Optimal designing of grating couplers for diffused waveguides in electro-optical crystals

Grating couplers for electro-optical diffused waveguides (such as Ti:LiNbO₃, Ti:LiTaO₃, etc.) are examined theoretically. A comparison of the improvements resulting from groove deepening, grating metallization, covering and multicovering of waveguide corrugation is made. Expressions for the calculation of the optimum parameters for multicovering are given and the method for optimization of the parameters is presented. It is shown that high-efficiency couplers for diffused electro-optical waveguides can be made using corrugations no longer than 1 mm and no deeper than 10 nm. A numerical illustration is presented to demonstrate the usefulness of the method for the improvement of other grating devices (distributed Bragg reflector, thin-film grating coupler).