The design of low-loss high-birefringence terahertz hollow-core anti-resonant fiber

Background Owing to the high absorption of terahertz waves by water vapor and certain materials, significant transmission loss occurs, seriously affecting the transmission quality of terahertz signals. High-birefringence characteristics can protect optical pulses from polarization coupling and polarization mode dispersion during transmission in optical fibers. The application of high- birefringence optical fiber helps to enhance the stability of optical fiber communication and sensing systems.

Purpose In order to ensure the stability of the polarization state of terahertz (THz) fibers in polarization-sensitive applications and maintain low loss, a terahertz hollow-core anti-resonant fiber configuration with low loss and high birefringence was proposed.

Methods Firstly, a nested structure for a hollow-core anti-resonant fiber configuration was designed by using COMSOL Multiphysics, where the nested structure is capable of significantly reducing fiber loss. The cladding employs a nodeless structure to prevent additional loss caused by nodal resonance. The finite element method was used to analyze the characteristics of the designed fiber in combination with the boundary conditions of the perfectly matched layer.

Results The results demonstrate that the fiber exhibits two main transmission windows in the frequency range of 1-2.8 THz: 1-1.4 THz and 1.85-2.8 THz. For the x-polarization mode, the lowest confinement losses of 0.074 dB/m and 0.032 dB/m are achieved at 1.2 THz and 2.1 THz, respectively. The corresponding y-polarization confinement losses at these frequencies are 0.078 dB/m and 0.079 dB/m, respectively. Furthermore, the fiber exhibits birefringence values of 1×10⁻⁴ and 1.53×10⁻⁵ at 1.2 THz and 2.1 THz, respectively. In addition, the minimum total losses for the x-polarization mode are 0.089 dB/m and 0.038 dB/m in two main transmission windows, respectively. Low-loss transmission is successfully realized while achieving a prominent birefringence effect.

Conclusions This paper presents a hollow-core anti-resonant fiber configuration for terahertz applications. By specially nesting the cladding tubes in the vertical direction, the structural asymmetry of the fiber is introduced, and the fiber birefringence is effectively improved. The fiber also exhibits favorable bending resistance, when the bending radius is 25 cm, the lowest bending losses of x-polarization mode in the two windows are 0.09 dB/m and 0.06 dB/m, and the highest birefringence values reach 1.86×10⁻⁴ and 5.48×10⁻⁵, respectively, which meets the requirements for high-birefringence terahertz transmission.