Shallow Ti:LiNbO ₃ strip waveguide

We report shallow single-mode Ti:LiNbO ₃ strip waveguides fabricated on Z-cut congruent LiNbO ₃ substrate by a two-step Ti-diffusion process, starting with the diffusion of a homogeneous Ti film at 1020°C for 2 h, followed by the diffusion of Ti strips at the same temperature for 2 h again. These waveguides with a loss as low as 0.3 dB/cm have smaller cross section sizes, larger surface refractive index increment, and a mode field depth that is half as shallow as the conventional Ti:LiNbO ₃ waveguide, satisfying the requirement by photonic crystal fabrication. The Ti ⁴⁺ -concentration profile was characterized by secondary ion mass spectrometry. The 2-D refractive index profile model is proposed for the peculiar waveguide structure. On the basis of the index model, we have simulated the mode field distribution using the beam propagation method. The results show that the theory is in good agreement with the experiment, verifying the validity of the proposed index model.