Laser damage in the sub-picosecond regime of transport mirrors and development of pulse compression gratings for the PETAL laser facility

The PETAL laser (Petawatt Aquitaine Laser) is a high-power laser located in the CEA-CESTA. This is part of the LMJ-PETAL research facility. This laser was designed to deliver sub-picosecond pulses (between 0.5 and 10 ps) with an energy of few kJ, allowing to reach a power of several PW. This power is today limited because of the problem of laser-induced damage to the final optical components of the laser chain. It is in particular at the level of the transport mirrors, located after the compression stage, that laser damage sites appears. These damage sites degrade the optical function of the components and can also degrade the beam propagation. Moreover, an evolution of the damaged surface was observed on the facility following several laser shots which drastically reduces the lifetime of these components. This phenomenon, called laser-induced damage growth, represents a major issue for the PETAL facility, particularly in terms of maintenance costs and the difficulty of replacing these components on the facility. One of the objectives of this thesis is to study the damage growth in the laboratory in order to improve our understanding of this phenomenon and to predict the lifetime of the components. A parametric study was performed to characterize the influence of different parameters: pulse duration, polarization state, spatial beam profile and component design. A numerical model has been developed to compare the numerical results with the trends observed experimentally. The second objective of this thesis concerns the PETAL vacuum compression gratings. In the next few years, a new generation of transport mirrors will be installed on PETAL facility with a better laser resistance. This new generation will allow an increase of the laser power. The fear is that the problem of laser damage will be transposed to vacuum compression gratings. The objective of this thesis is therefore to improve the laser resistance of PETAL gratings. These components are multilayer dielectric (MLD) gratings operating in reflection. A numerical code for the design of these components has been developed at the Fresnel Institute. This code allows to calculate the electric field distribution and the diffraction efficiency at different diffraction orders. The optimization procedure focused on reducing the electric field enhancement (a quantity directly linked to laser-induced damage) in the etching profile by modifying the thicknesses of the top layers of the stack (under the etch). Only one etching profile was considered. The results showed a strong impact of the multilayer stack on the electric field intensity peaks reducing the electric field enhancement. Samples were fabricated for laser damage tests. The results were in good agreement with the numerical calculations with deviations in laser flux withstand similar to the numerical deviations in electric field enhancement. The next generation of PETAL gratings would be designed following this optimization procedure. (author)