What do the AC and DC effective index changes refer to in Bragg Grating?

#1

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.

1. 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?
2. 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.

#2

Hi @esm

Thank you for your post. You are actually very right about the “effective index change dc” and “effective index change ac” definitions. The “effective index change dc” is the global linear offset for the index perturbation and the “effective index change ac” defines the highest and lowest effective indices after the offset. We use the sinusoidal waveform for the ac index change. The definitions are based on the following equation:

I hope this could help

#3

All right, this makes sense. Just as a basic test, increasing the DC term by 0.001 shifts the grating response towards longer wavelengths, as predicted by the Bragg condition.

Here, you can see what happens if you keep increasing the DC term a few times by 0.001.

#4

Hi @esm

Thank you for the validation!