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.

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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.

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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?

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