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Direct detection and characterization of a planet around a star by nulling interferometry, must be efficient in a large wavelength domain in order to detect simultaneously the infrared bio-tracers CO2, O3 and H2O. This condition requires that an achromatic phase shift of pi be implemented, with an accuracy sufficient for achieving a deep nulling at...
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... found several ways to distribute geometrically the cells on the mirror's surface, following those prescriptions. We show in Fig. 3 three of them, as well as, on Fig. 4, the corresponding residual focal plane image of a system of a star plus a planet one million times fainter. One can note for the configuration on right that the planet is showing up more clearly but however is more embedded in residues of the stellar light. This illustrates well the trade-off that ...
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Context. To characterize their atmospheres in order to find evidences of
life, one has to detect directly photons from the exoplanets to measure
their spectra. One possible technique is dark fringe interferometry that
needs an achromatic π phase shift in one arm of the interferometer.
We have conceived a phase shifter made of two cellular mirrors, in which
each cell position and phase shift is specific, so that the behaviour of
the nulling with respect to wavelength is flat within a broad range.
Aims. We want to validate experimentally two versions of this achromatic
phase shifter: a transmissive one in bulk optics and a reflective one
using a segmented deformable mirror. What we present in this paper are
the last results obtained in the lab. Methods. We built an optical bench
in the visible that allows us to test the principle and characterize the
performances and the limits of this phase shifter. Results. We tested
several transmissive and one reflective phase shifter and obtained, for
instance, an attenuation of about 2.10-3 for a white source
(from 430 to 830 nm) that proved the achromatic behavior of the phase
shifter. The preliminary performances and limitations are analyzed.
Context. Dark fringe interferometry in the thermal infrared is one way to detect directly a planet orbiting a star, and so to characterize the planet's atmosphere through spectroscopy. This method demands a phase shift of pi1 in one arm of the interferometer. In order to detect various bio-tracers gases, a broad wavelength range (6-18 mum)2-3 is necessary, therefore an achromatic phase shift of pi is required. The achromatic device presented here is a phase shifter made of two cellular mirrors, in which each cell induces a specific phase shift. Aims. We wish to demonstrate that this theoretical concept is experimentally valid. We present in this paper the setup and the very first results. Methods. In a first step, we have consolidated the theoretical ground and in a second step we developed an optical bench in the visible domain to test the concept and measure the performances of this device. Results. The preliminary experimental tests show evidences that such a device is working as expected in terms of nulling and achromatism: in spite of an error on one cell of the prototype, it provides a nulling of 2.10-3 at one wavelength, and this value is close to the expected value. Besides, a nulling of 1.10-2 in a 450 to 750 nm bandwidth: a hint that a perfect device should be achromatic.
We recently presented a new concept for designing an achromatic phase shifter. An APS is required in nulling interferometry, a technique that aims at directly detecting and characterizing planets around a star in the thermal infrared. Our solution is based on two cellular mirrors (alternatively, transparent plates can be used) where cells have thickness which introduce OPD that are respectively odd and even multiples of half the central wavelength, on the fraction of the wave it reflects. A destructive interference is thus produced on axis for the central wavelength when recombining the two beams. We have shown that if the thicknesses are distributed according to the Pascal triangle, a fair quasi-achromatism is also reached on typically one octave in wavelength, provided there is a suffcient number of cells. The major interest of this solution is that it allows a compact, simple and fully symmetric design, without complex sub-systems to adjust. In this paper, after reminding the basic concept, we first present the theoretical estimations for the expected performances in the two possible regimes of recombination: on axis and multi-axial (Fizeau). We then describe the laboratory setup of the demonstration bench we are developing, as well as the first results obtained.
