  The actual techniques allowing the fabrication of waveguides in lithium
niobate are not able to satisfy one of the demands of modern integrated
photonics namely well-controlled tunability of index contrast over a large
range of values from high-index contrast to low index contrast. This paper
presents a simple and reliable method allowing well-controlled index contrast
tunability while benefiting from optical nonlinearity of LiNbO$_3$ waveguides
fabricated using HiVac-VPE technique. The study involves LiNbO$_3$ planar
waveguides subjected to an annealing process for 1 to 5.5 hours. The waveguides
are characterized before and after the thermal treatment, in order to determine
the index contrast and to analyze the changes of index profile shapes.
Furthermore, it is found that the index profile after annealing is fitted by an
exponential function different from Gaussian fit found in APE technique.
Additionally, index contrast dependence of the annealing time is also fitted by
an exponential function. Our findings allow for well-controlled index contrast
tunability, between high-index contrast ($\Delta n_e=0.1$) and low-index
contrast ($\Delta n_e=0.035$), of LiNbO$_3$ waveguides offering the opportunity
for customization and enhancement of non-linear optical devices and photonics
applications.
