In practice, the environment and atmospheric conditions of free space influence the transmission of terahertz waves, leading to issues such as dispersion and large transmission loss (TL). Hollow core fibers emerge as promising for terahertz wave transmission because of their broad application potential. However, current hollow core photonic bandgap fibers (HC-PBGFs) for terahertz wave transmission have issues including poor mode purity, elevated surface scattering loss, and complex manufacturing processes. In contrast, hollow core anti-resonant fibers (HC-ARFs) have a simpler structure and are easier to fabricate, making them a research focus. Nevertheless, reported HC-ARFs still have room for improvement in TL and some fiber properties. To overcome these limitations, we have designed a novel terahertz HC- ARF structure. By carefully designing the geometric configuration of the nested structure in the cladding, increasing the number of nested layers, and using high-resistivity silicon with minimal absorption loss as the fiber material, we further reduce TL and dispersion and thus enhance the overall fiber performance. This fiber provides a valuable reference for the development of high-performance, low-loss terahertz wave transmission waveguides.

The anti-resonant reflection waveguide model combined with the suppression coupling theory comprehensively explains the guiding mechanism in HC-ARFs. These fibers mainly use the anti-resonant effect to confine energy within the fiber cores. First, we design the nested structure that constitutes the cladding of HC-ARF. Unlike typical fiber structures using circular nested tubes (fiber structure A), elliptical nested tubes (fiber structure B), and double-layer elliptical nested tubes (fiber structure C), the innovative nested configuration integrating ellipses, circles, and straight rods has superior performance in constructing the fiber cladding. In addition, the nine nested structures in the cladding are arranged without nodes, which avoids the resonance of nodes between nested structures affecting the fiber loss. High-resistivity silicon is selected as the fiber material, which helps reduce the effective material loss. Second, the control variable method is used to optimize the fiber structure parameters, including diameter D_c, elliptical major axis d₁, ellipticity η, and tube thickness t. Within the frequency range of 0.5?1.6 THz, the TL of the fiber is optimized. Finally, based on the optimal structural parameters of the fiber, the properties of the fiber such as mode field distribution, dispersion, bending resistance, and effective mode field area are analyzed.

First, we propose a combination of increasing the number of nested layers and changing the geometric shape of the nested structure. A terahertz HC-ARF with nine uniformly distributed multilayer nested structures is designed (Fig. 1). On the one hand, the anti-resonant reflecting effect is used to guide light, reducing the confinement loss. On the other hand, the use of high-resistivity silicon in the cladding structure helps reduce the effective material loss. Next, the structural parameters of the fiber are optimized. The results show that the best TL is achieved with a diameter D_c=6 mm, an elliptical major axis d₁=2.4 mm, an ellipticity η=1.104, and a tube thickness t=0.030 mm. That is, the proposed HC-ARF achieves a TL of less than 10^-1 dB/m within the transmission window of 0.84?1.56 THz. Moreover, in the frequency range of 0.98?1.44 THz, TL is as low as 10^-4 dB/m, and the lowest TL of 2.80×10^-4 dB/m is achieved at f=1.10 THz (Fig. 7). Finally, other performance parameters of the fiber are simulated. The results show that within 0.84?1.56 THz, near-zero and flat dispersion is achieved, with a dispersion variation of (0.02139±0.08824) ps·THz^-1·cm^-1 (Fig. 9). A large effective mode field area is obtained, with values reaching the order of 10⁷ μm² (Fig. 11). Excellent bending resistance is demonstrated, with bending losses less than 10^-2 dB/m at smaller R_b (35 and 45 cm) (Fig. 10), which is beneficial for more stable and effective transmission of terahertz waves. This makes the fiber have broad application value in the fields of terahertz wave sensing, detection, and terahertz communication systems.

We design a terahertz HC-ARF with low TL and wide bandwidth, using high-resistivity silicon (HRS) as the fiber material. Its nested structure, consisting of a combination of ellipses, circles, and straight rods, is advantageous for forming the cladding of the fiber. The results indicate that the HC-ARF achieves a low TL bandwidth of 0.72 THz within the transmission window. The lowest TL of 2.80×10^-4 dB/m is obtained at 1.10 THz. The dispersion variation is (0.02139±0.08824) ps·THz^-1·cm^-1. The effective mode field area remains above 10⁷ μm². The bending loss is less than 10^-2 dB/m at the smaller R_b (35 and 45 cm). We achieve more stable and high-performance terahertz wave transmission. The fiber has potential application value in the fields of terahertz wave sensing, detection, and terahertz communication systems.