Abstract
The arm length and the isolation in space enable LISA to probe for signals unattainable on ground, opening a window to the sub-Hz gravitational-wave universe. The coupling of unavoidable angular spacecraft jitter into the longitudinal displacement measurement, an effect known as tilt-to-length (TTL) coupling, is critical for realizing the required sensitivity of picometer$/\sqrt{\rm{Hz}}$. An ultra-stable interferometer testbed has been developed in order to investigate this issue and validate mitigation strategies in a setup representative of LISA, and in this paper it is operated in the long-arm interferometer configuration. The testbed is fitted with a flat-top beam generator to simulate the beam received by a LISA spacecraft. We demonstrate a reduction of TTL coupling between this flat-top beam and a Gaussian reference beam via introducing two- and four-lens imaging systems. TTL coupling factors below $\pm 25\,\mu$m/rad for beam tilts within $\pm 300\,\mu$rad are obtained by careful optimization of the system. Moreover we show that the additional TTL coupling due to lateral alignment errors of elements of the imaging system can be compensated by introducing lateral shifts of the detector, and vice versa. These findings help validate the suitability of this noise-reduction technique for the LISA long-arm interferometer.
Highlights
The direct sensing of gravitational effects by tracking the motion between freely moving masses is at the center of experimental gravitational physics, with various exciting results having been achieved in recent years
The coupling of angular noise into the longitudinal-pathlength readout is an aspect of the utmost importance in space interferometers such as Laser Interferometer Space Antenna (LISA) Pathfinder, GRACEFO, and LISA
We demonstrate the use of imaging systems to reduce TTL coupling in a setup representative of the LISA long-arm interferometer, a step toward validating this noise-reduction strategy for LISA
Summary
The direct sensing of gravitational effects by tracking the motion between freely moving masses is at the center of experimental gravitational physics, with various exciting results having been achieved in recent years. The discovery consolidated laser interferometry as a suitable technology for gravitationalwave detection and strengthened the revolutionary scientific value and discovery potential of a deep-space gravitational-wave observatory capable of listening to subhertz gravitational-wave signals, such as the Laser Interferometer Space Antenna (LISA) [3,4]. On May 22, 2018, the National Aeronautics and Space Administration (NASA) and the German Research Centre for Geosciences (GFZ) launched GRACE FollowOn (GRACE-FO), a twin-satellite gravity-exploration mission carrying a laser ranging instrument, successfully
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