Ultrahigh birefringent photonic crystal fiber with ultralow confinement loss using four airholes in the core

A simple photonic crystal fiber (PCF) design with a rectangular array of four airholes in the core region and a traditional circular-airhole cladding is proposed in this work. The modal birefringence is induced by the asymmetry of the rectangular distribution of four airholes and/or the elliptical shape of the holes. The traditional symmetric cladding structure results in good confinement loss performance by limiting the light in the core region. Therefore, the proposed design enables simultaneous realization of high birefringence and low confinement loss. Simulations based on the full-vector finite element method (FEM) with anisotropic (PML) show that ultrahigh single-mode birefringence ∼ 10^-2) and ultralow confinement losses (<0.002 dB/km) can be achieved at 1.55 μm wavelength. Dependence study of the birefringence and losses on several key parameters is also provided. Compared to previously studied PCF with asymmetric core or cladding structures, by having just four relatively large airholes in the core, this design could be much easier to be implemented with even better performance.