Here we have a silicon waveguide leading into a taper where there is a facet at the end
of the taper so light exits out of the taper into free space.
The taper is on top of a sapphire substrate with air cladding above.
The EME solver region has already been added and the ports and the y and z geometry of
the solver region has already been set up, but the cell group definition table has not
yet been set up properly.
Edit the EME solver region.
Since the waveguide starts at x=0, the x min value of 0 is appropriate.
Set the number of cell groups to 3, since there are three distinct regions of the device
where the first cell group is the uniform straight silicon waveguide, cell group 2 will
be the tapered waveguide region, and cell group 3 will be the region of free space beyond the
In the cell group definition table, set group span 1 to 85 um which is the length of the
straight waveguide segment.
Set group span 2 to 15 um which is the length of the taper, and set the final group span
to 4 um.
Since the first and last regions are uniform, only 1 cell needs to be used, but over the
taper region, we need to set an increased number of cells.
We will set this to 40, and in the later Convergence Testing section of the course, we'll discuss
how to check if enough cells are being used.
Select the CVCS subcell method for the tapered region.
By default, the same number of modes is used in all cell group regions, but we can select
the "allow custom eigensolver settings" option to be able to use different numbers of modes
in each region.
Typically, more modes are required in regions where light is propagating at steeper angles
away from the x-axis, so we'll use 20 modes over the waveguide and taper, and 100 modes
in free space, since the light will diverge from the facet and travel at steeper angles.
Additionally, I want to use custom settings for finding modes in the free space region.
To do this, select the third row of the cell group definition table corresponding to the
free space region, and click the "custom settings for cell group 3" button.
This opens up a new window.
When I click "calculate modes", and look at the mode profiles that are found in the mode
list, I can see that the modes all have fields that are strong inside the substrate, but
not in the air above where I expect most of the light to be propagating.
This is because when you use the default settings to find modes, it will try to find modes with
effective index close to the maximum refractive index of the structure cross section which
is the substrate index in this case.
In most cases, like where the waveguide is present, this is fine and it will find the
modes guided by the waveguide.
Since I actually want to find modes propagating in air, I can de-select the "use max index"
option and set the index to search near to 1.
Now if I calculate modes again, I can see that the effective index values are closer
to 1, and as I scroll down, I'll see modes where the fields are in the air region.
Click on the "take settings" button to use these settings, then click OK.
I have a visualizer window with the profile monitor plot from the simulation using these
settings that I have already run.
You can see that the light gets concentrated by the taper, before dispersing in the free