Optical characterization of high performance mirrors based on cavity ringdown time measurements with 6 degrees of freedom mirror positioning.

An instrument capable of measuring optical losses, transmission, and the radius of curvature of high reflectivity mirrors is presented. The measurement setup consists of two remote controlled hexapod systems with 6 degrees of freedom placed inside a vacuum enclosure. Mirror loss measurements are performed via the cavity ring-down time method using a linear resonant two-mirror Fabry-Perot cavity configuration. The use of high-precision positioning systems enables cavity loss mapping by transversely scanning the position of the cavity end mirror. Mirror surfaces of up to 30 mm in diameter can be scanned, and the cavity length can be tuned by 120 mm.

Progress in the measurement and reduction of thermal noise in optical coatings for gravitational-wave detectors.

Coating thermal noise is a fundamental limit for precision experiments based on optical and quantum transducers. In this review, after a brief overview of the techniques for coating thermal noise measurements, we present the latest worldwide research activity on low-noise coatings, with a focus on the results obtained at the Laboratoire des Matériaux Avancés. We report new updated values for the Ta2O5, Ta2O5-TiO2, and SiO2 coatings of the Advanced LIGO, Advanced Virgo, and KAGRA detectors, and new results from sputtered Nb2O5, TiO2-Nb2O5, Ta2O5-ZrO2, MgF2, AlF3, and silicon nitride coatings. Amorphous silicon, crystalline coatings, high-temperature deposition, multi-material coatings, and composite layers are also briefly discussed, together with the latest developments in structural analyses and models.

Mirrors used in the LIGO interferometers for first detection of gravitational waves.

For the first time, direct detection of gravitational waves occurred in the Laser Interferometer Gravitational-wave Observatory (LIGO) interferometers. These advanced detectors require large fused silica mirrors with optical and mechanical properties and have never been reached until now. This paper details the main achievements of these ion beam sputtering coatings.

Feedback control of thermal lensing in a high optical power cavity.

This paper reports automatic compensation of strong thermal lensing in a suspended 80 m optical cavity with sapphire test mass mirrors. Variation of the transmitted beam spot size is used to obtain an error signal to control the heating power applied to the cylindrical surface of an intracavity compensation plate. The negative thermal lens created in the compensation plate compensates the positive thermal lens in the sapphire test mass, which was caused by the absorption of the high intracavity optical power. The results show that feedback control is feasible to compensate the strong thermal lensing expected to occur in advanced laser interferometric gravitational wave detectors. Compensation allows the cavity resonance to be maintained at the fundamental mode, but the long thermal time constant for thermal lensing control in fused silica could cause difficulties with the control of parametric instabilities.

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.

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.