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Commissioning of advanced, dual-recycled gravitational-wave detectors: simulations of complex optical systems guided by the phasor picture


Malec,  Michaela
Laser Interferometry & Gravitational Wave Astronomy, AEI-Hannover, MPI for Gravitational Physics, Max Planck Society;
AEI-Hannover, MPI for Gravitational Physics, Max Planck Society;

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Malec, M. (2006). Commissioning of advanced, dual-recycled gravitational-wave detectors: simulations of complex optical systems guided by the phasor picture. PhD Thesis, Gottfried Wilhelm Leibniz Universität Hannover, Hannover.

Cite as: https://hdl.handle.net/11858/00-001M-0000-0013-4A8B-5
Currently, the data acquisition of an international network of large, laser-interferometric gravitational-wave detectors is about to begin. This preludes a new form of astronomy; gravitational-wave astronomy. The British-German gravitational-wave detector GEO600 is the only detector of this network featuring signal recycling (SR), an optical technique that allows for increasing the sensitivity in a particular frequency band, at the expense of other, surrounding frequencies. This is achieved by resonantly enhancing the gravitational-wave signals of an arbitrary frequency region inside the so-called signal-recycling cavity, formed by a Michelson interferometer (MI), and a SR mirror at the MI output port. The reflectivity of the SR mirror determines the bandwidth of the enhanced region, the microscopic position, the mid-frequency. The mid-frequency, or detector tuning, of GEO600 can already systematically be set during the operation within a range of 2 kHz. In future, the detector bandwidth may also be customised, replacing the conventional SR mirror by an etalon whose reflectivity is adjustable. A basic requirement for a reliable long-term operation of these observatories is a permanent control of all degrees of freedom. The respective control signals are gained by modulation/demodulation techniques applied to light. In an advanced optical system like GEO600, however, these control signals depend on several degrees of freedom at the same time. Altering, for example, the position or reflectivity of the SR mirror during detector operation instantaneously changes the properties of the control signals of other degrees of freedom. Within the scope of this thesis, the control signals of GEO600 and the corresponding light fields were investigated using the program Finesse. In order to yield operating points that agree with the experiment, the differential MI armlength, and the resonator lengths were determined with an accuracy of at least ±1mm. Beyond, we managed to calibrate the SR and MI demodulation phases with ±2◦ precision. Using these input parameters, the shape of the signal enhancement of GEO600 can be predicted with a deviation of less than 5% from the experiment, within a region of 2 kHz around the respective tuning frequency. Furthermore, a matrix was generated by simulation that contains demodulation phase and gain settings for the SR and MI control loops, enabling a quasi-continuous tuning of the detector. In comparison to an experimental parameter determination, the simulation allows for a more targeted and faster optimisation of the loop parameters. Another part of this thesis is dedicated to the improvement of the sensitivity of GEO600. On the one hand, employing the phasor picture allowed for a global examination of the sensitivity dependency on the resonance conditions of the MI control sidebands. Increasing the currently used sideband frequency, by 33Hz only, can globally enhance the sensitivity for low gravitationalwave frequencies by up to 30%. alOn the other hand, analysing the coupling mechanisms of noise sources into the detector output supported the identification of particular sources that limited the detector sensitivity. For the laser-amplitude noise coupling, for example, the noise-sideband resonances play a decisive role. This insight helped to find another source exhibiting similar coupling features, namely modulationindex noise. With an etalon taking the place of the SR mirror, the control signal features change in comparison with the conventional-mirror configuration. However, simulations indicate that these changes do not compromise the detector operation nor the process of tuning. When adjusting the reflectivity of the etalon in a GEO600 configuration similar to the current, the distance between the etalon surfaces should, due to control reasons, be increased rather than decreased.