CHARGE is a carrier transportation solver, which does not have optical waveform included. It has electric static fields by solving Possion equation. Optical signal waveform should be viewed in optical sovers, such as FDTD, INTERCONNECT etc.
When ssac is chosen, the equations are linear: https://optics.ansys.com/hc/en-us/articles/360034917693-CHARGE-solver-introduction
So it only needs a small variation for the perturbation. https://optics.ansys.com/hc/en-us/articles/360034918833-Boundary-Conditions-in-CHARGE-Simulation-Object
You may choose "apply ac small signal" and by default it will automatically use the perturbation amplitude of 1mV.
The "small signal as optical generation" is for optical generation: