This unit shows the workflow for a varFDTD simulation.
We start by setting up the 3D device geometry where the light will propagate in the xy plane
of the device, and add the solver region where we can select
the vertical slab mode of the core waveguide.
This slab mode is calculated given the cross section of the device along the z-dimension
at the specified x,y position.
1D finite-difference Eigenmode (or FDE) calculation is performed to find the supported slab mode,
and we can plot the calculated slab mode profile as a function of z in the Effective index
tab of the solver region.
The solver region settings will be discussed in more detail in the next section of the
The slab mode is used in the calculation of the effective 2D materials at each x, y position
of the solver region in order to collapse the 3D problem to a 2D problem, and this calculation
will be discussed in the following unit.
For broadband simulations, we can calculate the slab mode profile as a function of wavelength
and the effective material properties are calculated over the broadband range allowing
the simulation to account for material dispersion and waveguide dispersion.
Next, we can add sources to inject light and monitors to record the fields.
The mode source is a commonly used source, and it's important to select the mode to inject
from the mode source only after the slab mode has been selected in the varFDTD solver region,
since the modes are calculated given the effective 2D materials whose properties depend on the
Running the simulation will trigger the calculation of the all the effective material properties
required to represent the 3D geometry by a 2D set of effective materials, and a 2D FDTD
simulation is performed.
After the simulation has been run, the monitor results will be available.
The field data can be expanded along z by multiplying the result from the 2D simulation
with the vertical slab mode.
The key difference between varFDTD and a traditional 2D FDTD simulation is that varFDTD automatically
determines effective material properties for the 2D simulation that will give the closest
representation of the actual 3D device whereas a 2D FDTD simulation in FDTD Solutions will
simply take the cross section of the physical structure assuming that the structure is uniform
in the third dimension.