Diffusion kinetics and optical waveguiding properties of outdiffused layers in lithium niobate and lithium tantalate

Increases in the extraordinary refractive index sufficient to produce single and multimode optical waveguides have been produced by the outdiffusion of lithium oxide from the surfaces of lithium niobate and lithium tantalate crystals. The outdiffusion kinetics have been studied in detail by optical interferometry. The data fit a diffusion model for which the vaporizing surface flux is constant with time. For lithium niobate, the activation energy for diffusion is 68 +/- 1.2 kcal/mol and does not vary with orientation. However the gradient of refractive index change at the surface is larger for diffusion normal to the c-axis than parallel to the c-axis. Activation energies for vaporization of 71 kcal/mol and 59 kcal/mol were calculated from the model for diffusion perpendicular and parallel to the c-axis, respectively. The evaporation coefficient, alpha, was estimated to be less than 10(-4) with alpha perpendicular/alpha || approximately 3 so vaporization is surface reaction limited. For lithium tantalate, the activation energy for diffusion is 51 +/- 6 kcal/mol and also does not vary with orientation. The activation energy for vaporization, Q(upsilon), is approximately 63 kcal/mol and, within experimental error, is quite similar to that for lithium niobate. These values of Q(upsilon) indicate that the major vaporization species are probably lithium ions and oxygen ions. The characteristics of graded index waveguides are discussed and compared with those of slab guides. It is shown that in many respects the behavior of the two types is equivalent. We have produced a single-mode guide in lithium niobate with an effective thickness of 12 microm. Effective thicknesses as small as 6 microm are possible in lithium tantalate.