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

We report the near-stoichiometric Ti:LiNbO₃ stripwaveguides fabricated by vapour transport equilibration (VTE) at1060~^{circ}C for 12 h and co-diffusion of 4--8~mu m wide,115-nm thick Ti-strips. Optical studies show that these waveguidesare monomode at 1.5~mu m and have losses of 1.3 and 1.1~dB/cm forthe TM and TE modes, respectively. In the waveguide width/depthdirection, the mode field follows a Gauss/Hermite--Gauss profile.A secondary ion mass spectrometry study reveals that the Ti profilefollows a sum of two error functions along the width direction and acomplementary error function in the depth direction. Micro-Ramananalysis shows that the Li-composition in the depth directionalso follows a complementary error function. The mean Li/Nb ratio inthe waveguide layer is about 0.98. The inhomogeneous Li-compositionprofile results in a varied substrate index in the guiding layer,and the refractive index profile in the guiding layer is given.