Stimulated Raman backscattering with and without optical fiber smoothing technique in 0.53 μm laser-created plasmas

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Abstract
Backward stimulated Raman scattering (SRS) experiments have been performed using the optical fiber smoothing implemented on the high-power Phébus laser facility [Laser Part. Beams 4, 93 (1988)]. The interaction took place in low-Z plasmas presenting either exponential density profiles (solid targets) or Gaussian-type profiles. Raman instability was driven with 1.4 ns duration—0.53 μm laser pulses, containing an energy up to 1.8 kJ. Comparative and absolute measurements of SRS reflectivity are reported as a function of the incident laser intensity, both with and without beam smoothing. Near-backward time-resolved SRS spectra are also presented. Random phase plates are not used in these experiments. With or without smoothing, SRS reflectivities of the order of 10% have been measured. However, smoothing becomes effective in exponential profiles below 1015 W/cm2. Although the optical fiber efficiently reduces the contrast of the energy modulations in the focal spot, the spectral bandwidth may be insufficient to quench SRS development in these conditions. This last assumption is in agreement with theoretical predictions.