Correlation of refractive index profiles with substitutional hydrogen concentrations in annealed proton-exchanged LiNbO3 waveguides

We have studied the behavior of the extraordinary refractive index in a set of annealed z-cut proton-exchanged lithium niobate (LiNbO3) substrates by means of optical prism-coupling measurements and numerical simulations. Values of the index as a function of depth have been determined and fitted with a single Gaussian diffusion expression. At 400 °C, the effective index diffusivity Dn of 4.6×10−12 cm−2/s is nearly identical to the diffusivity of substitutional H in these samples. Furthermore, we find a linear relationship between the increase in the extraordinary refractive index and the substitutional H concentration. At a wavelength of 0.6238 μm, the coefficient for this relationship is ∼1.5×10−23 (H cm−3)−1.