Mode Source in Ansys Lumerical FDTD — Lesson 5

Introduction & Applications

A mode source is commonly used for injecting the eigenmode supported by the cross-section of a waveguide. In this video lesson, you will learn about various settings and options in the Mode source.

Alternate video link.

Note: In many instances, it is actually more convenient to use Port objects rather than Mode sources.

Setup & Tips

In this video lesson, we will demonstrate how to set up the Modes source using a directional coupler waveguide example. You can download the simulation file and follow the steps shown below.

Alternate video link.

Add a mode source:

1. Click the arrow next to the “Sources” button in the main toolbar to open the drop-down menu

2. Click the “Mode” option which is the fifth in the list in the drop-down menu

Open the source edit window:

1. Make sure that the source is selected in the Objects Tree window. If not, click the source in the Objects Tree to select it

2. Click the Edit button in the edit toolbar on the left (pencil icon), or use the 'E' keyboard shortcut to open the edit window

Set properties in the source edit window:

1. General tab:

  • Injection axis: x-axis
  • Injection direction: Forward
  • Mode selection: fundamental mode
  • Multifrequency mode calculation: selected
  • Bent waveguide: selected
  • Bend radius: 6 microns
  • Bend orientation: 0 degrees
  • Theta: -28.3576 degrees

2. Geometry tab:

  • x: -3.068 microns
  • y: 1.536 microns
  • y span: 2.5 microns
  • z: 0.11 microns
  • z span: 2 microns

Check the field polarization of the selected mode:

1. Check the calculated mode profile from the General tab of the edit window by clicking on the “Visualize Data” button

  • In the attributes table of the visualizer window, you can select the E or H field dataset to plot
  • Under the “vector operation” column you can select the field component to plot

2. Click the “OK” button at the bottom right of the source edit window to accept the settings and check the dipole orientation in the CAD view

Use “user select” mode selection to get more information about the mode:

1. In the General tab of the source edit window, set the “mode selection” to “user select.”

2. Click the “Select Mode” button and click “OK” in the dialogue box asking whether you would like to select a new mode.

3. Click the “Calculate Modes” button

4. After the modes have been calculated, additional information including effective index, loss, and polarization fraction can be viewed from the Mode List table.

5. Click “Cancel” and change the “mode selection” back to the fundamental mode.

6. Click “OK” to accept the settings.

Check the source in the CAD view:

  • The angled white line in the XY view indicates the angle of injection.
  • The line which is not angled shows the source injection plane.

Run the simulation and plot the transmission through the straight waveguide.

1. Click the “Run” menu from the simulation toolbar.

2. After the simulation has been completed, right-click the power monitor named “monitor” and select Visualize ->T from the context menu