Mirror suspensions for the Glasgow Sagnac speed meter

Hennig, Jan-Simon (2018) Mirror suspensions for the Glasgow Sagnac speed meter. PhD thesis, University of Glasgow.

Abstract

A new era of gravitational wave astronomy has begun with the first direct detections of gravitational waves from the collision of binary black holes and a binary neutron star system. The scientific outcomes from these detections have been magnificent, however in order to increase the event rates for known sources, to be sensitive to new sources, to detect sources at greater distances, and to increase the signal to noise ratio for better extraction of source parameters, further research is required to increase the detectors sensitivity. The Advanced LIGO and Advanced Virgo detectors that enabled these first detections will ultimately be limited in their sensitivity by reaching the standard quantum limit (SQL). One novel technique to reduce the influence of quantum radiation pressure noise in a measurement of strain between two test masses is the speed meter topology. As a proof of concept experiment the Glasgow Sagnac Speed Meter experiment aims to show a reduction in quantum radiation pressure noise compared to an equivalent Michelson interferometer at audio-band frequencies. Two triangular cavities are the core of the experiment and consist of two 100g end test masses and one 1g input test masses per cavity, all suspended from multistage pendulums. In this combination the whole Sagnac Speed Meter experiment should be limited by quantum radiation pressure noise from about 100Hz to 1kHz and it is expected to achieve a reduction of quantum radiation pressure noise by a factor of 3-5 compared to an equivalent Michelson interferometer.

This thesis presents the development, design, commissioning and testing of the three main types of suspensions in the Sagnac Speed Meter experiment. The longitudinal displacement noise requirement for both cavity suspension types is <1.5 x 10-18m/√Hz over the measurement band between 100Hz and about 1kHz. In order to isolate the mirrors from seismic ground motion in the Sagnac Speed Meter experiment, they are suspended from multistage pendulums, resulting ideally in a 1/f^2n response for n pendulum stages above the pendulums rigid body modes. Reduction of thermal noise in the suspension elements (suspension thermal noise) is achieved by the introduction of high quality-factor materials in the lowest pendulum stage, making it fully monolithic. The 100g end test mass suspension is based on an existing design, originally developed for the AEI 10m prototype, as a triple suspension with two stages of vertical blade springs and a fully monolithic lowest pendulum stage. The 1g input test mass suspension, designed as a quadruple pendulum with a fully monolithic lowest pendulum stage, utilises the same vertical blade springs and top mass as the 100g end test mass suspension. The quadruple pendulum design enables passive damping of test mass motion at the penultimate stage. As passive damping introduces force noise due to thermal noise, a switchable passive damping system was developed and tested to mitigate limitation by this force noise. The auxiliary suspension, a double pendulum, serves to suspend the mirrors in the experiment that guide the beam towards the Sagnac Speed Meter, in between the cavities, and towards the balanced homodyne detector. As these are not part of the cavities, the longitudinal displacement noise requirement can be relaxed to <8 x 10-15m/√Hz at 100Hz.

The pendulum dynamics of the auxiliary and 100g end test mass suspension were measured in an optical lever set up and, in case of the auxiliary suspension, additionally with a vibrometer. With these measurements, the models were adjusted and could be used to estimate the longitudinal displacement noise due to coupling from seismic ground motion and thus verify the required performance of the suspensions. The research conducted in this thesis is an important step towards establishing the speed meter topology for consideration in future gravitational wave detectors. The developments in the scope of the monolithic assembly for the 100g end test masses will be applied to the AEI 10m prototype in order to enable sub-SQL measurements.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Gravitational waves, speed meter, suspensions, bonding, interferometry, quantum non-demolition, seismic noise, thermal noise, fused silica fibres, laser, sagnac.
• Subjects: Q Science > Q Science (General); Q Science > QB Astronomy; Q Science > QC Physics
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Supervisor's Name: Hild, Professor Stefan and Strain, Professor Kenneth
• Date of Award: 2018
• Depositing User: Jan-Simon Hennig
• Unique ID: glathesis:2018-8984
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 20 Apr 2018 11:19
• Last Modified: 09 May 2018 07:31
• URI: https://theses.gla.ac.uk/id/eprint/8984