Effect of the measurement wavelength on a multi-dielectric mirror wavefront

摘要

LIL and LMJ are two French high power lasers dedicated to fusion and plasma experiments. These laser beams involve hundreds of rather large optical components, the clear aperture of the beams being 400×400 mm~2. Among these components are multi-dielectric mirrors designed to reflect more than 99% at the wavelength of 1053 nm. Measuring the phase effects due to slight thickness defects in thin films is a difficult problem when one cannot achieve the phase measurement at the wavelength for which the mirror is designed. We believe this problem to be general in the world of thin films. Despite the fact that we have an interferometer that can achieve wavefront measurements at the correct wavelength, we performed measurements with another standard 633 nm Fizeau interferometer. Indeed, this second interferometer has a much higher spatial resolution. The effect of the wavelength difference can be strongly dependent on the layer design; that is why we achieved spectrophotometric measurements in order to have the most accurate knowledge we could get for the coating parameters. The phase effects for different kinds of defects have been simulated at both wavelengths and have been compared to experimental results. This study leads to a better understanding of the limits and the trust we can have in such measurements performed at the "wrong" wavelength.