Marc Barillot

Realization of single-mode telluride rib waveguides for mid-IR applications between 10 and 20 μm.

The feasibility of all-telluride integrated optics devices based on waveguides presenting a single-mode behavior in the spectral range (10-20 μm) is demonstrated. These waveguides are constituted of a several micrometer thick Te(82)Ge(18) film deposited onto a Te(75)Ge(15)Ga(10) bulk glass substrate by thermal coevaporation and further etched by reactive ion etching under the CHF(3)/O(2)/Ar atmosphere. The obtained structures were proven to behave as channel waveguides with a good single-mode transmission over the whole spectral range. These results allowed validating our technological solution for the fabrication of integrated optics modal filters for spatial interferometry.

Stable deep nulling in polychromatic unpolarized light with multiaxial beam combination

In the context of the space-based nulling mission ESA-Darwin, Thales Alenia Space has developed a nulling breadboard for the European Space Agency (ESA): the multiaperture imaging interferometer (MAII) to demonstrate deep and stable nulling and to investigate various subsystems of the ESA-Darwin interferometer. Recently, we have extended our investigations to the multiaxial beam combination. This combination scheme presents many advantages: simplicity, compactness, and a high coupling efficiency for a three-beam combination. The near-infrared (lambda approximately 1.55 microm) MAII breadboard has been upgraded to the multiaxial beam combination. Polarization and stability issues have been thoroughly investigated. We report on the recent results we have obtained with the multiaxial configuration of MAII in unpolarized light with a relative spectral bandwidth of 5%: nulling ratios of mean value N=1.7 x 10(-5), stable over 1 h with a standard deviation sigma( N )=5.7 x 10(-7). These results indicate that the multiaxial beam combination has the potential to meet Darwin requirements.

Persee: a nulling demonstrator with real-time correction of external disturbances

Nulling interferometry is one of the most promising methods to study habitable extrasolar systems. Several projects, such as Darwin, TPF, Pegase, FKSI or Aladdin, are currently considered and supported by R&D programs. One of the main issues of nulling interferometry is the feasibility of a stable polychromatic null despite the presence of significant disturbances, induced by vibrations, atmospheric turbulence on the ground or satellite drift for spaceborne missions. To reduce cost and complexity of the whole system, it is necessary to optimize not only the control loop performance at platform and payload levels, but also their interaction. In this goal, it was decided in 2006 to build a laboratory demonstrator named Persee. Persee is mostly funded by CNES and built by a consortium including CNES, IAS, LESIA, OCA, ONERA and TAS. After a definition phase in 2006, the implementation of the sub-systems has now begun and the integration near Paris by GIS-PHASE (LESIA, ONERA and GEPI) is planned in 2009. This paper details the main objectives of PERSEE, describes the definition of the bench, presents the current status and reports results obtained with the first sub-systems.

PERSEE: description of a new concept for nulling interferometry recombination and OPD measurement

Nulling interferometry requires, among other things, a symmetric recombination module and an optical path difference control system. The symmetric recombination stage has been particularly studied over the last ten years and several concepts are now well known. One of them is the Modified Mach Zehnder (MMZ) interferometer, proposed by Serabyn and Colavita (2001) [1]. In this paper, we describe a new version of the MMZ beam combiner which provides a deep null signal in the science channel and, at the same time, phase-sensitive signals in the so-called co-phasing channel. From the latter, accurate optical path difference measurements can be derived. This beam combiner works in the 0.8 to 3.3 μm spectral range (0.8 to 1.5 μm for the co-phasing channel and 1.65 to 3.3 μm for the science channel). Both optical functions can be implemented in the same device thanks to an original optical design involving dedicated phase shifts. In this paper, we describe its principle and detail the optical and mechanical design.

Wide-range transmitting chalcogenide films and development of micro-components for infrared integrated optics applications

Development of micro-components for IR integrated optic devices requires the elaboration of IR waveguides. It is shown that amorphous chalcogenide films from the Ge-Se-Te system are well suited to such development. Thermal and optical characteristics of films elaborated by thermal co-evaporation are first measured. The Se-rich (> 60 at. %) region with a Ge content of about 25 at. % comprises films with a vitreous transition temperature, Tg, larger than 400K, a high thermal stability (ΔT > 100K) and a well-controlled refractive index, n, owing to a weak dependence of n with composition in this region. Films in this composition region are then profitably used to develop optical structures, such as straight or S-bend waveguides, spirals, Y-junction or Mach-Zehnder interferometer, by stacking and further etching of the films. The transmission region accessible to these structures lies from telecommunication wavelength up to 16-17 µm. When a higher transmission region is required, the use of pure Ge-Te films is mandatory. A modal filter allowing a light rejection efficiency of 6.10−5 to be a part of a spatial interferometer is then elaborated.

Current results of the PERSEE testbench: the cophasing control and the polychromatic null rate

Stabilizing a nulling interferometer at a nanometric level is the key issue to obtain deep null depths. The PERSEE breadboard has been designed to study and optimize the operation of cophased nulling bench in the most realistic disturbing environment of a space mission. This presentation focuses on the current results of the PERSEE bench. In terms of metrology, we cophased at 0.33 nm rms for the piston and 60 mas rms for the tip/tilt. A Linear Quadratic Gaussian (LQG) control coupled with an unsupervised vibration identification allows us to maintain that level of correction, even with characteristic vibrations of nulling interferometry space missions. These performances, with an accurate design and alignment of the bench, currently lead to a polychromatic unpolarised null depth of 8.9 × 10-6 stabilized at 2.7 × 10-7 on the [1.65 - 2.45] μm spectral band (37% bandwidth). With those significant results, we give the first more general lessons we have already learned from this experiment, both at system and component levels for a future space mission.

Fabrication and testing of all-telluride rib waveguides for nulling interferometry

The feasibility of two types of all-telluride integrated optics devices being able to single-mode guiding of light in the spectral ranges [6-11 µm] and [10-20 µm], respectively, has been demonstrated. The so-called “rib” waveguides show a several micron thick Te82Ge18 film deposited onto a Te75Ge15Ga10 bulk glass substrate by thermal co-evaporation and further etched by reactive ion etching in CHF3/O2/Ar atmosphere. The obtained structures were proved to behave as channel waveguides with a satisfactory confinement of light in the whole spectral ranges. These results allowed validation of our technological solution for the fabrication of micro-components for spatial interferometry.

Review of OCA activities on nulling testbench PERSEE

We present a review of our activities on PERSEE (Pégase Experiment for Research and Stabilization of Extreme Extinction) at Observatoire de la Côte dAzur (OCA). PERSEE is a laboratory testbench aiming at achieving a stabilized nulling ratio better than 10-4 in the astronomical bands K and M, in presence of flight-representative spacecraft perturbations. The bench has been jointly developed by a Consortium of six French institutes and companies, among which OCA was responsible for the star simulator and of the opto-mechanical studies, procurement and manufacturing of the optical train. In this communication are presented the alignment and image quality requirements and the optomechanical design of the illumination module and main optical train, including a periscope Achromatic Phase Shifter (APS), tip-tilt mirrors used to introduce and then compensate for dynamic disturbances, delay lines, beam compressors and fiber injection optics. Preliminary test results of the star simulator are also provided.

Fabrication of far infrared rib waveguides based on Te-Ge-Ga films deposited by co-thermal evaporation

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.

Design of a star, planet and exo-zodiacal cloud simulator for the nulling testbench PERSEE

On-going developments on the PERSEE nulling testbench include the realization of a focal plane simulator featuring one central star, an extra-solar planet orbiting around it, and an Exo-Zodiacal Cloud (EZC) surrounding the observed stellar system. PERSEE (Pégase Experiment for Research and Stabilization of Extreme Extinction) is a laboratory testbench jointly developed by a Consortium of six French institutes and companies, incorporating Observatoire de la Côte dAzur (OCA) who is in charge of the manufacturing and procurement of the future Star and Planet Simulator (SPS). In this communication is presented a complete description of the SPS, including general requirements, techniques employed for simulating the observed planet and EZC, opto-mechanical design and expected performance. The current status of the SPS activities is summarized in the conclusion, pending final integration on the PERSEE test bench in September 2011.