Alain Labèque

Instrumental stability requirements for exoplanet detection with a nulling interferometer: variability noise as a central issue

We revisit the nulling interferometer performances that are needed for direct detection and the spectroscopic analysis of exoplanets, e.g., with the DARWIN [European Space Agency-SCI 12 (2000)] or TPF-I [JPL Publ. 05-5, (2005)] missions. Two types of requirement are found, one concerning the mean value of the instrumental nulling function (nl(lambda)) and another regarding its stability. The stress is usually put on the former. It is stringent at short wavelengths but somewhat relaxed at longer wavelengths. The latter, which we call the variability noise condition, does not usually receive enough attention. It is required regardless of telescope size and stellar distance. The results from three nulling experiments performed in laboratories around the world are reported and compared with the requirements. All three exhibit 1/f noise that is incompatible with the performances required by the mission. As pointed out by Lay [Appl. Opt. 43, 6100-6123 (2004)], this stability problem is not fully solved by modulation techniques. Adequate solutions must be found that are likely to include servo systems using the stellar signal itself as a reference and internal metrology with high stability.

Nulling interferometry for the Darwin Mission: polychromatic laboratory test bench

The Darwin mission is a project of the European Space Agency that should allow around 2015 the search for extrasolar planets and a spectral analysis of their potential atmospheres in order to detect gases and particularly tracers of life. The basic concept of the instrument is a Bracewell nulling interferometer. It allows high angular resolution and high dynamic range. However, this concept, proposed 25 years ago, is very difficult to implement with high rejection factor and has to be demonstrated in laboratory before being applied in space. Theoretical and numerical approaches of the question highlight strong difficulties : - The need for very clean and homogeneous wavefronts, in terms of intensity, phase and polarisation distribution ; - The need for achromatic optical devices working in a wide spectral range (typically 6 to 18 microns for the space mission). A solution to the first point is the optical filtering which has been successfully experimentally demonstrated at 10 microns using a single mode laser. We focus now on the second point and operate a test bench working in the near infrared, where the background constraints are reduced. We present this test bench and the first encouraging results in the 2-4 microns spectral range. We particularly focus on recent optical developments concerning achromatic component, and particularly the beam combiners and the phase shifter, which are keypoints of the nulling interferometry principle. Finally, we present the future of this experimental demonstration, in the thermal infrared, covering the real and whole spectral range of Darwin.

Stabilising a nulling interferometer using optical path difference dithering

Context. Nulling interferometry has been suggested as the underlying principle for the Darwin and TPF-I exoplanet research missions. Aims. There are constraints both on the mean value of the nulling ratio, and on its stability. Instrument instability noise is most detrimental to the stability of the nulling performance. Methods. We applied a modified version of the classical dithering technique to the optical path difference in the scientific beam. Results. Using only this method, we repeatedly stabilised the dark fringe for several hours. This method alone sufficed to remove the 1/f component of the noise in our setup for periods of 10 min, typically. These results indicate that performance stability may be maintained throughout the long-duration data acquisitions typical of exoplanet spectroscopy. Conclusions. We suggest that further study of possible stabilisation strategies should be an integral part of Darwin/TPF-I research and development.

Opto-thermo-mechanical numerical simulations of three different concepts of infrared achromatic phase shifters

The Darwin/TPF mission aims at detecting directly extra solar planets. It is based on the nulling interferometry, concept proposed by Bracewell in 1978, and developed since 1995 in several European and American laboratories. One of the key optical devices for this technique is the achromatic phase shifter (APS). This optical component is designed to produce a π phase shift over the whole Darwin spectral range (i.e. 6-18 μm), and will be experimentally tested on the NULLTIMATE consortium nulling test bench (Labèque et al). Three different concepts of APS are being simulated: dispersive plates focus crossing and field reversal. In this paper, we show how thermal, mechanical and optical models are merged into a single robust model, allowing a global numerical simulation of the optical component performances. We show how these simulations help us to optimizing the design and present results of the numerical model.

Stabilising a nulling interferometer using optical path difference dithering: an update

Nulling interferometry has been suggested as the underlying principle for an instrument which could provide direct detection and spectroscopy of Earth-like exo-planets, including searches for potential bio-signatures. This paper documents the potential of optical path difference (OPD) stabilisation with dithering methods for improving the mean nulling ratio and its stability. The basic dithering algorithm, its refined versions and parameter tuning, are reviewed. This paper takes up the recently presented results1 and provides an update on OPD-stabilisation at significantly higher levels of nulling performance.

Nulling interferometry : Lommel's integrals applied to a Fresnel's diffraction effect

Nulling Interferometry applied to the search and characterization of earth-like ex- oplanets requires to eliminate the stars contribution at a rejection level (Rej = collected en- ergy/residual energy)larger than 10 6 over a large bandwidth (6 to 18 µm). Nulling test-benches are in development in several laboratories so as to master such high a rejection. One approach relies on a Mach-Zehnder set-up with Achromatic Phase Shifters (APS). One APS concept is based on the focus-crossing property, providing an intrinsically achromatic phase shift by π. Using a confocal configuration for the focus-crossing approach, a Fresnels diffraction effect de- grades the rejection. Usual optical engineering softwares fail in assessing rejection performance and an analytical approach is needed. We describe the bench optical configuration and the Fres- nels diffraction effect as well as a possible way for correction. Then we describe the analytical method, based on Lommels integrals, to evaluate the expectable rejection.

Nulling interferometry for the darwin mission: laboratory demonstration experiment

The DARWIN mission is a project of the European Space Agency that should allow around 2012 the search for extrasolar planets and a spectral analysis of their potential atmosphere in order to evidence gases and particularly tracers of life. The principle of the instrument is based on the Bracewell nulling interferometer. It allows high angular resolution and high dynamic range. However, this concept, proposed more than 20 years ago, has never been experimentally demonstrated in the thermal infrared with high levels of extinction. We present here a laboratory monochromatic experiment dedicated to this goal. A theoretical and numerical approach of the question highlights a strong difficulty: the need for very clean and homogeneous wavefronts, in terms of intensity, phase and polarisation distribution. A classical interferometric approach appears to be insufficient to reach our goals. We have shown theoretically then numerically that this difficulty can be surpassed if we perform an optical filtering of the interfering beams. This technique allows us to decrease strongly the optical requirements and to view very high interferometric contrast measurements with commercial optical pieces. We present here a laboratory interferometer working at 10,6 microns, and implementing several techniques of optical filtering (pinholes and single-mode waveguides), its realisation, and its first promising results. We particularly present measurements that exhibit stable visibility levels better than 99,9% that is to say extinction levels better than 1000.

The NULLTIMATE test bench: achromatic phase shifters for nulling interferometry

The NULLTIMATE project developed and realized three concepts of achromatic phase shifters for nulling interferometry. One of the concepts is based on dispersive plates made of three materials which where fully characterized regarding their refractive index and thermo-optic behavior between 100K and 330 K. The other two concepts are based on mirror optics, one of which uses the phase shift of π when crossing a focus, the other the reversal of electric fields at reflection. An optical bench has been set up to test and characterize these phase shifters at wavelengths 2 − 2.4 μm with the option of changing to the 10 μm domain. We summarize the development of the achromatic phase shifters and report on the current status of the test bench.

New polychromatic testbed for DARWIN/TPF nulling interferometer laboratory demonstration

Several concept of space missions dedicated to the direct detection and analysis of extrasolar planets are based on nulling interferometry principle. This principle, which is theoretically very promising requires the capability of propagating and combining beams with very high accuracy in term of amplitude phase and polarization. In order to validate the principle of nulling interferometry, it is necessary to develop laboratory techniques of recombination. In this paper, we present a new test bench that should allow measuring rejection rate up to 105 in a large spectral band between 2 and 4 microns.