Before running the simulation, the software is in Layout mode and the EME analysis window
is blank. Once the cell modes have been calculated, the EME analysis window becomes active and
is populated with various options. The Find Modes step which happens when you
click the run button has calculated the modes for each cell and the overlaps between modes
in neighboring cells. Now you can inject the mode from the input port and determine how
the fields propagate through the device. To visualize the fields and the refractive index profile,
add an eme profile monitor and eme index monitor. You can also add these monitors before finding
the modes. Use the eme propagate button to calculate
how the fields propagate through the device. A red marker on an object's icon will show that
results are available. Right click on the monitors to view the profile
monitor to view the field profile, and the index profile.
To see the conversion efficiency of the entire device, visualize the user s-matrix result
from the EME object and plot the absolute value squared. Using the "View data" button,
we can see that values of the user s matrix. Row 2 column 1 shows the absolute value
squared of S21 which is the conversion efficiency. If you want to obtain the results for a device
with a modified length, you do not need to modify the structures and redo the whole simulation.
Just modify the group span of the corresponding device section and repeat the Propagate step.
I'll increase group span 2 to 100 um and propagate again. If I plot the updated user s matrix,
I can see that that the conversion efficiency is increased.
To easily obtain the s-matrix parameters as a function of device length, use the built-in
Propagation sweep tool, rather than manually changing the group spans for each value.
Click the checkbox to enable the Propagation sweep feature. Set parameter to group span
2 to sweep taper length, set start and stop to 10 to 200 um with 201 points in sweep, then
click eme sweep. Note that this is fast since the modes and
overlaps have already been calculated and do not need to be repeated.
Once the sweep is done, click the visualize eme sweep button and view the absolute value of S21 squared versus span,
to get the conversion efficiency as a function of taper length. What the plot tells us is
that we need the taper to be at least 200 um long to achieve nearly 100% conversion
efficiency from the small silicon waveguide mode to a much larger spot at the SiON output waveguide.