Polarization-maintaining hollow-core anti-resonance optical fiber

Abstract

The invention relates to the technical field of special optical fibers, in particular to a polarization-maintaining hollow-core anti-resonance optical fiber which comprises an outer cladding, an inner cladding, a fiber core, a first-class anti-resonance unit and a second-class anti-resonance unit. The inner cladding is arranged in the outer cladding, and the fiber core, the first-class anti-resonance unit and the second-class anti-resonance unit are arranged in the inner cladding; the first-class anti-resonance unit does not comprise a coupling thin wall, the second-class anti-resonance unit is provided with a coupling thin wall close to the fiber core side, and the coupling thin walls are two thin walls which are extremely close to each other and are not in contact with each other. According to the invention, high birefringence and broadband low transmission loss can be realized at the same time.

Description

Technical Field

[0001] The present application relates to the technical field of special optical fibers, and in particular to a polarization-maintaining hollow-core antiresonant optical fiber. Background Art

[0002] Polarization-maintaining fiber (PMF) is a type of optical waveguide that maintains the stable transmission of linearly polarized light by suppressing coupling of orthogonal polarization states to the fundamental mode through high birefringence. It is primarily targeted at a wide range of optical applications requiring high coherence, including fully polarization-maintaining fiber laser systems, high-power laser transmission, high-performance fiber-optic sensing, and astronomical photonics. Traditional PMF uses quartz or doped quartz glass as its optical core. Its light transmission performance is limited by the inherent dispersion, nonlinearity, damage threshold, and environmental sensitivity (to temperature and magnetic fields) of the quartz glass material, which has gradually restricted the performance of cutting-edge optical fiber applications. In contrast, hollow-core antiresonant fiber (HCHF) uses a cladding microstructure to confine the light field within a hollow core (air / vacuum). Not only does it inherently possess optical properties superior to those of quartz, such as low nonlinearity, low dispersion, and a high photoinduced damage threshold, it also exhibits a lower loss limit than traditional solid-core quartz fiber. With the introduction of polarization-maintaining properties, HCHF is expected to replace traditional solid-core PMF in most passive coherent optical applications.

[0003] Since the mode field of hollow-core fiber mainly exists inside the hollow structure, it is not compatible with the stress-induced birefringence method of traditional solid-core fiber. Constructing birefringence in hollow-core fiber has always been a technical challenge in the field of optical fiber. The existing polarization-maintaining hollow-core antiresonant fiber still has great limitations in terms of technology and cannot achieve low loss, wide bandwidth and high birefringence at the same time. The mainstream quasi-quadruple symmetric fiber structure has reached 10 -4 High birefringence will be accompanied by a series of high-loss peaks introduced by the resonance of the cladding's high-order thin-wall mode within the working window, significantly deteriorating the minimum transmission loss and low-loss bandwidth. The polarization-maintaining hollow-core antiresonant fiber developed based on this technology does not yet have the ability to be used over long distances. Although hollow-core antiresonant fibers based on elliptical core designs can generate birefringence, the efficiency of birefringence introduction is extremely low, and it is difficult to reach 10 -4 The scale is also not feasible for application. Summary of the Invention

[0004] In order to overcome the above-mentioned defects, the present application provides a polarization-maintaining hollow-core antiresonant optical fiber, comprising an outer cladding, an inner cladding, a core, a first type of antiresonant unit, and a second type of antiresonant unit; the inner cladding is arranged in the outer cladding, and the core, the first type of antiresonant unit, and the second type of antiresonant unit are arranged in the inner cladding; the first type of antiresonant unit does not include a coupling thin wall, and the second type of antiresonant unit has a coupling thin wall close to the core side, and the coupling thin walls are two thin walls that are extremely close to each other but not in contact.

[0005] Furthermore, the total number of the first type anti-resonance units and the second type anti-resonance units is greater than or equal to 3.

[0006] Furthermore, the minimum spacing of the coupling thin walls is less than or equal to 1.5 times the operating wavelength.

[0007] Furthermore, the first type of anti-resonance unit includes a non-tubular structure or a tubular structure, and the total number of the non-tubular structure and the tubular structure is greater than or equal to two.

[0008] Furthermore, the second type of anti-resonance unit includes a non-tubular structure or a tubular structure, the coupling thin wall is composed of two tubular structures or two non-tubular structures or a combination of a tubular structure and a non-tubular structure, and the total number of non-tubular structures and tubular structures is greater than or equal to two.

[0009] Furthermore, the refractive index of the material of the tubular structure and the non-tubular structure ranges from 1 to 4.

[0010] Furthermore, the tubular structure is a circular tubular structure or an elliptical tubular structure.

[0011] Furthermore, the inner boundary shape of the outer cladding is circular, elliptical, polygonal, or elliptical polygonal.

[0012] The above technical solution of this application has the following advantages: The polarization-maintaining hollow-core antiresonant optical fiber provided herein is constructed by arranging first-type antiresonant units and second-type antiresonant units within the inner cladding. The first-type antiresonant unit does not include a coupling thin wall, while the second-type antiresonant unit has a coupling thin wall near the core side. The coupling thin walls are two thin walls that are extremely close to each other but not touching. The introduction of the coupling thin walls can significantly enhance the coupling difference between the core fundamental mode and the cladding thin wall mode in the orthogonal polarization states of the optical fiber near the resonant wavelength. High birefringence can be generated near the operating wavelength without excessively constructing orthogonal differentiated wall thicknesses around the core, thereby avoiding high loss peaks caused by resonance of high-order thin wall modes of the cladding. The optical fiber can simultaneously achieve low loss, wide bandwidth, and high birefringence. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the specific implementation methods or the description of the prior art. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0014] Figure 1 This is a design diagram of the optical fiber structure of Example 1 provided in this application; Figure 2 This is a technical rendering of Example 1 provided in this application; Figure 3 This is a diagram of the optical fiber structure design of Example 2 provided in this application; Figure 4 This is a technical rendering of Example 2 provided in this application; Figure 5 This is a design diagram of the optical fiber structure of other embodiments provided in this application. DETAILED DESCRIPTION

[0015] In the following description, specific details such as specific system structures and techniques are provided for purposes of illustration rather than limitation to facilitate a thorough understanding of the embodiments of the present application. However, it will be apparent to those skilled in the art that the present application may be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted to avoid obscuring the description of the present application with unnecessary detail.

[0016] It should be understood that when used in the present specification and the appended claims, the term "comprising" indicates the presence of described features, integers, steps, operations, elements and / or components, but does not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or collections thereof.

[0017] In the description of this application, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "back," "left," "right," "vertical," "horizontal," "top," "bottom," "inside," "outside," and the like, indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended only to facilitate the description of this application and simplify the description. They do not indicate or imply that the devices or components referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore should not be construed as limiting this application. In addition, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0018] In the description of this application, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integral connections; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal connections between two components. Those skilled in the art will understand the specific meanings of the above terms in this application based on the specific circumstances.

[0019] References to "one embodiment" or "some embodiments" in this specification mean that a particular feature, structure, or characteristic described in conjunction with the embodiment is included in one or more embodiments of the present application. Thus, phrases such as "in one embodiment," "in some embodiments," "in other embodiments," and "in yet other embodiments" appearing in various places in this specification do not necessarily refer to the same embodiment, but rather mean "one or more but not all embodiments," unless otherwise specifically emphasized.

[0020] This application aims to efficiently develop the polarization-maintaining low-loss optical transmission performance of hollow-core antiresonant optical fibers, sacrificing the lowest possible transmission loss and low-loss passband width in exchange for the birefringence properties of the optical fiber, that is, improving the birefringence introduction efficiency of the optical fiber.

[0021] The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0022] An embodiment of the present application provides a polarization-maintaining hollow-core antiresonant optical fiber, comprising an outer cladding, an inner cladding, a core, a first-type antiresonant unit, and a second-type antiresonant unit; the inner cladding is arranged within the outer cladding, and the core, the first-type antiresonant unit, and the second-type antiresonant unit are arranged within the inner cladding; the first-type antiresonant unit does not include a coupling thin wall, and the second-type antiresonant unit has a coupling thin wall near the core side, and the coupling thin walls are two thin walls that are extremely close to each other but not in contact.

[0023] In some embodiments, the total number of the first type anti-resonance units and the second type anti-resonance units is greater than or equal to 3.

[0024] In some embodiments, the minimum spacing of the coupling thin walls is less than or equal to 1.5 times the operating wavelength.

[0025] In some embodiments, the first type of anti-resonance unit includes a non-tubular structure or a tubular structure, and the total number of the non-tubular structure and the tubular structure is greater than or equal to two.

[0026] In some embodiments, the second type of anti-resonance unit includes a non-tubular structure or a tubular structure, the coupling thin wall is composed of two tubular structures or two non-tubular structures or a combination of a tubular structure and a non-tubular structure, and the total number of non-tubular structures and tubular structures is greater than or equal to two.

[0027] In some embodiments, the refractive index of the material of the tubular structure and the non-tubular structure ranges from 1 to 4.

[0028] In some embodiments, the tubular structure is a circular tubular structure or an elliptical tubular structure.

[0029] In some embodiments, the inner boundary shape of the outer cladding is circular, elliptical, polygonal, or elliptical polygonal.

[0030] This application proposes a new mechanism for enhancing birefringence in hollow-core antiresonant fibers, deploying two types of antiresonant units within the inner cladding. One type of antiresonant unit constructs a coupling wall near the core. The coupling wall consists of two thin walls that are extremely close but not touching, with the separation between the walls less than 1.5 times the operating wavelength. Due to the high proximity between the two thin walls, modes will couple, manifesting as eigenstates of the hybrid mode field in the fiber. The other type of antiresonant unit does not contain a coupling wall.

[0031] The introduction of coupling walls can further break the degeneracy of the optical fiber's thin-wall modes, enhancing the polarization-differential coupling between the core's orthogonal polarization fundamental mode and the thin-wall mode near the resonant wavelength, and improving birefringence performance in the low-loss band. More importantly, this effect enhances birefringence without introducing significant high-order thin-wall mode resonant loss peaks within the low-loss operating window, resulting in a high efficiency in birefringence introduction. The optical fiber structure proposed in this application can simultaneously achieve low loss, wide bandwidth, and high birefringence.

[0032] The key points of this application are the deployment of two types of antiresonant units within the inner cladding. The first type of antiresonant unit does not include a coupling thin wall, while the second type of antiresonant unit has a coupling thin wall near the core. The coupling thin wall assists in enhancing the birefringence characteristics of the optical fiber, thereby improving the broadband polarization-maintaining, low-loss optical transmission performance of the hollow-core antiresonant fiber. The antiresonant unit is composed of a tubular structure (circular or elliptical), a non-tubular structure (negative curvature wall, straight wall, positive curvature wall), or a combination of the two. The total number of structures (including both tubular and non-tubular structures) contained in the antiresonant unit is ≥2.

[0033] The coupling thin walls are two thin walls that are very close but not touching, with the minimum spacing between the first and second thin walls being less than or equal to 1.5 times the operating wavelength. The refractive index of the tubular or non-tubular structure forming the antiresonant unit in the inner cladding region of the optical fiber ranges from 1 to 4. The inner boundary shape of the optical fiber outer cladding can be not only circular but also elliptical, polygonal, and elliptical-polygonal.

[0034] Conventional designs for high-birefringence hollow-core antiresonant optical fibers require significant thickness differences between the first thin walls in directions orthogonal to the core within a four-fold rotationally symmetric structure. This results in numerous resonant loss peaks in the fiber's loss spectrum, making it difficult to further reduce transmission loss. The high-birefringence hollow-core antiresonant optical fiber designed in this application utilizes polarization-dependent interactions between core modes and coupled thin walls near the core (separated by ≤1.5 times the operating wavelength) to enhance the birefringence effect. The associated loss increase is far less than with conventional orthogonal differentiated wall thickness structures, enabling the fiber to simultaneously achieve high birefringence and broadband, low transmission loss.

[0035] The following describes the invention through specific embodiments.

[0036] Example 1 This embodiment designs a transmission loss in the dB / km range, a low-loss bandwidth > 100nm (loss ≤ 10dB / km), and a birefringence of 10 -4 Polarization-maintaining hollow-core antiresonant optical fiber of the order of magnitude, the optical fiber structure is as follows Figure 1 . In this embodiment, the optical fiber core diameter a=20μm, and the inner boundary diameter of the outer cladding tube e=66μm. The first type of anti-resonance unit is composed of a negative curvature non-tubular structure with a wall thickness of b11=1.42μm and an outer diameter of b12=40μm, and two symmetrically deployed tubular structures with a wall thickness of b21=0.37μm, an outer diameter of b22=13.6μm, and a spacing of b23=2μm. The second type of anti-resonance unit is composed of a negative curvature non-tubular structure with a wall thickness of c11=1.10μm and an outer diameter of c12=40μm, and a negative curvature non-tubular structure with a wall thickness of c21=1.17μm and an outer diameter of c22=32μm, forming a coupled thin wall with a minimum spacing of d=0.9μm, and a tubular structure with a wall thickness of c31=0.37μm and an outer diameter of c32=13.6μm is deployed inside.

[0037] The optical performance simulation results of the optical fiber structure of this embodiment are shown in Figure 2 ,in Figure 2 (a) shows the limiting loss of the fundamental modes of the two orthogonal polarization states of the optical fiber core. The loss of the low-loss polarization mode at a typical wavelength of 1550 nm is 4.6 dB / km, and the low-loss bandwidth is 130 nm (the loss spectrum bandwidth corresponding to loss ≤ 10 dB / km). Figure 2 (b) shows the simulated birefringence of the fiber. In the entire fiber working window, the birefringence is 10 -4 Optical fiber has both broadband, low loss and high birefringence properties.

[0038] Example 2 This embodiment designs a transmission loss of 0.1dB / km, a low-loss bandwidth > 100nm (loss ≤ 10dB / km), and a birefringence of 10 -4 Polarization-maintaining hollow-core antiresonant optical fiber of the order of magnitude, the optical fiber structure is as follows Figure 3In this embodiment, the optical fiber core diameter is a = 20 μm, and the inner diameter of the outer cladding tube is e = 70 μm. The first type of antiresonance unit is composed of a negative curvature non-tubular structure with a wall thickness of b11 = 1.42 μm and an outer diameter of b12 = 40 μm, and two symmetrically arranged nested tubular structures with a spacing of b23 = 2 μm. The nested tubular structure is composed of an outer tube with a wall thickness of b21 = 0.37 μm and an outer diameter of b22 = 15.6 μm and an inner tube with a wall thickness of b31 = 0.37 μm and an outer diameter of b32 = 9.6 μm. The second type of antiresonance unit consists of a negative curvature non-tubular structure with a wall thickness of c11 = 1.10μm and an outer diameter of c12 = 40μm and a negative curvature non-tubular structure with a wall thickness of c21 = 1.17μm and an outer diameter of c22 = 32μm, forming a coupled thin wall with a minimum spacing d = 0.9μm, and a nested tube structure is deployed inside. The nested tubular structure is composed of an outer tube with a wall thickness of c31 = 0.37μm and an outer diameter of c32 = 15.6μm and an inner tube with a wall thickness of c41 = 0.37μm and an outer diameter of c42 = 9.6μm.

[0039] The optical performance simulation results of the optical fiber structure of this embodiment are shown in Figure 4 ,in Figure 4 (a) shows the limiting loss of the fundamental modes of the two orthogonal polarization states of the optical fiber core. The loss of the low-loss polarization mode at the typical wavelength of 1550 nm is 0.36 dB / km, and the low-loss bandwidth is 110 nm (the loss spectrum bandwidth corresponding to loss ≤ 1 dB / km). Figure 4 (b) shows the simulated birefringence of the fiber. In the entire fiber working window, the birefringence is 10 -4 Optical fiber has both broadband, low loss and high birefringence properties.

[0040] In addition to Embodiment 1 and Embodiment 2, the present application also provides other possible embodiments. Including 1) a polarization-maintaining hollow-core antiresonant fiber structure with an outer cladding inner boundary shape of a rounded quadrilateral, see Figure 5 (a); 2) Polarization-maintaining hollow-core antiresonant fiber structure with two types of antiresonant units of the same composition in the inner protective layer, see Figure 5 (b) The optical fiber performance of the two embodiments is comparable to that of embodiment 1 and embodiment 2, respectively.

[0041] In the existing technology, the polarization-maintaining hollow-core antiresonant fiber technology with an elliptical core is relatively inefficient in introducing birefringence. In contrast, the polarization-maintaining hollow-core antiresonant fiber based on a four-fold rotational symmetric structure has better birefringence and low loss performance. However, this technology has problems in over-constructing the orthogonal differentiated wall thickness configuration around the core to achieve high birefringence (such as near-infrared wavelength 10 -4At the same time, it will introduce high-order thin-wall mode resonance loss peaks in the working window, significantly worsening the optical fiber transmission loss and low-loss passband width. Moreover, this type of loss peak cannot be suppressed by increasing the number of wall thickness layers in the anti-resonance unit. As the optical fiber birefringence is introduced, the efficiency is obviously insufficient.

[0042] The present invention proposes a polarization-maintaining hollow-core antiresonant fiber structure based on coupled-wall enhancement of fiber birefringence. Two types of antiresonant units are deployed within the inner cladding. One type of antiresonant unit comprises a pair of thin walls, located near the core, with a minimum spacing of less than 1.5 times the operating wavelength, but not touching each other. The leakage modes of these walls overlap, and their coupling affects the mode field and effective refractive index of the mixed-state mode. This enhances the orthogonal, differentiated coupling between the core mode and the cladding thin-wall mode at the resonant wavelength, thereby improving the birefringence of the fiber within the low-loss passband. This technology significantly improves the birefringence introduced by wall thickness differences and significantly avoids the high-order thin-wall mode resonances that accompany the birefringence introduction process. Under similar structural parameter conditions, the fiber designed in this application significantly outperforms polarization-maintaining hollow-core antiresonant fibers based on quasi-quadruple rotationally symmetric structures in terms of birefringence, loss, and bandwidth.

[0043] Those skilled in the art will clearly understand that, for the convenience and brevity of description, only the division of the above-mentioned functional units and modules is used as an example for illustration. In actual applications, the above-mentioned functions can be distributed and completed by different functional units and modules as needed, that is, the internal structure of the device can be divided into different functional units or modules to complete all or part of the functions described above. The functional units and modules in the embodiments can be integrated into one processing unit, or each unit can exist physically alone, or two or more units can be integrated into one unit. In addition, the specific names of the functional units and modules are only for the purpose of distinguishing each other and are not used to limit the scope of protection of this application.

[0044] It should be noted that the various embodiments in this specification are described in a progressive manner. Reference can be made to the same or similar parts between the various embodiments. Each embodiment focuses on the differences from other embodiments. This application is not limited to the specific structures described above and shown in the figures. Furthermore, for the sake of brevity, detailed descriptions of known methods and technologies are omitted here.

[0045] The above-described embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present application, and should all be included in the scope of protection of the present application.

Claims

1. A polarization-maintaining hollow-core antiresonant optical fiber, characterized in that: The fiber core comprises an outer cladding, an inner cladding, a fiber core, a first type antiresonance unit, and a second type antiresonance unit; the inner cladding is arranged in the outer cladding, and the fiber core, the first type antiresonance unit, and the second type antiresonance unit are arranged in the inner cladding; the first type antiresonance unit does not include a coupling thin wall, and the second type antiresonance unit has a coupling thin wall close to the fiber core side, and the coupling thin walls are two thin walls that are extremely close to each other but not in contact.

2. The polarization-maintaining hollow-core antiresonant optical fiber according to claim 1, wherein: The total number of the first type anti-resonance units and the second type anti-resonance units is greater than or equal to 3.

3. The polarization-maintaining hollow-core antiresonant optical fiber according to claim 1, wherein: The minimum spacing of the coupling thin walls is less than or equal to 1.5 times the operating wavelength.

4. The polarization-maintaining hollow-core antiresonant optical fiber according to claim 1, wherein: The first type of anti-resonance unit includes a non-tubular structure or a tubular structure, and the total number of the non-tubular structure and the tubular structure is greater than or equal to two.

5. The polarization-maintaining hollow-core antiresonant optical fiber according to claim 1, wherein: The second type of anti-resonance unit includes a non-tubular structure or a tubular structure, the coupling thin wall is composed of two tubular structures or two non-tubular structures or a combination of a tubular structure and a non-tubular structure, and the total number of non-tubular structures and tubular structures is greater than or equal to two.

6. The polarization-maintaining hollow-core antiresonant optical fiber according to claim 4 or 5, wherein: The refractive index of the material of the tubular structure and the non-tubular structure ranges from 1 to 4.

7. The polarization-maintaining hollow-core antiresonant optical fiber according to claim 4 or 5, characterized in that: The tubular structure is a circular tubular structure or an elliptical tubular structure.

8. The polarization-maintaining hollow-core antiresonant optical fiber according to claim 1, wherein: The inner boundary shape of the outer cladding is circular, elliptical, polygonal, or elliptical polygonal.

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