Due to the increase of astronomical projects and of their instruments, the request of large optics with higher optical performances does not stop growing. An important step in the manufacture of these optics is the deposition of high-precision optical coatings.
To answer to this request we developed coatings working at different angles of incidence and spectral ranges on large surface:
- anti-reflective coatings for large lenses with strong curvatures,
- dichroic coatings with sharp transition for large optics.
Main results will be presented on the basis of several examples of realization.
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].
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.