Spot size converters are important for photonic integrated circuits where a coupling is done between two different waveguide sizes or shapes. The most obvious place to find a spot size converter is between a waveguide of a PIC and a fiber coupling lens.
Spot size converters feature tapered layers on top of a ridge waveguide for instance, to gradually change the mode while preventing coupling loss.
The below RSoft example shows how an optical path is converted from a more narrow path (such as a waveguide) to a wider path (which could be for a fiber).
While the following simulation is designed in Silicon, similar structures are realized in other platforms such as InP or GaAs/AlGaAs.
RSoft Beamprop simulation, demonstrating conversion between two mode sizes. Optical power loss is calculated in the simulation for the structure.
This is the 3D structure. Notice the red section present carries the more narrower optical path and this section is tapered to a wider path.
The material layers are shown: