GEO600

GEO600 is a gravitational wave detector located near Sarstedt in the South of Hanover, Germany. This instrument, and its sister interferometric detectors, when operational, are some of the most sensitive gravitational wave detectors ever designed. They are designed to detect relative changes in distance of the order of 10⁻²¹, about the size of a single atom compared to the distance from the Sun to the Earth. GEO600 is capable of detecting gravitational waves in the frequency range 50 Hz to 1.5 kHz. Construction on the project began in 1995.

Hardware

GEO600 is a Michelson interferometer. It consists of two 600 meter long arms, which the laser beam passes twice, so that the effective optical arm length is 1200 m. The major optical components are located in an ultra-high vacuum system. The pressure is in the range of 10⁻⁸ mbar.

Suspensions and seismic isolation

For precise measurements, the optics must be isolated from ground motion and other influences from the environment. For this reason, all ground-based interferometric gravitational wave detectors suspend their mirrors as multi-stage pendulums. For frequencies above the pendulum resonance frequency, pendulums provide a good isolation against vibrations. All the main optics of GEO600 are suspended as triple pendulums, to isolate the mirrors from vibrations in the horizontal plane. The uppermost and the intermediate mass are hung from cantilever springs, which provide isolation against vertical movement. On the uppermost mass are six coil- magnet actuators that are used to actively dampen the pendulums. Furthermore, the whole suspension cage sits on piezo crystals. The crystals are used for an ‘active seismic isolation system’. It moves the whole suspension in the opposite direction of the ground motion, so that ground motion is cancelled.