The search for Earth-like exoplanets, orbiting in the habitable zone of stars other than our Sun and showing biological activity, is one of the most exciting and challenging quests of the present time. Nulling interferometry from space, in the thermal infrared, appears as a promising candidate technique for the task of directly observing extra-solar planets. It has been studied for about 10 years by ESA and NASA in the framework of the Darwin and TPF-I missions respectively .
Nevertheless, nulling interferometry in the thermal infrared remains a technological challenge at several levels. Among them, the development of the "modal filter" function is mandatory for the filtering of the wavefronts in adequacy with the objective of rejecting the central star flux to an efficiency of about 105. Modal filtering  takes benefit of the capability of single-mode waveguides to transmit a single amplitude function, to eliminate virtually any perturbation of the interfering wavefronts, thus making very high rejection ratios possible.
The modal filter may either be based on single-mode Integrated Optics (IO) and/or Fiber Optics. In this paper, we focus on IO, and more specifically on the progress of the on-going “Integrated Optics” activity of the European Space Agency.
In the present paper we focus on the fabrication of waveguides which will be able to work in the large infrared window
[6-20μm], compatible with the ESA requirements in the framework of the detection of Exo-solar planets by nulling
The first step in the fabrication of such components is the realization of planar waveguides being able to guide light in
this spectral range. In order to do so, telluride materials were selected: Te75Ge15Ga10 bulk glasses were chosen as
substrates and TeGe films as guiding layers. The Te75Ge15Ga10 bulk glasses were purified during their synthesis which
ensures an optimal transmission in the whole range from 6 to 20 μm. TeGe thick films with different compositions were
deposited by thermal co-evaporation. Homogeneous films with thickness up to 15 microns could be produced. The M-lines
measurement of their refractive index at λ = 10.6 μm highlighted a linear behavior versus the atomic percentage in
tellurium and confirmed their compatibility for the project.
First planar waveguides could be optically characterized after having prepared their input and output facets by an
appropriate polishing procedure. Guidance of light was demonstrated in the whole range [6-20 μm].
The development of integrated optics components being able to work in the mid-infrared is of major interest for specific
applications, such as the detection of pollutant gases for the environmental control or the detection of exoplanetary
systems. Chalcogenide glasses being characterised by a unique property of transmission in the infrared are very
promising materials for the realisation of such infrared micro-components. In order to realise and test channel
waveguiding structures, we studied the deposition and the etching of chalcogenide films. Both ARROW and RIB
waveguides were realised. In particular, all-chalcogenide RIB waveguides obtained by etching a Te-As-Se film deposited
on an As-S bulk glass substrate were characterised at 10.6 μm.
In the present paper we focus on the fabrication of rib waveguides being able to work in the large infrared window
[6-20μm], compatible with the Darwin mission requirements. The rib waveguides to be realized are based on etched thick
films of telluride materials deposited on telluride glass. The choice of the Te75Ge15Ga10 material as the substrate is
justified by its excellent transmission in the infrared region and its thermal stability. Films of the ternary system made of
Te, Ge and Ga were investigated as the core layer and the superstrate. Details are provided on the film deposition and
etching technologies: (i) Te-Ge-Ga films are prepared by co-thermal evaporation from the pure elements Te, Ge and Ga;
(ii) the geometry of the as-obtained films is modified by reactive ion etching under an atmosphere of CHF3 + O2 or CH4
+ H2. First results concerning Te-Ge binary films are particularly interesting.
Modal filtering is based on the capability of single-mode waveguides to transmit only one complex amplitude function to
eliminate virtually any perturbation of the interfering wavefronts, thus making very high rejection ratios possible in a
nulling interferometer. In the present paper we focus on the progress of Integrated Optics in the thermal infrared [6-20μm] range, one of the two candidate technologies for the fabrication of Modal Filters, together with fiber optics. In
conclusion of the European Space Agency's (ESA) "Integrated Optics for Darwin" activity, etched layers of chalcogenide
material deposited on chalcogenide glass substrates was selected among four candidates as the technology with the best
potential to simultaneously meet the filtering efficiency, absolute and spectral transmission, and beam coupling
requirements. ESA's new "Integrated Optics" activity started at mid-2007 with the purpose of improving the technology
until compliant prototypes can be manufactured and validated, expectedly by the end of 2009. The present paper aims at
introducing the project and the components requirements and functions. The selected materials and preliminary designs,
as well as the experimental validation logic and test benches are presented. More details are provided on the progress of
the main technology: vacuum deposition in the co-evaporation mode and subsequent etching of chalcogenide layers. In
addition, preliminary investigations of an alternative technology based on burying a chalcogenide optical fiber core into a
chalcogenide substrate are presented. Specific developments of anti-reflective solutions designed for the mitigation of
Fresnel losses at the input and output surface of the components are also introduced.
This paper presents the development and tests in the thermal infrared of Integrated Optics (IO) technology in preparation of ESA's space mission Darwin. This mission aims to detect and characterize earth-like planets orbiting solar-type stars, using nulling interferometry in the spectral range 6 - 20 μm. Since typically 1:1e6 rejection of starlight is required, wavefront modal filtering is mandatory. Thus, mid-infrared single-mode IO is being developed in the framework of the ESA-funded "Integrated Optics for Darwin" project. Beyond its wavefront filtering capabilities, an IO component may support various optical functions, and is thus likely to ease instrumental design. This paper addresses the manufacturing process and the characterization tests results of newly developed IO devices. Investigated solutions are dielectric waveguides based on Chalcogenide glasses and Hollow Metallic Waveguides. In a first phase, the pre-selected technological solutions were validated and modal behavior of the manufactured devices was demonstrated, both through polarization and spectral analysis. Preliminary nulling ratios up to 5000 have been obtained with an IO modal filter in the 6 - 20 μm range. In a second phase of the project, the development of more complex IO functions was attempted. The methods used to validate the waveguide behavior and interferometric capabilities are also discussed. After achieving 1:1e5 polychromatic extinctions with similar solutions in the near IR, the presented results further underline the credibility of a mid-infrared IO concept for Darwin.
To detect earth-like planets orbiting around solar-type stars in the mid-infrared spectral range, a typical rejection ratio of 106 of the stellar flux must be achieved. Space missions like Darwin/TPF aim at achieving such contrasts using nulling interferometry between 4 μm and 20 μm. The instrumental constraints on beam combination, spatial filtering, intensity and phase mismatches must then be accurately considered. This paper presents the first characterization results of mid-infrared waveguides for integrated optics (IO) developed in the frame of an ESA contract. Taking into account the scientific achievements already obtained with IO components in the near infrared range, results demonstrate that these technologies can also be used for future nulling devices as an alternative to bulk optics instrumentation in the mid-infrared spectral range. Good waveguiding behaviour has been obtained on dielectric waveguides based on Chalcogenide or Zinc Selenide glasses and Hollow Metallic Waveguides. The single-mode behavior, spatial filtering and polarization control capabilities of the hollow metallic channel waveguides have been also demonstrated. This paper focuses on the methods used to validate the waveguide behaviour and the first laboratory results obtained with the different technologies used in the mid-infrared.
The spectral properties of Er3+-doped As2S3 and Ge33As12Se55 chalcogenide glasses are presented and discussed. Bulk samples and thin films have been studied. Bulk samples have been obtained by melt-quenching. Thin films have been obtained by RF sputtering. Sputtering targets have been fabricated from home-made cut and polished doped bulk samples and from commercial undoped targets with erbium pieces on the surface. The film morphology has been analysed by AFM and a column-like structure has been observed for the Ge33As12Se55 films. The presence of Er3+ ions in As2S3 and Ge33As12Se55 films has been confirmed by PL emission at 1.55 µm. A PL lifetime of 4 ms has been measured in Er-doped As2S3 films. Single mode waveguides have been fabricated by wet etching in Ge33As12Se55 films.
Spatial filtering is a critical issue to achieve nulling interferometry in the framework of spatial missions aimed at the detection of exoplanets. Several working interferometric instruments take benefit of guided optics for spatial filtering in the near infrared wavelengths and thus provide accurate visibility measurements. Furthermore planar optics would also provide beam combining functions within a single compact and stable device.
Existing telecom technology allows beam combiner manufacturing for
0.8-1.8 micrometers wavelengths. Adaptation of these technologies
is required to cover the scientific domain of ground based interferometry in each atmospheric spectral band and of the spatial missions like IRSI/DARWIN and TPF dedicated to thermal infrared
wavelengths [4-20 micrometers]. We present here some of the most promising materials and their associated technologies for the thermal infrared range. For each of these solutions, based on chalcogenide glasses, semiconductor materials and hollow waveguides, we present some manufactured components with their optical characterizations. We also present a method and test-benches to measure the single-mode wavelength range of waveguides.