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In the framework of the ALOHA (Astronomical Light Optical Hybrid Analysis) project, dedicated to high resolution imaging in the L-band using optical fibre and nonlinear optics, we have implemented a servo controlled hectometric outdoor fibre link between two telescopes and the recombination beam facility of the CHARA telescope array. A two-stage servo system using optical fibre modulator, fibre delay line, and a metrology laser at 1064 nm allows to stabilise the optical path difference within 3 nm RMS over a 3000 s record. Using an internal source at 810 nm, the signal-to-noise ratio of the fringe modulation peak is enhanced by a factor better than two when the servo control is switched on. This study can be also considered as a seminal work towards very long base fibre linked telescope arrays and allows to scale the perturbative environment of an outdoor fibre link.

In the framework of the Astronomical Light Optical Hybrid Analysis (ALOHA) laboratory mid-infrared (MIR) up-conversion fibred interferometer in the L band, we report on the influence of the input-stage architecture. Using an amplitude division set-up in the visible or near-infrared is a straightforward choice in most cases. In the MIR context, the results are slightly different and we show that a wavefront division set-up is needed. These in-laboratory principle experiments allow us to measure a reliable 88 per cent instrumental contrast with high flux and to obtain fringes from faint sources at 3.5 μm with a spectral bandwith of 37 nm converted to 817 nm. An equivalent limiting L-band magnitude around 3.9, equivalent to 3.0 fW nm−1, could be demonstrated on 1 m class telescopes. This opens the possibility of planning future on-sky tests at the Center for High Angular Resolution Astronomy (CHARA) array and of predicting the performance attained.

Les moyens d’observation en astronomie n’ont cessé d’évoluer au fil des siècles. L’utilisation de réseaux de télescopes permet aujourd’hui d’avoir des détails de plus en plus fins dans les images obtenues (grande résolution). Dans ce contexte, le projet ALOHA propose une approche totalement originale consistant à changer la couleur de la lumière de l’objet observé au niveau de chaque télescope du réseau avant de la détecter. Cela permet d’éviter d’utiliser des systèmes de cryogénie très coûteux et complexes nécessaires lors de la collecte de lumière moyen et lointain infrarouge. Ce document présente quelques éléments sur nos activités de recherche sur le projet ALOHA.

This paper gives an overview of the Astronomical Light Optical Hybrid Analysis (ALOHA) project dedicated to investigate a new method for high resolution imaging in mid infrared astronomy. This proposal aims to use a non-linear frequency conversion process to shift the thermal infrared radiation to a shorter wavelength domain compatible with proven technology such as guided optics and detectors. After a description of the principle, we summarise the evolution of our study from the high flux seminal experiments to the latest results in the photon counting regime.

International audience

In the framework of the ALOHA project (Astronomical Light Optical Hybrid Analysis), devoted to high resolution imaging in the L band, we investigate in this paper the possibility to deploy fibre links between the telescopes of the CHARA Array. As a first step, we set up an outdoor hectometric fibre interferometer to measure the vibration environment at the Mount Wilson Observatory. The optical path difference of this interferometer has been actively stabilised within an accuracy better than 4 nmrms, a stability largely sufficient for any interferometric application.

In this paper, several methods suitable for real time on-chip temperature measurements of power AlGaN/GaN based high-electron mobility transistor (HEMT) grown on SiC substrate are presented. The measurement of temperature distribution on HEMT surface using Raman spectroscopy is presented. We have deployed a temperature measurement approach utilizing electrical I-V characteristics of the neighboring Schottky diode under different dissipated power of the transistor heat source. These methods are verified by measurements with micro thermistors. The results show that these methods have a potential for HEMT analysis in thermal management. The features and limitations of the proposed methods are discussed. The thermal parameters of materials used in the device are extracted from temperature distribution in the structure with the support of 3-D device thermal simulation. The thermal analysis of the multifinger power HEMT is performed. The effects of the structure design and fabrication processes from semiconductor layers, metallization, and packaging up to cooling solutions are investigated. The analysis of thermal behavior can help during design and optimization of power HEMT.

International audience; In this paper, we propose a multichannel spectral configuration of the Astronomical Light Optical Hybrid Analysis (ALOHA) instrument dedicated to high resolution imaging. A frequency conversion process is implemented in each arm of an interferometer to transfer the astronomical light to a shorter wavelength domain. Exploiting the spectral selectivity of this non-linear optical process, we propose to use a set of independent pump lasers in order to simultaneously study multiple spectral channels. This principle is experimentally demonstrated with a dual-channel configuration as a proof-of-principle.

GaN-based HEMTs’ high potential is deteriorated by self-heating during the operation, this has influence on the electrical properties as well as device reliability. This work is focused on an average channel temperature determination of power AlGaN/GaN HEMT prepared on SiC substrate using quasi-static and pulsed I-V characterization. There was analyzed the drain current change relation to temperature dependent electrical HEMT parameters such as source resistance, threshold voltage, saturation velocity, resp. leakage current which allows to calculate an average channel temperature versus dissipated power for various ambient temperature. Differential temperature of investigated device with and without heatsink was determined. Obtained results were discussed using simulated spatial temperature distribution.

This paper deals with the effect of the telescope size on accuracy of the data acquisition in a optical fibre linked stellar interferometer. In this context we introduce the concept of antenna diagram commonly used for microwaves antennae. This concept is essential to explain the contrasts and the phaseclosure acquisitions corruption in a stellar interferometer. The telescope pointing errors induces additional effects that are superimposed with the field limitation and could become critical.

International audience; High resolution stellar interferometers are very powerful efficient instruments to get a better knowledge of our Universe through the spatial coherence analysis of the light. For this purpose, the optical fields collected by each telescope Ti are mixed together. From the interferometric pattern, two expected information called the contrast Cij and the phase information φij are extracted. These information lead to the Vij, called the complex visibility, with Vij=Cijexp(jφij). For each telescope doublet TiTj, it is possible to get a complex visibility Vij. The Zernike Van Cittert theorem gives a relationship between the intensity distribution of the object observed and the complex visibility. The combination of the acquired complex visibilities and a reconstruction algorithm allows imaging reconstruction. To avoid lots of technical difficulties related to infrared optics (components transmission, thermal noises, thermal cooling...), our team proposes to explore the possibility of using nonlinear optical techniques. This is a promising alternative detection technique for detecting infrared optical signals. This way, we experimentally demonstrate that frequency conversion does not result in additional bias on the interferometric data supplied by a stellar interferometer. In this presentation, we report on wavelength conversion of the light collected by each telescope from the infrared domain to the visible. The interferometric pattern is observed in the visible domain with our, so called, upconversion interferometer. Thereby, one can benefit from mature optical components mainly used in optical telecommunications (waveguide, coupler, multiplexer...) and efficient low-noise detection schemes up to the single-photon counting level.

International audience; Astronomical Light Optical Hybrid Analysis (ALOHA@3.39 µm) is a new instrumental concept designed for high angular resolution astronomical imaging in the L Band. The originality of our experimental device is to include a frequency conversion process in each arm of the interferometer that frequency shifts the mid infrared signal to the near infrared domain. This method relaxes the stringent limitations due to the thermal noise background radiated by the instrument itself. The aim of this paper is to estimate the current sensitivity limit of the instrument and validate the ability to provide calibrated contrasts with a quasi monochromatic signal. We demonstrate the possibility to measure fringe contrasts greater than 94.5% with a signal-to-noise ratio close to 7. The flux level at the input of each arm of the interferometer is then equal to 100 fW. This would correspond to a zero magnitude star in the L Band for a 1 m 2 telescope surface and a 3 nm converted spectral bandwidth.

International audience; In this paper we describe an upconversion detector in the mid infrared (around 3.5 μm). We take advantage of the PPLN ridge waveguide technology to achieve single photon detection at room temperature on a single spatial mode. With a pump power of 192 mW we obtain a detection efficiency of 0.4% for 22k dark count per second, which corresponds to a noise equivalent power of 3.0 fW · Hz−1/2 and an internal conversion efficiency of 85 %/W of pump.

International audience; In the context of new generation instruments for astronomical interferometer, we have developed in laboratory a new kind of optical interferometer using the frequency conversion of the star light. We investigate the sensitivity limit and reliability of our so-called upconversion interferometer when operating in photon counting mode. For this purpose, we have implemented a laboratory test bench including two three-arm interferometers dedicated to high angular resolution imaging. The first classical one works at the same wavelength as the laboratory star (infrared) and operates with high optical flux levels (used as a reference interferometer). The second one, under test, uses sum frequency generation process in a non-linear optical waveguide placed on each arm to shift the wavelength of the infrared laboratory star to the visible domain. The observables are obtained in photon counting operation, involving signal processing to recover unbiased data. The reference measurement (high flux complex visibilities in infrared) and the upconversion interferometer observables (complex visibilities obtained in photon counting regime) are in good agreement. We notice a degradation on the phase closure terms reliability for low values of the triple product (bispectrum). We estimate from the experimental results the related limiting magnitude for several configurations using an upconversion interferometer.

In the aim of access the high angular resolution for mid infrared observations, our team propose to include non linear processes on each arm of an interferometer. This project called ALOHA is now adapted for the L band detection, specially at 3.39 μm. Our team has previously published the first contrast measured in laboratory with such an up-conversion interferometer. The fringe contrast we measured was closed to the theoretical maximum at 100%. In a second step, we investigated the stability of the instrument over several months. The residual drifts are mainly due to the non real-time photometry monitoring.

The interferometric concept named ALOHA (Astronomical Light Optical Hybrid Analysis) offers an alternative for high resolution imaging in the mid-infrared domain by shifting the astronomical light to shorter wavelength where optical guided components from telecommunications are available and efficient. A prototype with two arms converting a signal from 1.55 μm to 630 nm is used to validate the concept in laboratory and on-sky. Thanks to collaboration with the CHARA team, photometric tests were achieved with a single arm of the interferometer and have allowed to predict instrument performance in its interferometric configuration in order to obtain first fringes in H band.

The ALOHA research program aims to propose a breakthrough generation of instrument for high resolution imaging in astronomy. This fully innovative concept results from our unique skills with a simultaneous competence in nonlinear optics and high resolution imaging with telescope arrays. Acting like a mixer in a radio receiver, the nonlinear process (sum frequency generation) shifts the infrared radiations emitted by the observed astrophysical source to a visible spectral domain. This way, the light beam is more easily processed by mature optical devices and detectors. The compatibility of the nonlinear process with the spatial coherence analysis has been successfully tested through preliminary in lab experiments. Now it’s time to apply this technique in a real astronomical environment. First on-sky results have been observed during the last missions at the CHARA Array.

The Astronomical Light Optical Hybrid Analysis project investigates the combined use of a telescope array interferometer and nonlinear optics to propose a new generation of instruments dedicated to high-resolution imaging for infrared astronomy. The nonlinear process of optical frequency conversion transfers the astronomical light to a shorter wavelength domain. Here, we report on the first fringes obtained on the sky with the prototype operated at 1.55 μm in the astronomical H band and implemented on the Center for High Angular Resolution Astronomy telescope array. This seminal result allows us to foresee a future extension to the challenging midinfrared spectral domain.

In this paper, we report the first experimental demonstration of a Temporal HyperTelescope (THT). Our breadboard including 8 telescopes is firstly tested in a manual cophasing configuration on a 1D object. The Point Spread Function (PSF) is measured and exhibits a dynamics in the range of 300. A quantitative analysis of the potential biases demonstrates that this limitation is related to the residual phase fluctuation on each interferometric arm. Secondly, an unbalanced binary star is imaged demonstrating the imaging capability of THT. In addition, 2D PSF is recorded even if the telescope array is not optimized for this purpose.

International audience; In this paper, we report on a novel method to control the coherence behavior in a sum frequency generation interferometer powered by two independent pump lines. At the output of the interferometer, the two incoherent fringe patterns must be superimposed to maximize the contrast. The first step consists in canceling the differential group delay. The second one uses the phase control on one pump to synchronize the fringe patterns. This innovative method is experimentally demonstrated with a setup involving a 1544 nm signal and two pump lines around 1064 nm leading to a converted signal around 630 nm. It can be easily extended to a greater number of pump lines.

We present some experimental results on tunable sum-frequency generation in a periodically poled lithium-niobate waveguide using a multi-wavelength fiber laser pump stabilized by a nonlinear optical loop mirror. We are able to up-convert to about 629 nm a continuous-wave infrared signal varying from 1497 nm to 1525 nm. Such a wideband conversion efficiency is ensured by the multiple spectral components of the laser pump, which is controlled by an adjustable Fabry–Perot filter. Potential applications, in particular for stellar imaging, are discussed.

We analyze the temporal coherence of an optical infrared radiation in the visible domain by using a Mach–Zehnder interferometer and a wavelength conversion stage in each arm. We exploit a sum frequency generation process in bulk PPLN crystal to convert the infrared radiation at 1.55 μm into 0.63 μm before the interferometric mixing. The applicability of the Wiener–Kintchine theorem through up-conversion processes is here demonstrated by direct comparisons among visible and infrared measurements.

International audience; We report on the implementation of a mid-infrared (MIR) interferometer prototype for furthermore application in the framework of high-resolution imaging in astronomy. This paper demonstrates the possibility to extend to the L band our experimental study performed on the up-conversion interferometer in H band. This in-laboratory preliminary experiment allowed us to get the first fringes with the MIR Astronomical Light Optical Hybrid Analysis (ALOHA) @3.39 μm up-conversion interferometer with a bright quasi monochromatic source (HeNe 3.39 μm). A stable contrast greater than 98 per cent has been reached. This opens the possibility to propose an alternative instrument for the demanding domain of high resolution imaging in the MIR domain

We intend to implement the ALOHA at 1.55[Formula: see text][Formula: see text]m up-conversion interferometer on the CHARA Array. After a full laboratory investigation, a sensitivity evaluation is conducted on several stars using a single interferometric arm in a photometric mode. The on-sky photometric results allows us to calibrate a numerical simulation of the interferometric configuration, and to predict the future performance of ALOHA at 1.55[Formula: see text][Formula: see text]m as a function of the seeing conditions.

Most of space instruments and research facilities require test equipment with demanding opto-mechanical stability. In some specific cases, when the stability performance directly drives the final performance of the scientific mission and when feasibility is questionable, specific methods must be implemented for the associated technical risk management. In present paper, we will present our heritage in terms of methodology, design, test and the associated results for two specific systems : the SOPAC-POS and the MOTA, generating new references for future developments. From performance point of view, we will emphasis on following key parameters : design symmetry, thermal load management, and material and structural choices. From a method point of view the difficulties arise first during design, from the strong coupling between the thermal, mechanical and optical performance models, and then during testing, from the difficulty of conceiving test setup having appropriate performance level. We will present how these limitations have been overcome. SOPAC-POS is the target alignment system of the LMJ, Laser Mega Joule, the French inertial confinement fusion research center. Its stability has been demonstrated by tests in 2014 after 10 years of research and development activities, achieving 1μm stability @ 6m during one hour periods. MOTA is an Optical Ground Support Equipment aiming at qualifying by tests the Flexible Combined Imager (FCI). FCI is an instrument for the meteorological satellite MTGI, a program of and funded by the European Space Agency and under prime contractorship of Thales Alenia Space. Optimized design will allow to get better than 0.2 μrad stability for one hour periods, as required for MTF measurement.

Most of space instruments and research facilities require test equipment with demanding opto-mechanical stability. In some specific cases, when the stability performance directly drives the final performance of the scientific mission and when feasibility is questionable, specific methods must be implemented for the associated technical risk management. In present paper, we will present our heritage in terms of methodology, design, test and the associated results for two specific systems : the SOPAC-POS and the MOTA, generating new references for future developments. From a performance point of view, we will emphasis on following key parameters : design symmetry, thermal load management, and material and structural choices. From a method point of view the difficulties arise first during design, from the strong coupling between the thermal, mechanical and optical performance models, and then during testing, from the difficulty of conceiving test setup having appropriate performance level. We will present how these limitations have been overcome. SOPAC-POS is the target alignment system of the LMJ, Laser Mega Joule, the French inertial confinement fusion research center. Its stability has been demonstrated by tests in 2014 after 10 years of research and development activities, achieving 1μm stability @ 6m during one hour periods. MOTA is an Optical Ground Support Equipment aiming at qualifying by tests the Flexible Combined Imager (FCI). FCI is an instrument for the meteorological satellite MTG-I, a program of and funded by the European Space Agency and under prime contractorship of Thales Alenia Space. Optimized design will allow to get better than 0.2 μrad stability for one hour periods, as required for MTF measurement.

International audience; In the frame of the Optical Hawaiian Array for Nanoradian Astronomy (`OHANA) project, 300-m long silica fibers are necessary to coherently link the Canada France Hawaii and Gemini telescopes. In this paper, we report measurements of the differential chromatic dispersion between two 300-m long silica polarization maintaining fibers for the astronomical J band (1.10 1.40 mum) as a function of the temperature variation on one fiber arm of the interferometer. We demonstrate that the second order coefficient of chromatic dispersion increases linearly with the temperature. Consequently, we propose two methods to compensate this additional and variable differential chromatic dispersion: the first one using a dispersive optical fiber delay line and the second one using a set of CaF2 bulk plates.

In the frame of ’OHANA project (phase II), IRCOM institute characterizes the 300-m long silica fibres which will link the Canada–France–Hawaii-Telescope and the Gemini telescope. In this paper, we report a method to compensate the differential chromatic dispersion between two 300-m long silica polarization-maintaining fibres. We experimentally show that the differential dispersion is not the same on each neutral axis of the fibre. Moreover, thanks to a channeled spectrum analysis, we are able to foresee the evolution of the contrast as a function of the wavelength and the spectral resolution over a given spectral band. Consequently, we are able to adjust the fibre lengths in order to maximize the contrast over J spectral window.

This paper reports on an experimental validation of the photon noise correction applied on interferometric data, in the frame of a laboratory stellar interferometer. The main feature of this study comes from the quantitative comparison between the contrasts and phase closure measurements in photon counting regime and simultaneously, on the same interferometer, in high flux level regime. This experiment uses an end-to-end imaging demonstrator including a three-telescope array linked by optical fiber to the mixing station. Contrasts and phase closure are extracted from a temporal fringe pattern. We show that it is possible to correct properly the interferometric data with light power level reaching 0.7 femto-Watt at a 680 nm mean wavelength. This corresponds to a photon flux close to 2.5 photons/ms and to 25 photons for each snap shot.

Phase closure is a well-known technique able to eliminate phase biases in a three-arm interferometer. This property is very useful to avoid the phase effect of the atmosphere in stellar interferometry. In this paper we theoretically investigate the effect of differential dispersion in a three-arm interferometer. We demonstrate that phase closure can be corrupted by differential dispersion between the interferometric arms. We begin by a general demonstration of such effect and propose a few examples where symmetry properties avoid phase closure errors. An analytical model for gaussian shaped spectrum and second order dispersion development is proposed. This analytical solution is illustrated by experimental results on a fibre version of a stellar interferometer.

International audience; In this paper, we present a new concept of instrument for high resolution imaging in astronomy, involvingthe sum frequency generation in non-linear waveguides. The aim is to convert the infrared radiationemitted by an astronomical source to the visible spectral domain where the optical components aremature and efficient. We present the main experimental results obtained in laboratory, and propose a newdesign for this instrument for its implementation on the Center for High Angular Resolution Astronomy(CHARA) telescope array. Preliminary stability and photometric results obtained at CHARA are pre-sented. Using these last measurements, we estimate the limiting magnitudes which could be reached bythis interferometer in the H spectral band.

We propose an exotic use of sum frequency generation process (SFG) to develop a new kind of high resolution interferometer for astronomical imaging. SFG is well known to be intrinsically a noiseless non linear process of upconversion which permits a wavelength shift. Thereby we propose to shift astronomical MIR and FIR radiation to shorter wavelength where optical fibers and optical components are available and efficient. In order to demonstrate the validity of this method for high resolution imaging, we plan to set up a two-arm upconversion interferometer on the CHARA telescope array (California). Each arm would include an upconversion stage at the focus of telescope. The success of such a project is obviously conditioned by the quality of nonlinear components (waveguided PPLN) in term of efficiency and noise biases. Moreover, coherence study requires the use of identical non linear components which implies manufacturing constraints. To ensure the feasibility of this project, several studies have been conducted. By implementing an upconversion interferometer in laboratory we have recently demonstrated our ability to analyze the coherence properties of a 1550nm signal at visible wavelength. We also have successfully converted astronomical light using one arm of this interferometer at the Hawai observatory. It showed the capability of our instrument to astronomical observing conditions in photon counting regime. A preliminary mission at CHARA observatory allowed us to check the compatibility of our instrument with the environment onsite and expected photometric levels. From these data we estimate to be able to study the coherence of astronomical target at 1550nm using such an instrument.

Following the first demonstration of an all guided two-beam stellar interferometer designed for space missions, we report an experiment recombining the beams coming from three telescopes using only guided optics components. This additional aperture could give us the possibility to achieve an image reconstruction using the phase-closure technique. We focus this calibration experiment on the interference fringe contrast measurements and the evolution of the phase-closure term versus the differential dispersion effects induced by the stretching of fibre delay lines.

We report laboratory experimental results obtained with end to end image reconstruction breadboard including a 2D object, a highly birefringent fiber linked telescope array, an interferometric mixing assembly and an image restoration algorithm. The full control of birefringent effects allows the acquisition of the object complex Fourier spectrum (phase and modulus). The results consist of an accurate image reconstruction compared with the original source. The image restoration algorithm seeks for the object brightness distribution that satisfies two criteria: goodness of fit with the data and regularization. Future improvements of both the instrumental setup and image restoration algorithm are discussed.

International audience; In the field of high resolution imaging in astronomy, we experimentally demonstrate the spatial-coherence analysis of a blackbody using an up-conversion interferometer in the photon counting regime. The infrared radiation of the blackbody is converted to a visible one in both arms of the interferometer thanks to the sum-frequency generation processes achieved in Ti-diffused periodically poled lithium niobate waveguides. The coherence analysis is performed through a dedicated imaging stage which mimics a classical telescope array analyzing an astrophysical source. The validity of these measurements is confirmed by the comparison with spatial-coherence analysis through a reference interferometer working at infrared wavelengths.

International audience; Context. Amongst the new techniques currently developed for high-resolution and high-dynamics imaging, the hypertelescope architecture is very promising for direct imaging of objects such as exoplanets. The performance of this instrument strongly depends on the co-phasing process accuracy. In a previous high-flux experimental study with an eight-telescope array, we successfully implemented a co-phasing system based on the joint use of a genetic algorithm and a sub-aperture piston phase diversity using the object itself as a source for metrology.Aims. To fit the astronomical context, we investigate the impact of photon noise on the co-phasing performance operating our laboratory prototype at low flux. This study provides experimental results on the sensitivity and the dynamics that could be reached for real astrophysical observations.Methods. Simulations were carried out to optimize the critical parameters to be applied in the co-phasing system running in the photon-counting regime. We used these parameters experimentally to acquire images with our temporal hypertelescope test bench for different photon flux levels. A data reduction method allows highly contrasted images to be extracted.Results. The optical path differences have been servo-controlled over one hour with an accuracy of 22.0 nm and 15.7 nm for 200 and 500 photons/frame, respectively. The data reduction greatly improves the signal-to-noise ratio and allows us to experimentally obtain highly contrasted images. The related normalized point spread function is characterized by a 1.1 × 10-4 and 5.4 × 10-5 intensity standard deviation over the dark field (for 15 000 snapshots with 200 and 500 photons/frame, respectively).Conclusions. This laboratory experiment demonstrates the potential of our hypertelescope concept, which could be directly transposed to a space-based telescope array. Assuming eight telescopes with a 30 cm diameter, the I-band limiting magnitude of the main star would be 7.3, allowing imaging of a companion with a 17.3 mag.

This paper reports on the implementation of an integrated optics component acting as a multi-recombiner in an optical servo-system dedicated to control of a linear optical path modulation in a stellar interferometer. This laboratory experiment involves a two-fibre-armed interferometer with an integrated optics combiner providing two π/2 phase-shifted interferometric signals required for the error signal generation. Thanks to the control of the polarization status along the whole of the interferometric device, fringes with steady high contrasts are obtained. When operating, the servo-system allows the residual error on the optical path linear modulation to be kept lower than 3 nm RMS.

International audience; This paper reports on the experimental implementation of an interferometer featuring sum frequency generation (SFG) processes powered by a pump spectral doublet. The aim of this configuration is to allow the use of the SFG process over an enlarged spectral domain. By analyzing the converted signal, we experimentally demonstrate a frequency spectral compression effect from the infrared input signal to the visible one converted through the SFG process. Recently, such a compression effect has been numerically demonstrated by Wabnitz et al. We also verify experimentally that we fully retrieve the temporal coherence properties of the infrared input signal in the visible field. The experimental setup permits to demonstrate an experimental frequency spectral compression factor greater than 4. This study takes place in the general field of coherence analysis through second order non-linear processes.

We investigate the sensitivity of frequency conversion of starlight using a non-linear optical sum frequency process. This study is being carried out in the context of future applications of optical interferometry dedicated to high-resolution imaging. We have implemented a complete experimental chain from telescope to detector. The starlight frequency is shifted from the infrared to the visible using an optically non-linear crystal. To fulfil the requirements of interferometry, our experimental setup uses spatially single-mode and polarization maintaining components. Due to the small size of the collecting aperture (8 inches Celestron C8) with a 3 nm spectral bandwidth, on-sky tests were performed on bright stars in the H band. The detection was achieved in a true photon counting operation, using synchronous detection. Betelgeuse (HMag =− 3.9), Antares (HMag =− 3.6) and Pollux (HMag =− 1) were successfully converted and detected in visible light. Despite the low transmission of our experiment, our results prove that the efficiency of frequency conversion offers sufficient sensitivity for future interferometric applications.

International audience; Context. The diluted aperture synthesis is one of the most promising ways of obtaining direct images with an angular resolution in the milliarcsecond range. By applying apodization techniques to a hypertelescope, it is possible to discriminate between objects with a high contrast in intensity with a reasonable number of telescopes (

International audience; This paper demonstrates the use of a nonlinear upconversion process to observe an infrared source through a telescope array detecting the interferometric signal in the visible domain. We experimentally demonstrate the possibility to retrieve information on the phase of the object spectrum of an infrared source by using a three-arm upconversion interferometer. We focus our study on the acquisition of phase information of the complex visibility by means of the phase closure technique. In our experimental demonstration, a laboratory binary star with an adjustable photometric ratio is used as a test source. A real time comparison between a standard three-arm interferometer and our new concept using upconversion by sum-frequency generation demonstrates the preservation of phase information which is essential for image reconstruction.

International audience; We study the analytic solution to match in phase a particular type of low-noise vector four-wave frequency conversion in birefringent nonlinear waveguides. We confirm our predictions with a series of numerical simulations.

We describe an application of nonlinear optimization in interferometric optical astronomy. The aim is to find the relative positions of the output pupils and the modulus of the beams through each pupil of a linear array of telescopes in order to design an instrument capable of imaging exoplanets. The problem is modelized under the form of a semi-infinite nonlinear minimization problem. The model problem is transformed by a simple discretization into a minimization problem with a finite number of constraints, then it is solved by using a minimization solver. Numerical experiments are reported.

International audience; We theoretically and experimentally evidence that fiber systems are convective systems since their nonlocal inherent properties, such as the dispersion and Raman effects, break the reflection symmetry. Theoretical analysis and numerical simulations carried out for a fiber ring cavity demonstrate that the third-order dispersion term leads to the appearance of convective and absolute instabilities. Their signature is an asymmetry in the output power spectrum. Using this criterion, experimental evidence of convective instabilities is given in a fiber cavity pumped by a pulsed laser.

We experimentally demonstrate the possibility of retrieving the spatial coherence of an infrared source by using an up-conversion interferometer. Sum-frequency generation in Ti-diffused periodically poled lithium-niobate waveguides in both arms of the interferometer is used to convert the infrared into the visible domain. The fringe contrast of the interference pattern in the visible domain permits us to resolve the spatial separation of two uncorrelated pointlike infrared sources, which simulate a binary star. The validity of these measurements is confirmed through a simultaneous comparison with a reference interferometer working in the infrared domain.

Context. In the future, giant optical interferometric arrays will be developed with kilometric baselines and a large number of telescopes. Such arrays could have direct imaging capabilities if optimized beam combiners are used. Aims. This paper aims at studying the performance of an interferometric beam combiner using single mode fibers and in the frame of a hypertelescope. Methods. A laboratory testbed called SIRIUS was developed. We describe the general concept, the technical specifications and the results obtained. These results are analyzed with the help of a numerical simulator. Results. Direct images were obtained at the densified focus of SIRIUS. We show that the fibers greatly ease the pupil rearrangement. They also greatly improve the quality and the stability of the direct image. The computed images allow us to reproduce the effects of differential photometry and the influence of optical path difference variations. Optical path difference errors less than $\lambda/10$ and differential photometries less than 60% are required to keep the quality of the direct image. Conclusions. These results demonstrate the great potential of direct imaging interferometric beam combiners for future optical large arrays. The excellent comparison between experience and simulation clearly shows the simplicity of the fibered pupil densifier. It also gives us a great confidence in the extrapolation of these results and specifications for future arrays with a very large number of apertures.

We report laboratory tests of an all-guided stellar interferometer used for optical aperture synthesis. In anticipation of use of the interferometer in space missions, this research is focused especially on compactness of the recombining device. The coherent transport and delay lines are implemented with polarization-maintaining fiber. Beam recombination is achieved by means of an integrated optics component. This two-arm interferometer operates at 670-nm mean wavelength and allows for a 24-cm correction for the differential air path.