L-band few-mode erbium-doped optical fiber amplifier

Abstract

The utility model discloses an L-band few-mode erbium-doped optical fiber amplifier, which comprises a mode multiplexing device and a demultiplexing device, wherein the mode multiplexing device is connected with a first pumping coupler through a first few-mode isolator; the first pumping coupler is used for simultaneously coupling a signal from the mode multiplexing device and pumping light from the first pumping source into the few-mode erbium-doped optical fiber for transmission, the output end of the few-mode erbium-doped optical fiber is connected with the second pumping coupler, and the second pumping coupler is respectively connected with the second few-mode isolator and the second pumping source; according to the amplifier, mode gain balance can be achieved in the L wave band, the capacity of an optical fiber communication system can be further improved by combining wavelength division multiplexing and mode division multiplexing, and the amplifier is suitable for transmission of a long-distance space division multiplexing optical fiber communication system.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical fiber communication, and particularly relates to a kind of for L waveband amplifier. BACKGROUND

[0002] DMG: differential modal gain, which represents the maximum value and minimum value of mode gain in a few-mode erbium-doped optical fiber, and the calculation formula is: m is the number of modes existing in the optical fiber.

[0003] With the emergence of low-loss optical fiber, erbium-doped fiber amplifier, the application of wavelength division multiplexing technology and digital correlation technology, the communication capacity of a single optical fiber has approached the theoretical limit. In order to solve the problem of "capacity crunch", the use of space division multiplexing to expand the capacity of optical fiber communication system is proposed. Space division multiplexing includes optical fiber bundle composed of single-mode optical fiber, multi-core optical fiber and few-mode optical fiber. Among them, few-mode optical fiber has become the focus of research because of its relatively lower preparation process requirement and higher spatial density. Mode division multiplexing technology based on few-mode optical fiber is a technology that uses several independent orthogonal modes in few-mode optical fiber as independent channels to multiply the capacity of optical fiber communication. Compared with single-mode optical fiber, the mode field area of few-mode optical fiber is larger, so the tolerance of nonlinear effects will also be improved. Therefore, both the transmission capacity and the nonlinear Shannon limit of a single channel can be improved by relying on mode parallel transmission, thereby greatly improving the communication capacity of the optical fiber communication system.

[0004] To achieve long-distance transmission of few-mode fiber, a few-mode erbium-doped fiber amplifier (DMG) for repeater amplification was proposed. Since it needs to amplify multiple signal modes simultaneously, and each signal mode has a different mode field distribution, a new indicator, differential mode gain (DMG), was proposed in addition to traditional erbium-doped fiber indicators such as gain, noise figure, and bandwidth. DMG represents the difference between the maximum and minimum mode gain. During transmission, a large DMG can lead to signal degradation and distortion of the communication eye diagram, so it is necessary to balance the gains of different modes. Since the differential mode gain is determined by the overlap factor between the pump mode field, the signal mode field, and the rare-earth ion doping distribution, three methods were proposed to control DMG: (1) designing a special refractive index profile to change the mode field distribution of the signal mode; (2) adjusting the distribution of rare-earth doped ions in the fiber core; and (3) changing the pump mode method and structure. In most cases, DMG is minimized by combining the above three techniques. Existing research on few-mode erbium-doped fiber amplifiers mainly focuses on the C-band, while fully utilizing the L-band bandwidth resources is considered one of the most effective ways to improve the capacity of wavelength division multiplexing (WDM) systems. Therefore, research on L-band few-mode erbium-doped fiber amplifiers is essential. However, since the amplification of L-band few-mode erbium-doped fibers mainly relies on amplified spontaneous emission generated by the longer fiber, and this amplified spontaneous emission light cannot maintain its original mode during transmission, changing the pump mode content has a relatively small impact on L-band amplification. Therefore, this paper mainly considers changing the fiber refractive index profile and erbium ion doping distribution to achieve mode gain equalization.

[0005] World Patent WO2022122016AL proposes an optical amplification setup and a mode division multiplexing system including an optical amplification device. The mode transmitted in the middle of the first amplifier is amplified, and then the two modes transmitted in the optical fiber are switched by a mode switcher. Then, the switched signal mode is amplified by a second amplifier. Mode gain equalization is achieved through a method similar to gain compensation.

[0006] US Patent US20140063592AL proposes a few-mode erbium-doped fiber amplifier for mode division multiplexing system transmission. Since it uses a spatial optical structure, the high-order pump mode has problems such as mode impurity and large insertion loss when converted. Therefore, it proposes to use ring-doped erbium ions to achieve gain equalization of the two signal modes.

[0007] Chinese patent CN208589638U proposes an EDFA that supports six-line polarization mode signal amplification. It designs two types of erbium-doped optical fibers with erbium ion doping distributions. The erbium ions in these two erbium-doped fibers are biased towards the outer and inner sides of the fiber core, respectively. By cascading the two types of optical fibers, the six-line polarization mode signal light undergoes differential amplification and compensatory amplification in sequence, thereby achieving mode gain equalization.

[0008] Chinese patent CN112180499A proposes a three-layer core multi-layer erbium-doped four-mode optical fiber. The three-layer core trench-assisted refractive index distribution makes the optical fiber have a higher mode refractive index difference, can weaken the intermodal crosstalk, and reduce the bending loss in application. Moreover, the erbium ions are layered doped, and the mode gain balance is realized by optimizing the doping area and concentration ratio.

[0009] In summary, the conventional few-mode erbium-doped optical fiber and the reported invention patent cannot well achieve the requirements of the optical fiber on the working waveband and the differential mode gain. Therefore, the present application proposes a few-mode erbium-doped optical fiber amplifier for L-band amplification, which is used to solve the problem of narrow working waveband and high differential mode gain of the few-mode erbium-doped optical fiber amplifier in a mode division multiplexing system. SUMMARY

[0010] The utility model aims at overcoming the insufficient of above-mentioned background art, proposes an amplifier suitable for L waveband mode division multiplexing system relay amplification.

[0011] The technical scheme of the utility model is as follows:

[0012] An L waveband few-mode erbium-doped optical fiber amplifier, comprising a mode multiplexing device and a demultiplexing device, characterized in that: the mode multiplexing device is connected with a first pump coupling device through a first few-mode isolator, the first pump coupling device simultaneously couples the signal from the mode multiplexing device and the pump light from a first pump source into the few-mode erbium-doped optical fiber for transmission, the output end of the few-mode erbium-doped optical fiber is connected with a second pump coupling device, the second pump coupling device is respectively connected with a second few-mode isolator and a second pump source, and the second few-mode isolator is connected with the demultiplexing device. Both the mode multiplexing device and the demultiplexing device are mode selection photonic lanterns.

[0013] The few-mode erbium-doped optical fiber sequentially comprises a first core layer, a second core layer, a trench layer, an inner cladding layer and an outer cladding layer from inside to outside, and the first core layer and the second core layer are doped with erbium ions, the radius r1 of the first core layer and the radius r2 of the second core layer satisfy the functional relationship: , the value range of k1 is 0.012 to 0.038, and the value range of A is 0.138 to 2.018; the refractive index n1 of the first core layer is greater than the refractive index n2 of the second core layer, the refractive index n4 of the inner cladding layer and the refractive index n3 of the trench layer.

[0014] The refractive index difference between the refractive index n1 of the first core layer and the refractive index n4 of the inner cladding layer is 0.0034 to 0.0069, the refractive index difference between the refractive index n2 of the second core layer and the refractive index n4 of the inner cladding layer is 0.0014 to 0.0034, and the refractive index difference between the refractive index n3 of the trench layer and the refractive index n4 of the inner cladding layer is -0.0040 to -0.0020; in the wavelength range of 1565nm to 1615nm, there are four signal modes in the optical fiber.

[0015] The trench width is in the range of 2.5-10.0 μm, the second core layer radius r2 is in the range of 8.0-12.0 μm, and the inner cladding layer radius r4 is in the range of 50.0-100.0 μm.

[0016] The inner cladding layer radius r4 is 62.5 μm, and the trench width is 4.0 μm.

[0017] The few-mode erbium-doped fiber is doped with two layers of erbium ions, and the doping mode can be divided into two types: one is to dope the erbium ions according to the first core layer and the second core layer radius; and the other is to select two regions with similar mode field distributions according to the overall mode field distribution of the modes existing in the fiber to dope the erbium ions.

[0018] The cross section of the inner cladding layer is rectangular, D-shaped, plum blossom-shaped, hexagonal or octagonal.

[0019] In the 1565-1615 nm wavelength band, the few-mode erbium-doped fiber supports four signal modes, i.e. 01 , LP 11 , LP 02 , and LP 21 , and the minimum effective refractive index difference between the modes is greater than 1.0*10 -4 .

[0020] The L-band few-mode erbium-doped fiber amplifier is composed of a mode selection photonic lantern 1, a pump coupler 2, a pump source 3 (Pump LD), a few-mode isolator 4 (FM-ISO) and a few-mode erbium-doped fiber 5 (FM-EDF). The mode selection photonic lantern 1 serves as a mode multiplexer (4M-MUX) and a mode demultiplexer (4M-DeMUX), which converts the fundamental mode signal from the signal source into the required high-order signal mode at the signal input end, and converts the high-order signal mode into the fundamental mode signal at the signal output end. The pump coupler simultaneously couples the signal mode from the mode selection photonic lantern and the pump light from the pump source into the few-mode erbium-doped fiber for transmission, the pump source is used to generate a high-power pump signal, the few-mode erbium-doped fiber serves as a gain medium to amplify the signal mode in the fiber, and the few-mode isolator is used to prevent self-oscillation caused by radiation at the end face of the fiber.

[0021] The pump mode of the pump source is double-stage pumping, in which the forward direction adopts core pumping, and the backward direction adopts cladding pumping.

[0022] Compared with the prior art, the advantages of the present application are as follows:

[0023] (1) The amplifier can realize mode gain equalization in L band, and through combining wavelength division multiplexing with mode division multiplexing, the capacity of the optical fiber communication system can be further improved, and the transmission of the long-distance space division multiplexing optical fiber communication system is suitable.

[0024] (2) The double core layer structure is adopted, and the refractive index of the first core layer is higher than that of the second core layer, so that the minimum effective refractive index difference between modes is increased, and the crosstalk between modes is reduced.

[0025] (3) The auxiliary groove structure is used, compared with the optical fiber without the groove, the signal mode of the optical fiber has a similar overlap factor, mode gain equalization can be realized under cladding pumping, and the auxiliary groove structure can improve the bending resistance of the optical fiber, which is more conducive to practical application.

[0026] (4) In the few-mode optical fiber, the erbium ions are doped according to the intensity distribution of the mode, not only the mode gain equalization problem of a single wavelength is considered, but also the gain characteristics of the entire working wavelength band are considered, and the mode gain equalization in L band is realized.

[0027] (5) The refractive index profile and doping distribution designed have low preparation difficulty and good robustness, are suitable for the mode amplification demand of the space division multiplexing optical fiber communication system, and have good application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 is a structure schematic diagram of the few-mode erbium-doped optical fiber amplifier;

[0029] In the figure, 1 is a mode selection photonic lantern, 2 is a pump coupler, 3 is a pump source (Pump laser diodes, Pump LD), 4 is a few-mode isolator (Few mode isolator, FM-ISO), and 5 is a few-mode erbium-doped fiber (Few-mode erbium-doped fiber, FM-EDF). The mode selection photonic lantern 1 serves as a mode multiplexer (4 Mode multiplexer, 4M-MUX) and a demultiplexer device (4 Mode demultiplexer, 4M-DeMUX).

[0030] Figure 2 is a refractive index profile structure schematic diagram of the optical fiber of the application;

[0031] In the drawings, the components are not necessarily drawn to scale, and the components having similar related properties or characteristics can have the same or similar reference numerals. DETAILED DESCRIPTION

[0032] The application will be described in detail below with reference to the accompanying drawings and specific embodiments, the aspects described below with reference to the accompanying drawings and specific embodiments are only exemplary and should not be understood as limiting the protection scope of the application in any way.

[0033] Reference is made to the accompanying drawings and specific embodiments Figure 1 and Figure 2 The application proposes a few-mode erbium-doped fiber amplifier for L-band amplification, which is composed of a mode-selective photonic lantern, a pump coupler, a pump source and a few-mode erbium-doped fiber. The mode-selective photonic lantern serves as a 4 Mode multiplexer (4M-MUX) and a 4 Mode demultiplexer (4M-DeMUX), which converts the fundamental mode signal from the signal source into the required high-order signal mode at the signal input end, and converts the high-order signal mode into the fundamental mode signal at the signal output end. The pump coupler simultaneously couples the signal mode from the mode-selective photonic lantern and the pump light from the pump source into the few-mode erbium-doped fiber for transmission. The pump source is used to generate a high-power pump signal. The few-mode erbium-doped fiber serves as a gain medium to amplify the signal mode in the fiber. The few-mode isolator is used to prevent self-oscillation caused by radiation at the fiber end face.

[0034] The few-mode erbium-doped fiber is sequentially composed of a first core layer (r1), a second core layer (r2), a trench (r3), an inner cladding layer (r4) and an outer cladding layer from inside to outside. The inner cladding layer has a hexagonal structure in cross section. The radius r1 of the first core layer and the radius r2 of the second core layer satisfy the function relationship, , the value range of k1 is 0.012 to 0.038, the value range of A is 0.138 to 2.018, the value range of the second core layer radius r2 is 8.0 μm to 12.0 μm, and the first core layer and the second core layer are doped with erbium ions only. The value range of the trench width is 2.5 μm to 10.0 μm, and the trench width is preferably 4.0 μm. The value range of the cladding radius r4 is 50.0 μm to 100.0 μm, and the cladding radius r4 is preferably 62.5 μm. The refractive index difference between the refractive index n1 of the first core layer and the refractive index n4 of the inner cladding layer is 0.0034 to 0.0069, and the refractive index difference between the refractive index n2 of the second core layer and the refractive index n4 of the inner cladding layer is 0.0014 to 0.0034. The refractive index difference between the refractive index n3 of the trench layer and the refractive index n4 of the inner cladding layer is -0.0040 to -0.0020. In the wavelength range of 1565 nm to 1615 nm, there are four modes of LP 01 , LP 11 , LP 02 , LP 21 in the fiber. And the average gain of the modes is higher than 25 dB, and the differential modal gain between the modes is 0.5 dB to 2.0 dB.

[0035] Implementation Example 1:

[0036] The few-mode erbium-doped fiber consists of, from the inside out, a first core layer (r1), a second core layer (r2), a trench (r3), an inner cladding (r4), and an outer cladding. The inner cladding has a hexagonal cross-section. Erbium ions are uniformly doped into the first and second core layers at a doping concentration of 1 × 10⁻⁶. 25 m -3 The pumping method is forward cladding pumping, in which the pump light is uniformly distributed across the fiber cross-section. Each signal mode is injected individually into the mode selection photonic lantern 1 for transmission. In this case, the few-mode erbium-doped fiber amplifies only a single signal mode. Preferably, the radius r1 of the first core layer is 5.0 μm, the radius r2 of the second core layer is 10.0 μm, the radius r3 of the trench is 14.0 μm, and the radius r4 of the cladding is 62.5 μm. The refractive index difference between the first core layer n1 and the inner cladding n4 is 0.0069, the refractive index difference between the second core layer n2 and the inner cladding n4 is 0.0034, and the refractive index difference between the trench layer n3 and the inner cladding n4 is -0.0027. Within the wavelength range of 1565 nm to 1615 nm, LP... 01 LP 11 LP 02 LP 21 Four modes can coexist stably in this fiber, and the minimum effective refractive index difference between modes is greater than 1.0 × 10⁻⁶. -4 This effectively reduces crosstalk between modes. Furthermore, the gain of each mode is higher than 30dB, and the differential mode gain between modes is less than 2.0dB.

[0037] This is a few-mode erbium-doped fiber amplifier for L-band amplification. It consists of modules including a mode-selection photonic lantern 1, a pump coupler 2, a pump source 3, and few-mode erbium-doped fiber. The mode-selection photonic lantern 1 acts as a mode multiplexing and demultiplexing device. The pump coupler 2 simultaneously couples the signal mode and pump light into the few-mode erbium-doped fiber for transmission. The pump source 3 generates a high-power pump signal, and the few-mode erbium-doped fiber serves as the gain medium to amplify the signal mode within the fiber.

[0038] Implementation Example 2:

[0039] The application provides a few-mode erbium-doped fiber amplifier for L-band amplification, which is composed of a mode selection photonic lantern 1, a pump coupler 2, a pump source 3 and a few-mode erbium-doped fiber 5 and the like. The mode selection photonic lantern 1 serves as a mode multiplexing and demultiplexing device, the pump coupler 2 simultaneously couples signal modes and pump light into the few-mode erbium-doped fiber for transmission, the pump source 3 is used for generating high-power pump signals, and the few-mode erbium-doped fiber serves as a gain medium to amplify signal modes in the fiber. The few-mode erbium-doped fiber is sequentially provided with a first core layer (r1), a second core layer (r2), a trench (r3), an inner cladding layer (r4) and an outer cladding layer from inside to outside, the inner cladding layer has a hexagonal structure in cross section, the first core layer and the second core layer are doped with erbium ions in layers, the centers of the two layers of doping are respectively at 3.0 μm and 8.0 μm, and the doping concentration ratio is 1:1.66. The pump mode is forward cladding pumping, at this time, the pump light is uniformly distributed in the cross section of the fiber, the signal mode is injected into the mode selection photonic lantern 1 for transmission, at this time, the few-mode erbium-doped fiber only amplifies a single signal mode. Preferably, the radius r1 of the first core layer is 2.0 μm, the radius r2 of the second core layer is 10.0 μm, the radius r3 of the trench is 14.0 μm, and the radius r4 of the cladding layer is 62.5 μm; the refractive index difference between the refractive index n1 of the first core layer and the refractive index n4 of the inner cladding layer is 0.0069, the refractive index difference between the refractive index n2 of the second core layer and the refractive index n4 of the inner cladding layer is 0.0034, and the refractive index difference between the refractive index n3 of the trench layer and the refractive index n4 of the inner cladding layer is -0.0027; in the wavelength range of 1565 nm to 1615 nm, the four modes LP 01 , LP 11 , LP 02 , LP 21 Four modes can stably exist in the fiber, the minimum effective refractive index difference between the modes is greater than 1.0×10 -4 , which can effectively reduce the crosstalk between the modes. And the gain of each mode is higher than 25 dB, and the differential mode gain between the modes is less than 0.5 dB.

[0040] Embodiment three:

[0041] The application provides a few-mode erbium-doped fiber amplifier for L-band amplification, which is composed of a mode-selecting photonic lantern 1, a pump coupler 2, a pump source 3 and a few-mode erbium-doped fiber 5. The mode-selecting photonic lantern 1 serves as a mode multiplexing and demultiplexing device, the pump coupler 2 simultaneously couples signal modes and pump light into the few-mode erbium-doped fiber for transmission, the pump source 3 is used to generate high-power pump signals, and the few-mode erbium-doped fiber 5 serves as a gain medium to amplify signal modes in the fiber. The few-mode erbium-doped fiber 5 is sequentially composed of a first core layer (r1), a second core layer (r2), a trench (r3) and a cladding (r4) from inside to outside, and the first core layer and the second core layer are doped with erbium ions in layers, the doping concentration regions are 0.0 μm to 2.0 μm and 4.6 μm to 8.0 μm, and the concentrations of the two-layer doping concentrations are the same. The pump mode is forward core pumping, and the pump mode is 980 nm or 1480 nm fundamental mode pumping. All signal modes are injected into the mode-selecting photonic lantern 1 for transmission, at this time the few-mode erbium-doped fiber 5 simultaneously amplifies all signal modes, and mode competition exists in the few-mode erbium-doped fiber 5. Preferably, the radius r1 of the first core layer is 5.0 μm, the radius r2 of the second core layer is 8.0 μm, the radius r3 of the trench is 12.0 μm, and the radius r4 of the cladding is 62.5 μm; the refractive index n1 of the first core layer is equal to the refractive index n2 of the second core layer, the refractive index difference between the refractive index n1 of the first core layer and the refractive index n4 of the inner cladding is 0.0055, the refractive index difference between the refractive index n3 of the trench layer and the refractive index n4 of the inner cladding is -0.0027, and in the wavelength range of 1565 nm to 1615 nm, the four modes LP 01 , LP 11 , LP 02 , LP 21 Four modes can stably exist in the fiber, the minimum effective refractive index difference between the modes is greater than 1.0×10 -4 , which can effectively reduce the crosstalk between the modes. And the gain of each mode is higher than 25 dB, and the differential mode gain between the modes is less than 0.8 dB.

[0042] Embodiment four:

[0043] The application provides a few-mode erbium-doped fiber amplifier for L-band amplification, which is composed of a mode selection photonic lantern 1, a pump coupler 2, a pump source 3 and a few-mode erbium-doped fiber 5. The mode selection photonic lantern 1 serves as a mode multiplexing and demultiplexing device, the pump coupler 2 simultaneously couples signal modes and pump light into the few-mode erbium-doped fiber for transmission, the pump source 3 is used for generating high-power pump signals, and the few-mode erbium-doped fiber 5 serves as a gain medium to amplify the signal modes in the fiber. The few-mode erbium-doped fiber 5 comprises, from inside to outside, a first core layer (r1), a second core layer (r2), a trench (r3) and a cladding (r4), and the first core layer and the second core layer are doped with erbium ions in layers, the doping concentration regions are 0.0-2.0 μm and 4.6-8.0 μm, and the concentrations of the two-layer doping concentrations are the same. The pump mode is double-stage pumping, the front direction adopts core pumping, and the back direction adopts cladding pumping, so that the pump efficiency is improved, and higher mode gain is achieved. The signal modes are injected into the mode selection photonic lantern 1 for transmission, at this time, the few-mode erbium-doped fiber 5 simultaneously amplifies all the signal modes, and mode competition exists in the few-mode erbium-doped fiber 5. Preferably, the radius r1 of the first core layer is 5.0 μm, the radius r2 of the second core layer is 8.0 μm, the radius r3 of the trench is 12.0 μm, and the radius r4 of the cladding is 62.5 μm; the refractive index n1 of the first core layer is equal to the refractive index n2 of the second core layer, the refractive index difference between the refractive index n1 of the first core layer and the refractive index n4 of the inner cladding is 0.0055, the refractive index difference between the refractive index n3 of the trench layer and the refractive index n4 of the inner cladding is -0.0027, and in the wavelength range of 1565-1615 nm, the four modes LP 01 , LP 11 , LP 02 , LP 21 , LP -4 Four modes can stably exist in the fiber, the minimum effective refractive index difference between the modes is greater than 1.0*10 -4 , the crosstalk between the modes can be effectively reduced. And the gain of each mode is higher than 30 dB, and the differential mode gain between the modes is less than 1.0 dB.

[0044] The various embodiments in the specification are described in a progressive manner, and each embodiment focuses on the difference from other embodiments, and the same or similar parts of each embodiment can be referred to each other.

[0045] The prior description of this disclosure is provided to enable any person skilled in the art to make or use it. Various modifications to this disclosure will be apparent to those skilled in the art, and the general principles defined herein can be applied to other variations without departing from the spirit or scope of this disclosure. Therefore, this disclosure is not intended to be limited to the examples and designs described herein, but should be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0046] The above description is merely a preferred example of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application shall be included within the scope of protection of this application.

Claims

1. An L-band few-mode erbium-doped fiber amplifier, comprising a mode multiplexing device and a demultiplexing device, characterized in that: The mode multiplexing device is connected to the first pump coupler via a first few-mode isolator. The first pump coupler simultaneously couples the signal from the mode multiplexing device and the pump light from the first pump source into the few-mode erbium-doped fiber for transmission. The output end of the few-mode erbium-doped fiber is connected to a second pump coupler, which is connected to both the second few-mode isolator and the second pump source. The second few-mode isolator is connected to the demultiplexing device. The few-mode erbium-doped fiber consists of a first core layer, a second core layer, a trench layer, an inner cladding layer, and an outer cladding layer, from the inside out. Erbium ions are doped in the first and second core layers. The radii r1 of the first core layer and r2 of the second core layer satisfy the following functional relationship: The value of k1 ranges from 0.012 to 0.038, and the value of A ranges from 0.138 to 2.018; the refractive index of the first core layer n1 > the refractive index of the second core layer n2 > the refractive index of the inner cladding layer n4 > the refractive index of the trench layer n3.

2. The L-band few-mode erbium-doped fiber amplifier according to claim 1, characterized in that: Both the mode multiplexing device and the demultiplexing device are mode-selective photonic lanterns.

3. The L-band few-mode erbium-doped fiber amplifier according to claim 1, characterized in that: The refractive index difference between the first core layer (n1) and the inner cladding layer (n4) is between 0.0034 and 0.0069; the refractive index difference between the second core layer (n2) and the inner cladding layer (n4) is between 0.0014 and 0.0034; and the refractive index difference between the trench layer (n3) and the inner cladding layer (n4) is between -0.0040 and -0.0020. Within the wavelength range of 1565 nm to 1615 nm, four signal modes exist in the optical fiber.

4. The L-band few-mode erbium-doped fiber amplifier according to claim 1, characterized in that: The trench width ranges from 2.5 μm to 10.0 μm, the second core radius r2 ranges from 8.0 μm to 12.0 μm, and the inner cladding radius r4 ranges from 50.0 μm to 100.0 μm.

5. The L-band few-mode erbium-doped fiber amplifier according to claim 4, characterized in that: The inner cladding radius r4 is 62.5 μm, and the trench width is 4.0 μm.

6. The L-band few-mode erbium-doped fiber amplifier according to claim 1, characterized in that: Erbium-doped few-mode optical fibers are doped with erbium ions in two layers. There are two types of doping methods: one is to dope erbium ions according to the radii of the first core layer and the second core layer. Secondly, based on the overall mode field distribution of the modes present in the optical fiber, two regions with similar mode field distributions are selected for erbium ion doping in layers.

7. The L-band few-mode erbium-doped fiber amplifier according to claim 2, characterized in that: The cross-section of the inner cladding can be rectangular, D-shaped, plum blossom-shaped, hexagonal, or octagonal.

8. The L-band few-mode erbium-doped fiber amplifier according to claim 2, characterized in that: In the 1565nm to 1615nm wavelength band, few-mode erbium-doped fiber supports four signal modes, namely LP. 01 LP 11 LP 02 LP 21 The minimum effective refractive index difference between modes is greater than 1.0 × 10⁻⁶. -4 .

9. The L-band few-mode erbium-doped fiber amplifier according to any one of claims 1-8, characterized in that: The pump source uses a two-stage pumping method, with core pumping in the forward direction and cladding pumping in the backward direction.

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