Optical Suppression of Tilt-to-Length Coupling in the LISA Long-Arm Interferometer
Michael Chwalla, K. Danzmann, M. Dovale Álvarez, Juan José Esteban Delgado, Germán Fernández Barranco, E. D. Fitzsimons, Oliver Gerberding, Gerhard Heinzel, C. J. Killow, M. Lieser, M. Perreur-Lloyd, D. I. Robertson, Johann Max Rohr, Sönke Schuster, Thomas S. Schwarze, Michael Tröbs, Gudrun Wanner, R. L. Ward
Abstract
The arm length and the isolation in space enable the Laser Interferometer Space Antenna (LISA) to probe for signals unattainable on the ground, opening a window to the subhertz 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/Hz. An ultrastable interferometer test bed 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 test bed 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 the introduction of two- and four-lens imaging systems. TTL coupling factors below ±25μm/rad for beam tilts within ±300μrad 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. © 2020 authors. Published by the American Physical Society.