Stimulated Brillouin scattering (SBS) is a nonlinear phenomenon which happens when optical power, larger than a defined threshold value, is loaded to the fibre. SBS threshold depends on the width of the laser spectrum and its output power. SBS leads to scattering of large part of the optical power transmitted through the optical fibre and this is the reason for reducing SNR at the output of the optical channel. In general when using the sources with narrow spectral line and external modulation, the SBS threshold varies from 5 to 10 mW. If using lasers with direct modulation then this power varies between 20 – 30 mW.
The stimulated Raman scattering (SRS) is similar to SBS. SRS threshold power is much bigger than SBS and reaches values about 1 W. The final effect of SRS is transferring signal power from the channels with low wavelength to channels with large wavelength. That power transfer has a positive effect in some cases – for example Raman amplifier.