An ultra-low loss hollow antiresonant optical fiber with a birefringent structure

Abstract

This invention relates to an ultra-low loss hollow-core antiresonant optical fiber with a birefringent structure, comprising an outer cladding and an inner cladding. The inner cladding is composed of antiresonant structural units arranged circumferentially along and connected to the inner wall of the outer cladding. The inner cladding includes four antiresonant structural units, evenly spaced circumferentially, comprising two long-axis antiresonant structural units and two short-axis antiresonant structural units. The two long-axis antiresonant structural units are symmetrically arranged on both sides of the inner wall of the outer cladding, and the two short-axis antiresonant structural units are symmetrically arranged on the other two sides of the inner wall of the outer cladding. The central cavity enclosed by the inner cladding formed by the four antiresonant structural units forms an elliptical fiber core with both major and minor axes. This invention can reduce transmission loss while maintaining high birefringence performance. The microstructure cladding has regular shapes and is easy to fabricate.

Description

Technical Field

[0001] This invention relates to an ultra-low loss hollow anti-resonant optical fiber with a birefringent structure, which can realize low-loss single-polarization fundamental mode transmission and belongs to the field of special optical fiber technology. Background Technology

[0002] Polarization-maintaining fiber is widely used in various sectors of the national economy, including aerospace, aviation, marine, industrial manufacturing, and communications. Existing polarization-maintaining fibers are solid-core fibers. By controlling the core shape or applying stress units to the cladding, effective single-polarization fundamental mode transmission can be achieved, and they have been used for many years in fields such as fiber optic gyroscopes in aerospace. However, because the optical field transmission is within a solid core, the performance of solid-core polarization-maintaining fibers deteriorates significantly under external high / low temperature interference or strong radiation, thus reducing their application value. Furthermore, due to fluid density fluctuations during fiber drawing, the intrinsic Rayleigh scattering loss of solid-core fibers has a limit that cannot be further reduced, leading to higher backscattering, all of which impair their performance in practical applications.

[0003] Hollow-core antiresonant optical fibers can confine most of the optical field within the air core for low-loss transmission via antiresonant reflective waveguides, making them extremely insensitive to external factors such as temperature and radiation. The overlap between the fiber core optical field and the quartz portion can be as low as 10. -5 (Related to wavelength and core diameter). Therefore, compared to another type of hollow fiber in the hollow fiber family, hollow antiresonant fiber has lower backscattered light intensity and lower transmission loss. To date, the transmission loss of hollow antiresonant fiber in the near-infrared band has significantly surpassed that of solid single-mode fiber. However, due to the very low overlap between the fiber and the silica, achieving a birefringent structure in hollow antiresonant fiber presents a significant challenge.

[0004] Existing patents, such as CN118795594A, CN 119224919A, CN116840966A, and CN116699754A, achieve high birefringence at the target wavelength by altering the shape of one or more antiresonant elements in the microstructure cladding and / or the shape of the outer cladding and its internal boundary surfaces. It is generally agreed that current hollow-core fiber fabrication processes cannot produce structures where the antiresonant elements are elliptical (or other non-circular or non-arc shapes) or where the outer cladding and its internal boundary surfaces are irregularly shaped. Therefore, assessing fabrication feasibility is crucial before considering the optical performance of the fiber. CN117872524A and CN118068479A achieve high birefringence by changing the wall thickness of the antiresonant layer in two orthogonal directions using a four-unit circular arc structure, and their fabrication feasibility exists, but the loss is relatively high and difficult to further reduce.

[0005] CN111474628A discloses a polarization-maintaining hollow-core antiresonant optical fiber. The microstructure region comprises three thin-walled layers. High birefringence is introduced through the thickness difference between the first and second thin walls, and the birefringence effect resulting from the thickness difference is effectively amplified through the quasi-multiple symmetry structure of the first and second thin walls. While the aforementioned patent demonstrates fabrication feasibility and considerable birefringence performance, the fiber attenuation remains high, and the attenuation spectrum exhibits severe fluctuations, indicating significant instability in practical applications. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide an ultra-low loss hollow anti-resonant optical fiber with a birefringent structure that addresses the shortcomings of the prior art. It can reduce transmission loss while maintaining high birefringence and is easy to fabricate.

[0007] The technical solution adopted by the present invention to solve the above-mentioned problems is as follows: It includes an outer cladding layer and an inner cladding layer. The inner cladding layer is composed of anti-resonant structural units, which are arranged circumferentially along the inner wall of the outer cladding layer and connected to the inner wall of the outer cladding layer. The central cavity covered by the inner cladding layer forms a fiber core. The inner cladding layer comprises four anti-resonant structural units, which are evenly spaced circumferentially. Each of the four anti-resonant structural units includes two long-axis anti-resonant structural units and two short-axis anti-resonant structural units. The two long-axis anti-resonant structural units are symmetrically arranged on both sides of the inner wall of the outer cladding layer, and the two short-axis anti-resonant structural units are symmetrically arranged on the other two sides of the inner wall of the outer cladding layer. The central cavity covered by the inner cladding layer formed by the four anti-resonant structural units forms an elliptical fiber core with both long and short axes.

[0008] According to the above scheme, the long-axis anti-resonant structure unit includes at least three layers of anti-resonant tubes with different radii, including an outer anti-resonant tube and an inner anti-resonant tube. The outer anti-resonant tube is an arc-shaped anti-resonant tube, and the inner anti-resonant tube includes arc-shaped anti-resonant tubes and / or circular anti-resonant tubes with different radii. The outer periphery of the arc-shaped outer anti-resonant tube has a long distance from the center of the fiber core, which is the long axis of the elliptical fiber core.

[0009] According to the above scheme, in the long-axis anti-resonant structure unit, each arc anti-resonant tube intersects with the inner wall of the outer cladding or with the adjacent outer anti-resonant tube, and each circular anti-resonant tube is tangent to each other, or each circular anti-resonant tube is tangent to the inner wall of the outer cladding.

[0010] According to the above scheme, the short-axis anti-resonant structure unit includes at least three layers of anti-resonant tubes with different radii, including an outer anti-resonant tube and an inner anti-resonant tube. The outer anti-resonant tube is a circular anti-resonant tube, and the inner anti-resonant tube includes circular anti-resonant tubes with different radii. The outer periphery of the circular outer anti-resonant tube has a short distance from the center of the fiber core, which is the short axis of the elliptical fiber core.

[0011] According to the above scheme, the inner and outer circular anti-resonant tubes in the short-axis anti-resonant structure unit are tangent to a point and tangent to the inner cavity wall of the outer cladding.

[0012] According to the above scheme, both the outer cladding and the inner wall are circular, the two long-axis anti-resonant structural units are identical, and the two short-axis anti-resonant structural units are identical.

[0013] According to the above scheme, the major axis of the elliptical fiber core is 30±2μm, and the minor axis of the elliptical fiber core is 20±2μm.

[0014] According to the above scheme, the four anti-resonant structural units in the inner cladding layer maintain a circumferential gap of 4.5±0.5μm.

[0015] According to the above scheme, the short-axis anti-resonance structure unit includes three layers of circular anti-resonance tubes with different radii, including one circular outer anti-resonance tube and two circular inner anti-resonance tubes with different radii. The inner and outer circular anti-resonance tubes are tangent to a point and tangent to the inner wall of the outer cladding layer. The long-axis anti-resonance structure unit includes three layers of arc-shaped anti-resonance tubes with different radii, including one arc-shaped outer anti-resonance tube and two arc-shaped inner anti-resonance tubes with different radii. Each arc-shaped anti-resonance tube intersects with the inner wall of the outer cladding layer.

[0016] According to the above scheme, the short-axis anti-resonance structure unit includes three layers of circular anti-resonance tubes with different radii, including one circular outer anti-resonance tube and two circular inner anti-resonance tubes with different radii. The inner and outer circular anti-resonance tubes are tangent to a point and tangent to the inner wall of the outer cladding layer. The long-axis anti-resonance structure unit includes three layers of anti-resonance tubes with different radii, including one arc-shaped outer anti-resonance tube and two circular inner anti-resonance tubes with different radii. The arc-shaped outer anti-resonance tube intersects with the inner wall of the outer cladding layer, and the two circular inner anti-resonance tubes with different radii are tangent to a point and tangent to the inner wall of the outer cladding layer.

[0017] According to the above scheme, the short-axis anti-resonance structure unit includes three layers of circular anti-resonance tubes with different radii, including one circular outer anti-resonance tube and two circular inner anti-resonance tubes with different radii. The inner and outer circular anti-resonance tubes are tangent to a point and tangent to the inner cavity wall of the outer cladding. The long-axis anti-resonance structure unit includes three layers of anti-resonance tubes with different radii, including one arc-shaped outer anti-resonance tube, one arc-shaped inner anti-resonance tube, and one circular inner anti-resonance tube arranged inside the arc-shaped inner anti-resonance tube. The inner and outer arc-shaped anti-resonance tubes intersect with the inner wall of the outer cladding, and the circular inner anti-resonance tube is tangent to the inner cavity wall of the outer cladding.

[0018] According to the above scheme, auxiliary structural units are symmetrically arranged on both sides of the circular inner anti-resonant tube, and the auxiliary structural units are connected to the inner cavity wall of the outer cladding.

[0019] According to the above scheme, the auxiliary structural unit includes an arc tube or a circular tube, wherein the arc tube intersects with the inner wall of the outer cladding layer, and the circular tube is tangent to the inner wall of the outer cladding layer.

[0020] According to the above scheme, the wall thickness of the arc-shaped outer anti-resonant tube of the long-axis anti-resonant structure unit is inconsistent with the wall thickness of the circular outer anti-resonant tube of the short-axis anti-resonant structure unit, and both are different from the wall thickness of the inner anti-resonant tube of each anti-resonant structure unit.

[0021] According to the above scheme, the arc angle of the circular anti-resonant tube is 120±5°.

[0022] According to the above scheme, the substrate materials of the outer cladding layer and the anti-resonance structural unit are both pure quartz glass.

[0023] According to the above scheme, the core region and other cavity regions within the outer cladding are filled with gas.

[0024] According to the above scheme, the gas is argon, nitrogen, helium, air, or a mixture of multiple gases.

[0025] According to the above scheme, the diameter of the outer coating layer is 200~250μm; the outer coating layer is covered with a coating layer, and the diameter of the coating layer is 320~410μm.

[0026] According to the above scheme, the limiting loss of the optical fiber is <0.1dB / km.

[0027] According to the above scheme, the optical fiber has an 8×10 Ω·cm configuration in the low-loss band. -5 Its birefringence properties.

[0028] The beneficial effects of this invention are: 1. The optical fiber microstructure is maintained by four sets of nested anti-resonant units, through...

[0029] The inconsistent wall thickness of the anti-resonant elements near the fiber core in two orthogonal directions introduces birefringence. Furthermore, in the direction with a thinner wall thickness (radial distance between the anti-resonant structural unit and the inner wall of the cladding cavity), an arc-shaped nested anti-resonant structure is introduced as the anti-resonant element near the fiber core. This increases the fiber core diameter in this direction and further segments the air region formed by this structure, the circular tubular nested structure in the other orthogonal direction, and the boundary of the inner wall of the cladding cavity, thus hindering the risk of fundamental mode energy leakage along the gaps between the anti-resonant units. Furthermore, all four anti-resonant units contain multiple layers of anti-resonant elements, reducing the energy leakage of the fiber core's fundamental mode along the radial direction of the anti-resonant units. The entire microstructure cladding surrounds the fiber core into an elliptical core with both major and minor axes. The optical field along the minor axis has a high degree of overlap with the solid material of the microstructure cladding, enhancing the birefringence effect. The major axis effectively increases the fiber core diameter, thus significantly reducing the attenuation of the birefringent hollow-core anti-resonant fiber while maintaining high birefringence. The fiber's limiting loss is <0.1dB / km, and it has 8×10⁻⁶ Ω·cm in the low-loss band. -5 The birefringence property further increases the difference between the long and short axes of the fiber core, reaching 1×10⁻⁶. -4 1. **Birefringence performance.** 2. The long-axis anti-resonant structural unit is equipped with an auxiliary anti-resonant structural unit or an inner arc-shaped anti-resonant tube. Its function is to fill the excessive cavity area formed between the outermost layer and the inner cavity wall of the outer cladding due to the use of an arc-shaped structure, thereby reducing fiber core energy leakage. 3. Except for the outermost structure introducing the birefringence effect, the rest are anti-resonant elements. According to the corresponding light guiding theory, modifying the thin-wall thickness can change the low-loss operating band of the optical fiber. The target wavelength, for the wall thickness of the outermost tubular structure corresponding to the short axis of the fiber core, falls on the short-wavelength edge of the light guiding band. The target wavelength, for the wall thickness of the outermost arc-shaped structure corresponding to the long axis of the fiber core, falls on the long-wavelength edge of the light guiding band. 4. Since the microstructure cladding is composed of tubular and arc-shaped structures, and the outer cladding along with its internal boundary is a regular circle without irregular structures, all parts of the optical fiber cross-section can be prepared by the stacking-drawing method. Therefore, this invention has the characteristic of being easy to prepare. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the radial cross-section structure of an embodiment of the present invention.

[0031] Figure 2 This is the real part of the effective refractive index of the two polarization fundamental modes obtained by simulation in one embodiment of the present invention.

[0032] Figure 3 This is the confined loss of two polarization fundamental modes obtained by simulation in one embodiment of the present invention.

[0033] Figure 4 This is a birefringence curve obtained through simulation in one embodiment of the present invention.

[0034] Figure 5 This is a schematic diagram of the radial cross-section structure of the second embodiment of the present invention.

[0035] Figure 6 This is a schematic diagram of the radial cross-section structure of the third embodiment of the present invention. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments.

[0037] The first embodiment of the invention is as follows: Figures 1-4 As shown, Figure 1The horizontal and vertical directions are X and Y, respectively. In this embodiment, the fiber structure includes an outer cladding layer 2 and an inner cladding layer. The inner cladding layer includes four anti-resonant structural units, which are evenly distributed circumferentially. These four anti-resonant structural units include two long-axis anti-resonant structural units and two short-axis anti-resonant structural units. The two long-axis anti-resonant structural units are symmetrically arranged on the left and right sides of the inner wall of the outer cladding layer, i.e., on both sides of the inner wall of the outer cladding layer along the X direction. The two short-axis anti-resonant structural units are symmetrically arranged on the upper and lower sides of the inner wall of the outer cladding layer, i.e., on both sides of the inner wall of the outer cladding layer along the Y direction. Each short-axis anti-resonant structural unit includes three layers of different half-wavelengths. The circular anti-resonant tube includes one circular outer anti-resonant tube 7 and two circular inner anti-resonant tubes 8 and 9 with different radii. The inner and outer circular anti-resonant tubes are tangent to a point and tangent to the inner cavity wall of the outer cladding. The long-axis anti-resonant structure unit includes one arc-shaped outer anti-resonant tube 3 and two circular inner anti-resonant tubes 5 and 4 with different radii. The arc-shaped outer anti-resonant tube intersects the inner wall of the outer cladding. The two inner circular anti-resonant tubes with different radii are tangent to a point and tangent to the inner cavity wall of the outer cladding. Auxiliary structure units 6 are symmetrically spaced on both sides of the inner circular anti-resonant tubes. The auxiliary structure units are circular tubes and are connected to the inner cavity wall of the outer cladding. The central cavity enclosed by the inner cladding formed by four anti-resonant structural units forms an equivalent elliptical fiber core 1 tangent to the four anti-resonant structural units and possessing both major and minor axes. The outer periphery of the arc-shaped outer anti-resonant tube of the major axis anti-resonant structural unit has a longer distance from the center of the fiber core, constituting the major axis of the elliptical fiber core. The outer periphery of the circular outer anti-resonant tube of the minor axis anti-resonant structural unit has a shorter distance from the center of the fiber core, constituting the minor axis of the elliptical fiber core. A circumferential gap of approximately 4.5 μm is maintained between the four anti-resonant structural units in the inner cladding. The arc-shaped outer anti-resonant tube 3 of the major axis anti-resonant structural unit and the circular outer anti-resonant tube 7 of the minor axis anti-resonant structural unit have different wall thicknesses, thereby disrupting the phase accumulation of the fundamental mode in the two orthogonal solid layers adjacent to the fiber core and ultimately causing a birefringence effect of the fundamental mode in the fiber core. Since the overlap between the optical field and the solid layer is related to the fiber core diameter and wall thickness, the thickness of the arc-shaped external antiresonator in the long-axis antiresonator structure unit is less than the antiresonant wall thickness for the target wavelength, while the thickness of the circular external antiresonator in the short-axis antiresonator structure unit is greater than the antiresonant wall thickness for the target wavelength. Furthermore, because birefringence exacerbates fiber transmission loss, introducing a long axis can effectively increase the fiber core diameter. Combined with multiple layers of internal antiresonator elements added for the target wavelength, fiber attenuation can be significantly reduced. The auxiliary structural unit 6 of the long-axis antiresonator structure unit fills the large spatial region formed between the arc-shaped external antiresonator 3 and the circular internal antiresonator, preventing cavities that could cause energy coupling of the fiber core's fundamental mode.The arc-shaped external anti-resonant tube 3 not only changes the diameter of the fiber core along the X direction, but also further divides the air region between the circular external anti-resonant tube 7 and the inner cavity of the outer cladding 2 of the short-axis anti-resonant structural unit, reducing the energy leakage of the fiber core fundamental mode along the gap between the microstructure units.

[0038] In this embodiment, the fiber structure parameters for the 1550nm band are as follows: the minor axis diameter of the elliptical core 1 is 20μm; the major axis diameter is 30μm; the outer diameter of the circular outer anti-resonator tube 7 of the minor axis anti-resonator structural unit is 36μm, and the wall thickness is 1.42μm; the complete circle diameter of the major axis first circular arc structure 3 is 71.23μm, the arc angle is 117°, the cutting angle is 243°, and the wall thickness is 0.94μm; the outer diameters of the two circular inner anti-resonator tubes 5 and 4, and the auxiliary structural unit 6 are 22.6μm, 8μm, and 8μm, respectively; the outer diameters of the circular inner anti-resonator tubes 8 and 9 are 26μm and 12.7μm, respectively, and the wall thickness is 1.15μm, all designed for second-order anti-resonance at 1550nm. The substrate material of the outer cladding and the anti-resonator structural unit is pure quartz glass. The core region and other cavity regions within the outer cladding are filled with gas. It should be noted that the above structural parameters are established for the performance description of the first embodiment of the present invention, and the structural parameters can be adjusted for different wavelengths and fiber performance.

[0039] Figure 2 The figure shows the real part of the effective refractive index of the two polarization states of the fiber core fundamental mode obtained by simulation calculation based on the structural parameters of the above embodiment. Figure 3 The figure shows the confinement loss curve obtained by simulation calculation based on the structural parameters of the above embodiment. The Y-polarized fundamental mode loss at 1560nm can be less than 0.1dB / km, while the birefringence is 7×10⁻⁶. -5 . Figure 4 The figure shows the birefringence curve obtained from the simulation calculation based on the structural parameters of the above embodiment. In the low-loss band, the maximum birefringence occurs at 1520 nm, 8 × 10⁻⁶. -5 At this point, the polarization confinement losses of the fundamental mode X and Y are 0.3 dB / km and 0.18 dB / km, respectively.

[0040] A second embodiment of the invention, for example Figure 5As shown, its main difference from the previous embodiment is that the long-axis anti-resonant structure unit includes three layers of anti-resonant tubes with different radii, including one arc-shaped outer anti-resonant tube, one arc-shaped inner anti-resonant tube 10, and one circular inner anti-resonant tube arranged inside the arc-shaped inner anti-resonant tube. The inner and outer arc-shaped anti-resonant tubes intersect with the inner wall of the outer cladding, and the circular inner anti-resonant tube is tangent to the inner cavity wall of the outer cladding. Auxiliary structure units 11 are symmetrically spaced on both sides of the circular inner anti-resonant tube inside the arc-shaped inner anti-resonant tube. The auxiliary structure units are circular tubes and are tangent to the inner cavity wall of the outer cladding. The diameter of the circular tube of the auxiliary structure unit is the same as the diameter of the circular inner anti-resonant tube. The remaining structure is the same as the previous embodiment.

[0041] A third embodiment of the invention, for example Figure 6 As shown, its main difference from the first embodiment is that the long-axis anti-resonant structure unit includes three layers of arc-shaped anti-resonant tubes with different radii, including one arc-shaped outer anti-resonant tube and two arc-shaped inner anti-resonant tubes 10 and 12 with different radii, and each arc-shaped anti-resonant tube intersects with the inner wall of the outer cladding layer.

[0042] It should be noted that the structural parameter design principles based on the first embodiment are also applicable to the second and third embodiments. Based on design principles, theoretical expectations, and as... Figure 2-4 The simulation results show that the second and third embodiments have similar optical performance to the first embodiment.

Claims

1. An ultra-low loss hollow anti-resonant optical fiber with a birefringent structure, comprising an outer cladding and an inner cladding, wherein the inner cladding is composed of anti-resonant structural units, the anti-resonant structural units being arranged circumferentially along the inner wall of the outer cladding and connected to the inner wall of the outer cladding, and the central cavity covered by the inner cladding forming the fiber core, characterized in that... The inner cladding includes four anti-resonant structural units, which are evenly spaced circumferentially. Each anti-resonant structural unit comprises two long-axis anti-resonant structural units and two short-axis anti-resonant structural units. The two long-axis anti-resonant structural units are symmetrically arranged on both sides of the inner wall of the outer cladding, and the two short-axis anti-resonant structural units are symmetrically arranged on the other two sides of the inner wall of the outer cladding. The central cavity enclosed by the inner cladding formed by the four anti-resonant structural units forms an elliptical fiber core with both major and minor axes. Each long-axis anti-resonant structural unit includes at least three layers of anti-resonant tubes with different radii, including an outer anti-resonant tube and an inner anti-resonant tube. The resonant tube includes an outer anti-resonant tube that is arc-shaped, and an inner anti-resonant tube that includes arc-shaped inner anti-resonant tubes and / or circular inner anti-resonant tubes of different radii. The outer periphery of the arc-shaped outer anti-resonant tube has a relatively long distance from the center of the fiber core, which is the major axis of the elliptical fiber core. The short-axis anti-resonant structure unit includes at least three layers of anti-resonant tubes of different radii, including an outer anti-resonant tube and an inner anti-resonant tube. The outer anti-resonant tube is circular, and the inner anti-resonant tube includes circular inner anti-resonant tubes of different radii. The outer periphery of the circular outer anti-resonant tube has a relatively short distance from the center of the fiber core, which is the minor axis of the elliptical fiber core.

2. The ultra-low loss hollow antiresonant optical fiber with a birefringent structure according to claim 1, characterized in that... In the aforementioned long-axis anti-resonant structure unit, each arc anti-resonant tube intersects with the inner wall of the outer cladding or with an adjacent outer anti-resonant tube, and each circular anti-resonant tube is tangent to each other or to the inner wall of the outer cladding.

3. The ultra-low loss hollow antiresonant optical fiber with a birefringent structure according to claim 1, characterized in that... In the aforementioned short-axis anti-resonant structure unit, the inner and outer circular anti-resonant tubes are tangent to a single point and are also tangent to the inner cavity wall of the outer cladding layer.

4. The ultra-low loss hollow antiresonant optical fiber with a birefringent structure according to claim 1 or 2, characterized in that... The outer cladding and inner wall are both circular, the two long-axis anti-resonant structural units are identical, and the two short-axis anti-resonant structural units are identical.

5. The ultra-low loss hollow antiresonant optical fiber with a birefringent structure according to claim 1 or 2, characterized in that... The major axis of the elliptical fiber core is 30±2μm, and the minor axis of the elliptical fiber core is 20±2μm.

6. The ultra-low loss hollow antiresonant optical fiber with a birefringent structure according to claim 1 or 2, characterized in that... The four anti-resonant structural units in the inner cladding layer maintain a circumferential gap of 4.5 ± 0.5 μm.

7. The ultra-low loss hollow antiresonant optical fiber with a birefringent structure according to claim 1, characterized in that... The short-axis anti-resonance structure unit includes three layers of circular anti-resonance tubes with different radii, including one outer circular anti-resonance tube and two inner circular anti-resonance tubes with different radii. The inner and outer circular anti-resonance tubes are tangent to a point and tangent to the inner wall of the outer cladding layer. The long-axis anti-resonance structure unit includes three layers of arc-shaped anti-resonance tubes with different radii, including one outer arc-shaped anti-resonance tube and two inner arc-shaped anti-resonance tubes with different radii. Each arc-shaped anti-resonance tube intersects with the inner wall of the outer cladding layer.

8. The ultra-low loss hollow antiresonant optical fiber with a birefringent structure according to claim 1, characterized in that... The short-axis anti-resonance structure unit includes three layers of circular anti-resonance tubes with different radii, including one circular outer anti-resonance tube and two circular inner anti-resonance tubes with different radii. The inner and outer circular anti-resonance tubes are tangent to a point and tangent to the inner wall of the outer cladding layer. The long-axis anti-resonance structure unit includes three layers of anti-resonance tubes with different radii, including one arc-shaped outer anti-resonance tube and two circular inner anti-resonance tubes with different radii. The arc-shaped outer anti-resonance tube intersects with the inner wall of the outer cladding layer, and the two circular inner anti-resonance tubes with different radii are tangent to a point and tangent to the inner wall of the outer cladding layer.

9. The ultra-low loss hollow antiresonant optical fiber with a birefringent structure according to claim 1, characterized in that... The short-axis anti-resonance structure unit includes three layers of circular anti-resonance tubes with different radii, including one circular outer anti-resonance tube and two circular inner anti-resonance tubes with different radii. The inner and outer circular anti-resonance tubes are tangent to a point and tangent to the inner cavity wall of the outer cladding. The long-axis anti-resonance structure unit includes three layers of anti-resonance tubes with different radii, including one arc-shaped outer anti-resonance tube, one arc-shaped inner anti-resonance tube adjacent to the outer anti-resonance tube, and one circular inner anti-resonance tube arranged inside the arc-shaped inner anti-resonance tube. The arc-shaped inner and outer anti-resonance tubes intersect with the inner wall of the outer cladding, and the circular inner anti-resonance tube is tangent to the inner cavity wall of the outer cladding.

10. The ultra-low loss hollow anti-resonant optical fiber with a birefringent structure according to claim 8 or 9, characterized in that... The circular inner anti-resonant tube is provided with auxiliary structural units on both sides, and the auxiliary structural units are connected to the inner cavity wall of the outer cladding.

11. The ultra-low loss hollow anti-resonant optical fiber with a birefringent structure according to claim 10, characterized in that... The auxiliary structural unit includes an arc tube or a circular tube, wherein the arc tube intersects with the inner wall of the outer cladding layer, and the circular tube is tangent to the inner wall of the outer cladding layer.

12. The ultra-low loss hollow anti-resonant optical fiber with a birefringent structure according to claim 1, characterized in that... The wall thickness of the arc-shaped outer anti-resonant tube of the long-axis anti-resonant structure unit is inconsistent with the wall thickness of the circular outer anti-resonant tube of the short-axis anti-resonant structure unit, and both are different from the wall thickness of the inner anti-resonant tube of each anti-resonant structure unit.

13. The ultra-low loss hollow antiresonant optical fiber with a birefringent structure according to claim 1, characterized in that... The arc angle of the external anti-resonant tube and the internal anti-resonant tube is 120±5°.

14. The ultra-low loss hollow antiresonant optical fiber with a birefringent structure according to claim 1 or 2, characterized in that... The substrate materials for the outer cladding layer and the anti-resonance structural unit are all pure quartz glass, doped quartz glass, sulfide glass, fluoride glass, or plastic.

15. The ultra-low loss hollow antiresonant optical fiber with a birefringent structure according to claim 1 or 2, characterized in that... The core region and other cavities within the outer cladding are filled with gas.

16. The ultra-low loss hollow antiresonant optical fiber with a birefringent structure according to claim 15, characterized in that... The gas is argon, nitrogen, helium, air, or a mixture of multiple gases.

17. The ultra-low loss hollow antiresonant optical fiber with a birefringent structure according to claim 1 or 2, characterized in that... The outer coating has a diameter of 180~400μm; the outer coating is covered with a coating layer, and the coating layer has a diameter of 300~550μm.

18. The ultra-low loss hollow antiresonant optical fiber with a birefringent structure according to claim 1 or 2, characterized in that... The limiting loss of the optical fiber is <0.1dB / km.

19. The ultra-low loss hollow antiresonant optical fiber with a birefringent structure according to claim 1 or 2, characterized in that... The optical fiber described above has an 8×10 Ω·cm low-loss band. -5 Its birefringence properties.

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