Characterization of index profiles of Zn-diffused LiNbO3 waveguides

We characterize the refractive-index profiles of Z-cut and X-cut Zn-diffuse d LiNbO3 substrates from the vapor phase. We obtained these profiles by ref lectivity measurements of the samples, using the prism coupling technique a t each step of a systematic polishing, and then fitting the data with a pro gram that models a multilayer optical structure. For diffusions performed a t 800 degrees C, four different layers were dearly distinguished in the LiN bO3:Zn diffused samples, the first one being a very thin layer of polycryst alline ZnO. Two step-index layers were also detected: One was isotropic, an d the other showed birefringence depending on the crystal cut. These two la yers were correlated with Zn-rich phases previously detected in Zn-diffused LiNbO3 by x-ray techniques. The fourth layer detected presents a graded-in dex profile and corresponds to the formation of a Zn-containing solid solut ion with a LiNbO3-likestructure. These results are discussed and compared w ith the phases reported for Ti-diffused LiNbO3. (C) 1999 Optical Society of America [S0740-3232(99)01310-1].