Patrick Lanzoni

Characterization of MOEMS devices for the instrumentation of Next Generation Space Telescope

Instrumentation of the Next Generation Space Telescope (NGST) will include a Multi-Object Spectrograph (MOS) in order to record simultaneously several hundred spectra in a single observation run. The selection of the objects in the field of view will be done by a MOEMS-based device: a micro-shutter array (MSA). In Laboratoire d’Astrophysique de Marseille, we have developed since several years different tools for the modeling and the characterization of these MOEMS-based slit masks. We are now developing a new bench for the measurement of the contrast value. The contrast is the amount of non-selected flux from sky background and bright sources passing through the multi-slit device. Contrast measurement have been carried out on the micro-mirror array fabricated by Texas Instrument. We can address several parameters in our experiment, as the size of the source, its location with respect to the micro-elements, the wavelength, and the input and output pupil size. In order to measure the contrast, the micro-mirrors are tilted between the ON position (towards the spectrograph) and the OFF position (towards a light trap). Contrast exceeding 400 has been measured for a 10° ON/OFF angle and values exceeding 6000 for a 20° ON/OFF angle.

ZEUS, a cophasing sensor based on the Zernike phase contrast method

We describe the ZEUS phasing camera for future extremely large telescopes (ELTs) based on the Zernike phase contrast method. A prototype instrument is under construction for implementation in the Active Phasing Experiment (APE), a VLT test bed scheduled for operation in 2007. The paper describes theoretical aspects of the method and its experimental validation, as well as the instrumental implementation for APE. Aspects of its implementation in an ELT are also discussed. While the classical Zernike method uses a phase mask with diameter approximately equal to the Airy disk, we employ a mask the size of the seeing disk. This allows us to overcome the problems related to atmospheric turbulence, whose low spatial frequency phase errors are much larger than the co-phasing errors to be measured. The thickness (OPD) of the mask can be set to lambda/4 - as in the classical case - for maximum signal strength, but for initial phasing where phase errors are much larger than the sensors linear range (+/-lambda/4), a thinner mask produces a cleaner signal more easily exploitable, leaving the signal analysis more robust. A multi wavelength approach is implemented in order to extend the capture range of the sensor, and the ultimate precision is reached using an iterative approach. End-to-end simulations indicating an achievable precision within the required precision will be shown.

Interferometric characterization of MOEMS devices in cryogenic environment for astronomical instrumentation

Next generation of infra-red astronomical instrumentation for space telescopes as well as ground-based extremely large telescopes requires MOEMS devices with remote control capability and cryogenic operation, including programmable multi-slit masks for multi-object spectroscopy (MOS). For the complete testing of these devices, we have developed in parallel and coupled a high-resolution Twyman-Green interferometer and a cryogenic-chamber for full surface and operation characterization. The interferometer exhibits a nanometer accuracy by using phase-shifting technique and low-coherence source. The cryogenic-chamber has a pressure as low as 10e-6 mbar and is able to cool down to 60K. Specific interfaces minimizing stresses for vacuum and cryo have been set. Within the framework of the European program on Smart Focal Planes, micro-mirrors have been selected for generating MOEMS-based slit masks. A first 5×5 micro-mirror array (MMA) with 100×200μm2 mirrors was successfully fabricated using a combination of bulk and surface silicon micromachining. They show a mechanical tilting angle of 20° at a driving voltage below 100V, with excellent surface quality and uniform tilt-angle. The mirrors could be successfully actuated before, during and after cryogenic cooling. The surface quality of the gold coated micro-mirrors at room temperature and below 100K, when they are actuated, shows a slight increase of the deformation from 35nm peak-to-valley to 50nm peak-to-valley, due to CTE mismatch between silicon and gold layer. This small deformation is still well within the requirement for MOS application.

Active optics and modified-Rumsey wide-field telescopes: MINITRUST demonstrators with vase- and tulip-form mirrors

Wide-field astronomy requires the development of larger aperture telescopes. The optical properties of a three-mirror modified-Rumsey design provide significant advantages when compared to other telescope designs: (i) at any wavelength, the design has a flat field and is anastigmatic; (ii) the system is extremely compact, i.e., it is almost four times shorter than a Schmidt. Compared to the equally compact flat-field Ritchey-Chrétien with a doublet-lens corrector, as developed for the Sloan digital sky survey-and which requires the polishing of six optical surfaces-the proposed modified-Rumsey design requires only a two-surface polishing and provides a better imaging quality. All the mirrors are spheroids of the hyperboloid type. Starting from the classical Rumsey design, it is shown that the use of all eight available free parameters allows the simultaneous aspherization of the primary and tertiary mirrors by active optics methods from a single deformable substrate. The continuity conditions between the primary and the tertiary hyperbolizations are achieved by an intermediate narrow ring of constant thickness that is not optically used. After the polishing of a double vase form in a spherical shape, the primary-tertiary hyperbolizations are achieved by in situ stressing. The tulip-form secondary is hyperbolized by stress polishing. Other active optics alternatives are possible for a space telescope. The modified-Rumsey design is of interest for developing large space- and ground-based survey telescopes in UV, visible, or IR ranges, such as currently demonstrated with the construction of identical telescopes MINITRUST-1 and -2, f/5-2 degrees field of view. Double-pass optical tests show diffraction-limited images.

Variable curvature mirrors: implementation in the VLTI delay-lines for field compensation

As the result of an analysis pursued from the very beginning, today the VLT Interferometer is the only interferometer allowing to have a 2 arcsec interferometric field of view (f.o.v) available at the instruments entrance. This accessible interferometric field is the direct result of a careful pupil transfer from the individual telescopes to the central laboratory, unique feature of the VLTI. For this goal it has been necessary to develop a new optical device, the Variable Curvature Mirror (VCM.), using large deformation theory of elasticity, and advanced techniques in optical fabrication. The possibility with the VLTI to use various baselines, from 8 to 200 m with UTs or ATs, leads to severe conditions on the VCM curvature range. A given delay-line, and its associated VCM, should be able to transfer a pupil to the interferometric laboratory from a very far or relatively close position of an ATs. Considering the f.o.v required in the VLTI (2 arcsec), the delay-lines strokes or the OPD to compensate for, and the various locations of the UTs and ATs stations, the curvature of the VCM has to be continuously variable within a range from 84 mm-1 to 2800 mm-1. The location of the VCM in the delay-line system, on the piezo-translator used for small OPD compensation, led to minimize its dimensions and to realize a small active mirror with a 16mm diameter. With this small optical aperture, the VCM range of curvature corresponds to a f ratio from f/∞ to f/2.625. The two first VCM complete systems (mirror, mechanics and control command software) have been achieved in 2001/2002 and will be installed in the VLTI delay-lines during fall 2002. Their final performances (optical quality, pupil transfer accuracy, etc.) are reviewed.

Micromirrors for multiobject spectroscopy: optical and cryogenic characterization

We are developing micromirror arrays (MMA) for future generation infrared multiobject spectroscopy (MOS) requiring cryogenic environment. So far we successfully realized small arrays of 5×5 single-crystalline silicon micromirrors. The 100μm ×200μm micromirrors show excellent surface quality and can be tilted by electrostatic actuation yielding 20° mechanical tilt-angle. An electromechanical locking mechanism has been demonstrated that provides uniform tilt-angle within one arc minute precision over the whole array. Infrared MOS requires cryogenic environment and coated mirrors, silicon being transparent in the infrared. We report on the influence of the reflective coating on the mirror quality and on the characterization of the MMA in cryogenic environment. A Veeco/Wyko optical profiler was used to measure the flatness of uncoated and coated mirrors. The uncoated and unactuated micromirrors showed a peak-to-valley deformation (PTV) of below 10nm. An evaporated 10nm chrome/50nm gold coating on the mirror increased the PTV to 35nm; by depositing the same layers on both sides of the mirrors the PTV was reduced down to 17nm. Cryogenic characterization was carried out on a custom built interferometric characterization bench onto which a cryogenic chamber was mounted. The chamber pressure was at 10e-6 mbar and the temperature measured right next to the micromirror device was 86K. The micromirrors could be actuated before, during and after cryogenic testing. The PTV of the chrome/gold coated mirrors increased from 35nm to 50nm, still remaining in the requirements of < lambda/20 for lambda=1μm.

Optical characterization of fully programmable MEMS diffraction gratings

We have fabricated and characterized fully programmable diffraction gratings consisting of 64 silicon micro-mirrors. The mirrors are 700µm long and 50µm wide with a fill factor of 90%. They are actuated electrostatically and move down by 1.25μm while showing negligible cross-talk and bowing as small as 0.14μm over 700μm. Extinction ratio up to 100 has been achieved by adjusting only 3 adjacent micro-mirrors. The gratings could operate either as light modulators up to 5μm or spectra generators up to 2.5μm.

Large MEMS-based programmable reflective slit mask for multi- object spectroscopy fabricated using multiple wafer-level bonding

Multi-object spectroscopy (MOS) allows measuring infrared spectra of faint astronomical objects that provides information on the evolution of the Universe. MOS requires a slit mask for object selection at the focal plane of the telescope. We are developing MEMS-based programmable reflective slit masks composed of 2048 individually addressable micromirrors. Each micromirror measures 100 × 200 μm2 and is electrostatically tilted by a precise angle of at least 20°. The main requirements for these arrays are precise and uniform tilt angle over the whole device, uniformity of the mirror electromechanical behavior, a flat mirror deformation and individual addressing capability of each mirror. This capability of our array is achieved using a line-column algorithm based on an optimized tilt angle/voltage hysteresis of the electrostatic actuator. Micromirror arrays composed of 2048 micromirrors (32 × 64) and modeled for individual addressing were fabricated using fusion and eutectic wafer-level bonding. These micromirrors without coating demonstrated a peak-to-valley deformation less than 8 nm and a tilt angle of 24° for an actuation voltage of 130 V. A first experiment of the linecolumn algorithm was demonstrated by actuating individually 2 × 2 micromirrors. In order, to avoid spoiling of the optical source by the thermal emission of the instrument, the micromirror array has to work in a cryogenic environment. Therefore, these devices were characterized in a cryogenic environment at -111°C and several lines of micromirrors were tilted successfully under these conditions.

Programmable CGH on photochromic plates coded with DMD generated masks

Computer Generated Holograms (CGHs) are used for wavefront shaping and complex optics testing. Present technology allows for recording binary CGHs. We propose a Digital Micro-mirror Device (DMD) as a reconfigurable mask, to record rewritable binary and grayscale CGHs on a photochromic plate. Opaque at rest, this plate becomes transparent when it is illuminated with visible light of suitable wavelength. We have successfully recorded the very first amplitude grayscale CGH, with a contrast greater than 50, which was reconstructed with a high fidelity in shape, intensity, size and location. These results reveal the high potential of this method for generating programmable/rewritable grayscale CGHs, which combine DMDs and photochromic substrates.

Realization and characterization of a MEMS-based programmable slit mask for multi-object spectroscopy

Multi-object spectroscopy (MOS) is a powerful tool for space and ground-based telescopes for studying the formation of galaxies. This technique requires a programmable slit mask for astronomical object selection. A first sample of MEMS-based programmable reflective slit masks with elements of size 200×100 μm2 has been successfully tested in cryogenic conditions at 92 K. Devices of larger size were microfabricated, the largest chip measures 25×22 mm2 and is composed of 200×100 electrostatic actuated micromirrors. These devices are composed of two chips: the electrode chip and the mirror chip, which are processed separately and assembled consecutively. The mirror chip is bonded on top of the electrode chip and microfabricated pillars on the electrode chip provide the necessary spacing between the two parts. A process flow utilizing refilling techniques based on borophosphosilicate glass (BPSG) deposition and reflow was developed. Programmable reflective slit masks based on this fabrication process were microfabricated and characterized. These devices exhibit a micromirror deformation of 11 nm peak-to-valley and an actuation voltage of 145 V for a tilt angle of 9°. Preparation of samples for MOS experiments are underway.