Wave propagation, Brillouin scattering

Electro-optic and magneto-optic phenomena contain terms of nonlinear optics effects (see Eqs. (4.32), (4.33), (4.36), and (4.39)). On the other hand, acousto-optic effects which arise from a periodical density fluctuation of the medium, analogous to the Brillouin scattering phenomenon, do not contain terms of nonlinear optics, as a general rule.5) The perturbation of light propagation by sonic waves differs from that induced by electric and magnetic fields. As the electric susceptibility, xe, is a function of the density of the medium, it will be influenced by the periodical density fluctuation induced in a medium by sound waves. 

Brillouin scattering occurs as a result of an interaction between the propagating optical signal and thermally acoustic waves present in the silica fibre giving rise to frequency-shifted components, similar to a Doppler effect. The acoustic velocity is directly related to the medium density and depends on both temperature and strain. As a result, the so-called Brillouin frequency shift carries information about the local temperature and strain of the fibre. Furthermore, Briflouin-based sensing techniques rely on the measurement of a frequency as opposed to Raman-based techniques that are intensity based. 

Brillouin scattering is the inelastic scattering of light on propagating elementary excitations. For acoustic phonons considered here, the relation between the measured frequency shift of the scattered light Av, the sound velocity Y, (he sound frequency/, and the sound wave length A is... 

According to Landau and Placzdc (68,69), the first term represents local entropy fluctuations which do not propagate in normal liquids and are the source of the central (unshifted) component of the scattered light, while the second term represents the isentropic pressure fluctuations (Le., sound waves) which are the source of the Brillouin doublet. 

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