Development of an optical test facility with a toolset for adjustable picometer-stable interferometers for space-based gravitational-wave detectors

Abstract

Space-based gravitational-wave detectors, such as the Laser Interferometer Space Antenna (LISA), employ picometer-precision laser interferometry to detect gravitational waves from 1 Hz down to below 0.1mHz. These high-precision observatories enable diverse science cases, including detailed studies of massive black hole growth mechanisms and their pivotal role in galaxy formation and cosmic evolution. Ground-based prototypes of such interferometers are typically constructed by permanently bonding optics to an ultra-stable bench made of low-expansion glass ceramic, minimizing dominant temperature coupling at low frequencies. This thesis presents the study of an alternative optomechanical concept in which optical components are mounted on an ultra-stable benchusing adjustableand freely positionablemounts, whilemaintaining picometer-level length stability. The concept, referred to as toolset for adjustable picometer-stable interferometers (TAPSI), enables the realization of diverse interferometer configurations and facilitates a simplified and faster assembly process. In addition, an optical test facility was constructed to operate the interferometer setup within a seismically isolated and thermally insulated environment. Furthermore, a temperature sensor was developed that enables the characterization of the K-stable thermal conditions and investigations of temperature coupling to the interferometer. This thesis characterizes the length stability of the toolset by setting up a prototype cavity and measuring its displacement noise using two laser-cavity locking techniques, Heterodyne laser frequency Stabilization (HS) and Pound-Drever-Hall (PDH) locking. While the HS scheme was limited by demodulation phase noise, PDH demonstrated picometer-level stability, meeting LISA requirements. The interferometer toolset was subsequently employed to realize and investigate a low-frequency homodyne quadrature interferometer displacement sensing concept, a strong candidate for precursor missions of future gravitational-wave detection concepts like the Lunar Gravitational-Wave Antenna (LGWA), designed to measure gravitational waves via lunar seismic motion. The sensor achieved picometer-level performance that meets the precursor mission (LGWA Soundcheck) requirements and was limited only by electronic readout noise. Another interferometer, referred to as the Concealed Noise experiment, was set up to characterize the noise floor of sub-picometer-precision heterodyne interferometers operating at MHz frequencies. Combined theoretical modeling and experimental validation of this setup will contribute to a comprehensive noise budget, advancing understanding of individual noise sources essential for risk mitigation in space-based missions like LISA and for the design of future missions with higher sensitivity. In conclusion, the picometer-level stability and rapid experimental commissioning demonstrated in this thesis confirm the suitability of the toolset for realizing flexible optical ground support equipment setups for space-based gravitational-wave detectors and other applications requiring ultra-stable, low-frequency laser interferometry.