Ports are available for s-parameter extraction and although they are not strictly monitors,
we will discuss them here.
Ports act as a combination of a frequency domain power monitor, mode expansion monitor,
and mode source.
They are primarily used for extracting S-parameters of a device.
S-parameters are scattering parameters used to characterize the transmission and reflection
of devices, and are often used for performing circuit-level simulations where each element
of the circuit is represented by its S-parameters.
The image on the left shows a port in the CAD view, and the image on the right shows
the equivalent combination of source and monitors.
Since the port object has all of the functionality in a single object, it is simpler than setting
up a combination of the monitors and source and having to set up the properties for all
Port objects can be added from the ports menu in the top toolbar, and once you add a port,
the ports group will automatically be generated as a child of the FDTD simulation region object
in the Objects Tree.
The port group is a container which holds all of the port objects and it is where you
can specify which port will act as the source and what mode will be injected by the port
when the simulation is run.
Since ports are a child of the FDTD simulation region, this means the simulation region must
be added before a port can be added, which isn't the case for other monitors or sources.
The arrows in the CAD view show the input and output directions of the port.
The active port where the mode will be injected has a pink arrow in the source injection direction.
The other ports have a red arrow in the input direction of the port and the green arrows
indicate the output directions of each port.
In the port object, you can select the modes of interest the same way as mode expansion
monitors from the Modal Properties tab.
You can also choose to calculate the characteristic impedance of the selected modes in the Impedance
The characteristic impedance is calculated by the power over current squared where the
power is calculated by integrating the Poynting vector over the cross section area of the
mode, and the current is calculated by integrating the magnetic fields around a loop enclosing
Once the characteristic impedance calculation is enabled, you can specify the integration
region over which to calculate the P and I values.
The results neff (effective index of the modes), mode profiles, and Z0 (characteristic impedance)
are available before running the simulation.
Once the simulation has been run, you can also obtain the S result, as well as all of
the results that would be available from the mode expansion monitor, field power monitor,
and mode source under the port modal expansions, port monitor results, and port source results
in sections of the Result View window.
To get the S-parameters of a device, you will typically need to run multiple simulations.
If the device has no symmetry then one simulation is needed to obtain each column of the S-matrix
where in each simulation, one port mode is excited and the reflection and transmission
through the remaining ports are measured.
The running and collecting of the S results for the full S-parameters can be automated
using the S parameter sweep tool.
After running the sweep, you can choose to export the S-parameter data from the sweep
to a .dat file which can be loaded in INTERCONNECT, Lumerical's circuit simulation tool.
For a detailed tutorial on using the S-parameter sweep tool, see the related links below.