A Study on Motion Reduction for Suspended Platforms Used in Gravitational Wave Detectors

We report a reduction in motion for suspended seismic-isolation platforms in a gravitational wave detector prototype facility. We sense the distance between two seismic-isolation platforms with a suspension platform interferometer and the angular motion with two optical levers. Feedback control loops reduce the length changes between two platforms separated by 11.65 m to 10 pm Hz^-1/2 at 100 mHz , and the angular motion of each platform is reduced to 1 nrad Hz^-1/2 at 100 mHz . As a result, the length fluctuations in a suspended optical resonator on top of the platforms is reduced by three orders of magnitude. This result is of direct relevance to gravitational wave detectors that use similar suspended optics and seismic isolation platforms.