We launch an input signal into two versions of an SMF-28 fiber. In the top branch of the topology, SBS is included in the model. In the bottom branch, it is not. All other fiber parameters are identical, including the length of 20 km. This setup allows us to determine at what fiber launch power SBS begins to have an effect.
The fiber launch power is varied from 0 to 25 dBm, and when the simulation is over, we observe the output power of each fiber as a function of launch power. These two plots are shown below: