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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.

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.

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.

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.

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 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 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.

International audience; The influence of air turbulence on femtosecond laser filamentation is studied experimentally and numerically for laser powers of a few critical powers. Air turbulence in the path of the beam prior to filamentation induces a large pointing and formation instability attributed to an increase of the self-focusing distance and higher modulational instability in the presence of turbulence. By contrast, previously formed filaments are robust both in terms of beam pointing accuracy and survival when crossing turbulent air.

One of the major issues in new Extremely Large Telescopes is the phasing of their primary segmented mirror. A cophasing sensor is mandatory to achieve the ultimate resolution of the telescope. Phase diversity (PD) is a light-hardware cophasing technique. In this paper, we show that this technique is suited to segmented pupil instruments, such as the E-ELT.

Scintillation effects caused by the fluctuations of the refractive index of air are not negligible in the stratosphere. Recent experiments highlight the composite nature of optical turbulence in the stratosphere. We present an analytical model of scintillation based on a 3-D model of anisotropic and isotropic refractive index fluctuations spectrum that predicts scintillation rates inside the Rytov regime. This model uses a multi-layer decomposition of the turbulence profile. The effect of anisotropy leads to significant scintillation rates when a source is observed with a horizontal line of sight. Astronomical observations consisting in stellar scintillation made from balloon-borne spectrometer AMON-RA allow us to remotely probe statistical characteristics in the stratosphere, to validate the model of scintillation developed herein and to obtain refined values of its parameters. Data reduction from these observations brings out values of the inner scale of the anisotropic spectrum. We retrieve metric values of the inner scale that are compatible with space-based measurements. We find a major contribution of the anisotropic spectrum relatively to the isotropic contribution. This effect is particularly noticeable when the sight line plunges into the atmosphere, leading to strong scintillation as well as coupled chromatic refraction effects. This makes clear the presence of gravity waves in the stratosphere.

Different statistical properties of laser illumination through atmospheric turbulence have already been derived either in the small or strong perturbation regimes. However, modeling the instantaneous illumination remains useful for the estimation of optical system performances. using an end-to-end propagation model based on a split step algorithm leads up to heavy calculations. We propose a simplified model to provide instantaneous laser illuminations according to the C2/n profile. This simplified method has been implemented. Hypothesis taken into account and cross check with a numerical end-to-end model developed at Onera are presented.

The Cophasing Sensor (CS), which measures the disturbances between the sub-apertures, is a key component of multiple-aperture telescopes. As multiple-aperture telescopes become more ambitious, requirements for the CS become more demanding: low flux (for stellar interferometers), sub-nanometric accuracy (for interferometric nullers), image with very small contrast (for wide-field telescopes such as spaceborne Earth imagers), larger number of beams (for all applications). Focal-plane sensing is a solution to cope with all these requirements, with a very simple opto-mechanical setup. Two implementations have been investigated at ONERA: phase retrieval , using the sole focal-plane image, and phase diversity , based on the joint analysis of a focal and an extra-focal images. Phase diversity can measure any mode on any source, while phase retrieval is more suited to real-time piston/tip/tilt measurements on an unresolved (or partially resolved) source. To evaluate accurately the performance of CS or other high-resolution devices, ONERA has built a multipurpose bench called BRISE (Banc Reconfigurable d'Interferometrie sur Sources Etendues). BRISE mainly includes an extended scene and a reference point source, a deformable mirror, a focal-plane CS, afocal input/output ports to interface with other instruments, and a general purpose code MASTIC (Multiple-Aperture Software for Telescope Imaging and Cophasing). BRISE has already been (or will be) used for several applications, such as the validation of CSs for Earth imaging or nulling interferometry, or the exploration of advanced nulling techniques. This paper describes the bench and the investigated CSs, the experiments performed on BRISE, and reports main results such as nanometric accuracy or three-beam nulling.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Adaptive optics enable large telescopes to provide diffraction limited images, but their corrected field is restrained by the angular decorrelation of the turbulent wave-fronts. However many scientific goals would benefit a wide and uniformly corrected field, even with a partial correction. Ground Layer Adaptive Optics systems are supposed to provide such a correction by compensating the lower part of the atmosphere only. Indeed this layer is in the same time highly turbulent and isoplanatic on a rather wide field. In such a system the wave-front analysis is a critical issue. Measuring the ground layer turbulence requires multi-object wave-front analysis. Two multi-object wave-front sensing concepts have been proposed so far, derived from multi conjugate adaptive optics. They are the star oriented and the layer oriented approaches. A criterion for the analytical study of both concepts performance had been proposed in a previous presentation. First results on the behavior one can expect from one concept or the other had been given then. Here is presented a study made by improving the analytical model and completing its results with the ones of a numerical model which accounts for AO limitations that are uneasy to insert in an analytical formalism. Results are presented that highlight the advantages and drawbacks of each wave-front sensing concepts and the interest of optimizing them.

We propose a first orae performance estimation of multiconjugate adaptive optics (MCAO) systems. An important and restrictive parameter is the angular guide star (GS) separation for a field of view (FOV) of interest to be compensated. An analytical approach is proposed to estimate the residual variance of a MCAO systems for a given position in the FOV as a function of GS separation. This approach allows us to define expected isoplanatic angles for MCAO systems as a function of the atmospheric and observing conditions (turbulence profile, telescope diameter and GS separation). The analytical results are also compared with the numerical simulation of a MCAO system, in which a great care has been taken in the 3D wavefront reconstruction from 2D wavefront sensor (WFS) measurements. For a 8 m telescope, we show that 3 GS's and 3 deformable mirrors provides very good performance in a 200 arcseconds FOV at 2.2 micrometer.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Adaptive Optics systems provide a real time compensation for atmospheric turbulence, which severely limits the resolution of large telescopes. However, the correction is often only partial and a deconvolution is required to reach the telescope diffraction limit. The need for a regularized deconvolution is discussed, and a Maximum A Posteriori based deconvolution technique is presented. This technique incorporates a positivity constraint and the knowledge of the object Power Spectral Density. This method is then extended to the case of an unknown PSF. Deconvolution results are presented for both simulated and experimental data.

Adaptive optics and related techniques are now routinely used in the field of astronomical observations to compensate images for turbulence induced degradations and to retrieve the telescope diffraction limited resolution. However, for ground-to-ground applications, the effects of turbulence on both passive and active systems are more severe. The environmental constraints are also more important. Nevertheless, adaptive optics issues exist. The purpose of this paper is to review the evolutions of turbulence limitations according to the observing scenario. The turbulence anisoplanatism effects are emphasized. Furthermore, potential fields of application of adaptive optics and related techniques are presented.

Laser guide stars (LGSs) aim at increasing the sky coverage of adaptive optics (AO) as this is highly restricted when using only natural guide stars. With such three-dimensional extended objects, spot elongation may limit the measurement accuracy of wavefronts. We evaluate the effect of differential focal anisoplanatism, induced solely by the longitudinal extension of a side-launched LGS, on the slope measurements performed by a Shack-Hartmann for a 40 m class telescope. We also take this effect into account in the wavefront reconstruction and derive estimations of the resulting wavefront error in a multi-LGS AO system. We find an error of 100 nm in the worst case at the subaperture level and a small error of the order of 10 nm for six LGSs after wavefront reconstruction.

The objectives of the COME-ON project are to experiment a 19 actuator adaptive optics system planned to be used on a 4 m diameter telescope. This project has been worked out by 4 teams from: - Laserdot (Aerospatiale) - Office National d'Etudes et de Recherches Aerospatiales - Observatoire de Paris - European Southern Observatory (E.S.O.) The adaptive system is made of the following major components: - a 19 actuator adaptive mirror - a Shack-Hartmann wavefront sensor and its real time slope processor - a 68020 Motorola based real time control processor In the present paper we relate the works done for: - The interaction matrix identification. This matrix links the slope measurements and the controls of the adaptive mirror. A process to determine this matrix by a direct measurement method is described ; considerations on the matrix inversion and control mode cancellation are examined. - The control algorithm performing the real time computations needed to control the 19 actuators is described. Some results of the laboratory and observatory tests are given. Same paper has been presented during the SPIE's 90 Symposium on Astronomical Telescopes and Instrumentation for the 21st Century in February 1990.

Space optical communication requires high quality collimated laser diodes. A Hartmann-Shack wavefront sensor was developed at ONERA in order to test wavefront quality of such laser beams. For this application, the Hartmann-Shack wavefront sensor offers many advantages compared to an interferometer system (simple design, compactness, no moving part). Presently, the wavefront measurement precision is limited by the quality of the reference wavefront used to calibrate the Hartmann-Shack wavefront sensor (λ/100 rms). Preliminary experimental tests was achieved. The sensitivity of the Hartmann-Shack wavefront sensor was only limited by the experiment conditions.

We designed an optical bench to demonstrate full-wave precompensation for laser telecommunications. This technique requires a device performing time reversed waves. We propose and characterize a solution to realize such a function.