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

- January 29, 2024 at 4:36 am
  
  ddol410
  Subscriber
  
  Hello, everyone.
  
  I'm trying to follow the example, so-called bragg grating initial design(https://optics.ansys.com/hc/en-us/articles/360042304394-Bragg-grating-initial-design-with-FDTD), and would like to change some structure parameters.
  
  In this example, the group index is set in the model script.
  I thought that it meant the group index of the center wavelength for the Si waveguide with averaged width. When I compared two data, 4.3874 and my calculation value from the MODE group index calculation, there is a bit gap.
  
  Therefore, I would be grateful if anyone could explain the conditions of the group index in the example. I cannot be certain that it is a group index with the averaged Si waveguide.
  
  Thank you.
- January 30, 2024 at 1:12 pm
  
  Dimitris Polyzos
  Ansys Employee
  
  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