Aspects of suspension design for the development of advanced gravitational wave detectors

Field: Physics and Astronomy (Physical Sciences)

Gravitational waves are considered as ripples in the curvature of space-time and were predicted by Einstein in his general theory of relativity. Gravitational waves interact very weakly with matter which makes them very difficult to detect. However, research groups around the world are engaged in building a network of ultra sensitive ground and space based interferometers for the first detection of these signals. Their detection will open a new window in the field of astronomy and astrophysics.
The nature of gravitational waves is such that when incident on a particle, they stretch and squeeze the particle orthogonally thus producing a tidal strain. The strain amplitude expected for gravitational waves which may be detected on earth are of the order of hrms ~10-22 to 10-23 (over a frequency range from few Hz to a few kHz). A network of instruments based on the Michelson interferometer design currently exists around the world. These detectors are undergoing a major upgrade and once online by 2015-16 the improved sensitivity and increased sky coverage may lead to the first detection of the gravitational waves signals.
The Institute for Gravitational Research in the University of Glasgow in collaboration with the Albert Einstein Institute in Hannover, Golm and the University of Cardiff has been actively involved in the research for the development of instruments and data analysis techniques to detect gravitational waves. This includes construction of a long ground based interferometer in Germany called GEO 600 (upgraded to GEO-HF) having an arm length 600 m and strong involvement in the larger detectors of the LIGO (Laser interferometer gravitational wave observatory) project in USA having arm lengths of 4 km (Operated by MIT, Boston and CALTECH, Pasadena). An upgrade to LIGO called Advanced LIGO (aLIGO) is currently under construction with significant input from the University of Glasgow.
Thermal noise is one of the most significant noise sources affecting the sensitivity of the detector at a range of frequencies. Thermal noise arises due to the random fluctuations of atoms and molecules in the materials of the test mass mirrors and suspension elements, and is related to mechanical loss in these materials.
The work presented in chapter 3 of this thesis is devoted to the analysis of aspects of mechanical loss and thermal noise in the final stages of the GEO suspension. GEO-600 is currently undergoing an upgrade to GEO-HF targeting sensitivity improvements in the kiloHertz region. However, the planned upgrade requires access to the vacuum tanks enclosing the fused silica suspension system. There is a risk of damaging the suspension, which has led to a repair scenario being developed in Glasgow, to reduce the downtime of the detector. An optimised design of the fused silica fibre has been proposed. A study of mechanical loss has been undertaken through Finite Element Analysis (FEA) modeling techniques. The mechanical loss of the optimised fibre is estimated to be lower than the original GEO fibre by a factor of ~4. In terms of thermal noise performance the optimised fibre gives an improvement of ~1.8.
The repair scenario of the monolithic suspension has led to the development of tools and welding procedures. Three prototype suspensions involving metal masses were successfully built, before fabricating the monolithic fused silica suspension in Glasgow.

The work in chapter 4 focuses on the theory of photoelasticty and birefringence techniques. The production and use of various forms of polarised light has been discussed. A setup of plane and a circular polariscope using two polarisers and two-quarter wave plates has been shown. The retardation of light due to the birefringence in the sample can be measured using the Tardy method of compensation and a Babinet-Soleil compensator. Finally a discussion on the stress-optic law has shown that the relative stress in a sample can be measured once the retardance is known.

The silica fibres in the aLIGO detector would be laser welded using a 100 W CO2 laser. The laser welding would lead to high temperature and development of thermal gradients. This could result in residual thermal stress in fused silica, which could lead to an additional mechanical loss. A study of mechanical and thermal stress induced in fused silica has been discussed in chapter 5 of this thesis. To understand the working of photoelastic techniques learned in chapter 4, a study of mechanical stress was undertaken by applying a load on the sample to induce temporary birefringence. The estimated values of stress showed a good agreement when compared with the theoretical predictions and FEA modelling. Thermal stress was induced in fused silica by applying a 25 W CO2 laser beam for 10 seconds and the relative stress was measured using photoelastic birefringence techniques. Thermal modelling of the stressed sample was performed using the techniques developed in FEA. The experimental values show a good agreement with the estimated 1st principal stress (FEA model) and equivalent stress. A study of thermal stress in fused silica welds has also been presented in chapter 5. Two fused silica samples were welded using CO2 laser welding and the relative stress at different points were measured. The stress in the weld region was measured to be relatively lower than other areas. At a distance of 3 mm away from the weld line the maximum stress was measured which was greater than the stress in the weld region by a factor of ~5.
The work discussed in chapter 6 focuses on the study of the suspension thermal noise in aLIGO detector for applying incremental upgrades. To further enhance the sensitivity of the aLIGO detector, incremental upgrades could be applied to the suspension system to improve the thermal noise. The incremental upgrades focused on two aspects: improving the dissipation dilution factor, and obtaining a lower mechanical loss than the aLIGO baseline. Based on the results from FEA, two designs were compared, each having a suspension of length 100 cm but different stock diameter - 3mm and 5 mm. A comparison with the aLIGO baseline showed that these two models obtained a lower mechanical loss by a factor of 3.4 to 6.8. In terms of suspension thermal noise there was an improvement by factor of 2.5 to 3.7, which could lead to rise in the sensitivity of the detector by a factor of 2.5.