We show that two different phases can be present on top of the well an
nealed proton exchanged (APE) LiNbO3 optical waveguides depending on a
nnealing temperature. Moreover the crystal lattice parameters of APE L
iNbO3 waveguides which have been annealed at the temperatures 320-340
and 400 degrees C differ considerably. The relationships between the o
rdinary and extraordinary refractive indices and the crystal lattice d
eformations have been experimentally determined in both phases observe
d in APE LiNbO3 waveguides.