Near-stoichiometric Ti:LiNbO₃ strip waveguide with varied substrate refractive index in waveguide layer

We report the near-stoichiometric Ti:LiNbO₃ strip waveguides fabricated by vapour transport equilibration (VTE) at 1060 ^\circC for 12 h and co-diffusion of 4--8 \mum wide, 115-nm thick Ti-strips. Optical studies show that these waveguides are monomode at 1.5 \mum and have losses of 1.3 and 1.1 dB/cm for the TM and TE modes, respectively. In the waveguide width/depth direction, the mode field follows a Gauss/Hermite--Gauss profile. A secondary ion mass spectrometry study reveals that the Ti profile follows a sum of two error functions along the width direction and a complementary error function in the depth direction. Micro-Raman analysis shows that the Li-composition in the depth direction also follows a complementary error function. The mean Li/Nb ratio in the waveguide layer is about 0.98. The inhomogeneous Li-composition profile results in a varied substrate index in the guiding layer, and the refractive index profile in the guiding layer is given.