Fiber-to-chip grating couplers for lithium niobate on sapphire

Lithium niobate on sapphire (LNOS) is an emerging platform for photonic integrated circuits, offering unique properties such as a wide transparency window, high nonlinearity, and strong electro-optic, nonlinear, and acousto-optic effects. Efficient light coupling between optical fibers and on-chip waveguides is crucial for practical applications. We present the design, simulation, and experimental demonstration of high-efficiency fiber-to-chip grating couplers for LNOS. The grating coupler design employs a self-imaging approach with a fixed period and linearly diminished filling factor, enabling a negative diffracted angle to match the fiber array and suppress higher-order diffraction. Numerical simulations predict a coupling efficiency of 42% at a 1550 nm wavelength. The grating couplers are fabricated on an X-cut, 400 nm thick LN film with a 220 nm etching depth using electron beam lithography and inductively coupled plasma etching. Experimental characterization using a fiber array and a six-axis displacement stage reveals a single-end coupling efficiency exceeding 20%, confirming the effectiveness of the design. The demonstrated grating couplers pave the way for efficient light coupling in LN-on-sapphire photonic circuits, enabling diverse applications in classical and quantum information processing, sensing, and nonlinear optics.