A significant limiting factor on the sensitivity of interferometric gravitational wave detectors has been identified as thermal noise generated by mechanical loss in the high reflectivity dielectric mirror coatings on the test masses. The development of coatings which maintain high optical performance and minimize mechanical loss is therefore vital if the current designs of interferometers are to achieve adequate sensitivity. While the origins of the mechanical loss are yet to be fully elucidated, some progress has been made toward minimizing it, although there is still some way to go before specifications can be met. The work reported here is progress made toward achieving low mechanical loss coatings on behalf of the LIGO consortium. The current directions include attempts to reduce the loss in the coating materials by control of the coating stoichiometry and intrinsic stress. This includes such methods as ion bombardment of the growing films and optimization of post-deposition thermal treatments.
Einstein's General Theory of Relativity predicts waves in spacetime caused by oscillating masses. Such waves, known as gravitational waves, are predicted to be created by binary black hole or neutron star inspirals, super-nova, or other catastrophic astronomical events. Even with such large masses moving so repidly, the expected size of the waves is extremely small, typically of order 10<sup>-21</sup> in unitless strain as seen on Earth. LIGO, the Laser Interferometer Gravitational Wave Observatory, is a basic physics experiments designed to detect and study these waves. The next generation interferometers, known as Advanced LIGO, are currently being designed. Thermal noise from mechanical loss in the optical coatings of the mirrors is expected to be an important limiting noise source. Reducing this noise by developing lower mechanical loss coatings, while preserving optical and thermal properties needed in the interferometer, is an area of active research.
Gravitational waves are a prediction of Einstein's General Theory of Relativity. Astrophysical events like supernova and binary neutron star inspirals are predicted to create potentially detectable waves. The Laser Interferometer Gravitational-wave Observatory (LIGO) is an experiment to detect these waves using Michelson interferometers with 4 km long arms. The effect of gravitational waves, even on an interferometer with such a long baseline, is extremely, with mirror displacements around 10<sup>-18</sup>m. Reducing noise is thus a primary design criterion. For the next generation interferometers now being designed, thermal noise from the optical coatings of the interferometer mirrors could prove a problematic limiting noise source. Reducing the mechanical loss of these coatings to improve thermal noise, while preserving the sub-ppm optical absorption, low scatter, and high reflectivity needed in the interferometer is an important area of research.
To meet the overall isolation and alignment requirements for the optics in Advanced LIGO, the planned upgrade to LIGO, the US laser interferometric gravitational wave observatory, we are developing three sub-systems: a hydraulic external pre-isolator for low frequency alignment and control, a two-stage active isolation platform designed to give a factor of ~1000 attenuation at 10 Hz, and a multiple pendulum suspension system that provides passive isolation above a few hertz. The hydraulic stage uses laminar-flow quiet hydraulic actuators with millimeter range, and provides isolation and alignment for the optics payload below 10 Hz, including correction for measured Earth tides and the microseism. This stage supports the in-vacuum two-stage active isolation platform, which reduces vibration using force feedback from inertial sensor signals in six degrees of freedom. The platform provides a quiet, controlled structure to mount the suspension system. This latter system has been developed from the triple pendulum suspension used in GEO 600, the German/UK gravitational wave detector. To meet the more stringent noise levels required in Advanced LIGO, the baseline design for the most sensitive optics calls for a quadruple pendulum, whose final stage consists of a 40 kg sapphire mirror suspended on fused silica ribbons to reduce suspension thermal noise.
The LIGO project has completed the installation of large fused silica optical components in the vacuum systems of its observatories. Commissioning work on the Hanford 2 km interferometer has determined an upper limit to the optics losses, allowing comparison with design and pre-installation testing. Planning and development of sapphire optics for the next generation, advanced LIGO detector is now underway, including polishability, optical homogeneity, absorption, and birefringence. The advanced optics development also includes research aimed at lowering coating loss.