Hello, I have created 2D simulation of directional coupler, and I excite it using a beam from an input port. I know that in case of solving the problem using mode analisys - I am sepposed to get sinosodial oscilations of the power inside the each waveguide, as the mode "jumps" from one waveguide to the other. however, in my simulation (when the waveguides are exited using an input beam), I get the attached graph (power inside both waveguides vs coupler length). As can be seen, the amplitude of the oscilations changes. what is the different between solving with mode analisys and solving using input beam? why is my amplitode not constant?
I use port to input a gaussian beam, with total width of the entire coupler - so that both waveguides are exited simultanisly and equally. For bounderies I use Matched Boundary Condition.
Thank you for your help!