In this example, we will consider how Mode Expansion Monitor can be used for analyzing the fraction of power transmitted into the fundamental mode in FDTD, particularly for calculating the power reflection and transmission of a waveguide facet. Many calculations involving waveguide modes can also be accomplished with MODE.
We will consider a high index contrast waveguide on Silicon-on-insulator (SOI) at a wavelength of 1.55 microns. It is instructive to use a high index contrast waveguide since it is one of the cases where many assumptions commonly used by mode solvers, such as scalar fields or effective index models, break down. We will consider a waveguide that supports several modes. The waveguide is 200 nm in height and 800 nm wide.
The file waveguide_facet.fsp has the structure drawn and the fundamental mode is already selected in the mode source. The mode profile can be seen in the Edit Mode Source window by clicking on the "Visualize Data" button, as shown below. The mode expansion monitor must be placed over the cross section of the waveguide/fiber that supports the modal fields as shown in the simulation file. Please refer to the page Mode Expansion Monitors for more information.
After the simulation has finished, a set of expansion results can be obtained using the Mode Expansion monitor. Right click on the Mode expansion monitor object and visualize the expansion data.
The collected results include the forward transmission of the selected mode, the reflected power of the mode selected, and the net power of the selected power. To show a summary of the results run the script facet_calculation.lsf. We find the following results:
- T total ~0.509
- T forward ~0
- T backward ~0.247
- T net ~0.247
- Transmitted power ~0.475
- Reflected power ~0.509
- Unaccounted power at grazing incidence ~0.0167
- Total power in the system ~1.001