This video is taken from the FDTD Learning Track on Ansys Innovation Courses.
- Monitors - Frequency-domain field profile (information about frequency-domain field profile and frequency-domain field and power monitor settings)
- Near to far-field projections (far-field projections - also covered in the Far-field projections section of the course)
- Spectral averaging (details on total and partial spectral averaging which can be set under the "Spectral averaging and apodization" tab for frequency domain field monitors)
- Apodization (details on apodization settings which can be set under the "Spectral averaging and apodization" tab for frequency domain field monitors)
- PC Micro Cavity (example which demonstrates a similar photonic crystal cavity)
- Curved or angled monitors (information about the angled monitor analysis group from the Object Library)
We’ll use the photonic crystal cavity resonator structure as the starting point as with the
I’ll add a frequency domain monitor to record the fields at different frequency points and
plot the far field radiation pattern from the cavity.
Start by adding a frequency-domain field profile monitor from the Monitors menu.
This monitor will be used to measure the field profile across the center of the slab at z=0
at 10 different frequency points over the wavelength range.
Edit the monitor and select “override global monitor settings”, then set the frequency
to 10. in most cases, using the global monitor settings is a good idea. Overriding them is less common. However, we are overriding the settings here since we want different monitors to record different numbers of frequency points
In the geometry tab, make sure the monitor is centered and set the x span and y span
to 2.5 microns to match the span of the simulation region.
Set the monitor name to Profile.
Click OK to accept the settings.
Next, to add a frequency-domain field and power monitor, I can either add a new monitor
from the monitors menu, but I can also duplicate the existing monitor and change the type of
monitor from frequency-domain field profile to field and power by changing the interpolation
option that is selected.
I will do this by clicking on the duplicate button in the edit toolbar and editing the
I’ll change the name of the monitor to Power and under the Advanced tab, select “nearest
mesh cell” in the spatial interpolation drop down menu.
Under the General tab, select “override global monitor settings” and set frequency
points to 1 to record only at the center frequency of source range.
Under the geometry tab, change z to 0.2 microns so the monitor is located in the air above
Click OK to accept the settings.
In the CAD you can see that the icon for the monitor in the Objects Tree has automatically
switched to the frequency domain field and power monitor icon based on the chosen interpolation.
Next, run the simulation.
After the simulation has completed, right-click and visualize the electric field profile in
the near field from the profile monitor.
You can click on the lambda parameter in the Parameters table and plot the field profile
at the 10 different wavelengths that were recorded by the monitor.
We can see that the field profile doesn’t change much over this range.
We can see that the field profile doesn’t change much over this range.' is missing from the transcript Next, right-click on the power monitor and select the visualize farfield option.
Since the far fields have not yet been calculated, the select frequency window pops up.
If I recorded more than 1 frequency point using the monitor I would be able to choose
which frequency point to project, or to project all frequency points.
There are also additional far field settings that can be accessed using the far field settings
Click OK to calculate the far field angular distribution.
Here I can see the field intensity projected to the upper z hemisphere and I can see that
there the light gets radiated at 30 40 and 50-degree angles from the cavity.
Let’s review some tips for using frequency domain power or profile monitors.
The frequency-domain field and power monitor is the default frequency domain monitor we
recommend for collecting all frequency domain data.
This includes field profiles, far field projections, and net power transmission data.
Increasing the number of frequency points recorded does not change the simulation accuracy.
It only increases the number of data points in the frequency domain over which monitor
data is recorded and returned.
In other words, the raw time domain fields calculated as the simulation is being run
will be exactly the same.
It is only the number of frequency points over which fourier transform is calculated
by the monitor that will change.
The farfield result can be calculated and returned from frequency-domain field monitors
as a post-processing step after running the simulation.
The E and H field components need to be recorded by the monitor in order to be able to perform
the far field projection calculations.
Apodization can be applied if you want to get a resonant field profile – it allows
you to choose a time window over which to use the time domain data to get the frequency
However, apodization needs to be used with care since it can invalidate the normalization
of the results, so see the details in the apodization link below.
Monitors must be axis-aligned meaning that they must be aligned to the x, y, or z axis
and they cannot be angled.
However, it is possible to get results over a curved or angled plane.
One option is to add a large number of point monitors to recreate the desired line or surface.
A second option is to collect the data from an axis aligned monitor, then use interpolation
in a post-processing step.
An analysis group called “Angled monitor” is available in the Object Library which can
be used for this purpose.
See the link below for details.