Reply To: how to set the group index in this example, ‘Bragg grating initial design’?

[Dimitris Polyzos]

Hello Kim Kang Seok,

Thanks you for your post. 

The group index used in this example is a constant. Its calculation is group index of the Bloch mode, that emanates by taking the derivative of the bandstructure dispersion curve. More information about this calculation you can find in the script from this example: Bandstructure of photonic crystal waveguide - Line defect.

Normally, the group index should change when we sweep a parameter, such as length, of the structure. However, in our demo example we simplify it as a constant. Thus, you are correct by using MODE to calculate the group index, as it will return more accurate results of ng. After calculating it, then you can import into FDTD to do the following simulation.

I hope I helped. PLease let me know if you need further assistance.

Kind regards

Dimitris