Polarization splitter-rotator on thin-film lithium niobate via shortcuts to adiabaticity

We propose a compact and broadband polarization splitter-rotator (PSR) based on thin-film lithium niobate (TFLN), consisting of two adiabatic tapers and an adiabatic coupler. The TM0 mode is converted into the TE1 mode via the adiabatic taper and subsequently demultiplexed by the adiabatic coupler, thereby achieving polarization splitting and rotation. The fast quasiadiabatic approach is utilized to optimize the geometry of both the taper and the coupler, providing shortcuts to adiabaticity (STA). The length of the taper and the coupler can be reduced to 93 μm and 94 μm, respectively. The proposed PSR exhibits an ultra-low excess loss (EL) of less than 0.014 dB for the TE0 mode, and an EL of less than 0.83 dB for the TM0 mode, within the wavelength range of 1400 to 1700 nm. The polarization extinction ratio (PER) for the TE0 mode is greater than 37 dB across the same wavelength range, whereas for the TM0 mode, the bandwidth in which the PER exceeds 20 dB spans 124 nm (from 1494 to 1618 nm). Our research provides a valuable reference for realizing compact adiabatic devices on TFLN, and the proposed PSR could find application in high-density polarization diversity systems.