A double pendulum vibration isolation system for a laser interferometric gravitational wave antenna

Abstract

We have developed a nested double pendulum suspension system for the test masses of a laser interferometric gravitational wave antenna. The system consists of a mass hung as a pendulum inside a shell mass that is also hung as a pendulum. A set of two-degree-of-freedom reflective ‘‘shadow detectors’’ senses motion of the shell relative to ground. Identical sensors measure motion of the mass relative to the shell. The equations of motion were solved to find the resonances and mode shapes for all of the rigid body degrees of freedom. The predicted resonant frequencies agree well with the measured frequencies. A damping system has been implemented that damps the resonances by applying forces to the shell mass alone. The vibration transfer function along the optic axis was measured. It shows the steep f−4 decline expected of a double pendulum. We have also measured the vertical vibration transfer function and the cross coupling due to misalignment. A set of plates on the inner surface of the shell allows the application of low noise electrostatic forces directly to the test mass for high-bandwidth control such as interferometer fringe lock. We have measured the response of the system to this input, and compared it to that predicted by our model equations of motion. We have determined that there exist stable feedback loops that can maintain fringe lock. The possibilities of active isolation are discussed.