Photonic crystal-enhanced grating coupler on thin film lithium niobate

Thin-film lithium niobate (TFLN)-based photonic integrated circuits (PICs) require grating couplers that are not only efficient but also compact and compatible with standard fabrication processes to ensure optimal device performance. In this work, we present a novel, to our knowledge, design approach to improve the coupling efficiency of lithium niobate grating couplers. A photonic crystal structure is introduced on an ultra-thin silicon substrate to suppress optical leakage, thereby reducing its detrimental effect on coupling performance. Furthermore, a tapered structure is implemented on the TFLN layer to enhance mode matching between the fiber and the waveguide. Three-dimensional finite-difference time-domain simulations demonstrate that, under TE polarization, the proposed design achieves a maximum coupling efficiency of 69% at a central wavelength of 1549 nm, with a 1 dB bandwidth of 30 nm and a 3 dB bandwidth of 97 nm.