CN113341499A - Gain-balanced erbium-doped few-mode optical fiber and communication system thereof - Google Patents
Gain-balanced erbium-doped few-mode optical fiber and communication system thereof Download PDFInfo
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/02—Optical fibres with cladding with or without a coating
- G02B6/02042—Multicore optical fibres
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/02—Optical fibres with cladding with or without a coating
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/02—Optical fibres with cladding with or without a coating
- G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
- G02B6/03616—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference
- G02B6/03638—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 3 layers only
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- H—ELECTRICITY
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Abstract
The invention relates to the technical field of optical fiber communication, in particular to an erbium-doped few-mode optical fiber with balanced gain and a communication system thereof. An erbium-doped few-mode optical fiber with balanced gain comprises a fiber core and a cladding; the fiber core and the cladding form a cylindrical optical fiber; the fiber core is used as the center of the optical fiber, and the cladding is wrapped on the periphery of the fiber core; carrying out layered doping on the fiber core to obtain erbium-doped layers with different erbium-doped concentrations; the erbium-doped layer comprises a first erbium-doped layer and a second erbium-doped layer; wherein: the first erbium-doped layer is positioned at the center of the fiber core; the second erbium-doped layer surrounds the first erbium-doped layer; the second erbium doped layer has its body on the core and is partially doped into the cladding. The erbium ion distribution outside the core of the second erbium-doped layer can be better overlapped with a high-order mode, the overlapping integral of a light field and erbium ions among modes is reduced, a similar gain effect is generated for each mode, the mode gain difference is effectively reduced, and a good mode gain balance effect is achieved; the preparation difficulty is reduced by adopting the single-fiber core structure design.
Description
Technical Field
The invention relates to the technical field of optical fiber communication, in particular to an erbium-doped few-mode optical fiber with balanced gain and a communication system thereof.
Background
With global informatization, our lives are interconnected everywhere. A large number of emerging network technologies such as smart home, autopilot, and VR are gradually entering daily life, and with this, higher requirements are put forward on network rate and bandwidth. Because the single mode fiber in the existing fiber transmission system has nonlinear effect, the system capacity gradually approaches to the Shannon theoretical limit, the optical communication gradually faces the capacity crisis, and the space division multiplexing technology comes along with the process. Space division multiplexing is a novel multiplexing technology with great potential, the capacity of the existing optical fiber communication system can be improved by 2-3 orders of magnitude, and the capacity crisis problem can be effectively relieved. The mode division multiplexing technology with few-mode optical fibers as the core is an important branch of the space division multiplexing technology. The mode division multiplexing technology can transmit a plurality of modes in one fiber core, and the capacity of a single core is improved exponentially.
Optical signals experience corresponding losses with propagation distance, and the propagation distance of optical signals is still limited, although the optical fiber propagation loss has decreased to below 0.2dB/km with improvements in the optical fiber manufacturing process. Therefore, in long-haul optical fiber communication transmission systems, optical fiber amplifiers are required to boost the power of the optical signals that have been attenuated. In a mode division multiplexing transmission system, the parallel use of single mode fiber amplifiers results in higher system cost. For this reason, researchers have conducted extensive research on few-mode erbium-doped fiber amplifiers. In few-mode erbium-doped fiber amplifiers, the gain difference obtained for different modes determines the quality of the transmitted signal and the stability of the system. Therefore, mode gain equalization becomes a key measure for the performance of few-mode transmission systems.
The gain difference between different modes can be controlled by adjusting the distribution overlapping condition of the signal light field, the pumping light field and the doped ions. Currently there are mainly the following three strategies: adjusting the refractive index distribution of the optical fiber, optimizing the distribution of the pumping optical field and improving the distribution of the doped rare earth ions. In the prior art, the fiber core is made into a ring shape, and the mode gain difference can be reduced by doping in the ring core, but the refractive index distribution of the fiber is mismatched with that of a common step transmission fiber, so that larger connection loss is easily caused. In the prior art, different modes of pumping light fields of a core region are regulated, so that the overlapping degree of the pumping light fields and the signal light fields is higher, but the regulation effect is poor and higher cost is often needed. In the prior art, the multilayer doped rings in the core are utilized, the gain of a regulation mode is poor, more and more accurate doped rings are often needed along with the increase of the number of the modes, the manufacturing process difficulty is high, and the method is not easy to apply to a few-mode transmission system.
Chinese patent publication No. CN 112180499 a (published as 2021-01-05), discloses a three-core multilayer erbium ion-doped 4-mode optical fiber with very small gain difference, which comprises a core, a trench and a cladding, wherein the core is located at the center of the optical fiber, the cladding is located outside the core, and the trench is located in the cladding at a position closer to the core. The fiber core is divided into three layers, namely a central layer, an annular layer and an outer core layer which are tightly connected. The ion doping is mainly concentrated in a fiber core area, the fiber core is divided into three layers of rings for doping, and the three layers of ion filling areas are respectively the same as the refractive index distribution areas of a central layer, a ring layer and an outer core layer of the erbium ion-doped 4-mode optical fiber. The refractive index distribution assisted by the three-layer core groove ensures that the optical fiber has higher mode refractive index difference, can weaken the cross talk between modes and reduces the bending loss in application. Moreover, the refractive index distribution is beneficial to reducing the difference value of the power filling factors of all modules, the difficulty of optimizing the particle doping distribution can be greatly reduced, and the gain balance is easier to realize. The invention provides a multi-layer core doped optical fiber structure design for achieving mode gain balance. Erbium ions with different concentrations are doped in the multilayer core respectively, so that the overlapping degree of a signal optical field and the erbium ions is regulated and controlled, and the mode gain difference is reduced. The disadvantages of the invention are: the difficulty of the doping manufacturing process of the multilayer core is high, and the refractive index of different core layers is changed due to excessive doping layers, so that the propagation mode of the optical fiber is influenced.
Disclosure of Invention
The invention aims to overcome the defects of larger mode gain difference and larger manufacturing process difficulty caused by using multi-layer core doping in the prior art. In order to achieve the above object, the present invention provides an erbium-doped few-mode optical fiber with balanced gain and a communication system thereof.
An erbium-doped few-mode optical fiber with balanced gain comprises a fiber core and a cladding; the fiber core and the cladding form a cylindrical structure; the fiber core is used as the center of the optical fiber, and the cladding is wrapped on the periphery of the fiber core;
carrying out layered doping on the fiber core to obtain erbium-doped layers with different erbium-doped concentrations; the erbium-doped layer comprises a first erbium-doped layer and a second erbium-doped layer; wherein:
the first erbium-doped layer is positioned in the center of the fiber core; the second erbium-doped layer surrounds the first erbium-doped layer; the second erbium-doped layer has a main body located on the core and is partially doped into the cladding.
As a preferable scheme: the radius of the fiber core is 8.5 +/-1 mu m, the radius of the cladding is 62.5 +/-1 mu m, the refractive index of the fiber core is 1.452 +/-0.005, and the refractive index of the cladding is 1.444 +/-0.005;
the radius of the first erbium-doped layer is 2 mu m to 2.5 mu m, the radius of the second erbium-doped layer is 9.3 mu m to 9.5 mu m, and the doping concentration ratio of the first erbium-doped layer to the second erbium-doped layer is 1.05:1 to 1.25: 1.
As a preferable scheme: a third erbium-doped layer is also included; the third erbium-doped layer is positioned between the first erbium-doped layer and the second erbium-doped layer.
As a preferable scheme: the radius of the core is 11 +/-1 μm, the radius of the cladding is 62.5 +/-1 μm, the refractive index of the core is 1.452 +/-0.005, and the refractive index of the cladding is 1.444 +/-0.005.
As a preferable scheme: the radius of the first erbium-doped layer is 3.2 +/-0.5 mu m, the radius of the third erbium-doped layer is 8.5 +/-0.5 mu m, and the radius of the second erbium-doped layer is 12 +/-0.5 mu m; the doping concentration ratio of the first erbium-doped layer to the second erbium-doped layer is 0.92:1 to 0.96:1, and the doping concentration ratio of the third erbium-doped layer to the second erbium-doped layer is 0.74:1 to 0.78: 1.
As a preferable scheme: the refractive index loss is complemented by co-doping with aluminium in the third erbium doped layer.
As a preferable scheme: the refractive index loss is complemented by co-doping germanium in the third erbium doped layer.
As a preferable scheme: the second erbium-doped layer eliminates the surplus of the refractive index by co-doping fluorine, so that the refractive index of the optical fiber is still kept in a step type.
As a preferable scheme: the first erbium-doped layer eliminates the surplus of the refractive index by co-doping fluorine, so that the refractive index of the optical fiber is still kept in a step type.
A gain-balanced erbium-doped few-mode optical fiber communication system comprises an optical signal input unit, an optical signal modulation unit, a mode division multiplexer, a few-mode optical fiber, a mode decomposition multiplexer and an optical signal demodulation unit;
the optical signal input unit acquires an optical signal and transmits the optical signal to the optical signal modulation unit; the optical signal modulation unit modulates the optical signal to obtain a modulated optical signal and transmits the modulated optical signal to the mode division multiplexer; the mode division multiplexer processes the optical signal to obtain a mode division multiplexing optical signal and transmits the mode division multiplexing optical signal to the few-mode optical fiber; the few-mode optical fiber is an erbium-doped few-mode optical fiber with balanced gain, and transmits an optical signal of mode division multiplexing from the mode division multiplexer to the mode decomposition multiplexer; the mode decomposition multiplexer converts the optical signal of the mode division multiplexing into a basic mode optical signal and transmits the basic mode optical signal to the optical signal demodulation unit; the optical signal demodulation unit converts the fundamental mode optical signal into data information and finally outputs the data information.
Compared with the prior art, the invention has the beneficial effects that:
the erbium-doped few-mode optical fiber with balanced gain is provided, the distribution of erbium ions outside the core of the second erbium-doped layer can be better overlapped with a high-order mode, the overlapping integral of a light field and the erbium ions among modes is reduced, a similar gain effect is generated for each mode, the mode gain difference is effectively reduced, and a good mode gain balancing effect is achieved; the preparation difficulty is reduced by adopting the single-fiber core structure design.
The erbium-doped few-mode optical fiber communication system with balanced gain is provided, and the erbium-doped few-mode optical fiber with balanced gain is used between the mode division multiplexer and the mode decomposition multiplexer, so that the gain difference of different modes is reduced, and the quality of transmission signals and the stability of the system are greatly improved.
Drawings
Fig. 1 is a schematic diagram of a refractive index profile and erbium ion doping distribution of an erbium-doped few-mode fiber with balanced gain and a communication system in embodiment 1 of the invention.
Fig. 2 is a schematic diagram of a refractive index profile and erbium ion doping distribution of an erbium-doped few-mode fiber with balanced gain and a communication system embodiment 2 thereof according to the present invention.
Fig. 3 is a schematic diagram of a refractive index profile and erbium ion doping distribution of an erbium-doped few-mode fiber with balanced gain and a communication system embodiment 3 thereof according to the present invention.
Fig. 4 is a C-band gain spectrum obtained from preferred parameters of an erbium-doped few-mode fiber with balanced gain and a communication system embodiment 1 thereof according to the present invention.
The core-cladding-based optical fiber comprises a core 1, a cladding 2, a first erbium-doped layer 31, a second erbium-doped layer 32 and a third erbium-doped layer 33.
Detailed Description
The following detailed description of embodiments of the present invention is provided in connection with the accompanying drawings and examples. The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention.
In the description of the present invention, it should be noted that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate orientations or positional relationships based on those shown in the drawings, and are only for convenience of description and simplicity of description, but do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and thus, should not be construed as limiting the present invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.
In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.
In addition, in the description of the present invention, "a plurality" means two or more unless otherwise specified.
Specifically, as shown in fig. 1 to 4, embodiments of the invention relate to an erbium-doped few-mode fiber with balanced gain and a communication system thereof.
An erbium-doped few-mode optical fiber with balanced gain comprises a fiber core 1 and a cladding 2; the fiber core 1 and the cladding 2 jointly form an optical fiber with a uniform cylindrical structure; the fiber core 1 is used as the center of the optical fiber, and the cladding 2 is wrapped on the periphery of the fiber core 1;
carrying out layered doping on the fiber core 1 to obtain erbium-doped layers with different erbium-doped concentrations; the erbium-doped layer comprises a first erbium-doped layer 31 and a second erbium-doped layer 32; wherein:
the first erbium doped layer 31 is located at the center of the core 1; the second erbium-doped layer 32 surrounds the first erbium-doped layer 31; the second erbium doped layer 32 has its body on the core 1 and is partially doped into the cladding 2.
The optical fiber is a uniform cylindrical structure, and the refractive index cross section of the optical fiber is a concentric ring which is sequentially represented as a fiber core 1 and a cladding 2 from inside to outside. The layered doping cross section of the optical fiber is a concentric ring, the erbium-doped concentration in the same layer is equal, and the erbium-doped concentrations in the two layers are different from inside to outside.
Furthermore, the radius of the core 1 is 8.5 μm, the radius of the cladding 2 is 62.5 μm, the refractive index of the core 1 is 1.452, the refractive index of the cladding 2 is 1.444, and 4 modes can be accommodated;
in embodiment 1, as shown in fig. 1, the radius of the first erbium-doped layer 31 is 2.5 μm, the radius of the second erbium-doped layer 32 is 9.5 μm, and the doping concentration ratio of the first erbium-doped layer 31 to the second erbium-doped layer 32 is 1.22: 1.
A gain-balanced layered core-out erbium-doped few-mode optical fiber is shown in figure 1, the refractive index profile of the fiber is a step type, the fiber comprises a fiber core 1 and a cladding 2, and two concentric ring areas which are divided according to the erbium-doped concentration, namely a first erbium-doped layer 31 and a second erbium-doped layer 32 from inside to outside, wherein the erbium-doped concentrations of the first erbium-doped layer and the second erbium-doped layer are unequal.
The first and second erbium-doped layers 31 and 32 eliminate the refractive index surplus by co-doping fluorine, so that the refractive index of the optical fiber is still kept in a step type.
In the embodiment, gain equalization of each mode can be realized in the amplification process by using the method of erbium doping outside the layered core. Meanwhile, the number of supported modes can be matched with that of commercial few-mode transmission optical fibers, so that the optical fiber can be really applied to a few-mode transmission system.
In example 2, as shown in fig. 2, the radius of the first erbium-doped layer 31 is 2 μm, the radius of the second erbium-doped layer 32 is 9.3 μm, and the doping concentration ratio of the first erbium-doped layer 31 to the second erbium-doped layer 32 is 1.1: 1.
A gain-balanced layered core-out erbium-doped few-mode optical fiber is shown in figure 1, the refractive index profile of the fiber is a step type, the fiber comprises a fiber core 1 and a cladding 2, and two concentric ring areas which are divided according to the erbium-doped concentration, namely a first erbium-doped layer 31 and a second erbium-doped layer 32 from inside to outside, wherein the erbium-doped concentrations of the first erbium-doped layer and the second erbium-doped layer are unequal.
The first and second erbium-doped layers 31 and 32 eliminate the refractive index surplus by co-doping fluorine, so that the refractive index of the optical fiber is still kept in a step type.
The embodiment can effectively reduce mode gain difference, achieve good mode gain equalization effect, and simultaneously ensure that each mode obtains higher gain amplitude. Meanwhile, the optical fiber structure parameters can be selected to ensure that the optical fiber can accommodate a certain number of transmission modes, and the commercial few-mode transmission optical fiber can be better matched.
Embodiment 3, as shown in fig. 3, further includes a third erbium-doped layer 33; the third erbium-doped layer 33 is located between the first and second erbium-doped layers 31 and 32.
Further, the radius of the core 1 is 11 μm, the radius of the cladding 2 is 62.5 μm, the refractive index of the core 1 is 1.452, and the refractive index of the cladding 2 is 1.444, so that 6 modes can be accommodated.
A gain-balanced layered core-out erbium-doped few-mode optical fiber is shown in figure 3 and comprises a fiber core 1, a cladding 2, three concentric ring regions divided according to erbium doping concentrations, a first erbium-doped layer 31, a third erbium-doped layer 33 and a second erbium-doped layer 32 in sequence from inside to outside, wherein the erbium doping concentrations of the first erbium-doped layer 31, the third erbium-doped layer 33 and the second erbium-doped layer 32 are different from each other.
Further, the radius of the first erbium-doped layer 31 is 3.2 μm, the radius of the third erbium-doped layer 33 is 8.5 μm, and the radius of the second erbium-doped layer 32 is 12 μm. The doping concentration ratio of the first erbium-doped layer 31, the third erbium-doped layer 33, and the second erbium-doped layer 32 is 0.94:0.76: 1.
Further, the refractive index loss is complemented by co-doping germanium in the third erbium doped layer 33.
Further, the first erbium-doped layer 31 and the second erbium-doped layer 32 eliminate the refractive index surplus by co-doping fluorine, so that the refractive index of the optical fiber is still kept in a step type.
Embodiment 4, as shown in fig. 3, further includes a third erbium-doped layer 33; the third erbium-doped layer 33 is located between the first and second erbium-doped layers 31 and 32.
Further, the radius of the core 1 is 11 μm, the radius of the cladding 2 is 62.5 μm, the refractive index of the core 1 is 1.452, and the refractive index of the cladding 2 is 1.444, so that 6 modes can be accommodated.
A gain-balanced layered core-out erbium-doped few-mode optical fiber is shown in figure 3 and comprises a fiber core 1, a cladding 2, three concentric ring regions divided according to erbium doping concentrations, a first erbium-doped layer 31, a third erbium-doped layer 33 and a second erbium-doped layer 32 in sequence from inside to outside, wherein the erbium doping concentrations of the first erbium-doped layer 31, the third erbium-doped layer 33 and the second erbium-doped layer 32 are different from each other.
Further, the radius of the first erbium-doped layer 31 is 3.2 μm, the radius of the third erbium-doped layer 33 is 8.5 μm, and the radius of the second erbium-doped layer 32 is 12 μm. The doping concentration ratio of the first erbium-doped layer 31, the third erbium-doped layer 33, and the second erbium-doped layer 32 is 0.94:0.76: 1.
Further, the refractive index loss is complemented by co-doping with aluminum in the third erbium doped layer 33.
Further, the first erbium-doped layer 31 and the second erbium-doped layer 32 eliminate the refractive index surplus by co-doping fluorine, so that the refractive index of the optical fiber is still kept in a step type.
The refractive index can be increased due to the doping outside the core, and the fiber can be reduced by doping fluorine, and can be supplemented by doping aluminum or germanium at the part with insufficient refractive index. Thus, the entire refractive index of the optical fiber is maintained in a single-layer step type. Compared with other layered structures, the embodiment reduces the number of doped layers in the core by doping outside the core, can effectively reduce mode gain difference, achieves good mode gain equalization effect, and simultaneously ensures that each mode obtains higher gain amplitude. In addition, in the embodiment, by selecting the optical fiber structure parameters, the optical fiber can be better matched with a commercial few-mode transmission optical fiber under the condition of ensuring the number of transmission modes, and is of great importance to the development of an air-division multiplexing transmission system.
Fig. 4 is a C-band gain spectrum obtained by optimizing parameters according to the refractive index doping profile of example 1, wherein the average gain of the C-band can reach 24.82dB, and the maximum difference between the gains is 0.64dB, which illustrates that the layered erbium-doped optical fiber with a few layers doped in the core can achieve high gain amplitude and gain balance.
A gain-balanced erbium-doped few-mode optical fiber communication system comprises an optical signal input unit, an optical signal modulation unit, a mode division multiplexer, a few-mode optical fiber, a mode decomposition multiplexer and an optical signal demodulation unit;
the optical signal input unit acquires an optical signal and transmits the optical signal to the optical signal modulation unit; the optical signal modulation unit modulates the optical signal to obtain a modulated optical signal and transmits the modulated optical signal to the mode division multiplexer; the mode division multiplexer processes the optical signal to obtain a mode division multiplexing optical signal and transmits the mode division multiplexing optical signal to the few-mode optical fiber; the few-mode optical fiber is an erbium-doped few-mode optical fiber with balanced gain, and transmits an optical signal of mode division multiplexing from the mode division multiplexer to the mode decomposition multiplexer; the mode decomposition multiplexer converts the optical signal of the mode division multiplexing into a basic mode optical signal and transmits the basic mode optical signal to the optical signal demodulation unit; the optical signal demodulation unit converts the fundamental mode optical signal into data information and finally outputs the data information.
In summary, the embodiment provides an erbium-doped few-mode fiber with balanced gain, the distribution of erbium ions outside the core of the second erbium-doped layer can better overlap with a high-order mode, the overlap integral of the optical field and erbium ions between modes is reduced, and a similar gain effect is generated for each mode, so that the mode gain difference is effectively reduced, and a good mode gain balancing effect is achieved; the preparation difficulty is reduced by adopting the single-fiber core structure design.
The embodiment provides an erbium-doped few-mode fiber communication system with balanced gain, and the erbium-doped few-mode fiber with balanced gain is used between a mode division multiplexer and a mode decomposition multiplexer, so that the gain difference of different modes is reduced, and the quality of a transmission signal and the stability of the system are greatly improved.
Other persons skilled in the art will appreciate that the above parameters of the step-index profile and the doping profile are merely examples, and other similar applications, now existing or hereafter discovered, that may be applicable to the embodiments of the present invention, are also within the scope of the present invention.
The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and substitutions can be made without departing from the technical principle of the present invention, and these modifications and substitutions should also be regarded as the protection scope of the present invention.
Claims (10)
1. An erbium-doped few-mode optical fiber with balanced gain comprises a fiber core (1) and a cladding (2); the fiber core (1) and the cladding (2) form a cylindrical optical fiber; wherein the fiber core (1) is used as the center of the optical fiber, and the cladding (2) is wrapped on the periphery of the fiber core (1); the method is characterized in that:
carrying out layered doping on the fiber core (1) to obtain erbium-doped layers with different erbium-doped concentrations; the erbium-doped layer comprises a first erbium-doped layer (31) and a second erbium-doped layer (32); wherein:
the first erbium-doped layer (31) is positioned at the center of the fiber core (1); the second erbium-doped layer (32) surrounds the first erbium-doped layer (31); the second erbium-doped layer (32) has a main body located on the core (1) and is partially doped into the cladding (2).
2. A gain equalized erbium doped few-mode fiber as claimed in claim 1, wherein: the radius of the fiber core (1) is 8.5 +/-1 mu m, the radius of the cladding (2) is 62.5 +/-1 mu m, the refractive index of the fiber core (1) is 1.452 +/-0.005, and the refractive index of the cladding (2) is 1.444 +/-0.005;
the radius of the first erbium-doped layer (31) is 2 mu m to 2.5 mu m, the radius of the second erbium-doped layer (32) is 9.3 mu m to 9.5 mu m, and the doping concentration ratio of the first erbium-doped layer (31) to the second erbium-doped layer (32) is 1.05:1 to 1.25: 1.
3. A gain equalized erbium doped few-mode fiber as claimed in claim 1, wherein: further comprising a third erbium doped layer (33); the third erbium-doped layer (33) is located between the first erbium-doped layer (31) and the second erbium-doped layer (32).
4. A gain equalized erbium doped few-mode fiber as claimed in claim 3, wherein: the radius of the core (1) is 11 +/-1 mu m, the radius of the cladding (2) is 62.5 +/-1 mu m, the refractive index of the core (1) is 1.452 +/-0.005, and the refractive index of the cladding (2) is 1.444 +/-0.005.
5. A gain equalized erbium doped few-mode fiber as claimed in claim 4, wherein: the radius of the first erbium-doped layer (31) is 3.2 +/-0.5 mu m, the radius of the third erbium-doped layer (33) is 8.5 +/-0.5 mu m, and the radius of the second erbium-doped layer (32) is 12 +/-0.5 mu m; the doping concentration ratio of the first erbium-doped layer (31) to the second erbium-doped layer (32) is 0.92:1 to 0.96:1, and the doping concentration ratio of the third erbium-doped layer (33) to the second erbium-doped layer (32) is 0.74:1 to 0.78: 1.
6. A gain equalized erbium doped few-mode fiber as claimed in claim 5, wherein: the refractive index loss is complemented by co-doping aluminium in the third erbium doped layer (33).
7. A gain equalized erbium doped few-mode fiber as claimed in claim 5, wherein: the refractive index loss is complemented in the third erbium doped layer (33) by co-doping germanium.
8. A gain equalized erbium doped few-mode optical fiber according to any of claims 1 to 7, characterized in that: the second erbium-doped layer (32) eliminates the refractive index surplus by co-doping fluorine, so that the refractive index of the optical fiber is still kept in a step type.
9. A gain equalized erbium doped few-mode fiber as claimed in claim 8, wherein: the first erbium-doped layer (31) eliminates the refractive index surplus by co-doping fluorine, so that the refractive index of the optical fiber is still kept in a step type.
10. A gain-balanced erbium-doped few-mode fiber communication system is characterized in that: the optical fiber module comprises an optical signal input unit, an optical signal modulation unit, a mode division multiplexer, a few-mode optical fiber, a mode decomposition multiplexer and an optical signal demodulation unit;
the optical signal input unit acquires an optical signal and transmits the optical signal to the optical signal modulation unit; the optical signal modulation unit modulates the optical signal to obtain a modulated optical signal and transmits the modulated optical signal to the mode division multiplexer; the mode division multiplexer processes the optical signal to obtain a mode division multiplexing optical signal and transmits the mode division multiplexing optical signal to the few-mode optical fiber; the few-mode fiber is a gain-equalized erbium-doped few-mode fiber according to any one of claims 1 to 9, the few-mode fiber transmits mode-division-multiplexed optical signals from the mode division multiplexer to the mode division multiplexer; the mode decomposition multiplexer converts the optical signal of the mode division multiplexing into a basic mode optical signal and transmits the basic mode optical signal to the optical signal demodulation unit; the optical signal demodulation unit converts the fundamental mode optical signal into data information and finally outputs the data information.
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