Learning Forum

[Bartu Yaman]

I noticed that changing the size, mesh step and orientation (by orientation I mean dragging the region around the waveguide to include more air or cladding) of the simulation region greatly effects the propagation losses and I am unsure which value I should trust since there is 6 orders of magnitude difference between some configurations. How can I make sure that I design this step correctly?

[Guilin Sun]

May I know which product you are referring? 

If it is FDTD, it means the result is very senstive to mesh. You will need to do convergence testing and set the override region in the proper location and size and the mesh size. Please do not use coarse mesh to make the simulation volume small. In general we suggest mesh accuracy 2, the default accuracy.

If it is FDE, it may mean that the simulation region   and the mesh are not properly set. 

 

Anyway 6 order difference is too large to accept. Please elaborate more. You may post some screenshots here.  

 

[Bartu Yaman]

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.

[Guilin Sun]

FDE solves eigen values of the Maxwell Equations. There is no known relationship bettwen the loss (or neff) and the mesh size. If the material has no loss, then the loss is from the PML, which is very lossy. Therefore increasing simulation region can reduce the effect of PML. However, the eigen values are solved with beta square where beta is the propagation wave vector. When the loss is small, neff=beta/k0 where k0 is the wavelenumber in vacuum, the imaginary part of neff is very small. FDE uses single precision digit which can be accurate at most the 6th digits. Due to discritization it is easy to have numerical errors in the 5th digits. So as long as it is accurate in the 5th digit, it is ok. Further improvements may be done with careful settings of the simulation size, PML, and mesh size.