Thank you for the reply Guilin, let me elaborate further. I am using the FDE solver and I am trying to understand how the simulation region and the mesh size effects the loss on modes.

When I perform the convergence tests, I can still see that the loss keeps increasing with increasing mesh points and increasing simulation region even though the effective index stays pretty much the same. How do I decide the correct parameters in this case? I can just pick a combination of simulation region size and numbe rof mesh points where they seem to obey the convergence test but the loss values of the fundamental TE mode are wildly different.

Putting in simpler words, what is the relation between loss (dB/cm) vs number of mesh points and loss (dB/cm) vs simulation region size?

Let me know if I was able to make the problem clearer.