Learning Forum

[Dimitris Polyzos]

Hello Taketoshi,

In general, varFDTD is used for planar geometries where there is negligible coupling between vertical modes. varFDTD solver is based on collapsing a 3D geometry into a 2D set of effective indices that can be solved with 2D FDTD. This works best with waveguides made from planar structures, as the main assumption of this method is that there is little coupling between different supported slab modes. For many devices, such as SOI based slab waveguide structures, that only support 2 vertical modes with different polarization, this is an excellent assumption. varFDTD can achieve accuracy comparable to 3D FDTD with computation times comparable to 2D FDTD.  Also, varFDTD is able to approximately simulate rib waveguide as it gets the "effective index" anywhere is the simulation region. However, varFDTD is quick but 3D FDTD is advised to verify the varFDTD results. 

An alternative to varFDTD would be to use FDE and EME solver. With FDE you will be able to calculate the neff of the waveguide while with EME you will calculate the s-parametres transmission/reflection coefficients. For 3D geometries where there is coupling between vertical modes, EME is a frequency domain method that offers a good alternative to 3D FDTD because its computational requirements scale exceptionally well with distance. Please refer to the following articles:

Ring modulator - transient response (varFDTD)

When to Use varFDTD

I hope I helped.

Dimitris