We present the ELAN testbed, which aims at assessing laser-induced damage of thick and large silica optics for the Laser Megajoule (LMJ) facility. A custom and patented injection seeding system was implemented on our Nd:YAG laser: this allows us to use phase modulated pulses to annihilate backward stimulated Brillouin scattering during tests while guaranteeing a smooth temporal profile. Then, beam propagation simulations of the incident spatial profile with Miro code (CEA) allows us to correct for the surface narrowing of the beam on the exit face of the optic due to Kerr self-focusing. This testbed allows us to test LMJ 34 mm vacuum window at 351 nm in nanosecond range with fluence up to 30-35 J/cm² with a control of non-linear propagation effect.
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.
High-energy laser pulses in the nanosecond regime used to be spectrally broadened to mitigate the stimulated Brillouin scattering known to deteriorate the optical elements. Due to propagating effects, this spectrum broadening lead to FM-to-AM conversion, where the UV laser beam experiences an amplitude modulation at frequencies which are harmonics of the phase modulation frequency. We study the impact of the FM-to-AM conversion on the Brillouin backscattering by applying an amplitude modulation on the UV pump laser beam operating at 351 nm and with a 3 ns pulse duration.
Experimental measurements show that adding an amplitude modulation frequency on a phase-modulated laser beam could enhance the stimulated Brillouin scattering and lead to laser damage. Thanks to a theoretical and numerical analysis, we show that this singular behavior originates from a resonance between the frequency of the amplitude modulation and the low orders harmonic frequencies of the phase modulated laser beam.
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.
The paper aims to compare the results obtained with the same laser source with a large beam and with small beams. These
latest were shaped from phase objects implemented to obtain several small beams from a single larger beam. The
consistency of the results from both sets of measurements is shown. It validates the assumptions made and the specific
mathematical treatments implemented to establish the link between the two approaches. It also validates and strengthens
the approach developed from the rasterscan procedure used to measure damage densities from the scanning with beams
of small dimensions. This shows that small beam tests are reasonably representative of tests carried out with large beams.
The paper aims to compare the results obtained with the same laser source with a large beam and with small beams. These latest were shaped from phase objects implemented to obtain several small beams from a single larger beam. The consistency of the results from both sets of measurements is shown. It validates the assumptions made and the specific mathematical treatments implemented to establish the link between the two approaches. It also validates and strengthens the approach developed from the rasterscan procedure used to measure damage densities from the scanning with beams of small dimensions. This shows that small beam tests are reasonably representative of tests carried out with large beams.
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