1 High-Precision Measurement of a Propagation Loss 2 of Single-Mode Optical Waveguides on Lithium 3 Niobate On Insulator 4

We demonstrate fabrication of single-mode optical waveguides on lithium niobate on 18 insulator (LNOI) by optical patterning combined with chemo-mechanical polishing. The fabricated 19 LNOI waveguides have a nearly symmetric mode profile of a mode field size of ~2.5 μm (full-width 20 at half maximum). We develop a high-precision measurement approach by which the single mode 21 waveguides are characterized to have a propagation loss of ~0.042 dB/cm. 22