Birefringence is a key parameter to judge whether polarization-maintaining fiber can maintain polarization state, which has research significance. The birefringence of traditional single-mode fibers is very sensitive to subtle changes in the external environment, and two orthogonal polarization modes in fibers are easy to couple. Generally, polarization-maintaining fibers have strong birefringence, and two orthogonal polarization modes with different propagation constants are not easy to couple. The birefringence caused by external environment changes is far less than that of the fiber itself. Therefore, polarization-maintaining fibers have good polarization-maintaining ability and resistance to external interference, with a wide application prospect in optical fiber sensing, optical components, optical fiber communication, and other fields. We design a high birefringence elliptical-core pseudo-rectangle polarization-maintaining fiber with both shape birefringence and stress birefringence. The shape birefringence depends on the ellipticity of the core, while the stress birefringence depends on the pseudo-rectangle structure in the stress region, which has a simple structural design. The structural parameters are optimized by numerical simulations, and the birefringence of the designed fiber is nearly doubled compared with that of the traditional panda-type polarization-maintaining fiber. The designed high birefringence fiber will be helpful to practical engineering and provide the possibility for the sensing of high birefringence polarization-maintaining fibers.

We study the structure design and birefringence characteristics of numerical simulations in elliptical-core pseudo-rectangle polarization-maintaining fibers. Firstly, the birefringence characteristics of the circular-core pseudo-rectangle polarization-maintaining fiber model are studied and compared with the existing research results to verify the correctness of the proposed polarization-maintaining fiber model. Then, based on the circular-core pseudo-rectangle polarization-maintaining fiber model, the changes in core ellipticity and birefringence characteristics are studied when the core ellipticity changes from less than 1 to more than 1. Then, the von Mises stress distribution and stress-induced birefringence distribution in the x direction of the cross section of the elliptical-core pseudo-rectangle polarization-maintaining fiber are analyzed by simulation software. The influence of the length and width of the stress zone on the birefringence of elliptical-core pseudo-rectangle polarization-maintaining fiber is studied and compared with that of circular-core pseudo-rectangle polarization-maintaining fiber. Next, we research the relationship between the effective refractive index and birefringence of the core fundamental modes in the x and y polarization directions at different wavelengths. Finally, the designed birefringent fibers in other references at home and abroad in recent years are compared.

When the ellipticity of the designed elliptical core rectangular polarization-maintaining fiber is less than 1, the mode birefringence decreases with the increasing short semi-axis a of the core [Fig. 4(a)]. When the ellipticity of the core is greater than 1, the mode birefringence increases with the rising long semi-axis a of the core [Fig. 4(b)]. The relationship between elliptical-core pseudo-rectangle polarization-maintaining fiber and circular-core pseudo-rectangle polarization-maintaining fiber with the distance between the core and the stress zone is compared and analyzed. As the distance between the core and the stress zone increases, the birefringence decreases significantly (Fig. 5). The functional relationship between the birefringence of elliptical-core pseudo-rectangle polarization-maintaining fiber and circular-core pseudo-rectangle polarization-maintaining fiber is studied respectively. With the increasing length of stress zone, the mode birefringence of the two types of polarization-maintaining fibers tends to rise, but the birefringence is quite different [Fig. 7(a)]. With the increase in the width of the stress zone, the basic trend of the mode birefringence of the two types of polarization-maintaining fibers is the same, with a faster growth rate and smaller birefringence difference [Fig. 7(b)]. Additionally, the birefringence of the designed fiber reaches 8.0794×10^-4 at the wavelength of 1550 nm [Fig. 8(b)], which is nearly doubled compared with that of the traditional panda-type polarization-maintaining fiber. Thus it has a good polarization-maintaining ability.

We design an elliptical-core pseudo-rectangle polarization-maintaining fiber based on shape birefringence and stress birefringence, which has an elliptical core and two symmetric rectangular stress regions. The influence of the size of the rectangular stress region and the core ellipticity on the birefringence of the polarization-maintaining fiber is studied by numerical simulations. When the ellipticity changes from less than 1 to more than 1, the birefringence increases with better polarization-maintaining ability. Under other conditions unchanged, the width of the stress zone required by elliptical-core pseudo-rectangle polarization-maintaining fiber should be smaller than that of circular-core pseudo-rectangle polarization-maintaining fiber to obtain the same birefringence, which indicates a smaller area of stress zone. By optimizing the parameters, the birefringence of elliptical-core pseudo-rectangle polarization-maintaining fiber at 1550 nm is 8.0794×10^-4, which is nearly double that of the traditional panda-type polarization-maintaining fiber. Additionally, the relationship between birefringence and wavelength of the proposed fiber in the C communication band (1530-1565 nm) is also studied. The proposed elliptical-core pseudo-rectangle polarization-maintaining fiber has a simple structure and potential applications in optical fiber communication and sensing.