Oswald Wallner

Minimum length of a single-mode fiber spatial filter

Using the concept of leaky modes, we derive the minimum length of a single-mode fiber required to act as a spatial-mode filter of given quality. The degree of filter action is defined by the ratio of power carried by the fundamental mode to that carried by the leaky modes.

Alignment tolerances for plane-wave to single-mode fiber coupling and their mitigation by use of pigtailed collimators

We discuss the efficiency with which coherent plane waves can be coupled to single-mode fibers in the presence of deterministic or stochastic misalignments of the fiber relative to the focal point of a lens. We point out how the alignment demands can be relaxed by means of graded-index-lens fiber-pigtailed collimators.

Three telescope nuller based on multibeam injection into single-mode waveguide

Nulling interferometry of exo-solar planets requires as a minimum two telescopes, of which one is phase shifted by 180 degrees, such that the on-axis stellar object is cancelled, while the light from the off-axis planet interferes constructively. Improvement of the nulling performance and the introduction of chopping leads to space interferometers of four or more telescopes and a separate spacecraft dedicated to beam recombination, as currently baselined for Darwin and TPF. It has recently been demonstrated that the stellar leaks mainly affects the integration times for near-by target stars [o,c]. Considering that there are only a few near-by targets and that the integrations times for each of these is short compared to that of distant stars, it appears advantageous to simplify the interferometer, by accepting higher levels of stellar leaks for near-by targets. A simple, chopping nulling interferometer can be obtained by adding one equal size telescope to the basic two telescope nulling interferometer. Modulation is obtained by applying time-varying phase-shifts to the beams before recombination, i.e. inherent modulation [d]. The recombination of 3 multi-axial beams is achieved by coupling into a single mode waveguide, leading to high modulation and coupling efficiencies, and a single focal plane [i]. Linear and circular telescope configurations are proposed and investigated, including a discussion on the need of a separate spacecraft for beam recombination. The associated transmission and modulation maps and efficiencies are calculated and discussed.

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.

Application of single-mode fiber-coupled receivers in optical satellite to high-altitude platform communications

In a free-space optical communication system employing fiber-optic components, the phasefront distortions induced by atmospheric turbulence limit the efficiency with which the laser beam is coupled into a single-mode fiber. We analyze different link scenarios including a geostationary (GEO) satellite, a high-altitude platform (HAP), and an optical ground station (OGS). Single-mode coupled optically preamplified receivers allow for efficient suppression of background noise and highly sensitive detection. While GEO-to-OGS communication suffers from atmospheric turbulence, we demonstrate that GEO-to-HAP communication allows for close to diffraction-limited performance when applying tip-tilt correction.

Nulling interferometry for spectroscopic investigation of exoplanets: a statistical analysis of imperfections

In ESAs Infrared Space Interferometry mission, a multi-aperture interferometer fed by telescopes will serve to analyse exoplanets orbiting bright stars. Spectroscopy of the planets radiation could give hints on the possibility of the existence of life. However, for a Sun/Earth-like constellation, a star light rejection ratio of some 80 dB is required. This is the factor by which the star light is suppressed, when comparing the interferometer with a standard, wide-field-of-view telescope. We investigate the nulling capability of space-based interferometers, realized either in fiber or bulk optics, in the presence of imperfections of the structure and of optical components. Mismatch of amplitude, optical path length, and polarization among the interferometer arms is taken into account, as well as multiple reflections and telescope imperfections. The parameters describing the interferometers receive characteristic, which are actively controlled or influenced by environmental disturbances, are modeled stochastically. We analyse Sun/Earth-like constellations by numerical simulation for a wavelength range of 6 to 18 microns. The expected value of the star light rejection ratio is calculated for several interferometer configurations. The exemplary numerical results confirm the extreme requirements for interferometer uniformity and give a quantitative insight into the dependence of the attainable rejection ratio on individual and/or combined interferometer imperfections.

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.

Terrestrial exo-planet science by nulling interferometry: instrument design and scientific performance

The detection of terrestrial exo-planets in the habitable zone of Sun-like stars as well as the proof of biomarkers is one of the most exciting goals in Astrophysics today. A nulling interferometer operated in the mid-infrared wavelength regime allows for overcoming the obstacles of huge contrast ratio and small angular separation between star and planet. Dedicated missions, as ESAs DARWIN or NASAs TPF-I, are implemented as a closely controlled formation of free-flying spacecraft which carry the distributed payload. We discuss various implementation alternatives and present an optimized design of the DARWIN instrument including the science payload and the formation-flying subsystem. We analyze the achievable scientific performance of the DARWIN instrument by taking into account the target properties and the instrument performance. We show that the DARWIN mission is feasible and that the mission goals can be fulfilled.