Getting different neff and loss for same design


I have downloaded model from link:

When I am checking for modes and corresponding loss, I can see neff are having imaginary values and there is some loss present as well as shown in below image (for attached model waveguide_dispersion.lms)

waveguide_dispersion.lms (8.0 MB)

But when i have made similar structure by myself using new file, neff are not showing imaginary values and also there is no significant loss this time(all zero) as shown in below image(attached model waveguide_dispersion_1.lms)

waveguide_dispersion_1.lms (7.6 MB)

Both structures look same to me. Do I need to change some settings to get imaginary neff and can you clarify on loss as well.?


Hi @sunny.chugh

To calculate the loss, you need to use PML boundary conditions, while in your attached file, the boundary conditions are set to metal. That’s being said, for a confined mode the loss is expected to zero. You will also need to perform convergence testing to make sure that the FDE simulation span is large enough so that the mode fields are not truncated by the simulation boundaries.

If you are new to FDE, I recommend you to take our online course on FDE:
New FDE Solver online course available at Lumerical University