Low-loss fiber grating coupler on thin film lithium niobate platform

A grating coupler with a high coupling efficiency and low back reflections is designed and demonstrated on the thin film lithium niobate platform, which facilitates an efficient interface between a lithium niobate ridge waveguide and a standard single mode fiber. The excellent performances of the present grating coupler are enabled by a cavity-assisted grating structure and a top metal mirror, even though a uniform grating is adopted as the diffractive element. Experimentally, a coupling loss of −0.89 dB at 1552 nm is demonstrated with a 1-dB bandwidth of 45 nm. Low back reflections of <−16.5 dB to the waveguide and <−13.7 dB to the fiber are also achieved from 1536 to 1579 nm.