Hi Steve Dods..
Thank you for your great answer. I have found in a sample file of OptiBPM ie. Magnesium doped waveguide is placed on top of the Titanium doped lithium niobate. The waveguide model is working, but the output electric field is decreasing in magnitude if we increase the length of the waveguide. I also tried to configure as directional coupler, but there is no coupling between two waveguides. We are mainly focusing on MgO:LiNbO3 wafers to develop our optical devices, hence I require this Mg doped LN waveguide.