D. H. Shoemaker

LIGO: The Laser Interferometer Gravitational-Wave Observatory

The goal of the Laser Interferometer Gravitational-Wave Observatory (LIGO) Project is to detect and study astrophysical gravitational waves and use data from them for research in physics and astronomy. LIGO will support studies concerning the nature and nonlinear dynamics of gravity, the structures of black holes, and the equation of state of nuclear matter. It will also measure the masses, birth rates, collisions, and distributions of black holes and neutron stars in the universe and probe the cores of supernovae and the very early universe. The technology for LIGO has been developed during the past 20 years. Construction will begin in 1992, and under the present schedule, LIGOs gravitational-wave searches will begin in 1998.

Seismic isolation enhancements for initial and Advanced LIGO

A seismic isolation system for the proposed Advanced LIGO detector upgrade is under development. It consists of a two-stage in-vacuum active isolation platform that is supported by an external hydraulic actuation stage. A full-scale preliminary-design technology demonstrator of the in-vacuum platform has been assembled and is being tested at Stanfords engineering test facility. Unanticipated excess ground motion from local human activity at LIGO Livingston has prompted accelerated development of the external stage for installation and use in the initial Livingston detector. As an interim measure, active external isolation in the laser beam direction is implemented using existing PZT external actuators.

Low‐noise rf capacitance bridge transducer

We have designed a low‐noise servo system to both damp the motion of a high Q pendulum as well as measure the ground noise that drives it. The servo is based on a high‐frequency low‐noise capacitance bridge which is used as a displacement transducer for sensing the position of the pendulum. The bridge itself is servoed to minimize the need to adjust its components because of slow mechanical thermal drifts. Used as a displacement transducer we have achieved a position sensitivity of 4×10−11 cm/√Hz.

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.

Effects of electrical charging on the mechanical Q of a fused silica disk

We report on the effects of an electrical charge on mechanical loss of a fused silica disk. A degradation of Q was seen that correlated with charge on the surface of the sample. We examine a number of models for charge damping, including eddy current damping, and loss due to polarization. We conclude that rubbing friction between the sample and a piece of dust attracted by the charged sample is the most likely explanation for the observed loss.

Gravitational waves: from astrophysics to optics

Gravitational waves, predicted by Einsteins general theory of relativity, will be a unique probe of violent astrophysical events. The precision measurement challenges and successes for interferometric detectors, now starting to observe, take optics to extremes.