Qualification of IONIC (Integrated Optics Near-infrared Interferometric Camera)

Transcription

1 Qualification of IONIC (Integrated Optics Near-infrared Interferometric Camera) Karine Rousselet-Perraut a, Pierre Haguenauer a, Panayoti Petmezakis a, Jean-Philippe Berger b, Denis Mourard c, Sam Ragland c, Guillaume Huss d, Francois Reynaud d, Etienne Le Coarer a, Pierre Kern a, Fabien Malbet a a Laboratoire d Astrophysique de l Observatoire de Grenoble B.P. 53, F Grenoble Cedex 9, France b Laboratoire d Electromagnétisme, Micro-Ondes et Opto-électronique ENSERG, B.P. 257, F Grenoble Cedex 1, France c Observatoire de la Côte d Azur - Département Fresnel Plateau de Calern, Caussols, F Saint-Vallier-de-Thiey, France d Institut de Recherche en Communications Optiques et Micro-ondes Université de Limoges, 123 Rue Albert Thomas, F Limoges Cedex, France ABSTRACT We report the first technical results of the Integrated Optics Near-infrared Interferometric Camera (IONIC) developed and characterized at the Observatoire de Grenoble as well as the first tests carried out on the GI2T Interferometer (Observatoire de la Côte d Azur, France). This near-infrared interferometric camera dedicated to astronomical observations is implemented in a single dewar, which hosts the planar integrated optics beam combiner and a cooled HgCdTe infrared detector, with optical interfaces reduced to optical fibers for signal injection in the component. Thanks to its versatility, this concept allows to combine any number of telescopes in the near infrared range (J, H and K bands). The compactness of the integrated optics components allows various combining schemes (co- and multi-axial ones) and observations in different spectral bands simultaneously. Finally, a camera upgrade with a PICNIC chip is also described. This new set-up under integration at the Observatoire de Grenoble would allow to reach limiting magnitudes of H = 4-6 with the GI2T. Keywords: Planar integrated optics, interferometric camera, single-mode interferometry. 1. INTRODUCTION Several single-mode astronomical instrumentations have already proved that spatial filtering associated to photometric calibrations provides very accurate visibility measurements. For instance the accuracies on visibility obtained with FLUOR (IOTA, Arizona) or PTI (California) reach a fraction of 1% 1. These very attractive results lead the European Southern Observatory to foresee a near-infrared single-mode instrument to combine the Unit and Auxiliary Telescopes of the Very Large Telescope Interferometer, which leads to the study of the AMBER instrument 2. These existing or under study instruments use single-mode optical fibers to perform the spatial filtering. Another approach has been investigated by the Observatoire de Grenoble since 1996: the planar integrated optics 3. After studying how planar integrated optics can be attractive for astronomical interferometry 4,5, we validated our approach with several systematic interferometric tests on a dedicated lab workbench 6,7,8. The very encouraging results lead us to study and design an Integrated Optics Near-infrared Interferometric Camera (called IONIC). The latter consists in a single dewar hosting a planar integrated optics beam combiner and a cooled infrared detector, with optical interfaces reduced to optical fibers for light injection in the component. This camera is first dedicated to technically validate our approach on a stellar interferometer and this concept of compact camera, which requires to test the behavior of planar integrated optics components and connection fibers at low temperature, the solutions for Further author information: (Send correspondence to Karine Rousselet-Perraut) Karine Rousselet-Perraut: Karine.Perraut@obs.ujf-grenoble.fr

2 ensuring air-tightness of the dewar fiber feed-through, etc. Then, a camera based on this concept will be dedicated to scientific astronomical observations in the J, H and K bands. In this paper we first recall the attractive capabilities of integrated optics for astronomical interferometry (Sect. 2.1). By this way we illustrate how an instrumentation based on this technique allows to address several scientific key drivers that are outlined in Sect In Sect. 3, we present the concept of an integrated optics interferometric camera and finally we report the first results of the IONIC prototype qualification in laboratory and with the GI2T/REGAIN Interferometer (Sect. 4) Integrated Optics Scientific Advantages 2. SCIENTIFIC RATIONALE We outline below the intrinsic capabilities of integrated optics that are very attractive for an astronomical application. 1. As single-mode waveguides, planar integrated optics components can ensure the spatial filtering of the incoming stellar beams. These waveguides being manufactured by photolithographic technique, extraction of photometric calibration waveguides can be implemented on the photolithography mask, which allows a real-time calibration of intensity fluctuations. This ensures a high accuracy on visibility measurements. The single-mode behavior implies to operate in reduced spectral bands, typically in one atmospheric band. At present, planar integrated optics devices are available in the J, H and K bands and investigations are under progress for the thermal infrared wavelengths as reported in Schanen et al. in these proceedings Since all the waveguides are manufactured on a single chip of glass or silicon, the components are very stable and quite insensitive to the external parameters (as temperature and pressure). This ensures a high stability of the visibility measurements. 3. Birefringence of the planar integrated optics waveguides is perfectly controlled by manufacture, which allows to obtain visibility measurements without instrumental polarization bias. 4. Several interferometric configurations can be implemented in a single camera. Indeed thanks to the compactness of such components (a few cm 2 ), we can integrate several instruments in a limited space. For instance, a single camera can host a co-axial beam combiner AND a multi-axial one, or one beam combining device per polarization direction, or one beam combining device per spectral band to cover J, H and K bands for instance. 5. Planar integrated optics intrinsically confines light in one direction only: planar optics components can act as the slit of one spectrograph. Spectral capabilities can thus be easily implemented without specific cylindrical optics. 6. As reported by Berger et al. in these proceedings, the combination of up to 8 telescopes is quite possible with planar integrated optics. In fact, increasing the telescope number only implies a more complex photolithographic mask and does not induce more opto-mechanical complexities, more instabilities, more critical alignments or huge optical tables. Thus phase closure with several baselines and besides high angular resolution imaging is accessible with integrated optics devices Astrophysical Targets It is thus obvious that the scientific program of an integrated optics interferometric camera can be huge, even if we restrict the spectral range to the near-infrared one (J, H and K bands). Among the numerous items, we can mention (see also the science opportunities with AMBER in these proceedings 9 for examples): High accuracies on visibility measurements and spectral information enables the study of the circumstellar environments of young stellar objects with investigations on disk geometry and density distribution, jets and magnetic activity, velocity field, multiplicity, etc. 10

3 The control of the instrumental polarization and high accuracies on visibility measurements makes possible to fully investigate polarimetric interferometry technique described by Chesneau et al. in these proceedings 11. Simultaneously recording visibilities in two polarizations is a powerful tool for constraining scattering phenomena, temperature and density distributions in hot circumstellar environments, magnetic geometry of Ap and Bp stars, etc. Very high accuracies on visibility measurements allow to foresee the detection and the study (masses, spectra) of extrasolar giant planets around solar type stars 12. Finally, imaging capabilities (first by phase closure measurements, then by image restoration) obviously concern all the objects throughout the HR-diagram with the possibility to produce high angular resolution images of stellar surfaces of chemically peculiar and magnetic stars, of spots of cold giants and young stellar objects, etc. The number of astrophysical candidates clearly depends upon the sensitivity of the whole instrument (besides upon the collecting surfaces and the detector readout noise). This problem is not addressed in this paper since nowadays very large telescopes (8-10 m class) and low readout noise detectors exist, which leads to a large number of objects for each scientific item. 3. CONCEPT OF AN INTEGRATED OPTICS CAMERA: IONIC To address several of these scientific items, the Observatoire de Grenoble proposes the concept of an Integrated Optics Near-infrared Interferometric Camera, called IONIC, operating in the near-infrared range. Its principle is illustrated in Figure 1. Several interferometric functions are integrated on a single planar integrated optics chip (See Malbet et al. 4 for current integrations and developments). Such a chip or several ones are included in the dewar of the liquid-nitrogen cooled infrared array. The stellar light is injected in the planar integrated optics chip(s) either via high-birefringent fibers or directly Design 4. QUALIFICATION OF THE IONIC PROTOTYPE The IONIC prototype developed at Observatoire de Grenoble is described in Figure 2. It is based on planar optics components ensuring polarization control, beam combination, spatial filtering and photometric calibrations (See also Fig. 1). A single portable dewar is used to integrate an infrared HgCdTe array and the planar optics chip. The stellar light is injected into the dewar via fiber connectors feed-through in place of the dewar window. The component outputs are imaged onto the detector via relay lenses. Alignments are performed by cryogenic motors. There is no spectral dispersion but wavelengths are selected via a motorized filter wheel (this prototype can operate in the J, H and K bands). We choose to test it step by step, as described below Combiners Tests in Lab Workbench Two- and three-telescope beam combiners in co- and multi-axial mode operating in the H atmospheric band are now available at the Observatoire de Grenoble thanks to research collaborations with the Laboratoire d Electromagnétisme, Micro-Ondes et Opto-électronique (LEMO) and the CEA/LETI at Grenoble. We performed systematic interferometric tests of our planar integrated optics beam combiners. The high performances in terms of instrumental contrast, accuracy and stability of these components are reported in Haguenauer et al. in these proceedings 7. Improvements on beam combiners operating in the K bands are also in progress 14.

4 Beam Quality Control Light Collecting Beam Transportation Optical Path Delay Wavefront Correction Fringe Tracking Polarization Control Spatial Filtering Photometric Calibration Beam Combination Spectral Dispersion Detection Deformable Mirror Sensor Camera WFS WFS WFS Planar integrated optics Dewar Optical fibers Planar integrated optics Figure 1. Principle of an integrated optics camera: several interferometric functions (see the functional analysis of an interferometer at the top of the figure) are integrated on a single planar integrated optics chip. This chip can be placed inside a dewar (bottom) for an infrared application. Stellar light can then be injected in the planar integrated optics chip via optical fibers, which allows to operate with a blind dewar and thus to reduce the thermal noise. Note that only 3 interferometric ways have been represented but that this concept can be applied to a larger number of telescopes (See Berger et al. in these proceedings 13 ). Dewar feed-through Optical fibers Fiber connectors Integrated optics beam combiner Relay lenses Spectral filters Cryogenic motors Cold plate (77K) Dewar Infrared detector Figure 2. Design of the IONIC prototype developed at LAOG. See text for details.

5 4.3. Tests of the acquisition module The previous tests in lab workbench also allow to validate the capabilities of electronics and acquisition software. Dedicated developments on the IONIC prototype detector have been performed at the Observatoire de Grenoble, especially to optimize the frame rate: a zoom facility allowing a fast readout of any chosen pixel window (typically few pixels on a single line in the case of our planar integrated optics beam combiners) has been implemented, tested and validated in the laboratory testbench. The acquisition software monitors this pixel window and is versatile enough to allow the implementation of any planar integrated optics component. Standard motorized translation stages have been tested to ensure an Optical Path Difference (OPD) scan without loss of coupling efficiency (these losses remain of the order of 1% for typical scans of 20 mm). A piezo-actuator has also been linearized over its stroke of 60 µm. Thanks to its good characteristics we use it to determine the residual chromatic dispersion inside our fiber connected planar integrated optics components as explained in Ref Cryogenic and vacuum tests Cryogenic and vacuum tests have been performed at the Observatoire de Grenoble to check: the air-tightness of the fiber connectors feed-through. In fact, in the IONIC concept the dewar window is replaced by a fiber feed-through including air-tight joint (vacuum glue) at the fiber level and fiber connectors fixed onto its external plate as depicted in Fig. 3. The tests validate the air-tightness of our feed-through and allow to improve the concept (essentially for easier integrations and implementations). Dewar plate Fiber connectors Stellar light Figure 3. Fiber connectors feed-through used for the IONIC prototype. The feed-through (background) can be screwed onto the dewar in place of the dewar window. Stellar light is injected inside the IONIC prototype thanks to optical fibers (foreground) that are directly connected onto the external connectors of the feed-through. the behavior of the planar integrated optics components at low temperatures. In this first step, the component temperature reaches about -5 C and we do not notice any flux variation between ambiant and low temperatures, which proves the satisfying behavior of the planar integrated optics itself and also of its fiber connection (each planar integrated optics components are glued to a standard telecom fiber ribbon).

6 4.5. Preliminary Coupling Tests with the GI2T/REGAIN Interferometer We decided to technically validate our approach on the GI2T/REGAIN Interferometer 15 with our IONIC prototype. For this purpose, the Institut de Recherche en Communications Optiques et Micro-ondes (IRCOM) studied and manufactured the opto-mechanical GI2T/IONIC interface (Figure 4 bottom right). The stellar light coming from the GI2T is reflected by flat mirrors and then injected thanks to microscope objectives onto the input fibers of the IONIC prototype. The Optical Path Difference (OPD) scan is performed thanks to a motorized 25-mm translation stage combined with a 60-µm stroke piezoelectric actuator. Because of the large 1.5-m apertures of the GI2T, tip-tilt correctors are mandatory for a near-infrared single-mode applications. Such correctors have been designed and manufactured by IRCOM (Figure 4 bottom left). A dichroic directs the blue part of the spectrum (λ 0.5 µm) towards the photocenter detection by four photomultiplier tubes, allowing to servo the tip-tilt mirrors placed close to the pupil plane of REGAIN. Tip-Tilt corrector GI2T/IONIC interface Transport fiber From South telescope From North telescope IONIC electronics and test detector Microscope objectives Flat mirrors IONIC fibers OPD scan Detection and servoing device Tip-tilt mirror Figure 4. IONIC prototype implantation for preliminary coupling tests on the GI2T/REGAIN Interferometer. This includes the alignment tools (sources and a collimating-telescope), the GI2T/IONIC interface (photo at the bottom right), the tip-tilt correctors (photo at the bottom left) and a transport fiber to the test detector. Note that only one tip-tilt corrector has been represented.

7 Preliminary tests of coupling into single-mode fibers occured at the GI2T during Summer 1999 to quantify the performances of the tip-tilt correctors. The observations parameters were: Objects: αlyr (A0V, H=0), βpeg (M2II, H=-2) Seeing: 2.5 then 3 (with strong wind) Integration time: 50 ms τ 150 ms. Recorded points per frame: 500 Tip-tilt correction: frames are recorded without tip-tilt correction, with tip-tilt correction alone and with tip-tilt correction and video servo loop of telescope guiding Fig. 5 illustrates preliminary results obtained on αlyr. Table 1 quantifies the significant effect of the correction in the case of Fig. 5. It provides the detection number corresponding to a signal larger than the noise at 3σ (line 2), the maximal Signal-to-Noise Ratio (SNR max - line 3) and the maximal time during which the signal is larger than the noise at 3σ (τ max - line 4). The results show that the tip-tilt correction greatly improves the coupling efficiency in the single-mode fiber by more than a factor 3. Table 1. Detection number corresponding to a signal larger than the noise at 3σ, maximal Signal-to-Noise Ratio (SNR max ) and maximal time during which the signal is larger than the noise at 3σ (τ max ) obtained without tip-tilt correction and with tip-tilt correction and video servo loop of telescope guiding on αlyr on the GI2T during Summer The integration time is 150 ms and the seeing is 2.5 (see Fig. 5). Criteria Without tip-tilt correction With tip-tilt correction Detection number 25% 79% SNR max τ max 900 ms 3150 ms Flux (A.U.) Number of the recording Figure 5. Preliminary results of coupling into single-mode fibers obtained on αlyr on the GI2T during Summer 1999 without tip-tilt correction (dash line) and with tip-tilt correction and video servo loop of telescope guiding (solid line): flux at the fiber output versus the number of the recording. For each recording the integration time is 150 ms (the duration of the above recording is about 15 s) and the seeing is 2.5.

8 4.6. Complementary tests of the tip-tilt correctors with the GI2T/REGAIN Interferometer The previous coupling results allowed to quantify the tip-tilt correctors efficiency as well as the capabilities of the electronics of our prototype detector within the context of astronomical applications (low flux and high frame rate). These tests led us to perform efficient improvements on tip-tilt correctors and acquisition software of the IONIC prototype. The latter improvements will allow to reduce the integration time (the reported high integration times were implied by the acquisition limitations and the bad seeing) for the next observations so as to reach integration times in agreement with fringe recording (i.e. few milliseconds). Figs. 6 and 7 illustrates the results obtained after tip-tilt improvements, especially as regards to the detection and servoing device. These figures correspond to observations on Capella performed with the GI2T/REGAIN in January 2000 with a bad seeing of 3 to 4. These observations confirm that the tip-tilt correctors properly work and that the significant improvement they provide is stable over a long observation time. Figure 6. Short exposure photocentroid displacements along the two directions without tip-tilt correction (left) and with tip-tilt correction and video servo loop of telescope guiding (right). They are obtained with the GI2T/REGAIN on Capella in January The seeing is about 3 to 4 and the data with and without tip/tilt correction are recorded one after the other to ensure that the seeing remains the same for both the data sets.

9 Figure 7. Long exposure images of Capella obtained with the GI2T/REGAIN in January 2000 without tip-tilt correction (left) and with tip-tilt correction and video servo loop of telescope guiding (right). The seeing is about 3 to 4. The long exposure images are reconstructed from short exposure photocentroids (Fig. 6). 5. PERSPECTIVES During 2000, we first foresee to technically validate the IONIC concept with the GI2T/REGAIN Interferometer and our prototype, i.e. to measure calibrated visibilities and check the accuracy of these measurements. Because of the read-out noise of the detector prototype ( 700 e ), limiting magnitudes are first estimated to H = 0-2 (with the GI2T/REGAIN). This sensitivity is sufficient for technical validations on well known stars. During the same time, the Observatoire de Grenoble develops a low read-out noise camera based on a Rockwell PICNIC array, whose read-out noise is expected to remain smaller than 20 e, allowing a sensitivity gain of 4 mag. This future camera is dedicated to scientific observations with planar integrated optics components. First two-telescope beam combiners in near-infrared bands (J, H or K bands) could be implemented and could be used at the GI2T/REGAIN focus. Besides, three-telescope beam combiners would be integrated to allow phase closure measurements and imaging capabilities on existing instruments (IOTA 16, CHARA 17 for instance). Such a concept could also be contemplated for the next generation instruments of the VLTI. A future instrumentation could permit to combine all the VLTI telescopes (8-m and 1.8-m ones) with a single planar optics chip 13 and to implement a spectral dispersion facility, allowing to fully investigate the imaging capability of the ESO interferometric array 18. ACKNOWLEDGMENTS These works are funded by INSU/PNHRA, ULTIMATECH/CNRS, DRET/DGA and CNES. The authors are grateful to Eric Stadler, Yves Magnard, Colin Duport, Alain Dexet and Alain Delboulbé for their support for manufacturing the prototype. The planar integrated optics components are manufactured by GeeO company and by the CEA/LETI at Grenoble. For the HgCdTe detector of the IONIC prototype there is a contract with Sofradir (Grenoble, France). The authors are grateful to the GI2T team for their observing support.

10 REFERENCES 1. V. Coudé du Foresto, PhD Thesis, University of Paris, R. Petrov et al., AMBER: the near-ir focal instrument for the VLTI, in Interferometry in Optical Astronomy, in these proceedings, P. Kern, F. Malbet, I. Schanen-Duport, P. Benech, Integrated optics single-mode interferometric beam combiner for near infrared astronomy, in Integrated Optics for Astronomical Interferometry, P. Kern & F. Malbet, ed., AstroFib 96 Conference, pp , F. Malbet, P. Kern, I. Schanen-Duport, J.P. Berger, K. Rousselet-Perraut, P. Benech, Integrated optics for astronomical interferometry. I- Concept and astronomical applications, A&A Suppl. Series 138, p , P. Kern, J.P. Berger, P. Haguenauer, F. Malbet, K. Rousselet-Perraut, Planar integrated optics contribution to instrumentation for interferometry, in Interferometry in Optical Astronomy, in these proceedings, J.P. Berger, K. Rousselet-Perraut, P. Kern, F. Malbet, I. Schanen-Duport, F. Reynaud, P. Haguenauer, P. Benech, Integrated optics for astronomical interferometry. II- First laboratory white-light interferograms, A&A Suppl. Series 139, p , P. Haguenauer, J.P. Berger, K. Rousselet-Perraut, P. Kern, F. Malbet, I. Schanen-Duport, P. Benech, Integrated optics for astronomical interferometry. III- Optical validation of a planar optics two-telescope beam combiner, Applied Optics, in press, P. Haguenauer, M. Severi, I. Schanen-Duport, K. Rousselet-Perraut, J.P. Berger, Y. Duchêne, P. Kern, F. Malbet, P. Benech, Optical characterization of planar optics three telescope beam combiners, in Interferometry in Optical Astronomy, in these proceedings, A. Richichi, Science opportunities with AMBER, the near-ir VLTI instrument, in Interferometry in Optical Astronomy, in these proceedings, F. Malbet, C. Bertout, Detecting T Tauri disks with optical long-baseline interferometry, A&A Suppl. Series 113, p , O. Chesneau, K. Rousselet-Perraut, F. Vakili, D. Mourard, C. Cazalé, Polarimetric Interferometry: concept and applications, in Interferometry in Optical Astronomy, in these proceedings, V. Coudé du Foresto, J.-M. Mariotti, G. Perrin, Direct observation of extrasolar planets with an infrared interferometer, in Science with the VLT Interferometer, F. Paresce, ed., ESO Conf. Proc., Garching, Germany, p.86-87, J.P. Berger, P. Benech, I. Schanen-Duport, F. Malbet, F. Reynaud, Combining light of an array of up to 8 telescopes in a single chip, in Interferometry in Optical Astronomy, in these proceedings, I. Schanen-Duport, F. Malbet, G. Taillades, Optical infrared waveguides for astronomical interferometry, in Interferometry in Optical Astronomy, in these proceedings, D. Mourard, D. Bonneau, R. Dalla, A. Glentzlin, G. Merlin, M. Pierron, N. Thureau, L. Abe, A. Blazit, O. Chesneau, P. Stee, S. Ragland, F. Vakili, C. Vérinaud, GI2T/REGAIN Interferometer, in Interferometry in Optical Astronomy, in these proceedings, W. Traub, Recent results from the IOTA interferometer, in Astronomical Interferometry, R. D. Reasenberg, ed., Proc. SPIE, Vol. 3350, pp , H.A. McAlister, W.G. Bagnuolo, T.A. ten Brummelaar, R. Cadman, S.T. Ridgway, L. Sturmann, N.H. Turner, CHARA array on Mt Wilson, California, in Interferometry in Optical Astronomy, in these proceedings, A. Glindemann, V. Coudé du Foresto, F. Delplancke, F. Derié, M. Ferrari, A. Gennai, P. Gitton, P. Kervella, B. Koehler, S.A. Lévêque, G. de Marchi, S. Ménardi, A. Michel, F. Paresce, A. Richichi, M. Schoeller, A. Wallander, VLT Interferometer: an unique instrument for high-resolution astronomy, in Interferometry in Optical Astronomy, in these proceedings, 2000.