  We develop a technique for realizing lithium niobate on insulator (LNOI)
waveguides of a multi-centimeter-length with a propagation loss as low as 0.027
dB/cm. Our technique relies on patterning a chromium (Cr) thin film coated on
the top surface of LNOI into a hard mask with a femtosecond laser followed by
the chemo-mechanical polishing for structuring the LNOI into the waveguides.
The surface roughness on the waveguides is determined to be 0.452 nm with an
atomic force microscope (AFM). The approach is compatible with other surface
patterning technologies such as optical and electron beam lithographies or
laser direct writing, enabling high-throughput manufacturing of large-scale
LNOI-based photonic integrated circuits.
