Low-loss multi-nested anti-resonant fiber for terahertz communication

Abstract

This invention relates to a novel multi-nested anti-resonant optical fiber for reducing transmission loss in the terahertz band. The fiber structure includes: an air core, four sets of circular arc rectangular tube nested units, two sets of semi-circular arc hollow core nested tubes, and an outer cladding tube. The four sets of circular arc rectangular tube units are symmetrically connected to the inner wall of the outer cladding tube around the air core at an angle; the two sets of semi-circular arc hollow core nested tubes are symmetrically distributed about the x-axis and located between adjacent circular arc rectangular tube nested units. This invention introduces multi-layered nested negative curvature resonant units, utilizing the anti-resonance effect to confine terahertz waves within the air core for transmission. By optimizing the number of layers, size, and rotation angle of the circular arc rectangular tube nested units, as well as the layout of the semi-circular arc hollow core nested tubes, the transmission loss of the optical fiber is effectively reduced and higher-order modes are suppressed. This invention provides a waveguide solution for broadband, low-loss, single-mode transmission in the terahertz band, and has application potential in terahertz communication, imaging, and sensing fields.

Description

Technical Field

[0001] This invention relates to a low-loss multi-nested anti-resonant optical fiber for terahertz communication, belonging to the field of terahertz waveguide and optical fiber communication technology. Background Technology

[0002] Terahertz (THz) waves typically refer to electromagnetic waves with frequencies ranging from 0.1 to 10 THz and wavelengths from 30 μm to 3 mm, falling between microwaves and infrared light. Due to their unique penetrability, non-ionization, and characteristic absorption spectrum response, terahertz waves hold great promise in fields such as 6G communication, high-resolution imaging, biomedical sensing, and security detection. However, terahertz waves suffer significant transmission losses in traditional media, primarily due to high material absorption and waveguide structure losses. Traditional solid-core terahertz waveguides exhibit high losses and low birefringence, limiting the long-distance application of terahertz waves.

[0003] To address the aforementioned issues, hollow-core fiber, by confining the light field primarily within its air core and significantly reducing material absorption loss, has become an important development direction for terahertz waveguides. Hollow-core anti-resonant fiber, with its simple structure and low fabrication difficulty, utilizes the coherent reflection (anti-resonance effect) of the cladding dielectric film to confine light within the air core, achieving low loss across a wide wavelength range from ultraviolet to terahertz, making it a current research hotspot.

[0004] In recent years, scholars both domestically and internationally have proposed various terahertz hollow-core antiresonant fiber structures. In terms of materials, polymers such as COC, PMMA, and HDPE are widely used due to their low intrinsic absorption. Structurally, nested tubes have been proven to effectively reduce confinement loss, leading to the emergence of multi-layer nesting and elliptical tube nesting techniques. However, existing technologies still have shortcomings: most use single-shape nested tubes, limiting parameter adjustment and making it difficult to simultaneously achieve extremely low loss and stable single-mode transmission over a wide bandwidth; some high-performance designs employ rigid materials such as high-resistivity silicon, resulting in complex processes, high costs, and hindering flexibility and large-scale applications; and there is still room for improvement in high-order mode suppression.

[0005] Patent application number CN202411730261 discloses an asymmetric cladding structure single-polarization anti-resonant optical fiber operating in the terahertz band. This fiber features six longitudinally nested double-layer cladding tubes and two laterally nested double-layer cladding tubes within its cladding, achieving single-polarization transmission through the asymmetric cladding structure. At 1 THz, the patent achieves fundamental mode losses of 9.07 dB / m for X-polarization and 2.59 × 10⁻⁻⁻⁶ for Y-polarization. 5 The polarization loss ratio is as high as 349318 dB / m. However, this patent mainly focuses on single polarization characteristics and adopts a complex arrangement of eight double-layer nested tubes. It has many structural parameters and is difficult to manufacture. Moreover, its operating bandwidth is mainly concentrated around 1 THz (0.98~1.02 THz), and the narrow bandwidth limits its application in broadband terahertz communication systems.

[0006] Patent application number CN202410233905 discloses a hollow-core antiresonant optical fiber for the terahertz band. It introduces high birefringence by placing an elliptical tube in the center of the air core, and by making the diameters of the antiresonant circular tubes in the x and y directions different, increasing the optical path difference in the two directions and thus reducing transmission loss. While the patent's structural design is relatively simple, its main innovation lies in introducing birefringence through the elliptical central tube. It does not fully utilize the antiresonance enhancement effect of the multi-layer nested structure, and there is still room for improvement in loss performance.

[0007] The literature "Sun GR, Liu Q, Mu HW, et al. Anti-resonant fiber with nested U-shape tubes for low-loss terahertz waveguides[J]. Optics & Laser Technology, 2023, 109424" first used high-resistivity silicon (HRS) as the optical fiber material to design a double-layer nested U-shaped tube structure anti-resonant fiber to achieve low-loss terahertz wave transmission. The total loss at 1 THz was only 3.1 × 10⁻³ dB / m, and the low-loss transmission bandwidth in the range of 0.5~1.5 THz reached 0.44 THz. However, this fiber uses a rigid high-resistivity silicon material, which makes it difficult to achieve flexible fabrication and large-scale application; at the same time, its low-loss bandwidth is still somewhat inferior to that of this invention.

[0008] The paper "Liu Q, Sun GR, Mu HW, et al. Hybrid nested negative curvature fiber with ultra-low-loss in the terahertz band[J]. Infrared Physics & Technology, 2024" proposes a hybrid nested negative curvature fiber based on a three-layer anti-resonance effect. It employs high-resistivity silicon (HRS) and a three-layer nested structure to suppress confinement loss, achieving a total loss as low as 6.62 × 10⁻⁻⁻⁶ at 1.1 THz. 5 dB / m, with a loss of less than 10⁻ in both the 1~1.06 THz and 1.1~1.24 THz frequency bands. 4While this optical fiber offers significant advantages in loss level, its three-layer nested elliptical tube cladding structure is complex and difficult to adjust parameters. Furthermore, the use of rigid HRS material hinders flexible applications, and its low-loss bandwidth is relatively narrow, only about 0.2 THz. In contrast, this invention employs four arc-shaped rectangular tube units in a multi-layer nested structure, resulting in a simpler structure, more flexible parameter adjustment, and the use of a flexible polymer material, offering significant advantages in bandwidth and fabrication feasibility.

[0009] Compared with the prior art, the present invention has the following significant distinguishing features: First, the present invention combines two different shapes of cladding tubes—circular arc rectangular tube nested units and semi-circular arc hollow core nested tubes—instead of using a single-shape nested tube; Second, the present invention uses four sets of circular arc rectangular tube units rotated at a specific angle (33°~35°) to form a symmetrical distribution about the x-axis and y-axis, while simultaneously using two sets of semi-circular arc hollow core nested tubes symmetrical about the x-axis, together forming a quasi-elliptical air core. This structural layout combining symmetry and asymmetry is rarely seen in existing terahertz antiresonant fibers; Third, the present invention utilizes the aforementioned structural asymmetry to enhance the coupling between higher-order modes and cladding modes, thereby achieving broadband low-loss single-mode transmission without introducing rigid materials, while also considering flexible fabrication and excellent optical performance. In summary, existing terahertz hollow core antiresonant fibers still have shortcomings in terms of low-loss transmission, higher-order mode suppression, and structural flexibility. Based on the above problems, this invention proposes a multi-nested anti-resonant optical fiber with a simple structure, flexible parameter adjustment, extremely low fundamental mode transmission loss and excellent single-mode performance in the terahertz band, to meet the requirements of terahertz communication systems for high-performance waveguide devices. Summary of the Invention

[0010] The purpose of this invention is to provide a low-loss, multi-nested anti-resonant optical fiber for terahertz communication, so as to solve the problem that it is difficult to achieve broadband, ultra-low loss, single-mode transmission and flexible fabrication at the same time in the prior art.

[0011] The objective of this invention is achieved as follows:

[0012] A low-loss multi-nested anti-resonant optical fiber for terahertz communication includes: an air core, a circular arc rectangular tube nested unit, a semi-circular arc hollow core nested tube, and an outer cladding tube.

[0013] The air core is an air channel formed by the nested arc-shaped rectangular tube units and the nested semi-circular hollow tubes, and is the main light-guiding area of ​​the optical fiber. The nested arc-shaped rectangular tube units consist of four sets of composite tube structures symmetrical about the fiber core center and connected to the inner wall of the outer cladding tube. Each set of nested arc-shaped rectangular tube units comprises an outer large arc-shaped rectangular tube and three layers of smaller arc-shaped rectangular tubes nested within it. There are two sets of semi-circular hollow tubes, located on the inner wall of the outer cladding tube and situated between adjacent nested arc-shaped rectangular tube units, symmetrically distributed about the x-axis of the optical fiber. Each set of semi-circular hollow tubes comprises an outer large semi-circular tube and a smaller semi-circular tube nested within it. The outer cladding tube is the outer protective structure of the hollow optical fiber.

[0014] The core innovation of this invention lies in the following: by rotating four sets of nested circular arc rectangular tube units at a specific angle to form a symmetrical distribution about the x-axis and y-axis, and cooperating with two sets of semi-circular hollow nested tubes symmetrical about the x-axis, they together form a quasi-elliptical air fiber core. This structure utilizes the negative curvature effect of the circular arc rectangular tubes to enhance anti-resonance, and optimizes the mode constraint in the x-axis direction through the supplement of the semi-circular tubes. It also utilizes the structural asymmetry to enhance the coupling between higher-order modes and cladding modes, thereby achieving broadband, low-loss single-mode transmission without relying on rigid materials.

[0015] The working principle of this invention is based on the anti-resonance effect. When terahertz waves propagate in an air fiber core, they undergo partial reflection upon encountering the dielectric thin film (cladding wall). When the anti-resonance condition is met, the reflected light coherently enhances, effectively confining the light field within the low-refractive-index air fiber core. The anti-resonance wavelength is determined by the dielectric wall thickness, satisfying the formula: Where t is the dielectric thickness, n is the refractive index of the dielectric, and m is a positive integer (resonance order). When the operating wavelength λ is much greater than λres, it is in the anti-resonance region, limiting the loss to be relatively low. This invention selects a dielectric thickness t = 65~75 μm, so that the operating frequency band 0.9~1.5 THz (corresponding to wavelengths of 333~200 μm) falls within the anti-resonance window.

[0016] The transmission loss of optical fiber includes confinement loss and material absorption loss. Confinement loss is determined by the imaginary part of the effective refractive index of the mode, and is calculated using the following formula: Where f is the frequency and c is the speed of light. This represents the imaginary part of the effective refractive index of the mode. This invention, by employing a three-layer nested arc-rectangular tube structure, increases the anti-resonance reflection interface and significantly reduces confinement loss. Simultaneously, the special shape of the arc-rectangular tube, compared to a circular tube, allows for more flexible adjustment of the mode distribution in the cladding region, enhancing coupling leakage to higher-order modes. Furthermore, the introduction of two sets of semi-circular hollow nested tubes further optimizes mode confinement in the x-axis direction, working synergistically with the four sets of arc-rectangular tube units to improve the single-mode transmission performance of the optical fiber.

[0017] Higher-order mode suppression ratio (HOMER) is a key indicator for evaluating the single-mode performance of optical fibers, defined as: ,in, The loss is for the highest-order mode (usually LP11 mode). This represents the loss of the fundamental mode (LP01 mode). When HOMER > 100, it indicates that the optical fiber has good single-mode transmission characteristics. This invention optimizes the structural parameters to ensure that HOMER is greater than 100 within the operating frequency band.

[0018] The air fiber core of the present invention is an air hole formed by four sets of the aforementioned circular arc rectangular tube nesting units and two sets of the aforementioned semi-circular arc hollow core nesting tubes, with a width ranging from 2.0 to 3.0 mm.

[0019] The number of the circular arc rectangular tube nesting units of the present invention is four groups, which are symmetrically distributed about the x-axis and y-axis; each group of the circular arc rectangular tube nesting units includes three layers of nested small circular arc rectangular tubes; the medium thickness of the outer large circular arc rectangular tube and the inner nested small circular arc rectangular tubes is the same, and the range is 65~75 μm.

[0020] The outer large circular arc rectangular tube of this invention has a diameter of 2.0~2.5 mm for the semicircles at both ends, a width of 1.2~1.8 mm for the middle rectangular portion, a length of 2.0~2.4 mm for the rectangular portion, and a width-to-length ratio of 0.6~0.9. The arc diameters of the three nested small circular arc rectangular tubes are 0.3 times, 0.5 times, and 0.6 times the diameter of the outer large circular arc rectangular tube, respectively. The gap distance between the nested small circular arc rectangular tubes is 300~600 μm.

[0021] Each of the four sets of nested arc-shaped rectangular tube units provided by the present invention rotates around itself at a specific angle tr, the specific angle tr being in the range of 33°~35°, so that the four sets of units form a symmetrical distribution about the x-axis and y-axis.

[0022] The semi-circular hollow nested tube of this invention has a semi-circular ring structure, consisting of two groups symmetrically distributed about the x-axis, with its outer diameter connected to the inner wall of the outer sheath tube; each group has an embedded layer of small semi-circular tubes, and the inner diameter of the outer large semi-circular tube ranges from 7.2 to 8.0 mm, which is 1.5 times the inner diameter of the inner nested small semi-circular tubes; the medium thickness of the outer large semi-circular tube and the inner small semi-circular tube is the same as the medium thickness of the circular arc rectangular tube nesting unit.

[0023] The minimum gap between the circular arc rectangular tube nesting unit and the adjacent semi-circular hollow nesting tube of the present invention is 100 μm.

[0024] The materials of the air fiber core, the circular arc rectangular tube nesting unit, the semi-circular arc hollow core nesting tube, and the outer cladding tube described in this invention are all polymer materials with low absorption loss in the terahertz frequency band, including one of cyclic olefin copolymer (COC), polymethyl methacrylate (PMMA), or high-density polyethylene (HDPE), with a refractive index of 1.5 to 1.6; the remaining area is air with a refractive index of 1.0.

[0025] The optical fiber described in this invention operates in the frequency band of 0.9~1.5 THz, and its fundamental mode transmission loss within this frequency band is far less than 10⁻ 4 The transmission loss is on the order of dB / m, and less than 10⁻ at 1.2 THz. 5 The dB / m level; and the high-order mode rejection ratio HOMER is greater than 100, possessing single-mode transmission characteristics.

[0026] Due to the adoption of the above technical solution, the technical progress achieved by this invention mainly includes the following three aspects:

[0027] 1. Ultra-low transmission loss: By introducing a three-layer nested circular arc rectangular tube structure, the anti-resonance reflection interface is significantly increased, efficiently confining terahertz waves within the air fiber core and significantly reducing confinement loss. Combined with low-loss polymer materials, the fundamental modes in the x and y directions are achieved at significantly lower values ​​within a wide frequency band of 0.9–1.5 THz. A transmission loss of dB / m is achieved at 1.2 THz. dB / m y-direction fundamental mode loss, The fundamental mode loss in the x-direction of dB / m provides a feasible solution for long-distance terahertz communication.

[0028] 2. Excellent single-mode transmission characteristics: The symmetrical distribution formed by rotating four sets of circular arc rectangular tube units at a specific angle, in conjunction with two sets of semi-circular arc hollow nested tubes on the x-axis, optimizes the mode distribution in the cladding region and enhances the suppression coupling effect on higher-order modes. The higher-order mode suppression ratio (HOMER) of the optical fiber of this invention is greater than 100 in the operating frequency band, indicating that it has excellent single-mode transmission performance.

[0029] 3. Simple structure, adjustable parameters, and easy to manufacture: This invention uses only four sets of nested circular arc rectangular tube units and two sets of semi-circular arc hollow nested tubes, resulting in a regular structure and good symmetry. All media have a consistent thickness, and the material is a commonly used thermoplastic polymer, which can be prepared by stacking and stretching or extrusion molding, showing good prospects for industrialization. Attached Figure Description

[0030] Figure 1 A schematic diagram of the cross-sectional structure of a low-loss multi-nested anti-resonant optical fiber for terahertz communication provided by the present invention. Figure 1 The structural units corresponding to the labels shown are: S1-air fiber core, S2-circular arc rectangular tube nesting unit, S3-semi-circular arc hollow core nesting tube, and S4-outer cladding sleeve.

[0031] Figure 2 The mode field diagram of the multi-nested anti-resonant optical fiber provided in this embodiment of the invention at 1.2 THz;

[0032] Figure 3 The loss diagram shows the transmission loss of the multi-nested anti-resonant optical fiber in the 0.9~1.5 THz band as a function of frequency, as provided in the embodiments of the present invention.

[0033] Figure 4 The effective refractive index and phase birefringence diagrams of the multi-nested anti-resonant optical fiber provided in this embodiment of the invention are shown. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Those skilled in the art should understand that the following embodiments are only for explaining the invention and do not constitute a limitation on the scope of protection of this invention.

[0035] Example

[0036] Please refer to Figure 1 This embodiment provides a low-loss multi-nested anti-resonant optical fiber for terahertz communication, the cross-sectional structure of which includes: an air core (S1), four sets of circular arc rectangular tube nesting units (S2), two sets of semi-circular arc hollow core nesting tubes (S3), and an outer cladding tube (S4).

[0037] The specific parameters of each component of the optical fiber are as follows:

[0038] 1. Outer sheath (S4): Made of cyclic olefin copolymer (COC) material with a refractive index of 1.53. The inner diameter of the outer sheath is 10.0 mm, the outer diameter is 12.0 mm, and the wall thickness is 1.0 mm.

[0039] 2. Nested Arc-Rectangular Tube Units (S2): Four groups are symmetrically distributed around the air fiber core. Each nested arc-rectangular tube unit consists of an outer large arc-rectangular tube and three layers of smaller arc-rectangular tubes nested within it. The outer large arc-rectangular tube has a diameter of 2.2 mm at both ends of its semicircles, a width of 1.5 mm, and a length of 2.2 mm for the middle rectangular portion, with a width-to-length ratio of 0.68. The medium thickness is t = 70 μm. The arc diameters of the three nested smaller arc-rectangular tubes are 0.3 times (0.66 mm), 0.5 times (1.1 mm), and 0.6 times (1.32 mm) the diameter of the outer large arc-rectangular tube, respectively. The medium thickness of each layer is 70 μm. The gap between adjacent layers of smaller arc-rectangular tubes is 450 μm. Each of the four nested arc-rectangular tube units rotates around its own center at a specific angle tr = 34°, forming a symmetrical distribution about the x-axis and y-axis. All nested units of the circular arc rectangular tube are made of COC.

[0040] 3. Semi-circular hollow nested tubes (S3): Two sets are used, positioned symmetrically along the x-axis between adjacent circular arc rectangular tube nesting units. Each set consists of an outer large semi-circular arc tube and a smaller semi-circular arc tube nested within it. The outer diameter of the outer large semi-circular arc tube connects to the inner wall of the outer cladding sleeve (outer diameter 10.0 mm), and its inner diameter is 7.6 mm. The inner diameter of the nested smaller semi-circular arc tube is 1 / 1.5 times the inner diameter of the outer large semi-circular arc tube, approximately 5.07 mm. The medium thickness of both the outer large semi-circular arc tube and the inner smaller semi-circular arc tube is 70 μm. The minimum gap between the semi-circular hollow nested tube and the adjacent circular arc rectangular tube nesting unit is 100 μm. The material is COC.

[0041] 4. Air core (S1): An air channel formed by four sets of circular arc rectangular tube nesting units and two sets of semi-circular arc hollow core nesting tubes, with a width of about 2.5 mm. The medium is air with a refractive index of 1.0.

[0042] Working principle and simulation verification

[0043] This embodiment utilizes the anti-resonance effect to confine terahertz waves to the air fiber core for transmission. According to the anti-resonance condition formula... Substituting t=70μm, n=1.53, m=1, The corresponding frequency is approximately 1.85 THz. The operating band of 0.9~1.5 THz (wavelength 333~200 μm) is located within the anti-resonant window, and low-loss transmission is expected.

[0044] Simulations were performed using COMSOL software based on the finite element method. A perfectly matched layer (PML) absorbing boundary was set, and the outer wall of the cladding layer was set as a scattering boundary. The refractive index of the COC material in the terahertz band was set to 1.53, and the material absorption loss was set according to literature data; the refractive index of air was 1.0. The effective refractive index of the mode was calculated using a frequency domain solver, and the loss was determined according to the limiting loss formula. Calculate the limiting losses for each mode. The total loss is the sum of the limiting losses and the material absorption losses.

[0045] Please refer to Figure 2 The mode field distribution diagram at 1.2 THz in this embodiment shows that the energy of the terahertz wave is efficiently confined in the air core region, while the energy in the cladding region and nested structure is very low, indicating that the multi-nested anti-resonance effect is significant.

[0046] Please refer to Figure 3 The fundamental mode transmission loss curve of this embodiment varies with frequency. Within a wide bandwidth of 0.9~1.5 THz, the fundamental mode loss in both the x and y directions is significantly lower than that in other embodiments. The transmission loss is reduced to 1.2 THz. dB / m y-direction fundamental mode loss, The fundamental mode loss in the x-direction is measured in dB / m. This verifies that the present invention achieves ultra-wideband, ultra-low-loss transmission in the terahertz band.

[0047] Please refer to Figure 4 This embodiment presents the changes in effective refractive index and phase birefringence curves within the 0.9~1.5 THz band. Phase birefringence is defined as... As can be seen from the figure, it reaches its maximum value at approximately 1.2 THz. The birefringence remains constant throughout the entire operating frequency band. The magnitude indicates that the optical fiber of this invention has a certain polarization-maintaining capability.

[0048] Parameter adjustability and preparation method

[0049] In this embodiment, the optical fiber material is not limited to COC; it can also be terahertz low-loss polymer materials such as polymethyl methacrylate (PMMA) or high-density polyethylene (HDPE) with a refractive index between 1.5 and 1.6. By adjusting the rotation angle tr (33°~35°), the gap between adjacent units (300~600 μm), and the minimum gap (100 μm) between the semi-circular hollow nested tube and the circular rectangular tube, stable ultra-low loss transmission performance can be obtained in the 0.9~1.5 THz frequency band.

[0050] The optical fiber of this invention can be fabricated using either a stacked drawing method or an extrusion molding method. Taking the stacked drawing method as an example: First, according to the design dimensions, a COC material sleeve and various cladding tubes are customized, and then arranged according to... Figure 1 The structure shown is arranged into a preform, and each tube is positioned using a special mold; then, the preform is heated to an appropriate temperature in a drawing furnace (for COC material, the typical drawing temperature range is 200~250℃), and the feeding speed and air pressure are controlled to draw it into an optical fiber in one step.

[0051] Summary of technical effects

[0052] In summary, this embodiment achieves ultra-low transmission loss and a wide operating bandwidth (0.9~1.5 THz) in the terahertz band through the synergistic effect of multiple nested circular arc rectangular tube units (three layers nested, diameter ratio 0.3 / 0.5 / 0.6) and semi-circular arc hollow nested tubes (inner and outer double layers, outer diameter / inner diameter ratio 1.5). Simultaneously, the higher-order mode rejection ratio is far greater than 100, exhibiting excellent single-mode characteristics. This fiber structure is simple, its parameters are flexibly adjustable, and it utilizes flexible polymer materials, making it promising for applications in terahertz communication, high-resolution imaging, biomedical sensing, and security detection.

[0053] The specific parameters of this invention can be adjusted according to actual application requirements, but any modifications and changes based on the structural principles of this invention are within the protection scope of this invention.

Claims

1. A low-loss, multi-nested anti-resonant optical fiber for terahertz communication, characterized in that, The optical fiber includes: an air core, a circular arc rectangular tube nesting unit, a semi-circular arc hollow core nesting tube, and an outer cladding tube; The air core is an air channel formed by the circular arc rectangular tube nesting unit and the semi-circular arc hollow core nesting tube, and is the main light guiding area of ​​the optical fiber; The circular arc rectangular tube nesting unit consists of four sets of composite tube structures that are symmetrical about the fiber core center and connected to the inner wall of the outer sheath. Each set of the circular arc rectangular tube nesting unit consists of an outer large circular arc rectangular tube and three layers of small circular arc rectangular tubes nested inside it. The semi-circular hollow nested tubes are in two groups, set on the inner wall of the outer cladding sleeve and located between adjacent circular rectangular tube nesting units, and symmetrically distributed about the x-axis of the optical fiber. Each group of semi-circular hollow nested tubes consists of an outer large semi-circular tube and a small semi-circular tube nested inside it. The outer cladding sleeve is the outer protective structure of the hollow optical fiber.

2. The low-loss multi-nested anti-resonant optical fiber for terahertz communication according to claim 1, characterized in that, The air core is an air hole formed by four sets of the aforementioned circular arc rectangular tube nesting units and two sets of the aforementioned semi-circular arc hollow core nesting tubes, with a width ranging from 2.0 to 3.0 mm.

3. The low-loss multi-nested anti-resonant optical fiber for terahertz communication according to claim 1, characterized in that, The number of the circular arc rectangular tube nesting units is four, symmetrically distributed about the x-axis and y-axis; each group of the circular arc rectangular tube nesting units includes three layers of nested small circular arc rectangular tubes; the medium thickness of the outer large circular arc rectangular tube and the inner nested small circular arc rectangular tubes is the same, ranging from 65 to 75 μm.

4. The nested arc-shaped rectangular tube unit according to claim 3, characterized in that, The outer large circular arc rectangular tube has a diameter of 2.0~2.5 mm for the semicircles at both ends, a width of 1.2~1.8 mm for the middle rectangular section, a length of 2.0~2.4 mm for the rectangular section, and a width-to-length ratio of 0.6~0.

9. The diameters of the three nested small circular arc rectangular tubes are 0.3, 0.5, and 0.6 times the diameter of the outer large circular arc rectangular tube, respectively. The spacing between the nested small circular arc rectangular tubes is 300~600 μm.

5. The nested arc-shaped rectangular tube unit according to claim 3, characterized in that, Each of the four sets of nested arc-shaped rectangular tube units rotates around itself at a specific angle tr, which ranges from 33° to 35°, so that the four sets of units form a symmetrical distribution about the x-axis and y-axis.

6. The low-loss multi-nested anti-resonant optical fiber for terahertz communication according to claim 1, characterized in that, The semi-circular hollow nested tube is a semi-circular ring structure, consisting of two groups symmetrically distributed about the x-axis, with its outer diameter connected to the inner wall of the outer sheath tube; each group has an inner layer of small semi-circular tubes embedded within it, and the inner diameter of the outer large semi-circular tube ranges from 7.2 to 8.0 mm, which is 1.5 times the inner diameter of the inner nested small semi-circular tubes; the medium thickness of the outer large semi-circular tube and the inner small semi-circular tube is the same as the medium thickness of the circular arc rectangular tube nesting unit.

7. The low-loss multi-nested anti-resonant optical fiber for terahertz communication according to claim 1, characterized in that, The minimum gap between the circular arc rectangular tube nesting unit and the adjacent semi-circular hollow nesting tube is 100 μm.

8. The low-loss multi-nested anti-resonant optical fiber for terahertz communication according to claim 1, characterized in that, The materials of the air fiber core, the circular arc rectangular tube nesting unit, the semi-circular arc hollow core nesting tube, and the outer cladding tube are all polymer materials with low absorption loss in the terahertz frequency band, including one of cyclic olefin copolymer (COC), polymethyl methacrylate (PMMA), or high-density polyethylene (HDPE), with a refractive index of 1.5~1.6; the remaining area is air with a refractive index of 1.

0.

9. The low-loss multi-nested anti-resonant optical fiber for terahertz communication according to claim 1, characterized in that, The circular arc rectangular tube nesting unit and the semi-circular arc hollow core nesting tube together form a quasi-elliptical air fiber core on the inner wall of the outer cladding tube; the four sets of circular arc rectangular tube nesting units are symmetrically distributed about the x-axis and y-axis and rotate at an angle of 33°~35°, and the two sets of semi-circular arc hollow core nesting tubes are symmetrically distributed about the x-axis. The structural asymmetry between the circular arc rectangular tube nesting unit and the semi-circular arc hollow core nesting tube enhances the suppression of higher-order modes, realizing broadband low-loss single-mode transmission in the terahertz band; the operating frequency band of the optical fiber is 0.9 THz~1.5 THz, and the fundamental mode transmission loss in this frequency band is much lower than 10⁻ 4 The transmission loss is on the order of dB / m, and less than 10⁻ at 1.2 THz. 5 The order of magnitude is in the dB / m range.

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