Design of a polarization beam splitter based on a thin-film lithium niobate and grating-assisted contra-directional coupler.

A polarization beam splitter (PBS) is a key device for controlling the polarization state of light in photonic integrated circuits (PICs). In this paper, a compact TM-pass/TE-divide PBS based on a thin-film lithium niobate (TFLN) and grating-assisted contra-directional coupler (GACDC) is proposed. The device makes the contra-directional coupling satisfy the phase-matching condition by etching the corrugated strip waveguide on the waveguide sidewall. It is able to separate the two polarization modes of TE and TM, thus realizing the function of polarization beam splitting. The coupling length of the PBS designed in this paper is 209.5 µm. The simulation results at the center wavelength of 1555 nm show that the extinction ratio of the TE(TM) polarization mode is 42.8 dB (21.0 dB), and the insertion loss is 0.56 dB (0.01 dB). Besides, the crosstalk of the contra-directional coupling is ${-}21;{\rm dB}$. The waveguide grating is employed in the device, which makes it independent of the free spectral range, and it has a large tuning range. The design has the advantages of a high extinction ratio and low crosstalk, which is expected to be an indispensable component in future TFLN PICs.