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The first two sections (Create the structure, and Add EME Solver and monitors) describe how to setup the simulation from a blank simulation file. If you prefer to skip this section, a copy of the completed simulation file is provided on the first page of the tutorial.

Create the structure

The structure will consist of a substrate, the input high index waveguide with uniform y span, the tapered portion of the high index waveguide with varying y span, and the low index polymer waveguide.

 

Begin by starting MODE. You can save the MODE Simulation Project file (.lms) at any point in this process. To do so, choose SAVE in the FILE menu.

 

Add the substrate

Press on arrow on the STRUCTURES button icon_structures and select a RECTANGLE from the drop-down menu. Set the properties of the substrate rectangle according to the following table. To open the edit properties window for an object, click on the edit tool button icon_edit on the side toolbar, or right click on the object in the Objects Tree and select EDIT OBJECT from the right-click menu.

tab

property

value


name

substrate

Geometry

x (μm)

0


x span (μm)

20


y  (μm)

0


y span (μm)

10


z (μm)

-2.5


z span (μm)

5

Material

index

1.465

Graphical rendering

override colour opacity from material database

selected


alpha

0.3

 

Add the input high index waveguide

Press on arrow on the STRUCTURES button icon_structures and select a RECTANGLE from the drop-down menu. Set the properties of the rectangle according to the following table.

tab

property

value


name

input

Geometry

x (μm)

-7.5


x span (μm)

5


y  (μm)

0


y span (μm)

0.4


z (μm)

0.1


z span (μm)

0.2

Material

material

Si (Silicon) - Palik

 

Add the tapered section of the high index waveguide

Press on arrow on the COMPONENTS button icon_components and select EXTRUDED POLYGONS from the pull-down menu. This will open the object library window.

Select ISOSCELES TRAPEZOID from the list and press the INSERT button.

Set the properties of the isosceles trapezoid according to the following table.

tab

property

value


name

taper

Properties

x, y (μm)

0


z (μm)

0.1


material

Si (Silicon) - Palik


z span (μm)

0.2


y span (μm)

10


lx top (μm)

0.4


lx base (μm)

0.08

Rotations

first axis

z


rotation 1 (degrees)

90

 

Add the low index polymer waveguide

Press on arrow on the STRUCTURES button icon_structures and select a RECTANGLE from the drop-down menu. Set the properties of the rectangle according to the following table.

tab

property

value


name

SiON

Geometry

x (μm)

2


x span (μm)

16


y  (μm)

0


y span (μm)

3


z (μm)

1.5


z span (μm)

3

Material

index

1.5


override mesh order from material database

selected


mesh order

3

Graphical rendering

override colour opacity from material database

selected


alpha

0.3

 

By setting the mesh order of one material to be larger than that of another material, the material will have lower mesh priority in the regions where objects overlap.

Select the model analysis group in the Objects Tree. Click on the zoom extent button icon_zoomextent on the side view toolbar to zoom the view ports around the completed structure.

 

Add EME Solver and monitors

Press on the arrow on the on the SIMULATION button icon_mode_simulation and select the EME SOLVER from the drop-down menu. Set the properties according to the following table.

tab

property

value

General

background index

1.465


wavelength (μm)

1.5

EME setup

x min (μm)

-8


number of cell groups

3


cell group definition

See table below


display cells

selected


y (μm)

0


y span (μm)

5.5


z (μm)

0.5


z span (μm)

7

 

Set the properties of the table in the CELL GROUP DEFINITION section of the EME setup tab of the simulation region object according to the following table.


group spans (μm)

cells

subcell method

1

3

1

none

2

10

19

CVCS

3

3

1

none

 

The number of cell groups is set to 3 for the three distinct regions of the structure, the input waveguide, the tapered region, and the output waveguide. In each cell group we can specify the span of the region the cell group will cover, and the number of cells to use in the cell group. The number of cells corresponds to the number of locations where the modes of the device will be solved.

 

In the cell group regions where the cross section of the device does not change (ex. the input/output waveguide regions), it is not necessary to use more than 1 cell, and the subcell method should be set to "none". In regions where the cross section of the structure changes, more cells are required to resolve the change in the geometry. In cases where the cross section is changing continuously over the region, the CVCS subcell method is recommended since it will give better results by reducing the staircasing effect from using a finite number of cells.

 

The DISPLAY CELLS option in the EME setup tab shows the cell boundaries in the CAD view. We can ignore the section in the EME setup tab for periodicity since this structure does not include any periodic regions.

 

Set up Ports

Expand the EME object in the Objects Tree by clicking on the triangle symbol next to the object name. Then expand the Ports group under the EME object. Edit the properties of both port_1 and port_2 according to the following table.

tab

property

value

Geometry

use full simulation span

selected


y (μm)

0


y span (μm)

5.5


z (μm)

0


z span (μm)

7

EME port

mode selection

fundamental mode

 

The selected modes will be the ones the user S-matrix will return results for.

 

Add Mesh Override

The mesh override region is used to set a finer transverse mesh over the tapered section of the high index waveguide

Select the model analysis group at the top of the Objects Tree. Press on the arrow on the on the SIMULATION button icon_mode_simulation and select MESH icon_sims_override from the drop-down menu to add a mesh override region. Set the properties of the mesh override region according to the following table.

tab

property

value

General

Set mesh multiplier

selected


y mesh multiplier

5


z mesh multiplier

5

Geometry

x (μm)

0


x span (μm)

20


y (μm)

0


y span (μm)

0.45


z (μm)

0.1


z span (μm)

0.2

 

Press the VIEW MESH button icon_viewmesh in the side toolbar to display the transverse mesh in the CAD.

 

Add Monitors

Press on the arrow on Monitors button icon_monitors and select EME INDEX from the drop-down menu. Set the properties according to the following table.

tab

property

value


name

index

Geometry

x (μm)

0

 

x span (μm)

20

 

y  (μm)

0

 

y span (μm)

6


z (um)

0.1

 

Press on the arrow on Monitors button icon_monitors and select EME PROFILE from the drop-down menu. Set the properties according to the following table.

tab

property

value


name

profile_xz

Geometry

monitor type

2D Y-normal


x (μm)

0

 

x span (μm)

20

 

y (μm)

0

 

z (um)

0


z (um)

8

 

Calculate and extract results

Run

Press on the RUN button icon_run_parallel. This will calculate the supported modes in each cell and switch the simulation from the layout to analysis mode. When the simulation finishes running, the EME Analysis window will be opened.

 

Propagate fields

In the EME Analysis window, set the SOURCE PORT setting to PORT 1. This will use the fundamental mode from PORT 1 as the source when generating profile monitor results.

Note that since we are not using periodicity we can ignore the warning in the CELL GROUP SEQUENCE section of the EME Analysis window.

Press the EME PROPAGATE button.

 

Plot refractive index and field profile

Once the propagation is complete, the EME object and the monitors in the Objects Tree will be populated with data. The Results View window (which can be opened by enabling " Result View - EME" in the top menu View > Windows) will display all the results and their corresponding dimensions/values for the selected object. Plot the refractive index by right-clicking on the "index" monitor and selecting Visualize -> index profile. The field profiles can also be visualized in the same way.

To see the values of the user S-matrix, right click on the EME object, select Visualize -> user s-matrix. Then in the visualizer under the Attributes section, double click on the "VIEW DATA" cell for the EME:user s-matrix dataset. The values of the rows and columns correspond to the S-matrix. For example, the value in row 1, column 1 corresponds to S11, and the value in row1, column 2 corresponds to S12. And abs(S(21))^2 is the transmission. Since the device behaves symmetrically, S12=S21.

 

Change taper length

To recalculate results for a longer taper, go to the CELL GROUP DEFINITION section of the EME Analysis window and change the GROUP SPAN of the second cell group region. Change this from 10um to 100um, which will correspond to the same device geometry but now with a 100um long taper region.

Press the EME PROPAGATE button to re-calculate the results, then visualize the new results for the longer taper. Note that this calculation is almost instant since it does not require any additional mode calculations.

 

Scan taper length

To scan the taper length over a range of values, the propagation sweep widget in the EME analysis window can be used.

In the EME Analysis window, select the PROPAGATION SWEEP checkbox, and set the settings of the propagation sweep according to the following table.

setting

value

parameter

group span 2

start

10

stop

200

number of points

191

Press the EME SWEEP button to run the sweep over taper lengths.

 

Plot sweep results

After the sweep is complete, press the VISUALIZE EME SWEEP button. The S-parameters will be plotted in a new visualizer.

The transmission corresponds to |S12|^2. To plot this, remove the other results from the list of attributes in the Visualizer, and set the SCALAR OPERATION of the result to Abs^2.

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