Mr. Michael Uwakwe Profile

Michael Uwakwe

Sr. Process Engineer

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Proceedings Article | 12 September 2021 Presentation + Paper

A simple collimator design for measuring focal spots with high dynamical range

B. Le Garrec, M. Chabanis, F. Tinker, M. Uwakwe

Proceedings Volume 11873, 118730E (2021) https://doi.org/10.1117/12.2599953

KEYWORDS: Satellites, Collimators, Spatial frequencies, Polishing, Wavefronts, Standards development, Interferometry, Adaptive optics, National Ignition Facility, Manufacturing

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High intensity lasers in the 10-PW range require large optics (up to 600 mm diameter) with very high surface quality. In our case we have been facing troubles coming from very small wavefront defects at high spatial frequencies; defects that were not visible when checking with a Fizeau-type interferometer that was providing a PSD record. These very small defects were creating high-contrast Talbot fringes when propagating our laser beam. We decided to check carefully how the focal spot of our laser beam was affected and this was the reason why we decided to build collimators. Our collimator is a classical Newton-type telescope that can be used either off-axis or on- axis with a low central occultation (< 12%). The beam comes from a collimated laser source that is focused through a 30-μm pinhole by a microscope objective (x 20) in order to obtain a "clean" spatial distribution. In the collimated part, optical densities calibrated at the operating wavelength(s) are inserted in order to control the intensity in the focal spot. An operating range of 10 orders of magnitude is obtained with a source emitting 5 mW. The measurement is made in the focal plane of the component to be tested with a CCD camera. In cooperation with Aperture Optical Sciences Inc., the collimator was tested with known components and results compared with other available systems. Finally, we are able to identify and to measure periodic defects as low as l/2500 and to connect them with their PSD value.

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