Disagreements concerning a linear correlation between the H concentration and the extraordinary refractive index of proton-exchanged material were resolved here by partitioning the total H content into optically active substitutional H and optically inactive interstitial H. The H and Li spatial variations in both Z-cut and X-cut crystals were determined by means of secondary-ion mass spectrometry. The samples were proton-exchanged in benzoic acid at 185C, and were then annealed (400C, 0.1 to 4h) in wet flowing O. In the case of Z-cut crystals, fitting the secondary ion mass spectrometry profiles by using expressions which were obtained from the equations for diffusion from a finite layer, yielded a substitutional H diffusivity of 5.0 x 10-12cm2/s and an interstitial H diffusivity of 1.4 x 10-11cm2/s. The wet flowing O acted as a constant source of interstitial H at the surface and furnished an integrated H concentration, due to the flowing wet O, which increased as √t. The Li diffusivity was equal to 4.8 x 10-12cm2/s; a value which was almost equal to the substitutional H diffusivity in Z-cut crystals. In the case of X-cut crystals, the substitutional H diffusivity was equal to 3.4 x 10-12cm2/s, and the interstitial diffusivity was equal to 1.3 x 10-11cm2/s. Refractive index profiles were evaluated by means of optical prism-coupling measurements and numerical simulations. For both crystal orientations, the effective index diffusivity was almost equal to the diffusivity of substitutional H. It was found that there was an excellent linear relationship between the refractive index profile and the substitutional H distribution.

Correlation of Substitutional Hydrogen to Refractive Index Profiles in Annealed Proton-Exchanged Z- and X-Cut LiNbO3. J.M.Zavada, H.C.Casey, R.J.States, S.W.Novak, A.Loni: Journal of Applied Physics, 1995, 77[6], 2697-708