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