Gravitational Radiation Detection with Laser Interferometry

Gravitational-wave detection has been pursued relentlessly for over 40 years. With the imminent operation of a new generation of laser interferometers, it is expected that detections will become a common occurrence. The research into more ambitious detectors promises to allow the field to move beyond detection and into the realm of precision science using gravitational radiation. In this article, I review the state of the art for the detectors and describe an outlook for the coming decades.Comment: 38 pages typos, references update

Breaking the seismic wall: how to improve gravitational wave detectors at low frequency

The era of gravitational-wave astronomy was enabled by the incredible sensitivity of the LIGO and VIRGO detectors. However, they are still plagued by technical noises at frequencies below 30 Hz, driven in part by the limitations of the seismic isolation of the detector. To detect gravitational waves at low frequency, the isolation performance must be improved to reduce these technical noises. To improve the performance of seismic isolation systems, I have developed HoQI a new interferometrically sensor, that can be applied to both the isolation tables and suspensions. HoQI has a resolution a factor 1000 higher than sensors currently used in LIGO and I have quantified the level of non-linearity present in the sensor and shown this to not be a limiting factor. HoQIs impact on the performance of the seismic isolation system has also been quantified, through the use of a accurate model of an Advanced LIGO isolation platform that I have developed. Using the model I have shown that using using HoQI, the expected isolation platform motion can be reduced by a factor of 70 at 0.1 Hz and a factor of 10 at 2 Hz. I have shown that the control filters used in this model can be improved by up to 70% by designing them using particle swarm optimisation

Laser power stabilization via radiation pressure

This thesis reports a new active power stabilization scheme which can be implemented in high precision experiments, such as gravitational wave detectors. The novel aspect of the scheme is sensing laser power fluctuations via the radiation pressure driven motion they induce on a movable mirror. The mirror position and its fluctuations are determined by means of a weak auxiliary beam and a Michelson interferometer, which form an in-loop sensor for the proposed stabilization scheme. This sensing technique exploits the concept of a nondemolition measurement, since the power fluctuations are inferred by measuring the fluctuations in the phase observable of the auxiliary beam. This process results in higher in-loop signals for power fluctuations than what would be achieved by a direct detection, e.g. via the traditional scheme where a fraction of the laser power is picked off and sensed directly by a photodetector. Other advantages of this scheme are that the full beam power is preserved and available for further use, and that it enables the generation of a strong bright squeezed out-of-loop beam. An extensive theoretical investigation on the concept of the new sensing scheme is presented. In this investigation, different schemes in which power fluctuations are transferred to another observable of the light field, e.g. phase or polarization, are compared to each other, and the advantages of the radiation pressure scheme are highlighted. Furthermore, a complete calculation of the fundamental limit of the proposed radiation pressure scheme, set by the quantum noise in the interferometer and the thermal noise of the movable mirror, is performed. The calculations show that a bright squeezed beam with a power of 4W and up to 11 dB of squeezing might be achievable in the near future. Based on the results of the theoretical investigation, a proof-of-principle experiment was realized with microoscillator mirrors with masses ranging from 25 to 250 ng, and fundamental resonance frequencies from 150 to 210 Hz. Power stabilization in the frequency range from 1 Hz to 10 kHz was demonstrated. The results for the out-of-loop power stability are presented for different beam powers, and a relative power noise of 3.7 * 10^−7 Hz^−1/2 was achieved at 250 Hz for 267 mW. The stability performance was limited by the structural thermal noise of the micro-oscillators, which was particularly high due to operation at room temperature. The results from the investigations conducted in this thesis are a promising step towards generation of a strong bright squeezed beam, and towards an improved stabilization scheme to be used in the future generation of gravitational wave detectors

An active fiber sensor for mirror vibration metrology in astronomical interferometers

We present a fiber sensor based on an active integrated component which could be effectively used to measure the longitudinal vibration modes of telescope mirrors in an interferometric array. We demonstrate the possibility to measure vibrations with frequencies up to $\simeq 100$ Hz with a precision better than 10 nm.Comment: 7 pages, 6 figure

Laser power stabilization via radiation pressure

This Letter reports the experimental realization of a novel, to the best of our knowledge, active power stabilization scheme in which laser power fluctuations are sensed via the radiation pressure driven motion they induce on a movable mirror. The mirror position and its fluctuations were determined by means of a weak auxiliary laser beam and a Michelson interferometer, which formed the in-loop sensor of the power stabilization feedback control system. This sensing technique exploits a nondemolition measurement, which can result in higher sensitivity for power fluctuations than direct, and hence destructive, detection. Here we used this new scheme in a proof-of-concept experiment to demonstrate power stabilization in the frequency range from 1 Hz to 10 kHz, limited at low frequencies by the thermal noise of the movable mirror at room temperature

Arm length stabilisation for advanced gravitational wave detectors

Currently the Laser Interferometric Gravitational-wave Observatory (LIGO) is undergoing upgrades from Initial LIGO to become Advanced LIGO. Amongst these upgrades is the addition of a signal recycling mirror at the output port of the interferometer; upgrades of the mirror suspensions to quadruple pendulums; the implementation of less invasive and hence weaker test mass actuators; and the change of readout scheme from a heterodyne based RF readout to a homodyne based DC readout. The DC readout scheme requires the installation of an Output Mode Cleaner (OMC), to stop `junk light' generated in the interferometer from making its way to the DC photodetector where it can limit the sensitivity of the gravitational wave detector. The steering of the interferometer beam into the OMC will be handled by Tip Tilt mirrors designed at the Australian National University. The first core piece of work presented in this thesis was the characterisation of a prototype Tip Tilt mirror, which involved measuring the various eigenmodes of the mirror

A High-Finesse Suspended Interferometric Sensor for Macroscopic Quantum Mechanics with Femtometre Sensitivity

We present an interferometric sensor for investigating macroscopic quantum mechanics on a table-top scale. The sensor consists of a pair of suspended optical cavities with finesse over 350,000 comprising 10 g fused silica mirrors. The interferometer is suspended by a four-stage, light, in-vacuum suspension with three common stages, which allows for us to suppress common-mode motion at low frequency. The seismic noise is further suppressed by an active isolation scheme, which reduces the input motion to the suspension point by up to an order of magnitude starting from 0.7 Hz. In the current room-temperature operation, we achieve a peak sensitivity of 0.5 fm/Hz in the acoustic frequency band, limited by a combination of readout noise and suspension thermal noise. Additional improvements of the readout electronics and suspension parameters will enable us to reach the quantum radiation pressure noise. Such a sensor can eventually be utilized for demonstrating macroscopic entanglement and for testing semi-classical and quantum gravity models