In this subsection, we will use the spot size converter example to demonstrate the basic
EME simulation workflow.
The spot size converter is used for coupling light from a silicon waveguide into an optical
Directly coupling the light from an Si waveguide to an optical fiber is not efficient due to
the large difference in the modal area.
By using the spot size converter, you can expand the modal area of the output waveguide
to match that of the fiber, resulting in a higher coupling efficiency.
The device consists of an Si waveguide and a taper with another lower-index (n=1.5) SiON
waveguide surrounding the taper.
The whole structure is embedded in a glass cladding.
When the fundamental mode of the small, high index Silicon waveguide propagates through
the taper, the modal fields slowly leak out of the Silicon and into the larger SiON waveguide.
If the taper is gradual enough, the fields will smoothly transition to the fundamental
mode of the larger waveguide, with minimal back reflection.
The images here show the amplitude of electric field from monitors placed at various locations
in the propagation direction.
You can clearly see that the spot size becomes larger as it propagates.
The design goal of this spot size converter is to find the taper length that maximizes
the power conversion efficiency from the fundamental mode of the Si waveguide to the fundamental
TE mode of the SiON waveguide.
This can be tracked by monitoring the S-parameter results.
The conversion efficiency as a function of taper length can be calculated using the Propagation
Field profiles at various cross sections of the device can also be calculated and analyzed.