Accurate measurement of Cn2 profile with Shack-Hartmann data

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 8447-19

Optical ground station optimization for future optical geostationary satellite feeder uplinks

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)

Imagerie à travers la turbulence par déconvolution myope multi-trame

La déconvolution par analyse de fronts d'onde est une technique d'imagerie à haute résolution en présence de turbulence atmosphérique. Elle consiste en une déconvolution multi-trame d'images courte pose utilisant des mesures de fronts d'onde enregistrées simultanément. L'approche bayésienne proposée ici nous permet de construire un algorithme de déconvolution original pour prendre en compte le bruit sur les mesures de front d'onde. L'utilisation de connaissances a priori sur les objets observés et sur la statistique de la turbulence atmosphérique nous permet de plus de régulariser le problème. Une simulation numérique montre l'utilité de la régularisation sur les fronts d'onde

Polychromatic guide star: feasibility study

International audienceAdaptive optics at astronomical telescopes aims at correcting in real time the phase corrugations of incoming wavefronts caused by the turbulent atmosphere, as early proposed by Babcock. Measuring the phase errors requires a bright source located within the isoplanatic patch of the program source. The probability that such a reference source exists is a function of the wavelength, of the required image quality (Strehl ratio), of the turbulence optical properties, and of the direction of the observation. It turns out that the sky coverage is disastrously low in particular in the visible wavelength range where, unfortunately, the gain in spatial resolution brought by adaptive optics is the largest. Foy and Labeyrie have proposed to overcome this difficulty by creating an artificial point source in the sky in the direction of the observation relying on the backscattered light due to a laser beam. This laser guide star (hereinafter referred to as LGS) can be bright enough to allow us to accurately measure the wavefront phase errors, except for two modes which are the piston (not relevant in this case) and the tilt. Pilkington has emphasized that the round trip time of the laser beam to the mesosphere, where the LGS is most often formed, is significantly shorter than the typical tilt coherence time; then the inverse-return-of-light principle causes deflections of the outgoing and the ingoing beams to cancel. The apparent direction of the LGS is independent of the tilt. Therefore the tilt cannot be measured only from the LGS. Until now, the way to overcome this difficulty has been to use a natural guide star to sense the tilt. Although the tilt is sensed through the entire telescope pupil, one cannot use a faint source because $APEX 90% of the variance of the phase error is in the tilt. Therefore, correcting the tilt requires a higher accuracy of the measurements than for higher orders of the wavefront. Hence current adaptive optics devices coupled with a LGS face low sky coverage. Several methods have been proposed to get a partial sky coverage for the tilt. The only one providing us with a full sky coverage is the polychromatic LGS (hereafter referred to as PLGS). We present here a progress report of the R&D; program Etoile Laser Polychromatique et Optique Adaptative (ELP-OA) carried out in France to develop the PLGS concept. After a short recall of the principles of the PLGS, we will review the goal of ELP-OA and the steps to get over to bring it into play. We finally shortly described the effort in Europe to develop the LGS

Polychromatic guide star: feasibility study

International audienceAdaptive optics at astronomical telescopes aims at correcting in real time the phase corrugations of incoming wavefronts caused by the turbulent atmosphere, as early proposed by Babcock. Measuring the phase errors requires a bright source located within the isoplanatic patch of the program source. The probability that such a reference source exists is a function of the wavelength, of the required image quality (Strehl ratio), of the turbulence optical properties, and of the direction of the observation. It turns out that the sky coverage is disastrously low in particular in the visible wavelength range where, unfortunately, the gain in spatial resolution brought by adaptive optics is the largest. Foy and Labeyrie have proposed to overcome this difficulty by creating an artificial point source in the sky in the direction of the observation relying on the backscattered light due to a laser beam. This laser guide star (hereinafter referred to as LGS) can be bright enough to allow us to accurately measure the wavefront phase errors, except for two modes which are the piston (not relevant in this case) and the tilt. Pilkington has emphasized that the round trip time of the laser beam to the mesosphere, where the LGS is most often formed, is significantly shorter than the typical tilt coherence time; then the inverse-return-of-light principle causes deflections of the outgoing and the ingoing beams to cancel. The apparent direction of the LGS is independent of the tilt. Therefore the tilt cannot be measured only from the LGS. Until now, the way to overcome this difficulty has been to use a natural guide star to sense the tilt. Although the tilt is sensed through the entire telescope pupil, one cannot use a faint source because $APEX 90% of the variance of the phase error is in the tilt. Therefore, correcting the tilt requires a higher accuracy of the measurements than for higher orders of the wavefront. Hence current adaptive optics devices coupled with a LGS face low sky coverage. Several methods have been proposed to get a partial sky coverage for the tilt. The only one providing us with a full sky coverage is the polychromatic LGS (hereafter referred to as PLGS). We present here a progress report of the R&D; program Etoile Laser Polychromatique et Optique Adaptative (ELP-OA) carried out in France to develop the PLGS concept. After a short recall of the principles of the PLGS, we will review the goal of ELP-OA and the steps to get over to bring it into play. We finally shortly described the effort in Europe to develop the LGS

Conversion de frequence et compression d'impulsion d'un laser Alexandrite par diffusion Raman stimulee intracavite

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Maîtrise du front d'onde

My research work falls under the theme of "High Angular Resolution", defined as the study and development of methods and instruments for approaching the limit of diffraction, in spite of dynamic aberrations. They focus on the measurement of the wavefront for adaptive optics and active optics and on the effects of optical propagation in the lower atmosphere and the means of correcting it.In astronomy, the perturbations of the optical wave are modeled by the fluctuations of its phase. This approach is no longer valid as soon as more severe propagation conditions are encountered. The work carried out on the limits of classical adaptive optics, particularly in near-ground propagation conditions, and on the search for an alternative approach for optical links, will be particularly developed.Mes travaux de recherche s'inscrivent dans la thématique "Haute Résolution Angulaire", définie comme l'étude et le développement des méthodes et des instruments permettant d'approcher la limite de résolution théorique de la diffraction, en dépit des aberrations évolutives. Ils ont porté, d’une part sur la mesure de front d’onde pour l’optique adaptative et l’optique active, d’autre part sur les effets de la propagation optique dans la basse atmosphère et les moyens de les corriger.En astronomie, les perturbations de l'onde sont en général modélisées par les fluctuations de sa phase. Cette approche n’est plus valide dès que des conditions de propagation plus sévères sont rencontrées. Les travaux menés sur les limites d'application de l'optique adaptative classique, notamment dans les conditions de propagation proches du sol, et sur la recherche d'une approche alternative pour les liens optiques, seront particulièrement développés

Propagation optique et correction en forte turbulence

L optique adaptative multi-conjuguée (OAMC) a été proposée en astronomie pour augmenter le champ de correction de l optique adaptative. En imagerie terrestre, les effets de la turbulence atmosphérique sont plus importants : le champ de correction est encore plus faible et les effets diffractifs de la propagation ne peuvent être négligés. L objectif de cette thèse est de déterminer les performances et les limites de l OAMC lorsque les effets de la propagation sont pris en compte. Pour cette étude, nous nous sommes placés dans une configuration idéale où les effets de la turbulence sont simulés par deux couches parfaitement conjuguées avec deux miroirs déformables. Nous avons montré que l OAMC pourrait permettre d apporter une correction satisfaisante même dans les cas de très fortes perturbations. La limitation ultime de la correction est due à la troncature de la pupille qui provoque en particulier des fluctuations d intensité dans le plan image de la pupille. Nous avons proposé un modèle de réponse impulsionnelle prenant en compte ces effets. Nous avons montré que ces fluctuations représentent un problème majeur pour la mise en œuvre d une OAMC en perturbant la mesure de front d onde. Parallèlement, en se plaçant dans le cadre des faibles perturbations, nous avons évalué analytiquement les corrélations des mesures de l analyseur de Shack-Hartmann (pentes et intensités) en fonction du profil de Cn2. Nous avons utilisé ce formalisme pour étudier l erreur d anisoplanétisme sur les mesures de front d onde par Shack-Hartmann avec un objet étendu. Nous avons ensuite montré qu il était possible de l appliquer à la détermination du profil de Cn2 par inversion des mesures.Multi-conjugate Adaptive Optics (MCAO) has been suggested in astronomy to increase the correction field of adaptive optics. The effects of atmospheric turbulence are stronger in case of near to ground observations: the correction field is smaller and diffractive effects of the propagation can no longer be neglected. The main purpose of this work is to study MCAO performances and limitations when the diffractive effects of the propagation are taken into account. We have considered an ideal configuration of two layers and two exactly conjugated dynamical mirrors. We show that MCAO may provide a good correction even if perturbations are strong. We observe that the limitation to a perfect correction comes from pupil truncation effects. They cause intensity fluctuations in the conjugated plane of the pupil. We propose a model of the point spread function which takes into account those effects. Concerning intensity fluctuations, we demonstrate that they can strongly affect wavefront measurement and therefore jeopardise MCAO systems operation. Finally, in the framework of the small perturbations approximation, we have expressed analytically the correlations between Shack-Hartmann data (slopes and intensities) as functions of the Cn profile. This formalism is used to study anisoplanatism effects on wavefront measurement with a Shack-Hartmann operating on extended sources. We also demonstrate that it can be used to recover the Cn profile by data inversion.NICE-BU Sciences (060882101) / SudocSudocFranceF