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Optical gains measurement with a gain scheduling camera: On-sky demonstration with PAPYRUS and perspectives
No full textInternational audienceContext. Reaching the high angular resolution and contrast level desired for exoplanetary science requires us to equip large telescopes with extreme adaptive optics (XAO) systems to compensate for the effect of the atmospheric turbulence at a very fast rate. This calls for the development of ultra-sensitive wavefront sensors (WFSs), such as Fourier filtering wavefront sensors (FFWFSs), to be operated at low flux, as well as an increase in the XAO loop frame rate. These sensors, which constitute the baseline for current and future XAO systems, exhibit such a high sensitivity at the expense of a non-linear behaviour that must be properly calibrated and compensated for to deliver the required performance.Aims. We aim to validate on-sky a recently proposed method that associates the FFWFS with a focal plane detector - the gain scheduling camera (GSC) - to estimate in real time the first-order terms of the sensor non-linearities, known as modal optical gains.Methods. We implemented a GSC on the adaptive-optics (AO) bench PAPYRUS to be associated with the existing pyramid wavefront sensor (PWFS). We compared experimental results to expected results obtained with a high-fidelity numerical twin of the AO system.Results. We validated experimentally the method both in laboratory and on-sky. We demonstrated the capability of the GSC to accurately estimate the optical gains of the PWFS at 100 Hz, corresponding to the current limit in speed imposed by PAPYRUS hardware, but it could be applied at higher frequencies to enable frame-by-frame optical gains tracking. The presented results exhibit good agreement on the optical gains estimation with respect to numerical simulations reproducing the experimental conditions tested.Conclusions. Our experimental results validate the strategy of coupling a FFWFS with a focal-plane camera to master the nonlinearities of the sensor. This demonstrates its attractiveness for future XAO application
Three-Site Diversity Based on Q-Band Alphasat Satellite Propagation Campaign in Central Europe
No full textInternational audienceTo satisfy the increasing demand for ultra-high data rates, next-generation satellite communications will leverage the vast bandwidth resources available in the Q-band and higher frequencies. However, the signal propagation at these frequencies is highly susceptible to tropospheric impairments, in particular rain-induced attenuation, which can severely degrade Earthsatellite link availability and compromise service continuity. Site diversity has proven to be an effective technique for mitigating these impairments and guarantees high link reliability. In this letter, we present a three-site diversity configuration involving Alphasat satellite and ground stations located in Ljubljana (Slovenia), Graz (Austria), and Budapest (Hungary). Long-term slant-path rain attenuation time series, acquired from the beacon measurements of satellite at each ground stations, are statistically analyzed to evaluate individual site performance and joint diversity gains. Statistical data are subsequently compared with representative prediction models for three-site diversity scenario
Ionospheric scintillation modeling. I. Validity of the plane wave approximation for an electromagnetic wave illuminating the ionosphere
No full textInternational audienceLow Earth Orbit (LEO) satellites have a low altitude orbital track around the Earth. For a satellite to Earth link passing through the ionosphere, the approximation of the electromagnetic wave illuminating the ionosphere irregularities by a plane wave is questionable. We develop an analytical method for quantifying the error induced by the formalism assuming an incident plane wave illuminating a turbulent ionospheric layer. The method is applied to the special case of weak scattering under a thin-layer assumption. It is validated through comparison with the numerical approach solving the Parabolic Wave Equation (PWE) associated with Multiple Curved Phase Screens (MCPS). In addition, the correction of the classical incident plane wave approach by correcting the Fresnel distance from plane wave to spherical wave is considered. The analytic method allows quantifying the errors induced by an incident plane wave approximation or by the approach using the corrected Fresnel distance. For this, the amplitude and phase scintillation indices, respectively S 4 and σ φ , are compared when the transmitter – irregularities distance varies. Finally, in realistic configurations, the impact of satellite altitude on ionospheric scintillation is computed and discussed.Les satellites en orbite terrestre basse (OTB) ont une trajectoire orbitale de faible altitude autour de la Terre, et pour un lien satellite-sol traversant l’ionosphère, l’approximation de l’onde électromagnétique illuminant les irrégularités ionosphériques par une onde plane est discutable. Dans cet article, une étude théorique est proposée afin de quantifier l’erreur induite par le formalisme supposant une onde plane incidente illuminant une couche ionosphérique turbulente. L’approche analytique dans le régime de diffusion faible et sous l’hypothèse de couche mince est utilisée et validée par comparaison avec l’approche numérique résolvant l’Équation d’Onde Parabolique (EOP) associée à des écrans de phase courbes multiples (MCPS). De plus, la correction de l’approche classique d’onde plane incidente par une correction de la distance de Fresnel, passant de l’onde plane à l’onde sphérique, est envisagée. La méthode analytique permet de quantifier les erreurs induites par l’approximation d’onde plane incidente ou par l’approche utilisant la distance de Fresnel corrigée. Pour cela, les indices de scintillation d’amplitude et de phase, respectivement S4 et sigma_phi, sont comparés lorsque la distance entre l’émetteur et les irrégularités varie. Enfin, dans des configurations réalistes, l’impact de l’altitude du satellite sur la scintillation ionosphérique est calculé et discuté
On the Loewner Framework, the Kolmogorov Superposition Theorem, and the Curse of Dimensionality
No full textInternational audienceThe Loewner framework is an interpolatory approach for the approximation of linear and nonlinear systems. The purpose here is to extend this framework to linear parametric systems with an arbitrary number n of parameters. To achieve this, a new generalized multivariate rational function realization is proposed. Then, we introduce the n-dimensional multivariate Loewner matrices and show that they can be computed by solving a set of coupled Sylvester equations. The null space of these Loewner matrices allows the construction of the multivariate barycentric rational function. The principal result of this work is to show how the null space of the n-dimensional Loewner matrix can be computed using a sequence of 1-dimensional Loewner matrices, leading to a drastic reduction of the computational burden. Equally importantly, this burden is alleviated by avoiding the explicit construction of large-scale n-dimensional Loewner matrices of size N \times N. Instead, the proposed methodology achieves decoupling of variables, leading to (i) a complexity reduction from O(N^3) to below O(N^{1.5}) when n > 5 and (ii) to memory storage bounded by the largest variable dimension rather than their product, thus taming the curse of dimensionality and making the solution scalable to very large data sets. This decoupling of the variables leads to a result similar to the Kolmogorov superposition theorem for rational functions. Thus, making use of barycentric representations, every multivariate rational function can be computed using the composition and superposition of single-variable functions. Finally, we suggest two algorithms (one direct and one iterative) to construct, directly from data, multivariate (or parametric) realizations ensuring (approximate) interpolation. Numerical examples highlight the effectiveness and scalability of the method
Fast, accurate, and predictive method for atom detection in site-resolved images of microtrap arrays
No full textInternational audienceWe introduce a new method, rooted in estimation theory, to detect individual atoms in site-resolved images of microtrap arrays, such as optical lattices or optical tweezers arrays. Using labelled test images, we demonstrate drastic improvement of the detection accuracy compared to the popular method based on Wiener deconvolution when the inter-site distance is comparable to the radius of the point spread function. The runtime of our method scales approximately linearly with the number of sites, and remains well below 100 ms for an array of 100 x 100 sites on a desktop computer. It is therefore fully compatible with a real-time usage. Finally, we propose a rigorous definition for the signal-to-noise ratio of the problem, and show that it can be used as a predictor for the detection error rate. Our work opens the prospect for future experiments with increased array sizes, or reduced inter-site distances
Multiscale turbulence synthesis: Validation in 2D hydrodynamics
No full textInternational audienceContext . Numerical simulations can follow the evolution of fluid motions through the intricacies of developed turbulence. However, they are rather costly to run, especially in 3D. In the past two decades, generative models have emerged that produce synthetic random flows at a computational cost equivalent to no more than a few time steps of a simulation. These simplified models qualitatively bear some characteristics of turbulent flows in specific contexts (incompressible 3D hydrodynamics or magnetohydrodynamics) but generally struggle with the synthesis of coherent structures. Aims . We aim to generate random fields (e.g. velocity, density, magnetic fields) with realistic physical properties for a large variety of governing partial differential equations and at a small cost relative to time-resolved simulations. Methods . We propose a set of simple steps applied to given sets of partial differential equations: we filter from large to small scales, derive a first order time evolution approximation from Gaussian random initial conditions during a prescribed coherence time, and finally sum over scales to generate the fields. Results . We test the validity of our method in the simplest framework: 2D decaying incompressible hydrodynamical turbulence. We compare the results of 2D decaying simulations with snapshots of our synthetic turbulence. We first quantitatively assess the difference with standard statistical tools: power spectra, increments, and structure functions. These indicators can be reproduced by our method during up to about a third of the turnover timescale. We also consider recently developed scattering transforms statistics, which are able to efficiently characterise non-Gaussian structures. This reveals a more significant discrepancy; however, this can be bridged by bootstrapping. Finally, the number of Fourier transforms necessary for one synthesis scales logarithmically in the resolution, compared to linearly for time-resolved simulations. Conclusions . We have designed a multiscale turbulence synthesis (MuScaTS) method to efficiently short-circuit costly numerical simulations to produce realistic instantaneous fields
Méthode de Trefftz pour une classe de systèmes de Friedrichs harmoniques
No full textThis document presents a class of two-fields Friedrichs systems, allowing to encompass classic time-harmonic wave propagation problems into a unique formalism: notions of incoming and outgoing traces, in addition to a normal flux decomposition and a consistent numerical flux expression are defined in this general setting. Then, a Trefftz method is introduced for this class of problems: formulations based on the reciprocity formula or the Ultra Weak Variational Formulation are defined, allowing to prove weak-coercivity and contraction properties of the preconditioned system. Finally, we discuss the interpretation of the method from the point of view of Domain Decomposition methods, and recall classic error estimates results and discretisation by plane waves.Ce document présente une classe de systèmes de Friedrichs à deux champs, permettant d'inclure les problèmes d'ondes harmoniques classiques dans un unique formalisme : des notions de traces entrantes et sortantes, ainsi qu'une décomposition du flux normal et une expression d'un flux numérique consistant sont ainsi définies de manière générale. Une méthode de Trefftz est alors introduite pour cette classe de problèmes : des formulations basées sur la formule de réciprocité ou la formulation ultra-faible sont ainsi définies, permettant de prouver des propriétés de coercivité faible et de contraction du système préconditionné. Enfin, nous discutons l'interprétation de cette méthode à l'aune des méthodes de Décomposition de Domaines, et rappelons des résultats d'estimation d'erreur classiques et de discrétisation par ondes planes
Etude de la fissuration transverse sous conditions cryogéniques pour application aux réservoirs composites LH2
No full textInternational audienceEtude de la fissuration transverse sous conditions cryogéniques pour application aux réservoirs composites LH
