# Dielectric Waveguide with Mode Launcher using Point Permittivity (dielectricWaveguideModeLaunchPPT.pre)

Keywords:

Mode Loading, Photonic Waveguide, Unidirectional Mode Launcher, MAL,
Guided Mode, Semiconductor

## Problem description

The dielectric waveguide consists of a single, straight silicon waveguide that is parallel to the x-axis and centered at the origin. The waveguide is surrounded by silica. Matched Absorbing Layers (MALs) are used to dampen the E and B fields near the boundary of the simulation. This is a way to dampen reflected fields from the simulation boundaries.

The fundamental guided mode is launched in the silicon waveguide in the +x direction. The fundamental mode was extracted in the “Dielectric Waveguide Mode Calculation using Point Permittivty” example. The extractModesViaOperator.py analyzer produced the eigenmode vsh5 which is loaded into this simulation.

This simulation can be performed with a VSimEM license.

## Opening the Simulation

The dielectric waveguide example can be accessed from within VSimComposer through the following steps:

• In the resulting Examples window, expand the VSim for Electromagnetics option.
• Expand the Photonics (text-based setup) option.
• Select Dielectric Waveguide with Mode Launcher using Point Permittivity (text-based setup) and press the Choose button.
• In the resulting dialog, create a New Folder if desired, and press the Save button to create a copy of this example.

Some relvant parameters should now be visible as seen in Fig. 273. You can access more of the variables, functions, and geometries by clicking View Input File in the tool bar.

Fig. 273 The Setup window for the dielectric waveguide example showing some relevant constants.

## Simulation Properties

This example contains a number of constants defined to make the simulation easily modifiable. Some relevant constants are listed below.

PERMITTVITY_WAVEGUIDE and PERMITTVITY_BACKGROUND: Relative permittivities of silicon and silica. These constants are used in multiple parameters and in the accompanying Python file for solving the waveguide modes.

LENGTH_UNIT: The constant factor by which VSim will scale all simulation lengths.

WAVELENGTH_VAC: Wavelength of the input signal. This wavelength is also used for the calculation of the fundamental guided mode of the device.

NWAVELENGTH_MAL: Approximate number of wavelengths that can fit in a MAL region. The thickness of the MAL regions in this example are measured in wavelengths.

In photonics simulations, Matched Absorbing Layers (MALs) are the most stable boundary conditions for preventing reflections. The eigen mode is imported form a vsh5 file. This can be seen in the input file.

## Running the simulation

After performing the above actions, continue as follows:

• Proceed to the Run Window by pressing the Run button in the left column. You will be asked to Save. Click Save.
• To run the file, click on the Run button in the upper left corner of the right pane. You will see the output of the run in the right pane. The run has completed when you see the output, “Engine completed successfully.” The result is shown in Fig. 274.

Fig. 274 The output after a successful run.

## Visualizing the Results

Then proceed to the Visualize window by pressing the Visualize button in the left column.

A useful visualization of the dielectric waveguide would be to view the y-component of the E field to qualitatively see the mode propagate down the waveguide.

• Near the top left corner of the window, make sure Data View is set to Data Overview.
• Expand Scalar Data, expand E, and select E_y
• In the controls below the variables frame, select Clip All Plots.
• In the top of the screen, press the button that’s titled Colors, check the Fix Minimum and Fix Maximum buttons, and input {-.03, .03} for the min and max, respectively.

Fig. 275 shows an example of what one should expect if one has run the simulation for enough cycles.

Fig. 275 Visualization of the B field’s Z component

## Further Experiments

One can experiment by changing constants or introducing a different signal to drive the waveguide.