A hollow optical fiber and its preparation method

Abstract

This invention discloses a hollow-core optical fiber and its fabrication method. The hollow-core optical fiber includes a coating layer, an inwardly recessed negative curvature structure cladding, anti-resonant units, and an air-filled core. The coating layer is disposed outside the inwardly recessed negative curvature structure cladding. Twelve or more anti-resonant units are uniformly arranged on the inner wall of the inwardly recessed negative curvature structure cladding. The air-filled core is located at the center of the hollow-core optical fiber and is tangent to the outer wall of the surrounding anti-resonant units. The hollow-core optical fiber of this invention has the advantages of low confinement loss, low nonlinear coefficient, and flat dispersion. It can better maintain multiple nested structures and keep the wall thickness satisfying the anti-resonance condition, reducing the area of ​​the gap between nested tubes and reducing attenuation. Simultaneously, the reduction in substrate loss optimizes the bending performance of the hollow-core optical fiber, improves signal transmission efficiency, and enhances anti-interference capability. The use of a more heat-resistant and corrosion-resistant transparent alumina ceramic substrate optimizes the optical performance of the hollow-core optical fiber.

Description

Technical Field

[0001] This invention belongs to the field of optical communication technology, specifically relating to a hollow optical fiber and its preparation method. Background Technology

[0002] With the rapid development of emerging technologies such as big data, cloud computing, and the Internet of Things, the performance requirements for optical communication systems are increasing. To overcome the limitations of traditional optical fibers in terms of capacity, transmission distance, and latency, hollow-core optical fibers have attracted much attention due to their unique structure and superior performance. The structural design of hollow-core optical fibers has a significant impact on their performance; optimizing parameters such as the arrangement, size, and shape of the air holes to achieve lower loss is currently a hot research topic and a challenge.

[0003] Existing ultra-low loss antiresonant hollow-core optical fibers typically employ a ≥5-tube structural unit (5-fold symmetry structure), with the core mode field exhibiting a pentagonal, hexagonal, or near-circular shape. This structure is ideal for two-layer nested tube structures. However, to further reduce loss, the number of antiresonant layers needs to be increased. When designing a three-layer antiresonant structure, several problems arise: The air layer area is insufficient to filter out higher-order modes, making it difficult to achieve good single-mode characteristics. Typically, the fundamental mode loss differs from the higher-order mode loss by only about one order of magnitude, and higher-order modes are prone to crosstalk with the fundamental mode. To achieve a high higher-order mode suppression ratio, the area of ​​one air layer must be reduced, potentially causing the air layer to resonate, thus increasing confinement loss or bending loss. Simultaneously, due to the special structure of hollow-core optical fibers, their bending performance is relatively poor, easily generating significant loss during bending, affecting light transmission efficiency.

[0004] Chinese invention patent, publication number CN115745391A, discloses an integrated hollow optical fiber preform, optical fiber, and its preparation method. The initial preform is obtained by drilling, and the optical fiber with an anti-resonant ring structure is prepared by introducing gas into the hole during the fiber drawing process for pressure control. The focus is on simplifying the complex cross-section and designing a thinner wall thickness to improve attenuation, but no further improvement is shown for other parameter performance.

[0005] Chinese invention patent, publication number CN117388980A, discloses an ultra-low attenuation single-mode hollow fiber and its fabrication method. The radial cross-section of the outer cladding cavity is a regular quadrilateral or near-quadrilateral, maintaining a structure of four nested units and keeping the wall thickness to meet the anti-resonance condition. The near-quadrilateral cavity is formed by adding filling layers to the four sides of the circular base cavity. The focus is on reducing crosstalk between the fundamental mode and higher-order modes of the anti-resonant hollow fiber to maintain single-mode output characteristics. However, no further research or discussion was conducted on improving attenuation and bending performance.

[0006] None of the aforementioned patents mention any research on reducing bending loss. Summary of the Invention

[0007] To address the technical problems existing in the prior art, the present invention aims to provide a hollow optical fiber and its preparation method.

[0008] To achieve the above objectives and technical effects, the technical solution adopted by this invention is as follows:

[0009] A hollow optical fiber includes a coating layer, an inwardly recessed negative curvature structure cladding, anti-resonant units, and an air-filled core. The coating layer is disposed outside the inwardly recessed negative curvature structure cladding. There are 12 or more anti-resonant units, which are uniformly arranged on the inner wall of the inwardly recessed negative curvature structure cladding. The air-filled core is located at the center of the hollow optical fiber and is tangent to the outer wall of the anti-resonant units surrounding it.

[0010] Furthermore, the inwardly recessed negative curvature structure outer cladding is made of a highly transparent alumina ceramic substrate tube with a real part of refractive index of 1.2-1.8 μm.

[0011] Furthermore, the anti-resonance unit is formed by nesting several capillaries of different diameters from the inside out.

[0012] Furthermore, each anti-resonance unit is composed of three capillaries of different diameters nested from the inside out. The three capillaries of different diameters are referred to as the first capillary, the second capillary, and the third capillary from the outside in. The inner wall of the first capillary is tangent to the outer wall of the second capillary, and the inner wall of the second capillary is tangent to the outer wall of the third capillary.

[0013] Furthermore, the diameter of the first capillary is 18-27 μm, the diameter of the second capillary is 13-17 μm, and the diameter of the third capillary is 8-12 μm.

[0014] Furthermore, the diameter of the first capillary is 25.03 μm, the diameter of the second capillary is 16.86 μm, and the diameter of the third capillary is 11.36 μm.

[0015] Furthermore, the diameter of the first capillary is 19.87 μm, the diameter of the second capillary is 13.39 μm, and the diameter of the third capillary is 9.02 μm.

[0016] Furthermore, the total diameter of the hollow fiber is 240-250μm, the total diameter of the hollow fiber excluding the coating layer is 110-150μm, and the diameter of the air-filled fiber core is 33-45μm.

[0017] This invention also discloses a method for preparing hollow-core optical fiber, comprising the following steps:

[0018] S1: Several capillary tubes of different diameters are drawn using a tube drawing platform;

[0019] S2: Capillaries of different diameters are made into anti-resonance units;

[0020] S3: Evenly adhere all anti-resonant units to the inner wall of the recessed area of ​​the outer sleeve. Use a welding process to bond the capillary tube to the outer sleeve. Perform vacuum treatment on the assembled structure to achieve a vacuum level of 10. -6 mbar or even higher, which facilitates subsequent filling with special gases of low refractive index;

[0021] S4: After welding, the fiber is extended and drawn. During the drawing process, the negative curvature structure boundary is formed by the pressure difference and surface tension, thus forming an anti-resonant nested structure surrounded by an inwardly concave negative curvature structure cladding. Then, a low refractive index gas is filled to further reduce the transmission loss of light in the optical fiber.

[0022] Furthermore, the low-refractive-index gas is any one or a combination of several of compressed air, nitrogen, helium, and argon.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] 1) The radial cross section of the inner cavity of the inwardly concave negative curvature structure of the outer cladding structure disclosed in this invention has 12 sides and above that are negative curvature structures. The hollow fiber made therefrom has the advantages of low confinement loss, low nonlinear coefficient and flat dispersion.

[0025] 2) This invention can better maintain the nested structure of multiple capillaries and keep the wall thickness to meet the anti-resonance condition, reduce the area of ​​the gap between the nested tubes, i.e. the capillaries, reduce attenuation, and at the same time, the reduction of substrate loss optimizes the bending performance of hollow optical fiber, improves signal transmission efficiency, and enhances anti-interference ability.

[0026] 3) The inwardly concave negative curvature structure cladding disclosed in this invention is made of a highly transparent alumina ceramic substrate that is more resistant to high temperature and corrosion. The real part of the refractive index is 1.2-1.8μm, which makes it easier to better control the refractive index of the hollow fiber. At the same time, its good light transmittance can reduce loss and significantly optimize the optical performance of the hollow fiber. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the structure of the present invention. Detailed Implementation

[0028] The present invention will now be described in detail so that its advantages and features can be more easily understood by those skilled in the art, thereby providing a clearer and more explicit definition of the scope of protection of the present invention.

[0029] The following provides a brief overview of one or more aspects to offer a basic understanding of them. This overview is not an exhaustive summary of all conceived aspects, nor is it intended to identify key or decisive elements of all aspects, nor to define the scope of any or all aspects. Its sole purpose is to present some concepts of one or more aspects in a simplified form to prepare for the more detailed descriptions that follow.

[0030] like Figure 1 As shown, this invention discloses a hollow-core optical fiber, comprising a coating layer 1, an inwardly recessed negative curvature structure cladding 2, anti-resonant units 3, and an air-filled core 4. The coating layer 1 is disposed outside the inwardly recessed negative curvature structure cladding 2, and the radial cross-section of the cladding cavity has 12 or more sides with negative curvature. Twelve or more anti-resonant units 3 are uniformly arranged on the inner wall of the inwardly recessed negative curvature structure cladding 2. The air-filled core 4 is located at the center of the hollow-core optical fiber and is tangent to the outer wall of the anti-resonant units 3 surrounding it. This invention, by designing an inwardly recessed negative curvature structure cladding 2, can reduce fiber loss and bending loss, and solve the problem of high loss due to the circular boundary of ordinary claddings.

[0031] In some embodiments, the total diameter of the hollow fiber is 240-250 μm, the total diameter of the hollow fiber excluding the coating layer 1 is 110-150 μm, and the diameter of the air-filled fiber core 4 is 33-45 μm.

[0032] In some embodiments, coating layer 1 is made of MgAl2O4, which remains stable at a high temperature of 1200℃. The purpose of coating layer 1 is to prevent external substances from entering the structure of the hollow fiber and avoid interference with the optical performance of the fiber.

[0033] In some embodiments, the anti-resonance unit is formed by nesting several capillaries of different diameters from the inside out. Preferably, each anti-resonance unit 3 is formed by nesting three capillaries of different diameters from the outside in. The three capillaries of different diameters are referred to as the first capillary 5, the second capillary 6, and the third capillary 7 from the outside in. The inner and outer walls of the three capillaries are tangent to each other. Specifically, the inner wall of the first capillary 5 is tangent to the outer wall of the second capillary 6, and the inner wall of the second capillary 6 is tangent to the outer wall of the third capillary 7.

[0034] In some embodiments, the diameter of the first capillary 5 is 18-27 μm, the diameter of the second capillary 6 is 13-17 μm, and the diameter of the third capillary 7 is 8-12 μm.

[0035] In some implementations, the capillary is made of quartz.

[0036] In some embodiments, the outer sheath is made of a highly transparent alumina ceramic substrate with a real part of refractive index of 1.2-1.8 μm, used to form an inwardly concave negative curvature structure outer cladding 2.

[0037] This invention also discloses a method for preparing hollow-core optical fiber, comprising the following steps:

[0038] S1: Several capillary tubes of different diameters of the required specifications are drawn through the tube drawing platform, and various assembly materials or instruments, including the capillary tubes, are washed with water. During the washing process, M3 paper is inserted into the capillary tubes, and the glass shards inside the tubes are removed by water pressure to prevent breakage during the extension process.

[0039] S2: Assemble the capillaries into anti-resonance units 3 by using the assembly platform in a nested manner from the inside out, and make all the anti-resonance units 3 in the same way.

[0040] S3: Evenly adhere all anti-resonant units 3 to the inner wall of the recessed area of ​​the outer tube to maintain the overall shape and integrity of the subsequently drawn optical fiber, preserving the circular cross-sectional shape of the hollow fiber. Use a welding process to bond the capillary to the outer tube. Perform vacuum treatment on the assembled structure to achieve a vacuum level of 10. -6 mbar or even higher, which facilitates subsequent filling with a special gas of low refractive index. After welding the tail tube, the temperature is slowly reduced to prevent the tube structure from cracking due to stress caused by excessive heating. Tail tube welding helps to form a sealed structure in the hollow fiber intermediate, preventing external moisture, dust and other impurities from entering. It should be noted that if a voltage divider device has already been installed, tail tube welding is not necessary.

[0041] S4: After welding, the fiber is extended and drawn. During the drawing process, the pressure difference and surface tension create an inwardly concave negative curvature structural boundary, forming an anti-resonant nested structure surrounded by an inwardly concave negative curvature outer cladding 2. Then, a low-refractive-index gas is filled to further reduce light transmission loss in the optical fiber. Preferably, the gas filled in this step is any one or a combination of compressed air, nitrogen, helium, and argon. A nodeless anti-resonant cavity forms the inner cladding. The diameter of the gas-filled fiber core 4 is controlled between 33-45 μm, the minimum wall thickness of the anti-resonant nested structure is ≤1 μm, and the wall thickness of the anti-resonant unit 3, formed by sequentially nesting capillaries of different diameters, all satisfy the anti-resonant reflection condition.

[0042] The hollow-core fiber structure prepared using this invention exhibits low transmission loss, low confinement loss over a wide wavelength range of 1.87-4 μm, a large mode field area, and near-zero flat dispersion with a dispersion coefficient between 0 ± 0.45 ps / (nm / km) in the 1.46-1.65 μm wavelength range, meeting the requirements for optical communication and fiber optic sensing. This hollow-core fiber structure can reduce the fundamental mode transmission loss to below 0.08 dB / km while maintaining a loss difference of 2-3 orders of magnitude from higher-order modes, thereby avoiding inter-mode crosstalk and achieving single-mode transmission.

[0043] Example 1

[0044] like Figure 1 As shown, a hollow optical fiber includes a coating layer 1, an inwardly recessed negative curvature structure outer cladding 2, anti-resonant units 3, and an air-filled core 4. The coating layer 1 is disposed outside the inwardly recessed negative curvature structure outer cladding 2, and all twelve sides of the radial cross-section of the inner cavity of the outer cladding are negative curvature structures. There are twelve anti-resonant units 3, which are evenly arranged on the inner wall of the inwardly recessed negative curvature structure outer cladding 2. The air-filled core 4 is located at the center of the hollow optical fiber and is tangent to the outer wall of the anti-resonant unit 3. By designing the inwardly recessed negative curvature structure outer cladding 2, fiber loss and bending loss can be reduced, and the problem of high loss due to the circular boundary of ordinary outer cladding can be solved.

[0045] In this embodiment, the total diameter of the hollow fiber is 245 μm, the total diameter of the hollow fiber excluding the coating layer 1 is 110 μm, and the diameter of the air-filled fiber core 4 is 36 μm.

[0046] Coating layer 1 is made of MgAl2O4, which remains stable at a high temperature of 1200℃. The purpose of coating layer 1 is to prevent external substances from entering the structure of the hollow fiber and avoid interference with the optical performance of the fiber.

[0047] Each anti-resonant unit 3 is composed of three capillaries of different diameters nested together. The capillaries are made of quartz material. The three capillaries of different diameters are named from the outside to the inside as the first capillary 5, the second capillary 6, and the third capillary 7. The inner and outer walls of the three capillaries are tangent to each other. The inner wall of the first capillary 5 is tangent to the outer wall of the second capillary 6, and the inner wall of the second capillary 6 is tangent to the outer wall of the third capillary 7. The diameter of the first capillary 5 is d1 = 19.87 μm, the diameter of the second capillary 6 is d2 = 13.39 μm, and the diameter of the third capillary 7 is d3 = 9.02 μm.

[0048] The outer tube is made of a highly transparent alumina ceramic base tube with a thickness of 0.75μm and a real part of refractive index of 1.76μm. It is used to form an inwardly concave negative curvature structure outer cladding 2, and satisfies d1 / D=0.552, where D is the diameter of the air-filled fiber core 4.

[0049] A method for fabricating hollow-core optical fiber includes the following steps:

[0050] S1: The required specifications of the first capillary tube 5, the second capillary tube 6, and the third capillary tube 7 are drawn through the tube drawing platform, and the assembly materials including the first capillary tube 5, the second capillary tube 6, and the third capillary tube 7 are washed with water. During the washing, M3 paper is inserted into the capillary tube, and the glass shards in the tube are removed by the action of water pressure to prevent breakage during the extension.

[0051] S2: By assembling the third capillary tube 7, the second capillary tube 6, and the first capillary tube 5 into an anti-resonance unit 3 according to the nesting pattern from the inside to the outside through the assembly platform, 12 anti-resonance units 3 are made in the same way.

[0052] S3: Evenly adhere all anti-resonant units 3 to the inner wall of the recessed area of ​​the outer tube to maintain the overall shape and integrity of the subsequently drawn optical fiber, preserving the circular cross-sectional shape of the hollow fiber. Use a welding process to bond the capillary to the outer tube. Perform vacuum treatment on the assembled structure to achieve a vacuum level of 10. -6 mbar facilitates subsequent filling with a special low-refractive-index gas. After welding the tail tube, the temperature is slowly reduced to prevent the tube structure from cracking due to stress caused by excessive heating. Tail tube welding helps to form a sealed structure in the hollow fiber intermediate, preventing external moisture, dust and other impurities from entering. If a voltage divider device has already been installed, tail tube welding is not necessary.

[0053] S4: After welding, the fiber is extended and drawn. During the drawing process, the pressure difference and surface tension create an inwardly concave negative curvature structural boundary, forming an anti-resonant nested structure surrounded by an inwardly concave negative curvature outer cladding 2. Then, low-refractive-index nitrogen gas is filled to further reduce light transmission loss in the optical fiber. A nodeless anti-resonant cavity constitutes the inner cladding. The diameter of the gas-filled fiber core 4 is controlled at 36 μm, the minimum wall thickness of the anti-resonant nested structure is 1 μm, and the wall thickness of the anti-resonant unit 3, formed by sequentially nesting capillaries of different diameters, all satisfy the anti-resonant reflection condition.

[0054] The hollow-core fiber structure fabricated using this embodiment exhibits low transmission loss, low confinement loss over a wide wavelength range (1.87-4 μm), a large mode field area, near-zero flat dispersion (dispersion coefficient of approximately 0.26 ps / (nm / km) in the 1.46-1.65 μm wavelength range), and good single-mode characteristics at 3 μm, meeting the requirements for optical communication and fiber optic sensing. The loss in the 1550-1610 nm range is 0.182 dB / km, comparable to the performance of the latest generation of glass-core fibers. At a limiting bending radius of 0.25 cm, the bending loss of this hollow-core fiber is measured to be 3.56 × 10⁻⁶ per turn. -3 dB@1550nm.

[0055] Example 2

[0056] like Figure 1 As shown, a hollow optical fiber includes a coating layer 1, an inwardly recessed negative curvature structure outer cladding 2, anti-resonant units 3, and an air-filled fiber core 4. The coating layer 1 is disposed outside the inwardly recessed negative curvature structure outer cladding 2, and all twelve sides of the radial cross-section of the inner cavity of the outer cladding are negative curvature structures. A total of twelve anti-resonant units 3 are disposed, evenly arranged on the inner wall of the inwardly recessed negative curvature structure outer cladding 2. The air-filled fiber core 4 is located at the center of the hollow optical fiber and is tangent to the outer wall of the anti-resonant unit 3 surrounding it. By designing the inwardly recessed negative curvature structure outer cladding 2, fiber loss and bending loss can be reduced, and the problem of high loss due to the circular boundary of ordinary outer cladding can be solved.

[0057] In this embodiment, the total diameter of the hollow fiber is 250 μm, the total diameter of the hollow fiber excluding the coating layer 1 is 149 μm, and the diameter of the air-filled fiber core 4 is 44 μm.

[0058] Coating layer 1 is made of MgAl2O4, which remains stable at a high temperature of 1200℃. The purpose of coating layer 1 is to prevent external substances from entering the structure of the hollow fiber and avoid interference with the optical performance of the fiber.

[0059] Each anti-resonance unit 3 is composed of three capillaries of different diameters nested together. The capillaries are made of quartz material. The three capillaries of different diameters are named from the outside to the inside as the first capillary 5, the second capillary 6, and the third capillary 7. The inner and outer walls of the three capillaries are tangent to each other. The inner wall of the first capillary 5 is tangent to the outer wall of the second capillary 6, and the inner wall of the second capillary 6 is tangent to the outer wall of the third capillary 7. The diameter of the first capillary 5 is d1 = 25.03 μm, the diameter of the second capillary 6 is d2 = 16.86 μm, and the diameter of the third capillary 7 is d3 = 11.36 μm.

[0060] The outer tube is made of a highly transparent alumina ceramic base tube with a thickness of 0.4 μm and a real part of refractive index of 1.757 μm, which is used to form an inwardly concave negative curvature structure outer cladding layer 2.

[0061] A method for fabricating hollow-core optical fiber includes the following steps:

[0062] S1: The required specifications of the first capillary tube 5, the second capillary tube 6, and the third capillary tube 7 are drawn through the tube drawing platform, and the assembly materials including the first capillary tube 5, the second capillary tube 6, and the third capillary tube 7 are washed with water. During the washing, M3 paper is inserted into the capillary tube, and the glass shards in the tube are removed by the action of water pressure to prevent breakage during the extension.

[0063] S2: By assembling the third capillary tube 7, the second capillary tube 6, and the first capillary tube 5 into an anti-resonance unit 3 according to the nesting pattern from the inside to the outside through the assembly platform, 12 anti-resonance units 3 are made in the same way.

[0064] S3: Evenly adhere all anti-resonant units 3 to the inner wall of the recessed area of ​​the outer tube to maintain the overall shape and integrity of the subsequently drawn optical fiber, preserving the circular cross-sectional shape of the hollow fiber. Use a welding process to bond the capillary to the outer tube. Perform vacuum treatment on the assembled structure to achieve a vacuum level of 10. -6 mbar facilitates subsequent filling with a special low-refractive-index gas. After welding the tail tube, the temperature is slowly reduced to prevent the tube structure from cracking due to stress caused by excessive heating. Tail tube welding helps to form a sealed structure in the hollow fiber intermediate, preventing external moisture, dust and other impurities from entering. If a voltage divider device has already been installed, tail tube welding is not necessary.

[0065] S4: After welding, the fiber is extended and drawn. During the drawing process, the pressure difference and surface tension create an inwardly concave negative curvature structural boundary, thus forming an anti-resonant nested structure surrounded by an inwardly concave negative curvature structural cladding 2. Then, a low-refractive-index gas is filled to further reduce light transmission loss in the optical fiber. Preferably, the gas is any one or a combination of compressed air, nitrogen, helium, and argon. Furthermore, a nodeless anti-resonant cavity constitutes the inner cladding. The diameter of the gas-filled fiber core 4 is controlled at 44 μm, the minimum wall thickness of the anti-resonant nested structure is 1 μm, and the wall thickness of the anti-resonant unit 3, formed by sequentially nesting capillaries of different diameters, all satisfy the anti-resonant reflection condition.

[0066] The hollow-core fiber structure prepared using this embodiment exhibits low transmission loss, low confinement loss over a wide wavelength range (1.87-4 μm), a large mode field area, near-zero flat dispersion (dispersion coefficient of approximately 0.25 ps / (nm / km) in the 1.46-1.65 μm wavelength range), and good single-mode characteristics at 3 μm, meeting the requirements for optical communication and fiber optic sensing. The loss in the 1550-1610 nm range is 0.178 dB / km, comparable to the performance of the latest generation of glass-core fibers. At a limiting bending radius of 0.25 cm, the bending loss of this hollow-core fiber is measured to be 3.67 × 10⁻⁶ per turn. -3 dB@1550nm.

[0067] The rest is the same as in Example 1.

[0068] Any parts or structures not specifically described in this invention can be made using existing technologies or products, and will not be elaborated upon here.

[0069] 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 specification, 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 hollow-core optical fiber, characterized in that, The fiber includes a coating layer, an inwardly recessed negative curvature structure outer cladding, anti-resonant units, and an air-filled fiber core. The coating layer is disposed outside the inwardly recessed negative curvature structure outer cladding. There are 12 or more anti-resonant units, which are evenly arranged on the inner wall of the inwardly recessed negative curvature structure outer cladding. The air-filled fiber core is located at the center of the hollow fiber and is tangent to the outer wall of the anti-resonant units surrounding it. The radial cross-section of the inner cavity of the inwardly concave negative curvature structure has 12 sides and above that are all negative curvature structures; The inwardly concave negative curvature structure outer cladding is made of a highly transparent alumina ceramic substrate tube with a real part of refractive index of 1.2-1.8 μm; Each anti-resonance unit consists of three capillaries of different diameters nested from the inside out. These three capillaries, from the outside in, are designated as the first capillary, the second capillary, and the third capillary. The inner wall of the first capillary is tangent to the outer wall of the second capillary, and the inner wall of the second capillary is tangent to the outer wall of the third capillary. The diameter of the first capillary is 18-27 μm, the diameter of the second capillary is 13-17 μm, and the diameter of the third capillary is 8-12 μm.

2. The hollow-core optical fiber according to claim 1, characterized in that, The first capillary has a diameter of 25.03 μm, the second capillary has a diameter of 16.86 μm, and the third capillary has a diameter of 11.36 μm.

3. The hollow-core optical fiber according to claim 1, characterized in that, The first capillary has a diameter of 19.87 μm, the second capillary has a diameter of 13.39 μm, and the third capillary has a diameter of 9.02 μm.

4. A hollow-core optical fiber according to claim 1, characterized in that, The total diameter of the hollow fiber is 240-250μm, the total diameter of the hollow fiber excluding the coating layer is 110-150μm, and the diameter of the air-filled fiber core is 33-45μm.

5. A method for preparing a hollow-core optical fiber according to any one of claims 1-4, characterized in that, Includes the following steps: S1: Several capillary tubes of different diameters are drawn using a tube drawing platform; S2: Capillaries of different diameters are made into anti-resonance units; S3: Evenly adhere all anti-resonant units to the inner wall of the recessed area of ​​the outer sleeve. Use a welding process to bond the capillary tube to the outer sleeve. Perform vacuum treatment on the assembled structure to achieve a vacuum level of 10. -6 mbar or even higher, which facilitates subsequent filling with special gases of low refractive index; S4: After welding, the fiber is extended and drawn. During the drawing process, the negative curvature structure boundary is formed by the pressure difference and surface tension, thus forming an anti-resonant nested structure surrounded by an inwardly concave negative curvature structure cladding. Then, a low refractive index gas is filled to further reduce the transmission loss of light in the optical fiber.

6. The method for preparing a hollow optical fiber according to claim 5, characterized in that, The low-refractive-index gas is any one or a combination of several of the following: compressed air, nitrogen, helium, and argon.

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