Realistic Design of Ti-Diffused LiNbO3 Directional Couplers

Design criteria, fabrication techniques and the general features of titanium-diffused lithium niobate waveguides are illustrated. The diffusion constant and the index change at the surface are evaluated via a computer aided inverse WKB method, fitting the effective indices measured by a prism-film coupler technique. The mode calculation for non-homogeneous and anisotropic channel waveguides, both single and coupled, is then accomplished via an improved effective index method, where the slab analyses are carried out both by a numerical integration of Maxwell's equations and by the WKB technique. The complete coupling (crossover) length is reported in form of design charts as a function of wavelength, refractive index change at the surface, interguide separation and width of the channel-guides.