Interferometry developments for spaceborne gravitational wave detectors

Killow, Christian J (2006) Interferometry developments for spaceborne gravitational wave detectors. PhD thesis, University of Glasgow.

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The existence of gravitational waves is predicted by Einstein's General Theory of Relativity. They can be considered as a wave-like distortion of four dimensional spacetime. Gravitational waves are produced by systems with time varying quadrupole mass moment. Spacetime is a stiff elastic medium, implying that waves traveling through it will have small amplitudes and this makes their detection very challenging. There are well established efforts towards the detection of gravitational waves using ground-based systems. These detectors are limited by a lower frequency limit of ~ 10 Hz set by the gravity gradient wall, which is a consequence of being in a gravitationally noisy environment. However, there are many predicted sources of gravitational radiation of relatively large amplitude at lower frequencies. Thus to complement the ground based network of detectors a spaceborne detector, the Laser Interferometer Space Antenna (LISA), is planned. Gravitational wave detection by interferometry on Earth involves displacement measurements of order 10[-18]m/√Hz on tens of millisecond timescales and over arm lengths of kilometers. In contrast, LISA requires the monitoring of 5 million kilometer baselines at a noise level of 10[-12]m/√Hz and over 1000 second timescales. So while the displacement sensitivities required of LISA may appear routine in the context of current ground-based detectors, the frequency regime and distances involved introduce new challenges. In order to try and reduce some of the technological risks of LISA, a precursor mission (called LISA Pathfinder) will be flown to demonstrate performance of technologies that cannot be adequately demonstrated on Earth. LISA Pathfinder contains an experiment called the LISA Technology Package (LTP). The work presented in this thesis deals mainly with investigations of the interferometry that will be used in the LTP and in LISA, with particular emphasis on the identification of sources of excess noise and of methods to minimise their effects. LTP will use interferometry to monitor the distance between two inertial proof masses. The goal is to demonstrate the performance of the intertial sensors to within an order of magnitude of that required for LISA. To do this the interferometer sensitivity is relaxed an order of magnitude from the LISA goal but is still technically very challenging. The approach adopted to demonstrate the interferometry for LTP was to build a stable optical bench using hydroxide-catalysis bonding of optical components to a low thermal expansion baseplate. This is the construction approach to be used in LTP and likely to be adopted for LISA. The stability of the optical bench was then tested using an LTP style heterodyne interferometer arrangement and demonstrated to be stable to 10pm/?Hz from 3 mHz to 30 mHz, with the exception of a minor spectral feature of temperature driven excess noise when operated in a laboratory environment. The experience gained by constructing and testing the optical bench strongly influenced the techniques used to construct the engineering model LTP bench and the techniques that will be used for building the flight model.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Kevin Middleton
Keywords: Theoretical physics
Date of Award: 2006
Depositing User: Enlighten Team
Unique ID: glathesis:2006-74077
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 23 Sep 2019 15:33
Last Modified: 23 Sep 2019 15:33

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