Frederic Zamkotsian

Arrays of High Tilt-Angle Micromirrors for Multiobject Spectroscopy

Micromirror arrays are promising components for generating reflective slit masks in future multiobject spectrographs. The micromirrors, 100 mum times200 mum in size, are etched in bulk single crystal silicon, whereas a hidden suspension is realized by surface micromachining. The micromirrors are actuated electrostatically by electrodes located on a second chip. The use of silicon on insulator (SOI) wafers for both mirror and electrode chip ensures thermal compatibility for cryogenic operation. A system of multiple landing beams has been developed, which latches the mirror at a well-defined tilt angle when actuated. Arrays of 5times5 micromirrors have been realized. The tilt angle obtained is 20deg at a pull-in voltage of 90 V. Measurements with an optical profiler showed that the tilt angle of the actuated and locked mirror is stable with a precision of 1 arcmin over a range of 15 V. This locking system makes the tilt angle independent from process variations across the wafer and, thus, provides uniform tilt angle over the whole array. The surface quality of the mirrors in actuated state is better than 10-nm peak to valley and the local roughness is about 1-nm root mean square

The two-dimensional array of 2048 tilting micromirrors for astronomical spectroscopy

In multi-object spectrometers, field selectors, which are located in the focal plane of the telescopes, are required for the selection of astronomical objects, such as stars or faint galaxies. We present a two-dimensional micromirror array as a field selector. Object selection is achieved through the precise tilting of micromirrors, which reflect the light of objects toward the spectrometer. These arrays were composed of 2048 (32 x 64) electrostatically actuated silicon micromirrors that measured 100 x 200 mu m(2). The micromirrors were addressed either by lines or individually using a line-column addressing scheme. The fabrication process of these devices involved three wafers and two wafer-level bondings. Characterization, by white light interferometry, revealed a tilt angle of 25 degrees at a voltage of 121 V, and a coated micromirror deformation below 10 nm. A fill factor of 82% and a contrast of 1000:1 were also observed. Demonstration of the line-column addressing scheme was achieved through its application to a sub-array of 2 x 2 micromirrors.

Successful evaluation for space applications of the 2048×1080 DMD

Next-generation infrared multi-object spectrographs (MOS) for ground-based and space telescopes could be based on MOEMS programmable slit masks. This astronomical technique is used extensively to investigate the formation and evolution of galaxies. ESA has engaged a study for a technical assessment of using a DMD from Texas Instruments for space applications. The DMD features 2048 × 1080 mirrors on a 13.68μm pitch, where each mirror can be independently switched between an ON (+12°) position and an OFF (-12°) position. For MOS applications in space, the device should work in vacuum, at low temperature, and each MOS exposure would last for typically 1500s with micromirrors held in a static state (either ON or OFF). A specific thermal/vacuum test chamber has been developed for test conditions down to -40°C at 10-5 mbar vacuum. Imaging capability for resolving each micromirror has also been developed for determining degradation in any single mirror. Our first tests reveal that the DMD remains fully operational at -40°C and in vacuum. A 1038 hours life test in space conditions, Total Ionizing Dose radiation, thermal cycling and vibrations/shocks have also been successfully completed. These results do not reveal any concerns regarding the ability of the DMD to meet environmental space requirements. Detailed analysis of micromirror throughputs has also been studied for a whole set of tests, and shows a rather low variation and no impact of the space environment. We have also developed a bench for MOS demonstration using MOEMS devices. DMD chip has been successfully tested revealing good contrast values as well as good functionality for applying any mask pattern, demonstrating its full ability for space instrumentation, especially in multi-object spectroscopy applications.

Characterization by guided wave of instabilities of optical coatings submitted to high-power flux: thermal and third-order nonlinear properties of dielectric thin films

We study the reversible refractive index variations of optical thin films submitted to a high-power light flux. As a preliminary step, we study the thermorefractive coefficient ?n/?T and the laser damage threshold of our materials. From the hypothesis of a localized optical Kerr-type effect, we use the m-line technique to estimate the nonlinear refractive-index coefficients n(2) of TiO(2), Ta(2)O(5), and ZnS films with continuous illumination. Very large values of n(2) are found for the films obtained by conventional evaporation condensation. On the other hand, the study of the change in the decoupling direction of a low-power light guided in a layer disturbed by a pulsed YAG laser gives a first analysis of the phenomena versus time.

Surface characterization of micro-optical components by Foucault's knife-edge method: the case of a micromirror array.

As micro-optical components are introduced into optical systems, accurate surface characterization becomes important. We describe a method for quantitative evaluation of surface deformations based on Foucaults knife-edge test. By measurement of local slopes, the surface shape of each mirror in a micromirror array has been reconstructed with a subnanometer accuracy. In addition to low-order deformation (tilt, curvature, astigmatism), each mirror is seen to be palm-tree shaped. This may be explained by strain relaxation in the fabrication process. Measurement on a conventional concave mirror confirms our method.

New astronomical instrument using MOEMS-based programmable diffraction gratings

Programmable Micro-Diffraction Gratings (PMDG) are a new type of MOEMS, opening new observational capabilities in future astronomical instrumentation. Programmable gratings are based on a serial of parallel ribbons able to move out of the plane. By using electrostatic force, ribbons are actuated and a grating could be formed. A few ribbons are efficient enough to diffract the light; then, locally, this grating acts as a ON-OFF switch. If the spectrum is focused on this type of device, by setting ON and OFF a selected number of wavelengths, the spectral response of the spectrograph is programmable. Programmable gratings permit the design of programmable spectrometers, useful in space mission, like ESA Darwin mission. This mission will search, detect and characterize exo-planets, using high-contrast nulling interferometry, coupled with spectroscopic observation. We propose a new observational concept for Darwin using a programmable spectrometer. By tailoring the spectral response, sensitivity as well as signal to noise ratio of the instrument will be increased. A demonstrator breadboard with a PMDG device has been designed and built. This demonstrator, including adjustable sources (location, spectral type, brightness), permits the tailoring of spectral patterns by the PMDG component. Two parallel spectral and imaging channels are used for the optical analysis of the tailored signals. Typical exo-planet spectra have been generated and set by the PMDG. Simulated signatures of exo-planets with life forms are clearly revealed and characterized on the breadboard, demonstrating successfully our concept. Several new observational modes using PMDG devices in future astronomical instrumentation is then foreseen.

Space evaluation of 2048×1080 mirrors DMD chip for ESA's EUCLID Mission

Next-generation infrared astronomical instrumentation for ground-based and space telescopes could be based on MOEMS programmable slit masks for multi-object spectroscopy (MOS). This astronomical technique is used extensively to investigate the formation and evolution of galaxies. We are engaged in an ESA study for a technical assessment of using a DMD from Texas Instruments for space applications (for example in ESA EUCLID mission). The DMD features 2048×1080 mirrors on a 13.68μm pitch, where each mirror can be independently switched between an ON (+12°) position and an OFF (-12°) position. For MOS applications in space, the device should work in vacuum, at low temperature, and each MOS exposure would last for typically 1500s with micromirrors held in a static state (either ON or OFF). A specific thermal/vacuum test chamber has been developed for test conditions down to -40°C at 10-5 mbar vacuum. Imaging capability for resolving each micromirror has also been developed for determining degradation in any single mirror. Our first tests reveal that the DMD remains fully operational at -40°C and in vacuum. A 1038 hours life test in space conditions, Total Ionizing Dose radiation, thermal cycling and vibrations/shocks have also been successfully completed. These results do not reveal any concerns regarding the ability of the DMD to meet environmental space requirements. We have also developed a bench for MOS demonstration using MOEMS devices. DMD chip has been successfully tested revealing good contrast values as well as good functionality for applying any mask pattern, demonstrating its full ability for space instrumentation, especially in multi-object spectroscopy applications.

BATMAN: a DMD-based multi-object spectrograph on Galileo telescope

Next-generation infrared astronomical instrumentation for ground-based and space telescopes could be based on MOEMS programmable slit masks for multi-object spectroscopy (MOS). This astronomical technique is used extensively to investigate the formation and evolution of galaxies. We are developing a 2048x1080 Digital-Micromirror-Device-based (DMD) MOS instrument to be mounted on the Galileo telescope and called BATMAN. A two-arm instrument has been designed for providing in parallel imaging and spectroscopic capabilities. The field of view (FOV) is 6.8 arcmin x 3.6 arcmin with a plate scale of 0.2 arcsec per micromirror. The wavelength range is in the visible and the spectral resolution is R=560 for 1 arcsec object (typical slit size). The two arms will have 2k x 4k CCD detectors. ROBIN, a BATMAN demonstrator, has been designed, realized and integrated. It permits to determine the instrument integration procedure, including optics and mechanics integration, alignment procedure and optical quality. First images and spectra have been obtained and measured: typical spot diameters are within 1.5 detector pixels, and spectra generated by one micro-mirror slits are displayed with this optical quality over the whole visible wavelength range. Observation strategies are studied and demonstrated for the scientific optimization strategy over the whole FOV. BATMAN on the sky is of prime importance for characterizing the actual performance of this new family of MOS instruments, as well as investigating the operational procedures on astronomical objects. This instrument will be placed on the Telescopio Nazionale Galileo mid-2015.

Optical MEMS in space instruments for Earth observation and astronomy

Optical MEMS could be major candidates for designing future generation of space instruments. In addition to their compactness, scalability, and specific task customization, they could generate new functions not available with current technologies. We have listed new functions associated with several types of MEMS. Instrumental applications are derived and we propose two promising concepts using object selection and spectral tailoring techniques. In Earth Observation instruments, observation of scenes including bright sources leads to an important degradation of the recorded signal. We propose a new concept to remove dynamically the bright sources and obtain a field of view (FOV) with an optically enhanced SNR. Our concept consists in replacing the plain slit in classical designs by an active row of MOEMS. Experimental demonstration of this concept has been conducted on a dedicated bench: a scene with a contiguous bright area has been focused on a micromirror array and imaged on a CCD detector. After the programmable slit, the straylight issued from the bright zone is polluting the scene; the micromirrors located on the bright area are switched off, removing almost completely the straylight in the instrument. In Astronomy and Earth Observation, we propose an innovative reconfigurable instrument: a programmable wide-field spectrograph where both the FOV and the spectrum could be tailored thanks to a 2D micromirror array. The FOV is linear and each point spectrum could be modified dynamically along the second direction. A demonstrator has been designed and its realization is under way for testing the unique performances of this instrument.

The E-NIS instrument on-board the ESA Euclid Dark Energy Mission: a general view after positive conclusion of the assesment phase

The Euclid Near-Infrared Spectrometer (E-NIS) Instrument was conceived as the spectroscopic probe on-board the ESA Dark Energy Mission Euclid. Together with the Euclid Imaging Channel (EIC) in its Visible (VIS) and Near Infrared (NIP) declinations, NIS formed part of the Euclid Mission Concept derived in assessment phase and submitted to the Cosmic Vision Down-selection process from which emerged selected and with extremely high ranking. The Definition phase, started a few months ago, is currently examining a substantial re-arrangement of the payload configuration due to technical and programmatic aspects. This paper presents the general lines of the assessment phase payload concept on which the positive down-selection judgments have been based.