Observation of inhomogeneous spectral broadening of stimulated brillouin scattering in an optical fiber

We report experimental results which show that the stimulated Brillouin scattering (SBS) spectrum in an optical fiber is inhomogeneous, exhibiting spectral broadening and hole burning under cw monochromatic laser excitation. This phenomenon arises from the waveguide interaction of the pump and Stokes signals and is a fundamental property of SBS in waveguiding systems due to their ability to confine a fan of radiation wave vector directions.     

Modification of spatial and temporal gains of stimulated brillouin and raman scattering by polarization smoothing

Significant reductions of stimulated Brillouin (SBS) and Raman (SRS) scattering are measured by adding polarization smoothing (PS) to a random phase plate smoothed beam. The associated plasma waves, as measured by Thomson scattering, are spatially and temporally modified and reveal that the interplay between SBS and SRS has to be taken into account to understand the effect of the smoothing. The results also support the numerical simulations [S. Huller et al., Phys. Plasmas 5, 2706 (1998); R. Berger et al., Phys. Plasmas 6, 1043 (1999)] predicting a reduction of self-focusing with PS, resulting in a decrease of the maximum laser intensity and consequently of SBS and SRS gains.     

Direct measurements of an increased threshold for stimulated brillouin scattering with polarization smoothing in ignition hohlraum plasmas

We demonstrate a significant reduction of stimulated Brillouin scattering by polarization smoothing in large-scale high-temperature hohlraum plasma conditions where filamentation is measured to be negligible. The stimulated Brillouin scattering experimental threshold (defined as the intensity at which 5% of the incident light is backscattered) is measured to increase by a factor of 1.7+/-0.2 when polarization smoothing is applied. An analytical model relevant to inertial confinement fusion plasma conditions shows that the measured reduction in backscatter with polarization smoothing results from the random spatial variation in polarization of the laser beam, not from the reduction in beam contrast.     

Effects of spatial and temporal smoothing on stimulated brillouin scattering in the independent-hot-spot model limit

The influence of laser beam smoothing on stimulated Brillouin backscattering (SBBS) is studied analytically in the limit of the independent hot spot model. It is shown that the temporal beam smoothing can reduce the SBBS reflectivity significantly even though the laser bandwidth is smaller than the growth rate for the average intensity. The explanation of this reduction effect is given in terms of SBBS growth in the statistically significant hot spots. The minimum laser bandwidth corresponding to an important reduction of the reflectivity is thus determined properly. The dependence of this reduction effect on the acoustic damping for a given laser bandwidth is discussed.     

大型光学元件中的横向受激布里渊散射

研究大型光学元件中横向受激布里渊散射的特点和破坏机制,以及横向受激布里渊散射的抑制方案。在理论分析的基础上,进行数值模拟。当入射光强足够高,材料的横向尺寸足够大时,在受激散射过程中产生的弹性声波场有可能超出材料的抗拉强度,从而引起力学破坏;增加泵浦光带宽可以显著降低散射光的增益。强激光辐照下的大尺寸光学元件中很容易激发横向受激布里渊散射;对于大多数强激光系统,20GHz的带宽已经足以抑制TSBS带来的影响。     

Phase conjugated wavefronts by stimulated brillouin and Raman scattering

Experimental investigations were made of wavefront reproduction (WFR) by backward stimulated Raman scattering (SRS) and backward stimulated Brillouin scattering (SBS) in CS₂ with a linearly polarized ruby pump beam. The experimental studies were carried out as a function of the optical waveguide diameter and photographic records comparing the pump beam with the backscattered SBS and SRS beams are presented. In all of the cases studied there was a high degree of replication between the SBS and pump beams; however, a deterioration in the quality of the replicated SRS beam was observed, and it increased as the cross-sectional area of the waveguide increased.     

Controlling stimulated brillouin backscatter with beam smoothing in weakly damped systems

We derive an analytical estimate of the effect of temporal smoothing of laser beams on stimulated Brillouin scattering (SBS) in a regime relevant to indirect drive ignition. We predict a strong reduction of SBS in the gold plasma expanding from the Hohlraum wall with temporal smoothing. This is a new regime far above threshold where the time to reach convective saturation allows for an effective contrast reduction of the beam intensity driving the instability. This result agrees with three dimensional simulations. Polarization smoothing is shown to double the effective bandwidth.     

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.     

Reduction of stimulated brillouin scattering by the generation of ion acoustic harmonics

Ion wave harmonics generated by the primary ion acoustic wave excited in stimulated Brillouin scattering are shown to introduce heavy nonlinear damping of the process and to considerably reduce the anomalous reflectivity in an underdense plasma (n/n_c<0.5). The generation of ion wave harmonics acts similarly to strong linear damping and provides an alternative explanation to the heavy damping indicated by experiments.     

Multibeam stimulated brillouin scattering from hot,solid-target plasmas

We report on multibeam, laser-plasma interaction experiments in plasmas relevant to future direct-drive-ignition experiments. Six interaction beams are incident on preformed plasmas containing critical density. Stimulated Brillouin scattering (SBS) shows strong evidence of electromagnetic wave seeding of side- and backscattering. The data are also consistent with shared ion waves driven by the six symmetrically arranged interaction beams. Early SBS quenching is observed and attributed to the hydrodynamics of the plasma.     

Suppression of stimulated brillouin scattering by increased landau damping in multiple-ion-species hohlraum plasmas

We demonstrate that multiple-ion-species plasmas greatly reduce stimulated Brillouin scattering (SBS) in high-electron temperature inertial confinement fusion hohlraums. Landau damping is increased by adding hydrogen to a CO(2) gas filled hohlraum. We find that the SBS reflectivity decreases monotonically with increasing hydrogen fraction from 18% to 3% with a simultaneous increase of laser beam transmission. Detailed simulations with a 3D laser-plasma interaction code are in agreement with the experimentally observed reduction in backscattered light.     

圆柱形等离子体对微波散射的数值模拟与实验研究

模拟和实验研究了非均匀圆柱形等离子体及阵列对微波的散射作用。利用有限时域差分(FDTD)方法仿真得到了等离子体柱的密度、碰撞频率对微波传播系数的影响,并利用低气压放电产生的等离子体柱对微波的吸收和散射作用进行了验证。结果表明:电子密度中心高、周围低的非均匀等离子体柱可将微波散射至两个侧向;等离子体频率越大,散射的微波功率越强;增加碰撞频率使等离子体柱的微波散射功率减小、吸收增大。等离子体必须具有合适的密度,才能对微波反射产生较大影响。     

圆柱形等离子体对微波散射的数值模拟与实验研究

模拟和实验研究了非均匀圆柱形等离子体及阵列对微波的散射作用。利用有限时域差分(FDTD)方法仿真得到了等离子体柱的密度、碰撞频率对微波传播系数的影响,并利用低气压放电产生的等离子体柱对微波的吸收和散射作用进行了验证。结果表明:电子密度中心高、周围低的非均匀等离子体柱可将微波散射至两个侧向;等离子体频率越大,散射的微波功率越强;增加碰撞频率使等离子体柱的微波散射功率减小、吸收增大。等离子体必须具有合适的密度,才能对微波反射产生较大影响。     

Thermal component of stimulated brillouin scattering due to two-photon absorption

The thermal component of the SBS spectrum due to two-photon absorption is analyzed. It is shown that this component must manifest itself as a slight increase in the Brillouin shift depending on the pump intensity. A positive anti-Stokes gain is possible only when the pump intensity is sufficiently high.     

Compression of ultraintense laser pulses in inhomogeneous plasma upon backward stimulated raman scattering

The potential for time compression of ultraintense laser pulses in plasma based on nonlinear interactions between waves during backward stimulated Raman scattering is analyzed analytically and numerically. The optimal conditions for obtaining a maximal energy conversion coefficient of 10-ps laser pulses to the energy of 100-fs Stokes pulses are found.     

Effect of stimulated Brillouin scattering on the reflection of two large-amplitude waves of different frequencies by an inhomogeneous plasma

It is shown that, by using two pump waves of different frequencies, the SBS part of the radiation reflected by inhomogeneous plasmas can be measured. The possibility of using this as a diagnostic is discussed.     

Stimulated scattering and wavefront conjugation in an inhomogeneous plasma

In the context of the problem of stimulated scattering we discuss the mechanism of wavefront conjugation in an inhomogeneous plasma proposed relatively recently, associated with a difference in suppression of scattering (due to inhomogeneities) for the inverted and uninverted components. We analyze the solutions of the integro-differential equations describing this process both numerically and analytically for different sound attenuation lengths (v ⁻¹). It is shown that for this effect to exist it is necessary that n not be too small. We also consider extinction of the inverted wave in terms of this mechanism. Zh. éksp. Teor. Fiz. 113, 168–180 (January 1998)     

Pressure dependence of stimulated brillouin scattering from a hydrogen gas target

Backscattering of CO₂ laser radiation from underdense hydrogen plasma was found to increase from ～0.2 to ～12% as gas target filling pressure was increased from 20 to 130 torr. No marked change in growth constant was observed for increased radiation intensity from 1 × 10¹¹ to 3 × 10 ¹¹ W/cm².