A Coupled Multi-Core Optical Fiber and Its Fabrication Method

Abstract

This invention discloses a coupled multi-core optical fiber and its fabrication method. The coupled multi-core optical fiber includes a core unit and an optical cladding surrounding the core unit. Each core unit includes multiple transmission cores, each transmission core including a core layer and a core cladding surrounding the core layer. The multiple transmission cores are uniformly distributed within the optical cladding. The optical cladding surrounding the transmission cores includes an inner cladding and an annular outer cladding. The refractive index profile of the multi-core optical fiber is a step-index profile. No transmission core is located at the center of the multi-core optical fiber. This coupled multi-core optical fiber exhibits low transmission loss and spatial mode dispersion, as well as superior bending performance.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber technology, and specifically relates to a coupled multi-core optical fiber and its preparation method. Background Technology

[0002] Multi-core optical fibers based on Space Division Multiplexing (SDM) technology can achieve a significant increase in communication capacity, breaking through the communication capacity limits of single-mode optical fibers. Based on the inter-core coupling type, multi-core optical fibers can be divided into weakly coupled multi-core fibers and strongly coupled multi-core fibers. Weakly coupled multi-core fibers have a larger core spacing to achieve weaker inter-core coupling, sacrificing core density. Simultaneously, to accommodate more cores, the cladding diameter needs to be increased, affecting the fiber's strength and bending performance. Strongly coupled multi-core fibers have sufficiently close core spacing, causing crosstalk between the cores. However, this crosstalk can be decoded using Multiple-Input Multiple-Output (MIMO) technology to achieve mode-division multiplexing transmission.

[0003] Strongly coupled multi-core fibers form supermodes due to the superposition of the core mode fields, which increases the effective area of ​​the fiber and helps reduce nonlinear effects. At the same time, due to the small inter-core spacing of strongly coupled multi-core fibers, more fiber cores can be accommodated with the same cladding diameter in fiber design. The key design points of strongly coupled multi-core fibers based on SDM systems include the following aspects: (1) The effective refractive index difference between modes should be large enough to avoid mode coupling; (2) The mode loss of each mode should be the same or the difference should be small enough; (3) The presence of a depressed cladding is beneficial to improving macro bending performance; (4) The mode delay (DMD) and spatial mode dispersion (SMD) of the supermode should be as small as possible to reduce the complexity of the MIMO system.

[0004] In summary, reducing transmission loss and spatial mode dispersion in strongly coupled multi-core optical fibers, while improving macro-bending performance, are urgent issues that need to be addressed to realize the application of strongly coupled multi-core optical fibers.

[0005] Patent application CN107179581A proposes a method for fabricating coupled multi-core fibers, but it mainly focuses on the transmission loss of optical fibers and does not pay attention to the spatial mode dispersion and macro bending performance of optical fibers. Summary of the Invention

[0006] To address the aforementioned technical problems, the present invention aims to provide a coupled multi-core optical fiber and its fabrication method, wherein the coupled multi-core optical fiber has lower transmission loss and spatial mode dispersion as well as superior bending performance.

[0007] To achieve the above-mentioned technical objectives and effects, the present invention is implemented through the following technical solution:

[0008] A coupled multi-core optical fiber includes a core unit and an optical fiber cladding surrounding the core unit;

[0009] The fiber core unit includes multiple transmission fiber cores, each transmission fiber core including a core layer and a fiber core cladding surrounding the core layer; the multiple transmission fiber cores are uniformly distributed in the fiber cladding.

[0010] The fiber cladding surrounding the transmission fiber core includes an inner cladding and an annular outer cladding.

[0011] The refractive index profile of this multi-core optical fiber is a step-index structure.

[0012] No transmission core is placed at the center of this multi-core optical fiber.

[0013] Furthermore, the fiber core unit also includes a marker fiber core, which is located near any one of the transmission fiber cores.

[0014] Furthermore, the spacing between two adjacent transmission fiber cores is less than or equal to 35 μm, more preferably less than or equal to 25 μm and greater than or equal to 20 μm.

[0015] Furthermore, the radius of the core layer of the transmission fiber core is greater than or equal to 3.0 μm and less than or equal to 6.25 μm, more preferably greater than or equal to 4.0 μm and less than or equal to 6.0 μm; the ratio of the radius of the core cladding to the radius of the core layer of the transmission fiber core is 1.0 to 3.0.

[0016] Furthermore, the radius of the fiber cladding is 62.5 μm.

[0017] Furthermore, the refractive index difference ΔN1 between the core layer and pure silicon dioxide is -0.0005 to 0.0058; the refractive index difference ΔN2 between the core cladding and pure silicon dioxide is -0.0060 to -0.0005.

[0018] Furthermore, the refractive index difference ΔN3 of the inner cladding layer relative to pure silicon dioxide is smaller than the refractive index difference ΔN4 of the outer cladding layer relative to pure silicon dioxide; preferably, the refractive index difference ΔN3 of the inner cladding layer relative to pure silicon dioxide is -0.0080 to -0.0010; and the refractive index difference ΔN4 of the outer cladding layer relative to pure silicon dioxide is -0.0055 to 0.

[0019] Furthermore, the core layer is a germanium-doped silicon dioxide core layer or an alkali metal-doped silicon dioxide core layer; the fiber core cladding is a fluorine-doped silicon dioxide cladding.

[0020] Furthermore, the radius of the marking fiber core is greater than or equal to 2.5 μm and less than or equal to 4 μm; the marking fiber core is a germanium-doped silicon dioxide fiber core; the refractive index difference between the marking fiber core and pure silicon dioxide is 0.0030 to 0.0040.

[0021] The fabrication method of this coupled multi-core optical fiber includes the following steps:

[0022] (1) First, prepare a cylindrical blank rod consisting of an inner cladding layer and an outer cladding layer, as well as each core rod (transfer core and marking core) in the core unit;

[0023] (2) Extend the diameter of each core rod of the core unit to the target matching diameter;

[0024] (3) Using high-precision drilling equipment, holes are drilled sequentially on the sleeve blank according to the designed hole distribution diagram to obtain the sleeve;

[0025] (4) Assemble each core rod of the fiber core unit with the corresponding hole of the sleeve post to obtain a strongly coupled multi-core fiber rod, and then process the rod through a subsequent drawing process to obtain the coupled multi-core fiber.

[0026] The beneficial effects of this invention are:

[0027] The optical fiber structure of the present invention does not have a fiber core at the center, which allows for more effective coupling between the transmission fiber cores.

[0028] The refractive index profile of the optical fiber of the present invention adopts a step-index profile structure, which can improve the effective refractive index difference between modes and avoid coupling of higher-order modes.

[0029] The optical fiber of the present invention has an inner cladding and an outer cladding, the outer cladding being a ring structure, which can effectively improve the bending performance of the coupled multi-core optical fiber;

[0030] The optical fiber of this invention, through the structural design of the core and cladding and the design of the refractive index difference, can meet the requirements of high-capacity transmission, and has low transmission loss, spatial mode dispersion and superior bending performance. Attached Figure Description

[0031] Figure 1 This is a diagram showing the refractive index profile of the transmission core of the coupled multi-core optical fiber of the present invention.

[0032] Figure 2 This is a schematic diagram of the radial cross-sectional structure of the coupled multi-core optical fiber according to Embodiment 1 of the present invention.

[0033] Figure 3 This is a diagram showing the refractive index profile of the coupled multi-core optical fiber according to Embodiment 1 of the present invention.

[0034] Figure 4 This is a schematic diagram of the radial cross-sectional structure of the coupled multi-core optical fiber in Embodiment 2 of the present invention. Detailed Implementation

[0035] The technical solutions of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. 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.

[0036] like Figure 2 and Figure 4 As shown, the present invention provides a coupled multi-core optical fiber, comprising a core unit and an optical fiber cladding surrounding the core unit;

[0037] The fiber core unit includes a plurality of transmission fiber cores 11, each transmission fiber core 11 including a core layer 111 and a fiber core cladding 112 surrounding the core layer 111; the plurality of transmission fiber cores 11 are uniformly distributed in the fiber cladding 2.

[0038] The fiber cladding 2 surrounding the transmission fiber core includes an inner cladding 21 and an annular outer cladding 22;

[0039] The refractive index profile of this multi-core optical fiber is a step-index structure.

[0040] No transmission core is placed at the center of this multi-core optical fiber.

[0041] The spacing Λ between two adjacent transmission cores in the drawn optical fiber is less than or equal to 35 μm, more preferably less than or equal to 25 μm and greater than or equal to 20 μm.

[0042] After drawing, the radius R1 of the core layer of the transmission fiber core is greater than or equal to 3.0 μm and less than or equal to 6.25 μm, more preferably greater than or equal to 4.0 μm and less than or equal to 6.0 μm; the ratio of the radius R2 of the core cladding to the radius R1 of the core layer of the transmission fiber core is 1.0 to 3.0.

[0043] After drawing, the radius R3 of the fiber cladding (radius of the inner cladding) is preferably greater than or equal to 23.0 μm and less than 62.5 μm.

[0044] After drawing, the radius R4 (outer radius of the outer cladding) of the optical fiber is preferably 62.5 μm.

[0045] The core layer of the transmission fiber core is a germanium-doped silicon dioxide core layer or an alkali metal-doped silicon dioxide core layer; when the core layer is alkali metal-doped silicon dioxide, the average concentration of the alkali metal is greater than or equal to 5 ppm and less than or equal to 100 ppm; the alkali metal is preferably at least one of lithium, sodium, potassium and rubidium.

[0046] The core cladding of the transmission fiber is a fluorine-doped silica cladding; the inner cladding of the optical fiber cladding is a fluorine-doped silica cladding, and the outer cladding of the optical fiber cladding is a fluorine-doped silica cladding or a pure silica cladding.

[0047] Among them, such as Figure 1 As shown, the refractive index difference ΔN1 between the core layer and pure silicon dioxide is -0.0005 to 0.0058; the refractive index difference ΔN2 between the fiber core cladding and pure silicon dioxide is -0.0060 to -0.0005.

[0048] Wherein, the refractive index difference ΔN3 of the inner cladding layer relative to pure silicon dioxide is less than the refractive index difference ΔN4 of the outer cladding layer relative to pure silicon dioxide; preferably, the refractive index difference ΔN3 of the inner cladding layer relative to pure silicon dioxide is -0.0080 to -0.0010; the refractive index difference ΔN4 of the outer cladding layer relative to pure silicon dioxide is -0.0055 to 0.

[0049] The multiple transmission fiber cores are symmetrically distributed, and their distribution pattern can be a regular quadrilateral or a regular hexagon, or other uniform symmetrical distribution patterns.

[0050] The transmission loss at 1550nm in the transmission optical fiber of the present invention is less than or equal to 0.22dB / Km, more preferably less than or equal to 0.16dB / Km.

[0051] The fiber core unit also includes a marking fiber core 12, which is located near any one of the transmission fiber cores 11. The marking fiber core is used to confirm the position of each transmission fiber core in the multi-core optical fiber to facilitate fusion splicing. After drawing, the radius of the marking fiber core is greater than or equal to 2.5 μm and less than or equal to 4 μm; the marking fiber core is a germanium-doped silicon dioxide fiber core; the refractive index difference between the marking fiber core and pure silicon dioxide is 0.0030 to 0.0040.

[0052] The fabrication method of this coupled multi-core optical fiber includes the following steps:

[0053] (1) First, prepare each core rod in the core unit, namely the transmission core and the marking core, and prepare the cladding rod (fiber cladding).

[0054] (2) Extend the diameter of each core rod of the core unit to the target matching diameter;

[0055] (3) Using high-precision drilling equipment, holes are drilled sequentially on the sleeve blank rod according to the designed hole distribution diagram to produce the sleeve rod. The number of holes is equal to the number of core rods of the fiber core unit.

[0056] (4) Assemble each core rod of the fiber core unit with the corresponding hole of the sleeve post to obtain a strongly coupled multi-core fiber rod, and then process the rod through a subsequent drawing process to obtain the coupled multi-core fiber.

[0057] Example 1

[0058] like Figure 2 As shown, the coupled multi-core optical fiber of this embodiment 2 includes a core unit and an optical fiber cladding surrounding the core unit;

[0059] The fiber core unit includes four transmission fiber cores 11 and one marker fiber core 12; each transmission fiber core 11 includes a core layer 111 and a fiber core cladding 112 surrounding the core layer 111; the four transmission fiber cores 11 are distributed in a square around the center of the optical fiber in the fiber cladding 2; the marker fiber core 12 is close to any one of the transmission fiber cores 11; the fiber cladding 2 surrounding the fiber core unit includes an inner cladding 21 and an annular outer cladding 22; the refractive index profile of the multi-core optical fiber is a step-index profile structure; no transmission fiber core is set at the center of the multi-core optical fiber.

[0060] The core radius R1 of the drawn transmission fiber core is 4.3 μm; the core cladding radius R2 is 8.6 μm; the outer cladding radius R4 is 62.5 μm; and the spacing Λ between two adjacent transmission fiber cores is 20 μm.

[0061] The core of the transmission fiber is a germanium-doped silicon dioxide core layer; the refractive index difference ΔN1 between the core layer and pure silicon dioxide is 0.0050.

[0062] The core cladding of the transmission fiber is a fluorine-doped silica cladding; the refractive index difference ΔN2 between the core cladding and pure silica is -0.0010.

[0063] The inner cladding of the optical fiber is a fluorine-doped silica cladding, while the outer cladding is a pure silica cladding. The refractive index difference between the inner cladding and pure silica is equal to the refractive index difference between the core cladding and pure silica.

[0064] The fabrication method of the coupled multi-core optical fiber in Example 1 is as follows:

[0065] (1) A fluorine-doped core rod is prepared by VAD process. The core rod is processed into a target diameter target rod by extension and external cylindrical grinding process. Then, a pure silicon dioxide outer cladding layer is prepared by OVD process. Finally, it is extended to a cylindrical blank rod of 80-100 mm.

[0066] Transmission fiber core rods and marking fiber core rods with diameters of 90–120 mm were prepared using the VAD process.

[0067] (2) Extend each fiber core rod to the target matching diameter;

[0068] (3) Using high-precision drilling equipment, holes are drilled sequentially on the sleeve blank according to the designed hole distribution diagram to form a sleeve containing marked fiber core holes and transmission fiber core holes.

[0069] (4) Assemble the transmission fiber core rod and the marking fiber core rod with the sleeve post to obtain a strongly coupled multi-core fiber rod; and then process the rod through a subsequent drawing process to obtain the coupled multi-core fiber.

[0070] The strongly coupled four-core optical fiber prepared in Example 1 supports eight modes, with a transmission loss of 0.198 dB / km at 1550 nm for each transmission core and a spatial mode dispersion of 20.5 ps / km^0.5, making it suitable for preparing strongly coupled four-core optical fiber devices.

[0071] Example 2

[0072] like Figure 4 As shown, the coupled multi-core optical fiber of this embodiment 2 includes a core unit and an optical fiber cladding surrounding the core unit;

[0073] The fiber core unit includes six transmission fiber cores 11 and one marker fiber core 12; each transmission fiber core 11 includes a core layer 111 and a core cladding 112 surrounding the core layer 111; the six transmission fiber cores 11 are distributed in a regular hexagon around the center of the optical fiber in the fiber cladding 2; the marker fiber core 12 is close to any one of the transmission fiber cores 11; the fiber cladding 2 surrounding the fiber core unit includes an inner cladding 21 and an annular outer cladding 22; the refractive index profile of the multi-core optical fiber is a step-index profile structure; no transmission fiber core is provided at the center of the multi-core optical fiber.

[0074] The core radius R1 of the drawn transmission fiber core is 4.5 μm; the core cladding radius R2 is 9.1 μm; the outer cladding radius R4 is 62.5 μm; and the spacing Λ between two adjacent transmission fiber cores is 25 μm.

[0075] The core of the transmission fiber is a germanium-doped silicon dioxide core layer; the refractive index difference ΔN1 between the core layer and pure silicon dioxide is 0.0051.

[0076] The core cladding of the transmission fiber is a fluorine-doped silica cladding; the refractive index difference ΔN2 between the core cladding and pure silica is -0.0020.

[0077] The inner cladding of the optical fiber is a fluorine-doped silica cladding, while the outer cladding is a pure silica cladding. The refractive index difference between the inner cladding and pure silica is equal to the refractive index difference between the core cladding and pure silica.

[0078] The fabrication method of the coupled multi-core optical fiber in Example 2 is as follows:

[0079] (1) A fluorine-doped core rod is prepared by VAD process. The core rod is processed into a target diameter target rod by extension and external cylindrical grinding process. Then, a pure silicon dioxide outer cladding layer is prepared by OVD process. Finally, it is extended to a cylindrical blank rod of 80-100 mm.

[0080] Transmission fiber core rods and marking fiber core rods with diameters of 90–120 mm were prepared using the VAD process.

[0081] (2) Extend each fiber core rod to the target matching diameter;

[0082] (3) Using high-precision drilling equipment, holes are drilled sequentially on the sleeve blank according to the designed hole distribution diagram to form a sleeve containing marked fiber core holes and transmission fiber core holes.

[0083] (4) Assemble the transmission fiber core rod and the marking fiber core rod with the sleeve post to obtain a strongly coupled multi-core fiber rod; and then process the rod through a subsequent drawing process to obtain the coupled multi-core fiber.

[0084] The strongly coupled six-core optical fiber prepared in Example 2 supports twelve modes, with a transmission loss of 0.208 dB / km at 1550 nm for each transmission core and a spatial mode dispersion of 25 ps / km^0.5, making it suitable for preparing strongly coupled six-core optical fiber devices.

[0085] Example 3

[0086] The coupled multi-core optical fiber of this embodiment 3 includes a core unit and an optical fiber cladding surrounding the core unit;

[0087] The fiber core unit includes four transmission fiber cores 11 and one marker fiber core 12; each transmission fiber core 11 includes a core layer 111 and a fiber core cladding 112 surrounding the core layer 111; the four transmission fiber cores 11 are distributed in a square around the center of the optical fiber in the fiber cladding 2; the marker fiber core 12 is close to any one of the transmission fiber cores 11; the fiber cladding 2 surrounding the fiber core unit includes an inner cladding 21 and an annular outer cladding 22; the refractive index profile of the multi-core optical fiber is a step-index profile structure; no transmission fiber core is set at the center of the multi-core optical fiber.

[0088] The core radius R1 of the drawn transmission fiber core is 6.0 μm; the core cladding radius R2 is 6.0 μm; the outer radius R4 of the outer cladding is 62.5 μm; and the spacing Λ between two adjacent transmission fiber cores is 20 μm.

[0089] The core layer of the transmission fiber is an alkali-doped metal silica core layer; the refractive index difference ΔN1 between the core layer and pure silica is 0.0005.

[0090] The core cladding of the transmission fiber is a fluorine-doped silica cladding; the refractive index difference ΔN2 between the core cladding and pure silica is -0.0060.

[0091] The inner cladding of the optical fiber is a fluorine-doped silica cladding, and the outer cladding is also a fluorine-doped silica cladding; the refractive index difference between the inner cladding and pure silica is -0.0060; the refractive index difference between the outer cladding and pure silica is -0.0040.

[0092] The fabrication method of the coupled multi-core optical fiber in Example 3 is as follows:

[0093] (1) A fluorine-doped core rod is prepared by the VAD process. The core rod is processed into a target diameter target rod by the extension and outer cylindrical grinding process. Then, the outer cladding layer is prepared by the OVD process. Finally, it is extended to a cylindrical blank rod of 80-100 mm.

[0094] Prepare transport fiber core rods and marking fiber core rods with a diameter of 90-120 mm;

[0095] (2) Extend each fiber core rod to the target matching diameter;

[0096] (3) Using high-precision drilling equipment, holes are drilled sequentially on the sleeve blank according to the designed hole distribution diagram to form a sleeve containing marked fiber core holes and transmission fiber core holes.

[0097] (4) Assemble the transmission fiber core rod and the marking fiber core rod with the sleeve post to obtain a strongly coupled multi-core fiber rod; and then process the rod through a subsequent drawing process to obtain the coupled multi-core fiber.

[0098] The strongly coupled four-core optical fiber prepared in Example 3 supports eight modes, with a transmission loss of 0.158 dB / km at 1550 nm for each transmission core and a spatial mode dispersion of 8.5 ps / km^0.5, making it suitable for high-capacity transmission systems.

[0099] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A coupled multi-core optical fiber, characterized in that: It includes the fiber core unit and the fiber cladding surrounding the fiber core unit; The fiber core unit includes multiple transmission fiber cores, each transmission fiber core including a core layer and a fiber core cladding surrounding the core layer; the multiple transmission fiber cores are uniformly distributed in the fiber cladding. The fiber cladding surrounding the transmission fiber core includes an inner cladding and an annular outer cladding. The refractive index profile of this multi-core optical fiber is a step-index structure. No transmission core is placed at the center of this multi-core optical fiber.

2. The coupled multi-core optical fiber according to claim 1, characterized in that: The fiber core unit also includes a marker fiber core, which is located near any one of the transmission fiber cores.

3. The coupled multi-core optical fiber according to claim 1, characterized in that: The spacing between two adjacent transmission fiber cores is less than or equal to 35μm.

4. A coupled multi-core optical fiber according to claim 1, characterized in that: The radius of the core layer of the transmission fiber is greater than or equal to 3.0 μm and less than or equal to 6.25 μm; the ratio of the radius of the core cladding to the radius of the core layer of the transmission fiber is 1.0 to 3.

0.

5. A coupled multi-core optical fiber according to claim 1, characterized in that: The radius of the fiber cladding is 62.5 μm.

6. A coupled multi-core optical fiber according to claim 1, characterized in that: The refractive index difference ΔN1 between the core layer and pure silicon dioxide is -0.0005 to 0.0058; the refractive index difference ΔN2 between the core cladding and pure silicon dioxide is -0.0060 to -0.0005.

7. A coupled multi-core optical fiber according to claim 1, characterized in that: The refractive index difference ΔN3 of the inner cladding layer relative to pure silicon dioxide is -0.0080 to -0.0010; the refractive index difference ΔN4 of the outer cladding layer relative to pure silicon dioxide is -0.0055 to 0.

8. A coupled multi-core optical fiber according to claim 1, characterized in that: The core layer is a germanium-doped silicon dioxide core layer or an alkali metal-doped silicon dioxide core layer; the fiber core cladding is a fluorine-doped silicon dioxide cladding.

9. A coupled multi-core optical fiber according to claim 2, characterized in that: The radius of the marked fiber core is greater than or equal to 2.5 μm and less than or equal to 4 μm.

10. A method for fabricating a coupled multi-core optical fiber according to any one of claims 1 to 9, characterized in that, Includes the following steps: (1) First, prepare the cylindrical blank rod and the fiber core unit; (2) Extend the diameter of each core rod of the core unit to the target matching diameter; (3) Using high-precision drilling equipment, holes are drilled sequentially on the sleeve blank according to the designed hole distribution diagram to obtain the sleeve; (4) Assemble each core rod of the fiber core unit with the corresponding hole of the sleeve post to obtain a strongly coupled multi-core fiber rod. Then, process the fiber rod through a subsequent drawing process to obtain the coupled multi-core fiber.

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