Showing papers by "Ulrich Johann published in 2009"

Picometer and nanoradian optical heterodyne interferometry for translation and tilt metrology of the LISA gravitational reference sensor

Abstract: The Laser Interferometer Space Antenna (LISA) aims at detecting gravitational waves by referencing heterodyne interferometry to free-flying (inertial) proof masses, located at the corners of a triangle with 5 million kilometers arm length. The position of each proof mass with respect to the associated satellite must be measured with 1 pm Hz−1/2-sensitivity in translation measurement and below 10 nrad Hz−1/2-sensitivity in attitude. In this paper, we present a compact heterodyne interferometer utilizing polarizing optics combined with the method of differential wavefront sensing (DWS) serving as a demonstrator for a complete optical readout system of the proof mass translation and attitude aboard the LISA satellites. Our interferometer is based on a highly symmetric design, where reference and measurement beam have similar optical paths and equal polarization and frequency. Intensity stabilization of the laser radiation, phaselock of the laser frequencies at the fiber outputs and a digital phase measurement based on a field programmable gate array (FPGA) are implemented to achieve noise levels below 10 pm Hz−1/2 and 10 nrad Hz−1/2, respectively, for frequencies >10−2 Hz.

Matter wave explorer of gravity (MWXG)

Abstract: In response to ESA's Call for proposals of 5 March 2007 of the COSMIC VISION 2015-2025 plan of the ESA science programme, we propose a M-class satellite mission to test of the Equivalence Principle in the quantum domain by investigating the extended free fall of matter waves instead of macroscopic bodies as in the case of GAUGE, MICROSCOPE or STEP. The satellite, called Matter Wave Explorer of Gravity, will carry an experiment to test gravity, namely the measurement of the equal rate of free fall with various isotopes of distinct atomic species with precision cold atom interferometry in the vicinity of the earth. This will allow for a first quantum test the Equivalence Principle with spin polarised particles and with pure fermionic and bosonic atomic ensembles. Due to the space conditions, the free fall of Rubidium and Potassium isotopes will be compared with a maximum accelerational sensitivity of 5*10 − 16 m/s2 corresponding to an accuracy of the test of the Equivalence Principle of 1 part in 1016. Besides the primary scientific goal, the quantum test of the Equivalence Principle, the mission can be extended to provide additional information about the gravitational field of the earth or for testing theories of fundamental processes of decoherence which are investigated by various theory groups in the context of quantum gravity phenomenology. In this proposal we present in detail the mission objectives and the technical aspects of the proposed mission.

Advancing fundamental physics with the Laser Astrometric Test of Relativity The LATOR mission

Abstract: The Laser Astrometric Test of Relativity (LATOR) is an experiment designed to test the metric nature of gravitation—a fundamental postulate of the Einstein’s general theory of relativity. The key element of LATOR is a geometric redundancy provided by the long-baseline optical interferometry and interplanetary laser ranging. By using a combination of independent time-series of gravitational deflection of light in the immediate proximity to the Sun, along with measurements of the Shapiro time delay on interplanetary scales (to a precision respectively better than 0.1 picoradians and 1 cm), LATOR will significantly improve our knowledge of relativistic gravity and cosmology. The primary mission objective is i) to measure the key post-Newtonian Eddington parameter γ with accuracy of a part in 109. $\frac{1}{2}(1-\gamma)$ is a direct measure for presence of a new interaction in gravitational theory, and, in its search, LATOR goes a factor 30,000 beyond the present best result, Cassini’s 2003 test. Other mission objectives include: ii) first measurement of gravity’s non-linear effects on light to ∼0.01% accuracy; including both the traditional Eddington β parameter and also the spatial metric’s 2nd order potential contribution (never measured before); iii) direct measurement of the solar quadrupole moment J 2 (currently unavailable) to accuracy of a part in 200 of its expected size of ≃ 10 − 7; iv) direct measurement of the “frame-dragging” effect on light due to the Sun’s rotational gravitomagnetic field, to 0.1% accuracy. LATOR’s primary measurement pushes to unprecedented accuracy the search for cosmologically relevant scalar-tensor theories of gravity by looking for a remnant scalar field in today’s solar system. We discuss the science objectives of the mission, its technology, mission and optical designs, as well as expected performance of this experiment. LATOR will lead to very robust advances in the tests of fundamental physics: this mission could discover a violation or extension of general relativity and/or reveal the presence of an additional long range interaction in the physical law. There are no analogs to LATOR; it is unique and is a natural culmination of solar system gravity experiments.

Interferometry based high-precision dilatometry for dimensional characterization of highly stable materials

Abstract: We present an optical dilatometer for high-accuracy and high-resolution absolute measurement of the linear coefficient of thermal expansion (CTEl). Based on a highly symmetric differential heterodyne interferometer, dimensional changes of a tubular shaped specimen under controlled thermal conditions can be characterized. Our measurement facility is located in vacuum where the test specimen can be temperature controlled in a temperature range between 20 °C and 60 °C. A thermally stable support and two identical isostatic mirror clamps were specifically designed to fix a reference and a measurement mirror inside the tube enabling a measurement, where no load in the axial direction was applied to the device under test (DUT). We measured the linear CTE of two carbon-fibre reinforced plastic (CFRP) tubes with different predicted linear CTEs at room temperature: −0.647 × 10−6 K−1 and 0 ± 2.5 × 10−9 K−1, respectively. Currently, we are investigating the manufacture limitations of the CFRP and the limitations of our apparatus in terms of measurement accuracy. In the next step, we will characterize a specifically manufactured zero-class Zerodur™ tube with a CTEl value <10 × 10−9 K−1. Due to its high thermal stability and non-directional structural isotropy this material has been chosen for macroscopic calibration of the metrology system. The results of these measurements will thus provide the resolution limitations of our facility and can be taken as an absolute accuracy reference.

GAUGE: the GrAnd Unification and Gravity Explorer

Abstract: The GAUGE (GrAnd Unification and Gravity Explorer) mission proposes to use a drag-free spacecraft platform onto which a number of experiments are attached. They are designed to address a number of key issues at the interface between gravity and unification with the other forces of nature. The equivalence principle is to be probed with both a high-precision test using classical macroscopic test bodies, and, to lower precision, using microscopic test bodies via cold-atom interferometry. These two equivalence principle tests will explore string-dilaton theories and the effect of space–time fluctuations respectively. The macroscopic test bodies will also be used for intermediate-range inverse-square law and an axion-like spin-coupling search. The microscopic test bodies offer the prospect of extending the range of tests to also include short-range inverse-square law and spin-coupling measurements as well as looking for evidence of quantum decoherence due to space–time fluctuations at the Planck scale.

Advanced drag-free concepts for future space-based interferometers: acceleration noise performance

Abstract: Future drag-free missions for space-based experiments in gravitational physics require a Gravitational Reference Sensor with extremely demanding sensing and disturbance reduction requirements. A configuration with two cubical sensors is the current baseline for the Laser Interferometer Space Antenna (LISA) and has reached a high level of maturity. Nevertheless, several promising concepts have been proposed with potential applications beyond LISA and are currently investigated at HEPL, Stanford, and EADS Astrium, Germany. The general motivation is to exploit the possibility of achieving improved disturbance reduction, and ultimately understand how low acceleration noise can be pushed with a realistic design for future mission. In this paper, we discuss disturbance reduction requirements for LISA and beyond, describe four different payload concepts, compare expected strain sensitivities in the 'low-frequency' region of the frequency spectrum, dominated by acceleration noise, and ultimately discuss advantages and disadvantages of each of those concepts in achieving disturbance reduction for space-based detectors beyond LISA.

Alternative opto-mechanical architectures for the LISA instrument

Abstract: As part of the on-going LISA Mission Formulation study under ESA contract, EADS Astrium has recently suggested and investigated a variety of novel LISA payload architectures utilizing so-called "In-Field Pointing" for accommodation of seasonal constellation dynamics. Here, the annual variation in the angle between the interferometer arms of roughly ±1° is compensated by steering the lines of sight of the individual telescopes with a small actuated mirror located in an intermediate pupil plane inside the telescopes. This introduces a certain flexibility for the overall payload configuration and allows for very compact designs. In particular, it enables a "single active proof mass" mode with a true cold redundancy between a nominal and a backup GRS system on board each spacecraft, and thus enhances mission robustness.

A high sensitivity heterodyne interferometer as a possible optical readout for the LISA gravitational reference sensor and its application to technology verification

Abstract: The space-based gravitational wave detector LISA (Laser Interferometer Space Antenna) utilizes a high performance position sensor in order to measure the translation and tilt of the free flying proof mass with respect to the optical bench. Depending on the LISA optical bench design, this position sensor must have up to pm/ sensitivity for the translation measurement and up to nrad/ sensitivity for the tilt measurement. We developed a heterodyne interferometer, combined with differential wavefront sensing, for the tilt measurement. The interferometer design exhibits maximum symmetry where measurement and reference arm have the same frequency and polarization and the same optical path-lengths. The interferometer can be set up free of polarizing optical components preventing possible problems with thermal dependencies not suitable for the space environment. We developed a mechanically highly stable and compact setup which is located in a vacuum chamber. We measured initial noise levels below 10 pm/ (longitudinal measurement) for frequencies above 10 mHz and below 20 nrad/ (tilt measurement) for frequencies above 1 mHz. This setup can also be used for other applications, for example the measurement of the coefficient of thermal expansion (CTE) of structural materials, such as carbon fiber reinforced plastic (CFRP).

Picometer and nanoradian interferometry for the LISA gravitational reference sensor and its application to technology verification

Abstract: The spaceborne gravitational wave detector LISA (Laser Interferometer Space Antenna) utilizes free floating proof masses as inertial references for its 5 million km long interferometer arms. In the current baseline design their position and tilt with respect to the appropriate optical bench is measured by an optical readout which needs picometer sensitivity in translation measurement and nanoradian sensitivity in tilt measurement. For this purpose EADS-Astrium GmbH - in cooperation with HTWG Konstanz and the Humboldt-Universitat zu Berlin - developed a highly symmetric heterodyne interferometer [1] with differential wavefront sensing for the tilt measurement [2].