  We investigate the dependence of photonic waveguide propagation loss on the
thickness of the buried oxide layer in Y-cut lithium niobate on insulator
substrate to identify trade-offs between optical losses and electromechanical
coupling of surface acoustic wave (SAW) devices for acousto-optic applications.
Simulations show that a thicker oxide layer reduces the waveguide loss but
lowers the electromechanical coupling coefficient of the SAW device. Optical
racetrack resonators with different lengths were fabricated by argon plasma
etching to experimentally extract waveguide losses. By increasing the thickness
of the oxide layer from 1 {\mu}m to 2 {\mu}m, propagation loss of 2 {\mu}m (1
{\mu}m) wide waveguide was reduced from 1.85 dB/cm (3 dB/cm) to as low as 0.37
dB/cm (0.77 dB/cm), and, resonators with quality factor greater than 1 million
were demonstrated. An oxide thickness of approximately 1.5 {\mu}m is sufficient
to significantly reduce propagation loss due to leakage into the substrate and
simultaneously attain good electromechanical coupling in acoustic devices. This
work not only provides insights on the design and realization of low-loss
photonic waveguides in lithium niobate, but most importantly offers
experimental evidence on how the oxide thickness directly impacts losses and
guides its selection for the synthesis of high-performance acousto-optic
devices in Y-cut lithium niobate on insulator.
