As one of the important properties of the light field, polarization plays an important role in the interaction between light and matter. The modulation of polarization plays an indispensable role in optical communication systems, fiber sensors, fiber lasers, and other fields. However, in view of the twist, defects, environment perturbations, and other factors in the process of optical fiber manufacturing, the manufactured optical fiber is not completely uniform, which introduces random birefringence and leads to unpredictable polarization states. Therefore, it is of great practical value to study optical fibers with excellent polarization states. Although the existing single-polarization single-mode negative-curvature hollow-core fiber has the advantages of simple structure, easy preparation, endless single-mode transmission, and low loss, due to the limitation of research habits and optical materials, the current research mainly focuses on common communication bands. But obviously, the mid-infrared band will become the next hot band of the negative-curvature hollow-core fiber. Research shows that a wavelength of 3-5 μm plays an important role in national defense, medical care, communications, and other fields, especially near the wavelength of 4 μm, which is an ideal band for quantum cascade detectors to detect low-level light. Single-mode single-polarization light helps to provide a more pure light source for quantum cascade detectors. Therefore, it is of great practical significance to study the single-mode single-polarization negative-curvature hollow-core fiber with a wavelength of 4 μm.

A hollow-core anti-resonant fiber composed of six nested tubes working near 4 μm is designed, which can transmit single-mode single-polarization with low loss. The influence of structural parameters on fiber performance is calculated by using the control variable method. The capillary wall thickness will lead to an obvious change in the fiber loss with the working band, which is the key factor affecting the characteristics of the negative-curvature hollow-core anti-resonant fiber. Therefore, the capillary wall thickness is analyzed and optimized. Through the scanning study of the capillary wall thickness, the local optimal parameter values of the minimum fundamental mode loss and the maximum high-order mode extinction ratio in the 4 μm band are determined, and the design goal of the single-mode performance of the fiber is successfully realized. The second step is to optimize the capillary radius. This parameter mainly affects the polarization state of the fiber, and different parameter combinations of the six inner tube radii correspond to different implementation effects. The optimization of capillary radius successfully achieves single-polarization operation in a single-mode state. In the third step, the core diameter of the fiber is optimized. Although the study does not reflect the further optimization effect of the parameters that have been optimized and determined in the previous steps, the parameter design still retains the effective mode area and the maximum transmission power tolerance value of the fiber. The fourth step is to study and characterize the bending resistance of optical fiber. Research shows that this design fully meets the preset requirements for bending resistance and verifies that the natural advantages of negative-curvature hollow-core anti-resonant fibers, such as large effective mode field area and less substrate material coverage, can contribute to the bending resistance of the fiber.

A negative-curvature hollow-core fiber with low-loss single-mode single-polarization transmission is proposed and analyzed by the finite element method. By calculating the influence of fiber parameters on the fiber structure, the high-order mode extinction ratio reaches 163 (Fig. 3), and the fiber successfully realizes single-mode transmission. However, in order to further ensure the single polarization performance of the fiber, the size of the capillary radius is optimized, and the single polarization function is realized based on single-mode transmission (Fig. 4). In order to ensure that the fiber has good bending resistance, the critical bending radius of the fiber is defined, and it is found that the bending loss of the x-polarization fundamental mode of the fiber is always less than 10^-3 dB/m (Fig. 7). In addition, the fiber structure also has a large effective mode field area (Fig. 8), which meets the transmission requirements of high power lasers. The results show that the designed structure achieves both single-polarization performance and single-mode transmission.

In this paper, a single-mode, single-polarization, low-loss, negative-curvature, hollow-core, and anti-resonant fiber is proposed. The substrate material of the fiber is As₄₀S₆₀, which is specially studied and experimentally prepared by Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences. Its refractive index is 2.395 at 4 μm. It has low intrinsic loss and great chemical stability in the mid-infrared band, which is beneficial to realize the low loss performance of the fiber. The fiber structure adopts a six-nested, capillary-type, negative-curvature, hollow-core, and anti-resonant structure with relatively mature preparation technologies and a simple structure. After optimizing the parameters of the fiber, the single-mode single-polarization effect can be achieved from 3.99 μm to 4.00 μm. Especially at the wavelength of 4 μm, the polarization extinction ratio (PER) and high order mode extinction ratio (HOMER) reach 491 and 694, respectively, which meet the conditions of single-polarization single-mode transmission, and the loss is as low as 1.8×10^-4 dB/m. The fiber also has excellent bending resistance. At the wavelength of 4 μm, single-mode single-polarization transmission of the fiber can be achieved by selecting the appropriate bending radius at any bending angle. When the bending angle is equal to 0°, and the bending radius is from 1 cm to 10 cm, the confinement loss of the fiber is less than 5.3×10^-3 dB/m. The negative-curvature, hollow-core, and anti-resonant fiber proposed in this paper has the advantages of simple structure, single-mode single-polarization operation, low loss, and excellent bending resistance. It can not only be applied to the communication industry and medical system but also is expected to provide a more pure light source for quantum cascade detectors operating in the band of 4 μm.