MODE Solutions includes 3 solvers: FDE, EME, varFDTD.
The EME solver is suited for simulating bi-directional propagation of guided light where the propagation
is mainly along 1 axis (the x-axis).
It takes into account multiple reflection events at interfaces.
The propagation sweep tool allows length sweeping in a very efficient manner.
Results available from EME include S-parameters, field and index profiles.
Example devices include MMIs, spot size converters, y-branchs, Bragg waveguides and fibers, and polarization
You can find many other devices suited for EME, but it should be noted that EME has some
limitations as well.
EME is not suitable for simulating devices such as ring resonators, which exhibit steep-angle
or omnidirectional propagation.
It's also generally not suited for simulating propagation in free space, for example
Mie particle scatting where FDTD is more suitable.
However, if the propagation in free space is mostly bi-directional as in the edge coupler
example, EME works fine.
FDE finds modes of waveguides or fibers where the cross section of the structure does not
change along the propagation direction.
The EME solver, using the Eigenmode expansion method, can be seen as an extension of the
FDE simply finds the supported modes of devices with a fixed cross section, such as a waveguide.
The EME solver can simulate devices with a non-uniform cross section.
It uses the FDE solver to find the supported modes at many cross sections, then calculates
how an arbitrary mode will propagate through the system using the Eigenmode expansion method.
This method will be covered in more detail in the Solver Physics section.
varFDTD is a time-domain solver, so you can get a broadband results from a single simulation.
However, EME is a frequency-domain solver, so you will only get results for a single frequency.
varFDTD is based on collapsing a 3D geometry into a 2D set of effective indices.
An assumption of this method is that there is little coupling between different supported
slab modes, it works best with waveguides made from planar structures.
In contrast, EME does not make such approximations and assumptions.
So, it is suited for planar as well as non-planar structures.
While varFDTD can handle omni-directional propagation, EME is limited to bi-directional