First observed by French mathematical physicist, Marcel Louis Brillouin. Brillouin scattering is a form of nonlinear scattering which occurs when intensely modulated light, such as that from a laser, is launched into a crystalline substance, such as a fibre optic line. If this light is launched with sufficient energy (usually above the lines saturation point) it can cause some of it to be converted into acoustic waves, known as phonons. These phonons interfere with the incoming signal and cause them to be reflected away from their initial direction of propagation. This reflected light is usually red-shifted (by about 11GHz for a 1550nm signal).

Brillouin scattering can cause problems in fibe optic systems as some of the refracted signal can actually be reflected back to its source, creating heating problems for the signal generating laser, reducing its efficiency