Future space based missions for gravitational wave research call for an improved inertial reference sensor with acceleration noise levels of fm/s2. Spherical test masses can enable increased performance by suspension-free operation, contrary to cuboid solutions suffering from cross-coupling of attitude control noise. However, interferometric readout is affected by surface irregularities and test mass tumbling [1]. An accurate surface map for compensation must be established for compensation, either by characterisation a priori or in flight, when optical path length changes due to surface occur in the measurement band. We demonstrate a method for generating a surface map of a spherical body with optical point sensors using a differential method to suppress common mode errors present, taking advantage of the excellent performance of heterodyne interferometry at sub-nanometer levels. A measurement setup is proposed in which two beams of a Nd:YAG Michelson interferometer are used to scan the surface, which is afterwards reconstructed from the differential measurement. Such a method could potentially benefit other research areas, such as the precise determinations of the Avogadro constant [2] or aspheric surface metrology.
KEYWORDS: Mirrors, Retroreflectors, Space operations, Optical components, Ranging, Field programmable gate arrays, Beam steering, Satellites, Bragg cells, Digital signal processing
Interferometric laser ranging is an enabling technology for high-precision satellite-to-satellite tracking within the context of earth observation, gravitational wave detection, or formation flying. In orbit, the measurement system is affected by environmental influences, particularly satellite attitude jitter and temperature fluctuations, demanding an instrument design, which has a high level of thermal stability and is insensitive to rotations around the satellite's center of mass. Different design approaches for a heterodyne dynamic laser ranging instrument have been combined to a new improved design concept that involves the inherent beam tracking capabilities of a retroreflector into a mono-axial configuration with nanometer accuracy. In order to facilitate the accommodation onboard a future satellite mission, the design allows for a continuously adjustable flexible phase center position. To cover large inter-spacecraft distances, the instrument design comprises an active transponder system, featuring a two-dimensional beam steering mechanism to align a local, strong laser to the (weak) input beam without affecting the measurement path.
To this end, a dynamic laser ranging instrument is presented, which has compact dimensions and is fully integrated on a single Zerodur baseplate. The instrument performance will be evaluated in a dedicated test setup providing a flat-top beam simulating the laser beam received from a distant spacecraft, including a beam steering subsystem, which allows for monitoring of pathlength variations when the angle of incidence at the optical instrument is changing.
A variety of future space missions rely on the availability of high-performance optical frequency references with applications in fundamental physics, geoscience, Earth observation and global satellite navigation systems (GNSS). Examples are the gravitational wave detector LISA (Laser Interferometer Space Antenna), the Earth gravity mission NGGM (Next Generation Gravity Mission) and missions, dedicated to tests of Special Relativity, e.g. by performing a Kennedy-Thorndike experiment testing the boost dependence of the speed of light. In this context, we developed optical frequency references based on Doppler-free spectroscopy of molecular iodine where compactness and mechanical and thermal stability are main design criteria. We demonstrated a frequency instability of 6•10-15 at 1 s integration time and 3•10-15 for integration times between 100 s and 1.000 s. Furthermore, a very compact spectroscopy setup was realized for the sounding rocket mission JOKARUS which was successfully flown in May 2018. In a current activity, an integrated high-performance iodine-based frequency reference is developed which serves as a demonstrator for future GNSS using optical technologies.
We present the development of a compact optical frequency reference with a stability in the 10-15 domain at longer integration times utilizing Doppler-free spectroscopy based on molecular iodine. With respect to its future application in space, a setup on elegant breadboard (EBB) level was realized and successfully implemented and tested. A frequency stability of 5 • 10-15 at an integration time of 200 s was verified in a beat measurement with a ULE cavity setup. For ensuring high thermal and mechanical stability, the EBB utilizes a baseplate made of ultra-low CTE glass ceramics. The optical components are fixed to the baseplate using an adhesive bonding technology. In a current activity, a setup on engineering model (EM) level will be realized with increased compactness and stability compared to the EBB setup utilizing a very compact multipass gas cell.
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.
We realized ultra-stable optical frequency references on elegant breadboard (EBB) and engineering model (EM) level utilizing Doppler-free spectroscopy of molecular iodine near 532nm. A frequency stability of about 1•10-14 at an integration time of 1 s and below 5•10-15 at integration times between 10 s and 100 s was achieved. These values are comparable to the currently best laboratory setups.
Both setups use a baseplate made of glass material where the optical components are joint using a specific assembly-integration technology. Compared to the EBB setup, the EM setup is further developed with respect to compactness and mechanical and thermal stability. The EM setup uses a baseplate made of fused silica with dimensions of 380 x 180 x 40 mm3 and a specifically designed 100 x 100 x 30 mm3 rectangular iodine cell in nine-pass configuration with a specific robust cold finger design. The EM setup was subjected to thermal cycling and vibrational testing.
Applications of such an optical frequency reference in space can be found in fundamental physics, geoscience, Earth observation, and navigation & ranging. One example is the proposed mSTAR (mini SpaceTime Asymmetry Research) mission, dedicated to perform a Kennedy-Thorndike experiment on a satellite in a sunsynchronous low-Earth orbit. By comparing an iodine standard to a cavity-based frequency reference and integration over 2 year mission lifetime, the Kennedy-Thorndike coefficient will be determined with up to two orders of magnitude higher accuracy than the current best ground experiment. In a current study, the compatibility of the payload with the SaudiSat-4 host vehicle is investigated.
C. Dahl, A. Baatzsch, M. Dehne, F. Gilles, P. Hager, M. Herding, K. Nicklaus, K. Voss, K. Abich, C. Braxmaier, M. Gohlke, B. Guenther, J. Sanjuan, B. Zender, G. Barranco, A. Görth, C. Mahrdt, V. Müller, D. Schütze, G. Stede, G. Heinzel
The Gravity Recovery and Climate Experiment (GRACE) is a successful Earth observation mission launched in 2002 consisting of two identical satellites in a polar low-Earth orbit [1]. The distance variations between these two satellites are measured with a Micro Wave Instrument (MWI) located in the central axis. In data postprocessing the spatial and temporal variations of the Earth’s gravitational field are recovered, which are among other things introduced by changing groundwater levels or ice-masses [2, 3, 4, 5]. The Laser Ranging Interferometer (LRI) on-board the GRACE Follow-On (GFO) mission, which will be launched in 2017 by the joint collaboration between USA (NASA) and Germany (GFZ), is a technology demonstrator to provide about two orders of magnitude higher measurement accuracy than the initial GRACE MWI, about 80 nm/√Hz in the measurement band between 2 mHz and 0.1 Hz. The integration of the LRI units on both GFO S/C has been finished in summer 2016. The design as well as the functional, performance, and thermal-vacuum tests results of the German LRI flight units will be presented.
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