Wavefront measurements through optical diffraction interpretation

Abstract

The French Laser MégaJoule (LMJ) will include 176 square beams involving hundreds of large optical components. Wavefront performances of all these components are critical to achieve the desired focal spot shape and to limit hot spots that could damage the components. These specifications are usually checked with interferometric setups. This can be uneasy to achieve for specific components such as multi-dielectric mirrors or gratings because one has to use the exact nominal configuration (wavelength, incidence, geometry of the incident beam) to perform the measurement. For the smallest spatial periods, classical techniques like interferometric microscopes fail to measure the wavefront and propose a "surface" measurement that can lead to misinterpretations. We present in this paper measurement methods based on a laser beam diffraction interpretation that can efficiently replace the usual techniques. The first technique consists in measuring intensity level of the dim scattered "corona" around the focal spot. The second one is based upon image processing of near-field acquisitions by the means of Fourier analysis and the Talbot effect theory. Those techniques do not lead to a phase map as classical techniques do but they give access to the Power Spectral Density of wavefront defects over a large spatial frequency bandwidth. For many applications, this is enough information to estimate the component performance. We present results obtained by this way on LMJ components and a comparison with Fizeau interferometer measurement.

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

“…We study the component PSD 2D and we compare our measurement with the one obtained with an other technique at the laboratory: the focal spot acquisition [11].…”

“…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.…”