It seems the simulation time seems ok. BUt please use log scale and see if the signal is small enough at the end of simulaiton.
I believe the field changing behavior is physical as the deviceÂ
as this device is not uniform so there is round-reflections because of the taper structures.
As for fluctuation from the monitor location, it might be due to interpolation error: the monitor needs the result interpolated since the Yee cell does not allocate the field components in the same location:
You may need to use an override mesh for the monitor along the propagation direction only, and place the monitor exactly the same location relative to the mesh, for exmaple all monitors are in the mesh grid plane.
A simple suggestion for PML: to check the effect, 2 or 4 layer change might be small. I would suggest to use 8 layers.
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