Investigations and Developments Relevant to a Prototype Laser Interferometric Gravitational Wave Detector

Gravitational waves are travelling distortions in the curvature of space-time. They are a prediction of most relativistic theories of gravity, including general relativity. Their coupling to matter is extremely weak and the only sources likely to be detectable are violent astrophysical events where huge amounts of matter are accelerated very rapidly. Gravitational radiation is quadrupole in nature. It causes a tidal strain in space. With the prototype detector at Glasgow we aim to detect this strain by measuring the differential length change induced in two orthogonal reference arms. This is done using optical interferometry. Chapter 1 is a general introduction to gravitational radiation. The first part is a short review of the sources likely to be detectable by terrestrial interferometric detectors and bar detectors. This is followed by a review of the main features of both resonant bar detectors and interferometric detectors, including a discussion of the main noise sources in both types of detector. The frequency of the illuminating laser needs to be stabilised in interferometric gravitational wave detectors and this is dealt with in the next three chapters. Chapter 2 is a review of the methods used to improve the frequency stability of lasers. It concentrates on continuous, single mode gas lasers such as the argon ion laser used to illuminate the detector at Glasgow. In our prototype detector the laser is frequency locked to one of the 10m long Fabry-Perot cavities. To achieve the desired sensitivity the residual frequency noise must be 2 x 10e7 at 1kHz. Chapter 3 describes a new frequency stabilisation system which acheives this performance. The system is narrow band, as would be required for a large scale detector. It employs two nested servo loops to achieve the desired gain. Chapter 4 is an analysis of the gain and stability of the nested servo loop system described in the previous chapter. The next two chapters concern a 100 hour data run with the prototype detector. Chapter 5 describes the preparation for the run. It includes details of the timing system used and a description of each of the signals that were recorded. Chapter 6 describes the preliminary analysis of the data taken during the run. This analysis was carried out to look at the behaviour of the detector rather than to look for signals from astrophysical sources. The variation of the noise level with time, noise statistics of the data and possible contamination of the data by acoustic and seismic noise were investigated. Chapter 7 starts with a discussion of the results of some calculations of the seismic isolation and thermal noise in some single and double pendulum suspension systems. These show that thermal noise in the lowest stage of the suspension system is likely to be dominant. Measurements of the Q of the suspension of the test masses and of the test masses themselves are then described. Appendix A contains diagrams of the circuits used in the frequency stabilisation system described in chapters 3 and 4, some of their special features are commented on. In appendix B the results of intensity noise measurements in three large frame argon ion lasers are given. These show that the level of high frequency intensity noise varies widely depending on the type of plasma tube used. The measurements were taken in the range 0-7MHz. Appendix C contains two of the data analysis programs