Low-frequency active vibration isolation for advanced LIGO

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.