Discrepancy between 2.5D var FDTD and 3D FDTD
I am trying to simulating the transmission characteristics of a directional coupler with certain waveguide geometry. 3D FDTD is so time consuming and requires a lot of memories, so I want to use 2.5D FDTD to conduct initial optimization and then use 3D FDTD to finalize it. But one thing I notice is that, the results given by 2.5D FDTD are quite different from the ones given by 3D FDTD, and this difference seems to grow when the cross-section of the waveguide has smaller base angle (with less vertical sidewall). Does anyone come across this before? Is there a way to decrease this discrepancy?
Please let me know if you need more information, since they do not allow me to upload the fsp/lms files.
Thank you so much!