Question about improving group delay dispersion calculation in Lumerical
I have a question regarding the calculation of group delay dispersion (GDD) using a mode source and a mode expansion monitor.
Our simulation setup is as follows: we have a simple ridge (straight) waveguide surrounded by a lower refractive index material. The waveguide material is dispersive (user-defined). We inject a mode source (fundamental TM mode) and then use a mode expansion monitor at a different position in the waveguide, where we extract the acquired unwrapped phase (through the Angle() command) of the complex transmission coefficient. To get the GDD, we compute the second derivative of the phase (simply taking the difference of the phase between neighbor frequency points -> we use a frequency-dependent broadband mode source with more than 100 frequency points).
We have a reference COMSOL 2D eigenmode simulation, and when calculating the GDD in "transmission", so placing the mode expansion monitor a certain distance in front of the mode source, we can nicely reproduce the reference result. Here, the waveguide length extends beyond the boundaries ("infinite" waveguide).
We are also interested in "reflection", where the waveguide is now terminated a certain distance in front of the mode source (waveguide facet), and a part of the light is reflected back into the waveguide. When we compute the GDD using the phase in the mode expansion monitor (now in the backwards direction), the resulting GDD follows the general trend of the reference result, but is very noisy (oscillations).
See the attached plot for a comparison. We suspect that in reflection, there is some numerical noise/artifacts in the phase, giving rise to the large oscillations. Our question is how we should modify the simulation settings (for example monitor size/position, auto shutoff level, boundary conditions, ? , ...) to improve the phase in reflection? Could it be there is some numerical artifacts coming from the interaction between the injected mode source, the transmitted/reflected light or something close to the boundaries?
Thank you very much for any hints/help and have a nice day!