User:Hooh1/GEO600Draft

GEO600
GEO600 is a German-British gravitational wave detector located near Sarstedt, in the South of Hanover, Germany. It is operated by the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI). The goal of GEO600 is the direct observation of gravitational waves. Until today, gravitational waves have not been measured directly. However, they have been observed indirectly by Hulse and Taylor, and confirmed by observations on other binary systems (OJ287 and PSR J0737-3039).

Hardware
GEO600 is a Michelson (Laser-) 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 optic components are located in an ultra-high vacuum system.

Laser
GEO600 uses an YAG laser with an output power of about 10 W at a wavelength of 1064 nm.

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 pendulum resonances. 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.

Optics
The main mirrors of GEO600 are cylinders of fused silica with a diameter of 18 cm and a height of 10 cm. The beam splitter (with dimensions of 26 cm diameter and 8 cm thickness) is the only transmissive piece of optics in the high power path, therefore it was made from special grade fused silica. Its absorption has been measured to be smaller than 0.5 ppm.

Advanced features
GEO600 uses many advanced techniques and hardware that are planned to be used in the next generation of ground based gravitational wave detectors:


 * Signal recycling: An additional mirror at the output of the interferometer forms a resonant cavity together with the end mirrors and thus increases a potential signal.


 * Tuned SR


 * Monolithic suspensions: The mirrors are suspended as pendulums. While steel wires are used for secondary mirrors, GEO’s main mirrors are hanging from so called ‘monolithic’ suspensions. This means that the wires are made from the same material as the mirror: fused silica. The reason is that fused silica has less mechanical losses, and losses lead to noise.


 *  Electrostatic drives: Actuators are needed to keep the mirrors in their position and to align them. Secondary mirrors of GEO600 have magnets glued to them for this purpose. They can then be moved by coils. Since gluing magnets to mirrors will increase mechanical losses, the main mirrors of GEO600 use electrostatic drives (ESDs). The ESDs are a comb like structure of electrodes at the backside of the mirror. If a voltage is applied to the electrodes, they produce an inhomogeneous electric field. The mirror will feel a force in this field.


 *  Thermal mirror actuation system: A circular heater is sitting behind the far east mirror. Due to inhomogeneous thermal expansion the radius of curvature of the mirror changes. The heater allows thermal tuning of the mirror’s radius of curvature.


 * Output Mode Cleaner (OMC): An additional cavity at the output of the interferometer in front of the photodiode. Its purpose is to filter out light that does not potentially carry a gravitational wave signal.


 * Homodyne detection (also called ‘DC readout’)


 * Squeezing: Squeezed vacuum is injected into the dark port of the beam splitter. The use of squeezing improves the sensitivity of GEO600 above 700Hz by a factor of 1.5 [6].

A further difference to other projects is that GEO600 has no arm cavities.

The GEO-HF upgrade
The upgrade program ‘GEO-HF‘ started in 2009 and aims at a sensitivity improvement for frequencies above 100Hz. A big part of the upgrade focuses on increasing the circulating light power, since at high frequencies GEO is limited by the laser power. The update will feature a new laser with 30 W output, and new mirrors for the input mode cleaner with higher transmission. It is planned to reach about 20 kW of circulating laser power.

Sensitivity and measurements
The sensitivity for gravitational wave strain is usually measured in amplitude spectral density (ASD). The peak sensitivity of GEO600 in this unit is 10-22 1/sqrt(Hz) at 1 kHz. At high frequencies the sensitivity is limited by the available laser power. At the low frequency end, the sensitivity of GEO600 is limited by seismic ground motion.

Data/ Einstein@home
Not only the output of the main photodiode is registered, but also the output of a number of secondary sensors, for example photodiodes that measure auxiliary laser beams, microphones, seismometers, accelerometers, magnetometers and the performance of all the control circuits. These secondary sensors are important for diagnosis and to detect environmental influences on the interferometer output. With these sensors, GEO600 records about 100 GB of data per day. The data stream is partly analyzed by the distributed computing project ‘Einstein@home’, software that volunteers can run on their computer.

From the end of 2011, both VIRGO and the LIGO detectors will be shut down for upgrades, leaving GEO600 as the only operating large scale laser interferometer searching for gravitational waves.

Results
GEO600 has not detected gravitational waves yet. However, with the known sensitivity, it is possible to deduce upper limits on the gravitational wave emission of pulsars [3] and on the gravitational wave background (reference?).

Facts:

 * 600 m arm length
 * 400 m^3 vacuum at 10-8 mBar
 * 2.7 kW circulating laser power
 * 18 cm mirror diameter