The self-image effects of the multi-port multimode couplers based on weakly guided graded Ti:LiNbO<sub>3</sub> optical waveguides are analyzed through the three dimensional (3D) full-vector beam propagation method (FVBPM). By applying the perfect matching layer (PML) boundary condition, we can efficiently calculate the various performances such as insertion loss, crosstalk, and power splitting ratio of the multimode couplers. Through this model, the effect of the design and fabrication parameters such as the thickness and width of the Ti-strip and diffusion time and temperature on performances of the multimode couplers can be easily investigated. By comparing with traditional MMI couplers based on step-index optical waveguides, some salient features are observed and the related design rules should be revised. Finally, some related issues such as the bending loss and high order modes are discussed.
In this presentation, first we investigate the diffusion process analytically and numerically. Some important fabrication and design parameters are abstracted and used for the next level simulation. Then we applied the efficient method to analyze the modal properties of the diffused waveguides. The accuracy and the scope of validity for the analytical methods are investigated and the modal properties directly link to the fabrication parameters. Based on modal properties of optimized waveguides, the device performances of the related devices can be easily obtained. The relation between the waveguide modal performance and design parameters is built and the possible optimization methods can be applied. Finally, some related issues such as the bending loss and the coupling loss with the standard fiber are discussed.