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.
Specification of visual defects (scratch and digs) on optics used in high-power laser facilities has always been a headache. Indeed, the wave degradation and the ensuing laser performances losses with regard to focal spot or downstream laser induced damage seem hard to predict. Indeed, one often has only partial information on each of the (often numerous) defects whereas the light behavior downstream strongly depends on the defect nature and morphology. So, determining general rules seem to be an impossible task. Borderline cases are then generally processed through timeconsuming optical profilometer measurements that are used in complex numerical laser propagation. We show in this paper that a simple analytic model can predict light intensification (responsible for some fratricide laser damage) with a high reliability. Defects are modeled by concentric quasi-circular rings of different radii, transmissions and phase shifts. The accuracy of these predictions will depend on the degree of knowledge of the model parameters set. In any case, upper bounds of intensifications can be provided as well as safe areas where intensification has decreased enough. These results allowed to specify defects dimensions and nature. We show good agreement between observed diffraction patterns downstream of real defects and model predictions, in terms of “hot spots” generation.
The Laser MégaJoule (LMJ) is a French high power laser that requires thousands of large optical components. For all
those optics, scratches, digs and other defects are severely specified. Indeed, diffraction of the laser beam by such
defects can lead to dangerous “hot spots” on downstream optics. With the help of a near-field measurement setup, we
make quantitative comparison between simulated and measured near-fields of reference objects (such as circular phase
steps). This leads to a better understanding which parameters impact the diffracted field. In this paper, we proposed to
study two types of reference objects: phase disks and phase rings. We actually made these objects by CO2 laser ablation.
The experimental setup to observe the diffracted intensity by these objects will be described and calibrated. Comparisons
between simulations and measurements of the light propagation through these objects show that we are able to predict
the light behavior based on complete phase measurement of these objects.