This video is taken from the FDTD Learning Track on Ansys Innovation Courses.
Another factor that can affect the accuracy of frequency-domain monitor results is the
Since the frequency-domain results are obtained by taking a Fourier transform of the time-domain
fields, truncation of the signal can introduce artifacts in the Fourier transform.
The maximum simulation time is a setting of the FDTD solver region.
In this example, of a ring resonator, when the simulation time of 700 fs is used, the
simulation ends before the fields have a chance to fully propagate through the device leading
to artificial ripples in the transmission spectrum from the drop port.
You can also see that the peaks in the spectrum are slightly shifted and the amplitudes of
the peaks are also different from the case where a longer simulation time of 2000 fs
The early shutoff feature which is enabled in the FDTD simulation region object in the
"Advanced options" tab is enabled by default.
This feature monitors the auto shutoff level which gives a rough estimate of the energy
remaining in the simulation region as a fraction of the power injected into the simulation
Once the fraction of power remaining in the simulation falls below a specified threshold
value (usually 10^-5 by default), this triggers the simulation to end.
This is a time-saving feature since it makes sure that the fields have run for enough time to fully decay
and it doesn't run the simulation longer than necessary.
An additional benefit is that it allows you to check whether the simulation time setting
is long enough by checking whether the early termination of the simulation was triggered
If the maximum simulation time specified in the FDTD simulation region object is too short,
the simulation will terminate after reaching the maximum simulation time without triggering
the early termination.
There are several ways to tell whether the auto shutoff threshold was reached.
One is by monitoring the Job Manager window while the simulation is running.
You might see that the simulation ends before the progress bar reaches 100% and that indicates
that the simulation was ended early.
You can also tell by checking the "status" result which is returned by the FDTD solver
If the status is 1, this means that the simulation ran to the full time without reaching the
auto shutoff level, and if it is 2, then that indicates that the auto shutoff threshold
Otherwise, you can also check the simulation log file.
The log file is generated when the simulation is run and it's saved in the current working
The file name will have the same name as the simulation file with _p0 appended to the file name and the
file will have a .log extension.
This file can be opened using any text editor program.
If the early shutoff is triggered, it will contain the message "Early termination of
simulation, the autoshutoff criteria are satisfied."