A seven-core seven-mode mode division multiplexer based on mode coupling principle

Abstract

This invention belongs to the field of optical fiber communication technology, specifically relating to a seven-core, seven-mode mode divider multiplexer based on the mode coupling principle, comprising a substrate material, a central few-mode fiber core, and peripheral single-mode fiber cores; the central few-mode fiber core contains a high-refractive-index ring; the peripheral single-mode fiber cores include components for LP (Laser-Parse) coupling to the central few-mode fiber core. 11 The first single-mode fiber core for mode conversion is used for LP to the central few-mode fiber core. 31 The second single-mode fiber core for mode conversion is used for LP to the central few-mode fiber core. 21a The third single-mode fiber core for mode conversion is used for LP to the central few-mode fiber core. 02 The fourth single-mode fiber core for mode conversion is used for LP to the central few-mode fiber core. 21b The fifth single-mode fiber core for mode conversion and the LP for the central few-mode fiber core. 12 The sixth single-mode fiber core for mode switching. This invention features a large number of switching modes, high space utilization, low insertion loss, and high throughput.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber communication technology, specifically relating to a seven-core seven-mode mode divider multiplexer based on the mode coupling principle. Background Technology

[0002] With the rise of the internet age, bandwidth-intensive services such as mobile internet, e-commerce, video calls, and the Internet of Things are constantly increasing, putting enormous pressure on existing single-mode fiber optic communication systems. Traditional fiber optic communication systems are facing increasingly severe problems of bandwidth resource shortages and excessive network load. Against the backdrop of ever-increasing communication demands, developing technologies to improve fiber optic transmission capacity is particularly important, leading to the emergence of mode-division multiplexing (MDD) technology. The MMD multiplexer, as the core component of an MMD multiplexing system, has become a research focus in fiber optic communication systems. The concept of the MMD multiplexer originated in the early stages of optical communication technology exploration, but it experienced explosive growth as the transmission capacity of single-mode fiber approached the Shannon limit. Today, MMD multiplexers are gradually developing towards higher mode multiplexing efficiency, lower insertion loss, and lower crosstalk, laying the foundation for communication technology development over the next few decades.

[0003] The development of multi-core few-mode mode dividers (MDDs) stems from the integration of few-mode division multiplexing (FMD) and multi-core technologies. Early MMD technologies primarily focused on the multiplexing of a few modes within a single-core fiber. As the number of reusable modes in a single-core fiber reached saturation, researchers began exploring the combination of multi-core fibers and few-mode technology. From the late 20th to the early 21st century, with the gradual maturation of both technologies, researchers began exploring the integration of multi-core structures with few-mode multiplexing, leading to the emergence of multi-core few-mode MMDs.

[0004] In recent years, through continuous innovation and optimization of mode dividers (MDDs), multi-core few-mode MMDs have achieved certain results in terms of mode matching, crosstalk suppression, and loss control. Currently feasible mode conversion schemes include: mode converters based on photonic lanterns, mode converters based on long-period fiber gratings, and mode converters based on fused biconical tapered fibers.

[0005] Because these fiber-optic mode dividers (MDDs) achieve mode switching by disrupting the fiber waveguide structure (such as thermal tapering or cladding polishing), these solutions still suffer from high device design complexity, insufficient flexibility, high crosstalk, and significant manufacturing difficulty and cost. Furthermore, multi-core few-mode MMDs exhibit poor compatibility with existing communication systems, resulting in mode mismatch and high interface loss when interfacing with single-mode fibers or other types of multiplexers. These issues severely restrict the performance improvement and application expansion of multi-core few-mode MMDs. Therefore, developing high-performance, low-cost, and easily integrated multi-core few-mode MMDs has become a current research focus. Summary of the Invention

[0006] To overcome the problems existing in the prior art, this invention proposes a seven-core seven-mode mode division multiplexer based on the mode selective coupling principle. This device has advantages such as high mode coupling efficiency, low inter-mode crosstalk, high space utilization, and low insertion loss.

[0007] The technical solution of this invention to solve the above problems is: a seven-core, seven-mode mode-division multiplexer based on the principle of mode-selective coupling, which is special in that:

[0008] It includes a substrate material, a central few-mode fiber core located within the substrate material, and peripheral single-mode fiber cores; the central few-mode fiber core located within the substrate material includes a core substrate and a high-refractive-index ring; the peripheral single-mode fiber cores located within the substrate material are distributed around the central few-mode fiber, including: converting LP to the central few-mode fiber core. 11 The first single-mode fiber core of the mode, transitioning to the central few-mode fiber core LP 31 The second single-mode fiber core undergoes mode conversion, and the LP conversion to the center few-mode fiber core. 21a The third single-mode fiber core of the mode, the conversion to the central few-mode fiber core LP 02 The fourth single-mode fiber core of the mode, the conversion to a central few-mode fiber core LP 21b The fifth single-mode fiber core and the conversion to a center few-mode fiber core LP 12 The sixth single-mode fiber core of the pattern.

[0009] Furthermore, the angle between the center line connecting the first single-mode fiber core and the center few-mode fiber core and the center line connecting the second single-mode fiber core and the center few-mode fiber core is 30°; the angle between the center line connecting the second single-mode fiber core and the center few-mode fiber core and the center line connecting the third single-mode fiber core and the center few-mode fiber core is 60°; the angle between the center line connecting the third single-mode fiber core and the center few-mode fiber core and the center line connecting the fourth single-mode fiber core and the center few-mode fiber core is 90°; the angle between the center line connecting the fourth single-mode fiber core and the center few-mode fiber core and the center line connecting the fifth single-mode fiber core and the center few-mode fiber core is 45°; the angle between the center line connecting the fifth single-mode fiber core and the center few-mode fiber core and the center line connecting the sixth single-mode fiber core and the center few-mode fiber core is 45°; and the angle between the center line connecting the sixth single-mode fiber core and the center few-mode fiber core and the center line connecting the first single-mode fiber core and the center few-mode fiber core is 90°.

[0010] Furthermore, the distances between the center of the first single-mode fiber core, the second single-mode fiber core, the third single-mode fiber core, the fourth single-mode fiber core, the fifth single-mode fiber core, and the sixth single-mode fiber core and the center of the central few-mode fiber core are L1, L2, L3, L4, L5, and L6, respectively, where L3 = L5.

[0011] Furthermore, the above values ​​are as follows: L1 is 12.85 μm, L2 is 15.05 μm, L3 and L5 are both 13.9 μm, L4 is 14.1 μm, and L6 is 18.16 μm.

[0012] Furthermore, the diameter of the first single-mode fiber core is 5 μm, and the refractive index of the fiber core is 1.4624 at a wavelength of 1550 nm.

[0013] The diameter of the second single-mode fiber core is 4μm, and the refractive index of the fiber core is 1.4551 at a wavelength of 1550nm.

[0014] The diameter of the third and fifth single-mode fiber cores mentioned above is 6μm, and the refractive index of the fiber cores at a wavelength of 1550nm is 1.4563.

[0015] The diameter of the fourth single-mode fiber core is 5 μm, and the refractive index of the fiber core is 1.4557 at a wavelength of 1550 nm.

[0016] The diameter of the sixth single-mode fiber core is 3μm, and the refractive index of the fiber core is 1.45425 at a wavelength of 1550nm.

[0017] Furthermore, the diameter of the aforementioned central few-mode fiber core is 15 μm, and the substrate refractive index at a wavelength of 1550 nm is 1.456. The center of the high-refractive-index ring is the center of the optical fiber, the inner diameter of the high-refractive-index ring is 7 μm, the ring width is 1.5 μm, and the refractive index of the high-refractive-index ring at a wavelength of 1550 nm is 1.46.

[0018] Furthermore, the substrate material is silicon dioxide, with a refractive index of 1.44402 at a wavelength of 1550 nm; the mode multiplexer length is 4200 μm.

[0019] Advantages of this invention:

[0020] ① Based on the principle of mode-selective coupling, this invention proposes a mode division multiplexer that can simultaneously multiplex seven modes by combining multi-core optical technology and few-mode multiplexing technology. It has the advantages of high coupling efficiency, low insertion loss, and low inter-core crosstalk.

[0021] ② In the same fiber structure, LP can be achieved through an outer single-mode fiber core. 11 LP 31 LP 21a LP 21b LP 02 and LP 12 Simultaneous conversion of six high-order modes, and LP direct input to the center low-mode fiber core. 01 The system can reuse seven modes simultaneously, which, compared to traditional cascaded mode multiplexers, greatly improves the number of modes and space utilization.

[0022] ③ This invention effectively reduces LP by introducing a high-refractive-index ring structure into the central fiber core. 02 and LP21 The effective refractive index difference between modes can be distinguished to avoid serious crosstalk during mode switching, which is of great use for high-purity conversion of seven modes in a multiplexer.

[0023] ④ By reusing degenerate modules, this invention effectively avoids the use of higher-order modes to transmit information, thereby achieving the simultaneous reuse of seven modes using a smaller number of higher-order modes.

[0024] ⑤ The seven-mode modulus multiplexer designed in this invention operates at a wavelength of 1550nm. 01 LP 11 LP 21a LP 21b LP 02 LP 31 and LP 12 The mode conversion efficiencies were 99.35%, 96.44%, 93.96%, 94.83%, 90.03%, 83.58%, and 75.61%, respectively.

[0025] ⑥ The length of this device is 4200μm, and the optical fiber used is short, resulting in low manufacturing cost. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the cross-sectional structure and mode conversion of the modulus multiplexer of the present invention;

[0027] Figure 2 This is a schematic diagram showing the diameter and refractive index of the single-mode fiber core and the few-mode fiber core in the mode multiplexer of the present invention;

[0028] Figure 3 This is a schematic diagram showing the positional relationship and distance from the center of the single-mode fiber core and the few-mode fiber core in the modulus multiplexer of the present invention;

[0029] Figure 4 This is a schematic diagram illustrating the actual usage of the present invention.

[0030] Figure 5 This is a schematic diagram showing the change of extinction ratio of each mode as a function of coupling length at a wavelength of 1550nm for the mode division multiplexer of the present invention;

[0031] Figure 6 The mode divider multiplexer of this invention operates at a wavelength of 1550nm. 11 LP 21a LP 21b LP 02 LP 31 and LP 12 A schematic diagram showing how the coupling efficiency of the mode changes with the coupling length;

[0032] Figure 7This is a schematic diagram of the insertion loss of the mode divider multiplexer of the present invention at wavelengths of 1.50~1.60μm.

[0033] Among them: 1. Base material, 2. Core substrate, 3. High refractive index ring, 4. First single-mode core, 5. Second single-mode core, 6. Third single-mode core, 7. Fourth single-mode core, 8. Fifth single-mode core, 9. Sixth single-mode core. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings only represents selected embodiments of the present invention.

[0035] This invention provides a seven-core, seven-mode multiplexer based on the principle of mode-selective coupling, such as... Figure 1 As shown, it includes a base material 1, a central few-mode fiber core and an outer single-mode fiber core, with the central few-mode fiber core and the outer single-mode fiber core disposed inside the base material 1.

[0036] The central few-mode fiber core includes a core substrate 2 and a high-refractive-index ring 3. The core substrate 2 is the main material of the few-mode fiber core, and the high-refractive-index ring 3 is located inside the central few-mode fiber core substrate 2. The high-refractive-index ring 3 is used to increase the LP (low-mode flux) in the few-mode fiber core. 21 and LP 02 The effective refractive index difference of the mode.

[0037] The peripheral single-mode fiber core includes LPs that are converted to the central few-mode fiber core. 11 LP 31 LP 21a LP 02 LP 21b LP 12 The pattern consists of six single-mode fiber cores: the first single-mode fiber core 4, the second single-mode fiber core 5, the third single-mode fiber core 6, the fourth single-mode fiber core 7, the fifth single-mode fiber core 8, and the sixth single-mode fiber core 9. These six single-mode fiber cores are distributed outside the central few-mode fiber core.

[0038] In a preferred embodiment of the present invention, the third single-mode fiber core 6 and the fifth single-mode fiber core 8 have the same diameter and refractive index; the first single-mode fiber core 4 and the fourth single-mode fiber core 7 have the same diameter but different refractive indices.

[0039] As a preferred embodiment of the present invention, such as Figure 2 As shown, the diameter d1 of the first single-mode fiber core 4 is 5 μm, and the refractive index n1 of the fiber core at a wavelength of 1550 nm is 1.4624; the diameter d2 of the second single-mode fiber core 5 is 4 μm, and the refractive index n2 of the fiber core at a wavelength of 1550 nm is 1.4551; the diameters d3 and d5 of the third single-mode fiber core 6 and the fifth single-mode fiber core 8 are both 6 μm, and the refractive indices n3 and n5 of the fiber core at a wavelength of 1550 nm are both 1.4563; the diameter d4 of the fourth single-mode fiber core 7 is 5 μm, and the refractive index n4 of the fiber core at a wavelength of 1550 nm is 1.4557; the diameter d6 of the sixth single-mode fiber core 9 is 3 μm, and the refractive index n6 of the fiber core at a wavelength of 1550 nm is 1.45425. The diameter d0 of the central few-mode fiber core is 15μm, and the refractive index of the fiber core substrate 2 is 1.456 at a wavelength of 1550nm; the center of the high refractive index ring 3 is the center of the optical fiber, the inner diameter D of the high refractive index ring 3 is 7μm, the ring width a is 1.5μm, and the refractive index of the high refractive index ring 3 is 1.46 at a wavelength of 1550nm.

[0040] As a preferred embodiment of the present invention, such as Figure 3 As shown, the angle between the center line connecting the first single-mode fiber core 4 and the center few-mode fiber core is 30° with the center line connecting the second single-mode fiber core 5 and the center few-mode fiber core; the angle between the center line connecting the second single-mode fiber core 5 and the center few-mode fiber core is 60° with the center line connecting the third single-mode fiber core 6 and the center few-mode fiber core; the angle between the center line connecting the third single-mode fiber core 6 and the center few-mode fiber core is 90° with the center line connecting the fourth single-mode fiber core 7 and the center few-mode fiber core; the angle between the center line connecting the fourth single-mode fiber core 7 and the center few-mode fiber core is 45° with the center line connecting the fifth single-mode fiber core 8 and the center few-mode fiber core; the angle between the center line connecting the fifth single-mode fiber core 8 and the center few-mode fiber core is 45° with the center line connecting the sixth single-mode fiber core 9 and the center few-mode fiber core; and the angle between the center line connecting the sixth single-mode fiber core 9 and the center few-mode fiber core is 90° with the center line connecting the first single-mode fiber core 4 and the center few-mode fiber core.

[0041] like Figure 3 As shown, the distances from the center of the first single-mode fiber core 4, the second single-mode fiber core 5, the third single-mode fiber core 6, the fourth single-mode fiber core 7, the fifth single-mode fiber core 8, and the sixth single-mode fiber core 9 to the exact center of the central few-mode fiber core are L1, L2, L3, L4, L5, and L6, respectively, where L3 = L5; L1 is 12.85 μm, L2 is 15.05 μm, L3 and L5 are both 13.9 μm, L4 is 14.1 μm, and L6 is 18.16 μm.

[0042] In a preferred embodiment of the present invention, the substrate material 1 of the optical fiber is silicon dioxide, and its refractive index is 1.44402 at a wavelength of 1550 nm.

[0043] Preferably, such as Figure 4 As shown, the total length x of the module multiplexer is 4200μm.

[0044] The working performance of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0045] Example

[0046] A seven-core, seven-mode mode-division multiplexer based on the principle of mode-selective coupling, such as Figure 1 As shown, it includes a substrate material 1, a central few-mode fiber core, and peripheral single-mode fiber cores. The central few-mode fiber core includes a core substrate 2 and a high-refractive-index ring 3. The core substrate 2 is the main material of the few-mode fiber core, and the high-refractive-index ring 3 is located inside the core substrate 2. The high-refractive-index ring 3 is used to increase the LP (low-mode capacity) in the few-mode fiber core. 21 and LP 02 The effective refractive index difference between modes. The total length x of the mode multiplexer is 4200 μm.

[0047] refer to Figure 2 The diameters d1, d2, d3, d4, d5, and d6 of the first single-mode fiber core 4, the second single-mode fiber core 5, the third single-mode fiber core 6, the fourth single-mode fiber core 7, the fifth single-mode fiber core 8, and the sixth single-mode fiber core 9 are 5μm, 4μm, 6μm, 5μm, 6μm, and 3μm, respectively. At a wavelength of 1550nm, the core refractive indices n1, n2, n3, n4, n5, and n6 of the first single-mode fiber core 4, the second single-mode fiber core 5, the third single-mode fiber core 6, the fourth single-mode fiber core 7, the fifth single-mode fiber core 8, and the sixth single-mode fiber core 9 are 1.4624, 1.4551, 1.4563, 1.4557, 1.4563, and 1.45425, respectively, where d1=d4; d3=d5; and n3=n5. The diameter d0 of the central few-mode fiber core is 15 μm, and the refractive index n0 of the fiber core substrate 2 is 1.456 at a wavelength of 1550 nm. The center of the high refractive index ring 3 is the center of the optical fiber. The inner diameter D of the high refractive index ring 3 is 7 μm, the ring width a is 1.5 μm, and the refractive index n7 of the high refractive index ring 3 is 1.46 at a wavelength of 1550 nm.

[0048] The outer core is located in a hexagonal arrangement of six single-mode cores, with each core positioned at one of the vertices of the hexagon. Specifically, the angle between the center line connecting the first single-mode fiber core 4 and the center few-mode fiber core and the center line connecting the second single-mode fiber core 5 and the center few-mode fiber core is 30°; the angle between the center line connecting the second single-mode fiber core 5 and the center few-mode fiber core and the center line connecting the third single-mode fiber core 6 and the center few-mode fiber core is 60°; the angle between the center line connecting the third single-mode fiber core 6 and the center few-mode fiber core and the center line connecting the fourth single-mode fiber core 7 and the center few-mode fiber core is 90°; the angle between the center line connecting the fourth single-mode fiber core 7 and the center few-mode fiber core and the center line connecting the fifth single-mode fiber core 8 and the center few-mode fiber core is 45°; the angle between the center line connecting the fifth single-mode fiber core 8 and the center few-mode fiber core and the center line connecting the sixth single-mode fiber core 9 and the center few-mode fiber core is 45°; and the angle between the center line connecting the sixth single-mode fiber core 9 and the center few-mode fiber core and the center line connecting the first single-mode fiber core 4 and the center few-mode fiber core is 90°. (Reference) Figure 3 The distances from the center of the first single-mode fiber core 4, the second single-mode fiber core 5, the third single-mode fiber core 6, the fourth single-mode fiber core 7, the fifth single-mode fiber core 8, and the sixth single-mode fiber core 9 to the center of the central few-mode fiber core are L1, L2, L3, L4, L5, and L6, respectively, where L3 = L5; L1 is 12.85 μm, L2 is 15.05 μm, L3 and L5 are both 13.9 μm, L4 is 14.1 μm, and L6 is 18.16 μm.

[0049] Calculate the crosstalk and loss issues caused by mode conversion in the embodiments of the present invention:

[0050] The core of this invention is a multi-core optical fiber, and inter-core crosstalk in multi-core optical fibers is one of the important standards for evaluating device quality. Inter-core crosstalk specifically manifests as the mode coupling efficiency between the individual fiber cores. Mode extinction ratio and insertion loss, as key parameters in mode conversion and multiplexing processes, can also be used to evaluate the performance of mode divider multiplexers.

[0051] The invention was calculated and analyzed using the beam propagation method, and LP wavelengths of 1550nm were injected into the outer single-mode fiber core. 01 The mode was adjusted, and the power changes of all other fiber cores in the mode divider were observed, as shown in Table 1. Table 1 shows the output power of other fiber cores when each peripheral single-mode fiber underwent mode conversion at a wavelength of 1550nm in Example 1.

[0052] We analyzed the extinction ratio data given in Table 1. In this invention, we consider the seventh-order linear polarization mode (LP). 01 LP 11 LP 21a LP 21b LP 02 LP31 and LP 12 The impact on the desired mode. Simulations of mode coupling revealed that when only one core at the fiber input is lit, the extinction ratio of the corresponding mode to be converted at the fiber output is calculated. A higher extinction ratio indicates less crosstalk from undesired modes to the desired mode; conversely, a lower extinction ratio indicates greater crosstalk from all other undesired modes to the desired mode. For example... Figure 5 As shown, LP 01 LP 11 LP 21a LP 21b LP 02 LP 31 and LP 12 The extinction ratio of each mode increases with the increase of fiber length. When the fiber length increases to 4200 μm, each mode to be converted has a high extinction ratio, and the extinction ratio curve tends to flatten, indicating that the mode conversion is basically completed. Therefore, the total length of the fiber cut is 4200 μm.

[0053] Table 1

[0054]

[0055] Analysis of the conversion efficiencies of each mode in Table 1 clearly shows that the crosstalk is very small during the conversion process of each mode at a working wavelength of 1550nm, and high mode purity can be obtained for mode modulation and multiplexing.

[0056] The structural parameters designed in this invention exhibit high mode conversion efficiency at a wavelength of 1550nm. However, considering the adaptability and tolerance of the device in practical applications, Figure 6 The conversion efficiency curves of this mode-division multiplexer at a wavelength of 1550nm are presented.

[0057] This invention determines the optimal operating wavelength of the multiplexer by calculating the insertion loss in the C-band, and the results are as follows: Figure 7 As shown, the insertion loss of this mode divider is less than 1 dB in the wavelength range of 1.5211~1.585 μm, and the insertion loss reaches the minimum value (0.36 dB) at the wavelength of 1.55 μm.

[0058] The above description is merely an embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention specification and drawings, or direct or indirect applications in other related system fields, are similarly included within the scope of protection of the present invention.

Claims

1. A seven-core seven-mode mode division multiplexer based on the principle of mode coupling, characterized by: comprising a base material (1), and a central few-mode fiber core and a peripheral single-mode fiber core inside the base material (1); the central few-mode fiber core comprises a fiber core base (2) and a high refractive index ring (3) inside the fiber core base (2); the included angle between the center line of the first single-mode fiber core (4) and the central few-mode fiber core and the center line of the second single-mode fiber core (5) and the central few-mode fiber core is 30°, the included angle between the center line of the second single-mode fiber core (5) and the central few-mode fiber core and the center line of the third single-mode fiber core (6) and the central few-mode fiber core is 60°, the included angle between the center line of the third single-mode fiber core (6) and the central few-mode fiber core and the center line of the fourth single-mode fiber core (7) and the central few-mode fiber core is 90°; the included angle between the center line of the fourth single-mode fiber core (7) and the central few-mode fiber core and the center line of the fifth single-mode fiber core (8) and the central few-mode fiber core is 45°, the included angle between the center line of the fifth single-mode fiber core (8) and the central few-mode fiber core and the center line of the sixth single-mode fiber core (9) and the central few-mode fiber core is 45°, and the included angle between the center line of the sixth single-mode fiber core (9) and the central few-mode fiber core and the center line of the first single-mode fiber core (4) and the central few-mode fiber core is 90°; the distance between the center of the first single-mode fiber core (4), the second single-mode fiber core (5), the third single-mode fiber core (6), the fourth single-mode fiber core (7), the fifth single-mode fiber core (8) and the sixth single-mode fiber core (9) and the positive center of the central few-mode fiber core is L1, L2, L3, L4, L5 and L6 respectively, wherein L3=L5. The L1 is 12.85 μm, the L2 is 15.05 μm, the L3 and the L5 are both 13.9 μm, the L4 is 14.1 μm, and the L6 is 18.16 μm. 2.The seven-core seven-mode mode division multiplexer based on the principle of mode coupling according to claim 1, characterized by: the diameter of the first single-mode fiber core (4) and the fourth single-mode fiber core (7) is equal, and the fiber core refractive index is not equal; the diameter and the fiber core refractive index of the third single-mode fiber core (6) and the fifth single-mode fiber core (8) are equal. The outer single-mode core is distributed around the outer periphery of the central few-mode core: including a first single-mode core (4) that converts LP 11 modes to the few-mode central core, a second single-mode core (5) that converts LP 31 modes to the few-mode central core, a third single-mode core (6) that converts LP 21a modes to the few-mode central core, a fourth single-mode core (7) that converts LP 02 modes to the few-mode central core, a fifth single-mode core (8) that converts LP 21b modes to the few-mode central core, and a sixth single-mode core (9) that converts LP 12 modes to the few-mode central core. 3.The seven-core seven-mode mode division multiplexer based on the principle of mode coupling according to claim 2, characterized by: the diameter of the first single-mode fiber core (4) is 5 μm, and the fiber core refractive index at 1550 nm wavelength is 1.4624; The diameter of the second single-mode fiber core (5) is 4 μm, and the fiber core refractive index at 1550 nm wavelength is 1.4551; The diameter of the third single-mode fiber core (6) and the fifth single-mode fiber core (8) is 6 μm, and the fiber core refractive index at 1550 nm wavelength is 1.4563; The diameter of the fourth single-mode fiber core (7) is 5 μm, and the fiber core refractive index at 1550 nm wavelength is 1.4557; The diameter of the sixth single-mode fiber core (9) is 3 μm, and the fiber core refractive index at 1550 nm wavelength is 1.45425. 4.The seven-core seven-mode mode division multiplexer based on the principle of mode coupling according to claim 3, characterized by: ​ ​ ​ ​ ​ ​ The diameter of the central few-mode fiber core is 15 μm, and the substrate refractive index is 1.456 at a wavelength of 1550 nm.

5. A seven-core, seven-mode mode-division multiplexer based on the mode coupling principle according to claim 4, characterized in that: The center of the high refractive index ring (3) is the center of the optical fiber. The inner diameter of the high refractive index ring (3) is 7 μm, the ring width is 1.5 μm, and the refractive index of the high refractive index ring is 1.46 at a wavelength of 1550 nm.

6. A seven-core, seven-mode mode-division multiplexer based on the mode coupling principle according to claim 5, characterized in that: The substrate material (1) is made of silicon dioxide, and its refractive index is 1.44402 at a wavelength of 1550nm.

7. A seven-core, seven-mode mode-division multiplexer based on the mode coupling principle according to claim 6, characterized in that: The length of the module multiplexer is 4200μm.

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