Harald Lück, for the GEO team, Universität Hannover, Germany
The construction of the GEO600 gravitational-wave detector, a German-British collaboration of the University of Glasgow, the Max-Planck-Institut für Quantenoptik at Garching and Hannover, the Universität Hannover, the Laser-Zentrum Hannover, the Albert-Einstein-Institut Potsdam, and the University of Wales College of Cardiff is well on its way. Short summaries on some aspects of the current work may give an impression of the status of the project.
Civil engineering: All the civil engineering work for the buildings and the clean-rooms was finished about a year ago and gives a clean room class in the inner sections of the stations of < 1000. A clean tent for obtaining class <100 clean-room environment in the vicinity of the tanks is currently being set up.
Vacuum system: The vacuum system of GEO600 approaches completion. Both 600m long tubes are installed and evacuated. One of them has been baked by passing a DC current of up to 600A through the tube. The tube got an air bake for two days at about 200C and a vacuum bake at about 250C for a week. The current pressure in the baked tube is at about mbar limited by an air leak which remains to be found. The second tube where the pressure is still dominated by water vapour is already thermally insulated and will be baked in a few weeks time.
The vacuum system in the central station can be separated into two subsystems by a gate valve: 1) The mode-cleaner vacuum system is installed, evacuated, and baked. Currently it is disconnected from the main central cluster and pumped by a separate vacuum pump; 2) The central cluster, where all the tanks are on site and are currently being positioned and anchored.
Mirror suspensions: The stacks for the mode cleaner tanks, i.e. three legs per tank of alternating rubber and stainless steel layers that carry a frame structure from which the mirrors are suspended, are manufactured and will shortly be installed. The whole vacuum system of GEO600 is entirely made of metal. To avoid contamination of the vacuum system by the rubber of these stacks they are welded into stainless steel bellows that will be pumped seperately. The suspensions for the mode cleaner mirrors which will be hung as double pendulums is halfway through the workshop.
The stacks for those tanks that will contain the main optics will include an active anti-seismic stage consisting of piezo actuators and geophones in a digital feedback loop. All the hardware is purchased and a suppression factor of about 20dB has recently been demonstrated in the sub-Hertz frequency range.
The main optics will be suspended as triple pendulums with a monolithic fused silica suspension for the lowest stage. Sufficiently high mechanical Q-factors have been demonstrated by the Glasgow arm of the GEO600 collaboration.
Electronics: The local control boards are just moving from the construction into the manufacturing phase. Data communication via a 6Mbit/s radio link to Hannover and from there into the internet works to our satisfaction. The only difficulty the glass fibre data transport along the interferometer arms encountered were mice who in winter time, for the lack of other food, checked whether glass fibres provide a healthy diet.
Optics: The optical layout of GEO600 has been changed from external modulation to Schnupp (frontal) modulation hence avoiding the need of an additional Mach-Zehnder Interferometer at the Michelson output. In addition the control signal for the differential Michelson length control and also the control signal for the signal recycling mirror can be obtained with one modulation frequency by picking up the signals at different locations in the detector. This technique has successfully been demonstrated at the 30m prototype in Garching.
In the time it takes the 10W slave laser to come from a prototype stage to meet the specifications we will use a stabilised 1W Nd-YAG laser, which works well enough to start with.
Additional information about the status and the schedule of GEO600 can be found in the GEO600 web pages: http://www.geo600.uni-hannover.de.