Different properties can be set for each type of source, but we will first cover the settings
which are common to many of the sources before going into detail about individual source
All sources can be added from the sources menu in the main toolbar at the top of the
In the sources menu, you will also find the global properties option at the bottom of
This opens up the global source options window where you can set the default frequency/wavelength
settings or time domain pulse settings which are used whenever you add a new source.
Let's consider a Gaussian beam source.
This can be added from the sources drop down menu.
Click on the edit button or use the E keyboard shortcut to open the source edit window.
In the general tab, some of the settings here that are common to most of the sources are
the amplitude and phase.
The amplitude sets the maximum electric field amplitude of the source.
The amplitude is important when simulating non-linear materials.
It's not as important for linear systems since the default amplitude of 1 V/m can be used
to run the simulation, and results from monitors can be scaled as a post-processing step to
get the result for any desired source spectrum.
The phase setting sets the phase of the electric field at the center of the source injection
Usually this is used if you have multiple sources and you want to impose a relative
phase difference between the sources.
Under the direction and polarization box, the injection axis setting sets the main propagation
This can be set to the x, y, or z-axis.
The direction can be set to forward or backward and this determines whether the source is
injected towards the positive or negative axis direction.
The angle theta and angle phi settings can be used to rotate the source propagation direction
where theta is the angle from the injection axis and the phi is a rotation applied in
a right-hand context around the injection axis.
The polarization angle setting sets the direction of electric field polarization.
After setting up the source direction and polarization, I can check the CAD view ports
to verify the injection direction indicated by the pink arrow of the source, and the polarization
direction of the electric fields indicated by the blue arrow.
Under the geometry tab, we can set the position and spans of the source.
Depending on the injection direction and type of source, some of the span settings may be
For example, here we are injecting the Gaussian beam source in the x-direction so the source
injection plane will be in the YZ plane with no span in the x-direction so the x span setting
is greyed out.
The Frequency/wavelength tab is where you can set either the frequency or wavelength
range of the source, or the time domain settings of the source pulse.
These settings will override the global source settings.
When simulating linear systems, it is sufficient to use the "set frequency/wavelength" option
and specify the start and stop wavelength range of the source to use the automatically
generated source pulse rather than using the "set time domain" option to set properties
of the time domain pulse.
This is because the CW normalization will normalize away the spectrum of the pulse to
give result as though a uniform spectrum is injected.
For more information see the CW normalization link below.
On the right side of the panel you can see the spectrum of the source pulse as well as
the plot of the time signal of the pulse.
You can zoom into the plots by selecting a region of the plot with your mouse cursor.
Now that we have seen how to set the basic properties of sources, here are some useful
tips to know when you set up a source.
The source has a finite 2 to 3 cell wide injection region which is displayed as a grey shaded
region in the CAD view.
To avoid any injection errors or scattering at the source injection plane, the shaded
injection region should always be contained in one medium meaning that the source injection
region shouldn't intersect two different objects.
However this constraint doesn't apply when using the mode source to inject the mode of
a waveguide or fiber since the mode profile injected by the source is calculated for the
given cross section of the structure that the source intersects, so there won't be any
scattering when injecting this field profile.
In the Geometry tab of the source, the span settings are enabled based on the source injection
direction for plane wave, Gaussian beam, and mode sources, so be sure to set up the direction
first before setting up the geometry.
For example, if you have a plane wave source propagating the x-direction, the x span of
the source is not relevant and the setting will be greyed out in the edit window.
Under the Frequency/wavelength tab, you can use the plot of the signal vs time to make
sure that the simulation time setting of the FDTD simulation is long enough to include
the entire time signal of the source pulse.
This can be important when injecting at frequencies below the optical range where a longer source
pulse is generated.
It's also important to know that the simulation mesh size will adapt based on the source wavelength.
Smaller wavelengths result in a finer mesh and you can see that for a source wavelength
of 0.5 microns a finer mesh is generated compared to using a source wavelength of 1 micron.
Therefore, it's best to look at the simulation mesh after setting up the source.
A final tip is that the fields inside the source injection region may be unphysical,
so when setting up monitors to record field results, make sure to place the monitors outside
of the grey shaded source injection region.
Or, if using a monitor to record the field profile where the monitor intersects with
the source injection plane, be aware that the fields measured in the two cells around
the source injection plane may be unphysical.
In the Knowledge Base Sources chapter under the Sources others and advanced techniques
subsection, you will find additional advanced information such as tips for adjusting the
source bandwidth, simulating frequencies below the optical range, simulating coherence and
In the following units, we'll cover each source in detail.