Learning Forum

[Sangyeon_Cho]

Hi Lumerical,

I am Sangyeon Cho, a post-doc researcher at MIT and Harvard Medical School.

I am writing to ask you about the equation or method to calculate the group index of the waveguide mode.

I am trying to simulate the structure and calculate the confinement factor and group index.

https://www.nature.com/articles/ncomms5953

[Guilin Sun]

Thank you to bring this issue up. For FDE in MODE, the group refractive index is calculated from conventional definition of c/vg where the group velocity vg is defined like in the wiki:
The formula you gave is to consider the complicated dispersion of permittivity which could lead to different result from the simple definition. So just use this formula to get the result you want.
Actually, there are some other definitions about various velocities, such as energy velocity in addition to group velocity and phase velocity.

[Valentina Moskalenko]

Hi. Can you elaborate how you calculate effective ng when neff if calculated only at one wavelength in FDE solver?

[Guilin Sun]

It is based on the definition of group index. https://optics.ansys.com/hc/en-us/articles/360034396774-Frequency-Analysis-Modal-Analysis-Tab

[Valentina Moskalenko]

Thanks. 

In the page you sent me the group index comes after frequency analysis is done. 

In FDE even when a mode at a fixed wavelength is simulated the solver gives group index. How is it done in this case? 

[Guilin Sun]

The wesite is not updated after new feature added.

Internally it calculates another frequency close to the preset frequency in order to do the difference, very similar to frequency sweep. This feature is added in recent versions to save customer time wihtout doing the frequency sweep.