Nulling interferometry is one of the promising techniques for the study of extra terrestrial planets. This technique will be applied in the future space missions Darwin and TPF-I, and from the ground with GENIE. The nulling interferometry techniques require high symmetry of the interfering beams, to obtain the required contrast (typically $10^6$ to detect terrestrial exo-planets in the thermal infrared). In this paper we consider the polarization symmetry issue, such as polarization rotation and polarization phase shifts occurring on slightly misaligned optics. We study the consequences of these symmetry requirements on a nulling interferometer design. We find the relation between the misalignment tolerances and the achievable nulling, and we show that this tolerance is highly dependent on the interferometer configuration (the way beams turn right, left, up or down in the interferometer arms). It is typically of the order of the arcminute (not the arcsecond) for a $10^6$ contrast. We present a analytical and numerical analyses.

The Darwin and TPF-I missions are Infrared free flying interferometer missions based on nulling interferometry. Their main objective is to detect and characterize other Earth-like planets, analyze the composition of their atmospheres and their capability to sustain life, as we know it. Darwin and TPF-I are currently in study phase. A number of mission architectures of 3 and 4 free flying telescopes are evaluated on the basis of the interferometer's response, ability to distinguish multiple planet signatures and starlight rejection capabilities. The characteristics of the new configurations are compared also to the former, more complex Bowtie baseline architectures as well as evaluated on base of their science capability.

The search for life on extrasolar planets is based on the assumption that one can screen extrasolar planets for habitability spectroscopically. The first space born instruments able to detect as well as characterize extrasolar planets, Darwin and terrestrial planet finder (TPF-I and TPF-C) are scheduled to launch before the end of the next decade. The composition of the planetary surface, atmosphere, and its temperature-pressure profile influence a detectable spectroscopic signal considerably. For future space-based missions it will be crucial to know this influence to interpret the observed signals and detect signatures of life in remotely observed atmospheres. We give an overview of biomarkers in the visible and IR range, corresponding to the TPF-C and TPF-I/DARWIN concepts, respectively. We also give an overview of the evolution of biomarkers over time and its implication for the search for life on extrasolar Earth-like planets. We show that atmospheric features on Earth can provide clues of biological activities for at least 2 billion years.

Nulling Interferometry applied to the search and characterization of earth-like exoplanets requires to eliminate the star's contribution at a rejection level (Rej = collected energy/residual energy)larger than $10^{6}$ over a large bandwidth (6 to 18 $\mu$m). 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 $\pi$. Using a confocal configuration for the focus-crossing approach, a Fresnel's diffraction effect degrades 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 Fresnel's diffraction effect as well as a possible way for correction. Then we describe the analytical method, based on Lommel's integrals, to evaluate the expectable rejection.

The interferometry is the most promising way to directly observe exoplanets, their spectra and surfaces at optical or infrared domain. The complex imaging process can be described as the extraction of information from the data gathered by the interferometer. This information can be treated to be independend on any à priori knowledge or the integration process. In this case it is analyzed in a classical way. In fact, the imaging of exoplanets is not the classical way of the data reconstruction. The best extraction of information requires all accessible à priori knowledge. This is the bayesian way. The knowledge gathered during an integration is also contributing as à priori information for futher image reconstruction. We disscuss both approaches supporting the analysis with the estimates of information flow through the interferometer and examples of simulations of imaging.

The Planet Detection Testbed is designed to simulate the architecture and operation of a space-borne four-telescope nulling interferometer. Constructed in the form of a dual chopped Bracewell interferometer together with star and planet sources, it reproduces the principal features of the flight beam combiner designed at JPL for the proposed TPF-I Formation Flying Interferometer. The aims of the testbed are to demonstrate stable four-beam nulling and planet detection at representative star-planet contrast ratios. In the flight design, starlight between 7 and 17 micron wavelength is to be nulled, and 2 to 3 micron starlight is used for fringe tracking and phasing the interferometer. There are also metrology systems and alignment systems which are required for deep and stable nulling. The testbed reproduces these features; 2 to 3 micron light from a thermal source is used for fringe tracking, and nulling and planet detection is performed at 10 microns. The testbed also incorporates laser metrology and other systems enabling continuous control of beam alignment. The ultimate goal is to simulate planet detection at star-planet contrast ratios of order $10^{-7}$ during full rotations of the telescope array using the phase chopping method. The latest results from the testbed are presented including four-beam nulling experiments at null depths of $10^{-5}$ and planet signal detections at similar contrast ratios.

Our activities are focused upon the experimental demonstration of deep nulling in the mid-IR over a wide bandpass. Specifically, our near-term goal is demonstrating a contrast of 10$^{-6}$ at 10 $\mu$m with a 25% spectral bandwidth. To meet this goal, several areas of technology development are required. These include: single-mode infrared fibers, bright infrared sources, laser path-length and tip/tilt metrology, and improvements to detection sensitivity. Progress in each of these areas of technical development will be reviewed as well as their impact on the overarching technical milestones.

We present the current performances of the AMBER / VLTI instrument in terms of differential observables (differential phase and differential visibility) and show that we are already able to reach a sufficient precision for very low mass companions spectroscopy and mass characterization. We perform some extrapolations with the knowledge of the current limitations of the instrument facility.

We show that with the current setup of the AMBER instrument, we can already reach $3\sigma = 10^{-3}$ radians and have the potential to some low mass companions characterization (Brown dwarves or hypothetical very hot Extra Solar Giant Planets). With some upgrades of the VLTI infrastructure, improvements of the instrument calibration and improvements of the observing strategy, we will be able to reach $3\sigma = 10^{-4}$ radians and will have the potential to perform Extra Solar Giant Planets spectroscopy and mass characterization.

Many methods have been proposed for the detection of exoplanets. The minimum detectable planetary size and mass, and the maximum distance from us at which exoplanets are observable, are determined by both technological and environmental constraints. The unique atmospheric conditions found at Dome C offer significant advantages. Using what is now known about the turbulence profile of the atmosphere above Antarctic plateau sites, we explore the potential these sites offer for detecting exoplanets around nearby stars using various methods.

The limitation to ground based astronomy is the Earth's atmosphere. The atmosphere above the Antarctic plateau is fundamentally different in many regards compared to the atmosphere at temperate sites. The extreme altitude, cold and low humidity offer a uniquely transparent atmosphere at many wavelengths. Studies at the South Pole have shown additionally that the turbulence properties of the night time polar atmosphere are unlike any mid latitude sites. The lack of high altitude turbulence combined with low wind speeds presents favorable conditions for interferometry. More recent site testing at Dome C has revealed the most superb seeing of any site tested. The unique properties of the polar atmosphere can be exploited for Extra-solar Planet studies with differential astrometry, differential phase and nulling interferometers.

The ALADDIN concept is an integrated Antarctic-based L-band experiment whose purpose is to demonstrate nulling interferometry and to prepare the DARWIN mission. Because of their privileged location, the relatively modest collectors (1 m) and baseline (up to 40 m) are sufficient to achieve a sensitivity (in terms of detectable zodi levels) which is about twice better than that of a nulling instrument on a large interferometer (such as GENIE at the VLTI), and to reach the 20-zodi threshold value identified to carry out the DARWIN precursor science. These numbers are based on a preliminary design study by Alcatel Alenia Space and were obtained using the same simulation software as the one employed for GENIE. The integrated design enables top-level optimization and full access to the light collectors for the duration of the experiment, while reducing the complexity of the nulling breadboard.

Direct detection and spectroscopic characterization of earth-like exoplanets require to eliminate the starlight in the recorded signal at a rejection level (collected flux / residual flux) around 10$^{6}$ in the thermal infrared domain. Another need is to reach angular resolution in the range 0.1 to 0.01 arcsec. In this context, nulling interferometry is recognized as the preferred instrumental approach. In the framework of the ESA-Darwin mission, preliminary studies for nulling systems have been initiated, the prime goal to achieve being mastering nulling process. In this purpose, Alcatel Space has developed a nulling test-bench, operating in near infrared (1.55 $\mu$m), not only to demonstrate capability of obtaining an appropriate rejection level but also to study and characterize the use of emerging techniques. The Darwin system will ultimately benefit from these results. In this paper, we report the most recent configuration of the Alcatel Space nulling breadboard which uses the multi-beam recombination technique, current baseline for Darwin. The results obtained with this version shows rejection performance at nearly 10$^{5}$ (stable level) with a polychromatic source ($\lambda$ = 1.55 $\mu$m, $\Delta\lambda$= 80 nm).

This paper discusses the potential performances of interferometers at the Dome C site in the Antarctic. The work is focussed on the limiting performances for fringe tracking, considering different fringe trackers architectures. The fringe tracking limiting magnitude is combined with estimates of the isopistonic angle at Dome C to evaluate the possibilities of off axis fringe tracking. The first evaluations presented here indicate that the sky coverage with realistic off axis fringe tracking is quite high at Dome C even with modest individual apertures and this could finally the best and decisive advantage of this site for optical interferometry. This might make a Dome C interferometer an unique deep sky very high angular resolution machine.

Direct detection and spectral characterization of extrasolar planets is one of the most exciting but also one of the most challenging area in modern astronomy. For its second generation instrumentation on the VLT, ESO has supported two phase A studies for a so-called “Planet Finder” dedicated instrument. Based on the results of these two studies, a unique instrument is now considered for first light in early 2010, including a powerful extreme adaptive optics system, various coronagraphs, an infrared differential imaging camera, an infrared integral field spectrograph and a visible differential polarimeter. We will briefly summarize the science objectives and requirements, describe the proposed conceptual design and discuss the main limitations and corresponding instrumental issues of such a system. We will also derive the expected performance of the proposed Planet Finder and present the project organization.

We present a Japanese “roadmap” on direct extrasolar planet studies spanning from the current ground-based telescope to future IR/Opt space-based telescopes. Several searches for young planets and disks have been conducted with the Subaru 8.2m telescope with adaptive optics (AO) infrared coronagraph, CIAO. The instrument will be soon upgraded to a new AO and a coronagraph with simultaneous spectral and polarimetric differential imaging modes (HiCIAO), which will significantly improve the contrast performance and hence the capability of young planet detection. A sensitive unbiased survey for extrasolar zodiacal emission around nearby stars will be conducted with the ASTRO-F space mission (0.7m telescope, $\sim$2–200 $\mu$m) to be launched around the beginning of 2006. A successor space mission, SPICA (3.5m, 5–200 $\mu$m), is also planned; its high sensitivity will enable the detection and characterization of outer-most planets around nearby stars, if any. For the studies of extrasolar terrestrial planets, a high contrast space telescope (HCST; 3.5m, $\sim$0.3–2 $\mu$m). We are also seeking for collaborations with or are considering to join to foreign missions. We describe an outline, status, and role of each project on the extrasolar planet studies.

As of early $\sim$2010's, the Japanese SPace Infrared telescope for Cosmology and Astrophysics (SPICA) space observatory will be launched. This actively cooled, cryogenic (4.5K), 3.5m telescope will operate in the mid and far infrared spectral regions. With its very high sensitivity, one of SPICA's aims will be the direct detection and characterization of extra-solar outer planets of nearby stars. The goal contrast ranges from $10^5$ to $10^6$ up to an angular separation of ${\sim}5$ arcsec. The relatively low angular resolution at MIR (5 to 20 $\mu$m) requires an efficient and robust coronagraphic mode working at cryogenic temperatures. In this presentation we describe several envisaged preliminary designs and assess their performance against the science goals and host telescope specifications. These are compared against numerical simulations and instrumental environment considerations, such as the need for an actively corrected wavefront.

Creating an optical space telescope observatory capable of detecting and characterizing light from extra-solar terrestrial planets poses technical challenges related to extreme wavefront stability. The Terrestrial Planet Finder Coronagraph design team has been developing an observatory based on trade studies, modeling and analysis that has guided us towards design choices to enable this challenging mission. This paper will describe the current flight baseline design of the observatory and the trade studies that have been performed. The modeling and analysis of this design will be described including predicted performance and the tasks yet to be done.

Visible Nulling Coronagraphy (VNC) is the proposed method of detecting and characterizing exo-solar Jovian planets (null depth $10^{-9}$) for the proposed NASA's Extrasolar Planetary Imaging Coronagraph (EPIC) Clampin & Lyon 2004 and is an approach under evaluation for NASA's Terrestrial Planet Finder (TPF) mission. The VNC approach uses a single unobscured filled-aperture telescope and splits, via a 50:50 beamsplitter, its re-imaged pupil into two paths within a Mach-Zender interferometer. An achromatic PI phase shift is imposed onto one beam path and the two paths are laterally sheared with respect to each other. The two beams are recombined at a second 50:50 beamsplitter. The net effect is that the on axis (stellar) light is transmitted out of the bright interferometer arm while the off-axis (planetary) light is transmitted out of the nulled interferometer arm. The bright output is used for fine pointing control and coarse wavefront control. The nulled output is relayed to the science camera for science imagery and fine wavefront control. The actual transmission pattern, projected on the sky, follows a $\theta^2$ pattern for a single shear, $\theta^4$ for a double shear, with the spacing of the successive maxima proportional to the inverse of the relative lateral shear. Combinations of shears and spacecraft rolls build up the spatial frequency content of the sky transmission pattern in the same manner as imaging interferometer builds up the spatial frequency content of the image.

In order to detect directly faint planets around other stars it is important to develop techniques to cancel the central starlight. We report progress in terms of null depth on the visible nulling experiments at JPL, essential for NASAs future space missions as well as for an upcoming sounding rocket based experiment. A level of 1 Million to 1 nulling ratio (1e-6 rejection ratio) has been obtained for the first time for visible laser light, and a level of 100000 to 1 nulling ratio has been obtained for a relatively large (5%) bandpass at 650nm. Using a combination of a fiber array and deformable mirror a null of 1e-6 would correspond to a contrast of 1e-9 at a distance of 2-3$\lambda$ in the airy disk plane, only 10x away from TPFs goal. The configuration used was a fiber fed Mach Zender type interferometer, using 2 or 3 mirrors in each two arm, placed in an enclosed air container. For broadband light we used tilted dispersion glass plates in the nuller arms and an avalanche photon diode module to cover the large dynamic range. We describe a variety of conditions that have to be met and optimized to reach very deep nulls and sub-nanometer optical path difference stability, such as optical alignment, symmetry in the two arms, mechanical stability and vibration isolation.

We present two new phase mask coronagraphs implemented with subwavelength diffractive optical elements. The first one is an evolution of the four quadrant phase mask coronagraph (FQPM), which resolves the $\pi$ phase shift chromaticity issue: the four quadrant zeroth order grating (4QZOG). The second one is a totally new design consisting of an optical vortex induced by a space-variant grating: the annular groove phase mask (AGPM) coronagraph is fully symmetric and free from any “shaded zones”. The potential performances of the 4QZOG and AGPM coronagraph are very good, ensuring, for instance, a theoretical contrast of $1.4 \times 10^{-7}$ at 3$\lambda/D$ over the whole K band. These coronagraphs could be used alone on single-pupil telescopes either in space or on the ground (with an adaptive optics system) to detect exoplanets.