Optimization design of a polarization-independent grating coupler on lithium-niobate-on-insulator

We have designed a novel, to the best of our knowledge, polarization-independent grating coupler with non-uniform periods, assisted by plasmonic modes on a lithium-niobate-on-insulator platform. Through simulations, we demonstrate that depositing metal into the LN grating trenches excites plasmonic modes, which effectively tune the effective refractive index difference between the TE and TM modes. Unlike previous studies, the etching depth of the LN is optimized to maximize the coupling efficiency for both polarization modes simultaneously within the C-band. The coupling efficiency, polarization-dependent loss (PDL), and back reflection were further optimized using inverse design. The simulated coupling loss of the proposed grating is −3.18dB for the TE mode and −3.22dB for the TM mode. The PDLs are less than 0.09 dB, and the back reflection to the waveguide is below −11.9dB for both polarization modes over an 80 nm wavelength range. Additionally, we designed a taper that connects the gratings and single-mode waveguides to reduce transmission loss for TM modes by mitigating the influence of mode hybridization, resulting in a PDL of less than 0.03 dB. This novel grating coupler has potential applications in polarization multiplexing and polarization-independent systems on LNOI chips.