Claus Braxmaier

The LTP interferometer and phasemeter

The LISA Technology Package (LTP), to be launched by ESA in 2006/2007, is a technology demonstration mission in preparation for the LISA space-borne gravitational wave detector. A central part of the LTP is the optical metrology package (heterodyne interferometer with phasemeter) which monitors the distance between two test masses with a noise level of 10 pm Hz−1/2 between 3 mHz and 30 mHz. It has a dynamic range of >100 µm without any actuators for the pathlength. In addition to the longitudinal measurements, it provides alignment measurements with an expected noise level of <10 nrad Hz−1/2. While the basic design has been described previously by Heinzel et al (2003 Class. Quantum Grav. 20 S153–61), this paper gives new details on the laser stabilization, the phasemeter and recent prototype results.

Successful testing of the LISA Technology Package (LTP) interferometer engineering model

The LISA Technology Package (LTP), to be launched by ESA in 2008, is a technology demonstration mission in preparation for the LISA space-borne gravitational wave detector. A central part of the LTP is the optical metrology package (heterodyne interferometer with phasemeter) that measures the distance between two test masses with a noise level of 10 pm Hz−1/2 between 3 mHz and 30 mHz and also the test mass alignment with a noise level of <10 nrad Hz−1/2. An engineering model of the interferometer has been built and environmentally tested. Extensive functionality and performance tests were conducted. This paper reports on the successful test results.

Interferometry for the LISA technology package LTP: an update

This paper gives an update on the status of the LISA technology package (LTP) which is to be launched in 2009 by ESA as a technology demonstration mission for the space- borne gravitational wave observatory LISA. The dominant noise source in the interferometer prototype has been investigated and improved such that it is now comfortably below its budget at all frequencies.

Line of Sight Alignment Algorithms for Future GravityMissions

Laser interferometry is viable candidate to microwave interferometry as it exploits bet- nter performances in terms of range measurements. Preliminary studies on future gravity nmissions show that, before addressing their metrology target, these laser terminals of- nten require an intermediate alignment and signal acquisition phase whose complexity can nvary according to instruments layout and the environmental constrains. The line of sight nalignment algorithms here presented are designed to autonomously perform spatial scans nthrough laser beam pointing corrections and, if necessary, laser frequency tunings in order nto overcome initial pointing and frequency errors. The laser beam guidance algorithms an- nalyzed use randomly generated pointing and/or continuous curve patterns while frequency nchanges (if necessary) are operated through Piezo-electric and thermal tuning. The al- ngorithms here presented are tailored for a two-satellite-based heterodyne interferometer nand have been tested through numerical simulations. Nevertheless, being the formulation nparametric allows the algorithms to be readapted for a di�erent mission concept.

The Space-Time Asymmetry Research (STAR) program

Space-Time Asymmetry Research (STAR) is a proposed satellite mission that aims for significantly improved tests of fundamental space-time symmetry and the foundations of special and general relativity. In the current concept, STAR comprises a series of three subsequent missions with increasingly advanced instruments performing clock to clock comparisons. While the first STAR missions will perform Kennedy-Thorndike (KT) and Michelson-Morley (MM) experiments, later missions will focus on fundamental gravitational physics by precision measurement of gravitational redshift, time dilation and Local Position Invariance (LPI). Compared to previous experimental accuracy, STAR aims for an improvement of at least two orders of magnitude. The STAR1 mission will measure the constancy of the speed of light to one part in 10-17 and derive the Kennedy Thorndike coefficient of the Mansouri-Sexl test theory to 7 × 10-10. The KT experiment will be performed by comparison of an atomic or molecular frequency reference with a length reference (highly stable cavity made e.g. from ultra low expansion (ULE) glass ceramics) during flight around Earth with an orbital velocity of 7 km/s. The corresponding sensitivity to a boost dependent violation of Lorentz invariance as modeled by the KT term in the Mansouri-Sexl test theory or a Lorentz violating extension of the standard model (SME) will be significantly enhanced as compared to Earth-based experiments. The space environment will enhance the measurement precision such that an overall improvement by a factor of 400 over current Earth bound experiments is expected. The STAR1 philosophy is to realize a fast, small - and therefore cheap - mission with a high scientific output, also providing the instrument technology and the spacecraft for the subsequent STAR missions, which plan to use different optical frequency standards. The 180 kg small satellite will be attitude, vibration and temperature controlled. The power consumption of the whole spacecraft will be less than 185 W. The launch of STAR1 is foreseen for 2015, the follow-on missions will be flown with an overlap with the previous mission by two to three years. Each mission has a maximum duration of 5 years (from mission set up to data acquisition) which permits students to experience the full mission lifecycle. Education and training of undergraduate and graduate students is a specific mission goal.

Optical Design of the LISA Interferometric Metrology System

Within the context of the LISA Mission Formulation Study, we have developed a detailed concept for the optical layout of the LISA payload, which consists of two movable assemblies per spacecraft, each pointing to its respective remote spacecraft to form a constellation triangle of 5 million kilometer arm length. The movable assemblies comprise a Cassegrain telescope, an optical bench, and the gravity reference sensor with its free floating proof mass, which delimits the respective arm. Differential changes in the distances between the two proof masses of each arm, caused by the passage of a gravitational wave, are detected by a combination of heterodyne interferometry and differential wavefront sensing. The optical architecture is further characterized by a “strap‐down” approach with proof mass optical readout, as well as a “frequency swap” between transmitted and local reference beams. The nominal performance and sensitivity of the complete system is verified by extensive optical modeling.

mSTAR: Testing Lorentz invariance in a low Earth orbit with high performance optical frequency standards

fundamental physics test, Kennedy-Thorndike, clocks, nultra-stable cavities, iodine spectroscopy, space instrumentation

Dual absolute and relative high precision laser metrology

Design, integration, test setup, test results, and lessons-learnt of a high precision laser metrology demonstrator for dual absolute and relative laser distance metrology are presented. The different working principles are described and their main subsystems and performance drivers are presented. All subsystems have strong commonalities with flight models as of LTP on LISA Pathfinder and laser communication missions, and different pathways to flight models for varying applications and missions are presented. The setup has initially been realized within the ESA project High Precision Optical Metrology (HPOM), originally initiated for DARWIN formation flying optical metrology, though now serves as demonstrator for a variety of future applications. These are sketched and brought into context (PROBA-3, IXO onboard metrology, laser gravimetry earth observation missions, fundamental science missions like LISA and Pioneer anomaly).

Interferometric surface characterization of laser mirrors with sub-nanometer reproducibility for satellite-to-satellite optical metrology

Path length errors caused by beamwalk over the surface topography of optical components can have a detrimental influence on the accuracy of highly sensitive translational metrology, that is of particular relevance for In-Field Pointing payload concepts, investigated for the LISA space mission. This paper presents the results of our experimental and theoretical investigations in surface induced path length errors with a detailed characterisation of their magnitudes.