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To ensure highest intensity, one has to accurately control spatial phase to get the smallest focused spot. The spatial phase is controlled using adaptive optics systems with a wavefront sensor to measure spatial phase and a deformable mirror to correct it. This adaptive optics system is commonly placed at the output of the laser chain (just before or just after the compressor) and it now becomes a standard feature on high-power laser chains. The usual strategy of adaptive optics correction is to separate a small fraction of the main beam and to measure its wavefront using a wavefront sensor. However such strategy only ensures that the laser beam is free from aberrations at the location of the wavefront sensor. Aberrations induced by the optical elements located downstream of the wavefront sensor, for instance focusing optics, are not measured and therefore are not corrected by the adaptive optics loop. These aberrations contribute to final focal spot degradation. In order to get the highest intensity on the target, an aberration-free wavefront in the interaction chamber after the focusing optics is required. We will present a simple, direct and automated method using a standard focal spot camera and phase retrieval algorithms in order to measure and correct wavefront directly on the focal spot itself. This method is simple as it does not require additional hardware and can be used with spectral bandwidth larger than 200 nm. |
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