Reinhold Flatscher

Design of spatial and modal filters for nulling interferometers

Spatial or modal filters are essential parts of highly rejecting nulling interferometers. We review the principle of operation of both types of filters and explain the fundamental physical difference. We point out the filters individual properties and potentials, and analyze practical limitations. For modal filters we discuss implementation alternatives, also with regard to their suitability for mid-infrared operation. For a single-mode fibre filter we analyze the broadband performance and the minimum length ensuring a prescribed filter action. We further present simulation results of a DARWIN-representative nulling interferometer breadboard which confirm the distinct improvement in rejection ratio due to spatial or modal filtering.

Development of silver-halide single-mode fibers for modal filtering in the mid-infrared

Modal filtering is mandatory in nulling interferometers dedicated to direct detection of extrasolar terrestrial planets. However, up to date no appropriate waveguides to act as wavefront filter were available for the mid-infrared wavelengths in question. We present the development of silver-halide fibers and chalcogenide fibers to be used for modal filtering within the European DARWIN mission. We give a trade-off of suitable waveguides geometries, possible materials, and fabrication technologies and present measurements of the beam profiles, the insertion loss, and of the modal filtering capability of the developed fiber samples.

Realization of a 10-µm homodyne receiver

We have realized a homodyne receiver for phase-modulated laser radiation at \lambda = 10.6 \mu m, based on a dc-coupled optical phase-locked loop (PLL). A control loop maintains the output power of the CO 2 laser local oscillator at 0.7 mW. Fine frequency tuning of this laser is achieved by an acoustooptic frequency shifter whose inherent time delay has been taken into account in the design of both the PLL and the power control loop. The PLL rms phase error is some 3° for a received optical power of 4 nW. The HgCdTe photodiode serving as phase detector and as demodulator, as well as the preamplifier are operated at 80 K by means of a Stirling cooler. Clear domination of shot noise is achieved for the data rate of 140 Mbit/s. A microcomputer performs dc-offset compensation and frequency acquisition. The system is part of a breadboard of an intersatellite data link.

EADS Astrium nulling interferometer breadboard for DARWIN and GENIE

Within the context of the ESA TRP programme for DARWIN, a Nulling Interferometer Breadboard for the Near-Infrared was developed and tested. Its basic principle is recombining two light beams relying on a highly symmetric optical design (autobalanced Sagnac Core). Two different star simulators have been implemented, based on a) amplitude division and b) on wavefront division. The required achromatic Pi phase shift was implemented using a) dispersive phase shifter, and b) periscopes (geometrical pupil and field rotation). Due to the extremely symmetric optical design, very good star suppression up to 400 000 has been achieved. OPD control better than 1 nm RMS has been demonstrated over hours.

DARWIN mission and configuration trade-off

The European DARWIN mission aims at detection and characterization of Earth-like exo-planets as well as at aperture synthesis imaging. The method to be applied is nulling interferometry in the mid-infrared wavelength regime. The DARWIN instrument consists of a flotilla of free-flying spacecraft, one spacecraft carrying the optics for beam recombination and three or more spacecraft carrying the large collector telescopes. We provide a trade-off of different configuration, payload, and mission concepts. We discuss various two and three-dimensional aperture configurations with three or four telescopes, beam routing schemes, phase modulation methods, and beam recombination and detection schemes as well as different launch vehicle configurations, launch scenarios, and orbits. We trade the different DARWIN concepts by assessing the performance in terms of science return, development risk, and planning.

Manufacturing of chalcogenide and silver-halide single-mode fibres for modal wavefront filtering for DARWIN

Modal wavefront filtering is mandatory in nulling interferometers dedicated to detect extrasolar planets. Several activities have been initiated by ESA for developing single-mode waveguides for the mid-infrared. We present the development of fibres to be used for modal filtering within the European DARWIN mission and its scientific precursor GENIE: Chalcogenide fibres fit the wavelength range up to about 11 microns, while silver halide fibres can cover the full DARWIN wavelength range from 6.5 to 20 microns. A wide range of different manufacturing methods have been applied for producing step-index fibres. We also present the first results of manufacturing photonic crystal silver halide fibres. We tested the modal wavefront filtering capability of the fibres in a Mach-Zehnder interferometer fed by a CO2-laser. In addition we recorded the transverse output beam profile for each fibre. The results of both measurements are strong indicators for single-mode operation. We identified the critical issues experienced in the course of this manufacturing activity. The efficient removing of cladding modes and the required length of the fibres, commonly strongly underestimated, turned out as the keys for successful demonstration of singlemode behaviour. We found dedicated and compatible materials acting as mode stripper for both fibre materials used. We highlight the required steps for further improvement of the manufactured fibres and for a reasonable continuation of the fibre development activities for DARWIN.

X-Array aperture configuration in planar or non-planar spacecraft formation for DARWIN/TPF-I candidate architectures

The missions DARWIN and TPF-I (Terrestrial Planet Finder-Interferometer) aim at the search and analysis of terrestrial exo-planets orbiting nearby stars. The major technical challenge is the huge contrast ratio and the small angular separation between star and planet. The observational method to be applied is nulling interferometry. It allows for extinguishing the star light by several orders of magnitude and, at the same time, for resolving the faint planet. The fundamental performance of the nulling interferometer is determined by the aperture configuration, the effective performance is driven by the actual instrument implementation. The x-Array, an aperture configuration with 4 telescopes allowing for phase chopping and decoupling of the nulling and imaging properties, provides highest instrument performance. The scientific goals necessitate an instrument setup of high efficiency and utmost symmetry between the beams concerning optical path length, beam profile and state of polarization. Non-planar spacecraft formations allow for a simpler spacecraft design which comes at the cost of inherent constellation and beam asymmetry, of increased complexity of the beam relay optics and of instrumental errors synchronous to the planet signal demodulation frequency. Planar formations allow for perfect efficiency and symmetry but need deployable structures for the secondary mirror and the sunshield due to launcher accommodation constraints. We present a discussion of planar and non-planar implementations of the x-Array aperture configuration and identify for both the critical items and design drivers. We compare the achievable instrument performance and point out the constraints for each spacecraft formation.

Exo-zodi detection capability of the Ground-Based European Nulling Interferometry Experiment (GENIE) Instrument

The Ground-Based European Nulling Interferometry Experiment (GENIE) is intended as an Earth-based precursor for the European Darwin mission that will prepare the Darwin science program and demonstrate the required technology at system level. We propose a compact nulling interferometer design consisting of a two-telescope aperture configuration, an optional split-pupil add-on, and only four active control loops for counteracting environmentally induced disturbances. We show by simulation that the proposed instrument is able to detect, within a few minutes of observation time, exo-zodiacal dust clouds around Sunlike stars at 20 parsecs that are 20 times stronger than the local zodiacal dust cloud density.

Nulling Breadboard for DARWIN

This work is funded by ESA under ESTEC/Contract No. 14827/00/NL/CK. Astrium Germany has been awarded this first ESA breadboarding towards nulling interferometry. Interferometric nulling devices are essential ingredients in the TPF and DARWIN missions for suppressing the star light by a factor of 106 over a wide wavelength range in the mid infrared. The current DARWIN baseline concept comprises six telescopes. The coherent combination scheme in the nulling mode operation foresees three nulling assemblies in parallel. The breadboard serves to demonstrate the deep and stable null required for an operational instrument. The demonstrator operates in the near infrared to save costs but its principle is fully applicable to the mid infrared. The nulling device is based on an autobalancing Sagnac core offering just one critical beam combiner. Two different ways of achieving the required π phase shift are implemented: a) arbitrary phase shift by dispersive phase shifter plates b) phase shift of π using periscopes (image flip) The target simulator features two point sources of adjustable radiometry and angular separation, representing a strong star and a weak planet. In addition, the sources can be also used to simulate a double star for demonstrating the basic DARWIN imaging mode. The simulator can be operated in two styles, namely as wavefront dividing star/planet source and, alternatively, as an amplitude dividing source, providing highly symmetric wavefronts to both interferometer arms. Because of its representativity for the DARWIN situation, the latter mode is the preferred simulator for quantitative nulling experiments. The breadboard design has been finalized in January 2002 and verified by detailed simulations. The entire hardware has been manufactured by end of July. Currently, nulling and imaging measurements are in progress to validate the per-formance of the selected approach. The project is part of ESA´s technology preparatory program for DARWIN, paving the way for a collaborative ESA/ESO guest instrument at ESOs VLTI with scientific implications.

Mechanically Cooled Receiver Front-End For High Data Rate CO 2 Laser Communication

A cooler/detector/preamplifier unit to be used as the front end in coherent optical receivers for X = 10pm was designed, fabricated, and tested. Both the HgCdTe-detector and the preamplifier were cooled to 80 K with the aid of a Stirling cooler. The three-stage high-impedance preamplifier realized employs field effect transistors and can hence be operated at low temperatures, too. Receiver tests were performed at a data rate of 140 Mbit/s using a quasi-homodyne receiver setup, with the bit-error-rate as the criterion of performance. Receiver operation very close to the quantum limit was demonstrated.