These two properties exist both in the “Bragg Grating” and “Sampled Bragg Grating” elements. The definitions from the Knowledge Base are
effective index change ac: The effective index change over the grating length.
effective index change dc: The background effective index change over the grating length.
But I’m not sure what these are referring to exactly.
- It sounds like the ac change is referring to the difference between the highest and lowest effective indices within the grating, as specified by the grating period. But it doesn’t explain what kind of a waveform is used to modulate this effective index. Is it a square wave? Or is it assumed to be a sinusoidal?
- The dc change on the other hand makes less sense to me. Are we adding a linear offset to the effective index as we go through the grating? Then, is the group index property specified at the beginning or at the middle of the grating? Also, I don’t quite see how a single kappa value (the grating coupling coefficient) can specify this, since when that option is chosen, these ac and dc changes are grayed out.
Thanks in advance!