Anti-resonance hollow-core optical fiber with cavity embedded in outer cladding

Abstract

The invention discloses an anti-resonance hollow-core optical fiber with a cavity embedded in an outer cladding layer. The optical fiber comprises: a hollow core (101); the main anti-resonance tube (102) is distributed around the hollow core, and the main anti-resonance tube can comprise nested anti-resonance tubes (103 and 104); and an outer cladding layer (301) surrounding the main anti-resonance tube. The anti-resonance tube is characterized in that the cavity structure (201) is completely embedded into the solid material of the outer cladding layer (301) and is not communicated with an air area defined by the inner wall (301a) of the outer cladding layer (301), the continuous solid material between the front wall (201a) of the cavity and the inner wall (301a) of the outer cladding layer forms an anti-resonance layer, the cavity structure is located at the corresponding position between the adjacent main anti-resonance tubes (102), light field gap leakage is effectively restrained, and the anti-resonance effect is improved. And the transmission loss is obviously reduced. Due to the fact that the cavity is embedded into the solid material and is rigidly restrained by 360 degrees, the manufacturing problem that the position of a suspension element in a traditional'Tube-in-Air 'structure is unstable can be solved, and the manufacturing yield is increased.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical fiber communication technology, in particular to a kind of anti-resonant hollow core optical fiber with embedded cavity structure, especially in the outer cladding solid material embedded cavity structure to suppress gap light field leakage Anti-resonant hollow core optical fiber, suitable for optical communication data transmission, high-power laser energy transmission, wide spectrum light transmission and gas sensing and other applications. BACKGROUND

[0002] Anti-resonant hollow core optical fiber is a new type of optical fiber, and its light transmission is mainly in the air core. Compared with traditional solid core optical fiber, it has the advantages of low delay, low nonlinearity, low dispersion, etc., and has great application potential in optical communication, high-power laser transmission, precision sensing and other fields.

[0003] The working principle of anti-resonant hollow core optical fiber is based on the anti-resonance effect: multiple tubular anti-resonance units are arranged in the cladding region of the optical fiber. The wall thickness of these anti-resonance units satisfies the specific anti-resonance condition, thereby forming a strong reflection effect on light in a specific wavelength range, and confining the light in the air core for transmission. Compared with traditional photonic bandgap hollow core optical fiber, anti-resonant hollow core optical fiber has a wider working bandwidth, simpler structure design and lower transmission loss.

[0004] In recent years, anti-resonant hollow core optical fiber technology has made significant progress. In particular, the emergence of double nested anti-resonant nodeless fiber (DNANF) structure, by nesting one or more concentric tubular structures inside the main anti-resonant tube, further reduces the transmission loss. In 2022, the University of Southampton and other research institutions in the UK achieved a loss of 0.174 dB / km, see: Gregory T Jasion, et al. "0.174dB / km Hollow Core Double Nested Antiresonant Nodeless Fiber (DNANF)". Optical Fiber Communication Conference (OFC) 2022. Paper Th4C.7. 2022.”).

[0005] In 2025, the team further reported a record loss of 0.091 dB / km in Nature-Photonics, surpassing the loss limit of traditional solid core optical fiber in the C-band, see: Marco Petrovich, et al. "Broadband optical fibre with an attenuation lower than 0.1 decibel per kilometre". Nature Photonics, Vol. 19, pp. 1203-1208, 2025.”.

[0006] However, these reported super low-loss hollow-core fibers usually have a large core diameter (28-35 pm) and a large mode field diameter (20-25 pm), about twice that of a standard single-mode fiber (SMF), while their outer diameter also needs to be increased to about 175-250 pm accordingly. This makes it difficult for such fibers to be directly spliced with existing standard single-mode fibers, and also cannot use standard fiber connectors and cables, greatly increasing the complexity and cost of system integration. In order to solve the compatibility problem with standard single-mode fibers, researchers have developed various mode field matching and splicing techniques. However, the use of intermediate transition fibers or special splicing devices not only increases the system cost, but also introduces additional insertion loss and reliability risks. Therefore, it is of great practical significance to develop a hollow-core fiber that can not only maintain low transmission loss, but also be directly compatible with standard single-mode fibers.

[0007] Recently, a research team from the University of Southampton, UK, reported a new design of 4-tube double-nested anti-resonant nodeless hollow-core fiber (4T-DNANF). Unlike traditional 5-tube or 6-tube designs, this 4-tube design reduces the number of main anti-resonant tubes, allowing each main anti-resonant tube to increase in size, thereby pushing the outer cladding tube further away from the fiber center, significantly reducing the confinement loss. This design achieves a core diameter of 14.7-16.1 pm, a mode field diameter of about 10.82 pm (close to the 10.4 pm of a standard single-mode fiber), and an outer diameter of 125 pm (the size of a standard single-mode fiber), allowing it to be directly spliced and interfaced with a standard single-mode fiber, see: “Ghafour Amouzad Mahdiraji, et al. "SMF-Matched, Hollow-Core DNANF with <2dB / km Loss in the O and C-bands for Intra-Datacentre Applications". ECOC 2024; 50th European Conference on Optical Communication. Frankfurt, Germany, 2024.” (hereinafter referred to as "Reference 1"), In this kind of design, the confinement loss occupies a high proportion in the total loss, which is a technical bottleneck restricting the further reduction of transmission loss.

[0008] In addition, the drawing problem often occurs in the preparation, which leads to a large gap between adjacent main anti-resonant tubes, see: “Gregory T. Jasion, et al. "Fabrication of tubular anti-resonanthollow core fibers: modelling, draw dynamics and process optimization".Optics Express, Vol. 27, Issue 15, pp. 20567-20582, 2019.”, In these gap directions, the light field lacks effective anti-resonant suppression layers, leading to light leakage from these gap directions to the outer cladding, which is one of the important sources of limiting loss.

[0009] In order to solve the above problem of high confinement loss, the prior art attempts to introduce auxiliary structures in the gap area between the main anti-resonant tubes. According to the differences in their functional targets, these technical solutions are mainly divided into two categories:

[0010] The first type of solution attempts to increase the units with anti-resonant function in the gap between the main anti-resonant tubes, in order to suppress light field leakage. For example, prior art document one (such as US12117646B2) discloses a scheme of introducing fourth tubular elements in the gap between the main anti-resonant tubes, which are attached to the inner wall of the outer cladding. Although this structure has a certain optical anti-resonant function, these tubular elements are located in the inner cladding air area and are attached in the form of "sticking to the wall", which is difficult to maintain accurate design position in the high-temperature drawing process due to the influence of surface tension and airflow.

[0011] For another example, prior art document two (such as Chinese patent application CN120065409A) proposes to set auxiliary nested structure units in the gap, which are connected with the inner wall of the outer cladding to form a circular leak mode cavity. Although this design helps to optimize the mode quality through the leak mode effect, these nested structure units are also located in the inner cladding air, which belongs to the typical "Tube-in-Air" structure and lacks the all-around rigid constraint from the surrounding solid material in the manufacturing process.

[0012] The second type of solution introduces auxiliary structures to fix the position of the main tube or to mark it. For example, prior art document three (such as US patent US11215751B2) proposes to use spacing elements to support the main anti-resonant tube; prior art document four (such as Chinese patent application CN120722488A) and prior art document five (such as Chinese patent application CN120871332A) propose a connecting sheet structure and a self-marking unit, respectively. However, the auxiliary structures in these solutions are either strictly limited in cross-sectional area to avoid optical loss, or are only used for visual marking, and generally do not have the active anti-resonant optical function of actively suppressing the leakage of the gap light field.

[0013] In summary, as Figure 17 (a) shown, the above prior art solutions for gap leakage or structure fixation generally adopt the "Tube-in-Air" structure paradigm, i.e. the auxiliary elements are mainly located in the inner cladding air region. This structure paradigm faces two main challenges in actual engineering applications:

[0014] 1. Manufacturing process challenge: The auxiliary elements are only fixed by point or line contact in the high-temperature drawing hydrodynamic environment, and the position holding ability is relatively weak, which is prone to position drift or deformation, affecting the yield of finished products.

[0015] 2. Optical performance challenge: Due to the difficulty of position control, it is difficult to accurately ensure the design gap between the auxiliary elements and the main tube in large-scale manufacturing, which may lead to fluctuations in the consistency of optical performance.

[0016] Therefore, there is an urgent need for a new anti-resonant hollow core fiber structure that can break through the process limitations of the "Tube-in-Air" structure and effectively suppress the gap leakage without introducing additional manufacturing instability. SUMMARY

[0017] The purpose of the present application is to provide an anti-resonant hollow core fiber with an embedded cavity structure to overcome the problems of high gap leakage loss and poor manufacturing stability of auxiliary elements in the prior art, effectively reduce the transmission loss while maintaining compatibility with standard single-mode fibers, and ensure manufacturing yield and process stability.

[0018] To solve the above technical problems, the present application adopts the following technical solutions:

[0019] An anti-resonant hollow core fiber with an embedded cavity in the outer cladding, comprising:

[0020] Hollow core (101): located in the center of the fiber, serving as the main channel for optical signal transmission;

[0021] Main anti-resonant tubes (102): multiple are provided, which are distributed around the hollow core (101), the main anti-resonant tubes (102) are tubular structures, which optionally contain zero, one or more nested anti-resonant tubes (103, 104) inside;

[0022] Outer cladding (301): surrounds the main anti-resonant tubes (102), is composed of solid material, has an inner wall (301a);

[0023] Cavity structure (201): at least one is provided, which is completely embedded in the solid material region of the outer cladding (301), and the front wall (201a) of the cavity structure (201) is separated from the inner wall (301a) of the outer cladding (301) by a continuous solid material layer, forming an anti-resonant layer; the cavity structure (201) is located at a corresponding position between adjacent main anti-resonant tubes (102), used to suppress light field leakage between the adjacent main anti-resonant tubes (102).

[0024] Further, in order to accurately regulate the optical performance and mechanical structure, the profile of the cavity structure (201) is defined as two parts: the inner wall near the hollow core (101) side constitutes an anti-resonant reflection surface, defined as the front wall (201a); the profile of the remaining part is defined as the back wall (201b). The solid layer defined between the front wall (201a) and the inner wall (301a) of the outer cladding (301) has a thickness t.

[0025] Further, the number of main anti-resonant tubes (102) is not limited to a specific number, which can be 4, 5, 6, 8 or other numbers, and is uniformly or non-uniformly distributed around the hollow core (101). Correspondingly, the number and position of the cavity structure (201) correspond to the gap distribution between the main anti-resonant tubes: one or more cavity structures (201) can be provided in the gap region between each adjacent main anti-resonant tube.

[0026] Further, as a preferred embodiment capable of realizing direct compatibility with standard single-mode optical fibers, the number of main anti-resonant tubes (102) is 4, which is distributed in a 90-degree rotational symmetry; correspondingly, the number of cavity structures (201) is 4, one cavity structure (201) is provided in each gap region. In this preferred embodiment, the outer diameter of the optical fiber is configured to be 125µm, matching the outer diameter of the standard single-mode optical fiber; the diameter of the hollow core (101) is configured to be 14µm to 16µm, and the mode field diameter is 10µm to 11µm, which is highly matched with the mode field diameter of the standard single-mode optical fiber.

[0027] Further, the cross-sectional shape of the cavity structure (201) is selected from a circle, an ellipse, a concave circular arc, a rectangle, a triangle, a polygon, a racetrack, a crescent, or other special shapes. The geometric curvature feature of the front wall (201a) can be selected from a positive curvature (the arc top is directed towards the outer cladding direction), zero curvature (flat), or negative curvature (the arc top is directed towards the center direction of the hollow core); preferably, a positive curvature or zero curvature design is adopted. The rear sidewall (201b) can be selected from a circular arc, a polyline, or a combination thereof. When the cavity structure (201) is constructed based on a circle, the diameter d of the circle is preferably 1 µm to 1000 µm, more preferably 10 µm to 50 µm, and most preferably 12 µm to 30 µm; the central angle θ of the circle ranges from 1° to 359°, and in the embodiment in which one cavity structure is provided for each gap, it is preferably 10° to 180°, and more preferably 15° to 90°.

[0028] Further, the front wall (201a) can have different geometric curvature features in the cross-section, including: positive curvature (exhibiting a circular arc shape of the front wall with the arc top directed towards the outer cladding direction), zero curvature (exhibiting a straight line shape of the front wall), or negative curvature (exhibiting a circular arc shape of the front wall with the arc top directed towards the hollow core direction). Different curvature forms affect the coupling characteristics of the cavity structure (201) and the light field in the gap region, and can be optimized according to the specific waveband, loss requirement, and manufacturing process convenience. Preferably, a positive curvature or zero curvature design is adopted, which can achieve optimal optical performance while maintaining manufacturing feasibility.

[0029] Further, in order to achieve the best optical suppression effect and take into account the manufacturing process, the solid layer thickness t (i.e., the anti-resonance layer thickness) between the front wall (201a) of the cavity structure (201) and the inner wall (301a) of the outer cladding (301) is configured to be 0.1 µm to 200 µm; preferably, the thickness t is 0.4 µm to 6.4 µm. Within this preferred range, the limited loss of the optical fiber shows extremely high insensitivity to the process fluctuations of the t value. In addition, the solid layer thickness t can be uniformly distributed or non-uniformly distributed along the front wall (201a), or different cavity structures (201) have different solid layer thicknesses t to achieve a wider working waveband.

[0030] Further, the solid layer thickness t preferably satisfies the anti-resonance matching condition to enhance the suppression of the leaked light field. The anti-resonance matching condition can adopt one of the following two forms:

[0031] First form (simplified formula):

[0032]

[0033] where λ is the working wavelength, n is the refractive index of the outer cladding material, and m1 is a non-negative integer, preferably 0 or 1.

[0034] Second form (exact formula):

[0035]

[0036] where λ is the operating wavelength, n is the refractive index of the outer cladding material, and m2 is a positive integer, preferably 1 or 2. The above two forms correspond to different theoretical derivation methods, and both can be used in engineering applications.

[0037] Further, to ensure effective coverage of the wide gap region between the main anti-resonant tubes (102), define θ_gap as the central angle between the lines connecting the centers of the nearest two points on the outer walls of two adjacent main anti-resonant tubes (102) and the center of the optical fiber, and define θ_cavity as the central angle of the cavity structure (201) with respect to the geometric center of the optical fiber. The ratio k = θ_cavity / θ_gap is called the coverage coefficient, which ranges from 0.2 to 10.0, preferably from 0.8 to 3.0, and more preferably from 1.2 to 2.5, to achieve moderate coverage.

[0038] Further, define d as the diameter of the circular cross-section of the cavity structure (201); the ratio of the solid layer thickness t to d (t / d) is preferably 0.01 to 100. Further, define S1 as the net solid cross-sectional area of the outer cladding (301) (i.e. the area of the outer cladding after removing all cavity structures), and S2 as the sum of the cross-sectional areas of all the cavity structures (201); the ratio of S1 to S2 is 0.01 to 1000; preferably, the ratio is 0.1 to 100.

[0039] Further, the cavity structure (201) can further contain one or more inner anti-resonant units (202) inside. The inner anti-resonant unit (202) can be a bridge structure connected to the inner wall of the cavity, or a nested tubular structure suspended inside the cavity. By setting multiple layers of nesting, a double-layer, triple-layer or more auxiliary anti-resonant system can be formed, and the anti-resonant layers can be one or a combination of positive curvature, negative curvature, straight line, etc., thereby further expanding the operating bandwidth of the optical fiber while reducing loss.

[0040] Further, the cavity structure (201) can be arranged sparsely, i.e. not every gap between adjacent main anti-resonant tubes (102) is provided with a cavity structure (201); or arranged in an interleaved manner, with cavity structures (201) interleaved at different positions to adapt to different application requirements and manufacturing conditions.

[0041] Further, the outer diameter of the optical fiber is 125 µm, matching the outer diameter of a standard single-mode optical fiber; the diameter of the hollow core (101) is 10 µm to 20 µm, matching the core diameter of a standard single-mode optical fiber.

[0042] Further, the outer cladding (301), the main anti-resonant tube (102), and the material of the outer wall (201a) of the cavity structure (201) are selected from pure quartz glass, doped quartz glass, or silicon. The hollow core (101), the main anti-resonant tube (102), and the cavity structure (201) can be filled with air, a specific gas, a gas mixture, or kept in a vacuum state. The cavity structure (201) is obtained by pre-setting a hole structure inside the outer cladding during the preform rod stage and keeping or deforming it during the drawing process.

[0043] Compared with the prior art, the present application has the following significant beneficial effects:

[0044] 1. The manufacturing stability is greatly improved: see Figure 17 In contrast, the present application first proposes the "Hole-in-Cladding" structure paradigm, which completely embeds the cavity structure inside the solid material of the outer cladding. Unlike the "Tube-in-Air" structure in the prior art, in which the auxiliary tubular element is attached to the inner wall of the outer cladding (point contact or tangential contact) ( Figure 17 (a)), the cavity structure (201) of the present application is rigidly fixed in all directions by the surrounding solid layer ( Figure 17 (b)). The anti-resonant layer formed between the front wall (201a) of the cavity and the inner wall (301a) of the outer cladding also has the function of suppressing gap leakage, but since the cavity structure is firmly surrounded by solid material, it is not affected by air flow disturbance, gravity, and surface tension instability during high-temperature drawing, and will not drift, rotate, flip, or stick to the main tube unexpectedly, greatly improving the manufacturing yield compared to the traditional "Tube-in-Air" structure.

[0045] In addition, since the cavity structure (201) is embedded in the outer cladding (301), the local geometry of the main anti-resonant tube (102) connection root area is changed, which helps to suppress the lateral expansion of the contact node during drawing, thereby facilitating the acquisition of smaller node size, and further reducing the potential Fano resonance loss.

[0046] 2. Effectively suppresses gap leakage loss: the cavity structure is located at the corresponding position between the adjacent main anti-resonant tubes, actively suppressing the leakage of the optical field in the gap direction through the anti-resonance effect. Simulation results show that the confinement loss at 1310 nm and 1550 nm can be significantly reduced.

[0047] 3. Excellent polarization maintaining performance synergy effect: In the polarization maintaining design embodiment (see Example 5), the introduction of the cavity structure presents an asymmetric optimization effect of "fast axis loss reduction and slow axis loss enhancement", with a fast axis loss reduction of about 70% and an extinction ratio exceeding 20 dB, while the birefringence remains stable. Combined with the advantage of high manufacturing yield, it provides a solution for high-performance polarization maintaining hollow core fiber.

[0048] 4. Excellent single-mode transmission characteristics: The present application significantly improves the high-order mode suppression performance. Through simulation verification, taking 1310 nm wavelength as an example, after introducing the cavity structure, the dominant high-order mode suppression ratio is improved from 109 times to 221 times, ensuring excellent single-mode transmission characteristics and mode field quality.

[0049] 5. High tolerance: Through simulation verification, when the solid layer thickness t of the cavity structure and the inner wall of the cladding changes in a wide range of 0.4-6.4µm, the loss fluctuation of the fiber at 1550nm wavelength is <12%. This means that even with larger manufacturing deviations (such as ±50% or even larger), the fiber performance remains stable. This tolerance insensitivity is an important advantage of the "Hole-in-Cladding" structure, reducing the difficulty and cost of precision manufacturing.

[0050] 6. Maintaining standard single-mode fiber compatibility: The present application is based on a 4-tube double-embedded anti-resonant nodeless hollow core fiber structure with an outer diameter of 125µm and a core diameter of 10-20µm, directly compatible with standard single-mode fiber, allowing low-loss direct fusion splicing. At the same time, thanks to the filtering effect of the cavity structure on the cladding modes, the present application has excellent high-order mode suppression capability, ensuring robust single-mode characteristics in long-distance transmission.

[0051] 7. Flexible design scalability and wideband characteristics: The present application provides a high degree of design freedom. By adjusting the shape (such as circular, racetrack, crescent, etc.), size (diameter d, central angle θ, solid layer thickness t) and arrangement (full coverage, sparse, staggered) of the cavity structure, it can be optimized for different wavelength bands and application requirements. Further, by introducing an inner anti-resonant unit inside the cavity structure, a double or multi-layer anti-resonant structure can be formed, further expanding the operating bandwidth and reducing transmission loss (see Example 7).

[0052] 8. Breakthrough existing technical difficulties: The present application solves the "two difficulties" faced by existing technology, achieving both the suppression of gap light field leakage (the goal of the first type of solution) and ensuring a robust manufacturing process (the demand of the second type of solution), providing a new technical path for high-performance, high-yield single-mode compatible hollow core fiber. BRIEF DESCRIPTION OF DRAWINGS

[0053] Figure 1 is the schematic diagram of the overall structure cross-section of the anti-resonant hollow optical fiber of the present application.

[0054] Figure 2 is the schematic diagram of the local enlargement of the cavity structure (201).

[0055] Figure 3 is the schematic diagram of the key geometric parameters of the cavity structure (201), and the dashed part represents the reference profile of the complete circle based on which the cavity structure (201) is formed.

[0056] Figure 4 shows the geometric definition of the coverage coefficient k, where θ_gap is the central angle between the two closest points on the outer walls of the two adjacent main anti-resonant tubes (102) and the center line of the optical fiber, and θ_cavity is the central angle of the cavity structure (201) to the geometric center of the optical fiber. The coverage coefficient k is defined as k = θ_cavity / θ_gap.

[0057] Figure 5 is the schematic diagram of the simulation results of the energy flow distribution comparison, showing a 1 / 4 symmetric model of a 90-degree sector. Among them, (a) is model A (traditional 4T-DNANF structure without cavity structure (201)); (b) is model B (structure of the present application containing cavity structure (201)).

[0058] Figure 6 is the loss comparison curve of the structure of the present application (containing cavity structure (201)) and the traditional structure (not containing cavity structure (201)), and the dashed box represents the loss curve of the traditional structure (model A), and the solid circle represents the loss curve of the structure of the present application (model B).

[0059] Figure 7 is the parameter scanning simulation result of the solid layer thickness t.

[0060] Figure 8 is the parameter scanning simulation result of the diameter d when the cavity structure is a circular cross-section.

[0061] Figure 9 is the schematic diagram of different cross-sectional shapes of the cavity structure (201): (a) circular; (b) rectangular; (c) elliptical; (d) triangular; (e) polygonal; (f) petal-shaped.

[0062] Figure 10 is the schematic diagram of the double-layer anti-resonant structure formed by the cavity structure (201) containing the bridging anti-resonant unit (202).

[0063] Figure 11 is Figure 10Loss spectrum comparison of the structure shown. The dotted box represents the loss curve of a single-layer auxiliary anti-resonance structure containing only cavity structures (201), and the solid circle represents the loss curve of a double-layer auxiliary anti-resonance structure containing cavity structures (201) and inner-layer anti-resonance units (202).

[0064] Figure 12 Schematic diagram of a double-layer auxiliary anti-resonance structure formed by embedding anti-resonance units (202) in cavity structures (201).

[0065] Figure 13 Schematic diagram of sparse or staggered arrangement of cavity structures (201).

[0066] Figure 14 Schematic diagram of geometric comparison of cavity structures with different curvatures. (a) Positive curvature; (b) Zero curvature; (c) Negative curvature.

[0067] Figure 15 Schematic diagram of energy flow distribution simulation under different curvature forms of cavity front walls. (a) Zero curvature front wall; (b) Negative curvature front wall.

[0068] Figure 16 Loss comparison curve of cavity structures with different curvatures.

[0069] Figure 17 Schematic diagram of manufacturing stability comparison between the present application and traditional structures. (a) Schematic diagram of traditional "air-suspended tube" structure; (b) Schematic diagram of "cladding-embedded cavity" structure of the present application.

[0070] Figure 18 Field intensity distribution comparison between the fundamental mode and the dominant high-order mode in the optical fiber structure of the present application. (a) Fundamental mode; (b) Dominant high-order mode. DETAILED DESCRIPTION

[0071] The specific embodiments of the present application will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the protection scope of the present application.

[0072] I. Basic structure description:

[0073] As shown in FIG. 1, the optical fiber structure of the present application comprises a cladding (100) and a core (200) embedded in the cladding (100). Figure 1As shown, the anti-resonant hollow core fiber provided by the present application comprises: a hollow core (101) located in the center of the fiber, serving as the main channel for optical signal transmission; four main anti-resonant tubes (102) distributed around the hollow core (101), each of which contains nested anti-resonant tubes (103, 104) inside, forming a double-nested structure; an outer cladding (301) composed of solid materials such as quartz glass, surrounding the main anti-resonant tubes (102); four cavity structures (201) completely embedded in the solid material of the outer cladding (301), and the air regions defined by the cavity structures (201) and the inner wall (301a) of the outer cladding (301) are not connected to each other, and each cavity structure (201) is located at the corresponding position of the gap between the adjacent two main anti-resonant tubes (102).

[0074] In order to accurately control the optical performance and mechanical structure, such as Figure 2 As shown, the profile of the cavity structure (201) is defined as two parts: the inner wall near the hollow core (101) constitutes the anti-resonant reflection surface, defined as the front wall (201a); the profile of the remaining part is defined as the back wall (201b). The solid glass layer defined between the front wall (201a) and the inner wall (301a) of the outer cladding (301) is defined as the solid wall.

[0075] The core innovation of the present application lies in the adoption of the "Hole-in-Cladding" structure paradigm, embedding the cavity structure inside the outer cladding, rather than suspending auxiliary elements in the air region of the inner cladding as in the prior art. This embedded structure ensures the positional stability during the manufacturing process, and at the same time effectively suppresses the leakage of light field in the gap direction through the anti-resonant effect of the solid wall between the front wall (201a) and the inner wall (301a).

[0076] II. Anti-resonance condition

[0077] As shown in Figure 3 The thickness t of the solid layer between the front wall (201a) of the cavity structure (201) and the inner wall (301a) of the outer cladding (301) is a key parameter affecting the anti-resonance effect. In order to enhance the reflection suppression effect on the leaked light field, the thickness t of the solid layer preferably satisfies the anti-resonance condition.

[0078] The thickness t of the solid layer between the front wall (201a) of the cavity structure (201) and the inner wall (301a) of the outer cladding (301) is a key parameter affecting the anti-resonance effect. In order to enhance the reflection suppression effect on the leaked light field, the thickness t of the solid layer preferably satisfies the following anti-resonance condition:

[0079] First form (simplified formula):

[0080]

[0081] Second form (exact formula):

[0082]

[0083] where λ is the operating wavelength, n is the refractive index of the outer cladding material, m1 is a non-negative integer, and m2 is a positive integer.

[0084] It should be particularly noted that the simulation results shown in Figure 7 indicate that the loss at 1550 nm fluctuates by <12% over a wide range of solid layer thickness t (spanning over 15 times), from 0.4 µm to 6.4 µm. This high degree of tolerance insensitivity means that even if the t value deviates significantly from the theoretical optimum value (±50% or even more), the fiber performance remains stable, which is an important advantage of the "Hole-in-Cladding" structure in actual manufacturing.

[0085] III. Definition and design of coverage coefficient

[0086] In order to ensure that the cavity structure (201) can effectively cover the gap region between the main anti-resonant tubes (102), the present application defines a coverage coefficient k as a design index.

[0087] As shown in Figure 4 , define θ_gap as the central angle between the two closest points on the outer walls of the two adjacent main anti-resonant tubes (102) and the line connecting the fiber center. Define θ_cavity as the central angle of the cavity structure (201) to the geometric center of the fiber. The coverage coefficient k is defined as the ratio of the two: k = θ_cavity / θ_gap. The coverage coefficient k ranges from 0.2 to 10.0, preferably from 0.8 to 3.0. By adjusting the size and position of the cavity structure (201), the coverage coefficient k can be flexibly controlled to adapt to different fiber structure parameters and performance requirements.

[0088] IV. Explanation of simulation method

[0089] The optical performance of all embodiments of the present application is simulated and verified using COMSOL Multiphysics 6.0 finite element analysis software.

[0090] Since Figure 1 the 4-tube design shown has 4-fold rotational symmetry (90-degree rotational symmetry), in order to improve computational efficiency, a 1 / 4 model, i.e. a 90-degree sector model, is established in the simulation.

[0091] In order to fairly and rigorously verify the technical effect of the present application, this specification sets "Model A (traditional structure)" as the comparative reference. The geometric parameters of Model A directly refer to the recently published 4-tube double-nested anti-resonant nodeless hollow fiber (4T-DNANF) literature data (see "Reference 1"). "Model B (structure of the present application)" is based on Model A, only adding a cavity structure (201) in the outer cladding, and all other structural parameters (including hollow core diameter, main anti-resonant tube size, nested anti-resonant tube size and distribution position) are completely consistent with Model A.

[0092] V. Design range of main structural parameters and scaling principle

[0093] The design principle of the cavity structure (201) of the present application has good size scalability and can be applied to anti-resonant hollow core fibers of different outer diameter sizes.

[0094] (1) Fiber outer diameter

[0095] The outer diameter of the outer cladding (301) of the present application is not strictly limited, and the typical range is 20 µm to 2000 µm, preferably 80 µm to 300 µm, and more preferably 100 µm to 200 µm. In particular, when the outer diameter is 125 µm ± 1 µm, it can be completely matched with a standard single-mode fiber (SMF) (see Example 1).

[0096] (2) Size scaling design principle

[0097] When the fiber outer diameter changes, the solid layer thickness t remains unchanged (determined by the working wavelength), and the other parameters are scaled proportionally. Let the scaling factor α = new outer diameter / reference outer diameter, then the hollow core diameter, main anti-resonant tube outer diameter, and cavity diameter are multiplied by α.

[0098] (3) Other main parameters

[0099] Number of main anti-resonant tubes (102): 2 to 20, preferably 4, 5, 6, or 8;

[0100] Number of nested layers: 0, 1, 2, or more;

[0101] Diameter d of the circle on which the cavity structure (201) is based: 1 µm to 1000 µm, preferably 10-50 µm;

[0102] Cavity center angle θ_cavity: 1° to 359°, preferably 15°-90°;

[0103] Example 1: Low-loss structure compatible with standard single-mode fiber;

[0104] This example corresponds to Figure 1 , Figure 5 ,Figure 6 and Figure 18 , shows a design of anti-resonant hollow core fiber optimized for O-band to L-band (especially 1310 nm and 1550 nm windows).

[0105] (I) Configuration of structural parameters (control variables):

[0106] As shown in Figure 1 , the fiber includes a hollow core (101), 4 main anti-resonant tubes (102) (each containing 2 layers of nested anti-resonant tubes inside), an outer cladding (301), and 4 circular cavity structures (201) embedded therein. The cavity structure (201) is located at the corresponding position of the gap between the adjacent main anti-resonant tubes (102), and the front wall (201a) and the inner wall (301a) of the outer cladding form an anti-resonant layer therebetween, which is used to suppress the leakage of light field in the gap direction.

[0107] In order to ensure the single variable principle of performance comparison, the basic parameters of the fiber of the present embodiment strictly reproduce the design in the prior art document (see "Reference 1"):

[0108] Fiber outer diameter: 125 µm (consistent with standard single-mode fiber);

[0109] Main anti-resonant tubes (102): 4 in number, uniformly distributed in a cross shape around the hollow core (101) (i.e. azimuth angles of 0°, 90°, 180° and 270°), with an outer diameter of 22.8 µm;

[0110] Nested anti-resonant tubes (103, 104): the outer diameter of the first layer of nested anti-resonant tubes (103) is set to 17.9 µm, and the outer diameter of the second layer of nested anti-resonant tubes (104) is set to 12.1 µm;

[0111] Tube wall thickness: the wall thickness of the main anti-resonant tubes (102) and the nested anti-resonant tubes (103, 104) is uniformly set to 0.4 µm;

[0112] Hollow core diameter: 16.1 µm;

[0113] (II) Cavity structure design (feature of the present invention)

[0114] On the basis of the above structure, the present embodiment introduces embedded cavity structures (201) between the adjacent main anti-resonant tubes (102). Specifically:

[0115] (1) Cavity position and distribution: In the present embodiment, the 4 main anti-resonant tubes (102) are symmetrically distributed at 0°, 90°, 180° and 270°, and the gaps between the adjacent main tubes are located at 45°, 135°, 225° and 315°. One cavity structure (201) is introduced at each gap (seeFigure 1 ).

[0116] (2) Cavity shape and cross section: The cavity structure (201) is embedded in the solid material region of the outer cladding (301), which is circular arc-shaped in cross section (see Figure 3 ); the cavity structure (201) is constructed based on a circle with a diameter d = 22 pm, the center of the circle is arranged along the 45°, 135°, 225°, 315° azimuth angle, and the distance from the center of the circle to the center of the fiber is equal to the sum of the inner radius of the outer cladding (301) and the solid layer thickness t; the solid layer thickness t between the cavity front wall (201a) and the outer cladding inner wall (301a) is 0.4 pm.

[0117] (3) Geometric parameters and coverage characteristics:

[0118] Solid layer thickness t: 0.4 pm (the wall thickness of the anti-resonant layer formed between 201a and the outer cladding inner wall (301a));

[0119] Cavity center angle θ cavity: about 40.5° (the center angle of the cavity arc covering the outer cladding inner wall (301a)); the calculation method of the cavity center angle θ cavity: the projection arc segment of the cavity structure (201) on the outer cladding inner wall (301a) to the center angle of the fiber geometric center (i.e. the center of the hollow core (101));

[0120] Gap center angle θ gap: about 18.6° (the center angle of the two closest points on the outer wall of the two main anti-resonant tubes (102) to the center line connecting the fiber center);

[0121] Coverage coefficient k = θ cavity / θ gap ≈ 2.18.

[0122] (Three) Performance verification and comparison:

[0123] This embodiment sets the structure of "Reference Literature 1" without the cavity structure (201) as Model A, and the structure of the application containing the cavity structure (201) as Model B.

[0124] (1) Energy flow distribution visualization analysis (corresponding to Figure 5 )

[0125] Figure 5 The intuitive display of the energy flow distribution comparison of the 1 / 4 model at 1310 nm in the simulation calculation is shown in the figure, and the black arrows in the figure represent the transmission path and density of the optical field energy flow.

[0126] Model A (Model B Figure 5(a): For the traditional 4-tube double-nested anti-resonant nodeless hollow fiber (4T-DNANF) structure (i.e. without the embedded cavity structure (201)), it can be clearly observed from the simulation figure that the optical field energy presents a significant outward leakage in the gap region between the adjacent main anti-resonant tubes (102). The black energy flow arrows penetrate through the gap into the outer cladding, resulting in a relatively high background energy density.

[0127] Model B (with the embedded cavity structure (201)) Figure 5 (b): After introducing the embedded cavity structure (201) of the present application, the energy flow distribution changes. The anti-resonant layer formed by the cavity structure (201) and the inner wall (301a) of the outer cladding effectively suppresses the leakage channel that originally leads to the outer cladding. As can be seen from the figure, the energy flow lines at the gap are suppressed, and the background energy density leaked to the outer cladding (301) is significantly reduced compared to Model A.

[0128] (2) Confinement loss spectrum analysis (corresponding to Figure 6 )

[0129] Figure 6 Further quantifies the loss reduction effect brought by the above-mentioned energy flow blocking effect. The simulation results show that:

[0130] At 1310nm: The confinement loss is reduced from 4.7dB / km of Model A to 0.28dB / km of Model B, with a reduction of about 94%;

[0131] At 1550nm: The confinement loss is reduced from 4.8dB / km of Model A to 0.68dB / km of Model B, with a reduction of about 86%.

[0132] This embodiment proves that by introducing the embedded cavity structure of the present application in the outer cladding, the confinement loss can be effectively and significantly reduced.

[0133] (3) High-order mode suppression performance evaluation (corresponding to Figure 18 )

[0134] To fully verify the single-mode transmission characteristics of the present application, the confinement loss of the dominant high-order mode at 1310nm is further calculated. The simulation results show that:

[0135] For Model A (without the cavity structure (201)): The fundamental mode loss is 4.7dB / km, the dominant high-order mode loss is 512.3dB / km, and the high-order mode suppression ratio is 109 times.

[0136] For Model B (with the cavity structure (201)): The fundamental mode loss is 0.28dB / km, the dominant high-order mode loss is 61.9dB / km, and the high-order mode suppression ratio is 221 times.

[0137] After introducing the cavity structure, the high-order mode suppression ratio is improved from 109 times to 221 times, about 2 times, and the single-mode transmission characteristics are significantly improved. Figure 18 The light field distribution of the fundamental mode and the dominant high-order mode is shown.

[0138] Embodiment 2: Optimization design based on tolerance analysis

[0139] This embodiment verifies Figure 7 the tolerance-insensitive characteristics shown. The same basic structure as in Embodiment 1 is adopted, only the thickness t of the solid layer is changed: design A takes t = 0.4 µm, design B takes t = 6.4 µm, and the difference between the two is 16 times. The simulation results show that at 1550 nm, the loss of design A is 0.68 dB / km, and the loss of design B is 0.76 dB / km, with a difference of only 12%. This proves that even if there is a large deviation in the thickness t of the solid layer, the loss can still remain stable.

[0140] Embodiment 3: Optimization design based on the diameter d of the cavity (circular cross-section)

[0141] This embodiment corresponds to Figure 8 , aiming to explore the influence of the diameter d of the cavity structure (201) based on the circular cross-section on the performance of the optical fiber.

[0142] On the basis of the structure in Embodiment 1, keeping other parameters unchanged, only changing the value of the diameter d of the circular cross-section of the cavity (201) (from 12 µm to 22 µm), the confinement loss of the optical fiber at 1550 nm is calculated.

[0143] Simulation result analysis: as shown in Figure 8 , the abscissa is the diameter d (µm) of the circle based on which the cavity structure (201) is constructed, and the ordinate is the confinement loss (dB / km) at 1550 nm. According to the change in the slope of the loss decrease trend with the increase of the diameter d, the design space can be divided into three typical characteristic regions:

[0144] Fast decline region (d = 12 ~ 16 µm): in this region, with the increase of the diameter d, the confinement loss decreases sharply.

[0145] Medium-speed decline region (d = 16 ~ 19 µm): in this region, the trend of loss decrease slows down.

[0146] Slow decline region (d > 19 µm): when d exceeds 19 µm, further increasing the diameter enters the stage of diminishing returns in terms of loss reduction.

[0147] Embodiment 4: Optimization design of anti-resonant wall curvature form

[0148] This embodiment corresponds to Figure 14 , Figure 15 andFigure 16 The specific influence of different curvature patterns (positive curvature, zero curvature, and negative curvature) of the front wall (201a) of the cavity structure (201) on the loss characteristics of the optical fiber is analyzed.

[0149] (1) The definition and classification of the curvature pattern are shown in Figure 2 The solid wall between the front wall (201a) of the cavity structure (201) and the inner wall (301a) of the outer cladding constitutes an anti-resonance layer. When the thickness of the solid layer remains uniform and constant, according to the bending direction of the front wall (201a), the curvature pattern is divided into three categories:

[0150] Positive curvature (as shown in Figure 14 (a)): The front wall (201a) is in the form of a circular arc, and the arc top is directed towards the outer cladding (301).

[0151] Zero curvature (as shown in Figure 14 (b)): The front wall (201a) is in the form of a straight line.

[0152] Negative curvature (as shown in Figure 14 (c)): The front wall (201a) is in the form of a circular arc, and the arc top is directed towards the center of the hollow core (101).

[0153] (2) Simulation parameter setting

[0154] This embodiment uses the same basic structure parameters as in Embodiment 1, and only changes the curvature of the front wall (201a) of the cavity structure (201), which is divided into zero curvature and negative curvature.

[0155] (3) Comparison and analysis of energy flow distribution

[0156] In combination with the comparison between the cavity-free and positive curvature cavity shown in Figure 5 , this embodiment further shows the energy flow distribution characteristics of the zero curvature and negative curvature cavities at 1310 nm. Figure 15 The zero curvature design (a) has a leakage suppression effect comparable to that of the positive curvature design (b). The negative curvature design (b) shows that, compared with the cavity-free design, the gap leakage is suppressed, and the background energy density leaked to the outer cladding is significantly reduced. The above comparison shows that the cavity structure with different curvature patterns can effectively suppress the gap light field leakage. Figure 15 Figure 5 (4) Quantitative comparison of confinement loss Figure 15 As shown in

[0157] , the confinement losses of the four designs at 1550 nm are quantitatively shown:

[0158] Figure 16

[0159] ​​​No cavity design: 4.78 dB / km (baseline);

[0160] Positive curvature design: 0.68 dB / km (85.8% reduction);

[0161] Zero curvature design: 0.69 dB / km (85.6% reduction);

[0162] Negative curvature design: 1.84 dB / km (61.5% reduction);

[0163] The simulation results show that the optical performance of the positive curvature and zero curvature designs is optimal, and the negative curvature design is second but still significantly better than the no cavity structure.

[0164] Example 5: Low-loss polarization maintaining optical fiber design

[0165] This example is based on Example 1, and the polarization maintaining function is introduced by differentially configuring the wall thickness of the main anti-resonance tube (102).

[0166] Specifically, the wall thickness of the main anti-resonance tube (102) in the vertical axis direction (90° and 270°) is increased from 0.4 µm to 0.8 µm, and the remaining structure parameters are exactly the same as in Example 1. This differential design forms an asymmetric anti-resonance condition in the X and Y axis directions, introducing phase birefringence.

[0167] (2) Performance comparison

[0168] To verify the effect of the embedded cavity structure on the polarization maintaining design, two groups of comparison models are established: a control group without a cavity structure (201) and an inventive group with a cavity structure (201). The wall thickness of the main anti-resonance tube (102) in the two groups is configured exactly the same (0.4 µm in the horizontal axis and 0.8 µm in the vertical axis), and the only difference is whether the cavity structure (201) is introduced. The base mode transmission characteristics of the two orthogonal polarization states at a wavelength of 1310 nm are simulated and calculated, and the results are shown in Table 1.

[0169] Table 1: Comparison of polarization maintaining performance

[0170]

[0171] Note: The fast axis refers to the polarization state with lower loss, and the slow axis refers to the polarization state with higher loss. Extinction ratio = 10 x log 10 (Slow axis loss / fast axis loss).

[0172] As can be seen from Table 1, after introducing the cavity structure:

[0173] The fast axis loss is reduced from 0.96 dB / km to 0.285 dB / km, and the slow axis loss is increased from 39.05 dB / km to 75.99 dB / km, which shows that the cavity structure forms a beneficial synergistic effect by changing the light field distribution.

[0174] The extinction ratio is increased from 16.09 dB to 24.26 dB, and the polarization maintaining performance is significantly improved; the birefringence remains stable at 6.38*10-5 (with a beat length of about 2.05 cm), which proves that the cavity structure does not affect the birefringence mechanism.

[0175] (3) Technical significance

[0176] This embodiment proves that the cavity structure of the present application has a good synergistic effect with the polarization maintaining design: while reducing the fast axis loss, the slow axis suppression is enhanced, and the extinction ratio is increased to 24.26 dB. Combined with the high manufacturing yield advantage of the present application (Example 11), a solution is provided for high-performance polarization maintaining hollow core optical fiber. Those skilled in the art can understand that in addition to the difference in wall thickness of the main anti-resonance tube, birefringence can also be introduced by the following methods: difference in wall thickness or outer diameter of the main anti-resonance tube (102), difference in wall thickness or outer diameter of the nested anti-resonance tube (103, 104), different number of nested layers in different directions, non-circular cross-section of the outer cladding (301), etc., all of which are within the scope of the present application.

[0177] Example 6: Cavity structure with different cross-sectional shapes

[0178] Example 1 uses a cavity based on a circular cross-sectional design. In addition to this, the cavity can also use the following complete geometric shapes:

[0179] (1) Rectangular cross-sectional cavity ( Figure 9 (b));

[0180] (2) Elliptical cross-sectional cavity ( Figure 9 (c));

[0181] (3) Triangular cross-sectional cavity ( Figure 9 (d));

[0182] The above cavity structures with different cross-sectional shapes have their own characteristics in terms of manufacturing process, gas filling characteristics, mechanical strength, etc.

[0183] Example 7: Double-layer structure containing a bridged inner-layer anti-resonance unit

[0184] This embodiment corresponds to Figure 10 and Figure 11A solid arc layer is added inside the cavity structure (201) (diameter 22 pm) as a bridging inner-layer anti-resonance unit (202). The first layer of anti-resonance layer formed between the front wall (201a) of the cavity and the inner wall (301a) of the outer cladding, together with the second layer of anti-resonance layer formed by the inner-layer anti-resonance unit (202), constitutes a double-layer anti-resonance system. The wall thickness of the inner-layer unit 202 is 1 pm. As shown in Figure 11 Compared with the single-layer anti-resonance structure, the working bandwidth of the double-layer anti-resonance structure is expanded by about 100 nm in the short-wave region, and the loss at the center of the bandwidth is reduced by about 10-15%.

[0185] Example 8: Double-layer structure containing nested inner-layer anti-resonance unit

[0186] This embodiment corresponds to Figure 12 A conventional stacked nested annulus is suspended inside the cavity structure (201) (diameter 22 pm) as an inner-layer anti-resonance unit (202). The nested annulus has an outer diameter of 10 pm and a wall thickness of 400 nm, and is tangent to the outer wall of the cavity (close to the outer wall of the outer cladding). The bandwidth expansion effect of this design is similar to that of Example 7, and the loss is reduced by about 5%.

[0187] Example 9: Sparse arrangement of cavity structure

[0188] This embodiment corresponds to Figure 13 This embodiment shows a flexible arrangement of the cavity structure (201). The basic structure of Example 1 is used, but not all four gaps are provided with the cavity structure (201), but only part of the gaps (such as every other gap) are provided with the cavity structure. This sparse arrangement design can be used for specific mode control.

[0189] Example 10: Extension of different wavebands

[0190] The present application can be extended to different wavelength bands. Based on the anti-resonance condition, the structure size and the working wavelength λ have a linear scaling relationship:

[0191] Mid-infrared waveband (e.g. 3 pm): the hollow core diameter is adjusted to 20 pm to 50 pm, and the solid layer thickness t is adjusted to 1 pm to 2 pm;

[0192] Far-infrared waveband (e.g. 10.6 pm): the hollow core diameter is expanded to 80 pm to 150 pm;

[0193] Ultraviolet waveband (e.g. 355 nm): the hollow core diameter is reduced to 3 pm to 10 pm, and the anti-resonance layer thickness is correspondingly thinned.

[0194] Example 11: Manufacturing feasibility

[0195] The cavity structure (201) of the present application can be realized by various mature processes, including but not limited to: pre-setting a hole structure inside the outer cladding (301) at the preform rod stage, and keeping or deforming during the drawing process. The pre-set hole structure can be formed by conventional methods in the art such as machining, chemical etching, assembly forming, etc. The specific process parameters can be optimized and adjusted according to the actual equipment and material conditions.

[0196] The specific advantages of the cladding-in-cavity manufacturing process of the present application are detailed in the "beneficial effects" section. Figure 17 The essential difference between the present application and the prior art in terms of manufacturing stability is shown. As Figure 17 (a) shown, the auxiliary element (401) of the "tube-in-air" structure of the prior art is attached to the inner wall (301a) of the outer cladding only by point / line contact; as Figure 17 (b) shown, the cavity structure (201) of the present application is rigidly constrained by the solid material of the outer cladding (301) 360°, and the position can be kept stable during the high-temperature drawing process.

Claims

1. An anti-resonant hollow core optical fiber with an overclad inlaid cavity, characterized in that, The application relates to a hollow-core fiber, comprising: a hollow core (101) located at the center of the fiber; a plurality of main anti-resonant tubes (102) distributed around the hollow core; an outer cladding (301) surrounding the main anti-resonant tubes (102), the outer cladding (301) having an inner wall (301a); at least one cavity structure (201) completely embedded in a solid material region of the outer cladding (301), the cavity structure (201) having a front wall (201a) on the side close to the hollow core (101), the front wall (201a) being separated from the inner wall (301a) of the outer cladding (301) by a continuous solid material layer and forming an anti-resonant layer, and the cavity structure (201) and the air region defined by the inner wall (301a) of the outer cladding (301) being not in communication with each other; the cavity structure (201) is located at a corresponding position between adjacent main anti-resonant tubes (102).

2. The anti-resonant hollow core optical fiber according to claim 1, characterized in that, The cavity structure (201) presents a closed hollow core region in a cross section, and the contour is sequentially connected by two part interfaces: a first part is an inner wall on the side close to the hollow core (101), which constitutes one side of the anti-resonant layer and is defined as the front wall (201a); a second part is the remaining part of the contour, which is used for defining the embedding depth and coverage width of the cavity structure (201) in the outer cladding (301) and is defined as the back side wall (201b); the front wall (201a) and the back side wall (201b) are seamlessly connected and jointly form the cavity structure (201).

3. The anti-resonant hollow core optical fiber according to claim 2, characterized in that, The geometric curvature feature of the front wall (201a) in the cross section is selected from the following: positive curvature, which is manifested as the arc top of the front wall (201a) being directed to the outer cladding (301); zero curvature, which is manifested as the front wall (201a) being in a straight line shape; negative curvature, which is manifested as the arc top of the front wall (201a) being directed to the center of the hollow core (101).

4. The anti-resonant hollow core optical fiber according to claim 3, characterized in that, The geometric curvature feature of the front wall (201a) is positive curvature or zero curvature.

5. The anti-resonant hollow core optical fiber according to claim 2, characterized in that, The geometric shape of the back side wall (201b) in the cross section is selected from the following: a circular arc shape, an elliptical arc shape, an irregular curve, a broken line shape or a combination thereof; the two ends of the back side wall (201b) are directly connected with the front wall (201a) in a head-tail mode, and the connection position is in smooth transition or forms an inflection point with an included angle.

6. The anti-resonant hollow core optical fiber according to claim 5, characterized in that, The overall cross-sectional shape formed by the front wall (201a) and the back side wall (201b) includes the following: a circular shape, an elliptical shape, a crescent shape, a triangular shape, a rectangular shape, a racetrack shape, a polygonal shape, a combination of the above shapes or other special shapes.

7. The anti-resonant hollow core optical fiber according to claim 2, characterized in that, The solid layer thickness (denoted as t) defined between the front wall (201a) of the cavity structure (201) and the inner wall (301a) of the outer cladding (301) is configured in the following distribution form in the cross section of the fiber: constant distribution: the solid layer thickness t between the front wall (201a) of all the cavity structures (201) and the inner wall (301a) of the outer cladding (301) is equal and uniform everywhere. The solid layer thickness varies periodically, gradually, or different cavity structures (201) have different solid layer thickness values.

8. The anti-resonant hollow core optical fiber according to claim 7, characterized in that, The solid layer thickness t ranges from 0.1 µm to 200 µm; preferably, the solid layer thickness t ranges from 0.4 µm to 6.4 µm.

9. The anti-resonant hollow core optical fiber according to claim 8, characterized in that, The solid layer thickness t satisfies the anti-resonance condition: Wherein: λ is the operating wavelength of the optical fiber; n is the refractive index of the solid material between the cavity structure (201) and the outer cladding (301); m is a non-negative integer, which is 0, 1, 2, 3 or higher; preferably, m is 0 or 1.

10. The anti-resonant hollow core optical fiber according to claim 8, characterized in that, The solid layer thickness t satisfies the anti-resonance condition: Wherein: λ is the operating wavelength of the optical fiber; n is the refractive index of the solid material between the cavity structure (201) and the outer cladding (301); m is a non-negative integer, which is 0, 1, 2, 3 or higher; preferably, m is 0 or 1.

11. The anti-resonant hollow core optical fiber according to claim 9 or 10, characterized in that, The deviation of the solid layer thickness t from the theoretical value of the anti-resonance condition is within ±50%; preferably, the deviation is within ±30%; more preferably, the deviation is within ±20%.

12. The anti-resonant hollow core optical fiber according to claim 2, characterized in that, The cavity structure (201) is constructed based on a circular shape with a diameter d in the cross-section, and has a circular arc cross-section; the diameter d is 1 µm to 1000 µm; preferably, the diameter d is 10 µm to 50 µm; more preferably, the diameter d is 12 µm to 30 µm.

13. The anti-resonant hollow core optical fiber according to claim 12, characterized in that, The cavity structure (201) has a central angle of 1° to 359° with respect to the geometric center of the optical fiber; preferably, the central angle is 10° to 180°; more preferably, the central angle is 15° to 90°.

14. The anti-resonant hollow core optical fiber according to claim 1, wherein, The coverage coefficient k of the cavity structure (201) to the gap between the adjacent main anti-resonance tubes (102) is defined as: k=θ_cavity / θ_gap, wherein θ_cavity is the central angle of the cavity structure (201) with respect to the geometric center of the optical fiber, and θ_gap is the central angle between the two closest points on the outer wall of the adjacent two main anti-resonance tubes (102) and the connecting line of the optical fiber center, and the ratio k is 0.2 to 10.0; preferably, the ratio k is 0.8 to 3.0; more preferably, the ratio k is 1.2 to 2.

5.

15. The anti-resonant hollow core optical fiber according to claim 12, characterized in that, The ratio (t / d) of the solid layer thickness t to the diameter d is 0.01 to 100; preferably, the ratio is 0.05 to 10.

16. The anti-resonant hollow core optical fiber according to claim 1, characterized in that, The ratio of the outer diameter of the main anti-resonance tube (102) to the diameter of the cavity structure (201) is 0.5 to 20; preferably, the ratio is 1.0 to 10.

17. The anti-resonant hollow core optical fiber according to claim 1, characterized in that, The ratio of S1 to S2 is 0.01 to 1000; preferably, the ratio is 0.1 to 100.

18. The antiresonant hollow core fiber of claim 1, wherein, The number of the cavity structures (201) is equal to the number of the main anti-resonance tubes (102).

19. The antiresonant hollow core fiber of claim 1, wherein, A plurality of the cavity structures (201) are arranged between at least one pair of adjacent main anti-resonance tubes (102).

20. The antiresonant hollow core fiber of claim 1, wherein, The number of the cavity structures (201) is less than the number of the main anti-resonant tubes (102).

21. The anti-resonant hollow core optical fiber according to claim 20, characterized in that, The cavity structures (201) are arranged in a staggered or spaced manner; preferably, one cavity structure (201) is arranged at a corresponding position between every other adjacent main anti-resonant tube (102); more preferably, one cavity structure (201) is arranged at every other two or more corresponding positions.

22. The antiresonant hollow core fiber of claim 1, wherein, Compared with a hollow-core anti-resonant fiber without the cavity structures (201), the fundamental mode loss of the fiber is significantly reduced.

23. The antiresonant hollow core fiber of claim 1, wherein, The ratio of the high-order mode loss to the fundamental mode loss of the fiber is greater than 10; preferably, the ratio is greater than 100; more preferably, the ratio is greater than 1000.

24. The antiresonant hollow core fiber of claim 1, wherein, The main anti-resonant tubes (102) are single-layer tubular structures without containing nested anti-resonant tubes inside.

25. The antiresonant hollow core fiber of claim 1, wherein, At least one of the main anti-resonant tubes (102) contains at least one nested anti-resonant tube (103) inside.

26. The anti-resonant hollow core optical fiber according to claim 25, characterized in that, At least one of the nested anti-resonant tubes (103) further contains at least one nested anti-resonant tube (104) inside, forming a double-layer nested structure.

27. The anti-resonant hollow core optical fiber according to claim 26, characterized in that, At least one of the nested anti-resonant tubes (104) further contains at least one nested anti-resonant tube inside, forming a triple-layer nested structure.

28. The anti-resonant hollow core optical fiber according to claim 1, characterized in that, The number of the main anti-resonant tubes (102) is 2 to 20.

29. The anti-resonant hollow core optical fiber according to claim 28, characterized in that, The number of the main anti-resonant tubes (102) is 4, 5, 6, or 8.

30. The anti-resonant hollow core optical fiber according to claim 29, characterized in that, The number of the main anti-resonant tubes (102) is 4.

31. The anti-resonant hollow core optical fiber according to claim 1, characterized in that, The main anti-resonant tubes (102) are uniformly distributed around the hollow core (101).

32. The anti-resonant hollow core optical fiber according to claim 1, characterized in that, The main anti-resonant tubes (102) are non-uniformly distributed around the hollow core (101).

33. The anti-resonant hollow core optical fiber according to claim 1, characterized in that, The cross-sectional structure of the fiber is configured to be non-rotationally symmetric to introduce phase birefringence, so that the fundamental mode losses of two orthogonal polarization states exhibit differences, wherein the low-loss polarization state is defined as the fast axis, and the high-loss polarization state is defined as the slow axis.

34. The hollow-core anti-resonant fiber according to claim 33, wherein, The non-rotationally symmetric is realized by one or a combination of the following ways: (a) the main anti-resonant tubes (102) distributed in different directions have different wall thicknesses or outer diameters; (b) the nested anti-resonant tubes (103, 104) distributed in different directions have different wall thicknesses, outer diameters, or nested layer numbers; (c) the cavity structures (201) distributed in different directions have different sizes or shapes; (d) the outer cladding (301) has a non-circular cross-section.

35. The anti-resonant hollow core optical fiber according to claim 34, characterized in that, said phase birefringence is not less than 1x10 -5 ; preferably, said phase birefringence is not less than 5x10 -5 ; corresponding beat length is not more than 3cm; extinction ratio is not less than 10dB; preferably, said extinction ratio is not less than 15dB; more preferably, said extinction ratio is not less than 20dB.

36. Use of the anti-resonant hollow core optical fiber according to any one of claims 33 to 35 in polarization maintaining, characterized in that, The polarization state maintenance is realized by using the phase birefringence and polarization-dependent loss difference of the fiber.

37. The use according to claim 36, wherein The fiber is used for polarization state maintenance transmission in a fiber-optic gyroscope, a coherent optical communication system, an interference measurement system, a quantum communication system, or a polarization-sensitive sensing system.

38. Use of the anti-resonant hollow core optical fibre according to any one of claims 33 to 35 in polarisation, characterised in that, The polarization function is realized by using the differential loss characteristics of different polarization states of the fiber.

39. The use according to claim 38, wherein The single-polarization output is realized by using the high-extinction ratio characteristics of the fiber to attenuate the slow-axis polarization component to a negligible level within a preset transmission length.

40. The antiresonant hollow core fiber of claim 1, wherein, The spacing between adjacent main anti-resonant tubes (102) is 0 µm to 100 µm; preferably, the spacing is 1 µm to 20 µm.

41. The anti-resonant hollow core optical fiber according to claim 1, characterized in that, The outer diameter of the outer cladding (301) is 20 µm to 2000 µm.

42. The anti-resonant hollow core optical fiber according to claim 41, characterized in that, The outer diameter of the outer cladding (301) is 80 µm to 300 µm.

43. The anti-resonant hollow core optical fiber according to claim 42, characterized in that, The outer diameter of the outer cladding (301) is 100 µm to 200 µm.

44. The anti-resonant hollow core optical fiber of claim 1, wherein, The optical fiber is configured to achieve low-loss fusion splicing with a standard single-mode fiber (SMF), and the structural parameters thereof satisfy the following conditions: the outer diameter of the outer cladding (301) is 120 µm to 130 µm; and the diameter of the hollow core (101) is 10 µm to 20 µm.

45. The anti-resonant hollow core optical fiber of claim 1, wherein, The inner part of the cavity structure (201) contains at least one inner-layer anti-resonance unit (202), forming a double-layer or multi-layer auxiliary anti-resonance structure.

46. The anti-resonant hollow core optical fiber according to claim 45, wherein, The inner-layer anti-resonance unit (202) is a bridging structure connected to the inner wall of the cavity structure (201).

47. The anti-resonant hollow core optical fiber according to claim 46, wherein, The bridging structure is an arc-shaped solid partition, and the center of the arc-shaped solid partition is concentric with the center of the outer cladding (301).

48. The anti-resonant hollow core optical fiber according to claim 47, wherein, The distance from the center of the arc-shaped solid partition to the center of the optical fiber is 1% to 99% of the inner radius of the outer cladding (301).

49. The anti-resonant hollow core optical fiber of claim 46, wherein, The bridging structure is a linear solid partition.

50. The anti-resonant hollow core optical fiber of claim 46, wherein, The thickness of the bridging structure is 0.1 µm to 100 µm; preferably, 0.3 µm to 10.0 µm.

51. The anti-resonant hollow core optical fiber according to claim 46, wherein, The thickness t2 of the bridging structure satisfies the anti-resonance matching condition: wherein λ is the operating wavelength of the optical fiber; n is the refractive index of the solid material of the bridging structure; and m2 is a non-negative integer, taking a value of 0, 1, 2, 3, or higher; preferably, m2 takes a value of 0 or 1.

52. The anti-resonant hollow core optical fiber of claim 46, wherein, The thickness t2 of the bridging structure satisfies the anti-resonance matching condition: wherein λ is the operating wavelength of the optical fiber; n is the refractive index of the solid material of the bridging structure; and m2 is a positive integer, taking a value of 1, 2, 3, or higher; preferably, m2 takes a value of 1 or 2.

53. The anti-resonant hollow core optical fiber according to claim 51 or 52, characterized in that, The deviation of the thickness t2 of the bridging structure from the theoretical value of the anti-resonance condition is within ±50%; preferably, the deviation is within ±30%; more preferably, the deviation is within ±20%.

54. The anti-resonant hollow core optical fiber of claim 46, wherein, The thickness of the bridging structure gradually changes along the extension direction thereof.

55. The anti-resonant hollow core optical fiber of claim 46, wherein, At least one side edge of the bridging structure is wavy.

56. The anti-resonant hollow core optical fiber of claim 45, wherein, The inner-layer anti-resonance unit (202) is a nested structure, including at least one independent tubular structure suspended in the inner part of the cavity structure (201).

57. The anti-resonant hollow core optical fiber according to claim 56, characterized in that, The cavity structure (201) contains two or more layers of nested inner-layer anti-resonance units (202).

58. The anti-resonant hollow core optical fiber of claim 56, wherein, The wall thickness of the independent tubular structure is 0.1 µm to 50 µm; preferably, 0.3 µm to 10 µm.

59. The anti-resonant hollow core optical fiber according to claim 56, wherein, The wall thickness t3 of the independent tubular structure satisfies the anti-resonance matching condition: wherein λ is the operating wavelength of the optical fiber; n is the refractive index of the solid material of the independent tubular structure; and m3 is a non-negative integer, taking a value of 0, 1, 2, 3, or higher; preferably, m3 takes a value of 0 or 1.

60. The anti-resonant hollow core optical fiber according to claim 56, wherein, The wall thickness t3 of the independent tubular structure satisfies the anti-resonance matching condition: Wherein: λ is the working wavelength of the optical fiber; n is the refractive index of the solid material of the independent tubular structure; m3 is a positive integer, taking 1, 2, 3 or higher; preferably, m3 takes 1 or 2.

61. The anti-resonant hollow core optical fiber according to claim 59 or 60, characterized in that, The deviation of the tube wall thickness t3 of the independent tubular structure relative to the theoretical value of the anti-resonance condition is within ±50%; preferably, the deviation is within ±30%; more preferably, the deviation is within ±20%.

62. The anti-resonant hollow core optical fiber of claim 56, wherein, The outer wall of the independent tubular structure is tangent to the inner wall of the cavity structure (201).

63. The anti-resonant hollow core optical fiber of claim 56, wherein, The independent tubular structure is located at the center position of the cavity structure (201).

64. The anti-resonant hollow core optical fiber of claim 45, wherein, Through the double-layer or multi-layer auxiliary anti-resonance structure, one or more of the following technical effects are achieved: working bandwidth expansion; further reduction of fundamental mode loss; enhanced high-order mode suppression capability.

65. The anti-resonant hollow core optical fiber of claim 1, wherein, The base material of the outer cladding layer (301) is a solid transparent material capable of transmitting optical signals, selected from any one of the following: pure quartz glass, doped quartz glass, multi-component glass (such as chalcogenide glass, fluoride glass), silicon, semiconductor material, crystal material, or high polymer material.

66. The anti-resonant hollow core optical fiber according to claim 1, wherein, The materials of the main anti-resonance tube (102) and the nested anti-resonance tubes (103, 104) possibly contained therein are selected from: pure quartz glass, doped quartz glass, multi-component glass, silicon, semiconductor material, crystal material, or high polymer material.

67. The anti-resonant hollow core optical fiber according to claim 1, wherein, The material forming the wall surface of the cavity structure (201) is the same as the base material of the outer cladding layer (301); or the wall surface of the cavity structure (201) is composed of a heterogeneous solid material different from the base material of the outer cladding layer (301).

68. The anti-resonant hollow core optical fiber of claim 67, wherein, When the wall surface is composed of a heterogeneous solid material, the inner surface of the cavity structure (201) is provided with one or more functional anti-resonance layers, and the material of the functional anti-resonance layer is selected from doped quartz glass, high-refractive-index glass, semiconductor material or polymer material, for enhancing anti-resonance reflectivity or achieving gain in a specific waveband.

69. The anti-resonant hollow core optical fiber of claim 1, wherein, The inner regions of the main anti-resonance tube (102) and the nested anti-resonance tubes (103, 104) possibly contained therein, and the inside of the cavity structure (201), are respectively configured as controllable gas environments; the gas environments are selected from: air, inert gas, functional gas mixture or vacuum state.

70. The anti-resonant hollow core optical fiber of claim 69, wherein, The gas environment inside the cavity structure (201) is configured independently of the gas environment inside the hollow core (101) and the main anti-resonance tube (102); the gas pressure or gas composition inside the cavity structure (201) is the same as or different from that inside the hollow core (101).

71. The anti-resonant hollow core optical fiber of claim 70, wherein, Differential gas configuration is adopted to achieve specific functions: the hollow core (101) is configured to transmit a gas to be measured or maintain a vacuum; and the cavity structure (201) is configured to be filled with high-pressure inert gas or gas with a specific thermo-optic coefficient.

72. The anti-resonant hollow core optical fiber of claim 1, wherein, The cavity structure (201) is obtained by pre-setting a hole structure inside the outer cladding layer (301) in the preform rod stage, and maintaining or deforming during the drawing process.

73. The anti-resonant hollow core optical fiber of claim 72, wherein, The pre-defined hole structure is formed by one or more of mechanical machining, chemical etching, pre-fabricated assembly, or heat treatment shaping.

74. An optical transmission system or apparatus substantially as herein described with reference to any one of the preceding embodiments. The system or device comprising the anti-resonant hollow core optical fiber as claimed in claim 1 is configured for one or more of the following application categories: Optical communications and data transmission: including data center interconnections, long-haul fiber communications, time and frequency transmission, quantum communications, or submarine and underwater optical communications; Laser energy transmission: including high-power laser transmission, industrial laser processing, laser radar, fiber laser, or space laser energy transmission; Medical applications: including laser surgery, molecular surgery, medical endoscopy, optical coherence tomography, photodynamic therapy, or minimally invasive intervention; Sensing and detection: including gas sensing, spectral analysis, environmental monitoring, chemical analysis, or biological sensing; Navigation and positioning: including fiber-optic gyroscopes, inertial navigation systems, or attitude measurement; Industrial automation: including industrial robot sensing and communication, automated production lines, or intelligent manufacturing systems; Semiconductor manufacturing and detection: including lithography system light transmission, wafer detection, or mask detection; Scientific research and special applications: including nonlinear optics, ultrafast optics, optical frequency combs, extreme environment transmission, or fundamental physics experiments.

75. The system or device of claim 74, wherein, The system or device is used for transmission of mid-infrared band laser with a working wavelength range of 2 µm to 20 µm, applied to one or more of molecular surgery, tissue cutting, tumor ablation, or neurosurgery precision operations.

76. The system or apparatus of claim 74, wherein, The anti-resonant hollow core optical fiber has an outer diameter of 120 µm to 130 µm and a mode field diameter of 8 µm to 15 µm, configured to be directly fused or interfaced with standard single-mode optical fibers.

77. The system or apparatus of claim 74, wherein, The system or device is used for transmission of high-power laser beams with an average power greater than 100 W or a peak power greater than 1 MW.

78. The system or device of claim 74, wherein, The anti-resonant hollow core optical fiber is configured to operate within a temperature range of -40 °C to +85 °C, used for submarine cable communications, underwater sensor networks, or deep-sea scientific data transmission.

79. The system or device of claim 74, wherein, The anti-resonant hollow core optical fiber is used for transmission of control signals or sensing signals with a working wavelength of 500 nm to 2400 nm, applied to industrial robots, automated production lines, or flexible manufacturing cells.

80. The system or device of claim 74, wherein, The system or device is used for transmission of ultraviolet, deep ultraviolet, or extreme ultraviolet light with a working wavelength range of 10 nm to 400 nm, applied to lithography systems, wafer detection, or mask detection.

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