Design of a compact and broadband terahertz polarization splitter based on gradient dual-core photonic crystal fiber

We report a broadband polarization splitter based on polyethylene photonic crystal fiber with microstructured dual refractive index gradient cores. These dual cores consist of a properly optimized arrangement of air holes such that for individual fibers $x$x-polarized modes have large effective indices difference, while this index difference is almost zero for their $y$y-polarized modes, leading to efficient coupling between the $y$y-polarized modes. We have shown that by proper optimization of gradience created in the arrangement of air holes, efficient polarization splitting can be achieved for a broad range of terahertz frequencies. Device length and extinction ratio have been calculated numerically for the proposed configuration. Device length of $\sim{1.96}$∼1.96 to $\sim {60}\;{\rm cm}$∼60cm was found to be appropriate for frequencies in the 0.4-1.0 THz range to have high extinction ratios: $ - {38}$-38 to $ - {49}\;{\rm dB}$-49dB and $ - {15}$-15 to $ - {23}\;{\rm dB}$-23dB for the $x$x and $y$y polarizations, respectively. The bending loss for the proposed design is quite low: $\sim{0.05}\;{\rm dB/m}$∼0.05dB/m at 1 THz for the bend radius of 1 cm. These results suggest that a compact, low-loss, and broadband polarization splitter with very high extinction ratios can be achieved by wrapping the fiber around a small mandrel.