Non-linear Scattering represented mainly by Stimulated Brillouin Scattering (SBS) or Stimulated Raman Scattering (SRS) may be regarded as the modulation of light through thermal molecular vibrations within the fiber. SRS is similar to SBS except that high frequency optical phonon rather than acoustic phonon is generated in scattering processes. Phonon – collective excitation in a periodic arrangement of atoms or molecules in solid.
High irradiance/intensity of light beam is the main reason of non-linear scattering to occur.