In this paper we describe the experimental validation of the technique of correction of wavefront aberration in the middle of the laser amplifying chain. This technique allows the correction of the aberrations from the first part of the laser system, and the pre-compensation of the aberrations built in the second part. This approach will allow an effective aberration management in the laser chain, to protect the optical surfaces and optimize performances, and is the only possible approach for multi-Petawatt laser system from the technical and economical point of view. This approach is now possible after the introduction of new deformable mirrors with lower static aberrations and higher dynamic than the standard devices.
Since the beginning of the 90's the generation of high-intense laser pulses has known an unprecedented evolution thanks to the conjunction of the possibility of the Chirped Pulse Technique and the availability of spectrally broad-band laser media. Lasers capable of producing petawatt pulses can now be built on few optical tables in a small laboratory. We review the generation and the amplification of ultra-short pulses by the Chirped Pulse Amplification technique.
The major bottleneck for the development of robust and cost-effective femtosecond amplification systems is the uncertainty concerning the damage threshold of Ti: Sapphire crystals. Up to now, Ti: Sapphire is the only material that supports the generation of temporally short pulses (few femtosecond) at high repetition rates, and overcoming this bottleneck will represent a major advance in laser performance for all the femtosecond community. Currently, when pumped at 532nm, the uncertainty on Ti:Sapphire damage threshold, is about a factor of ten. The empirically estimated threshold is 10J/cm<sup>2</sup> but for safety reasons the femtosecond laser community (especially the companies producing the lasers) uses the conservative value of 1J/cm<sup>2</sup>. Such a low pumping fluency means low extraction efficiency during the amplification process and a great waste of pumping energy, the most expensive part of a Ti:Sapphire amplifier. In order to remove this bottleneck, we launch a complete analysis of all the factors that influence the damage threshold in Ti:Sapphire Crystals. Our program is to first measure the bulk threshold to define the upper threshold limit, and the influence of Ti ion concentration in the crystal garnet. Then, we will analyze all the surface effects that influence the value of the threshold. These effects depend on the polishing, on the cleaning process, as well as the type of anti-reflective coating. Only a complete understanding of all the mechanisms involved in threshold limitation will allow us to produce Ti:Sa crystals with the best performances. The study of the characteristics of the Ti:Sapphire damage threshold will not be complete and reliable without a complete characterization of the pump beams (temporal and spatial modulations), and this analysis will be done with nanosecond and picosecond pulses at 532nm. Finally, to complete the exploration of the the behavior of the titanium doped sapphire crystal, we will characterize the damage threshold with femtosecond pulses, at 800nm to reach the deterministic dielectric threshold and validate fundamentals models and simulation results. To our knowledges this is the first time that such a complete characterization is done for Ti:Sapphire laser crystals. We will present the first conclusions about the experiments as well as the methods we will employ in our systematic analysis.