High birefringence terahertz fibers have a strong polarization preservation ability for linearly polarized light. They can be used for polarization-maintaining transmission of terahertz waves, polarization control, and modulation of terahertz signals. Currently, the most common high birefringence terahertz fibers include photonic crystal fibers and hollow core anti-resonant fibers (HC-ARFs). The former generally introduces structural asymmetry by arranging air holes or changing the core shape to achieve mode birefringence (B). However, their structures are relatively complex, leading to higher fabrication difficulties and significant effective material losses. The latter utilizes the anti-resonant reflection effect for light guidance, which results in low confinement loss and a simple structure. This not only simplifies the fabrication process but also minimizes effective material loss, which makes it a focal point of current research. However, most of the reported high birefringence terahertz HC-ARFs fail to achieve both high mode birefringence and low loss across a broad bandwidth. Based on this, a new structure is designed for high birefringence terahertz HC-ARF. This design combines the introduction of non-circular tubes and nested structures in the cladding. Additionally, four gap-compensated circular tubes are introduced, and high-resistivity silicon with low absorption loss is used as the fiber material to further reduce transmission loss (TL) and improve fiber performance.

Firstly, we present the design of the structure for a high birefringence terahertz HC-ARF. Nested structures are incorporated in both the x and y directions, and non-circular tubes are introduced within the cladding tubes to induce asymmetry in the fiber structure, thereby achieving high birefringence. Secondly, four gap compensated circular tubes are added to the interstitial spaces within the cladding tubes, with the aim of reducing confinement loss in the fiber. Furthermore, high resistivity silicon is selected as the fiber material to minimize the effective material loss within the fiber. Subsequently, the control variable method is used to optimize the fiber structure parameters, including the outer diameter of the circular tube in the y-direction (d₁), the outer diameter of the nested circular tube in the y-direction (d₂), the major axis of the outer elliptical tube in the x-direction (d₃), the ellipticity η, the major axis of the nested elliptical tube in the x-direction (d₄), the outer diameter of the gap compensated circular tubes (d₅), and tube thickness t. The objective is to achieve optimal values for both the TL and the B of the fiber. Finally, based on the optimal structural parameters of the fiber, within the frequency range of 0.7 to 1.4 THz, the properties of the fiber are analyzed such as TL, B, and dispersion.

Firstly, a high birefringence terahertz HC-ARF is designed in this paper. On one hand, high birefringence is achieved by incorporating non-circular tubes and nested structures within the cladding tubes. On the other hand, the introduction of four gap compensated circular tubes in the interstitial spaces of the cladding tubes reduces confinement loss. Additionally, the use of high resistivity silicon materials further decreases the effective material loss (Fig. 1). Secondly, at an operating frequency of 1 THz, the structural parameters of the fiber are optimized. The results show that the best TL and B are achieved with d₁=3.0 mm, d₂=1.0 mm, d₃=3.1 mm, η=0.525, d₄=d₂=1.0 mm, d₅=1.2 mm, and t=0.035 mm. Next, we analyze the properties of the fiber, such as TL, B, and dispersion, within the frequency range of 0.7 to 1.4 THz. The results indicate that within the bandwidth ranging from 0.92 to 1.32 THz, B exceeds 1.09×10^-3 [Fig. 8(a)]. TL remains as low as 1 dB/m, with the minimum transmission loss of 0.09 dB/m occurring at a frequency of 1.18 THz. Finally, the dispersion performance of the fiber is simulated and analyzed. The results show that in the bandwidth of 0.92 to 1.32 THz, the designed high birefringence terahertz HC-ARF exhibits near-zero and flat waveguide dispersion, with a dispersion variation of (0.13513±0.22014) ps^-1·THz^-1·cm^-1 (Fig. 9). Meanwhile, the lowest polarization mode dispersion of 3.68×10^-12 s is achieved (Fig. 10).

In this paper, we design a high birefringence terahertz HC-ARF with both high B and low TL across a wide bandwidth. To accomplish this, we incorporate non-circular tubes into the cladding, introduce nested structures in both the x and y directions, and incorporate gap-compensated circular tubes within the cladding gaps. The results demonstrate that, within the bandwidth ranging from 0.92 to 1.32 THz, the high birefringence terahertz HC-ARF exhibits B greater than 10^-3 and TL as low as 1 dB/m. Notably, the lowest transmission loss is achieved at a frequency of 1.18 THz, where the TL for the x-polarization mode is 0.14 dB/m and the TL for the y-polarization mode is 0.09 dB/m. Additionally, it possesses excellent dispersion characteristics. This fiber has broad prospects in areas such as polarization-maintaining transmission of terahertz waves and polarization control of terahertz signals. It also provides a reference for the design of high birefringence terahertz fibers.