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

Abstract

The invention belongs to the technical field of optical fiber communication, and particularly relates to a seven-core seven-mode mode division multiplexer based on a mode coupling principle, which comprises a substrate material, a central few-mode fiber core and a peripheral single-mode fiber core, a high-refractive-index ring is arranged in the central few-mode fiber core; the peripheral single-mode fiber core comprises a first single-mode fiber core used for carrying out LP11 mode conversion on the central few-mode fiber core, a second single-mode fiber core used for carrying out LP31 mode conversion on the central few-mode fiber core, a third single-mode fiber core used for carrying out LP21a mode conversion on the central few-mode fiber core, and a fourth single-mode fiber core used for carrying out LP02 mode conversion on the central few-mode fiber core; the fifth single-mode fiber core is used for carrying out LP21b mode conversion on the central few-mode fiber core; and the sixth single-mode fiber core is used for carrying out LP12 mode conversion on the central few-mode fiber core. The invention has the characteristics of large number of conversion modes, high space utilization rate, low insertion loss, high flux and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber communication, and in particular relates to a seven-core seven-mode mode division multiplexer based on the mode coupling principle. Background Art

[0002] With the rise of the internet era, bandwidth-intensive services such as mobile internet, e-commerce, video calls, and the Internet of Things (IoT) have continued to grow, placing immense pressure on existing single-mode fiber optic communication systems. Traditional fiber optic communication systems face increasingly severe bandwidth resource constraints and network overload. Given the ever-increasing demand for communications, developing technologies that increase fiber optic transmission capacity is crucial, leading to the emergence of mode division multiplexing (MDM). As the core component of MDM systems, MDMs have become a research focus for fiber optic communication systems. The concept of MDMs dates back to the early days of optical communication technology exploration, but their development exploded as single-mode fiber transmission capacity approached the Shannon limit. Today, MDMs are evolving toward high mode multiplexing efficiency, low insertion loss, and low crosstalk, laying the foundation for decades of communication technology advancement.

[0003] The development of multi-core few-mode mode division multiplexers stems from the integration of few-mode division multiplexing and multi-core technologies. Early mode division multiplexing focused on multiplexing a few modes within single-core optical fibers. As the number of multiplexable modes in single-core optical fibers reached saturation, researchers began exploring the integration of multi-core optical fibers and few-mode technology. From the late 20th century to the early 21st century, as both technologies matured, researchers began exploring the integration of multi-core structures with few-mode multiplexing, resulting in the development of multi-core few-mode mode division multiplexers.

[0004] In recent years, continuous innovation and optimization of mode division multiplexers (MDMs) have yielded significant results in mode matching, crosstalk suppression, and loss control for multi-core, few-mode MDMs. Currently available mode conversion solutions include those based on photonic lanterns, long-period fiber gratings, and fused-tapered fibers.

[0005] Since these fiber-optic mode division multiplexers achieve mode conversion by destroying the fiber waveguide structure (adiabatic tapering, cladding polishing, etc.), these solutions still have problems such as high device design complexity, lack of flexibility, high crosstalk, difficulty in manufacturing, and high cost. In addition, multi-core few-mode mode division multiplexers have poor compatibility with existing communication systems. When docking with single-mode fiber or other types of multiplexers, there are problems such as mode mismatch and high interface loss. These problems have greatly restricted the performance improvement and application expansion of multi-core few-mode mode division multiplexers. Therefore, the development of high-performance, low-cost, and easy-to-integrate multi-core few-mode mode division multiplexers has become a current research focus. Summary of the Invention

[0006] In order to overcome the problems existing in the above-mentioned prior art, the present invention proposes a seven-core seven-mode mode division multiplexer based on the mode selective coupling principle. The device has the advantages of high mode coupling efficiency, low inter-mode crosstalk, high space utilization, and low insertion loss.

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

[0008] The invention comprises a base material, a central few-mode fiber core and a peripheral single-mode fiber core located in the base material; the central few-mode fiber core located in the base material comprises a fiber core base and a high refractive index ring; the peripheral single-mode fiber core located in the base material is distributed on the periphery of the central few-mode fiber, including: converting LP to the central few-mode fiber core 11 The first single-mode core of the mode is converted to the central few-mode core LP 31 The second single-mode core of the mode conversion, the LP conversion to the central few-mode core 21a The third single-mode core of the mode is converted to the central few-mode core LP 02 The fourth single-mode core of the mode is converted to the central few-mode core LP 21b mode of the fifth single-mode core and the transition to the central few-mode core LP 12 mode of the sixth single-mode core.

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

[0010] Furthermore, the distances between the centers 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-mentioned 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 core refractive index is 1.4624 at a wavelength of 1550 nm.

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

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

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

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

[0017] Furthermore, the diameter of the central few-mode core is 15 μm, and the base 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 made of silicon dioxide, and its refractive index is 1.44402 at a wavelength of 1550 nm; the length of the mode division multiplexer is 4200 μm.

[0019] Advantages of the present invention:

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

[0021] ② In the same section of optical fiber structure, the peripheral single-mode fiber core can be used to achieve LP 11 LP 31 LP 21a LP 21b LP 02 and LP 12 Simultaneous conversion of six higher-order modes and direct input to the central few-mode core LP 01 A total of seven modes can be multiplexed simultaneously. Compared with traditional cascade mode multiplexers, it can multiplex seven modes simultaneously, greatly improving the number of modes and space utilization.

[0022] ③ The present invention effectively converts LP into 02 and LP21 The effective refractive index difference of the modes can be distinguished to avoid serious crosstalk during mode conversion, which is extremely effective for the high-purity conversion of seven modes simultaneously multiplexed by the multiplexer.

[0023] ④ The present invention effectively avoids using too high-order modes to transmit information by multiplexing degenerate modes, thereby using a smaller number of high-order modes to achieve simultaneous multiplexing of seven modes.

[0024] ⑤ The seven-mode mode division multiplexer designed by the present invention is LP at a wavelength of 1550nm 01 LP 11 LP 21a LP 21b LP 02 LP 31 and LP 12 The mode conversion efficiencies are 99.35%, 96.44%, 93.96%, 94.83%, 90.03%, 83.58% and 75.61% respectively.

[0025] ⑥The length of this device is 4200μm, the optical fiber used is short, and the production cost is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of the cross-sectional structure and mode conversion of the mode division multiplexer of the present invention;

[0027] Figure 2 Schematic diagram of the diameter and refractive index of the single-mode fiber core and the few-mode fiber core in the mode division multiplexer of the present invention;

[0028] Figure 3 Schematic diagram of the positional relationship between the single-mode fiber core and the few-mode fiber core and the distance to the core center in the mode division multiplexer of the present invention;

[0029] Figure 4 This is a schematic diagram of the method of using the present invention in actual operation;

[0030] Figure 5 Schematic diagram showing the variation of the extinction ratio of each mode of the mode division multiplexer of the present invention with the coupling length at a wavelength of 1550nm;

[0031] Figure 6 The mode division multiplexer of the present invention is LP at a wavelength of 1550nm 11 LP 21a LP 21b LP 02 LP 31 and LP 12 Schematic diagram of the mode coupling efficiency changing with coupling length;

[0032] Figure 7The figure is a schematic diagram of the device insertion loss of the mode division multiplexer of the present invention at a wavelength of 1.50 to 1.60 μm.

[0033] Among them: 1. base material, 2. fiber core base, 3. high refractive index ring, 4. first single-mode fiber core, 5. second single-mode fiber core, 6. third single-mode fiber core, 7. fourth single-mode fiber core, 8. fifth single-mode fiber core, 9. sixth single-mode fiber core. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work 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 drawings only represents selected embodiments of the present invention.

[0035] The present invention provides a seven-core seven-mode multiplexer based on the mode selective coupling principle, such as Figure 1 As shown, it includes a base material 1, a central few-mode fiber core and a peripheral single-mode fiber core, and the central few-mode fiber core and the peripheral single-mode fiber core are arranged inside the base material 1.

[0036] The central few-mode core comprises a core substrate 2 and a high refractive index ring 3. The core substrate 2 is the main material of the few-mode core. The high refractive index ring 3 is located inside the central few-mode core substrate 2. The high refractive index ring 3 is used to increase the LP 21 and LP 02 The mode effective refractive index difference of the mode.

[0037] The peripheral single-mode fiber cores include LP fiber cores that convert to the central few-mode fiber cores 11 LP 31 LP 21a LP 02 LP 21b LP 12 There are six single-mode cores in total, namely, the first single-mode core 4, the second single-mode core 5, the third single-mode core 6, the fourth single-mode core 7, the fifth single-mode core 8, and the sixth single-mode core 9. The six single-mode cores are distributed outside the central minority-mode core.

[0038] As a preferred embodiment of the present invention, the diameters and core refractive indices of the third single-mode fiber core 6 and the fifth single-mode fiber core 8 are equal; the diameters of the first single-mode fiber core 4 and the fourth single-mode fiber core 7 are equal, but the core refractive indices are different.

[0039] As a preferred embodiment of the present invention, Figure 2 As shown, the diameter d1 of the first single-mode fiber core 4 is 5 μm, and the core refractive index n1 is 1.4624 at a wavelength of 1550 nm; the diameter d2 of the second single-mode fiber core 5 is 4 μm, and the core refractive index n2 is 1.4551 at a wavelength of 1550 nm; 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 core refractive indices n3 and n5 are both 1.4563 at a wavelength of 1550 nm; the diameter d4 of the fourth single-mode fiber core 7 is 5 μm, and the core refractive index n4 is 1.4557 at a wavelength of 1550 nm; the diameter d6 of the sixth single-mode fiber core 9 is 3 μm, and the core refractive index n6 is 1.45425 at a wavelength of 1550 nm. The diameter d0 of the central few-mode fiber core is 15 μm, and the refractive index of the core base 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 of the high refractive index ring 3 is 1.46 at a wavelength of 1550 nm.

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

[0041] like Figure 3 As shown, the distances between the centers 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 exact center of the central few-mode fiber core are: L1, L2, L3, L4, L5 and L6, wherein L3=L5; the above L1 is 12.85μm, L2 is 15.05μm, L3 and L5 are both 13.9μm, L4 is 14.1μm, and L6 is 18.16μm.

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

[0043] Preferably, if Figure 4 As shown, the total length x of the mode division multiplexer is 4200 μm.

[0044] The working performance of the present invention is 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 base material 1, a central few-mode core and a peripheral single-mode core. The central few-mode core includes a core base 2 and a high-refractive-index ring 3. The core base 2 is the main material of the few-mode core. The high-refractive-index ring 3 is located inside the core base 2. The high-refractive-index ring 3 is used to increase the LP in the few-mode core. 21 and LP 02 The effective refractive index difference between the modes. The total length x of the mode division multiplexer is 4200 μm.

[0047] refer to Figure 2 The diameters d1, d2, d3, d4, d5, and d6 of the first, second, third, fourth, fifth, and sixth single-mode fiber cores 4, 5, 6, 7, 8, and 9 are 5 μm, 4 μm, 6 μm, 5 μm, 6 μm, and 3 μm, respectively. At a wavelength of 1550 nm, the core refractive indices n1, n2, n3, n4, n5, and n6 of the first, second, third, fourth, fifth, and sixth single-mode fiber cores 4, 5, 6, 7, 8, and 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 base 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 six single-mode fiber cores in the peripheral fiber core position distribution are arranged in a hexagonal structure, wherein each fiber core is orderly distributed on one of the vertices of the hexagon. Specifically, the angle between the center line of the first single-mode fiber core 4 and the central minority-mode fiber core and the center line of the second single-mode fiber core 5 and the central minority-mode fiber core is 30°; the angle between the center line of the second single-mode fiber core 5 and the central minority-mode fiber core and the center line of the third single-mode fiber core 6 and the central minority-mode fiber core is 60°; the angle between the center line of the third single-mode fiber core 6 and the central minority-mode fiber core and the center line of the fourth single-mode fiber core 7 and the central minority-mode fiber core is 90°; the angle between the center line of the fourth single-mode fiber core 7 and the central minority-mode fiber core and the center line of the fifth single-mode fiber core 8 and the central minority-mode fiber core is 45°; the angle between the center line of the fifth single-mode fiber core 8 and the central minority-mode fiber core and the center line of the sixth single-mode fiber core 9 and the central minority-mode fiber core is 45°; the angle between the center line of the sixth single-mode fiber core 9 and the central minority-mode fiber core and the center line of the first single-mode fiber core 4 and the central minority-mode fiber core is 90°. Reference Figure 3 The distances between the centers 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 center of the central few-mode fiber core are: L1, L2, L3, L4, L5 and L6, where L3=L5; the above L1 is 12.85μm, L2 is 15.05μm, L3 and L5 are both 13.9μm, L4 is 14.1μm, and L6 is 18.16μm.

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

[0050] The present invention is designed around multi-core optical fibers, where inter-core crosstalk is a key quality criterion. This crosstalk is specifically measured as the mode coupling efficiency between the individual cores. Modal extinction ratio and insertion loss, key parameters in mode conversion and multiplexing, can also be used to evaluate the performance of mode division multiplexers.

[0051] The present invention is calculated and analyzed by the beam propagation method, and LP at a wavelength of 1550nm is injected into the peripheral single-mode fiber core. 01 mode, and observe the power changes of all other fiber cores in the mode division multiplexer at the same time, as shown in Table 1. Table 1 shows the output power in other fiber cores when each peripheral single-mode optical fiber performs mode conversion at a wavelength of 1550 nm in Example 1.

[0052] We analyze the extinction ratio data given in Table 1. In this invention, the seventh-order linear polarization mode (LP 01 , LP 11 , LP 21a , LP 21b , LP 02 , LP31 and LP 12 ) on the desired mode. Through the simulation of mode coupling, it is found that when only one of the cores at the input end of the optical fiber is passed through, the extinction ratio of the corresponding mode to be converted at the output end of the optical fiber is calculated. The larger the extinction ratio, the smaller the crosstalk of the undesirable conversion mode on the desired conversion mode; the smaller the extinction ratio, the greater the crosstalk of all other undesirable modes taken into account on the desired mode. Figure 5 As shown, LP 01 , LP 11 , LP 21a , LP 21b , LP 02 , LP 31 and LP 12 The extinction ratios of all modes increase with increasing fiber length. When the fiber length reaches 4200 μm, each mode to be converted has a high extinction ratio, and the extinction ratio curve flattens, indicating that mode conversion is essentially complete. Therefore, the total length of the fiber cut is 4200 μm.

[0053]

[0054] By analyzing the conversion efficiency of each mode in Table 1, it is obvious that the present invention has very small crosstalk during each mode conversion process at the working wavelength of 1550nm, and can achieve high mode purity for mode modulation and multiplexing.

[0055] The structural parameters designed by the present invention have a high mode conversion efficiency at a wavelength of 1550nm, but in practical applications, considering the adaptability and tolerance of the device, Figure 6 The conversion efficiency variation curves of each mode of the mode division multiplexer at a wavelength of 1550nm are given.

[0056] The present invention determines the most suitable operating wavelength of the multiplexer by calculating the insertion loss of the C band of the multiplexer. The results are as follows: Figure 7 The insertion loss of the mode division multiplexer is less than 1dB in the wavelength range of 1.5211 to 1.585um, and reaches the minimum value (0.36dB) at the wavelength of 1.55um.

[0057] The above descriptions are merely embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention description and drawings, or directly or indirectly applied to other related system fields, are also included in the scope of protection of the present invention.

Claims

1. A seven-core, seven-mode mode division multiplexer based on the mode coupling principle, characterized by: It comprises a base material (1), and a central few-mode fiber core and a peripheral single-mode fiber core located inside the base material (1); The central few-mode fiber core comprises a fiber core base (2) and a high refractive index ring (3), wherein the high refractive index ring (3) is located inside the fiber core base (2); The peripheral single-mode fiber core is distributed around the central few-mode fiber core: including converting LP to the few-mode central fiber core 11 The first single-mode fiber core (4) of the mode is converted to the LP core of the minority mode center 31 The second single-mode fiber core (5) of the mode is converted to the LP core of the minority mode center 21a The third single-mode fiber core (6) of the mode is converted to the LP core of the minority mode center 02 mode of the fourth single-mode core (7), to the minority-mode center core conversion LP 21b The fifth single-mode fiber core (8) of the mode is converted to the LP core of the minority mode center 12 mode of the sixth single-mode fiber core (9).

2. The seven-core seven-mode mode division multiplexer based on the mode coupling principle according to claim 1, characterized in that: The angle between the center line of the first single-mode fiber core (4) and the central minority-mode fiber core and the center line of the second single-mode fiber core (5) and the central minority-mode fiber core is 30°, the angle between the center line of the second single-mode fiber core (5) and the central minority-mode fiber core and the center line of the third single-mode fiber core (6) and the central minority-mode fiber core is 60°, and the angle between the center line of the third single-mode fiber core (6) and the central minority-mode fiber core and the center line of the fourth single-mode fiber core (4) and the central minority-mode fiber core is 90°. The angle between the center line of the fourth single-mode fiber core (7) and the central minority-mode fiber core and the center line of the fifth single-mode fiber core (8) and the central minority-mode fiber core is 45°, the angle between the center line of the fifth single-mode fiber core (9) and the central minority-mode fiber core and the center line of the sixth single-mode fiber core (9) and the central minority-mode fiber core is 45°, and the angle between the center line of the sixth single-mode fiber core (9) and the central minority-mode fiber core and the center line of the first single-mode fiber core (4) and the central minority-mode fiber core is 90°.

3. The seven-core seven-mode mode division multiplexer based on the mode coupling principle according to claim 2, characterized in that: The distances between the centers 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.

4. The seven-core seven-mode mode division multiplexer based on the mode coupling principle according to claim 3, characterized in that: The 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.

5. The seven-core seven-mode mode division multiplexer based on the mode coupling principle according to any one of claims 1 to 4, characterized in that: The diameters of the first single-mode fiber core (4) and the fourth single-mode fiber core (7) are equal, but the core refractive indices are unequal; the diameters and core refractive indices of the third single-mode fiber core (6) and the fifth single-mode fiber core (8) are equal.

6. The seven-core seven-mode mode division multiplexer based on the mode coupling principle according to claim 5, characterized in that: The diameter of the first single-mode fiber core (4) is 5 μm, and the core refractive index is 1.4624 at a wavelength of 1550 nm; The diameter of the second single-mode fiber core (5) is 4 μm, and the core refractive index is 1.4551 at a wavelength of 1550 nm; The diameter of the third single-mode fiber core (6) and the fifth single-mode fiber core (8) is 6 μm, and the core refractive index is 1.4563 at a wavelength of 1550 nm; The diameter of the fourth single-mode fiber core (7) is 5 μm, and the core refractive index is 1.4557 at a wavelength of 1550 nm; The diameter of the sixth few-mode fiber core (9) is 3 μm, and the core refractive index is 1.45425 at a wavelength of 1550 nm.

7. The seven-core seven-mode mode division multiplexer based on the mode coupling principle according to claim 6, characterized in that: The diameter of the central few-mode fiber core is 15 μm, and the base refractive index is 1.456 at a wavelength of 1550 nm.

8. The seven-core seven-mode mode division multiplexer based on the mode coupling principle according to claim 7, 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.

9. The seven-core seven-mode mode division multiplexer based on the mode coupling principle according to claim 8, characterized in that: The base material (1) is made of silicon dioxide, and its refractive index is 1.44402 at a wavelength of 1550 nm.

10. The seven-core seven-mode mode division multiplexer based on the mode coupling principle according to claim 9, characterized in that: The length of the mode division multiplexer is 4200 μm.

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