  In this contribution we investigate the impact of lateral leakage for linear
and nonlinear optical waveguides in lithium niobate on insulator (LNOI).
Silicon nitride (SiN) loaded and direct patterned lithium niobate
cross-sections are investigated. We show that lateral leakage can take place
for the TE mode in LNOI ridge waveguides (X-cut lithium niobate), due to the
birefringence of the material. This work gives guidelines for designing
waveguides in LNOI that do not suffer from the lateral leakage effect. By
applying these design considerations, we avoided the lateral leakage effect at
the second harmonic wavelength of a nonlinear optical waveguide in LNOI and
demonstrate a peak second harmonic generation conversion efficiency of ~1160%
W-1cm-2.
