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Scattered light studies for the LISA optical metrology system

Abstract : The Laser Interferometer Space Antenna (LISA) is a space-based gravitational wave observatory now in Phase A. The measurements of the gravitational waves are performed by precise heterodyne interferometric measurements. If any light, which was not intended into the design (stray light), couples to the interfering beams, the measurements will be perturbed. Scattered light differs from other types of stray light (such as diffraction at apertures, stray reflection or transmission) in that it results from an unknown roughness profile or dust contamination distribution: no exact evaluation is possible. The thesis is dedicated to the studies of the consequences of the scattered light on interferometric measurements. When a rough surface is illuminated by a coherent, monochromatic beam of light, a scattering process takes place, and the scattered light shows a grainy structure called speckle. A similar pattern is also observed in the case of scattering from particulate contamination, or due to the irregularities in the structure of optical fibers. This thesis is devoted to the study of the scattering of coherent light, and the perturbation of the readout of an interferometer due to the presence of scattered light. For these studies of coherent light scattering, I use two approaches: numerical modeling and experimental measurements. I have developed a numerical model of coherent scattering due to microroughness. It is in agreement with the Harvey-Schack model of the Bidirectional Reflectance Distribution Function (BRDF). From the other side, it correctly describes the observed features of coherent scattering: amplitude and intensity distribution, the spatial dimension of the single speckle grain. Another numerical model, which I have developed, is used to describe coherent backscattering in optical fibers. The result of the model coincides with the conventional, incoherent model. In addition to this, it correctly describes the features of coherent scattering observed on an experiment made at the Albert Einstein Institut in Hannover: intensity distribution and temperature change rate.Two fibered, homodyne interferometric setups (at 1.55 µm and 1.06 µm) were built for experimental studies of the coherent scattering. These studies' necessity is driven by the need for an accurate description of coherent scattering effects in interferometric setups, such as LISA. Both setups have demonstrated the presence of a speckle type response. A signal processing algorithm was specially developed to measure low backscattering values from the optical surfaces. The measurement floor of the 1.06 µm setup reaches 10-13 in relative power, and 10-5 1/sr in BRDF, which matches modern, state-of-the-art BRDF meters.The same experimental setups were used to study coherent scattering due to contamination. The results of the measurements were compared with the Mie scattering theory. Besides this, I have used conventional methods to study scattered light due to micrometeoroid damage. The impact on an optical surface by a micrometeoroid gives rise to a specific type of stray light inherent only in space optical instruments. This causes a double source of light scattering: the impact crater, and the ejected contamination. I propose a method of stray light estimation and apply it to the case of the LISA telescope. I have estimated upper limits for the backscattering fraction for nominal (4 years) and extended (10 years) mission durations.This work brings an ensemble of experimental and modeling studies that improve the knowledge of the properties of coherently scattered light, and its consequences in high precision interferometric instruments.
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Vitalii Khodnevych. Scattered light studies for the LISA optical metrology system. Instrumentation and Methods for Astrophysic [astro-ph.IM]. Université Côte d'Azur, 2020. English. ⟨NNT : 2020COAZ4044⟩. ⟨tel-03177562⟩

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