Bug report: Incorrect sign of effective indices

- July 6, 2022 at 5:16 pm
  
  langrock
  Subscriber
  
  Running the modal analysis on a simple silicon wire waveguide surrounded by air results in a sign reversal of the calculated effective indices. This is a reproducible bug and results in the incorrect calculation of derivative-based values such as the group index and dispersion. I was able to trace it back to the order of the start and stop wavelength inside the FDE object. Setting the shorter wavelength as start and the longer as stop value results in a sign reversal compared to the other way around. This is a big bug and should be fixed.
- July 7, 2022 at 10:46 pm
  
  Guilin Sun
  Ansys Employee
  
  Please search “correct back propagation modes when pml is present” and tick this option.
- July 11, 2022 at 5:32 pm
  
  Guilin Sun
  Ansys Employee
  
  Thank you for pointing this. However it is not a bug, but one of the solutions of sqrt(beta^2), since FDE solver solves beta^2 internally. Negative real neff can happen, which means the mode propagates in the negative direction.

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