a multi-core optical fiber having the same core arrangement as the optical amplification fiber 27, and has a double clad structure. The core portion of the output-side multi-core optical fiber 24a is connected to any one of the core portions of the optical amplification fiber 27. The output-side multi-core optical fiber 24 a propagates the signal light in the single mode by the core portion, propagates the pump light in the multi mode by the inner cladding portion, and outputs the signal light to the optical amplification fiber 27. In the third embodiment, the WDM coupler 24 is a fiber type, but the present invention is not limited to this, and a spatially coupled WDM coupler may be used.

In the optical fiber amplifier 200, the optical multiplexer/demultiplexer 23, the WDM coupler 24, and the optical amplification fiber 27 are arranged so that the signal light input from the input optical fiber 1 is input to the core portion 27aa of the optical amplification fiber 27. The connection is made.

The optical amplification fiber 27 propagates the excitation light in the multimode by the inner clad portion 27b. Then, Er added to each of the core portions 27aa to 27ar of the optical amplification fiber 27 is optically excited. The signal light is propagated there by the core portion 27aa in the single mode, so that the stimulated emission phenomenon occurs and the signal light is optically amplified. The optical amplification fiber 27 outputs the amplified signal light to the optical multiplexer/demultiplexer 29.

As described above, since the inner sub-cladding portion 27bb has a higher refractive index than the inner sub-cladding portion 27ba in the inner cladding portion 27b, the pumping light is generated in the inner sub-cladding portion 27bb more than in the inner sub-cladding portion 27ba. , Uneven distribution with stronger power. Similarly, since the inner sub-clad portion 27bc has a higher refractive index than the inner sub-clad portion 27bb, the excitation light is unevenly distributed in the inner sub-clad portion 27bc with a stronger power than in the inner sub-clad portion 27bb. As a result, the core parts 27ag to 27al of the core part group 27e surrounded by the inner sub-clad part 27bb are more than the core parts 27aa to 27af of the core part group 27d surrounded by the inner sub-clad part 27ba. It is in a state of being more strongly excited by the excitation light. Further, the core portions 27am to 27ar of the core portion group 27f surrounded by the inner sub-cladding portion 27bc are excited more than the core portions 27ag to 27al of the core portion group 27e surrounded by the inner sub-clad portion 27bb. It is more strongly excited by light.

The optical multiplexer/demultiplexer 29 is a fiber bundle type optical multiplexer/demultiplexer, and in the third embodiment, includes one input-side multi-core optical fiber 29s and 18 output-side optical fibers 29a to 29r. .. The input-side multi-core optical fiber 29s is a multi-core optical fiber having the same core arrangement as the optical amplification fiber 27. Each of the core portions of the input-side multi-core optical fiber 29s is connected to one of the core portions of the optical amplification fiber 27.

The output-side optical fibers 29a to 29r all propagate the signal light in a single mode. The cores of the output side optical fibers 29a to 29r are connected to any of the cores of the input side multi-core optical fiber 29s. In the optical fiber amplifier 200, the optical amplification fiber 27 and the optical multiplexer/demultiplexer 29 are connected so that the signal light output from the core portion 27aa of the optical amplification fiber 27 is output from the output side optical fiber 29a. .. As a result, the optical multiplexer/demultiplexer 29 outputs the signal light output from the core portion 27aa of the optical amplification fiber 27 to the output side optical fiber 29a. In the third embodiment, the optical multiplexer/demultiplexer 9 is a fiber bundle type, but the present invention is not limited to this, and a spatially coupled optical multiplexer/demultiplexer may be used.

The input side multi-core optical fiber 29s and the optical amplification fiber 27 are fusion-spliced at the connection point CP2. The residual excitation light processing unit 8 is provided at the connection point CP2.

The optical isolator 10 is connected between the output side optical fiber 29 a of the optical multiplexer/demultiplexer 29 and the connection optical fiber 11. The connection optical fiber 11 connects the output side optical fiber 29a of the optical multiplexer/demultiplexer 29 and the input side optical fiber 23k of the optical multiplexer/demultiplexer 23, and propagates the signal light in a single mode. The connection optical fiber 11 inputs the signal light output from the output side optical fiber 29a into the input side optical fiber 23k of the optical multiplexer/demultiplexer 23.

In the optical fiber amplifier 200, the optical multiplexer/demultiplexer 23, the WDM coupler 24, and the WDM coupler 24 are provided so that the signal light input to the input side optical fiber 23k of the optical multiplexer/demultiplexer 23 is input to the core portion 27ak of the optical amplification fiber 27. The optical amplification fiber 27 is connected. As a result, the signal light propagates in the single mode by the core portion 27ak in which the added Er is optically excited, so that the stimulated emission phenomenon occurs and the signal light is optically amplified again.

Further, in the optical fiber amplifier 200, the optical amplification fiber 27 and the optical multiplexer/demultiplexer 29 are connected so that the signal light output from the core portion 27ak of the optical amplification fiber 27 is output from the output side optical fiber 29k. ing. As a result, the optical multiplexer/demultiplexer 29 outputs the signal light output from the core portion 27ak of the optical amplification fiber 27 to the output side optical fiber 29k.

The optical isolator 30 is connected between the output side optical fiber 29k of the optical multiplexer/demultiplexer 29 and the connection optical fiber 31. The optical isolator 30 preferably has a wavelength-dependent optical loss characteristic that prevents residual pumping light that could not be removed by the residual pumping light processing unit 8 from passing toward the connecting optical fiber 31.

The connection optical fiber 31 is a single-mode optical fiber that connects the output-side optical fiber 29k of the optical multiplexer/demultiplexer 29 and the input-side optical fiber 23r of the optical multiplexer/demultiplexer 3 and propagates the signal light in a single mode. The connection optical fiber 31 inputs the signal light output from the output side optical fiber 29k into the input side optical fiber 23r of the optical multiplexer/demultiplexer 3.

The ASE cut filter 32 is inserted in the middle of the connection optical fiber 31. The ASE cut filter 32 has a function of transmitting the signal light and blocking the ASE light generated in the core portion 7ak of the optical amplification fiber 27, and is configured by, for example, a bandpass filter. However, the insertion position of the ASE cut filter 32 is not limited to this, and may be, for example, between the output side optical fiber 29k and the optical isolator 30.

In the optical fiber amplifier 200, the optical multiplexer/demultiplexer 23, the WDM coupler 24, and the WDM coupler 24 are provided so that the signal light input to the input side optical fiber 23r of the optical multiplexer/demultiplexer 23 is input to the core portion 27ar of the optical amplification fiber 27. The optical amplification fiber 27 is connected. As a result, the core portions 27aa, the core portions 27ak, and the core portions 27ar belonging to the different core portion groups 27d, 27e, and 27f are connected in series to form the multistage optical amplification fiber structure 25 having three stages. As a result, the signal light is propagated in the single mode by the core portion 27ar in which the added Er is photoexcited, so that the stimulated emission phenomenon occurs and the signal light is optically amplified again.

Here, as described above, the core portion 27ak is more strongly excited by the excitation light than the core portion 27aa, and the core portion 27ar is more strongly excited by the excitation light than the core portion 27ak. It is in the state of being As a result, the gain given to the signal light is different between the core portions 27aa, 27ak, 27ar belonging to the different core portions 27d, 27e, 27f, respectively. Specifically, the core section 27ak gives a higher gain to the signal light than the core section 27aa, and the core section 27ar gives a higher gain to the signal light than the core section 27ak. The gain difference between the core portion 27aa and the core portion 27ak and the gain difference between the core portion 27ak and the core portion 27ar are, for example, about 3 to 4 dB, preferably 5 dB or more, but the value of the gain difference is It is not limited to this. The gain difference can be adjusted by the difference in refractive index between the inner sub-clad portion 27ba, the inner sub-clad portion 27bb, and the inner sub-clad portion 27bc, the pump light intensity, and the like.

Further, in the optical fiber amplifier 200, the optical amplification fiber 27 and the optical multiplexing/demultiplexing device are arranged so that the signal light output from the core portion 27ar of the optical amplification fiber 27 is output from the output side optical fiber 29r of the optical multiplexing/demultiplexing device 29. The device 29 is connected. As a result, the signal light optically amplified by the core unit 27ar is output from the output side optical fiber 29r of the optical multiplexer/demultiplexer 29, passes through the optical isolator 13, and propagates the signal light in the single mode. Is output from.

As described above, in the cladding-pumped optical fiber amplifier 200 according to the third embodiment, the optical amplification fiber 27 includes the core portions 27aa, 27a, and 27ak that belong to different core portion groups 27d, 27e, and 27f, respectively. 27ar are configured so that the gains given to the signal light are different from each other, and the core portion 27aa, the core portion 27ak, and the core portions 27ar are connected in series, and the core portion 27aa has the first stage and the core portion 27aa. The multi-stage optical amplification fiber structure 25 has a second stage 27ak and a third stage core 27ar. As described above, since the core portion 27aa, the core portion 27ak, and the core portion 27ar included in the optical amplification fiber 27 which is a multi-core fiber are connected in series, the optical fiber amplifier 200 has a three-stage optical amplification fiber structure. Can be miniaturized. Furthermore, since the core portion 27aa, the core portion 27ak, and the core portion 27ar are collectively pumped by cladding pumping, the number of pumping light sources 5 may be one while having a three-stage optical amplifying fiber structure. Can reduce power consumption.

As in the case of the optical fiber amplifier 100A shown in FIG. 7, an input optical fiber, an optical isolator, a connecting optical fiber, an ASE cut filter, and an output optical fiber are appropriately added to the optical fiber amplifier 200 according to the third embodiment. By doing so, an optical fiber amplifier having six 3-stage optical amplification fiber structures can be constructed. As a result, it is possible to realize an optical fiber amplifier with further reduced size and lower power consumption. In such an optical fiber amplifier, the number of stages in the multistage optical amplification fiber structure is 3, the structure has 6 multistage optical amplification fiber structures, and the number of core portions in the optical amplification fiber is 3×6=18. ..

(Example of optical amplification fiber configuration)
Next, a configuration example of an optical amplification fiber that can be used instead of the optical amplification fiber 27 in the optical fiber amplifier 200 will be described.

(Structure example 5)
FIG. 10 is a schematic cross-sectional view of Configuration Example 5 of the optical amplification fiber. The optical amplification fiber 27A is formed on the outer periphery of the six core portions 27Aaa to 27Aaf to which Er is added, the twelve core portions 27Aag to 7Aar to which Er and Yb are co-added, and the core portions 27Aaa to 27Aar. The optical amplification fiber has a double clad structure including an inner clad portion 27Ab and an outer clad portion 27c formed on the outer circumference of the inner clad portion 27Ab.

The core portions 27Aaa to 27Aar include a core portion group 27Ad composed of six core portions 27Aaa to 27Aaf, a core portion group 27Ae composed of six core portions 27Aag to 27Aal, and six core portions 27Aam to It is composed of a core group 27Af composed of 27Aar. The core portions 27Aaa to 27Aaf of the core portion group 27Ad, the core portions 27Aag to 27Aal of the core portion group 27Ae, and the core portions 27Aam to 27Aar of the core portion group 27Af are arranged in a hexagonal shape.

Moreover, the inner cladding portion 27Ab has a lower refractive index than the core portions 27Aaa to 27Aar. The outer cladding portion 27c has a lower refractive index than the inner cladding portion 27Ab. An example of the constituent material of the optical amplification fiber 27A for realizing such a relationship of the refractive index is the same as that of the optical amplification fiber 7A of the configuration example 1.

Here, in the optical amplification fiber 27A, the core diameter and the relative refractive index difference with respect to the inner cladding portion 27Ab are substantially the same in all the core portions 27Aaa to 27Aar. Further, the concentration of Er added is substantially the same in all of the core portions 27Aaa to 27Aaf. Regarding the added concentrations of Er and Yb, all the core portions 27Aag to 27Aal are substantially equal, all the core portions 27Aam to 27Aar are substantially equal, and Er and Yb of the core portions 27Aam to 27Aal are smaller than those of the core portions 27Aam to 27Aal. The added concentration of is higher.

A case in which the optical amplification fiber 27 is used instead of the optical amplification fiber 27 in the optical fiber amplifier 200 will be described. The optical multiplexer/demultiplexer 23, the WDM coupler 24, and the optical amplification fiber 27A are connected so that the signal light input from the input optical fiber 1 is input to the core portion 27Aaa of the optical amplification fiber 27A. The optical amplification fiber 27A and the optical multiplexer/demultiplexer 29 are connected so that the signal light output from the core unit 27Aaa is output from the output side optical fiber 29a of the optical multiplexer/demultiplexer 29. Further, the optical multiplexer/demultiplexer 23, so that the signal light propagated through the connection optical fiber 11 and input to the input side optical fiber 23k of the optical multiplexer/demultiplexer 23 is input to the core portion 27Aak of the optical amplification fiber 27A. The WDM coupler 24 and the optical amplification fiber 27A are connected. Further, the optical amplification fiber 27A and the optical multiplexer/demultiplexer 29 are connected so that the signal light output from the core unit 27Aak is output from the output side optical fiber 29k of the optical multiplexer/demultiplexer 29. In addition, the optical multiplexer/demultiplexer 23, so that the signal light propagated through the connection optical fiber 31 and input to the input side optical fiber 23r of the optical multiplexer/demultiplexer 23 is input to the core portion 27Aar of the optical amplification fiber 27A. The WDM coupler 24 and the optical amplification fiber 27A are connected. Further, the optical amplification fiber 27A and the optical multiplexer/demultiplexer 29 are connected so that the signal light output from the core unit 27Aar is output from the output side optical fiber 29r of the optical multiplexer/demultiplexer 29. As a result, the core portions 27Aaa, 27Aak, and 27Aar belonging to different core groups 27Ad, 27Ae, and 27Af are connected in series to form a multistage optical amplification fiber structure.

The optical amplification fiber 27A propagates the excitation light in a multimode by the inner clad 27Ab. Since the refractive index of the inner clad 27Ab is substantially uniform in the cross section perpendicular to the longitudinal direction of the optical amplification fiber 27A, the intensity distribution of the excitation light in that cross section is also substantially uniform. Then, Er added to each of the core portions 27Aaa to 27Aaf of the optical amplification fiber 27A absorbs the excitation light and is photoexcited. On the other hand, in the core portions 27Aag to 27Aar, Yb added to the core portions mainly absorbs excitation light and photoexcites, and energy is transferred from Yb to Er, so that Er is excited. The signal light is propagated there in the single mode by the core portion 27Aaa, so that the stimulated emission phenomenon occurs and the signal light is optically amplified. The optical amplification fiber 27A outputs the amplified signal light to the optical multiplexer/demultiplexer 29.

Further, the signal light propagates through the connection optical fiber 11 and is input to the core unit 27Aak. Then, the signal light is propagated in the single mode by the core portion 27Aak, the stimulated emission phenomenon occurs, and the signal light is optically amplified again. Further, the signal light propagates through the connection optical fiber 31 and is input to the core unit 27Aar. Then, the signal light is propagated in the single mode by the core portion 27Aar, so that the stimulated emission phenomenon occurs, and the signal light is optically amplified again. Finally, it is output from the output optical fiber 14.

Here, in the optical amplification fiber 27A, the absorption coefficient of the excitation light in the core portions 27Aag to 27Aal is larger than the absorption coefficient of the excitation light in the core portions 27Aaa to 27Aaf. Further, the absorption coefficient of the excitation light in the core portions 27Aam to 27Aar is larger than the absorption coefficient of the excitation light in the core portions 27Aag to 27Aal. As a result, the core portion 27Aak absorbs more excitation light than the core portion 27Aaa, and is in a more strongly excited state. Similarly, the core portion 27Aar absorbs more excitation light than the core portion 27Aak, and becomes more strongly excited. As a result, the core portions 27Aaa, 27Aak, and 27Aar belonging to the different core portion groups 27Ad, 27Ae, and 27Af respectively have different gains to signal light. Specifically, the core portion 27Aak gives a higher gain to the signal light than the core portion 27Aaa, and the core portion 27Aar gives a higher gain to the signal light than the core portion 27Aak. The gain difference between the core portion 27Aaa, the core portion 27Aak, and the core portion 27Aar is, for example, about 3 to 4 dB, preferably 5 dB or more, but the value of the gain difference is not limited to this. The gain difference can be adjusted by the Er addition concentration in the core group 27Ad, the Er and Yb addition concentrations in the core groups 27Ae and 27Af, or the excitation light intensity.

As described above, in the optical fiber amplifier that uses the optical amplification fiber 27A in place of the optical amplification fiber 27 in the optical fiber amplifier 200, the optical amplification fiber 27A has core parts belonging to different core part groups 27Ad, 27Ae, and 27Af, respectively. 27Aaa, the core portion 27Aak, and the core portion 27Aar are configured so that the gains given to the signal light are different from each other, and the core portion 27Aaa, the core portion 27Aak, and the core portion 27Aar are connected in series. The section 27Aaa constitutes the first stage, the core section 27Aak constitutes the second stage, and the core section 27Aar constitutes the third stage, forming a three-stage optical amplification fiber structure. As a result, this optical fiber amplifier can be downsized while having a three-stage optical amplification fiber structure. Further, since the core portion 27Aaa, the core portion 27Aak, and the core portion 27Aar are collectively pumped by the cladding pumping, this optical fiber amplifier can have low power consumption.

Note that the present invention is not limited to the optical amplification fiber having the configuration of the configuration example 5 described above, and Er, or Er and Yb is added to each of the core portions that belong to different core portion groups. The gains given to the signal light may be made different from each other by making the addition concentrations of the above different from each other among the core groups.

(Structure example 6)
FIG. 11 is a schematic cross-sectional view of Configuration Example 6 of the optical amplification fiber. The optical amplification fiber 27B includes 18 core portions 27Baa to 27Bar to which Er is added, an inner clad portion 27Bb formed on the outer circumference of the core portions 27Baa to 27Bar, and an outer clad formed on the outer circumference of the inner clad portion 27Bb. And an optical amplification fiber having a double-clad structure having a portion 27c.

The core portions 27Baa to 27Bar include a core portion group 27Bd composed of six core portions 27Baa to 27Baf, a core portion group 27Be composed of six core portions 27Bag to 27Bal, and six core portions 27Bam to. It is composed of a core group 27Bf composed of 27 Bar. The core portions 27Baa to 27Baf of the core portion group 27Bd, the core portions 27Bag to 27Bal of the core portion group 27Be, and the core portions 27Bam to 27Bar of the core portion group 27Bf are arranged in a hexagonal shape.

The inner cladding portion 27Bb has a lower refractive index than the core portions 27Baa to 27Bar. The outer clad 27c has a lower refractive index than the inner clad 27Bb. An example of the constituent material of the optical amplification fiber 27B for realizing such a relationship of the refractive index is the same as that of the optical amplification fiber 7A of the configuration example 1.

Here, in the optical amplification fiber 27B, the addition concentration of Er and the relative refractive index difference with respect to the inner cladding portion 27Bb are substantially the same in all the core portions 27Baa to 27Bar. However, regarding the core diameter, all the core portions 27Baa to 27Baf are substantially equal, all the core portions 27Bag to 27Bal are substantially equal, and all the core portions 27Bam to 27Bar are substantially equal, but the core portions 27Baa to 27Baf and the core portion 27Bag are substantially equal. .About.27Bal and core portions 27Bam to 27Bar are different from each other. Specifically, the core portions 27Bag to 27Bal have a larger core diameter than the core portions 27Baa to 27Baf, and the core portions 27Bam to 27Bar have a larger core diameter than the core portions 27Bag to 27Bal.

A case will be described in which the optical amplification fiber 27 is used in place of the optical amplification fiber 7 in the optical fiber amplifier 200. The optical multiplexer/demultiplexer 23, the WDM coupler 24, and the optical amplification fiber 27B are connected so that the signal light input from the input optical fiber 1 is input to the core portion 27Baa of the optical amplification fiber 27B. The optical amplification fiber 27B and the optical multiplexer/demultiplexer 9 are connected so that the signal light output from the core unit 27Baa is output from the output side optical fiber 29a of the optical multiplexer/demultiplexer 9. In addition, the optical multiplexer/demultiplexer 23, so that the signal light propagated through the connection optical fiber 11 and input to the input side optical fiber 23k of the optical multiplexer/demultiplexer 3 is input to the core portion 27Bak of the optical amplification fiber 27B. The WDM coupler 24 and the optical amplification fiber 27B are connected. Further, the optical amplification fiber 27B and the optical multiplexer/demultiplexer 9 are connected so that the signal light output from the core portion 27Bak is output from the output side optical fiber 29k of the optical multiplexer/demultiplexer 9. In addition, the optical multiplexer/demultiplexer 23, so that the signal light propagated through the connection optical fiber 31 and input to the input side optical fiber 23r of the optical multiplexer/demultiplexer 23 is input to the core portion 27Bar of the optical amplification fiber 27B. The WDM coupler 24 and the optical amplification fiber 27B are connected. Further, the optical amplification fiber 27B and the optical multiplexer/demultiplexer 29 are connected so that the signal light output from the core unit 27Bar is output from the output side optical fiber 29r of the optical multiplexer/demultiplexer 29. As a result, the core portions 27Baa, 27Bak, and 27Bar belonging to the different core portions 27Bd, 27Be, and 27Bf are connected in series to form a multistage optical amplification fiber structure.

The optical amplification fiber 27B propagates the excitation light in multimode by the inner clad portion 27Bb. Since the refractive index of the inner cladding portion 27Bb is substantially uniform in the cross section perpendicular to the longitudinal direction of the optical amplification fiber 27B, the intensity distribution of the excitation light in that cross section is also substantially uniform. Then, Er added to each of the core portions 27Baa to 7Bar of the optical amplification fiber 27B absorbs the excitation light and is optically excited. The signal light is propagated there in a single mode by the core portion 27Baa, so that the stimulated emission phenomenon occurs and the signal light is optically amplified. The optical amplifying fiber 27B outputs the amplified signal light to the optical multiplexer/demultiplexer 9.

Further, the signal light propagates through the connection optical fiber 11 and is input to the core unit 27Bak. Then, the signal light is propagated in the single mode by the core portion 27Bak, the stimulated emission phenomenon occurs, and the signal light is optically amplified again. Further, the signal light propagates through the connection optical fiber 31 and is input to the core unit 27Bar. Then, the signal light is propagated in the single mode by the core portion 27Bar, so that the stimulated emission phenomenon occurs, and the signal light is optically amplified again. Finally, it is output from the output optical fiber 14.

Here, as described above, the core portions 27Bag to 27Bal have larger core diameters than the core portions 27Baa to 27Baf, and the core portions 27Bam to 27Bar have larger core diameters than the core portions 27Bag to 7Bal. As a result, the core portion 27Bak absorbs more excitation light than the core portion 27Baa, and is in a more strongly excited state. Similarly, the core portion 27Bar absorbs more excitation light than the core portion 27Bak, and becomes more strongly excited. As a result, the gains given to the signal light are different between the core portions 27Baa, 27Bak, and 27Bar belonging to the different core portion groups 27Bd, 27Be, and 27Bf, respectively. Specifically, the core portion 27Bak has a higher gain given to the signal light than the core portion 27Baa, and the core portion 27Bar has a higher gain given to the signal light than the core portion 27Bak. The gain difference between the core portion 27Baa, the core portion 27Bak and the core portion 27Bar is, for example, about 3 to 4 dB, preferably 5 dB or more, but the value of the gain difference is not limited to this. The gain difference can be adjusted by the Er addition concentration, the core diameter, the excitation light intensity, or the like.

As described above, in the optical fiber amplifier using the optical amplification fiber 27B in place of the optical amplification fiber 27 in the optical fiber amplifier 200, the optical amplification fiber 27B has core portions belonging to different core portion groups 27Bd, 27Be, 27Bf, respectively. 27Baa, the core section 27Bak, and the core section 27Bar are configured so that the gains given to the signal light are different from each other, and the core section 27Baa, the core section 27Bak, and the core section 27Bar are connected in series. The section 27Baa constitutes the first stage, the core section 27Bak constitutes the second stage, and the core section 27Bar constitutes the third stage, forming a three-stage optical amplification fiber structure. As a result, this optical fiber amplifier can be downsized while having a three-stage optical amplification fiber structure. Furthermore, since the core portion 27Baa, the core portion 27Bak, and the core portion 27Bar are collectively pumped by the cladding pumping, this optical fiber amplifier can have low power consumption.

(Structure example 7)
FIG. 12 is a schematic cross-sectional view of Structural Example 7 of the optical amplification fiber. The optical amplification fiber 27C is formed on the outer periphery of the six core portions 27Caa to 27Caf to which Er is added, the twelve core portions 27Cag to 27Car to which Er and Yb are co-added, and the core portions 27Caa to 27Car. It is an optical amplification fiber having a double clad structure having an inner clad portion 27Cb and an outer clad portion 27c formed on the outer periphery of the inner clad portion 27Cb.

The core portions 27Caa to 27Car include a core portion group 27Cd including six core portions 27Caa to 27Caf, a core portion group 27Ce including six core portions 27Cag to 27Cal, and six core portions 27Cam to 27Cam. The core unit group 27Cf is composed of 27Car. The core portions 27Caa to 27Caf of the core portion group 27Cd, the core portions 27Cag to 27Cal of the core portion group 27Ce, and the core portions 27Cam to 27Car of the core portion group 27Cf are arranged in a hexagonal shape.

The inner cladding portion 27Cb has a lower refractive index than the core portions 27Caa to 27Car. The outer cladding portion 27c has a lower refractive index than the inner cladding portion 27Cb. An example of the constituent material of the optical amplification fiber 27C for realizing such a relationship of the refractive index is the same as that of the case of the optical amplification fiber 27A.

Here, in the optical amplification fiber 27C, the core diameter and the relative refractive index difference with respect to the inner cladding portion 27Cb are substantially the same in all the core portions 27Caa to 27Car. Further, the concentration of Er added is substantially equal in all the core portions 27Caa to 27Caf. The addition concentrations of Er and Yb are substantially equal in all of the core portions 27Cag to 27Cal, substantially equal in all of the core portions 27Cam to 27Car, and higher than the core portions 27Cag to 27Cal.

Comparing the optical amplification fiber 27A and the optical amplification fiber 27C, in the optical amplification fiber 27A, the hexagon formed by the core portions 27Aaa to 27Aaf, the hexagon formed by the core portions 27Aag to 27Aal, and the core portions 27Aam to 27Aar are shown. The formed hexagon is concentric with the vertices aligned in the radial direction with the central axis of the optical amplification fiber 27A as the center. On the other hand, in the optical amplifying fiber 27C, the hexagons formed by the core portions 27Caa to 27Caf and the hexagons formed by the core portions 27Cam to 27Car are centered on the central axis of the optical amplifying fiber 27C and the vertices have a diameter. Although arranged concentrically along the direction, the hexagons formed by the core portions 27Cag to 27Cal are arranged by rotating the hexagons clockwise by 30° around the central axis. ..

In this way, by arranging the core parts so that at least one of the hexagons formed by the core parts belonging to each core part group 27Cd, 27Ce, 27Cf is rotated around the central axis, It becomes easy to set the distance such that the crosstalk of the signal light is -40 dB or less.

An optical fiber amplifier using an optical amplification fiber 27C instead of the optical amplification fiber 27 in the optical fiber amplifier 200 can be downsized while having a three-stage optical amplification fiber structure, and can be reduced in power consumption. is there.

(Structure example 8)
FIG. 13 is a schematic cross-sectional view of Structural Example 8 of the optical amplification fiber. The optical amplification fiber 27D includes 18 core portions 27Daa to 27Dar containing Er, an inner clad portion 27Db formed on the outer circumferences of the core portions 27Daa to 27Dar, and an outer clad formed on the outer circumference of the inner clad portion 27Db. And an optical amplification fiber having a double-clad structure having a portion 27c.

The core portions 27Daa to 27Dar include a core portion group 27Dd including six core portions 27Daa to 27Daf, a core portion group 27De including six core portions 27Dag to 27Dal, and six core portions 27Dam to. It is composed of a core group 27Df composed of 27Dar. The core portions 27Daa to 27Daf of the core portion group 27Dd, the core portions 27Dag to 27Dal of the core portion group 27De, and the core portions 27Dam to 27Dar of the core portion group 27Df are arranged in a hexagonal shape.

Further, the inner clad portion 27Db has three inner sub clad portions 27Dba, 27Dbb, 27Dbc, and the core portions 27Daa-27Daf, 27Dag-27Dal, 27Dam-27Dar belonging to different core portion groups 27Dd, 27De, 27Df are, It is surrounded by different inner sub-cladding portions 27Dba, 27Dbb, 27Dbc. Specifically, the core parts 27Daa to 27Daf belonging to the core part group 27Dd are surrounded by the inner sub-cladding part 27Dba having a circular cross section, and the core parts 27Dag to 27Dal belonging to the core part group 27De are inner sub-cladding parts 27Dba. The core parts 27Dam to 27Dar, which are surrounded by the ring-shaped inner sub-cladding part 27Dbb formed on the outer periphery of the core part group 27Df, are ring-shaped inner sub-clad parts formed on the outer periphery of the inner sub-cladding part 27Dbb. It is surrounded by the portion 27Dbc.

Further, like the optical amplification fiber 27, the inner cladding portion 27Db has a lower refractive index than the core portions 27Daa to 7Dal. The outer cladding portion 27c has a lower refractive index than the inner cladding portion 27Db. The inner sub-cladding portions 27Dba, 27Dbb, 27Dbc have different refractive indexes. Specifically, the inner sub-clad portion 27Dbb has a higher refractive index than the inner sub-clad portion 27Dba, and the inner sub-clad portion 27Dbc has a higher refractive index than the inner sub-clad portion 27Dbb. An example of the constituent material of the optical amplification fiber 27D for realizing such a relationship of the refractive index is the same as that of the case of the optical amplification fiber 27.

In addition, in the optical amplification fiber 27D, the core diameter, the Er addition concentration, and the relative refractive index difference with respect to the enclosed cladding portion are substantially the same in all the core portions 27Daa to 27Dar. In addition, for example, the core diameter is 2 to 6 μm, the concentration of Er added is 300 to 3000 ppm, and the relative refractive index difference with respect to the enclosed clad portion is 0.3 to 1.5%.

Comparing the optical amplification fiber 27 and the optical amplification fiber 27D, the outer diameters of the inner sub-cladding portions 27Dba and 27Dbb of the optical amplification fiber 27D are larger than the outer diameters of the inner sub-cladding portions 27ba and 27bb of the optical amplification fiber 27, respectively. large. Further, the distances of the core portions 27Daa to 27Daf and the core portions 27Dag to 27Dal from the central axis of the optical amplification fiber 27D are smaller than the distances of the core portions 27aa to 27af and the core portions 27ag to 27al from the central axis of the optical amplification fiber 27. Each is big. Further, the radial width of the inner sub-cladding portion 27Dbc of the optical amplification fiber 27D is smaller than the radial width of the inner sub-cladding portion 27bc of the optical amplification fiber 27. The inner sub-cladding portion 27Dbc is made of, for example, pure silica glass, and its width is freely selected to be 10 to 90% of the diameter of the optical amplification fiber 27D (outer diameter of the outer cladding portion 27c). it can. In the optical amplification fiber 27, the hexagons formed by the core portions 27aa to 27af, the hexagons formed by the core portions 27ag to 27al, and the hexagons formed by the core portions 27am to 27ar are the same as those of the optical amplification fiber 27. The vertices are concentric with the center axis as the center, and the vertices are arranged along the radial direction. On the other hand, in the optical amplification fiber 27D, the hexagons formed by the core portions 27Daa to 27Daf and the hexagons formed by the core portions 27Dam to 27Dar are such that the vertices are centered on the central axis of the optical amplification fiber 27D. Although arranged concentrically along the direction, the hexagons formed by the core portions 27Dag to 27Dal are arranged by rotating the hexagons clockwise about the central axis by 30°. ..

The core parts are arranged such that the outer diameters of the inner sub-cladding parts 27Dba and 27Dbb are adjusted and at least one of the hexagons formed by the core parts belonging to each core part group 27Dd, 27De, and 27Df is rotated around the central axis. By doing so, it becomes easy to set the distance such that the crosstalk of the signal light between the adjacent core portions becomes −40 dB or less.

The optical fiber amplifier using the optical amplification fiber 27D in place of the optical amplification fiber 27 in the optical fiber amplifier 200 can also be downsized while having a three-stage optical amplification fiber structure and low power consumption. is there.

(Configuration example 9)
FIG. 14 is a schematic sectional view of Configuration Example 9 of the optical amplification fiber. The optical amplification fiber 27E includes 18 core portions 27Eaa to 27Ear containing Er, an inner clad portion 27Eb formed on the outer periphery of the core portions 27Eaa to 27Ear, and an outer clad formed on the outer periphery of the inner clad portion 27Eb. And an optical amplification fiber having a double-clad structure having a portion 27c.

The core portions 27Eaa to 27Ear include a core portion group 27Ed including six core portions 27Eaa to 27Eaf, a core portion group 27Ee including six core portions 27Eag to 27Eal, and six core portions 27Eam to. It is composed of a core part group 27Ef composed of 27Ear. The core portions 27Eaa to 27Eaf of the core portion group 27Ed, the core portions 27Eag to 27Eal of the core portion group 27Ee, and the core portions 27Eam to 27Ear of the core portion group 27Ef are arranged in a hexagonal shape.

The inner cladding portion 27Eb has a lower refractive index than the core portions 27Eaa to 27Ear. The outer cladding portion 27c has a lower refractive index than the inner cladding portion 27Eb. An example of the constituent material of the optical amplification fiber 27E for realizing such a relationship of the refractive index is the same as the case of the optical amplification fiber 27B of the configuration example 6.

Here, in the optical amplification fiber 27E, the doping concentration of Er and the relative refractive index difference with respect to the inner cladding portion 27Eb are substantially equal in all the core portions 27Eaa to 27Ear. However, regarding the core diameter, all the core portions 27Eaa to 27Eaf are substantially equal, all the core portions 27Eag to 27Eal are substantially equal, and all the core portions 27Eam to 27Ear are substantially equal, but the core portions 27Eaa to 27Eaf and the core portion 27Eag are substantially equal. .About.27Eal and core portions 27Eam to 27Ear are different from each other. Specifically, the core portions 27Eag to 27Eal have larger core diameters than the core portions 27Eaa to 27Eaf, and the core portions 27Eam to 27Ear have larger core diameters than the core portions 27Eag to 27Eal.

Comparing the optical amplification fiber 27B and the optical amplification fiber 27E, the distances from the central axis of the optical amplification fiber 27E of the core portions 27Eaa to 27Eaf and the core portions 27Eag to 27Eal are the core portions 27Baa to 27Baf and the core portions 27Bag to 27Bal. It is larger than the distance from the central axis of the optical amplification fiber 27B. Further, in the optical amplification fiber 27B, the hexagon formed by the core portions 27Baa to 27Baf, the hexagon formed by the core portions 27Bag to 27Bal, and the hexagon formed by the core portions 27Bam to 27Bar are the same as those of the optical amplification fiber 27B. The vertices are concentrically arranged along the radial direction around the central axis. On the other hand, in the optical amplification fiber 27E, the hexagons formed by the core portions 27Eaa to 27Eaf and the hexagons formed by the core portions 27Eam to 27Ear are such that the vertices are centered on the central axis of the optical amplification fiber 27E. Although arranged concentrically along the direction, the hexagons formed by the core portions 27Eag to 27Eal are arranged by rotating the hexagons clockwise about the central axis by 30°. ..

By arranging the core parts so that at least one of the hexagons formed by the core parts belonging to each of the core part groups 27Ed, 27Ee, and 27Ef is rotated around the central axis, the signal light between the adjacent core parts is increased. It becomes easy to set the distance such that the crosstalk of -40 dB or less.

An optical fiber amplifier using an optical amplification fiber 27E instead of the optical amplification fiber 27 in the optical fiber amplifier 200 can be downsized while having a three-stage optical amplification fiber structure and can be reduced in power consumption. is there.

Further, in the above embodiment, the number of stages of the multi-stage optical amplification fiber structure included in the optical fiber amplifier is two or three, and the number of the multi-stage optical amplification fiber structure is six. If an optical amplifying fiber designed to have more fiber structures is used, an optical fiber amplifier having more multi-stages and many multi-stage optical amplifying fiber structures can be realized. For example, n and m are integers of 2 or more, the number of stages in the multistage optical amplification fiber structure is n, the number of core portions in the optical amplification fiber is n or more, and the number of core portions in the optical amplification fiber is n×m. Then, the optical fiber amplifier can have m multi-stage optical amplification fiber structures. At this time, if the number of pumping light sources is smaller than m, the number of pumping light sources can be made smaller than the number of multi-stage optical amplification fiber structures, which is preferable in terms of low power consumption.

Further, in the above-described embodiment, the core part having a larger gain is connected to the subsequent stage to form the multistage optical amplification fiber structure, but the present invention is not limited to this. That is, depending on the design and specifications of the optical fiber amplifier, a core unit with a smaller gain may be connected in a later stage, or the core units may be connected in an order that does not depend on the magnitude of the gain. May be configured.

Further, the present invention is not limited to the above embodiment. The present invention also includes those configured by appropriately combining the above-described components. For example, core portions belonging to different core portion groups in the optical amplification fiber are (1) surrounded by a plurality of inner sub-cladding portions having different refractive indices, (2) different doping concentrations of rare earth elements, (3) ) The characteristics of (1) to (4) in which the types of the added rare earth elements are different from each other and (4) the core diameters are different from each other can be appropriately combined. Further, further effects and modified examples can be easily derived by those skilled in the art. Therefore, the broader aspects of the present invention are not limited to the above embodiments, and various modifications can be made.

1, 1A Input optical fiber 2, 2A, 10, 10A, 13, 13A, 30 Optical isolator 3, 9, 23, 29 Optical multiplexer/demultiplexer 3a to 3l, 23a to 23r Input side optical fiber 9m, 29s Input side multi-core light Fibers 3m, 4a, 23s, 24a Output side multi-core optical fibers 9a to 9l, 29a to 29r Output side optical fibers 4, 24 WDM coupler 5 Excitation light source 6 Multimode optical fibers 7, 7A, 7B, 7C, 7D, 27, 27A , 27B, 27C, 27D, 27E optical amplification fibers 7aa to 7al, 7Aaa to 7Aal, 7Baa to 7Bal, 7Caa to 7Cal, 7Daa to 7Dal, 27aa to 27ar, 27Aaa to 27Aar, 27Baa to 27Bar, 27Caa to 27Car, 27Daa to 27Da. , 27Eaa to 27Ear core parts 7b, 7Ab, 7Bb, 7Cb, 7Db, 27b, 27Ab, 27Bb, 27Cb, 27Db, 27Eb inner cladding parts 7d, 7e, 7Ad, 7Ae, 7Bd, 7Be, 7Cd, 7Ce, 7Dd, 7De, 27d, 27e, 27f, 27Ad, 27Ae, 27Af, 27Bd, 27Be, 27Bf, 27Cd, 27Ce, 27Cf, 27Dd, 27De, 27Df, 27Ed, 27Ee, 27Ef Core part groups 7ba, 7bb, 7Cba, 7Cbb, 27ba, 27bb. 27bc, 27Dba, 27Dbb, 27Dbc Inner sub-cladding portion 7c, 27c Outer cladding portion 8 Residual pumping light processing portion 11, 11A, 31 Connection optical fiber 12, 12A, 32 ASE cut filter 14, 14A Output optical fiber 15, 15A, 25 Multi-stage optical amplification fiber structure 100, 100A, 200 Optical fiber amplifier CP1, CP2 Connection points P1, P2 Refractive index profile

Claims (8)

A plurality of core portions to which a rare earth element is added, formed on the outer periphery of the plurality of core portions, an inner clad portion having a lower refractive index than the plurality of core portions, and formed on the outer periphery of the inner clad portion, An optical amplification fiber having an outer clad having a lower refractive index than the inner clad,
The inner cladding of the optical amplification fiber, at least one pumping light source for supplying pumping light for optically pumping the rare earth element,
A clad pumped optical fiber amplifier comprising:
The plurality of core portions is composed of a plurality of core portion groups,
The optical amplification fiber is configured so that the gains are different from each other among the core parts belonging to different core part groups,
An optical fiber amplifier, wherein core parts belonging to the different core part groups are connected in series to each other to form a multistage optical amplification fiber structure. The inner clad portion has a plurality of inner sub-clad portions having different refractive indexes, and core portions belonging to the different core portion groups are surrounded by different inner sub-clad portions, respectively. 1. The optical fiber amplifier according to 1. The optical fiber amplifier according to claim 1, wherein the core portions belonging to the different core portion groups have different rare earth element doping concentrations. The optical fiber amplifier according to any one of claims 1 to 3, wherein the core parts belonging to the different core part groups have different kinds of the added rare earth elements. The optical fiber amplifier according to any one of claims 1 to 4, wherein the core parts belonging to the different core part groups have different core diameters. n, as two or more integer m, the number of stages in the multistage optical amplifier fiber structure is n, the number of core portions groups before Symbol optical amplifying fiber as above n, the number of core portions in the optical amplifying fiber n The optical fiber amplifier according to any one of claims 1 to 5, wherein m has the multistage optical amplification fiber structure. The optical fiber amplifier according to claim 6, wherein the number of the pumping light sources is smaller than m. A plurality of core portions to which a rare earth element is added, formed on the outer periphery of the plurality of core portions, an inner clad portion having a lower refractive index than the plurality of core portions, and formed on the outer periphery of the inner clad portion, An optical amplification fiber having an outer clad having a lower refractive index than the inner clad,
The plurality of core portions is composed of a plurality of core portion groups,
The optical amplification fiber is configured so that the gains are different from each other among the core parts belonging to different core part groups,
A multistage optical amplification fiber structure, wherein core parts belonging to the different core part groups are connected in series.

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