Abstract

The Quasi-Phase-Matching technique (QPM) [1], in which the ferroelectric domains of a nonlinear material are periodically inverted in order to compensate for dispersion, has become widely used for a number of nonlinear processes [2,3,4]. While bulk experiments in such structures have yielded results close to theoretical predictions, waveguided versions, which offer additional possibilities (higher theoretical efficiencies, electrooptic tuning, etc.) have not attained the predicted efficiencies. In this paper we compare the parametric fluorescence efficiency in both bulk and Annealed Proton Exchange (APE) waveguide configurations in an electric-field poled LiNbO₃ sample, in which the inverted domains exist throughout the entire sample thickness.The results indicate, that even after annealing, the proton exchange process appears to erase the periodic domain inversion, to a depth comparable to that of the exchanged structure, leading to a lack of QPM in that region. Such an effect had already been proposed by Webjorn [5] concerning domains inverted by Ti diffusion at the surface of LiNbO₃. He also noted however, that if an inverted "seed" remained below the surface, annealing could permit regrowth of the inverted structure above the seed. This model would have led one to expect that in materials poled throughout their entire thickness, as in E-field poled LiNbO₃, the seed would always be present allowing appropriate regrowth of the domains during annealing. However, we shall demonstrate that this does not appear to be the case.

© 1996 Optical Society of America