Polarization-independent symmetrical directional coupler utilizing orientation-engineered method on the x-cut lithium-niobate-on-insulator

Abstract Due to the strong birefringence, symmetrical directional couplers (DCs) are typically polarization-sensitive in the high index contrast platforms. In this letter, we demonstrate polarization-independent directional couplers utilizing orientation-engineered method on the x-cut lithium-niobate-on-insulator (LNOI) platform. We show that the polarization-dependent coupling of a DC can be greatly engineered by controlling the orientation of the waveguides in LNOI utilizing the anisotropic property of lithium niobate (LN) material. Based on the design method, a polarization-independent DC of the representative 3dB coupling ratio is realized using the ultra-sample symmetrical structure. The DC exhibits high performance in excess losses (EL