Stimulated Brillouin Scattering Reflectivity Research Articles

Modeling of a stimulated Brillouin scattering experiment with statistical distribution of speckles

The results of experimental studies of stimulated Brillouin scattering (SBS) from a laser beam smoothed with a random phase plate (RPP) in a preformed expanding plasma are compared with the statistical theory of SBS from an inhomogeneous plasma. Many observed features can be explained assuming that SBS occurs in randomly distributed speckles. This hypothesis naturally combines observed pump depletion effects within a speckle, with relatively minor SBS reflectivity of the whole interaction beam.

Effects of plasma long-wavelength hydrodynamical fluctuations on stimulated Brillouin scattering

Two important effects of long wavelength velocity and density perturbations on stimulated Brillouin scattering (SBS) have been identified: (i) nonlinear interaction of the SBS-produced ion acoustic wave with velocity perturbations provides detuning of the SBS interaction, formation of ion acoustic wave satellites and its additional damping; (ii) harmonic generation of long wavelength density perturbations enhances the spectral density of ion acoustic fluctuations over a wide range of wavevectors, including SBS resonant sound waves. The first effect reduces the magnitude of the SBS gain, and the second one enhances the nonthermal noise level for SBS. Long wavelength ion acoustic perturbations reduce SBS reflectivity more dramatically than short wavelength harmonics produced by the SBS driven nonlinear sound wave.

Beam smoothing effects on the stimulated Brillouin scattering (SBS) instability in Nova exploding foil plasmas

The effects of beam smoothing on the stimulated Brillouin and Raman backscattering instabilities (SBBS and SRBS) in a long scale length exploding foil plasma are experimentally investigated using the Nova laser. We observe strongly time-dependent and blueshifted SBS emission and broadband SRS emission in time-resolved spectral measurements of the backscattered light. The results of this paper focus on the SBS scattering, where we find that random phase plate spatial beam smoothing leads to a decrease in the total SBS reflectivity. Adding temporal smoothing to this spatial smoothing leads to an even greater decrease in the total reflectivity, as well as changes in the spectral nature of the backscattered light. Beam smoothing induced changes to the instabilities are limited to plasmas with a peak density below about 0.2ncr; beam smoothing causes little change in the backscattered emission for higher densities. We describe the measurements and discuss models that may explain certain aspects of the results.

Laser hot spots and the breakdown of linear instability theory with application to stimulated Brillouin scattering.

Convective instabilities in the strongly damped regime are shown to exhibit essential nonlinear behavior due to laser hot spots when the average laser intensity [l angle][ital I][r angle] approaches a critical threshold value [ital I][sub [ital c]]. The onset of this nonlinear regime is formally signaled by the divergence of the average convective amplification [l angle][ital A][r angle]as [l angle][ital I][r angle][r arrow][ital I][sub [ital c]]. An independent hot spot model of random phase plate optics predicts that [l angle][ital A][r angle][similar to]1/([ital I][sub [ital c]][minus][l angle][ital I][r angle])[sup 2]. A saturated hot spot model of nonlinear stimulated Brillouin scattering (SBS) predicts a rapid turn on and saturation of SBS reflectivity with laser intensity and optic [ital f] number.

Stimulated Brillouin scattering with a 1 ps laser pulse in a preformed underdense plasma.

An experimental study of stimulated Brillouin scattering (SBS) with a 1 ps laser pulse in a large underdense preformed plasma is presented. It is shown that the SBS reflectivity increases from ${10}^{\mathrm{\ensuremath{-}}4}$ to ${10}^{\mathrm{\ensuremath{-}}1}$ as the intensity of the interaction beam rises from 5\ifmmode\times\else\texttimes\fi{}${10}^{14}$ to ${10}^{16}$ W/${\mathrm{cm}}^{2}$; for intensities between ${10}^{16}$ and ${10}^{17}$ W/${\mathrm{cm}}^{2}$, a saturation level at 10% is experimentally established. This experimental result is compared with theoretical estimates of time-integrated SBS reflectivity in the transient phase of the so-called modified decay regime. The latter results are seen to be consistent with the experimental data provided that there exists a nonthermal noise in the plasma depending on the laser intensity.

SBS reflection of broad band XeCl excimer laser radiation: comparison of suitable liquids

Stimulated Brillouin scattering (SBS) may be useful to improve the beam quality of excimer laser radiation by phase conjugation. The SBS reflectivity of 26 organic liquids using a broad band XeCl laser has been investigated, while narrow bandwidth radiation has been applied up to now. Beam diameter and focusing conditions are important to achieve sufficient coherence in the scattering region. Maximum reflectivities of about 30% were observed. The influence of molecular size and purity was studied.

Noise initiation of stimulated Brillouin scattering.

We describe a theoretical model that shows how stimulated Brillouin scattering (SBS) is initiated by thermally excited acoustic waves distributed within a Brillouin-active medium. This model predicts how the SBS reflectivity, Stokes linewidth, and fluctuations in Stokes intensity depend upon the laser intensity and upon the physical properties of the SBS medium. This model also leads to the prediction that the value of the single-pass gain (i.e., G=gIL) at the threshold for SBS is not a universal number, but depends upon the laser frequency and on the properties of the SBS medium. For typical organic liquids at room temperature, G is in the range 20--25.

Stimulated Brillouin scattering and phase conjugation of multiline hydrogen fluoride laser radiation

Stimulated Brillouin scattering (SBS) of pulsed multiline hydrogen fluoride laser radiation has been demonstrated from a single focal volume. The SBS reflection efficiency was consistent with the postulate that the individual lines are scattered independently. Phase-conjugate fidelity of the multiline SBS was demonstrated by correction of an aberrated beam. Experiments comparing SBS efficiency and phase-conjugate fidelity for common and spectrally dispersed focal volumes produced similar results.

Ion heating as a limiting process in CO2 laser induced Brillouin scattering

Two complementary models invoking ion heating are presented which, together with pump depletion, quantitatively account for the principal observations (reflectivity, spectral shape, saturation) of stimulated Brillouin scattering (SBS) in a CO2 laser/plasma interaction experiment. A simple energy balance model, which incorporates ion heating and includes a consistent calculation of ion wave damping, is applicable for steady state convective growth of the ion waves (long interaction times). A more refined model, which includes a calculation of the trapped ion fraction (along with heating), is applicable for SBS in a transient regime. Strong ion wave damping has been consistently observed in both cases in our high intensity CO2 laser/plasma scattering experiments. A variety of measurements have been made to investigate the basic features of SBS and associated density fluctuations for both long pulse (40 ns) and short pulse (2 ns) CO2 laser irradiation. Measured parameters reported here include plasma density, temperature, density scalelength, SBS reflectivity, and spectral characteristics. Together with previous measurements of density fluctuations (amplitude, wavenumber spectra, spatial and temporal information), the observed behavior confirms that Brillouin scattering is dominated by strong ion trapping, heating and ion wave damping for the laser/plasma parameters prevailing in these experiments.

Experimental investigation of phase conjugation in stimulated Brillouin scattering of beams with mild cylindrical aberration

We monitored transverse intensity profiles of Nd:YAG laser beams that had undergone stimulated Brillouin scattering (SBS) after being mildly aberrated by a cylindrical lens (CL). Poor phase conjugation was confirmed by the presence of this aberration in the SBS reflected beam after it hadretraced the incident beam's path through the CL. The results are consistent with the occurrence, during this SBS, of spatial filtering near line foci typical of cylindrical waberration. When two such beams with overlapping focal regions undergo SBS together, the distortion of one beam after SBS reflection is insensitive to that of the other.

Beam combination by stimulated Brillouin scattering

Phase conjugation of two beams by stimulated Brillouin scattering (SBS) was studied with a computer model and investigated experimentally. The relative phase of the return beams was found to vary randomly if the pump beams were well separated in the SBS cell but remained fixed and equal to the inverse of the pump relative phase if the beams overlapped. Backward Stokes injection was used to lower the threshold and increase the reflectivity of SBS.