This is an FDTD Solutions simulation of a Y-branch with 3 ports, where each port supports 2 modes.
The S-parameter sweep here is used to run a set of simulations where an individual mode
is excited in each simulation and the transmission through the ports is measured to get the S-parameters.
Because the device is symmetric however, we have set up the sweep
only to only excite ports 1 and 2 and infer the S-parameters based on symmetry.
If I edit the S-parameter sweep, I can see in the INTERCONNECT Export Setup tab,
where the mode labels, mode ID and port location information for the data export can be set.
This S-parameter sweep has already been run.
Generating the S-parameter data file is as simple as right-clicking on the sweep
and selecting the "Export for INTERCONNECT" option.
This opens a file browser window where you can specify the name of the file.
I get a warning here about not being able to check the S-parameter reciprocity
which is related to using symmetry to infer the S-Parameters for port 3.
The data will be written to the required format for loading into an Optical N Port S-Parameter element in INTERCONNECT.
In INTERCONNECT, I have a blank simulation file.
Add an Optical N Port S-Parameter element from the S Parameters -> Optical folder of the Element Library.
When you add the element, it has 2 ports by default.
Under the Standard properties, set "load from file" to "true", and click on "s parameters filename"
to open the file browser where you can choose the data file that we just exported from FDTD Solutions.
Once the data has been loaded, the element reads the data from the file
and automatically sets up 3 ports with port 1 on the left and ports 2 and 3 on the right,
as was indicated in the S-parameter sweep in FDTD Solutions.
From the FDTD Solutions simulations, when port 2 mode 1 was excited,
about 42.7% of the power was transmitted through port 1 mode 1 at a wavelength of 1.55 um.
We can quickly confirm that we get the same result here by adding an Optical Network Analyzer.
Connect the output to port 2 and the input to port 1,
and set the "center frequency" property of the ONA to 1.55 um, and the "plot kind" to wavelength.
Run the simulation. Let's visualize the mode 1 transmission.
The transmitted power agrees with what we would expect from our S-parameter file.