Accurate laser damage testing for Laser MégaJoule (LMJ) fused silica optics is crucial for predicting their lifetimes. However, beam propagation in optics is usually neglected and yet, damage sites are mostly initiated on the exit face of optics. The MELBA testbed in CEA CESTA (France) delivers nanosecond UV laser pulses representative of LMJ optics conditions on a centimeter scale. Our particular imaging system measures the beam spatial profile before and after propagation in samples, enabling quantification of self-focusing induced by the Kerr effect. This metrology is necessary for laser damage parametric studies, particularly when these laser parameters influence the Kerr effect. We present here a study of the impact of a linear-to-circular polarization conversion on laser damage, which highlights the importance of accurately assessing non-linear beam propagation for laser damage tests.
The poster introduces the MELBA setup located at CEA CESTA (France). The MELBA laser delivers a nanosecond UV centimeter-sized laser beam and is dedicated to the study of laser-induced damage and damage growth within the Laser MégaJoule framework. Laser pulses are spatially, temporally and spectrally both shaped and characterized. A dedicated imaging system can measure the non-linear propagation in samples and its consequence on surface damage and filamentation. Recently, it was made possible to adjust the beam polarization from linear to circular.
Possible linear-to-circular polarization conversion had been studied for the Laser MégaJoule. We measured the consequences of such polarization conversion on laser-induced damage using the MELBA testbed. The MELBA laser is located in CEA CESTA (France) and delivers a nanosecond UV centimeter-sized laser beam. Experimental comparison of polarizations states showed a significant decrease of damage densities in circular polarization. Thanks to the particular imaging setup, we were able to explain this by both a reduction of the Kerr effect (supported by theory) and a reduction of the intrinsic absorption of silica optics defects.
The MELBA testbed located at CEA CESTA (France) is a nanosecond UV centimeter-sized beam aiming at studying laser damage. Previous work carried out on MELBA showed a dependency of laser-induced damage to power modulation of the laser pulse in linear propagation regime. We extended this study to the non-linear propagation regime. Thanks to dedicated imaging setup, we carried out an experimental study of Bespalov-Talanov gain and laser-induced damage with different temporal modulations. The tuning of phase modulation parameters made it possible to annihilate backward stimulated Brillouin scattering and consequently focus on the impact of the Kerr effect.
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