L. Ju

Parametric instabilities and their control in advanced interferometer gravitational-wave detectors.

A detailed simulation of Advanced LIGO test mass optical cavities shows that parametric instabilities will excite 7 acoustic modes in each fused silica test mass, with parametric gain R up to 7 and only 1 acoustic mode with R approximately 2 for alternative sapphire test masses. Fine-tuning of the test mass radii of curvature causes the instabilities to sweep through various modes with R as high as approximately 2000. Sapphire test mass cavities can be tuned to completely eliminate instabilities using thermal g-factor tuning. In the case of fused silica test mass, instabilities can be minimized but not eliminated.

Vibration isolation performance of an ultra-low frequency folded pendulum resonator

Abstract We present an analysis of the transfer function of a very low frequency folded pendulum resonator. It is shown that performance depends critically on centre of percussion tuning of the pendulum arms. Experimental measurements of the transfer function are shown to agree well with theory. The isolator achieves 90 dB isolation at 7 Hz.

Performance of an ultra low-frequency folded pendulum

Abstract A folded pendulum consisting of the sum of a conventional compound pendulum and an inverse pendulum can be tuned to create a pendulum with the dynamic response of a kilometre scale pendulum. Results are presented demonstrating the stability and Q -factor of the system down to a resonant frequency of 17 mHz. It can be scaled up to form a pre-isolation stage in the vibration isolation system for laser interferometer gravitational wave detectors.

Near-shore ocean wave measurement using a very low frequency folded pendulum

A very low frequency folded pendulum has been used to measure the horizontal ground displacement at a site near the Indian Ocean. The results show a close correlation between the displacement and ocean wave height. This suggests that such a pendulum can provide a simple land-based wave measurement system and, in an array, can provide tomographic images of the wave height along extended shorelines.

Ultrahigh Q pendulum suspensions for gravitational wave detectors

Pendulum suspensions for laser interferometer gravitational wave detectors need to have an extremely high Q factor to minimize Brownian motion noise. In this paper we analyze the limits to the Q factor of the compound pendulum. We show that the observed acoustic loss of niobium can allow pendulum Q factors of 1010 to be achieved. This should enable a 3 km terrestrial laser interferometer detector to achieve strain sensitivity of 10−22/√Hz at frequencies as low as 10 Hz. At cryogenic temperatures Q factors up to 1012 should be achievable.

Observation of Parametric Instability in Advanced LIGO

Parametric instabilities have long been studied as a potentially limiting effect in high-power interferometric gravitational wave detectors. Until now, however, these instabilities have never been observed in a kilometer-scale interferometer. In this Letter, we describe the first observation of parametric instability in a gravitational wave detector, and the means by which it has been removed as a barrier to progress.

Low resonant frequency cantilever spring vibration isolator for gravitational wave detectors

An all metal multistage low frequency vibration isolator for a laser interferometer gravitational wave detector has been built and tested with a very sensitive accelerometer. Curved cantilever springs are used to form a low frequency compact form of isolation element. Near center of mass suspension of the isolation stages reduces cross coupling between orthogonal directions. The isolator has a low‐pass corner frequency of 6.5 Hz vertically and 1.8 Hz horizontally. The system shows low temperature coefficients and absence of creep. Results demonstrate that steel curved springs near their elastic limit do not generate broadband acoustic noise at the 10−15 m /√Hz level.

Sapphire beamsplitters and test masses for advanced laser interferometer gravitational wave detectors

We present a feasibility study of using sapphire beamsplitters and test masses in laser interferometer gravitational wave detectors. The internal thermal noise, optical losses and birefringence effects are analysed. Suspension losses are investigated. Experimental data on birefringence is presented. The conclusions are generally positive.

Compensation of Strong Thermal Lensing in High-Optical-Power Cavities

In an experiment to simulate the conditions in high optical power advanced gravitational wave detectors, we show for the first time that the time evolution of strong thermal lenses follows the predicted infinite sum of exponentials (approximated by a double exponential), and that such lenses can be compensated using an intracavity compensation plate heated on its cylindrical surface. We show that high finesse approximately 1400 can be achieved in cavities with internal compensation plates, and that mode matching can be maintained. The experiment achieves a wave front distortion similar to that expected for the input test mass substrate in the Advanced Laser Interferometer Gravitational Wave Observatory, and shows that thermal compensation schemes are viable. It is also shown that the measurements allow a direct measurement of substrate optical absorption in the test mass and the compensation plate.

Tests on a low-frequency inverted pendulum system

The authors report the performance and mechanical properties of an inverted pendulum as a vibration isolator. The advantages of such a system for vibration isolation in an interferometric gravitational wave detector is demonstrated through modelling of the isolators response. An experimental investigation using a prototype inverted pendulum was also performed where the behaviour of the stability, resonant frequency and Q factor with varying load mass was examined. The pendulum was also used as a seismometer to examine low-frequency seismic activity.