question about overlap function
I am currently studying the overlap between single-mode fiber and waveguide,
First, I ran the single-mode fiber structure (SMF28) in MODE solution to find the required mode,
Then I also ran FDE on the waveguide structure I designed to find the required mode and used the overlap script to calculate a value.
Considering about the area problem, the FDE area is 80um*80um
Then use FDTD, under the same waveguide structure, re-use FDE to calculate the same mode as the light source and let the light run into the air, put a monitor for each distance and also use the overlap script to calculate the value.
It was originally expected that the light field of the waveguide will be smaller than the light field of the fiber at the beginning. As the distance travels in the air, the larger the light field, so the value of overlap will start to decrease after reaching a peak, as shown in the figure.
But the actual result is
I don't know what went wrong
Thank you in advance!
Note: If there is any necessary information, I will attach it again