Guilin Sun
Ansys Employee
This is normal in FDTD in the discrete world, due to injection error of the source, and PML reflection. If you use DFT monitor to check the amount of reflection is very small, on the scale of 1e-5 , which matches with the mesh accuracy.
To reduce injection error, you can use a local finer mesh; to reduce PML reflection, you can use more number of PML layers.

Please write your post at the specific Photonics forum here Photonics ÔÇö Ansys Learning Forum