Measuring a laser focal spot on a large intensity range: effect of optical component laser damages on the focal spot

Abstract

LIL and LMJ are two French high power lasers dedicated to fusion and plasma experiments. Mastering the characteristics of the focal spots focused on the targets during the experiments is very important. In order to analyze the focal spots in its high power lasers, the CEA has developed an independent set-up that enables to measure energy spatial profiles over a 5 decade dynamic range by the means of several acquisitions taken at different power levels. The different data sets are then stitched to obtain a high dynamic picture of the beam. The experiment can also be used as a photometer enabling to measure the energy transmitted by an optical component. We used this set-up to study the effect of different parameters on the energy spatial profile of the focal spots. We have measured the effect of laser damages (on the optical components of the beam) on the energy scattered around the main focal spot. We also demonstrated that the level of this scattered power can be calculated from a near-field picture of the beam or even with pictures of the damaged components taken with an appropriate lighting.

Intermediate field measurement to characterize the wavefront of high power laser large optics

“…We compare the focal spot with each configuration: the collimated intermediate field, the divergent intermediate field and the near intermediate field, as describe in the Figure 6. The figure caption contains the distance L eq , defined in the equation (10) and (11) for, respectively, the collimated and the divergent intermediate field. The relation between the normalized intensity profile P(r) in the corona focal spot and the PSD 1D , as function of the radius r, the wavelength λ and the focal length f, is described as follow [11]:…”

“…We have also developed a method to characterize the specific component defects thanks to the measure of the focal spot [10,11]. Now, we have adapted the setup for both techniques and it consists in a double pass setup: a plane wave is transmitted through the component twice thanks to the reflection on a mirror located behind the component, in order to simplify the setup and improve the measurement.…”