Low loss directly etched uniform grating couplers on thin film lithium niobate platform

We report a low-loss uniform grating coupler (GC) directly etched on a thin film lithium niobate (TFLN) platform. The GC is characterized to have a low coupling loss of 3.2 dB around 1550 nm and a 3-dB bandwidth of 76 nm. Fabrication of such GC employs just a single-step etching process without any complex grating structure, mirror reflector, or incorporation of hybrid materials. Our work demonstrates that directly etched uniform GCs can realize high coupling efficiency on the TFLN platform by optimizing the fiber coupling angle, grating parameters, and upper cladding thickness, holding the potential to benefit a wide range of applications across diverse fields such as high-speed transmitters, frequency combs, and hybrid integration.