Optimization of smoothing by spectral dispersion with a sinusoidal phase modulation

Abstract

In order to improve the propagation of a laser in a plasma, the intensity profile should be as uniform as possible. For this purpose, smoothing techniques are used. The most commonly used, smoothing by spectral dispersion (SSD), induces temporal fluctuations of the effective amplification length of Stimulated Brillouin scattering (SBS) or Stimulated Raman scattering (SRS). These fluctuations are random from shot to shot and may thus induce unpredictable levels of the efficient laser power used in laser-plasma experiments. Increasing the modulation frequency f_m while maintaining the modulation bandwidth strongly reduces these fluctuations and even reduces the average SBS level. However, increasing f_m has also an impact on laser propagation within the laser optical components. We show that, as long as chromatic dispersion is precompensated in the front-end, FM-to-AM conversion, a detrimental propagation effect, may remain sufficiently low and that anormal FM-to-AM conversion is not sensitive to the modulation frequency. Hence, there is no important bottleneck to an increase of the modulation frequency.