Design for a high birefringence photonic crystal fiber with multimode and low loss

In this work, a novel design of a high birefringence photonic crystal fiber (HB-PCF) with multimode and low confinement loss is proposed. To achieve high birefringence, the core is designed as an elliptical region, which is enclosed by twelve small holes. Based on this design, replacing the two circular holes at the top and bottom of the core region with two elliptical holes can further improve the birefringence. At the wavelength of 1.55 μm, the birefringence of the fundamental mode (LP₀₁) and the second-order mode (LP₁₁) are 1.70×10^-2 and 1.85×10^-2, respectively. Meanwhile, the confinement losses maintain on orders of 1×10^-5  dB/km (LP₀₁) and 1×10^-1  dB/km (LP₁₁). After the effective refractive indices of two types of the proposed HB-PCF are calculated by the finite element method, the birefringence, confinement loss, bending loss, dispersion, and nonlinear coefficient are studied. These results reveal that the HB-PCF might be applied for polarization-maintaining and nonlinear optics.