Polarization rotator with large bandwidth and low insert loss based on lemon core photonic crystal fiber

A photonic crystal fiber (PCF) polarization rotator, featuring a large bandwidth and low insertion loss (IL) and based on the mode hybridization principle, is proposed. By reducing the size of two air holes in the 45 deg inclined direction of the fiber core, a tilted lemon-shaped fiber core is constructed. This design breaks the vertical and horizontal symmetry in the cross section of the PCF, achieving efficient mode conversion between transverse magnetic and transverse electric modes. The characteristics are numerically analyzed using a full-vectorial finite-difference method with a perfectly matched layer boundary condition. The results show that, at a working wavelength of 1.55 μm, the device achieves nearly 100% polarization conversion efficiency, an extinction ratio of 43.3 dB, and an IL of 0.025 dB, with a mere device length of 291 μm. Additionally, the operational bandwidth exceeds 825 nm (1.096 to 1.921 μm), covering the entire range of current optical fiber communications, including the O, E, S, C, and L bands. It demonstrates a good tolerance and significant potential for engineering applications in informational photonics.