Guilin Sun
Ansys Employee

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.