Effect of spectral bandwidth on the stimulated scattering of laser beams in plasma

The effect of pump bandwidth on the excitation of three-wave parametric instabilities is considered by comparing a monochromatic pump, a phase-modulated pump, and a pump with random phase modulations. It is found that for the instability growth in the time domain, phase-modulated and random pumps produce a qualitatively similar effect, reducing the temporal growth rate approximately inversely proportional to the bandwidth, in agreement with theoretical predictions. In the case of convective instability, a significant enhancement of the saturation level is found for the pump with random phases if its bandwidth is smaller than the temporal growth rate. For larger bandwidths, the amplification is suppressed, but to a lesser extent than theoretically predicted. The random phase pump and the phase-modulated pump result in qualitatively similar suppression of amplification. The one-dimensional analysis of parametric instabilities is extended to speckled laser beams, assuming that each speckle generates scattered light independently. The laser bandwidth may suppress scattering from low-intensity speckles, but the high-intensity speckles may still dominate the response. A criterion for the bandwidth suppression of stimulated scattering in a speckled beam is proposed.