As an ideal transmission medium of terahertz (THz) wave, THz photonic crystal fiber (THz-PCF) has attracted extensive attention. The structure of THz-PCF is usually composed of periodically arranged air holes whose size is in the same order of the THz wavelength. Therefore, the transmission of THz wave in PCF can be flexibly controlled by adjusting the size and shape of the air hole. However, when the THz wave is transmitted in ordinary PCFs, birefringence caused by stress and other factors is inevitable, which will result in polarization crosstalk, polarization dependent loss, and polarization mode dispersion in THz link, leading to the degraded performance of the entire THz system. High birefringence optical fiber can ensure that the polarization state of the incident THz wave remains unchanged when it is transmitted in the fiber. The birefringence of the THz fiber can be greatly improved by changing the symmetry of the cross-section structure of the fiber, which is of great value for achieving polarization maintaining transmission and polarization manipulation of THz wave in PCFs.

Topas is usually employed as the substrate material of porous core THz-PCF in present study. Firstly, an initial PCF structure is designed. Then, the characteristics of the proposed PCF are numerically analyzed based on the finite difference time domain (FDTD) method. Next, the control variable method is adopted to investigate the polarization characteristics of THz-PCF through the following three ways: optimizing the core structure, adjusting the cladding structure, and changing the parameters of the core and cladding at the same time to obtain the optimal structure. Meanwhile, the crucial performance parameters of PCF are analyzed to evaluate the performance of the designed THz-PCF, which include birefringence, confinement loss, effective absorption loss, bending loss, power fraction, waveguide dispersion, and polarization mode dispersion.

According to available literature, this paper leverages cyclic olefin copolymer as the substrate and employs the FDTD method to design and analyze a high birefringence THz-PCF based on a porous core structure. The anisotropy and high polarization characteristics of THz-PCF are introduced by changing the arrangement of the air holes in the core area. The specific description is as follows. By adjusting the structural parameters of the fiber core and cladding air holes, the paper designs the porous core high birefringence THz-PCF with Ferris-wheel-like and calculates the birefringence characteristics numerically. The results show that when the Ferris-wheel-like porous core fiber structure parameters are r₁=1.7 μm, r₂=2.3 μm, d₁=27 μm, d₂=24 μm, d₃=35.5 μm, l=8 μm, Λ=79 μm, and R=38.5 μm (Fig. 2) at f=4 THz, an ultra-high birefringence of 0.1085 (Fig. 3) and an ultra-low confinement loss of 10-16 dB/cm (Fig. 4) are obtained. The porous core PCF structure has achieved an ultra-high birefringence (Fig. 3), low confinement loss (Fig. 4), low bending loss (Fig. 6), and near-zero flattened dispersion (Figs. 8 and 9) in THz technology, which provides important reference value for the design of high birefringence and low loss THz-PCFs in the future.

In this paper, a THz-PCF with Ferris-wheel-like porous core is proposed. The Topas is adopted as the substrate material of the designed PCF. The FDTD method is employed to numerically analyze the birefringence, loss, and dispersion of the proposed optical fiber. After structural optimization, the THz-PCF can provide a birefringence of the magnitude of 10^-1 in the 3-6 THz operating frequency band. Ultra-high birefringence of 0.1085, low confinement loss of 10^-16 dB/cm, and low bending loss of 2.4×10^-14 dB/cm are achieved at 4 THz, which is very competitive even compared with the previous results: the reported PCF does not achieve 10^-1 order of magnitude high birefringence and low confinement loss at the same time. Additionally, the proposed THz-PCF exhibits a low and flattened dispersion in the range of 3-5.5 THz, and the dispersion value is within ±0.11 THz^-2·cm^-1. The study shows that such kind of PCF with high excellent birefringence and low confinement loss can be obtained through the optical fiber structure design, which combines multi-layer cladding and novel Ferris-wheel-like porous core structure. The excellent properties of the proposed THz-PCF will promote the development of THz optical devices and polarization sensing.