Abstract
Brillouin light scattering describes the diffraction of light waves by acoustic phonons, originating from random thermal fluctuations inside a transparent body, or by coherent acoustic waves, generated by a transducer or from the interference of two frequency-detuned optical waves. In experiments with optical fibers, it is generally found that the spontaneous Brillouin spectrum has the same frequency dependence as the coherent Brillouin gain. We examine the origin of this similarity between apparently different physical situations. We specifically solve the elastodynamic equation, giving displacements inside the body, under a stochastic Langevin excitation and in response to a coherent optical force. It is emphasized that phase matching is responsible for temporal and spatial frequency-domain filtering of the excitation, leading in either case to the excitation of a Lorentzian frequency response solely determined by elastic loss.
Highlights
Brillouin light scattering (BLS) [1] and stimulated Brillouin scattering (SBS) [2] are extensively studied in optical fibers and guided optics for fundamentals [3,4,5,6] and applications [7,8,9]
This model has been enriched because of new observations involving the polarization of elastic waves in solids: the whole family of normal modes of an optical fiber are involved in guided acoustic wave Brillouin scattering (GAWBS) [14,15,16]; hybrid phonons having coupled longitudinal and shear polarization have been observed in photonic crystal fibers [17]; anisotropic guided acousto-optical diffraction has been observed with polarized optical waves [18]; and all-optical generation of surface acoustic waves have been observed in microwires, tapered optical fibers and optical waveguides [19,20,21]
The usual model of stimulated Brillouin scattering is based on the works of Kroll [11], Tang [12], and Boyd et al [10,13], among others
Summary
Brillouin light scattering (BLS) [1] and stimulated Brillouin scattering (SBS) [2] are extensively studied in optical fibers and guided optics for fundamentals [3,4,5,6] and applications [7,8,9]. Brillouin gain and the exponential growth of SBS [11,12], with noise initiation of SBS described by random fluctuations generating thermal acoustic phonons [13] This model has been enriched because of new observations involving the polarization of elastic waves in solids: the whole family of normal modes of an optical fiber are involved in guided acoustic wave Brillouin scattering (GAWBS) [14,15,16]; hybrid phonons having coupled longitudinal and shear polarization have been observed in photonic crystal fibers [17]; anisotropic guided acousto-optical diffraction has been observed with polarized optical waves [18]; and all-optical generation of surface acoustic waves have been observed in microwires, tapered optical fibers and optical waveguides [19,20,21]. The vector polarization of elastic waves had long before been considered for the description of BLS and SBS in solids and high viscosity liquids interrogated with freely propagating light [22,23,24]
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