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Optical fiber sensors based on Fiber Bragg Grating (FBG) technology are used to monitor the mechanical behavior of the magnet's coils from its assembly at room temperature to its powering at cryogenic temperatures. The development of this instrumentation required several years of research and development effort to validate the precision and accuracy of FBGs based measurements in cryogenic conditions. FBGs are bonded on the coils in different locations along with the longitudinal and azimuthal directions that allow studying the average stress levels. The stainless-steel shells of the magnets and each extremity of the four rods are equipped with resistive strain gauges (RSG). This paper provides a description of the instrumentation used to monitor the strain profile of superconducting magnets. Strain measurements during magnet assembly, cool-down, and powering are presented to confirm the agreement between electrical and optical sensors.

Context: The detection and characterization of Earth-like exoplanets (exoEarths) from space requires exquisite wavefront stability at contrast levels of $10^{-10}$. On segmented telescopes in particular, aberrations induced by cophasing errors lead to a light leakage through the coronagraph, deteriorating the imaging performance. These need to be limited in order to facilitate the direct imaging of exoEarths. Aims: We perform a laboratory validation of an analytical tolerancing model that allows us to determine wavefront error requirements in the $10^{-6} - 10^{-8}$ contrast regime, for a segmented pupil with a classical Lyot coronagraph. We intend to compare the results to simulations, and we aim to establish an error budget for the segmented mirror on the High-contrast imager for Complex Aperture Telescopes (HiCAT) testbed. Methods: We use the Pair-based Analytical model for Segmented Telescope Imaging from Space (PASTIS) to measure a contrast influence matrix of a real high contrast instrument, and use an analytical model inversion to calculate per-segment wavefront error tolerances. We validate these tolerances on the HiCAT testbed by measuring the contrast response of segmented mirror states that follow these requirements. Results: The experimentally measured optical influence matrix is successfully measured on the HiCAT testbed, and we derive individual segment tolerances from it that correctly yield the targeted contrast levels. Further, the analytical expressions that predict a contrast mean and variance from a given segment covariance matrix are confirmed experimentally., Comment: 16 pages, 16 figures, accepted for publication in A&A

Coupled slope and scintillation detection and ranging (CO-SLIDAR) is a very promising technique for the metrology of near ground C n 2 profiles. It exploits both phase and scintillation measurements obtained with a dedicated wavefront sensor and allows profiling on the full line of sight between pupil and sources. This technique is applied to an associated instrument based on a mid-IR Shack–Hartmann wavefront sensor coupled to a 0.35 m telescope, which observes two cooperative sources. This paper presents what we believe is the first comprehensive description of the CO-SLIDAR method in the context of near-ground optical turbulence metrology. It includes the presentation of the physics principles underlying the measurements of our unsupervised C n 2 profile reconstruction strategy together with the error bar estimation on the reconstructed values. The application to data acquired in a heterogeneous rural landscape during an experimental campaign in Lannemezan, France, demonstrates the ability to obtain profiles with a sampling pitch of about 220 m over a 2.7 km line of sight. The retrieved C n 2 profiles are presented and their variability in space and time is discussed.

Effective and accurate in-vivo diagnosis of retinal pathologies requires high performance imaging devices, combining a large field of view and the ability to discriminate the ballistic signal from the diffuse background in order to provide a highly contrasted image of the retinal structures. Here, we have implemented the Partial-Field Illumination Ophthalmoscope, a patterned illumination modality, integrated on a high pixel rate adaptive optics full-field microscope. This non-invasive technique enables us to mitigate the low signal-to-noise ratio, intrinsic of full-field ophthalmoscopes, by partially illuminating the retina with complementary patterns to reconstruct a wide field image. This new modality provides an image contrast spanning from the full-field to the confocal contrast, depending on the pattern size. As a result, it offers various trade-offs in terms of contrast and acquisition speed, guiding the users towards the most efficient system for a particular clinical application., 6 pages, 5 figures

We present recent laboratory results demonstrating high-contrast coronagraphy for future space-based large segmented telescopes such as the Large UV, Optical, IR telescope (LUVOIR) mission concept studied by NASA. The High-contrast Imager for Complex Aperture Telescopes (HiCAT) testbed aims to implement a system-level hardware demonstration for segmented aperture coronagraphs with wavefront control. The telescope hardware simulator employs a segmented deformable mirror with 36 hexagonal segments that can be controlled in piston, tip, and tilt. In addition, two continuous deformable mirrors are used for high-order wavefront sensing and control. The low-order sensing subsystem includes a dedicated tip-tilt stage, a coronagraphic target acquisition camera, and a Zernike wavefront sensor that is used to measure low-order aberration drifts. We explore the performance of a segmented aperture coronagraph both in “static” operations (limited by natural drifts and instabilities) and in “dynamic” operations (in the presence of artificial wavefront drifts added to the deformable mirrors), and discuss the estimation and control strategies used to reach and maintain the dark zone contrast. We summarize experimental results that quantify the performance of the testbed in terms of contrast, inner/outer working angle and bandpass, and analyze limiting factors by comparing against our end-to-end models.

Quasi-static aberrations in coronagraphic systems are the ultimate limitation to the capabilities of exoplanet imagers both ground-based and space-based. These aberrations - which can be due to various causes such as optics alignment or moving optical parts during the observing sequence - create light residuals called speckles in the focal plane that might be mistaken for a planets. For ground-based instruments, the presence of residual turbulent wavefront errors due to partial adaptive optics correction causes an additional difficulty to the challenge of measuring aberrations in the presence of a coronagraph. In this paper, we present an extension of COFFEE, the coronagraphic phase diversity, to the estimation of quasi-static aberrations in the presence of adaptive optics-corrected residual turbulence. We perform realistic numerical simulations to assess the performance that can be expected on an instrument of the current generation. We perform the first experimental validation in the laboratory which demonstrates that quasistatic aberrations can be corrected during the observations by means of coronagraphic phase diversity., Comment: This is a pre-copyedited, author-produced PDF of an article accepted for publication in the Monthly Notices of the Royal Astronomical Society following peer review

Focal plane wavefront sensing is an elegant solution for wavefront sensing since near-focal images of any source taken by a detector show distortions in the presence of aberrations. Non-Common Path Aberrations and the Low Wind Effect both have the ability to limit the achievable contrast of the finest coronagraphs coupled with the best extreme adaptive optics systems. To correct for these aberrations, the Subaru Coronagraphic Extreme Adaptive Optics instrument hosts many focal plane wavefront sensors using detectors as close to the science detector as possible. We present seven of them and compare their implementation and efficiency on SCExAO. This work will be critical for wavefront sensing on next generation of extremely large telescopes that might present similar limitations., Comment: SPIE proceeding : Astronomical Telescopes + Instrumentation 2020. arXiv admin note: text overlap with arXiv:1912.10179

Focal plane wavefront sensing is an appealing technique to cophase multiple aperture telescopes. Phase diversity, operable with any aperture configuration or source extension, generally suffers from high computing load. In this Letter, we introduce, characterize and experimentally validate the LAPD algorithm, based on a fast linearized phase diversity algorithm \rev{with a capture range comparable to classic phase diversity.} We demonstrate that a typical performance of lambda/75 RMS wavefront error can be reached., Comment: Accepted for publication in JATIS

The goal of the High-contrast imager for Complex Aperture Telescopes (HiCAT) testbed is to demonstrate coronagraphic starlight suppression solutions for future segmented aperture space telescopes such as the Large UV, Optical, IR telescope (LUVOIR) mission concept being studied by NASA. The testbed design has the flexibility to enable studies with increasing complexity for telescope aperture geometries starting with off-axis telescopes, then on-axis telescopes with central obstruction and support structures. The testbed implements the Apodized Pupil Lyot Coronagraph (APLC) optimized for the HiCAT aperture, which is similar to one of the possible geometries considered for LUVOIR. Wavefront can be controlled using continuous deformable mirrors, and wavefront sensing is performed using the imaging camera, or a dedicated phase retrieval camera, and also in a low-order wavefront sensing arm. We present a progress update of the testbed in particular results using two deformable mirror control to produce high-contrast dark zone, and preliminary results using the testbed’s low order Zernike wavefront sensor.

International audience; Multiphoton (MP) microscopy of thick biological tissues is limited by the presence of aberrations and scattering. In addition, living samples impose an additional limit due to uncontrolled movements. To overcome these constraints, deconvolution methods are proposed as an alternative to well-established adaptive optics procedures. A marginal blind deconvolution approach for the reconstruction of MP images of both ex-vivo and living ocular tissues is shown and discussed. The quality of the images reconstructed by this method is compared to that of the original ones in terms of the achieved resolution and inherent artefacts, among other criteria.

Direct imaging of exo-Earths and search for life is one of the most exciting and challenging objectives for future space observatories. Segmented apertures in space will be required to reach the needed large diameters beyond the capabilities of current or planned launch vehicles. These apertures present additional challenges for high-contrast coronagraphy, not only in terms of static phasing but also in terms of their stability. The Pair-based Analytical model for Segmented Telescope Imaging from Space (PASTIS) was developed to model the effects of segment-level optical aberrations on the final image contrast. In this paper, we extend the original PASTIS propagation model from a purely analytical to a semi-analytical method, in which we substitute the use of analytical images with numerically simulated images. The inversion of this model yields a set of orthonormal modes that can be used to determine segment-level wavefront tolerances. We present results in the case of segment-level piston error applied to the baseline coronagraph design of LUVOIR A, with minimum and maximum wavefront error constraint between 56 pm and 290 pm per segment. The analysis is readily generalizable to other segment-level aberrations modes, and can also be expanded to establish stability tolerances for these missions., Comment: 15 pages, 10 figures; published

Direct imaging of Earth-like planets from space requires dedicated observatories, combining large segmented apertures with instruments and techniques such as coronagraphs, wavefront sensors, and wavefront control in order to reach the high contrast of 10^10 that is required. The complexity of these systems would be increased by the segmentation of the primary mirror, which allows for the larger diameters necessary to image Earth-like planets but also introduces specific patterns in the image due to the pupil shape and segmentation and making high-contrast imaging more challenging. Among these defects, the phasing errors of the primary mirror are a strong limitation to the performance. In this paper, we focus on the wavefront sensing of segment phasing errors for a high-contrast system, using the COronagraphic Focal plane wave-Front Estimation for Exoplanet detection (COFFEE) technique. We implemented and tested COFFEE on the High-contrast imaging for Complex Aperture Telescopes (HiCAT) testbed, in a configuration without any coronagraph and with a classical Lyot coronagraph, to reconstruct errors applied on a 37 segment mirror. We analysed the quality and limitations of the reconstructions. We demonstrate that COFFEE is able to estimate correctly the phasing errors of a segmented telescope for piston, tip, and tilt aberrations of typically 100nm RMS. We also identified the limitations of COFFEE for the reconstruction of low-order wavefront modes, which are highly filtered by the coronagraph. This is illustrated using two focal plane mask sizes on HiCAT. We discuss possible solutions, both in the hardware system and in the COFFEE optimizer, to mitigate these issues., Comment: 12 pages, 15 figures, Astronomy & Astrophysics

International audience; The design and performance of an adaptive optics flood illumination ophthalmoscope (AO-FIO) platform, based on eye motion and dynamic aberrations experimental analysis, are described. The system incorporates a custom-built real-time controller, enabling up to 70 Hz loop rate without jitter, and an AO-corrected illumination capable of projecting high-resolution features in the retina. Wide-field (2.7°×5.4°) and distortionless images from vessel walls, capillaries, and the lamina cribrosa are obtained with an enhanced contrast and signal-to-noise ratio, thanks to careful control of AO parameters. The high spatial and temporal resolution (image acquisition up to 200 Hz) performance achieved by this platform enables the visualization of vessel deformation and blood flow. This system opens up the prospect of a return to favor of flood illumination adaptive optics systems provided that its high pixel rate and structured illumination capabilities are exploited.; La conception et la performance de l’ophtalmoscope plein champ assisté par Optique Adaptative (OA), basés sur des analyses expérimentales des mouvements et aberrations oculaires, sont décrites. Le système est composé d’un calculateur en temps-réel qui permet d’atteindre une vitesse de boucle d’asservissement de 70Hz. Le système permet aussi la projection de motifs dans la rétine. Grâce au contrôle et la maîtrise d’un système d’Optique Adaptative, des images à grand champ (2.7x5.4°) sans distorsion des différentes régions de la rétine présentent une amélioration de contraste et de rapport signal sur bruit. La haute résolution spatio-temporelle permet également la visualisation des déformations vasculaire et du flux sanguin. Ce système ouvre des nouvelles perspectives en ophtalmoscopie plein-champ, qui pourrait s'imposer dans les années à venir à condition que sa haute résolution temporelle et sa capacité de projeter des motifs d’illumination soient exploitées.

Context. The next generation of space-borne instruments dedicated to the direct detection of exoplanets requires unprecedented levels of wavefront control precision. Coronagraphic wavefront sensing techniques for these instruments must measure both the phase and amplitude of the optical aberrations using the scientific camera as a wavefront sensor. Aims. In this paper, we develop an extension of coronagraphic phase diversity to the estimation of the complex electric field, that is, the joint estimation of phase and amplitude. Methods. We introduced the formalism for complex coronagraphic phase diversity. We have demonstrated experimentally on the Tr\`es Haute Dynamique testbed at the Observatoire de Paris that it is possible to reconstruct phase and amplitude aberrations with a subnanometric precision using coronagraphic phase diversity. Finally, we have performed the first comparison between the complex wavefront estimated using coronagraphic phase diversity (which relies on time-modulation of the speckle pattern) and the one reconstructed by the self-coherent camera (which relies on the spatial modulation of the speckle pattern). Results. We demonstrate that coronagraphic phase diversity retrieves complex wavefront with subnanometric precision with a good agreement with the reconstruction performed using the self-coherent camera. Conclusions. This result paves the way to coronagraphic phase diversity as a coronagraphic wave-front sensor candidate for very high contrast space missions., Comment: Reproduced with permission from Astronomy & Astrophysics, Copyright ESO

International audience; The alignment of the subapertures is a major challenge for future segmented telescopes and telescope arrays. We show here that a focal plane wave-front sensor using only two images can fully and efficiently align a multiple aperture system, both for the alignment (large amplitude tip/tilt aberrations correction) and phasing (piston and small amplitude tip/tilt aberrations correction) modes. We derive a new algorithm for the alignment of the subapertures : ELASTICS. We quantify the novel algorithm performance by numerical simulations. We show that the residues are within the capture range of the fine algorithms. We also study the performance of LAPD, a recent real-time algorithm for the phasing of the sub-apertures. The closed-loop alignement of a 6 sub-aperture mirror provides experimental demonstration for both algorithms.; L'alignement des sous-pupilles est une défi majeur pour les futurs télescopes segmentés ou réseaux de télescopes. Nous montrons ici qu'un analyseur de surface d'onde en plan focal n'utilisant que deux images peut corriger un système multi-pupille de manière complète et efficace, aussi bien pour l'alignement initial (correction des basculements de grande amplitude) que le cophasage (correction des perturbations de piston et basculement de petite amplitude). Nous établissons un nouvel algorithme (ELASTICS) pour l'alignement des sous-pupilles, dont nous quantifions les performances par simulation numérique et montrons que les résidus sont inclus dans la zone d'accrochage des algorithmes fins. Nous étudions aussi les performances de LAPD, a algorithme temps-réel récent pour le cophasage des sous-pupilles. Une validation expérimentale de ces deux algorithmes a été effectuée par l'alignement en boucle fermée d'un miroir à 6 sous-pupilles.

International audience; New space missions dedicated to exoplanet imaging will rely on coronagraphs to address the high contrast between the stars and their environments. In order to avoid the image of planets to be lost in post-coronagraphic starlight residuals, high precision wavefront sensing and control is a key element to these missions. We present recent results of simultaneous post-coronagraphic phase and amplitude sensing obtained on the THD bench using the coronagraphic phase diversity. We also present results of simulation studies on the non-linear dark hole technique to assess the main limitations of this technique. Finally, we present a first experimental validation of its principle and corroborate expectations on its speed of convergence. These results suggest that the non-linear dark hole is a good candidate for wave-front control for future space-based exoplanet imaging missions, where fast techniques to produce deep dark holes are of paramount importance.

International audience; A key issue in a sparse-aperture imaging system is the relative arrangement of apertures, or aperture configuration. Transposing previous works into a discrete setting, we perform a systematic search for maximal resolution configurations with 9 to 21 apertures. From the catalog of found solutions, we derive a procedure to simply optimize the main free parameters of the aperture from high-level constraints such as the sought resolution and the minimum MTF level or the fill-factor.; Un problème fondamental dans un système imageur à pupille clairsemée réside dans le positionnement relatif des sous-pupilles, ou configuration pupillaire. En adaptant des travaux antérieurs à des coordonnées discrètes, nous effectuons une recherche systématique des configurations de résolution maximale avec 9 à 21 sous-pupilles. A partir du catalogue des solutions trouvées, nous établissons une procédure pour optimiser simplement les principaux paramètres libres de la pupille à partir de contraintes de haut-niveau comme la résolution souhaitée et le minimum de la fonction de transfert ou le facteur de remplissage.

The retina is the only optically accessible neurovascular network in the human body. After a brief state of the art, the latest results obtained with our Adaptive Optics Flood-Illumination Ophthalmoscope will be presented.

This paper is devoted to the presentation of a new technique of characterization of the intra-pixel sensitivity variations (IPSVs) of astronomical detectors. The IPSV is the spatial variation of the pixel response function (PRF). In the case of under-sampled instruments for high quality imaging and accurate photometry, IPSV can contribute to the instrument global error and it should be considered carefully. Our measurement technique is based in the Fourier transform (FT) approach. It consists into the sampling of the pixel transfer function (PTF) by projecting high-resolution periodic patterns onto the whole sensor without classic optics but using the self-imaging property (the Talbot effect) of a continuously self imaging grating (CSIG) illuminated by a plane wave. The PRF is determined by computing the inverse FT. Our measurement technique permits to determine the PRF with a resolution of pixel/10 (10 times Nyquist frequency).

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.

Earth-imaging or Universe Science satellites are always in need of higher spatial resolutions, in order to discern finer and finer details in images. This means that every new generation of satellites must have a larger main mirror than the previous one, because of the diffraction. Since it allows the use of larger mirrors, active optics is presently studied for the next generation of satellites. To measure the aberrations of such an active telescope, the Shack-Hartmann (SH), and the phase-diversity (PD) are the two wavefront sensors (WFS) considered preferentially because they are able to work with an extended source like the Earth's surface, as well as point sources like stars. The RASCASSE project was commissioned by the French spatial agency (CNES) to study the SH and PD sensors for high-performance wavefront sensing. It involved ONERA and Thales Alenia Space (TAS), and LAM. Papers by TAS and LAM on the same project are available in this conference, too [1,2]. The purpose of our work at ONERA was to explore what the best performance both wavefront sensors can achieve in a space optics context. So we first performed a theoretical study in order to identify the main sources of errors and quantify them — then we validated those results experimentally. The outline of this paper follows this approach: we first discuss phase diversity theoretical results, then Shack-Hartmann’s, then experimental results — to finally conclude on each sensor’s performance, and compare their weak and strong points.

We propose and apply two methods to estimate pupil plane phase discontinuities for two realistic scenarios on VLT and Keck. The methods use both Phase Diversity and a form of image sharpening. For the case of VLT, we simulate the `low wind effect' (LWE) which is responsible for focal plane errors in the SPHERE system in low wind and good seeing conditions. We successfully estimate the simulated LWE using both methods, and show that they are complimentary to one another. We also demonstrate that single image Phase Diversity (also known as Phase Retrieval with diversity) is also capable of estimating the simulated LWE when using the natural de-focus on the SPHERE/DTTS imager. We demonstrate that Phase Diversity can estimate the LWE to within 30 nm RMS WFE, which is within the allowable tolerances to achieve a target SPHERE contrast of 10$^{-6}$. Finally, we simulate 153 nm RMS of piston errors on the mirror segments of Keck and produce NIRC2 images subject to these effects. We show that a single, diverse image with 1.5 waves (PV) of focus can be used to estimate this error to within 29 nm RMS WFE, and a perfect correction of our estimation would increase the Strehl ratio of a NIRC2 image by 12\%, Comment: 16 pages, 8 figures, Accepted for publication in JATIS

The final performance of current and future instruments dedicated to exoplanet detection and characterization is limited by intensity residuals in the scientific image plane, which mainly originate in uncorrected optical aberrations. In order to reach high contrasts, these aberrations needs to be compensated for. We have proposed a focal-plane wave-font sensor called COFFEE (for COronagraphic Focal-plane wave-Front Estimation for Exoplanet detection), which consists in an extension of conventional phase diversity to a coronagraphic system. We have recently shown that COFFEE could be extended to jointly estimate both phase and amplitude of the complex field in a pupil plane, which is mandatory in the context of very high contrast space-based coronagraphic instruments, e.g. for exo-Earth imaging. In this communication, we perform the first experimental validation of the estimation of the complex field with COFFEE, on a high-contrast imaging bench representative of a coronagraphic space instrument, namely the THD2 bench of LESIA/Observatoire de Paris. We first calibrate precisely the diversity that is effectively introduced by the pupil-plane deformable mirror (DM) by using classical (i.e., non coronagraphic) phase diversity, which is actually relatively far from a pure defocus due to the DM nonlinearities. Then, we introduce either phase aberrations using the pupil-plane DM or amplitude aberrations using the out-of-pupil-plane DM, we measure several coronagraphic images with varying diversity, and we reconstruct the complex wavefront from these images. We show that COFFEE is able to distinguish between amplitude and phase aberrations and we assess the measurement precision. A short-term perspective is to close the loop using these measurements in order to perform a dark-hole in the focal plane.

Hyper Rayleigh scattering (HRS) was used to measure the second-order nonlinear susceptibility, 𝝌(2), for liquid exfoliated WS2 monolayers. To the best of our knowledge, it is the first reported application of the HRS technique to assess the bulk-like 𝝌(2) of a two-dimensional (2D) material. The concentration-dependent HRS signal indicated a 4.90±0.30×10−25 esu first hyperpolarizability for 42 nm WS2 monolayers under 1064 nm laser irradiation using para-nitroaniline as an external reference. The corresponding value of 𝝌(2)𝒙𝒙𝒙 was calculated to be 460±28 pm V−1. This was within 46% of independent density functional theory predictions. Agreement with theory was improved over related microscopy-based approaches. These results support the use of HRS to evaluate 2D materials for nonlinear frequency mixing applications.

International audience; The intrapixel response is the signal detected by a single pixel illuminated by a Dirac distribution as a function of the position of this Dirac inside this pixel. It is also known as the pixel response function (PRF). This function measures the sensitivity variation at the subpixel scale and gives a spatial map of the sensitivity across a pixel. The variation of the pixel sensitivity finds its origin in the physical properties of the pixel. The measurement of this function gives information on effects which occur after the absorption of the incoming photons by the material. Therefore the knowledge of this function is of great interest, on the one hand for the technologist, who could check the quality of the pixels and study the phenomena related to the detection; and on the other hand, for the instrument scientist, who could evaluate the impact of the spatial filtering of the pixel on image quality.The knowledge of the intra-pixel sensitivity is also important to control and reduce the systematic errors in under-sampled instruments. Indeed, in these instruments, the variation of pixel sensitivity limits the performances of the system and contributes to the instrument error budget [1,2]. All these reasons brings ESA to require the measurement of the intra-pixel sensitivity with a resolution close to the tenth of the pixel in detector technology development activities.The techniques used to measure the intra-pixel response are based on the projection of optical probes on the detector. These techniques can be divided in two main approaches: the direct approach and the indirect approach.

The resolution of coronagraphic high contrast exoplanet imaging devices such as SPHERE is limited by quasistatic aberrations. These aberrations produce speckles that can be mistaken for planets in the image. In order to design instruments, correct quasi-static aberrations or analyze data, the expression of the point spread function of a coronagraphic telescope in the presence of residual turbulence is useful. We have derived an analytic formula for this point spread function. We explain physically its structure, we validate it by numerical simulations and we show that it is computationally efficient.

Adaptive optics PSF can be described as a weighted sum of modes. These weights are estimated using a marginal blind deconvolution algorithm. Results in the context of turbulence-degraded observations are shown. Our method can be extended to anisoplanatic imaging.

The alignment of the subapertures is a major challenge for future segmented telescopes and telescope arrays. We show here that a phase diversity sensor using two near-focus images can fully and efficiently align a multiple aperture system, both for the alignment (large amplitude tip/tilt aberrations correction) and phasing (piston and small amplitude tip/tilt aberrations correction) modes. We derive a new algorithm for the alignment of the subapertures : ELASTIC. We quantify the novel algorithm performance by numerical simulations and we demonstrate it experimentally on a test bench. We also study the performance of LAPD, a recent real-time algorithm for the phasing of the sub-apertures. This work should simplify the design of future multiple aperture systems.

We study the impact of standard active double-clad (DC) fiber characteristics and laser architectures on the generation of long-wavelength Yb-doped high-power fiber lasers around 1154 nm. Both theoretical and experimental works have been performed highlighting that Yb-doped fiber lasers can emit efficiently in multiwatts operation at wavelengths higher than 1150 nm. This study has been focused on the use of standard all-fiber active and passive components as well as on the use of standard DC Yb-doped fibers without any spectral filtering except cavity mirrors.

International audience; We have investigated nanocrystals of Fe(IO3)3 by polarization-sensitive second harmonic generation (SHG) microscopy. As the nonlinear optical properties of this material were only poorly characterized, we have first determined the relative values of the elements of its second-order susceptibility tensor, by the global fitting of the polarization-resolved SHG response of an ensemble of nanocrystals. This information allows one to optically retrieve the orientation of individual particles in the sample. The high SHG efficiency measured for nanocrystals of Fe(IO3)3 and their polar nature could make them very attractive for nonlinear microscopy of biological samples.

Terbium doped Y2O3 planar waveguides were fabricated by sol–gel process and dip-coating using yttrium acetate as precursor. Two different doping modes were compared, i.e. introduction in the sol of dispersed Tb3+ions from dissolved Tb(NO3)3, or of nanoparticles of Tb2O3 or [Y2O3:50% Tb] from an alcoholic suspension. The chemical and nanostructural properties were analyzed by infrared spectroscopy, transmission electron microscopy and X-ray diffraction. The Tb3+ fluorescence properties were studied as a function of temperature and atmosphere of the thermal treatments, and of the Tb3+ concentration. The fluorescence properties are discussed in relation to the quenching mechanisms induced by Tb4+ transformation, residual OH groups, and crystallites size and structure. Optical propagation losses of the Tb doped Y2O3 planar waveguides related to the doping modes and Tb3+ concentration are presented. Doping sol–gel films by nanoparticles is shown to be a valuable alternative to the conventional doping from dissolved terbium salt.

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.

response, often referred to as harmonicnanoparticles (HNPs). HNPs feature a series of properties which distinguish them from otherphotonics nanoprobes (quantum dots, up-conversion nanoparticles, noble metal particles). HNPsemission is inherently nonlinear and based on the efficient generation of harmonics as opposed tofluorescence or surface plasmon scattering. In addition, the fully coherent signal emitted byHNPs together with their polarization sensitive response and absence of resonant interactionmake them appealing for several applications ranging from multi-photon (infrared) microscopyand holography, to cell tracking and sensing.Keywords: multiphoton microscopy, second harmonic generation, third harmonic generation,nonlinear optics, nanophotonics, perovskites(Some figures may appear in colour only in the online journal)1. IntroductionSince the advent of ultrafast lasers in the early nineties andrelated availability of high peak power pulses at compara-tively low energies, the investigation of nonlinear opticalresponse have become a widespread activity resulting ininnovative applications. In particular, in the field of imagingthis has led to the introduction of multi-photon microscopy byW Webb [1]. This technique, as compared to one-photonexcited fluorescence microscopy, features increased imagingdepth, no out-of-focus bleaching, and spatial resolutioncomparable to confocal techniques without compromise insensitivity. The latest technological advancements in this fieldare related to its extension towards longer wavelengths,facilitated by the timely introduction of novel tunable sourcesin the 0.7–1.3 μm spectral range and new optical componentswith optimized performances in this region (microscopeobjectives, acousto-optic modulators, ...). Although these newinfrared tunable devices are supposedly more adapted forimaging through thick (living) samples because of reducedscattering [2–4], the available choice of imaging targets(fluorescent molecules, dyes) with two-photon absorptionbands >1200nm is very limited.Nanoparticle-based labelling, which is sometimes pre-ferred over other approaches for its superior photo-stability,brightness, and chemical selectivity usually requires two-photon interactions in the 700–900nm range, with a fewnotable exceptions [5]. However most optically active labelsmay suffer from bleaching and blinking and their use inbiological research might be prevented because of low bio-compatibility. For these reasons, since 2006, several researchgroups worldwide have proposed a complementary approach

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%.

We apply computational diffractive imaging to propose a new method for 3D imaging by using a diffractive optical component called Continuously Self Imaging Grating (CSIG).

The sensitivity of a gravitational wave detector based on a laser interferometer is limited by photon shot noise for signals with frequencies above a few hundred hertz. The amplitude of this noise depends on the interferometer fringe contrast. By means of a complete simulation, we show that using a short optical cavity placed at the interferometer output port, the contrast can be significantly improved and the optimal sensitivity recovered. Such an output mode-cleaner has been realized and tested. The adopted design and the results of the tests are presented.

A new acrylate-modified silica incorporating tantalum is synthesised via a sol–gel route. After deposition, the few micron-thick films can be photo-polymerised by UV-irradiation. FTIR spectroscopy gives a first insight into the curing process and the chemical structure of the hybrid material. The refractive index of the layers can be modulated by varying the concentration of tantalum in the material. Coatings exhibit a good transparency in the blue range of the optical spectrum. Finally, the propagation losses are low enough to make the material suitable for applications in integrated optics.

International audience; We present a new method based on a maximum likelihood (ML) estimation of the sub-pixel shift between images of a given object observed with a single instrument. We first study the case of two noisy images and give the ML approach of the registration problem. By means of simulations, we show the gain obtained with this ML solution compared to a classical registration method with an academic noise model (stationary white Gaussian), and then demonstrate the relevance of this ML estimation with a more realistic noise model. We then address the problem of a sequence of low signal frames of the same object. We develop a joint ML approach in which we simultaneously estimate the reference (i.e. the noiseless) image and the shift parameters. The registration accuracy is increased at low photon levels as the number of frames grows, reaching the sub-pixel domain at very low SNR (about 1), when considering 100 frames. When applied to experimental data (thermal IR images of a faint galaxy), both ML methods show their efficiency to recover the resolution in averaged frames and totally outperform the classical cross-correlation.

Nulling interferometers such as Darwin or TPF will require a rather sophisticated data processing in order to perform a reliable planet detection and characterization. We propose a Bayesian method, which follows the maximum a posteriori (MAP) approach, to solve this problem. Our method accounts for the noise statistics and optimally combines the data from a nulling interferometer at all observed wavelengths to perform reliable planet detection. The problem to be solved is however multi-modal. We show how, in practice, the global optimum of the MAP criterion can be found by our method; the latter also provides the most likely spectral energy distributions of all planets. Additionally, we show that a proper regularization allows us to achieve an improved robustness of the detection and could lead to shorter observation times.

Y 3 Al 5 O 12 (YAG) thin films are prepared by the sol-gel process using two different synthesis routes: acetate-alkoxide method (YAG-I) and two-alkoxides method (YAG-II). The sols and films elaborations are described. The film behavior is studied by X-ray diffraction (XRD) and conventional transmission electron microscopy (CTEM) for different annealing temperatures. Differential thermal analysis (DTA) measurements are used for crystallization kinetics investigations. The refractive index and thickness of the as-prepared films are analyzed by m-lines spectroscopy (MLS) and their waveguiding properties are characterized. The relationships between structural evolutions and waveguiding properties of YAG thin films are discussed. These results show that the structural and optical performances of the elaborated YAG thin films strongly depend on the starting materials and their associated annealing treatments.

International audience; A Laponite clay-based lithium iodate nanocomposite has been synthesized for non-linear optical applications. The surface morphology of dip-coated optical waveguides were analysed by Scanning Force Microscopy (SFM) techniques. Individual Laponite particles were first imaged by using Intermittent-Contact Atomic Force Microscopy on pure Laponite layers. Electrostatic Force Microscopy was then used on nanocomposite waveguides in order to obtain a "chemical contrast" of sample surface. The strong electrical contrast observed was attributed to sample topography and not to the different properties of each nanocomposite constituent. Finally, in ambient conditions, dip-coated layers were also shown to be sensitive to humidity which leads to the occurrence of surface lithium iodate crystals with sub-micrometer size.

A new clay-based nanocomposite has been developed for non-linear optical waveguiding applications. Thin layers are deposited on glass substrates by using dip coating technique from an aqueous solution made of a Laponite suspension mixed with a lithium iodate (LiIO3) aqueous solution. Transparent layers with waveguiding properties are obtained with an effective non-linear coefficient of about 1.4 pm/V for a 55% vol. LiIO3 composite. Control of nanocrystals orientation has been performed by applying an electric field with Corona discharge or deposited gold electrodes. Characterisation of nanocrystals orientation was done by Second Harmonic Generation (SHG) measurements and polarised light optical microscopy.

We present here the progress on thin scintillating films of Lu2O3 and Gd2O3 doped with Eu3+ ions deposited on various substrates. Pulsed Laser Deposition (PLD) as well as sol–gel processes are used. Properties and performances of the different films are described in this contribution.

This paper presents a complete description of Virgo, the French-Italian gravitational wave detector. The detector, built at Cascina, near Pisa (Italy), is a very large Michelson interferometer, with 3 km-long arms. In this paper, following a presentation of the physics requirements, leading to the specifications for the construction of the detector, a detailed description of all its different elements is given. These include civil engineering infrastructures, a huge ultra-high vacuum (UHV) chamber (about 6000 cubic metres), all of the optical components, including high quality mirrors and their seismic isolating suspensions, all of the electronics required to control the interferometer and for signal detection. The expected performances of these different elements are given, leading to an overall sensitivity curve as a function of the incoming gravitational wave frequency. This description represents the detector as built and used in the first data-taking runs. Improvements in different parts have been and continue to be performed, leading to better sensitivities. These will be detailed in a forthcoming paper. © 2012 IOP Publishing Ltd and Sissa Medialab srl.

Y2O3 optical planar waveguides are fabricated by the sol–gel method and the dip-coating technique. High purity yttrium alkoxide (yttrium 2-methoxyethoxide) is used as starting material for the Y2O3 sol and acetylacetone is added to improve the stability of the sol. Highly concentrated sol up to 0.8 M is prepared and six stacked layers are enough to support four propagation modes at wavelength from 488 to 632.8 nm. The cubic phase of Y2O3 is crystallized after an annealing treatment at 700 °C. X-ray diffraction is conducted on the films for their structural analysis. Optical properties of the waveguides are determined by multi-wavelength m-lines spectroscopy and attenuation coefficient measurement. The attenuation coefficient of the prepared Y2O3 waveguiding thin films is in the range between 1 and 2 dB/cm at 632.8 nm.

ZnO waveguiding layers deposited onto soda lime glass were produced from the thermal oxidation of ZnS thin films elaborated by a carefully conducted chemical bath deposition (CBD) method. X-ray diffraction indicated that ZnO films possessing a polycrystalline hexagonal structure were obtained. M-line spectroscopy was used to determine the refractive index and the thickness of ZnO films which were 1.90 at wavelength λ =632.8 and 130 nm, respectively. These analyses were also conducted on the ZnS elaborated films before annealing treatment. The ZnO films presented waveguiding properties and optical losses of 3.0±0.5 dB cm −1 at λ =632.8 nm were measured. The waveguide Raman spectrum of the ZnO films mainly exhibited a broad band at 558 cm −1 which corresponds to longitudinal optical phonon and a large band which is assigned to amorphous ZnO material.

Wedescribe and evaluate the performance of a phase diversity wavefront sensor used to measure the staticaberrations of the VLT instrument NAOS-CONICA. The main limitations of this phase diversity technique are compiled. We investigate the systematic errors due to the experimental implementation and the design restrictions. Further error sources stem from the imperfect knowledge of the system, and from limitations of the algorithm. The influence of these errors on the wavefront estimation is evaluated on numerical and experimental data. This study highlights the essential verifications and calibrations needed to obtain accurate results and gives a practical guideline for the application of a phase diversity wavefront sensor. The comprehensive calibration results and the final gain in optical performance are presented and discussed in a complementary paper (Hartung et al. 2003).

NAOS is the first adaptive optics (AO) system of the Very Large Telescope and will provide CONICA with diffraction-limited images. CONICA is a near infrared camera that offers a variety of imaging and spectroscopic observing modes. A technique will be described to benefit of the AO system to correct not only for atmospheric turbulence but also for the internal optical aberrations of the high-resolution camera and the beam splitters of NAOS. The aberrant optical components in the light path of CONICA as well as the beam splitters are outside of the AO loop and therefore no self-acting correction is possible. Independently of the AO wavefront sensor, a separate measurement of these aberrations using a method called phase diversity allows one to predict for a certain instrument configuration the corresponding aberrations. They are quantified by sets of Zernike coefficients that are rendered to the adaptive optics. This technique turns out to be very flexible and results in a further improvement of the optical overall performance. The application of phase diversity to the instrument is investigated in a preceding paper (Blanc et al. 2003). In the present paper we present in detail the instrumental implementation of phase diversity, the obtained calibration results, and the achieved gain in optical performance.