In OptiBPM, one can define waveguides created by diffusion of specific metals into specific crystals. These features follow the technical process for each diffused waveguide technology. Examples can be found in OptiBPM_Tech_Background_and_Tutorials.pdf,Lesson 7: applying predefined diffusion processes. For example the substrate can be lithium niobate, the required process parameters are already defined in OptiBPM. One cannot have a substrate of dielectric material, since the diffusion process is not defined. I recommend following lesson 7 and modifying it to suit your application as far as possible.
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