LIGO Publication Abstract
A Study of Thermal Noise
Kovalik, J.M.
PhD Thesis, MIT (1994)

Thermal noise will be a fundamental limit to the sensitivity of the Laser Interferometer Gravitational Wave Observatory (LIGO) in the frequency band where astrophysical sources should be detected. A study of thermal noise in mechanical systems helps to predict the noise floor of high sensitivity experiments such as LIGO and also gives insight to the loss mechanisms in macroscopic systems. This thesis investigates the thermal noise in wires that support the test masses of a gravity wave detector and in the internal normal modes of the test masses themselves. The thermal noise of a pendulum is calculated by considering the losses in the flexure of the thin fibres that support the pendulum mass. An experimental investigation of thermoelastic damping was done by measuring the Q's of thin fibres made of tungsten, sapphire, silicon and fused quartz. Tungsten had the highest losses with Q's on the order of 10^3. Fused quartz had the lowest losses with Q's between 10^5-10^6. The results indicate that thermoelastic damping is at best only an upper limit for the Q of a wire. The internal thermal noise of the gravity wave detector test masses depends upon the frequency dependence of the loss mechanism in the test mass material (in this case, fused quartz-SiO_2). The design and noise sources of a high sensitivity special purpose interferometer to measure the thermally excited motions in a thin disk of fused quartz are presented. The RMS thermally driven motion of the mechanical resonances from normal modes between 8 kHz and 20 kHz was 4x10^-13 cm and the typical Q between 5x10^3 and 10^5. The measured mechanical noise of the system was 2x10^-15 cm/rHz between 1 kHz and 20 kHz which was too large to be attributed to the off-resonance thermal noise from one of the measured mechanical modes of the plate. Various candidates for this noise are presented. Future experiments that would lead to a better understanding of the measured noise are discussed. A possible microscopic model for the loss mechanisms in fused quartz is presented. Finally, a method to monitor the internal thermal noise directly in future advanced gravity wave detectors is discussed. The optical experiment performed in this thesis is a prototype for such a technique.


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