Secondary-ion-mass spectrometry study on near-stoichiometric LiNbO3 strip waveguide fabricated by vapour transport equilibration and Ti co-diffusion

摘要

Secondary-ion-mass spectrometry was applied to study Ti-concentration profiles in the depth direction and on the surface of near-stoichiometric (NS) Ti:LiNbO3 strip waveguides fabricated using vapour transport equilibration (VTE) and co-diffusion of Ti-metal strips. Results show that the profile of Ti concentration along the width direction on the waveguide surface can be well fitted by a sum of two error functions, while, in the depth direction, the Ti concentration follows either a complementary error function or a generalized Gaussian function. Surface Ti concentration, 1/e width and depth, mean diffusivities along the width and depth directions of the guide are, respectively, 1.04 x 10(21) cm(-3), 8.5 mu m, 6.3 mu m, 0.18 and 0.1 mu m(2)/h. Two-dimensional refractive index profile in the NS waveguides is indirectly constructed by assuming linearity between Ti-induced index change and Ti concentration. The surface refractive index increments at 1545 nm, Delta n(o) and Delta n(e), are evaluated to be 3.132 x 10(-3) and 1.186 x 10(-2), respectively. All of the above-mentioned diffusion and optical parameters are compared with the corresponding data of the common congruent Ti: LiNbO3 waveguide or NS waveguide fabricated starting from a NS substrate. The rationality of the assumed linear relationship between the index change and the Ti concentration is discussed. The results show that the assumed linearity remains controversial, and all expressions and data with regard to the refractive index are the approximate results and need to be verified by direct measurement on refractive index.