A non-uniform grating coupler with high coupling efficiency on LNOI

A non-uniform segmented grating coupler with a metal reflective layer is designed by finite difference time domain simulation. The grating coupler is on a 400 nm thick z-cut lithium niobate thin film waveguide. A metal reflective layer is added to reduce the leakage to the substrate, and a non-uniform grating structure is constructed to improve the mode mismatch. The non-uniform grating coupler consists of three segmented grating regions, with a gap between two adjacent grating regions. The grating periods of these three grating regions are not completely the same, namely Λ1, Λ2, and Λ3, Λ1 is shorter than Λ2 and Λ3. According to grating coupling principle, the key structural parameters of the grating coupler such as the gap between the grating regions, the grating period, the duty cycle are discussed with respect to the grating coupling efficiency. Through FDTD simulation calculation, the coupling efficiency under theoretical simulation (TM mode, at 1550 nm) can reach nearly 76%. The proposed non-uniform segmented grating coupler is expected to be fabricated and applied to the input-output coupling of waveguide devices.