Optical and Acoustic Phonons?

[narra]

Hello. I'm trying to develop a clear image of Raman and Brillouin scattering in my head, but one uncertainty I have is what are the physical differences between the generation, propagation, and any other characteristics between Optical and Acoustic Phonons in silica fibre (such as, what is it about optical phonons that allows Stimulated Raman Scattering to have bi-directional photon scattering; but not with SBS's acoustic phonons?) . So far I have been unable to find a good source of information which describes these best, can anyone help me out here?

Many thanks.

 

[BigJDubs]

Optical phonons are higher energy excitations of the lattice, generated by photons and involve the displacement of atoms. They have very short lifetimes and can be scattered in both directions due to their wave nature. Acoustic phonons are lower energy excitations of the lattice, generated by sound waves and involve the compression and expansion of the material. They have longer lifetimes and can only be scattered in one direction due to their particle-like nature. In silica fibre, stimulated Raman scattering is caused by optical phonons, while stimulated Brillouin scattering is caused by acoustic phonons.