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.
LIGO is dedicated to the detection of gravitational waves. To achieve the design sensitivity of the proposed Advanced LIGO detectors, the seismic isolation system is required to isolate the interferometer mirrors from ground motion above 0.1 Hz. The dominant source of motion above 0.1 Hz is the microseismic peaks near 0.15 Hz. The system needs to isolate the payload from this motion by at least a factor of five in all three translational degrees of freedom.
Tilt-horizontal coupling is the most challenging problem for
seismic isolation below 1 Hz. Tilt-horizontal coupling results from the principle of equivalence: inertial horizontal sensors cannot distinguish horizontal acceleration from tilt motion. Tilt-horizontal coupling rises dramatically at low frequencies, which makes low frequency isolation difficult.
Several techniques are used to address the tilt-horizontal
coupling problem. The isolation platform is designed to separate
horizontal motions from tilt motions. Feedback control to
displacement sensors is used to command the platform in all
degrees of freedom. These sensors are "corrected" by ground
seismometers, using an optimal FIR filtering technique to separate
tilt noise from horizontal acceleration. With these techniques, we
obtained isolation factors of 10 to 20 simultaneously in all three
degrees of freedom at 0.15 Hz.