The GEO 600 detector is a 600m baseline interferometer in Hannover, Germany. We begin by discussing the history of this detector and go on to describe the techniques used to achieve initial target sensitivity. We then describe a series of upgrades and advanced techniques that will increase the sensitivity of this instrument for frequencies above 500 Hz. We finish by summarising the future plans of GEO 600.

A bit of history

GEO 600 is a British/German gravitational wave detector (see Grote for the LIGO Scientific Collaboration, 2008) located in Germany close to the city of Hannover. The scientific goal of GEO 600, beside taking data for gravitational wave detection, is the demonstration and testing of techniques for advanced detectors. GEO evolved from a collaboration between the groups working on the Garching 30m and the Glasgow 10m prototypes. In 1989 these groups proposed to build an underground 3 km gravitational wave detector, ‘GEO’, in the Harz mountains in northern Germany (Hough et al., 1989) based on earlier proposals (Maischberger et al., 1985; Leuchs et al., 1987). Although reviewed positively, a shortage of funds on both ends, in the British Science and Engineering Research Council and after the German reunification also in the German funding bodies, made the realisation of this large project impossible. The collaborators decided to try obtaining funds for a shorter detector and compensate for the shortness by implementing more advanced techniques. A suitable stretch of land to build a 600m instrument was found 20 km south of the city of Hannover, owned by the Universität Hannover and the state of Lower Saxony.