Open Access. © 2019 S. Di Pace et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License Open Mater. Sci. 2019; 5:12ś18 Research Article Sibilla Di Pace*, Arwa Dabbech, Vitalii Khodnevych, Michel Lintz, and Nicoleta Dinu-Jaeger Particle contamination monitoring in the backscattering light experiment for LISA https://doi.org/10.1515/oms-2019-0002 Received Dec 10, 2018; accepted Mar 25, 2019 Abstract: In the context of space-based optics, contamina- tion due to particle deposition on the optics is inevitable and constitutes a critical issue. This gets more challenging for the sensitive heterodyne measurements of the Laser In- terferometer Space Antenna (LISA), the space-based grav- itational wave observatory to be launched in 2034. There- fore, table-top experiments need to be developed for a bet- ter understanding of how micrometer to millimeter sized dust particles, present on optical surfaces, afect LISA mea- surements. In this work, we present an experimental set- up for the simultaneous measurement of the coherent backscattering and the monitoring of particles deposition on the optics to be tested. The results of the frst measure- ments are presented and discussed in this article. Keywords: Straylight, backscattering, heterodyne, space, interferometer 1 LISA and the back-scattered light issue The Laser Interferometer Space Antenna (LISA) is a L-class ESA mission, starting the phase A. It will be a space-based gravitational wave (GW) observatory composed of a con- stellation of three spacecrafts (S/C) in equilateral confgu- ration, orbiting at 1A.U. around the Sun [1, 2]. Each space- craft will host and constantly follow two free-falling test masses (TMs) only subjected to the gravitational force. The *Corresponding Author: Sibilla Di Pace: Laboratoire ARTEMIS, Université Côte d’Azur, Observatoire de la Côte d’Azur and CNRS, Nice, France; now at Università degli Studi di Roma La Sapienza e INFN Sezione di Roma, Rome, Italy; Email: sibilla.dipace@roma1.infn.it Arwa Dabbech: Institute of Sensors, Signals and Systems, Heriot- Watt University, Edinburgh, United Kingdom Vitalii Khodnevych, Michel Lintz, Nicoleta Dinu-Jaeger: Labo- ratoire ARTEMIS, Université Côte d’Azur, Observatoire de la Côte d’Azur and CNRS, Nice, France presence of seismic noise prevents the observation of GW sources below 1Hz on Earth [3]. This issue can be over- come by going to space. LISA sensitivity curve will cover the frequency range from 0.1mHz to 1Hz, that is a window of observation complementary to those of ground based GW interferometers. Therefore, LISA will be able to observe other important sources, such as GWs from cosmological stochastic background, and sources detectable from Earth years or months before they enter in the frequency range of the ground based GW detectors [1, 2]. Moreover, thanks to its orbital confguration, LISA will be able to detect GW sources coming from all directions in the sky. The LISA mission has two main conceptual basic functions and technological challenges [1, 2]: ś drag-free system: μ-thrusters to adjust the S/C posi- tion to keep the test masses (TMs) in free-falling con- dition; ś interferometric measurement to monitor arm-length changes within a noise of ∼ 10pm/ √ Hz in the range 0.1 mHz-1 Hz. The former has been successfully tested by the technol- ogy demonstrator of LISA, the LISA Pathfnder (LPF), while the interferometric measurement requirements of LISA [2] could not be tested with the single satellite of LPF. In LISA, S/Cs will be identical and each one will host two laser links, two optical benches and two telescopes. Each telescope will transmit the beam towards the distant S/C and will receive back the signal from the distant S/C. The transmit- ted power will be T x ∼ 2W while the received power will be R x ∼ 700pW only. Therefore, if the transmitted power is partially backscattered (due to optics imperfections: roughness, dusts, etc.) into the optical bench and reaches the detection photodiodes, this may seriously afect the sensitive heterodyne measurements of LISA. The inter- ferometric measurement to monitor arm-length changes should have a noise lower than 10pm/ √ Hz [2], which im- plies a noise allocation of less than 1 μcycle / √ Hz. This is- sue sets an important requirement: even stray beams with a relative power as low as a few 10 −12 can signifcantly de- grade the noise in the arm-length measurement. This crit- ical issue should be investigated in detail at the present time as well as in the next years. Therefore, table-top ex-