C. D. Blair

All-sky search for long-duration gravitational wave transients with LIGO

We present the results of a search for long-duration gravitational wave transients in two sets of data collected by the LIGO Hanford and LIGO Livingston detectors between November 5, 2005 and September 30, 2007, and July 7, 2009 and October 20, 2010, with a total observational time of 283.0 days and 132.9 days, respectively. The search targets gravitational wave transients of duration 10 - 500 seconds in a frequency band of 40 - 1000 Hz, with minimal assumptions about the signal waveform, polarization, source direction, or time of occurrence. All candidate triggers were consistent with the expected background; as a result we set 90% confidence upper limits on the rate of long-duration gravitational wave transients for different types of gravitational wave signals. We also report upper limits on the source rate density per year per Mpc^3 for specific signal models. These are the first results from an all-sky search for unmodeled long-duration transient gravitational waves.

Parametric instability in long optical cavities and suppression by dynamic transverse mode frequency modulation

Three-mode parametric instability has been predicted in advanced gravitational wave detectors. Here we present the first observation of this phenomenon in a large scale suspended optical cavity designed to be comparable to those of advanced gravitational wave detectors. Our results show that previous modeling assumptions that transverse optical modes are stable in frequency except for frequency drifts on a thermal deformation time scale is unlikely to be valid for suspended mass optical cavities. We demonstrate that mirror figure errors cause a dependence of transverse mode offset frequency on spot position. Combined with low-frequency residual motion of suspended mirrors, this leads to transverse mode frequency modulation which suppresses the effective parametric gain. We show that this gain suppression mechanism can be enhanced by laser spot dithering or fast thermal modulation. Using Advanced LIGO test-mass data and thermal modeling, we show that gain suppression factors of 10–20 could be achieved for individual modes, sufficient to greatly ameliorate the parametric instability problem.

Radiation pressure excitation of test mass ultrasonic modes via three mode opto-acoustic interactions in a suspended Fabry-Perot cavity

Abstract Three-mode parametric instabilities may compromise stable operation of gravitational wave detectors. Instabilities manifest as varying radiation pressure distributions, derived from beating between two optical modes, exciting mirror acoustic modes in Fabry–Perot cavities. Here we report the first demonstration of radiation pressure driving of ultrasonic acoustic modes via pairs of optical modes in gravitational wave type optical cavities. In this experiment ∼ 0.4 W of TEM 01 mode and ∼ 1 kW of TEM 00 mode circulated inside the cavity, an ∼ 181.6 kHz excitation was observed with amplitude ∼ 5 × 10 − 13 m . The results verify the driving force term in the parametric instability feedback model (Braginsky et al., 2001) [1] . The interaction parametric gain was ( 3.8 ± 0.5 ) × 10 − 3 and mass-ratio scaled opto-acoustic overlap 2.7 ± 0.4 .

Three mode interactions as a precision monitoring tool for advanced laser interferometers

Many thousands of three mode opto–acoustic interactions are expected to be observable in the advanced laser interferometer gravitational wave detectors now under construction. Each interaction represents a high-Q acoustic resonance interacting with high order optical modes inside the interferometer. This paper shows that this huge set of signals between 10–100 kHz have high sensitivity to changes in the optical wavefronts within the interferometer and can be used to create a powerful probe of the entire interferometer. We show that 3MI signals can be used to monitor thermal distortions corresponding to wavefront changes ~3 × 10−12 m. Observations can be used at low optical power to predict parametric instabilities that could occur at higher power. In addition, the observed mode amplitudes could be used to control the interferometer operating point against slow environmental perturbations. Data on 80 m cavities and modelling results are used to demonstrate the sensitivity of 3MI monitoring. Experimental observations on advanced interferometers are suggested as a means to turn 3MI monitoring into an effective tool.

Modular suspension system with low acoustic coupling to the suspended test mass in a prototype gravitational wave detector

Low acoustic loss suspension systems are essential components in low thermal noise instruments including gravitational wave detectors. Monolithic fused silica suspensions have been used successfully with fused silica test masses but may not be suitable in next generation detectors that may use sapphire or silicon test masses. Here we report a study of a modular suspension system with high replaceability. The system is based on high pressure gravitationally attached mechanical contacts which have been previously shown to contribute low acoustic losses to sapphire resonators. Here we combine high pressure contacts with cantilevers and fibres to create sets of four suspension modules which are shown to have low loss contributions to fused silica test masses in a 74-m high-finesse optical cavity. Results are combined with finite element simulations to estimate the strain energy distributions of the eigenmodes. By combining the simulations and measurement results, the test mass loss angle due to the coupling to the suspension system was estimated. The modular suspension system is shown to contribute <10% to the total test mass acoustic loss. Such suspension systems could have applications for test masses or subsystems in next generation gravitational wave detectors.

The development of ground based gravitational wave astronomy and opportunities for Australia–China collaboration

This paper begins by reviewing the development of gravitational wave astronomy from the first predictions of gravitational waves to development of technologies across the entire gravitational wave spectrum, and then focuses on the current status of ground based gravitational wave detectors. With substantial improvements already demonstrated in early commissioning it is emphasised that Advanced detectors are on track for first detection of gravitational waves. The importance of a worldwide array of detectors is emphasised, and recent results are shown that demonstrate the continued advantage of a southern hemisphere detector. Finally it is shown that a north–south pair of 8 km arm length detectors would give rise to a dramatic improvement in event rate, enabling a pair of detectors to encompass a 64-times larger volume of the universe, to conduct a census on all stellar mass black hole mergers to z > 1 and to observe neutron star mergers to a distance of ∼800 Mpc.