PISTIL interferometry (PISton and TILt interferometry), dedicated to the measurement of regular segmented wave surfaces such as segmented mirrors or the near-field output of coherent combined lasers, consists on making replicas of the central part of the wavefronts of each surface segment interfere with its nearest neighbors. The resulting interference pattern is made of an apposition of surfaces containing two-wave interference whose phase, frequency and orientation provide direct information on the respective heights and tilts of the segments in relation to each other. This technique has evolved into a new device called Full Light PISTIL. More compact and more efficient, this new version overcomes the drawbacks of previous PISTIL interferometer version by taking into account the whole surface of the incident wavefront. The performance obtained, estimated on a segmented mirror, is state of the art, and points the way towards a plug and play product adapted to the specific measurement needs of these surfaces, even in low light conditions.
The aim of this lecture is to show how the consideration of very specific measurement requirements, related to segmented surfaces and metasurfaces, has enabled a generalist multilateral shearing interferometer to evolve into new innovative devices. This has enabled state-of-the-art performance to be achieved, in particular by exploiting in the design a priori knowledge of the objects to be measured. Performance is illustrated here by a comparison of measurements on a canonical object, a PTT-111 deformable mirror from the company Iris AO, with a reference Phase Shifting Interferometer from the community.
Meta-optics allow the realization of new optical functions that are increasingly complex to realize and characterize locally. This is why we propose an interferometric method of systematic wavefront metrology of the meta-elements constituting a metasurface. This technique will allow the design of a library of nano-antennas, characterized in phase and amplitude. Once constituted, this library will allow the design of more complex optical functions. Tested for MIM (Metal-Isolating-Metal) metasurfaces, this technique can be applied to all metasurfaces.
KEYWORDS: Fiber lasers, Target detection, Turbulence, Laser development, Signal detection, Signal analyzers, Near field optics, Near field, Laser beam propagation, Fiber amplifiers
Coherent beam combining (CBC) by active phase control is an efficient way to power scale fiber amplifiers. Most often, CBC operates from measuring the phase differences between the lasers at their outputs, hence resulting in efficient combination of the laser beams in the very near-field. We developed a laser testbed coherently combining seven 1.5-µm fiber lasers through active phase control, using frequency-tagging to assess the phase fluctuations to be compensated for. The testbed can operate in a target-in-the-loop (TIL) configuration, with a detection sub-system designed to analyse the optical signal back-scattered by a remote target, in order to achieve coherent combining on the target rather than at the output of the lasers. In this paper, we present the testbed and its components, as well as the results obtained in direct coherent combining, operated at the output of the lasers, during the preliminary tests of the setup. Then, we present the results of the outdoor experimental campaign where the testbed is operated in a TIL-CBC configuration. Measurement of TIL-CBC efficiency when distance to the target is progressively increased from 15 meters to 1 km is detailed. As the experimental campaign took place in hot weather, with a close to the ground horizontal path of propagation for the laser beams, very strong turbulence conditions were encountered. However, efficient atmospheric turbulence compensation was demonstrated, confirming that TIL-CBC can be achieved, even under such detrimental turbulence conditions.
PISTIL (PISton and TILt) interferometry is a segmented wavefront metrology technique that can fulfill the role of being an independent phase analyzer for tiled laser arrays used in coherent beam combining (CBC). It presents a plug-and-play characteristics enabling others research or industrial applications such as metrology of segmented mirrors, MOEMS or measurement standards. It can operate onto complex optical benches. Alongside the PISTIL concept, we developed methods for phase extraction and meta-analysis, with best accuracy to rightfully address an end user needs in term of segmented wavefront diagnosis. We demonstrate those functionalities onto the HIBISCUS optical testbed equipped with a segmented mirror, specifically designed test data analysis pipelines and improve the control-command based on PISTIL wavefront analysis. In the current configuration, it can emulate CBC near field piston and tilt variations.
PISTIL (Piston and Tilt) is a recent interferometric system that computes the absolute piston and tip/tilt map of a segmented wavefront. Its high precision makes it usable as a metrology tool for wavefront sensing of coherently-combined laser arrays for example. This interferometer needs to correctly address high dynamic piston sensing, while dealing with fringes wrapping that leads to ambiguous phase estimations. We derived a mathematical combination for two measurements at different wavelengths and did a technical demonstration of it, using a IRIS-AO PTT111 Deformable Mirror as a segmented wavefront generator. We have verified that the loss of accuracy is slightly increased for a larger piston compared to a previous study, and we got a standard error of λ/160 with a Peak-to-valley of λ/50. This technique could be extended to a broader spectrum.
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