Highly Integrated Polarization Beam Splitter with MMI Structure Based on LNOI-SiN Rib Waveguide

We present a highly integrated, high-performance polarization beam splitter (PBS) based on heterogeneous integration of X-cut thin-film lithium niobate platforms, the device forms a ridge waveguide by depositing Silicon nitride (SiN) on lithium niobate. The device size is reduced by relying on the high refractive index contrast birefringence of lithium niobate. The PBS based on multimode interference coupling (MMI) structure not only achieves good polarization separation characteristics, but also ensures large fabrication tolerances. The structural parameters of the device are optimized using the eigenmode expansion (EME) method and the three-dimensional time-domain finite-difference (FDTD) method. The experimental results show that in the wavelength range of 1550nm, the device achieves a polarization extinction ratio (ER) of more than 16dB, an interpolation loss of 3.5dB, and an operating bandwidth of 3dB up to 300nm.