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L’optique adaptative est une technique qui s’impose dans des domaines de plus en plus varies. Developpee initialement pour corriger les effets de la turbulence atmospherique, elle est aujourd’hui appliquee a l’imagerie biomedicale comme aux grandes chaines laser. Cet article en presente son principe, les techniques et composants sur lesquels elle s’appuie et donne quelques regles pour les mettre en œuvre. Enfin, il introduit les principaux defis qui s’ouvrent dans les annees a venir pour lever les verrous qui limitent encore son utilisation.

International audience; Space Situational Awareness has become a key issue both for defense and civilian/industrial applications. Identification of potential or active threats and monitoring of key assets and operations are at stake. This activity also includes the follow up of dedicated satellites (such as telecommunication, observation), traffic handling, debris identification and tracking. We present current work performed at Onera on Adaptive Optics (AO) for Low Earth Orbit (LEO) satellite imaging. We describe briefly our AO system and its performance. We also address recent developments on image post-processing based on marginalized blind deconvolution combined to parsimonious PSF modelling. Finally, we present on sky results.; La connaissance de la situation spatiale est devenue un enjeu clé tant pour la défense que pour les applications civiles / industrielles. L'identification des menaces potentielles ou actives et la surveillance des actifs et des opérations clés sont en jeu. Cette activité comprend également le suivi des satellites dédiés (tels que les télécommunications, l'observation), la gestion du trafic, l'identification et le suivi des débris. Nous présentons les travaux actuels réalisés à l'Onera sur l'optique adaptative (AO) pour l'imagerie par satellite en orbite terrestre basse (LEO). Nous décrivons brièvement notre système AO et ses performances. Nous abordons également les développements récents sur le post-traitement d'image basé sur la déconvolution aveugle marginalisée combinée à une modélisation parcimonieuse de la PSF. Enfin, nous présentons les résultats sur le ciel.

International audience; The need for space-ground data transmission systems with high data throughput has encouraged the development of new satellite-to-ground laser links, using adaptive optics to compensate for the deleterious effects of atmospheric turbulence. On the ground segment, the use of single-mode fiber components, and thus the coupling of the propagating signal into a single-mode fiber, is strongly favored. However, the single-mode fiber coupling efficiency and the resulting data rate are strongly hampered by the turbulenceinduced phase distortions and amplitude fluctuations (called "scintillation"). The effects of scintillation in the pupil are the strongest at low elevations of Low Earth Orbit satellites-toground links and are not compensated by traditional adaptive optics, which correct only the phase of the distorted signal. In this paper, we assess the effects of AO correction on the statistical properties of single-mode fiber coupled flux based on experimental data obtained in the presence of medium and strong scintillation. We show preliminary results of coupled flux, as well as their temporal variations and probability distribution, for different adaptive optics correction levels.

International audience; An optical communication link performance between the ground and a geostationary satellite can be impaired by scintillation, beam wandering and beam spreading due to its propagation through atmospheric turbulence. These effects on the link performance can be mitigated by tracking and error correction codes coupled with interleaving. Precise numerical tools capable of describing the irradiance fluctuations statistically and of creating irradiance time series are needed to characterize the benefits of these techniques and optimize them. The wave optics propagation method have proven their capability of modeling the effects of atmospheric turbulence on a beam but it is known to be computationally intensive. We present an analytical-numerical model that provides good results on the probability density functions of irradi-ance fluctuations as well as time series with an important saving of time and computational resources.

The payloads for Earth Observation and Universe Science are currently based on very stiff opto-mechanical structures with very tight tolerances. The introduction of active optics in such an instrument would relax the constraints on the thermo-mechanical architecture and on the mirrors polishing. A reduction of the global mass/cost of the telescope is therefore expected. Active optics is based on two key-components: the wave-front sensor and the wave-front corrector.

International audience; High throughput ground to GEO satellite telecommunication systems will require very capacitive links for the uplink. When diffraction limited, a few optical Watts per channel are sufficient to close the link budget. In the presence of atmospheric turbulence, higher emission powers are necessary to cope with the subsequent disruptions of the beam. Pre-compensation by adaptive optics has been identified as a key strategy to keep the requested power at reasonable levels. However its performance is limited by point-ahead anisoplanatism. Fine trade-offs shall be carried out to overcome this limitation while considering the specificities of optical telecommunication requirements (low fading probably, short fading duration, manageable cost and complexity). The expected performance of the pre-compensation by adaptive optics for optical feeder links is investigated through numerical simulations. A ground-to-ground experimental demonstration is proposed. The selected slant line of sight and the beam geometry is shown to induce turbulence effects, including angular decorrelation, that are representative of the GEO feeder link scenario when considering an appropriate setup.

Active coherent beam combination (CBC) of femtosecond pulses requires knowledge of the absolute phase between all the optical pulses: pulse synchronization and phase locking are both mandatory and must be measured and controlled. To assess controller requirements, we study fiber amplifier phase noise for various packaging and power levels. The main contributors are thermal noise (with large amplitude phase noise and frequencies below 5 Hz) and vibrations (with low amplitude phase noise and frequencies from 5 Hz to 2 kHz), leading to a woofer/tweeter pair of actuator. The amplifier packaging is of major importance: a simple PID model shows that in order to achieve a residual phase error of λ/100, the required controller bandwidth ranges from 0.5 Hz to 1 kHz, mostly depending on amplifier packaging. We also shortly review common phase locking techniques either based on pupil-plane sensor or focal-plane sensor. Common focal-plane techniques as SPGD or LOCSET are not directly compatible with pulsed femtosecond operation. Pupil-plane fringe-sensor techniques are more promising and route towards collective synchronization for femtosecond regime are proposed. Following the example of telescope interferometry, the final system might include both a pupil-plane, fringe-sensor and a focal-plane phase-diversity sensor, with pupil-plane sensor for synchronization and phase lock, and focal-plane sensor to control and maintain required focalized beam quality.

The ICAN beam combining system may require a control with thousands of degrees of freedom. The methods developed for the control of adaptive optics are presented. The recent issues raised by the European-Extremely Large Telescope are introduced as well as the solutions presently studied. The possible benefits which could be drawn for ICAN are analysed.

International audience; The James Webb Space Telescope (JWST) Optical Simulation Testbed (JOST) is a tabletop experiment designed to study wavefront sensing and control for a segmented space telescope, such as JWST. With the JWST Science & Operations Center co-located at STScI, JOST was developed to provide both a platform for staff training and to test alternate wavefront sensing and control strategies for independent validation or future improvements beyond the baseline operations. The design of JOST reproduces the physics of JWST's three-mirror anastigmat (TMA) using three custom aspheric lenses. It provides similar quality image as JWST (80% Strehl ratio) over a field equivalent to a NIRCam module, but at 633 nm. An Iris AO segmented mirror stands for the segmented primary mirror of JWST. Actuators allow us to control (1) the 18 segments of the segmented mirror in piston, tip, tilt and (2) the second lens, which stands for the secondary mirror, in tip, tilt and x, y, z positions. We present the most recent experimental results for the segmented mirror alignment. Our implementation of the Wavefront sensing (WFS) algorithms using phase diversity is tested on simulation and experimentally. The wavefront control (WFC) algorithms, which rely on a linear model for optical aberrations induced by misalignment of the secondary lens and the segmented mirror, are tested and validated both on simulations and experimentally. After alignment, JOST achieves 23 nm RMS WFE at 633 nm (RMS = lambda/28), which after accounting for the difference in design wavelengths, is comparable to the budgeted post-alignment WFE expected for JWST. We present the performance of the full active optic control loop in presence of perturbations on the segmented mirror, and we detail the quality of the alignment correction.

International audience; An optical link based on a multiplex of wavelengths at 1.55µm is foreseen to be a valuable alternative to the conventional radio-frequencies for the feeder link of the next-generation of high throughput geostationary satellite. Considering the limited power of lasers envisioned for feeder links, the beam divergence has to be dramatically reduced. Consequently, the beam pointing becomes a key issue. During its propagation between the ground station and a geostationary satellite, the optical beam is deflected (beam wandering), and possibly distorted (beam spreading), by atmospheric turbulence. It induces strong fluctuations of the detected telecom signal, thus increasing the bit error rate (BER). A steering mirror using a measurement from a beam coming from the satellite is used to pre-compensate the deflection. Because of the point-ahead angle between the downlink and the uplink, the turbulence effects experienced by both beams are slightly different, inducing an error in the correction. This error is characterized as a function of the turbulence characteristics as well as of the terminal characteristics, such as the servo-loop bandwidth or the beam diameter, and is included in the link budget. From this result, it is possible to predict intensity fluctuations detected by the satellite, both statistically (mean intensity, scintillation index, probability of fade, etc.) and temporally (power spectral densities, time series). The final objective is to optimize the different parameters of an optical ground station capable of mitigating the impact of atmospheric turbulence on the uplink in order to be compliant with the targeted capacity (1Terabit/s by 2025).

The James Webb Space Telescope (JWST) Optical Simulation Testbed (JOST) is a tabletop experiment designed to study wavefront sensing and control for a segmented space telescope, including both commissioning and maintenance activities. JOST is complementary to existing testbeds for JWST (e.g. the Ball Aerospace Testbed Telescope TBT) given its compact scale and flexibility, ease of use, and colocation at the JWST Science and Operations Center. The design of JOST reproduces the physics of JWST’s three-mirror anastigmat (TMA) using three custom aspheric lenses. It provides similar quality image as JWST (80% Strehl ratio) over a field equivalent to a NIRCam module, but at 633 nm. An Iris AO segmented mirror stands for the segmented primary mirror of JWST. Actuators allow us to control (1) the 18 segments of the segmented mirror in piston, tip, tilt and (2) the second lens, which stands for the secondary mirror, in tip, tilt and x, y, z positions. We present the full linear control alignment infrastructure developed for JOST, with an emphasis on multi-field wavefront sensing and control. Our implementation of the Wavefront Sensing (WFS) algorithms using phase diversity is experimentally tested. The wavefront control (WFC) algorithms, which rely on a linear model for optical aberrations induced by small misalignments of the three lenses, are tested and validated on simulations.

International audience; To match the increasing need for high data rate between high altitude platforms and ground free space optics links are investigated. Part of the growing interest is motivated by the possibility to reap the benefits of the technological maturity of the fibered components. This requires the injection of the received wave into a single mode fiber. To reduce injection losses on the ground terminal the use of adaptive optics (AO) is investigated. The AO system must work for a wide variety of turbulence conditions: by daytime and nighttime, at potentially very low elevations for LEO satellites, with localizations of optical ground stations that could be unfavorable regarding atmospheric turbulence. Contrary to astronomy where the quantity optimized is the average Strehl ratio, for free space communications statistical and temporal characteristics of the injection losses must be taken into account. The consequences of a partial correction are investigated here by numerical simulation for both GEO and LEO to ground links.; Pour répondre à l'accroissement du besoin en débit des communications entre les satellites et le sol, les liens optiques sont envisagés. Cet intérêt est en partie motivé par la possibilité de disposer des technologies de communication développées pour les liens fibrés. Pour cela, l'injection de l'onde optique reçue du satellite au foyer du segment sol est nécessaire.OPTIQUE ADAPTATIVE, LIENS OPTIQUES, COMMUNICATIONS OPTIQUES

International audience; Near-infrared wavefront sensing allows for the enhancement of sky coverage with adaptive optics. The recently developed HgCdTe avalanche photodiode arrays are promising due to their very low detector noise, but still present an imperfect cosmetic that may directly impact real-time wavefront measurements for adaptive optics and thus degrade performance in astronomical applications. We propose here a model of a Shack-Hartmann wavefront measurement in the presence of residual fixed pattern noise and defective pixels. To adjust our models, a fine characterization of such an HgCdTe array, the RAPID sensor, is proposed. The impact of the cosmetic defects on the Shack -Hartmann measurement is assessed through numerical simulations. This study provides both a new insight on the applicability of cadmium mercury telluride (CMT) avalanche photodiodes detectors for astronomical applications and criteria to specify the cosmetic qualities of future arrays. (C) 2015 Optical Society of America

We describe an adaptive optics approach for single-mode fiber coupling, developed jointly for interferometry and space communications. The aberrations downstream the wavefront sensor are estimated and corrected, statically or dynamically with atmospheric turbulence.

This paper draws on the reciprocity principle to quantify turbulence effects on ground-space adaptive optics assisted optical links. Illustrations are given in the case of high precision frequency transfer and of high data rate telecommunications.

Over the past decade, adaptive optics (AO) has become an established method for overcoming the effects of atmospheric turbulence on both astronomical imaging and spectroscopic observations. These systems are now beginning to make extensive use of laser guide star (LGS) techniques to improve performance and provide increased sky coverage. Sodium LGS AO employs one or more lasers at 589-nm wavelength to produce an artificial guide star through excitation of sodium atoms in the mesosphere (90 km altitude). Because of its dependence on the abundance and distribution of sodium atoms in the mesosphere, this approach has its own unique set of difficulties not seen with natural stars. The sodium layer exhibits time-dependent variations in density and altitude, and since it is at a finite range, the LGS images become elongated due to the thickness of the layer and the offset between the laser projection point and the subapertures of a Shack‐Hartmann wavefront sensor (SHWFS). Elongation causes the LGS image to be spread out resulting in a decrease in the signal-to-noise ratio which, in turn, leads to an increase in SHWFS measurement error and therefore an increased error in wavefront phase reconstruction. To address the problem of elongation, and also to provide a higher level of readout performance and reduced readout noise, a new type of charge-coupled device (CCD) is now under development for Shack‐Hartmann wavefront sensing called the polar coordinate CCD. In this device, discrete imaging arrays are provided in each SHWFS subaperture and the size, shape and orientation of each discrete imaging array are adjusted to optimally sample the LGS image. The device is referred to as the polar coordinate CCD because the location of each imager is defined by a polar coordinate system centred on the laser guide star projection point. This concept is especially suited to Extremely Large Telescopes (ELTs) where the effect of perspective elongation is a significant factor. In this paper, we evaluate the performance of centroiders based on this CCD geometry by evaluating the centroid error variance and also the linearity issues associated with LGS image sampling and truncation. We also describe how we will extend this work to address the problems presented by the time variability of the sodium layer and how this will impact SHWFS performance in LGS AO systems.

Next generation satellite-to-ground laser link systems, either for telemetry or satcom will request very high data rate. This goal could be achieved with a manageable cost if the benefits from the fibered technologies are reaped. For downlink the wave therefore needs to be coupled into a single mode fiber. Due to atmospheric turbulence its spatial coherence is compromised. This causes strong coupling losses that result into deep fadings. To cope with this, adaptive optics (AO) is envisaged. Thanks to a real time compensation of atmospheric induced optical path differences, it might enable to reach average coupling efficiencies as high as 0,5 (3 dB average losses). AO is now a mature technology, mostly brought to market by astronomy or biomedical applications. Usual correction bandwidth and available flux to perform the wavefront measurement are rather small (typically 50 Hz bandwidth and tenth of photons per subaperture and per frame). The specificity of AO for LEO satellite to ground optical links resides into higher requested bandwidth, optimal operation for a wide variety of atmospheric conditions (daytime and nighttime) with potentially low elevations that might severely affect wavefront sensing due to scintillation. In addition to this the performance criterion of the correction is different from usual imaging applications, with appropriate constraints on coupling statistics and temporal characteristics. To address these specificities, AO dimensioning approach needs to be adapted and consolidated by in situ measurements. We report here the first AO results on a LEO microsatellite as far as we know. The AO bench located at Coude focus of the MeO telescope, designed for imaging applications, is used to correct for optical aberrations on a 976 nm laser beam provided by SOTA terminal. AO performances are investigated and confronted to state of the art performance evaluations for satellite to ground laser links.

Free space laser communications make transmissions at high data rates possible. But they suffer from atmospheric turbulence. Based on modelling and experimental acquisitions, we characterize the propagation channel influence on the optical links.

We present a novel method to control the phase and amplitude of a femtosecond laser beam using a linearized version of the transport-of-intensity equation. Simulations show a peak power improvement better than 30%.

International audience; For satellite to ground laser links, atmospheric turbulence is a major cause of impairments. The induced phase perturbations along the propagation path cause beam scintillation in the receiver plane and they can also severely compromise the coupling of the flux into a receiver of limited size. To address these impairments, dedicated mitigation strategies must be developed. This requires accurate understanding of the perturbation origin. Beam propagation models have demonstrated their ability to reproduce statistical characteristics of optical perturbations on a satellite to ground laser link for elevations as low as 20°. For smaller elevations, measurements performed on stars illustrated the limits of analytical approaches and the interest for end-to-end models. We report here the first propagation channel measurements performed on a LEO microsatellite with a Shack-Hartmann wavefront sensor (WFS). The laser beam at 976 nm provided by SOTA optical terminal have been analyzed with a Shack- Hartmann wavefront sensor located at Coudé focus of the French ground station (1,55 m MéO telescope) in July 2015. Wavefront characteristics and scintillation patterns recorded with the WFS are analyzed and compared to atmospheric turbulence perturbations model fed with in situ measurements of atmospheric parameters retrieved from GDIMM.; La turbulence atmosphérique est la principale cause de dégradation des liens laser sol satellite. Les perturbations de phase induites le long du cana de propagation conduisent à de la scintillation dans le plan du récepteur et peuvent sévèrement compromettre le couplage du flux dans un récepteur de petite dimension. Pour résoudre ces problèmes, différentes stratégies dédiées doivent être développées. Cela nécessite une compréhension précise de l'origine des perturbations. Les modèles de propagation de faisceau ont démontré leur capacité à reproduire les caractéristiques statistiques des perturbations optiques affectant un lien laser satellite sol pour des élévations descendant à 20°. Pour des élévations plus petites des mesures réalisées sur étoiles ont démontrées toutefois les limites de ces approches analytiques et l'intérêt de modèles de type Monte-Carlo. Nous faisons état ici des premières mesures sur un canal de propagation établi avec un microsatellite LEO grâce à un analyseur de Shack Hartmann. Le faisceau laser à 976nm est fourni par le terminal optique SOTA et analysé grâce à un Shack Hartmann situé au foyer coudé d'une station française (télescope MéO de 1.55m) en juillet 2015. Les caractéristiques du front d'onde et les structures de scintillation enregistrées avec l'analyseur sont analysées et comparées aux modèles de perturbation de la turbulence atmosphérique alimentés par les mesures in situ des paramètres atmosphériques tels que fournis par un GDIMM.

The practical implementation of a phase diversity sensor for high resolution measurement of disturbed laser beams is detailed here. Numerical simulations show that, with simple material, a 60×60 map of complex amplitude with λ/100 accuracy can be obtained.

Analytical theory is combined with extensive numerical simulations to compare different flavours of centroiding algorithms: thresholding, weighted centroid, correlation, quad cell (QC). For each method, optimal parameters are defined in function of photon flux, readout noise and turbulence level. We find that at very low flux the noise of QC and weighted centroid leads the best result, but the latter method can provide linear and optimal response if the weight follows spot displacements. Both methods can work with average flux as low as 10 photons per subaperture under a readout noise of three electrons. At high-flux levels, the dominant errors come from non-linearity of response, from spot truncations and distortions and from detector pixel sampling. It is shown that at high flux, centre of gravity approaches and correlation methods are equivalent (and provide better results than QC estimator) as soon as their parameters are optimized. Finally, examples of applications are given to illustrate the results obtained in the paper.

The problematic of wavefront sensing in Multiconjugate Adaptive Optics (MCAO) is addressed in this article. We first focus on the sky coverage estimation which drives, in particular, the choice between natural and laser guide stars and therefore has a major impact on wavefront sensor design. Then a comparison between star oriented and layer oriented concepts is proposed. Analytical developments and optimization of the concepts are derived in the simplest MCAO case: the ground layer AO system. From this study, advantages and drawbacks of each concept are highlighted. To cite this article: T. Fusco et al., C. R. Physique 6 (2005).

Near-Infrared wave front sensing allows to enhance sky coverage with adaptive optics. The recently developed HgCdTe avalanche photodiode arrays are promising, but still present an unperfect cosmetic. After the fine characterization of such a HgCdTe array, we propose a statistical model of the pixel response. Assuming the cosmetic is partially corrected, we simulate the impact of the residual error on a Shack-Hartmann wavefront sensor measurement. The study shows the operability of such a real time correction, and of the HgCdTe APD array characterized in this framework: the RAPID camera.

International audience; We propose here a novel way to analyze Shack-Hartmann wavefront sensor images in order to retrieve more modes than the two centroid coordinates per sub-aperture. To do so, we use the linearized focal-plane technique (LIFT) phase retrieval method for each sub-aperture. We demonstrate that we can increase the number of modes sensed with the same computational burden per mode. For instance, we show the ability to control a 21 x 21 actuator deformable mirror using a 10 x 10 lenslet array. (C) 2014 Optical Society of America

The application of petawatt lasers to scientific and technological problems is advancing rapidly. The usefulness of these applications will depend on being able to produce petawatt pulses at much higher repetition rates than is presently possible. The International Coherent Amplification Network (ICAN) consortium seeks to design high repetition rate petawatt lasers using large scale coherent beam combination of femtosecond pulse amplifiers built from optical fibres. This combination of technologies has the potential to overcome many of the hurdles to high energy, high average power pulsed lasers, opening up applications and meeting societal challenges.

The control of the optical quality of a laser beam requires a complex amplitude measurement able to deal with strong modulus variations and potentially highly perturbed wavefronts. The method proposed here consists in an extension of phase diversity to complex amplitude measurements that is effective for highly perturbed beams. Named CAMELOT for Complex Amplitude MEasurement by a Likelihood Optimization Tool, it relies on the acquisition and processing of few images of the beam section taken along the optical path. The complex amplitude of the beam is retrieved from the images by the minimization of a Maximum a Posteriori error metric between the images and a model of the beam propagation. The analytical formalism of the method and its experimental validation are presented. The modulus of the beam is compared to a measurement of the beam profile, the phase of the beam is compared to a conventional phase diversity estimate. The precision of the experimental measurements is investigated by numerical simulations., Comment: Accepted for publication in Optics Express

The landscape of biomedical research in neuroscience has changed dramatically in recent years as a result of spectacular progress in dynamic microscopy. However, the optical accessibility of deep brain structures or deeper regions of the surgically exposed hippocampus (a few 100 microns typically) remains limited, due to volumic aberrations created by the sample inhomogeneities. Adaptive optics can correct for these aberrations. Our goal is to realize a novel adaptive optics module dedicated to in vivo two-photon calcium imaging of the hippocampus. The key issue in adaptive optics is the ability to perform an accurate and reliable wavefront sensing. In twophoton microscopy indirect methods are required. Two families of approaches have been proposed so far, the modal sensorless technique and a method based on pupil segmentation. We present here a formal comparison of these approaches, in particular as a function of the amount of aberrations.

The point spread function (PSF) of an adaptive optics system evolves in the Field Of View (FOV). This variation strongly limits the conventional deconvolution methods for the processing of wide FOV images. A theo- retical expression of this PSF variation is derived. This ex- pression is both validated on simulations and experimental data. It is then applied to the a posteriori processing of stellar elds. Using the available prior information about the object (point-like sources), this technique allows the restoration of the star parameters (positions and intensi- ties) with a precision much better than the conventional methods, in a FOV much larger than the isoplanatic eld.

Long-range free space optical communications suffer from atmospheric turbulence effects. To mitigate them, a bidirectional full-wave compensation technique seems promising. We present an experimental implementation and characterization of this concept on a laboratory breadboard. Experimental results confirm former numerical results for similar propagation conditions. The effects of measurement and control errors are analyzed by numerical modeling.

International audience; In order to address the high throughput requested for both downlink and uplink satellite to ground laser links, adaptive optics (AO) has become a key technology. While maturing, application of this technology for satellite to ground telecommunication, however, faces difficulties, such as higher bandwidth and optimal operation for a wide variety of atmospheric conditions (daytime and nighttime) with potentially low elevations that might severely affect wavefront sensing because of scintillation. To address these specificities, an accurate understanding of the origin of the perturbations is required, as well as operational validation of AO on real laser links. We report here on a low Earth orbiting (LEO) microsatellite to ground downlink with AO correction.We discuss propagation channel characterization based on Shack–Hartmann wavefront sensor (WFS) measurements.Fine modeling of the propagation channel is proposed based on multi-Gaussian model of turbulenceprofile. This model is then used to estimate the AO performance and validate the experimental results. While AO performance is limited by the experimental set-up, it proves to comply with expected performance and further interesting information on propagation channel is extracted. These results shall help dimensioning and operating AO systems for LEO to ground downlinks.; Le besoin croissant de très haut débit dans les liens sol-espace montants ou descendants a fait de l'Optique Adaptative (OA) une technologie clé. Bien que cette technologie soit aujourd'hui mature, son application aux liens sol-satellite se heurte toutefois à diverses difficultés telles que le besoin de fréquences d'échantillonnage accrues, une exploitation en conditions opérationnelles dans une large gamme de conditions atmosphériques (diurne ou nocturne), avec des élévations potentiellement très basses qui affectent sévèrement la détection en raison de la scintillation. La gestion de ces problématiques spécifiques requière une compréhension aigüe de l'origine des perturbations ainsi que des validations de l'OA sur lien optiques réels en conditions opérationnelles. Nous décrivons dans ce papier les résultats d'un lien optique établi entre un microsatellite LEO et une station sol avec correction par OA. Nous discutons de la caractérisation du canal de propagation sur la base des mesures fournies par le senseur de front d'onde de l'OA , de type Shack-Hartmann. Une modélisation fine de ce canal est proposée sur la base d'un modèle multi-gaussien du profil de turbulence. Ce modèle est ensuite exploité pour estimer la performance de l'OA et valider les résultats expérimentaux. Bien la performance de l'OA soit actuellement limitée par le système expérimental, elle se révèle cohérente avec la performance simulée et permet d'extraire des informations critiques sur le canal. Ces résultats devraient permettre dans le futur de mieux dimensionner et opérer des systèmes d'OA dédiés aux liens espace-sol pour les satellites LEO.

Adaptive optics provide real-time compensation for atmospheric turbulence. The correction quality relies on a key element: the wavefront sensor. We have designed an adaptive optics system in the mid-infrared range providing high spatial resolution for ground-to-air applications, integrating a Shack-Hartmann infrared wavefront sensor operating on an extended source. This paper describes and justifies the design of the infrared wavefront sensor, while defining and characterizing the Shack-Hartmann wavefront sensor camera. Performance and illustration of field tests are also reported.

International audience; Atmospheric turbulence perturbates to a great extent the optical path of incoming light from outer space thereby limiting the resolution power of capturing devices. One of the most common techniques used in astronomical imaging to compensate for this perturbation is Adaptive Optics (AO). In this paper we explore the potential of Microcanonical Multiscale Formalism (MMF) for the reconstruction of the perturbated wavefront, from the low-resolution acquisition of the turbulent phase by a Hartmann-Shack wavefront sensor used in AO. In fact, turbulent flows, although chaotic in nature, are characterised by scale hierarchy and develop cascade like structures where transfer of energy takes place from one scale to the other. We make use of MMF to infer properties along the scales of the complex signal consisting of optical phase perturbation and perform reconstruction using an appropriate wavelet decomposition associated to the cascading properties of the turbulent flow.

The precise reconstruction of the turbulent volume is a key point in the development of new-generation Adaptive Optics systems. We propose a new Cn2 profilometry method named CO-SLIDAR (COupled Slope and scIntillation Detection And Ranging), that uses correlations of slopes and scintillation indexes recorded on a Shack-Hartmann from two separated stars. CO-SLIDAR leads to an accurate Cn2 retrieval for both low and high altitude layers. Here, we present an end-to-end simulation of the Cn2 profile measurement. Two Shack-Hartmann geometries are considered. The detection noises are taken into account and a method to subtract the bias is proposed. Results are compared to Cn2 profiles obtained from correlations of slopes only or correlations of scintillation indexes only., Comment: 10 pages, 8 figures, SPIE Conference "Astronomical Telescopes and Instrumentation" 2012, Amsterdam, paper 8447-194

International audience; Adaptive Optics (AO) refers to (servo-control) methods used in astronomical imaging to compensate in real time for the loss of spatial resolution power of the instrument caused by the earth's upper atmosphere turbulence, which produces inhomogeneties of air refractive index and hence modifies in a chaotic manner the optical phase of incoming light from space. In this paper we present the first results of a radically new approach to phase reconstruction in AO based on non linear methods for the processing of complex signals having multiscale properties. We make use of the Microcanonical Multiscale Formalism (herein referred to as MMF) to infer properties along the scales of the complex signal made of the perturbated optical phase, and we perform the reconstruction of the phase using appropriate wavelet decomposition associated to the cascading properties of fully developped turbulence.

With the advent of high resolution imaging through the atmosphere, turbulence distribution measurement has become a key issue. The possibility to measure C2n profile from Shack-Hartman data, slopes and intensities, acquired on a unique point source has recently been demonstrated numerically.1 This method, called SCOSLIDAR, is exploited here on experimental data. From slopes and intensities of a mid-infrared wavefront sensor, C2n profiles along an oblique line of sight are estimated and compared to local measurements. Time averaged profiles are confronted to a profile deduced from similitude law.

The future of adaptive optics includes laser guide stars. While they are a great solution to sky coverage, they do introduce additional errors in the adaptive optics system. In particular, because of the finite thickness of the sodium layer, there is reduced centroiding accuracy due to elongated spots in the wavefront sensor. These become even more pronounced on large telescope apertures. In this paper we focus on the performance of a Shack-Hartmann wavefront sensor for 30m-plus large aperture telescopes studying the consequences of both the decrease in signal to noise ratio due to the spot elongation and the variations of the sodium density variations in the mesosphere. We incorporate real on-sky measurements of the return from the sodium layer using images of the laser guide star taken at Lick Observatory and simulate the expected wavefront reconstruction performance in the case of a Thirty Meter Telescope. Using this ensemble of data, we compare performance for various Hartmann centroiding methods, including correlation and weighted least square algorithms.

Adaptive Optics (AO) relies on aWave Front Sensor (WFS) to measure properly the perturbations induced by the turbulence on the wavefront. Yet, source extension may limit its performance. In the case of a Shack-Hartmann (SH) WFS associated to one or more Laser Guide Stars (LGS), this extension becomes all the more problematic as the diameter of the telescope tends to increase. In order to mitigate the effects induced by the spot elongation on the quality of the WFS measurements, various algorithms are used. Most of them are non-linear, and require a reference. The design of the SH WFS itself has an influence on the measurement quality, as it also induces non-linearities. In addition, the time-evolving structure of the LGS, due to the fluctuations of the Sodium layer, has an impact on the measurements as well. Eventually, both influences are coupled in the wavefront sensing process. The present study is aimed at optimizing a whole set of carefully chosen parameters defining the design of a SH WFS, in the particular case of a LGS on an ELT. The relative impact of these parameters are studied first at the subaperture level, and then on the reconstructed wavefront.

ATLAS is a generic Laser Tomographic AO (LTAO) system for the E-ELT. Based on modular, relatively simple, and yet innovative concepts, it aims at providing diffraction limited images in the near infra-red for a close to 100 percent sky coverage.

Data-rates of long-range free-space optical communication links are deteriorated by atmospheric turbulence which causes power in the bucket fluctuations. In order to compensate for those effects the use of adaptive optics is envisioned. Different solutions have been proposed for the correction. We study here the performances of several compensation methods, encompassing both amplitude and phase and phase-only precompensation. In the case of phase-only precompensation we studied two system designs, one which is dedicated to symmetrical communication systems and the other to dissymmetric systems. In the dissymmetric case we studied two ways of driving the deformable mirror: the use of a Shack-Hartmann wavefront sensor and a model-free phase modulation. For each compensation architecture simulation results covered weak, moderate and strong turbulence conditions.