I am simulating a silicon nitride ring resonator with a radius of ~10 microns coupled to two optical waveguides (the same as the one of the examples of the FDTD solver, but with different dimensions and material). First, I tried 3-D simulation, but when I looked at the transmission or coupling frequency response, I saw so many ripples. Therefore, I tried 2-D FDTD for a quick estimation. However, still the ripples exist. I even tried to make simulation parameters more accurate (like FDTD time around 20000 fs, extra mesh in the gap region, increasing mesh accuracy (5-6) and mesh type (conformal variant 2)), but I still see the ripples and even the fluctuations become more severe.
My question is that is this phenomenon an intrinsic feature of the time-domain solver (because I don’t see the same problem with FEM solvers), or can I address this problem?
I appreciate any comment or response. Thank you in advance.