Low frequency vibration isolation and alignment system for Advanced LIGO

The Laser Interferometer Gravitational-Wave Observatory (LIGO) is dedicated to the observation of astrophysical sources through detection of gravitational waves. In order to reach the sensitivity requirements for Advanced LIGO, the isolation and alignment system for the optics must reduce the RMS of seismic motion at all frequencies. At the LIGO observatories, the seismic motion has peaks around 0.15 Hz in all three translational degrees of freedom which dominate the differential RMS motion of the ground. The isolation system needs to simultaneously reduce the seismic peak magnitude by at least a factor of five in all three degrees of freedom. At frequencies from 1 Hz to 10 Hz, the isolation system needs to achieve an isolation factor of 1000 to 2000.; Tilt-horizontal coupling is the most challenging problem for low-frequency seismic isolation systems. Tilt-horizontal coupling comes from the principle of equivalence: horizontal inertial sensors cannot distinguish between horizontal acceleration and tilt motion. The magnitude of tilt-horizontal coupling rises very rapidly at low frequencies, and this makes low frequency isolation difficult. A variety of techniques, including sensor correction and sensor blending, are used to address the tilt-horizontal coupling problem. Optimal FIR complementary filters were designed to separate efficiently tilt motion from horizontal acceleration. A nonlinear analysis technique was developed to study the nonlinear tilt-horizontal coupling effect. With these techniques, our prototype vibration isolation systems obtained isolation performance that is very close to the requirement of Advanced LIGO.