Compact Adiabatic Polarization Splitter-Rotator on Thin-Film Lithium Niobate

Polarization controlling devices on thin-film lithium niobate (TFLN) are critical components for photonic integrated circuits (PICs) on the platform because the birefringence of lithium niobate intrinsically determines its polarization-dependent property. Here, we propose and demonstrate a compact polarization splitter-rotator (PSR) on TFLN based on two adiabatic tapers and an asymmetric directional coupler with an effective length of only 240 $\mu$m. The device is fabricated via a one-step electron beam lithography (EBL) and inductively coupled plasma (ICP) etching process. Our device shows efficient polarization extinction at the telecom C-band, providing a polarization extinction ratio (PER) of 26.6 dB and 12.2 dB for TE and TM mode, respectively. The highest PER both exceed 30 dB at 1544 nm and the insertion loss both less than 0.91 dB. The device is expected to apply to efficient polarization multiplexing and polarization modulating, an indispensable component of the PICs on TFLN in the future.