Performance enhancement of nonlinear lithium niobate couplers via double titanium and magnesium diffusion

The effects of double Ti and Mg diffusion into lithium niobate couplers exploiting cascaded second-order nonlinearity are theoretically investigated. We demonstrate that this technology may be employed to optimize the performance of a new type of coupler made by uniaxial crystal having an unusual dielectric tensor configuration, i.e, equatorial. An extended version of the simple effective index method is developed in order to take into account the complex nature of hybrid modes. A home-made computer code is utilized in the electromagnetic analysis of rotated anisotropic channel waveguides. As an example, the simulation results show that a magnesium diffusion, made in the external region of a Ti : LiNbO3 coupler which induces negative extraordinary and ordinary changes of the refractive indices of LiNbO3 Delta n(e(Mg)) = -0.002 and Delta n(o(Mg)) = -0.001, decreases the linear coupling length from L-c = 73 to 44 mm.