Christoph Mahrdt

Laser ranging interferometer on Grace follow-on

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.

Laser link acquisition demonstration for the GRACE Follow-On mission

We experimentally demonstrate an inter-satellite laser link acquisition scheme for GRACE Follow-On. In this strategy, dedicated acquisition sensors are not required-instead we use the photodetectors and signal processing hardware already required for science operation. To establish the laser link, a search over five degrees of freedom must be conducted (± 3 mrad in pitch/yaw for each laser beam, and ± 1 GHz for the frequency difference between the two lasers). This search is combined with a FFT-based peak detection algorithm run on each satellite to find the heterodyne beat note resulting when the two beams are interfered. We experimentally demonstrate the two stages of our acquisition strategy: a ± 3 mrad commissioning scan and a ± 300 μrad reacquisition scan. The commissioning scan enables each beam to be pointed at the other satellite to within 142 μrad of its best alignment point with a frequency difference between lasers of less than 20 MHz. Scanning over the 4 alignment degrees of freedom in our commissioning scan takes 214 seconds, and when combined with sweeping the laser frequency difference at a rate of 88 kHz/s, the entire commissioning sequence completes within 6.3 hours. The reacquisition sequence takes 7 seconds to complete, and optimizes the alignment between beams to allow a smooth transition to differential wavefront sensing-based auto-alignment.

Subtraction-noise projection in gravitational-wave detector networks

In this paper, we present a successful implementation of a subtraction-noise projection method into a simple, simulated data analysis pipeline of a gravitational-wave search. We investigate the problem to reveal a weak stochastic background signal which is covered by a strong foreground of compact-binary coalescences. The foreground, which is estimated by matched filters, has to be subtracted from the data. Even an optimal analysis of foreground signals will leave subtraction noise due to estimation errors of template parameters which may corrupt the measurement of the background signal. The subtraction noise can be removed by a noise projection. We apply our analysis pipeline to the proposed future-generation space-borne Big Bang Observer mission which seeks for a stochastic background of primordial gravitational waves in the frequency range � 0: 1H z— 1H zcovered by a foreground of black-hole and neutron-star binaries. Our analysis is based on a simulation code which provides a dynamical model of a time-delay interferometer network. It generates the data as time series and incorporates the analysis pipeline together with the noise projection. Our results confirm previous ad hoc predictions which say that the Big Bang Observer will be sensitive to backgrounds with fractional energy densities below � ¼ 10 � 16 .

Simulating laser interferometers for missions such as (E)Lisa, Lisa pathfinder and Grace follow-on

Sensing tiny distance variations interferometrically will be a key task in several future space missions. Interferometric detectors such as (e)LISA will observe gravitational waves from cosmic events such as for instance super novae and extreme mass ratio inspirals. The detection principle of such detectors is sensing phase variations due to tiny distance variations between two free floating test masses aboard two remote spacecraft originating from passing gravitational waves. This detection principle will be tested for the first time by LISA Pathfinder (launch ~2015), where the interferometric readout of two free floating test masses aboard one single spacecraft will be demonstrated. Future geodesy missions will map Earths Gravity field, by interferometrically measuring distance variations between two spacecraft in low Earth orbit. This will be tested for the first time by the Laser Ranging Instrument (LRI) aboard GRACE Follow-On (launch ~2017). The low noise laser interferometry of all these missions provides a number of challenging tasks. We will present optical simulations performed for the missions above. The interferometry of LISA Pathfinder is purely local (there do not exist any received beams from remote spacecraft), such that all beams can be approximated by fundamental Gaussian beams. We will present simulations regarding the coupling of residual test mass jitter (longitudinal and lateral as well as angular) to the phase readout, including Monte Carlo simulations to predict how misalignment affects resulting phase noise and estimate in-flight alignment of the test masses. In all of the mentioned missions, the local laser beams are delivered to the optical bench by fibers, resulting in laser beams in fiber modes. Besides local laser beams, the interferometry of missions such as (e)LISA and LRI involves also received beams from remote spacecraft. These beams have diameters in the range of tens of meters (LRI) or kilometers (LISA / eLISA and alike), before being clipped down to centimeter scale by the receiving aperture. The resulting top hat beams show strong diffraction effects and are therefore imaged on the optical benches. Key elements for simulations are therefore the propagation with diffraction of top hat beams and fiber modes in vacuum, as well as imaging optics causing aberration and astigmatism, with the central task to characterize the coupling of test mass or spacecraft jitter to optical readout noise, in presence of realistic alignment errors. A recurring and often limiting noise in the length measurement originates from the cross coupling of angular component jitter. This cross coupling will be briefly introduced with strategies for its mitigation in the various missions. To overcome the limitations of existing and commercial software, we have written and used for the simulations above as well as for general interferometer design purposes a dedicated software package called IfoCAD which is publicly available and will be presented as well.

GRACE-Follow On Laser Ranging Interferometer: German contribution

The Gravity Recovery and Climate Experiment (GRACE) is a joint US/German mission that has been mapping the Earths gravity �eld since 2002 by measuring the distance variations between two spacecraft using a micro-wave link. GRACE is reaching the end of its lifetime. For this reason and in order to minimize data gaps, an almost identical mission will be launched in 2017. This mission is called GRACE-Follow On (GRACE-FO) and it will include an additional instrument as a technological demonstrator to monitor distance changes between the spacecraft. This instrument is the Laser Ranging Interferometer (LRI), which is based on heterodyne laser interferometry at 1064nm and takes advantage of many technologies developed for LISA. In this paper a short overview of the current status of the German contribution is presented.