Noise characterisation, analysis and mitigation at the AEI 10 m prototype
Creators: name: Lehmann, Johannes
Publisher: name: Hannover : Institutionelles Repositorium der Leibniz Universität Hannover
Year: 2024
Subjects: subject: Ground-based gravitational wave detector
subject: AEI 10 m prototype
subject: noise investigation
subject: optical layout
subject: contains research data
subject: Erdgebundene Gravitationswellendetektoren
subject: AEI 10 m Prototyp
subject: Rauschuntersuchung
subject: optisches Layout
subject: enthält Forschungsdaten
subject: Dewey Decimal Classification::500 | Naturwissenschaften::530 | Physik

description: The direct measurement of gravitational waves offers an entire new way to detect astronomic events and is extremely valuable for scientific research. It is made possible by highly sensitive ground based laser interferometers detecting small length changes in km-scale arms. A further improvement of those detectors would allow to measure features of already detectable sources more precisely and increase their detection range, but also to find new types of sources. The Albert Einstein Institute (AEI) 10m prototype is a facility where new technologies for gravitational wave detectors can be developed and tested. Its core instrument is an interferometer whose sensitivity is foreseen to be limited by the standard quantum limit (SQL) between 65 Hz-200 Hz. To achieve this sensitivity, numerous subsystems are required to suppress all other noise sources. In order to investigate the performance of those subsystems in the absence of the high-quality interferometer mirrors, a linear suspended cavity called single arm test (SAT) was set up. In the scope of this thesis, a detailed characterisation of the 10m prototype with the help of the SAT was performed. The initial noise budget of the SAT was calculated based on known noise sources. An experimental investigation showed other disturbances affecting the SAT and its sensitivity was gradually improved by reducing their influence. It was estimated for each excess noise source, if a further reduction is necessary on the way towards a Sub-SQL interferometer. Stray light from view ports of the vacuum system and from detection optics outside of it was identified to have a strong coupling path to the readout of the interferometer. It was reduced effectively by attenuating the beams as much as possible on the isolated optical tables in the vacuum system before they pass the view ports. Another critical disturbance is alignment jitter of the input beam due to mechanical resonances of mirror holders and of the optical table which they are attached to. An input mode cleaner (IMC) was installed to attenuate the alignment jitter and the design choices and requirements are explained. The IMC is a short linear cavity consisting of two separately suspended mirrors, resulting in a strong passive isolation in the measurement band of the Sub-SQL interferometer. Since alignment jitter of the input beam only causes length noise for a statically misaligned interferometer, different approaches to improve the alignment stability were also investigated. Apart from better wavefront sensors, compact interferometric sensors can improve the isolation system by enhancing the sensitivity of inertial sensors or by improving the damping of multi-stage suspensions. The usability of homodyne quadrature interferometers (HoQIs) for this purpose was demonstrated in a vertical inertial sensor and at the triple suspension of the main beam splitter. The analysis of the inertial sensor shows that the simple mechanical design of its test mass suspension leads to a temperature dependent cross coupling from tilt motion. The suspended steering mirrors in the 10m prototype are not influenced by this effect and could be converted to potent inertial sensors without adding mass to the system. Based on the results of the SAT, a new optical layout of the 10m prototype is presented with a detailed explanation of the choices. This includes the new IMC and multiple mechanical improvements which will further reduce the input beam alignment jitter and use the limited payload of the isolation tables more efficiently. The layout is designed with the goal to minimise the delay before commissioning the Sub-SQL interferometer and to simplify gradual upgrades based on measured requirements. It also gives a starting point for the addition of a balanced homodyne detection (BHD), pointing out challenges and possible solutions.