Ultra-Low-Loss Hollow-Core Anti-Resonant Fiber Combining Double-Tube Nesting and a Single-Layer Anti-Resonant Wall

Abstract

This study innovatively presents a hollow-core anti-resonant fiber integrating double-tube nesting and a single-layer anti-resonant wall. Featuring an exclusive two-layer cladding configuration along with an outer cladding circular ring, it differs significantly from traditional fibers. After careful parameter optimization, at 1.55 μm wavelength, the fiber shows excellent performance. Its confinement loss drops to 0.00088 dB/km, 1–2 orders lower than traditional ones. The proportion between the loss of the lowest higher-order mode and that of the fundamental mode reaches 19,900, indicating excellent single-mode performance. In the case of a bending radius of 11–14.2 cm, the x-polarization loss is below 0.001 dB/km, showing good bending resistance. Through structural comparisons, this paper quantitatively reveals the effects of the anti-resonant wall, cladding tube, and outer cladding ring on fiber performance. From the practical fiber-drawing process, it thoroughly analyzes the impact of the outer connecting tube’s offset angle on fiber performance. This research provides crucial theoretical support for new hollow-core fiber design, manufacture, and application, and is expected to drive technological innovation in this field.

1. Introduction

Traditional solid-core optical fibers confine light to propagate within the light-guiding material. Consequently, they suffer from relatively high material losses. In contrast, hollow-core optical fibers confine light to propagate in air, which significantly reduces the transmission losses. Due to their unique structures and light-guiding mechanisms, hollow-core optical fibers have attracted the attention of numerous researchers [1,2]. Hollow-core optical fibers have achieved important research results in many fields, such as the study of nonlinear optics [3,4], optical fiber sensing [5], high-power pulsed laser transmission [6], long-distance optical communication [7], and radiation resistance [8]. This indicates that they have broad application prospects. Among various classifications of hollow-core optical fibers, the anti-resonant structure has undoubtedly become a current research hotspot. It combines the anti-resonant principle with the mode-coupling effect [9,10]. By using the anti-resonant wall, it traps light within the air core. By modifying the structural variables, the effective refractive indices of various modes inside the fiber can be changed, enabling interaction among modes, thereby obtaining various high-performance optical fibers. In addition, since the thickness of the light-guiding material can be adjusted according to the wavelength of the transmitted light, this structure exhibits a high degree of design freedom. That is, this structure can achieve a relatively wide transmission bandwidth [11,12,13,14], making it of great research value.

Since changes in the structure can exert a substantial influence on various performance aspects of optical fibers, researchers have carried out many innovative designs of the fiber structure and achieved remarkable results. In 2016, the research by Wang et al. demonstrated that a cladding tube is placed within the anti-resonant tube, which in turn raises the quantity of anti-resonant layers, could further decrease the transmission loss of hollow-core anti-resonant fibers [15]. Subsequently, scientists and researchers focused on reducing losses by adjusting the structure of the nested tube. Adding connecting tubes to the nested structure was an important attempt. In 2018, Gao et al. [16] developed an extremely low-loss hollow-core anti-resonant fiber. Its outer cladding featured six connecting tubes. The addition of this structure increased the number of reflective walls, effectively confining the light field. The results showed that this approach reduced the loss to 2 dB/km. Based on this, in 2021 Shaha et al. proposed a nested elliptical hollow-core fiber. It achieved a loss of 0.0007 dB/km at 1.06 μm wavelength and had a 670 nm bandwidth with a loss of below 0.003 dB/km [17]. In 2019, Jasion et al. [18] found that gaps between cladding tubes can repel power streamlines to reduce leakage. They proposed an improved structure with a large cladding tube housing two nested small tubes. The radial overlap of the small tubes’ gap with the large tube’s center curbed central power leakage, potentially cutting confinement loss by two orders of magnitude. Furthermore, in 2021, Akosman [19] added small circular tubes outside the cladding tube. This enhancement improved the fiber’s single-mode performance and reduced the inner cladding tube size.

Based on the above-mentioned design process of the optical fiber structure, this study presents a hollow-core anti-resonant optical fiber integrating double-tube nesting with a single-layer anti-resonant wall. It is noted that the anti-resonant wall not only further reduces the transmission loss but also collaborates with the nested tube structure to modify the in-tube mode shape and effective refractive index, thereby achieving superior single-mode performance. This also enhances the design freedom and flexibility of the structure. Meanwhile, to achieve better performance, an extra cladding-tube layer and an outer cladding ring are incorporated on its outside. Through numerical simulations, the results indicate that at a wavelength of 1.55 μm, the CL of this structure drops to 8.8 × 10⁻⁴ dB/km, the HOMER reaches 1.99 × 10⁴, and when the bending radius is 11 cm, the bending loss stays below 1 × 10⁻³ dB/km. Moreover, in the wavelength range of 1.2–1.7 μm, the HOMER is greater than 100, indicating that this structure has excellent characteristics such as low loss, high single-mode performance, and bending resistance. In contrast to the aforementioned anti-resonant hollow-core fibers of various structures, the fiber structure presented in this paper exhibits a loss reduction of one to two orders of magnitude.

2. Model for Structural Design and Simulation

The internal structure diagram of the hollow-core fiber that combines double-tube nesting and a single-layer anti-resonant wall, along with the enlarged view of the area of its inner cladding tube, is shown in Figure 1.

In this figure, the silica is denoted by the purple region, while the air is represented by the white region. The first-layer cladding of the proposed optical fiber consists of five cladding tubes, each combining a single-layer anti-resonant wall and two nested small capillary tubes. The second layer is composed of five cladding tubes with smaller dimensions. In addition, to obtain lower confinement loss, we use connecting rods with a length of L = 5 to support a cladding ring with the same thickness t outside the second-layer cladding tubes [20]. For this design, the outer-circle of the first-layer cladding has a diameter of dt, and the gap between the outer-circles is the clearance g. According to the geometric shape, g and dt satisfy the following calculation formula:

$${\mathrm{d}}_t={\displaystyle \frac{\mathit{sin}\left(\frac{\pi }{N}\right)\times Dc-g}{1-\mathit{sin}\left(\frac{\pi }{N}\right)}}-2t$$

(1)

Among them, N represents the number of outer tubes in the first layer, which is taken as five in this structure. Dc represents the core diameter, while t denotes the thickness of each cladding tube.

The angle p is formed between the line connecting the center of the circle of the nested part with the center of the outer-circleand the line connecting the centroid of the outer-circle with the centroid of the air core. We can simulate the alteration in the position of the inserted tube by varying the value of $P$. Inside the cladding tube, the semi-elliptical glass wall has a thickness of $t$, with the semi-major axis radius $dx$ and semi-minor axis radius $dy$. They satisfy the following operations:

$$r_y=d_t∕2$$

(2)

$$r_x=r_y\times k$$

(3)

Here, $k$ represents the ratio of $dx$ to $dy$. By adjusting the value of $k$, we can simulate and control the degree of curvature of the single-layer anti-resonant wall, thereby identifying the optimal position of it. In addition, the diameter of the second-layer cladding tube is $d_z$.

Among the performance indicators of optical fibers, the confinement loss (CL) is of particular importance and will be the focus of the following discussion. The formula that follows can be employed to work it out [20]:

$$CL=40\pi \times n_{imag}∕\left[\mathit{ln}\left(10\right)\lambda \right]$$

(4)

λ stands for the light wavelength, and nimag represents the imaginary part of the effective refractive index of the fundamental mode. Besides, the anti-resonant condition decides the size of the thickness t of the cladding tube [11,21]:

$$t={\displaystyle \frac{\left(m-0.5\right)\lambda }{2\sqrt{n_1^2-n_2^2}}}$$

(5)

In the formula, the refractive index corresponding to silica is represented by n1, and it’s possible to calculate it using the Sellmeier relation [22], and n2 stands for the refractive index of the air medium, which equals 1. In the subsequent analysis, we set the anti-resonant order m = 1. The core diameter is specified as 30 μm, while the wall thickness t for the first order anti-resonance is determined to be 0.4 μm.

In this paper, the finite-element methodology is applied to examine the behavior of optical fibers, and COMSOL Multiphysics 5.4 is utilized to construct various structures. The numerical simulation results of each structure are obtained through simulation. The mesh dimensions play a crucial role in determining the simulation results. The mesh size is determined by the material refractive index and the incident wavelength. For an incident wavelength of 1.55 μm, with the refractive index of the air region being 1, the maximum mesh size of the air part should not exceed λ/4, and that of the glass part should not exceed λ/6, so as to obtain more accurate calculation results. In addition, to simulate a more ideal simulation environment, a perfectly matched layer (PML) is affixed to the exterior of the structure.

3. Determination of Structural Variables

3.1. Diameter dz of Second Tube

In this section, at a 1.55 μm wavelength, we analyze the effect of varying the diameter dz of the second cladding tube on the fiber confinement loss and the high-order mode elimination ratio (HOMER)—a key metric for gauging optical fiber single-mode traits, calculated as the ratio of minimum high-order mode loss to fundamental mode loss [20,23]. Meanwhile, the other relevant parameters such as DC = 30 μm, t = 0.4 μm, dp = 9.32 μm, p = 45° and k = 0.56 are kept unchanged.

As can be observed from Figure 2, when the parameter $dz$ is within the range of 6–15 μm, the fundamental mode loss of the optical fiber generally exhibits a decreasing trend. When dz = 12.2 μm, the confinement loss reaches 0.00024 dB/km, a value that is significantly lower than many of the known minimum values obtained from loss simulations. At the same time, the HOMER value at this point is 450, indicating good single-mode characteristics at this time. When dz increases to 13.6 μm, the CL further drops to 0.000047 dB/km, and the HOMER value reaches 1209. This convincingly demonstrates that, with this parameter setting, the structure not only has an extremely low confinement loss but also exhibits exceptionally excellent single-mode performance.

As depicted in Figure 3, we studied how the confinement loss and HOMER fluctuate with wavelength under the specified conditions of $dz=1.35 \mathsf{\mu }\mathrm{m}$, $dz=1.36 \mathsf{\mu }\mathrm{m}$, and $dz=1.37 \mathsf{\mu }\mathrm{m}$. As clearly shown in Figure 3a, all three loss curves exhibit oscillations of varying degrees with the increase in wavelength; it is initiated due to the Fano resonance [16] created by the nodes between the two claddings. However, compared with the other two curves, the loss curve with $dz=0.36 \mathsf{\mu }\mathrm{m}$ has a smaller oscillation amplitude. Moreover, when $\lambda =1.55 \mathsf{\mu }\mathrm{m}$, its minimum loss value reaches 0.000047 dB/km, at the lowest point among the three loss curves. In light of these findings, we have ultimately selected dz = 13.6 μm as the key parameter for follow-up research.

Figure 3b illustrates the variation pattern of HOMER with respect to the wavelength. The reason for the oscillation of the curves is the same as described above. Overall, the three curves change from a stable state to a sharp decline, then a sharp increase, and finally become stable again. This indicates that minor variations in the size of the second-layer cladding tube do not alter the overall trend of the performance curves. When $dz=1.36\mathsf{\mu }\mathrm{m}$ and $\lambda =1.55\mathsf{\mu }\mathrm{m}$, the HOMER curve reaches its maximum value of 1209. However, when $\lambda =1.54\mathsf{\mu }\mathrm{m}$and $\lambda =1.56\mathsf{\mu }\mathrm{m}$, the value of HOMER is less than 100. Therefore, it is necessary to further regulate other dimensions of the structure to expand the bandwidth capable of single-mode transmission.

3.2. Angle p of the Nested Tube

The alteration in the angle p within the nested tube causes two main effects. First, it changes the size of the gap between the two nested tubes. This directly affects the ability to confine the fundamental-mode optical power, thus causing changes in fiber loss. Second, as the angle g varies, both the mode shape within the cladding tube and its effective refractive index change. Such alterations further impact the coupling efficiency among the cladding modes and the higher order modes, thereby causing changes in the single-mode performance of the fiber. Therefore, choosing the right angle p is very important for improving the fiber performance.

Figure 4 illustrates the changing rules of the CL and HOMER as they relate to the nested-tube angle p under the conditions of dz = 1.36 μm and λ = 1.55 μm. According to Figure 4, when the value of p is between 43°and 75°, the value of the CL for the fundamental mode remains below 0.01 dB/km. Moreover, when p = 45°, it is consistent with the previous simulation results; the FL drops as low as the order of 10–5. Additionally, when it is in the range of 59°to 69°, the magnitude of the confinement loss mostly aligns with a value similar to 10⁻⁴. Conversely, as the value of p rises, the higher-order mode loss typically demonstrates a pattern of initially declining and subsequently ascending. In addition, the HOMER curve has a relatively large oscillation amplitude. It only reaches the order of 10³ when p = 45°. When p = 53°, the value of HOMER also reaches 706, indicating that a good single-mode performance is also achieved at this time. However, the loss at this moment is 0.0012 dB/km, which is two orders of magnitude higher than that when p = 45°. Therefore, in this paper, p = 45° is still selected for subsequent research.

3.3. Diameter dq of the Nested Tube

Once the nested-tube angle p is fixed, the diameters dq of the two nested tubes will directly affect the size of the gap between them. This, in turn, influences the ability to confine the optical power. Therefore, it is necessary to conduct numerical simulations on their diameters to select the optimal dimensions of the nested tubes. In this context, we define k₁ as the ratio of the nested tube’s diameter dq to the cladding tube’s diameter dt. By conducting a meticulous scrutiny of the alterations in optical fiber characteristics induced by the change in k₁, the most appropriate value of k₁ can be pinpointed. Therefore, the magnitude of dq for the nested tube is established. Figure 5 illustrates how the fiber CL and HOMER change as k₁ increases, with the nested-tube angle fixed at p = 45°. When k₁ lies within the range of 0.26 to 0.32, the troughs of the oscillating CL curve are all below 0.001 dB/km. However, the HOMER curve attains the order of 103 only at k₁ = 0.3, corresponding to dp = 9.32 μm. In all other ranges of k₁, its value remains less than 100. Thus, the diameter of dp = 9.32 μm of gap g between the cladding tubes is selected for the subsequent research in this paper.

3.4. The Gap g Between the Cladding Tubes

As the dimension of the gap among the cladding tubes varies, the diameters of these cladding tubes will adjust correspondingly. Additionally, the change in the diameters of the cladding tubes will have an immediate impact on the transmission characteristics of the optical fiber. As a result, under the condition of keeping Dc = 30 μm, dz = 13.6 μm, p = 45° and dp = 9.32 μm constant, in this section, we are going to explore the impact of the cladding gap g upon the CL and HOMER.

Figure 6 illustrates the alterations in the magnitudes of the confinement losses related to the LP01 and LP11 modes and the behavior of HOMER as g varies. The figure reveals that with the growth of g, the CL of the fiber initially shows a downward trend and later an upward one. In contrast, the HOMER value typically drops as g gets larger. Among them, when g = 3 μm, the CL is measured to be 0.00081 dB/km, while the HOMER attains a value of 13,900. This indicates that the optical fiber exhibits not only an extremely low confinement loss but also excellent single-mode characteristics. When g = 4.25 μm, the loss is 0.000104 dB/km, which is lower than the former value. However, at this time, the HOMER is 63, indicating relatively poor single-mode performance. When g = 4.5 μm, similar to the previous situation, although the magnitude of CL attains a value around 10⁻⁵, and the HOMER reaches a value around 103, however, according to the CL and HOMER curves of dz = 13.6 μm in Figure 3, this structure only has good single-mode performance at $\lambda$ = 1.55 μm, and the HOMER is less than 100 in other wavelength bands, so the overall performance is relatively poor. Therefore, in this paper, g = 3 μm is selected as the cladding tube gap for subsequent research.

3.5. The Curvature k of the Single-Layer Anti-Resonant Wall

The change of the curvature k of the single-layer anti-resonant wall will cause the shape of the cladding mode and the effective refractive index to change, thereby affecting the coupling effect between the core mode and the cladding mode, and thus, altering the fiber performance. Therefore, the selection of the curvature k is very important.

Figure 7 shows the variation law of the confinement losses and HOMER of the fiber LP01 and LP11 modes with the change of the curvature k of the single-layer anti-resonant wall. Evidently, as k increases, the fundamental mode loss demonstrates a pattern of initially declining and subsequently rising, whereas the HOMER curve first ascends and then descends.

Among them, for k values within the interval from 0.46 to 0.62, the fundamental mode loss consistently remains below 0.01 dB/km. Specifically, when k is in the range of 0.56 to 0.62, except for when k = 0.58 and the CL is 0.0025 dB/km, the CL in the remaining bands is lower than 0.001 dB/km, reaching as low as the order of 10⁻⁴. Whereas, when k lies within the range of 0.5 to 0.62, the magnitude of HOMER is always higher than 100, and when k falls within the range from 0.54 to 0.58, the HOMER values are all greater than 1000. Finally, when k is within the range from 0.568 to 0.58, the HOMER attains a value of 10⁴.

Furthermore, when k = 0.568, it pertains to the previously simulated structure, with a loss of 0.00081 dB/km and a HOMER value of 13,900. When k = 0.57, the fundamental mode confinement loss amounts to 0.00088 dB/km and the HOMER value reaches 19,900, whereas when k = 0.58, the CL is 0.0025 dB/km and the HOMR is 47,400. For the purpose of selecting an index that satisfies both low-loss and high single-mode performance requirements, we therefore opt for k = 0.57 for subsequent investigations.

3.6. Bending Loss

In the course of calculations, it becomes imperative to comprehensively analyze the performance of optical fibers under diverse states, prominently including the bent and twisted configurations. When an optical fiber undergoes bending, the inner part of the fiber core is under compressive stress, whereas the outer part is under tensile stress. Relying on the well-established photoelastic effect theory, these distinct stress states differentially affect the refractive indices in the inner and outer regions of the fiber core [24]. Typically, we convert a bent optical fiber into an equivalent straight optical fiber for numerical simulation purposes. The relevant conversion formulas are as follows [25]:

$$n_{e_q}=n\left(x,y\right)\times \mathit{exp}\left(x∕R_b\right)$$

(6)

Among them, n_eq is the equivalent refractive index of the bent fiber, n(x,y) is the equivalent refractive index of the straight fiber, x is the bending direction, and R_b is the bending radius.

Figure 8 illustrates the functional curves with the bending radius (Rb) as the independent variable in the x-polarization and y-polarization directions. Through precise experimental measurements and data analysis, the research results clearly demonstrate that within the entire effective variation range of the bending radius, the losses for polarizations along the x-axis and y-axis generally show a decreasing trend. However, at a specific position where the value of Rb is approximately 3.8 cm, there is a remarkable peak phenomenon in the losses of the two mutually orthogonal polarization directions. This unique experimental phenomenon strongly implies that under this specific bending radius condition, a mode-coupling phenomenon occurs in the optical waveguide structure, where the core mode and the cladding mode are engaged in coupling. This mode coupling results in additional losses of optical wave energy during the transmission process, thereby causing a sharp increase in the bending loss. Moreover, when Rb = 7.8 cm, a second protrusion appears on the loss curve. At this time, the interaction between the core mode and the cladding-tube interspace via coupling raises the bending loss. Through a detailed comparative analysis, it is noted that when the bending radius is below 5 cm, the loss in the y-polarization state is significantly lower than that in the x-polarization state. When the bending radius exceeds 5 cm, the y-polarization loss gradually declines as the radius increases. Overall, throughout the entire bending radius variation range, the cumulative loss of x-polarization is lower than that of y-polarization. Moreover, additional investigation shows that the loss value of y-polarization ultimately stabilizes at an order of magnitude around 10⁻², whereas the loss value of x-polarization stabilizes at an order of magnitude around 10⁻³. Specifically, for bending radii in the interval from 11.8 to 12.6 cm, the y-polarization loss remains below 0.01 dB/km. When the bending radius ranges from 11 to 14.2 cm, the x-polarization loss is less than 0.001 dB/km, corresponding to an order of magnitude of 10⁻⁴. This result provides more precise quantitative data for a deeper understanding of the transmission characteristics of optical waveguides under different bending radii.

4. Comparison with Alternative Structures

In this subsection, to elucidate the pre-eminent performance of the devised structure, a comparative analysis will be conducted between it and three distinct fiber configurations. These configurations are the conventional five-tube nested two-circle structure; the five-tube nested two-circle structure incorporating a single-layer anti-resonant wall; and the five-tube nested two-circle hollow-core anti-resonant fiber structure furnished with an outer cladding tube and an anti-resonant wall, as graphically presented in Figure 9. Each parameter within (1), (2), (3), and (4) in the figure are identical, specifically, Dc = 30 μm, dz = 13.6 μm, p = 45°, dq = 9.32 μm, g = 3 μm, k = 0.57.

Figure 10 depicts the changing patterns of the CL and HOMER as a function of wavelength for these four structures. In Figure 10a, the loss of structure (1) stably remains at the order of 10⁰. The losses of structure (2) and structure (3) are similar, fluctuating within the range of the order of 10⁻² to 10⁻¹; compared to structure (1), they are 1–2 orders of magnitude lower. The loss of structure (4) fluctuates between the order of 10⁻⁴ and 10⁻², and at the trough of the loss, it shows a two-order of magnitude decrease compared to those of structure (2) and structure (3). It can be concluded that adding a layer of anti-resonant wall can effectively reduce the fiber loss, while adding a second-layer cladding tube outside the first-layer cladding has no significant effect on reducing the loss. In addition, adding a layer of outer cladding ring at the outermost layer can effectively reduce the loss.

In Figure 10b, the HOMER of structure (1) remains consistently at the 10¹ order. When the wavelength is in the range of 1.2–1.52 μm, the HOMER of structure (2) is at the order of 10³, and that of structure (3) is at the order of 10⁴. After the wavelength exceeds a certain value, the HOMER of structure (4) is approximately one order of magnitude higher than that of structure (2), but lower than that of structure (3). After 1.52 μm, the HOMER of structure (2) and structure (3) decreases overall and finally tends to be around the order of 10³, while the HOMER of structure (4) is relatively stable, and the peak always exceeds the order of 10⁴. It can be obtained that adding a layer of anti-resonant wall or a layer of outer cladding ring can effectively improve the single-mode performance. Adding a layer of cladding tube has no obvious improvement in the single-mode performance, but it can ensure the stability of the structure’s overall single-mode operational state. Overall, within the 1.2–1.36 μm band, structure (4) has a fundamental mode loss of below 0.002 dB/km, with a loss of 0.00088 dB/km at 1.55 μm. Meanwhile, within 1.2–1.7 μm band, the HOMER of structure (4) are all larger than 100, and the maximum value reaches 158,818 at $\lambda$ = 1.4 μm. Therefore, the structure proposed in this paper exhibits outstanding performance, characterized by extremely low loss and highly superior single-mode attributes.

Table 1. Performance Comparison of Various Structures.

	Minimum Loss Order of Magnitude	Band Length LOSS < 0.01 dB/km(nm)	Bands with Homer > 100 (μm)
Structure (1)	10−1	0	0
Structure (2)	10−2	0	1.2–1.63
Structure (3)	10−2	0	1.2–1.7
Structure (4)	10−4	300	1.2–1.7

shows the comparison results among these structures. It clearly demonstrates that, compared with the other three structures, the structure proposed in this paper has a low-loss transmission bandwidth of 300 nm in the 1.2–1.7 μm band, and excellent single-mode characteristics with HOMER greater than 100 across the entire interval.

However, The HOMER curve shows drastic fluctuations, indicating unstable high-order mode suppression that may affect practical applications. In communications, this could cause signal instability, bandwidth fluctuations, and reduced system reliability; in sensing, it may lead to larger measurement errors, lower precision, and insufficient reliability. An optimization of the node resonance section is needed for improvement.

5. Discussion

For this paper, the simulation is conducted within an ideal situation where the two connecting tubes are opposite to each other. However, in the actual situation, the angle between the two connecting tubes is not strictly 180°. Minor angular deviations can have a significant impact on the functionality of the fiber optic system. Therefore, the angle w of the upper connecting tube is adjusted here in order to observe the variations within the performance of fiber optics.

Figure 11a depicts the schematic of w varying from −10° to 10°, and Figure 11b illustrates the variation laws of the fiber CL and HOMER with w. The figures clearly show that as the value of w varies, the loss curve and the HOMER curve do not change significantly. The fundamental mode loss always remains around the order of magnitude of 10⁻³. For w spanning from 0° through 10°, the loss of the fundamental mode stays below 0.0001 dB/km. Simultaneously, the HOMER consistently hovers around the 10⁻⁴ order of magnitude and is far larger than 100. Therefore, it can be concluded that a small angular deviation of the connecting tube does not cause a drastic impact on the fiber loss and single-mode characteristics. Obviously, this relaxes the requirement for angular accuracy during actual drawing, thus greatly reducing the difficulty of drawing.

In other ways, during fabrication, current technology allows for the adjusting of the size of nested tubes by controlling air pressure and regulating pressure differences in various intra-tube regions to manipulate the curvature of anti-resonant walls. However, challenges remain, such as the requirement for simultaneous alignment of two cladding tubes with the core, which imposes high precision demands on angular positioning—an issue for which no particularly accurate solution currently exists. Therefore, it is necessary to improve the preparation process to obtain a more accurate structure.

6. Summary

Overall, our study introduces a HC-ARF that combines a double-tube nested structure with a single-layer anti-resonant boundary. After careful parameter optimization, at 1.55 μm wavelength, the fiber shows excellent performance. Its confinement loss drops to 0.00088 dB/km and the HOMER reaches 19,900, indicating excellent single-mode performance. For bending radii ranging from 11 cm to 14.2 cm, the x-polarization loss remains under 0.001 dB/km, showing good bending resistance. Through structural comparisons, this paper quantitatively reveals the effects of the anti-resonant wall, cladding tube, and outer cladding ring on fiber performance. From the practical fiber-drawing process, it thoroughly analyzes the impact of the outer connecting tube’s offset angle on fiber performance. This research provides crucial theoretical support for new hollow-core fiber design, manufacture, and application, and is expected to drive technological innovation in this field.