Dynamics of absolute Brillouin scattering in the presence of ion wave harmonics

The effect of ion acoustic harmonic production on stimulated Brillouin scattering is studied by solving numerically the coupled wave equations. Time and space dependent model includes the detuning and linear damping of the ion harmonic cascade and the mismatch due to density gradient. Several new interesting aspects in the behaviour of stimulated Brillouin scattering are discovered.     

光学材料破坏的超声模型

给出了光学玻璃材料在强泵浦光作用下由光弹耦合激发超声波的瞬态受激布里渊散射理论模型，通过几个合理的近似利用数值计算给出该过程激发的超声波强度对泵浦光强度以及脉宽的依赖关系，计算表明光泵浦光强度超过一定阈值后该超声波能引起光学材料的破坏。     

Brillouin gain analysis for fibers with different refractive indices

We studied an anomalous result in the measured stimulated Brillouin scattering threshold for single-mode optical fibers with different refractive index profiles and effective areas. It was found that, in addition to the optical effective area, the refractive-index-dependent interaction between the optical and the acoustic fields must be considered in order to account for the differences in the stimulated Brillouin scattering threshold between fiber types. A detailed analytical formalism describing the acousto-optic interaction was derived to accommodate the influence of the index profile design on Brillouin scattering.     

激光等离子体相互作用中的受激布里渊散射

本文的研究表明,在亚毫微秒脉宽的高强度激光等离子体相互作用中,如入射激光包含有明显的预脉冲,则能从激光的后向反射率的剧增、后向反激光光谱的红移并加宽、离子总动能与快离子发射的明显减少以及大角度散射激光的增强等方面,自洽地证实受激布里渊散射的发生。 关键词：     

Soliton generation via stimulated Brillouin self-scattering under slow light conditions

Acousto-optic soliton generation via stimulated Brillouin self-scattering is predicted for light propagating at the speed of sound under electromagnetically induced transparency conditions. As in stimulated Raman self-scattering, the frequency of the electromagnetic component is gradually Stokes shifted as its intensity increases; the acoustic component has no carrier frequency. This phenomenon is explained by the possibility of forward stimulated Brillouin scattering, which is forbidden in nondispersive media. In contrast to stimulated Raman self-scattering, the Stokes shift of the electromagnetic component approaches a constant limit after the pulse has propagated to a certain distance. It is shown that the predicted soliton generation does not involve any threshold condition and can occur at extremely low input pulse intensities.     

Thomson-scattering study of the subharmonic decay of ion-acoustic waves driven by the Brillouin instability

Thomson scattering (TS) has been used to investigate the two-ion decay instability of ion acoustic waves generated by stimulated Brillouin scattering in an underdense CH plasma. Two complementary TS diagnostics, spectrally and spatially resolved, demonstrate the occurrence of the subharmonic decay of the primary ion acoustic wave into two secondary waves. The study of the laser intensity dependence shows that the secondary ion acoustic waves are correlated with the SBS reflectivity saturation, at a level of a few percent.     

Harmonic ion acoustic waves in stimulated Brillouin scattering

Ion acoustic harmonic generation is a heavy damping mechanism of stimulated Brillouin scattering. The theory of first harmonic ion wave production is extended to frequencies four times the fundamental. Besides, the corresponding set of 6 differential equations is solved numerically for the steady state, and spatial solutions of the ion wave amplitudes are obtained. By varying typical parameters of laser plasma interaction experiments some characteristics properties are gained which might be of use for comparison with experiments.     

激光辐照等离子体中的受激布里渊侧向散射

本文在预先已形成较大尺寸的稀薄等离子体的条件下,研究了亚毫微秒脉宽的高强度激光与等离子体相互作用中的受激布里渊散射问题。通过细致地测量后向散射激光的能量、光谱以及侧向散射激光的角分布,特别是它在空间取向上的各向异性,证实了受激布里渊后向散射与侧向散射不稳定性的存在。 关键词：     

Tomographic Description of Stimulated Brillouin Scattering of Light

The process of stimulated Brillouin scattering is described by the twodimensional oscillator model. Photons of the Stokes mode are described by one mode of the oscillator, and acoustic phonons are described by the other mode. The interaction of photons and acoustic phonons is assumed to be quadratic in the creation and annihilation operators of photons and phonons. The laser is considered as a classical light source and its depletion is neglected. New timedependent integrals of motion and the photon–phonon probability distribution function are found. The mean Stokes photon number and the mean acoustic phonon number are expressed as functions of the medium parameters (initial dispersions) and the interaction parameter (coupling constant). The classical propagator for stimulated Brillouin scattering and tomograms of the photon and phonon states are investigated within the framework of the symplectictomography scheme.     

Stimulated Brillouin scattering frequency-domain analysis in a single-mode optical fiber for distributed sensing

A numerical and experimental analysis of the stimulated Brillouin scattering in a single-mode optical fiber for distributed sensing applications is carried out in the frequency domain. The theoretical model describing the Brillouin interaction is solved by taking into account the temporal dynamics of the acoustic wave that is involved. The simulations and the experimental results reveal the role played by the ac component of the acoustic wave, which is responsible for significant changes of the small-signal stimulated Brillouin scattering transfer function that occur when the modulation frequency rises above the natural Brillouin gain spectrum linewidth. One should take these effects into account to perform accurate signal processing of frequency-domain signals in high-resolution distributed sensing applications.     

Inelastic Light Scattering Near the Ferroelectric Phase-Transition Point in Bismuth Vanadate Crystals

The Raman and Brillouin spectra in bismuth vanadate crystals near the ferroelectric phase-transition point are studied. The intensity maximum corresponding to the soft mode in light scattering spectra is found. This mode is responsible for the instability of the bismuth vanadate crystal lattice in the vicinity of the ferroelectric phase-transition point. Strong interaction of the soft optical mode with an acoustic mode of the corresponding symmetry is revealed by an analysis of the Raman and Brillouin spectra observed. The theory of light scattering by coupled lattice modes based on the model of two strongly coupled oscillators is developed. The introduction of an additional oscillator strongly interacting with the fundamental soft mode is shown to result in a temperature shift of the structural phase transition to higher temperatures. The results obtained confirm the possibility of changing the phase-transition temperature by modification of the vibrational spectrum through introduction of additional degrees of freedom or comminution of the macroscopic sample with formation of the ordered superdispersed structure (globular crystal). These results are general and can be subsequently used to increase the phase-transition temperature in ferroelastics, ferroelectrics, and superconductors.     

Characteristic properties of ion acoustic harmonics produced by stimulated brillouin scattering

The theory of ion acoustic harmonic production with stimulated Brillouin scattering is extended up to the third harmonic. Stationary spatial solutions are given for normalized ion acoustic wave amplitudes and characteristic properties derived, which may serve experimentalists for future detailed investigations.     

光纤中基于双布里渊增益线的受激布里渊散射快光

为了解决受激布里渊散射快光在高吸收区产生损耗的问题,通过分析普通单模光纤中双线泵浦产生的双布里渊增益线特性及在增益峰间实现脉冲的超光速传输理论,利用有限元法数值模拟了双布里渊增益线处受激布里渊散射引起的快光特性。结果表明,当频率分离因子大于0.596时,可以观察到双增益峰;当频率分离因子在1~5.25范围内时,两个泵浦波产生的双增益峰之间可以明显地产生快光;当频率分离因子为1.75时,在双布里渊增益线之间的最大时间提前可达25 ps。当频率分离因子为2.42时,三阶色散所对应的归一化色散长度为无穷大,三阶色散可以得到完全补偿;当频率分离因子大于2.464时,脉冲展宽因子趋近于1,可以实现无畸变传输,但时间提前量小于13.52 ps。本文的研究结论对于在布里渊增益区实现快光具有一定的理论意义,并对设计基于受激布里渊散射快光器件具有理论指导作用。     

Resonant Brillouin scattering near the a exciton in CdS

We have studied resonant Brillouin scattering in CdS by tuning a narrow-band cw dye laser through the A-exciton transition at helium temperatures. We observed besides the resonance enhancement a pronounced asymmetry of the Stokes and anti-Stokes shift and a dramatic increase of the Brillouin shifts as a function of laser frequency. These observations are consistent with a model of exciton-polariton scattering by longitudinal acoustic phonons. For incident frequencies higher than the longitudinal exciton frequency additional features are found which cannot be explained by the two-branch model for exciton-polaritons.     

Brillouin scattering studies of rare gas solids

Abstract

The scattering of light by elementar excitations in the matter is results in two phenomena, discriminated by the zero wavevector frequency of the excitation: if this frequency is zero, one deals with Brillouin scattering, and with Raman scattering in the other case. Brillouin scattering results from the interaction of light with thermal excitations (acoustic phonons in a crystal) of a material, or, from a classical point of view, with density waves. Contrary to Raman scattering, the selection rules allow always the observation of at least one mode. It is a powerful technic in the study of rare gases under pressure: at ambient temperature, rare gases crystallize in the face centered cubic structure (except helium which structure was recently found to be hexagonal) and are therefore Raman and infrared inactive.

Experimental results will be reviewed on rare gases and rare gas mixtures in the fluid phase, like He-Ne and He-H₂. These results will be discussed in relation with recent measurements of the frequency of global oscillations of Jupiter.     

Modulation instability of the stimulated Brillouin scattering in fibers in a presence of acoustic diffraction

An influence of acoustic wave diffraction on the modulation instability of the backward stimulated Brillouin scattering in fibers is numerically investigated, when a small feedback for the Stokes wave is present. The cases of acoustic waveguide and anti-waveguide fibers are considered. It is shown that a presence of acoustic diffraction may expand the region of occurrence of modulation instability and may lead to 2–5 times increasing the peak amplitude of output Stokes pulses. In this case, a stable train of short powerful Stokes pulses of nanosecond duration is produced at the output.     

Stimulated Raman and Brillouin Scattering of a Circularly Polarized Gaussian EM-Beam in a Magnetoplasma

In the present paper we have studied the nonlinear interaction of a Gaussian EM-beam with the electron and ion waves in a collisionless magnetoplasma, and the resulting stimulated Raman and Brillouin scattering. The EM-beam is assumed to be propagating along the static magnetic field as a right/left handed circularly polarized mode. The spatial amplification factors and the temporal growth rates are found to be significantly modified in contrast to a uniform pump.     

Brillouin scattering study on elastic and piezoelectric properties of laser irradiated ZnO single crystals

Brillouin light scattering technique can be successfully used to determine the whole set of elastic and piezoelectric constants of a ZnO single crystal irradiated by different laser energy densities,into a micron range (radiation layer thickness).It is found that the scattering intensity,the linewidth and the Brillouin scattering shift of acoustic phonons are all strongly dependent on laser energy density.Based on the sound propagation equations and these results,the directional dependences of the compressional and shear moduli of the irradiated ZnO sample in the (001) plane are investigated.It is found that under an appropriate laser condition,248 nm KrF excimer laser irradiation can significantly improve the surface quality and increase the elastic properties of ZnO single crystal.This procedure has potential applications in the fabrication of ZnO-based surface acoustic wave and optic-electronic devices.     

Soft modes below the incommensurate transition in K2SeO4

A coordinated series of Raman and Brillouin scattering measurements were carried out in K₂SeO₄ with emphasis on temperatures near and below the incommensurate structural phase transition (T_i=129 K). Below the lock-in phase transition (T_c=99 K) Raman scattering from both the amplitude mode and the phase mode analog was observed. However, only the amplitude mode could be followed into the incommensurate phase. Details of the amplitude mode frequency and damping are reported for temperatures considerably closer to T_i than previously possible. Our results do not confirm the diverging linewidth extrapolated from earlier Raman work. We also report here the first Brillouin scattering results in K₂SeO₄. Although substancial interaction effects between the acoustic phonons and the soft amplitude mode are evident near T_i, the coupled mode lineshapes can be well described without the introduction of a relaxing self energy for the soft mode. Finally, limits on the characteristics of the phason are drawn from our failure to observe it directly in the incommensurate phase.     

Enhanced forward scattering in the case of two crossed laser beams interacting with a plasma

The nonlinear enhancement of large-angle forward scattering of two identical laser beams propagating in a preformed plasma has been observed experimentally. The spectral analysis of the forward-scattered light shows two components, one which is unshifted with respect to the initial laser light frequency, and the other which is redshifted by a few angstroms. The redshifted component is found to be strongly enhanced in the case of crossed beam interaction in comparison with that of one beam illumination. Two-dimensional numerical simulations show that this enhancement is due to large-angle forward stimulated Brillouin scattering in which each beam serves as seed for the forward scattering of the other.