In recent years, phase mask coronagraphy has become increasingly efficient in imaging the close environment of stars,
enabling the search for exoplanets and circumstellar disks. Coronagraphs are ideally suited instruments, characterized by
high dynamic range imaging capabilities, while preserving a small inner working angle. The AGPM (Annular Groove
Phase Mask, Mawet et al. 20051) consists of a vector vortex induced by a rotationally symmetric subwavelength grating. This technique constitutes an almost unique solution to the achromatization at longer wavelengths (mid-infrared). For this reason, we have specially conceived a mid-infrared AGPM coronagraph for the forthcoming upgrade of VISIR, the
mid-IR imager and spectrograph on the VLT at ESO (Paranal), in collaboration with members of the VISIR consortium.
The implementation phase of the VISIR Upgrade Project is foreseen for May-August 2012, and the AGPM installed will
cover the 11-13.2 μm spectral range. In this paper, we present the entire fabrication process of our AGPM imprinted on a
diamond substrate. Diamond is an ideal material for mid-infrared wavelengths owing to its high transparency, small
dispersion, extremely low thermal expansion and outstanding mechanical and chemical properties. The design process
has been performed with an algorithm based on the rigorous coupled wave analysis (RCWA), and the micro-fabrication
has been carried out using nano-imprint lithography and reactive ion etching. A precise grating profile metrology has
also been conducted using cleaving techniques. Finally, we show the deposit of fiducials (i.e. centering marks) with
Aerosol Jet Printing (AJP). We conclude with the ultimate coronagraph expected performances.
The Vector Vortex Coronagraph (VVC) is one of the most attractive new-generation coronagraphs for ground- and
space-based exoplanet imaging/characterization instruments, as recently demonstrated on sky at Palomar and
in the laboratory at JPL, and Hokkaido University. Manufacturing technologies for devices covering wavelength
ranges from the optical to the mid-infrared, have been maturing quickly. We will review the current status of
technology developments supported by NASA in the USA (Jet Propulsion Laboratory-California Institute of
Technology, University of Arizona, JDSU and BEAMCo), Europe (University of Li`ege, Observatoire de Paris-
Meudon, University of Uppsala) and Japan (Hokkaido University, and Photonics Lattice Inc.), using liquid
crystal polymers, subwavelength gratings, and photonics crystals, respectively. We will then browse concrete
perspectives for the use of the VVC on upcoming ground-based facilities with or without (extreme) adaptive
optics, extremely large ground-based telescopes, and space-based internal coronagraphs.
Phase-mask coronagraphs are known to provide high contrast imaging capabilities while preserving a small inner
working angle, which allows searching for exoplanets or circumstellar disks with smaller telescopes or at longer
wavelengths. The AGPM (Annular Groove Phase Mask, Mawet et al. 20051) is an optical vectorial vortex coronagraph
(or vector vortex) induced by a rotationally symmetric subwavelength grating (i.e. with a period smaller than λ/n, λ being
the observed wavelength and n the refractive index of the grating substrate). In this paper, we present our first midinfrared
AGPM prototypes imprinted on a diamond substrate. We firstly give an extrapolation of the expected
coronagraph performances in the N-band (~10 μm), and prospects for down-scaling the technology to the most wanted L-band
(~3.5 μm). We then present the manufacturing and measurement results, using diamond-optimized microfabrication
techniques such as nano-imprint lithography (NIL) and reactive ion etching (RIE). Finally, the subwavelength grating
profile metrology combines surface metrology (scanning electron microscopy, atomic force microscopy, white light
interferometry) with diffractometry on an optical polarimetric bench and cross correlation with theoretical simulations
using rigorous coupled wave analysis (RCWA).
In the context of exoplanet detection, a large majority of the 400 detected exoplanets have been found by indirect
methods. Today, progress in the field of high contrast and angular resolution imaging has allowed direct images of
several exoplanetary systems to be taken (cf. HR 8799, Fomalhaut and β Pic).1-4 In the near future, several new
instruments are going to dramatically improve our sensitivity to exoplanet detection. Among these, SPHERE
(Spectro Polarimetric High contrast Exoplanet REsearch) at the VLT, MIRI (Mid Infra-Red Instrument) onboard
JWST and EPICS at the ELT will be equipped with coronagraphs to reveal faint objects in the vicinity of nearby
stars. We made use of the Lyon group (COND) evolutionary models of young (sub-)stellar objects and exoplanets
to compare the sensitivities of these different instruments using their estimated coronagraphic profiles. From this
comparison, we present a catalogue of targets which are particularly well suited for the different instruments.
The use of a rotating-baseline nulling interferometer for exoplanet detection was proposed several decades ago, but the
technique has not yet been fully demonstrated in practice. Here we consider the faint companion and exozodiacal disk
detection capabilities of rotating-baseline nulling interferometers, such as are envisioned for space-based infrared
nullers, but operating instead within the aperture of large single telescopes. In particular, a nulling interferometer on a
large aperture corrected by a next-generation extreme adaptive optics system can provide deep interferometric contrasts,
and also reach smaller angles (sub λ/D) than classical coronagraphs. Such rotating nullers also provide validation for an
eventual space-based rotating-baseline nulling interferometer. As practical examples, we describe ongoing experiments
with rotating nullers at Palomar and Keck, and consider briefly the case of the Thirty Meter Telescope.
The presence of large amounts of exozodiacal dust around nearby main sequence stars is considered as a potential
threat for the direct detection of Earth-like exoplanets (exoEarths) with future space-based coronagraphic and
interferometric missions. In this paper, we estimate the amount of exozodiacal light that can be tolerated around
various stellar types without jeopardizing the detection of exoEarths with a space-based visible coronagraph or a
free-flying mid-infrared interferometer. We also address the possible effects of resonant structures in exozodiacal
disks. We then review the sensitivity of current ground-based interferometric instruments to exozodiacal disks,
based on classical visibility measurements and on the nulling technique. We show that the current instrumental
performances are not sufficient to help prepare future exoEarth imaging missions, and discuss how new groundor
space-based instruments could improve the current sensitivity to exozodiacal disks down to a suitable level.
A unique statistical data analysis method has been developed for reducing nulling interferometry data. The idea
is to make use of the statistical distributions of the fluctuating null depths and beam intensities to retrieve the
astrophysical null depth in the presence of fluctuations. The approach yields an accuracy much better than is
possible with standard data reduction methods, because the accuracy of the null depth is not limited by the
sizes of the phase and intensity errors but by the uncertainties on their statistical distributions. The result is
an improvement in the instrumental null depth measurement limit of roughly an order of magnitude. We show
in this paper that broadband null depths of 10-4 can be measured in the lab with our infrared Fiber Nuller
without achromatic phase shifters. On sky results are also dramatically improved, with measured contrasts up to
a couple of 10-4 with our instrument mounted on the Hale telescope at the Palomar Observatory. This statistical
analysis is not specific to our instrument and may be applicable to other interferometers.
The small angular distance (<100 mas) and the huge flux ratio (107) between an Earth-like exoplanet in the socalled
habitable zone and its host star makes it very difficult to direct image such systems. Nulling interferometry
consists of a very powerful technique that combines destructively the light from two or more collectors to dim
the starlight and to reveal faint companions in its vicinity. We have developed a new nulling experiment based
on the fiber nuller principle. This fully symmetric reflective nulling bench aims at testing broadband nulling
in both H and K bands as well as characterizing photonic fibers for modal filtering. We present in this paper the
design, the development as well as preliminary results of the experiment.
We report on high-accuracy, high-resolution (< 20mas) stellar measurements obtained in the near infrared (
2.2 microns) at the Palomar 200 inch telescope using two elliptical (3m x 1.5m) sub-apertures located 3.4m
apart. Our interferometric coronagraph, known as the "Palomar Fiber Nuller" (PFN), is located downstream
of the Palomar adaptive optics (AO) system and recombines the two separate beams into a common singlemode
fiber. The AO system acts as a "fringe tracker", maintaining the optical path difference (OPD) between
the beams around an adjustable value, which is set to the central dark interference fringe. AO correction
ensures high efficiency and stable injection of the beams into the single-mode fiber. A chopper wheel and a fast
photometer are used to record short (< 50ms per beam) interleaved sequences of background, individual beam
and interferometric signals. In order to analyze these chopped null data sequences, we developed a new statistical
method, baptized "Null Self-Calibration" (NSC), which provides astrophysical null measurements at the 0.001
level, with 1 σ uncertainties as low as 0.0003. Such accuracy translates into a dynamic range greater than 1000:1
within the diffraction limit, demonstrating that the approach effectively bridges the traditional gap between
regular coronagraphs, limited in angular resolution, and long baseline visibility interferometers, whose dynamic
range is restricted to 100:1. As our measurements are extremely sensitive to the brightness distribution very
close to the optical axis, we were able to constrain the stellar diameters and amounts of circumstellar emission
for a sample of very bright stars. With the improvement expected when the PALM-3000 extreme AO system
comes on-line at Palomar, the same instrument now equipped with a state of the art low noise fast read-out near
IR camera, will yield 10-4 to 10-3 contrast as close as 30 mas for stars with K magnitude brighter than 6. Such
a system will provide a unique and ideal tool for the detection of young (<100 Myr) self-luminous planets and
hot debris disks in the immediate vicinity (0.1 to a few AUs) of nearby (< 50pc) stars.
A rotating nulling coronagraph has been built for use on ground-based telescopes. The system is based on the concept of sub-aperturing the pupil of the telescope with two elliptical apertures and combining the resulting two input beams on a single-mode fiber. By a relative π phase shift of the beams, the starlight can be nulled and a relatively faint companion star can be detected. Rotation of the aperture mask on the telescope pupil results in a signal similar to that expected from a space-borne telescope system such as the proposed TPF/Darwin interferometer. The design of the nulling coronagraph and the ancillary systems that are needed, such as the fringe tracker, are described and the potential for observations on telescopes such as the Palomar 200" is discussed. Results of a nulling experiment using a single mode fiber as a beam combiner for broadband light between 1.50 μm and 1.80 μm are shown.
The Darwin and TPF-I space missions will be able to study the atmosphere of distant worlds similar to the Earth.
Flying these space-based interferometers will however be an extraordinary technological challenge and a first step
could be taken by a smaller mission. Several proposals have already been made in this context, using the simplest
nulling scheme composed of two collectors, i.e., the original Bracewell interferometer. Two of these projects, viz.
Pegase and the Fourier-Kelvin Space Interferometer, show very good perspectives for the characterisation of hot
extra-solar giant planets (i.e., Jupiter-size planets orbiting close to their parent star). In this paper, we build on
these concepts and try to optimise a Bracewell interferometer for the detection of Earth-like planets. The major
challenge is to efficiently subtract the emission of the exo-zodiacal cloud which cannot be suppressed by classical
phase chopping techniques as in the case of multi-telescopes nulling interferometers. We investigate the potential
performance of split-pupil configurations with phase chopping and of OPD modulation techniques, which are
good candidates for such a mitigation. Finally, we give a general overview of the performance to be expected
from space-based Bracewell interferometers for the detection of extra-solar planets. In particular, the prospects
for known extra-solar planets are presented.
One of the most critical units of nulling interferometers is the Achromatic Phase Shifter. The concept we propose
here is based on optimized Fresnel rhombs, using the total internal reflection phenomenon, modulated or not.
The total internal reflection induces a phase shift between the polarization components of the incident light.
We present the principles, the current status of the prototype manufacturing and testing operations, as well as
preliminary experiments on a ZnSe Fresnel rhomb in the visible that have led to a first error source assessment
study. Thanks to these first experimental results using a simple polarimeter arrangement, we have identified the
bulk scattering as being the main error source. Fortunately, we have experimentally verified that the scattering
can be mitigated using spatial filters and does not decrease the phase shifting capabilities of the ZnSe Fresnel
rhomb.
The Annular Groove Phase Mask coronagraph (AGPM) is an intrinsically achromatic vectorial vortex. It consists
of integrated subwavelength optical elements whose space-variant polarization properties can be engineered and
optimized to synthesize one of the theoretically most efficient coronagraphs. This paper briefly recalls the
principles of the AGPM, presents the benefit of its implementation inside a polarimetric differential imager,
realistic numerical simulations assessing its performances, as well as the current status of the near-infrared and
visible prototype manufacturing operations.
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