Intensity and closed-loop wavefront correction in the 8-TW Ti:S-laser system ATLAS

In the 8-TW Ti:S-laser system ATLAS 10, a 40-mm diameter Ti:S-crystal is used as the final four-pass amplifier delivering up to 1 J in 130-fs pulses at 10 Hz. The crystal has serious growth defects and shows inhomogeneous thermal lensing when pumped by up to 4 J of a frequency-doubled Nd-YAG laser. The near-field fluence distribution in the plane of the first compressor grating is rather inhomogeneous, too, showing typically two main peaks with a fluence of up to 300 mJ/cm2, which is beyond the damage threshold of the gratings. To flatten the profile, a 17-electrode bimorph piezoelectric deformable mirror (DM) is used. This procedure differs from the usual application of adaptive optics, which only aims at the correction of the deformed wavefront so that the Strehl ratio gets close to unit. In ATLAS 10 this is done in a further step using a second DM of the same type but with 33 electrodes which is mounted downstream from the compressor and which is since the beginning of 2001 routinely operated in a closed-loop using a modified Shack-Hartmann wavefront sensor. With the first DM the peak fluence in the plane of the first compressor grating is reduced by a factor of 3 and the second DM allows to focus the beam by using a F/3-parabola to a spot which contains 65% of the total energy in a diffraction limited diameter, corresponding to a Strehl ratio of 0.8. Thus the peak intensity in the focus is raised from 5x1018 W/cm2 to 2x1019 W/cm2.

fluence distribution in plane of short pulse grating fluence distribution in focus DM1 in operation
Elaser pulse=1.3 J

DM1 not in operation
fluence Jmax=300 mJ/cm²

DM2 in flat position Strehl ratio S<0.1

DM1 in operation
fluence Jmax=90 mJ/cm²

DM2 with closed loop in operation
Strehl ratio S=0.8