This video is taken from the FDE 100 course on Lumerical University.

## Transcript

The Overlap analysis tab allows you to calculate the overlap of a selected mode with a reference

mode or beam.

For details about the overlap calculation, see the Solver physics section.

Here's the edge coupler example from Knowledge Base to show overlap between fiber and waveguide.

The goal of this specific example is to optimize the fiber position to maximize the overlap

between the fiber mode and the waveguide mode.

To do that, first disable the fiber in the object tree

and let the FDE see only the waveguide to calculate the mode of waveguide. After finding the waveguide modes, select the mode you are interested in and copy it

to the DECK. Then switch back to layout mode, enable

the fiber and disable the waveguide to calculate the modes of the fiber.

Now open the Overlap analysis tab, and select the waveguide mode saved in the DECK to perform

the overlap calculation.

The overlap calculation shows about 84 % of overlap between the two modes.

This at first looks a bit strange considering the apparently very small mode size of the

waveguide.

But if you plot the modes in log scale, you can see that the waveguide mode is actually

very broad.

The tiny spot in the linear scale is just due to the high field intensity in the center

of the waveguide.

Shift d-card center allows you to shift the position of one mode relative to the other.

Use optimize position to find the position corresponding to the maximum overlap

You can also generate a Gaussian beam field profile for overlap calculations to calculate

the coupling efficiency between a beam and fiber mode.

Note that beam can be specified using the fully vectorial thin lens, or scalar approximation,

where scalar approximation is valid if the beam waist is much larger than the wavelength