Lesson 14 presents an example of waveguides made by diffusion into lithium niobate. The definitions for horizontal and vertical diffusion lengths can be found in the OptiBPM User Reference on pages 423 – 424. The electric field is strongest between electrodes, not on them. Therefore, to obtain the strongest field in the waveguide, the electrodes are placed on each side of the waveguide. I’m not sure why the buffer was considered necessary for the simulation. However, it seems it appears often enough that OptiBPM should be able to incorporate it in simulations.