Design of an X-cut thin-film lithium niobate waveguide as a passive polarization rotator

Abstract

The transverse-electric and transverse-magnetic modes of an X-cut thin-film lithium niobate waveguide vary in effective indices and exchange power when the waveguide makes an oblique angle with its crystallographic Z-axis, i.e. its optics axis. We leverage this phenomenon to design a passive fundamental-mode polarization rotator. In our design, the lithium niobate waveguide is tilted at an optimum angle with respect to its Z-axis, such that material anisotropy induces phase-matched polarization conversion. We discuss the rotator’s ideal-device length, crosstalk, and bandwidth. The proposed design yields compact (shorter than 1 mm), low-loss, passive polarization rotators for telecom wavelengths.