The excitation port should be as large as possible to not truncate the source mode;
it seems the PML mightnot be effective as there are interference patterns in the input/output straight waveguides. Please check the PML;
please extend the device outside of the PMLs, but not let them end inside PML;
You may also need to run long simulation time, with smaller autoshutoff min, say 1e-6.
Please add at least one point time monitor at the output waveguides and make sure the signal is completely decayed. You should observe a series of pulses with smaller and smaller amplitude due to the MMI cavity.
This post can be focused on using FDTD.
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