High-quality waveguides by reverse-proton-exchange in stoichiometric lithium tantalate

This paper reports the development and modelling of the reverse-proton-exchange process for the realization of high quality optical waveguides in MgO doped stoichiometric lithium tantalate, a promising nonlinear material due to its low coercive field and high damage threshold. Reverse-proton-exchange gives rise to a buried refractive index-profile providing significant advantages in terms of attenuation, insertion losses and overlap of the fields interacting through the nonlinear susceptibility. By characterizing several samples fabricated under different experimental conditions, we identified a fabrication procedure which is simpler than the conventional one used for lithium niobate: the annealing and reverse-exchange processes are indeed performed at the same temperature so that the diffusion of hydrogen ions towards the substrate during the reverse-exchange occurs in the same conditions as during the annealing. This results in very simple empirical laws relating the fabrication to the optical parameters. By such a modeling we defined fabrication parameters giving rise to a single-mode waveguide at lambda=1.55mum with good fiber mode-matching and high efficiency when used as a nonlinear device for telecom applications.