The original damage ring pattern at the exit surface of fused silica induced by highly modulated nanosecond infrared laser pulses demonstrates the time dependence of damage morphology. Such a damage structure is used to study the dynamics of the plasma issued from open cracks. This pattern originates from electron avalanche in this plasma, which simultaneously leads to an ionization front displacement in air and a silica ablation process. Experiments have shown that the propagation speed of the detonation wave reaches about 20 km/s and scales as the cube root of the laser intensity, in good agreement with theoretical hydrodynamics modeling. During this presentation, we present the different phases and the associated mechanisms leading to this peculiar morphology: • During an incubation phase, a precursor defect provides energy deposit that drives the near surface material into a plasma state. • Next the silica plasma provides free electrons in the surrounding air, under laser irradiation an electron avalanche is initiated and generates a breakdown wave. • Then this breakdown wave leads to an expansion of the air plasma. This latter is able to heat strongly the silica surface as well as generate free electrons in its conduction band. Hence, the silica becomes activated along the breakdown wave. • When the silica has become absorbent, an ablation mechanism of silica occurs, simultaneously with the air plasma expansion, resulting in the formation of the ring patterns in the case of these modulated laser pulses. These mechanisms are supported by experiments realized in vacuum environment. A model describing the expansion of the heated area by thermal conduction due to plasma free electrons is then presented. Next, the paper deals with the two damage formation phases that are distinguished. The first phase corresponds to the incubation of the laser flux by a subsurface defect until the damage occurrence: an incubation fluence corresponds to this phase. The second is related to the damage expansion that only refers to the energy deposit feeding the activation mechanism up to the end of the pulse: an expansion fluence corresponds to this phase. A striking feature is that the damage diameters are proportional to the fluence of expansion at a given shot fluence. Indirectly, the fluences of incubation by the precursors are then determined.
The influence of vacuum on nanosecond laser-induced damage at the exit surface of fused silica components is investigated at 1064 nm. In the present study, as previously observed in air, ring patterns surrounding laserinduced damage sites are systematically observed on a plane surface when initiated by multiple longitudinal modes laser pulses. Compared to air, the printed pattern is clearly more concentrated. The obtained correlation between the damage morphology and the temporal structure of the pulses suggests a laser-driven ablation mechanism resulting in a thorough imprint of energy deposit. The ablation process is assumed to be subsequent to an activation of the surface by hot electrons related to the diffusive expansion of a plasma formed from silica. This interpretation is strongly reinforced with additional experiments performed on an optical grating in vacuum on which damage sites do not show any ring pattern. Qualitatively, in vacuum, the intensity-dependent ring appearance speed <i>V</i> ∝ <i>I</i><sup>1/2</sup> is shown to be different than in air where <i>V</i> ∝ <i>I</i><sup>1/3</sup> . This demonstrates that the mechanisms of formation of ring patterns are different in vacuum than in air. Moreover, the mechanism responsible of the propagation of the activation front in vacuum is shown to be outdone when experiments are performed in air.
The rasterscan procedure, developed to test large components, is an efficient method that allows measuring extremely low surface damage density (until 0.01 site/cm<sup>2</sup> for large optics). This procedure was improved in terms of accuracy. The equipment, test procedure and data analysis to perform this damage test of large aperture optics are described. The originality of the refined procedure is that a shot to shot correlation is performed between the damage occurrence and the corresponding fluence by recording beam parameters of hundreds of thousands of shots during the qualification. Because tests are realized with small Gaussian beams (about 1mm @ 1/e), beam overlap and beam shape are key parameters which have to be taken into account in order to determine damage density. After complete data analysis and treatment, a repeatable metrology has been reached. The measurement is destructive for the sample. However the consideration of error bars on defects distributions allows us to compare data obtained on a same batch of optical components. This will permit to reach reproducible metrology. Then this procedure provides a straightforward means of comparing the experimental results obtained from several facilities using different lasers. Recently, an additional step has been added to the procedure, a growth step that permits considering only growing damage sites. To the end the lifetime of large optics on high power laser can be predicted.
The morphology of laser-induced damage sites at the exit surface of fused silica is tightly correlated to the mode
composition of the nanosecond laser pulses at 1064 nm. In the single longitudinal mode (SLM) configuration, a molten
and fractured central zone is surrounded by a funnel-shaped surface modification. Ring patterns surround the damage
sites when these are initiated by multiple longitudinal modes (MLM) laser pulses. In this last mode configuration, the
pulses temporal profiles as well as the damage ring patterns differ from pulse to pulse. The appearance chronology of the
rings is found to be closely related to the temporal shape of the laser pulses. This supports that the damage morphology
originates from the coupling of a laser-supported detonation wave propagating in air with an ablation mechanism in
silica. In our experiments, the propagation speed of the detonation wave reaches about 20 km/s and scales as the cube
root of the laser intensity, in good agreement with theory.
The laser induced damage densities measured on fused silica surface are found to be higher when produced with multiple longitudinal mode pulses than those produced by single longitudinal mode pulses at 1064 nm. The enhancement of the three-photon absorption due to the intensity spikes related to longitudinal mode beating might favor the damaging process at this wavelength. At 355 nm the picture is different. The absorption is supposed to be linear and an opposite behavior occurs. Evidences of a process leading to the possible annealing of a part of absorbent defects are discussed in this paper.
With the purpose of understanding nanosecond laser induced damage mechanisms when working with multiple longitudinal mode pulses, an accurate measurement of the temporal profiles is required. In this study, the use of a streak camera with a wide bandwidth is justified through the knowledge of the Nd:YAG spectral characteristics. A statistical and phenomenological analysis of multiple longitudinal modes intensity profiles is then performed through experiments and modeling. The resolution limitation of our photodiodes is also discussed.