Learning Forum

[Guanhui Wang]

Here I wanna show how to start creating a parameterized model for your FDTD optical component and start using it on the circuit level in INTERCONNECT. I will use a simple example of a Taper waveguide.

Goal: To have a circuit component (TE taper) in INTERCONNECT that has some parameters (w1, w2, L). You can choose any values for those parameters and INTERCONNECT checks the S parameters of the component in a look up table (.xml file). If available, the S parameters file is loaded. If not, a message pops up giving you one of 2 options: 1) Use the nearest S parameters file 2) Generate S parameters file using FDTD.

In general, I always have 2 directories; one for the S parameters files and look up table, the other is for the FDTD files and scripts.

• Steps in FDTD:

1. In FDTD folder, create a folder called “taper”. Then create a file called “taper.fsp”.
2. Define w1, w2, L as variable in MODEL.
3. Write a script to the setup tab of MODEL such that all the components, the FDTD region, the meshes and the monitors are all automatically changed with the design parameters (w1, w2, L).
4. Since the taper is a 2-port network, define a variable in MODEL called “port”. This variable is responsible for creating the source either at port 1 or port 2.
5. Write a script that set the PORT = 1 and calculates the S-parameters. Then sets PORT =2 and calculates the S-parameters again. This script writes the calculated S-param to a file named

sfilename = “w1=” + num2str(w11e6) + “um_w2=” + num2str(w21e6) + “um_length=” + num2str(L*1e6) + “um.dat”;

• It is all straight forward except the definition of the S12 and S21 where the change in the waveguide cross section should be considered:
• S11=Port1.b/Port1.a;
• S21=(Port2.asqrt(Port2.N))/(Port1.asqrt(Port1.N));

When we run the script above, a .dat file that has all the S-parameters ready to be used in INTERCONNECT is automatically created in the same directory.

The next step is to create a look up table which is easily done through script. This script creates a look up table file that has one record for whatever combination of (w1,w2,L) you choose. Of course, you can do a sweep and create a long look up table for the different combinations but for the sake of illustration one case is enough. We will rely on INTERCONNECT to prompt FDTD and simulate any missing cases.

• Steps in INTERCONNECT

1. Insert an Optical N-port S-parameter from the Library. Right-click >> create compound element