Polarization modification of a spatially randomized picosecond-pulse beam during its amplification by a nanosecond pump

Modifications of the spatial distribution of polarization in the focal spot of a picosecond-pulse beam after its amplification by a nanosecond pump in a plasma have been observed experimentally. The spatially resolved measurement of beam polarization was achieved thanks to the simultaneous imaging of the focal spot with two cameras, one of which was equipped with a polarizer, permitting to determine the polarization characteristics. The mean modification of the picosecond-pulse beam polarization after crossing the nanosecond beam depends on its mean amplification by crossed-beam energy transfer. A fine analysis of the spatial polarization modification reveals a large dispersion of the speckle polarization modification after their amplification. This dispersive modification originates from the spatial smoothing of the two crossing beams.