Structural design of a polarization beam splitter based on a thin-filmlithium niobate photonic crystal and a multimode interferencecoupler

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 thin-film lithium niobate (TFLN) and a photonic crystal-multimode interference (PhC-MMI) is proposed. This device achieves different polarization propagation characteristics by etching photonic crystals in the multimode region of the MMI. It is able to separate the two polarization modes of TE and TM, thus realizing the function of polarization beam splitting. The length of the structure proposed in this paper is 243 µm. The simulation results at the center wavelength of 1550 nm show that the extinction ratio of the TE(TM) polarization mode is 27.7 dB (26.3 dB), and the insertion loss is 0.78 dB (0.83 dB). This design has the advantages of high extinction ratio and low crosstalk. In addition, the device adopts a photonic crystal structure, making it free from the inherent TE/TM mode length dependency of traditional MMI devices. It is expected to become an indispensable component in future TFLN PICs.