Calibration of the power-recycled gravitational wave detector, GEO 600

GEO 600 is an interferometric gravitational wave detector which, when completed, will employ a novel, dual-recycled optical scheme allowing its optical response to be tuned over a range of frequencies. The accurate calibration of the sensitivity of GEO to gravitational waves is a critical part of preparing GEO data for exchange with other detectors worldwide and for the extraction of scientific results. The final optical scheme that will be used in GEO will make it necessary to combine more than one output signal to optimally recover a calibrated strain channel. An on-line calibration scheme has been developed with a view to covering both the power-recycled and dual-recycled interferometer configurations of GEO. The recent worldwide coincident science run (S1) proved an excellent opportunity to test the calibration scheme of GEO in a power-recycled mode. The ability to track and study the stability of the calibration over long time scales was invaluable. A number of areas have been highlighted where improvement is necessary, but the scheme proved successful and produced a continuous real-time h(t) for the entire duration of the S1 science run. Consideration of the errors discussed throughout this article lead to a calibration accuracy for this science run of around 4% at high frequencies (>200 Hz) and about 6% for frequencies between about 50 and 200 Hz. This article presents the on-line calibration scheme that was developed for, and employed during, the recent S1 science run. The hardware signal injection is presented in detail followed by the on-line analysis method that allows real-time calibration of this interferometer configuration. The results of the calibration process are discussed and attention is drawn to the successes and to the points where improvement can be made. (C) 2003 American Institute of Physics.