Full-wave analysis of coplanar waveguides for LiNbO3 optical modulators by the mode-matching method considering nonideal conductors on etched buffer layers

Rigorous analysis of traveling-wave coplanar waveguide electrodes for LiNbO₃ optical modulator applications is presented by using an extended full-wave mode-matching method. The microwave propagation characteristics under the composite influence of substrate anisotropy, uniform or etched buffer layers, finite electrode thickness and conductivity, and metallization undercutting are accurately assessed by employing a network equivalent formulation. Variations of the coplanar waveguide microwave effective index and the characteristic impedance at low frequencies due to finite electrode conductivity are illustrated, and are important even though the mode is quasi-TEM in nature. The effect of etching the SiO₂ buffer layer is shown to be one possible method for lowering the microwave effective index while keeping the conductor loss at a fixed level