There are three analysis tabs in the Eigensolver Analysis window, modal, frequency and overlap
analyses.
We will go through each of these in details, starting with the modal analysis tab.
Once the modes are calculated, they are listed in the Mode list.
The modes are arranged in descending orders in terms of the real part of the effective
index.
In this specific example, we are looking at the modes of a silicon waveguide on a silica
substrate.
If you look at the mode 1 and 2, the fields are concentrated in or around the Si region.
On the other hand, the third and higher order modes has a broad field distribution in the
low-index silica region.
These are not actually physical modes.
They are the result of our choice to use metal boundaries in this simulation.
The effective indices of these modes are lower than 1.444 for silica, which is the lowest
index material where the modes are residing.
Another indication that the modes are not real.
So, from the investigation of the mode list, you can conclude that this waveguide supports
only two modes, one TE and one TM.
After finding modes, click on the desired mode in the mode list to plot it the plot
area.
The default plot is Modal fields.
You can visualize the amplitude, phase, real and imaginary part of the field, where applicable.
You can also choose to plot specific component of the fields.
If you select the cylindrical coordinate, the available field components will also be
changed accordingly.
You can plot the mode data in linear or log scale.
Plotting the field in log scale can be sometimes useful to get a rough picture of how far the
mode is spreading out and to adjust the simulation spans appropriately.
If you want to add or remove the outline of your structures, use the superimpose structure
option.
For a wider choice of settings for the plot, use plot in New Window option.
Also note that you can right-click the modes in the data analysis group under FDE and visualize
the mode data.
To calculate the modes of a bent waveguide, select the bent waveguide option and specify
the bend radius and the orientation.
For this calculation, the FDE solver considers the boundaries as part of the simulation region
and the center of the simulation region is defined accordingly.
As the bend radius is defined as the distance between the axis of rotation and the center
of the simulation region, you need to use a bend radius that is equal or larger than
half of the simulation region.
If you use a bend radius that is smaller than the half of the simulation span, the solver
will use the half of the simulation span as its bend radius.
Modal analysis tab also provides Power and Impedance Integration, Far Field Settings
and Data Export.
The Power and Impedance Integration option allows you to spatially integrate the electric
field intensity or power.
It can also calculate the characteristic impedance of a waveguide by integrating the magnetic
field along the path you specify in the GUI.
Note that you can do the integration on the nearfield as well as the farfield.
The Farfield Setting option allows you to specify the farfield projection setting for
the farfield plot.
Data Export allows export to text file, Matlab file, Lumerical data file (LDF).
You can select data to export (effective index, material properties, modes, far field), and
click on the Export.