A few-mode erbium-doped fiber amplifier system based on mode routing and its application

Abstract

The present invention discloses a few-mode erbium-doped fiber amplification system based on mode routing selection and its application. The system includes two sections of few-mode erbium-doped fiber, a pump source, a tunable laser, a variable optical attenuator, a wavelength division multiplexer, a mode multiplexer and demultiplexer, and a circulator. Using the same erbium-doped fiber, different mode signals are divided into high-gain and low-gain groups based on their gain capabilities. The mode signals in the high-gain group undergo single-stage amplification through a section of few-mode erbium-doped fiber, while the mode signals in the low-gain group undergo cascade amplification through two sections of few-mode erbium-doped fiber. This further increases the gain of the low-gain group mode signals, reduces the gain difference with the high-gain group, and achieves good mode gain balancing. The few-mode erbium-doped fiber in the device can adopt a single core, a common step-index refractive index, and a uniformly doped structure within the core, requiring no special design and requiring low fabrication difficulty.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber communication, and in particular to a few-mode erbium-doped optical fiber amplification system based on mode routing selection and applications thereof. Background Art

[0002] The booming development of the Internet of Things (IoT) is placing ever-increasing demands on optical fiber transmission capacity. The capacity of standard single-mode optical fiber communication transmission systems is approaching the theoretical limit set by Shannon's theorem, making it increasingly difficult to increase single-mode optical fiber transmission capacity using existing technologies. However, the physical dimension of space remains a subject of considerable research and exploration, and the enormous capacity increases it will bring will become one of the key development directions for ultra-high-capacity optical transmission. Space-division multiplexing (SDM) is a highly promising new multiplexing technology that can increase the capacity of existing optical fiber communication systems by two to three orders of magnitude, effectively alleviating the capacity crisis. Mode-division multiplexing (MDM), centered around few-mode fiber, is a key branch of SDM technology. This technology fully utilizes the fiber's cross-section to provide multiple, parallel, and independent channels, potentially significantly increasing the transmission capacity of a single optical fiber. Furthermore, channels can share components such as light sources and optical amplifiers, resulting in lower energy consumption and cost per unit of information. It is also insensitive to signal format and wavelength, making it compatible with existing signal modulation and wavelength-division multiplexing systems.

[0003] Few-mode optical signals experience losses in long-haul fiber-optic communication transmission systems, necessitating the use of few-mode fiber amplifiers (FMFs) to boost the attenuated optical signal power. Traditional FMFs offer varying gains for different signal modes, easily causing transmission interruptions and other issues. These amplifiers are crucial for signal quality and system stability. To address this, researchers have conducted extensive research and optimization of FMFs, aiming to minimize the gain differences between modes. Consequently, modal gain equalization has become a key performance metric for FMF systems.

[0004] Current methods for achieving mode gain equalization include: 1. Fabricating erbium fibers with specific refractive index structures, such as ring fibers, or multi-layer grooved fibers, to change the signal light fields of each mode. However, the refractive index distribution of such fibers is mismatched with commonly used step-index transmission fibers, which can easily cause large connection losses. 2. Designing special doping structures: such as multi-layer doping and extra-core doping to balance the degree of overlap between each mode and the erbium particles. This places high demands on the fiber preparation process, and too many doping layers will cause the refractive index of different core layers to change, thereby affecting the propagation mode in the fiber. 3. Optimizing the pump light field distribution, such as using high-order mode pumping to increase the signal gain of high-order modes, but the adjustment effect is poor and often requires higher costs.

[0005] Comparative Document 1: Chinese invention patent application number 202110600015.1: "A Gain-Balanced Erbium-Doped Few-Mode Fiber and Its Communication System", whose technical solution is specifically as follows: A gain-balanced erbium-doped few-mode fiber includes a core and a cladding; the core and the cladding form a cylindrical optical fiber; wherein the core is the center of the optical fiber, and the cladding wraps around the outer periphery of the core; layered doping is performed on the core to obtain erbium-doped layers with different erbium doping concentrations; the erbium-doped layer includes a first erbium-doped layer and a second erbium-doped layer; wherein: the first erbium-doped layer is located at the center of the core; the second erbium-doped layer surrounds the first erbium-doped layer; the main body of the second erbium-doped layer is located on the core and is partially doped into the cladding. The erbium ion distribution outside the core of the second erbium-doped layer can better overlap with the higher-order modes, reduce the overlap integral of the light field and erbium ions between the modes, and thus produce a similar gain effect for each mode. The technical solution provided by this invention patent achieves gain balance for each mode by designing a multi-layer doping structure in the fiber core. However, the preparation of multi-layer doped optical fibers using this technical solution is difficult and has limited widespread application.

[0006] Comparative Document 2: Chinese invention patent application number 202210697588.5: "A Gain-Balanced Few-Mode Fiber Amplifier Based on Bidirectional Hybrid Pumping", whose technical solution is as follows: In the bidirectional hybrid pumping structure, cladding pumping is mainly used for signal gain, while bidirectional core pumping is used to compensate for mode differences, and the power ratio and mode selection of each pump source are adjusted to achieve precise amplification between different signal modes in the amplifier, effectively reduce the inter-mode gain difference of the amplifier, and achieve good gain balancing characteristics. In the few-mode fiber amplifier based on bidirectional hybrid pumping, a single-mode pump source, a multi-mode pump source, and a single-mode pump source are cascaded, and combined pumping is used to implement targeted amplification of each signal mode. The number of amplification modes is controlled by selecting an appropriate gain fiber, and the signal gain and inter-mode gain difference are regulated by optimizing the pump and fiber length. This invention patent utilizes double-clad optical fibers, including erbium-doped optical fibers, ytterbium-doped optical fibers, erbium-ytterbium co-doped optical fibers, thulium-doped optical fibers, and other double-clad optical fibers that can provide gain for signal modes. It also optimizes the pump light field distribution. The overall technical solution is relatively complex, the optical fiber materials used are difficult to prepare, and the implementation cost is high. Summary of the Invention

[0007] In order to solve the technical problems of large connection loss, high requirements for optical fiber preparation process, and poor gain adjustment effect in the existing mode gain equalization technology, the present invention provides a few-mode erbium-doped fiber amplifier system based on mode routing selection and its application. The technical solution adopted by the present invention is:

[0008] A first aspect of the present invention provides a few-mode erbium-doped fiber amplification system based on mode routing selection, comprising a first few-mode erbium-doped fiber, a second few-mode erbium-doped fiber, a first mode multiplexer and demultiplexer, a second mode multiplexer and demultiplexer, a first circulator, a second circulator, a first tunable laser for forward transmission, a first variable optical attenuator, a first wavelength division multiplexer, and a first pump source, and a second tunable laser, a second variable optical attenuator, a second wavelength division multiplexer, and a second pump source for reverse transmission;

[0009] The first tunable laser is connected to the first variable optical attenuator, and the first variable optical attenuator and the first pump are both connected to the first wavelength division multiplexer; the first wavelength division multiplexer is connected to the first mode multiplexer and demultiplexer, the first mode multiplexer and demultiplexer is connected to the second port of the first circulator, and the third port is connected to the first few-mode erbium-doped fiber. The other end of the first few-mode erbium-doped fiber is connected to the first port of the second circulator, and the third port is connected to the first port of the first circulator to form a loop. The second port of the second circulator is connected to the second few-mode erbium-doped fiber and then connected to the second mode multiplexer and demultiplexer; the second tunable laser is connected to the second variable optical attenuator, and the second variable optical attenuator and the second pump are both connected to the second wavelength division multiplexer; the second wavelength division multiplexer is connected to the second mode multiplexer and demultiplexer.

[0010] Compared with the existing technology, the present invention provides a few-mode erbium-doped fiber amplification system based on mode routing selection, which adopts a few-mode erbium-doped fiber with a single-core ordinary step-index structure to reduce the difficulty of preparation; a low-gain mode signal is selected and cascaded amplified through two sections of few-mode erbium-doped fiber to further improve the gain of the mode signal, while a high-gain mode signal is amplified only through one section of few-mode erbium-doped fiber, thereby effectively reducing the mode gain difference and achieving a good mode gain equalization effect.

[0011] As a preferred solution, the first few-mode erbium-doped fiber and the second few-mode erbium-doped fiber both include a core and a cladding; the core is wrapped in the cladding to form a cylindrical optical fiber; and erbium ions are doped inside the core.

[0012] As a preferred solution, the core refractive index of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber is the same, and both are preset values.

[0013] As a preferred solution, the cladding refractive index of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber are the same, and both are preset values.

[0014] As a preferred solution, the core radius of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber are the same, and are both preset values.

[0015] As a preferred solution, the cladding radius of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber are the same, and are both preset values.

[0016] As a preferred solution, the core erbium ion doping concentrations of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber are the same, and are both preset values.

[0017] As a preferred solution, the first few-mode erbium-doped fiber and the second few-mode erbium-doped fiber have different lengths.

[0018] As a preferred solution, the forward pump wavelength of the first pump source in forward transmission is 1480nm, the pump mode is LP01, and the pump power is 193mW; the reverse pump wavelength of the second pump source in reverse transmission is 1480nm, the pump mode is LP11, and the pump power is 171mW.

[0019] The second aspect of the present invention provides an application of a few-mode erbium-doped fiber amplifier system based on mode routing selection, specifically:

[0020] When a mode signal with weak gain capability is selected for forward transmission, the signal is attenuated by the first variable optical attenuator after passing through the first tunable laser. The attenuated signal and the forward pump generated by the first pump source are then coupled into the first mode multiplexing and demultiplexing device using the first wavelength division multiplexer for mode multiplexing. The mode-multiplexed signal is then cascade-amplified through the first few-mode erbium-doped fiber and the second few-mode erbium-doped fiber in sequence. Finally, the signal is demultiplexed and output through the second mode multiplexing and demultiplexing device.

[0021] When a mode signal with a strong gain capability is selected for reverse transmission, the signal is attenuated by the second variable optical attenuator after passing through the second tunable laser. Subsequently, the second wavelength division multiplexer couples the attenuated signal and the reverse pump generated by the second pump source into the second mode multiplexing and demultiplexing device for mode multiplexing processing. The mode-multiplexed signal is then amplified only through the second few-mode erbium-doped fiber. Finally, the signal is demultiplexed and output through the first mode multiplexing and demultiplexing device.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] The present invention provides a few-mode erbium-doped fiber amplification system based on mode routing selection. Low-gain mode signals are selected and cascaded amplified through two sections of few-mode erbium-doped fiber, further improving the gain of the mode signals. High-gain mode signals are amplified through only one section of few-mode erbium-doped fiber, thereby effectively reducing the mode gain difference and achieving a good mode gain equalization effect. The few-mode erbium-doped fiber is a single-core ordinary step-index structure, thereby reducing the difficulty of preparation. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A schematic diagram of a few-mode erbium-doped fiber amplification system based on mode routing selection provided by an embodiment of the present invention;

[0025] Figure 2 A gain diagram showing changes in input signal after optimizing parameters of a few-mode erbium-doped fiber amplifier system based on mode routing selection provided in an embodiment of the present invention;

[0026] Figure 3 This is a C-band gain spectrum diagram after optimizing parameters for a few-mode erbium-doped fiber amplifier system based on mode routing selection provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0027] The accompanying drawings are for illustrative purposes only and are not to be construed as limiting this patent;

[0028] It should be clear that the embodiments described are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments of the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the embodiments of the present application.

[0029] The terms used in the embodiments of the present application are for the purpose of describing specific embodiments only and are not intended to limit the embodiments of the present application. The singular forms "a," "the," and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise. It should also be understood that the term "and / or" used herein refers to and includes any or all possible combinations of one or more associated listed items.

[0030] When the following description refers to the accompanying drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present application. On the contrary, they are merely examples of devices and methods consistent with some aspects of the present application as detailed in the appended claims. In the description of the present application, it should be understood that the terms "first", "second", "third", etc. are only used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence, nor can they be understood as indicating or implying relative importance. For those of ordinary skill in the art, the specific meanings of the above terms in the present application can be understood according to the specific circumstances.

[0031] In addition, in the description of this application, unless otherwise specified, "plurality" refers to two or more. "And / or" describes the relationship between associated objects, indicating that three relationships can exist. For example, "A and / or B" can mean: A exists alone, A and B exist simultaneously, and B exists alone. The character " / " generally indicates that the associated objects are in an "or" relationship. The present invention is further described below with reference to the accompanying drawings and examples.

[0032] The present invention is further described below with reference to the accompanying drawings and embodiments.

[0033] Example 1

[0034] Please refer to Figure 1 A few-mode erbium-doped fiber amplification system based on mode routing selection includes a first few-mode erbium-doped fiber, a second few-mode erbium-doped fiber, a first mode multiplexer and demultiplexer, a second mode multiplexer and demultiplexer, a first circulator, a second circulator, a first tunable laser for forward transmission, a first variable optical attenuator, a first wavelength division multiplexer, and a first pump source, and a second tunable laser, a second variable optical attenuator, a second wavelength division multiplexer, and a second pump source for reverse transmission;

[0035] The first tunable laser is connected to the first variable optical attenuator, and the first variable optical attenuator and the first pump are both connected to the first wavelength division multiplexer; the first wavelength division multiplexer is connected to the first mode multiplexer and demultiplexer, the first mode multiplexer and demultiplexer is connected to the second port of the first circulator, and the third port is connected to the first few-mode erbium-doped fiber. The other end of the first few-mode erbium-doped fiber is connected to the first port of the second circulator, and the third port is connected to the first port of the first circulator to form a loop. The second port of the second circulator is connected to the second few-mode erbium-doped fiber and then connected to the second mode multiplexer and demultiplexer; the second tunable laser is connected to the second variable optical attenuator, and the second variable optical attenuator and the second pump are both connected to the second wavelength division multiplexer; the second wavelength division multiplexer is connected to the second mode multiplexer and demultiplexer.

[0036] In a specific embodiment, the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber each include a core and a cladding; the core is wrapped in the cladding to form an optical fiber with a cylindrical structure; and erbium ions are doped inside the core.

[0037] In a specific embodiment, the core refractive index of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber is the same, and both are preset values.

[0038] Specifically, the core refractive index of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber is the same, which is 1.451.

[0039] In a specific embodiment, the cladding refractive index of the first few-mode erbium-doped optical fiber and the cladding refractive index of the second few-mode erbium-doped optical fiber are the same, and are both preset values.

[0040] Specifically, the cladding refractive index of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber are the same, both being 1.444.

[0041] In a specific embodiment, the core radius of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber are the same, and are both preset values.

[0042] Specifically, the core radius of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber is the same, which is 9.5 μm.

[0043] In a specific embodiment, the cladding radius of the first few-mode erbium-doped optical fiber and the cladding radius of the second few-mode erbium-doped optical fiber are the same, and are both preset values.

[0044] Specifically, the cladding radius of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber is the same, which is 62.5 μm.

[0045] In a specific embodiment, the core erbium ion doping concentrations of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber are the same, and are both preset values.

[0046] Specifically, the first few-mode erbium-doped fiber and the second few-mode erbium-doped fiber have the same erbium ion doping concentration in their cores, both of which are 1.09×10 25 m -3 .

[0047] In a specific embodiment, the first few-mode erbium-doped fiber and the second few-mode erbium-doped fiber have different lengths.

[0048] Specifically, the lengths of the first few-mode erbium-doped fiber and the second few-mode erbium-doped fiber are different. The length of the first few-mode erbium-doped fiber is 1.56 m, and the length of the second few-mode erbium-doped fiber is 3.94 m.

[0049] In a specific embodiment, the forward pump wavelength of the first pump source in forward transmission is 1480 nm, the pump mode is LP01, and the pump power is 193 mW; the reverse pump wavelength of the second pump source in reverse transmission is 1480 nm, the pump mode is LP11, and the pump power is 171 mW.

[0050] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0051] A few-mode erbium-doped fiber amplification system based on mode routing selection is provided, which selects low-gain mode signals and performs cascade amplification through two sections of few-mode erbium-doped fiber to further improve the gain of the mode signal, while the high-gain mode signal is amplified only through one section of few-mode erbium-doped fiber, thereby effectively reducing the mode gain difference and achieving a good mode gain equalization effect; the few-mode erbium-doped fiber is a single-core ordinary step-type structure, thereby reducing the difficulty of preparation. The present invention further reduces the gain difference of each mode and achieves mode gain equalization by selecting a method of performing cascade amplification on a mode signal with lower gain and performing single-stage amplification on a mode signal with higher gain. At the same time, there is no need to prepare specially designed optical fibers, and the number of modes supported can match the currently commercial few-mode transmission optical fibers, so that it can be truly applied to few-mode transmission systems, which is crucial to the development of space-division multiplexing transmission systems.

[0052] Example 2

[0053] An application of a few-mode erbium-doped fiber amplifier system based on mode routing selection, specifically:

[0054] When a mode signal with weak gain capability is selected for forward transmission, the signal is attenuated by the first variable optical attenuator after passing through the first tunable laser. The attenuated signal and the forward pump generated by the first pump source are then coupled into the first mode multiplexing and demultiplexing device using the first wavelength division multiplexer for mode multiplexing. The mode-multiplexed signal is then cascade-amplified through the first few-mode erbium-doped fiber and the second few-mode erbium-doped fiber in sequence. Finally, the signal is demultiplexed and output through the second mode multiplexing and demultiplexing device.

[0055] When a mode signal with a strong gain capability is selected for reverse transmission, the signal is attenuated by the second variable optical attenuator after passing through the second tunable laser. Subsequently, the second wavelength division multiplexer couples the attenuated signal and the reverse pump generated by the second pump source into the second mode multiplexing and demultiplexing device for mode multiplexing processing. The mode-multiplexed signal is then amplified only through the second few-mode erbium-doped fiber. Finally, the signal is demultiplexed and output through the first mode multiplexing and demultiplexing device.

[0056] Example 3

[0057] Please refer to Figure 2 as well as Figure 3 This embodiment verifies and analyzes the method, more specifically:

[0058] Please refer to Figure 3 , Figure 3 The present invention provides a few-mode erbium-doped fiber amplifier system based on mode routing. After optimizing parameters, the resulting C-band gain spectrum shows an average gain of 22.79 dB in the C-band, and an inter-mode gain difference of less than 0.86 dB in the 1540-1565 nm range. These results demonstrate that this few-mode erbium-doped fiber amplifier device can achieve high gain amplitude and gain balance while maintaining a simple erbium fiber structure.

[0059] Obviously, the above embodiments of the present invention are merely examples for the purpose of clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the claims of the present invention.

Claims

1. A few-mode erbium-doped fiber amplifier system based on mode routing selection, characterized in that: The optical fiber comprises a first few-mode erbium-doped optical fiber, a second few-mode erbium-doped optical fiber, a first mode multiplexer and demultiplexer, a second mode multiplexer and demultiplexer, a first circulator, a second circulator, a first tunable laser for forward transmission, a first variable optical attenuator, a first wavelength division multiplexer and a first pump source, and a second tunable laser for reverse transmission, a second variable optical attenuator, a second wavelength division multiplexer and a second pump source. The first tunable laser is connected to the first variable optical attenuator, and the first variable optical attenuator and the first pump are both connected to the first wavelength division multiplexer; the first wavelength division multiplexer is connected to the first mode multiplexer and demultiplexer, the first mode multiplexer and demultiplexer is connected to the second port of the first circulator, and the third port is connected to the first few-mode erbium-doped fiber. The other end of the first few-mode erbium-doped fiber is connected to the first port of the second circulator, and the third port is connected to the first port of the first circulator to form a loop. The second port of the second circulator is connected to the second few-mode erbium-doped fiber and then connected to the second mode multiplexer and demultiplexer; the second tunable laser is connected to the second variable optical attenuator, and the second variable optical attenuator and the second pump are both connected to the second wavelength division multiplexer; the second wavelength division multiplexer is connected to the second mode multiplexer and demultiplexer; The low-gain mode signal is selected and cascade-amplified through two sections of few-mode erbium-doped fiber to further improve the gain of the mode signal, while the high-gain mode signal is amplified only through the second few-mode erbium-doped fiber.

2. The few-mode erbium-doped fiber amplifier system based on mode routing selection according to claim 1, characterized in that: The first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber both include a core and a cladding; the core is wrapped in the cladding to form an optical fiber with a cylindrical structure; and erbium ions are doped inside the core.

3. The few-mode erbium-doped fiber amplifier system based on mode routing selection according to claim 2, characterized in that: The core refractive index of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber is the same, and both are preset values.

4. The few-mode erbium-doped fiber amplifier system based on mode routing selection according to claim 2, characterized in that: The cladding refractive index of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber are the same, and are both preset values.

5. The few-mode erbium-doped fiber amplifier system based on mode routing selection according to claim 2, characterized in that: The core radius of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber are the same, and are both preset values.

6. The few-mode erbium-doped fiber amplifier system based on mode routing selection according to claim 2, characterized in that: The cladding radius of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber are the same, and are both preset values.

7. The few-mode erbium-doped fiber amplifier system based on mode routing selection according to claim 2, characterized in that: The core erbium ion doping concentrations of the first few-mode erbium-doped optical fiber and the second few-mode erbium-doped optical fiber are the same, and are both preset values.

8. The few-mode erbium-doped fiber amplifier system based on mode routing selection according to claim 1, characterized in that: The first few-mode erbium-doped fiber and the second few-mode erbium-doped fiber have different lengths.

9. The few-mode erbium-doped fiber amplifier system based on mode routing selection according to claim 1, characterized in that: The forward pump wavelength of the first pump source in forward transmission is 1480 nm, the pump mode is LP01, and the pump power is 193 mW; the reverse pump wavelength of the second pump source in reverse transmission is 1480 nm, the pump mode is LP11, and the pump power is 171 mW.

10. An application based on the system according to claim 1, characterized in that: Specifically: When a mode signal with weak gain capability is selected for forward transmission, the signal is attenuated by the first variable optical attenuator after passing through the first tunable laser. The attenuated signal and the forward pump generated by the first pump source are then coupled into the first mode multiplexing and demultiplexing device using the first wavelength division multiplexer for mode multiplexing. The mode-multiplexed signal is then cascade-amplified through the first few-mode erbium-doped fiber and the second few-mode erbium-doped fiber in sequence. Finally, the signal is demultiplexed and output through the second mode multiplexing and demultiplexing device. When a mode signal with a strong gain capability is selected for reverse transmission, the signal is attenuated by the second variable optical attenuator after passing through the second tunable laser. Subsequently, the second wavelength division multiplexer couples the attenuated signal and the reverse pump generated by the second pump source into the second mode multiplexing and demultiplexing device for mode multiplexing processing. The mode-multiplexed signal is then amplified only through the second few-mode erbium-doped fiber. Finally, the signal is demultiplexed and output through the first mode multiplexing and demultiplexing device.

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