Optical and Acoustic Phonons?

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.

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