Ultrafast smoothing scheme for improving illumination uniformities of laser quads.

The beam smoothing technology, smoothing by spectral dispersion, plays an important role in improving the illumination uniformities of the lasers in inertial confinement fusion facilities. However, due to the limitations of the modulation frequency of the electro-optic modulator, the uniformity of the lasers approaches an asymptotic value after tens of picoseconds that are much longer than the response time of laser plasma instabilities. To the best of our knowledge, this is the first time that an ultrafast smoothing approach for improving the uniformity of a laser quad in both radial and azimuthal directions in the picosecond scale was proposed. Among the four individual beams in a quad, two of them were smoothed by independent ultrafast focal zooming, and the rest were transformed into Laguerre-Gaussian (LG) beams that carry same topological charges with opposite signs. The focal spots of these two LG beams were coherent superposed, and their intensity distributions rotated rapidly in a period of several picoseconds. As a result of the focal zooming and rotation, an ultrafast and significant improvement of the uniformity of a laser quad was achieved.