ABSTRACT
Until this point we have focused on Raman scattering when describing scattering of light as it propagates through a material. However, in the following we direct attention to Brillouin scattering. Both scattering mechanisms deal with interaction between phonons/sound waves and photons/electrical fields. Where Raman scattering is an interaction between optical phonons and electrical fields, Brillouin scattering is an interaction between acoustical phonons and electrical fields. In a more detailed description Brillouin scattering is the diffraction of optical waves on sound waves created by electrostriction. More specifically, due to electrostriction the forward propagating optical wave creates a Bragg grating that propagates in the same direction as the launched optical wave. This causes a back reflection of an optical wave at the frequency of the launched wave only corrected by the doppler shift due to the forward propagating Bragg grating. The Bragg grating, i.e. the
sound wave, is caused by vibrations of the crystal (or glass) lattice. When these vibrations are associated with optical phonons i.e. high frequency phonons, the effect is called Raman scattering, whereas acoustical phonons i.e. low frequency phonons, are associated with Brillouin scattering.
