Techniques in Laser Interferometry for the Detection of Gravitational Radiation

Strain, Kenneth Alexander (1990) Techniques in Laser Interferometry for the Detection of Gravitational Radiation. PhD thesis, University of Glasgow.

Abstract

General relativity leads to the expectation of travelling gravitational waves which manifest themselves as strains in space. Massive relativistic systems are the only sources of gravitational radiation which are expected to be detectable on earth. A range of such sources is reviewed in the first chapter in order to estimate the likely gravitational wave amplitudes at the earth. It is seen that in the frequency range accessible to ground-based detectors (tens of hertz to several kilohertz) the expected strain amplitudes are at most ~ 10 -21. At present the most promising technique for the detection of gravitational radiation is laser interferometry between nearly-free test masses. The signal and noise properties of the simplest configuration of this system are considered to show that the performance can approach that required to detect the predicted sources of gravitational radiation. In order to reach this level of performance more advanced optical techniques are needed and these are discussed in Chapter 2. The lasers used to illuminate the interferometers have excess intensity noise at low frequencies and this forces all measurements to be made using high frequency (at least several MHz) modulation techniques. Two modulation and signal recovery schemes for laser interferometers (internal and external modulation) are compared. It is seen that external modulation should provide the same signal as internal modulation but without the need to add lossy modulation components to the arms of the interferometer. The effect of the modulation technique on the signal to noise ratio of the interferometer is evaluated and it is found that both the technique used and the particular modulation waveform can alter the potential signal to noise ratio slightly. There is an optimum length of the arms of an interferometer designed to detect gravitational waves at a given frequency. The time the light spends in each of the arms should be half of the period of the gravitational wave (i. e. several milliseconds). This can be achieved by multiple reflection of the light along arms (perhaps several km in length) using optical delay-lines or Fabry-Perot cavities. Some aspects of both of these schemes are considered in this thesis. The application of internal modulation to an interferometer with cavities in its arms is considered. The signals which can be expected from this system are investigated to reveal if there should be any problems with the control of such an interferometer. It is found that there should be no unexpected control problems. The very important techniques of power and dual recycling should both allow the performance of a basic interferometer to be enhanced considerably. Both of these are discussed in Chapter 2 in order to enable an experiment to be done to test the optical properties of these techniques and to test suitable control systems. Power recycling is concerned with the maximisation of the light amplitude in the interferometer for a given laser power by making the laser light resonant in the system. Dual recycling provides a method of enhancing the frequency response of the interferometer, especially at low frequencies or in a narrow range of frequencies, by resonating the signal sidebands. In this respect it can replace the need for cavities in the arms of the interferometer and also provides a method of optimising the detector response if the frequency of an expected signal is known. The techniques presented in Chapter 2 were tested as described in Chapter 3. The results suggested that the behaviour of the optical systems were understood and that control systems could be made to operate these relatively complicated arrangements. The effects of internal modulation on measurement noise were also confirmed. The above systems require extremely low loss mirrors. A method based on the measurement of the storage time of a Fabry-Perot cavity was adapted to allow the losses of such mirrors to be evaluated. This has revealed that sufficiently low loss mirrors are now becoming available in the large sizes required for gravitational wave detectors. (Abstract shortened by ProQuest.).

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Astrophysics, Optics
• Date of Award: 1990
• Depositing User: Enlighten Team
• Unique ID: glathesis:1990-78244
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 30 Jan 2020 15:35
• Last Modified: 30 Jan 2020 15:35
• URI: https://theses.gla.ac.uk/id/eprint/78244